Page 149 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH11.qxd 04/04/2005 19:48 Page 11.6

WET ETCHING

11.6 WAFER PROCESSING

The basic chemistry in wet chemical etching of group III-V materials involves oxidation of the sub-
strate surface and subsequent removal of the soluble reaction product. The etch rate can be limited either
by the diffusion process, i.e., diffusion of active etchant to the substrate surface, or the diffusion of sol-
uble product away from the surface, or the chemical reaction at the substrate surface. The diffusion-
limited etching process can be highly anisotropic, but difficult to control in a reproducible fashion. The
reaction-limited etch is preferred in the device fabrication. It is generally insensitive to agitation, but
shows a marked temperature dependency illustrated by the Arrhenius equation. Depending on the etch
mixtures and actual application, the solution can either be heated or cooled to obtain controlled etch
rates. For group III-V materials with at least two different sublattices, a wet etch can produce a degree
of anisotropy during pattern transfer, because the solution usually etches different crystal orientations
with different rates. For example, the As faces in GaAs typically etch faster than the Ga faces.
Table 11.1 summarizes the most common etchant mixtures for GaAs, InP, InGaP, and other group
III-V semiconductor materials. These solutions usually contain strong acids such as phosphoric acid
(H PO ), sulfuric acid (H SO ), nitric acid (HNO ), HCl, HF, acetic acid, and citric acid. They some-
3 4 2 4 3
times also contain hydrogen peroxide (H O ), which is used to oxidize the substrate surface and dis-
2 2
solve the oxidized products.
For a GaAs substrate, both acidic and basic etchant mixtures can be used since Ga and As oxides
8
can be dissolved in both media. An ammonium hydroxide (NH OH)-H O -water mixture with
4 2 2
1:700 v/v ratio of NH OH:H O affords a very controlled etch rate of approximately 3000 Å/min. For
4 2 2
H SO -H O -water mixtures, the concentrations of sulfuric acid and peroxide have a major impact
2 4 2 2
on the etch process. A mirror-smooth after-etch surface can be obtained over a wide range of tem-
peratures using mixtures with either high sulfuric acid or peroxide concentration. When both concen-
trations are high, the etch rate is extremely rapid, resulting in a rough surface. A 4:1 H SO :H O ratio
2 4 2 2
has an etch rate of approximately 5000 Å/min.
The mixture of H PO -H O -water can be used to etch GaAs at slow, controlled rates. For mix-
3 4 2 2
tures with high phosphoric acid content, the etching process becomes diffusion-limited due to the
high viscosity of H PO .
3 4
Bromine-methanol solutions have very rapid etch rates for GaAs and GaSb (typically more than
5 µm/min at 25°C) and are used for polish etching. A photoresist is not a suitable mask due to the
attack by bromine, but patterned etching using SiO leads to grooves that have rounded bottoms.
2
Therefore they are often used for V-groove formation.
TABLE 11.1 Summary of Wet Etchant Solutions for Various Group III-V Materials
Group III-V materials Etchant mixtures Comment
GaAs Acid-H O -H O Acid: H SO , HNO , H PO ,
2 2 2 2 4 3 3 4
Base-H O -H O acetic acid with HNO
2 2 2 3
HNO -HF-H O Base: NH OH
3 2 4
KI-I -H O Selective for AlGaAs at low pH
2 2
Br -methanol Chemical polishing
2
InP HCl or HCl acid mixture-H O Acid mixture: mixtures of HCl with
2
HCl or HCl acid mixture-H O -H O HNO , H PO , acetic acid
2 2 2 3 3 4
HBr/HCl Selective for InP over InGaAsP
InGaP H PO -HCl-H O Highly selective over GaAs for low H PO
3 4 2 3 4
AlInP HCl-H O Selective over GaAs
2
GaSb HCl-H O, HCl-H O -H O HF-HNO
2 2 2 2 3
GaN NaOH-H O -H O Must be heated (80°C)
2 2 2
GaP Acid-H O -H O Etched in a similar mixture as GaAs
2 2 2
HF-HNO -H O
3 2
InSb Acid-H O -H O
2 2 2
HF-HNO -H O
3 2
InAs HCl or HCl acid mixture-H O
2
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.