Advances in Electronics and Electron Phisics. Vol. 43

ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS

VOLUME 43

CONTRIBUTORS TO

THISVOLUME

G. Donelli L. Fiermans John M. Houston John Kelly L. Paoletti Robert K. Swank J. Vennik Kirby G. Vosburgh

Advances in

Electronics and Electron Physics EDITEDBY L. MARTON Smithsonian Institution, Washington, D.C. Assistant Editor

CLAIRE MARTON EDITORIAL BOARD E. R. Piore T. E. Allibone M. Ponte H. B. G. Casimir A. Rose W. G. Dow L. P. Smith A. 0.C. Nier F. K. Willenbrock

VOLUME 43

1977

ACADEMIC PRESS New York San Francisco London A Subsidiary of Harcourt Brace Jovanovich, Publishers

COPYRIGHT 0 1977, BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN A N Y FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDINQ, O R ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRlTlNQ FROM THE PUBLISHER.

ACADEMIC PRESS, INC.

111 Fifth Avenue,New York. New York 10003

United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road. London NWI

LIBRARY OF CONGRESS CATALOG

CARD

NUMBER: 49-7504

ISBN 0-12-014643-6 PRINTED IN THE UNITED STATES OF AMERICA

CONTRIBUTORS TO VOLUME43 . . . . . FOREWORD . . . . . . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . .

vii

ix

Electron Micrograph Analysis by Optical Transforms G . DONELLI A N D L. PAOLEITI I . Introduction . . . . . . . . . . . . . . I1 . Image Formation in an Electron Microscope . . I11. Optical Processing of Electron Microscopic Data IV. Applications . . . . . . . . . . . . . . References . . . . . . . . . . . . . . .

. . . . .

. . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

1 2 12 23 40

Recent Advwes in Electron Beam Addreesed Memories JOHNKELLY

.

I Introduction . . . . . . . . . . . I1. Background . . . . . . . . . . . 111. StorageMedia . . . . . . . . . . IV. Surface Charge Storage . . . . . . V. Bulk Charge Storage . . . . . . . VI. Alternative Storage Media . . . . . VII. Electron-Optical Systems . . . . . VIII. Beam Memory Systems . . . . . . References . . . . . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

43

44

. . . . .

51 56 81

99 112 129 135

Electron Beam as Analytical Tods in Surface Research: LEED and AES L. FIERMANS AND J. VENNIK I. Introduction . . . . . . . . . . . . I1. Low Energy Electron Diffraction (LEED) . I11 Auger Electron Spectroscopy (AES) . . . References . . . . . . . . . . . . .

.

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . .

. . . . . . . . .

139 141 164 198

X-Ray Image Intensifiers KIRBYG . VOSBURGH.ROBERTK. SWANK. AND JOHNM . HOUSTON I . Introduction . . . I1. Input Phosphors . 111. Electron Optics . . IV. Output Phosphors . V XRII Performance .

.

. . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

. . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

205

207 216 226 231

vi

CONTENTS

VI . FutureTrends

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Selected Bibliography . References . . . . .

AUTHOR INWX SUBJECT INDEX

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

238 242 243 245 253

CONTRIBUTORS TO VOLUME 43 Numbers in parentheses indicate the pages on which the authors’ contributions begin.

G. DONELLI, Physics Laboratory, Istituto Superiore di Sanita, Rome, Italy (1)

L. FIERMANS, Laboratorium voor Kristallografie en Studie van de Vaste Stof, Rijksuniversiteit Gent, Gent, Belgium (139) JOHNM. HOUSTON, Corporate Research and Development, General Electric Company, Schenectady, New York (205) JOHNKELLY,* Stanford Research Institute, Menlo Park, California (43)

L. PAOLEITI,Physics Laboratory, Istituto Superiore di Sanita, Rome, Italy (1) ROBERTK.SWANK, Corporate Research and Development, General Electric Company, Schenectady, New York (205) J. VENNIK, Laboratorium voor Kristallografie en Studie van de Vaste Stof, Rijksuniversiteit Gent, Gent, Belgium (139) KIRBYG. VOSBURGH, Corporate Research and Development, General Electric Company, Schenectady, New York (205)

* Present address: HP Laboratories, Hewlett-Packard Company, 1501 Page Mill Road, Palo Alto, California, 94304. vii

This Page Intentionally Left Blank THIS PAGE INTENTIOALLY LEFT BLANK .

FOREWORD Due to the entirely different physical processes governing the image formation in light and electron microscopes, new methods are needed for the interpretation of electron micrographs. G. Donelli and L. Paoletti review the application of optical transforms to their analysis. They discuss the numerous applications of optical processing, such as contrast transfer functions, defects of the images, periodical structure analysis, three-dimensional reconstruction, and optical spatial filtering. In the past we have published only one review on memory devices [in Volume 21 (1965)l.That review was limited to magnetic core technology. J. Kelly presents in this volume a review of electron beam addressed memories. These latter may have originated earlier than the magnetic core memories but were supplanted for a while by the magnetic ones. The trend has now reversed and electron beam addressed memories have taken a great step forward. The many aspects and devices are described in detail. L. Fiermans and J. Vennik examine the use of electron beams as analytical tools, in particular in low-energy electron diffraction and in Auger electron spectroscopy. The emphasis is on the practical applications of LEED and AES, such as the interpretation of a LEED pattern or the analysis of an Auger spectrum. Whereas ample space was devoted in earlier volumes to image intensifiers for visible infrared and ultraviolet light, practically nothing was said about X-ray image intensifiers. This gap is now filled by the review written by K. G. Vosburgh, R. K. Swank, and J. M. Houston. They discuss the components of such devices, by breaking them down to input phosphors, electron optics, and output phosphors. The performance of one particular design is analyzed and predictions are offered for the future. Following a long established practice we would like to list here the reviews, with their authors, which we expect to publish in future volumes: Time Measurements on Radiation Detector Signals In Situ Electron Microscopy of Thin Films Electron Bombardment Semiconductor Devices

Atomic Photoelectron Spectroscopy. I1 Light-Emitting Devices. 11: Applications ix

S. Cova A. Barna, P. B. Barna, J. P. Pocza and I. Pozsgai D. J. Bates, R. Knight and S. Spinella S. T. Manson H. F. Matarl:

X

FOREWORD

Nonlinear Atomic Processes Righ Injection in a Two-Dimensional Transistor Semiconductor Microwave Power Devices. I1 Basic Concepts of Minicomputers Physics of Ion Beams from a Discharge Source Physics of Ion Source Discharges Auger Electron Spectroscopy High Power Electron Beams as Power Tools Terminology and Claskification of Particle Beams On Teaching of Electronics Wave Propagation and Instability in Thin Film Semiconductor Structures The Gunn-Hilson Effect A Review of Applications of Superconductivity Minicomputer Technology Digital Filters Physical Electronics and Modeling of MOS Devices Measurement and Application of Precise Time Thin Film Electronics Technology Characterization of MOSFETs Operating in Weak Inversion Electron Impact Processes Sonar Microchannel Electron Multipliers The Negative Hydrogen Ion Electron Attachment and Detachment Noise in Solid State Devices Radar Signal Processing Electron Beam Controlled Lasers Amorphous Semiconductors Electron Beams in Microfabrication. I and I1 Photoacoustic Spectroscopy Design Automation of Digital Systems. I and I1

J. Bakos W. L. Engl S.Teszner and J. L. Teszner L. Kusak G. Gautherin and C. Lejeune G. Gautherin and C. Lejeune N. C. Macdonald and P. W. Palm berg B. W. Schumacher 8. W. Schumacher and J. H. Fink H. E. Bergeson and G. Cassidy A. A. Barybin M. P. Shaw W. B. Fowler C. W. Rose S. A. White J. N. Churchill, T. W. Collins, and F. E. Holmstrom G. M. R. Winkler T. P. Brody R. J. Van Overstraeten

S. Chung F. N. Spiess R. F. Potter R. Geballe R. S. Berry E. R. Chenette and A. van der Ziel Merrill I. Skolnik Charles Cason H. Scher and G. Pfister P. R. Thornton A. Rosencwaig W. G. Magnuson and Robert J. Smith

Supplementary Volumes:

Sequency Theory Computer Techniques for Image Processing in Electron Microscopy High Voltage and High Power Applications of Thyristors

H. F. Harmuth W. G. Saxton G. Karady

We wish to express our best thanks to the many friends whose help makes it possible to produce these volumes. As in the past we would be very grateful for further advice and constructive criticism.

L. MARTON C. MARTON

ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS

VOLUME 43

This Page Intentionally Left Blank

Electron Micrograph Analysis by Optical Transforms G. DONELLI

AND

L. PAOLETTI

Physics Laboratory Istituto Superiore di Sanita Rome, Italy

.........................................................

1

D. Three-Dimensional Reconstruction ............................................ E. Optical Spatial Filtering ............... ......

29

I. Introduction.

B. Defects of Electron Im

References..

......................................................................

35 40

I. INTRODUCTION An electron micrograph, because of the great depth of field of an electron microscope, is the projection on the image plane of the three-dimensional structure of the specimen. It consequently contains all of the information relative to the various levels of the specimen itself. In the case of a specimen for which a low resolution is required, the interpretation of the electron micrograph, i.e., the extraction of structural information relative to the specimen, is a fairly easy operation which generally leads to correct results even when the electron images are considered as common photographic images. The analysis of high resolution micrographs, on the other hand, is extremely complex and subject to gross misinterpretation. In this case, to trace back to a correct structural definition of the specimen based on the image contrast distribution it must be remembered that the image is due to different contrast formation processes-scattering contrast, phase contrast, and amplitude contrast-which depend critically on the specimen observed and on the working conditions. This problem can be solved by using two alternative ways of describing 1

2

G. DONELLI AND

L. PAOLETTI

contrast formation: the method of weak object of phase and amplitude or the incoherent method. It must be emphasized, however, that we are dealing with approximate methods. Furthermore, the theory of the formation of contrast in neither one nor the other approximation leads to a simple linear relationship between the structure of the specimen (mass thickness or projected density) and the image contrast. Another factor that makes interpretation of high resolution images of biological specimens particularly difficult is that such specimens are usually made up of light atoms. These are capable of producing only an extremely low contrast in the image, and staining or shadowing techniques must be used to produce enough contrast for observation. It is generally difficult to determine the exact interaction of the stain and the specimen (degree of penetration of the stain into the structure, degree of symmetry and uniformity with which different sides of the specimen have been stained) and thus to decode the structural information provided by the electron image without a “staining theory” which allows us to know exactly how the stain “ changes ” the specimen. In the past ten years a number of techniques for analysis of high resolution micrographs have been proposed and widely used; in particular, methods of Fourier analysis of images have been developed. Among the methods for obtaining the Fourier transform (Bracewell, 1965) of an electron micrograph we would like to discuss the optical transform technique (Taylor and Lipson, 1964; Markham, 1968; Klug, 1971; Horne and Markham, 1972; Donelli and Paoletti, 1972; Lipson, 1972). Applied for the first time by Klug and Berger (1964) as a method of micrograph analysis of periodical structures, this technique turns out to be extremely useful because of its ease and rapidity. However, this method has the disadvantage that the quantity that can be effectively recorded and’measured is the square modulus of the transform, and thus any information relative to the phase of the spatial frequencies present in the transform itself is lost. IN AN ELECTRON MICROSCOPE 11. IMAGE FORMATION

In a transmission electron microscope, information concerning the structure of the specimen is transferred to the electron beam by means of the interaction between the specimen and the beam itself (Cosslett, 1965; Glick, 1965; Lenz, 1965; Marton, 1965; Scherzer, 1965; Zeitler, 1965, 1968; Hawkes, 1973). The signal carrying the information (the transmitted beam) is then transferred by the electron-optical channel to the image plane. In this transfer, the signal is modified by the characteristics of the electron-optical channel, and this modified signal then becomes available on the image plane in the form of a distribution of intensity.

ELECTRON MICROGRAPH ANALYSIS

3

The problem of interpreting an electron micrograph thus consists in deducing information about the structure of a given specimen based on the distribution of intensity available on the image plane, in addition to adequate knowledge of details concerning the specimen-beam interaction. In practice, this problem can be resolved (Hall, 1953; Heidenreich, 1964; Burge, 1973; Misell, 1973) by reducing the specimen-beam interaction process to one of the following approximations: a. The “scattering contrast” or “intensity absorption” approximation (Crick and Misell, 1971), which starts from the hypothesis that incoherent scattering phenomena are prevalent: the production of the image contrast is then described using an exponential absorption law. b. The approximation of the weak object of phase and amplitude (Lenz, 1970; Hanszen, 1971), in which coherent scattering phenomena are considered to be prevalent: the production of the contrast is then described using a small variation of the amplitude and phase of the incident electron wave. A . Transfer Function of an Electron-Optical Channel

1. Scattering Contrast Images The structure of the specimen is represented by the variation of its mass thickness w = pt, where p is the density and t the thickness of the specimen itself. Each element of the specimen (object point) transmits a fraction j , of within the objective lens the incident beam intensity j , equal to j, = j , ,-‘Iw aperture, where c, is the total cross section for scattering outside of the objective lens. The distribution of intensity on the image plane is obtained by summing the intensities transmitted from all of the object points. The approximtion is essentially correct if it is assumed that the incident beam scattering due to the specimen gives an incoherent transmitted beam. Because of the great depth of field of an electron microscope, the specimen can be described by a bidimensional function o(xo, y o ) = o(ro), defined as projected density, expressed in atoms per unit area. For a specimen having an atomic weight A, the image produced by an ideal electron-optical system will be, at a magnification M , given by

with N being Avogadro’s number. In reality, because of aberrations of the objective lens and other instrumental factors, such as instability in high voltage and in the excitation

4

G. DONELLI A N D L. PAOLETTI

current of the lenses, the observed imageji is given rather by

in which the convolution function S(ri)represents the intensity distribution which, on the image plane, corresponds to an object point. The function S(ri)can be calculated when the angle and energy distribution of each of the three components, elastic, inelastic, and transmitted, of the electron beam scattered by the specimen are known (Crick and Misell, 1971) and it is taken into account that electrons scattered with an angle 3 and with an energy spreading equal to AU/U are focused on the image plane at a distance r from the ideal gaussian point given by r = M ( C , 9’

+ C, 9 AUIU)

(3) with C, being the coefficient of spherical aberration and C, the coefficient of chromatic aberration of the objective lens (Heidenreich, 1964). More strictly, the image must be considered as the sum of three integrals such as that in Eq. (2), corresponding to the three components of the scattered beam, with the convolution function in each integral appropriate to the relative component. Such functions evidently will then also depend on the specimen, in addition to the characteristics of the electronoptical system, and in particular on the thickness and atomic number of the specimen. In the less strict description of using only the integral (2), with the convolution function Smax(ri)corresponding to the “disk of confusion whose radius is obtained assuming, in Eq. (3), the angular semiaperture of the objective in place of 3 and with the more probable fraction ofenergy lost by the inelastic electrons in place of AU/U, the contrast Ci(ri)on the image will turn out to be ”

where (ji) indicates the average value of the intensity on the image plane, c, indicates the total cross section for elastic and inelastic scattering outside the objective, and A a indicates the variations of the projected density of the specimen from the average. The operations of the electron-optical system can be better described by the Fourier transform of Eq. (4). In this case we have A

3[Ci(ri)]= - - c, Tm,,(v)S N

in which Tma.(vx, \I,.) = Tmax(v) = T[Smax(ri)].

ELECTRON MICROGRAPH ANALYSIS

5

If the contrast transfer function TmaX(v) were constant for all of the spatial frequency values, the electron microscope would be an ideal electron-optical in Eq. (4) is differsystem; in practice, since the convolution function Smax(ri) ent from zero on the “disk of confusion” having a finite radius rmpx,the acts as a low-pass filter, attenuating the high transfer function Tmax(v) frequencies and transferring, with negligible contrast, the frequency band that falls beyond ,v N 1/rmax. The experimental conditions under which the approximation of the scattering contrast can be considered good, i.e., the conditions in which inelastic scattering is the dominant process, correspond to the observation of relatively thick amorphous objects (thickness > 500 A with an incident beam energy between 20 and 100 keV) with variations in their thickness of small magnitude with respect to the average thickness (Burge, 1973). 2 . Phase Contrast Images The interaction between the incident electron wave Yo(xo,y o , z o ) = Yo(ro)and the specimen is described as a small phase shift qo(ro)and a small attenuation of the incident wave amplitude Eo(ro). The wave function immediately following interaction with the specimen can thus be written ’Y,(ro) = 1 + ivo(r0) - co(r0)

(6)

where qo 4 272 and E~ 4 1 (weak object of phase and amplitude). The functions qo and c0 depend on the structure of the specimen. Assuming that qo is always 4 272 and neglecting the phase shift due to the thickness f of the specimen, the phase shift qo of Yo(ro) after passing through the specimen can be written proportionally (Lenz, 1970) to the projection of the potential distribution V ( x o , y o , z o ) of the specimen in the zo direction of the wave propagation:

where h is Planck’s constant and m, e, 1,are respectively mass, charge, and wavelength of the beam electrons. The potential distribution V(ro) can be written, at least in the tight binding approximation, V ( x o , y o , zo)

=

<(XI,

y’, z’) d(xo - x’, yo - y‘, zo - z‘) dx‘ dy’ dz‘

(8)

where d ( x o , y o , zo) is the density in atoms per unit of volume of the y’, z’) is the atomic potential. From specimen at the point ro and <(XI,

6

G. DONELLI A N D L. PAOLETTI

Eq. (8) it is seen that Eq. (7) can be written

.I‘

qo(ro)= - wa(r’)a(ro - r’) dr’

(9)

where a(r0)is the density of the specimen projected in the direction of the zo axis and o,(r’) is the atomic potential projected in the same direction and normalized by the constant 2nrneA/hz. The effect of scattering on the incident beam can be represented by a complex potential (Yoshioka, 1957) V = V + iV’in which the real part V is that considered in Eqs. (7) and (8) and represents elastic scattering; the imaginary component iV‘ represents the effect of absorption and can be calculated from inelastic effects such as the excitation of phonons (Radi, 1970), plasmons (Radi, 1970), and internal electrons (Whelan, 1965). As for Eq. (8), it can be written Vi(ro) = J Va(r‘) d(ro - r’) dr’, having introduced an “imaginary atomic potential” iVi ; we then have Eo(ro)=

-[ w;(r’)a(r, - r’) dr’

(10)

the symbols having the same meanings as in Eq: (9). If Y,(ro)given by Eq. (6) is the wave function after interaction with the specimen, the diffracted wave function on the second focal plane of the objective will be = 6(v)

+ iA(v) - E(v)

(11) where A(v) and E(v) are respectively the Fourier transforms of qo and c0 and the delta function represents the unscattered beam. Considering Eqs. (9) and (10) we then have Yd(V)

+

Yd(v)= 6(v) - iF[wa(ro)] * F[o(ro)] F[o:(r,)] * F[a(ro)] (12) The effect of the electron-optical channel (aberrations, defocusing, objective aperture) can be described (Born and Wolf, 1959; Lenz, 1970; Hanszen, 1971) using the transfer function T,,(v) of the microscope. The wave function on the second focal plane is then modified to: (13)

W V ) =~ d ( V ) L ( V )

where (Lenz, 1970; Hanszen, 1971):

M with (2a/A)W(v)representing the phase shift due to aberrations; B(v) = 1 on the objective aperture and B(v) = 0 elsewhere. W(v), in the presence of spherical aberration C,, of defocusing A$ and of astigmatism C, , can be written (Born and Wolf, 1959; Lenz, 1970)

cs A4v4 W ( v )= 4

Af A 2 V Z - Ca +2 2

-

- v;)

(15)

7

ELECTRON MICROGRAPH ANALYSIS

In conclusion, for the modified wave function on the diffraction plane: %(v)

=

T,m(v)'(v) + iT,m(V)A(v)- T,m(v)E(v)

and for the wave function ",(Ti)

Yi(ri)= Y [ Y i ( v ) = ] <,(O) 1 =M

+ iF[T,(v)

*

on the image plane,

+ i F [ T m ( v ) . A(v)]- Y [ x , ( v ) - ~ ( v ) ] A(v)]- F [ T m ( v )E(v)] .

(17)

where it is held that, for the amplitude T,,(O) of the background,

1 T,,(v)G(v)exp [ -i2n (v .

s)]

dv = T,,(O)

1

=-

M

The intensity on the image plane will be given byj,(r,) = [Yi(ri)I2. Considering that (ji) = 1/M2 and neglecting second-order terms, the contrast on the image will be C,(r,) = [lYi(ri)12- 1 / M 2 ] / ( 1 / M 2 ) = i M { F [ T , ( V ) . A(v)]- S * [ T , ( V ) . A(v)]} - M { ~ [ T , ( V ) E ( V ) I+ s * [ T , ( v ) E ( v ) ] )

= iMF[Tm(v)A(v) - Tzm(-v)A*( - v ) ]

- MF[T,,(v)E(v)

+ T:, ( - v)E*(- v ) ]

(19)

Since A(v) = A( -v)* and E(v) = E( -v)*, assuming that the transfer function has cylindrical symmetry around the optical axis, T,,(v) = T,,( - v ) , and considering Eq. (14), Eq. (19)can be finally rewritten

(20)

From Eq. (20) we can obtain a simple relationship between the contrast transform and the specimen transform: F[Ci(ri)] = A(v)2 sin

B(v) - E(v)2 cos

8

G. DONELLI AND L. PAOLETTI

and considering Eqs. (9) and (lo), y[Ci(ri)] = -2 sin

r: r:

-

+ 2 cos

1 1

W(v) B(v)s[wa(r,)]s[o(r,)]

-

W(v) B ( v ) . ~ [ ~ ~ ( r , ) ] ~ [ o ( r , ) ]

In conclusion, the image transform is simply the specimen transform multiplied by a transfer function T(v) given by

+ T[oi,(ro)] cos (23) The function T(v) is essentially determined by the two terms sin[(2n/A)W(v)] and cos[(2n/A)W(v)] since both .T[w,] and F[oa]are slowly varying functions of v (Eisenhandler and Siegel, 1966; Burge, 1973). Assuming that the astigmatism is correct, sin[(2n/l)W(v)] and cos[(2lt/A)W(v)] have the shape shown in Figs. 1 and 2. As can be seen, the amplitude contrast (cos[(2n/A)W(v)]) will contribute significantly only at low resolutions, while the phase contrast (sin[(2n/A)W(v)])will be the main mechanism for the production of contrast at high and medium resolutions.

004

aoe

012

016

020

024

02e

SPATIAL FREQUENCIES

032

036 v ( 8-l)

FIG. 1. The contrast transfer function (CTF) in the case ofa weak phase object for defocus values A f = 0 and Af = -800 A. Electron energy = 100 keV; spherical aberration coefficient = 1.6 mm.

9

ELECTRON MICROGRAPH ANALYSIS

c

SPATIAL FREQUENCIES

2 h

v &’I

FIG.2. The contrast transfer function (CTF) in the case of a weak amplitude object for defocus values Af = 0 and Af= -800 A. Electron energy = 100 keV; spherical aberration coefficient = 1.6 m m .

In order to valuate the relative contribution of phase and amplitude contrast to the final image we can rewrite T ( v ) in the form T ( v ) = 2 B ( v ) . F [ ~ o ~ ( r ,, ) ]s~i n p W ( v ) ] + P ( v ) cos

I

W ( v ) ] ) (24)

where P ( v ) now represents the “percentage” of amplitude contrast in relation to the total contrast. Some experimental data indicate that P ( v ) should have a value of about 0.1 for light elements and reach values of about 0.4 for the heavier atoms (Erickson and Klug, 1971). The experimental conditions under which the approximation of the weak object of phase and amplitude can be considered satisfactory correspond to those conditions in which the incoherent scattering processes are negligible and the image is formed essentially by the elastic component, i.e., the observation of a thin specimen ( c100-200 A) whose density variations are fast and relatively large (Burge, 1973). In these conditions, the image depends critically on focus and lens aberrations. B. Photographic Recording of Electron-Optical Data Information concerning the observed specimen is available on the output plane of the electron-optical channel in the form of a distribution of electron intensity j ( r ) or rather as electron image contrast, C , ( r )= [ j ( r )- ( j ) ] / < j ) . Photographic recording of electron-optical data makes the information concerning the specimen available in the form of a modulation of the optical density D(r) of a photographic plate, or, more precisely, as the micrograph contrast C,(r) = [ D ( r )- ( D ) ] / ( D ) , where ( D ) indicates the average value of the optical density of the micrograph itself.

10

G. DONELLI AND L. PAOLETTI

The “large area” response of photographic emulsion to the incident electrons (Zeitler and Hayes, 1965; Burge et al., 1968; Valentine, 1969) is characterized, under the working conditions used in electron microscopy, by an approximately linear relationship between the optical density D(r) of the exposed and developed plate and the exposure e(r), defined as the product of the intensity j of the incident electron beam for the time t of exposure: e = j t ; ie.,

with the exact value of the constant y depending on the characteristics of the particular emulsion utilized and on the energy of the incident electrons. The simple linear relationship of Eq. (25) between optical density and exposure can be considered rigorously valid (Burge et al., 1968; Valentine, 1969) up to an optical density equal to D,/4,D, being the saturation density of the considered emulsion; and it can be considered to be valid with good approximation up to a density equal to D,/2; i.e., Eq. (25) can be considered satisfied in the usual applications of an electron microscope. If the “small area” response of a photographic emulsion could be i.e., if the latter continued to be described still using the relationship (3), satisfied even in the presence of rapid fluctuations of exposure from one point to another of the emulsion between the image contrast C,(r) and the photographic contrast C,,(r), we would obviously find

and the information concerning the specimen contained in the micrograph would turn out to be exactly equivalent to that contained on the output plane of the electron-optical channel. Nonetheless, Eqs. (25) and (26) are not adequate to describe the “small area” response of the emulsion (Burge and Garrard, 1968; Zeitler and Hayes, 1965; Zeitler, 1968); in the transfer through the photographic channel the signal which contains information can be degraded by various factors. Each electron incident on the emulsion can make more than one silver halide grain developable during its passage in the emulsion. The electron path is not straight because of the multiple scattering that the electron itself undergoes. During development the halide grains change form and dimension in a random way. During fixation and drying of the emulsion, developed grains can change from their original position due to deformations that the gelatine support may undergo. Random variations in the developer concentration can cause variations in contrast. All of these effects contribute to modifying the photographic channel response with respect to the ideal situation which would correspond to

ELECTRON MICROGRAPH ANALYSIS

11

Eq. (26) and can be adequately described only by substituting this latter with the more general

where Sp(r’ - r ) indicates the spread function of the photographic channel, i.e., its response to an impulsive input signal. We then see that for Eq. (27) the photographic contrast at a point r of the micrograph also depends on the image contrast in the nearby points in a way that depends on the function S(r’ - r), On the basis of Eq. (27), the information contained in the image is modified in the transfer through the photographic channel since the spatial frequency spectra of Ci(r) and of C,(r) do not turn out to be equal on their entire bandwidth. Taking the Fourier transform of the two members of Eq. (27) we have, in fact, F[Cp(r)l = Tp(v) * S[Ci(r)l

(28)

having indicated by T,(v) the transfer function of the photographic channel, i.e., the transform of S p ( t ) . For emulsions exposed to electrons through narrow slits, the experimental data (Zeitler and Hayes, 1965) relative to the contrast produced qear the slit on the zone of the emulsion not exposed can be adequately described by

I

S,b) = exp[ - x I /KI

(29)

where 1 x 1 represents the distance from the edge of the slit and the constant K is the radius at which the “spurious” contrast is reduced to l/e of its initial value. The value of the constant K depends essentially on the energy of the incident electrons and the thickness of the sensitive emulsion. In conditions normally used in electron microscopy, K can be assigned a value of about 10 p (Zeitler, 1968). The Fourier transform of Eq. (29) can be used to evaluate the transfer function T ( v ) in Eq. (28). The result of the transform can be written: 2K/(4rc2K4v2 + 1); this function rapidly decreases at high frequencies and for v ‘v 1/60 pm- it is already reduced to 50% of its original value, having assumed K = 10 pm. This means that 60-pm details found in the image plane of the electron-optical system are recorded on the micrograph with a contrast of about half of the “electron contrast.” Finally, the photographic channel acts like a low-pass filter, and the higher spatial frequencies of the electron image are transferred with reduced contrast or else not transferred at all in the micrograph.

12

G. DONELLI A N D L. PAOLETTI

If details of the observed specimen are to be maintained on the micrograph, it is necessary that the spatial frequency band corresponding to those details not be filtered by Tp(v).In the usual working conditions of electron microscopy this limitation is not excessively pressing. As long as a 20-A detail of the observed specimen, for example, is found on the micrograph with a contrast not less than 50% of the contrast on the electron image, it is sufficient to work with an instrumental magnification of not less than 3 x lo4, which is a relatively low magnification for electron microscopes presently in use. Thus, for sufficiently high instrumental magnifications, the transfer function Tp(v) of the photographic channel, at least in the spatial frequency band that effectively transports information concerning the specimen, can be considered as a constant, with not more than 20-30”/, error, and Eq. (28) can be replaced by the simpler expression r[Cp(r)] = F[Ci(r)] 111. OPTICAL PROCESSING OF ELECTRON MICROSCOPIC DATA

The processing of electron microscopic data has been successfully carried out using both computers (Klug, 1971; Frank, 1973; Hawkes, 1973) and coherent optical systems (Klug and Berger, 1964; Markham, 1968; Berger, 1972; Donnelli and Paoletti, 1972; Horne and Markham, 1972; Lake, 1972). Nonetheless, the techniques of optical processing hold a fundamental advantage over other methods of processing since the data are, in most cases, already available in suitable form to be introduced in the optical system. Optical processing of data with coherent systems, in fact, requires that the input data be in the form of an amplitude transmittance (Vander Lugt, 1968; Thompson, 1972); the light modulator can be constituted directly by an electron micrograph. Being able to avoid the “translation of data into a suitable “format for processing has obvious advantages, saving both time and equipment and, in addition, avoiding possible degradation of the data in the translation process. The purpose of processing electron microscopic images is that of restoring data degraded by aberrations of the electron-optical channel and disturbed by noise coming from various sources, or at least of evaluating some parameters contained in the data themselves. To realize this purpose, a coherent optical system (Born and Wolf, 1959; Stroke, 1966; Collier et al., 1971) is such that it is possible to carry out a frequency analysis of the image through its Fourier transform, or to carry out spatial filtering of the image itself through control of the transfer function of the optical system with suitable filters. Data processed in this way are then available in the form of a light intensity distribution on the output plane of the optical system. ”

”

13

ELECTRON MICROGRAPH ANALYSIS

A . Coherent Optical Data-Processing Systems 1. Elements of a

Coherent Optical System and

Their Operational

Characterization

In the discussion of a coherent optical data-processing system in terms of the light intensity distribution produced on any one of its optical planes, it is useful to characterize from an operational point of view all of the elements of the system itself. This latter, in general, along with one or more spherical lenses and a coherent light source, will be made up of a light modulator through which the data to be processed are introduced into the system and free spaces which separate the lenses, the modulator, and the planes of the optical system on which the light intensity distribution is to be observed. In a first-order analysis (i.e., neglecting aberrations) and supposing an infinite lens aperture, a thin spherical lens (Born and Wolf, 1959) can be described by a quadratic phase factor given by

“

”

wherefis the focal length of the lens and I the wavelength of the light. The light wave function immediately after the lens is the product of the phase factor in Eq. (31) and the wave function incident on the lens. Furthermore, the light wave function is changed by its propagation through a “free space” along the axis of the optical system. From the Fresnel-Kirchhoff integral, in the paraxial approximation, it follows that if Y(x, y) is the wave function on a given plane ofthe system, the new wave function Y(u, v ) on a plane at a distance 1 from the first is given (Born and Wolf, 1959) by i

1

Y ( uU) , = - Y(x, y) exp

I1

[(u - x)’

+

(1)

I

- Y ) ~ ]dx dy

(32)

where u, v are the coordinates in the new plane. The propagation of a wave function Y(x, y ) through a distance 1 can then be described, for Eq. (32), using the convolution of the latter with the phase term T‘:

14

G . D O N E L L I A N D L. PAOLETTI

Because of the similarity between Eqs. (31) and (33) it would be convenient to define the operator O ( x , y ; Q ) (Vander Lugt, 1966) using O(X, y ; Q ) = exp

(34)

in which Q is a parameter that can assume suitable real values. For Eq.,(34), then, the phase factor that describes the operations ofa lens can be represented by the operator @(x, y; - F), having F = 1/1; while the propagation of the wave function Y ( x , y ) through a distance 1 can be described using the convolution of Y(x, y ) itself with the operator (iL/A)@(x,y; L), assuming L = 1/1. 2 . Optical Systems for Frequency Analysis and Spatial Filtering The fundamental system for optical data processing as shown in Fig. 3, is made up of: a light modulator, characterized by a complex amplitude transmittance a,(x,, y,) and through which the input signal is introduced into the system; a free space lI that separates the modulator from a spherical lens with a focal lengthf; and a free space 1, that separates the lens from the output plane on which the signal is available as a light intensity distribution.

.L

11

I

FIG.3. An optical system for frequency analysis and spatial filtering of the data.

If the modulator is then illuminated by a monochromatic wave of unit amplitude, diverging from a point source at a distance l o , the wave function Y , ( x l ,y l ) transmitted by the modulator itself will be:

having Lo = l / l o

15

ELECTRON MICROGRAPH ANALYSIS

The convolution of Y , ( x , , y , ) with (iL,/A)Q,(x,,y,; L l ) , where L, = 1/11, gives the wave function Y i ( x 2 y, z ) incident on the lens:

The product of Y i ( x , , y 2 ) with @ ( x 2 y, , ; - F ) provides the wave function Y f ( x 2 , y,) coming out of the lens: Y f ( x 2 ~ 2 =) Yi(x2 Y ~ ) Q , ( x~ ~ 2 - ;F ) (37) and finally the convolution of Y f ( x 2 y,) , with (iL,/A)Q,(x,,y , ; L,), where L, = l/lz, gives the wave function Y u ( x 3 y, 3 ) on the output plane of the optical system: 7

3

Yu(x3

9

y3)

=

iL2 11

yf(x2

9

Y2)Q,(x3

- xZ

3

Y3

- y2;

L2) dx2

dy2

(38)

Explaining Eq. (38) through Eqs. (35k(37) and keeping Eq. (34) in mind, the output signal can be written

If the conditions for imaging obtained from geometrical optics are valid, i.e., if L2

= F - L1

(40) it is easy to verify that the output signal of Eq. (39) (Vander Lugt, 1966), aside from a phase factor, has the form of the input signal a,(x,, y , ) . More exactly, in this case it can be seen that

(41)

From Eq. (41) the output signal turns out to be equal to the input signal inverted with respect to the origin of the coordinates, enlarged by M = - L1/L2, and multiplied by a spherical phase factor dependent on the

16

G. DONELLI A N D L. PAOLETTI

cuivature of the wave function incident on the modulator. Since, in general, the signal is recorded as an intensity distribution YuY,*,the operations of the optical system, at least as far as the imaging of the input signal is concerned, are independent of the curvature Lo of the illuminating wave. A fundamental property of the optical system under discussion is that of being able to produce the Fourier transform of the input signal; it can in fact be verified that if the following condition is valid:

the wave function of Eq. (39), assumes the form

where A,(u, u ) indicates the Fourier transform of a,(x,, y,). The transformable properties of the optical system are satisfied independently of the distance 1/L, between the modulator and spherical lens. The phase factor can be reduced to 1 if the further condition L , = F is satisfied. In addition to the frequency analysis of the input signal the optical system allows for spatial filtering of the signal. In fact, on the basis of Eqs. (40) and (42) the plane of the optical system on which the input signal is reproduced is always situated after the plane on which the transform is produced. Inserting appropriate filters (Vander Lugt, 1968; Thompson, 1972) on the transform plane it is possible to obtain an output signal for which the intensity or the phase of the spatial frequencies has been changed on the basis of selected filter. This discussion of the operations of the optical system is not completely general insofar as it neglects the effects of aberrations and of the finite aperture of the lenses; the latter limitation, in particular, has important consequences on the operations of the system itself (Vander Lugt, 1966). In order to evaluate such consequences on the Fourier transformable properties let us consider the very simple optical system of Fig. 4. A light modulator a,(x,, y , ) is illuminated by a spherical wave converging in a point, at a distance 1, situated after the modulator itself. If the system has a limited aperture, the incident wave is described on the plane of the modulator by the wave function

Y b , ?Y , ) = b b , ,

Y,)@(Xl?

y,; -L)

(44)

ELECTRON MICROGRAPH ANALYSIS

17

FIG.4. An optical system for performing the Fourier transform ofdata. The finite aperture of the system implies an indeterminancy in the representation on the output plane of the system itself of the spatial frequencies contained in the input signal.

where b(x,, y , ) indicates a function other than zero on the aperture and equal to zero elsewhere. On the basis of Eq. (32), it is then seen that the wave function on the x z y z plane at a distance 1 from the modulator is given by

(45 )

where B(u, v ) and A,(u, u ) indicate respectively the transforms of b(x,, y , ) and Ofa,(xi, ~ 1 ) . Because of the convolution operation indicated in Eq. (45), a spatial frequency found in the spectrum of a,(x,, y , ) is no longer represented on the x , y , plane by a delta function, i.e., by a geometrical point but rather by a light distribution represented by B[( - L/A)x, ,( - L/A)y,] whose spreading on the plane of the transform is inversely proportional to the aperture b(x,, y , ) of the optical system. B[( - L / A ) x , , (- L/A)y,] itself can thus be considered a measure of the indeterminancy with which a point on the output plane of the optical system represents a spatial frequency present in the spectrum of the input signal because of the finite dimensions of the elements of the optical system itself. B. Fourier Transforms of Electron Micrographs

In a coherent optical system, the electron micrograph is used as a wave amplitude modulator characterized by its complex amplitude transmittance a, = lal/eie. Bearing in mind that we want to obtain the transform of the electron image 9 C i , it would be appropriate to consider as parasite signal or noise anything that is different from or additional to 9 C i . From this

18

G. DONELLI A N D L. PAOLETTI

point of view, assuming as eliminated or negligible bad alignment, mechanical instability, dust on the lens of the optical bench, etc., the main factors to be taken into account can be described as follows: a. The modulus la,/ of the micrograph’s amplitude transmittance (Collier et al., 1971 ; Shamir, 1972) is not a linear function of the photographic contrast C , but rather depends exponentially on this latter. b. The “photographic channel” (Zeitler and Hayes, 1965; Burge and Garrand, 1968) acts as a low-pass filter, that is, its transfer function Tp goes to zero with high spatial frequencies; c. The amplitude transmittance of the micrograph has a phase 0 not related to the information contained by the contrast on the micrograph itself. d. The optical density and the contrast of the micrograph (Zeitler and Hayes, 1965; Valentine, 1969) are subjected to fluctuations which are due to the statistical distribution of the electrons. In spite of this discouraging list of factors contributing to degradation of the output signal, the optical elaboration of the data obtained from a micrograph can, nevertheless, provide a good approximation to the spectrum of the electron image, i.e., of the F C i , as can be seen from a discussion of the factors just mentioned. 1. Optical Analysis of Electron Microscopic Data

The modulus I a, 1 of the complex amplitude transmittance is defined in such a way that i, = (a,12i,

(46)

where io and i, are respectively the intensity of the incident light and that of the light transmitted from the micrograph. Since the optical density D of the micrograph is given by D = -loglo(il/io)

(47)

it can then be seen from Eqs. (46) and (47) that

la,( = 10-D’2

(481

Taking into account the definition of the photographic contrast C,, we then have

19

ELECTRON MICROGRAPH ANALYSIS

where I a, I depends exponentially on C, . Expanding I a, 1 into a series of C, powers, the modulus of the amplitude transmittance is lull = 10-(D)/2[1- f(D)(ln lO)C,

+ $(D)’(ln

10)2C:

+ ...I

(50)

Ignoring the phase of the amplitude transmittance, the output signal of the optical system is given by S[la,1] and can be written

Y[ I a, I] = 1 0 - ( ~ ) / ~ [ 6 ( uu), - j(D>(ln 1o)SC,

+ +(D)’(ln

10)2FC, 0 YC,

+ .**I

(51)

The optical system thus provides the micrograph’s contrast transform, with the approximation that corresponds to neglecting all of the terms following the second one in the expansion of I a, I in a power series. In fact, higher terms such as F C , 0 SC,,, Y C , 0 YC, @ YC,, etc., can be found in the output signal of the optical system. Since the width of the spectral band of these terms is two or three times the width of the SC, (Bracewell, 1965), these terms generate parasite signals. Nevertheless, these signals are smaller, by at least one order of magnitude, than those which correspond to YC, . This is true for electron micrograph density values included in the usual range. Considering that Eq. (51) represents the wave amplitude on the output plane of the optical system, the light intensity on this same plane, that is to say, the quantity that can actually be observed, will be given by

I S [ la,l] 1’

= 10-‘D’{6(u,

u)

+ q 2 I F C , l2 + iq41SC, 0 SC,12

- f q ~ S * C , ( Y C ,0 YC,) - $q3SCp(YC, @ YC,)*

+ ...}

(52)

where 6(u, u ) represents the component scattered in the incidence direction and the cross products, like

the interference between the terms of Eq. (51); the constant q is defined by q = $(D) In 10. The total power W on the output plane uu of the optical system, aside from a proportionality constant, is given by W = f 1 S[la, I ] l2 du du. The integral must be considered to be extended over the entire bandwidth of F [la, I], i.e., over the entire area of the output plane in which S[Ia, 11 # 0. It can be seen from Eq. ( 5 2 ) that, apart from the component diffused in the direction of incident light represented by the delta function, the power

20

G . DONELLI A N D L . PAOLETTI

W can be thought of as a sum of terms. The first two of these terms correspond respectively to

w,= ). IFC,12 Wz-. -

du du

I IYC, @ FC,Iz du du

(53) (54)

Since the function under the integral sign can be expressed by the autoconvolution of F C , , it is evident that the integral W, is extended over a bandwidth twice as large as the one that corresponds to W,. On the other hand, it is easy to verify that W, 2 W, . In fact,

w,= 1 IFC,(2 du d" = C(C,I2dxdy2

( l C , Z 1 2 d x d y = CJFC,Z12dud~= W,

(55)

having taken into account Rayleigh's theorem and considering the fact that C , is a real function such that - 1 < C, < + 1. It is also clear that the smaller W, is with respect to W, and the more extended its bandwidth with respect to that of W,, the more negligible is the term of Eq. (52) corresponding to W, (i.e., the term in which we find autoconvolution) with respect to the first-order term I3C,I2. The terms of Eq. (52) in which convolutions of higher order are present will be similarly negligible with respect to I Y C , 1.' Limiting ourselves to a unidimensional case, we see, for example, that a micrograph whose contrast C, varies sinusoidally with a period p will have C, = cos 2nx/p. Considering that 3 C , = ;S(u + l/p) + ;S(u - l/p) and that F C , @ F C , = *6(u) + $6(u + 2/p) + $S(u - 2/p), from Eq. (51) and ignoring the finite dimensions of the micrograph, we find

For Eq. (52) the ratio r between the intensity of the components 6(u + 2/p) and 6(u - 2/p) of the output signal, corresponding to 3 C , @ YC,, and the intensity of the components 6 ( u + l/p) and 6 ( u - l/p), corresponding to F C , , is r = 16q 1 2 = n(D)2(ln 1 lo),. For an average optical density, (D)= 1, it will be r 2 804;for (D)= 0.6, it will be r z 5%. We thus see that the intensity of the parasite signals corresponding to the term YC, @ YC, in the expansion of Eq. (5 1 ) is less, by a factor of 10 or 20, than the intensity of the signal corresponding to YC,.

ELECTRON MICROGRAPH ANALYSIS

21

If we then keep in mind that the spectrum of the micrograph F C p can be considered with the approximation of Eq. (30) equal to the spectrum of the electron image 9-Ci we can suppose that

is sufficiently verified, at least in the optical Fourier analysis of electron micrographs obtained in standard working conditions (average value of 1.0 and instrumental magnification not optical density not higher than lower than 3 * lo4.

-

-

2. Phase Noise

As a light modulator, the electron micrograph acts either by attenuating the wave amplitude or by modulating the phase of the transmitted wave. Since the phase 0 of the complex amplitude transmittance a, = la,leie of the micrograph does not in any way depend on the electron image C i , but rather depends on factors such as variations in thickness and microdefects of the photographic plate, the corresponding modulation of phase gives rise to components in the output signal of the optical system which represent parasite signals and background noise superimposed on the image spectrum. A notable phase effect is the asymmetry of the diffraction pattern; in fact, a real amplitude transmittance would have a hermitian transform (Bracewell, 1965), and the square of the modulus of such a transform, i.e., the corresponding diffraction pattern, would be centrosymmetric. The phase modulations can be partially avoided using such techniques as the “optical sandwich” (Harburn and Ranniko, 1971), which allow for the elimination of phase effects due to variations in the thickness of the plate glass or of the emulsion. The phase effects due to microdefects cannot be eliminated, however, and they thus represent a constant noise component present on the optical transform of an electron image. Such microdefects, distributed randomly over the plate, act as (Goldfischer, 1965; Enloe, 1967; Collier et al., 1971) scattering centers that diffuse the coherent incident light with random phase angles; the plate can thus be considered as a random distribution of spherical wave sources whose relative phase differences are such that any value from 0 to 2n is equally probable. In order to discuss noise due to phase effects, let us consider the optical system described in Fig. 4, assuming the micrograph situated on the x 1 y 1 plane.

22

G. DONELLI A N D L. PAOLETTI

The wave function on the x 2 y 2 plane resulting from the scattered spherical waves will be given by

where ( x ! , y ! ) are the coordinates and a,, the relative phase of the h th scattering center. The intensity on the x 2 y , plane is then the square modulus of the wave function. If we indicate by (I) the average intensity and by T ( x 2 , y , ) the variable part of intensity we have

If we consider the summation of the second member of Eq. (59), which represents the intensity fluctuations, it is seen that for each pair of scatterers there is a series of interference fringes that go along the x 2 y 2 plane in directions perpendicular to the vector of components x: - x: and y: - y : . Equation (59) gives rise to an intensity pattern whose aspect is that of a distribution of light speckles all over the x 2 y , plane. Information on the statistical characteristics (Goldfischer, 1965) of this speckle pattern can be obtained from the autocorrelation R(u, 0) of the variable part T ( x 2 , y 2 ) of Eq. (59). Averaging over the variables x 2 and y 2 , we then find

+ u, y , + c))

(60) In the likely hypothesis that the scattering centers due to microdefects in the plate are extremely numerous, the preceding relation is to be considered in the limit when the number N of the scatterers approaches infinity, and the power scattered by each of these approaches zero. In such a way the average intensity on the x , y 2 plane remains constant, and from Eqs. (58), (59), and (a),it can thus be seen that R(u, 0) = ( r 7 . 2 ,

y,)T(x,

ELECTRON MICROGRAPH ANALYSIS

23

and that R(u,~)=($!ilL S ( ~ Lu , ~ u ) I where B(u, u ) = Y [ b ( x , ,y , ) ] and /lis the coherently illuminated surface of the plate. In conclusion we can say, based on Eq. (61), that the mean square value of the intensity fluctuations on the output plane of the optical system is equal to the square of the average intensity on the same plane. Based on Eq. (62), it can then be seen that the autocorrelation function of phase noise is proportional to the diffraction pattern of the system aperture. Qualitatively, this means that the average dimensions of the speckles constituting the pattern defined by Eq. (59) are inversely proportional to the dimensions of the system aperture, i.e., to the dimensions of b ( x , , yl). IV. APPLICATIONS A . Contrast Transfer Function of an Electron Microscope

A correct interpretation of the phase contrast image is subordinate to adequate knowledge of contrast transfer function (CTF) of the electron-optical channel under the specific operating conditions. For a weak object of phase the CTF can be written (Lenz, 1970)

2n T ( v )= sin - W ( v )= sin ;1

where the astigmatism is supposed correct and the effects of partial coherence and chromatic aberration are considered negligible. Experimental determination of the CTF is a relatively simple operation using the method introduced by Thon (1966). Thin carbon films can be considered phase objects whose spectrum, due to the presence of local and random variations in the electron refraction index, contains every possible spatial frequency. The Fourier spectrum of the electron image of such films can thus be described as a “white” spectrum (i.e., containing every spatial frequency) modulated by Eq. (63). On the optical transform of a carbon film micrograph it is possible to measure the spatial frequencies in correspondence with which the CTF reaches maximal and zero values. For a fixed defocusing Af of the objective lens the spatial frequencies in correspondence with which the CTF gives maximal contrast are those that satisfy the equation 271 II - W ( v )= ( 2 n - 1 ) fl 2 with n a whole number.

24

G . DONELLI AND L. PAOLETTI

Figure 5 shows the curves corresponding to Eq. (64) for some values of n along with the curves corresponding to the zero contrast values. For a given defocusing, the contrast on the image changes sign when the C T F passes through its zero values. Consequently, we should expect to find on the image artifacts that reduce the resolution to the spatial frequency band that precedes the first zero of the CTF. A technique for the critical evaluation of the information contained in a phase image thus consists in the study of the optical transform of a carbon film micrograph obtained under the same instrumental conditions. The spatial frequencies in correspondence with which the contrast becomes inverted

overbcus

d f ( ~ l underfocus

FIG.5. The dependence on defocusing of the contrast with which the spatial frequencies in a phase image are transferred.The contrast is maximal in correspondence with the solid lines; zero contrast corresponds to the dashed lines. Electron energy = 100 keV; spherical aberration coefficient = 1.6 mm.

are clearly observable on the optical transform, corresponding to low intensity zones. This technique can be used also in the determination of optimal conditions in phase contrast observation (Johansen, 1973). These conditions correspond to the degree of underfocusing for which the widest spatial frequency band is transferred with the same sign. For a weak object of phase and amplitude, the CTF can be written

where P(v) represents the percentage of amplitude contrast with respect to the total contrast. It is difficult to give a sufficiently exact evaluation of P(v). Its weight will depend both on the atomic number of the elements present in the sample and on instrumental factors such as acceleration voltage, lens

25

ELECTRON MICROGRAPH ANALYSIS

aperture, etc. A theoretical estimate (Erickson, 1973), confirmed by experimental results (Erickson and Klug, 197 l), obtained under normal working conditions valuates P ( v ) as included in the interval between 0.1 and 0.4, going from light elements (carbon) to heavy elements (uranium). Since P ( v ) is generally small, Eq. (65) can be written in the alternative form

1

where the effect of amplitude contrast is described as a further phase shift q ( v ) . The zero and maximal values of Eq. (63), just as the curves in Fig. 5, must then be thought of as shifted toward the low spatial frequencies. Similarly to the case of carbon film, in a phase and amplitude image the random fluctuations in beam absorption and in the refraction index of the sample generate a background noise whose spectrum contains every possible spatial frequency. This spectrum modulated by Eq. (65) can be observed on the optical transform of the image. Considering the theoretical uncertainty with which Eqs. (65) and (66) can be valuated because of limited knowledge regarding the beam-specimen interaction, it is particularly interesting to observe that using the optical transform of the image background noise it is possible to valuate experimentally the CTF in the actual observation conditions.

B. Defects of Electron Images 1 . Astigmatism

In the presence of objective residual astigmatism, the CTF of the electron-optical channel does not depend only on the radial coordinate I v I on the spatial frequency plane, but must be written, more generally, as a function of I v 1 and of the azimuthal coordinate w (Lenz, 1970). Due to aberrations and objective defocusing, the CTF in this case will be 211

I“

~ ( l v lw, ) = sin-- W( I v J ,w ) = sin - -R4v4 2K 1 4 R

-

c.2

A2V’

+ Af -1’v2 2

1

cos(20)

(671

where the symbols have the same meaning as in Eq. (15). Theopticaldiffraction pattern will be modulated by [sin(2n/A)W( I v 1, w)]’. Consequently, the zones of the spatial frequency plane on which the contrast has maximum and minimum values no longer have rotational symmetry

26

G . DONELLI A N D L. PAOLETTI

around the origin of the coordinates (Thon, 1966). For example, the zones corresponding to the zeros of the CTF, which on the diffraction pattern are found as areas of weak intensity, will be those corresponding to the condition

and to W ( I v l , w ) = nA/2

(69)

Equation (69) can be satisfied by suitable whole number values of n. For each of these values the optical transform presents intensity minima which are described by the equation

c.2n

13v4

+ A-?Av2 n

c.n

1v2

cos(2w) = 1

If the residual astigmatism is completely correct we shall find C , = 0 and the points of the v w plane whose coordinates satisfy Eq. (70) will be found on circumferences centered on the origin. In the presence of astigmatism, from Eq. (70) it can be verified that there are two perpendicular directions on the v w plane along which the zero values of the CTF have respectively maximum and minimum distances from the origin. It is for this reason that the bandwidth of the frequencies transferred with the same contrast is different according to the direction on the spatial frequency plane. 2. Mechanical Instability and Magnetic Disturbance

The effects of electron-optical image drift, due to movement of the specimen, vibrations of the microscope column, or varying magnetic fields, can be difficult to recognize if, for example, they are of small magnitude and the image already possesses directional characteristics. Observation of the image optical transform can then be a simple method for measuring the direction and magnitude of such drifts. A blurred image M ( x ) can be considered as the superimposition of two identical images m ( x ) reciprocally translated by a distance 2d, and the corresponding micrograph can be represented by M(x)=fm(x d ) fm(x - d) (71) Equation (7 1) can more conveniently be written

+ +

M ( x ) = m ( x )0 [ @ ( x

+ d ) + @(x - d)]

(72)

ELECTRON MICROGRAPH ANALYSIS

27

The Fourier transform of Eq. (72) is then simply F [ M ( x ) ]= F [ m ( x ) ]cos(2nvd) and the intensity on the diffraction pattern will be given by

I F [ M ( x ) ]I*

(73)

1 F [ m ( x ) ]1’

cos’(2nvd) (74) If, more generally, the image is blurred by a linear drift of amplitude 2d, the micrograph can be represented using =

M ( x ) = m ( x )0 D ( x ) (75) where D ( x ) indicates a rectangular function, constant in the interval -d. + d and zero elsewhere. The transform of Eq. (75) can thus be written sin(2nvd) F [ M ( x ) ]= F [ m ( x ) ] 2nvd

and the intensity on the diffraction pattern will be (77) To determine the amplitude and direction of the drift, Eq. (74) or (77), respectively, must be considered; the continuous intensity distribution on the optical transform pattern will turn out to be modulated by a series of parallel fringes, perpendicular to the direction of the drift. The width of such fringes in the first case, Eq. (74), is uniform and equal to Av = 1/2d. In the second case, Eq. (77), the intensity minima will be found at distances from the center of the diffraction pattern: Avl = & 1/2d, Av, = +2/2d, Av3 = i~ 3/2d, etc., and consequently we shall find a central fringe with a width double that of the secondary lateral fringes.

C . Periodical Structure Analysis The use of optical transforms is a particularly potent method when applied to the analysis of a periodical structure (Klug and Berger, 1964; Donelliand Paoletti, 1972; Horne and Markham, 1972).The purpose of such an analysis can be summarized in the singling out and measuring of all of the periodicit ies. The value of each spacing should be considered as a nonlocal property that depends on the total characteristics of the image. The best way to measure this parameter is then simply a spatial frequency analysis of the entire image using its Fourier transform. A periodical image m(x, y) can be described as the repetition of a

G . DONELLI AND L. PA~LETTI

28

“motif ” according to a given mono- or bidimensional scheme and consequently can be represented by the following equation :

where g(x, y) indicates the repeated “motif ”, r(x, y) is the lattice plane that describes the periodicities present on the image, and b(x, y) is a function describing the actual extension of the image. Taking into consideration Eq. (78), the Fourier transform will be

m(vs v,) 9

[r(vr

=

3

v,) 0 &J< > v,)]

. d v r v,)

(79)

Y

where vt and v,, indicate the coordinates in the spatial frequency plane, and m(ve, v,,), i‘(vs,v,), 6(vc, v,,), and g(vr, v,) indicate respectively the Fourier transforms of m(x, y), r(x, y), b(x, y), and g(x, y). The optical transform thus turns out to be

I m(vs

9

I2

v,)

I

= [fly{

9

v,) 0 QV,

9

v,)l l2 I B(vr

9

v,)

I2

(80)

This equation can be more easily discussed by explicating the convolution. More exactly, if a and b are two vectors that describe the unit cell of the lattice r(x, y), a point of the latter is singled out by the vector v = ha + kb where h and k are whole numbers. The term [r(x, y) . b(x, y)] in Eq. (78) can then be written in the form r(x, y ) * b(x, Y)

=

0,111-1

0 . n ~ - 1

h

k

1

c

S(X- 0,; Y - 0,)

0.n l - 1 0, n 2 - 1

=

1

1

h

S[X - (ha, + kb,); y - (ha, + kb,)]

(81)

k

where the h and k indices vary respectively from 0 to n , - 1, and from 0 to n, - 1, describing the actual extension of the periodical image. For Eq. (81), we have F[r(x, y ) . b(x, y)] = i‘(v< v,) 0 &(V< , v,) 3

0.n,

=

-1

1

0 . nz- 1

exp[i27rh(vca,

h

+ v,,a,)] 2

exp[i2nk(vrb,

+ v,b,)]

(82)

k

and for the second term of Eq. (80) we have vs)o 6(Vr’

vv)12

+

sin2[n,n(v,:a, v,a,)] sin2[n27r(vsbx+ v,,b,)] = sin2[n(vFa, + v,a,)] sin2[n(vcb, v,,b,)]

+

ELECTRON MICROGRAPH ANALYSIS

29

If we assume that n , and n, are sufficiently large, it can be seen that the second term of Eq. (83) is different from zero only at those points of the v5 v,, plane whose coordinates satisfy the equations v5 a ,

+ v,,a y = I,

v5:b,

+ v,,by = m

(84)

with 1 and rn whole numbers. It can be verified that the solutions to the equations in (84) are given by those vectors v that can be written in the form v = la

where the vectors a and

+ mb

(85)

are determined by the conditions

a-a=B*b=l,

a*b=b*a=O

(86)

The vectors defined in Eq. (85) individuate a lattice7 on the v5 v,, plane whose elementary translations a and B depend, through Eqs. (86), on the translations of the r(x, y) lattice. This lattice is generally defined as the “reciprocal” of the r ( x , y) lattice and has some well-known geometric properties (Donelli and Paoletti, 1972). Going by Eq. (83) we can thus see that the intensity on the optical ‘transform is different from zero only on the points of the reciprocal lattice 7. The intensity at these points, however, is weighted by the term Ig(vtE’ ~ ,,)1~, i.e., by the “motif” transform of the periodical image. Measuring the highest spatial frequencies on the optical transform, it is possible to valuate the bandwidth of g(vC,v,) and thus the image resolution. This information is particularly significant for biological specimens in that the resolution with which these specimens are observed depends both on the characteristics of the electron-optical channel and, even more, on the preparation methods, staining, and damage undergone by the specimens during interaction with the electron beam. It is important to emphasize that generally the factors that effectively limit resolution are, in the case of biological samples, precisely those mentioned above. Nonetheless, it is extremely difficult to evaluate a priori the limits imposed on the resolution by such factors; they are, in fact, limits that can vary greatly with the type of sample studied and with the preparation techniques used.

