> I am process restricted to using a 400 um thick silicon wafer but
> require a thickness of 300 um.  Is there a possibility of using some
> kind of post-processing "grinding" step or something similar
> to achieve
> this? What would be the practical concerns of performing this if it is
> possible?

Craig,

   You can definitely grind and polish silicon wafers to (almost)
whatever thickness you desire.  I've used Aptek Industries
(http://www.aptekindustries.com/) with very good results (thinned 525um
to 200um).  They were able to thin my wafers on one side without
damaging features on the other side.  Good luck,

John Maloney
Senior MEMS Engineer
MicroCHIPS, Inc.
http://www.mchips.com