I recently grew a thermal oxide on a standard silicon wafer, then milled a
number of holes through the oxide layer with a FIB. The oxide was 200nm and
the holes were 100nm-2um wide. After milling the holes, I tried to do a wet
etch of the silicon layer underneath, but in many cases nothing happened. In
those cases where etching did succeed, I was left with a disc in the hole in
the shape of the pattern I milled. I tried this with non-circular holes to
be sure. The discs are not attacked by piranha, HCl or aqua regia, so I
don't think they are any kind of Ga product or leftover Pt residue from the
system.

My theory is that they are SiO2, created when the incoming ions knocked
oxygen from the original oxide layer deeper into the substrate. Has anyone
seen this before? Any reason why that is or is not a decent explanation?
Anything I can do about it? Maybe mill at a lower voltage? Any suggestions
would be appreciated

-Morgan