D. Three-Dimensional Reconstruction The electron image of a specimen must be considered as the projection of the structure of the specimen itself onto the image plane. Information coming from all of the structure levels thus appears superimposed,and it can be difficult to deduce at which level observed details are located and to

30

G. DONELLI AND L. PAOLETTI

reconstruct a three-dimensional model. Serial sections (Williams and Kallman, 1955; Sjostrand, 1958; Donelli et al., 1970), shadow techniques (Hall, 1953; Bradley, 1967; Henderson and Griffiths, 1972; Ageno er al., 1973),and stereomicrographs (Garrod and Nankivell, 1958 ; Wells, 1960; Helmcke, 1965; Ageno et al., 1973) have been used successfully for low resolution structural studies. At higher resolutions these methods become less accurate, if not totally useless, for three-dimensional reconstruction. A useful method in high resolution studies is that of first reconstructing the three-dimensional Fourier transform of specimen structure using the “projection theorem ” and then, on this basis, calculating the requested three-dimensional structural parameters (De Rosier and Klug, 1968 ; Crowther et d., 1970; Klug, 1971; Lake, 1972). The “projection theorem” can be stated as follows: the transform of the projection of a three-dimensional structure onto a plane coincides with a plane section, passing through the center of the three-dimensional Fourier transform of the structure itself. In fact, if s(x, y, z ) is a three-dimensional structure, its projection a(y, z ) on the yz plane is given by

Its transform is then

=

5 s(x,

y , z ) exp[ - i2n(yvI

+ zvr)] dx dy dz = s(0, vI , vy)

(88)

where the transform of s(x, y, z ) is indicated by S ( v r , v,,, v~). The method of reconstruction of the transform is applicable to any specimen for which a sufficient number of micrographs from different and known directions are available. The very large quantity of data to be evaluated generally requires the use of electronic computers (Frank, 1973). For specimens whose structural symmetries are known, such as crystal lattices and helical structures, it is nonetheless possible to reconstruct the three-dimensional transform directly with the data obtained from the optical transforms of micrographs. In this case, the resolution with which the sample is reconstructed turns out to be much lower; the most meaningful structural parameters can, however, be obtained in a simple and rapid way. 1. Crystal Lattice A three-dimensional crystal lattice M ( x , y, z ) can be described using M ( x , y, z ) = [ R ( x , y, z ) . B(x, Y , 211 0 G(x9 Y , z )

(89)

31

ELECTRON MICROGRAPH ANALYSIS

where C(x, y,z ) indicates the content of the unit cell, R ( x , y, z ) the threedimensional lattice, and B(x, y, z ) a function describing the effective form and dimension of the crystal lattice. The transform of Eq. (89) can thus be written

where M = F M , R = F R , B = .FB, and G = F G . If the vectors a, b, c are three elementary translations of the R(x, y, z ) lattice, the latter can be described by a vector of the form V = ia + hb kc, with i, h, k whole numbers. The transform R(v,,, v,,, vi) of the lattice still has the form of a three-dimensional lattice, the reciprocal lattice, which can be described by a vector v = la rn’ ny, with I, m, and n whole numbers and the vectors a, p, and y related to the vectors a, b, and c through

+

+

a=-

bxc a * (b x c) ’

+

’

cxa = a * (b x c ) ’

axb = a (b x c)

’

-

(91)

The optical transform of a crystal lattice micrograph is a section through the origin of the reciprocal lattice of the crystal itself (Berger, 1969; Reimer et al., 1973; Donelli et al., 1975). If the crystal lattice projection is chosen at random, any one of the following situations may be found: (1) the section intersects the reciprocal lattice only in the origin; (2) the section intersects the reciprocal lattice along a lattice line (unidimensional section); (3) the section intersects the reciprocal lattice along a lattice plane (bidimensional section). If we were to consider the reciprocal lattice as made up of geometric points without dimension, the most probable of the above-mentioned possibilities would be the first, and we would rarely find sections passing through more than one lattice point. Nonetheless, because of the finite dimensions of the crystal lattice M ( x , y , z ) its transform M(v,, v,,, vi) is given by Eq. (W), and it is made up of points [the terms B(v,, v,,, vJ] “expanded” in the reciprocal space in the direction in which M ( x , y , z ) is limited. For this reason optical transforms of M ( x , y, z ) micrographs will, effectively, provide a collection of uni- and bidimensional sections of A7(vt, v,,, vs). This latter can thus be reconstructed taking into account that any two different bidimensional sections must have a lattice line in common, while there is only one way to orient three different bidimensional sections reciprocally. This is achievable given the hypothesis that it is possible to recognize and measure the spacings present in the reciprocal lattice M . We must consider, in fact, that because of a series of sources of error, each spacing of M will be present on the optical transforms with a distribution o f values. We must include in the causes of error the different inclinations of

32

G. DONELLI A N D L . PAOLETTI

sections that intersect a given lattice plane of M (Ohlendorf et al., 1975) the degree of indeterminancy in the knowledge of the actual magnification of each section of A,and the compressions and distortions of the lattice as a consequence of the specimen preparation procedures. These are all causes that will make for different values for the same spacing. In spite of the limitations due to these sources of error, it is generally possible to reconstruct the reciprocal lattice M and from this to calculate the lattice M . This method makes it possible to trace back to a series of structural parameters, such as dimensions and volume of the crystal unit cell. If the resolution on the electron micrograph is sufficient to distinguish, in the unit cell, an asymmetric subunit from its specular image or from the rotated subunit itself, further structural data such as the presence of screw axes or minor planes in the spatial group of the crystal lattice, can be derived also from (Donelli et al., 1975) systematic absences of reflexes on the optical transforms. 2. Helical Structures

The parameters that are necessary to describe the three-dimensional structure of a specimen made up of subunits regularly distributed along a helix (in the approximation of disregarding the content of the repeated asymmetric subunit) are the following: the radius p of the helix, the period c after which the structure repeats itself exactly along the helical axis, the number of turns t that the helix makes over the period c, and the number u of asymmetric subunits contained in t turns. Another parameter, the pitch p of the helix, is obviously related to the ones mentioned above by the relation p = c/t. It is convenient, to treat the helix transform problem in cylindrical coordinates. In this case it can be seen that the transform of a helical structure (Cochran et al., 1952), whose axis coincides with the axis z, is different from zero only on the reciprocal space planes perpendicular to the axis Z , which satisfy 2 = l/c with 1 a whole number. On these planes the transform F ( R , (D, I/c) of a helix passing through the point ( p o , 'po, zo) can be written

(92) where f indicates the transform of the asymmetric subunit, J , the Bessel function of order n, and the summation is extended to all the numbers n that satisfy the selection rule 1 = tn

+ urn

(93)

33

ELECTRON MICROGRAPH ANALYSIS

+

where rn may assume any integer value from - 03 to 03. A geometric interpretation of the selection rule can be given considering the reciprocal lattice of the radial projection of the helix. It has been shown (Klug et al., 1958) that if we introduce a coordinate system onto the reciprocal lattice in such a way that each point is characterized by a pair of whole numbers n, 1, these latter satisfy Eq. (93). This means that the existence of a point (n, 1) on the reciprocal lattice implies the presence of the Bessel function of order n on the lth plane of the transform F ( R , 0,l/c). When the helical structure consists of many different asymmetric subunits that are not all at one radius, the transform will be the sum of the contributions of each of the subunits:

(94) where the index n takes on the values provided by Eq. (93) and the summation is extended over all the asymmetric subunits. If we rewrite the transform separating the part that depends on the azimuthal coordinate @ from the rest, Eq. (94) takes on the form F ( R , 0,l/c) =

1 Tn,, ( R ) exp

(95)

n

where the part that depends on the radial coordinate is given by

An important case is that in which the helical structure also possesses, among its elements of symmetry, an N-fold parallel rotation axis (Klug et al., 1958; Donelli and Paoletti, 1972), i.e., when the structure can be described as a family of N identical parallel helices. In this case it is easy to verify that in Eqs. (95) or (92) the terms whose order n is not an integer multiple of N become zero. We thus see that in this case the transform is still given by Eqs. (95) and (96) or by Eq. (92), where the index n, however, is selected by the two rules

1 = tn + urn,

n

=

KN

(97)

with K a whole number. The micrograph of a helical structure (Moody, 1967, 1971; De Rosier and Moore, 1970; Donelli et al., 1972; Donelli and Paoletti, 1972) can be considered as a projection perpendicular to the axis of the helix, and its transform can be obtained from Eqs. (95) or (92) for a fixed value of the azimuthal coordinate @. What can be observed on the optical transform of

34

G. DONELLI A N D L. PAOLETTI

the micrograph is the square of the modulus of such a section of the threedimensional transform. Limiting ourselves to the simplest case of Eq. ( 1 13), the optical transform is then given by

1F(R

0,l/c) 1’

=

C f2Jn(2npnR)Jp(2nppR)e x ~ [ i ( n- P ) ( @ + n/i2 - ~ 0 ) l n. P

(98)

where the n and p indices take on all of the integer values provided by Eqs. (97). Separating‘ in Eq. (98) the terms with the same values for the n and p indices from the other terms, we obtain a more meaningful form for the square of the modulus:

I F ( R 0,l/c l2

=

Cf’ 1 Jn(27rp,R)l2 n

+ 2 1 f 2 J , ( 2 x P f l R ) J p ( 2 V pcos[(n R) - P) (0+ 4

2 - V0)l

n f p

(99)

where the second summation is extended to all possible combinations of values for n and p on the condition that n # p . As can be seen from Eq. (99), a simple inspection of the optical transform can furnish the axial period c of the helical structure measuring the spacing between the layer lines on which the transform is different from zero. Disregarding the second summation in Eq. (99), which represents the interference between the various diffraction orders, it can be seen that the strongest intensity maxima on the layer lines of the optical transform correspond to the principal maxima ofthe Bessel functions present on those layer lines; then such maxima fall near the points of the reciprocal lattice of the axial helical projection. If we take into account the cylindrical symmetry of the term In f 2 I Jn(2nRp)l2 of Eq. (99), it can be seen that the optical transform is actually formed by two reciprocal lattices related to each other by a mirror plane along the meridian of the optical transform itself. To index the optical transform, the most efficient system is the direct reconstruction of the reciprocal lattice on the basis of the criteria just discussed. Once the lattice itself is known, the Bessel function orders can be estimated from the position of the maxima on each layer line. Since the Bessel function orders must satisfy Eqs. (97), it is generally possible to obtain one or at most a few alternatives. The indexing of the reciprocal lattice of the helical axial projection allows us to obtain, aside from the axial period c, other geometric parameters of the structure: the number t of turns, the number u of subunits

ELECTRON MICROGRAPH ANALYSIS

35

contained in c, and the degree of symmetry N of the rotational axis. The interference between the various diffraction orders described by the second summation of Eq. (99) is generally negligible in the analysis of an optical transform on which only the Bessel functions of lower order are observable. The higher orders, in fact, fall in zones of the transform that correspond to a higher resolution than that obtainable on electron micrographs. We must keep in mind, however, the possibility of observing phase cancellations between Bessel functions of close order, but of opposite sign, which are found on the same layer line. The reconstruction of the reciprocal lattice may help in recognizing the presence of Bessel functions that have been attenuated by such effects.

E . Optical Spatial Filtering Beyond the analysis in spatial frequency of an electron image a coherent optical system can be used for restoration of the image itself with the technique of optical spatial filtering. This technique consists in processing the transform with suitable filters and reconstructing the image with the relative spatial frequency spectrum modified correspondingly to the filter used. A filter may work (Vander Lugt, 1968; Thompson, 1972) both on the amplitude of the spatial frequencies and the phase with which these contribute to the image spectrum. Such a filter can be described on the spatial frequency plane by a complex function H ( v , , v,) = I H ( v , , v,) 1 exp icp(v,, v,) whose modulus I H ( v , , v,) I and whose phase q ( v c , v,) represent respectively the amplitude and phase response of the filter itself. It may then be held, since filters are normally passive elements, that for the modulus I H ( v c , v,)I < 1 is valid. If we then consider M ( x , y ) as representing the image to be elaborated and A ( v c , v,) = F [ M ( x , y ) ] as its spatial spectrum, the operation of the H ( v c , ' v , ) filter is described by the product H ( v , , v,) . M ( v , , v,), while the reconstructed image is M , ( x , , y , ) = . F - l [ H ( v , v,) . q v < , v,)] 1

=

F ' - - l [ H ( V , ,v,)] 0 M ( x , y ) .

For this reason, the spatial filtering operation may be alternatively considered a convolution operation of the image with a suitable function R(x, y ) = F - - l [ H ( v , , v,)]. The optical spatial filtering technique can be used for the restoration of electron micrographs for three purposes: improvement of the signalhoise ratio; restoration of phase images; and separation of superimposed images.

36

G . DONELLI AND L. PAOLETTI

The filters used in each case are simple binary amplitude filters, i.e., the function H ( v g , v,) takes on only real values equal to zero or equal to one. Filters of this type are particularly easy to construct since they are opaque screens with apertures arranged on the diffraction plane of the optical system. The construction of a filter is possible only if we have some a priori information concerning the specimen structure and the causes of degradation of its image. The image information content will probably be changed by using the filter. With this technique it is not the quantity of the available information concerning the specimen structure represented in the image that is increased, but rather its accessibility, i.e., the ability to utilize this same information. 1. Noise Filtering for Periodical Structures

Among many factors that can contribute to the degradation of the image recorded on an electron micrograph, those that have the common characteristic of being generated by random processes are generally described as image “noise.” The main noise sources we may consider are the following: the structure of the film support whose image is superimposed on the micrograph over that of the specimen; the damage that the specimen itself undergoes due to its interaction with the electron beam ; the electron noise from which arises the micrograph granularity. The noise spectrum of an image disturbed by the preceding factors contributes to the “white” and continuous part of the spectrum of the image itself. This allows for a noticeable improvement of the signal/noise ratio (Fraser and Millward, 1970; Donelli and Paoletti, 1972; Frank, 1973) in the image of a periodical structure for which we know the spacing values. In fact, the spectrum of a periodical structure is different from zero only on the points of the reciprocal lattice of the structure itself, while it is practically equal to zero elsewhere. A binary amplitude filter, constructed in such a way as to have its apertures in correspondence with the reciprocal points, allows for the reconstruction of the image after having filtered out most of the noise components. If M ( r )represents the electron-optical image of a periodical structure m(r) disturbed by a noise n(r) we can write M ( r ) = m(r) + n(r), with the hypothesis that the noise can be considered additive, i.e., independent and superimposed on the signal. We consequently have &f(v) = ~ ( v+) n(v), having respectively defined M(v) = T [ M ( r ) ] ,~ ( v=) Y [ m ( r ) ] ,and ~ ( v=) Y [ n ( r ) ] . Before spatial filtering the signal/noise ratio can be written (100)

ELECTRON MICROGRAPH ANALYSIS

37

where W,(v) = lfi(v)I2 and Wn(v) = Ifi(v)lz indicate respectively the power spectrum of the structure and of the noise. After the use of the H(v) filter we shall have fi(v).H(v) = M(v).H(v)+ n(v). H(v)and the signal/noise ratio becomes

If we then consider that IH(v)lzis equal to one in correspondence with the reciprocal lattice m(v)and equal to zero elsewhere, Eq. (100) can also be written

where A i denote the integration areas around the points of the reciprocal lattice and whose size is a property of the particular filter chosen. The signal/noise ratio turns out to be clearly improved by the factor

As can be seen from Eq. (103), such a factor can reach high values when the A iareas of the spatial frequency plane are very small with respect to the frequency band on which the power spectrum of the noise is different from zero. As we have already observed, the use of an H(v)filter on the spatial frequency plane is equivalent, on the image plane, to a convolution operaa periodical tion of the image itself with a function h(r) = Y-'[H(v)].For structure, the H(v)filter is different from zero only on the points of the reciprocal lattice, and the convolution function in fact becomes the lattice of the structure itself. In this case the convolution is simply the superimposition of the image on itself, successively translated by whole multiples of its spacing. The result of this filtering procedure consists in obtaining an idealized version of the image (Taylor and Ranniko, 1974) in which the imperfections and irregularities are almost eliminated. When carrying out this type of image processing, however, it is reasonable to remember that the results may be considered satisfactory only if we have good reason to believe that the studied structure is effectively regular and ordered and that the defects d o not have a structural meaning. 2 . Zonal Filter'ing of Phase Images

The spectrum of the electron-optical image produced by a weak object of phase is adequately described (see Section II,A) by

.F[Ci(r)] = A(v).T(v)

(104)

38

G. DONELLI AND L. PAOLETTI

where A(v) is the Fourier spectrum of the object of phase and T ( v )is the C T F of the electron-optical channel. In the band of spatial frequencies that fall within the objective aperture the CTF usually has several zeros, in correspondence with which it changes sign (see, for example, Fig. 1). The frequencies transferred on the image with an inverted sign limit the resolution within the spatial frequency band that precedes the first zero of the CTF. We must then expect that the contrast of structural details, the size of which corresponds to higher frequencies, is greatly lowered if not actually inverted in sign. Consequently, one way of improving image resolution is that ofeliminating the frequencies transferred with inverted contrast (Hoppe, 1963, 1971 ; Mollenstadt et al., 1968). The filtering of these frequencies can be obtained by a binary amplitude filter H ( v ) designed in such a way as to have its apertures in correspondence with the spatial frequencies transferred with the same sign (Thon and Siegel, 1970; Hanszen, 1973). The corrected image, as it results from zonal filtering on the focal plane, is then given by ci(r) = F-' [ H ( v ). T(v) ~ ( v ) ]

(105) In the case of a point object, the restoration of the image thus performed optimizes the contrast (Lenz, 1970). In effect the spectrum of a point object is a constant, A(v) = c 0 , and its reconstructed image after zonal filtering, from Eq. (126), is given by ci(r) = e0 * r ' [ H ( v )* T ( v ) ] (106) +

The contrast at the center of the image is then simply Cl(0) = const. x ( H ( v ) . T ( v ) ) (107) taking into account that the Fourier transform in the origin is proportional to the mean value of the function on which it operates. Obviously, if H ( v ) = 1 for all of the frequencies present on the image (no filtering), the contrast Ci(0) will turn out to be lowered because of the contributions of the opposite signs in the average. On the other hand, Cl(0) will be a maximum if the filter H(v) is designed in such a way as to eliminate either all of the negative contribution (maximum positive contrast) or else all of the positive contributions (maximum negative contrast) from the average given by Eq. (107). For the correct design of a zonal filter it is necessary to know the CTF in the working conditions in which the image was obtained. This is because the positions of the zeros of the CTF depend on the particular image defocusing. Furthermore, when the astigmatism is not exactly compensated the CTF no longer has rotational symmetry and this introduces the critical problem of the azimuthal orientation of the filter.

39

ELECTRON MICROGRAPH ANALYSIS

3. Separation of Superimposed Images It has already been discussed in the preceding sections that an electron micrograph is a projection of the specimen on the plane of the photographic plate. For example, in the case of a specimen observed in negative contrast, for which the image is that provided from the distribution of the negative stain around the specimen, both the near and far surfaces with respect to the supporting film contribute to the final image on which they turn out to be superimposed (Klug and De Rosier, 1966). Considering a case like the preceding one in which the final image M ( x , y) is provided by two independent components spatially superimposed M , ( x , y) and M , ( x , y) (this type of reasoning could be extended to any number of components), we can write M(x9 Y ) = M l b , Y) + M , ( x , Y ) The spatial spectrum of the image will be

F[M(x, Y)] = =

7

+ M,(V< v,) I f=P[icpl(V< > VJI

v,)

I Ml(V<

>

(108)

9

VJ

(109) + I M,(vt v,) I exP[icp2(v< 4 1 in which F M l = I A?, I exp icpl and F M , = I M , I exp icp, . The intensity distribution on the opticil transform is then given by 7

IS[M(x,

Y ) l l2 = 1 A&

2

7

v,)

+ 2IMAve

I' + I @,(Vt v,)l lM,(VC

3

v,)

3

v,)l

x COS['Pl ( V < ? v,) - 432(V< 9

v,)l

9

l2 (110)

If the transforms of the two component images are spatially separate, i.e., if the term of interference 21M,I IM,I cos(cp, - cp,) of Eq. (110) is everywhere zero on the plane of spatial frequencies, it is possible to contruct an amplitude filter that completely eliminates the contribution of one of the component images, for example 1 A,12, leaving the contribution of the other 1 M , l2 unchanged. In this case, the resulting reconstructed image contains only the M , ( x , y) component of the original image. We must observe, nonetheless, that the interference term in Eq. (110) can never be supposed identically zero. In fact, in the origin of the spatial frequency plane, the transforms MI and M 2 turn out to be necessarily different from zero and the reconstructed one-side image will have a continuous component increased in amplitude. This will bring about a reduction of contrast, especially for the higher frequencies. Also assuming this effect as negligible, it is possible to realize the separation of the component images only if the latter are periodical and very regular (Klug and De Rosier, 1966;

40

G. DONELLI AND L. PAOLETTI

Donelli and Paoletti, 1972; Horne and Markham, 1972; Lake, 1972). In such conditions, in fact, the transforms correspond to lattices which, being constituted by discrete points, can result as separate. Even in this case, however, we can imagine situations in which there is notable interference (Taylor and Ranniko, 1974). In spite of its limitations, the described technique can be useful when the filtering is carried out correctly, i.e., when the reciprocal lattices of the two images are exactly indexed. The reconstructed image, however, must be used with great caution, keeping in mind that such an image is substantially predetermined by the filter used (Taylor and Ranniko, 1974). In cases in which the superimposed images do not present elements of periodicity, the interference cannot in any way be considered negligible and a simple binary filter will not allow for a correct separation of the image.

REFERENCES Ageno, M., Donelli G, and Guglielmi F. (1973). Micron 4, 376. Berger, J. E. (1969). J . Cell B i d . 43, 442. Berger, J. E. (1972). In “Optical Transforms” (M. Lipson, ed.), p. 401. Academic Press, New York. Born, M., and Wolf, E. (1959). “Principles of Optics.” Pergamon, Oxford. Bracewell, R. (1965). “The Fourier Transform and its Applications.” McGraw-Hill, New York. Bradley, D. E. (1967). In “Techniques for Electron Microscopy” (D. Kay, ed.), p. 96. Blackwell, Oxford. Burge, R. E. (1973). J . Microsc. (Oxford) 98, 251. Burge, R. E., and Garrard, D. F. (1968). J . Phys. E 1, 715. Burge, R. E., Garrard, D. F., and Browne, M. T. (1968). J. Phys. E 1, 707. Cochran, W., Crick, F. H. C., and Vand, V. (1952). Acta Crystallogr. 5, 581. Collier, R. I., Burckhardt, C. B., and Lin, L. H. (1971). “Optical Holography.” Academic Press, New York. Cosslett, V. E. (1965). In “Quantitative Electron Microscopy” (G. F. Bahr and E. H. Zeitler, eds.), p. 24. Williams & Wilkins, Baltimore, Maryland. Crick, R. A., and Misell, D. L. (1971). J. Phys. D 4, 1. Crowther, R. A., De Rosier, D. J., and Klug, A. (1970). Proc. R. Soc. London, Ser. A 317, 319. De Rosier, D. J., and Klug, A. (1968). Nature (London)217, 130. De Rosier, D. J., and Moore, P. B. (1970). J. Mol. Biol. 52, 355. Donelli G., and Paoletti L. (1972). Ann. 1st. Super. Sanitd 8, 197. Donelli G., Guglielmi F., Rosati Valente, F., and Tangucci, F. (1970). Ann. 1st. Super. Sanitd 6, 88. Donelli G., Guglielmi, F., and Paoletti L. (1972). J. Mol. Biol. 71, 113. Donelli G., DUva, V., and Paoletti, L. (1975). J. Ultrastruct. Res. 50, 253. Eisenhandler, C. B., and Siegel, 9. M. (1966). J . Appl. Phys. 37, 1613. Enloe, L. H. (1967). Bell Syst. Tech. J . 46, 1479. Erickson, H. P. (1973). Adu. Opt. Electron Microsc. 5, 163. Erickson, H. P., and Klug, A. (1971). Phil. Trans. R . SOC.London, Ser. B 261, 105.

ELECTRON MICROGRAPH ANALYSIS

41

Frank, J. (1973). In “Advanced Techniques in Biological Electron Microscopy” (I. K. Koehler, ed.), p. 215. Springer-Verlag, Berlin and New York. Fraser, R. D. B., and Millward, G. R. (1970). J. Ultrastruct. Res. 31, 203. Garrod, R. J., and Nankivell, J. F. (1958). Brit. J. Appl. Phys. 9, 214. Glick, A. J. (1965). In “Qualitative Electron Microscopy” (G. F. Bahr and E. M. Zeitler, eds.), p. 49. Williams & Wilkins, Baltimore, Maryland. Goldfischer, L. I. (1965). J . Opt. SOC.Am. 55, 247. Hall, C. E. (1953). “Introduction to Electron Microscopy.” McGraw-Hill, New York. Hanszen, K. J. (197 1). Adu. Opt. Electron Microsc. 4, 1. Hanszen, K. J. (1973). In “Image Processing and Computer-aided Design in Electron Optics” (P. W. Hawkes, ed.), p. 16. Academic Press, New York. Harburn, G., and Ranniko, J. K. (1971). J . Phys. E 4, 394. Hawkes, P. W. (1973). “Image Processing and Computer-aided Design in Electron Optics.” Academic Press, New York. Heidenreich, R. D. (1964). ‘‘ Fundamentals of Transmission Electron Microscopy.” Wiley (Interscience), New York. Helmcke, J. C. (1965). I n “Quantitative Electron Microscopy” (G. F. Bahr and E. M. Zeitler, eds.), p. 195. Williams & Wilkins, Baltimore, Maryland. Henderson, W. J., and Griffiths, K. (1972). I n “Principles and Techniques of Electron Microscopy” (M. A. Hayat, ed.), p. 151. Van Nostrand-Reinhold, New York. Hoppe, W. (1963). Optik 20, 599. Hoppe, W. (1971). Phil. Trans. R. SOC.London, Ser. B 261, 71. Horne, R. W, and Markham, R. (1972). I n “Practical Methods in Electron Microscopy” (A. M. Glauert, ed.), vol. 1, Part 11, p. 327. North-Holland Publ., Amsterdam. Johansen, B. V. (1973). Micron 4, 446. Klug, A. (1971). Phil. Trans. R. SOC.London, Ser. B 261, 173. Klug, A,, and Berger, J. E. (1964). J . Mol. Biol. 10, 565. Klug, A., and De Rosier, D. J. (1966). Nature (London)212, 29. Klug, A., Crick, F. H. C., and Wyckoff, H. W. (1958). Acta Crystallogr. 11, 199. Lake, J. A. (1972). I n “Optical Transforms” (H. Lipson, ed.), p. 153. Academic Press, New York. Lenz, F. (1965). In “Quantitative Electron Microscopy” (G. F. Bahr and E. H. Zeitler, eds.), p. 70. Williams & Wilkins, Baltimore, Maryland. Len& F. (1970). Lect. Inr. Summer Sch. Electron Microsc., Erice, April, 1970. Lipson, H. (1972). “Optical Transforms.” Academic Press, New York. Markham, R. (1968). Methods Yirol. 4, 503. Marton, L. L. (1965). I n “Quantitative Electron Microscopy” (G. F. Bahr and E. M. Zeitler, eds.), p. 1. Williams & Wilkins, Baltimore, Maryland. Misell, D. L. (1973). Adu. Electron. Electron Phys. 32, 64. Mollenstadt, G., Speidel, R., Hoppe, W., Langer, R., Katerban, K. K., and Thon, F. (1968). Electron Microsc., Proc. Eur. Reg. Con& 4th, Rome 1, 125. Moody, M. F. (1967). J . Mol. Biol. 25, 167. Moody, M. F. (1971). Phil. Trans. R. SOC.London, Ser. B 261, 181. Ohlendorf, D. H., Collins, M. L., and Banaszak, L. J. (1975). J . Mol. Biol. 99, 143. Radi, G. (1970). Acta Crystallogr., Sect. A 26, 41. Reimer, L., Roessner, A., Themann, M., and von Bassewitz, D. B. (1973). J. Ultrastruct. Res. 45, 356.

Scherzer, 0. (1965). In “Quantitative Electron Microscopy” (G. F. Bahr and E. M. Zeitler, eds.), p. 59. Williams & Wilkins, Baltimore, Maryland.

42

G. DONELLI AND L. PAOLETTI

Shamir, J. (1972). I n “Optical Transforms” (H. Lipson, ed.), p. 299. Academic Press, New York. Sjostrand, F. S . (1958). J . Llltrastruct. Res. 2, 122. Stroke, G. W. (1966). “An Introduction to Coherent Optics and Holography.” Academic Press, New York. Taylor, C. A., and Lipson, H. (1964). “Optical Transforms.” Bell, London. Taylor, C. A., and Ranniko, J. K. (1974). J . Microsc. (Oxford) 100. 307. Thompson, B. J. (1972). I n “Optical Transforms” (H. Lipson, ed.), p. 267. Academic Press, New York. Thon, F. (1966). 2. Naturforsch. A 21, 476. Thon, F., and Siegel, B. M. (1970). Ber. Bunsenges. Phys. Chem. 74, 1116. Valentine, R. C. (1969). Adv. Opt. Electron Microsc. 1, 180. Vander Lugt, A. (1966). Proc. I E E E 54, 1055. Vander, Lugt, A. (1968). Opt. Acta 15, 1. Wells, 0. C. (1960). Brit. J. Appl. Phys. 11, 199. Whelan, M. J. (1965). J. Appl. Phys. 36,2099. Williams, R., and Kallman, F. (1955). J . Biophys. Biochem. Cytol. 1, 301. Yoshioka, H. (1957). J. Phys. Soc. Jpn. 12, 618. Zeitler, E. (1965). I n “Quantitative Electron Microscopy” (G. F. Bahr and E. H. Zeitler, eds.), p. 36. Williams & Wilkins, Baltimore, Maryland. Zeitler, E.(1968). Adv. Electron. Electron Phys. 25, 277. Zeitler, E., and Hayes, J. R. (1965). I n “Quantitative Electron Microscopy” (G. F. Bahr and E. M. Zeitler, eds.), p. 586. Williams & Wilkins, Baltimore, Maryland.

Recent Advances in Electron Beam Addressed Memories JOHN KELLY* Stanford Research Institute, Menlo Park, California

........................

43

................................

44

I. Introduction .............................. B. Motivation ........................................... 111. Storage Media . . . . . . A. Requirements . . . B. Media Types . . . . . . . IV. Surface Charge Storage A. Williams Storage ...........................

............ VI. Alternative Storage A. Amorphous Sem B. Magnetic Storag

. . . . . . . . . . . . . . . . . . 56

. . . . . . . . . . . . . 92 ............. 99 .............

D. Ferroelectric Media .................................. VII. Electron-Optical Systems ............................................

............................

112 112

......................

135

ents .........

.I. INTRODUCTION

The promise of the electron beam as a means of addressing large quantities of digital information with high speed and low systems cost has intrigued many workers since the early days of modern computing machines. The major advantage of any beam memory, whether photons, electrons, or

* Present address: HP Laboratories, Hewlett-Packard Company, 1501 Page Mill Road, Palo Alto, California, 94304. 43

44

JOHN KELLY

ions, lies in its ability to address a location (or locations) on a storage surface randomly from the third dimension. The beam is a flexible interconnection that eliminates the need for extensive hard wiring and, as such, introduces considerable flexibility into the memory design itself. Additionally, relatively large amounts of data can be stored, using few components, with the consequent advantage of low cost. This review is limited to electron beam addressed memories (EBAMs) with emphasis focused on write/read systems rather than on data recording. The objective is to review the current status of research and, in so doing, stimulate activity in an area that appears fundamentally attractive. The references are extensive but not exhaustive. Unfortunately, because much of the work in this field has not been published, it is not generally available to the researcher. It is also significant that a memory device that achieves the fundamental limits of beam physics is still a long way from being realized. It is now possible, however, to predict the appearance of 10’ ’-lO1’-bit memory systems, with access times in the range of 1-10 p e c for the smaller systems and on the order of 1 msec for the larger. Systems of 107-109 bits are already in commercial development.

11. BACKGROUND A . Early Developments

In 1948, F. C. Williams and T. Kilburn ( I ) of the University of ManChester, England, reported the storage of digital information as a charge pattern on the screen of a cathode-ray tube. Eckert, Lukof€, and Smoliar published similar findings in 1950 (2) and referred to work dating back to 1946 at the University of Pennsylvania. At MIT, J. W. Forrester discussed high-speed electrostatic storage at a symposium in January 1947 (3). The tube developed used charge stored on conducting islands on a dielectric plate and was described by Dodd et al. in 1950 (4). These tubes were employed in the first Whirlwind computer in 1951 (5). During the following years, considerable activity was directed toward the application of the so-called “ Williams tube ” to information storage. In many cases, the tube was no more than a standard oscillograph tube, such as the 3KP1 (6).An improved Williams tube was developed by Mutter (7) and used in the IBM 701 computer in 1953 (8, 9 ) .The reason cited by Buchholz for the use of the electrostatic memory was the “practical combination of high operating speed and adequate storage capacity,” plus a short randomaccess time that “permitted instructions to be stored at any location and in any order without affecting speed or performance.” The IBM 701 electrosta-

45

ELECTRON BEAM ADDRESSED MEMORIES

tic memory consisted of 72 tubes, making a total of 2048 words of 36 bits each, and the effective cycle time was 12 psec. The Williams tubes (IBM 85s) were mounted in pairs in 36 pluggable drawers. The basic components of nearly all EBAM tubes can be illustrated by the Williams tube shown in Fig. 1. An electron gun generates a beam of elec-

OUTPUT READOUT SYSTEM ELECTRON GUN

LENS

STORAGE TARGET

FIG. 1. The basic components of an electron beam addressed memory (EBAM) tube.

trons, which is focused by a lens into a small diameter on the storage target. The beam size in the target plane is usually close to the actual data-bit size. The beam stores information by causing some physical change in the storage medium that can be detected at a later time by the reading beam. The location of information in the target plane is determined by directing the beam by means of an appropriate deflector to a particular bit site. This is the random-access mode. Alternatively, scanning in a TV rasterlike mode is also possible and can be used to achieve very high operating speeds in a block-oriented mode. For readout, a means for detecting the stored physical change must be provided. In the Williams tube, charge was stored on the insulating phosphor surface (often willemite). The presence of this charge could be determined by monitoring the ac target current on a collector electrode located on the faceplate of the tube. Signals were detected by capacitative coupling through the glass of the faceplate. A number of major problems were associated with the basic Williams tube. The major difficulty was the effect of “spill ” or read-around ratio, which is a form of bit interference in which repeated addressing of a particular storage location can modify nearby information. This occurs because of electron scattering and redistribution effects within the tube and because the beam itself may have long tails (a small but finite current outside its nominal radius). In the 701, this problem was largely eliminated by scattering sequential addresses throughout the memory. Other problems often encountered were the result of the relatively high incidence of phosphor defects and the occurrence of extraneous noise. The fact that information was stored for only a few tenths of a second motivated several ingenious schemes for regenerating the stored data. In the 701, regeneration was designed in as part of each instruction cycle; one spot

46

JOHN KELLY

in each of the 72 tubes was regenerated during each cycle, in accordance with a counter that specified the location of the next word to be regenerated. Normal operation ensured the regeneration of three locations (216 bits) per instruction executed. Other computers using the Williams tube storage system included the National Bureau of Standards computers SWAC (10)and SEAC (11).These systems used a number of tubes operating in parallel. Typical access times were in the range of 12-16 psec. In addition to the Williams tube developments, there were many related activities, such as the development of the Selectron by Rajchman (12), which, although an extremely complex tube, obviated the need for a precise deflection system. A flood beam uniformly illuminated a matrix consisting of two orthogonal sets of bars. Voltages applied to these bars suppressed the electron emission except in the one area addressed. The information was stored on an “eyelet” located behind the bars. This could be charged to either of two stable states, one more negative than the cathode, so that when subsequently addressed in the read mode, electrons were either permitted to pass or not, depending on the stored charge. The transmitted beam was collected in an array of Faraday cells for readout, or could be viewed on a phosphor screen for monitoring. The selection mechanism appears to be the most significant aspect of the tube design because it is unique. Figure 2 illustrates how electrons are transmitted only when the four address bars surrounding the selected window are made positive. If one bar (or more) is negative, the electrons are either repelled or collected on the positive bars. The number of leads required to operate this address system is 4N’’4,where N is the number of windows. As a result, 16 leads are required for a 256-bit tube, 32 for a 4096-bit tube, or 128 for 1 Mbit. Other advantages of this tube included indefinite information storage by means of a holding beam, an access time of approximately 5 psec, and the possibility of nondestructive readout. In practice, the tube capacity is severely limited by the complexity and hence by the ultimate bit cost. Another development was the Radechon, or barrier grid storage tube, which was developed initially for the comparison of successive signals. [For a review, see Kazan and Knoll (13).]The storage of binary information was investigated by several workers, including Hines et al. ( 2 4 ) and Greenwood and Staehler (15). This tube uses a dielectric target with a fine-mesh barrier grid in contact with the surface, and it operates in the region for which the secondary emission coefficient is greater than unity. In binary storage, the surface is charged either to ground or slightly negative potential. Readout is accomplished by capacitative coupling to the target or by the collection of secondary electrons from the front of the target. In operation, it was found possible to store considerably more information (16,OOO bits) per tube than

47

ELECTRON BEAM ADDRESSED MEMORIES

SELECTING GRID Y DIRECTION

SELECTING G R I D

) t t t t t t t I, \

V

/

O V E R A L L ELECTRON B O M B A R D M E N T

FIG. 2. The unique address mechanism as used in the Selectron. Individual windows are opened by the biasing of the four neighboring selection bars [Rajchman (12)].

in the Williams devices, and a memory cycle time of 2.5 psec, including regeneration, was achieved. In certain applications, however, read-around problems became a major limitation. B. Motivation

In the early 1950s there were few good memory devices. The magnetic core [see Forrester (16)] was just beginning to appear, and the only other systems in regular use were magnetic drums and acoustic delay lines. As a result, the development of the Williams tube as a random-access high-speed

48

JOHN KELLY

memory was significant; memories of 2000 words, as in the IBM 701, were considered large. Today, a wealth of on-the-shelf memory technologies is available. This is illustrated in Fig. 3, where memory capacity is shown as a function of access time for a variety of technologies; at the slow-access end are the mechanical

1014

1012

OPTICAL-

1o4

102

10-8

10-6

o-~

1 10-2 ACCESS TIME - seconds

1

102

FIG.3. Memory technologies; capacity and access time.

devices (disks, drums, and the laser type of film storage memory) and a t the fast-access end are the mainframe memories (bipolar, MOS, and core). The emerging memory technologies (EBAMs, charge-coupled devices, and bubbles) appear in the so-called “access gap” ( 1 7 ) between 1 psec and 10 msec. The EBAM “bubble” represents 10’- to 109-bit systems currently in development or preproduction phases. Not shown are the fast-access optical memories that have reached the preproduction stage (18-20), but

49

ELECTRON BEAM A D D R E S S E D MEMORIES

may eventually compete directly with EBAMs below a capacity of 10"bits. Figure 4 illustrates the development of the beam-addressed memory technology, from the first Williams tubes at the bottom, to the projected developments at the top. The zone between lo9 and 10'' bits represents memory systems with totally electronic access that may be developed

10'2

-

10'0

-

1014

c n .n

I

Y

108

> 0

106

lo4

1

FUTURE EBAMS (with mechanical motion)

FUTURE EBAMS (no mechanical motion)

-

-

-

-

-

E

!

1

I

-

> k

$

1

EBAMS I N DEVELOPMENT

-

I

FIRST WILLIAMS

102

10-8

10-6

1o

-~

ACCESS TIME

10-2

1

102

- seconds

FIG.4. EBAM technology: past, present, and future?

through emerging technologies. The use of mechanical motion must be invoked somewhere between 10' and loi3 bits, thereby slowing the access time by several orders of magnitude. EBAMs have the largest advantage for systems of 1010-1015bits because it appears practical to fabricate these systems a t low cost and with an absolute minimum of mechanical movement. Such systems could also have low volume, high data rates, and excellent reliability.

50

JOHN KELLY

The major reasoning for this projection is based on the knowledge that the present concepts still d o not approach the basic physical limits of an electron beam system. Consider the properties of a 1-keV electron beam: equivalent wavelength: 12 A ; beam velocity: 1.88 cm/nsec; beam current in 100-h; spot at 500 A/cmZ (already approached in some commercially available scanning electron microscopes): 3.95 x 10- l o A, or 25 electrons/lO nsec; deflection speed: an amplifier with a slew rate of loo0 V/psec (some EBS devices already achieve lo5 V/psec) and a deflector sensitivity of 10 V/cm would produce a scan rate of lo* bits/psec for 100-h; bits; depth of focus: 100r (or better), where r is the spot radius. These considerations lead one to believe that systems with 100-A bit size, achieving 10' bits/cm2 and access times of less than 1 psec, may be possible in the foreseeable future. However, the technology has not reached that level; and it is an objective of this review to indicate the level of current achievement, study some projections, and point out a few of the hazards. It is interesting to consider what it is that makes this technology appear much more attractive now than it was in the late 1950s.The fact is that there have been major advances in the following supporting technologies that greatly enhance the performance, reliability, and other specifications of these systems: Electronics. An EBAM relies heavily on highly stable electronic components, such as have only recently become available. For example, it is now possible to purchase, off the shelf and at low cost, the required high-voltage power supplies with stabilities of better than 1 part in lo4. It is also possible to obtain sufficiently precise digital-to-analog converters with settling times in the microsecond range. Electron optics. The art of gun-and-lens design has improved substantially with the use of computers to aid in the theoretical design. The factors necessary to produce reliable stable performance are now understood ; this knowledge has been gained in the tube industry and from the development of such instruments as the scanning electron microscope. In addition, there have been major improvements in deflector design, such as the octupole (21, 22) and the deflectron by Schlesinger (23). Cathode life. Dispenser cathodes with a proven life in excess of 10,000 hr and a potential of 50,000 hr are now available. Historically, the cathode has been a limiting factor in the operational life of a tube. The availability of these cathodes has resolved this problem. Silicon technology. Advances in methods of fabrication and understanding of material properties have not only aided the construction of better

ELECTRON BEAM ADDRESSED MEMORIES

51

support electronics, but have revealed new opportunities for the storage medium itself. Vacuum technology. This technology has advanced steadily over the past ten years. Economical and rugged construction methods, in addition to better materials, are now available.

MEDIA 111. STORAGE A. Requirements

The selection of a suitable storage medium is probably the most important factor in the design of an EBAM because the storage target determines most of the parameters of memory construction and performance, including volatility, error rate, temperature range, beam diameter and energy, current density, electronics requirements, and readout configuration. It may also govern the life of the tube. A storage medium should be composed of inherently good vacuum materials that will not contaminate the other components in the system. It must operate efficiently over a wide temperature range, preferably centered on room temperature. It should represent only a small fraction of the overall cost of the memory tube and should have an operational life of at least five to ten years of continuous use. Even in a write-once system, the need for any type of development process should be avoided so that information is available immediately after being written. An adequate shelf life for unused media is also desirable. An ideal storage target would present a structureless surface to the electron beam so that an address location would be determined by the impact point of the deflected beam and not by the structure on the target. The alternatives are to make the structure fine in comparison t o the beam dimensions and then treat the surface as continuous (the quasi-continuous mode) or provide some fiducial mark system on the target to assist in locating the stored information. The structure on the target itself might be used for addressing and data tracking. Many materials and operating systems have been proposed for use in EBAMs. The principal efforts, however, are currently focused on silicon targets and electrostatic charge storage. The properties of an ideal storage target are described below. 1. Volatility

Any medium used for a very large memory should offer nonvolatile storage. If an archival capability is not required, however, a storage time of

52

JOHN KELLY

several hours may constitute nonvolatility, particularly if a .means can be provided for regenerating or refreshing the stored information. 2. WritelReadlErase Cycling Very large memories (such as 10" bits and more) may be acceptable as write-once systems; however, the ability to use the medium in a read-mostly situation adds considerable flexibility. In systems on the order of 108-10' bits, erasable memory is very desirable. There is increasing emphasis on the achievement of high data rates (100 Mbitlsec or more), and the medium should be capable of high operating speeds or parallel usage. Access times should also be considered. Clear distinction must be made between any limitations imposed by the electronics and the electron-optical system and those related to the beam-interaction properties of the medium itself. A major application of the intermediate size of memory (108-10'obits) appears to be in secondary mainframe storage, and therefore an access time of approximately 1 psec appears desirable. 3. Discrimination and Signal-to-Noise Ratio

Any storage medium in combination with the readout system must provide adequate discrimination between binary ONES and ZEROS and an adequate signal-to-noise ratio to obtain acceptably low error rates. When very high bit densities are required and beams of very high resolution are used, device performance will often be limited by quantum statistics. As a result, in the limiting system, a design should evolve that would obtain an optimal error rate with the minimum number of signal particles; this can be achieved when the contrast ratio is maximized. To further illustrate, the expected error rate can be calculated under the following conditions ( 2 4 ) : Elements are either ONE or ZERO, with a probability of 0.5 for each state. The mean numbers ofelectrons arriving in the readout system are n, from the ONE state and no from the ZERO state, n, > n o . The readout system, including the amplifier, contributes negligible noise compared to n o . Based on the number of electrons arriving in the readout system, a decision must be made concerning the ONE or ZERO condition. If a Poisson distribution is assumed, the probability of collecting & electrons is P ( k 1 ni) = ( ni)'ee-"'I&! where i = 0 or 1 depending on the stored information.

(1)

53

ELECTRON BEAM ADDRESSED MEMORIES

An appropriate decision strategy is to select the state with the highest posterior probability. A level A can be determined so that the following choice can be made:

ONE

if k 2 A

ZERO

if k c A

The decision mechanism is illustrated in Fig. 5. The curves represent the I 0.3

I

I

I

1

I

*A f’ A

CHOOSE

ZERO

:i:

1

I

I

1

CHOOSE ONE

m

g

0.2

g W

F

4,

=

0.1

0

2

FIG. 5. Decision strategy employed in determining ZEROs and ONEs. The curves plotted are for equal occurrences of ONEs and ZEROs; an expected count of 10 for a ONE, and 3 for a ZERO.

probability that a given number of counts will be made. The shaded areas therefore denote the probability of an error. The probability that a ONE will be read as a ZERO is

A similar probability exists that ZERO will be interpreted as ONE:

P(k > A 1 no) =

x

C ni k = ~

e-no ~

k!

54

JOHN KELLY

Because it has been assumed that the two states occur with equal frequency,

Values of P , are plotted in Fig. 6 as a function of the contrast ratio n, /no. lo-loo

a

0

a

IT:

10-4

w

U

0

10-2

> k

J

m

0.1

20

0.2

E

0.3 0.5 1

2

5

10

20

CONTRAST R A T I O

50

100

200

500

- nl/nO

FIG.6. The probability of error in making a ONEjZERO decision, plotted as a function of contrast ratio for a given number of ONE state counts.

Each curve indicates a given number of signal events n,. The value of these calculations is that they show dramatically how the contrast ratio n, /no becomes increasingly significant as the number of events is reduced. The curves indicate that an error rate of is possible with only 100 signal electrons if a contrast ratio of 100: 1 is achieved. These requirements may be relaxed if error-correcting codes are used. 4. Destructive and Nondestructive Readout

Stored information should not be destroyed during readout. This is obviously true in archival systems, and it can also be significant in nonarchival systems when extremely low error rates are required because it is then possible to reread data on the detection of a parity error.

ELECTRON BEAM ADDRESSED MEMORIES

55

5 . I n Situ Gain The signals derived from storage media used at high bit density are apt to be small. To optimize the statistics of the readout system, a localized gain mechanism is desirable. In some systems, such gain can be provided by hole-electron pair production from the incident beam or by a built-in electron multiplier. 6 . Beam Energy

The choice of beam energy is largely determined by the storage target and its mode of operation. It is generally better, however, to minimize the required voltage because high voltages are a source of unreliability, and even microdischarges may be detected by a sensitive readout system. On the other hand, one of the factors that limits current density at the target is beam brightness. The Langmuir limit is J , = J,[(eV/kT) = /3(

+ 11 sin2 a,

V) n sin2 a,

where J o is the cathode current density, V the beam voltage, T the effective cathode temperature, and a, the beam half-angle at the target. In most cases, the brightness is proportional to the voltage, and the current density and the writing rate increase proportionately. This is an oversimplification and will be discussed further in Section VII.

B. Media Types In principle, a wide selection of storage media is available. Most media fall into two categories: those suitable for read/write systems and those suitable for archival memories. In read/write systems, there are three principal methods for storing information: charge storage in a dielectric surface (used directly or to modify some other physical property, such as conductivity or optical properties); the use of thermal effects to cause physical change (in semiconductor or magnetic surfaces); radiation damage (e.g., inhibition of color center luminescence in the alkali halides). The actual mode of usage depends on the selection of a readout technique, such as secondary electron emission, induced target current, photon emission, or optical absorption properties. In archival storage, it is possible to consider the following additional processes :

56

JOHN KELLY

selective removal by: sputtering removal of the surface films or adsorbed layers; electron-stimulated chemical reaction; electron desorption; preferential deposition: activated nucleation by preactivation or simultaneous bombardment with ionization; ion deposition, at low energy, or involving some endothermal reaction to assist in localization; physical change: ion-bombardment damage or implantation; radiation damage of molecules or structures. Although there are many possibilities for archival storage, nearly all the research to date has centered on the use of film (e.g., the IBM 1360 Photo Store) or thermoplastic materials (25). The most thoroughly explored area for read/write erasable stores has been that of charge storage. It is also significant that much of the recent work in this area involves charge storage in or on the dielectric grown on a siIicon wafer. This is due to both the excellent silicon technoldgy available and the fine qualities of thermally grown silicon dioxide (dielectric strength greater than 10’ V/cm, loss factor resistivity up to 10l6Q cm). In related, but unpublished work at CBS Labs, charge storage in SiO, deposited on a variety of substrates, including metal foils and glass, has created the possibility of high-density storage (- lo7 bits/cm2) over large surface areas.

-

IV. SURFACE CHARGE STORAGE The basis of charge storage has evolved from the Williams and Radechon storage concepts. These will be described briefly as an introduction to the later microcapacitor and MOS storage media. A . Williams Storage

In the Williams concept, the charge pattern is stored on the phosphor of a CRT (usually willemite) (1). It is well known (13, 26) that many materials have a total secondary emission coefficient 6 that is greater than unity for primary beam energies of 1-2 keV. Thus when a surface is bombarded with a primary beam for which 6 > 1, more electrons leave than arrive and the target tends to charge positively. If the beam is stationary, it creates a “well” for electrons as shown in Fig. 7. The negative potential ring around the well is created by some of the slower secondary electrons which are drawn back to the phosphor by the positive potential of the well. If they have an energy that produces a secondary emission coefficient of less than unity (6 < l), they will produce a net negative charge. This particular condition results in a stable state because the depth of the well is determined by the secondary emission coefficient and

ELECTRON BEAM ADDRESSED MEMORIES

57

PRIMARY BEAM

DIELECTRIC SURFACE

+ + + + +

FIG.7. The creation of a potential well (for electrons) by the bombardment of a dielectric surface having 6 > 1 [Williams and Kilburn ( I ) ] .

the associated velocity spread. As the well becomes increasingly positive, progressively more secondary electrons are prevented from escaping. The final depth is reached when the effective secondary emission coefficient drops to unity at a typical potential of 3.eV. Readout of this state is illustrated in Figs. 8a and 8b. As stated above, this potential “well” is a stable condition and, on being “interrogated” by the read beam, only a small negative pulse is detected by the output amplifier. This was attributed by Williams to the arrival of negative charge in the vicinity of the screen when the beam was turned on.

58

JOHN KELLY

The provision of a second state relies on the interaction between an existing potential well and the creation of a nearby potential well (Fig. 8c). Because the existing well at site 1 is positive, it attracts some of the lowvelocity secondary electrons from the well being created at site 2. These electrons charge the first well negatively (fill it). Consequently, a reading beam that addresses site 1 first, then site 2 produces a positive going pulse as

TIME

-

FIG.8. Stored potentials and readout process used in the Williams tube: (a) ZERO state, single charged potential well; (b) output detected during bombardment of a single isolated (ZERO state) potential well; (c) ONE state, double spot potential distribution; (d) output detected during bombardment of locations ( 1 ) then (2). After Williams and Kilburn ( I ) .

a result of the re-creation of the well at site 1, then a longer low-level negative pulse caused by the refilling of the well as site 2 is bombarded (Fig, 8d). The negative pulse shown in Fig. 8b is swamped by the positive pulse. Williams found that the effect was maximized at a separation of approximately l.ld, where d is the beam diameter. In memory operation the ONE state can be generated by a moving beam that progresses from site 1 to site 2, thereby writing a dash; therefore, ZEROS and ONES correspond to dots and dashes. The reading process can be used to regenerate the stored information because the existence of a dot or a dash is detected whenever site 1 is addressed. If a dash is detected, the beam is left on as it moves to site 2, thereby refilling the initial well and rewriting the. ONE.

ELECTRON BEAM A D D R E S S E D MEMORIES

59

B. The Radechon

In the Radechon information is stored on a bare dielectric (often mica) with a so-called “barrier grid in coincidence with the surface, as illustrated in Fig. 9 (13). As in the Williams tubes, 6 > 1; however, the presence of the ”

ELECTRON BEAM

G R I DER BARRl

U

M E T A L BACKPLATE

FIG.9. The Radechon storage target. A barrier grid in contact with the dielectric target surface creates a stable potential to which the dielectric surface is always returned under electron bombardment [Kazan and Knoll (13)].

barrier grid modifies the charging process. Under bombardment the isolated surface tends to equilibrium at the potential of the most positive electrode in the vicinity of the target (the barrier grid). If the dielectric surface is negative with respect to the grid, secondary electrons will emerge from the surface and, if 6 > 1, it will charge positively. On the other hand, if the surface is positive with respect to the grid, secondary electrons from the dielectric surface and from the grid will be drawn back to the dielectric, thereby reducing 6 below 1 and causing the surface to charge negatively. Both operations equilibrate with the dielectric at a small positive voltage, at which point the effective secondary emission coefficient 6 is unity. The actual voltage (V,,), typically 2 V, depends on the velocity spread of the emission and the geometry. The dielectric surface is coupled capacitatively to the backplate such that a positive potential applied to the backplate sets an initially uncharged dielectric surface positive, and locations bombarded with the electron beam are set back to t,.Subsequent removal of the backplate bias leaves the

60

JOHN KELLY

surface with a pattern of charges-negative where bombardment occurred and zero elsewhere. The beam therefore tends to set the surface to a potential ( - Vbias), and both positive and negative charges can be stored depending on the sign of I/bias. Readout is accomplished by monitoring the capacitative current flowing through the backplate as the beam discharges the charge pattern to Kq. In digital storage, ZEROs and ONEs are typically represented by stored potentials of Kq and - Kimso that a ZERO gives essentially no output and a ONE produces a positive pulse (14). An alternative method of readout depends on the collection of secondary electrons by means of a collector placed in front of the target. Because more secondary electrons escape the target when it is negatively charged than , and ZEROs can be distinguished by means of a when it is at t qONEs discriminator. Typical ratios achieved are only 1.5 : 1 or 1.3 : 1 [Greenwood and Staehler (15)], thus making adequate discrimination difficult. Typical problems with the Radechon were caused by variations in the secondary emission coefficient across the surfaces. Mica was often used as the dielectric target, and it was found that the nonuniformity could be improved by evaporating a thin layer of magnesium fluoride onto the surface (27). Other problems encountered included “shading,” or variation of the signals across the target surface, and relatively poor read-around numbers. Many of these difficulties have been resolved by the application of modern technology and materials. C . The Microcapacitor (or “ Mucap”) Storage Medium

The microcapacitor (“ mucap ”) storage medium was originally proposed by Shoulders in 1964 (28); and although it is somewhat similar to the Radechon target, it avoids many of the problems. Recent work on this storage MUCAP (MICROCAPACITOR) STORAGE ELECTRODES METAL GATE

1.4 pm

FIG. 10. The microcapacitor or “ mucap” storage medium. Typical mucap dimensions are 0.5-6 pm, hence the name “ mucap.”

