Professor N Cheung, U.C. Berkeley
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Lecture 15
Examples
HF solution
mask SiO2/Si etched by HF solution SiO2 Si
SSiO2, Si Selectivity is very large ( ~ infinity)
(c) Overetching around step
h2 step h1 h2
Overetch time (fraction)
h1 + h2 = h2
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Professor N Cheung, U.C. Berkeley
EE143 F05
Lecture 15
total
∴ tT =
vf ( φf 1
Ufm = uniformity factor
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Professor N Cheung, U.C. Berkeley
EE143 F05
Lecture 15
Wet Etching
1 2 3
1 2 3
Reactant transport to surface Selective and controlled reaction of etchant with the film to be etched Transport of by-products away from surface
Goal : Minimize W
Small W
θ → 90 o
v f >> ν m⊥ hf small
Professor N Cheung, U.C. Berkeley
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Lecture 15
Etching Selectivity Sfm between film material and mask material
step
Mask film
Substrate (a) Film thickness variation: target thickness value
hf (max) = hf ( 1+ δ )
variation factor
Professor N Cheung, U.C. Berkeley
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Lecture 15
(1) Bias
dm hf substrate df dm etching mask film
Bias B ≡ d f d m B can be > 0 or < 0.
substrate
df
Professor N Cheung, U.C. Berkeley
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(phosphoric acid)
(180oC: ~100 A/min etch rate) Typical selectivities:
– 10:1 for nitride over oxide – 30:1 for nitride over Si
Professor N Cheung, U.C. Berkeley
EE143 F05
Lecture 15
Etching
Etching Terminology Etching Considerations for ICs Wet Etching Reactive Ion Etching (plasma etching)
Professor N Cheung, U.C. Berkeley
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Professor N Cheung, U.C. Berkeley
EE143 F05
Lecture 15
Effect of Slow {111} Etching with KOH or EDP
Lecture 15
Complete Isotropic Etching Vertical Etching = Lateral Etching Rate B = 2 × hf Complete Anisotropic Etching
Lateral Etching rate = 0 B=0
Professor N Cheung, U.C. Berkeley
Etching of Steps with a Slope
vv t
θ
vv t
x1 x2
θ
start final film
x1 = vv t cot θ x2 = vl t x = x1 + x 2
substrate
Let etching time = t v v = vertical etch rate vl = lateral etch rate ∴ To minimize x make θ large
SiO2/Si etched by RIE (e.g. CF4)
SSiO2, Si Selectivity is finite ( ~ 10 )
Professor N Cheung, U.C. Berkeley
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Lecture 15
For a given allowable W/2 , what is the minimum selectivity required?
= ( vv cot θ + vl ) t
cot θ = 1/ tan θ = b / a
Professor N Cheung, U.C. Berkeley
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Lecture 15
Example
start
θ = 90
o
vvt vlt film sub final
θ =0
o
film sub
Professor N Cheung, U.C. Berkeley
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Si wet etching mechanisms
Professor N Cheung, U.C. Berkeley
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Lecture 15
Wet Etch Processes
(~30oC)
6H+ + 2Al → 3H2 + 2Al3+
(Al3+ is water-soluble)
Professor N Cheung, U.C. Berkeley
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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)
h f (1 + δ
)
)
(1 + )
h1 = h2
Let v m⊥ , v m / / be the vertical and lateral etching rates of the mask. Let v f be the vertical etching rate of the film. (ignoring lateral film rate for simplicity)
Professor N Cheung, U.C. Berkeley
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Lecture 15
Wet Etching (cont.)
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
S fm ≡
Vf ⊥ Vm ⊥
(vertical components only )
Wet Etching
Sfm is controlled by: chemicals, concentration, temp.
RIE
Sfm is controlled by: plasma parameters, plasma chemistry, gas pressure, flow rate & temperature.
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(2) Degree of Anisotropy
Af
B ≡1 2h f
anisotropic
0 ≤ Af ≤ 1
isotropic
∴ B = 2hf
B =0
Professor N Cheung, U.C. Berkeley
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Lecture 15
(1) Silicon Dioxide To etch SiO2 film on Si, use HF + H2O
18 26 T (oC) Etch rate (A/min) 6:1 BOE 1200 650
SiO2 + 6HF → H2 + SiF6 + 2H2O
Note: HF is usually buffered with NH4F to maintain [H+] at a constant level (for constant etch rate)