Hi Jeeva
You can use following solution
         HNO3 (69%)  :  H2O : NH4F (40%)
                 64% :  33% :  3%
             189 ml : 96 ml : 7.5 ml
The etch rate is about 100 A/sec but I will suggest you to optimize the etch
rate after making the solution of above mentioned compositions. You can also go
through the following references:

Williams et al, "Etch Rates for Micromachining part II", Journal of
Microelectromechanichs, Vol 12, No. 6, December 2003.

Williams et al, "Etch Rates for Micromachining", Journal of
Microelectromechanichs, Vol 5, No. 4, December 1996.etch

With best regards
Prem Pal
Yonsei University Seoul, South Korea

On Tue, 24 Jan 2006 jeeva S wrote :
>hi all,
>    We have a few silicon wafers with teflon coating on it. We would like to
>etch few holes into silicon (as we already did few holes onto the teflon).
>we need depths of 400nm in these silicon wafers. can anyone suggest a few
>isotropic etchants and etch rates for this silicon with teflon coating on
>it.
>