3 cleaning wet etch
TRANSCRIPT
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Cleaning and wet etching
Contents
Cleaning of Si wafers
Mechanism of wet etching
Etching chemistry of Si and III-V semiconductors
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Cleaning
The most frequent use of wet etching Cleaning of a Si wafer
Target
particles
alkaline metals, heavy metals
organicsnative oxide of Si
Method
dissolution into a solvent
lift-off
prevention of re-adsorptionformation of complex ions such as CuCu2+Cu(NH3)4
2+
etching
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Typical cleaning
RCA cleaning SC1 H2O2+NH4OH
Oxidizer: H2O2
Complex formation: NH4OH
Metal dissolution under high pH
but some metals such as Al, Fe cannot dissolve Particle removal etching + control of Zeta potential
SC2 H2O2+HCl
Removal of residual metals
dissolution under low pH
SPMsulfonic acid and hydrogen peroxide mixture
Oxidation of organics
H2SO4 + H2O2 H2SO5 + H2O
Strong removal of resists
Caution: sulfur tends to remain on the surface
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Typical solutions for etching
Name conditions Target of
removal
Side effects pH Surface
oxideSC1
APM
NH4OH:H2O2:H2O
=1:1:5
7080, 10 min
Particles
Organics
Metal
contamination
10-12 formed
SC2
HPM
HCl:H2O2:H2O
=1:1:5
7080, 10 min
Metals Particles
adsorption
0-2 formed
SPM H2SO4:H2O2=4:1
100120, 10 min
Organics
Metals
Particles
adsorption
0-2 formed
Diluted HF HF 0.510% Native oxide ofSi
Metals
(except for Cu)
Particles
adsorption
Cu deposition
(CuF2)
0-2 Removal
of SiO2
Buffered HF HF:NH4F
=7:1
Native oxide of
Si
Particles
adsorption
Cu deposition
0-2 Removal
of SiO2
Concentration of solutions above (approximated values):
NH4OH 28%, H2O2 30%, HCl 36%, H2SO4 98%, HF 50%, NH4F 40%
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Hydrogen termination of Si surface
HF: removal of SiO2
Hydrogen termination stable under atmosphere
HF reacts with bones and damage tissues
(with extreme pain)
If HF attaches your skin, wash intensively and treat with calciumgluconate gel (this must be equipped aside any draft chamber)
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Merit of wet etching and its applications
Low damage, large area Wrapping of a wafer surface, cleaning
Removal of surface damage induced by dry etching
Dependence on crystallographic orientation
Anisotropic shape suitable for MEMS etc.
High selectivity
Precise depth control by using etch-stop layer
Etch pits Evaluation of dislocation density
Doping dependence Characterization of a p-n junction
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Anisotropic wet etching
(100) (111) (110)
(001) surface
(111) limiting
isotropic
(110) limiting
with IPA 250 ml/L
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Anisotropic wet etching
The crystallographic surface with the minimum etchingrate appears
Etching solutions
KOH
TMAH (tetramethyl ammonium hydroxide);(CH3)4NOH
With KOH, the etching rates for Si crystal planes
(110) > (100) >> (111)
With IPA (100)>(110)>>(111)
The mechanism for different etching rates
A hypothesis: atomically flat and dense surfaces are
etched more slowly
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KOH etching of Si (anisotropy)
Wind RA, Jones H, Little MJ, Hines MA. J Phys Chem B 2002;106(7):155769.
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Category of etching mechanism
Anodic dissolution GaAs + 6h+(VB)Ga3+ + As3+ More likely for p-type semiconductors
Application of positive bias (hole supply) etching enhancement
For n-type semiconductors, light irradiation is necessary.
Electroless dissolution
Band alignment condition (along the electron energy axis)
Valence band edge > redox potential of a reaction
Chemical dissolution
Etching reagent: H2O2, Cl2, Br2, I2, OCl-, HCl, HBr
Etching rate is independent of the surface electric potential
No progress with the surface is covered with native oxide.
