Nano Lithography and Etching Technologies Y.J. Chan, C.K. Lin, H.C. Chiu and S.C. Yang Department of Electrical Engineering National Central University
Solid State and Microwave Laboratory
The Semiconductor Device Revolution
Solid State and Microwave Laboratory
Moore’s Law
Solid State and Microwave Laboratory
Integrated Circuits (ICs) Scale of Integration In practice, many gates are manufactured on a single IC chip. Although there are no universally accepted definitions for level complexity, we define the level of complexity as follows
•
SSI (Small-Scale Integration)
1-10 gates on a chip
Simple gates, flip-flops, decoders, multiplexers, etc.
•
MSI (Medium-Scale Integration)
10-100 gates on a chip
Counters, shift-registers, 4-bit adders, etc.
•
LSI (Large-Scale Integration)
100-1000 gates on a chip
ALUs, simple microprocessors, higher-bit adders, etc.
•
VLSI (Very-Large-Scale Integration)
• •
ULSI (Ultra-Large Scale Integration) above 10000 gates on a chip WSI (Wafer-Scale Integration)
1000-10000 gates on a chip ALUs, 8-bit microprocessors, microcomputers, Digital signal processors, etc.
Solid State and Microwave Laboratory
Lemma of New Technology “ The principal applications of any sufficiently new and innovative technology always have been — and will continue to be — applications created by that technology ” — Herbert Kroemer
Solid State and Microwave Laboratory
Examples • The Transistor（1947） – was not just a replacement for vacuum tubes, it created the modern computer and the new industrial revolution.
• The Semiconductor Laser（1962） – has revolutionized the optoelectronics technology, it created optical fiber communication, CD, and DVD.
• The Nonvolatile Semiconductor Memory（1967） – has revolutionized the information storage technology, it created numerous portable electronic products. Solid State and Microwave Laboratory
From: S.M. Sze
Solid State and Microwave Laboratory
From: S.M. Sze
Computer Technology Development
Solid State and Microwave Laboratory
From: S.M. Sze
Progress in Microelectronics Year
1959
19701970
1999
Ratio
Design Rule (μm)
25
8
0.18
140↓
VDD (V)
5
5
1.5
3↓
Wafer diameter (mm)
5
30
300
60↑
Devices per chip
6
2 × 103
2 × 109
DRAM density (bit)
－
1k
1G
Nonvolatile memory density (bit)
－
2k
256 M
> 105 ↑
Microprocessor clock rate (Hz)
－
750 k
800 M
> 103 ↑
Transistor shipped / year
107
1010
1017
1010 ↑
Average transistor price ($)
10
0.3
10-6
10-7 ↓
Solid State and Microwave Laboratory
3 × 108 ↑ 106 ↑
From: S.M. Sze
Fundamental Challenge — Lithography • Wavelength limit of optical lithography – Can 193 nm ArF support 100 nm generation – λ ≈ minimum feature length
• Nonoptical lithography techniques – – – –
EPL EUV EBDW IPL
• Lithography-independent nanotechnology – Edge-defined MOSFET – Self-assemble – Self-organization Solid State and Microwave Laboratory
From: S.M. Sze
Economic Challenge
Solid State and Microwave Laboratory
From: S.M. Sze
Intellectual Power Rules the IT Society
Source of Power
Muscle Violence Money
2nd Wave Muscle
Money Wealth
IT Revolution
3rd Wave
The Rising Second Wave
Muscle
on i ut l vo Re al t gi Di ital
Money
Mind Mind
Mind
(Knowledge)
g Analo Wave
R
1980 2010
Solid State and Microwave Laboratory
ig D t s F ir C
>
c
1st Wave
Industrial Revolution
Se
Agricultural Revolution
1990
2000
The IT Revolution
Semiconductor Revolution
(Intelligence) ＝ (Size)×(Cost)×(Power)
IT Revolution
Figure of Merit
•1948: Invention of the Transistor •Digitization of Information •Moving from a Leading Industry to the Soil for all Industry
Internet Revolution •Self-developing Information System •An Ownerless Information System
Mobile Revolution •Ultimate Personalization of Information •Accelerates Human-oriented IT
Solid State and Microwave Laboratory
TOP 30 WORLD MARKETS IN YEAR 2020 Market
Sales ($Billions)
Market
Sales ($Billions)
★
Portable Data Communications
630.
