> at the moment, I am trying to deposit silicon oxynitride
> films by PECVD, using TEOS-N2-O2-NH3 gaz mixtures. In order
> to characterise the samples, I have only a prism coupler,
> which enables me to get the index of refraction and the
> thickness.Could anyone help me to find some relation between
> the residual stress and/or the index of refraction . I am
> looking also for a relation between the stoicheometry and the
> index of refraction.

You may find the following papers helpful:

Gunde et al, "The relationship between the macroscopic properties of
PECVD silicon nitride and oxynitride layers and the characteristics of
their networks," Appl Phys A74, pp181-186, 2002.

Jozwik et al, "Interferometry system for the mechanical characterization
of membranes with silicon oxynitride thin films fabricated by PECVD,"
SPIE 4945, pp79-84, 2003.

The second paper compares residual stress and index of refraction.
These papers should only be used to get an idea of the techniques, since
both films were silane-based and will likely differ from yours.  Good
luck,

John Maloney
Sr MEMS Engineer
MicroCHIPS, Inc.
www.mchips.com
www.john.maloney.org