Etching Bulk GaAs Wafers for Use as Cathodes


Wear gloves, hair net and an apron that has been wiped clean of dust
and fibers. DO NOT allow gloves to touch inside surfaces or  lips of containers used for degreasing, etching and hot DI water preparation.
The surfaces of areas nearby should be free of dust as well as sources
of particle-laden air.

1. Clean glassware:
 a. Fill glassware to be used with hot UVTOC water from Millipore    system dispenser. Add heated Millipore water from one of four
  liter beakers on hot plate.
 b. Sprinkle a little Alconox into vessels with clean water.
 c. Ultra-sonic clean in plastic dish so that edges of clean glassware do
  NOT touch edges of ultrasonic cleaner.
 d. 15 minutes ultrasonic then pour out Alconox+water and rinse with
  Millipore water.
 e. Repeat steps a. - d. two more times.
 f. Rinse beakers out with at least three changes of Millipore water.
 g. A clean 1 liter beaker can now be filled with Millipore water,
  heated, and used as ‘leach’ step for 100 ml beaker (400 ml    beaker used if 500 kV wafer). (Leach beakers by filling with    near-boiling Millipore water and pour off after 15-30 sec.)
  Rinse 100 ml beaker with fresh methanol to rid surfaces of    water.
 h. Cover cleaned glassware with a loosely-fitting tent of Al foil
  to keep dust from settling onto cleaned surfaces.

2. Prepare etch solution.
 Late in the glassware cleaning procedure or early in the degreasing procedure prepare the etch solution as follows.
 The etch solution is 1:1:100 volume ratio of reagents* 30% NH4OH:
30% H2O2: H2O (Millipore UVTOC-grade polished water, 18.2 M?-cm
4 ppb or better T.O.C.). Measure 3 ml of NH4OH and 3 ml of H2O2 into a 400 ml graduated beaker. Use a new polypropylene small graduated cylinder. Fill to 300 ml level with D.I. water. Cover beaker to avoid contamination due to air-borne or splashed contamination.

3. Use only stainless tweezers (that have been degreased in clean Acetone)
 to put wafer into a 100 ml beaker.
4. Fill beaker to half full with Trichloroethlyene.
5. Add enough hot water to small ultrasonic basin (with cold or cooling
 water) to make temperature 55 C (NO hotter; since wafers will
 shatter if ultrasonically cleaned in hot Acetone due to cavitation
 and uncontrolled agitation). Larger basin underneath small basin can be
 60-65 C.
6. Ultrasonically clean at 55 C for 3 minutes with the voltage setting
 (on an autotransformer supplying the ultrasonic device) turned
 down to 30 V or 40 V AC or so--look for signs of excessive agitation
 that should be avoided!

 Never measure the temperature of the degreasing solvent directly
 with the thermometer since this will simply foul the solution
 directly exposed to the wafer. Prepare for drying step by filling small  dewar with liquid nitrogen.

7. Decant (pour off) solvent into appropriate waste solvent beaker
 for appropriate disposal later.

8. Repeat steps 3 through 6 for Acetone then Methanol (3 minutes each
 at 55 C).

9. Immediately following last degrease step (decant methanol from beaker  containing wafer) fill beaker half full with etch solution prepared in  step 2. above.
 Allow wafer to etch, face (shiny side) up in the beaker, for 7 minutes;  gently agitate beaker occasionally.

 Decant used etch solution (into waste etch solution container for later  disposal). Rinse and decant three times with Special Anhydrous  methanol, taking care to drain as much of the methanol as possible  without risking wafer. Use clean stainless steel tweezers to help retain  wafer in beaker.

 On last decant step, leave enough methanol in beaker to keep wafer  immersed. Move beaker with immersed wafer to source of filtered  N2(g) blow-off from L N2 dewar.

10. Remove wafer from a small puddle of methanol with stainless tweezers
 directly into the stream of dry nitrogen (blow-off from small dewar
 with a 0.22 micron filter in the blow-off line). Dry the wafer
 completely, taking care to force any liquid methanol away from the  sensitive area (shiny surface of polished wafer). A receding, advancing,  then receding droplet of methanol will leave a stain with traces of  contamination on the surface of the wafer.

 One method:  tip the beaker at 45 degrees or so, push the wafer into a  position (the edge of the wafer is off the side and bottom of the beaker)  then grasp the very edge of the wafer with the tweezers. DO NOT  allow gloved fingers to touch methanol! Practice with a used wafer  before trying this with a properly cleaned wafer.

11. Put wafer into carrier and close lid with dry filtered nitrogen blowing
 gently across the wafer carrier.

Brief Summary of Procedure

Glassware Prep
 3 each 15 minutes in hot DI water and Alconox
 3 minutes each in 55 C Trichloroethylene, Acetone and Methanol

Etch Solution Prep
 1:1:100 30% NH4OH: 30% H2O2: H2O (Millipore UVTOC-grade  polished water, 18.2 M?-cm 4 ppb T.O.C. or better).

 7 minutes in etch solution at room temperature
Rinse and dry
 3 changes each in Special Anhydrous Methanol
 blow dry in dry, filtered nitrogen blow-off from dewar

List of Supplies

The R&D Chem Room has been used at CEBAF for the etch preparations
and procedures. This room has a wet bench, DI water (center lab supply)
a UVTOC Millipore water polishing system, and large-load ultrasonic cleaners.

Below is a list of supplies and appliances one needs to efficiently carry out
the procedures listed above.

A small desk-top ultrasonic cleaner
A 1000 W hot plate
1 ea thermometer 110C max.
A supply of aluminum foil, lint-free lab wipes, gloves (UHV-quality), a timer
or clock, Alconox glassware cleaner.

4 ea 1 liter Pyrex beakers (used only for heating DI water)
1 ea 1 liter new Pyrex beaker (after cleaned, for heating leach step water)
2 ea 400 ml Pyrex beakers (for 500 kV wafer etch) or 1 ea 400 ml beaker
 and 1 ea 100 ml beaker (for 100 kV wafer)
1 ea stainless steel tweezers
1 ea unopened bottle of each of ammonium hydroxide, hydrogen peroxide,
 special anhydrous methanol, trichloroethylene.
 1 ea bottles of Acetone and Methanol.

* List of reagents used in CEBAF’s Polarized Electron Source group’s cleaning procedures.

30% NH4OH Baker Analyzed A.C.S. Reagent Ammonium Hydroxide 28.0-30.0% 500 ml (J. T. Baker product code 9721-01).

30% H2O2 Baker Analyzed Hydrogen Peroxide (Stabilized) 30% C.P. 500 ml (J. T. Baker product code 2189-01).

Trichloroethylene Baker Analyzed A.C.S. Reagent, Stabilized 500 ml(J. T. Baker product code 9458-01).

Acetone Baker Analyzed Low Sodium CMOS Electronic Grade Acetone 4 liter (J. T. Baker product code 9005-05).

Methanol (for methanol rinse during glassware cleaning procedure). Baker Analyzed Low Sodium CMOS Electronic Grade Acetone 4 liter (J. T. Baker product code 9073-05).

Methanol (for methanol degreasing steps and final rinse and decant steps after etch procedure). Methanol GR Special Anhydrous 500 ml (EM Chemical product code MX0487-6).