ELECTRON BEAM ADDRESSED MEMORIES

61

medium and its application to an experimental memory system have been described by Kelly et al. (22, 29, 128, 129). The mucap target (Fig. 10)consists of a metal-oxide-silicon sandwich in which holes have been etched through the metal gate and part way through the dielectric layer. In the bottom of each hole is an isolated metallic electrode, the mucap. The base is low-resistivity silicon and acts solely in a conductive mode. As in the Radechon, the secondary emission coefficient must be greater than unity (6 > 1) for the writing beam (approximately 2 keV). The base potential VB determines the final mucap potential because the mucap is more closely coupled capacitatively to the base than to the gate. The ONE state corresponds to a negative stored charge (with respect to the gate), and a ZERO state corresponds to a small positive charge. The writing process is illustrated in Fig. 11. Initially, the application ofa negative bias V,, (typically -50 V) to the base causes the uncharged mucaps to be set to negative value V, (typically -40 V). This is the ONEstate condition. The write-ZERO sequence is shown in Fig. 1la. Bombardment of the negatively charged mucap with a beam having 6 > 1 charges the target positively until it stabilizes at V,, which is just a few volts positive with respect to the gate potential; V,, therefore represents the ZERO state. This operation is exactly parallel to the stabilization in the Radechon. The second sequence shown in Fig. 1lb is the process of writing a ONE. The first step is to switch the base potential to V , , (0 V). The ZERO-state elements at V, (- 2 eV) initially are thus switched positive by Vo + V, or AV, as given by

A V = (bl - VBO)cb/ca (6) where C, and C, are the capacitances from the mucap to base and mucap to gate, respectively. For typical target geometries, the ratio Cb/Ca has a value close to 0.84, giving A V = 42 V for V,, = -50 V and V,, = 0. The beam is then turned on and, because the mucaps are positive, charge is collected until the stable potential of V, (- 2 V) is reached, as above. At this point, the beam is turned off and the base potential is switched' back to VBo TABLE I READ-WRITEOPERATION REQUIREMENTS FOR MUCAP OPERATION

Operation

Target bias (volts)

Beam on (relative time)

Write ONE Write ZERO Read

0 -50 - 50

100t, 100t,

',

VBo (-50V ) ’1’ STATE

VBO

VBO

‘0’ STATE

BEAM ON

(a)

WRITE ZERO

I “e

VBo (-50 V )

‘ 0 STATE

VB1 = 0

v

VBl = 0

SWITCH BASE

v

BEAM ON

VBo (-50 V )

’1‘ STATE

(b) W R I T E O N E

FIG.11. Writing sequences for ZEROS and ONES: (a)shows the write-ZERO sequence; (b) shows the write-ONE sequence. A secondary emission coefficient of greater than unity is assumed (6 > 1).

63

ELECTRON BEAM ADDRESSED MEMORIES

WRITE READ

vss-.,

o,

ZERO

R,

1 I

WRITE ZERO

-

WRITE ONE

-v* A

A

-

A

- -

==ca

FIG.12. The equivalent circuit of the mucap storage medium.

(-50 V). The mucaps are switched through - A V and will be set to V, (-40 V) which is the ONE state. This sequence appears to be rather complex, but in operation it is rather simple. The required switching is illustrated in Table I. A simple analysis of these writing processes is instructive. The equivalent circuit for this target is shown in Fig. 12 in which the principal capacitances are: C,, between the microcapacitor and gate; C b ,between the microcapacitor and base; and C,, between the gate and base. Associated with these capacitances are leakage resistances R , , R, ,and R, . In operation, C, and C , are the most important. Rb and R, are normally high as compared to R, which includes any surface leakage along the walls of the hole. The mucap potential is represented by V, and the capacitance to be charged is the combined capacitances C, and C, (Cab).The beam injects or removes current at point s. 1. Write-ONE Process In the following discussion, a careful distinction must be observed between the emission coefficients for the elastically scattered electrons with roughly primary energy q and the true secondary electrons with low energy 6,. The total secondary emission coefficient is denoted by 6. Assume a beam with current density p, at the storage target surface. As described above, because the mucap is initially positive after switching the base to 0 V (V,,), it collects nearly all the secondaries generated. Elastics and high-velocity secondaries, however, will still escape. The net charge being collected per unit area is - p , t( 1 - q), where t is time and q is the backscattering coefficient. As a result, if the target is initially at a potential V,, the potential changes according to

64

JOHN KELLY

& is determined by the initial mucap potential V, and by the change resulting from switching the base from V,, to VB1,and A V is defined by Eq. (6); therefore,

As V, approaches Vo, an increasing number of secondary electrons escape from the mucap until equilibrium is achieved when V, is reached. (This is implicit in the definition of Vo .) Assume that all secondaries with energy greater than eV, escape. The number of secondaries with energy between eV and e( V + 6 V ) can be represented by N e ( V ) SV, and the fractional number of electrons with energy greater than eV, is

The function F( V,) reduces the net charge being delivered to the mucap at a given time to - p,[l - q - 6, F ( V,)]. It can be seen that the nature of F( V,) ultimately determines the limiting value of V, and the rate at which it is approached. The full equation is

Unfortunately, V, is implicit in this equation, but it can be solved by successive approximations. Applicable data concerning the secondary electron energy distribution are difficult to obtain; however, Stehberger's data for gold (30) can be used and scaled to produce a reasonable approximation. Recent mucaps were made with a platinum surface. The total secondary electron emission and the elastically scattered yield for platinum are plotted in Figs. 13 and 14, respectively. There is a large scatter in the data from Bronshtein and Segal (31, 35), Sternglass (32), Warnecke (33), Copeland ( 3 9 and Palluel (36), and the following representative values were used in the calculation: 6 = 1.60 at 2 keV (total secondaries) q

= 0.43

at 2 keV (elastics)

6, = 1.17 at 2 keV (true secondaries) Cab= 6.27 x F/cm2 p, = 1 A/cm2

Figure 15 shows the calculated solution for the fill1 equation. The straight line illustrates how the mucap potential V, varies with time if F ( V , ) is

65

ELECTRON BEAM ADDRESSED MEMORIES 2.0

I

I

I

I

I

a

1.8

-

1.6

1.4 6

1.2

I

1.o

o Bronshtein (Ref. 31) o Sternglass (Ref. 32) 0Warnecke (Ref. 33) ACopeland (Ref. 34)

0.8

0.6

0

6

12

18

-

24

30

36

PRIMARY ENERGY ( x 100) eV

FIG.13. Total secondary electron emission for platinum, plotted as a function of primary beam energy. Data abstracted from refs. (31-34).

ignored. The final potential V, is 2.7 V. The write time is 0.5 psec for the ONE state (measured to the 10% level). This figure is inversely proportional to the beam current density at the target so that 10 A/cmZ should permit 20-MHz write rates under laboratory conditions and probably 10 MHz in practice. 2. Write-ZERO Process

Analysis of the write-ZERO process is similar to the write-ONE process; however, the mucap is initially negative and, as a result, more electrons will escape than are delivered (6 > 1). The net charge delivered per unit time is +p,(q + 6, - 1 ) or + p , ( 6 - 1). For a target initially at V,, the potential

0.5

0.4

0.3 rl

0.2

0.1

0 0

500

1000

1500

2000 2500 3000 PRIMARY ENERGY - e V

FIG. 14. Elastically scattered electrons for platinum. Electron yield of from Sternglass ( 3 2 ) . Bronshtein and Segal (35),and Palluel (36).

3500

4000

4500

5000

plotted as a function of primary voltage. Data drawn

ELECTRON BEAM A D D R E S S E D MEMORIES 50

I

I

40

I

pt

-

=

q = 0.43

B

6 = 1.60

I

6 , = 1.17

i 9

c,

I

1A/crn2

2

>- 30

67

= 6.27

1 0 - 9 ~ / ~ ~ 2

Vo = 2.7 V

I2

w I-

2

20

fi

U 0

!i 10

VO

0 TIME

- wsec

FIG. 15. Write ONE curve. The solution of Eq. (12). plotted as a function of time for the inset parameters.

changes according to .

c

v, = v, + p,r q + 0 , - 1

*

cab

As V, approaches V , , however, fewer true secondaries escape until, when V, is a few volts positive, the effective secondary electron emission coefficient is reduced to unity. This can be described in terms of the same function F ( K). The instantaneous charge density delivered to the target is p , [ q 6, F( V,) - 13; therefore, the full equation becomes

+

As before, V, is implicit in this equation and it must be solved by successive approximations. The solution is shown in Fig. 16. The mucap potential

68

JOHN KELLY +lo

"(

C

v)

-

: I

'

-10

1

L 5

+ 2

pt = 1A/crn2

w b

II = 0.43 h, = 1.17

n -20

cab = 6.27 x

w

F

/

~

~

~

a

0

6

-30

-40

0.2

0.4 TIME - psec

0.6

0.8

FIG.16. Write-ZERO curve. The solution to Eq. (14) plotted as a function of time for the inset parameters.

increases in a positive direction until it reaches approximately 2.7 V. The write time for ZEROS, again measured to the 10%level, is 0.42 p e c which is slightly faster than the write time for ONES.This implies that 10 MHz write rates should also be practicable if 10 A/cm2 can be obtained. 3. Discussion It is informative to compare the initial charging rates for the mucap medium:

ELECTRON BEAM A D D R E S S E D MEMORIES

Write ONE: Write ZERO:

69

-Pt(l - q)/cab

pt(q + 6, - l)/Cab In most write-read random-access systems, it is desirable to have approximately equal writing rates for ONES and ZEROs. This requires

6, = 2(1 - q) (13) and the highest writing rates will be obtained with q = 0 and 6, = 2. In the special case of transient storage, however, it would be possible to prime the target to the ONE state and then write ZEROs only, using a high 6, to obtain fast writing. Because secondary emission coefficients as high as 1000 can be obtained with specially prepared surfaces, data rates up to 1 GHz should be obtainable. Restricting the discussion to systems with equal write rates results in a conflict. In practice, the number of elastics q is a well-defined function of atomic number (2)( 3 2 ) and, for 2 > 30, tends to increase with primary beam energy in the region 0-5 keV, as shown in Fig. 14. For 2 < 30, q is fairly constant above 500 eV. To minimize q, a low-atomic-number material is required. On the other hand, Sternglass (37) has shown that, among the elements, the maximum true secondary emission yield can be obtained with the higher atomic number materials and bears a strong relationship to the atomic shell structure. Platinum appears to offer the best life stability and vacuum compatibility of all the metals despite the rather large elastic coefficient. The relative performance figures for platinum, molybdenum, gold, silver, and tungsten are listed in Table 11. One possibility considered is a sandwich construction with a thin secondary-emission-producing layer over a low-atomic-number material. Because the elastics are produced deeper in the material than the true secondaries, it is to be expected that q can be reduced without affecting the true secondary emission significantly. Because it may be advantageous to reduce the number of elastically scattered electrons (for other reasons such as background-noise level), this multilayer construction is very attractive. The concept of a two-layer secondary-emission surface has been used as a means for measuring electron-penetration depths and for studying secondary electron emission (31, 35). 4. Readout

Information stored on mucaps can be read out in several ways, including the capacitative pickup and secondary electron collection methods used in the Radechon. Energy analysis of the secondary electron emission, however, offers particular advantages.

JOHN KELLY

70

TABLE 11 SECONDARY EMISSION PARAMETERS OF VARIOUS MATERIALS AND THE CORRESPONDING WRITING EFFICIENCES (FOR MUCAPOPERATION)

Material Platinum Platinum Platinum Molybdenum Molybdenum Molybdenum Gold Gold Gold Silver Silver Silver Tungsten Tungsten Tungsten

Electron energy (keV) 2 1 0.5 2 1 0.5 2 1 0.5 2 1 0.5 2 1 0.5

Elastics

Total secondaries

Write-ONE efficiency

(d

(4

(1 - d

Write-ZERO efficiency ( 6 - 1)

0.43 0.36 0.24 0.30 0.22 0.17 0.34 0.28 0.20 0.32 0.25 0.21 0.35 0.28 0.20

1.68 1.80 1.56 0.8 1.15 1.26 1.30 1.40 1.35 1.20 1.48 1.42 ? 1.30 1.30

0.57 0.64 0.16 0.70 0.78 0.83 0.66 0.12 0.80 0.68 0.75 0.79 0.65 0.72 0.80

0.68 0.80 0.56 -.0.20 0.15 0.26 0.30 0.40 0.35 0.2 0.48 0.42 ? 0.30 0.30

~

The mucaps are charged to different potentials ( f 2 and -40 V) for ZEROS and ONES, and secondary electrons created a t the mucap surface therefore emerge through the grounded gate electrode with corresponding energy, as illustrated in Fig. 17. The secondary emission from the target will consist, therefore, of the following groups: elastically scattered electrons with energy approximating the primary beam ; secondary electrons from the gate electrode with a mean energy of approximately 2 eV; secondary electrons from the mucap storage elements having the following most probable energies with respect to ground: ONE state, 2 - V, eV ( - 42 eV); ZERO state, 2 - V, eV ( - 0 eV). One method of readout is to use a bandpass filter that accepts only the 42-eV ONE-state electrons and rejects all others, as illustrated in Fig. 18 which shows the relative energy distributions for the two states. A small noise signal will be contributed by the electrons in the tail of the energy distribution from ZERO-state elements. Several schemes have been used (38, 39), but the most successful is the energy-barrier system (22) illustrated in Fig. 19. Electrons from the storage target first pass through the ZERO collector which is normally operated a

PRIMARY READ BEAM 12 V)

PRIMARY READ BEAM ( 2 KeVl

rn

42 eV

G ctrn

SIGNAL ELECTRON

(-0 e V )

/ \ \ / \\ I Y 1 /- m\\\ - -

0

z

SIGNAL ELECTRON

W

rn

I

ov

2 v

*d

\

GATE-DIELECTRIC-

~/////////////////////' / V B (-50 V )

BASE

}

-4ov

f

...................... V B (-50 V )

(a)

ZERO

Ibl

ONE

FIG.17. The ZERO and O N E states, illustrating the differences in secondary emission used in readout. In the ONEstate secondary electrons are accelerated through a 40-V potential change, and emerge with approximately 42 eV energy.

I

/

E V

v)

72

JOHN KELLY v)

2

0 K I-

2J

PASS BAND

u

B K

w m

Iz

2

"1

(a) READ ONE

v)

2

-1

2

'VP

"I

ENERGY

Ib) READ ZERO

FIG.18. Energy spectra created by the bombardment of ONE and ZERO state mucaps.

V, represents the primary beam energy. few volts positive with respect to the gate electrode. This electrode collects a large percentage of the slow electrons ( < 10 eV) and ensures that relatively few return to the storage target to cause redistribution problems. Electrodes A and B form an electron " mirror " that reflects all electrons below approximately 50 eV. Elastically scattered electrons, however, pass through this mirror. The barrier electrode B produces a potential barrier in front of the predynode D, thereby excluding all secondaries below approximately 20 eV. Most of the electrons in the 20-50-eV range pass the barrier and strike the predynode D. Elastically scattered electrons from the storage target are screened geometrically from entering the predynode ; however, doubly reflected elastics-particularly from the deflector shield-contribute to the background noise. Electrons passed by the analyzer system enter an electron multiplier that provides high bandwidth gain where it is most needed. The performance of this analyzer system is illustrated in Fig. 20 which

B (BARRIER ELECTRODE)

A (MIRROR ELECTRODE)

TARGET

r

D IPREDYNODE)

II I

L

'p--80 V \ r

\

I

\

-

DEFLECTOR SHIELD

I

E (MULTIPLIER FRONT END)

MU LTlP LI ER

nOUTPUT COLLECTOR FIG.19. The energy barrier system used for readout of binary data stored in a mucap target [Kelly el al. (29)]

V,

=

-150

vc

=

+2

-100

h

VD = 0 V E = +50

-40

0

20

40 ELECTRON VOLTAGE

60

80

- eV

FIG.20. Multiplier output shown as a function of electron energy with V,, the barrier electrode bias as parameter [Kelly et al. (29)].

74

JOHN KELLY

shows the multiplier output as a function of electron energy. The parameter used is the barrier-electrode potential V,. It can be seen that the threshold potential (the low-energy cutoff of the analyzer) has an approximately linear relationship to V, as might be expected; the measured cutoff across the threshold was always in excess of 100 : 1 and usually in excess of 500 : 1. Apart from the potentially high contrast of this type of readout, there are other significant advantages. An electron multiplier provides high signal gain with low noise and, because it is a current amplifier, it eliminates the usual problems associated with amplifier input capacitance. This readout is also good in a random-access mode because it is completely isolated from the writing bias circuit which can necessitate a delay when detecting target currents [see, e.g., Speliotis (58)].The only delays encountered in the analyzer system are associated with the time required to address a particular location and the time of flight of the signal electrons through the analyzer and electron multiplier. This is estimated to be sec in typical practical geometries. Readout involves just the determination of the potential stored. It is only necessary, therefore, to sample the stored charge to obtain readout, and operation in a nondestructive readout mode is possible. This enables the recovery of certain statistical errors by rereading in the event of parity failure. If N , electrons are required to enter the readout system to obtain the required error rate, the mucap potential will be changed by N , e(6 - 1)/6,Cab(mr,2)ec,where E, is the analyzer collection efficiency, e is the electronic charge, and is the effective mucap area per bit. If the potential stored is allowed to drop through A V before the error rate becomes too high, the number of reads ( R ) available before refreshing is necessary, is given by

Assuming an effective bit area of 4 pm’, a collection efficiency of 0.1, a permissible potential drop of 20 V, a target capacitance of F/m’, 6 = 1.5, 6, = 1.1, and N , = 100 electrons per read, Eq. (14) gives R = 110 reads per write. Up to 100 reads/write have been obtained in the laboratory. 5. Mucap Targets and Their Mode of Usage The criticism most often raised against the mucap target is the existence of a microstructure; however, this structure is repetitive and is not difficult or expensive to construct. Mucap targets have been made and used in two modes: the bit-per-mucap and the quasi-continuous modes. In both, an accurate highly regular near-perfect array of holes is required. To achieve this, Westerberg et a f .(38,40,41) developed the screen-lens technique for the exposure of repetitious patterns in an electron-sensitive resist.

15

E L E C T R O N BEAM A D D R E S S E D MEMORIES

In this technique, a fine-mesh grid creates an array of miniature lenses, each of which can focus its own image of an object mask or crossover onto the target, as illustrated in Fig. 21. The lens action occurs when an accelerating field is applied between the screen and target so that the equipotentials

OBJECT POSITION

300 eV ELECTRONS F R O M OBJECT

SCREEN 1300 V )

TARGET I 2 5 kVI

(a)

U

lb)

21. Formation of multiple electron images by the screen lens technique. The field between the screen and target creates an array ofaperture lenses which reproduce an image o f a single aperture or crossover as in (a), or a more complex image such a s an electron transparent mask as in (b) [Heynick er a / . (42)]. FIG.

76

JOHN KELLY

bulge through the apertures in the screen as displayed in the inset. Figure 21a shows the formation of many images of a single aperture or crossoverone image per lenslet-and Fig. 21b shows the formation of more complex images by each lenslet. This second technique has been used to produce the exposure illustrated in Fig. 22. Each block of 100 mucaps is a field exposed

FIG.22. Arrays of mucaps made using the screen lens technique. Each block of 100 represents a single lenslet field. Holes are 0.5 pm on 1.2-hlm centers. The screen was a standard loo0 Ipi grid.

by a single lenslet. These mucaps are 0.5 pm in diameter and 1.2 pm center to center. Each subfield is very precise within itself because it is a single parallel exposure and is a demagnified image of a precise object mask. The accuracy between subfields is less good, being on the same order as the screen itself. This technique is promising for making targets suitable for “one bit per mucap” storage because a fiducial mark (Fig. 23) can be included in the mucap field (42). The marker can be made as a permanent ONE-state element by etching completely through the silicon dioxide to the silicon base; it can also be made a part ofeach subfield, thereby providing an accurate method for locating the data. A single scan across the marker can determine the beam location with sufficient accuracy. This method requires reasonable alignment between the beam deflection and the mucap array.

77

ELECTRON BEAM A D D R E S S E D MEMORIES

loooo

FIRST/ BIT

I

P

00000000000 00000 0000 0000 0 MATRIX 0 0

0 0 FIG.23. The use of a fiducial mark to locate the first bit in a data block. The mark in this case is a permanent ONE-state element.

Storing a single bit/mucap requires a moderate degree of target perfection even if encoding techniques employing a high degree of redundancy are used. The principal advantage is the high data-packing density achievable. An alternative is to use a group of mucaps for each bit by making the structure fine as compared to beam size [see, e.g., Dodd et al. ( 4 ) ] . By using this " quasi-continuous mode," infrequent imperfect elements can be ignored if approximately nine elements are covered by the beam. This technique has the further advantages that it is no longer necessary to make the deflection system precisely linear and that data can be written as an encoded string of transitions. Random access to the start of the track can be achieved digitally, and the track can be scanned on the fly with the beam remaining on continuously. This is illustrated in Fig. 24 in which the upper part displays a data pattern written as a string of individual bits and the corresponding charge pattern; the lower section displays the same data written with MFM or Miller encoding [see, e.g., Tamura et al. (130)]. In this case, the ONES are represented by a transition (in either direction) and ZEROs by the lack of a transition, as in NRZ (non-return-to-zero) writing on a magnetic surface. When there is a string of ZEROs, transitions are placed between bits to assist in timing or synchronizing the data flow. As noted earlier, reading partially destroys the stored information. A succession of reads on ONE-state elements will convert it into the ZERO

78

JOHN KELLY 1

MFM

1

-

0

0

0

-

0

1

0

0

-

1

1

0

-

RECORD I N G

FIG.24. Operation in the quasi-continuous mode, using approximately nine elements per bit.

state. This is shown schematically in Fig. 25, where the discharge curve from the ONE state into the ZERO state is a function of the number of reads. In the simplest system, a single comparator is used to distinguish between ONES and ZEROS; however, the incorporation of a second comparator level enables the need to refresh the data to be determined. Pulses falling in the regions between levels 1 and 2 would logically be assigned ONE, but would cause refresh of that particular piece of data. An automatic refresh technique, probably combined with a periodic refresh, has the following implications for this type of memory device: The memory becomes effectively NDRO (nondestructive readout). The loss of information through charge leakage is eliminated. Bit interference or read-around problems are minimized. The effects of small drifts in the beam-positioning system (which cause reduction in the ONE-level sigiials and eventual loss of information) are reduced because the refresh action rewrites the data in the new (drifted) location.

6 . Recent Experimental Data Bit-by-bit storage was demonstrated in a recent experimental system developed to show the operation of the mucap medium in a memory mode (22, 29, 42). Targets were prepared on a relatively coarse scale, using the screen-lens exposure technique. The high quality of the targets is illustrated by the scanning electron micrograph in Fig. 26. The tube was an electronoptical bench type described by Kelly, Moore, and Thornton (29). A standard commercial memory exerciser was used to operate the tube through an interface and control unit (see Section VIII). Write-read operation in the quasi-continuous mode was demonstrated with this system. Typical data-storage times of approximately 2 hr were

79

ELECTRON BEAM A D D R E S S E D MEMORIES NUMBER OF R E A D S

ZERO

-

LEVEL 1

-LEVEL

2

FIG.25. ONE-state readout signal as a function of the number of reads. Two comparator levels are shown illustrating the possibility of automatic refresh.

regularly achieved (42), although it was later determined that some areas of the target could store for as long as 65 hr (43). This is not completely surprising because some silicon vidicon targets can retain information for several days with power on and up to one month under power-off conditions.

FIG.26. A scanning electron micrograph of a mucap target. The holes are 6-pm diameter and are on 8.8-pm centers.

80

JOHN KELLY

The possible factors that could limit the useful life of this medium were considered in detail by Kelly and Moore ( 4 2 ) . The chief uncertainty is related to the life of the secondary emitting surface under electron bombardment. Over an extended exposure to the electron beam, the secondary emission coefficient of the surface is gradually reduced and this can be attributed to the formation of a polymerized layer of organic material over the emitting surface (29, 42-44). The most likely form of contamination is hydrocarbons, either present on the target from a previous processing step or present in the vacuum as a result of inadequate cleaning. In recent work in which degradation of the target was known to proceed at a fairly high rate, targets were removed from the memory tube and subjected to Auger analysis. It was found (42, 4 3 ) that all areas of the target, whether degraded or not, displayed large carbon and small platinum peaks, typically in the ratio of 27 : 1 ; in degraded areas, the ratio was even higher (55 : 1). The source of this contamination is not known, but it appears likely 0.20

I

I

I

RUN RUN 0 RUN A RUN 0

0.15

811 812 812 9

h 0

v1

&2

w0

rr.

0.10

LL

w

u

zc

I 0.05

0

0

0.5

1 .o

1.5

2.0

TARGET EXPOSURE C/crn2

FIG.27. Writing efficiency plotted as a function of electron exposure. The targets were platinum-coated molybdenum with 6-pm mucaps on 8.8-pm centers. The target was known to be contaminated with organic material (see text) [Kelly and Moore (42)].

ELECTRON BEAM ADDRESSED MEMORIES

81

that most of the contamination was introduced during fabrication of the target because the actual partial pressures measured by a quadrupole residual gas analyzer were quite low. Further attempts to clean up the targets have not been made. Some metal oxides and compound semiconductors are very good secondary emitters and could possibly be used to improve writing efficiency; however, certain oxide (A1,0,) surfaces exhibit a greatly reduced secondary yield after prolonged irradiation in good vacuum probably because of beam enhanced reduction of the oxide. Care should be taken to select a material, such as the lead glasses, in which this effect is small. The net result of this form of degradation is progressive reduction in the readout signals and in writing efficiency. Writing-efficiency data obtained from early targets are plotted in Fig. 27 as a function of exposure to the electron beam. The gradual reduction in the readout contrast results in an eventual sharp increase in the measured error rate, corresponding to the end of the useful life of a target. Further details are discussed in (42). This whole area is extremely complex because there are so many variables. Clearly, degradation can be controlled by the nature and source of the contaminant, the nature of the secondary emitting surface, sticking factors, and the effect of the beam on the surface. V. BULKCHARGE STORAGE

The discussion in the previous section was concerned with the storage of charge on surfaces. In this section, the storage of charge in the bulk of a dielectric film will be considered. The storage of positive charge in the dielectric of a metal-insulatorsemiconductor (MIS) sandwich has been observed by many researchers (45-49, 52). The first detailed study was made by MacDonald and Everhart (50) who measured the buildup of positive charge in MOS transistors with a positive gate bias by measuring the threshold voltage for turn-on after bombardment under different conditions of bias. The bombarding electron beam produces electron-hole pairs within the oxide and, with a positive gate bias, the electrons move toward the gate electrode and are collected. The holes, however, appear to be captured at deep trapping centers in the oxide, close to the Si/SiO, interface. This accumulation of positive charge continues to increase ‘until the full gate-bias potential is dropped close to the silicon interface and the field over the bulk of the oxide is neutralized. This phenomenon is illustrated in Fig. 28. The reversal of this process (apparent removal of the stored positive charge) occurs under conditions of negative bias. A typical charge/discharge curve for biases of k0.8 V is plotted in Fig. 29. The charge was measured as

82

JOHN KELLY INCIDENT ELECTRON BEAM

lA#k--

FIG.28. Charge storage in the oxide ofan MOS sandwich under conditions ofpositive bias (qualitative): (a) the geometry, (b) qualitative space charge distribution, (c) electric field in the oxide, (d) electric potential in the oxide. After MacDonald and Everhart (50).

83

E L E C T R O N BEAM ADDRESSED MEMORIES

0

1

2

3

TIME - sec

Charge/discharge curve for an MOS target, bombarded with a 200-pA. 12-kV electron beam (experimental data). Curve A is the charge buildup with positive bias V,, = +0.8 V. Curve B is the charge decay V , , = -0.8 V. After MacDonald and Everhart (50).

the corresponding charge induced in the silicon beneath the gate. Beam exposures to achieve saturation are on the order of 10- C/cm2. MacDonald and Everhart ( 5 1 ) considered the possible use of this mechanism for digital information storage, as illustrated in Fig. 30. The device is basically an extended MOS transistor biased in such a manner that positive charge must be stored in both locations a, and b j , where i = j , for a current to flow. Information is stored in a, as illustrated, and addressing is achieved by temporarily bombarding bj . In an experiment using continuous electron bombardment, an MOS transistor used in this mode revealed no deterioration after 600 cycles; however, a 30% reduction in signal amplitude occurred after lo6 cycles. This was attributed, in part, to possible degradation of the storage process. This storage technique is relatively complex and wasteful of packing density because space must be provided not only for the stored bit, but also for the source, drain, island, and address bit. A. The Depletion-Mode Medium

Huber, Cohen, and Smith ( 5 2 ) suggested an alternative method for readout. The presence of a positive charge in the oxide close to the silicon interface causes inversion in a p-type silicon substrate. In the presence of stored positive charge in the oxide the bombarding electron beam creates electron-hole pairs in the silicon, which are separated by the depletion field. This charge separation results in a current flow between gate and substrate. If there is no depletion layer, the electrons and holes recombine rapidly and

84

JOHN KELLY ,SOURCE

METALIZATION

D R A I N METALIZATION,

INFORMATION LOCATIONS

P+ SO?IRCE

"

Pi

Pt

P+ ISLANDS

DRAIN

FIG. 30. MOS storage concept, using charge stored in the gate oxide. Charge stored at sites a irepresents the stored data. The information stored at site aiis addressed by bombarding site

bi, thereby completing the channel connection if positive charge is stored at site from MacDonald and Everhart (51).

a , . Adapted

a negligible current flows through the silicon base. This storage technique is illustrated in Fig. 31. This type of storage medium has a number of important advantages. Not only is the information relatively nonvolatile, but the target is structureless and therefore potentially very cheap. Each bombarding electron produces many electron-hole pairs in the silicon dioxide and in the silicon. This provides a useful gain mechanism in both writing and reading processes. It is well established that the penetrating beam produces one pair for approximately every 3.7 eV dissipated in the silicon at room temperature (53,5 4 ) . Thus gains on the order of lo00 can be achieved. Cohen and Moore ( 5 5 ) estimated a gain of 540 could be produced by a 10-kV beam in a target having a 4OOO-Aaluminum gate and a 2500-A thermally grown oxide layer. Hughes el al. ( 5 6 ) calculated the electron-hole pair gain in the silicon as

85

ELECTRON BEAM A D D R E S S E D MEMORIES ELECTRON BEAM

w+++++

DEPLETION REGION

P-TYPE Si

SIGNAL OUT

FIG.31. The depletion-mode MOS target. Stored positive charge causes inversion of the p-type silicon substrate. Subsequent electron bombardment causes differences in the current Rowing in the base circuit because of the carrier separation across the depletion field. Adapted from Speliotis (58).

a function of beam voltage and the effective oxide thickness T, (the equivalent thickness of oxide having the same beam retardation as the combined gate and oxide layers). This information is shown in Fig. 32, together with the average penetration into the silicon TA. Cohen and Moore have considered the behavior of the depletion-mode target in a format suitable for a practical memory application (55). To obtain a large readout signal, it was determined that the target must first be set to the ONE condition, with positive charge stored everywhere. This charge must be large enough to ensure a highly conductive inversion layer at the surface of the silicon. ZEROS are then represented by isolated areas of zero or small positive charge. During the readout of a ONE, the electronhole pairs are separated across the depletion region and the electrons travel toward the Si/SiO, interface. Instead of piling up at the interface causing flattening of the bands and subsequent reduction of the signal current, these electrons flow laterally through the interconnected inversion layer. Figure 33 represents the equivalent circuit for this type of MOS storage medium. The beam action on the target can be represented as a current

JOHN KELLY

86 0.6

5 I

0.5

U

c

I

0.4

2

0

c a 0.3 a

L

2 a Lu

0.2

Lu (3

Q

5

>

0.1

U

0

0.2

0

0.6

0.4

0.8

E FFE CTIV E OXIDE THICKNESS - T o x p r n

FIG.32. The calculated relationship between the maximum gain G,, beam energy, average penetration depth into the silicon ( T J , and effective oxide thickness T,. From Hughes et ol. (56).

,,

---

hCo

IiIi.

Ic

*E

*E

WA

D

Rl

RL

--

€ ?L I--

ELECTRON BEAM ADDRESSED MEMORIES

87

generator feeding into a circularly symmetric RC transmission line. The magnitude of the current is the beam current I , multiplied by the effective gain G . The inversion layer is represented by a sheet of resistivity pE (R/square). The oxide layer is represented by the distributed capacitance SC, (F/cm2), and the depletion layer by SC, (F/cm2) and the equivalent diodes D ;also shown is a finite resistance R, corresponding to the beaminduced conductivity in the oxide layer. Not shown is the potential distribution created by the stored positive charge which will tend to cause a current flow in R, in the direction indicated by the arrow. During bombardment with the electron beam, the electrons from electron-hole pairs created near the depletion field flow toward the silicon-silicon dioxide interface. They cannot flow through the oxide even though bombardment-induced conductivity (BIC) provides a conductive path because the potential across the oxide created by the stored charge is in the wrong direction. The BIC electron current also tends to flow toward the interface. It is important, therefore, to consider the lateral electron flow in the inversion layer. Cohen and Moore ( 5 5 ) divide the problem into two distinct phases. First, a lateral flow of carriers, so rapid that space charge does not build up and the diodes D do not become forward biased. Second, the capacitors become charged and the diodes are forward biased, thereby short-circuiting the current generator. In the first phase the existence of the diodes and R, are ignored, and R, is assumed to be small so that C , and C , act in parallel. The current flowing in the inversion layer I(r, t ) is given by I(r, t ) = I,Gu(t) exp(-pEC,r2/4t)

(15)

where r is the radial distance from the origin, t is the time since the beam was turned on, ~ ( tis) the unit step function, and C , is the combined capacitances ( C , = SC, + K,). The solution to Eq. (15) is shown in Fig. 34 where the normalized inversion layer current is given as a function of radius. It will be seen that the wavefront travels outward with a steadily decreasing velocity. In the second phase, the capacitors C , become charged and the diode action must be considered. Using the lumped equivalent circuit in Fig. 35, the following expression for the load current i, has been attributed to Fu and Sah [see Cohen and Moore ( 5 5 ) ] : i,

=

(I

+ &)( C o" + Cw )I.Gu(t) IGB

JOHN KELLY

88 I

I

I

I

10-2

10-1

1

1 .o

-u .

; ; 0.5 i

-

-

0

10-~

10-~

10

RADIUS -- rlcrn)

FIG.34. Normalized inversion-layer current as a function of radial distance from the origin, for a selection ofdifierent times after beam turn-on. C, = 8 x lo-' F/cm2, pE = 3 x lo3 fI/square. From Cohen and Moore (55).

EQUIVALENT GENE RATOR

FIG.35. Equivalent circuit used to calculate the load current i, during the second phase [Cohen and Moore ( 5 5 ) ] .

where m is a constant between 1 and 2, T is the absolute temperature, and I, is the diode current defined as i, = I,[exp(eV/mkT) - 11

(17)

in which V is the inversion-layer potential and ID is a constant. Equation (16) represents a transient current, initially large and then gradually decreasing as the capacitors become charged and the diodes forward biased. Theoretical and experimental data are plotted in Fig. 36. It can be seen that the peak readout current is approximately proportional to the beam current and that the length of the transient decreases correspondingly. Cohen and

89

ELECTRON BEAM ADDRESSED MEMORIES

Moore noted that a similar negative transient exists on turn-off as C, and C , discharge through the equivalent diode. The relatively long time constants associated with the forward biasing of the depletion diode do not limit the readout rate. This rate is limited by the rise and fall times of the readout current i, as seen in the load resistor. If R , is fairly low, this becomes

to a good approximation. Typically, the parallel combination of C, and Cw F, giving a time constant of 1.0 psec with a 100-R load resistor. is

-

----

--

-\

-- 0

200

iB

I -

60 n A

iB =

29 n A

iB =

5 nA

z I

400

-s.

600

TIME -- psec

FIG.36. Readout current as a function of time for 5-, 29-, and 60-nA beam currents. Solid line calculated from Eq. (18). Points are experimental data: read bias, + 10 V ; target, 0.5 R cm Si; 2300-A oxide; 600-A molybdenum gate. After Cohen and Moore (55).

Cohen and Moore also measured write sensitivity. The data in Fig. 37 were measured by monitoring the readout current as a function of read bias for a prescribed write exposure. This set of curves is extremely informative. There is a threshold read bias for each write exposure. Below this voltage, a

90

JOHN KELLY

readout current is not detected because the inversion layer has insufficient conductivity to prevent charge buildup close to the beam landing position. The equivalent diode rapidly becomes forward biased and the output shortcircuits. At more positive bias, the inversion is stronger and the readout current can build up.

t

---

I

I

I

-

2.4 x 10-8C/crn2

--4.9

+2

x 10-9~1crn~

o

C/crn2

I----

w

E3 4

I

I

1.5 x 10-8Clcrn2

A-

u

3 0

.2

9

w

U

0 -40

-30

-20

-10

0

10

20

R E A D B I A S - volts

FIG.37. Readout current as a function of read bias voltage for different levels of beam exposure. Write bias, +20 V; beam current, 2 nA; target, 0.5 R cm Si; 2500-A oxide; 1600-A polysilicon gate. After Cohen and Moore (55).

For high write exposures, the output becomes saturated at a level that depends on the bombarding beam current. A beam current of 20 nA yields a saturation current of approximately 6.1 pA for the target under investigation-a gain of 305. This statement is supported by the data in Fig. 38 where the charge density stored is shown as a function of the writebeam exposure. These data were calculated from the readout threshold bias by the relation Q = C , V,, where V, is the read bias. The stored charge is strictly proportional to the write dose in this graph and shows no sign of saturating. The write-ONE and erase (write-ZERO) doses are plotted as a function of bias in Fig. 39. Here, the write dose was defined as that necessary to produce a readout signal of 50% the saturation value at 0-V read bias. This target required a write exposure approximately one order of magnitude larger than the target used in obtaining the data for Figs. 37 and 38. The erase exposure was measured in a similar manner and is seen to be smaller than the write-ONE dose.

91

ELECTRON BEAM A D D R E S S E D MEMORIES

The only factor that has not been considered in depth is the read-ZERO process. The original explanation offered by Huber et al. ( 5 2 ) appears inadequate because, even after erase, a small positive charge left in the oxide causes a slight inversion to remain. If the diameter is sufficiently larger than the read-beam diameter, the inversion-layer impedance will be high enough to cause forward biasing of the diodes and short-circuiting of the readout signal. N

5 . C

e

"

-

6

x

10l2

U

E X

0

f

a

4

U

lx

0

m l-

w

c7

a: a

2

I

V

U

1 L

m

B

0

0

0.5

1 .o

1.5

2.0

x 10-8

W R I T E EXPOSURE -- C/crn2

FIG.38. Positive charge buildup as a function of write exposure. Details as in Fig. 36. After Cohen and Moore (55).

The normalized readout signal SZERO/SONE is shown in Fig. 40 as a function of ZERO diameter. These measurements were made using a nominal 3-pm diameter read beam. The logarithmic dependence of the readout current on the ZERO diameter can be related directly to the logarithmic dependence of the inversion-layer resistance on the ZERO diameter. Care must be taken when optimizing the design parameters because the ZERO diameter required to achieve a 3 : 1 discrimination ratio must be eight times the nominal beam diameter (at 0-V read bias). Careful optimization is required to maximize the data-packing density unless an alternative operating mode can be found.

92

JOHN KELLY

10-6

N

i . u I Lu

\

BO

WRITE

W v)

Q

a

UJ

a

0 Lu

L

5

10-~

ERASE

0

20

W R I T E OR E R A S E B I A S

40

-

volts

FIG.39. Write and erase exposures as a function of write (erase) bias. Beam current, 20 nA; target, 0.5 i2 cm Si; 2300-A oxide; 600-Amolybdenum gate. After Cohen and Moore (55).

B. The Accumulation-Mode Medium

An interesting alternative that resolves some of the difficulties with the depletion-mode medium is the three-terminal accumulation-mode target described by Ellis et al. (57). The target has a thin n-type silicon layer

93

ELECTRON BEAM A D D R E S S E D MEMORIES 1 .o

k

'

z

W

a a:

\\ -\ \ \

3 V

I-

3

0

'\ \\B

\

0 Q

Lu

\

a

\

0

a:

\

W

N

0

-

0.1

J

a

\

2

a 0 z

\

'

\

\

\ \ B\

'\ \ ' \

' \

'

\

\

\

R E A D BIAS

- o vv ----- v -- +lo -10

+

\

-\

@ \

-20. -30 V

0

20 ZERO DIAMETER

\" 40

- prn

FIG.40. Normalized readout current as a function of Z E R O diameter, for various value of read bias. Beam diameter, 3 pm; target, 0.5 R cm Si; 2500-A oxide; 1600-A polysilicon gate. After Cohen and Moore (55).

94

JOHN KELLY

between the oxide and the p-type silicon substrate (see Fig. 41). Positive charge is again stored in the insulator layer; however, the positive charge is now used to modulate the surface recombination velocity and hence the number of minority carriers collected by the reverse-biased p-n junction. The excess holes created by the electron beam in the silicon can recombine with electrons in the bulk of the silicon, i.e., in the thin n-layer or at the ELECTRON BEAM

METAL\

I)

c

+++++

N - T Y P E SI

0

0

$ 8 77 DIODE BACK BIAS

P-TYPE Si

FIG.41. Three-terminal accumulation-mode medium. The target operation relies on the change in junction collection efficiency of minority carriers when a positive charge is stored in the oxide. Adapted from Hughes er a/. (56, 225).

Si-Si02 interface, or they can be collected by the junction and thereby produce an output current. Ellis et al. (57) concluded that the bulk recombination of the carriers in the silicon is insignificant when using an n-type layer of 10-pm thickness or less. The effective surface-recombination velocity S, on the other hand, varies considerably with the charge stored in the oxide. In the flatband condition or in depletion, S can be on the order of lo6 cm/sec for an interface state density of 10'2/cm2; however, when a positive charge stored in the oxide causes accumulation in the n-type layer, S is on the order of lo3 cm/sec. This large reduction occurs because most of the interface states are occupied by electrons and hence are unavailable as recombination centers.

95

ELECTRON BEAM ADDRESSED MEMORIES

Ellis derived the following equation from a one-dimensional diffusion calculation for the probability P that a hole created a distance x from the Si-SiO, interface will be collected and hence contribute to the output current: P=

1 1

+ (S/D)x + (S/D)d

where D is the hole-diffusion constant and d is the thickness of the n-type layer. Assuming D = 10 cm2/sec for 1 R cm n-type silicon, x = 0.4 pm, and d = 2 pm, Eq. (19) gives P = 0.98 for accumulation and P = 0.24 for the flatband condition. The actual gain is P times G,, where G , is the number of electron-hole pairs created in the silicon (see Fig. 42) and is clearly the maximum gain obtainable for S = 0 ( P = 1). Early measurements of the gain have been reported (57) for a target with a 10-pm n-type layer and a 0.6-pm oxide layer. The gate was 800-A of aluminum. The measured diode gain is plotted in Fig. 42 as a function of electron exposure under two conditions of gate bias, V, = 45 and 0 V. These curves, therefore, are a direct measurement of writing sensitivity in addition

I

I

I

4

5

I

I

I

7

8

9

I

400 t

u

a.

100

0 0

1

2

3

6

-

COULOMBS~C~

1 0 1 1

Xio-*

FIG.42. Diode gain in the accumulation mode MOS target as a function of electron exposure. (a) Bias = 45 V; (b) bias = 0 V. Target, 0.6 pm oxide; gate, 800-A aluminum; silicon 1 R cm, n-type, 10-pm thick on p-type substrate; beam, 10 k V [Ellis er al. (57)].

96

JOHN KELLY

to contrast and output signal. Gains of 1400 have been reported more recently (56) using a 10-kV beam on a target with a 0.4-pm oxide layer and a thin gate. C/cm2 is It can be seen in Fig. 42 that a net exposure of 1.2 x required to write the ONE state to 50% of saturation level and 2.4 x lo-' C/cm2 to write the ZERO state. Comparative data for the depletion and accumulation modes are shown in Table 111. The data are somewhat scattered because of the many variables and differences in measuring methods. Given the identical conditions, it appears however that both modes require similar write exposures. The significant physical difference between . the depletion- and accumulation-mode MOS media, as described here, lies in the contrastcreating mechanisms. Charge carrier separation occurs in both modes. In the depletion mode, the output is limited by the conductivity in the inversion layer; however, in the accumulation mode, the n-type surface layer can have low resistivity ( - 1 R cm) and does not limit the output. The bit size and packing density are set by different factors in the two modes. The ZEROS in the depletion mode target must be significantly larger than the beam size so that adequate contrast can be achieved during readout. The accumulationmode target, however, appears to be limited only by electron scattering in the target and by the lateral diffusion of the holes created in the n-layer. Measurements of the electron scattering (56) indicate that the effective increase in beam size is approximately 0.5 pm. Calculations of the lateral diffusion indicate that the optimal n-layer thickness is less than one-third of the bit spacing. Considerations of this type indicate that a bit spacing of 1 pm (perhaps less) should be achievable by using appropriately thin oxide and n-layers. For example, a 0.6-pm beam at 5 keV might be used with an 0.1-pm effective oxide thickness and an 0.3-pm n-type layer. Measurements of readout response made by Hughes et al. ( 5 6 ) on the accumulation-mode target are largely independent of frequency up to 20 MHz. It is anticipated that the ultimate limit will be determined by the hole diffusion time in the n-type layer, d2/D, which is approximately 1 nsec for a 1-pm-thick n-type layer. This appears to be rather faster than the depletion mode. Both the depletion- and accumulation-mode targets can be used in a partially destructive readout mode (PDRO). The number of reads before refresh becomes necessary depends on the write exposure and bias. The normalized readout gain is shown in Fig. 43 as a function of the number of reads. Perhaps the major limitation on both the depletion- and accumulationmode MOS media is associated with the delay required between writing and reading operations. In both cases, readout involves the detection of a small

ELECTRON BEAM ADDRESSED MEMORIES

97

98

JOHN KELLY

+

current flowing in the silicon substrate. Switching the gate from the 45 V write-ONE bias to 0 V for reading involves charging the MOS capacitance through the associated impedances. This introduces in the readout circuit a disturbance that must be reduced to a low level before readout can begin. The problem can be largely circumvented by dividing the silicon target into a number of reasonably sized pieces and by careful design of the switching and readout circuitry. Typical delay times are in the range of 10-30 p e c (56, 58).

lo-“ C/crn2, + 15 V

1.0

0.8 0

$ . z

u

c7

0.6 C / c m 2 , + 40 V

0.4 lo-’

C/cm2, t 60

-

v

0.2 0 0

10

20

30

40

NO. READS

FIG.43. Signal reduction as a function of the number of read operations. The parameters are write exposure and write bias. Read exposure, 5 x lo-’ C/cmz/read [Hughes et al. ( 5 6 ) ] .

In the discussion of the mucap medium, a fatigue or degradation mechanism was described that involved the decrease of the secondary emission coefficient with exposure to electrons. The MOS media discussed above store charge in the oxide and suffer from a different form of fatigue (55, 57). The bombardment of the oxide layer by electrons introduces localized states, and those close enough to the silicon substrate to interact electrically, are known as interface states. These interface states alter the bias conditions at the silicon surface and, consequently, the readout gain. At approximately 1 C/cm2, this damage reaches a point at which the read signals have significantly decreased. It is reported however that the fatigue can be removed by annealing at 320°C for a short time (57). The importance of this level of fatigue depends on the mode of target usage. In a block-oriented mode where the target can be used uniformly, a 30,000-hr life has been predicted for a 32 x lo6 bit tube operating at lo7 bits/sec. In a random-

ELECTRON BEAM ADDRESSED MEMORIES

99

access mode where it is impossible to make the usage uniform, fatigue may become a problem more rapidly. It is reasonable to ask if there is any useful degree of annealing at room temperature or if the beam itself could be used to heat and anneal the target locally.

VI. ALTERNATIVESTORAGE MEDIA This section describes some of the alternative storage media that are generally less fully developed than the charge-storage media described previously. They appear also to be more ideally suited to read-mostly or archival memories. Relative performance of the various media is summarized in Table VI. A . Amorphous Semiconductor

In 1971, Chen et al. ( 5 9 ) proposed the use of the transformation from the amorphous state to the crystalline state in the chalcogenide glasses as a means of storing data. Such information would be nonvolatile and suitable for archival storage. The basic storage mechanism involves raising the temperature of the amorphous thin film to the crystallization temperature. Localization of the required heating effect and crystallization within a microsecond have been demonstrated with laser (60) and electron beams (59). The possibility of a fast erase or write-ZERO process also exists. To achieve this, however, the chalcogenide glass must be heated to above' the melting point and then rapidly cooled. 1. Electron Beam Writing

The major questions concerning electron beam writing are the time necessary to heat the amorphous film locally to the transition temperature, the time for the transition to occur, and the time to subsequently dissipate the heat. Fortunately, the transverse conductivity in the glassy material Ge-Te-As is relatively low, while the heat conduction out through the substrate can be relatively high. The problem of electron beam heating has been considered by several authors (61-63) but without consideration of the variations in the thermal properties with temperature. Chen ( 6 4 ) used a detailed computer-simulation model and derived a set of heating and cooling curves for a thin amorphous layer located on top of a conducting substrate. Electron penetration is included, but the energy loss is assumed to be uniform with penetration depth. The beam is also assumed to have a uniform current density. As may be expected, the data concerning the thermal properties are inadequate, and

100

JOHN KELLY

simplifying assumptions were necessary. Nevertheless, the resulting heat-flow curves are very instructive. The calculated surface temperature of a silicon substrate coated with 4OOO A of molybdenum and 3750 A of chalcogenide glass is shown in Fig. 44,as a function of time. The 5-kV 2-pm beam is turned on for 1 psec at t = 0. The transition temperature was assumed to be 315"C, and the melting 700

I

I

1

I

I

1

1

-

-

500 -

-

600

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

TIME - psec

FIG.44. The surface temperature in a chalcogenide glass film (3750-A thick) as a function of time for various beam currents. Beam, 5 keV; 2 pm diameter; exposure, 1 psec; substrate, 4000-A molybdenum on silicon [Chen (64)].

point was 425°C. Because writing occurs a t the lower temperature, it would appear possible to achieve the required heating in a tenth of a microsecond. Cooling generally takes slightly longer. The 0.2-pA beam current corresponds to approximately 6 A cm2. Writing currents are necessarily fairly high because the maximum temperature that can be reached by the target under the beam is determined by a balance between the energy delivered by the beam and the heat conduction away from the beam location. As a result, current and time are not completely interchangeable as with electrostatic targets. The other parameter of interest is the spread of the " hot spot" that will limit the final bit size. The temperature distribution in the target is shown in Fig. 45 for the time t = 1.0 psec. Even with a 1-pA beam, the spread is less than 0.5 pm. This spread will continue after the beam is turned off; however, the effect is not large because the transverse conductivity is relatively poor in the chalcogenide layer.

ELECTRON BEAM ADDRESSED MEMORIES 700

I

I

I

I

I

10s I

600

500

0

400

I

c

300

200

100

n 0

1 .o R A D I A L DISTANCE

2.0

3.0

- krn

FIG.45. The surface temperature in a chalcogenideglass film (3750-Athick) after exposure to beam currents of various magnitudes for 1 psec. Beam, 5 keV; 2 pm diameter; substrate, 4000-A molybdenum on silicon [Chen ( 6 4 ) )

2. Electron Beam ErasurelWrite ZERO As explained earlier, this process involves heating the chalcogenide layer above its melting point and subsequent rapid cooling. This was assumed to be 425°C in calculating the curves in Figs. 44 and 45. These figures demonstrate that a beam current approaching 1 pA must be used to heat the full 2-pm spot to this temperature in 1 psec. Subsequent cooling requires approximately 1 psec, largely because the latent heat must first be dissipated. Very few data on the erase process have been published. 3. Readout One proposed method for readout relies on the change in the secondary electron emission coefficient between the amorphous and crystalline phases

102

JOHN KELLY

(64). This has been investigated by Chen et al. (65), who showed that, in many cases, the actual change is on the order of 10%. Because this modulation is rather low, readout and discrimination are difficult. A method that improves the signals uses the surface deformation that results from the phase change (66, 67). The secondary emission yield increases as the primary beam strikes the surface at smaller glancing angles. The relationship was derived by Bruining (68) as 6,/& = exp[aX,( 1 - cos O)] (20) where a is the secondary electron absorption coefficient, x,,, is the mean depth from which secondary electrons can escape into the vacuum, and O is the angle with the normal to the surface. Chen and Wang (66) measured the modulation efficiency for several beam voltages (see Table IV) using a target coated with a 100-A layer of molybdenum before exposure to the beam. This metallization enhances the signal output. Chen et al. (67) calculated that to achieve a modulation efficiency of 50% a surface deformation of 45" will be required. TABLE IV MODULATION EFFICIENCY OBTAINED FROM A GIVENREAD-BEAM VOLTAGE FOR AN AMORPHOUS SEMICONDUCTOR TARGE~ Beam voltage

(kV)

Modulation efficiency

10 5 3

0.14 0.17 1.43

The measurements were made in a scanning electron microscope with a 1-pm diameter, &nA, 10-keV write beam. From Chen and Wang (66).

Chen and Wang (66)also calculated the maximum readout rate based on beam and secondary emission limitations at an error rate of lo-'. The calculated rate depends on bit size and cathode loading as may be expected. The data are listed in Table V. The amorphous semiconductor memory mechanism appears promising for archival or read-mostly systems but is not currently suitable for write/read operation. Small bits (1.4 pm) have been observed (66),and high densities are possible but at the expense of the readout rate. From published information, the major limitation appears to be the relatively poor signal-tonoise and discrimination ratios.

103

ELECTRON BEAM ADDRESSED MEMORIES TABLE V

CALCULATED READOUTRATEAND OTHER PARAMETERS FOR AN AMORPHOUS SEMICONDUCTOR ELECTRON BEAMMEMORY' Cathode loading Bit size (pm) Density ( b / h 2 ) Data rate (Mb/sec) Target current readout Secondary electron readout

3 A/cm2 0.5 109

1 .o 2.5 x 108

10 A/cm2

0.5 109

0.2

1.o

1.o

10.0

80.0

50.0

1 .o 2.5 x 108

7 .O

100 A/cm2

lo9

1 .o 2.5 x 108

10.0

60.0

0.5

300.0

Assumptions: error rate = lo-'; spherical aberration coefficient C, = 25 cm; target capacitance C, = 25 pF; beam voltage V = 3-10 kV;modulation coefficient M = 0.1 + 0.4.

B. Magnetic Storage Media The storage of information in a magnetic medium has many advantages, particularly nonvolatility and archival possibilities. The writing and reading of magnetically stored information by means of electron beams have been considered by many workers (69, 71, 78-81), and methods for writing (controlling the direction of magnetization) are fairly well established. Unfortunately, most readout techniques result in rather poor output signals with low signal-to-noise ratios. 1. Curie-Point Writing

In 1958, Mayer (69) described Curie-point writing in a MnBi film deposited on cover glass disks. The MnBi film was deposited with an easy direction of magnetization normal to the film surface. The beam was used to heat the film locally to the Curie point (350°C)at which temperature the magnetic film reverses through the demagnetized region to minimize the magnetic energy. As the demagnetized region cools, the magnetization is locked in, in reverse polarity. Mayer was able to demonstrate this reversal by means of the Kerr magnetooptic effect and electron mirror microscopy. He also determined that erasing could be carried out by saturating the magnetic field in the forward direction. The Kerr effect has also been considered by Mee and Fan (70) as a means of readout for a laser-beam memory. The major disadvantage is the weak magnetooptical interaction. Typical conversion efficiencies are 10-4-10-

104

JOHN KELLY

unless the user is prepared to cool the magnetic material to cryogenic temperatures when efficiencesof the order of 0.01-0.1 may be obtained (e.g., with EuO at 60°K).