Ultra-thin Monitor
170.
★
★
PC
470.
IC Card
165.
★
★
Mobile Phone Service
380.
Ground-Wave Broadcasting
160.
★
★
CPU
300.
DNA Agricultural Products
160.
★
Digital Contents Products
270.
Multi-Purpose Communication Equip.
155.
★
★
Magnetic Memory
250.
Semiconductor Equip.
150.
★
★
Electronic Commerce
250.
Electrical Vehicle
150.
★
Network Information Service
230.
Wall Ultra-thin TV
145.
★
★
High Density Mag. Memory Mat.
230.
Mobile TV
140.
★
★
Systems-On-Chip
210.
Direct inject. Vehicle
140.
★
Home Medical Equip.
210.
ITS Equipment
140.
★
Internet
200.
DNA Processed Food
135.
★
CATV
200.
LCD
120.
Intelligent Transportation Syst.
190.
Clone
115.
Agents Software
180.
Fuel-Cell Car
110.
★
★
Microelectronics related 22 Markets: $5 trillions.（Nikkei Business 1999）.
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The IC manufacturing
Solid State and Microwave Laboratory
Semiconductor Manufacturing Process
Solid State and Microwave Laboratory
Semiconductor Manufacturing Process
Solid State and Microwave Laboratory
Semiconductor Manufacturing Process
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Semiconductor Manufacturing Process
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IC Packages DIP
PGA
Dual Inline Package
SOJ
Pin Grid Array
PLCC
Small outline J-Leaded Package
Plastic Leadless Chip Carrier
SOIC
Small Outline IC
PQFP
Plastic Quad Flat Package
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TSOP
Thin Small outline Package
BGA
Ball Grid Array
高科技產業發展趨勢
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Functional device scales Nano-scale
Micro-scale
SETs GMR layers nanotubes
quantum dots in lasers
atoms
transistors
molecules
0.1
1.0
Field emitters
10
100
Nanometers Solid State and Microwave Laboratory
1000
10,000
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Atomic Image by Scanning Tunneling Microscopy
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Atomic Image by STM
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微影(lithography) 之定義 以光子束、電子束或離子束經由光罩(Mask) 或直接對晶圓上之光阻(resist)照射，使光阻產 生極性變化、主鏈斷鏈或主鏈交接等化學作 用，經顯影後將光罩或直寫之特定圓案轉移至 晶圓上。
Solid State and Microwave Laboratory
微米-微影技術(Micro-lithography)
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微影(lithography) 之定義 TSMC 0.15 CMOS Technology
一個製程所需要之微影次數或所需要光罩之 數量來代表其半導體加工製程的難易程度 Solid State and Microwave Laboratory
微米-微影技術(Micro-lithography)
Commercial Step and Scan Exposure System
Solid State and Microwave Laboratory
微米-微影技術(Micron-lithography)
低壓及高壓汞(Hg)或汞-氙(Hg-Xe)弧燈(ArcLamp)在近紫外光波長範圍(350~450nm)有2 條光強度甚強之發射光譜線
1. 436 nm ---- G line 2. 365 nm ---- I line 解析度 1~0.5 微米
Solid State and Microwave Laboratory
微米-微影技術(Micron-lithography)
波長248 nm氟化氪(KrF)準分子雷射，解析度在 0.25~0.18 um
波長193 nm氟化氬(ArF) 雷射，解析度在 0.18 ~0.15um
末代光學之波長157 nm氟 (F2) 雷射，解析度在 0.1 um
Solid State and Microwave Laboratory
光學微影系統的解析能力
