I try to thin 4" wafer with TMAH 5% and 18% and I get bad surface at the 5%
solution and at the 18%.

These are the steps in my process :

1.To make those TMAH concentration I purchse TMAH 25% and dilute with
    water .
2. Before the etching I dip the wafer for 20 sec. in BOE to strip the
     native oxide and still I get a bad surface.
3. The process temp. is 85 cel. degree
4. The aim is to thin the wafer from 500 micron to 450 micron

I'm looking for someone who has done this procees successfully .

Your answers are very important for me.

Thank you, Moshe