2. Photon and Electron Beam Access (PEBA) In 1967, Smith ( 7 1 ) proposed a magnetic-film memory addressed by combined photon and electron beams. The basic concept required the use of an electron beam to heat the selected bit, thereby lowering the switching threshold. The final direction of magnetization was controlled by the direction of an external magnetic field. For readout, a magnetooptical output (modulated thermally by a fast series of electron beam pulses) was proposed. The bit address was then determined by the point of impact of the electron beam on the target, whereas the photon beam could illuminate a much larger area. This ingenious scheme requires a magnetooptical coefficient and coerciveforce that are appropriately dependent on the temperature. The magnetooptical temperature coefficient 6k is required to be large so as to obtain good readout signals. Typically, 6k becomes large as the Curie point is approached ;however, the coercive force must remain high enough to ensure data retention. Normally, this force tends to zero at the Curie point and, as a result, these two requirements are in opposition. Smith ( 7 1 ) proposed to achieve the required performance by using composite films consisting of several layers of different materials. The effect is illustrated in Fig. 46 which shows the combined temperature dependence of two films having magnetizations MI and M , . The film M I produces a high temperature coefficient 6k at V,, suitable for reading purposes, and M , maintains a level of magnetization sufficient for data stability. In a subsequent paper, Smith ( 7 2 ) discusses the problems of achieving a satisfactory output signal and signal-to-noise ratio in this type of memory. He notes that the proposed photon and electron beam access technique provides an adequate bit-selection mechanism and adequate discrimination against surface noise ; however, it is deficient in that no provision was made to reduce the shot noise associated with the photon illumination of the unselected bits to an acceptable level. Smith then suggests a modified approach, using the magnetooptical spectra in the rareearth iron garnets (REIG). These materials have a window in the near infrared from 1.5 to 5.0 pm in which the adsorption is zero, apart from a line spectrum associated with electronic transitions in the rareearth ions (73, 74). By exchange interaction with the iron sublattices, these line spectra cause magnetooptical effects (75, 76). Smith considers two possible methods for reducing background

105

ELECTRON BEAM ADDRESSED MEMORIES

"b

vc 1

vc2

TEMPERATURE

FIG.46. The magnetization of a two-layer structure as a function of temperature, qualitative [Smith (7f)I.

or array shot noise, both based on the use of temperature effects to control the REIG spectra : thermal shifting of the spectral lines, and thermal pumping to a state at which absorption is to take place. He rules out the first alternative but concludes that thermal pumping could be effective and practical. A three-state system is proposed in Fig. 47. As in the original proposal, bit selection is performed by the electron beam, and the photon beam covers a larger area. In this modified version, however, the electron beam also heats the selected site locally and pumps it into the excited state from which absorption can occur. Smith calculates that qz more bits may be illuminated through thermal pumping, where q = N , / N , ; N , and N , are the populations of the storage and read levels

106

JOHN KELLY

READOUT (Optical absorption1

A € > > kT,

3

ES

t

\

-

BIT SELECTION (Thermal Pumping)

FIG.47. The use of thermal pumping to achieve bit selection with reduced background shot noise. After Smith (72).

respectively, and q is defined as

where AE is the energy difference between the storage and read states. With a storage temperature of 77°K and a read temperature of 377°K q can be computed to be 3 x if a value of AE = 500 cm-' is used (77). By means of these figures, approximately lo7 more bits could be illuminated through thermal pumping. This storage technique is interesting; however, there are many practical materials problems, and the hybrid system is relatively complex. Perhaps the major limitation is the need to cool the target. 3. Lorentz Readout The possibility of readout using the action of Lorentz forces on the electron beam has been considered by Cohen (79) who examined and rejected the following four methods: (1) electron transmission through a magnetic film; (2) secondary electron analysis; (3) mirror microscopy, using a retarded beam; (4) reflection microscopy at shallow angles of incidence. His conclusions were based on the impracticality of achieving an adequate readout signal. In the transmission mode the power density required is prohibitive; secondary electron scanning requires more current density than can be obtained, and mirror microscopy has limited resolution. Cohen also

ELECTRON BEAM ADDRESSED MEMORIES

107

ruled out reflection microscopy because a high-intensity field-emission source would be required, and many other factors were marginal. Despite Cohen’s negative conclusions, the Lorentz approach should be reanalyzed as new methods and materials appear. For example, Cohen’s analysis of the transmission mode assumes a target without a substrate so that all the energy is dissipated in a thin film. Berkowitz (80),however, has proposed a magnetic film memory in which, on reading, the beam actually passes through a hole in the magnetic film, thereby eliminating Cohen’s principal objection. Writing is achieved by heating an annular ring around the hole above the Curie point. A magnetic field applied during cooling determines the direction of magnetization that can deflect the readout beam through a small angle. Berkowitz demonstrated a deflection of 2.5 x rad, using l-mil holes in a CrO, magnetic tape. Reading and writing rates up to 1 MHz are predicted. The major difficulty appears to be in fabricating a target with the required structure. C . Cathodoluminescence

In 1940, the use of color centers in reversible information storage was suggested by Rosenthal (82) who employed the F-centers created in alkali halides by electron irradiation for dark-trace display. Recent improvements have used sodalites as the storage medium in preference to the alkali halides (83, 84), and related work (85-87) uses these or similar materials in optical storage. In 1972, Bishop et al. (88) described a storage medium based on the inhibition of cathodoluminescence by the introduction of radiation-induced defects in the alkali halides. Most of the work used potassium iodide doped with thallium [KI(Tl)] although many of the alkali halides exhibit related phenomena. Potassium iodide containing thallium ions (TI’) luminesces at room temperature when excited by ultraviolet light or by electron irradiation (89, 90). Electron irradiation, however, also creates defects that inhibit the luminescence associated with the TI+ ions. The initial radiation occurs in a broad band centered on 420 nm and has a fast component with a half-life of 200 nsec useful for readout; a slow component with a half-life of 200 psec or more must be ignored. Following our terminology (ONES represented by signal output), the decrease in luminescence with radiation dose is the write-ZERO process. The procedure is approximately exponential and is illustrated in Fig. 48. These measurements were made with a 200-keV beam at 17 W/cm3. If these values are scaled for a medium penetration depth of 1 pm, the equivalent exposure in coulombs per square centimeter can be determined. As a result,

108

JOHN KELLY 1 .o

1

>

y w u

::

I -

\

0.2

w

g E

I

0.5

0.1

$

0.05

z =3

0.02

w

1

0.01

4

0.005

I-

w

a 0.002

t 0

20

40

60

ao

100

RADIATION DOSE - k J c r K 3

FIG.48. Relative luminescence efficiency of KI(T1) as a function of electron exposure. Beam at 200 keV, 17 W/cm3 [Bishop et al. (88)].

the radiation efficiency drops to 10% after a dose of 32 kJ cm-3 or an equivalent of 1.6 x lo-’ C cm-’. The write-ZERO process (inhibition) is attributed to the creation of interstitial iodine atoms. These atoms are reported to be very mobile at room temperature (91, 92) and attach themselves to the T1’ ions, thereby inhibiting the luminescence. The inhibition process is reversible. The radiation-induced defects can be annealed out by heating the alkali halide for a suitable time. Annealing occurs very slowly at room temperature (approximately 2000 years are required) but at very much higher rates at elevated temperatures ;however, thermal annealing at the melting point of the crystal still requires 2.5 msec. When annealing with an intense beam of electrons, the process occurs up to six orders of magnitude faster, presumably because of the effectively high temperatures of the electrons that are not in equilibrium with the lattice. Measurements of the thermal annealing properties indicate an activation energy of 1.1 eV, which is considerably less than the average energy loss by the beam per electron-hole pair (3.7 eV). In practice, anneal times of 2 p e c have been achieved (88,132). This medium is ideal in many ways because it offers good performance with a clean separation of writing and reading functions. For example, writing is achieved by bombardment of the crystal with the electron beam, and readout can be obtained by observation of the photon emission from the reverse side. Unfortunately, only several hundred write/read cycles or 10’

TABLE VI SUMMARY OF STORAGE MEDIAPERFORMANCE

Mucap (42)

Depletion mode MOS (55)

Accumulation mode (56)

Amorphous semiconductor (59)

Write-ONE sensitivity

Type

Read rate

Negative charge storage on surface

> 100 MHz (using electron multiplier)

Positive charge storage in oxide

2 MHz

-

Positive charge storage in oxide

20 M H ~

-

-

-

Medium

Amorphous to crystal-

C/cm2 4 x lo-’ C/cm2 1 day (calculated) (calculated)

10-7-10-8

10-7-10-8

1 MHz at lo-’ C/cm2 3 A/cm2 at 10 A/cmZ

-

10-8

10-8

Life

> 1 C/cm2

-

1 month

1 C/cm2 (anneala ble above 320’C)

-

-

1 month

I C/cm2 (annealable above 320’C)

-

-

Archival

?

C/cm2 a t I A/cm2

-

Archival

?

1 . 2 ~

Long

C/cm2 at 10 A/cmZ

Bit size

Write rate

- 4 pm (9 per bit) - 1 pm

I MHz at 1 A/cm2 - I kMHz (1 per bit) after priming

Limitation on Probable access time applications

-

10 nsec (time of flight)

Read-write systems

- 4 pm

-

10 MHz

15-30 psec Read-write (Switching and systems readout amplifier stabilization

< 1 pm

-

10 MHz

-

-

15-30 psec Read-write (Switching and systems readout amplifier stabilization)

1 MHz

Time of flight

Archival

1 MHz

Time of flight

Archival?

Fast

Read-write ?

Fast

Read mostly

I pm

line phase change

-

Beam de- I MHz a t lo-‘ Cicrn’ flection in 3 A/cmz a t 1 A/cm’ hole in film Curie point writing

Ferroelectrics

Switching bismuth titanate

Luminescent KI (TI) (88)

Storage time

5x

Magnetic film (80)

(94)

Write-ZERO sensitivity

Radiation damage in

KI

?

-

1 MHz

1 . 2 ~

- 4 pm

m

centers

Archival

?

lo5 cycles

2x2pm

- 1 pm

?

-100 kHz

110

JOHN KELLY

read/refresh cycles have been obtained, before fatigue starts to develop. This level of fatigue is much too high for the construction of an acceptable memory. The fatigue has not been explained in the literature, but it is to be hoped that alternative materials could be found or developed not exhibiting the same phenomenon. Even within the alkali halides there is a large selection of possible candidates from which to choose.

D. Ferroelectric Media Several attempts have been made to apply ferroelectrics to information storage (91-99). Storage in bismuth titanate has been proposed by Cummins (93) and demonstrated (94).The storage principle relies on the fact that there is a large difference in extinction direction on switching the polarization. This change is approximately 40"as opposed to the 180" switch (not optically detectable) that occurs in barium titanate and triglycine sulfate. This unique switching process has been described by Cummins and Cross (95). During normal switching, with a field applied along the c axis, the large polarization vector rocks through an angle of lo", causing the large change in optical properties. In Fig. 49, two states are shown that correspond to the two possible polarization states. The X and Y axes represent the two indicatrix positions. It can be seen that the light path is along the a, axis; and, if optical polarizers are aligned on one of the two possible extinction positions, the other will produce maximum transmission. Cummins (96) has also determined that domains of opposing polarity are distinguishable when the light is incident at 10"-15" with the co direction. Cummins and Hill (94)were able to write optically detectable domains in bismuth titanate by scanning the (001) surface with approximately C/cm2 at 23 kV. Domains with a 2-pm width in the a direction 1.2 x were written by scanning in the b direction; however, resolution for adirection scanning was believed to be limited to approximately 12 pm unless thinner crystals or an alternative writing scheme could be found. The major advantage of this technique is the permanence of the stored information which is in the form of a switched domain and does not rely on holding the electron charge. Other work is based on the electrooptic light-valve approach (98, 99) that relies on the birefringence introduced in a ferroelectric crystal (such as KDP) when an electric field is applied. Wieder et al. (98)made simple spatial filters by writing charge patterns on a dielectric surface deposited on a KDP crystal. Figure 50 is a schematic of the system. Polarized light from the laser passes through an object pattern, and the ferroelectric storage tube is positioned in the Fourier plane. The dielectric behind the KDP layer acts as a mirror. The birefringence in the KDP causes rotation of the plane of polarization; this can be analyzed by the polarizer, and the resulting filtered image

'DOMAIN WALL

FIG.49. A bismuth titanate crystal viewed along the a. axis. The axes x-y indicate the two possible positions of the optical indicatrix for opposite saturation states on the hysteresis loop [Cummins (9311. LASER INPUT

OBJECT

TRANSPARENT CONDUCTOR

DIELECTRIC

I OUTPUT PLANE (IMAGE)

POLARIZER

KDP' BEAM SPLITTER ELECTRON BEAM UNIT

FIG. 50. Method of generating and using electrooptic spatial filters using a ferroelectric beam memory tube [Wieder et ol. (98)].

112

JOHN KELLY

appears in the output plane. Recently, Casasent and Casasayas (99) used a transmission-mode device with separate write and erase guns. Unlike Cummins’ domain-switching technique, the information is not permanent and must be retained by refresh techniques.

SYSTEMS VII. ELECTRON-OPTICAL A . Basic Limitations

In this section, the limitations imposed by electron optics are discussed. A typical TV tube can scan approximately 500 to 600 lines; however, a high-quality CRT can scan perhaps 1000 lines, and 2000-line tubes are available. Electron-optical systems have been constructed with over 10,OOO lines in a single image field (100, 115), but the technology becomes increasingly difficult and expensive. To achieve increased resolution, it is necessary to limit the beam current severely and often to impose dynamic corrections. In a memory tube, this also limits the writing and reading rates. Difficulties in the electronics area also limit the number of bits addressable in open-loop fashion. For example, 16-bit D/A systems (65,536 lines) are relatively slow, and the necessary high-stability power supplies are very expensive. On the other hand, 12-bit D/A systems (4096 lines) are much faster, and the necessary power supplies are available at reasonable cost. It therefore can be concluded that it is practical to store between one and four million bits per image field with the current state-of-the-art electronics technology. The factors that limit beam current in the on-axis position are chromatic and spherical aberrations, space-charge repulsion, and diffraction. Because, generally, diffraction can be ignored in beams of the size range under discussion (approximately 100 A to 10 pm), the beam current is usually determined by a compromise between the Langmuir limitation (101) and spherical aberration. In some cases, space-charge repulsion is also significant. The Langmuir limitation (as noted in Section 111) imposes an upper limit on the current density attainable in the focused spot. Basically, this limit implies that the brightness of any focused image in a system is invariant although changes in refractive index (or velocity) must be taken into account.* The standard form for the Langmuir limitation is

(,“T

Ji=Jc -+1

sin2 a, )sin2 a. ~

Consequently, a brightness measurement should always be quoted with the appropriate voltage .

ELECTRON BEAM ADDRESSED MEMORIES

113

where Ji is the current density in the probe formed from a cathode having a current loading J , , temperature T, and the accelerating voltage V,; ai is the half-angle subtended at the focused spot, and a. is the half-angle at the cathode prior to acceleration. a. is usually taken to be W",but this is not necessarily the case for field emission cathodes. This equation indicates that the largest values of Ji can be obtained when ai is maximized. However, ai must be limited because of the lens aberrations. The spherical aberration gives rise to a circle of least confusion, with diameter

d, = fC,a?

(23)

where C, is the spherical aberration constant. The optimal semiangle can be determined by combining Eqs. (22) and (23) and solving to derive the condition for maximum density. Einstein et al. (102) defined the optimal halfangle as

and the corrresponding current density as

J,,,

= 0.34(5, V,/kT)(d/C,)z'3

The approach used is a modification of the method developed by Mulvey (103) and Castaing (104). Small differences in the final equations depend on the detailed assumptions made and, in particular, on the method used for compounding the spherical aberration with the Langmuir limited beam. Other differences occur because cathodes impose different constraints on the system (105,206). Equation (25) is often expressed in terms of cathode or gun brightness rather than current density. The most useful form depends on the type of cathode. Because the life of most cathodes can be related directly to the current density required, J, is often a limiting factor. It will be noted that the maximized current density increases slowly with beam diameter but decreases with increased values of the spherical aberration constant. In some instances, however, one basic assumption in the derivation of Eq.(25) is invalid; C, is not a constant but is a function of the focal length and conjugate positions. Typically, a value of C, for one conjugate at infinity is used; however, when approaching unit magnification, it is more correct to use a value of

c, = C,( a)(1 + M)4

(26)

where M is the magnification (38, 233, 134). In a beam-memory device, the primary concern is beam performance across a finite field and not just on-axis. This leads to other restrictions on

114

JOHN KELLY

the design. The actual deflection of a ray is determined by the third-order equation as Y = 3 a Y i bY& cY,az (271 where 5 is the gaussian deflection and aiis the beam half-angle; the principal aberration constants a, b, and c for the deflector are complex integrals of the deflection field and are constant for a particular geometry and deflection distance. The term aYp” produces a raster distortion or nonlinearity; bYia, results in deflection defocusing that appears as an astigmatismlike aberration, and cU,a; gives rise to a comalike term. The deflection parameters and the formation of distortion and deflection defocusing are illustrated in Fig. 51. The distortion term a Y i is equivalent to an additional deflection in the Y direction, which results in the typical pincushion or barrel shape of a raster scan. The astigmatismlike or deflection-defocusing term b Yiai results in the beam developing an elliptical shape that is larger than the focused spot. Two line foci are formed (F, and F,) on the deflector side of the target, and between them is a circle of least confusion. The ellipse in the gaussian deflection plane has semiaxes b, Y i ai, and b, Y i ai,when the gaussian or paraxial image dimensions are ignored. The comalike term cY,az can usually be neglected, unless the beam enters the deflector off-axis. These parameters restrict the number of lines in an image field. In many situations, a certain degree of raster distortion can be tolerated, and the field is limited by spot growth caused by deflection defocusing. This latter term is proportional to ai, and the number of lines can be increased by reducing ai.

+

+

+

ASTl GMATl SM OR DEFLECTION DEFOCUSING

+VV

I

I

\

I I

ISSIAN LECTION

CONFUSION

~

-VV

FIG. 51. A deflected electron beam showing the character of deflection defocusing and raster distortion. F , and F , are the two line foci associated with the astigmatismlike deflection defocusing.

ELECTRON BEAM ADDRESSED MEMORIES

115

This will result in a deviation from the optimal conditions defined by Eq. (25) and a consequent reduction in beam current. The increased number of lines is N 2 , where N Z / N , = (ail/ai2)1’2 (28) Because the maximum permissible astigmatism will be the same in both cases (Yil ail = Yi2ai2), the beam current density will be reduced by (ai2/ail)’ unless the system is reoptimized. The relative importance of spherical aberration and deflection defocusing depends on the values of C, for the focusing system and bi for the deflector. Fortunately, both raster distortion and astigmatism are amenable to dynamic correction signals and the use of a stigmator generally in the form of an octupole lens. It is apparent that the choice of cathode, the deflector design, and the lens system are of key importance in the design of a beam-addressed memory. In the following sections, these critical factors are considered, with emphasis on recent developments. Traditional electron optics are described in such standard texts as Klemperer and Barnett (107).

B. Cathodes The selection of a cathode in a beam-addressed memory is critical because beam brightness (A/cm2 sr) and source current density are of fundamental importance; in most systems, lifetime is also significant. Brightness data are often misapplied because brightness is not the only parameter that controls current density in a focused spot. For example, field emission produces extremely high brightness ; however, being a very small source, it cannot always be used in an optimal configuration. Typically, therefore, it is possible to apply only a small portion of the beam and, for on-axis beams above 1 pm, the field emitter offers little or no advantage over the conventional thermionic cathode (106).Recent advances in field emission (108, 109) and such considerations as off-axis performance modify these arguments and tend to improve the relative performance of the field emitter. In many cases, device lifetime is the overriding consideration, and it is now accepted that the dispenser cathode (1 10) offers excellent performance. Lifetimes of up to 50,000 hr at 3 A/cm2 are claimed, and higher current densities are available if a shorter life is acceptable (111, 112). The osmiumcoated cathode produces a further significant increase in available current density (113); however, the osmium gradually reacts with the tungsten matrix and the advantage is eventually lost (114). These complexities make it clear that, in defining the performance of a cathode, the brightness at a particular beam voltage and the corresponding lifetime should be quoted together. In many cases, the source size or specific emission should also be included. Some representative figures are listed in Table VII.

116

JOHN KELLY

TABLE VII REPRESENTATIVE CATHODE PERFORMANCE

Cathode

Brightness (A/cm2 sr at 10 kV)

Life (hr)

Current density (A/cm2)

Source size

Oxide

104

10,000

1

Tungsten hairpin Dispenser type

105

500

10

3 x 104

50,000

3

25-50pm crossover 25 pm crossover 12-25 pm

106-109

300-io,~ depending on conditions

-lo7

200-1000A

100

5-50 pm crossover

Field emitter

lo6

1000

-

Comments Well established Bright but short-lived Reliable in clean vacuum Requires special care and handling Problems of instability; requires good vacuum

C. Deflector Developments Over the past ten years, there have been several significant developments in deflector design. The deflection requirements are emphasized in beam memory devices because off-axis performance is as important as on-axis. In such systems, it is also convenient to have x, y deflection with equal sensitivities and a common center of deflection. The theory of deflection has been considered by many workers; however, the results are not generally adaptable to design methods. Haanjes and Lubben (116, 1 1 7 ) considered a magnetic deflector and its application to CRT design for color television. This theory only considers third-order aberrations but is amenable to design methods because the aberrations are expressed as a function of the deflection field on-axis E,(O, 0, z) and the second derivative of the field dZE,(O, 0, z)/dx2, where y is the direction of deflection. These parameters express the problem completely for the third-order solution because E, defines the field on-axis and its strength at all points and d Z E , / d x Z defines the distribution in the x, y plane. This theory was extended to electrostatic deflection by Kelly [see Heynick (21, 38)] and by Wang (118). The major difficulty in electrostatic deflection is the limited scope for shaping the field distribution. In magnetic deflection, it is possible to overlap coils to adjust the distribution in the x, y plane.

ELECTRON BEAM ADDRESSED MEMORIES

117

Recent work with the octupole deflector was stimulated by the desire to achieve a degree of field shaping to minimize the aberrations and the need for high sensitivity and a common center of deflection. Multielectrode deflectors have been known for many years (1 19) but have received relatively little attention until recently. In 1969, Kelly proposed the use of the octupole to minimize aberrations (21), and he designed and tested one to reduce raster distortion (38). Figure 52 shows a cross section of the general octupole deflector. The deflection voltages applied for x deflection and the summed voltages

(C)

FIG.52. The octupole deflector: (a) voltages required for y deflection; (b) summed voltages required for simultaneous x and y deflection; (c) a method of providing the summed voltages

. ...

.L..:J--

118

JOHN KELLY

required for simultaneous x-y deflection are illustrated in Figs. 52a and 52b; one method of providing the necessary voltages by means of a resistive bridge is shown in Fig. 52c. The ratio parameter ( a ) sets the fraction of the primary deflection voltage applied to the side electrodes and, therefore, the distribution of the field in the x, y plane. This field varies across a complete spectrum from a = 0 (corresponding to a quadrupole deflector) to a = 1 (the split-cylinder case). For a = 0.414 (actually - l), the system is symmetrical about the x = 0, y = 0, and x = y directions, and the field is very close to uniform over more than one-half the deflector diameter. Generally, the optimal configuration uses a ratio-parameter value close but not equal to 0.414,and both the aberration constants and the deflector sensitivity vary with this parameter. Figure 53 shows the relative field distributions for different values, and Fig. 54 illustrates the linear relationship between the ratio parameter and a Z E , / a x z for an infinitely long cylindrical octupole. These data were derived from the computed solution to Laplace’s equation, which was obtained by relaxation methods. Also plotted is relative deflection sensitivity. Figure 55 illustrates the variation in raster distortion with the ratio parameter a. Similar relationships can be plotted for other aberrations. Some details of the minimization of astigmatism have been discussed by Wang (118). An alternative approach to deflector design is the deflectron developed by Schlesinger (120). Originally proposed to achieve a common center of deflection and equal x-y sensitivities, this deflector has proved to offer very good aberration performance. The deflectron has interleaved x-y electrodes arranged in a periodic pattern on the inside of a cone or cylinder. The deflection field is not constant along the z axis but varies in a periodic fashion. A typical deflector pattern is illustrated in unrolled form in Fig. 56. It is difficult to find actual performance data in the literature ;however, the deflectron has been applied successfully in a number of tubes. A detailed comparison of octupole and deflectron performance would be interesting, but adequate data are not available at this time. Much of the work on EBAM devices has focused on electrostatic deflection despite the fact that magnetic systems.generallyhave lower aberrations. Most arguments for the electrostatic deflector are based on the relative speeds required for driving the deflector. A better argument is the need to produce a small, low-weight, low-power device with adequate mechanical stability. It is acceptable therefore to use a deflector made as part of the tube structure, but it is debatable whether a separate coil hung on the outside of the vacuum envelope can be satisfactory. [For a recent review of deflection studies, see Hutter and Ritterman (121).]

fi

119

ELECTRON BEAM ADDRESSED MEMORIES 1.8

1.6

1.4

1.2

1 .o

E" 0.8

0.6

0.4

0.2

t

0 0

I

0.2

0.4

0.6

0.8

1 .o

1.2

TRANSVERSE DISTANCE

axis

FIG.53. Variation of the deflection field as a function of transverse position for various values of the ratio parameter a.

D. Matrix Lenses Having considered the performance of single-lens systems, it can be concluded that it is possible to store between one and four million bits per image field with the current state-of-the-art technology. The simplest approach is to create a memory system from a number of tubes working either serially or in parallel if higher data rates are required. The next level is to construct a number of channels within a single

120

JOHN KELLY 1.4

N

1.2

1.4

1.0

1.3

0.8

1.2

t 2 k

0.6

1.1

:

. fi X

w

>

>

N

,=

I

2 wLL

h

v)

2

w

0.4

1.0

w

I-

a

u w

-I

LL

w

:

4

0.2

0.9

0

0.8

0 LL

0 w

I

c

Ia

-0.2

E

w

0

0 -0.4 2

8w

-0.6

-0.8

0

0.2

0.4

0.6

0.8

1.0

RATIO PARAMETER a

FIG.54. Normalized second derivative of the deflection field (l/V)a'E,/~x2and relative deflector sensitivity as a function of the ratio parameter a.

vacuum envelope. The multiplicity of lenses has been explored by Newberry et al. (222) and, more recently, by Lemmond et al. (223). The problem of lenslet interaction has been considered by Harte (124). The fly's eye (or

matrix-lens) system consists of an array of lenses that can be illuminated either individually or simultaneously by a double deflection system (Fig. 57). Deflection within each lens field is achieved by means of two sets of deflecting bars connected alternately to positive and negative amplifiers. Hughes et al. have applied a modified version of the fly's-eye lens to the design of an EBAM and reported (56, 125) a 32-Mbit tube that uses an

121

ELECTRON BEAM ADDRESSED MEMORIES 0.014

I

0.012

3

OCTUPOLE DEFLECTOR 2.5 cm LONG, 0.5 cm DIAMETER

0.010

0.006 c

n

0.004

2

0.002

a li 8 2

0 L

b, = +0.0004

-

0

a I

-

-

0.008

I-

4

-0.002

-

-0.004

-

-0.006

-

-0.008

-

-0.01 0

-

n

-

-0.01 2

0.3

0.4

0.5 0.6 RATIO PARAMETER a

0.7

FIG.55. Raster distortion constant b , as a function of ratio parameter a for a deflector 2.5-cm long and O S t m diameter, working at a throw distance of 7 cm.

18 x 18 matrix of lenses. The basic matrix-lens system is illustrated in Fig. 58. The storage target is at the left. The beam coming from the electron gun is focused and directed through a double-deflection system to a particular lenslet that further focuses the beam onto the target with appropriate demagnfication. Deflection within the lenslet field is achieved by the deflectors located between the lens and the target. In this way the address requirements imposed on the electronics and electron optics are divided between the two sets of deflectors. The first set of deflectors (lenslet selector) moves the beam off-axis in such a way that it enters the desired lenslet normally and on its optic axis. The accuracy with which the lenslet address

JOHN KELLY

122

t

BEAM

FIG.56. The deflectron-deflector patterns for a beam traveling toward the top of the page. The patterns shown are formed on the inside of a cylinder or cone [Schlesinger (12011.

must be achieved is relatively low because it is movement of the source or object position that causes a shift of the image, not movement of the illumination alone. This is illustrated in Fig. 59. The postlens deflector still carries the burden of reproducibly scanning the data within the addressed lenslet field. In the present tube with 110 kbits per lenslet, this requires only an 800-line resolution (9- to 10-bit accuracy). It is anticipated that it will be possible to increase the number of bits per lenslet considerably and that it is reasonable to consider a matrix of 32 x 32 (1024) lenslets, each storing lo6 bits (lo9 bits/tube). The possibility of using a number of lenslets simultaneously has been considered by Wang et al. (126),who designed a collimating lens to provide parallel illumination of such an array. The so-called Kan lens is capable of alignment within 0.005 mrads but has rather high chromatic aberration and necessitates a beam with low energy spread. The major problem with this type of lens structure is the difficulty in fabricating and aligning the lens plates with sufficient accuracy for all (or nearly all) of the lenslets to function satisfactorily. Earlier matrix or fly’s-eye lenses were threeelectrode Einzel lenses, which meant the accurate alignment of four plates-the electrodes plus the aperture plate (122). Later arrays have been made in the form of an immersion lens with only two

ELECTRON BEAM ADDRESSED MEMORIES

-RECORDING

I

123

PLATE

I

FLY‘S EYE LENS

-

UPPER DEFLECTRON

-LOWER

DEFLECTRON

CONDENSER LENS

LIMITING APERTURE/ FARADAY CUP

BLANKING PLATES

STEERING PLATES IMAGE APERTURE ELECTRON GUN

FIG.57. Electron-optical system for a fly’seye lens artwork camera [Lemmond (123)i.

et

01.

electrodes. A further advantage of the twoelectrode lens is that it has lower spherical aberration for a given midfocal length (the distance from the physical center of the lens to the focal plane). This is illustrated in Fig. 60 in which the normalized spherical aberration constant C, / R is plotted as a function of normalized midfocal length Z , / R . Except a t very short focal lengths, C, is reduced by a factor of approximately 5.

124

JOHN KELLY OXIDE

PAGE

LENS

RECORD AMP

-1

I DISPENSER CATHODE

AMP

IO U l l

BEAM

ELECTRON BEAM SOURCE

LENSLET SELECTOR SELECTOR MEMORY PLANE

FIG.58. Electron-optical system of the BEAMOS storage tube. A double deflectron illuminates a single element of the 18 x 18 matrix lens, at normal incidence [Hughes et a!. (56, 125)].

TARGET OBJECT

I

c

ADDRESSING BEAM

FIG.59. Illustrating the relaxed deflection stability requirement obtained by using a two-stage deflection system. If the illuminating beam is misaligned with the lenslet by S r , , the object is also displaced Sr,, and the image by Sr, = MSr, . The magnification M is usually much less than unity, and hence Sr, 4 Sr,. (a) Accurately aligned beam, (b) beam misaligned by Sr, .

ELECTRON BEAM ADDRESSED MEMORIES

125

1o4

. [r

0"

I

z

2 I-

a

E

103

2 -I

uaa W

X

v) 0.

102

10 10 FOCAL LENGTH

102

- Z,lR

FIG.60. Spherical aberration for electrostatic matrix lenses plotted as a function of normalized midfocal length: (A) three-electrode Einzel lens, center plate aperture radius R = 1.5 mils, gap S = 60 mils, outer plate aperture 2-mils radius; (B) two-electrode lens; ( + ) aperture lens. Adapted from Hughes er a/. (56) and Heynick er a / . (41).

126

JOHN KELLY

10-1

10-2

10-6

0

0.2

0.4

0.6

0.8

1.o

aIR

FIG.61. Radius of the circle of least confusion for an aperture lens versus radius of the beam-limiting aperture, both normalized to the aperture of the lens. 2, is the distance between the aperture lens and the target. C,is the calculated third-order spherical aberration constant. Adapted from Heynick et al. (41).

ELECTRON BEAM A D D R E S S E D MEMORIES

127

A further extension of the matrix lens approach is the so-called “screen lens” referred to earlier (Section IV,C) and illustrated in Fig. 21. This scheme can be used, in principle, to address a very large array of data, using the lenslets either singly or in groups. The lens system offers the same advantages as those described above, and requires essentially no alignment. The principle by which focusing is achieved results in the name “aperture lens.” The spherical aberration of these lenses has been calculated by Westerberg ( 4 1 ) and is shown in Fig. 61 as a function of the ray position in the aperture lens. Rays close to the axis and up to approximately one-half the aperture radius are subject to third-order aberration only; beyond, no further increase in spherical aberration appears because the fifth-order term is negative. Toward the edge of the aperture, seventh order aberration takes over and adds to the overall aberration. Westerberg’s data have been analyzed and superimposed on Fig. 60. It can be seen that, for a given midfocal length, the spherical aberration (third order) is almost identical to that of the three-electrode Einzel lens. Because the “center” of the lens is in the plane of the screen, however, the spherical aberration for a given free working distance is somewhat lower. Although this lens system has not been applied to the addressing of a memory plane, it does appear very attractive. A resolution of about 500 A

FIG.62. A conceptual digital deflector scheme, using a sequence of deflectors scaled in length and driven from a set of fixed voltage sources which are selected by a set of fast transistor switches.

0

OTONS

ADDRESSING BEAM (Electrons or Ions)

SUBSTRATE ACTIVE SURFACE

\

INFORMATION PATTERN

ADDRESSING BEAM

>I

I

__

-

i

-SOURCE

-' MULTIPLIER

DEFLECTOR SYSTEM

STORAGE TARGET (lo6 Subfields Each of lo6 Units]

SCREEN LENS (106 Lenslets)

FIG.63. A proposed beam memory concept using a screen or aperture lens, addressed by a double deflector. A second double deflector scans the target within each lenslet.

ELECTRON BEAM ADDRESSED MEMORIES

129

has already been demonstrated and lenslets spaced on 12.5- and 25-pm centers investigated. Potentially a screen of lo6 lenslets, each addressing a subfield of lo6 bits, appears possible although the practical difficulties are considerable. If the bits are on 100-A centers, the lenses would be approximately 10 pm apart and the bit density 1012/cm2.Bit and lenslet addressing could be determined through a sequence of electrostatic deflectors; the first two would address a single lenslet, and the last two would scan the beam within the subfield. This form of sequential deflection could be practical, using octupole deflectors in sequence. The deflectors themselves could be of the digital type illustrated in Fig. 62. Each deflector is subdivided into a series of subdeflectors, probably made as thin film conductors on the inside of a ceramic tube. The deflection voltages are fixed (V, to V') and are selected by fast switches controlled by the address logic. The success of a complex scheme such as this depends on many factors. The biggest uncertainty is the availability of a suitable storage medium, despite the wide variety available for investigation. An archival memory using a cathodoluminescent target with the information pattern superimposed is used to illustrate the principle of address using the screen lens in Fig. 63. It is interesting to note that this scheme bears a strong relationship to that proposed by Bryan and Focht in 1962 (127) for the addressing of 4 x lo6 bits! SYSTEMS VIII. BEAMMEMORY

This article has been largely concerned with the technology required to fabricate practical EBAM systems. The survey would not be complete without some discussion on recent systems work. Kelly et al. (29) described an experimental memory module, designed for testing at the USAF Avionics Laboratory at Wright-Patterson AFB. The system uses the electrostatic mucap storage medium in a single-channel demountable tube. The module was constructed as a laboratory tool so that such significant parameters as writing speeds, target life, and error rates could be measured. The tube is a modular construction, some key components of which are illustrated in Fig. 64. The single tube is operated from a standard off-the-shelf memory exerciser and is set up so that random access to any bit can be achieved. Figure 65 is a schematic of the electronics. Under microprogram control, the memory exerciser can provide data patterns, address information, control commands, and read and compare operations in parallel. The 16-bit address word from the exerciser is decoded into two %bit addresses corresponding to the x, y coordinates, and these are

130

JOHN KELLY

FIG.64. Components of an experimental beam addressed memory tube: (a) the assembly, (b) the target plane, (c) the octupole deflector [Kelly (12811.

passed to the D/A converters and deflection amplifiers. At the same time, the read/write control and the information to be written determine the target bias required. After suitable settling times, the beam is pulsed on and the operation is carried out. During the read operation, the information from the readout amplifier and comparator is stored in a buffer before being passed back to the exerciser for comparison to the data written originally. All timing operations are conducted in the digital interface. The tube control unit produces the necessary analog signals from the interface inputs.

m

z

0

E

B

E

2

m

132

JOHN KELLY

This simple system is capable of gathering a large amount of data in a short period of time-including measurements of practical operating speeds-and of providing a first look at error rates. The latter was particularly encouraging. In measurements on small data blocks, error rates of better than lo-’ were encountered; however, the system was not adequately isolated from line noise, and there were periods when errors were accumplated through switching transients. This problem was eliminated by programming the memory exerciser to reread whenever an error was encountered. This enabled the system to run free of error up to the point at which target fatigue had reduced the output signal quality to a marginal condition. This terminated the useful life of that area of the target. Typical data are illustrated in Fig. 66. Further details of the system and its operation are available in reports by Kelly and Moore (42) and Nicastri (43), and a summary can be found in (128). In a second development, Speliotis described a single-channel approach to the EBAM system, using the MOS depletion-mode storage target (58). Details of this work have not been published; however, a 1.2-Mbit system consisting of nine 128-kbit tubes operating in parallel was delivered to Control Data early in 1975 and has been reported operating on a CDC Star 1B computer. In the third development, Hughes et al. (56, 125) described the development and testing of a 32-Mbit tube. This tube uses an accumulation-mode MOS storage target and is addressed by means of a double deflectron and an 18 x 18 matrix lens. The data are block-oriented in such a way that each lenslet field is divided into 13 8448-bit pages. In a proposed multimodule system (Fig. 67) separate signals are provided for lenslet selection, page selection, and data scan. Representative performance figures are as follows : tube capacity system capacity access time data transfer rate

32 x lo6 bits 32-600 x lo6 bits 30 psec 10 Mbits/sec

Based on a (32 x 106)-bit tube operated in a block-oriented mode with uniform target usage at a 10-MHz data rate, a 30,000-hr target life is predicted. This is comparable to the 30,000-hr MTBF predicted for the digital circuits and to only 20,000 hr for the power supplies. The life situation for small blocks of random-access data is much more severe. The problem becomes less significant as the memory is made larger, provided that the bit size remains the same. In itself, however, the life of the tube is not vital-it is much more critical to be able to detect deterioration so as to make replacements before system failure.

133

ELECTRON BEAM ADDRESSED MEMORIES 100,000

I

I

I

)

I

’

10,000

v1

U 0 U U

1000

w

n

RUN 5

w U w

RUN

1

,

>

0

YU

z

100

3

(

10

1

0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

109

READlREFRESH CYCLES

FIG.66. Unrecoverederrors as a function of the number of read-refresh cycles as measured on the EBAM module. Useful life was terminated by fatigue of the target due to organic contamination.

The activity represented in this article corresponds to a considerable renewed interest in EBAM technology. There is still plenty of scope for invention. One feature of nearly all EBAMs has not yet been given the attention it deserves. The structure of most memories is determined in the detailed wiring and in the way it is fabricated. In the EBAM the information is stored in a two-dimensional plane. The format and data organization can be determined by the supporting hardware and software.

OXIDE BIAS

aX z

n

CONTROL REGISTER

I

n CONTROL ClRCUlTS

t1

FUNCTION READ

CHARACTER lSTlCS ACCESS TIME DATA RATE D A T A CAPACITY

30 psec 10 MBIsec 30 x lo6

IINTERFACE

FIG.67. A multimodule system for serial operation, using up to 20 BEAMOS matrix lens tubes of 3 x lo7 bits each [Hughes e r a / . (125)].

ELECTRON BEAM ADDRESSED MEMORIES

135

In this review actual applications for these memories have not been discussed. For a discussion on this subject, the reader is referred to a discussion on the impact of “electronic disks” by Wensley (131). ACKNOWLEDGMENTS The author would like to acknowledge the help of J. S. Moore and P. R. Thornton for reading and commenting on various parts of the manuscript. Thanks are also due to I. Brodie for suggesting the review and for his support and encouragement throughout; also to Joyce Garbutt for coordinating and typing the manuscript and to Hortense Shirley for her editorial comments. Part of the work on the “mucap” medium at SRI was supported by the U.S. Air Force Avionics Laboratory under Contrast F33615-72-C-1906.Early work was supported by the US. Army Electronics Command under Contract DA28-043 AMC 01261(E).

REFERENCES 1 . F. C. Williams, and T. Kilburn, Proc. Inst. Electr. Eng. 96, Part 111, 81-100 (1949). 2 . J. P. Eckert, Jr, H. LukolT, and G. Smoliar, Proc. I R E 38, 498-510 (1950). 3 . J. W. Forrester, Proc. Symp. Large Scale Digifal Calculating Mach., 1947 16, 125-129. Ann. Computation Lab. Harvard Univ., Harvard Univ. Press, Cambridge, Massachusetts (1948). 4. S. H. Dodd, H. Klemperer, and P. Youtz, Electr. Eng. (Amer. Instr. Electr. Eng.) 69, 990-995 (1950). 5. R. R. Everett, East. Jr. Comput. Conf. p. 70 (1951); Electr. Eng. 71, 681-686 (1952). 6. D. J. Wheeler, and J. E. Robertson, Proc. I R E 41, 1320-1325 (1953). 7. W. E. Mutter, Electr. Eng. ( A m . Inst. Electr. Eng.) 71, 352-356 (1952). 8. W. Buchholz, Proc. IRE 41( lo), 1262-1275 (1953). 9. C. E. Frizzell, Proc. I R E 41( lo), 1275-1287 (1953). 10. H. D. Huskey, R. Thorenson, B. F. Ambrosio, and E. C. Yowell, Proc. I R E 41(10), 12941299 (1953). 11. S. Greenwald, R. C. Haueter, and S . N. Alexander, Proc. IRE 41(10), 1300-1312 (1953). 12. J. Rajchman, R C A Rev. 12, 53-97 (1951). 13. B. Kazan and M. Knoll, “Electronic Image Storage.” Academic Press, New York, 1968. 14. M. E. Hines, M. Chruney, and J. A. McCarthy, Bell Sysr. Tech. J . 34, 1241-1264 (1955). 15. T. S. Greenwood, and R. E. Staehler, Bell Syst. Tech. J . 37, 1195-1220 (1958). 16. J. W. Forrester, J. Appl. Phys. 22, 4 4 4 8 (1951). 17. E. W. Pugh, IEEE Trans. Magn. 7, 810 (1971). 18. J. Rajchman, J . Appl. Phys. 41, 1376 (1970). 19. G. Knight, Appl. O p t . 13, 904 (1974). 20. A. Vander Lugt, Appl. Opr. 12(7), 1675 (1973). 21. L. N. Heynick, ed., “ High-Information-Density Storage Surfaces,’’ 17th Quart. Rep., ECOM Contract DA 28-043 AMC-01261 (E), SRI Proj. 5444. Stanford Res. Inst., Menlo Park, California (1969). 22. J. Kelly, IEDM Tech. Dig. 592-595 (1974). 23. K. Schlesinger, Elecrronics 25, 105 (1952). 24. D. G. Cogswell, unpublished observations (1967). 25. W. E. Glenn, J . SMPTE 74, 663-665 (1965).

136

JOHN KELLY

26. H. Bruining, “Physics and Applications of Secondary Emission.” Pergamon, Oxford, 1954. 27. A. S. Jensen, and R. L. Stow, I R E PGED Meet., Washington, D.C., 1958. 28. K. R. Shoulders, US. Patent 3,398,317 (1968). 29. J. Kelly, J. S. Moore, and P . R. Thornton, Proc. IEEE Natl. Aerosp. Electron. Con$ p. 55 (1974). 30. K. H. Stehberger, Ann. Phys. (Leiprig) 86, 825 (1928). 31. 1. M. Bronshtein, and R. B. Segal, Sou. Phys.-Dokl. 3(6), 1185- 1 I87 (1958). 32. E. J. Sternglass, Phys. Rev. 95(2), 345 (1954). 33. R. Warnecke, J. Phys. Radium 7 , 270 (1936). 34. P. L. Copeland, Phys. Rev. 40, 122 (1932). 35. I. M. Bronshtein, and R. B. Segal, Sou. Phys. Solid State 1( 10). 1365 (1960). 36. P. Palluel, C . R . Acad. Sci. 224, 1492 (1947). 37. E. J. Sternglass, Phys. Reu. 80(2), 925 (1950). 38. L. N. Heynick, ed., High-Information-Density-Storage Surfaces,” Final Rep., ECOM 01261-F, Contract DA 28-043 AMC 01261(E).Stanford Res. Inst., Menlo Park, California (1970). 39. K. R. Shoulders, K. T.Rogers, and J. Kelly, US. Patent 3,500,112 (1970). 40. E. R. Westerberg, US. Patent 3,619,608 (1971). 41. L. N. Heynick, E. R. Westerberg, C. C. Hartelius, Jr., and R. E. Lee, IEEE Trans. Electron Deoices 22(7), 399-409 (1975). 42. J. Kelly, and J. S. Moore, Tech. Rep. AFAL-TR-74-176. U.S. Air Force Avionics Lab., Wright-Patterson AFB, Ohio (1974). (Unlimited distribution.) 43. E. D. Nicastri, M.S. Thesis, GE/EE74-58, Air Force Inst. Tech. Sch. Eng., WrightPatterson AFB, Ohio (1974). (Unlimited distribution.) 44. H. Seiler, and M. Stark, Z . Phys. 183, 527 (1965). 45. J. R. Szedon, and J. E. Sandor, Appl. Phys. Len. 6, 181 (1965). 46. N. C. MacDonald, and T. E. Everhart, Appl. Phys. Lett. 7,267-269 (1965). 47. E. H. Snow, A. S. Grove, and D. J. Fitzgerald, Proc. IEEE 55, 1168 (1967). 48. K. H. Zaininger and A. G . Holmes-Siedle, R C A Reo. 28, 208 (1967). 49. J. P. Mitchell and D. K.Wilson, Bell Syst. Tech. J . 46, 1 (1967). 50. N. C. MacDonald and T. E. Everhart, J. Appl. Phys. 39(5), 2433-2447 (1968). 51. N. C. MacDonald and T. E. Everhart, Proc. IEEE 56(2), 158 (1968). 52. E. E. Huber, Jr., M. S. Cohen, and D. 0. Smith, Appl. Phys. Left. 16(4), 147-149 (1970). 53. D. W. Aitken, D. W. Emerson, and H. R. Zulliger, IEEE Trans. Nucl. Sci. 15(1), 456 (1967). 54. R. H. Pehl, F. S. Goulding, D. A. Landis, and M. Lenslinger, Nucl. Inst. Methods 59(45) (1968). 55. M. S. Cohen and J. S. Moore, J. Appl. Phys. 45(12), 5335-5348 (1974). 56. W. C. Hughes, C. Q. Lemmond, H. G. Parks, G.W.Ellis, G. E. Possin, and R. H. Wilson, Proc. I E E E 63(8), 1230-1240 (1975). 57. G. W. Ellis, G. E. Possin, and R. H. Wilson, Appl. Phys. Lett. 24(9), 419-421 (1974). 58. D. E. Speliotis, AFIPS Con$ Proc., 1975 Natl. Comput. Coni pp. 501-508 (1975). 59. A. C. M. Chen, J. F. Norton, and J. M. Wan& J . Non-Cryst. Solids p. 917 (1971). 60. J. Feinleib, J. de Neufville, S.C. Moss, and S. R. Ovshinsky, Appl. Phys. Lett. 18, 254-257 (1971). 61. L. G. Pittaway, Brir. J . Appl. Phys. 15, 967-982 (1964). 62. T. P. Lin, IBM J. Res. Deu. 11, 527-536 (1967). 63. J. A. Morrison and S. P. Morgan, Bell Sysr. Tech. J . 45, 661-684 (1960). 64. A. C. M. Chen, IEEE Trans. Electron Devices 20(2), 160-169 (1973). ‘I

ELECTRON BEAM ADDRESSED MEMORIES

137

65. A. C. M. Chen, J. F. Norton, and J. M. Wan& Appl. Phys. Lett. 18, 443-444 (1971). 66. A. C. M. Chen and J. M. Wang, IEEE. Trans. Magn. 8(3), 312-314 (1972). 67. A. C. M. Chen, A. Dunham, and J. M. Wang, J . Appl. Phys. 44(4), 1936-1937 ( 1 973). 68. J. Bruining, Physica (Utrechr) 3, 1046 (1936). 69. L. Mayer, J . Appl. Phys. 29, 1454-1456 (1958). 70. C. D. Mee and G. J. Fan, I E E E Trans. Mag. 3, 72-76 (1967). 7 1 . D. 0. Smith, I E E E Trans. Magn. 3(4), 593-599 (1967). 72. D. 0. Smith, IEEE Trans. M a g . 4(4), 634-639 (1968). 73. R. C. LeCraw, D. L. Wood, J. F. Dillon, Jr., and J. P. Remeika, Appl. Phys. Leu. 7,27-28 (1965). 74. D. L. Wood and J. P. Remeika, J. Appl. Phys. 38, 1038-1045 (1967). 75. M. V. Chetkin and A. N. Shalygin, Sou. Phys.-JETP 25, 580-581 (1967). 76. G. S. Krinchik and G. K. Tjutneva, J. Appl. Phys. 35, 1014-1917 (1964). 77. H. M. Crosswhite and H. W. Moos, eds., “Optical Properties of Ions in Crystals.” Wiley (Interscience), New York, (1967). 78. D. 0. Smith, I E E E Trans. M a g . 3, 433-452 (1967). 79. M. S. Cohen, IEEE Trans. M a g . 4, 639-645 (1968). 80. A. Berkowitz, W. C. Hughes, J. L. Lawson, and W.H. Meiklejohn, 1 1 t h Symp. Electron, Ion Laser Beam Techno/. pp. 1-8 (1971). 81. D. E. Speliotis, 27th Ann. E M S A Meet. p. 152 (1969). 82. A. H. Rosenthal, Proc. IRE 28(5), 203-212 (1940). 83. W. Phillips and Z . J. Kiss, Proc. IEEE 56( 1I), 2072-2073 (1968). 84. H. F. Ivey, Proc. IEEE 57. 853 (1969). 85. I. Schneider, Appl. Opt. 6( 12). 2197-2198 (1967). 86. I. Schneider, M. Marrone, and M . N. Kabler, Appl. Opt. 9(5), 1163-1 166 (1970). 87. Z. J. Kiss, IEEE J . Quantum Electron. 5(1), 12-17 (1969). 88. H. E. Bishop, R. P. Henderson, P. Iredale, and D. Pooley, Appl. Phys. Lett. 20(12), 504-506 (1972). 89. R. Illingworth, Phys. Rev. 136, 508A (1964). 90. W. B. Hadley, S. Polich, R. G. Kaufmann, and H.N. Hersh, J . Chem. Phys. 45, 2040 (1966). 91. D. Pooley, Proc. Phys. Soc., London 87, 245 (1966). 92. L. W. Hobbs, A. E. Hughes, and D. Pooley, Phys. Rev. Lett. 28(4). 234-236 (1972). 93. S. E. Cummins, Proc. 1 / 3 3 55, 1536-1537 (1967). 94. S. E. Cummins and B. H. Hill, Proc. IEEE 58, 938, 939 (1970). 95. S. E. Cummins and L. E. Cross, Appl. Phys. Lett. 10, 14-16 (1967). 96. S. E . Cummins, J . Appl. Phys. 37, 2510 (1966). 97. D. 0. Smith, US.Patent 3,710,352 (1973). 98. H. Wieder, R. V. Pole, and P. F. Heidrich, IBM J . Res. Deu. 13, 169-171. 99. D. Casasent and F. Casasayas, Appl. Opt. 14(6), 1364-1372 (1975). 100. 0. H. Schade, Sr., RCA Reu. 31, 60- I19 (1970). 101. D. B. Langmuir, Proc. IRE 25, 977 (1937). 102. P. A. Einstein, D. R. Harvey, and P. J. Simmons, J. Sci. Instrum. 40, 562-567 (1963). 103. J. Mulvey, J . Sci. Instrum. 36, 350 (1959). 104. R. Castaing, Adu. Electron. Electron Phys. 13, 317 (1962). 105. V. E. Cosslett and M. E. Haine, Proc. Int. Con/.Electron Microsc. p. 639. R. Microsc. SOC.,London (1954). 106. M. Drechsler, V. E. Cosslett. and W. C. Nixon, Proc. Int. Conf Electron Microsc., 4th p. 13 (1958).

138

JOHN KELLY

107. 0 . Klemperer and M. E. Barnett, “Electron Optics,” 3rd Ed. Cambridge Univ. Press, London and New York, 1971. 108. L. W. Swanson and A. E. Bell Adv. Electron. Electron Phys. 32, 194 (1973). 109. C. A. Spindt, J . Appl. Phys. 39(7), 3504 (1968). 110. R. Levi, J. Appl. Phys. 26, 639 (1955). 111. W. C. Hughes, Symp. Electron Ion Laser Beam Technol., 10th p. 441-452 (1967). 112. 1. Brodie, unpublished observations (1964). 113. P. Zalm and A. J. A. van Stratum, Phillips Tech. Rev. 27(3/4), 69 (1966). 114. A. J. A. Stratum and P. N. Kuin, J . Appl. Phys. 42(11), 4436 (1971). 115. R. F. Wolter, Symp. Electron Ion Laser Beam Technol., 10th p. 435 (1967). 116. J. Haanjes and G. J. Lubben, Philips Res. Rep. 12, 46 (1957). 11 7. J. Haanjes and G. J. Lubben, Philips Res. Rep. 13, 65 (1959). 118. C. C. T. Wang, IEEE Trans. Electron Devices 18(4), 258-274 (1971). 119. H. W. G. Salinger and H. W. Beach, U.S. Patent 2,472,727 (1949). 120. K. Schlesinger, Proc. I R E 44, 659-667 (1956). 121. R. G. E. Hutter and M. B. Ritterman, IEEE Trans. Electron Devices 19(6), 731-745 (1972). 122. S. P. Newberry, T. H. Klotz, Jr., and E. C. Buschmann, Prc. Natl. Electron. Conf:23,76 (1967). 123. C. Q. Lemmond, E. C. Buschmann, T. H. Klotz and G. M. White, IEEE Trans. Electron Devices 21, 598 (1974). 124. K. J. Harte, Symp. Electron Ion Photon Beam Technol., 13th p.1160 (1975). 125. W. C. Hughes, C. Q. Lemmond, H. G. Parks, G. W. Ellis, G. W. Possin, and R. H. Wilson, AFIPS Con$ Proc. 1975, Natl. Comput. Con$ pp. 541-548 (1975). 126. C. C. T. Wan& K. J. Harte, N. Curland, R. K. Likuski, and E. C. Dougherty, J. Vac. Sci. Technol. 10(6), 1 1 10-1 112 (1973). 127. J. S. Bryan and L. R. Focht, In “Large Capacity Memory Techniques for Computing

Systems” (M. Yovits, ed.), Macmillan, New York. 128. J. Kelly, Computer 8(2), 32 (1975). 129. J. Kelly, Proc. Con$ Appl. Small Accelerators, 3rd CONF-741040-P2. Natl. Tech. Inform. Serv. (1975). 130. T. Tamura, M. Tsutsumi, H. Aoi, H. Matsubhi, K. Nakagoshi, S. Kawano, and M. Makita, IEEE Trans. Mag. 8, 612-614 (1972). 131. J. H. Wensley, Computer 8(2), 44-48 (1975). 132. H. E. Bishop, P. Iredale, and D. Pooley, Intl. Symp. Colour Cent., Reading, Engl., 1971. 133. D. P. R. Petrie, Electron Microsc., Proc. Int. Congr., 5 t h Philadelphia 1, KK-2 (1962). 134. P. W. Hawkes, “Electron Optics and Electron Microscopy.” Barnes & Noble, New York, 1972.

Electron Beams as Analytical Tools in Surface Research: LEED and AES L. FIERMANS

AND

J. VENNIK

hboratorium tioor Kristallografe en Studie van de Vaste StoJ Rijksuniversiteit Gent, Gent, Belgium;

I. Introduction., .................................................................... 11. Low Energy Electron Diffraction (LEED) A. Introduction .......................................... B. Experiments .......

.............

139

142

sities ........ 148

.

...................................

155

E. Extra Features in Auger Spectra. .............................................. F. Lifetime Broadening and Coster-Kronig Transitions e Shape ......................... H. Crossover Transitions . ..................................

181

E. Results Obtained with LEED

A. Introduction

........................

C. Basic Principles of the Auger Process

.........................

.....................................

184 185 185 198

I. INTRODUCTION A space lattice, i.e., a three-dimensional periodic array of points indefinitely extended in space, is the geometrical analog of a perfect crystal. Departures from that mathematically perfect model are known as lattice defects. A discontinuity such as a surface is therefore a lattice defect on purely formal grounds. But it is a!so so on physical grounds because it confers to the solid, due to the presence of unsaturated bonds, properties that are solely due to its presence, just as other lattice defects do. The surface, the agent through which the solid reacts with is surroundings, has been intensively studied; but we still can state that it is the least known defect. The main reason for this was the impossibility of obtaining a clean surface and of keeping it uncontaminated for a sufficiently long period 139

140

L. FIERMANS A N D J. VENNIK

of time in order to make reliable measurements. The possibility of obtaining clean vacua of the order of lo-'' torr changed this situation. Examples of processes where the surface plays a major role are found in a wide variety of phenomena; adsorption is one example that is important in oxidation, corrosion, lubrication, catalysis, etc. In the semiconductor field, the surface to bulk ratios are increasing and so consequently does the importance of surface contamination. Surface treatments and cleaning procedures are therefore important factors in device fabrication. In a general paper on methods for the investigation of surfaces, Duke and Park ( 1 ) state that, when problems and phenomena related to a surface are considered, the following questions have to be answered : What is its structure? What is the origin of this structure, i.e., what are the bonding characteristics at the surface? What impurities and imperfections are present? What are the atomic motions experienced by the surface species? The two-dimensional structure of the clean surface is determined by its bonding characteristics. The asymmetrical forces can produce a displacement of the surface atoms relative to their normal sites, giving rise to structures different from the two-dimensional structure of a parallel lattice plane in the bulk. These structures are usually determined using low energy electron diffraction (LEED), although ion scattering spectroscopy (ISS) has recently also been used for this purpose. Auger electron emission spectroscopy (AES) is the most widely used method for determining cleanliness. The four questions that we mentioned above can, in principle, be answered by combining LEED and AES only. It is nevertheless desirable to have other analysis techniques in the same instrument. In choosing a technique to answer the four basic questions, one has in mind a series of criteria to which it should comply regarding sensitivity in general, resolution, sensitivity to elements and their compounds, capacity to separate isotopes, possibility for obtaining information concerning the chemical bonding of the species at the surface, and most important of all, limitation of the analysis to the surface, i.e., the uppermost layers of a solid. Another very important point is the fact that the method should give not only qualitative, but also quantitative data. In this review the emphasis is put on the practical applications of LEED and AES; i.e., How is a LEED pattern interpreted? How is an Auger spectrum analyzed, etc. Both LEED and AES remain the most widely used methods in surface research because of their relative instrumental simplicity.