(a) Ox+Red + h+(VB)
(b) GaAs + 6 h+ (VB) Ga3+ + As3+
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Examples of etching mechanism
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Band bending at semiconductor-liquid interface
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Surface charge of a semiconductor in a solution
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Mott-Schottky plot
fbDSC
VVeNC
-
0
2
21
This method is sometimes
used for the characterizationof dopant ion concentration.
(probably the same as carrier
concentration)
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The Energy level of a redox system
A chemical redox reaction
Red = Ox+ + e- equilibrium potential: E0Applied bias E>E0more positive Red Ox
+ + e- (e- extraction from liq.)
Applied bias EE
0positive
EF
E (111)B > (100) > (111)A
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Polarization dependence of GaN wet etching rate
Etching rate
N polar >> Ga polar
Etching mechanism
OH- attacks a Ga atom
Oxidation of Ga
Dissolution of Ga oxide Why N-polar surface is etched faster
Dangling bond of N (filled with
electrons) are relatively sparse on
the surface
(charge repulsion between the
dangling bond and OH-
More Ga bonds with OH- are
exposed after N removal
D. Zhuang, J.H. Edgar / Materials Science and Engineering R 48 (2005) 146
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Rate-limiting processes of wet etching
Transport of etching reagent to the surface Surface reaction
Electrochemical
Chemical
Dissolution of etching products
Transport-limited viscous solution, high temperature
Isotropic (no dependence on crystallographic
orientation)
Rate dependence on pattern density
Surface-reaction limited less viscous, low temperature
Dependence on crystallographic orientation
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Typical etching solutions for Si
Solution Rate
(mm/min)
comment Ref.
Si3 HF (50%) + 5 HNO3 (70%) + 3CH3COOH
35 1
1 HF (50%) + 5 HNO3 (70%) +2CH3COOH
+0.3g I2/250ml H2O
7 1
100HF + 0.1%HNO3Light irradiation
Visualization of p-njunctions
1
50% KOH @70 (110) 1.0(100) 0.9(111)
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Typical etching solutions for III-V semiconductors
GaAs
4 H2SO4 (98%) + 1 H2O2 (30%) + 1H2O@50
3 5
1 NaOH (1N) + 1 H2O2 (0.8 N) 0.2 53 H3PO4 (85%) + 1 H2O2 (30%) + 50 H2O 0.1 7
1 H3PO4 (85%) + 9 H2O2 (30%) + 1 H2O 5 Dependent on crystallographicorientation
7
Br2 (1% ) + CH3OH 9 (111)Aplane tends to appear 6InP
HCl (12N) 12 Nonlinear dependence on HCl conc. if diluted 4
1 HCl (12N) + 1 CH3COOH (17N) 6 Nonlinear dependence on HCl conc. if diluted
4
1 HCl (12N) + 1 H3PO4 (17N) 4 41 HCl (12N) + 1 HNO3 (15N) 7 4HBr (9N) 6.5 4Br2 (1% ) + CH3OH 12 (111)Aplane tends to appear 4
GaNKOH - Very slow. Only N-polar surfaces areetched.
2
AlNKOH 2.3 Faster for N-polar surfaces, but Al-
polar surfaces are also etched with areduced rate.
2
SiCK3Fe(CN)6 100 Only Si-polar surfaces are etched. 2
Room temperature is assumed if no temperature is specified. N for concentration stands for mol/L.
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References
[1] S. M. Sze
[2] D. Zhuang, J.H. Edgar, Materials Science and Engineering R 48 (2005) 146[3] Wind RA, Jones H, Little MJ, Hines MA. J Phys Chem B 2002;106(7):155769[4] S. Adachi and H. kawaguchi, J. Electrochemical. Soc. 128 (1981) 1342-1349.[5] I. Shiota, K. Motoya, T. Ohmi, N. Miyamoto and J. Nishizawa, J. Electrochem. Soc. 124 (1977) 155-157.[6] Y. Tarui, Y. Komiya and H. Harada, J. Electrochem. Soc. 118 (1971) 118.[7] Y. Mori and N. Watanabe, J. Electrochem. Soc. 125 (1978) 1510-1514.
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Etching solutions for oxides and metals