HF solutions etch LPCVD Six Ny films only slowly,
but etch most PECVD SiN films, which are lower quality and also have a lot
of hydrogen, rapidly and can therefore be patterned with photoresist.
I'm not sure where sputtered SiN falls in relation to these, but it's worth
trying 5:1 BHF with your sputtered SiN.
    --Kirt Williams