Though I have not yet tried etching with TMAH, I recently looked into the
prospect of replacing our current EDP process with TMAH and found a couple of
papers that may be of interest to you:

J. Vac Sci. Technol A 18(2), Mar/ April 2000 page 738
J. Vac Sci. Technol A 16(2), Mar/ April 1998 page 868

The first paper indicates that when etching with TMAH in concentrations less
then 15% one must add an oxidizer such as ammonium persulfate in order to
maintain a constant etch rate and avoid hillock formation.

If you etch with a higher concentration of TMAH, a constant etch rate without
hillock formation can be maintained without adding ammonium persulfate.  However
the etch rate is much lower.