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Microfabrication Technology
Section 3
Etching
Greg Tikhomirov
EE143 Fall 2024
EE143 F2023
Etching
Etching
• Etching Terminology
• Etching Considerations for ICs
• Wet Etching
• Reactive Ion Etching (plasma etching)
Professor J. Bokor, U.C. Berkeley
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Etching
Etch Process - Figures of Merit
• Etch rate
• Etch rate uniformity
• Selectivity
• Anisotropy
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Etching
dm
Bias and anisotropy
hf
etching mask
film
substrate
df
dm
Bias b = (df – dm)/2
Complete Isotropic Etching
Vertical Etching = Lateral Etching Rate
b = hf
Complete Anisotropic Etching
Lateral Etching rate = 0
b=0
substrate
df
Professor J. Bokor, U.C. Berkeley
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Etching
(2) Degree of Anisotropy
b
B
Af ≡ 1 −
2h f
0 ≤ Af ≤ 1
isotropic
∴ b = hf
Professor J. Bokor, U.C. Berkeley
anisotropic
b =0
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Etching
(3) Etching Selectivity S
v A ( vertical etching velocity of materal A )
S AB =
v B ( 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.
Professor J. Bokor, U.C. Berkeley
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Etching
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
Professor J. Bokor, U.C. Berkeley
Selectivity is finite ( ≥ 10 )
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Etching of Steps with a Slope
Etching
* Etching velocity has vertical component vv and lateral component vl
Let etching time = t
vv = vertical etch rate
vl = lateral etch rate
vv ⋅ t
θ
vv ⋅ t
θ
film
x1 x2
x1 = vv ⋅ t cot θ
x2 = vl ⋅ t
x = x1 + x 2
start
= ( vv cot θ + vl ) ⋅ t
final
substrate
cot θ = 1/ tan θ = b / a
Professor J. Bokor, U.C. Berkeley
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EE143 F2023
Etching
Worst-Case Design Considerations for Etching
Film thicker here due
to surface topography
Mask material
can be eroded
during film
etching
Variation of film
thickness across
wafer due to
deposition method Etching
Mask
step
Film
Substrate
Etching rate of film can vary from run-to-run
Professor J. Bokor, U.C. Berkeley
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Etching
(a) Film thickness variation across wafer
h f (max ) = h f ⋅ ( 1 + δ )
Nominal thickness
Thickness variation factor
•The variation factor δ is dictated by the deposition method,
deposition equipment, and manufacturing practice.
•Run-to-run variation of thickness data are recorded. Once
the deposition process is under control, the maximum/minimum
values will be used to define δ.
Professor J. Bokor, U.C. Berkeley
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Etching
(b) Film etching rate variation
v f (min ) = v f (1 − φ f )
variation factor
Worst − case etching time required to etch the film
h f (1 + δ )
=
=
⋅
v f (min ) v f (1 − φ f )
h f (max )
Professor J. Bokor, U.C. Berkeley
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Etching
(c) Over-etch around step
film hf
step
h1
hf
Fractional over-etch time to
completely remove film
= h1 / hf
substrate
Total worst-case etching time
∴ tT =
h f (1 + δ )
v f (1 − φ f )
Professor J. Bokor, U.C. Berkeley
⋅ (1 + ∆ )

h1 
∆ = 

h 
f
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Etching
Example 1: Worst-case Consideration for substrate erosion
hf (1-φf)
Worst-case
substrate erosion
film
substrate
hf (1+φf)
Film thickness has
variation factor = δ
Film etching rate has
variation factor = φf
h f (1 − δ )
Thinnest part of film with be completely removed in t 1 =
v f (1 + φ f )
Thickest part of film with be completely removed in t 2 = h f (1 + δ )
v f (1 − φ f )
Worst-case substrate erosion = vsubstrate • (t2 - t1)
Professor J. Bokor, U.C. Berkeley
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Etching
Example 2: Worst-case Design With Mask Erosion
State-of-problem:
Mask material can be eroded during film etching.
Top of film will be smaller than original mask size by an amount W/2.
Before
θ
Let v m⊥ , v m / / be the vertical
and lateral etching rates of
the mask.
hf
W/2
After
film
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film
Let v f be the vertical
etching rate of the film.
(lateral film etch rate is ignored
In this example for simplicity)
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Etching
W
= ( v m⊥ cot θ + v m / / ) ⋅ t T
2
 v m⊥ 
vm/ / 
(1 + δ )(1 + ∆ ) 
=
cot θ +
 ⋅ hf ⋅


