To get a high etch rate of Si and low etch rate of SiO2 try SF6 with low
power density.

To get a high etch rate of SiO2 and low etch rate of Si try CF4 with H2,
vary the ratio of H2 to CF4 to get the selectivity you want.

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-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org]
On Behalf Of Vikram
Sent: Thursday, August 24, 2006 11:30 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] Hi

Hi Guys,

I am looking for an etch process which is selective to silicon over oxide
and the reverse. I have just summed up what I have done and the problem that
I get. If anyone of you have experienced the same problem or/and have
solution to the same your suggestion is appreciated.

Test:
  Ebeam Lithography on SOI wafer with the thickness of stack as below.
Patterns are exposed which are 50nm wide lines spaced 300nm apart and 5µm
long. The patterns are defined by etching silicon using HSQ as mask. Then I
want to etch silicon oxide up to a depth of 200nm without etching silicon.
Process that I used to etch silicon is HBr chemistry. During oxide etching
using C4F8 chemistry I etch HSQ completely and I end up etching also the
silicon which I want to avoid. If anyone has used a etch process for etching
silicon selective to silicon-oxide and then the reverse it would be nice if
you can help me.

  The last resort will be to increase the thickness of HSQ which I don’t
want to do now as I will have to do dose variation of my patterns.

  Top-Silicon 85nm
  Buried Oxide 400nm
  Bulk Silicon