3 Cleaning Wet Etch

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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

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3 Cleaning Wet Etch