I agree with Mr. Larsson, the additional PR coating will only reduce the
pattern resolution. I have been doing glass etch for a while, hard mask
is definitely better choice. Some paper mentioned PR coating can fill
the local pin-hole and will protect the sample from local HF
penetration. I believe how to select the right etch mask really depends
on your application requirement.

Hope this helps

Michael ZHU


-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of Michael Larsson
Sent: Friday, November 16, 2007 12:57 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] Best resist for HF etching

Hi Mike,

Why use photoresist at all? You could just use a patterned layer of a
Cr as the mask. In my experience, even buffered HF attacks
photoresist. It may stick, but it will degrade the longer it remains
in the etchant. Of course there is no harm in leaving the photoresist
layer on the Cr, but as it absorbs liquid from the solution, the edges
will soften, expand and warp, affecting the resolution of pattern
transfer. Also, if you wet-etch the Cr, the Cr etchant will attack the
photoresist so you will no longer have a pristine photoresist mask for
the subsequent HF step.
 Pattern the Cr, strip off the resist, run an O2 plasma descum, then
etch the Pyrex in your chosen HF mixture. Good luck! :)

Regards,

Michael