With the selection of gases you have available there may be several
options.  A vertical profile in Si should be possible with SF6 and O2.
You might start with a 1:1 mixture.  You can then smooth the etched
surfaces with the addition of CHF3.

Look for papers on The Black Silicon Method by Henri Janson, e.g. J.
Micromech. Microeng. 5 (1995) 115-120.

Roger Shile

-----Original Message-----
From: mems-talk-bounces+rshile=nanoink.net@memsnet.org
[mailto:mems-talk-bounces+rshile=nanoink.net@memsnet.org] On Behalf Of
Nate Lawrence
Sent: Monday, August 23, 2010 7:55 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] Anisotropic RIE Etching of Si and SiN

Hi,

I'm trying to find a good RIE recipe for anisotropically etching Si
(crystalline and poly-Si) and SiN while maintaining straight and as
smooth
as possible sidewall profile. I'm using a metal etch mask (have tried
both
Cr and Ni) and would like some selectivity over SiO2, though high
selectivity is not crucial. I would like to be able to etch vertically
as
much as possible, my current goal is 2um. I have a plasma therm RIE
system
with the following gases

Channel1 Ar, CH4, He
Channel2 O2, H2
Channel3 SF6, CBrF3, CHF3, CF4
Channel4 Cl2

All combinations of gases I have tried with O2 seem to give poor
sidewall
profile. My latest attempt was CF4/H2 with the hope of forming some
polymer
passivation on the sidewalls though this the etch rate of crystalline Si
seems too slow in this process and my mask is not holding up to as deep
an
etch as I want (also no selectivity to SiO2).

If anyone has any suggestions I would greatly appreciate it! I have read
that etching with Cl or Br may improve the process and give selectivity
to
SiO2 though I have no experience myself with those gases.

Thanks!

Nate Lawrence
PhD Candidate
Department of Electrical and Computer Engineering,
Boston University