Depth of Focus=λ /2(NA)2 其中， NA為光學微影系統之數值孔徑(numerical aperture)；k1為一數值，與設備系統、光阻製程、光罩類 型、特殊技術之運用等有關。由上式可知：波長愈短時則 解析度愈佳，而設備的成本與光源的波長成反比，因此若 想得到較小的圖案定義能力，勢必要投資更多的資本購置 波長更短的微影設備。當所採用的光源波長固定時，該設 備之解析能力有一定範圍
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科學發展月刊
Phase Shift Mask Using the phase interference to improve the diffraction and resolution
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科學發展月刊
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光子與光學儀裝工程師學會(The Society of Photooptical instrumentation Engineers, SPIE)出版之光學 工程報導指出:
半導商業化量產之微影解析度將停留在0.15~0.13 um 相當長之時間，因為線路更細線化、晶片更微小 化，相當不易，且不易降低量產成本
Solid State and Microwave Laboratory
Molecular Circuit Micro-machine
MEMS
IR Filter
static electricity motor
Solid State and Microwave Laboratory
Solid State and Microwave Laboratory
Solid State and Microwave Laboratory
Solid State and Microwave Laboratory
Carbon Nanotube Field-effect Transistors
Solid State and Microwave Laboratory
Solid State and Microwave Laboratory
奈米微影 (Nano-lithography Techniques)
To circumvent the limitation caused by the diffraction effects of visible light, the following are being developed • X-ray lithography • Electron Beam Lithography
Solid State and Microwave Laboratory
Trend in DRAM production
Solid State and Microwave Laboratory
X-ray lithography
Why is this radiation of interest in lithography? In short, because of its ability to define very high resolution images in thick materials. The resolution comes from the extremely short wavelength, of the order of 0.01-1.0 nm, and the high penetration ability.
Solid State and Microwave Laboratory
X-ray lithography
Solid State and Microwave Laboratory
X-ray lithography X-ray 之來源 1. 電子束撞擊金屬靶極 : 電子束撞擊金屬靶極X-光：傳統X-光 是在真空中以電子束撞擊金屬靶(Cu, Mo, A1等)而得。
2. 同步幅射X-光:電子或帶電粒子進行加速度或減速度運動時， 即產生輻射。電子繞圓周運動，方向改變，速度減慢，此時有負 加速運動，可產生輻射，釋出能量。但因受磁場作用力增速，可 維持速度不變。電子運動速度較低時，幾乎在任何方向皆產生輻 射。速度接近光速，即相對論速度時，在電子圓周運動平面沿其 切線方向，有輻射產生，發散度甚低，且限制在一小張角、錐體 形之範圍內。此輻射稱之同步輻射，波長涵蓋範圍甚廣，其中含 X-光
Solid State and Microwave Laboratory
X-ray 之來源 3. 雷射誘發脈衝電漿X-光 :釔鋁紅榴石(Nd:YAG, Neodymium: Yttrium Aluminum Garnet)雷射，因其脈衝能量、脈衝間隔、功 率等能符合誘發電漿之需要。其未處理波長為1064奈米，可利用 倍頻器使操作波段改變，如二倍頻則為532奈米，目前最高有5倍 頻212.8奈米。 常見誘發電漿原理是利用雷射聚焦於板狀靶材，雷 射提供能量，切除靶材表面的原子，並使原子離子化 (Ionization)，在靶材表面形成一電漿柱(Plume)。雷射不斷提供能 量，並與電漿柱耦合(Coupling)，使電漿柱升溫並擴大。在電漿柱 溫度夠高時，可激出原子K, L內層軌域之電子，當外層軌域之高 能電子填補此缺位時，釋出對應之能量，發出X-光
http://pilot.mse.nthu.edu.tw/micro/ Solid State and Microwave Laboratory
X-光微影半陰影效應
非同步輻射X-光，或稱點光源，因準直性 不佳，會在圖罩上圖案邊緣擴散(Diffusion)， 在阻劑表面產生模糊陰影。如晶圓因應力、 受熱等而變型，圖罩至阻劑表面之間隙(Gap) 較設計間隙為大，半陰影效應將更嚴重。半 陰影效應常以二參數表示。一為半陰影模糊 (Penumbral Blur) p，二為側向偏移(Lateral Shift, Run-Out) d。同步輻射X-光準直性良 好，在平坦晶圓上，幾無半陰影效應。但在 不平之變型晶圓上，仍有輕微半陰影效應。
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同步幅射X-光
A Picture of CNTech Beamlines Solid State and Microwave Laboratory
A Picture of CNTech Beamlines
Solid State and Microwave Laboratory