v
v
−
φ
1
 f 
( f) 
m⊥ 
To minimize W
θ → 90 o
v f >> ν m⊥
hf small
Professor J. Bokor, U.C. Berkeley
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Etching
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.)
Etching
• Wet etch processes are generally isotropic
• Wet etch processes can be highly selective
• Acids are commonly used for etching:
HNO3 <=> H+ + NO3HF <=> H+ + FH+ is a strong oxidizing agent
=> high reactivity of acids
Professor J. Bokor, U.C. Berkeley
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Etching
Wet Etch Processes
(1) Silicon Dioxide
To etch SiO2 film on Si, use
HF + H2O
Etch rate (A/min)
6:1 BOE
650
18
1200
26 T (oC)
SiO2 + 6HF → H2 + H2SiF6 + 2H2O
Note: HF is usually buffered with NH4F to maintain [H+]
at a constant level (for constant etch rate)
NH4F → NH3 + HF
Professor J. Bokor, U.C. Berkeley
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Etching
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
Professor J. Bokor, U.C. Berkeley
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Etching
Wet Etch Processes (cont.)
(3) Aluminum
To etch Al film on Si or SiO2, use
H3PO4 + CH3COOH + HNO3 + H2O
(phosphoric acid) (acetic acid)
[dissolve Al2O3] [wetting/buffering]
(nitric acid)
[oxides Al]
(~30oC)
6H+ + 2Al → 3H2 + 2Al3+
(Al3+ is water-soluble)
Professor J. Bokor, U.C. Berkeley
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Etching
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)
(EDP: ethylene diamine pyro-catechol)
Professor J. Bokor, U.C. Berkeley
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Etching
Effect of Slow {111} Etching with KOH or EDP
Mask opening aligned in <110> direction => {111} sidewalls
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Etching
[110]-Oriented Silicon
{111} planes oriented perpendicular to the (110) surface
=> possible to etch pits with vertical sidewalls!
Bottom of pits are
• flat ({110} plane) if KOH is used
{100} etches slower than {110}
• V-shaped ({100} planes) if EDP is used
{110} etches slower than {100}
Professor J. Bokor, U.C. Berkeley
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Etching
Drawbacks of Wet Etching
• Lack of anisotropy
• Poor process control
• Excessive particulate contamination
=> Wet etching used for noncritical feature
sizes
Professor J. Bokor, U.C. Berkeley
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Etching
Reactive Ion Etching (RIE)
RF
13.56
MHz
~
plasma
Parallel-Plate
Reactor
wafers
Sputtering
Plasma generates (1) Ions
(2) Activated neutrals
Enhance chemical reaction
Professor J. Bokor, U.C. Berkeley
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Etching
Remote Plasma Reactors
Plasma Sources
(1) Transformer
Coupled
Plasma
(TCP)
(2) Electron
Cyclotron
Resonance
(ECR)
Professor J. Bokor, U.C. Berkeley
e.g. quartz
plasma
coils
wafers
-bias
Pressure
pump 1mTorr 10mTorr
bias~ 1kV
≤
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Etching
Processes Occurring in Plasma Etching
Professor J. Bokor, U.C. Berkeley
EE143 F2023
Etching
REMOVAL of
surface film
and DEPOSITION
of plasma reaction
products can
occur
simultaneously
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Etching
RIE Etching Sequence
gas flow
5
1
2
diffusion of
reactant
absorption
diffusion of by product
desorption
4
3
X
chemical
reaction
gaseous by products
Substrate
Professor J. Bokor, U.C. Berkeley
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Etching
Volatility of Etching Product
⇒
* Higher vapor pressure
higher volatility
*
e.g . Si + 4 F → SiF4 ↑ (high vapor pressure)
e.g . Cu + Cl → CuCl (low vapor pressure)
Example
Difficult to RIE Al-Cu
alloy with high Cu content
mask
Al-Cu Metal
Do not want CuCl residues
Professor J. Bokor, U.C. Berkeley
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Etching
Vapor pressure of by-product has to be high
P = P0 e
− ∆H v
