Hi James,
May be you might want to consider E-beam evaporation
of chrome on your silicon. As you know Sputtering tend
to create pin holes that will enable HF to penetrate.
Best,
Haroon

James writes:

I am using a sputtered chrome mask to etch amorphous silicon 7000A thick
with HNA system. For the HNA system I am using 32 parts Acetic acid, 25
parts nitric, 1 part Hydrofluoric acid. I have tried two different
thicknesses for the chrome mask: 1200A and 2400A. The  total etch time in
HNA was for 1min and 45 secs. After the chrome mask is stripped off with
CR-7, the surface of the amorphous silicon is evidentally
damaged, with what appears to be pits etched in the surface.

My question to anyone out there is, what thickness of
a chrome mask is required to stop the HNA from getting through and
damaging the amorphous silicon?