X-ray lithography 優點： 1. 波長短，解像度高，具有奈米製程能力。 2. 對0.10 ~ 0.05微米線幅而言，其繞射現象甚小，可以忽略。 3. 聚焦深度大，製程寬容度佳。 4. 穿透能力強，圖罩上的污染、微粒不會轉印在晶圓上。可用較厚 的阻劑，適用深寬比(Aspect Ratio)甚大之圖案。 5. 可大範圍的有效照射。 6. 晶圓在X-光波長之折射率趨近於1，近於空氣或真空，X-光可近 直線穿透晶圓，無駐波效應，故不需抗反射塗佈。 7. 小型同步輻射X-光功率高，光準直性甚佳，幾乎無半陰影效應， 且可開出多道光束線，成本降低，可與光學照射競爭。
Solid State and Microwave Laboratory
X-ray lithography
缺點： 1. 傳統金屬靶及雷射誘發X-光光源功率低，準直性差。 2. 傳統金屬靶及雷射誘發X-光有半陰影效應。 3. 縮小步進機製作不易。 4. 晶圓對準問題較不容易解決。 5. 圖罩製作不易。 6. X-光能量高，會加熱圖罩。
http://pilot.mse.nthu.edu.tw/micro/ Solid State and Microwave Laboratory
Solid State and Microwave Laboratory
E-beam lithography
電子束微影技術乃利用帶高 能量的電子群，經電磁裝置控制 方向後，照射於塗布感光性材料 (阻劑)的基板上，此時電子與阻 劑產生化學反應，在經過烘烤及 顯影步驟後，使阻劑內有／無電 子束反應的區域得以被區隔，阻 劑圖案因而顯現出來
Solid State and Microwave Laboratory
E-beam lithography
電子源 1. 熱游離發射(Thermionic Emission) 通常以鎢或六硼化鑭(LaB6)單晶體為電子源將該 材料置於陰極並且直接加熱，而所產生的電子束經 由電場加速後獲得能量。鎢電流亮度(A/cm2 sr)低， 真空度要求低(10-6 torr)，壽命短。六硼化鑭之電流 亮度較鎢高，真空度要求亦較高(10-8 torr)，壽命較 長，故目前廣為使用。
Solid State and Microwave Laboratory
E-beam lithography 電子源 2.場發射(Field Emission) 使用形狀尖銳的材料，並置於高電場環境下，所 以非常適合產生直徑極小的電子束，場發射以電場 吸出電子，較熱離子之電流亮度高，真空度要求亦 較高(10-9 ~ 10-10torr)，壽命亦長。較新的發展是使 用鋯/氧/鎢(Zr/O/W)合金以熱(Thermal)或冷(Cold) 場發射提供電子束，較LaB6之電流亮度可提高100 ~ 1000倍之多。 。
Solid State and Microwave Laboratory
E-beam lithography 電子散射效應
Solid State and Microwave Laboratory
Filament types
source type
brightness [A/cm²/sr] ~ 105
tungsten thermionic LaB6 thermionic ~ 106 thermal (Schottky) ~ 108 field emitter cold field emitter ~ 109
source energy spread Vacuum size [eV] [torr] 25µm 2-3 10-6 10µm 20nm
2-3 0.9
10-8 10-9
5nm
0.22
10-10
SPIE HANDBOOK OF MICROLITHOGRAPHY, MICROMACHINING AND MICROFABRICATION Volume 1: Microlithography, Chapter 2.2
Solid State and Microwave Laboratory
Beam Energy Influence 100 keV + Small scattering in resist + Small proximity effect
– High beam damage – Strong sample heating
20 keV + Small beam damage + Small sample heating
– Scattering in thick resist – Strong proximity effect
+ Best electron-optical performance 2 keV + No beam damage – High scattering in resist + No proximity effect – Needs very thin resists + High throughput (high resist sensitivity)
Solid State and Microwave Laboratory
Beam-blanking
Beam-blanker off
Beam-blanker on
Filament Anode +250V
Beam-blanker Aperture
Blank Frequency = 10MHz Solid State and Microwave Laboratory
GND
Beam deflection - electrostatic and magnetic Either magnetic or electrostatic fields can be used to focus electrons just as glass lenses are used to focus rays of light. Electro-magnetic
Electro-static
F = q · (E + v × B) (Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithograph 1999, SPIE's International Symposium on Microlithography 14-19 March, 1999; p. 22)
Solid State and Microwave Laboratory
EBL Writing Strategies Round (Gaussian) beam
Shaped beam
versus
Raster scan
Vector scan versus
(Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithogr 1999, SPIE's International Symposium on Microlithography 14-19 March, 1999; p.63)
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E-beam lithography 常用之光阻
