If you incorporate one or both of the following tricks, you will definately
improve the outcome: get a thicker thermal SiO2 underneath the nitride
and/or adding a layer of Cr/Au ontop of the nitride.

hv


Temmuz CEYHAN writes:

Hello,

I am trying to do back side etching to Si based SOI wafer entirely
(500um etch).  As etchant, I am using %30 KOH at 80 degrees for 7-8
hours.  In order to protect the topside, I am using 500nm thick nitride
layer, however, because of the void formations during PECVD SiN
deposition process, KOH etches everything on the topside.  DRIE is not
an option for me.  Finally I decided to paste my sample to glass in
order to etch only the backside.  Does anyone know of a paste, like
silver paste or epoxy, which can resist to extended hours of KOH etching
without getting etched at all?

OR!

Do anyone know of a way which I can protect the topside of my sample
during etching?

P.S.  As I read the archives, I learned that Protek B3 coating or
annealing to high temperatures (800 degrees) can work, however, as some
of the members say, coating is hard to remove and I can not anneal up to
800 or more degrees since my features include Al (it has melting point
of 660 degrees)..

Thank you.

Temmuz CEYHAN