ELECTRON BEAMS IN SURFACE RESEARCH

141

11. Low ENERGY ELECTRON DIFFRACTION (LEED) A . Introduction

The famous experiment in 1927 of Davisson and Germer by which the wave nature of moving electrons was demonstrated can be considered to be the first low energy electron diffraction experiment. Technical difficulties prevented the further development of this technique until the late 1950s, when a direct display apparatus was built by Germer and co-workers. A decisive factor in this development was the advent of ion-getter pumps, to obtain and sustain a vacuum in the lO-’’-torr range for periods sufficient to study clean surfaces. LEED has led to the discovery of new surface properties. One of the most interesting has already been mentioned, namely the fact that surfaces do not necessarily have the same structure as parallel planes in the bulk. Surface structure transformations, known in the LEED literature as surface reconstructions, can occur. Another important result concerns adsorption. It has been found that ordered adsorption occurs, i.e., well-defined two-dimensional structures can be formed depending on the coverage and type of impurity. Structural information alone however is generally not sufficient to describe surface phenomena completely. This is the reason LEED is used mostly in conjunction with other surface techniques, for instance Auger electron spectroscopy (AES). Until recently LEED was mainly used to determine surface periodicities because a complete structural analysis requires the interpretation not only of the positions of the LEED spots, but also of their intensities. An adequate theory to account for the latter has emerged only recently. It can now be stated that LEED has matured into a tool with which “surface crystallography” can be adequately practiced. The emphasis in this section is put on the information that can readily be obtained from LEED data, namely surface periodicity. This will be illustrated by a summary of the results obtained on clean surfaces (metals, semiconductors, oxides, insulators), on adsorption and chemisorption (and related subjects such as corrosion), on catalysis, and on a range of miscellaneous topics (surface melting, magnetic structures, etc.). The interpretation of spot intensities will also be discussed; a complete account of the details of the dynamical theory however falls beyond the scope of this review. B. Experiments

A LEED instrument consists of a spherical fluorescent screen, a grid system, and an electron gun (see Fig. 1). The electron gun produces a

142

L. FIERMANS A N D J. VENNIK

1

kV

FIG.1. Schematic representation of LEED electron optics: 1, specimen; 2, 3, grids; 4, screen; 5, third anode; 6, second anode; 7, first anode; 8, Wehnelt; 9, filament.

focused beam of low energy electrons ( < 500 eV), the diameter of which is approximately 1 mm. The electrons impinge on the specimen which, mounted on a specimen holder allowing rotations and translations, is positioned in the center of the fluorescent screen. The grid system is in fact a high-pass filter, repulsing the major part of the inelastically scattered electrons by biasing the first grid at a potential slightly less than that of the accelerator. The second grid is at ground potential, while the screen is set at high voltage in order to postaccelerate the electrons contributing to the diffraction pattern. The latter is directly visible through a viewing port of the ultrahigh vacuum (UHV) system. Figure 2 shows a typical LEED pattern recorded on a V,O,(OlO) surface, cleaved in UHV. Specimen cleaning in situ usually consists of one, or a combination of, the following operations: cleaving in UHV, flash heating, argon-ion bombardment, annealing. The cleanliness of the examined surface is determined either by an auxiliary technique such as AES or on the basis of LEED spot intensities. When the latter remain constant after repetitive cleaning cycles of different natures, the surface is considered to be virtually clean. C . Geometrical Interpretation of a LEED Pattern

1. Dejnitions and Notation The surface of a cleaved crystal is a net plane of which the unit mesh is determined by the smallest parallelogram reproducing the lattice using only

ELECTRON BEAMS IN SURFACE RESEARCH

143

FIG.2. LEED pattern recorded on a V,O,(OlO) surface, cleaved in ultrahigh vacuum, at 6 3 e V primary beam energy.

translations. The surface structure consists of this mesh, a Bravais lattice in two dimensions, and a base. The latter can be one atom or a more complicated molecule. The determination of the base is the problem of structure analysis. The unit or primitive mesh is determined by two unit vectors al, a2 (see

FIG.3. Direct and reciprocal unit meshes of a two-dimensional lattice.

Fig. 3). For a plane lattice, only a limited number of allowed rotations and reflections in the plane leave the lattice invariant. They are the 10 crystallographic two-dimensional point groups: 1,2, lrn, 2mm, 4,4rnrn, 3, 3m, 6,6mrn ( m represents a mirror reflection). This restricts the number of primitive

144

L. FIERMANS AND J. VENNIK

meshes to the five Bravais lattices. Their corresponding symmetries are 2, 2mm, 4mm, and 6mm (see Fig. 4). Adding the base to the five Bravais lattices

/7 f-----a =goo

d

c2

52

7 FIG.4. The five two-dimensional Bravais lattices.

results in the different surface structures. There are 17 space groups in two dimensions to describe the symmetries of these structures: P1, P2, Pm, Pg, Cm, Pmm, Pmg, Pgg, Cmm, P4, P4m, P4g, P3, P3m1, P31m, P6, P6m (g is the notation for a glide mirror plane or glide line). Next to the primitive lattice or direct net, a reciprocal lattice with unit vectors a: satisfying the conditions is defined. The construction of the reciprocal net is also represented in Fig. 3. A reciprocal lattice vector is defined by ghk

= h,a:

+ k,at,

h , = nh, k , = nk

(2)

It can easily be shown that in this case

I g h k I = 27Ln/dhk

(3) where d h k represents the spacing between the atom rows with Miller indices (hk), and that &hk is perpendicular to the direction of these rows. The reciprocal space of a surface consists of the set of perpendicular lines to the surface, constructed at the reciprocal lattice points. These are the so-called reciprocal lattice rods. From ( 1 ) it follows that if upon adsorption the dimensions of the direct lattice increase in both directions, the reciprocal lattice dimensions will decrease accordingly.

145

ELECTRON BEAMS IN SURFACE RESEARCH

In the LEED literature two kinds of notation are used to describe this phenomenon. The simplest notation includes the multiplication factors explicitly, e.g., Si(111)-(7 x 7) signifies a structure of the Si(l11) plane corresponding to a unit mesh which is seven times larger in both dimensions than the primitive one. One usually adds the symbol for the adsorbate responsible for the structure, e.g., Ni( 100)-0-(2 x 2) signifies a (2 x 2) structure occurring as a consequence of oxygen adsorption on a clean Ni(100) surface. The x f i ) R 45" notation implies a rotation over 45" of the new mesh with respect to the original one. It is often replaced by the notation c(2 x 2), meaning a centered (2 x 2) structure. The former notation was introduced by Wood (2). It is however inadequate to describe all kinds of adsorption structures, i.e., those that are not simply related or coherent with the substrate structure. In this case the transformation matrix relating the adsorbed layer unit vectors with those of the substrate is useful. If b,, b, are the unit vectors of the adsorbed layer, and

(fi

b, = h l a , + h2a,, b2 = h2,a1 + h2, a, (4) the value of the determinant of the transformation matrix determines the nature of the adsorbed layer (3). If det H = integer, the nets a and b are simply related and the superposition is sometimes called a true superstructure. The notation for the previous example, Ni(100)-0-(2 x 2) for instance, now becomes Ni(100)-0and ($

x

1 1,

f i )45" in this notation becomes

If det H is a rational fraction, the nets a and b are said to be rationally related, and the superposition is called a coincidence-site structure (4, 5 ) . The Si(ll1)-(7 x 7) structure, which we shall discuss in more detail in what follows, is considered by some authors to be a simple superstructure, for which det H = 1: I, while other authors assume it to be a coincidence structure, for which det H,for instance, could be equal to I 'h5 I. If det H is an irrational fraction, the adsorbed layer is unrelated to the substrate and the superposition is called incoherent; it is not describable by a net (6). As we shall see in what follows, adsorbed layers tend to arrange in register with the substrate because at low coverage the substrate-adsorbate interaction is usually much larger than the adsorbate-adsorbate interaction.

s,,

146

L. FIERMANS A N D J. VENNIK

2. Difiaction Conditions. Bragg’s Law and Ewald Construction The diffraction conditions can easily be deduced from a purely kinematical analysis (7). If a plane wave A(r) = A, exp i(k * r - at)

(5)

is incident on a surface, as represented by Fig. 5, the amplitude of the

FIG.5

scattered wave in R is proportional to

JJ dS n(r) exp i(k - r - kr cos(r, R))

(6)

where n(r) represents the electron density at the point r. If k represents the wave vector of the scattered wave in the direction R, the phase factor reduces to - i r . A k , where A k = k - k and Ikl = Ikl =2n/A (as a result of energy conservation in elastic diffraction). Introducing the atomic scattering factorfand taking into account that the scatterers on a surface are located at the lattice points, one obtains

A(R) = A,f

c c exp -i(ma, + .a2) m

n

*

Ak

(7)

and

assuming there are M x N unit meshes on the surface. The intensity shows sharp maxima when

-

a 1 Ak = 2nh,

-

a2 Ak = 2nk

(9)

ELECTRON BEAMS IN SURFACE RESEARCH

which reduces to

-

a, A&

a2 * Ak, = 2nk

= 2nh,

147

(10)

where Aks is the component of Ak parallel to the surface. These are the two Laue diffraction conditions. Taking into account that ghk

*

r = 271 x integer

(11)

-r

(12)

one obtains exp ig,,

=1

Consequently A k = ghk satisfies the Laue conditions, i.e.,

kl = ks + ghk

(13) This is the diffraction condition, which states that the value of the component of the wave vector parallel to the surface is constrained to within a reciprocal lattice vector. This is also the condition that assures momentum conservation. The diffraction process can be described by a simple Ewald construction in the reciprocal space, as shown in Fig. 6 for perpendicular incidence

To /-

I0

P

FIG.6. Ewald construction in the reciprocal space.

(which is normally very close to actual experimental conditions). The Ewald sphere is constructed with radius A 0 1 = I k I, where 0 is chosen as the origin of the reciprocal lattice. The vectors k; , . . . are wave vectors of diffracted waves because the component of k;, parallel to the surface is

I

,

148

L. FIERMANS AND J. VENNIK

equal to g l o , etc. The directions of the diffracted waves are therefore obtained by joining A with the intersections of the reciprocal lattice rods and the Ewald sphere. It is easy to show that dhk

sin 6 h k = n)L

(14)

which is the two-dimensional Bragg law. It follows that to each reciprocal lattice vector g h , corresponds a diffraction direction, denoted accordingly (hk). The specular diffraction is denoted (00). Since the specimen is usually mounted in the center of the spherical fluorescent screen, the difraction pattern is a direct image of the reciprocal lattice. The direct lattice thus can easily be obtained from the observed pattern. The Ewald construction shows how the positions of the diffraction spots change as a function of the primary energy. By increasing the latter they converge toward the center of the screen. Since only two Laue conditions have to be satisfied, a variation in primary energy and angle of incidence does not necessarily lead to an extinction of the spots. However, the third Laue condition, due to the periodicity perpendicular to the surface, becomes more and more important as the primary energy is increased, resulting in intensity modulations of the spots. The interpretation of these intensity variations as a function of the primary energy, commonly known as Z(hk)/V curves, is the problem discussed in LEED theory. In most cases the geometrical interpretation of the LEED pattern readily produces the direct lattice, i.e., the periodicity of the surface structure. The actual positions of the atoms in this lattice can often be assumed from other physicochemical arguments. Most practical applications of LEED were, until very recently, restricted to this purely geometrical aspect. Next to the theoretical difficulties described above, uncertainty concerning the cleanliness of the surface also was responsible for this state of affairs. It has been observed that LEED patterns are rather insensitive to considerable amounts of surface contamination (maybe as much as one-tenth of a monolayer in some cases), while spot intensities on the other hand show a marked dependence on this factor. Only since the moment LEED began to be commonly used in conjunction with a surface cleanliness monitor such as AES have reliable intensity data been obtained. D. Structure Analysis with LEED: Interpretation of LEED Spot Intensities

It was rapidly recognized that LEED owes its surface sensitivity to the very high scattering cross sections of atoms for low energy electrons. These are roughly lo8 times larger than for X rays. This implies strong interaction

ELECTRON BEAMS IN SURFACE RESEARCH

149

-

with matter, small mean free path length (,lee 3-10 A), and multiple scattering of the electrons before leaving the surface (8).A conventional LEED beam (with primary energy between 10 and 150 eV) therefore “sees” only the first few atomic layers. The diffraction information it carries comes from these few layers, and it is fully justified to speak of two-dimensional diffraction phenomena. It was also rapidly recognized that any suitable LEED theory would have to include the full details of multiple scattering. After this complex scattering phenomena had been adequately described, using reasonable assumptions and approximations, another problem had to be solved before the intricate theory could be numerically applied, namely the development of sufficiently fast perturbation schemes to reduce computer time to within acceptable limits. This evolution of the dynamic LEED theory is thoroughly described by many authors (9-16). In this section only a summary of the procedure followed to interpret LEED intensities is given. Before starting this description, it is useful to recall the elementary kinematical treatment of the LEED spot intensities. Assuming ri = ujal + via, are the vectors describing the positions of the atoms in the unit mesh, one obtains from (7) A(R) = A , fi exp -i(r + rj) Ak (15)

cc

-

r n n

The factor

F(hk) =

c fi exp -irj

ghk

i

=

C fj exp -i2a(uih + v j k )

(16)

i

is called the structure factor of the (hk) diffraction. The intensity of the spot is proportional to F(hk)F*(hk).Taking into account atom layers other than the surface layer, this structure factor becomes

1

F(hk) = fi eXp -i2a[hUj + kuj -k ( 1 -k COS e h k ) z j / n ] (17) where Zr represents the distance from the surface. The periodicity .perpendicular to the surface is thus not explicitly taken into account. Expression (17) is the kinematic structure factor, which has been used in some cases with success (17). As stated above this has to be seen as a rare exception however. In practically all cases, intensity curves calculated using (17) only barely resemble the experimental curves. A possible exception to this rule are the heavier noble-gas adsorption structures on metals. Indeed, the larger the unit mesh becomes the more multiple scattering can be neglected. An example of this behavior is described by Ignatiev and Rhodin (18).

150

L. FIERMANS AND J. VENNIK

Finally, it should be mentioned that multiple scattering does not influence the periodicity of the LEED pattern. The geometrical interpretation of surface periodicities is consequently not invalidated by the fact that multiple scattering is not taken into account explicitly (19). 1. Multiple Scattering Theory Multiple scattering has been described using different mathematical formalisms. The most important are the layer-by-layer theory of McRae (20) and the t matrix theory of Beeby (21, 22). We shall confine ourselves to a brief description of the latter, mainly based on a recent review by Holland (16) and Nuyts’s thesis (23). We define hk, and hk, as the linear moments of the incoming and scattered electrons (with energy E, = (h2/2rn)I ki 1’ = (h2/2rn)I k, 12). The intensity Z(k, + k,) scattered in the direction k, is given by

- 15 5

Z(k, + b)

dr

dr’ exp( -ik,

*

rf)T(r’, r; Ei) exp(ik, r)

1

2

(18)

where T(r’, r; Ei)is the total scattering matrix and all integrations are over the entire three-dimensional space. T satisfies the equation T(r‘, r; E,) = V(r‘)d(r’ - r) +

5 V(r’)Go(r’ - r”; Ei)T(r”,r; E,) dr” (19)

where V(r) is the crystal potential in the Schrodinger equation

[ - (h2/2rn)Vz + V(r)]$(r)

(20)

= E$(r)

and Go(’; E,) is a free space Green’s function: ~ o ( r Ei) ; =

1 8n3

~

eik. r

,

-

dk,

(h2/2m)(kl2 + i&

E-+

+O

(21)

Beeby assumes the crystal potential to be given by V(r) =

c vIl(r - R),

vIl(r) = 4( I r I ) W P , - I r

Il

I1

(22)

i.e., consisting of nonoverlapping central potentials centered at the lattice points R. [ H ( x ) is the Heaviside unit function.] The potential used in this model is also called the muffin-tin potential. The total scattering matrix T is furthermore expressed in terms of individual scattering matrices t of each atom : tIl(r‘,r; Ei) = uR (r’)a(r’ - r)

+

s

4(r’)Go(r’ - r”; Ei)t,(r”, r; E,) dr” (23)

ELECTRON BEAMS IN SURFACE RESEARCH

151

Defining subplanes as those parts of the lattice planes parallel to the surface having identical structures and containing only one atom per unit mesh, one can express T as a function of tv(rf,r; Ei), where

5

tv(r’,r; E,) = uv(r’)d(r’- r) + uv(r’)GO(r’ - r”; Ei)tv(rw, r; E i ) dr” (24) and where uv(r)= uR(r),with R defining a lattice point in the subplane with index v. Finally, calculations are performed in the angular momentum representation because the potential uv(r)is central. To obtain satisfactory agreement between theory and experiment it is necessary to introduce phase shifts, calculated from realistic ion-core potentials. Accurate calculations require seven to eight phase shifts (24). The inner potential can be taken into account by replacing E, by Ei - Vo,. A major contribution to this theory has been made by Duke and Laramore (25). These authors introduced thermal vibrations and inelastic scattering in this formalism, and this constitutes one of the major breakthroughs of LEED theory. The significance of inelastic scattering for LEED was first recognized by Duke and Tucker (8). This is formally done by replacing

1 5 dr’tR(r,r’; E i ) exp[i(k, r’ - k,r)]

tR(kl, k,; E i ) = dr

(25)

by exPC-

u6(kl - kZ)ltR(kl,kZ; Ei)

(26)

where

W,(d= f < [ s* U(R)I2)T

(271

and where u(R) is the vector Schrodinger operator for the displacement of an atom from its equilibrium position R.( )T signifies that a thermal average is taken. Furthermore, Ei is replaced by Ei - Vo(Ei),where ‘Vo(Ei) = br(Ei) + iVoi(Ei)

(28)

V,, is the real inner potential and Voi(Ei)= -h/27(Ei), where T(E,) is the lifetime of the electron with energy Ei . The normal components of ki and k, now have complex values. This theory leads to excellent agreement with experimental results as shown in Fig. 7, where results for the Al( 100) surface are shown. The upper

152

L. FIERMANS AND J. VENNIK

0 0

L(

30 60

90

120 150 18

E (ev)

FIG.7. Experimental [Jona (26)]and theoretical [Laramore et al. (27)] intensity curves for AI( 100) showing excellent agreement. Copyright 0 1970 by International Business Machines Corporation; reprinted with permission of the publisher.

part of the figure represents experimental data of Jona (26X the lower part calculated data of Laramore et al. (27). Similar agreement has been obtained on Ni and Cu [see, e.g., Duke (28) for more references, also concerning adsorption structures]. From expression (18) it follows that the result of a scattering experiment is determined by a single T matrix element specified by the incident and scattered wave vectors. The theoretical problem of LEED is thus reduced to the calculation of such T matrix elements. This involves a considerable amount of mathematical manipulation which will not be discussed here, but can be found in the literature [e.g., Holland (16)]. The direct solution of the equations obtained requires considerable computing facilities, e.g., the inversion of a matrix of order N(lmax + 1)2,where N is the number of layers and lmax+ 1 is the number of partial waves needed to describe adequately the ion core scattering properties. For simple cases I,,, 4 and N 10, a storage requirement of 60K is already necessary. Also the times taken even for the simple cases are too long (16).Therefore perturbation theories have been developed that are both fast and economical in storage requirements. The first iterative scheme developed by Pendry (29) is called the renor-

-

ELECTRON BEAMS IN SURFACE RESEARCH

153

malized forward scattering (RFS) scheme. Because backscattering tends to be weak it is treated by perturbation theory. The layer scattering amplitude is calculated exactly, but the scattering between layers is treated in terms of perturbation theory. This method is applicable to simple cases, e.g., where there is only one subplane per layer. Good agreement, e.g., for the Cu(100) surface, is obtained (see Fig. 8).

Energy ( c v

I

FIG.8. Results of calculations on Cu(100) [Pendry (29)l.The full curve represents the exact calculation; the dashed line corresponds to the first-order result and the crosses to third order [Holland (16)].

Zimmer and Holland (30) found that the RFS scheme could still be used if the ion cores are replaced by giant symmetric scatterers having equal forward and backward scattering power (see Fig. 9). The same authors furthermore devised a method for making numerical calculations on more complex structures (several inequivalent scatterers in a layer for instance) (31). Only a finite number of subplanes are considered because of the damping of the electron waves. The theory was tested for the Ni( 111) surface, and excellent results were obtained requiring only _+ 4% of the computing time as compared to the exact calculations (16). Holland concludes that this scheme, called the reverse scattering (RS) scheme, extends the main advantages of RFS to structures with several subplanes per layer and hence opens the door to realistic LEED calculations for complex structures. Finally, as a conclusion to this section on LEED theory, we would like to mention a paper by Duke (28) wherein this author gives a complete overview both of recent theoretical developments and of their applications to

L. FIERMANS AND J. VENNIK

154

-

‘8

W

0

a

V

w

0

‘I

U 10

20

30

50

-

90

70 eV

110

FIG.9. Intensity of the 00 beam for “quasi-copper” (100). Exact calculation is full line. Crosses represent first order and are again in close agreement with the exact result [Holland (1611.

practical surface structure determination. It seems worthwhile to quote Duke literally when he writes (in December 1974): “the past two years have been the scene of the introduction of LEED to characterize the geometrical, vibrational and electronic structure of planar, single crystal solid surfaces: i.e. the initiation of quantitative surface-sensitive electron scattering spectroscopies.”

2. Applications of the LEED Theory a. Clean surfaces. As mentioned above, the validity of the theory has been checked on clean metal surfaces, e.g., Al, Cu, Ni. Other metals studied are Ag (32) and W (33). b. Adsorption structures. The importance of the development of the LEED theory is also reflected in the growing number of adsorption structures studied. This topic has been reviewed recently by Rhodin and Tong (24). It has now become possible to obtain accurate chemisorption bond lengths of adsorbed atoms, as is illustrated by the analysis of the Ni( 100)-c(2 x 2) structures induced in chalcogen adsorption, discussed by these authors. Other examples are Ni( 100)-c(2 x 2)-Na (34), Cu( lOO)-c(2 x 2)-0 (35),Rh( 100)-c(2 x 8)-0 (36),Ag( 11l)-(fl x d ) 3 O 0 - I

ELECTRON BEAMS IN SURFACE RESEARCH

155

(37), Ni(100)-c(2 x 2)-Te (38), etc. More references can be found in (28) and (24). The development of the LEED theory has thus made it possible to determine quite accurately not only lateral adsorption distances, but also adsorbate-substrate distances. This is of course of great importance for chemisorption theory. We shall now review the results obtained from geometrical interpretations of LEED data, which up to now still constitute the bulk of LEED work. E. Results Obtained with LEED

1. Clean Surfaces

a. Metals. Most metal surfaces examined to date exhibit structures that are identical to those of parallel planes in the bulk. There are however a few exceptions that make it difficult to establish a general rule. These are the Au, Pt, and Ir (100) surfaces which exhibit a ( 5 x 1) structure. This is explained by the presence of a reconstructed hcp layer on top of the fcc bulk lattice (39, 40). It is now clearly established that this structure is not impurity stabilized but is an intrinsic property of the surface. It remains stable almost up to the melting point in contrast with, e.g., the Si(111)-(7 x 7) structure. As we shall see in what follows, reconstructions are commonly observed on semiconductor surfaces, but these are modified at temperatures well below the melting point. The metal surface reconstruction is thus essentially different from the ring or chain formation due to broken covalent bonds as was pointed out by Phillips (41). This author also draws attention to the fact that the reconstructed Au or Pt surfaces have a greater atomic density as compared to rearranged covalent surfaces. This suggests that metal surface planes tend to have higher electron densities. The question that arises is why Au, Pt, and Ir reconstruct and not the other, generally lighter metals. Phillips suggests that the core polarizabilities could play a role in this phenomenon. For large ion cores, the greater polarizability would give rise to greater core-valence exchange and correlation energies. Together with oscillations of s-p electron densities, this might just tip the balance in favor of surface reconstruction. The problem is important and more work is needed to understand these structures in full detail. Another important aspect of LEED work on clean metal surfaces concerns contraction and expansion of the first layers. This point has recently been discussed and summarized by Rhead (42). This author points out that as a result of the different, asymmetrical environment of surface atoms, interplanar spacings normal to the surface can be altered. This is called

156

L. FIERMANS A N D J. VENNIK

surface relaxation. Early theoretical results, e.g., for Cu (43), predict an expansion of 5-15% for the top layer, rapidly decreasing to 0.1% for the fourth plane. In this theory a pairwise interaction model and potential energy functions such as the Morse function were used. Recent LEED data on the other hand generally indicate a surface contraction, e.g., on Al( 110) (44) and Mo(100) ( 4 9 in agreement with a theoretical approach explicitly introducing electronic effects, such as electronic redistribution at the surface, altering the force constants (46). Summarizing the actual situation, Rhead (42) points out that the problem of surface relaxation is still in the forefront of current theoretical work, but that there already is general agreement concerning the fact that contraction seems to be the rule for most metal surfaces. b. Semiconductors. Contrarily to metals, clean semiconductor surfaces generally exhibit complex structures. Freshly cleaved Si( 11 1) and Ge( 11 1) surfaces show (2 x 1) structures (47) which upon heating change into Si(11 1)-(7 x 7) and Ge(11 1)-(8 x 8) (48,49). Whether these are due to true superstructures or to coincidence structures is still not completely clear. The difference in behavior of metals and semiconductors can be understood in terms of differences in chemical bonding. The nearly free electron gas in metals is easily redistributed at surfaces in order to compensate for broken bonds. On semiconductors and the above-mentioned heavy noble metals it is assumed that the broken bonds produce periodically buckled surfaces, arising from the relaxation of surface atoms (50). If this relaxation occurs in a periodic manner, new surface periodicities arise. This process is very sensitive to the presence of impurities and surface defects. Generally speaking, surface reconstruction can be understood when one considers that surface atoms may have more, or less, unpaired valence electrons available for binding than bulk atoms since they have fewer neighbors. Since the structure of a solid has been shown to depend on the number of unpaired s, p, and d electrons available, surface structures different from the corresponding bulk structures can indeed be expected for semiconductors. The most intensively studied semiconductor surface certainly is Si( 11 1). Lander and Morrison ( 5 1 ) originally proposed a warped benzenelike ring structure to explain the rearrangement .of the bonds into a (7 x 7) superstructure; other authors proposed different rearrangements (52,53).Optical simulation, however, contradicts these models (54). In the section on adsorption studies, the discussion of the Si(ll1)-(7 x 7) will be continued. Undoubtedly, this superstructure, together with the heavy noble metal ones, will be extensively studied in the near future now that a thorough theoretical approach has become feasible. . Other important semiconductors are the 111-V compounds GaAs, InSb, GaSb, and InAs (55). The cleaved (1 10) surfaces exhibit ( 1 x 1) structures.

ELECTRON BEAMS IN SURFACE RESEARCH

157

The (111) surfaces on the other hand reconstruct into various superstructures, e.g., (2 x 2) and (3 x 3) on GaAs, GaSb, and InSb (56-60). c . Insulators, oxides, and alkali halides. Insulators show charging when bombarded with a low energy electron beam. This difficulty can however be overcome by working at a primary energy where the secondary emission equals the primary beam intensity. Another difficulty posed by these materials is their instability under the beam. It has been found that most alkali halides decompose under the influence of the beam (62).The Al,O,(Wl) surface transforms, when heated, from a (1 x 1) structure into a complex x ,/%)-9" one (62). The V,O,(OIO) surface transforms into the V,O,,(OlO) under the influence of the beam only (63, 64). The BaTi0,(001) surface also exhibits a superstructure when heated (65). Summarizing, most insulator and oxide surfaces studied so far exhibit initially (1 x 1) structures, which however rapidly change into more complex ones either under the influence of the beam only, or when heated to elevated temperatures, with or without electron bombardment.

(fl

2. Adsorption Studies a. General aspects. Adsorption phenomena can be studied using the changes they induce in the LEED pattern periodicities and in LEED spot intensities. The latter point was discussed earlier. In this section we shall confine ourselves to geometrical interpretations. In many cases the manner in which the adsorbed layer is formed can be deduced and some information concerning adsorption sites and adatom-adatom and adatom-substrate interactions can be obtained. One of the most important discoveries made with LEED concerns the ordering of adsorbed species. They usually produce extra spots in the LEED pattern from which a model for the surface periodicity, and ultimately for the surface structure, can be deduced. This interpretation is however not always straightforward. A famous example is the superstructure observed on Ni(100) when oxygen is adsorbed (Fig. 10). It was initially assumed that the LEED pattern was entirely determined by the Ni atoms. Reconstruction giving rise to mixed Ni-0 surfaces had to be accepted to explain the observed features (66). It is however well established now that light elements, especially when they are ionized, are equally good scatterers. Therefore adsorption on top of the Ni substrate (Fig. 11) is now accepted to explain the observed (2 x 2) and x superstructures. Upon further exposure to oxygen, the Ni( 100) surface however does reconstruct, and the pattern characteristic for NiO( 100) is obtained (67). Another famous example is the Si(111)-(7 x 7) structure. It can be easily

(fi f i )

158

L. FIERMANS A N D J. VENNIK 0

0

.

0

0

0

.

0

.

.

0

.

0

.

.

1

0

.

0

.

.

a

0

.

0

.

.

f

0

.

0

.

.

0

:a (b)

0 0 . 0 .

-0-

1

2a

0

0 0 . 0 . (d ) 0

0 1

. 1

0

0 0 . 0 .

.

.

1

o . . . . 0

.

0

.

0

.

FIG.10. Diffraction patterns and proposed surface structures for oxygen adsorption on Ni(100) [May and Germer (66)].

FIG.11. “Nonrnixed” (2 x 2) and ($

x

,/2) structures on Ni(100) [Bauer (68)].

shown that the true one-dimensional superstructure and the coincidence site superstructure represented in Fig. 12 (68) give rise to LEED patterns with identical periodicities. Therefore the Si(111)-(7 x 7) structure need not necessarily correspond to a true superstructure with a large unit mesh. Alternative explanations based on a coincidence site structure (assuming for instance the existence of a Fe,Si, layer on top of the surface) have been put forward (69).

159

ELECTRON BEAMS IN SURFACE RESEARCH

1

C

i

T T T T .’ 7 1’ T T T ‘I

‘’

FIG. 12. True superstructure (upper part) and coincidence structure.

The AES data on which this interpretation is based are however ambiguous and not generally accepted. As stated before, it will be the task of detailed intensity interpretations to elucidate this until now enigmatic (7 x 7) superstructure. A helpful observation in the discrimination between true superstructures and coincidence site superstructures is the fact that in the latter the supplementary spots are generally visible principally in the vicinity of the primitive ones. This is sometimes explained on the basis of a double-diffraction picture [which is a first approximation to describe multiple scattering (20)] whereby every primitive diffraction is considered to be a primary beam for a second diffraction by the overlayer. An example of this behavior is shown by the Si(l1l)-P-(6G x 6 f i ) structure (70). This explanation, however, overlooks the fact that as far as the periodicity of the LEED pattern is concerned the lateral periodicity of the layer formed by the substrate and adsorbate atoms has to be taken into account as a whole. A more complete understanding of these data again must come from a thorough theoretical interpretation. Finally, the interpretation of supplementary spots is sometimes complicated by the presence of domains. Since the coherence diameter of a low energy electron beam is about 500 A, and the diameter of the beam is 1 mm2, these domains can be rather small. A simple (2 x 2) structure, for instance, can be produced by two domains each having a (2 x 1) structure and rotated over 90” with respect to each other. Of course in this superposition many “normal” (2 x 2) spots are missing, but since this can be due to other causes, such as intensity modulation, discrimination between the two is not always straightforward.

-

L. FIERMANS AND J. VENNIK

160

Figure 13 represents a typical example of domains present on a V,O,(OlO) surface. Lower oxides, characterized by V,O,,(OlO)-(l x 1) and V,O,,(OlO)-(1 x 2) structures were present on the surface. The latter structure could correspond to a distorted V 4 0 , or a VO, surface (64).

0

0

Y

.

O X

X

0

.

. X

0.

m X

0 X

m

X

0

0

0

0

m

. 0

.

o

a

. . . .

0

8 X

0

o0

0

'Ip x . O

0

X

0

c

X

m 0

.

0

0

mX0

FIG.13. LEED pattern observed on a V,05(010) surface, with lower oxides also present (upper part). The lower part of the figure represents the reciprocal nets for this LEED pattern: x , V,O,(OlO); ~ , V , 0 , , ( 0 1 0 ) ; B, V,Ol,(O1o)-(1 x 2); 0,V,Ol,(O1O)-(1 x 2).

b. Chemisorption. Chemisorption has been extensively studied with LE.ED. A comprehensive discussion has been presented by Somorjai and Szalkowski (71). Tables 1-111 in that paper and Table I1 in Somorjai and Kesmodel (6) contain a summary of the chemisorption superstructures ob-

ELECTRON BEAMS I N SURFACE RESEARCH

161

served on substrates with sixfold, fourfold, and twofold rotational symmetry. From these tables Somorjai et al. derive a set of empirical rules applicable to most of the observed structures and which allow the prediction with some confidence of the structure of a chemisorbed system. They conclude that chemisorption leads to the formation of surface structures that exhibit maximal adsorbate-adsorbate and absorbate-substrate interaction. The empirical rules are the following (citing Somorjai and Szalkowski):

rule of close packing: adsorbed atoms or molecules tend to form surface structures characterized by the smallest unit cells permitted by molecular dimensions, adsorbate-adsorbate and adsorbate-substrate interactions ; close-packed arrangements are preferred ; rule of rotational symmetry: adsorbed atoms or molecules form ordered structures that have the same rotational symmetry as the substrate plane; rule of similar unit uectors: adsorbed atoms or molecules in monolayer thickness tend to form ordered surface structures characterized by unit mesh vectors closely related to the substrate unit vectors; thus, the adsorption structure bears a greater similarity to the substrate structure than to the structure of the adsorbate in condensed form. The first rule illustrates the importance of adsorbate-adsorbate interactions; the second seems generally valid also for epitaxial growth. The third rule implies that considerable mismatch should be present in layer-by-layer growth of an adsorbate, which might be important in such fields as superconductivity and catalysis. c. Coadsorption and catalysis. Coadsorption structures are formed during the simultaneous adsorption of two gases (and not with one gas only), for instance CO and N, on W( 100) (72). Only simple catalytic reactions have been studied with LEED so far, e.g., oxidation of CO on Pd(ll0) (73). In this kind of study LEED is used in conjunction with other techniques, mainly AES and XPS. Especially Ertl's group at the Physikalisch-Chemisches Institut der Universitat Munchen, Germany has been active in this field (74). This kind of research is of considerable practical importance although the actual conditions under which catalytic reactions take place cannot be reproduced in a LEED instrument. In a detailed investigation of catalysis with LEED, one could in principle obtain information concerning the four important steps in a catalytic reaction : adsorption, surface migration, surface reaction, desorption. Indeed, LEED is capable of providing information concerning adsorption structures, e.g., by following the evolution of the LEED pattern as a function of time and temperature, and thus should in principle be a valuable tool in the study of catalysis. A drawback, however, is the unpredictable

162

L. FIERMANS A N D J. VENNIK

influence of the beam on the reaction studied, making any conclusion dubious as long as this factor is not under control. An important aspect of catalysis, i.e., the influence of defects and steps on a surface (possibly acting as active sites), has been studied by looking at vicinal metal surfaces (75). This subject is extensively reviewed by Somorjai and Kesmodel(6).These stepped, high Miller index surfaces are indicated by the postscript S,e.g., Pt(S). The ordered step array is indicated by giving the width and orientation of terraces and the height and orientation of the steps, e.g., Pt(S)-[6(111) x (loo)] means a terrace of (1 11) orientation, six atomic rows wide, and steps in the (100) orientation, one layer high. These surfaces exhibit a great reactivity as compared to low index crystal faces and are therefore important for catalysis. 3. Miscellaneous Applications

a. Imperfect surfaces. The degree of order of a clean surface or of an adsorbed layer can be estimated from the spot size and shape and from the appearance of streaks, rings, or arcs in the pattern. The higher the order in all directions, the smaller is the spot size. In the ideal case the spot size and shape are determined entirely by instrumental factors. Streaks indicate disorder in one direction. Rings appear when the surface is rotationally disordered with a rotation axis perpendicular to the surface. In the nonperpendicular case, arcs are observed. Finally, a completely disordered surface does not give diffraction patterns. A high background intensity is observed in this case. Another factor reducing the spot intensity and increasing the background, is the thermal vibration of the surface atoms. Intensity measurements can indicate something about the magnitude and anisotropy of these thermal vibrations. A surface Debye-Waller factor can be deduced from such measurements (76). The surface Debye factors for instance for Pb and Pd are about two times smaller than the corresponding bulk values, and conversely the rms displacements perpendicular to the surface are larger. It was therefore expected that surface melting should occur at temperatures below the bulk melting point. This is, however, not the case (50). 6. Epitaxy. Different systems have been investigated: metals on metals [e.g., Ag on Au, Cu(100), and Cu(ll1) (77-79)]; metals on semiconductors [e.g., Pb, Sn, In, Al, Ag, Au, Ni, and Fe on Si(ll1)-(7 x 7) (80, Sl)]; metals on insulators [e.g., Ag and Au on mica, KCl and MgO and Si on cr-Al,O, (82-85)]; and semiconductors on semiconductors [e.g., Si on Si (86, 87)]. LEED is exceptionally well suited for this kind of problem because one readily can observe whether the condensate is ordered or not, what its crystallographic orientation is, and how it relates to the substrate.

ELECTRON BEAMS IN SURFACE RESEARCH

163

c. Magnetic surface structures. In many magnetic materials, the magnetic unit cell is not identical to the atomic unit cell, e.g, NiO at its N k l temperature should have a Ni0(100)-(2 x 1) magnetic unit cell. This has actually been observed (88), proving that in some cases magnetic ordering can be studied with LEED. F. Evaluation of the Capacities of LEED and Future Prospects In evaluating the usefulness of a surface analytical technique, one often asks the following questions (89): Is the interpretation of the data unambiguous? Is this interpretation simple and straightforward? Is the technique not too complicated from a technological point of view? How are its sensitivity and resolution? What is its capability for identifying elements (isotopes) and compounds ? LEED obviously is as yet not an analytical technique, but it is a powerful tool for the study of surface structures, especially since the theoretical and computational progress of recent years has made surface crystallography possible. Its limitations are evident, i.e., a relative insensitivity to large amounts of disordered contamination as far as periodicity is concerned, its use in conjunction with AES is imperative; only applicable to crystalline material; a rather poor resolution: what one observes is an average over 1 mm2 of the surface; it can until now not identify elements or compounds; this might become possible only if scattering factors can be accurately determined from intensity data. When LEED was reintroduced in the late 1950s, it was hoped that it would rapidly yield important results in the fields of catalysis, corrosion studies, lubrication, oxidation, etc. It is now evident that LEED is not capable of solving the important problems arising in these fields alone. A combination with other surface techniques is imperative. The conditions under which LEED and related experiments are performed are basically different from those encountered in most practical cases. This and some other limitations (89) (e.g., the material must have low vapor pressure a t room temperature, the specimen should not melt below 200°C to allow a bake-out of the system, the surface may have only a limited number of facets, and gas pressures cannot exceed torr because of electron-gas atom scattering) are encountered in other surface techniques as well. Finally, the influence of the beam is a disturbing factor.

164

L. FIERMANS AND J. VENNIK

In conclusion, it can be said that LEED on the one hand will continue to be a useful supplementary technique in surface analytical methods, and on the other hand will be used more and more intensively in surface crystallography. It is indeed now possible, quoting Pendry (15), “to put LEED to work.”

111. AUGERELECTRON SPECTROSCOPY (AES) A . Introduction

Auger peaks are features appearing in the secondary electron spectrum of a solid excited with energetic electrons (in the keV range), X rays, ions, protons, etc. Figure 14 shows an example of a secondary emission distribution curve containing Auger peaks.

FIG.14. Secondary electron emission distribution with three typical regions: I, primary peak and loss peaks; 11, Auger peaks; 111, true secondary peak.

They are a consequence of radiationless rearrangements of the electrons in excited atoms in which a core hole has been created by the exciting radiation. The energy released during this process is transferred to another electron, the Auger electron, which is emitted with a well-defined kinetic energy, leaving the atom in a doubly ionized state (see Fig. 15). Already in 1953, Lander (90) pointed out that Auger peaks could be used to determine surface compositions. This work had practically no response for about a decade, because of the lack of ultrahigh vacuum (UHV) equipment. Furthermore, Auger peaks are normally small features situated on the background of the secondary emission. The real breakthrough came in 1967 when

ELECTRON BEAMS IN SURFACE RESEARCH

voc,

M

K-

165

fI 1:: --- - -

- --

FIG. 15. Elementary level scheme showing a typical Auger transition (KLM).

Weber and Peria (91) and Tharp and Scheibner (92) showed that secondary electron emission spectra could be recorded using a LEED instrument. Finally, in 1968 Harris (93a) introduced the technique of recording the derivative of the distribution curve and showed that by doing so, the sensitivity could be dramatically increased. In 1969, Palmberg et al. (94) introduced a high-sensitivity cylindrical mirror Auger analyzer (CMA) which allows the direct display of Auger spectra. Consequently, AES has, over the past few years, rapidly become a major tool in surface analyses. This success is primarily due to its high sensitivity. It is also essentially a nondestructive method and to some extent the apparatus is relatively simple. Using AES, it can now be established whether a surface is clean, the sensitivity being approximately 0.1 % of a monolayer; and this sensitivity will probably still be increased. Recent theoretical and experimental developments have made AES a quantitative technique for surface elemental analyses. The accuracy, however, may vary within broad limits depending on the case. B. Experiments 1. Retarding Field Analyzer

Auger spectra can be recorded with LEED optics used as a retarding field analyzer (91, 92). The normal setup is shown in Fig. 16. The LEED gun produces the necessary energetic primary beam. The secondary electrons are collected by the fluorescent screen, after passing the grid system. The retarding potential is applied to the two central grids. A small modulating voltage

166

L. FIERMANS A N D J. VENNIK

FIG.16. Schematic representation of an Auger spectrometer: 1, specimen; 2, oscillator (1050 Hz); 3, amplifier; 4, sweep generator; 5, digital voltmeter; 6, averager; 7, frequencydependent amplifier; 8, lock-in amplifier; 9, computer.

is applied to the same grids. The outer and inner grid are at ground potential. The modulated collector current that contains the information is amplified and demodulated using well-established lock-in amplifier techniques. The collector current is given by Tracy (95):

LIdE) = 1,

j

EP

E

N ( E ) dE

(29)

where N ( E ) is the secondary emission distribution; E,, E are the primary energy and the retarding potential; and I, is the primary current. It can easily be shown that for a smalf modulating voltage E - Eo = k sin ot (30) the first harmonics coefficient in the Taylor expansion of the current, is proportional to N ( E ) , the second harmonics coefficient to dN(E)/dE, etc. In the second harmonics mode, the background is greatly reduced and electronic amplification can be considerably increased (93a). The limitations and extensions of this modulation technique have recently been discussed in detail by Hanisch et al. (93b). Figure 17 represents a typical [ d N ( E ) / d E ] / EAuger spectrum, recorded on a V,O,(OlO) surface freshly cleaved in UHV. The peaks observed are due to vanadium and oxygen Auger transitions and to carbon contamination. Some other peaks (labeled C.I.) will be discussed in what follows(96).

167

ELECTRON BEAMS IN SURFACE RESEARCH

I

I

230

1

1

310

I

1

390

I

I

470

1

I

550

E(eV)

FIG.17. Auger peaks observed on V,O,. The peaks labeled C.I.are characteristic ionizations.

The high SIB values obtained in this manner are the major advantage of this mode of operation which however also has certain drawbacks, namely: when the Auger peak is asymmetrical, it is often difficult to recognize the original shape in the derivative peak; exact peak positions are not always easily determined; amplitude ratios of different peaks are ambiguous. In practice it has been agreed upon to determine the peak position at the high energy minimum of the double wing-shaped peaks. The peak width is usually given as an energy difference between high and low energy extrema. Amplitudes are given as peak-to-peak values. An alternative method to reduce the background consists in computeraided background substraction, after noise reduction through averaging of directly recorded N ( E ) curves (first harmonics mode of operation) (97). In this manner, the primary electron dose can be reduced and hence the damage due to the beam. Sickafus (98), Musket and Ferrante (99), and more recently Houston (100) have discussed different methods of background substraction to retrieve optimized Auger spectra. The latter method, called dynamic background subtraction, involves multiple differentiation followed by integration of the same order. The multiderivative scheme usually results in low SIN values, as demonstrated by Fig. 18, representing the N ( E ) , N’(E),and N ” ( E ) spectra of minute amounts of oxygen on Ta. This can, however, be overcome by performing first- and second-order background substraction, as shown in Fig. 19 where R , represents the integral of the data shown in Fig. 18b and R , the double integral of that’given in Fig. 18c. In this manner the oxygen

168

L. FIERMANS A N D J. VENNIK

-2

1

I

1

1

500

1

560

ELECTRON ENERGY - eV

FIG. 18. Auger spectra from minute amounts of oxygen on tantalum [Houston (loo)]:(a) measured electron energy distribution, (b) first derivative (first harmonic), (c) negative second derivative (second harmonic).

Auger spectrum is retrieved from the experimental data. For more details the reader is referred to Houston (100). A different approach has been proposed by Mularie and Peria (101). These authors retrieve high-resolution Auger spectra from normally recorded [dN(E)/dE]/E curves, by integration and deconvolution with a suitable instrument function (also containing the undesired inelastic effects present in Auger spectra, such as plasmon losses). This method has until now not known very intensive follow-up, mainly because it remains difficult to eliminate all inelastic effects.

ELECTRON BEAMS IN SURFACE RESEARCH

169

s w

d

a

6

0 440

560 ELECTRON ENERGY

- eV

FIG. 19. Results of first- and second-order dynamic background subtraction [Houston (IOO)].R , represents the integral of Fig. 18b, R, the double integral of Fig. 18c.

A detailed analysis of resolution and sensitivity of the LEED-AES instrument has been given by Taylor (102) and Bishop and Riviere (103). Resolutions of 0.5% can be obtained. Sensitivity in terms of noise is also discussed by Taylor. A practical limit to sensitivity is the time needed to collect the data. A limit of 0.1 % of a monolayer can be reached, the limiting factor being the high background current inherent in a high-pass filter (104).

-

2. Cylindrical Mirror Analyzer ( C M A ) The CMA was first used in AES by Palmberg et al. (94) in an arrangement shown in Fig. 20. It is a deflection type band-pass instrument with high transmission (10%) for moderate resolution (0.3%). At approximately the same resolution the transmission of the instrument is thus still more than half that of the LEED-AES instrument (17%). Its SIN ratio is 100 times larger than for a retarding field instrument, enabling a direct display of the Auger spectrum on an oscilloscope screen.

170

L. FIERMANS A N D J. VENNIK

COAXIAL CYLINDRICAL ANALYZER

FIG.20. The cylindrical mirror analyzer [Chang (106)].

The current collected in a CMA can be written as (95) I(E) = CEN(E)

(31)

where C is a constant, containing I,. It is easy to show that in this case the first harmonics coefficient is proportional to E d N ( E ) / d E , for sufficiently small k values (modulating voltage). If true distribution curves are desired, an integration has to be per formed. Figure 21 shows a typical CMA curve, recorded on a gold specimen, prior to cleaning. In such spectra the true secondary peak at low energy is drastically reduced as a consequence of the energydependent transmission [factor E in Eq. (31)]. High modulation frequencies are normally used to decrease the llfnoise. High gain electron multipliers allow for counting or synchronous detection with modulation to be used. High scan rates are possible and an Auger spectrum can continuously be monitored. If on the other hand the sample is beam sensitive, low primary beam currents can be used. Optimal values for modulation amplitude and primary beain energy are mentioned in the literature (95, 102, 103). High primary excitation energies are advised for the CMA ( - 3 keV). This has the additional advantage of flattening N ( E ) ; the same amplifier gain can consequently be used throughout the spectrum. Modulation amplitudes are chosen as a function of the problem. The effect of the modulation amplitude has been studied by Grant et al. (105). Their

171

ELECTRON BEAMS I N SURFACE RESEARCH

1

300

400

I

500

eV

FIG.21. Auger spectrum of an as-mounted gold sample. The main impurity peaks are indicated on the figure. The other peaks are due to Auger transitions of gold.

results are illustrated by Fig. 22. On the left-hand panel are shown Auger spectra of Ti recorded at various modulation amplitudes e o . The right-hand panel shows the result of integration of the derivative data. Attenuating factors are indicated in order to obtain an accurate relative scaling. From the left-hand side of this figure it is clear that peak-to-peak strengths and peak positions can be very sensitive to the modulation amplitude (93b). The integrated spectra on the other hand are less dependent on the modulation amplitude and are therefore better suited for quantitative analysis (see Section 111, 1, 2). Chang (106) has recently calculated the theoretical sensitivity limit of such an instrument. The largest measured Auger currents are Ii 10-4Z, (where I, is the primary beam current). I, A which gives a shot-noise current of 5 x typically is 5 x A, hence a maximum SIN ratio of lo5 is obtained. Practically speaking a ratio of lo6 might be attainable, corresponding to I ppm sensitivity. This sensitivity is limited however by background peaks and spectral overlap. A detection limit of 10-2-10-3 % is usually imposed.

-

-

-

3. Energy Calibration of a LEED-Type Auger Spectrometer

The Auger energies measured are kinetic energies referred to the vacuum level of the spectrometer (see Fig. 23). Since binding energies generally are referred to the Fermi level, it is necessary to add the spectrometer work

172

L. FIERMANS AND J. VENNIK

4'

e0

0.78 eV

4.7eV

1

1.0

i'

.

8.50

L

22.9 eV

1.0

ELECTRON ENERGY - eV

FIG.22. L M M spectra of Titanium [Grant er al. ( I O S ) ] . e, is the modulation amplitude. The right-hand figures represent integrated curves. Attenuation factors are indicated.

function Qana, to the measured Auger energy to obtain values comparable with the calculated values (see Sections 111, C, 2 and 111, D, 1). The spectrometer work function is an unknown factor and can furthermore change from one experiment to another. It can be determined by measuring contact potential differences between a speclliien with a known work function and the spectrometer. This is usually done by measuring the true zero of a secondary electron emission curve. The difference between the true zero and zero retarding-grid potential is equal to the contact potential difference in question. Normally, the energy scale is calibrated using suitably chosen primary peaks. Here one has to take the contact potential difference between the cathode and the spectrometer into account. The latter can be determined by

ELECTRON BEAMS IN SURFACE RESEARCH

173

E

@anal

EF

X L specimen

onalyzer

FIG.23. Energy-level diagram showing the relationship between actual and measured kinetic energies. .

measuring the difference between the beam energy (referred to the vacuum level of the cathode) and the retarding potential at which a maximum signal at the collector of the spectrometer is observed. Once the system is calibrated it is tacitly assumed that it is not modified in the course of experiments. All experimental AES values are therefore correct only within certain limits ( - 1 eV).

C . Basic Principles of the Auger Process 1. The Auger Notations

To arrive at the notation used in AES, let us consider the electrons in an atom, arranged in the shells K, L, M, . . ..Spin-orbit splitting gives rise to the subshells L,, L,, L,, M,, M , , M,, . .. obtained from the X-ray notation. When, for example, an electron is removed from the K shell, internal rearrangements of the electrons occur almost instantaneously. Radiative transitions such as KL,,, can give rise to the well-known X-ray photons. When the rearrangement is radiationless, the energy released is transferred to a

174

L. FIERMANS A N D J. VENNIK

second electron, e.g., also in the L2,3 shell, which in the case cited, results in an Auger process described as KL2,,L2.,. The kinetic energy of the Auger electron is determined by the energy conservation law. It is independent of the primary energy and in a first approximation determined only by the electron binding energies. The Auger spectrum therefore is a fingerprint of the composition of the particular material. Radiationless transitions explain why X-ray fluorescence yields are smaller than l. Auger processes are much more probable if small energy transitions are involved. In practice one can say that transitions below a few keV show a marked tendency to be radiationless. The Auger process is furthermore not governed by the dipole transition probability function ; and its probability, when considering only transitions between outer shells, remains practically constant with 2. Chang has pointed out that the high efficiency of the Auger process is illustrated by the sec, which is approximately the fact that the transiton takes only time it takes an electron to complete one orbit (104). AES is especially suited for surface research because the main Auger transitions produce electrons with relatively low kinetic energies and thus small escape depths. The sensitivity is highest for the light elements, where practically all electronic relaxations are Auger processes. As for the notation, we have already mentioned the initially used KLL notation, based on X-ray levels. In view of the limited resolution of most of the earlier AES spectrometers, this notation was adequate. Lately considerable importance has been attached to the study of the fine structure and line shape of Auger peaks. The electronic configurations of a KLL process, for example, give rise to different spectroscopic terms, and each term has its multiplet splitting. The terms to be considered from a theoretical point of view depend on the coupling scheme adopted. In pure L-S coupling (Russell-Saunders) the configurations 2s02p6,2s12ps, and 2s22p4give rise to 10 terms:

-

2s02p6:

'so

2s12pS: IP,, 3P,,,,,

including multiplet splitting. Parity conservation, however, limits this number to seven (2s22p4: is forbidden). In pure j-j coupling where spinorbit interactions prevail over the Coulomb and exchange interactions, only six possible energy states are obtained, namely those described by the notation KL,L,, KL,L,, KL,L,, KL,L,, KL,L,, KL3L3. In practice one

ELECTRON BEAMS IN SURFACE RESEARCH

175

often finds a set of terms described in intermediate coupling. In this case up to nine terms are obtained and experimentally verified. Figure 24 illustrates this. Since most Auger processes can be described with L-S and intermediate coupling, it is unfortunate that the typical j-j coupling notation, e.g., KL2,3L2,3,has been adopted (106).It is, however, now customary to include in this notation the symbol of the spectroscopic term considered, e.g.,

ATOMIC NUMBER

FIG.24. Relative positions of KLL Auger energies normalized to the 1S,-3P, interval [Siegbahn et al. (110) and Chang (106)].

KL,L,('S), KL2,3L2,3('D),KL2,3L2,3('S),etc. The same notation principles are used for Auger processes with initial holes in L, M, N, ... shells, e.g.9 L3M4*5M4,5(1mL3M4.SM4,5(3F)The above notation becomes somewhat less straightforward when one or more levels participating in the process are situated in a composite valence band. One then usually notes these transitions using the symbol V for the valence band, e.g., KVV, L3VV, L,M,,3V, . . .. The Auger processes involving transitions between sublevels of the same shell, e.g., between L, and L2.3 levels, are called Coster-Kronig transitions They have a pronounced effect on the and are described as, e.g., L1L2,3M2,3. relative intensities of the different Auger peaks of a given series or group. They clearly occur only for initial core holes in L, M, . . . shells, and their rate is strongly dependent on the energy separation between the sublevels considered.

176

L. FIERMANS A N D J. VENNIK

McGuire (107) recently introduced the term “super Coster-Kronig ” to designate processes occurring in the same shell, e.g., M,M2M4,, , M2M4,5M4,5

....

2. Energies of the Auger Transitions The kinetic energy of the ejected Auger electron is given in a general way by Ekin= Ei - E, (32) where f and i stand for final and initial, respectively. In the free atom approximation Ekincan in principle be calculated using the energy values of the different free atom configurations, as given by Moore (108). If the atom considered is part of a lattice, solid state corrections have to be taken into account. a. Semiempirical Auger energy calculations. The initial state energy is given by the ionization potential or binding energy of the corresponding core electron, e.g., EK, EL, . . .. Tables of binding energies have been published by Moore (108), Bearden and Burr (109), Siegbahn et al. (IIO),and Sevier (111 ) . The final state, assuming it is a ground state of the ion, is given by the energy needed to create two holes in the electron configuration, e.g., for a KL,L, process,

Ef = EL2 + EL3(L2) (33) where is the binding energy of an electron in the L, shell in an atom already ionized in the L, shell. All binding energies mentioned are referred to the Fermi level. One thus obtains Ekin

= EK

-

- EL3(L2)

(34)

In a first approximation, one could use the expression Ekin = EK - EL3 (35) This expression in general gives very inaccurate results. A better approximation is obtained when using the expression (112, 113)

-

- EL3(Z + ‘1

(36) where EK(Z),.... are binding energies for elements with atomic number Z and Z 1. Since, however, a core hole does not modify the binding energies in exactly the same way as an additional positive nuclear charge, a still better approximation is Ekin

= EK(Z)

+

Etin

= EK(Z) - ELz(Z) - EL3(z + A = EK(Z)

- EL2(Z)

4

- EL3(Z) - AZ[EL3(Z

-k l) - EL3(Z)I

(37)

177

ELECTRON BEAMS IN SURFACE RESEARCH

Values of AZ are determined experimentally. Depending on the Auger transition considered, the following values have been proposed:

-0.55 < AZ < 0.16; - A Z = 0.69 for Z 2 71 and LM,,,M,,, transitions; AZ = 0.69 + 0.85(71 - Z)/35 for 36 < Z ,< 7 1 ; - A Z x 1 for LMN transitions (114). Self-consistency is introduced in these expressions by considering two identical transitions, e.g., KL,L, and KL,L, , which are quantum-mechanically indistinguishable. Chung and Jenkins (1 1 5 ) have proposed using the expression E k i n = EK(Z) - f[ELz(Z)+ ELz(Z + 111 - f[EL3(Z)+ &3(Z + 111 ( 3 8 ) to take this into account. The values calculated with (38)are generally in fair agreement with experimental values. In the case of Auger transitions involving valence electrons, it has often been assumed that screening of the extranuclear charge would make any corrections superfluous. In the case of metals one often applies the expressions proposed by Coad and Riviere (116): EMz.3Ma,5V

= EM2.a(Z)

- f [ E M 4 , s ( Z ) + EM4,5(Z

= 'MZ,3(')

- 2EV

+ ')I

- EV

(39)

and EMz.3VV

(4)

where Ev represents an energy level situated in the valence band. It should be stressed that all these expressions are semiempirical and do not take solid state effects and splitting into account. b. Theory of the Auger Effect. Ab initio calculations of Auger energies and transition probabilities have mainly been published by Burhop (1 12), Asaad (117-119), McGuire (120), and Mehlhorn and Asaad (121). A brief summary is given by Chang (106).

