Please check the paper published on mems2000 conference.
"An improved TMAH Si-etching solution without attacking exposed aluminum".
The solution works very well.
Good luck

> Dear friends,
>
> i am trying to do TMAH etching of silicon under the
> cantilever beam  made of (silicon
> dioxide+polysilicon).
>
> We have used the solution of TMAH(10% w/w in
> water)+SA(40% aqueous solution)+Ammonium peroxy
> disulphate(APODS about 1.25gms).
>
> we have heated the entire solution @ 80 degree
> centigrade and chips were etched in this solution.
>
> But,we could see that Al bond pads,which were opened
> thru silicon dioxide passivation layer(9000 Angs)were
> attacked by the etching solution.
>
> Can anyone of u please suggest me any good etching
> technique for silicon that should not attack Aluminum.
>
> thanks
> Chakry