Transcript Section 3: Etching Jaeger Chapter 2 Reader

Section 3: Etching
Jaeger Chapter 2
Reader
Etch Process - Figures of Merit
• Etch rate
• Etch rate uniformity
• Selectivity
• Anisotropy
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Bias and anisotropy
dm
hf
etching mask
film
substrate
df
dm
Bias B  d f - dm
Complete Isotropic Etching
Vertical Etching = Lateral Etching Rate
B = 2 × hf
Complete Anisotropic Etching
substrate
df
Lateral Etching rate = 0
B=0
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Degree of Anisotropy
rlat: lateral etch rate
rver: vertical etch rate
Af: degree of isotropy
rlat
 1rver
Af
0  Af  1
isotropic
anisotropic
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Etching Selectivity S
S AB
rA (vertical etching velocity of materal A)

rB (vertical etching velocity of materal B)
Wet Etching
S is controlled by:
chemicals, concentration, temperature
RIE
S is controlled by:
plasma parameters, plasma chemistry,
gas pressure, flow rate & temperature.
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Selectivity Example
SiO2
Si
SiO2/Si etched by HF solution
SSiO2, Si Selectivity is very large ( ~ infinity)
SiO2/Si etched by RIE (e.g. CF4 plasma)
SSiO2, Si
Selectivity is finite ( ~ 10 )
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Uniformity
(a) Film thickness variation across wafer
h f max   h f   1  d

Thickness variation factor
Nominal thickness
•The variation factor d is dictated by the deposition method,
deposition equipment, and manufacturing practice.
(b) Film etching rate variation
rf min   rf 1 -  f 
variation factor
Worst - case etching time required to etch the film
h f max  h f 1  d 

 
rf min  rf 1 -  f 
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Wet Etching
1
3
2
1
Reactant transport to surface
2
Selective and controlled reaction of etchant with
the film to be etched
3
Transport of by-products away from surface
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Wet Etching (cont.)
• Wet etch processes are generally isotropic
• Etch rate is governed by temperature, concentration,
chemicals, etc.
• Wet etch processes can be highly selective
• Acids are commonly used for etching:
HNO3 <=> H+ + NO3HF <=> H+ + F-
H+ is a strong oxidizing agent
=> high reactivity of acids
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Wet Etch Processes
Etch rate (A/min)
(1) Silicon Dioxide
6:1 BOE
To etch SiO2 film on Si, use
1200
650
HF + H2O
18
SiO2 + 6HF  H2 + SiF6 + 2H2O
26 T (oC)
Note: HF is usually buffered with NH4F to maintain [H+] at a constant
level (for constant etch rate). This HF buffer is called Buffered Oxide
Etch (BOE)
NH4F  NH3 + HF
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Wet Etch Processes (cont.)
(2) Silicon Nitride
To etch Si3N4 film on SiO2, use
H3PO4
(phosphoric acid)
(180oC: ~100 A/min etch rate)
Typical selectivities:
– 10:1 for nitride over oxide
– 30:1 for nitride over Si
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Wet Etch Processes (cont.)
(3) Aluminum
To etch Al film on Si or SiO2, use
H3PO4 + CH3COOH + HNO3 + H2O
(phosphoric acid) (acetic acid)
(nitric acid)
(~30oC)
6H+ + 2Al  3H2 + 2Al3+
(Al3+ is water-soluble)
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Wet Etch Processes (cont.)
(4) Silicon
(i) Isotropic etching
Use HF + HNO3 + H2O
3Si + 4HNO3 3SiO2 + 4NO + 2H2O
3SiO2 + 18HF 3H2SiF6 + 6H2O
(ii) Anisotropic etching (e.g. KOH, EDP) for single crystalline Si
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Drawbacks of Wet Etching
• Lack of anisotropy
• Poor process control
• Excessive particulate contamination
=> Wet etching used for noncritical feature sizes
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Reactive Ion Etching (RIE)
RF
13.56 ~
MHz
plasma
Parallel-Plate
Reactor
wafers
Plasma generates (1) Ions
(2) Activated neutrals
Enhance chemical reaction
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Remote Plasma Reactors
Plasma Sources
(1) Transformer
Coupled
Plasma
(TCP)
(2) Electron
Cyclotron
Resonance
(ECR)
e.g. quartz
plasma
coils
wafers
-bias
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Pressure
pump1mTorr 10mTorr
bias~  1kV
RIE Etching Sequence
gas flow
5
1
diffusion of by product
desorption
4
diffusion of
reactant
2
3
X
absorption
chemical
reaction
Substrate
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gaseous by products
Volatility of Etching Product
* Higher vapor pressure

e.g . Si  4 F  SiF4 
*
higher volatility
(high vapor pressure)
e.g . Cu  Cl  CuCl (low vapor pressure)
Example
Difficult to RIE Al-Cu
alloy with high Cu content
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Examples
Use CF4 gas
For etching Si

CF4  e  CF3  F *  2e
Si  4 F *  SiF4 
F* are Fluorine radicals (highly reactive, but neutral)
Aluminum

CCl4  e  CCl3  Cl *  2e
Al  3Cl *  AlCl3 
Photoresist
C x H y Oz  O2
COx
HO x
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How to Control Anisotropy ?
1) ionic bombardment to damage expose surface.
2) sidewall coating by inhibitor prevents sidewall etching.
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How to Control Selectivity ?
E.g. SiO2 etching in CF4+H2 plasma
Rate SiO2
S
Rate Si
S
Rates
P.R.
SiO2
Si
SiO2
H 2%
Si
%H2 in (CF4+H2)
F *  H  HF  F * content
Reason:
 SiF4 
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Example: Si etching in CF4+O2 mixture
Reason:
Rates
1
(1)O  CFx  COFx  F *
Si
F * increases
Si etching
(2)Si  O2  SiO2  rate
2
%O2 in CF4
Poly-Si
Oxide
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rate
Example: RIE of Aluminum Lines
* It is a three-step sequence :
1) Remove native oxide with BCl3
2) Etch Al with Cl-based plasma
3) Protect fresh Al surface with thin oxidation
2
Cl2-based RIE
1 BCl3
P.R.
native Al2O3
Al
3
Form oxide again (gently)
Al
Al
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