Hi Julie,

This sort of thing has happened to me while patterning a nitride/oxide
structure. After I thought I had patterend the nitride (which was on the
top) using RIE, I put my wafers in BOE and nothing happened for a long
time. The problem quite obviously was that the nitride wasn't completely
gone, although there was no way to tell. A few additional seconds in the
RIE did the job. This could be going on in your case too, so perhaps you
can try extending the first DRIE process by a few more cycles to make sure
that all the silicon is etched away.

Another suggestion, if you process allows is to do both the DRIE steps
first and etch the oxide last.

good luck,

MANDAR DESHPANDE
http://www.uic.edu/~mdeshp1
Microsystems and Devices Laboratory, Mech. & Ind. Eng. Dept., University of
Illinois at Chicago
Phone: 312-413-2318
Fax: 312-413-0447