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

Thanks for your help.
vicky