Hi Andrea,

I have simply used 40% HF in a container and suspended my wafer
(etch-side down) on top of it. Some references prescribe heating it to
approx. 40 degrees celsius, but I haven't had any problems with it at
room temperature. Of course, I was etching SiO2 and not SiGe, so chances
are that it's not your solution. Since you potentially need 250µm
underetching with a 20 nm device layer you need to have extreme selectivity.

If you use liquid etchants, I'm afraid that you will have problems with
capillary forces and/or wetting. The real problem of liquid etchants
probably is to get it out after etching without bonding your newly
created Si film to the substrate because of trapped water. This is the
main reason I went with gas-phase etching. Alternatives include critical
point drying, but I'm sure people on mems-talk have other ideas.

// Morten

Andrea Mazzolari wrote:
> Hi Morten,
> the structure will stay in vertical position, so i should not have
> problems related to gravity.
>
> I think gas-phase etching is a very good possibility. Does someone know
> some etchant for SiGe which will preserve the Si layer ?
>
> Thanks,
> Andrea