Ken,

For a deposited dielectric material you might use a CVD SiO2 or better
yet one of the low k dielectric materials used in the semiconductor
industry (I'm not very familiar with these).

Someone else brought up the issue of stress in the dielectric causing
problems.  If you deposit the material by PECVD the stress can be
tailored by using an appropriate combination of low and high frequency
excitation.

Roger Shile

-----Original Message-----


Thanks to everyone for their suggestions.

Roger -

Yes, I'm proposing to add a thick oxide under the nitride. But, adding a

thick oxide layer on the back side of the wafer would be equally
suitable, so long as it covers the etched 111 silicon faces, but not the

suspended membranes. (I can't see a way to achieve this though, can
you?).

The SiO2:SiN selectivity for CHF3/O2 can be up to ~2:1 according to [1],

but you're right, this is not good enough. I think etching 90% through
the oxide with CHF3/O2 and then the final 10% with BOE would get close
to the desired features, but the results with BOE alone should be
tolerable and simpler to achieve.

Do you have any suggestions for an alternative dielectric material that
might be easier to incorporate into the membrane fabrication process?