>     can any one help me with wet etching of silicon. the
> etchant should etch
> apprx at nm/min. For the etchant, positive Photoresist should
> act as mask.

At U.C. Berkeley's Berkeley Sensor & Actuator Center (BSAC)
many theses and disserations are online.
Open the file
http://www-bsac.eecs.berkeley.edu/archive/kirt_williams_phd.zip
and see the file ChA4c_etch.table.pdf for a table of etch rates.
Also see the file ChA4_etching.pdf for the accomanying documentation.

        --Kirt Williams Agilent Technologies