Cleaning and wet etching
Contents
•Cleaning of Si wafers
•Mechanism of wet etching
•Etching chemistry of Si and III-V semiconductors
1
Cleaning
• The most frequent use of wet etching
– Cleaning of a Si wafer
• Target
particles
alkaline metals, heavy metals
organics
native oxide of Si
• Method
dissolution into a solvent
lift-off
prevention of re-adsorption
formation of complex ions such as Cu→Cu2+→Cu(NH3)42+
etching
2
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
• SPM（sulfonic acid and hydrogen peroxide mixture）
– Oxidation of organics
• H2SO4 + H2O2 → H2SO5 + H2O
– Strong removal of resists
– Caution: sulfur tends to remain on the surface
3
Typical solutions for etching
Name
conditions
Target of
removal
Side effects
pH
Surface
oxide
SC1
APM
NH4OH:H2O2:H2O
=1:1:5
70～80℃, 10 min
Particles
Organics
Metal
contamination
10-12
formed
SC2
HPM
HCl:H2O2:H2O
=1:1:5
70～80℃, 10 min
Metals
Particles
adsorption
0-2
formed
SPM
H2SO4:H2O2
=4:1
100～120℃, 10 min
Organics
Metals
Particles
adsorption
0-2
formed
Diluted HF
HF 0.5～10%
Native oxide of
Si
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%
4
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 calcium
gluconate gel (this must be equipped aside any draft chamber)
5
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
6
Anisotropic wet etching
(001) surface
(111) limiting
△
▲
isotropic
(110) limiting
(100)
(111)
(110)
with IPA 250 ml/L
7
Anisotropic wet etching
• The crystallographic surface with the minimum etching
rate 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
8
KOH etching of Si (anisotropy)
Wind RA, Jones H, Little MJ, Hines MA. J Phys Chem B 2002;106(7):1557–69.
9
KOH etching of Si (mechanism)
2-step reaction
Si (100) surface
Si (111) surface
(1) OH- attacks H-terminated Si
H
H
H
Si
Si
Si
Si
Si
Si
Si
Si-H is more instable
(2) OH termination of Si weakens SiSi bond
dissolution as Si(OH)2 into water
Si-H is regenerated
Etching rate
Step >> terrace (surface atomic structure)
- Si atoms at steps are instable
- Steps on (100) surface is less stable
than the steps on (111) surface
high etching rate on (100) surface
J.J. Kelly, H.G.G. Philipsen / Current Opinion in Solid State and Materials Science 9 (2005) 84–90
10
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
(a) Ox+→Red + h+(VB)
(b) GaAs + 6 h+ (VB)→ Ga3+ + As3+
– 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.
11
Examples of etching mechanism
12
Band bending at semiconductor-liquid interface
13
Surface charge of a semiconductor in a solution
14
Mott-Schottky plot
1
2


V - V fb 
2
 0 N De
CSC
This method is sometimes
used for the characterization
of dopant ion concentration.
(probably the same as carrier
concentration)
15
The “Energy level” of a redox system
•
A chemical redox reaction
Red = Ox+ + eequilibrium potential: E0
Applied bias E>E0（more positive）： Red  Ox+ + e- (e- extraction from liq.)
Applied bias E<E0（more negative）： Red  Ox+ + e- (e- injection to liq.)
Energy of an electron (more negative to the upward direction)
Vacuum
level
eEF
E0
e-
E0
e-
E0
EF
EF
E=E0
E>E0（positive）
E<E0（negative）
16
Equilibrium potentials for typical redox systems
17
Band-edge positions for typical semiconductors
18
Charge neutrality levels in a semiconductor
Van de Walle,
Nature 423,
p. 626 (2003)
E0 for
H- in solid =
H+ in solid +
e-
■Charge neutrality level agrees with the potential for normal hydrogen electrode
H+ + e- = ½ H2
19
Redox system with a semiconductor electrode
20
Molecular density of states in a solution
Energy diagram for a reduced/oxidized molecule
Rearrangement of
water molecules
Oxidation of
a reduced
molecule
Drawing energy changes with
respect to a single molecule
Reduction of
an oxidized
molecule
Energy change for a molecule
Redox potential is actually the energy difference for an molecule in reduced/oxidized state
stabilized by surrounding water molecules.
=
21
Charge transfer at a semiconductor/liquid interface
Equilibrium
(a) Ox+ + e- (CB) → Red
(b) Red → Ox+ + e- (CB)
Rate of reduction and oxidation
are equal.
