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: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org] 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.