D . Solid State Effects on Auger Spectra 1. Relaxation Effects in AES

Relaxation effects have been extensively described in photoelectron spectroscopy (XPS)(122-129). We shall summarize here the main definitions and theoretical and experimental results. When E6 is the binding energy of an electron in the ith orbital as

178

L. FIERMANS A N D J. VENNIK

measured by XPS and 6, is the calculated orbital energy, we have, according to Koopmans’ theorem E b = -8,

(41)

Taking into account the relaxation of the entire electron system due to the creation of a core hole, and assuming that this relaxation is adiabatic and complete, Hedin and Johansson (128) have derived the expression

The second term on the right-hand side is a polarization contribution in which

where V,* and V, are the sum of the Coulomb and exchange operators of the kth orbital, with and without omission of the ith orbital, respectively. Physically this means that when an electron from the ith orbital is excited, the outer orbitals relax toward the created hole in order to screen the extra positive charge. The photoelectron consequently acquires extra energy. Following these ideas, Shirley (122) has calculated core level binding energies. Instead of making hole state calculations, this author uses the “equivalent core” scheme proposed by Jolly and Hendrickson (130). In this scheme the outer orbitals of an atom with nuclear charge Ze and a core hole are assumed to be identical to the corresponding orbitals of the monovalent cation with nuclear charge (Z + 1)e. Values obtained on the noble gases for the atomic relaxation are of the order of a few tens eV for core levels and less than 10 eV for the outer levels (122).

In solids, molecules, or condensed phases in general, not only atomic relaxation, but also extraatomic relaxation have to be taken into account (122, 123, 125).

In order to calculate the extraatomic relaxation in metals, Ley et al. (125) have used the exciton-screening model of Friedel. In this model it is assumed that the core hole is screened by a semilocalized exciton state consisting of an electron-hole pair created by the capture of a conduction electron in an empty level, which, due to the very presence of a core hole, has dropped below the Fermi level. In a first approximation the first unoccupied orbital is identified with the screening orbital. The calculations are again based on the equivalent core model (130). Recently, Hoogewijs er al. (131) and Watson er al. (132) have proposed SCF hole state calculations of extraatomic relaxation energies, giving better agreement with experimental values.

179

ELECTRON BEAMS IN SURFACE RESEARCH

For the 3d transition metal series, values between 5 and 10 eV are obtained (131). Due to the two-hole final state, relaxation effects in AES are even more pronounced than in XPS. Following Kowalczyk et al. (126), the kinetic energy of an Auger electron for a K L L process, is written as E(KLL'; X ) = E(K) - E(L) - E(L') - 9(X)+ R,

+ Re,

(44)

where X is the particular multiplet state of the KLL' transition considered; E(K), E(L), E(L') are experimental XPS binding energies [including the atomic relaxation energy E,( Y)]; - 9 ( X ) is a linear combination of F and G Slater integrals, describing the coupling of the two holes in the final state [in Kowalczyk et al. (126)] only F terms are considered; it has however been shown by Nicolaides and Beck that G integrals have to be included (133)l; R , , called the atomic static relaxation energy (126),describes the lowering of the binding energy of the L electron due to the fact that all electrons belonging to the atom considered have already relaxed toward the positive hole left by the L electron; Re, describes the same effect as R, but due to the relaxation of the electrons of the surrounding lattice. In a first approximation Kowalczyk et al. (126) assumed that R , = 2E, and Re, = 2Ee,, i.e., that the static relaxation contributions are equal to twice the dynamic relaxation energies. [The latter are defined as the relaxation energies corresponding to the one hole (XPS)final state.] Hoogewijs et al. (134) recently have shown that improved agreement with experiments can be obtained by using the exact expression (valid both for Ra and Re,): Rie)a = E(e)a + E;C,)a=

+ AE(e,a

where E and E* respectively correspond to the ground and once-ionized states. Hole state calculations have shown that the assumption E = E* (OF R = 2 E ) may give rise to an error of several eV. For example, for the Zn(L,M,,,M,,,;'G) transition [experimental value is 992.3 eV (135)],the value calculated from experimental binding energies and calculated relaxation energies using nonrelativistic Optimized HartreeFock-Slater hole state calculations is E(ZnL,M,,,M,,,;'G)

= 1021.87

- 2 x 9.60

(ELs- 2EM4,s)

- 30.18

(RW)

+ 9.46 + 10.76

(Ra + AEa)

= 992.79 eV

(Re, + AEea) (45)

180

L. FIERMANS AND J. VENNIK

Next to the exciton-screening model, others have been proposed to calculate extraatomic relaxation energies, e.g., the plasmon model of Laramore and Camp (136).A detailed account of this theory falls however beyond the scope of this review. Finally, we would like to mention briefly the ideas developed by Matthew (137).This author writes the kinetic energy of an Auger process ABC in the general form

WA - wBC (46) where WAis the energy necessary to create a hole in the A level, and WBcis the energy to create the two-hole final state: EABC

=

WBC= WB-k wc -k AW

(47)

and

AW=H-P (48) In the last expression H contains the multiplet coupling in the final state, P is related to the polarization of the electron gas. P values of the order of 20 eV are obtained, in agreement with the theoretical results described above. 2. Valence Bands in AES

One of the fundamental questions arising in AES concerns the influence of valence band structure on the Auger peak shape for transitions involving once (e.g., KLV) or twice (e.g., K W ) the valence band (see Fig. 25). This question is still a point of controversy. In practical Auger spectra the band structure cannot be recognized at first sight. Even in the simplest cases with no possible splitting effects and transitions involving only once the valence band, the spectra have to be deconvoluted for instrumental effects and loss features (see Section 111,E). Furthermore, since the intensity at a given energy within the peak is a function of the local density of states within the valence band and of the transition probability, the latter has to be known accurately in order to calculate the band structure from the peak shape. Some studies have been performed in this direction (138-141); the results are, however, not convincing and alternative explanations are often possible. Auger transitions in ionic crystals are thought to be nearly atomic in nature, i.e., the initial hole localizes the levels in its vicinity, creating a short-lived defect center (142a).The main lattice influence is thought to be a broadening of the lines and a shift of their positions relative to the free-atom case. Similar studies have recently been performed on metals (142b). In the case of lithium the breadth of the KVV spectrum could qualitatively be explained by its band structure (143a). In other cases, however, the spectra

ELECTRON BEAMS IN SURFACE RESEARCH

1

181

KVV

"OC.--

K

KLV

----t--

--I-

E

FIG.25. Energy levels of a solid. Transitions involving the valence band may show structure due to variations in the density of states D ( E ) within this band.

are "quasi-atomic." Silver and indium (143b) and cadmium (143c) show quasi-atomic line shapes that can be simulated starting from broadened atomic spectra. Whether this remains true for more covalent materials is not clear yet, e.g., the Si(LVV) Auger spectrum. E. Extra Features in Auger Spectra

In an Auger spectrum, different extra features can be present, namely, plasmon loss peaks, characteristic ionization loss peaks (also called electron loss peaks), doubly ionized initial state Auger transitions, effects due to multiplet splitting and multiple excitations (shake-up and/or shake-off phenomena) in the initial state and diffraction effects. Another consequence of their presence is the fact that most Auger peaks are asymmetrical. The degree of asymmetry depends on the nature of the compound, i.e., on the relative intensity of the different loss mechanisms, e.g., Si and SiO, (106). 1. Plasmon Losses

Before emission into the vacuum, Auger electrons can interact with the ,electrons of the solid. Discrete amounts of energy can be lost as a result of

182

L. FIERMANS A N D J. VENNIK

the excitation of plasmons in the electron gas, which results in the presence of low energy satellites. It has also been suggested that Auger electrons could experience plasmon gain (144-148), giving rise to high-energy satellites. Alternative explanations for the latter can however be given by considering Auger transitions in doubly ionized atoms (124), or internal photoemission (149). 2. Characteristic Ionization Losses Some of the peaks in Fig. 17 (labeled C.I.) are due to characteristic ionization losses. They result from primaries having lost a certain amount of energy corresponding to the excitation of core electrons into unoccupied levels. The least energetic transition is the one where the core electron is excited into the first unoccupied level just above the Fermi level. In a metal this corresponds to the binding energy and these peaks can then easily be identified. The lattice electron can, however, take up larger amounts ofenergy from the primary and be excited into higher unoccupied levels. The interpretation thus is not straightforward if the band structure is not known in detail (150). 3. Multiple Excitations

In XPS two kinds of satellites of the photolines, multiplet splitting satellites and shake-up and/or shake-off satellites, are particularly well studied. The former are observed when incomplete valence shells couple with the core photohole. Typical examples are the 3s photolines of the transition metal oxides MnO and COO (151, 152). The latter are a result of a nonadiabatic relaxation of the electron cloud upon photoionization. Indeed, part of the relaxation energy can be used to promote valence electrons into unoccupied levels, instead of being entirely transferred to the ejected photoelectron. The latter will consequently leave the solid with a smaller kinetic energy, and a satellite is thus observed at the high binding energy side of the photoline. Very recently Sen (152), in a study of the shake-up satellites in the 2p X-ray photoemission spectra of the transition metal ion complexes, found that the experimental data could be explained assuming that: (1) The so-called shake-up satellites arise only due to covalency. (2) Pure ionic compounds will not exhibit shake-up satellites. (3) The shake-up satellites arise from monopole charge transfer excitation of the ligand-centered electron mainly to the empty and/or singly occupied metal antibonding 3d orbitals, occurring simultaneously with the photoionization process.

ELECTRON BEAMS IN SURFACE RESEARCH

183

Multiplet splitting and multiple excitations in the initial state could in principle give rise to additional structure in Auger spectra. They can however be obscured by the spectroscopic fine structure in the final state (129). A correlation between shake-up satellites and fine structure in the Auger spectra of NiO has been found. The details of this behavior are however unclear and need further study.

F . Lifetime Broadening and Coster-Kronig Transitions The width of an Auger line is determined by different factors: initial state effects due to the intrinsic width of the levels or bands involved in the process; crystal-field effects, i.e., the chemical state of the element and its environment; lifetime broadening; thermal broadening; instrumental broadening. In this section we consider lifetime broadening, which is thought to be one of the major factors, at least for quasi-atomic transitions which are very often encountered. As an example we consider the width of the Zn(L3M4,,M4,,) Auger lines in the zinc chalcogenides (129). The broadening of this line is determined by the lifetime of the two-hole state in the Zn 3d level. This final state can be destroyed in different ways: by capture of an electron or through separation whereby a hole jumps onto a neighboring Zn. Which of the two mechanisms, electron capture or hole separation, is the most probable depends on the chemical environment of the atom considered and on the nature of energy levels wherein the two final holes are located: hole separation will have a very low probability if both holes are located in core levels but might be very probable in the case of XVV Auger transitions. Whenever allowed, an energetically favorable interatomic or cross transition thus might contribute appreciably to reducing the lifetime of the two-hole . the Zn(L,M,,,M,,,) transition a constant width for all state ( 1 4 3 ~ ) For chalcogenides is observed. The latter is probably determined by lifetime broadening through M2,3M4,5M4,5Coster-Kronig transitions, which exhibit very high transition rates. It should be mentioned that Coster-Kronig transitions are also responsible for the fact that Auger transitions, e.g., based on L2 and L, initial holes, are much weaker than corresponding L3 transitions, as a result of rapid L1L3 and L,L, deexcitation. Relative intensities of Auger lines are consequently strongly influenced by Coster-Kronig transitions.

184

L. FIERMANS A N D J. VENNIK

A typical example where Coster-Kronig transitions play an important role are the Auger spectra of the transition metals (153). The L, components of the first 3d transition metals are extremely weak, while for Zn and Cu the experimentally observed ratio is close to the theoretical L, /L2 = 2/1 value. This is a result of rapid LzL3Coster-Kronig transitions which as borne out by theory (154) are the more probable, the smaller the energy gap between L, and L,. G . Chemical Environment-Energy

Shijl and Line Shape

Depending on the degree of oxidation of the ion in which the transitions occur, shifts in the positions of Auger peaks are commonly observed. The study of these chemical shifts is one of the main subjects in XPS.XPS shifts and their corresponding AES shifts in general are not identical because of the different relaxation energies involved in both processes (155). In exceptional cases they are found to be equal (156). Using expression (48X one can generally state that AExyz = AEX - AEy

- AEZ - AH -+ A P

(49)

Assuming in a first approximation AEx = AEy = AEz = AE and AH = 0, one obtains AExyz =

-A E -I-A P

(50)

The observed shift is consequently a sum of the binding energy shift (the true chemical shift) and the change in total relaxation energy. The latter is up to now not accurately known for most compounds. A study of binding energy shifts is not usually performed using AES. As was already mentioned not only the position but also the shape of an Auger line is strongly dependent on the chemical environment. This is for instance illustrated by the shape of the C(KLL) spectrum in different carbides [see Fig. 20.16 in Chang (10611. An extensive discussion of line shapes has been given by Sickafus (157, 158). This author has studied Auger spectra of a Ni(ll0)-c(2 x 2)-S surface, and compared these data with ion-neutralization data of Becker and Hagstrum (159). The same structural features are found in both types of spectra, i.e., fine-structure features in AES dN(E)/dE curves correspond to similar structure in INS spectra. Auger line shapes are calculated on the basis of simple density-of-states models, assuming that the density-of-states function of the surface is inherently convolved into the Auger line shape. In another paper (158) Sickafus concludes that an XVV transition wherein the valence DOS function plays a role is characterized by a definite threshold, a definite width (twice

ELECTRON BEAMS IN SURFACE RESEARCH

185

the width of the valence band), a certain shift, and peak multiplication due to convolution. All these factors have to be taken into account when discussing Auger peak fine structure. As will be discussed further on, the change in line shape and linewidth with the chemical environment of a given element are of very high practical importance. Indeed very often the peak to peak amplitude of the derivative line is taken as a measure for the Auger intensity. In order to compare intensities between different compounds it is clear that one should take care to choose an Auger transition for which the line shape is independent of the chemical environment. H . Crossover Transitions Crossover transitions involve levels of neighboring atoms in compounds. It is, for example, conceivable that core holes in compounds are filled by electrons from nearest neighbor atoms. In principle they are observed for upper levels only in ionic materials or materials where the initial hole strongly localizes the upper levels. Their probability in general is rather small and they are observed only when no other transitions are possible. They have, for example, been observed on LiF by Gallon and Matthew (160) and on oxidized Mg by Janssen et al. (161). Another type of crossover transition is considered in MgO by Bassett et al. (162). In the oxygen KLL spectrum a peak is observed attributed to a final state involving a 2p hole on the central oxygen originally ionized and a second 2p hole shared among neighboring oxygens in the second nearest neighbor positions.

I . Applications of AES The practical applications of AES can be subdivided in two main categories: qualitative determination of the elemental composition and accurate quantitative analyses. Before dealing with these subjects, we shall first give a short discussion of the practical interpretation of Auger spectra. 1. Practical Interpretation of AES Data

Most peaks in dN(E)/dE spectra have easily recognizable major features, together with more or less pronounced fine structure (see Fig. 17). The identification on energetic grounds of the major Auger lines usually is straightforward. Many binding energy tables (108-111), a catalog of calculated Auger transitions for the elements (163), and a handbook of Auger spectra (164) are readily available. The catalog contains data for all the

186

L. FIERMANS A N D J. VENNIK

Calculated Auger spectra. Silicon 14 Levels used for 1441 Orbital population

K LI L I1 L 111

MI M 23

Z energy (eV)

Z + 1 energy (eV)

1839.00 149.00 100.00 99.00 8.00 3.00

2149.00 189.00 136.00 135.00 16.00 10.00

2 2 2 4 2 2

List of Auger lines between 10.00 and 3000.00 for initial vacancies up to 6000.00 eV which have normalized multiplicities greater than 0 for surface containing 1441, E(X) and E(Y) APPRCX O.SO(2) 0.50 (2 + 1) levels.

+

Element

Vacancy level

14-SI 1441 1441

LI LI LI LI L 111 L I1 L 111 L I1 L 111 L I1 LI LI LI K K K K K K K K K K K K K K K

14-SI 1441 14-SI 14-SI 1441 1441 14-SI 14-SI 1441 1441

14-SI 1441 1441 1431 1441 14-SI 1431 14-SI 14-SI 1441 14-9 1441 1441 1441 144

Interaction levels

L I1 L 111 L I1 L 111 MI MI MI MI M 23 M 23 MI

MI M 23 LI LI LI L I1 L I1 L 111 LI I L I1 L 111 L I1 L 111 MI MI M 23

MI MI M 23 M 23 MI MI M 23 M 23 M 23 M 23 MI M 23 M 23 LI L I1 L 111 L I1 L 111 L 111 MI M 23 MI MI M 23 M 23 MI M 23 M 23

Energy (eV)

Normmultiplicity

19.00 20.00 24.50 25.50 75.00 76.00 80.50 81.50 86.00 87.00 125.00 130.50 136.00 1501.00 1552.00 1553.00 1603.00 1604.00 1605.00 1658.00 1663.50 1709.00 1710.00 1714.50 1715.50 1815.00 1820.50 1826.00

25 50 25 50 50 25 50 25 50 25 25 25 25 25 25 50 25 50 100 25 25 25 50 25 50 25 25 25

FIG.26. Calculated data for Si [Coghlan and Clausing (163)].

ELECTRON BEAMS IN SURFACE RESEARCH

187

elements calculated using expression (38). The handbook is a reference book of standard data for identification and interpretation of AES data and contains spectra for the elements Be through Th (4 < Z < 90). Figures 26 and 27 are examples from the catalog and the handbook concerning the element Si. The multiplicities mentioned in the catalog are obtained by multiplying the number of electrons in the shells involved in the transition and putting the maximum equal to 100. In this manner a crude estimate of the expected intensity is obtained. The energy values in the catalog and in the handbook differ for two reasons: firstly, in the handbook the value of the highenergy minimum is given instead of the N ( E ) peak value; and secondly, a work-function and relaxation correction has to be applied to the calculated values. Nevertheless, a rapid identification of the major features usually is no problem. Fine structure can then be interpreted on the basis of the spectroscopic terms of the final two-hole state. Care must, however, be taken to distinguish between inelastic losses, described above, and the spectroscopic fine structure. A more complete interpretation of the spectra then usually can be given. Sometimes it is necessary to take unpaired spins in the valence band into account. This is illustrated in Fig. 28, showing the N ( E ) Auger spectra of V 2 0 , and vanadium metal. A difference in fine structure of the L3M3,3M2,3is clearly observed and can be explained only by inferring unpaired spins in the valence levels of the metal. Here, as for many other metals and compounds (129, 143c), the transition is thought to be quasiatomic. Formally, the valence bapd of V20, is completely filled (02p6),but under the influence of the primary electron beam V 2 0 , readily loses oxygen resulting in a high density of V4+ centers at the surface. This explains why the L 3 M l M l peak, which should consist of one spectroscopic term ' S o only, clearly is a doublet, both on V,O, and V (129). Summarizing, an elemental analysis with AES usually presents no particular problems. The question is different however when a quantitative analysis is required.

2. Quantitative Analysis with AES a. Quantitative analysis of qdsorbed layers. The quantitative aspects of AES can best be discussed using the general expression of the Auger current for an adsorbed layer (165): I A

= 4Ep)(1

- w)lpF(~p)QT1 Niriqi

(51)

i

where Ep is the primary energy, a(Ep)the cross section for primary ionization, 1 - w the Auger yield after primary ionization, I, the primary current,

188 L. FIERMANS AND J. VENNIK

FIG.27. Si Auger spectrum from Palmberg et al. (164).

189

ELECTRON BEAMS IN SURFACE RESEARCH

R the aperture of the detector, T the transmission of the detector, N t the number of atoms in site i, ri the backscattering coefficient at site i, q1 the screening factor for Auger emission, and F(cp,) a geometrical factor describing the primary beam efficiency for a given type of analyzer. We shall now discuss each factor in detail. ( i ) Cross section a(Ep). At present, values for the K, L, and M shells are available. The latter two have recently been determined by Vrakking and

336

376

L16

1

I

456

4 96

536

E

FIG.28. Auger spectra ofvanadium and V,O,, recorded in an XPS experiment. Notice the difference in fine structure of the central peak.

Meyer (166), the former were already known from earlier work (167). Vrakking and Meyer measured L and M shell ionization cross sections by performing Auger measurements on gaseous samples containing molecules with at least one atom with a known K shell ionization cross section and another of which the ionization cross section was to be determined. The values obtained in this manner differ significantly from the estimated values used before. Figure 29 represents data obtained by Vrakking and Meyer (168) on S, P, Cl. The curve represents normalized cross sections as a function of the ratio primary energy versus ionization energy of the inner level (Ep/Ei). Absolute magnitudes at E,/Ei x 4 vary as 1/E:.6 and are shown in Fig. 30. Vrakking and Meyer have corrected their data for Coster-Kronig transitions, using McGuire's calculated data (120). ( i i ) Back-scattering factor r. Auger electrons are produced not only by primary electrons, but also by inelastically scattered electrons and secon-

L. FIERMANS A N D J. VENNIK

190 I

0

s

b0,25Y ‘0

2

L

6

;Z

8 O;-

I’C

;1

1’8

;O

Ep/E,

FIG.29. Ionization cross sections obtained by Vrakking and Meyer (168) on S, P, CI. The curve represents normalized cross sections as a function of the ratio primary energy versus ionization energy of the inner level (Ep/.Ei).

daries with energies greater than Ei,the ionization energy for the level considered. Values of r calculated by Bishop (169) are shown in Fig. 31. According to these calculations r increases with atomic number and with Ep. Vrakking and Meyer (168) point out that for submonolayer coverage from expression (51), it follows that

Once the ionization cross section is known, r can consequently be determined, for instance in a manner proposed by Gallon (1 70) and extended by Jackson et al. (1 71 ). Gallon assumes the Auger current to be given by

+

I , = CZp(0 azp) (53) where C is constant, mainly instrumental in nature, and fi is a geometrical factor. a2 is an average cross section for ionization by secondaries, given by u2(EP)=

5

EP

a ( E ) N ( E )d E

(54)

Ei

where the distribution N ( E ) is normalized such that J p N ( E )dE = 6, i.e., the secondary yield. This leads to the expression

191

ELECTRON BEAMS IN SURFACE RESEARCH

FIG.30. Absolute values of u at E , / E , z 4 [Vrakking and Meyer

I

20-0 ‘ 0

I

1

I

I

I

I

-

192

L. FIERMANS A N D J. VENNIK

In this way r is calculated as a function of E, /Ei by either changing the lower limit of the integral in one N ( E ) versus E curve, or by using different N ( E ) versus E curves (different values of EP)and keeping the lower limit of the integral constant. The results obtained by Vrakking and Meyer (168) are shown in Fig. 32, where r values are plotted versus U - '( U = E, /Ep).

0

01

02

03

OL

05

FIG.32. Backscattering values obtained by Vrakking and Meyer (168). Open circles are experimental points for S, filled circles for 0.

Vrakking and Meyer have also determined r using their calibration experiments with ellipsometry. The experimental points for S and 0 adsorbed on Si are also shown in Fig. 32. The agreement between the results of both methods is evident. The r values obtained in this manner for Si are in fair agreement with those obtained by Smith and Gallon (172) and by Bishop and Riviere (173). Goto et a1. (174) have recently studied the influence on Auger and secondary electron distribution curves of backscattered electrons for the system Be evaporated on Cu. Values of r for Be are of the order of those obtained for Si by the authors cited above (1.1 < rg, < 1.25 for 800 < E , < 2000). Neave et al. ( 1 7 9 , on the other hand, for glancing incidence on Si, reported an overwhelming contribution of backscattered electrons. This could however be due to the use of a theoretical cross-section curve (168). (iii) Other factors in expression (51). The other factors in (51) are easily

ELECTRON BEAMS IN SURFACE RESEARCH

193

understood. The Auger yield is given by empirical expressions, such as (where 2 is the atomic number)

where a = 1.12 x lo6 for K-shell ionization, 6.4 x lo7 for L-shell ionization (112).

The screening factor q is a measure of the probability an Auger electron has of reaching the detector without appreciable loss. Considering that Auger emission is omnidirectional, the problems encountered in the theoretical evaluation of q are similar to those found in LEED theory (176). F(cp,), L?,and T are instrumental factors. The influence of the angle of incidence has been investigated first by Harris (177) and by Bishop and Riviere (178). Diffraction effects (175, 179) were not considered by these authors and the results thus are valid only for amorphous materials. For retarding field analyzers a csc ‘pp dependence is found if it is assumed that the mean free path of primary electrons is much larger than that of the Auger electrons. The effect can be understood as a consequence of the longer time spent by the primary electron in the overlayer at smaller angles. Deflection type instruments probe only a very small surface area in optimal conditions. With smaller angles of incidence, the intersection of the beam with the surface becomes greater by a factor csc ‘pp and consequently, the primary current density in the active area is divided by the same factor. From purely geometrical considerations it is thus seen that the sensitivity of deflection type instruments is independent of the angle of incidence. It should be stressed here that due to the extremely small mean free paths of the electrons considered, surface roughness even at submicron scale might produce considerable deviations from these simple laws. The angle of incidence furthermore has its influence on the backscatter coefficient r. Indeed, the distribution function of secondaries within the active surface layer, and consequently the externally measured N ( E ) which is generally assumed to represent the internal distribution, are a function of the mgle of incidence. This is due to the fact that primaries have to be scattered through an “effective scattering angle before reappearing at the surface. The smaller the angle of incidence, the smaller the effective scattering angle has to be. It furthermore is a well-known fact that higher energies are favored at small scattering angles, thus the secondary emission spectrum N ( E ) will change accordingly. For a fixed angle of incidence, N ( E ) will furthermore depend on the angle of detection. This point has been verified experimentally by Meyer and Vrakking (165). ”

194

L. FIERMANS AND J. VENNIK

In the study of adsorbates or surface segregation, glancing incidence can be used to increase the intensity ratio of adlayer to bulk peaks. It has been shown, for example, that the intensity ratio of Pd and Ag lines of a Pd-Ag alloy changed after segregation and was ‘ppdependent while this was not the case for as-prepared surfaces (180). b. Thick overlayers. The foregoing discussion dealt essentially with monolayer adsorbates. In the case of “thick” overlayers, bulk alloys or compounds, the expression (51) for I A has to be modified. It is general practice to introduce the concept of average escape depth (or inelastic mean free path) of Auger electrons. Escape depths cannot yet be calculated from first principles. The only way to determine these quantities is from relative intensity measurements of adsorbate and substrate Auger yields as a function of overlayer thickness. An expression valid in the case of layer-by-layer growth has been developed by Gallon (181). It can be shown that the Auger current from a deposit of n layers is given by

where I, is the Auger current from a monolayer and I, is the Auger current from bulk material. Both have to be determined experimentally and corrected for the backscattering which might be different in the two cases of a monolayer on a foreign substrate and of the pure bulk material. The escape depth is then given by

or

A few examples found by Gallon are: no = 3.8 layers for 360-eV electrons in Ag and no = 2.5 layers for 46-eV electrons in Fe. Results from different authors have recently been compiled by Brundle (182) and are represented in Fig. 33. From this curve it is clear that the escape depth is rather independent of the matrix. This is due to the fact that the main loss mechanisms liable to remove electrons from the Auger peaks involve excitations of valence band electrons whose density does not vary very considerably from one material to another. Other losses such as plasmon losses and core-level excitations

2 Xa

X

8

4

X,n

4

2

3

4

4

I"

L. FIERMANS AND J. VENNIK

196

contribute far less. Phonon losses are too small and still fall within the observed Auger peak. The small values ( < 20 A) of the escape depth clearly demonstrate the surface sensitivity of AES. As will be discussed in the next section, if extreme accuracy is desired, for each particular matrix the exact value of the escape depth should be used. Recently the effective sensitivity formalism treated in Section 111,1,2,chas been extended to layered structures by Chang (183).This method has proven in practice to be extremely useful. c. Homogeneous Solids. The case of homogeneous solids can best be discussed using a modified version of (51) : Ii

= / l I , X i p D i ~ ( E , ) ( l- w J R T

(60)

where A is the area which is irradiated by the primary beam (cm2),I, is the primary beam current, Xi is the atomic concentration of element i (0 < X i < l), Di is the escape depth (cm), p is the atom density (atoms/cm3), r is the backscattering factor, a(Ep)is the ionization cross section, 1 - oi is the Auger yield, R is a surface-roughness factor, and T is the instrument transmission [corresponds to RT in (51)]. The critical factors determining the accuracy with which an analysis can be performed using (60) are the escape depth Di(Ei)and r(E,), which are both matrix dependent, and the surface roughness factor R, which depends on the sample preparation and in situ treatment. The backscattering factor is an increasing function of the weight density of the matrix. A heavy element matrix thus in general will increase the Auger current as compared to the light matrix case (168). In a first approximation, for Di the values from Fig. 33 can be taken; in order to improve the accuracy of AES, however, more experimental data on a wide variety of materials are required. It will then perhaps be possible to classify materials and to predict with a certain degree of accuracy the escape depth as well as the backscattering factor of an unknown alloy or compound. Furthermore it is clear that the Auger yield 1 - oi and the ionization cross section should also be known accurately. From the above it is seen that, since matrix effects at present cannot be dealt with on a firm theoretical basis, the application of (60) is not practical for routine analysis. The use of standard samples of approximately the same composition and structure as the test sample actually is the most reliable way to achieve a high degree of accuracy. From (60) it follows that

ELECTRON BEAMS IN SURFACE RESEARCH

197

which reduces to

if the matrix effects are identical for the test and standard samples. In a formalism proposed by Palmberg (164, 184) and extended by Chang (183) elemental sensitivity factors or inverse sensitivity factors u = l/s (184) are used. It can easily be seen that

xi = U i l i 1 U j l j l

j

where the summation covers all the elements of which the sample is composed. The inverse elemental sensitivity factor is defined from u;r; = 1p

(64)

where the superscript 0 indicates the pure element and s is an arbitrarily chosen standard element. is often made in applying (63). A direct conThe assumption ui = sequence of this approximation is that matrix effects are neglected completely. Indeed, it follows from (60)and (63) that except for I i , ui contains all the remaining terms of (60) of which some are matrix dependent. The expected accuracy therefore will not be better than 5 2 0 % . The accuracy can be improved to within a few percent if the sensitivity factors tli are determined directly from standard samples of approximately identical composition and structure as the test sample. An important advantage of the method is that it is insensitive to surface roughness. Indeed, all Auger lines from a given sample are affected in the same way by the roughness factor R . In determining the sensitivity factors however different samples are often used, and extreme care should be taken in preparing the surfaces. A general remark concerns diffraction effects in AES. In the discussion of the accuracy of quantitative AES it was stressed that the samples should be homogeneous. Elemental or compound single crystals can be made extremely homogeneous and thus can be used as standards. In AES two diffraction processes are possible, first diffraction of the primary beam (175, 179) and secondly diffraction of the Auger electrons in leaving the surface layer (179, 185, 186). Both effects give rise to a variation of the detected Auger current as a function of the angle of incidence and the angle of detection, respectively. If extreme accuracy is required, care should be taken to average these effects or to carry out the experimental determination of the sensitivity factors on a crystal of similar structure as the test sample

UP

198

L. FIERMANS A N D J. VENNIK

and under identical conditions as far as surface preparation and orientation are concerned. It should be mentioned that the sensitivity factor formalism, which as has been shown gives rise to a very useful method for the analysis of homogeneous solids, has been extended by Chang (183) to the case of layered structures. d. Independent calibration techniques. One of the most straightforward methods in combination with retarding field analysers is LEED. Partly unordered material is however not detected in this way and relatively important errors are therefore possible. Ellipsometry has been used extensively by Meyer and Vrakking (165). Thomas and Haas (187) used ion-counting techniques in their study of alkali adsorption. Radiotracers can be used wherever applicable (188).Work function data can also be used if available as a function of the coverage (189). e. Measurement of the Auger current. In many practical cases relative amplitudes of the different elemental components are measured starting from derivative spectra. The quantity actually measured is the peak-to-peak, the maximum negative, or positive amplitude of the differentiated peaks. These amplitudes are proportional to I i bat are a function of line shape as well. In order to compare peak amplitudes, line shapes should thus be identical. Consequently, peaks liable to show spectroscopic splitting, excessive lifetime broadening, or important losses, should not be used for this purpose. Best results are obtained by comparing maximum positive amplitudes. Absolute values can in principle be obtained by integration in N ( E ) spectra after background substraction. Only in a few particular cases can background substraction be performed in an accurate way. This results from the fact that in general the exact shape of the background is not known. Only for dilute solid solutions and low coverages of light elements on heavy substrates, i.e., where it may be assumed that the background is not affected by the presence of the “impurity,” is such an analysis possible. The integration techniques for background elimination developed by Houston (loo),Grant et al. (190) and the “tailored modulation techniques of Springer et al. (191) however show promising results for quantitative analysis. Background substraction in the sense of spectrum substraction, e.g., of a clean surface from the spectrum of a contaminated surface, is also described by Grant et al. (192). REFERENCES ”

1. C. B. Duke and R. L. Park, Phys. Today 25, August 23 (1972). 2. E. A. Wood, .I. Appl. Phys. 35, 1306 (1964). 3. S. Andersson, in “Electronic Structure and Reactivity of Metals” (E. G. Derouanne and A. A. Lucas, eds.), NATO Adv. Study Inst., Namur, Belgium, 1975. To be published by Plenum. New York.

ELECTRON BEAMS IN SURFACE RESEARCH

199

4. E. G. McRae, Fundam. Aspects Semicond. Suqaces, N A T O Ado. Study I n s t , Unio. Gent, 1968. (Notes). 5. P. J. Estrup and E. G. McRae, Surface Sci. 25, 1 (1971). 6. G. A. Somorjai and L. L. Kesmodel, Prepr. LBL-3135(1975).To be published in “MTP International Review of Science,” University of California, Berkeley. 7. C. Kittel, “Introduction to Solid State Physics,” 3rd Ed. Wiley, New York, 1967. 8. C. B. Duke and C. W. Tucker, Jr., Surface Sci. 15, 231 (1969). 9. G.E.Laramore, J . Vac. Sci. Technol. 9, 625 (1972). 10. C. B. Duke, in “LEED: Surface Structure of Solids” (L. Laznicka, ed.), Vol. 2,pp. 7-291, 361-387.Union Czech. Mathematicians Physicists, Prague, 1972. 11. K. Kambe, in “LEED: Surface Structure of Solids” (L. Laznicka, ed.), Vol. 2, pp. 293-303. Union Czech. Mathematicians Physicists, Prague, 1972. 12. J. B. Pendry, in “LEED: Surface Structure of Solids’’ (L. Laznicka, ed.), Vol. 2, pp. 305-345.Union Czech. Mathematicians Physicists, Prague, 1972. 13. C. B. Duke, in “Electron Emission Spectroscopy” (W. Dekeyser, L. Fiermans, G. Vanderkelen, and J. Vennik, eds.), pp. 1-148.Reidel Publ, Dordrecht, Netherlands, 1973. 14. C. B. Duke, A d a Chem. Phys. 27, l(1974). 15. J. B. Pendry, “Low Energy Electron Diffraction.” Academic, New York, 1974. 16. B. W.Holland, in “Electronic Structure and Reactivity of Metals” (E. G. Derouanne and A. A. Lucas, eds.), NATO Adv. Study Inst., Namur, Belgium, 1975.To be published by Plenum, New York. 17. J. J. Lander and J. Morrison, J . Appl. Phys. 36, 1706 (1965). 18. A. Ignatiev and T. N. Rhodin, Phys. Rev. B 8,893 (1973). 19. T. N. Rhodin, in “Electronic Structure and Reactivity of Metals” (E. G. Derouanne and A. A. Lucas, ed.), NATO Adv. Study Inst., Namur, Belgium, 1975.To be published by Plenum, New York. 20. E. G. McRae, J . Chem. Phys. 45, 3256 (1966). 21. J. L. Beeby, Proc. Phys. SOC.,London 1, 84 (1968). 22. J. L. Beeby, J . Phys. C I, 82 (1968). 23. R. Nuyts, Ph.D. Thesis, Univ. of Ghent, Belgium, 1975. 24. T. N. Rhodin and D. S . Y. Tong, Phys. Today 28, 23 (1975). 25. C. B. Duke and G. E. Laramore, Phys. Reo. B 2, 4765 (1970). 26. F. Jona, I B M J . Res. Deo. 14, 444 (1970). 27. G. E. Laramore, C. B. Duke, A. Bagchi, and A. B. Kunz, Phys. Rev. B 4, 2058

(1971). id. C. B. Duke, Jpn. J . Appl. Phys. Suppl. 2, Part 2, p. 641 (1974). 29. J. B. Pendry, Phys. Rev. Lett. 27, 856 (1971). 30. R. S. Zimmer and B. W. Holland, Surface Sci. 47, 717 (1975). 31. R. S.Zimmer and B. W. Holland, J. Phys. C to be published (1976).Cited as a reference in Holland (16). 32. T. C. Ngoc, M. G. Lagally, and M. B. Webb, Surface Sci. 35, 117 (1973). 33. R. M. Stern and S . Sinharoy, Surface Sci. 33, 131 (1972). 34. S.Andersson and J. B. Pendry, J . Phys. C 6, 601 (1973). 35. C. B. Duke, N. 0. Lipari, and G. E. Laramore, J. Vac. Sci. Technol. 11, 180 (1973);see also Elecrron. Fis. Apl. 17, 139 (1974). 36. C.W. Tucker, Jr., and C. B. Duke, Surface Sci. 29, 237 (1972). 37. F. Fortsmann, W. Berndt, and P. Biittner, Phys. Rev. Lett. 30, 17 (1973). 38. J. E. Demuth, D. W. Jepsen, and P. M. Marcus, Phys. Reo. Lerr. 31, 540 (1973). 39. A. M. Mattera, R. M. Goodman, and G. A. Somoqai, Surface SCI.7, 26 (1967). 40. T. A. Clarke, R. Mason, and M. Texari, Surface Sci. 4, 1 (1973). 41. J. C. Phillips. Surface Sci. 40,459 (1973).

200

L. FIERMANS AND J. VENNIK

42. G. E. Rhead, in “Electronic Structure and Reactivity of Metals” (E. G. Derouanne and A. A. Lucas, eds.), NATO Adv. Study Inst., Namur, Belgium, 1975. To be published by Plenum, New York. 43. P. Wynblatt and N. A. Gjostein, Surface Sci. 12, 109 (1968). 44. D. Aberdam, R. Baudoing, C. Gaubert, and Y. Gauthier, Surface Sci. 47, 181 (1975). 45. A. Ignatiev, F. Jona, H. D. Shih, D. W. Jepsen, and P. M. Marcus, Phys. Rev. B 11,4787 (1975). 46. G. Allan and M. Lannoo, Surface Sci. 40,375 (1973). 47. J. J. Lander, G. W. Gobeli, and J. Morrison, J. Appl. Phys. 34, 2298 (1963). 48. R. E. Schlier and H. E. Farnsworth, J. Chem. Phys. 30, 917 (1959). 49. B. A. Joyce, Surface Sci. 35, 1 (1973). 50. G. A. Somorjai and H. H. Farrell, Adv. Chem.Phys. 20, 215 (1971). 51. J. J. Lander and J. Morrison, J. Appl. Phys. 34, 1403 (1963). See also J. J. Lander and J. Morrison, J. Chem. Phys. 37, 729 (1962). 52. R. Seiwatz, Surface Sci. 2, 473 (1964). 53. N. R. Hansen and D. Haneman, Surface Sci. 2, 566 (1964). 54. D. G. Fedak, T. E. Fischer, and W. D. Robertson, J . Appl. Phys. 39, 5658 (1968). 55. A. U. MacRae and G. W. Gobeli J . Appl. Phys. 35, 1629 (1964). 56. F. Jona, I B M J. Res. Dev. 9, 375 (1965). 57. A. U. MacRae, Surface Sci. 4, 247 (1966). 58. D. Haneman, Phys. Rev. 121, 1093 (1961). 59. D. Haneman, J . Phys. Chem. Solids 14, 162 (1960). 60. D. L. Heron and D. Haneman, Surface Sci. 21, 12 (1970). 61. T. E. Gallon, I. G. Higginbothom, M. Prutton, and H. Tokutaka, Surface Sci. 21, 224 (1970). 62. T. M. French and G. A. Somorjai, J. Phys. Chem. 74, 2459 (1970). 63. L. Fiermans and J. Vennik, Surface Sci. 9, 187 (1968). 64. L. Fiermans and J. Vennik, Surface Sci. 18, 3 17 (1969). 65. D. Aberdam and C. Gaubert, Surface Sci. 27, 571 (1971). 66. J. W. May and L. H. Germer, Surface Sci. 11, 443 (1968). 67. A. U. MacRae, Science 139, 379 (1963). 68. E. Bauer, Bull. SOC.Fr. Mineral. Cristallogr. 94, 204 (1971). 69. E. Bauer, Phys. h t t . A 26, 530 (1968). 70. A. J. Van Bommel and F. Meyer, Surface Sci. 8, 381 (1967). 71. G. A. Somorjai and F. J. Szalkowski, J . Chem. Phys. 54, 389 (1971). 72. P. J. Estrup and J. Anderson, J . Chem. Phys. 49, 523 (1968). 73. G. Ertl and P. Rau, Surface Sci. 15, 443 (1969). 74. G. Ertl and J. Kiippers, Surface Sci. 24, 104 (1971); K. Christmann and G. Ertl, Surface Sci. 33, 254 (1972); H. Conrad, G. Ertl, and E. E. Latta, Surface Sci. 41, 435 (1974); G. Doyen and G. Ertl, Surface Sci. 43, 197 (1974); H. Conrad, G. Ertl, J. Koch, and E. E. Latta, Surface Sci. 43, 462 (1974); H. Conrad, G. Ertl, J. Kuppers, and E. Latta, Surface Sci. 50, 296 (1975). 75. B. Lang, R. W. Joyner, and G. A. Somorjai, Surface Sci. 30, 440 (1972). 76. R. M. Goodman and G. A. Somorjai J. Chem. P b s . 52, 6325 (1970). 77. H. E. Farnsworth, Phys. Rev. 43, 900 (1933). 78. H. E. Farnsworth, Phys. Rev. 49, 605 (1936). 79. E. Bauer, Surface Sci. 7, 351 (1967). 80. P. J. Estrup and J. Morrison, Surface Sci. 2, 465 (1964). 81. J. J. Lander and J. Morrison, Surface Sci. 2, 553 (1964). 82. K. Muller, 2.Phys. 195, 105 (1966). 83. K. Muller and H. Viehaus, Z. Naturforsch., Teil A 21, 1726 (1966).

ELECTRON BEAMS IN SURFACE RESEARCH

20 1

84. P. W. Palmberg, T. N. Rhodin, and C. J. Todd, Appl. Phys. Lett. 10, 122 (1967). 85. C. C. Chang, in “The Structure and Chemistry of Solid Surfaces” (G. A. Somorjai, ed.), p. 771, Wiley, New York, 1969. 86. F. Jona, Sugace Sci. 8,478 (1967). 87. B. A. Joyce, J. P. Neave, and B. E. Watt, Surface Sci. 15, 1 (1969). 88. P.W.Palmberg, R. E. De Wames, and L. A. Vredevoe, Phys. Rev. Len. 21,682(1968). 89. E. Bauer, in “Techniques of Metals Research” (R. F. Bunshah, ed.), Vol. 2, pp. 559ff, Wiley, New York, 1969. 90. J. J. Lander, Phys. Rev. 91, 1382 (1953). 91. R. E. Weber and W. T. Peria, J . Appl. Phys. 38, 3455 (1967). 92. L. N. Tharp and E. J. Scheibner, J . Appl. Phys. 38, 3320 (1967). 93a. L. A. Harris, J . Appl. Phys. 39, 1419 (1968). 93b. R. S. Hanisch, G. P. Hughes, and J. R. Merrill, Rev. Sci. Instrum. 46, 1262 (1975). 94. P. W.Palmberg, G. K. Bohn, and J. C. Tracay, Appl. Phys. Lett. 15, 254 (1969). 95. J. C. Tracy, in “Electron Emission Spectroscopy” (W. Dekeyser, L. Fiermans, G. Vanderkelen, and J. Vennik (eds.), p. 331, Reidel Publ., Dordrecht, Netherlands, 1973. 96. L. Fiermans and J. Vennik, Surface Sci. 24,541 (1971);35,42 (1973). 97. L. Fiermans and J. Vennik, Surface Sci. 38, 237 (1973). 98. E. N. Sickafus. Rev. Sci. Instrum. 40, 792 (1969). 99. R. G. Musket and J. Ferrante, J. Vac. Sci. Technol. 7 , 14 (1970). 100. J. E.Houston, Appl. Phys. Lett. 24,42 (1974). 101. W. M. Mularie and W. T. Peria, Surface Sci. 26, 125 (1971). 102. N. J. Taylor, Rev. Sci. Instrum. 40,792 (1969). 103. H. E. Bishop and J. C. Riviere, J . Appl. Phys. 40, 1740 (1969). 104. C. C.Chang, Surface Sci. 25, 53 (1971). 105. J. T. Grant, T. W. Haas, and J. E. Houston, Surface Sci. 42, 1 (1974). 106. C. C. Chang, in “Characterization of Solid Surfaces” (P. F. Kane and G. B. Larrabee, eds.), p. 509.Plenum, New York, 1974. 107. E. J. McGuire, Phys. Rev. A 5, 1043, 1048, 1052 (1972). 108. C. E. Moore, “Atomic Energy Levels,” NBS Circ. No. 467,Vols. 1-3.U.S. Gov. Printing Office, Washington, D.C., 1949-1958. 109. J. A. Bearden and A. F. Burr, Rev. Mod. Phys. 39, 125 (1967). 110. K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, G. Johansson, T. Bergmark, S-E. Karlsson, I. Lindgren, and B. Lindberg, ESCA, Nova Acta Regiae SOC.Sci. Ups. [ 4 ] 20,234 (1967). I l l . K.Sevier, “Low Energy Electron Spectroscopy.” Wiley (Interscience), New York, 1972. 112. E. H. S . Burhop, “The Auger Effect and Other Radiationless Transitions.” Cambridge Univ. Press, London and New York, 1952. 113. J. Bergstrom and R. H. Hill, Ark. Fys. 8,21 (1954). I 1 4. S. K. Haynes, Proc. Int. Con$ Inner Shell loniz. Phenom. p. 559.USAEC, Tech. Inform. Cent., Oak Ridge, Tennessee (1972). 115. M. F. Chung and L. H. Jenkins, Surface Sci. 22, 479 (1970). 116. J. P.Coad and J. C. Riviere, Surface Sci. 25,609 (1971). 117. W. N. Asaad and E. H. S. Burhop, Proc. Phys. Soc.. London 71,369 (1958). 118. W. N. Assad. Nucl. Phys. 66, 494 (1965). 119. W. N. Asaad, Nucl. Phys. 44,399,415 (1963);63, 337 (1965). 120. E. J. McGuire, Phys. Rev. 185,1 (1969);Phys. Rev. A 2,273(1970); 3,587,1801(1971);5, 1043, 1048, 1052 (1972);2, 1840 (1974). 121. W. Mehlhorn and W. N. Asaad, Z . Phys. 191, 231 (1966). 122. D. A. Shirley, Chem. Phys. Lett. 16, 220 (1972). 123. D. W. Davis and D. A. Shirley, Chem. Phys. Lett. 15, 185 (1972).

202

L. FIERMANS AND J. VENNIK

124. D. A. Shirley, Phys. Rev. A 7, 1520 (1973). 125. L. Ley, S . P. Kowalczyk, F. R. McFeely, R.A. Pollak, and D. A. Shirley, Phys. Rev. B 8, 2392 (1973). 126. S. P. Kowalczyk, R. A. Pollak, F. R. McFeely, L. Ley, and D. A. Shirley, Phys. Rev. 8 8 , 2387 (1973). 127. S. P. Kowalnyk, L. Ley, F. R. McFeely, R. A. Pollak, and D. A. Shirlej, Phys. Rev. B 9, 381 (1974). 128. L. Hedin and A. Johansson, J . Phys. B 2, 1336 (1969). 129. L. Fiermans, R. Hoogewijs, and J. Vennik, Surface Sci. 47, 1 (1975). 130. .W. L. Jolly and D. N. Hendrickson, J. Am. Chem. Soc. 92, 1863 (1970). 131. R. Hoogewijs, L. Fiermans, and J. Vennik, Chem. Phys. Lett. 37,87 (1976). 132. R. E. Watson, M. L. Perlman, and J. F. Herbst, Phys. Rev. B 13, 2358 (1976). 133. C. A. Nicolaides and D. R. Beck, Chem. Phys. Lett. 27, 269 (1974). 134. R. Hoogewijs, L. Fiermans, and J. Vennik, Chem. Phys. Lett. 38, 192 (1976). 135. L. Fiermans, Hoger aggregaatsverhandeling, Univ. Gent, 1974. 136. G. E. Laramore and W. J. Camp, Phys. Rev. B 9, 3270 (1974). 137. J. A. D. Matthew, Surface Sci. 40,451 (1973). 138. G. F. Amelio, Surface Sci. 22, 301 (1970). 139. E. J. Scheibner, Fundam. Processes Semicond. Surfaces, N A T O Adv. Summer Inst., Univ. Gent, 1968. (Lecture notes.) 140. G. F. Amelio and E. J. Scheibner, Surface Sci. 11, 242 (1968). 141. H. G. Maguire and P. D. Augustus, J. Phys. C 4, L174 (1971). 142a. T. E. Gallon and J. A. D. Matthew, Rev. Phys. Techno/. 3, 31 (1972). 142b. N. J. Shevchik and C. M. Penchina, J . Phys. F 5,2008 (1975). 1430. A. J. Jackson, C. Tate, T. E. Gallon, P. J. Bassett, and J. A. D. Matthew, J. Phys. F 5,363 (1975). 143b. P. J. Bassett, T. E. Gallon, J. A. D. Matthew, and M. Prutton, Surface Sci. 35,63 (1973). 143c. T. E. Gallon and J. D. Nuttall, Surface Sci. to be published (1976). 144. L. H. Jenkins and M. F. Chung, Surface Sci. 26, 151 (1971). 145. M. F. Chung and L. H. Jenkins, Surface Sci. 26, 649 (1971). 146. L. H. Jenkins ans M. F. Chung, Surface Sci. 28, 409 (1971). 147. L. H. Jenkins and M. F. Chung, Surface Sci. 33, 159 (1972). 148. L. H. Jenkins, D. M. Zehner, and M. F. Chung, Surface Sci. 38, 327 (1973). 149. C. J. Powell, Appl. Phys. Lett. 20, 335 (1972). 150. L. Fiermans and J. Vennik, Surface Sci. 38, 257 (1973). 151. C. S. Fadley, in “Electron Emission Spectroscopy” (W. Dekeyser, L. Fiermans, G. Vander-kelen, and J. Vennik, eds.), p. 151. Reidel Publ, Dordrecht, Netherlands, 1973. 152. S. K. Sen, Rep. LSE-75-3-9. Univ. of Namur, Belgium (1975). 153. T. W. Haas, J. T. Grant, and G. J. Dooley, Phys. Rev. B 1, 1445 (1970). 154. M. H. Chen, B. Crasemann, and V. 0. Kostroun, Phys. Rev. A 4 1 (1971). 155. C. D. Wagner and P. Biloen, Surface Sci. 35, 82 (1973). 156. A. Fahlman, K. Hamrin, R. Nordberg, C. Nordling, and K. Siegbahn, Phys. Lett. A 20, 159 (1966). 157. E. N. Sickafus, Phys. Rev. B 7, 5100 (1973). f58. E. N. Sickafus, Surface Sci. 36, 472 (1973). 159. G . E. Becker and H. D. Hagstrum, Surface Sci. 30, 125 (1972). 160. T. E. Gallon and J. A. D. Matthew, Phys. Status Solidi 41, 343 (1970). 161. A. P. Janssen, R. C. Schoonmaker, A. Chambers, and M. Prutton, Surface Sci. 45, 45 (1975). 162. P. J. Bassett, T. E. Gallon, M. Prutton, and J. A. D. Matthew, Surface Sci. 33,213 (1972).

ELECTRON BEAMS IN SURFACE RESEARCH

203

163. W. A. Coghlan and R. E. Clausing, “A Catalog of Calculated Auger Transitions for the

Elements,” ORNL-TM-3576. Oak Ridge Nat. Lab., Oak Ridge, Tenn., operated by

Union Carbide Corp. for the Energy Research and Development Administration (1971). 164. P. W. Palmberg, G. E. Riach, R. E. Weber, and N. C. MacDonald, “Handbook of

165. 166. 167. 168. 169. 170. 171. 172. 173. 174. 175. 176. 177. 178. 179.

180. 181. 182. 183.

184. 185. 186. 187.

188. 189. 190.

191. 192.

Auger-electron Spectroscopy,” Physical Electronics Industries, Inc., Edina, Minnesota (1972). F.Meyer and J. J. Vrakking, Surface Sci. 33, 271 (1972). J. J. Vrakking and F. Meyer, Phys. Rev. A 9, 1932 (1974). G. Glupe and W. Mehlhorn, Phys. Lett. A 26, 274 (1967); J . Phys. (Paris), Colloq. 440 (1971). J. J. Vrakking and F. Meyer, Surface Sci. 47, 50 (1975). H. E. Bishop, Brit. J. Appl. Phys. 18, 703 (1967). T. E. Gallon, J. Phys. D 5, 822 (1972). D. C. Jackson, T. E. Gallon, and A. Chambers, Surface Sci. 36, 381 (1973). D. M. Smith and T. E. Gallon, J . Phys. D 7, 151 (1974). H. E. Bishop and J. C. Riviere, J. Appl. Phys. 4, 1740 (1969). K. Goto, K. Ishikawa, T. Koshikawa, and R. Shimizu, Surface Sci. 47, 477 (1975). J. H. Neave, C. T. Foxon, and B. A. Joyce, Surface Sci. 29, 41 1 (1972). L. McDonnell, D. P. Woodruff, and B. W. Holland, Surface Sci. 51, 249 (1975). L. A. Harris, Surface Sci. 15, 77 (1969). H. E. Bishop and J. C. Riviere, J . Appl. Phys. 40, 1740 (1969). T. W. Rusch, J. P.Bertino, and W. P. Ellis, Appl. Phys. Lett. 23, 359 (1973). K. Christmann and G. Ertl, Surface Sci. 33, 254 (1972). T. E. Gallon, Surfme Sci. 17, 486 (1969). C. R. Brundle, Surface Sci. 48, 99 (1975). C. C. Chang, Surface Sci. 48, 9 (1975). P. W. Palmberg, Anal. Chem. 45, 549A (1973). L. McDonnell, B. D. Powell, and D. P. Woodruff, Surface Sci. 40,669 (1973). G. E. Becker and H. D. Hagstrum, J. Vac. Sci. Technol. 11,284 (1974). S. Thomas and T. W. Haas, J . Vac. Sci. Technol. 9, 840 (1971). M. Perdereau, Surface Sci. 24, 239 (1971). J. H. Pollard, Surface Sci. 20, 269 (1970). J. T. Grant, T. W. Haas, and J. E. Houston, Phys. Lett. A 45, 309 (1973); J . Vac. Sci. Technol. 11, 227 (1974); Proc. Int. Con$ Solid Surfaces, 2nd, Kyoto, 1974; Jpn. J . Appl. Phys. Suppl. 2, Pt. 2, 8 1 1 (1974). R. W. Springer, D. J. Pocker, and T. W. Haas, Appl. Phys. Lett. 27, 368 (1975). J. T. Grant, M. P. Hooker, and T. W. Haas, Surface Sci. 51, 318 (1975).

This Page Intentionally Left Blank

X-Ray Image Intensifiers KIRBY G. VOSBURGH, ROBERT K. SWANK, AND JOHN M. HOUSTON Corporate Research and Development General Electric Company Schenectady, New York

..............................................

205

............ 207

...........................

209

.............. 216 .................. 217

.................................................... ..........................................

221

.............................................

226

IV. Output Phosphors

.......... 231 B. Resolution

.....

........................ 233 ................................................. 237 D. Design " Trade-offs " VI. Future Trends ................................ .................. 238 B. Alternative Technical Approaches C. Other Applications

.........................

Selected Bibliography

............................................................

,243

I. INTRODUCTION

It has been known for over 70 years that a phosphor screen of suitable composition will make an X-ray image visible. In fluoroscopy (the observation of dynamic X-ray images) such phosphor screens were originally viewed by the dark-adapted naked eye. This technique has the disadvantages that the dark-adapted eye has poor detail resolution and that a long time period 205

K. G . VOSBURGH ET AL.

206

is required for dark adaptation. Hence, when reliable X-ray image intensifiers became available in the 1950s, they replaced simple phosphor screens in this application. The development of high quality video imaging systems has further stimulated medical X-ray image intensifier development, and promoted the use of these devices in applications other than medical fluoroscopy. An X-ray image intensifier (XRII) is a device that converts an incident X-ray pattern to yield a visible-light image of brightness substantially higher than that of a simple phosphor screen. In most XRIIs the final image is considerably smaller than the incident X-ray pattern, which facilitates the coupling of optical lenses to transfer the image to the final image receptor (video pickup, eye, cine film, etc.). The brightness gain is achieved in two .FOCUSING ELECTRODES

E L E C ~ R O NTRAJECTORIES

VACUUM WINOOW

PHOSPHOR

CONVE~SION TO LIGHT

CONVERSION TO LIGHT

FIG.1. A typical X-ray image intensifer with crossover electron-optical system. The device is enclosed in a vacuum bottle. The operation of the tube is shown schematically; incident X rays are absorbed by the input phosphor to make light, which causes the photocathode to emit electrons. The electrons are accelerated and focused by the applied electrostatic field, and pass through the anode aperture to strike the output phosphor, where they make the final light image. Recent research and development efforts have been directed toward improving the performance of the phosphors, the quality of the electron-optical imaging, and the contrast of the system, as discussed in the text.