Solid State and Microwave Laboratory
The Tri-layer PR System and The Dose Test
LO100nm
PMMA2% 1.0
Co-PMMA Normalized Development Depth(%)
HI500nm
LO180nm
PMMA5%
PMMA2 PMMA4 CoPMMA
0.8
0.6
0.4
0.2
0.0
0
50
100
150 2
Electron Dose(uC/cm )
Solid State and Microwave Laboratory
200
E-beam lithography 鄰近效應(proximity effect)
Solid State and Microwave Laboratory
E-beam lithography 鄰近效應(proximity effect) 修正方法
Solid State and Microwave Laboratory
Raith-150 e-beam Writer
Solid State and Microwave Laboratory
Raith-150 e-beam Writer - Beam resolution (30µm aperture) : 2nm @ 20 KeV - 150pA, 4nm @ 1 KeV - 70pA => > 3000 A/cm² beam current density - Energy range 200 eV - 30 KeV, continuous - Probe current 4 pA - 10 nA ( 6 x electromagnetic aperture selection ) probe current drift < 0.5 %/h - Electrostatic beam blanker >10 MHz switching time - Distortions << 50nm (in 100µm field)
Solid State and Microwave Laboratory
The Column of the Raith-150
Solid State and Microwave Laboratory
Raith-150 Writing Stage
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Focussing - Contamination Dots
top view
side view
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Solid State and Microwave Laboratory
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The Profile of 3-layer Pr System
Solid State and Microwave Laboratory
Improved Device Breakdown Voltage by using Selective Gate-Drain Recess
Solid State and Microwave Laboratory
≈ 2 mm
MMIC-HEMT with 200nm-gates fabricated by mix & match technique, ETH Zürich
200 nm
Monolithic integrated microwave oscillator working at 60 GHz (!), ETH Zürich
Solid State and Microwave Laboratory
Solid State and Microwave Laboratory
Solid State and Microwave Laboratory
Etching Process !Basic Concepts of Etching !Wet Etching Process !Dry Etching Process !Summary
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Pattern Transfer Methods Lift-off Process (a) Define resist
Etching Process (a) Deposit layer
Resist
Substrate
Substrate
(b) Deposit layer
(b) Define resist
Substrate
Substrate
(c) Strip resist Substrate
(c) Etch layer
Substrate
Solid State and Microwave Laboratory
(d) Strip resist
Substrate
Introduction • After deposition, thin films are usually selectively removed by etching to leave the desired film pattern • Parts of the substrate may be etched to form trenches for capacitors or isolation structures • The mask is usually photo-resist or oxide/nitride may be used as hard mask • Multi-layer structures can be etched sequentially using same masking layer • Etching can be done in either “wet” or “dry” environment
Light Mask photoresist
photoresist
Deposited film
Deposited film
Deposited film
Substrate
Substrate
Substrate
Film deposition
Photoresist coating
Exposure
Etch mask
Deposited film
–Wet etching uses liquid etchants with wafers immersed in etchant solution. –Dry etching uses gas phase etchants in a plasma.
