Duan, papers have been published on this topic, see

1) Experimental investigation of high Si/Al selectivity during
anisotropic etching in tetra-methyl ammonium hydroxide
 J. Vac. Sci. Technol. A 16(2), Mar/Apr 1998 pp 868-872

2) Silicon Anisotropic Etching without Attacking Aluminum with Si and
Oxidizing Agent Dissolved in TMAH Solution.
 IEEE 12th International conference on solid state sensors,actuators,
and microsystems, Boston, June 8-12 2003 pp1667-1670

Both have data about different concentrations of etchant and buffer, and
even the order of adding the chemicals. If you can't get the papers,
email me.
Andrew

QUOTE:
;Hi, dear all

;I am planning to do TMAH etching of silicon wafer with aluminum
;pattern on it.  I want to achieve as low etch rate for Al as possible.
; Some people add some silicon powder and some other additive, like
;amonium nitrate to do that.  Does anyone have some experience in doing
;so?  How about the experimental setup and the condition?
;
;Thanks ahead.
;
;Duan