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Xing Sheng, EE@Tsinghua
1
Xing Sheng盛兴
Department of Electronic EngineeringTsinghua University
xingsheng@tsinghua.edu.cn
Etching 刻蚀
Principles of Micro- and Nanofabrication for Electronic and Photonic Devices
Xing Sheng, EE@Tsinghua
Pattern Formation
3
Xing Sheng, EE@Tsinghua
Etching vs. Corrosion
4
Etching (刻蚀) Corrosion (腐蚀)
wanted unwanted
Xing Sheng, EE@Tsinghua
Wet vs. Dry
5
liquid source gas source
Xing Sheng, EE@Tsinghua
Chemical vs. Physical
6
Xing Sheng, EE@Tsinghua
Etching
7
Process Parameters Time
Temperature
Etchant type
Etchant concentration
Mask type
...
Control Parameters Etch rate
Selectivity
Anisotropy
Uniformity
...
diffusion - reaction - diffusion
- chemical reactions occur- products should be disposable
Xing Sheng, EE@Tsinghua
Selectivity
8
1 2 1 2 2
112
R
RS
Xing Sheng, EE@Tsinghua
SiO2 / Si wet etch by HF solution very large selectivity SSiO2/Si ~ infinity
SiO2 / Si dry etch by CF4 plasma selectivity SSiO2/Si ~ 10
Selectivity - Example
9
SiO2
Si
Xing Sheng, EE@Tsinghua
Anisotropy (各向异性)
10
vertical
lateral
R
RA 1degree of anisotropy
isotropicA = 0
fully anisotropicA = 1
anisotropic0 < A < 1
Xing Sheng, EE@Tsinghua
11
Isotropic vs. Anisotropic
chemical physical
Xing Sheng, EE@Tsinghua
Uniformity
12
Xing Sheng, EE@Tsinghua
13
Trends of Etching
Xing Sheng, EE@Tsinghua
Etching Methods
14
Wet Etching 湿法刻蚀
Dry Etching 干法刻蚀
CMP and other methods
Xing Sheng, EE@Tsinghua
References
15
Wet Etching Recipes
Guide to references on III-V semiconductor chemical etching
http://ieeexplore.ieee.org/iel4/84/11954/00546406.pdfhttp://ieeexplore.ieee.org/iel4/84/11954/01257354.pdf
http://www.sciencedirect.com/science/article/pii/S0927796X00000279
https://cleanroom.byu.edu/chemical_etching.html
Xing Sheng, EE@Tsinghua
Metal Dissolution in Acids
16
hydrogen
easy
hard
Xing Sheng, EE@Tsinghua
17
Metal Dissolution in Acids
Strong Acids + Strong Oxidants
Aqua Regia (王水) HCl : HNO3 = 3:1even dissolves Au, Pt
Piranha H2SO4 : H2O2 = 3:1dissolves most metals and organics
However, difficult to obtain ideal selectivity ...
Xing Sheng, EE@Tsinghua
18
SiO2 etching
Alkali (NaOH, etc) slowly etches SiO2
SiO2 + 2NaOH = Na2SiO3 + H2O
HF strongly etches SiO2
SiO2 + 6HF = H2SiF6 + 2H2O
Buffered HF (BHF/BOE) HF + NH4F
lower etch rate
safer for use
glass art by HF etch
Xing Sheng, EE@Tsinghua
19
Cu etching
Cu + 2FeCl3 = CuCl2 + 2FeCl2
Xing Sheng, EE@Tsinghua
20
Selectivity for Wet Etch
mask
mask SiO2KOH + K3[Fe(CN)6]
GaP
PRFeCl3Cu
PRKI + I2Au
PRH3PO4 + H2O2
GaAs
Si3N4
PR
MaskEtchantFilms
KOHSi
HFSiO2
most wet etch recipes are isotropic, except KOH etch for Si
Xing Sheng, EE@Tsinghua
Isotropy for Wet Etch
21
isotropicA = 0
Wet etch is usually isotropic
Exceptions some etching for single crystals
KOH etch Si
Xing Sheng, EE@Tsinghua
22
Si etching
HNO3 + HF isotropic etch
Xing Sheng, EE@Tsinghua
23
Si etching
KOH anisotropic etch
etch rate (111):(110):(100) ~ 1:600:400
mask: SiO2, Si3N4, Cr/Au, ...
Other chemistries TMAH: Tetramethyl ammonium hydroxide
EDP: Ethylene diamine pyrochatecol
Q: why?
