For SiO2 surfaces, the positive resist won't stick in a flouride
solution unless it is buffered to around pH 7. You need to use BOE,
Buffered Oxide Etch. This is an old standard in wet oxide etching in the
semiconductor industry. It is a solution of ammonium flouride and HF.
Almost, any semiconductor chemical vendor makes them. They come with
surfactants and are called Superwet, etc. This is important for wetting
the wafer and getting good oxide etch, especially for contacts and other
small features and high aspect ratio features.

There is a classic paper on BOE etching back in the 70s on all this
where they did SiO2 etching with positive photoresist under a series of
pHs. It is one of the breakthroughs that enabled semiconductor
processing.

The problem has nothing to do with the type of resist. Any positive
resist at an adequate thickness to cover your topology and not have
pinholes should do, if the solution is buffered. None will stick, unless
there is some radically different positive resist chemistry out there,
with out pH buffering.

Also, you need to make sure your surface is properly prepared. If you
have monolayers of hydrocarbons on it your resist may not stick even
during develop.

The best method is sulfuric peroxide or an acid peroxide solution, good
rinsing and drying, and then Vapor Priming. Use HMDS, Hexamethyldisilane
(?) and it abstracts the OH groups off the surface, the other popular
theory is that it coats a monolayer of bonded organic groups to the
wafer surface, either way it works.

However, if your wafer has metal or something, just a good clean in
peroxide or just plain water will do. Vapor Prime is thought to be
restricted to SiO2 or Silicon, however, I think it works on metal
surfaces by being a form of chemical drying. I used it on metal layers
in photomasking for years. Though it may have been just an extra set of
suspenders.

What concerns me most about this question, which I have nothing against
in itself, is that we are building on what is already known. How is the
MEMS profession using what is already known and building knowledge in
general.

The Semiconductor industry progressed since almost everyone was running
hugh volumes of similar technologies and drove the technology, Moores
law and all that. MEMS has diverse technologies. Are we to be merely
derivative or largely derivative of semiconductor technology? Can we
progress technologically like semiconductor processing/

Ed


-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of Michael A Gingras
Sent: Thursday, November 15, 2007 11:13 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] Best resist for HF etching

Hello,

Me and my colleagues are trying to etch a 4 micron Pyrex layer with HF
using photoresist as a mask. We've seen delamination of the mask as seen
by others. We are currently using a Shipley 7 micron resist and 4:1
HF:H2O etch.

Any recommendations on the best resist and HF concentration to help us
get the whole film etched before delamination? Current the Pyrex is
etching about 300A/s, so we'd need something that would last 2-3
minutes.

Thanks
-Mike