Below are some approach to your inquiry.

1) Use of SOI wafer : high cost
2) Use of Si CMP process : complicated process
3) Use of etch stop (i.e. boron) : simple and low cost
    To achieve good uniformity of thin membrane having thickness
    of few microns, the following factors are important.
      A. Low oxygen concentration of wafer
      B. Optimum boron doping condition (temperature & time)
      C. The preceding process temperature after
           boron doping should be lower than the doping temp.
       D. High uniformity Si wafer thickness
       E. Contamination-free TMAH/KOH bath
       F. Highly controlled TMAH /KOH temp.
       G. Bath w/ mixer or N2 bubbler (almost negligible　for TMAH)

Best regards,
Regan

-----------------------------------------------------
>Date: Thu, 8 May 2003 12:02:34 -0400 (EDT)
>From: Sriram Akella 
>Subject: [mems-talk] Thin Membrane etching
>To: [email protected]
>Message-ID: <[email protected]>
>Content-Type: text/plain
>
>Hi Everyone,
>
>   I am trying to etch 2 micron thick membrane. I am
>using TMAH as the etchant, but the etchrate of TMAH
>maintained at 80C is varying a lot.
>
> Can anyone suggest a procedure that is reproduce able
>to a fairly large extent.
>
>regards,
>sriram