Xing Sheng, EE@Tsinghua
24
Si etching
Single Crystalline Si Solar Cells KOH anisotropic etch
optical trapping and antireflection
world record efficiency: 26%
Xing Sheng, EE@Tsinghua
25
Si etching
Si cantilever beam KOH anisotropic etch
Xing Sheng, EE@Tsinghua
26
Si etching
Micro-Electro-Mechanical Systems (MEMS)
Xing Sheng, EE@Tsinghua
27
III-V etching
GaAs, AlGaAs, InGaAs H3PO4 + H2O2
NH4OH + H2O2
AlGaAs when Al > 70%, HF and HCl etch
InP, InGaP, InAlP HCl
GaN, InGaN no reliable wet etchants ...
Xing Sheng, EE@Tsinghua
28
Etch Stops
InGaP / GaAs lattice matched epitaxy
H3PO4 + H2O2 only etches GaAs
HCl only etches InGaP
Xing Sheng, EE@Tsinghua
29
Etch Stops
highly p-dope Si is resistant to KOH
Xing Sheng, EE@Tsinghua
30
Etch Stops
Silicon based 'Michigan Probe' for neuroscience
Xing Sheng, EE@Tsinghua
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Electrochemical Etch
Xing Sheng, EE@Tsinghua
32
Anodization (阳极氧化) - Porous Si
Xing Sheng, EE@Tsinghua
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Anodization - Porous Al2O3
H. Masuda and K. Fukuda, Science 268, 1466 (1995)
Xing Sheng, EE@Tsinghua
Thin-Film Si from SOI wafers
34H. C. Ho, et al., Nature 454, 748 (2008)
human eyes compound eyes
Y. M. Song, et al., Nature 497, 95 (2013)
'epidermal' electronics
D. Kim, et al., Science 333, 838 (2011)
Xing Sheng, EE@Tsinghua
Thin-Film Si from Si (111)
35J. Yoon, et al., Nature Mater. 7, 907 (2008)
KOH etches faster for Si (100) than (111)
Thin-film Si solar cells
High efficiency (Single Crystal)
Flexible
Low cost (wafer reuse)
Xing Sheng, EE@Tsinghua
GaN on Si (111)
36H. Kim, et al., Proc. Natl. Acad. Sci. 108, 10023 (2011)
KOH etches faster for Si (100) than (111)
Xing Sheng, EE@Tsinghua
GaAs and AlAs lattice matched growth
AlAs is selectively etched by HF
flexible III-V devices
GaAs Device Liftoff
37S. I. Park, et al., Science 325, 977 (2009)J. Yoon, et al., Nature 465, 329 (2010)
solar cells LED
Xing Sheng, EE@Tsinghua
Epitaxy Liftoff
38
NH4OH wet etch
KOH wet etch
HCl wet etch
FeCl3 wet etch
HCl wet etch
HF / HCl wet etch
HF wet etch
HF wet etch
KOH wet etch
HF wet etch
Release methods
sapphireZnOGaN
InPInGaAsInGaAs / InP
SiSiO2SiC
Si (111)-GaN
SiSiO2Ge
GaAsAlAsGaAs / InGaP
GaSb
GaAs
Si (111)
Si
SubstratesSacrificial layersMaterials
InAlPGaAs / InGaP
InGaSbInAs
-Si (111)
SiO2Si
H. Xu, et al., Adv. Mater. 30, 1800156 (2018)
Xing Sheng, EE@Tsinghua
Bio-degradable Materials
39
Materials that can be dissolved in the body.
Biodegradable Suture Cardiovascular Stent Bone Scaffold
Biocompatible and Degradable Materials Organic: PLGA, PLA, silk, ...
Metals: Mg, Ca, Zn, Fe, ...
Semiconductors: Si, Ge, ...
Xing Sheng, EE@Tsinghua
Bio-degradable Electronics
40
Si devices that can be dissolved by body fluids.
S. Hwang, et al., Science 337, 1640 (2012) Video
Xing Sheng, EE@Tsinghua
Etching Methods
41
Wet Etching 湿法刻蚀
Dry Etching 干法刻蚀
CMP and other methods
Xing Sheng, EE@Tsinghua
Challenges for Wet Etching
42
Most wet etching processes are chemical, isotropic
For features < 3 m, dry etching has much better resolution
Xing Sheng, EE@Tsinghua
Dry Etching
43
plasma etch
Xing Sheng, EE@Tsinghua
2XeF2 (g) + Si (g) = 2Xe (g) + SiF4 (g) very isotropic and selective
Dry Etch without Plasma
44
SiF4 boiling point 4 oC
Xing Sheng, EE@Tsinghua
Heavy ions (e.g. Ar)
Highly anisotropic
Poor selectivity for Au, Pt, ...
