Probably HF-vapor phase etching would be best for this application.
Unfortunately I don't know of anyone that makes a system like that anymore. The
ability to produce HF vapor at reduced pressure allows the etching to take place
in tight areas like you have described. Even so it might take a long time to
etch the oxide away and then you might have to deal with residual HF being left
behind. A water clean will probably not work for the same reason wet HF doesn't
reach far enough under you layer and then you have to dry it. I do have some
other ideas if you want to contact me directly.

Bob Henderson
480-968-8818 x 11