Richard,
I would consider adding a layer of polysilicon on top of the nitride to
prtect the surface. The polysilicon will dissolve when immersed in KOH.

Michael Pedersen

Haigh, Richard wrote:

> I am currently trying to etch a Si wafer with hot KOH. There is a Si3N4
> mask (1 um) on the back face wafer. There is a ceramic film on the other
> face, this film requires several high temperature treatments up to 750 C
> (drying and sintering), before the etching can be started. I do not
> believe that these thermal treatments damage the nitride. However
> continued removal and replacement of the nitride mask onto a hotplate
> might result in tiny scratches in the nitride and lead to pinholes in
> the etched sample.
>
> After etching of the back face for >12 hours, there are large pinholes
> (300 um wide by 600 um deep) in my samples. How can I protect the
> nitride from scratching / damage?