Professor N Cheung, U.C. Berkeley Lecture 15 EE143 F2010 1 Reactive Ion Etching (RIE) ~ plasma wafers RF 13.56 MHz Parallel-Plate Reactor Plasma generates (1) Ions (2) Activated neutrals Enhance chemical reaction Sputtering
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
1
Reactive Ion Etching (RIE)
~ plasma
wafers
RF13.56MHz
Parallel-PlateReactor
Plasma generates (1) Ions(2) Activated neutrals
Enhance chemical reaction
Sputtering
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
2
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
3
Remote Plasma Reactors
Plasma Sources(1) Transformer
CoupledPlasma(TCP)
(2) ElectronCyclotronResonance(ECR)
-bias
pump
e.g. quartz
coilsplasma
wafers
Pressure1mTorr 10mTorrbias~ 1kV
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
Processes Occurring in Plasma Etching
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
5
• Synergism of ion bombardment AND chemical reactiongive the high RIE rates.
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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REMOVAL ofsurface filmand DEPOSITIONof plasma reactionproducts canoccursimultaneously
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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RIE Etching Sequence
gas flow
1
2 3
5
4X
diffusion ofreactant
diffusion of by productdesorption
chemicalreaction gaseous by productsabsorption
Substrate
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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Volatility of Etching Product
* Higher vapor pressure higher volatility
)(..4.. 4
*
pressurevaporlowCuClClCugeSiFFSige
mask
Al-Cu Metal
(high vapor pressure)
Example
Difficult to RIE Al-Cualloy with high Cu content
Do not want CuCl residues
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
9
kTH v
ePP
0
1/T
P1500oC
AlCl3CuCl
[Al-Cu alloy]
Cl2 as etching gas.200oC
1~2% typical
Example
Difficult to RIE Al-Cualloy with high Cu content
Vapor pressure of by-product has to be high
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
10
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
11
Examples
4*
*34
3*
4
4
2
SiFFSieFCFeCF
CFFCF
Use CF4 gasFor etching Si
F* are Fluorine atoms with electrons
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
13
x
xzyx HO
COOOHC
AlClClAl
eClCCleCCl
2
3*
*34
3
2
Photoresist
Aluminum+
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
14
How to Control Anisotropy ?
1) ionic bombardment to damage expose surface.2) sidewall coating by inhibitor prevents sidewall etching.
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
15
Sidewall passivation filmsHCl/O2/BCl3 chemistry
Si
Photoresiston top of Si
This exaggerated picture shows a passivation layer so thickthat it can peel off from the sidewall
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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How to Control Selectivity ?
E.g. SiO2 etching in CF4+H2 plasma
SiO2
Si
Rates
%H2 in (CF4+H2)
S Rate SiORate Si
2
H2%
P.R.
SiSiO2
Reason:
4
**
SiFcontentFHFHF
S
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
17
1 2
Si
SiO2
Poly-Si
Oxide
Reason:
rateSiOOSirateetchingSiincreasesF
FCOFCFO xx
22
*
)2(*)1(
%O2 in CF4
Rates
Example: Si etching in CF4+O2 mixture
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
18
For reference only
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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effect
Controlvariable
Effect of RIE process variables on etching characteristics
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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Temperature Dependence of Selectivity
R A e
R A e
S RR
AA
e
QkT
QkT
Q QkT
1 1
2 2
1
2
1
2
1
2
1 2
77oK if Q1<Q2
1/T
S
R= etching ratesA = proportional constantsQ = activation energies
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
21
P.R.
Al
BCl31
2 Cl2-based RIE
native Al2O3
3 Form oxide again (gently)
Al Al
* It is a three-step sequence :1) Remove native oxide with BCl32) Etch Al with Cl-based plasma3) Protect fresh Al surface with thin oxidation
Example: RIE of Aluminum Lines
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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Example: Etching of Deep Trenches
~1m
mask
ballooning
mask
mask erosion
trenching
by-productresidue
Si
“ideal” “problems”
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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• Ballooning:– Use chemistry with a good sidewall inhibitor.
• Trenching:-Use high pressure to increase ion-neutral scattering(ion trajectory less directional)
• Bottom Roughness:– Increase vapor pressure of etching byproduct.
Approaches to minimize deep trench etching problems
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
Hard Mask for Etching
Photoresist
oxide
RIE 1
RIE 2
poly
To minimize CD distortion, sometimes a two-step RIE processis used. Example: Process 1 to transfer pattern from resist;followed by Process 2 to transfer pattern from oxide to poly.
24EE243S2010 Lec22
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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A better Solution:Multiple step RIE sequence
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
26EE243S2010 Lec22
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
27* Can etch through whole Si wafer thickness
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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Local Loading Effect
Wsmall
Wlarge
More etchant consumptionLess etchant consumption
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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* smaller trenches etch at a slower rate than larger trenches.
CCl2F2/O2 RIE
RIE Lag
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
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Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
Etching Profile Simulation(http://cuervo.eecs.berkeley.edu/Volcano/)
Many features such as mask erosion, sidewall angles,undercut, etc. can be predicted from the simple etchingmodels.
Material and Isotropic and Directional Etch rates in nm/s
Resist 0.83 0.0
Oxide 0.0 8.0
Silicon 11.7 5.0
Substrate 0.0 0.0
Q = sin-1 (5.0/(11.7 + 5.)) = 17.4o
Angle appears larger due to unequal scales in x and y.
For reference only
Professor N Cheung, U.C. Berkeley
Lecture 15EE143 F2010
SUMMARY OF ETCH MODULE
• Etch Bias, Degree of Anisotropy, Etch Selectivity• Worst-case considerations for etching• Wet etch – qualitative• KOH/EDP etch of Si (anisotropic)• Reactive Ion Etch equipment- qualitative• Synergism of ion bombardment and chemical etching• Selectivity Control - Gas mixture, Temperature• Anisotropy Control – Inhibitor deposition, Substrate
bombardment• RIE examples: Aluminum, deep trench etching.• Pattern and Aspect ratio Dependence - qualitative