Hi,
Oxide and Nitride are good masking layers for TMAH
long etching. If you wish to try metal masking, you
may want to try Aluminum. There are two very
interesting literature collections you may want to
refer:

1) "Hybrid Postprocessing Etching for CMOS-Compatible
MEMS". Journal of MEMS, Vol.6, No.4, Dec 1997.

2) "Micromachined Intrumentation Systems". Chapter 2
of Erno Hilbrand Klaassen's May 1996 Ph.D.
dissertation (Stanford University).

thanks,
Srikoundinya P.
OSD-M
SITAR
Bangalore, India