Dear Roger

Yes you're right - anisotropic etch is just asking for trouble.
But it is very hard to control HF:AC:HNO3 solution.
Secondly I have  trenches on the wafer and it must etch by anisotropic etch .
I have to tell you that TMAH 25% thin the wafer well but when the concentration
decrease  I can't fix it because 25% is the highest TMAH concentration .

I know that some people thin silicon wafer with TMAH.

If you have some ideas for TMAH thinning by etching I will glad to hear.

Thank you, Moshe