I have been using RIE to etch Silicon to release the diaphragm (various thickness: 50 micron to 10 micron), Etching recipe that I am using is SF6: O2 (50:5), pressure 15 mtorr, power 200W. I have used following combinations as a hard mask to etching silicon 200+ micron but Hard mask is also getting etched with the silicon.
1. S1813+ Al (1 micron)
2. S1813+ Mo (500 nm)
3. S1813+ Cr (100 nm)
4. S1813+ Au (100 nm)
Please suggest me what should I do to release the diaphragm (like change in recipe, change in hard mask layer)
It might be better to use DRIE (Bosch process) for deep Si etching. A soft mask made of AZ resist will give 1:10 - 1:20 selectivity, so for your 200 um of Si, 10-20 microns of resists could be enough. Alternatively, you can use PECVD oxide as a hard etch mask. In that case the selectivity can be as high as 1:100, but to be on the safe side you can assume 1:50 or even 1:25 selectivity. So, a couple of microns of oxide should do the job.
Thanks Sergey but we don't have DRIE, so I have to do the releasing by RIE only.
As Sergey Gorelick suggested, PECVD deposition of SiNx would be my suggestion.
You may find the attached article very helpful, there are plenty of options but it will very much depend on what equipment you have access to:
Etch Rates For Micromachining Processing - Part Two
Kind regards,
Jules