Substrate Development
Substrate Etching
Substrate Resist removal
Wet Etch chemical process only Dry Etch chemical and physical (sputtering) process
Solid State and Microwave Laboratory
Basic Concepts Diffusion Reactants
Diffusion
Diffusion Boundary layer
Reaction Products
Reaction Diffusion Film
Substrate Etching in principle is very similar to chemical vapor deposition (CVD) except that –material is removed instead of deposition • Etching process consists of three steps –Mass transport of reactants (through a boundary layer) to the surface to be etched –Reaction between reactants and the film(s) to be etched at the surface –Mass transport of reaction products from the surface through thesurface boundary layer •Film etch rate determined by any of three steps—reaction rate limited preferred –Allows the control of etch rate with temperature •Etching is usually done using liquid phase or gas phase reactants –liquid phase (wet) etching—reaction products must be soluble in solvent or gaseous –gas phase (dry) etching—reaction products must have be gaseous or have a low sublimation temperature Solid State and Microwave Laboratory
Isotropic & Anisotropic
b. anisotropic
More directional etching
a. isotropic
Etch Directionality
Measure of relative etch rates in different directions usually vertical vs. lateral
Isotropic Etching
Etch rates are same in all directions. It is usually related to chemical processes
Isotropic Etching
Highly directional etching with different etch rates in different directions. It is usually related to physical processes such as ion bombardment and sputtering
• Anisotropic etching is the prefered process because lithographically defined feature sizes are transferred by etching to the underlying films – Advantageous for today’s shrinking feature sizes • Vertical structures may cause step coverage problems during deposition–Less steep steps are easier to fill during later deposition
c. completely anisotropic
Solid State and Microwave Laboratory
Etch Figures of Merit Etch Rate (R)
Rate of film removal, typically 1000 A/min. Critical related issues are throughput and control
Etch Uniformity (U)
% change in etch rate across a wafer or from wafer to wafer, within a lot or from lot to lot
Selectivity(Sfm,Sfs)
Measure of directionality of the etch. A=1 corresponds to perfect anisotropic etch A=0 corresponds to isotropic etch
Anisotropy, A
Measure of the lateral extent of the etch per side
Undercut
Physical and/or chemical damage of the substrate. E.eg chemical attack, displacement damage or abrasive damage
Substrate Damage
Uniformity
U=
Selectivity S fs =
Ratio of the etch rate of various materials such as the film to photoresist or the film to substrate / underlying filmSfm: Film to mask selectivity;Sfs: Film to substrate selectivity
Rhigh − Rlow
Rlow= minimum etch rate
Rhigh + Rlow Rf Rs
S fm =
Rhigh= maximum etch rate
Anisotropy A =
Rvertical Rlateral
Rf
Rf= film etch rate
Rvertical= vertical etch rate
Rm
Rs= substrate etch rate Rm= mask etch rate
Rlateral= lateral etch rate
Solid State and Microwave Laboratory
Chemical Etch !Chemical etching is very selective because free radicals etch by chemical reaction which can be very specific !Chemical component of plasma etching when acting by itself, is isotropic, but need to consider also –Distribution of arrival angles –Sticking coefficient !Free radicals in plasma systems have isotropic arrival angles and low sticking coefficients !Purely chemical etching is isotropic or nearly isotropic – Etching occurs fast in all directions – Undercuts mask – Deviates if sticking coefficient is high or if there is shadowing from topography or etch mask erosion is significant
Radical species
Mask
Solid State and Microwave Laboratory
Film
Physical Etch !Ions are accelerated towards each electrode by electric field created by the voltage drop between the plasma and the electrode (wafer) !The ionic species such as Cl2+, CF4+, CF3+(or Ar+ in a purely physical sputter etch) strike the wafer surface with high momentum resulting in the physical component of etching – Atoms are dislodged into the gas stream – Mass transport from the boundary layer into the gas stream !The ions (and hence the etch) are very directional due to the electric field –Ions have a cosnθdistribution (narrow arrival angle distribution) !Etching is not very selective – Sputtering yield from most materials do not vary much !It is assumed that once an ion strikes the surface, it does not have sufficient energy to strike again – Sticking coefficient Sc= 1
+
Ionic species
+ + + + Mask
Solid State and Microwave Laboratory
Film
Ion Enhanced Etch Radical species
Radical species +