X-RAY IMAGE INTENSIFIERS

207

ways: first, by increasing the number of light quanta generated from a given region of the X-ray pattern (as compared with a simple phosphor screen); and second by reducing the size of the final image, so that the quanta are emitted from a smaller area. In almost all XRIIs the X-ray pattern is converted to a light pattern using a phosphor, and the brightness of the light pattern is then intensified. Many systems employ electron-optical processes to provide the quantum gain, even though this requires additional stages of quantum conversion : the light from the input phosphor is converted to an electron image by a photoemitter which forms the cathode of an electron-optical system. After gaining energy by passing through an electrostatic field, the electrons are converted back into a light image in another phosphor. As discussed in the following sections, these additional conversion processes contribute substantially to the loss of resolution and contrast in the final image. Thus far, electronoptical XRII systems of the type shown in Fig. 1 have been the most widely utilized. They are relatively inexpensive to manufacture with the large input diameters (15-23 cm) required for medical applications, and they provide fluoroscopic images of excellent quality. Compared with a conventional static radiograph made by placing a photographic film between two thin phosphor layers, the XRII images have markedly lower contrast and somewhat poorer resolution. While the basic concept of the device has changed little since it was originally developed by Coltman (I), subsequent workers have made substantial improvements in the phosphors, electron optics, and methods of performance quantification and measurement. In the following sections we discuss recent developments in these areas, and conclude with a description of alternative technical approaches to X-ray image intensification and its applications. 11. INPUTPHOSPHORS A. Choice of Phosphor Materials The input phosphor is the detecting element of the X-ray image intensifier system, and hence determines the level of quantum noise seen in the output. It also can be a limiting factor in determining the resolution and the conversion factor. Hence, the selection and preparation of the input phosphor are of great importance. Input phosphor design requires a compromise between greater thickness, which increases X-ray absorption, and smaller thickness, which improves resolution. Hence, in selecting a phosphor one wants to obtain the maximum X-ray absorption per unit thickness, or the maximum linear absorp-

K . G . VOSBURGH ET AL.

208

tion coefficient for the X-ray energy range of interest. This requires a combination of high density and composition of atoms of high atomic number. Earlier X-ray image intensifiers used input phosphors consisting of Zn,Cd,-,S (x x 0.5) particles in a resin binder. More recently, rare-earth phosphors ( 2 ) and evaporated CsI (3) have been used. Table I shows some X-ray absorption coefficients for three kinds of phosphor. One sees that the Gd,O,S screen is about equal to Zn,,,Cd,,,S below the K edge of Gd (50.24 keV), but is dramatically superior above this energy. Evaporated CsI, on the other hand, is superior to Zno,5Cdo,5Sover the whole energy range and only moderately inferior to Gd,O,S above 50.24 keV. If a screen of Gd,O,S could be prepared with a packing factor x 1, its superiority above 50.24 keV would be more dramatic, but it would still be inferior to evaporated CsI below this energy. TABLE I Linear X-ray absorption coefficient (cm-I).

Phosphor Zno,&do, 5 s powder in resin Gd,O,S powder in resin CSI evaporated

Crystal density (gm/cm2)

Packing factor

40

4.52

0.6b

25.7

13.9

1.34

0.6*

22.8

12.1

41.0

19.1

4.5 1

1 .o

101.0

55.9

33.9

15.2

(energy in keV) 50 60

8.35

80

3.66

a X-ray absorption data taken from Storm and Israel (4). Linear coefficientsobtained by multiplying ( p / p ) * ,, for the molecule by the “effective screen density.” The latter is obtained by multiplying the crystal density of the phosphor by the packing factor (the ratio of phosphor volume to screen volume). X-ray absorption by the resin binder is neglected. The factor of 0.6 used is an estimate of the state-of-the-art. For a discussion of the packing of granular materials, see Hudson ( 5 ) .

Other factors need to be considered in making a total judgment between phosphors, and these will be discussed below: (1) escape of K X-ray fluorescence radiation impairs the signal-to-noise ratio just above the K edge of a phosphor; this factor alone will make CsI superior to Gd,O,S at 60 keV, for example; (2) the signal/noise ratio of a powder screen will be poorer than for a transparent screen, owing to greater light absorption in the former; and (3)

X-RAY IMAGE INTENSIFIERS

209

resolution (modulation transfer) will be better for light-diffusing (powder) screens than for transparent screens. In summary, the classical Zn,Cd,-,S screen is now obsolete, but the comparison between Gd,O,S and CsI is too complex to rule out either one. In our opinion, Gd,02S will have a role to play at high kVP ( > loo), but for the general medical range the trend will be toward evaporated CsI. Development of optimal light-gathering structures in CsI will be the major innovative effort in the near future.

B. Physics of Phosphor Performance 1. Conversion Factor Although increasing the brightness of the fluoroscopic image was the reason for development of the X-ray image intensifier, the required specifications in this regard are easily met in most modern systems, and so are not a subject of intense development. For 80-kVP X rays, filtered by 1-in. Al, a 125-pm thick evaporated CsI(Na) phosphor yields approximately 33 % X-ray energy absorption and 15% luminescent energy efficiency, i.e., converts 15% of the absorbed X-ray energy into visible light.

2. Spatial Resolution The input phosphor screen limits the spatial resolution of the imaging system because of the lateral spreading of light in the layer. Screen thickness is the major factor in determining resolution, but it is also affected by bulk scattering and absorption of light and by reflection of light at the surfaces. Spatial resolution is usually described by the modulation transfer function (MTF), which specifies the relative frequency response of the imaging system to a sinusoidal, spatially modulated input pattern (6).In a few simple cases, the MTF of a phosphor can be calculated. Although these cases do not correspond exactly to typical circumstances, they do give some insight into the problem. Gasper has treated the case of a transparent phosphor in detail (7). Swank has derived formulas for a diffusing phosphor from a first order diffusion theory, and compared these results with a few simple cases of the transparent phosphor which can be solved in closed form (8).Figure 2 shows a comparison of some limiting cases of transparent and diffusing phosphors. The following information is relevant to Fig. 2: (1) The abscissa is spatial frequency times phosphor thickness.

210

K. G . VOSBURGH ET AL.

uT

FIG.2. Modulation transfer functions for special cases of scattering and nonscattering phosphors. The abscissa is spatial frequency times phosphor thickness. Curve A, transparent phosphor, reflective backing; B, transparent phosphor, black backing; C, diffusion limit, no light absorption in bulk or backing; D, diffusion limit, reflective backing, 50%light absorption in bulk; E, diffusion limit, black backing, no bulk absorption.

(2) The “backing” is the side of the phosphor screen opposite the photocathode. (3) The photocathode is assumed to be in optical contact with the phosphor and is nonreflecting. (4) The diffusion limit means that the scattering mean free path is vanishingly small compared to the phosphor thickness. ( 5 ) Weak X-ray absorption is assumed. Figure 2 shows several things. The presence of light absorption in the bulk or in the backing enhances the MTF. This is easily understood since such absorption selectively removes those photons that travel the longest distances to reach the photocathode. However, as will be shown below, the use of light absorption to enhance spatial resolution must be done with care since the signal-to-noise ratio will be degraded. When all factors are quantitatively assessed, light absorption may be a useful tool in performance optimization. Figure 2 also shows that with or without light absorption the diffusing phosphor has a spatial resolution superior to that of the transparent phosphor under the conditions assumed. This fact is less obvious; and it is often stated, incorrectly, that the transparent phosphor has superior resolution. The transparent phosphor would indeed be superior if it were being viewed by the eye or a lens-coupled optical system. But for a photocathode in optical contact, the situation is different. The light spreads radially from the source, and in the absence of scattering the amount reaching an element

X-RAY IMAGE INTENSIFIERS

211

of area of the photocathode some lateral distance away is determined by the solid angle subtended. In the case of intense scattering, the light diffuses radially from the source, and its average movement is also governed by solid angle considerations. However, superimposed on the diffusion movement is a random walk, so that a photon, which according to ray tracing would have diffused to a certain point on the photocathode, may have in fact struck the photocathode sooner as a result of the random walk. This fact makes it very improbable that photons will travel large lateral distances in the presence of intense scattering. The greatest limitation to the calculated curves shown in Fig. 2 is that an infinitesimal scattering length was assumed. Figure 3 shows a calculation

uT

FIG.3. Modulation

transfer function for case where scattering mean free path = x phosphor thickness (solid line). Dotted lines are limiting cases of 0 and co for the scattering mean free path. (No bulk absorption; cathode and backing totally absorbing.)

where this limitation has been removed. In Fig. 3 it is assumed that the photocathode and backing are both perfectly absorbing and there is no bulk absorption. The two dashed curves are for the limiting cases of 0 and 00 for the scattering mean free path, while the solid curve is for the case where the scattering mean free path is one-tenth the phosphor thickness. One sees a transition between the two limiting cases: at low spatial frequencies the finite-scattering case is asymptotic to the diffusion limit, while at high spatial frequencies it is asymptotic to the transparent case. Hence the finitescattering phosphor will be superior to the transparent phosphor at low spatial frequencies and equal to it at high spatial frequencies. The superiority of light-scattering phosphors at low spatial frequencies has been observed experimentally. However, verification of the dependence

212

K. G . VOSBURGH ET AL.

of MTF on scattering mean free path is complicated by the fact that unavoidable light absorption increases as the scattering mean free path is reduced since a greater path length is traversed by the photon. A detailed experimental investigation in this area has not been reported. Qualitatively, the diffusing phosphor model corresponds to a powder imbedded in resin, while the transparent model corresponds to evaporated CsI. However, in practice, the low frequency performance of CsI may be improved by light-reflecting cracks that arise in the course of processing. This is discussed further below. 3. X-Ray Absorption and Quantum Noise

The temporal and spatial fluctuations in brightness that are observed in the fluoroscopic image are caused by statistical fluctuations in the number of X-ray photons absorbed in a resolvable picture element within the resolving time of the observational system. In order to examine the role of the input phosphor in this noise phenomenon, we need to consider an experiment where the spatial and temporal intervals are set arbitrarily larger than the system resolution, and hence the latter does not influence the results. In this way we can discuss the signal-to-noise performance of the phosphor independently of its spatial or temporal resolution. Hence, consider the following experiment. We repeatedly expose a small portion of the input screen to a given X-ray flux for a given time at widely spaced intervals of time and measure the total amount of light given off the output screen for each exposure. If each X-ray absorption event produces the same amount of light, then Poisson statistics will apply, and the relative standard deviation in the measured results will be equal to l/N"', where N equals the average number of X-ray quanta converted by the input screen in one exposure. If we define the signal-to-noise ratio as the inverse of this, then (1) signal/noise = N"' If the number of X-ray photons incident on the screen per exposure is N o , then

signal/noise = ( N oAQ)l/'

(2)

where A , is the quantum absorption of the phosphors for the X rays used. In general, the assumption that each absorbed X-ray photon produces the same amount of light will not be valid because of variable X-ray energy, variable absorption processes, and variable optical absorption in the phosphor. It has been shown ( 9 ) that signal/noise

= (N,AQ1)''2

(3)

X-RAY IMAGE INTENSIFIERS

213

I =Mf/MoM2

(4)

where and M, are the moments of the scintillation pulse size distribu ion. If we define AN = A, I, then signal/noise

=

( N oAN)’”

Equation ( 5 ) states that the signal-to-noise ratio is determined by the number of X-ray quanta incident on the detecting element and a property of the phosphor, AN, which is called the noise-equivalent absorption to distinguish it from the quantum absorption used above. Since I G 1, AN G A,. Experimentally, the value of I has been found to be such that 0.5 < I < 0.9. One cause for I < 1 is the escape of X-ray fluorescence from the phosphor after absorption of the primary X ray. The effect is greatest just above the Kedge, where the fluorescence radiation carries away the largest fraction of the energy. Another cause of small I values is optical attenuation in the

ENERGY k E V ~

FIG.4. Noise-equivalent absorption for five phosphors versus X-ray energy at a thickness of 100 pm (full crystalline density).

K. G. VOSBURGH ET AL.

214

phosphors. This can be quite important for diffuse-scattering phosphors in certain cases. Swank has calculated the noise-equivalent absorption of a number of phosphors (9) and has presented formulas for calculating I and A, for diffusing phosphors (8).Figure 4 shows the noise-equivalent absorption vs. quantum energy for several phosphors 100 pm thick at full crystalline density. If powder in resin is used, the thickness would be increased accordingly.

C . Measurement of Phosphor Performance 1. Spatial Resolution

The measurement of the MTF of an input phosphor outside the image intensifier poses a special problem since the detector must be in optical contact with the phosphor. Measurements made with the usual air-spaced lens-coupled optical systems give results that are of little use in predicting performance inside an image intensifier. Figure 5 shows an arrangement

X

X-R4 IY SLIT JAWS

I I PHOSPHOR

I

.IER

WINDOW G L A S S DIFFUSER

u t w i r t u UN I ~ L A S S

FIG.5. Arrangement for measuring MTF of phosphors for XRII tubes.

used by one of the authors (Swank) for measuring CsI screens. X rays from the source are formed into a sheet beam by tantalum jaws. The beam is incident on the phosphor from the left in the figure. An optical analyzing slit is defined by opaque material deposited on the surface of a glass diffuser, made of glass having an index matched to CsI. The phosphor is coupled to the diffuser with an index-matching fluid. The diffuser is in turn coupled to a photomultiplier. The phosphor-diffuser-photomultiplier assembly is translated in a direction perpendicular to the planes of the slits. In this way the photomultiplier response vs. displacement yields a line spread function, which is transformed mathematically to obtain the MTF. A correction is made for the finite slit widths. The function of the diffuser is to allow a

2 15

X-RAY IMAGE INTENSIFIERS I

Y

=

I-

0.1

0.0I

01

I VT

FIG.6. MTF of three screens of CsI(Na) evaporated on A1 at a substrate temperature of 300°C. having the thicknesses indicated (solid curves). The dashed curve is calculated for a transparent phosphor with reflective backing. The abscissa is spatial frequency times phosphor thickness.

constant fraction of the light passing through the slit to reach the photomultiplier, independent of angle of incidence. The diffuser is made of two parts. The first half is transparent. The second part is made by grinding the glass to a powder, pressing into a pellet, and firing to produce the desired amount of diffusion. After grinding into a flat wafer, it is fused onto a clear wafer. The resulting disk is then ground, and polished on the clear face, where the optical slit is deposited. Figure 6 shows measurements made on some CsI(Na) screens evaporated onto A1 substrates at 300°C. Such screens have relatively little light scattering. The solid lines show the results for three screens of different thicknesses, while the dashed line is a calculated curve from Swank (8).One reason the experimental curves lie higher at low frequencies is the truncation of the line spread function. Measurements are made only down to 4% of the maximum value of the line sprread function. It is then truncated by fitting an exponential to the lower tail. This underestimates the tail of the line spread function and produces an MTF that is slightly high at low frequencies. Experimental curves usually fall more rapidly at high frequencies than the theoretical curves, probably due to the finite thickness of the optical contact fluid, which is not accounted for in the computations. 2. Absorption and Noise Quantum absorption can be measured at a given energy by measuring the transmission of the phosphor for X rays of that energy, either by using a

216

K. G. VOSBURGH ET AL.

monoenergetic source, or by using an X-ray tube with a spectrometer detector. However, this does not determine the noise-equivalent absorption. Swank (10) has described three methods for measuring noise-equivalent absorption in phosphors and X-ray image intensifiers. Although all three methods have particular virtues for certain applications, the simplest and most direct is the “pulse burst” method. In this method a continuous X-ray source is modulated into short bursts by a rotating shutter. The scintillation light from the phosphor is measured by a photomultiplier which produces a signal pulse for each burst of X rays. These pulses are analyzed by a pulseheight analyzer, and the standard deviation of the pulse-height distribution is determined. Then, using Eq. (9, the noise-equivalent absorption is determined. The value of N o is obtained by substituting a thick scintillation crystal for the test phosphor. For a thick crystal, A, will be close to unity. If an X-ray tube is used as a source, the supply voltage must be stable since fluctuations in X-ray output will invalidate the results. Alternatively, a monitor can be used to correct the data for source fluctuations. D . Structured Phosphors

It has long been realized ( 1 1 ) that the performance ofthe input phosphor could be improved by making it in the form of a “fiber-optic” layer, in which lateral light spreading is prevented while transmission to the photocathode is unimpeded. Recently, reports have appeared describing at least partial realization of this idea. Reifsnider and Brown (1 2) describe the use of fiber-optic plates made of luminescent glass. The atomic constituency of the glass was not reported, but evidently both X-ray absorption and luminescent yield were low. However, very high resolution was obtained at low X-ray energies. Stevels and Schrama-de Pauw (13) report detailed investigations of the formation in evaporated CsI films of cracks that act as barriers to the lateral propagation of light. These occur at least in part as a result of shrinkage which occurs when a film deposited at low temperature is subsequently fired at a higher temperature. The cracks occur naturally in a random pattern along grain boundaries, but can also be produced in a regular pattern by the use of suitably structured substrates. 111. ELECTRON OPTICS A . Introduction

The electron-optical system transmits the image from the photocathode to the output phosphor. The electrons are accelerated and the image is demagnified ; both of these effects increase image brightness.

X-RAY IMAGE INTENSIFIERS

217

In “crossover electron-optical systems, the electron trajectories pass through (or near) the XRII axis near the anode aperture (see Fig. 1). The elements of the crossover system can be deduced from concentric spherical electrode systems as analyzed by Schagen, Bruining, and Francken (14). Denote the radius of the cathode by rc and the radius of the anode by rc In. The gaussian image (the image for small initial electron velocities) is located on the system axis a distance of ”

5n - 2

z, = ( n - 4). ~

from the center of the electrode system, under the assumption that the anode aperture is small. The magnification of the electron-optical system is

M

=

3 / ( n - 4)

(7)

These equations may be used to guide the preliminary layout of the electronoptical system for an XRII. In practice, the concentric sphere geometry has the limitations that the image is curved and that the cathode electrode is usable over only a small area compared with a full hemisphere. In addition, the spherical geometry does not permit the adjustment of magnification. The spherical image produced by the concentric spherical electrode system is made more concave by the compression of the tube electrodes toward the axis, which is done to make the device more compact. The compensation for this effect generally involves the addition of electrodes and other changes in the simple spherical diode structure, and the resultant designs have become too complex to permit detailed analytic analysis. Various investigators have been successful in improving XRII performance by working with tube models in demountable vacuum test systems and by the careful study of specially designed prototype tubes. In the past ten years, digital computer techniques have been employed by several XRII design groups.

B. X R I I Design b y Computer Computer simulation techniques have been successful because they yield precise information in a short period of time. The computer simulation gives useful design criteria (such as the focal surface shape) which are difficult to measure accurately and systematically on a prototype device or demountable vacuum tube. Computer programs for XRII design often have in common these basic assumptions: (1) The electrostatic fields may be adequately represented by a grid of finite elements, with the potential at each grid node being a linear

218

K. G . VOSBURGH ET AL.

combination of that at the surrounding nodes, computed as an approximation to Laplace’s equation. (Space charge is always neglected since beam currents rarely exceed one microampere.) (2) The use of a few thousand grid points and predictor-corrector ray tracing techniques will yield accuracy sufficient for the determination of primary imaging characteristics (image magnification, paraxial focus location). (3) An adequate indicator of a point on the “true ” focal surface can be obtained by tracing a few (typically four) rays from the object to the image. In many cases, the computer programs and programming methods have been borrowed from studies of microwave electron tubes, ion engines, electron microscope lenses, and similar devices. The adequacy of the assumptions given above is best determined by direct comparison with prototype tube performance; several laboratories have reported success in the development of workable computer design programs for XRIIs. The most difficult conditions to satisfy are the accurate prediction of the focal properties of the lens, given the small electron velocity spread from the photocathode and the close proximity of many electron trajectories to electrodes, particularly near the anode aperture. Design tolerances are often stringent: the focal surface must be positioned within 0.15 mm out of a total length of 220 mm for a typical design. The initial velocities of electrons emitted from the photocathode vary from 0 to 2 V, with an energy distribution that depends on the optical emission spectrum of the phosphor and the work function of the photocathode. In calculating electron imaging performance, it is desirable to pick a relatively large initial electron velocity to reduce computational errors. The angular emission of electrons varies as the cosine of the angle from the normal to the surface (Lambert’s law) ( 1 9 and there is no significant correlation between the energy and angle of emission. A comprehensive analysis of computer methods for XRII design may be found in the recent paper of Schwierz (26). C. Focus UniJormity

Much ofthe recent work in XRII electron-optical design has been directed toward the attainment of focus uniformity across a planar image. The early diode designs, such as investigated by Schagen et al. (24), have the disadvantage that the focal surface is spherical. The reduction of envelope diameter and length are desirable from a practical standpoint; but, as pointed out by Wreathall (17), such deviations from monocentricity and spherical electrode surfaces cause the focal surface to become more sharply concave. We discuss here several approaches to correcting this condition, and conclude with a discussion of the tolerances necessary in electron-optical design for the image to appear “flat.”

X-RAY IMAGE INTENSIFIERS

219

The principal contribution to electron focusing is made by the relatively weak fields near the photocathode. The strength of the focusing action varies inversely with the field strength; i.e., a stronger potential gradient will cause the electrons to be focused further away from the photocathode. Electronoptical designers attempt to achieve an optimal photocathode field strength profile (as a function of the distance from the tube axis) to give the “flattest” focal surface, but without introducing astigmatism and distortion.

1 . Triodes and Tetrodes Additional electrodes were introduced into diode designs to improve focusing performance ( 1 7 ) and to provide a “zoom” capability (see below). The use of a simple focusing electrode is described by Niklas (It?), who also analyzes the effects of anode placement and anode aperture construction for optimal reduction of field curvature in triode designs. Recent designs of fixed-size-input tubes have tended toward the use of two focusing electrodes, one being on the tube wall with a metal projection adjacent to the cathode, and the other electrode surrounding the anode. Designs of this form allow field curvature to be essentially corrected across the image, even if the photocathode is purely spherical. The triode, and more particularly the tetrode designs have the additional advantage over the diode structure that the focal surface may be adjusted to give the best match with the output phosphor position. Thus construction tolerances for triodes and tetrodes can be somewhat less stringent than for diodes. 2. Nonspherical Photocathode Surfaces The electric field at the cathode surface can be adjusted to give the desired flat focal surface simply by shaping the cathode appropriately. Such shaping is normally done as “fine tuning” to a tetrode or pentode design, and the deviations from a spherical photocathode surface are small. The outer portions of the cathode (away from the tube axis) are made less concave; i.e., the cathode is bent toward the anode more than a spherical cathode at large distances from the axis. Thus the field at large distances from the tube axis on the cathode is stronger, and the electrons are focused further along their paths. Examples of designs incorporating this feature may be found in U.S.Patents 3,697,795 (Braun et a/.) and 3,784,830 (Schwierz and Wulff), and in the recent paper of Kuhl (19). The necessity of making a precise, smooth, nonspherical surface over the large sizes (up to 23-cm diameter) is a drawback in implementing these designs, particularly if the input phosphor-photocathode assembly is deposited directly on the glass vacuum window. It is possible to achieve a

220

K. G . VOSBURGH ET AL.

somewhat similar effect by locating a small “tuning” electrode adjacent to the outermost edge of the cathode, with this electrode being maintained at a fraction of the main focusing electrode (gl) potential (see Fig. 1). 3. Fiber-optic Phosphor Substrate

By replacing the plano-plano glass output phosphor substrate with a fiber-optic plate, the designer is free to abandon a plane image surface, and instead, to optimize the design for minimal astigmatism and distortion. The phosphor surface can be given a complex curvature, and the fiber optic will transfer the image to a plane surface on its opposite side. (The plane output surface is necessary for effective optical coupling of the subsequent image receptors-television camera lenses, film cameras, etc.). Such fiber optic substrates are used in high quality light amplifiers, but their use in XRIIs has been limited. The construction of fiber-optic plates with the proper aspheric contour to match an XRII focal surface is subject to the problems of grinding the proper aspheric surface and the alignment of the substrate in the tube. Plano-plano fiber-optic substrates have utility in improving image contrast, as discussed below. 4. Mesh Electrodes

As noted by Mayo and Bennett (20) and by Schleschinger (21), mesh electrodes may be used to correct the focal surface of an image tube to a more planar contour. In practice, such designs have not been widely used because of the difficulty of making, supporting, and aligning the mesh electrodes and the possibility of secondary emission from the meshes themselves.

5. Nonuniformity of Cathode Potential The effective field at the photocathode may be adjusted by making the photocathode in such a fashion that its potential is not constant, but varies uniformly as a function of radius from the tube axis. This approach has been followed by various investigators in the construction of parallel plane diode systems that provide a spherical potential gradient. An image intensifier employing this principle has been patented by Verat et al. (22) and electron radiographic systems have been developed along similar lines (23). The method of Verat et al. involves imbedding in the cathode substrate a thin conductor that is wound in a spiral out from the center of the cathode. A current is passed through the conductor, and the photocathode potential is determined by the resistive voltage drop along the conductor. For a given

X-RAY IMAGE INTENSIFIERS

22 1

application, the spiral windings must be on a sufficiently small scale that they are not resolved by the electron-optical system.

6. Tolerances for Field Flatness In the design of an XRII electron-optical system, a tolerance on field flatness must be set. In practice, the designer is faced with two complicating factors: ( 1 ) It is difficult to compute the exact “waist” in the electron trajectory pattern corresponding to the image of a point with the highest resolution. ( 2 ) For points off the tube axis, astigmatism and coma are demonstrably present (in addition to chromatic aberrations). Consider first the imaging of a point on the tube axis. The principal contribution to unsharpness is chromatic aberration, which is due to differences in the imaging of electrons with different longitudinal (along the tube axis) components of initial velocity. Various investigators (19, 2 4 ) have noted that a “typical ” electron crosses the focal surface at an angle of about 4“ to the axis. Taking a desired resolution, or “blur radius,” of 0.01 mm [ 5 line pairs per millimeter (lp/mm) at the tube input with a demagnification of ten], the electron could travel O.Ol/tan(@) = 0.14 mm along the tube axis before exceeding the blur radius. Thus the design tolerance would be to position the focal surface within 0.14 mm of its desired location. An independent measurement and computation was made to verify this tolerance. The limiting resolution of a prototype XRII was measured using conventional Emil-Funk lead bar patterns as a function of the potential of the focusing (gl) electrode. The focal properties of this tube were then computed using a digital computer simulation for the same potential variations. Figure 7 shows the measured limiting resolution of the XRII as a function of the computed electron trajectory “axis crossing point determined by the computer program. It is seen that the depth of focus is on the order of 0.3 mm, which agrees well with the approximate computation given above. The electron-optics designer, then, must attempt to make the computed focal surface flat within about 10.15 mm. For off-axis imaging, the focal surface computation is complicated by differences in the imaging of tangential and saggital rays [see, e.g., the paper by Robbins et al. (241. In general, the designer attempts to minimize this astigmatism, while adopting an average of several computed trajectory intersections as his indicator of focal surface position. D . Brightness Uniformity ”

The image of a uniformly illuminated XRII of conventional design is generally 20-30% less bright a t the edges of the image than at the image

222

K. G . VOSBURGH ET AL.

COMPUTED AXIS CROSSING

(mm)

FIG. 7. Limiting resolution measured experimentally for a prototype XRII using an Emil-Funk lead bar pattern is plotted versus the computed position that a “typical” electron trajectory would cross the axis if originally emitted on the axis at the photocathode. The XRII was a conventional tetrode design with 15-cm input field; its performance was simulated on a digital computer as discussed in the text. Note that the axis crossing point (which corresponds to the focal position or output phosphor location) must be specified within a few tenths of a millimeter.

center. Principal contributors to this brightness nonuniformity are electron-optical distortion and the distortion caused by projecting X-ray images onto the curved faceplate of the XRII. In fluoroscopic applications, the X-ray tube is located about one meter from the XRII, and the faceplate radius of curvature is roughly equal to the usable receptor diameter. For purpose of illustration in the following, we discuss an XRII with a 15-cm input diameter. We consider the imaging of an object plane adjacent to the XRII faceplate, and assume that it is uniformly illuminated with X rays. The brightness of the image is proportional to the square of the areal demagnification, which is analyzed in terms of “geometrical distortions (those due to the finite source distance and curved XRII receptor) and “ electron-optical distortions ” (due to nonlinearity in the electron imaging). The relative areal magnification due to the geometrical distortion increases by about 10% from the tube axis to the image edge for typical conditions. This would be the total distortion contribution to brightness nonuniformity if electron-optical imaging were perfect. The electron-optical distortion must be computed for each tube design, or it can be measured by moving the X-ray source to a large distance from ”

X-RAY IMAGE INTENSIFIERS

223

the XRII to eliminate geometrical effects. In manufacturer's specifications, a parameter often recorded is the distortion (local linear magnification) as defined by D(r) = [s(r)- s(O)]/s(O), where s(r) is the size of a small test object evaluated at radius r from the tube axis. D(r) is generally greater than unitythe distortion is of the pincushion type. Tube specifications frequently call for the measurement of both radial and tangential distortion. The tangential distortion is given by

(8) where M o is the radial magnification on the tube axis and M ( R ) is the radial magnification at the measurement radius (R). The radial distortion is DT

= M(R)/Mo

=( I ; - r;)/MO(r2

-

(9) where r; and r', are the measured output radii for two adjacent object points rl and r 2 . If the magnification at r l is given by M , then the radial magnification at r 2 (to first order) is ( M dM/dr),=,, Ar, where Ar = r2 - rl. The radial distortion DR can then be written DR

+

The relative brightness of the image is inversely proportional to the areal distortion D A = DR D T , or

where R again denotes a given object plane radius. This term was computed for a typical 15-cm input XRII, as shown in Fig. 8. The design used is representative of the performance of current systems with a demagnification of about 11. The brightness nonuniformity arising from these distortion terms is somewhat compensated by the higher absorption probability for X rays that traverse the input phosphor at large distances from the tube axis. As the distance from the axis increases, the X rays strike the phosphor at progressively more oblique angles, and thus have a longer path through the phosphor in which to convert. Depending on the geometry and the X-ray absorption probability, the brightness gain at the edges of the image from this effect can be as large as 10% of the on-axis edges. Figure 9 shows the computed object brightness for the geometrical conditions mentioned above and the electron-optical performance of Fig. 8. The distortion is not a strong function of the faceplate radius, i.e., there is little utility in attempting to increase the radius to reduce geometrical distortions.

K. G . VOSBURGH ET AL.

224

I

;:::I

AREAL DISTORTION (ELECTRON OPTICS)

I

I. 18

I

I.l6!-

[.:I 1.14

* 0

1.12

1.10

1.02 1.00

0

20

I

40 60 80 OBJECT RADIUS (%)

I

100

FIG.8. A plot of areal distortion versus object radius for a typical 15-cm input field XRII. This is the distortion due solely to the nonlinearities in electron-optical magnification. It would be measured by moving the X-ray source to a very large distance from the XRII input.

OBJECT RADIUS (%)

FIG.9. Computed object brightness of a typical 15-cm input field XRII as a function of object radius. The electron-optical contribution to the distortion is given by Fig. 8; the source was assumed to be 80 cm from the XRII faceplate; X-ray absorption of the phosphor was taken as 0.25; and the phosphor radius of curvature was 17.5 cm. Small changes in these parameters have little effect on the brightness profile, which is predominately determined by the electron-optical magnification nonlinearity.

225

X-RAY IMAGE INTENSIFIERS

Rather, the most promising direction is in continuing to improve the uniformity of the electrooptical imaging.

E . Multiple-Field and Zoom X R I I s It is advantageous to be able to change the magnification of an XRII electronically. When it is not necessary to use the full input field, higher object resolution can be achieved by reducing the demagnification, so that the contributions of the electron-optical system and the output phosphor (as well as subsequent system components) to the overall unsharpness are reduced. Reducing the irradiated field diameter also improves image contrast by reducing X-ray scatter from the object (e.g., patient), and “zooming” then allows the reduced-size input to fill the entire output screen. Zooming also obviously reduces patient dose. Two approaches to electronic “zooming ” have been developed. Vine (25) calls these “cathode space ” and “anode space ” zooming. Both methods have been developed in recent years to yield images of excellent quality (flat focal surface, low distortion). Cathode space zooming is accomplished by changing the potentials of grid electrodes in the main body of the tube. Generally, a pentode design is used. The potential on the first grid, which is adjacent to the photocathode, is fixed since this determines the electric field at the photocathode, and hence the focal plane location. Electrodes g, and g, (Fig. 1) are adjusted to vary the magnification. In an oversimplified description, g, is used to move the electron crossover point to the proper location along the tube axis to give the desired magnification, and the potential of g, is adjusted to shield the photocathode from effects due to the variation of the g, potential. FOCUSING SECTION

r-;--

- 1

-

I

ANODE SECTION

7

-

-

_

T

-

-

OUTPUT IMAGE S I Z E FOR WEAK ANODE , -SECTION F I E L D

1

OUTPUT IMAGE SIZE .‘FOR STRONG ANODE SECTION F I E L D

I

L

\

CATHODE

I

-1

GI

62

63

SCREEN

FIG.10. An electron-optical system that provides variable magnification and high image quality with the adjustment of only one potential. After Robbins et al. (24).

226

K. G. VOSBURGH ET AL.

One disadvantage of the cathode space zooming system is that at least two voltages must be changed to change the magnification. This may be overcome in an anode zoom design, as has been demonstrated by Robbins et al. (24). Their design, which is shown in Fig. 10, has the feature that the focusing electrodes are connected to a single voltage divider, with one variable element. The electrons are brought to a crossover at a boundary determined by the g, electrode. Once they pass through the small g, aperture, they come under the influence of the anode potential, with the fields in this region being set by the g,-to-anode potential difference. The focusing of the electrons is performed by the fields near the cathode, which are not perturbed by changes in the absolute level of the g, potential, since the potentials on g, and g, are scaled with it. As constructed by Robbins et al. ( 2 4 ) this design has shown excellent imaging performance. IV. OUTPUTPHOSPHORS Since the output image is reduced in size, usually to about one-tenth the diameter of the input image, the resolution requirements of the output phosphor are very severe. Many approaches have been taken to the achievement of high resolution output screens. A . Methods of Preparation

1 . Thin Film Phosphors Very high resolution should be obtainable with a transparent thin film phosphor. This has led a number of investigators to consider this approach (26-29). Unfortunately, even when the thin film phosphor has intrinsic Xrayenergy-to-light efficiency nearly equal to that of a good powder phosphor, the practical efficiency is lower by about a factor of 5 as a result of light trapping, i.e., high-angle light cannot escape from the phosphor layer because the phosphor has a higher index of refraction than the subsequent glass, vacuum, and air path. Any attempt to reduce light trapping by introducing light scattering in the phosphor layer degrades the resolution accordingly. Unless an observational system can be devised not requiring the light to pass through a medium of low optical index (such as air or vacuum), there appears to be no practical way to utilize the high intrinsic resolution of a thin film phosphor. 2. Settled (or Centrifuged) Screens The most commonly used material for particulate output screens is the P-20 phosphor, (Zn, Cd)S, having a density of about 4.3 gm/cm3. A 30-keV

X-RAY IMAGE INTENSIFIERS

227

cathode ray beam will have a practical range of about 3 p n in a dense crystalline layer of this material. Hence, for best resolution and good screen uniformity, the particles should have diameters smaller than 3 pm. Particles in this size range are slow to settle from liquid suspension under the action of gravity alone, and hence the use of the centrifuge is necessary. A detailed study of centrifuged screens has been given by Pakswer (30). With proper control of particle size, suspension stability, and screen weight, high resolution screens can be fabricated by this method. 3. Electrophoretic Screens

Use of electrophoretic deposition to fabricate scfeens of small particle size has been extensively investigated. Several experimenters report excellent results with these screens (31-33), while others describe various shortcomings (29, 34). A possible improvement in uniformity by self-healing of voids has been suggested. One property of electrophoretic screens which is unique is the bonding of particles to the substrate by metal hydroxides produced electrolytically. This provides good adhesion, but may also increase the area of optical contact with the substrate, leading to contrast degradation. 4. Screens Fabricated with Photosensitive Polymers

The use of polyvinyl alcohol (PVA) photobinder to determine the thickness of a screen has been described by Stone (35). After settling or centrifuging from a solution containing PVA, the screen is exposed to uv through the substrate, hardening the PVA to a depth determined by the amount of uv exposure. The excess phosphor is washed off, leaving the phosphor layer bonded to the substrate by the PVA, which is subsequently burned off. Improved screen uniformity is said to be achieved with this technique.

5 . Screens Fabricated with Thermoplastic Layers A patent by Lehmann (36) describes the formation of uniform layers of luminescent powders by brushing or vibrating the powder over a heated glass substrate coated with a thermoplastic layer, such as cellulose acetate. The thickness of the thermoplastic layer, together with the brushing technique, determines the thickness of the final layer, and also effects a segregation of particle sizes since the larger particles do not adhere to the tacky layer. After fabrication of the phosphor the thermoplastic binder is burned off. A further development is reported by Diakides (29), who used a high velocity “air brush” to apply the powder to the tacky layer, and

228

K.

G. VOSBURGH

ET AL.

worked with submicron powders. Resolution in excess of 400 lp/mm was obtained for 6-kV cathode rays. 6. Lungmuir Flotation Technique Beesley and Norman (28) have described the preparation of high resolution screens using this technique (37). Fine phosphor particles are coated with a water repellent coating, and a layer of these is spread onto water. The layer is compressed by moving a mechanical vane across the surface until essentially a perfect monolayer is formed. By draining the water the phosphor layer is lowered onto the surface of a glass slide previously placed in the tank. The process can be repeated, adding additional monolayers. A problem observed by Prener (38) is that upon drying there is a tendency for the monolayer to form a number of folds” or clusters, where the monolayer increases in thickness locally. “

7 . Spray Technique Spraying the phosphor onto the substrate in liquid suspension was one of the earliest methods of producing cathode ray screens. For high resolution screens it was superseded by the methods described above. Recently Bates and Anderson (39) have described a more sophisticated engineering of the spray method which results in screens of high uniformity and resolution. They claim a packing density of 0.7, a remarkably high value.

8 . Other Methods The above enumeration is not exhaustive. For example, Francz et al. ( 3 4 ) give results for a “rotational” technique, but details are not given. 9. Comparison

Aside from the transparent thin films for which the limitations are obvious, it is not possible to arrange the various fabrication methods in order of excellence of performance. Part of the problem stems from the fact that each technique has its advocates, in whose hands superior performance is obtained, i.e., development and execution of a particular technique seems to be more important than which technique is chosen. Another part of the problem is a lack of uniformity in measuring techniques, particularly of MTF. In some cases observers report improved performance for a “ new technique, but publish MTF curves that are poorer than other measurements reported for the “old” techniques. Good MTF measurements of cathodoluminescence at 40 lp/mm and above are difficult to make.

”

X-RAY IMAGE INTENSIFIERS

229

B. Measurement of Performance 1. Brightness

Screen brightness is nearly proportional to cathode ray current density and is approximately proportional to voltage up to the voltage at which the cathode rays begin to penetrate the screen. Above this voltage a maximum brightness is achieved and then a decline with further increase in voltage, It is customary to adjust the thickness of the phosphor layer so that the operating voltage will be near the top of the linear range. Within the linear range a fine-grain ( - 2-pm particle diameter) aluminized P-20 output screen can produce 15-20 candela/W along the normal direction. If the angular distribution were Lambertian, this would correspond to a luminous efficiency of 47-63 I m p . However, Pakswer (30) has found the distribution for his centrifuged screens to be much more peaked along the normal than a Lambertian radiator, so that the luminous efficiency was found to be less than half the above amount. In any event, it is the luminous intensity near the normal direction that is of interest for most applications. 2. M T F

A method for measuring the MTF of a cathode ray phosphor was given by Catchpole (40) and is illustrated in Fig. 11. It is important that the SCREEN

FIG.11. Experimental arrangement for measuring the spatial frequency response (MTF) of output phosphor screens. After Catchpole (40).

modulating slit be as close to the phosphor as possible. The “ system response is obtained by illuminating the slit with light with the phosphor absent, and focusing on the slit. Catchpole used a variable frequency bar pattern at the focal plane of the microscope objective to scan the bar pattern response curve, from which the sine wave response was calculated. An inverted version of Catchpole’s apparatus, designed and built by Webster (41), has been used by the authors for comparing output screens from various sources. In this apparatus an electroformed nickel bar pattern ”

230

K. G . VOSBURGH ET AL,.

(42) is placed in the cathode ray tube in place of the slit. A pattern-tophosphor distance of 12 pm is used. A commercially available scanning eyepiece (43) with a slit aperture is used with the microscope. Aside from the use of commercially available components, this system has the advantage that a larger area of the output screen is sampled. However, a different region is sampled for each spatial frequency. In this apparatus only the frequencies 10, 20, and 40 lp/mm were employed. One factor that is usually not discussed in connection with published MTF data is the extent to which long-range contrast degradation is included in the data. Ideally, the MTF is defined for an infinite phosphor illuminated by an infinite, sinusoidally modulated cathode ray beam. Reduced to best practical terms this would mean bar pattern modulation over the entire usable area of the output screen, or in the Catchpole apparatus, a bar pattern reticle and photodetector which covers the entire image of the screen. Such a procedure is clearly impractical, and usually only a small portion of the pattern is utilized, so that long-range effects are suppressed. The only problem is that MTF curves cannot be compared unless the fraction of the total screen (or image) area measured is the same. In the apparatus used by the authors, for example, about 1% of the output screen is illuminated. This results in considerable suppression of long-range effects. Under these conditions, a typical commercial screen gives a bar pattern response of 0.95 at 10 lp/mm, 0.85 at 20 lp/mm, and 0.4 at 40 lp/mm.

3. Large Area Contrast Since MTF data do not usually include the effects of long-range light propagation in the output screen, it is customary to measure these effects separately. One method is to measure the brightness at the center of the screen with and without an obstruction that prevents excitation of the central 10% of the screen area. The ratio of these two measurements is referred to as the “contrast ratio” (CR), usually expressed as a number greater than unity. The inverse ratio (l/CR) is also useful since it will normally be additive for different parts of the system; i.e., the inverse contrast ratio of the system is equal to the sum of the inverse contrast ratio of each part. The principal contribution to 1/CR in the output phosphor is the effect of halation (44). This is caused by internal reflection of light coming from phosphor particles that are in optical contact with the substrate. It can be minimized in three ways: (1) Utilize fabrication techniques that minimize optical contact. (2) Make the substrate partially light-absorbing. (3) Deposit the phosphor directly onto a fiber-optic output window.

X-RAY IMAGE INTENSIFIERS

23 1

Another contribution is Fresnel reflection or scattering at any glass surfaces that are traversed in reaching the final detected image. This includes components of the observational system, even though these are, in principle, separate from the image intensifier tube. 4. Graininess

An important property of the output phosphor, but one difficult to measure, is the inhomogeneity caused by irregular distribution of phosphor grains. This can be particularly troublesome if one attempts to maximize resolution by minimizing the screen thickness, resulting in regions of variable cathode ray penetration and hence brightness fluctuations. It is for this reason that one is forced to operate well below the knee of the brightnessvoltage curve. However, the main problem is proper sizing of particles and complete dispersal of particles in suspension. V. XRII PERFORMANCE A. Conversion Factor

The conversion factor or gain of an XRII is the result of several stages of energy and quantum conversion. The X rays are absorbed by the input phosphor, which then drives the photocathode. Electrons from the photocathode are accelerated and focused by the electron-optical system to strike the output phosphor, where they produce a light image that is a few orders of magnitude brighter than the image from a simple phosphor screen. The conversion factor may be expressed in units of nit/(mR/sec) (45), where 1 nit = 1 candela/m2 = 0.292 fL. For a typical XRII, we calculate the conversion factor as follows: An initial X-ray spectrum is assumed to have been created with 80 KVp, and filtered with 2.5 cm of aluminum. The X rays strike a 125-pm thick evaporated CsI(Na) phosphor yielding 33% X-ray energy absorption and 15% luminescent energy efficiency. Light photons from the phosphor strike the photocathode, which will typically have a sensitivity of 30 mA/W for the CsI(Na) spectrum. This sensitivity is lower than achievable under optimal conditions because the photocathode is deposited on a surface that is rough and may be partially discontinuous, leading to lower sensitivity and higher resistance. The problem of high cathode resistance may be overcome by introducing a transparent conducting layer between the cathode and the phosphor. The electrons emitted by the photocathode are accelerated to 30 keV by the electron-optical system, and focused onto the output phosphor. Before

K. G . VOSBURGH ET AL.

232

entering the active region of the output phosphor, they pass through a thin metallic coating, thus losing a few keV. If the output phosphor has a luminous efficiency of 15 candela/W, and the electron-optical systems have a demagnification of 10 (so that the areal demagnification is a factor of loo), the total conversion factor is 130 nit/(mR/sec). This is roughly twice the value normally required to operate the detection devices (e.g., vidicon) used in a typical medical system, so that even considering losses not included in the calculation and fluctuations encountered in production, excess conversion factor can be realized. B. Resolution

Resolution is normally specified in terms of the modulation transfer function (MTF) at the input plane of the system, which is adjacent to the input phosphor, but outside the vacuum tube. Table I1 shows the contributions to the MTF for a typical 15-cm input XRII of conventional design with demagnification of 11. It is seen from the table that the input phosphor, output phosphor, and electron optics all contribute about equally to the composite MTF at each frequency. These data are based on measured electron-optical performance and input and output phosphor measurements as discussed above. TABLE I1 MODULATION TRANSFER FUNCTION CONTRIBUTIONS FOR

Spatial frequency (Ip/mm at input) 1 2 4

A

TYPICAL XRII

Composite MTF

Input phosphor MTF

Electron optics MTF

Output phosphor MTF

0.90

0.95 0.75 0.50

0.95 0.85

0.65 0.30

0.40

0.8 1 0.41 0.06

The values quoted in Table I1 are indicative only of the general resolution levels attainable. As discussed below, various other tube properties (such as gain) may be sacrificed to give higher resolution. Also, the contributions from the electron optics and the output phosphor are dependent on the magnification of a particular design. This point has been discussed by Kuhl (19), who also emphasizes the necessity for including the properties of the final image receptor (such as a video pickup) in any realistic assessment of imaging performance.

X-RAY IMAGE INTENSIFIERS

233

C . Contrast Contrast, or “ large area contrast,” is a measure of the relative intensity of adjacent light and dark fields at a low spatial frequency. Specific numerical values are dependent on the measurement procedure; a typical procedure is the insertion of an occluding disk (lead is often used) in the center of the XRII input field. The image brightness at the disk center is measured relative to a uniformly illuminated input. The measured contrast increases as the area of the occluding disk is increased; several manufacturers have adopted a standard disk area of 10% of the input field. The contrast range of XRIIs is much lower than the available contrast in the normal medical radiologic image. Conventional screen-film cassette radiographs with resolution comparable to today’s XRIIs will give a contrast ratio in excess of 50 : 1. Recently, several investigations have been made to determine the mechanisms of contrast loss in XRIIs, and the application of these studies has raised the XRII contrast ratio to about 12 : 1. Many mechanisms contribute to the degradation of contrast in an XRII. Their contributions to the contrast ratio add reciprocally; i.e., if various mechanisms denoted by subscript i each have a contrast ratio C i , the overall contract C will be given by

Among the mechanisms that cause contrast degradation are X-ray scattering in the input window, light scattering inside the tube, and light scattering and reflections in the output phosphor assembly. The following analysis of these effects draws heavily on the work of V. C. Wilson of the General Electric Research and Development Center (46). Present day XRIIs have large-area contrast ratios ranging between 10 and 15. These values are a substantial improvement over those quoted in the late 1960s (5-10). Factors contributing to this improvement have included the design of higher contrast output phosphor assemblies and the reduction of light scatter inside the tubes. The principal contributors to contrast degradation in a typical 15-cm input XRII with a large area contrast of 12.6 are shown in Table 111. These values are derived from direct measurement of prototype XRIIs and individual XRII components using 65-KVp excitation. We discuss each mechanism in turn. 1. Phosphor

Depending on the detailed construction of the phosphor material, it is possible for light to travel considerable distances along the phosphor layer.

K. G . VOSBURGH ET AL.

234

TABLE 111

MEASUREDRECIPROCAL CONTRASTVALUESFOR VARIOUS MECHANISMS” Phosphor Phosphor curvature X-ray scattering by front window Light reflections from tube interior Output phosphor and window Sum of 1/C values Total contrast ratio (C)

0.0060 0.0158 0.0064

0.0062 0.0450 0.0794 12.6

After Wilson and Argersinger (46).

However, just a small amount of light will be directed along the layer, and it will be absorbed or scattered by irregularities and inhomogeneous regions of the phosphor. For a phosphor layer to have good contrast properties, then, the light must be scattered or reflected out of the phosphor close to its point of origin, and the “trapping” of light (by not providing a good refractive index match at the phosphor boundary, for example) should be avoided. If these principles are followed, the input phosphor is a minimal contributor to contrast loss. 2. Curvature of Phosphor

The light emitted from the phosphor strikes the photocathode. However, up to 30% of the light may pass through the photocathode, and travel across the phosphor chord (Fig. 12) to strike another region of the photocathode

DIAGONALLY THROUGH THE PHOTOCATHODE

FIG.12. A schematic cross section of the input phosphor and photocathode structure of an XRII, showing how light may be transmitted through the photocathode and strike the photocathode surface at a point well removed from the light source. This “cross-chord light” eNect is strongly dependent on the optical transmission of the photocathode, as discussed in the text.

X-RAY IMAGE INTENSIFIERS

235

surface. This “cross-chord light can be reduced by making the photocathode more light absorbent or by increasing the phosphor radius of curvature. The amount of cross-chord light is also affected by the optical properties of the phosphor and by the smoothness of the surface on which the photocathode is deposited. ”

3. Front Window

X rays are scattered in the front window (Compton effect). For the X-ray energies typically involved (40-120 KeV), the X rays are scattered over a large range of angles, thus causing large area contrast degradation. This effect can be alleviated by lowering the mass of the input window, for example, by using a beryllium window (47).

4. Light Rejections Light that passes through the photocathode from the input phosphor should be absorbed if possible; if not, it can be diffusely reflected back onto the photocathode. The interior tube walls, electrodes, and cathode deposition fixturing should be made nonreflective (48).

5. Output Phosphor and Window The output phosphor-window combination is the principal contributor to contrast degradation for the XRII analyzed in Table I. The construction of this assembly was conventional: the phosphor was deposited on a 1.0mm-thick glass substrate, which was separated by 0.25 mm from a 2.5-mmthick output window. The phosphor was covered with a thin evaporated aluminum film to keep its light from illuminating the photocathode. A major problem is reflections from the four glass surfaces. These may be alleviated by putting antireflection coatings on the glass and by making the phosphor substrate out of partially absorbent glass (the reflected rays must pass through the glass twice, and are thus relatively attenuated) or fiber-optic glass, although these latter options reduce brightness. The value of reciprocal contrast given in Table I is typical of a carefully designed substratewindow combination using an absorbent glass substrate. Additional gains may be made by replacing the entire substrate-window assembly with a fiber-optic plate. The contribution of the output phosphor combination would then be approximately equal to the reciprocal contrast of the phosphor itself, which is on the order of 0.01. With this value, the terms in Table I would yield a total reciprocal contrast of 0.044, and thus a contrast of 22.5.

236

K. G . VOSBURGH ET AL.

The contrast of an XRII with a fiber-optic output has been reported by Bates (49) to be between 27 and 51, depending on X-ray beam hardness, although these numbers are not strictly comparable to those of Table I because the tube studied by Bates was dissimilar in several respects to that investigated by Wilson (for example, Bates’s tube had a demagnification of three, whereas Wilson’s had a demagnification of 11). Once the effects of the output phosphor-window combination are eliminated, the principal contributors to contrast degradation are the input phosphor and the XRII front window. Fiber optics may be employed here as well, as noted by Fenner (50) and studied by Glascock and Davis (51), who recognized that the introduction of a fiber-optic input window would eliminate the degradation of contrast from cross-chord light, as well as alleviating the effects of Compton scattering of X rays in the window. There will still be some “ backscattered ” X-ray photons from the fiber-optic plate, but at a much reduced level than in the conventional geometry. Glascock and Davis constructed an XRII by placing a rare-earth phosphor layer on a commercial light amplifier having a 80-mm-diameterfiber-optic input and a 40-mmdiameter fiber-optic output, and measured the large area contrast. Measurements were made of the relative brightness of the image of a 10%PAR SCREEN 6.0 MILS 65 kVP X RAYS 15- kV LIGHT AMPLIFIER 1/4 in.-THICK Pb DISK

20 -

FIG. 13. Measured large area contrast of a fiber-optic input XRII as a function of the location in the image of the sensor. The input was occluded by a 10% area lead disk of image radius R . After Glascock and Davis (51).

X-RAY IMAGE INTENSIFIERS

237

area-occluding lead disk and of the unshadowed field using both direct photometer readings and calibrated photographic emulsions that were pressed directly onto the output fiber-optic plate. The results from the two techniques agreed. The measured brightness ratio was found to be 150 in the center of the image of the occluding disk, and values in excess of 100 were obtained at more than 0.6 of the disk radius from the disk image center (Fig. 13). A similar experimental XRII was evaluated by Bates (49), although he does not report contrast measurements. Fenner et a/. (50) reported a contrast of 5.7 for an XRII with a fiber-optic input, which was an improvement over the standards of the time (1971). Despite their contrast advantages, fiber-optic input windows will not be extensively used if their costs remain high and if methods are not found to make them more blemish-free. However, the use of fiber-optic output windows is expected to become considerably more prevalent. D . Design ‘‘ Trade-offs’’

Several trade-offs must be evaluated to produce an XRII with optimal sensitivity and brightness gain: 1. Phosphor Thickness As the input phosphor is made thicker, the X-ray absorption probability increases, but the spatial resolution deteriorates because the light spreads laterally to a greater extent before striking the photocathode. As the output phosphor is made thicker, the amount of light generated by the electrons is increased (up to the limit of full electron energy absorption), but the resolution is reduced.

2. Phosphor Optical Transmission The resolution of an input phosphor may be improved by absorbing light within the phosphor, at the expense of lower brightness and signal-tonoise ratio. 3. Photocathode Work Function As the photocathode work function is lowered, it is possible to gain in electron emission, thus improving brightness. However, the emitted electrons will have an increased initial velocity (on the average), which leads to poorer resolution because of the large chromatic aberrations of the electron-

238

K. G. VOSBURGH ET AL.

optical system. A similar effect may be achieved by modifying the spectral characteristics of the input phosphor. 4. Electron-Optical MagniJication

As the image is reduced in size, it will become brighter, but the resolution will become worse due both to electron-optical effects and to the poorer resolution of the output phosphor as evaluated at the object plane.

5 . Electron Accelerating Voltage As the voltage is increased, chromatic aberration effects in the electron optics are reduced, and the final image will be brighter because the electrons gain more energy. However, the thickness of the output phosphor must be increased to stop these more energetic electrons, and thus output phosphor resolution will deteriorate. Present operating voltages (25-30 kV) seem to be a good compromise, particularly in view of the practical difficulties (added insulation, etc.) of increasing the voltage. 6. Output Phosphor Substrate Transmission If the output phosphor substrate is made partially light absorbing, the contrast of the image is improved, at the expense of brightness. VI. FUTURE TRENDS A . Improvements to Conventional Designs

We expect that there will be continued progress in the development of better components for XRII construction. Recent interest in the construction of structured input phosphors (phosphors that “pipe” light, thus avoiding resolution degradation due to lateral light spreading) will certainly result in improved resolution at high spatial frequencies, and may yield better low frequency contrast as well. Theoretically, additional improvements can be made in making thinner (hence, higher resolution) output phosphors. For both phosphors, artifacts and blemishes remain problems. In many commercial XRIIs the photocathode sensitivity is lower than in high-quality phototubes with similar cathode composition, the difference being that in the phototube the photocathode is deposited on a smooth inert glass surface, rather than on a phosphor layer. Additional studies of cathode substrates may yield improvements, although making good cathodes on the surface of

X-RAY IMAGE INTENSIFIERS

239

structured XRII input phosphors may be difficult. The contributions of the electron-optical system to overall resolution can be reduced if the gain of the system is improved, so that a higher photocathode work function can be tolerated. The substitution of fiber-optic plates for clear glass vacuum windows appears worthwhile, particularly for improving image contrast. Although the cost of fiber-optic input windows may be prohibitive, fiber-optic output windows should certainly be more widely used, particularly if the blemish level can be reduced. Additional improvements in imaging performance can be realized by decreasing the electron-optical demagnification, although this may complicate the design of subsequent optical detection devices. A salient problem in medical fluoroscopy is the need to use several alternative light receptors (video, spot film, and cine) to look at a single intensifier output. If a single all-purpose receptor were designed, the optical coupling would be considerably simpler and more efficient. Direct fiber-optic coupling could even be used, giving more than a factor of ten gain in light to the receptor, and this excess light could be " traded-off " to improve XRII performance, as discussed above.

