Scott,

You don't mention the process envelope acceptable to your base device.
Presuming you have the world available to you I'd recommend MgF for the
thick layer deposited by evaporative coating however you'll need to heat
your substrate to get a robust film.  Then 2 to 3 microns is a hell of a
lot to deposit by evaporation.

A second approach would be a meso-porous silica deposited via sol-gel.
Of course this is porous which, in a aqueous environment could be a
problem.

Finally, there are a number of spin on fluorinated polymers that will
get you to 1.35 or lower but achieving acceptable adhesion for the SiO2
capping layer would be a challenge.

Good luck,
Matt

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Scott McWilliams
Sent: Wednesday, January 24, 2007 8:42 AM
To: General MEMS discussion
Subject: [mems-talk] low index SiO2 or other material

Hello All, I need your help in figuring out how to fabricate the
following stack:

Substrate:  Silicon Wafer
First layer:  A material that is 2-3 microns thick that has an index of
diffraction at 785 nm of around 1.33-1.39 (as close as water as
possible).
Second Layer:  10 microns or thicker of SiO2 with an index of around
1.46.