Dear all,
  I'd like to share the information I recieved about isotropic wet
silicon etching with you. Thanks all those that give me advice.

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Questions:

Dear all:
  Could you please share some isotropic wet silicon etching experience
with me? I want to know the volume ratio of the HF:HNO3:HAC ,the etching
rate and the mask. According to my reference, the etching rate of silicon
is too quick (7 um/min) with SiO2 mask. I wonder if there is another good
volume ratio to provide slower (less than 1um/min) silcion etching still
with SiO2 as the mask.

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Answers:

1. hi:
you can try:
   (1) LPCVD Silicon Nitride (Si3N4)
   (2) PECVD Silicon Carbide (SiC)

I did succeful Si isotropic etching with (1) as mask

2. Li,
  Schwartz and Robbins published 4 papers back in the 60's and 70's on this
subject in the Journal of Electrochemical Society vols 106, 107, 108, and
123.  Should be able to find them in the library.  They are excellent papers
and should answer all of your questions.  Additionally, Mark Madow has
published an excellent summary of this and other such topics called
"Fundamentals of Microfabrication", published by CRC press.  Hope this
helps.

3. HI, Dear Gang,

You can try HF:HNO3:CH3COOH at 10ml:30ml:80ml. Professor Kovacs reported
etch rate is about 0.7 to 1 microns. I tried the formula once, the etch rate
is less than 2 microns.
(Note: According to the textbook, this recipe only etch heavily doped
silicon).

Thanks.

4. We use to use 2000 Angstroms of SiO2 as a mask and etch 5 micron cavities
(1/8 inch in diameter) to form pressure sensing diaphragms. We used to mix
7000 ml Nitric Acid (HNO3) and 60 ml Hydrofluoric Acid (HF) at room
temperature. To improve the etch rate repeatability we would drop a diced
raw chip of Silicon into the mixture about a minute before we would etch,
and we would bubble Carbon Dioxide through the mixture to provide agitation.
Etch rate was about 0.6 microns per minute and etch depth was very
controllable. This was for a commercial application and yield was VERY
important.

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Thanks againg to those that give me advice.

Regards,

Li Gang



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