Ion Milling
45
Xing Sheng, EE@Tsinghua
Heavy ions (e.g. Ar)
Highly anisotropic
Poor selectivity
Ion Milling
46
Xing Sheng, EE@Tsinghua
Chemically reactive ions improved selectivity
lower power
Example: Si etch CF4 gas does not react with Si
energized F- plasma can react with Si
SiF4 is volatile (boiling point 4 oC)
Very isotropic no direction
Plasma Etch
47Si
Xing Sheng, EE@Tsinghua
Improved directionality by applied fields more anisotropic
Reactive Ion Etching (RIE)
48
Xing Sheng, EE@Tsinghua
Si SF6 plasma
SiO2
CF4 / CHF3 plasma
Photoresists can be used as masks F ions etch PR very slowly
RIE - Si and SiO2
49
Xing Sheng, EE@Tsinghua
Si SF6 plasma
SiO2
CF4 / CHF3 plasma
Photoresists can be used as masks F ions etch PR very slowly
RIE - Si and SiO2
50
SF6 is heavier than air
Xing Sheng, EE@Tsinghua
O2 plasma C-H-O + O- = CO2 + H2O
O2 plasma does not etch Si, SiO2, or metals SiO2 / metal oxides are non-volatile
RIE - Organics
51
Xing Sheng, EE@Tsinghua
Cl2 / BCl3 / SiCl4 plasma GaAs/AlGaAs, InP, GaN/InGaN, ...
RIE - III-Vs
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130AsCl3
200GaCl3
180AlCl3
> 1000lnF3
boiling point (oC)product
> 1000AlF3
1000GaF3
Q: why?
GaAs trenches
Xing Sheng, EE@Tsinghua
RIE - Recipes
53
Xing Sheng, EE@Tsinghua
Inductively Coupled Plasma (ICP) higher power
mostly for III-Vs
ICP-RIE
54
Xing Sheng, EE@Tsinghua
alternative etch / passivation
Deep RIE for Si
55
'Bosch process'
Xing Sheng, EE@Tsinghua
Atomic Layer Etching (ALE)
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1. Si + 2Cl2 = SiCl42. SiCl4 removed by plasma3. Repeat 1 and 2
K. J. Kanarik, et al., J. Vac. Sci. Tech. A 33, 020802 (2015)
Xing Sheng, EE@Tsinghua
Issues in Dry Etch
57
Xing Sheng, EE@Tsinghua
Summary of Dry Etch
58
Xing Sheng, EE@Tsinghua
Etching Methods
59
Wet Etching 湿法刻蚀
Dry Etching 干法刻蚀
CMP and other methods
Xing Sheng, EE@Tsinghua
Pattern Formation
60
Xing Sheng, EE@Tsinghua
Planar Layers are Desirable
61non-planarized IC planarized IC
Xing Sheng, EE@Tsinghua
62
Issues with Depth of Focus (DOF)
smaller , larger NA ----> smaller DOF
depth of focus(DOF)
Xing Sheng, EE@Tsinghua
Damascene Process
63P. C. Andricacos, et al., IBM J. Res. Develop. 42, 567 (1998)
Cu
SiO2
ancient art work
Xing Sheng, EE@Tsinghua
CMP: Chemical Mechanical Polishing
64
Chemical selectivity + Mechanical Planarization
applied for Cu, W, SiO2, ...
Video
Xing Sheng, EE@Tsinghua
3D IC
65
Electroplating + CMPdirtiest process for the most advanced IC
Xing Sheng, EE@Tsinghua
Other Methods for Etching
66
Laser Lift-Off
FIB: Focused Ion Beam
Laser Milling
...
Xing Sheng, EE@Tsinghua
GaN devices grown on sapphire low cost
low thermal conductivity
electrically insulating
sapphire is very difficult to etch
Release by laser liftoff KrF excimer UV laser (248 nm)
GaN = Ga + N2 (gas)
bonding onto new substrates
GaN Laser Liftoff
67W. S. Wong, et al., Appl. Phys. Lett. 72, 599 (1998)
Xing Sheng, EE@Tsinghua
Flexible GaN blue LEDs
68T. Kim, et al., Science 340, 211 (2003)
Xing Sheng, EE@Tsinghua
69
FIB: Focused Ion Beam
Etch: GaDeposition: Pt
- nanostructures- SEM/TEM sample
preparation- doping
Xing Sheng, EE@Tsinghua
Types
CO2 laser 10 m
Near-IR laser 1064 nm
UV laser 365 nm
Excimer laser 248 nm, 193 nm
ps/fs laser ...
Applications die cut, PCB cut, ...
Laser Milling
70
shorter wavelengthshorter pulse
better resolution
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