22
Electroless dissolution (p-GaAs)
(a) Ox+→Red + h+(VB)
(b) GaAs + 6 h+ (VB)→ Ga3+ + As3+
Open circuit  ia = ib
Anodic current
(hole injection to a solution)
Overall reaction
GaAs + 6Ox+ → Ga3+ + As3+ + 6Red
Dissolution of GaAs proceeds
E
GaAs
Fe(CN)63h+
Fe(CN)64Ga3+ + As3+
(dissolution)
23
Electroless dissolution (n-GaAs)
(a) Ox+→Red + h+(VB)
(b) GaAs + 6 h+ (VB)→ Ga3+ + As3+
No hole (h+) at VB edge (n-type)
Surface accumulation of h+ injected by
reaction (a)
 h+ immediately promotes reaction (b)
24
Dissolution by chemical reactions
• Reaction without charge transfer
• Symmetric molecules are necessary
such as Br2 and H2O2
25
Dissolution by chemical reaction
impact of reagent concentration
• Only for HCl and HBr
– Below 5 mol L-1
 no progress of etching
HCl or HBr exists as ions,
not as neutral molecules
– Addition of acetic acid
 linear dependence on
reagent concentration
Suppressed ionization of HCl
or HBr
26
Etching with H2O2 and acid
• Acidic environment promotes dissolution of reaction products
etching rate also enhanced
• Rate-limiting step
– Acid conc. ＞ H2O2 conc.  surface reaction of H2O2
– H2O2 conc. ＞ acid conc.  dissolution of reaction product
27
Anisotropic etching of III-V compound
• Etching rate for different crystallographic surfaces
(110) > (111)B > (100) > (111)A
28
Polarization dependence of GaN wet etching rate
•
•
•
D. Zhuang, J.H. Edgar / Materials Science and Engineering R 48 (2005) 1–46
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
29
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
30
Typical etching solutions for Si
Solution
Si
3 HF (50%) + 5 HNO3 (70%) + 3
CH3COOH
Rate
(mm/min)
comment
Ref.
35
1
1 HF (50%) + 5 HNO3 (70%) +
2CH3COOH
+0.3g I2/250ml H2O
100HF + 0.1%HNO3
Light irradiation
7
1
50% KOH @70℃
(110) 1.0
Anisotropic
(100) 0.9
(100) etched faster
(111) <0.01 with isopropyl alcohol
Visualization of p-n
junctions
1
3
Room temperature is assumed if no temperature is specified.
31
Typical etching solutions for III-V semiconductors
GaAs
4 H2SO4 (98%) + 1 H2O2 (30%) + 1H2O 3
@50℃
1 NaOH (1N) + 1 H2O2 (0.8 N)
0.2
3 H3PO4 (85%) + 1 H2O2 (30%) + 50 H2O
0.1
5
5
7
1 H3PO4 (85%) + 9 H2O2 (30%) + 1 H2O
5
Br2 (1% ) + CH3OH
InP
HCl (12N)
9
12
Nonlinear dependence on HCl conc. if 4
diluted
1 HCl (12N) + 1 CH3COOH (17N)
6
Nonlinear dependence on HCl conc. if 4
diluted
1 HCl (12N) + 1 H3PO4 (17N)
1 HCl (12N) + 1 HNO3 (15N)
HBr (9N)
Br2 (1% ) + CH3OH
GaN
KOH
4
7
6.5
12
AlN
KOH
SiC
K3Fe(CN)6 100℃以上
Dependent
on
crystallographic 7
orientation
(111)A plane tends to appear
6
(111)A plane tends to appear
4
4
4
4
-
Very slow. Only N-polar surfaces are 2
etched.
2.3
Faster for N-polar surfaces, but Al- 2
polar surfaces are also etched with a
reduced rate.
Only Si-polar surfaces are etched.
2
Room temperature is assumed if no temperature is specified. “N” for concentration stands for “mol/L”.
32
References
[1] S. M. Sze：「半導体デバイス」（南日康夫，川辺光央，長谷川文夫 訳）産業図書
[2] D. Zhuang, J.H. Edgar, Materials Science and Engineering R 48 (2005) 1–46
[3] Wind RA, Jones H, Little MJ, Hines MA. J Phys Chem B 2002;106(7):1557–69
[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.
33
Etching solutions for oxides and metals
34