Xiaochuan,
you can try to do a multi-stage oxidation. you will still have a small
step height that may cause problems with spinning resist but it will be
reduced. there is also a type of photoresist that is sputterable which
is a better solution, but I do not have specific info on this. for the
oxidation you can do...
1. oxidation
2. pattern structure A
3. oxidation
4. pattern structure B
5. oxidation
6. pattern structure C

this will leave no oxide on structure C, 1 layer on structure B, 2
layers on structure A, and 3 layers everywhere else. it requires careful
etching of the oxide to stop at the right time and I would keep each
layer about 0.1um thick at the most. at lower temps, koh is more
selective between oxide and silicon. I have done room temp koh etches
150um deep while consuming about 0.1um of oxide. you may try doing the
two shallow structures this way and getting the through holes with deep
rie.

Chris Foreman

-----Original Message-----
From: Xiaochuan Zhou [mailto:xczhou@xeotron.com]
Sent: Saturday, August 14, 1999 12:43 AM
To: MEMS@ISI.EDU
Subject: Help on processes


Dear colleagues:

I would like to get suggestions on fabrication processes.  I am
designing a
process for fabricating a device containing three levels of etched
features
on a silicon wafer.  The features include basins of about 10 micron
deep,
trenches of about 50 micron wide and 50 micron deep, and narrow
through-holes (etched all-the-way through the silicon wafers of 300 to
450
micron thick).  I am considering the use of Si(110) substrates and KOH
wet
etching for achieving the arrow through-holes.  It seems that I have to
go
through at least three etching steps in order to achieve three levels of
depth.  My difficulty is at the photoresist coating between the steps.
Any
one of my device features would cause sufficient corrugations on the
substrate surface and make uniform photoresist coating impossible.  I
would
like to hear any suggestions on the overall process design, suitable
etching
processes (wet or dry), and any tricks and/or materials that may make
the
fabrication easier.

Thank you very much.