kT
Example
P
Difficult to RIE Al-Cu
alloy with high Cu content
1500oC
AlCl3
CuCl
1~2% typical
200oC
1/T
[Al-Cu alloy]
Cl2 as etching gas.
Professor J. Bokor, U.C. Berkeley
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Professor J. Bokor, U.C. Berkeley
Etching
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Etching
Examples
Use CF4 gas
For etching Si
*
CF4 → F + CF3
+
*
CF4 + e ⇔ CF3 + F + 2e
*
Si + 4 F → SiF4 ↑
F* are Fluorine atoms with electrons
Professor J. Bokor, U.C. Berkeley
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EE143 F2023
Etching
Aluminum
+
*
CCl 4 + e ⇔ CCl3 + +Cl + 2e
*
Al + 3Cl → AlCl3 ↑
Photoresist
C x H y Oz + O2
Professor J. Bokor, U.C. Berkeley
COx
HOx
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Etching
How to Control Anisotropy ?
1) ionic bombardment to damage expose surface.
2) sidewall coating by inhibitor prevents sidewall etching.
Professor J. Bokor, U.C. Berkeley
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Etching
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)
Reason:
F * + H → HF ∴ F * content↓
∴ SiF4 ↓
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Etching
Example: Si etching in CF4+O2 mixture
Rates
1
Reason:
Si
(1)O + CFx → COFx + F *
2
F * increases
Si etching
rate
(2)Si + O2 → SiO2 ∴ rate↓
SiO2
%O2 in CF4
Poly-Si
Oxide
Professor J. Bokor, U.C. Berkeley
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EE143 F2023
Etching
Temperature Dependence of Selectivity
R1 = A1e
− Q1
R2 = A2 e
kT
− Q2
R= etching rates
A = proportional constants
Q = activation energies
kT
R1 A1 − ( Q1 − Q2 ) kT
∴S =
=
e
R2 A2
S
if Q1<Q2
77oK
1/T
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Etching
Hard Mask for Etching
RIE 1
RIE 2
Photoresist
oxide
poly
To minimize CD distortion, sometimes a two-step RIE process
is used. Example: Process 1 to transfer pattern from resist;
followed by Process 2 to transfer pattern from oxide to poly.
EE243S2010 Lec22
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Etching
Local Loading Effect
Less etchant consumption
Wsmall
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More etchant consumption
Wlarge
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Etching
Example: Etching of Deep Trenches
~1µm
mask erosion
mask
mask
ballooning
Si
trenching
by-product
residue
“ideal”
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“problems”
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Etching
RIE Lag
* smaller trenches etch at a slower rate than larger trenches.
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Etching
(c) Over-etch around step
film hf
step
h1
hf
Fractional over-etch time to
completely remove film
= h1 / hf
substrate
Total worst-case etching time
∴ tT =
h f (1 + δ )
v f (1 − φ f )
Professor J. Bokor, U.C. Berkeley
⋅ (1 + ∆ )

h1 
∆ = 

h 
f
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Professor J. Bokor, U.C. Berkeley
Etching
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Stringers and Sidewall Spacers
step
h1
h
Etching
Without over-etch:
“Stringer” results on the
sidewall
substrate
Professor J. Bokor, U.C. Berkeley
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Etching
Lightly Doped Source/Drain MOSFET (LDD)
CVD oxide
spacer
n+
n
n
n+
SiO2
p-sub
The n-pockets (LDD) doped to medium conc (~1E18) are used to
smear out the strong E-field between the channel and heavily doped n+
S/D, in order to reduce hot-carrier generation.
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“Spacer lithography”
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Etching
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Etching
“Self-Aligned double patterning (SADP)”
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Etching
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, SADP
• Pattern and Aspect ratio Dependence
Professor J. Bokor, U.C. Berkeley