+
+ +
Mask
Mask
Film
Film
+ +
Inhibitor
Chemical etched enhanced by ion bombardment
Inhibitor removed by ion bombardment
!Ion enhanced etching has both chemical and physical components that act synergistically to provide excellent anisotropic profiles and selectivity !To explain the phenomenon it is believed that ion bombardment enhances one of the steps in chemical etch process such as – Surface adsorbtion, etching reaction, by-product formation, by-product removal (inhibitor layer) and removal of un-reacted etchants !One example is the formation of inhibitor layer which consists of polymers formed from C2F6during reactive ion etch of SiO2 – Polymer coats sidewalls – Enhancement only occurs where the polymer has been removed by ion bombardment – Care must be taken to prevent too fast a deposition rate of inhibitor – Sidewall profile can be tailored by controlling relative inhibitor deposition rate
Solid State and Microwave Laboratory
Wet Etching How? Simply place the wafer in solution that attacks the film to be etched but not the mask (resist). !Diffusion reactive species from the Reaction Reactants liquid bulk through the boundary Products layer to the surface of wafer !Reaction of species at the surface to form solvable species Reaction !Diffuse reaction products away from Boundary the surface through the boundary layer layer into the bulk of the liquid Film
Substrate Advantages
High selectivity because it is based on chemical processes
Disadvantages Isotropic, poor process control and particulates
Solid State and Microwave Laboratory
Anisotropic Etching of Silicon Solution Reaction
2d w = 2a = tan θ w θ = 54.74o
23.4 KOH : 13.5 IPA : 63 H2O Si + H2O + KOH -> K2SiO3 + 2H2
(100)
d (111) a
! Orientation selective etch of silicon occur in hydroxide solutions because of the close packing of some orientations relative to other orientations –Density of planes :<111> > <110> > <100> –R(100)< R(110)< R(100) !<100> direction etches faster than <111> direction –R(100)= 100 R(111) –It is reaction rate limited
2003 J.P. Krusius, D.G. Ast, Cornell Univesity
Solid State and Microwave Laboratory
Summary of Wet Etches
Wet etches are selective isotropic and fast –Usually reaction rate limited Advantages
Disadvantages
Simple & Fast
Undercutting
Selective
Strain at interface can enhance undercutting and lift-off film
Reproducible
Solid State and Microwave Laboratory
Why Dry Etching
Dimensional control in etching small geometries---necessary for advanced semiconductor devices and micromaching---is an important topic in micro-technology. To etch these structures, dry plasmaassisted etching increasingly used due to (1) the achievement of etch directionality without using the crystal orientation as in the case of wet etching of single crystal like silicon or GaAs (2) the ability to faithfully transfer lithographically defined photoresist patterns into underlying layers (3) high resolution and cleanliness.
Solid State and Microwave Laboratory
Dry Chemical Etching Mechanisms
Plasma flowing gas
1. Reactive species generation 1. Generation of etchant species
2. Diffuse to the solid 3. Adsorption at the surface 4. Reaction at the surface 5. Reactive cluster desorption
2. Diffusion to surface 3. Adsorption
6. Diffusion into bulk gas 5. Desorption
6. Diffusion away from the surface 4. Reaction
Solid State and Microwave Laboratory
Sample
Gas Phase (Dry) Etching Plasma etching has largely replaced wet etching in IC technology because of the directional etching possible with plasma etch systems ! Directional etching is due to presence of ionic species in the plasma and the electric field that direct them normal to the wafer ! Systems can be designed so that reactive chemical components or th ionic components dominate ! Inmost cases plasma systems use a combination of ionic and reactive chemical species acting in a synergistic manner leading to an etch rate that is much faster than the sum of individual etch rates when they are acting alone !Reactive chemical component of plasma etching often has high selectivity !Ionic component of plasma etching often has directionality !Utilizing both components, directionality could be achieved while maintaining an acceptable selectivity
Solid State and Microwave Laboratory
Dry Etching Types and Equipment Dry Etching Ion beam methods -Triode Set-ups
Glow discharge Methods -Diode Set-up
Physical etching Plasma etching Reactive gas plasma
Reactive ion etching sputtering Reactive gas plasma
Inert gas
Ion milling Inert gas ion
Ion beam assisted chem.etch Inert gas ion
Reactive ion beam etching Reactive gas ion beam
0.01-0.2 Torr
0.2 – 2 Torr Low energy bombard.
