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.

I realize the challenge is the material with the index of < 1.4.  I am wondering
if spin-on-glass would be a solution for this layer?  Also, can layers of 10
microns and thicker of SiO2 be grown in PECVD without stress or cracking
problems?

I tried to to shift the index of SiO2 below 1.46.  I only have N20, Silane, and
N2 available.  I was unsuccessful and only obtained an index of 1.44 at 785nm.
Is it possible to adjust the index a little lower?

Is sol-gel or other material  a solution?

Thank you,

Scott