Hi Duan,

What type of photoresist are you using? TMAH is a developer for DQN-based
positive photoresist, which is the common type you find in AZ resists
(don't know about Shipley's because I haven't used it, but I won't be
surprised they are the same type). Depends on your TMAH concentration and
temperature during wet etching, perhaps your photoresist is being
developed at the same time. Remember, unexposed resist is also soluble in
developer, even though they are slower. If TMAH concentration is too high,
even unexposed resist is highly sensitive to the solution. Step coverage
of your resist in a 50 micron trench may also add to the problem. If
that's the case, you must find other types of masks for your wet etch
process.

Yours sincerely,

Isaac Chan, Ph.D.
University of Waterloo