Stability of low refractive index PECVD silicon oxynitride layers

Low refractive index Silicon Oxynitride (SiON) layers were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) using SiH4/N2 and N2O. At an index less than 1.473 the as-deposited layers appeared to be unstable in time and sensitive to moisture as could be observed by a spectroscopic ellipsometer. The stability, probably due to partly open structures, could be improved by deposition at pressures as low as 650 mTorr. Stable very low index layers comparable to thermally grown stoichiometric SiO2 (n=1.457) can be obtained with post deposition heat treatment.