Sputter etching
High energy
No reactive neutrals
High energy
Solid State and Microwave Laboratory
Reactive neutrals
Some
-3 Torr 10-4-10 reactive
Review of RF Plasmas
!Application of electric field across two electrodes gas !Atoms/molecules are ionized, producing positive ions and free electrons, creating a plasma !Voltage bias develops between the plasma and electrodes because of the difference in mobilities (masses) of electrons and ions !Plasma is positively biased with respect to the electrodes Pressure:1 mTorr –1 Torr Energy:RF Source @ 13.56 MHz
Solid State and Microwave Laboratory
RF Plasma Potential Profile !Sheaths form next to electrodes and voltage drops occur at sheaths corresponding to dark region !Electrodes capacitively couple to plasma !Ions respond to the average sheath voltage while the electrons respond to instantaneous voltage !If electrodes have equal areas, the voltage drop at the sheaths are symmetrical !If the electrodes have un-equal areas, the voltage drop between the sheaths and the electrodes are asymmetrical with a much larger voltage drop occuring at the smaller electrode –Two capacitors in series
Solid State and Microwave Laboratory
Review of Plasma Processes
For a plasma with inlet flow of molecule AB, Plasma processes are
Dissociation
e* + AB →A + B + e
Atomic Ionization
e* + A → A++ e + e
Molecular Ionization e* + AB →AB++ e + e Atomic Excitation
e* + A → A* + e
Molecular Excitation e* + AB → AB* + e
Solid State and Microwave Laboratory
Plasma Etch Methods for Various Films
Compounds
! Most reactant gasses contain halogens – Cl, F, Br, or I ! Exact choice of reactant gasses to etch each specific film depends on – Ability to form volatile by-products – Etch selectivity to resist and underlying films – Anisotropicity ! Boiling points are good indicators of volatility of species –Lower boiling point, higher tendency to evaporate
Vapor Pressure (Torr)
AlF3
1 (1238 ℃)
InCl3
10-8 (100 ℃) 10-4 (180 ℃) 10-2 (250 ℃)
GaCl3
2 (50 ℃)
AsCl3
40 (25 ℃) 290 (100 ℃)
AlCl3
2 x 10-4 (25 ℃) 1 (100 ℃)
PCl3
400 (57 ℃)
AsF3
750 (56 ℃)
Solid State and Microwave Laboratory
Comparing Wet vs. Dry Etching (1)
Parameter
Dry Etching
Wet Etching
Directionality
Can be highly directional with most materials
Only directional with single crystal materials
Production-line automation
Good
Poor
Environmental impact
Low
High
Masking film adherence
Not as critical
Very critical
Cost chemicals
Low
High
Selectivity
Poor
Can be very good
Materials that can be etched
Only certain materials can be etched
All
Radiation damage
Can be severe
None
Solid State and Microwave Laboratory
Comparing Wet vs. Dry Etching (2) Parameter
Dry Etching
Wet Etching
Cleanliness
Good under the right operational conditions
Good to very good
CD control
Very good (< 0.1µm)
Poor
Equipment cost
Expensive
Inexpensive
Sub micron feature
Applicable
Not applicable
Typical etch rate
Slow (0.1 µm/min, isotropic )
Fast (1 µm/min, anisotropic)
Theory
Very complex, not well
Better understood
Operating Parameters
Many
Few
Control of etch
due to slow etching
Difficult
Solid State and Microwave Laboratory
Micrograph of Silicon Plates with a Gap of 2µm.
Solid State and Microwave Laboratory
Isotropic Etch of Silicon
Solid State and Microwave Laboratory
0.25-µm-diameter InP Dots
0.4-µm-wide InP Slabs
Solid State and Microwave Laboratory
Nano-structure Etching on 0.2-mm Photonic-crystal Pattern
Pattern: 0.2-µm holes Etched thickness: 5000 Å
Material: GaAs Mask: Si3N4 Etching Time: 150~200 Å/min Gas species: BCl3, CH4, Ar Working Pressure: 10 mTorr RF power: 50 Watts
Solid State and Microwave Laboratory
Deep Trench Etching on Si
Solid State and Microwave Laboratory
