Page 150 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH11.qxd 04/04/2005 19:48 Page 11.7

WET ETCHING

WET ETCHING 11.7

Wet etchants for InP are mainly based on HCl, which is combined with H O, H PO HNO , H O 2,
3
4,
3
2
2
9
or HBr. The etch rate depends on HCl concentration with etch rates as high as 5 µm/min. Dilution
with water is used to provide etch rates in the hundreds of Å/min range for device fabrication. Etch
rates with HCl-H PO -water are in the range of 900 to 10000 Å/min for a 1:4 acid to water ratio.
3 4
For other group III-V materials, they are generally etched with the combination of acids and
water or acids, peroxide, and water. For example, InGaP is etched with HCl-H PO -water. AlInP can
3 4
be selectively etched from an underlying GaAs layer with an HCl-H O mixture. GaSb and GaP can
2
be etched in HNO -HF, HNO -HCl, H SO -H O, or bromine-methanol. InAs is typically etched in
3 3 2 4 2
HCl based mixtures, HNO -HF, or HNO -HCl-H O.
3 3 2
For selective etching, the H PO -H O -water and H SO -H O -water mixtures will etch GaAs or
3 4 2 2 2 4 2 2
InGaAsP, but stop on InP, whereas HCl/H O will etch InP, not InGaAs or InGaAsP. For nonselective
2
etching, an HCl-HNO -H O mixture will remove both GaAs and InP with equal rates.
3 2
11.5 EQUIPMENT FOR WET ETCH
Immersion processors, or wet benches, are typically used for wet etching. A cassette of wafers is
immersed in the etching solution for a specific period of time, after which it is rinsed in DI water.
Immersion baths can be heated to achieve the desired temperature. Physical agitation can be used to
flush away contaminants and provide fresh solutions to the surface, particularly with viscous etch-
ing solutions. The agitation is accomplished in various ways including ultrasonic agitation, nitrogen
bubbling through the solution, or mechanical agitation. Filtered recirculation of chemicals can be
used to improve chemical cleanliness and reduce chemical consumption.
Rinse tanks and dryers can be major sources of particulate contamination. The proper design of
the rinse and dry steps is especially critical in HF-type wet etching where hydrophobic surfaces are
produced. There are three types of rinse systems—quick-dump rinse, cascade overflow rinse, and
spin rinse dry (SRD).
The quick-dump rinse tanks with top spray rapidly remove chemicals from the wafer surfaces and
periodically drain the chemical solution. The disadvantage of this method is bacterial growth in parts
of the system, especially the nozzle, and the inherent turbulence that can move particles through the
solution.
The cascade overflow system is less susceptible to particle contamination. Hot water is often used
for cascade rinses when viscous chemicals such as phosphoric acids need to be removed.
For SRD process, water is sprayed on the wafers as they rotate in a chamber. After the rinse, the
wafers are spun dry, often with nitrogen flowing through the chamber. The chamber needs to be
designed such that the water remaining on the walls after the rinse is not deposited on the wafer.
Several other techniques for drying have been used, such as hot nitrogen drying, vacuum drying,
slow pull drying, and the popular isopropyl alcohol (IPA) vapor drying.

11.6 ENVIRONMENTAL, HEALTH, AND SAFETY ISSUES

There are a lot of environmental, health and safety (EH&S) issues associated with the wet etching
process, which uses some of the most directly hazardous materials. Strong oxidizers such as hydro-
gen peroxides are highly reactive and cause damage similar to acid burns. HF is a particularly cor-
rosive hazard. Dilute HF causes delayed damage undetected till hours after exposure.
Strong acids and oxidizers are often mixed together and heated to elevated temperatures. Fume
generation can occur and spread highly corrosive droplets. Some metal etching solutions are partic-
ularly hazardous. For example, cyanide solutions used for gold etching can react with acid, generat-
ing the deadly gas HCN. Proper ventilation and personal protective equipment (PPE) such as face
shields, aprons, and safety goggles are required while working with these solutions. Disposal of large
quantities of these hazardous solutions can also be very costly.

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