B. Alternative Technical Approaches 1. Diode Electrostatic Devices The simple diode intensifier (Fig. 14a) may be used to make an X-ray image visible, with a moderate gain. Goodson et al. (52)quote gain figures of 20-30 for a light amplifier of this structure, which is considerably lower than required for X-ray applications, given the large size of the output image and consequent intensity losses in coupling subsequent image receptors. As a result, several investigators (53-56)have constructed proximity-focused devices which include channel plate electron multipliers to boost the gain (Fig. 14b). Balter er al. (56)report measurements on a prototype microchannel X-ray converter which is 250 times brighter than a conventional (CB-2) phosphor screen, with a resolution in excess of four line pairs per millimeter. Their system had a usable diameter of 2.5 cm, which would need to be increased to at least 12 cm to be useful in general fluoroscopy. Millar et al. (54) describe such a device. 2. Solid State Image Conversion Panels These devices (Fig. 14c) employ a photoconductive layer to absorb X radiation. The photoconductive layer is electrically in series with an adjacent

240

K. G . VOSBURGH ET AL.

electroluminescent phosphor layer, and the two are contacted by constantpotential electrodes. When an X ray is absorbed in the photoconductive layer, its resistance decreases, which causes an increase in the voltage applied to the adjacent electroluminescent screen, and thus causes an increase in the

FIG.14. X-ray image intensificationsystems that do not employ crossover electron optics. Here P denotes a phosphor, PC a photocathode, CP a channel plate array, and V an applied potential. The systems are: (a) a simple proximity-focused diode; (b) a proximity-focuseddiode with microchannel plate amplification; (c) a solid state imaging panel of the photoconductor-electroluminescent type; and (d) an optically coupled phosphor system.

electroluminescent light output. Szepesi (57) gives encouraging results for these panels in terms of contrast and resolution, but notes that the threshold of response is 10-100 mR/min and that the temporal response is 1-10 sec, which probably precludes medical fluoroscopic applications. 3. X-Ray Sensitive Television Pickups

Several attempts have been made to construct X-ray image intensifiers that provide a video output directly, instead of a light image. Such systems would be lighter and more compact than existing fluoroscopic detectors, and hence more convenient to use. If a sufficiently high quality video signal can be obtained, such a system might be usable for spot films, and thus replace the cumbersome light-coupled systems for all applications that do not require high speed cine recording. Three approaches may be followed. First,

X-RAY IMAGE INTENSIFIERS

24 1

one may absorb the incident X rays directly in a photoconductive video target, which is then read out by a scanned electron beam on its opposite side (58). Such systems were marketed commercially in the 1960s, but were at a competitive disadvantage due to their larger time lag and problems with target decomposition. Secondly, one may modify a conventional XRII structure by replacing the output phosphor with a video target. Such systems would be similar to intensifier video systems currently being marketed, except that a phosphor would be added to absorb the X rays and make a light image for video detection. It is not clear whether such a system can be manufactured with a sufficiently high production yield to be viable commercially. Thirdly, one may use a self-scanning solid state video array (59, 60) to absorb and read out the X-ray pattern. While this may be possible in principle, it will certainly be difficult in view of the large image receptor diameter (15-23 cm) required for medical applications. 4. Optically Coupled Phosphor Systems

An X-ray intensifier can be constructed by otically imaging a phosphor onto a light amplifier (Fig. 1 4 ) . Such systems have been discussed by Webster (61), and systems using these principles have been marketed commercially (62). An advantage of this approach is that the light amplifier can be made smaller than the X-ray pattern, although optical coupling losses are prohibitive if the optical demagnification exceeds a factor of about four. Since the phosphor is separate from the amplifier structure, the amplifier is much simpler to manufacture than a conventional XRII. Also, phosphors may be changed readily for different applications. Disadvantages of the system include relatively large size and the need for rigid construction of the optical system elements. An additional disadvantage is the need to make compromises (particularly in the optical design) between quantum noise level and system resolution. This occurs because very wide aperture optics must be used if the “quantum sink” is to be kept at the input phosphor, and such optics degrade resolution, particularly at the image edges. Optically coupled systems tend to have lower gain than conventional XRIIs; less than 5 % of the light from the phosphor is focused on the light amplifier by the optics for a typical design. On the other hand, the large area contrast of such systems should be superior to conventional XRIIs because some of the XRII contrast loss mechanisms (Table 111) are absent, and the only new component, the optics, will have high contrast. The resolution of optically coupled systems should be roughly equivalent to that of conventional XRIIs, although, in a particular design, high resolution may be sacrificed to achieve low quantum noise and adequate gain (63).

K. G. VOSBURGH ET AL.

242

C . Other Applications

1 . Nondestructive Testing Static film radiography is extensively used in nondestructive testing applications. Many attempts have been made to replace the cumbersome and expensive film process with electronic imaging systems, but the inherently better resolution and contrast of the film images have ensured their predominance. In the past few years, however, XRII systems have been employed successfully in engineering research and test applications that utilize dynamic images (64-66). For example, Stewart (67) describes the use of XRII systems to examine the motion of internal components in operating aircraft jet engines.

2. Radiography If high quality images can be made .by an XRII system with an active receptor greater than 40 cm in diameter, it may be possible to replace large format film radiographic systems with XRII systems yielding a reduced size image. Such small images would be less costly to make and to store than conventional large format films. In a recent study, Vosburgh (68)concluded that such radiographic miniaturization systems were technically feasible (63), but not economically attractive for American medical applications at present. 3. X-Ray Diffraction

It has been demonstrated by Minor et al. (69) that XRII techniques can give radical improvements in the rate of data acquisition in X-ray diffraction studies. This is important in analyzing the structure of crystallized biological materials, which are often unstable in configuration. In contrast to conventional XRII applications, particular care must be taken to reduce spurious noise in the image. SELECTED BIBLIOGRAPHY Without attempting a complete list of works related to X-ray intensification and its applications, we note here a few general publications which have been of use to the authors.

L. Biberman, ed., “The Perception of Displayed Information.” Plenum, New York, 1973. L. Biberman and S. Nudelman, eds., “Photoelectric Imaging Devices,” 2 vols. Plenum, New York, 1971.

S. Hilal, ed,

“

Small Vessel Angiography.” See Kuhl (55).

X-RAY IMAGE INTENSIFIERS

243

R. D. Moseley, Jr. and J. H. Rust, eds, ‘‘ Diagnostic Radiologic Instrumentation-Modulation Transfer Function.” Thomas, Springfield, Illinois 1965. Proceedings of the Society of Photo-Optical Engineers Vols. 35, 43, 56. Palos Verdes Estates, California. Proceedings of the Symposia on Photo-Electronic Image Devices (published by Academic Press as parts of the series Advances in Electronics and Electron Physics Vols. 12, 16,22A, 22B, 33A, 33B). M.Ter-Pogossian, “The Physical Aspects of Diagnostic Radiology.” Harper, New York, 1967.

REFERENCES 1 . J. W. Coltman, Radiology 51, 359 (1948). 2. S. P. Wang, 0. Laudi, H. Lucks, K. A. Wickersheim, and R. A. Buchanan, IEEE Trans. Nucl. Sci. 17, 49 (1970); K. A. Wickersheim et a/., IEEE Trans. Nucl. Sci. 17, 57 (1970). 3. C. W. Bates, Jr, Adu. Electron. Electron Phys. UIA, 451 (1969). 4. E. Storm and H. 1. Israel, Report LA-3753 (1967), distributed by Clearinghouse Fed. Sci. Tech. Inf., N.B.S., US. Dep. Commer., Springfield, Virginia 22151. 5 . D. R. Hudson, J. Appl. Phys. 20, 154 (1949). Transfer Function” 6. See, e.g., “Diagnostic Radiological instrumentation-Modulation (R. D. Moseley, Jr. and J. H. Rust, eds.). Thomas, Springfield, Illinois, 1965. 7. J. Gasper, J . Opt. SOC. Am. 63, 714 (1973). 8. R. K. Swank, Appl. Opt. 12, 1865 (1973). 9. R. K. Swank, J . Appl. Phys. 44, 4199 (1973). 10. R. K. Swank, J. Appl. Phys. 45, 3673 (1974). 11. J. L. Hilton and R. K. Squire, US. Patent 3,225,193 (1961). 12. K. L. Reifsnider and J. J. Brown, Jr., J. Electrochem. SOC. Solid State Sci. Technol. 120, 282 (1973). 13. A. L. N. Stevels and A. D. M. Schrama-de Pauw, Philips Res. Rep. 29, 340, 353 (1974). 14. P. Schagen, H. Bruining, and J. C. Francken, Philips Res. Rep. 7, 119 (1952). 15. 0. Klemperer and M. E. Barnett, “Electron Optics,” Cambridge, London (1971). 16. G. Schwierz, Siemens Forsch. Entwicklungsber. 2, 305 (1973). 17. W. M. Wreathall, Adu. Electron. Electron Phys. 22A, 583 (1966). 18. W. F. Niklas, Am. J . Roentgenol. 85, 323 (1961). 19. W. Kuhl, Proc. SOC. Photo-Opt. Instrum. Eng. 56, 80 (1975). 20. B. J. Mayo and A. W. Bennett, Adu. Electron. Electron Phys. 33A, 571 (1972). 21. K. Schleschinger, IEEE Trans. Electron Devices 16, 284 (1969). 22. M. Verat, L. Guyot, and B. Driard, US. Patent 3,894,231. 23. A. Morsell, U.S. Patent 3,828,192 (1974). See also A. Prondian, T. Azzarelli, and M. Welkowsky, US.Application S/N 388,212 (1974). 24. D. Robbins, R. Enck, Jr., and J. Sackinger, Proc. SOC. Photo-Opt. Instrum. Eng. 23, 23 (1972). 25. J. Vine, Adu. Electron. Electron Phys. 28A, 537 (1969). 26. F. J. Studer and D. A. Cusano, J. Opt. SOC.Am. 45, 493 (1955). 27. L. R. Koller and H. D. Coghill, J. Electrochem. SOC. 107,973 (1960). 28. J. Beesley and D. J. Norman, Ado. Electron. Electron Phys. 22A, 551 (1966). 29. N. A. Diakides, Proc. SOC.Photo-opt. f n w w n . Eng. 42, 83 (1973). 30. S. Pakswer. Electrochem. Technol. 2, 207 (1964). 31. J. D. McGee, R. W. Airey, and M. Aslam, Adu. Electron. Electron Phys. 22A, 571 (1966). 32. P. F. Grosso, R. E. Rutherford, Jr., and D. E. Sargent, 1. Electrochem. SOC.: Electrochem. Technol. 117, 1456 (1970).

244 33. 34. 35. 36. 37. 38. 39.

40. 41. 42. 43. 44.

45. 46.

47. 48.

49.

50. 51. 52. 53.

54. 55.

56.

57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69.

K. G. VOSBURGH ET AL. B. P. Varma and C. Ghosh, Indian J. Pure Appl. Phys. 12, 670 (1974). K. Franz, G . Kochmann, and R. Lahmann, Adu. Electron. Electron Phys. 33A,483 (1972). H. D. Stone, Adu. Electron. Electron Phys. 22A,565 (1966). W. Lehmann, U.S. Patent 2,798,821 (1957). L. R: Koller, J. Opt. SOC.Am. 43,620 (1953). J. S. Prener, personal communication (1975). C. W. Bates, Jr., Proc. SOC.Photo-Opt. Instrum. Eng. 47, 152 (1974). C. E. Catchpole, Adu. Electron. Electron Phys. 22A, 425 (1966). H. F. Webster, personal communication (1975). Made by Dynamic Research Corp., 50 Concord St., Wilmington, Massachusetts 01887. Made by Gamma Scientific, Inc., 3777 Ruffin Road, San Diego, California, 92123. R. R. Law, Proc. I R E 27, 511 (1939). Handbook 89, “Methods of Evaluating Radiological Equipment and Materials,” Section 11. N.B.S., U.S. Dep. Commer., Washington, D.C., 1963. V. C. Wilson and R. E. Argersinger, “ Measurements of Contrast Ratio of General Electric X-ray Image Intensifiers.” Unpublished reports and personal communication (1973). E. Fenner, in “Diagnostic Radiologic Instrumentation-Modulation Transfer Function” (R. D. Moseley, Jr. and J. H. Rust, ed.), p. 160. Thomas, Springfield, Illinois, 1965. W. F. Niklas, in “Diagnostic Radiologic Instrumentation-Modulation Transfer Function” (R. D. Moseley,Jr. and J. H. Rust, eds.), p. 3 12. Thomas, Springfield,Illinois, 1965. C. W. Bates, Proc. SOC.Photo-Opt. Instrum. Eng. 47,512 (1974). E. Fenner, F. Franz, F. Gudden, H. Heinrich, and F. W. Hofman, Adu. Electron. Electron Phys. 338, 1049 (1972). H. H. Glascock, Jr. and V. L. Davis, to be published (1976). J. Goodson, A. J. Woolgar, J. Higgins, and R. F. Thumwood, Adu. Electron. Electron Phys. 33A, 83 (1972). J. Adams, Adu. Electron. Electron Phys. 28A, 471 (1969). I. C. P. Millar, D. Washington, and D. L. Lamport, Adu. Electron. Electron Phys. 33A. 153 (1972). W. Kuhl, in “Small Vessel Angiography, Imaging, Morphology, and Clinical Applications” (S. Hilal, ed.), p. 68. Mosby, St. Louis, Missouri, 1973. S. Baker, J. S. Laughlin, L. N. Rothenberg, N. C. Thomas, and J. Zandmanis, Radiology 110,673 (1974). Z. Szepesi, Proc. SOC.Photo-Opt. Instrum. Eng. 29, 31 (1972). H. Berger and A. L. Pace, Nondestr. Test. (Chicago) 15, 26 (1957). P. K. Weimer, R C A Rev. 36, 385 (1975). G. J. Michon, H. K. Burke, D. M. Brown, and M. Gezzo, Proc. Int. Con$ Appl. ChargeCoupled Devices, San Diego, p. 93 (1975). E. W. Webster, in “Small Vessel Angiography-Imaging, Morphology, Physiology and Clinical Applications” (S. Hilal, ed.), p. 61. Mosby, St. Louis, Missouri, 1973. Old Delft Corporation of America, Fairfax, Virginia. K. G. Losburgh and H. H. Glascock, Jr., IEEE Trans. Biomed. Eng., in press (1976). W. J. Oosterkamp, J. Proper, and M. C. Teves, Philips Tech. Rev. 21,281 (1960). T. D. Elsip and R. J. Lewis, Brit. J. Non-Destr. Test. 6, 1 1 (1964). R. L. Durant, Brit. J. Non-Destr. Test. 12, 125 (1970). P. A. E. Stewart, Proc. Int. Gas Turbine Con$, 20th, Houston, 1975, unpublished. K. G. Vosburgh, “An Assessment of the Storage and Retrieval of Radiographic Images,” Final Rep., Grant No. APR74-20095. US. Nat. Sci. Found. (unpublished) (1975). T. C. Minor, J. R. Milch, and G. T. Reynolds, J. Appl. Crystallogr. 7,323 (1974). See also J. R. Milch, Ph.D. Thesis, Princeton Univ., Princeton, New Jersey, 1974.

AUTHOR INDEX Numbers in parentheses are reference numbers and indicate that an author's work is referred to although his name is not cited in the text. Numbers in italics show the page on which the complete reference is listed. Bergstrom, J., 176(113), 201 Berkowitz, A., 103(80), 107, 110(80), 137 Berndt, W.,155(37), 199 Bertino, J. P., 193( 179), 197(179), 203 Bileon, P., 184(155), 202 Bishop, H.E., 107, 108, 110(88), 137, 138,

A

Aberdam. D., 156(44), 157(65), 200 Adams, J., 239(53), 244 Ageno, M.,30.40 Aitken, D.W.,84(53), 136 Alexander, S. N., 46(11). 135 Allan, G., 156(46), 200 Ambrosio, B. F.,46(10), 135 Amelio, G. F.,18q138, 140), 202 Anderson, J., 161(72), 200 Anderson, L. J., 228, 244 Anderson, S., 145(3), 154(34), 198, 199 Aoi, H.,77(130), 138 Argersinger, R. E., 233(46), 244 Asaad, W.N., 177, 201 Augustus, P. D., 180(141), 202

169, 170(103), 190, 191, 192, 193, 201, 203

Bohn, G. K.,165(94), 169(94), 201 Born, M.,6, 12, 13,40 Bracewell, R., 2, 19, 21, 40 Bradley, D. E., 30, 40 Brodie, I., 115(112), 138 Bronshtein, I. M.,64, 65(31), 66, 69(31, 39, 136

Brown, J. J., 216, 243 Brown, D. M.,241(60). 244 Browne, M.T., 1 4 4 0 Bruining, H.,56(26), 136, 217, 218(14), 243 Bruining, J., 102, 137 Brundle, C. R., 194, 195, 203 Bryan, J. S., 129, 138 Buchholz, W.,44,135 Buttner, P., 155(37), 199 Burckhardt, C. B., 12, 18, 21,40 Burge. R. E., 3, 5, 8, 9, 10, 18, 40 Burhop, E. H.S., 177(117), 193(112), 201 Burke, H.K.,241(60), 244 Burr, A. F.,176, 185(109), 201 Buschmann, E. C., 120(122, 123), 122(122),

B Bagchi, A., 152(27), 199 Baker, S.,239, 244 Banaszak, L. J., 32, 41 Barnett, M.E., 115, 138, 218(15), 243 Bassett, P. J., 180(143a), 181(143b), 18S, 202 Bates, C. W.,Jr., 208(3), 228,236,237,243,244 Baudoing, R., 156(44), 200 Bauer, E., 158(69), 162(79), 163(89), 200,201 Beach, H.W.,117(119), 138 Bearden. J. A., 176, 185(109), 201 Beck, D. R., 179, 202 Becker, G. E., 184, 197(186), 202,203 Beeby, J. L., 150. 199 Beesley, J., 226(28), 228, 243 Bell, A. E., 115(108), 138 Bennett, A. W.,220, 243 Berger, H.,241(58), 244 Berger, J. E., 2, 12, 27, 31, 40, 41

123(123), 138

C

Camp, W.J., 180, 202 Casasayas, F.,109(99), 112,137 Casasent, D., 109(99), 112, 137 Castaing, R., 113, 137 Catchpole, C.E.,229,230, 244 Chambers, A., 185(161), 190(171), 202, 203 245

246

AUTHOR INDEX

Chang, C. C., 162(85), 169(104), 170, 171, 174, 175, 177, 181(106), 184, 196, 197, 198, 200,201,203 Chen, A. C. M., 99, 100, 101, 102(64), 110(59), 136,137 Chen, M. H., 184(154), 202 Chetkin, M. V.. 104(75),137 Christmann, K., 161(74), 194(180),200,203 Chruney, M., 46(14), 60(14), 135 Chung, M. F., 177, 182(144, 145, 146, 147, 148), 201,202 Clarke, T. A., 155(40), 199 Clawing, R. E., 185(163),186, 202 Coad, J. P., 17.7, 201 Cochran, W., 32, 40 Coghill, H. D., 226(27), 243 Coghlan, W . A., 185(163), 186,202 Cogswell, D. G., 52(24), 135 Cohen, M. S., 81(52), 83, 84, 85, 86, 87, 88, 89, 90, 91(52), 92, 93, 97, 98(55), 103(79), 106, 110(55), 136, 137 Collier, R. I., 12, 18,21,40 Collins, M. L., 32, 41 Coltman, J. W., 207, 243 Conrad, H., 161(74), 200 Copeland, P. L., 64,65(34), 136 Cosslett, V. E., 2,40, 113(105), 113(106), 115(106), 137 Crasernann, B., 184(154), 202 Crick, F. M. C., 32, 33, 40, 41 Crick, R. A., 3, 4,40 Cross, L. E., 109,137 Crosswhite, H. M., 106(77), 137 Crowther, R. A., 30,40 Cummins, S . E., 109, 110(94), 111, 112, 137 Curland, N., 122(126), 138 Cusano, D. A., 226(26),243

D Davis, D. W., 177(123), 178(123),201 Davis, V. L., 236, 244 Demuth, J. E., 155(38), 199 de Neufville, J., 99(60), 136 De Rosier, D. J., 30, 33, 39, 40, 41 De Wames, R. E., 163(88),201 Diakides, N. A., 226(29), 227, 243 Dillon, Jr., J. F., 104(73),137

Dodd, S. H., 44,77, 135 Donelli, G., 2, 12, 27, 29, 30, 31, 32, 33, 36, 40,40 Dooley, G. J., 184(153), 202 Dougherty, E. C., 122(126), 138 Doyen, G., 161(74), 200 Drechsler, M., 113(106), 115(106), 137 Duke, C. B., 140, 149(8, 10, 13, 14), 151, 152(27), 153, 154(35, 36), 155(28), 198, 199 Dunham, A., 102(67),137 Durant, R. L., 242(66), 244 DUva, V., 31, 32, 40

E Eckert, Jr., J. P., 44,135 Einstein, P . A., 113, f 37 Eisenhandler, C. B., 8, 40 Ellis, G. W., 84(56), 86(56), 92, 94(56, 125), 95. 96(56), 97, 98(56, 57), 110(56), 120(56, 125), 124(56, 125), 125(56),132(56), 134(125),136, 138 Ellis, W. P., 193(179),197(179), 203 Elsip, T. D., 242(65), 244 Emerson, D. W., 84(53), I36 Enck, Jr., R., 221(24), 225(24), 226(24), 243 Enloe, L. M., 21, 40 Erickson, H. P., 25.40 Erickson, M. P., 9, 25, 40 Ertl, G., 161(73),194(180), 200, 203 Estrup, P. J., 145(5), 161(72), 162(80),199 Everett, R. R., 44(5), 135 Everhart, T. E., 81(46), 82, 83, 84, 136

F Fadley, C. S., 182(151), 202 Fahlman, A., 184(156), 202 Fan, G. J., 103, 137 Farnsworth, H.E., 156(48),162(77, 78), 200 Farrell, H.H.,156(50), 162(50), 200 Fedak, D. G., 156(54),200 Feinleib, J., 99(60), 136 Fenner, E., 235(47), 236, 237, 244 Ferrante, J., 167, 201 Fiermans, L., 157(63, 64). 160(64), 166(96), 167(97), 177(129), 178(131),179(131, 134, 135), 182(150), 183(129), 187(129), 200, 201, '202

247

AUTHOR INDEX

Fischer, T. E., 156(54),200 Fitzgerald, D. J., 81(47), 136 Focht, L. R., 129,138 Forrester, J. W., 44, 47, 135 Fortsmann, F., 155(37), 199 Foxon, C. T., 192(175), 193(175), 197(175), 203 Francken, J. C., 217, 218(14),243 Frank, J., 12, 30, 36, 41 Franz, K., 227(34),228, 244 Fraser, R. D. B., 36, 41 French, T. M.,157(62),200 Frizzell, C. E., 44(9), 135

G Gallon, T. E., 157(61), 180(142a, 143a), 181(143b, 143c), 183(143c), 185(162), 187(143c), 190(171), 192, 194,200, 202, 203 Garrard, D. F., 10, 18.40 Garrod, R. J., 30, 41 Gasper, J., 209, 233(7), 243 Gaubert, C., 156(44), 157(65), 200 Gauthier, Y., 156(44),200 Germer, L. H., 141, 157(66), 158, 200 Gezzo, M.,241(60),244 Ghosh, C., 227(33), 243 Gjostein, N. A., 156(43), 199 Glascock, Jr., H. H., 236, 241(63),242(63), 244 Glenn, W. E., 56(25), 135 Glick, A. J., 2, 41 Glupe, G., 189(167), 203 Gobeli, G. W., 156(47, 55), 200 Goldlischer, L. I., 21, 22, 41 Goodman, R. M.,155(39), 162(76),199, 200 Goodson, J., 239,244 Goto, K., 192, 203 Goulding, F. S., 84(54), 136 Grant, J. T., 170, 172, 184(153), 198, 201, 202, 203 Greenwald, S., 46(1l), 135 Greenwood, T. S., 46, 60, 135 Griffiths, K., 30, 41 Grosso, P. F., 227(32),243 Grove, A. S., 81(47), 136 Guglielmi, F., 2, 12, 27, 29, 30, 33, 40

H Haanjes, J., 116, 138 Haas, T. W., 170(105),172(105), 184(153), 198(190, 191, 192), 201,202,203 Hadley, W. B., 107(90),137 Hagstrum, H. D., 184, 197(186),202, 203 Haine, M.E., 113(105), 137 Hall, C. E., 3, 30, 41 Hamrin, K., 184(156),202 Haneman, D., 1.56(53), 157(58, 59, 60), 200 Hanisch, R. S., 166, 171(93b), 201 Hansen, N. R., 156(53), 200 Hanszen, K. J., 3, 6, 38, 41 Harburn, G., 21, 41 Harris, L. A., 165, 166(93a), 193, 201, 203 Harte, K. J., 120, 122(1263,138 Hartelius, Jr., C. C., 74(41), 75(41), 125(41), 126(41), 127(41), 136 Harvey, D. R., 113(102), 137 Haueter, R. C., 46(11), 135 Hawkes, P.W., 2, 12, 41, 113(134),138 Hayes, J. R., 10, 11, 18, 42 Haynes, S. K., 177(114), 201 Hedin, L., 177(128), 178,202 Heidenreich, R. D., 3, 4, 41 Heidrich, P. F., 109(98), 111(98),137 Helmcke, J. C., 30, 41 Henderson, R. P., 107(88), l08(88), 110(88), 137 Henderson, W. J., 30, 41 Hendrickson, D. N., 178,202 Herbst, J. F., 178(132), 202 Heron, D. L., 157(60),200 Hersh, H. N., 107(90),137 Heynick, L. N., 50(21), 70(38), 74(38, 41), 75, 113(38), 116, 117(21, 38), 125, 126, 127(41), 135, 136 Higginbothom, I. G., 157(61),200 Higgins, J., 239(52),244 Hill, B. H., 109, 110(94),137 Hill, R. H., 201 Hilton, J. L., 216(11),243 Hines, M. E., 46, 60(14), 135 Hobbs, L. W., 108(92), 109(92),137 Holland, B. W., 149(16), 150, 152, 153(31), 154, 193(176), 199,203 Holmes, Siedle, A. G., 81(48), 136 Hoogewijs, R., 177(129),178, 179(131), 183(129), 187(129), 202

248

AUTHOR INDEX

Hooker, M. P., 198(192), 203 Hoope, W., 38,41 Horne, R. W., 2, 12, 27, 40,41 Houston, J. E., 167, ,168, 169, 170(109, 172(109, 198(190), 201,203 Huber, Jr., E. E., 81(52), 83, 91, 136 Hudson, D. R., 208,233(5), 243 Hughes, A. E., 108(92), 109(92),137 Hughes, G. P., 166(93b), 171(93b), 201 Hughes, W.C., 84, 86, 94, 96, 98, 103(80), 107(80), 1lO(56, 80). 115(111), 120, 124, 125, 132, 134,136,137,138 Huskey, H.D., 46(10), 135 Hutter, R. G. E., 118, 138

I Ignatiev, A., 149, 156(45), 199, 200 Illingworth, R., 107(89), 137 Iredale, P., 107(88), 108(88), 110(88), 137, 138 Ishikawa, K., 192(174), 203 Israel, H.I., 208, 243 hey, H.F., 107(84), 137

J Jackson, A. J., 180(143a), 202 Jackson, D. C., 190, 203 Janssen, A. P., 185, 202 Jenkins, L. H., 177, 182(144, 145, 146, 147, 148), 201,202 Jensen, A. S., 60(27), 136 Jepsen, D. W., 155(38), 156(45), 199, 200 Johansen, B. V., 24.41 Johansson, A., 177(128), 178, 202 Jolly, W. L., 178, 202 Jona, F., 152, 156(45), 157(56), 162(86), 199, 200,201 Joyce, B. A., 156(49), 162(87), 192(175), 193(175), 197(175), 200, 201,203 Joyner, R. W., 162(75),200

K Kabler, M. N., 107(86), 137 Kallman, F., 30.42

Kambe, K., 149(11), 199 Katerban, K. K., 38, 41 Kaufmann, R. G., 107(90),137 Kawano, S., 77(130), 138 Kazan, B., 46, 56(13), 59, 135 Kelly, J., 50(22), 61, 70(22, 39), 73, 76(42), 79(22,42), 80(29), 81(42), 110(42), 117, 129, 130, 131, 132(128),135, 137, 138 Kesmodel, L. L., 145(6), 160, 162, 199 Kilburn, T., 44,56(1), 57, 58, 135 Kiss, Z . J., 107(83, 87), 137 Kittel, C., 146(7), 149(7), 199 Klemperer, H., 44(4), 77(4), 135 Klemperer, O., 115, 138, 218(15), 243 Klotz, Jr., T . H., 120(122, 123). 122(122), 123(123), 138 Klug, A., 2, 9, 12, 25, 27, 30, 33, 39, 40, 41 Knight, G., 48(19), 135 Knoll, M., 46, 56(13), 59, 135 Koch, J., 161(74), 200 Koller, L. R., 226(27), 228(37), 243, 244 Koshikawa, T., 192(174),203 Kostroun, V. O., 184(154),202 Kowalczyk, S. P., 177(125, 126, 127), 178(125), 179, 201,202 Krinchik, G. S., 104(76), 137 Kuppers, J., 161(74), 200 Kuhl, W., 219,221(19), 232,239(55), 242, 243, 244 Kuin, P. N., 115(114), 138 Kunz, A. B., 152(27),199

Lagally, M. G., 154(32), 199 Lake, J. A., 12, 30, 40, 41 Lamport, D. L., 239(54), 244 Lander, J. J., 149(17), 156(47), 162(81), 164, 199, 200,201 Landis, D. A., 84(54), 136 Lang, B., 162(75),200 Langer, R., 38,41 Langmuir, D. B., 55, 112, 137 Lannoo, M., 156(46),200 Laramore, G . E., 149(9), 151, 152, 154(35), 180,199,202 Latta. E. E.. 161t741 200 Laughlin, J.. S., 239i56). 244

249

AUTHOR INDEX

Law, R. R., 230(44), 244 Lawson, J. L., 103(80), 107(80), 110(80), 137 LeCraw, R. C., 104(73), 137 Lee, R. E.,74(41), 75(41), 125(41),126(41), 127(41),I36 Lehmann, W., 227, 244 Lemmond, C. Q., 84(56),86(56),94(56, 125), 96(56),98(56), llO(56), 120(56, 125), 123, 124(56, 125), 125(56), 132(56), 134(125), 136,138 Lenslinger, M., 84(54), I36 Lenz, F.,2,3, 5, 6, 23, 25,38, 41 Levi, R., 115(1 lo), 1 38 Lewis, R. J., 242(65), 244 Ley, L., 177(125, 126, 127), 178, 179(126), 201,202 Likuski, R. K.,122(126), 138 Lin, L. H.,12, 18,21,40 Lin, T.P.,99(62), 1 36 Lipari, N. O.,154(35), 199 Lipson, H.,2,41, 42 Lubben, G. J., 116,138 Lukoff, H.,44,135

M McCarthy, J. A., 46(14), 60(14), 135 MacDonald, N. C., 81(46), 82,83,84,136, 185(164), 188(164), 197(164),203 McDonnell, L., 193(176), 197(185), 203 McFeely, F. R., 177(125, 126, 127). 178(125), 179(126),201,202 McGee, J. D., 227(31), 243 McGuire, E.J., 176, 177, 190,201 MacRae, A. U.,156(55), 157(57,67), 200 McRae, E. G., 145(4,5), 150, 159(20), 199 Maguire, H.G.,180(141), 202 Makita, M., 77(130). I38 Marcus, P.M., 155(38), 156(45), 199,200 Markham, R., 2, 12,27,40,41 Marrone, M., 107(86), 137 Marton, L., 2,41 Mason, R., 155(40), 199 Matsuishi, H.,77(130), 138 Mattera, A. M., 155(39), 199 Matthew, J. A. D., 180(142a,143a), 181(143b), 185(162). 202

May, J. W., 157(66), 158,200 Mayer, L., 103,137 Mayo, B. J., 220,243 Mee, C. D., 103,137 Mehlhorn, 177, 189(167),201,203 Meiklejohn, W.H.,103(80), 107(80), 110(80), 137 Merrill, J. R., 166(93b), 171(93b),201 Meyer, F., 159(70), 187(165), 189, 190, 191, 192, 193(168), 194, 196, 198,200,203 Michon, G.J., 241(60), 244 Milch, J. R., 242(69),244 Millar, 1. C. P., 239,244 Millward, G.R., 36,41 Minor, T.C., 242,244 Misell, D. L., 3,4.40, 41 Mitchell, J. P., 81(49), 136 Mollenstadt, G.,38,41 Moody, M. F., 33,41 Moore, C. E.,176, 185(108), 201 Moore, J. S.,61(29), 73(29), 76(42), 79(42), 80(29), 81(42), 84,85, 86,87,88,89,90, 91, 92,93,97,98(55). llO(42, 55), 129(29), 132, 136 Moore, P. B., 33,40 Moos, H.W.,106(77), 137 Morgan, S. P.,99(63),I36 Morrison, J., 149(17), 156(47), 162(80, 81), 199,200 Morrison, J. A., 99(63), 136 Morsell, A., 220(23), 243 Moss, S. C., 99(60),136 Miiller, K.,162(82, 83),200 Mularie, W.M., 168,201 Mulvey, J., 113, 137 Musket, R. G.,167,202 Mutter, W.E.,44(7), 135

w.,

N Nakagoshi, S.,77(130), 138 Nankivell, J. F.,30,41 Neave, J. H.,192,203 Neave, J. P.,162(87), 193(175), 197(175), 201,203 Newberry, S. P., 120, 122(122),138 Ngoc, T.C.,154(32), 199 Nicastri, E. D., 79(43),80(43), 132, 136

AUTHOR INDEX

250

Nicolaides, C. A., 179,202 Niklas, W. F., 219, 243 Nixen, W. C., 113(106), 115(106), 137 Nordberg, R., 184(156), 202 Nordling, C., 184(156), 202 Norman, D. J., 226(28), 228, 243 Norton, J. F., 99(59), 102(65), 110(59), 136, 137 Nuttall, J. D., 181(143c), 183(143c), 187(143c), 202 Nuyts, R., 150, 199 0

Ohlendorf, D. H., 32,41 Oosterkamp, W.J., 242(64), 244 Ovshinsky, S. R., 99(60), 136

Pollard, J. H.,198(189), 203 Pooley, D., 107(88), lOS(88, 91, 92), 109(91, 92), 110(88), 137,138 Possin, G. E., 84(56), 86(56),92(57), 94(56, 57), 95(57), 96(56), 97(57), 98(56, 57), llO(56), 120(56), 124(56), 125(56), 132(56), 136 Possin, G. W., 94(125), 120(125), 124(125), 134(125), 138 Powell, B. D., 197(185). 203 Powell, C. J., 182(149), 202 Prener, J. S.,228, 244 Proper, J., 242(64), 244 Prutton, M., 157(61), 181(143b), 185(161, 162), 200,202 Pugh, E. W., 48(17), 135 R

P Pace, A. L., 241(58), 244 Pakswer, S., 227, 229, 243 Palluel, P., 64, 66, 136 Palmberg, P. W., 162(84), 163(88), 165, 169, 185(164), 188, 197,200,201,203 Paoletti, L., 2, 12, 27, 29, 31, 32, 33, 36, 40, 40

Park, R. L., 140, 198 Parks, H. G., 84(56), 86(56), 94(56, 125), 96(56), 98(56), 110(56), 120(56, 125), 124(56, 125), 125(56), 132(56), 134(125), 136,138 Pehl, R. H.,84(54), 136 Penchina, C. M., 180(142b), 202 Pendry, J. B., 149(12, 15), 152, 153, 154(34), 199 Perdereau, M., 198(188), 203 Pena, W. T., 165, 168, 201 Perlman, M. L., 178(132), 202 Petrie, D. P. R., 113(133), 138 Phillips, J. C., 155, 199 Phillips, W., 107(83), 137 Pittaway, L. G., 99(61), 136 Pitts, E.,41 Pocker, D. J., 198(191),203 Pole, R. V., 109(98), 111(98), 137 Polich, S., 107(90), 137 Pollak, R. A., 177(125, 126, 127), 178(125), 179(126), 201, 202

Radi, G., 6,41 Rajchman, J., 46,47( 12), 48(18). 135 Ranniko, J. K., 21, 37, 40, 41, 42 Rau, P., 161(73), 200 Reifsnider, K., 216, 243 Reimer, L., 31, 41 Remeika, J. P., lM(73, 74). 137 Reynolds, G. T., 242(69), 244 Rhead, G. E., 155, 156, 199 Rhodin, T. N., 149, 150(19), 151(24), 154, 155(24),162(84), 199,200 Riach, G. E., 185(164), 188(164), 197(164), 203 Ritteqnan, M. B., 118,138 Rivibre, J. C., 169, 170(103), 177, 192, 193, 201, 203 Robbins, D., 221,225, 226,243 Robertson, J. E., 44(6), 135 Robertson, W. D., 156(54), 200 Roessner, A., 31, 41 Rogers, K. T., 70(39), 136 Rosati Valente, F., 30, 40 Rosenthal, A. H., 107,137 Rothenberg, L. N., 239(56), 244 Rusch, T. W.,193(179),197(179),.203

s Sackinger, J., 221(24), 225(24), 226(24), 243 Salinger, H.W. G., 117(119),138

AUTHOR INDEX

Sandor, J. E., 81(45), 136 Schade, Sr.,O. H., 112(100), 137 Schagen, P., 217,218, 243 Scheibner, E. J., 165, 180 (139, 140), 201, 202 Scherzer, O., 2, 41 Schlesinger, K., 50, 118, 122, 135, 138, 220, 243 Schlier, R. E., 156(48), 200 Schneider, I., 107(85, 86). 137 Schoonmaker, R. C., 185(161), 202 Schrama-de Pauw, A. D. M., 216,243 Schwierz, G., 218, 219, 243 Segal, R. B., 64,65(31), 66, 69(31, 35), 136 Seiler, H., 80(44), 136 Seiwatz, R., 156(52), 200 Sen, S. K., 182, 202 Sevier, K.,176, 185(111),201 Shalygin, A. N., 104(75), 137 Shamir, J., 18, 42 Shevchik, N. J., 180(142b), 202 Shih, H. D., 156(45), 200 Shimizu, R., 192(174),203 Shirley, D. A., 177(122, 123, 124, 125, 126, 127), 178(122, 123, 125), 179(126), 182(124). 201, 202 Shoulders, K.R., 60,70(39), 136 Sickafus, E. N., 167, 184, 201,202 Slegbahn, K., 175, 176, 184(156), 185(110), 201,202 Siegel, B. M.,8, 38, 40, 42 Simmons, P. J., 113(102), 137 Sinharoy, S., 154(33), 199 Sjostrand, F. S., 30, 42 Smith, D. M.,192, 203 Smith, D. O., 81(52), 83,91(52), 103(71, 78), 104, 105, 106, 109(97), 136,137 Smoliar, G., 44,I35 Snow, E. H., 81(47), 136 Somorjai, G. A., 145(6), 155(39), 156(50), 157(62), 160, 161, 162(50. 75,76), 199,200 Speidel, R., 38, 41 Speliotis, D. E., 74, 85, 98(58), 103(81), 132(58), 136, 137 Spindt, C. A., 115(109), 138 Springer, R. W., 198, 203 Squire, R. K., 216(11), 243 Staehler, R. E., 46, 60, 135 Stark, M., 80( 144), 136 Stehberger, K. H., 64, 136

25 1

Stem, R. M.,154(33), 199 Sternglass, E. J., 64, 65(32), 66, 69(32), 136 Stevels, A. L. N., 216, 243 Stewart, P. A. E., 242, 244 Stone, H.D., 227,244 Storm, E., 208, 243 Stow, R. L., 60(27), 136 Stroke, G. W., 12, 42 Studer, F. J., 226(26), 243 Swank, R. K., 209, 212(9), 214, 215,216, 233(8, 9, lo), 243 Swanson, L. W.,115(108), 138 Szalkowski, F. J., 160, 161, 200 Szedon, J. R., 81(45), 136 Szepesi, Z., 240, 244

T Tamura, T., 77, 148 Tangucci, F., 30,40 Tate, C., 180(143a), 202 Taylor, C. A,, 2, 37, 40, 42 Taylor, N. J., 169, 170(102), 201 Teves, M.C., 242(64), 244 Texari, M.,155(40),199 Tharp, L. N., 165, 201 Themann, M.,3 1,41 Thomas, N. C., 239(56), 244 Thomas, S., 198, 203 Thompson, B. J., 12, 16, 35, 42 Thon, F., 23, 26, 38, 41, 42 Thorenson, R., 46(10), 135 Thornton, P. R., 61(29), 73(29), 79, 80(29), 126(29), 136 Thumwood, R. F., 239(52), 244 Tjutneva, G. K., 104(76), 137 Todd, C. J., 162(84),200 Tokutaka, H.,157(61), 200 Tong, D. S.Y.,151(24), 154, 155(24), 199 Tracy, J. C., 165(94), 166, 169(94), 170(95), 201 Tsutsumi, M.,77(130), 138 Tucker, C. W., Jr., 149(8), 151, 154(36), 199

V

Valentine. R. C., 10, 18, 42 Van Bommel, A. J., 159(70), 200 Vand, V., 32,40

252

AUTHOR INDEX

Vander Lugt, A., 12, 14, 15, 16, 35, 42, 48(20), 135 Van Stratum, A. J. A., 115(113, 114), 138 Varma, B. P.,227(33), 243 Vennik, J., 157(63, 64),160(64), 166(96), 167(97), 177(129), 178(131), 179(131, 134), 182(150), 183(129), 187(129),200,201, 202 Verat, M.,220, 243 Viehaus, H.,162(83), 200 Vine, J., 225, 243 von Bassewitz, D. B., 31,41 Vosburgh, K. G., 241(63), 241(63), 244 Vrakking, J. J., 187(165), 189, 190, 191, 192, 193(168), 194, 196, 198,203 Vredevoe, L. A., 163(88), 201

W Wagner, C. D., 184(155),202 Wang,C. C.T., 116, 118, 122,138 Wan& J. M.,99(59), 102(65,67), 110(59), 136,137 Wang, S. P.,208(2), 243 Warnecke, R., 64,65(33), I36 Washington, D., 239(54), 244 Watson, R. E., 178,202 Watt, B. E., 162(87), 201 Webb, M.B., 154(32), 199 Weber, R. E., 165, 185(164), 188(164), 197(164),201,203 Webster, E. W., 241, 244 Webster, H.F., 229, 244 Weimer, P.K., 241(59), 244 Wells, 0. C., 30, 42 Wensley, J. H.,135, 138 Westerberg, E. R., 74, 75(41), 125(41), 126(41), 127,136 Wheeler, D. J., 44(6), 135

Whelan, M.J., 6, 42 White, G.M.,120(123), 123(123), 138 Wickersheim, K. A., 208(2), 243 Wider, H.,109, 111, 137 Williams, F.C., 44,45, 56, 57, 58, 135 Williams, R.,30, 42 Wilson, D. K., 81(49), 136 Wilson, R. H.,84(56), 86(56), 92(57), 94(56, 57, 125), 95(57), 96(56), 97(57), 98(56, 57), 110(56), 120(56, 125). 124(56,125), 125(56), 132(56), 134(125), 136, 138 Wilson, V. C., 233,234, 236, 244 Wolf, E.,6, 12, 13, 40 Wolter, R. F., 112(115), 132(115), 138 Wood, D. L., 104(73, 74), 137 Wood, E. A., 145,198 Woodruff, D. P.,193(176), 197(185), 203 Woolgar, A. J., 239(52), 244 Wreathall, W.M.,218, 219(17), 243 Wyckoff, M.W., 33, 41 Wynblatt, P.,156(43), 199

Y Yoshioka, H., 6,42 Youtz, P.,44(4), 77(4), 135 Yowell, E. C., 46(10), 135

Z

Zaininger, K. H., 81(48), 136 Zalm, P., 115(113), 138 Zandmanis, J., 239(56), 244 Zehner, D. M.,182(148),202 Zeitler, E., 2, 10, 11, 18, 42 Zimmer, R. S., 153(31), 199 Zulliger, H. R., 84(53), I36

SUBJECT INDEX solid state effects on Auger spectra in,

A

177-181

Accumulation-mode medium, in bulk charge charge storage, 92-99 Adsorption studies, LEED and, 157-162 AES, see Auger electron spectroscopy Amorphous semiconductor storage medium, 99-103

thick overlayers in, 194 valence bands in, 180-181 Auger peaks defined, 164 fine structure and line shape of, 174 Auger transition, defined, 165

readout in, 101-102 Astigmatism, in electron microscopes,

B

15-26

Auger circuit, measurement of, 198 Auger electron spectroscopy, 148, 164-198 applications of, 185-198 Auger current measurement in, 198 back-scattering factor r in, 190-193 basic principles of, 173-1 77 characteristic ionization losses in, 182 chemical environment in, 184-185 Coster-Kronig transitions in, 175 crossover transitions in, 185 cylindrical mirror analyzer in, 169-171 for homogeneous solids, 196-198 energies of Auger transitions in, 176 energy shift and line shape in, 184-185 equivalent core in, 178 experiments in, 165-173 extra features of, 181-184 independent calibration in, 198 KLL notation in, 174-175 lifetime broadening and Coster-Kronig transitions in, 183-184 multiple excitations in, 182 notations used in, 173-176 peaks in, 185-187 plasma losses in, 181-182 practical interpretation of data in,

Beam energy, for electron beam addressed memory, 55 BEAMOS storage tube, electron-optical register for, 124 Bragg’s law, LEED pattern and, 146-148 Bravais lattices, two-dimensional, 144 Bulk charge storage, 81-99 accumulator-mode medium in, 92-99 depletion-mode medium in, 83-92 C Cathode ray phosphor, modulation transfer function and, 299-230 Cathodes, in electron beam addressed memory, 115-116 Cathodoluminescence storage medium, in electron beam addressed memory, 107-109

Coherent optical system, elements of, 13-14 Contrast transfer function, of electron microscope, 23-25 Coster-Kronig transitions, in Auger electron spectroscopy, 175 Crystal lattice micrograph, optical transform

185-187

Of,

quantitative analysis with, 187-188 relaxation effects in, 177-180 retarding field analyzer in, 165-169 semiempirical Auger energy calculations in, 176

31-32

CTF, see Contrast transfer function Curie point writing, for electron beam addressed memory, 103-104 Cylindrical mirror analyzer, in Auger electron spectroscopy, 169-171 253

254

SUBJECT INDEX

D Deflector developments, in electron beam addressed memory, 116-1 19 Depletion-mode medium, charge storage and, 83-92 Depletion-mode MOS target, 85-87 Diode electrostatic devices, vs. X-ray image intensifiers, 239 Disk and drum memories, 48 Dynamic background subtraction, in Auger electron spectroscopy, 167

E EBAM, see Electron beam addressed memory Electron beam@) as analytical tools in surface work, 139-198 in digital information addressing, 43-44 Electron beam addressed memory advances in, 43-135 alternative storage media in, 99-112 amorphous semiconductor storage for, 99-102 background of, 44-51 basic components of, 45 beam energy for, 55 bulk charge storage for, 81-99 cathode life in, 50 cathodes in, 115-116 cathodoluminescence storage medium in, 107-109 Curiapoint writing for, 103-104 deflector developments in, 116-1 19 deflectron-deflector patterns in, 122 depletion-mode MOS target in, 85-87 depletion- vs. accumulation-mode MOS media in, 96 early developments in, 44-47 electron beam erasure/write ZERO in, 101 electron beam writing in, 99-101 electronics of, 50 electron-optical systems for, 112-129 electrostatic deflection in, 118-119 experimental data for, 79-81 experimental memory module for, 129-132

ferroelectric storage media in, 109-1 12 future of, 49-50 inversion-layer current in, 88 Lorentz readout in, 106-107 magnetic storage media in, 103-107 motivation in, 47-51 mucap medium in, 60-65, 77,98 multimodule system for serial operation in, 134 nondestructive readout in, 78 ONE state in, 58 partially destructive readout mode in, 96 photon add electron beam access in, 104-106 Radechon in, 46-47,59-66 readout in, 69-74, 101-102 readout current in, 89-90,93 recent work in, 129-135 relaxed deflection stability requirement for, 124 ‘‘screen lens” in, 127-128 screen lens beam memory concept for, 128 Selectron in, 46 silicon technology in, 50-51 single-channel approach to, 132-135 storage media for, 51-56, 110 surface charge storage for, 56-8 1 three-terminal accumulation-mode medium in, 94 vacuum technology in, 51 Williams tube for, 44-46 write and erase exposures in, 92 write-ONE sequence in, 61-65, 90 write-ZERO process in, 61-68, 90 ZERO diameter in, 91 ZERO state in, 58 Electron beam addressed memory bank, development of, 49 Electron beam erasure/write ZERO, 101 Electron beam writing, in electron beam addressed memory, 99-101 Electron image, “staining theory” in, 2 Electron image defects, in electron microscope, 25-27 Electron micrograph of crystal lattice, 30-32 electron contrast in, 11 of helical structures, 32-35 noise filtering for, 36-37 ’

255

SUBJECT INDEX

optical spatial filtering for, 35-40 phase noise and, 21-23 photograph emulsion response in, 10-12 “projection theorem” for, 30 separation of superimposed images in, 39-40 Electron micrograph analysis applications of, 23-40 Fourier transform in, 2, 17-23 by optical transforms, 1-40 Electron microscope see also Electron micrograph; Electronoptical channel applications of, 23-40 astigmatism in, 25-26 blurred image in, 26 electron image defects in, 25-27 contrast transfer function and, 23-25 image formation in, 2-12 intensity absorption approximation for, 3 mechanical instability and magnetic disturbance in, 26-27 microdefects in, 21 noise filtering for periodical structures in, 36 optical analysis of data in, 18-21 optical processing of data in, 12-23 “optical sandwich” technique in, 21 optical spatial filtering in, 14-17, 35-40 optical system for frequency analysis in, 14-17 periodical structure analysis with, 27-29 photographic recording of electron-optical data in, 9-12 scattering contrast approximation for, 3 zonal filtering of phase images in, 37-38 Electron microscope data, optical analysis Of, 18-21 Electron microscope image, threedimensional reconstruction of, 29-30 Electron-optical channel phase contrast images of, 5-9 scattering contrast images in, 3-5 transfer function of, 3-9 Electron-optical data, photographic recording of in electron microscope, 9-12 Electron-optical image drift, causes of, 26 Electron-optical systems basic limitations of, 112-115

for BEAMOS storage tube, 124 cathodes in, 115-1 19 deflector developments in, 116-1 19 deflectron-deflector patterns in, 122 “disk of confusion” in, 4 in electron beam addressed memory, 112-129 for fly’s-eye lens artwork camera, 123 Fourier transform in, 4 Langmuir limitation in, 112 matrix lines in, 119-129 “screen lens” in, 127-128 for X-ray image intensifiers, 216-226 Electrooptic light valve, in ferroelectric storage media, 109-110 Electrooptic spatial filters, in electron beam addressed memory, 1 1 1 Equivalent core, in Auger electron spectroscopy, 178 Ewald construction, LEED pattern in, 146-148

F Ferroelectric storage media, in electron beam addressed memory, 109-112 Fly’s eye lens artwork camera, electron optical system for, 123 Focus uniformity, in X-ray image intensifier, 218-22 1 Fourier transform of electron micrographs, 17-23 of electron optical system, 4 Frequency analysis, optical systems for, 14-17 Fresnel-KirchhotT integral, 13

H Helical structures, electron micrographs of, 32-35 High resolution images, interpretation of, 1-2 1

Image intensifiers, X-ray, see X-ray image intensifiers

256

SUBJECT INDEX

Input phosphor choice of materials for, 207-209 defined, 207

K

energy calibration for, 171-173 resolution of, 169 Lenses, matrix, see Matrix lenses Lorentz readout, in electron beam addressed memory, 106-107 Low energy electron diffraction, see LEED

Kerr effect, in electron beam addressed memory, 103-104 KVV Auger transitions, 180

L Langmuir limitation, in electron-optical systems, 112-113 Lattice, defined, 139 LEED (low energy electron diffraction), 140-164

adsorption studies with, 157-162 capacities of, 163-164 chemisorption and, 160-161 on clean metal surfaces, 155-157 for coadsorption and catalysis studies, 161-162

experiments with, 141-142 future prospects for, 163-164 inelastic scattering for, 151 in insulators, oxides, and alkali halides, 157 miscellaneous applications of, 162-164 new surface properties and, 141 results obtained with, 155-164 in semiconductors, 156 surface structure analysis with, 148-155 LEED-AES instrument, resolution of, 169 LEED instrument, 141-142 LEED pattern Bragg’s law and, 146-148 definitions and notation in, 142-145 diffraction conditions for, 146-148 Ewald construction and, 146-148 geometrical interpretation of, 142-148 multiple scattering theory and, 150-154 periodicity of, 159 LEED spot intensities, interpretations of,

M Magnetic film memory, combined photon and electron beams in, 104-106 Magnetic storage media, for electron beam addressed memories, 103-106 Mainframe memories, 48 Matrix lenses, 119-129 beam-limiting aperture for, 126 beam memory concept and, 128 “screen lens” and, 127-128 spherical aberration for, 125 Memory technologies, availability of, 48 Metal-insulator-semiconductor sandwich, storage of positive charge in, 81 Metal oxide semiconductor sandwich, charge storage in, 82-84 Microcapacitor (mucap) storage medium, 60-65

Modulation transfer function, in spatial resolution, 209-212, 214-215, 229-230, 232

MOS transistor, 82-85 MTF, see Modulation transfer function Mucap storage medium, 60-65, 77, 98 lenslets in, 74-76 readout in, 69-74 usage mode for, 75-79 Multiple scattering theory, in LEED pattern interpretation, 150-154

N Noise filtering, for periodical structures, 36-37 Nondestructive readout, in electron beam addressed memory, 79

148- 155

LEED theory applications of, 154-155 evolution of, 149 LEED-type Auger spectrometer

0

Optical processing of electron microscope data, 12-23

SUBJECT INDEX coherent systems in, 13-17 purpose of, 12 Optical spatial filtering, in electron microscope, 35-40 Optical system aberrations and lens aperture in, 16 for frequency analysis and spatial filtering, 14-17 Optical transforms, electron micrograph analysis by, 1-40 Outputphosphors, for X-ray imageintensifiers, 226-23 1 Output phosphor-window combination, in X-ray image intensifiers, 235-237

257

“Projection theorem,” in electron micrograph analysis, 30

R Radechon (barrier grid storage tube), in electron beam addressed memory, 46.59, 69 Readout, in amorphous semiconductor storage, 101-102 Relaxation effects, in Auger electron spectra, 177-180

P

S

Partially destructive readout mode, in electron beam addressed memory, 96 PEBA, see Photon and electron beam access Periodical structures analysis of with electron microscope, 27-29 ’ noise filtering for, 36-37 Phase contrast images, for electron optical channel, 5-9 Phase images, zonal filtering of in electron microscope, 37-38 Phase noise, in electron micrographs, 21-23 Phosphor input, see Input phosphor modulation transfer function for, 229-230 thin film, 226 for X-ray image intensifiers, 226-23 1 Phosphor curvature, 234-235 Phosphor optical transmission, 237 Phosphor performance conversion factor for, 209 measurement of in X-ray image intensifiers, 2 14-2 15 spatial resolution in, 209-215 X-ray absorptionand quantum noisein,212 in X-ray image intensifiers, 209-214 Phosphor systems, optically coupled, 241 Phosphor thickness, 237 Photocathode work function, in X-ray image intensifiers, 237-238 Photon and electron beam access, in electron beam addressed memory, 104-106 Potential “well,” in Williams storage, 56-57

Scatteringcontrast images, for electron-optical channel, 3-5 Screen lens technique, in electron beam addressed memory, 74-76, 128 Selectron, in electron beam addressed memory, 46-47 Signal-to-noise ratio, in storage media, 52-54 Space lattice, defined, 139 Spatial filtering of electron image, 3 5 4 0 optical systems for, 14-17 Spatial resolution, modulation transfer function for, 209-215 Staining theory, in electron image formation, 2 Storage media alternative, 99-112 amorphous semiconductor, 99- 103 beam energy of, 5 destructive and nondestructive readout in, 54 discrimination and signal-tohoise ratio in, 52-54 for electron beam addressed memories, 5 1-56 in situ gain for, 55 performance of, 110 requirements for, 51-55 types Of, 55-56 volatility of, 51-52 write/read/erase cycling of, 52 Superimposed images,separation ofin electron micrographs, 39-40

258

SUBJECT INDEX

Surface as lattice defect, 139 origin of, 140 structure of, 140 Surface charge storage for electron beam addressed memories, 56-58 microcapacitor (or mucap) storage medium in, 60-65 Williams storage in, 56-58 Surface research see also LEED electron beams as analytical tools in, 139-198 LEEDinstrumentsand patterns in, 141-148 T

Thermoplastic layers, for X-ray image intensifiers, 227-228

V Volatility, of storage media, 51

W Williams tube, for information storage, 44-49, 56-58 Write-ONE process, 63-65 Write-ZERO process, 61-63, 65-68 " h

X-ray image, phosphor screen for, 205 X-ray image intensifiers, 205-242 absorption and noise measurement in, 215-216 alternative technical approaches in, 239-291 blur radius in, 221 brightness uniformity in, 221-225 brightness measurement for, 229 cathode space zooming in, 225 contrast in, 233 conversion factor for, 231-232 defined, 206

design improvement in, 238-239 design of by computer, 217-218 design trade-offs in, 237-238 and diode electrostatic devices, 239 electron accelerating voltage for, 238 electron-optical distortion in, 222-223 electron-optical magnification in, 238 electron optics for, 216-226 electrophoretic screens in, 227 fiber-optic phosphor substrate in, 220 focus uniformity for, 218-221 front window in, 235 future trends in, 238-242 graininess in, 23 1 input phosphors for, 207-209 Langmuir Rotation technique for phosphors in, 228 large area contrast in, 23C23 I light reflections in, 235 mesh electrodes in, 220 modulation transfer function in, 209-215, 229-232 multiple-field and zoom types, 225-226 nondestructive testing with, 242 nonspherical photocathode surfaces in, 219-220 nonuniformity of cathode potential in, 220-22 1 optically coupled phosphorsystemsand,241 output phosphors in, 226-23 1 output phosphor-window combination in, 235-237 performance of, 231-238 performance measurement for, 229-23 1 phosphor content in, 233-234 phosphor curvature in, 234-235 phosphor optical transmission in, 237 phosphor performance and, 209-216 phosphor thickness in, 237 photocathode work function for, 237-238 in radiography, 242 resolution in, 232 screens fabricated with photosensitive polymers in, 227 spray technique for phosphors in, 228 schematic diagram of, 206 tolerance for field flatness in, 221 triodes and tetrodes for, 219 VS. solid state image conversion panels, 239-240

259

SUBJECT INDEX

X-ray absorption and quantum noise in, 2 12-2 14 in X-ray diffraction studies, 242 and X-ray sensitive television pickups, 240-241 X-ray sensitive television pickups, 240-241

A 6 7 C 6 D 9

€ 0 F G H 1 1

1 2 3 4 5

XRII, see X-ray image intensifiers XVV Auger transitions, 183

2 Zonal filtering, in electron microscope, 37-38

259

SUBJECT INDEX

and quantum noise in, ThisX-ray Pageabsorption Intentionally Left Blank 2 12-2 14 in X-ray diffraction studies, 242 and X-ray sensitive television pickups, 240-241 X-ray sensitive television pickups, 240-241

A 6 7 C 6 D 9

€ 0 F G H 1 1

1 2 3 4 5

XRII, see X-ray image intensifiers XVV Auger transitions, 183

2 Zonal filtering, in electron microscope, 37-38