Rahul,

There are very good papers characterizing KOH etching available. These should
get you good initial estimates for etch rates.

1) Temperature: You must control the temperature well since the etch rate can
double for every 10 degree change. You should be able to achieve plus or minus 1
degree control with an etch tank.

2) Concentration: If you have a cover on the tank and you have a large volume of
etchant compared to the amount of silicon you are removing, then your
concentration change should not be a problem at 40%. You can look at the
equation for the etch from the papers and calculate how many OH groups will be
consumed during the etch and estimate the concentration change. Just estimating
I would say that you could etch 20 wafers, where the cavity is the only thing
etching, in a two gallon tank without worrying about the concentration change
due to etching. If you lose water during the etch the concentration will go up
and the etch rate will go down, between 25-50% KOH. Consuming too large an
amount of silicon per volume of KOH will cause the concentration to go down and
the etch rate to increase, between 25-50% KOH. If you use a lower concentration
then adjitation can become important to achieve a uniform etch. The paper will
show that there is a maximum etch rate around 25% KOH concentration. KOH becomes
less selective to the 111 sidewalls if you use low concentrations but 40% is
about as good as you can get.

3) Etch stop: If you are not using an etch stop such as the oxide layer of an
SOI wafer or a P+ layer then you must worry about the initial thickness control
of your wafer. With an etch stop you can achieve less than 1 um of thickness
control in a diaphragm.

4) Diaphragm size: The alignment of the mask to the crystal plane is important
for diaphragm size.

5) Additives: The etch rates, need for adjitation, etch rates of different
crystal planes, and surface morphology can be changed by using additives such as
alcohol. My advice would be to stick to KOH and water with no additives unless
you cannot achieve your goals.

6) Protecting the top side of the wafer: There are commercially available etch
fixtures and commercially available protective spin-on films

Dan Chilcott
ITT


-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org] On
Behalf Of rahul goswami goswami
Sent: Thursday, October 01, 2009 4:39 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] KOH etching

hi!
           How would I standardize KOH etching for 40% koh.  I have to realize
membrane for a pressure sensor of dimension: 500 by 500 and 500 by 1000..

rahul