FTIR phase-modulated ellipsometry was used to characterize the time evolution of the optical properties of hydrogenated amorphous silicon nitride thin films with embedded nanoparticles (ns-SiNx:H). Films were produced by PECVD at room temperature using both continuous wave and square wave r.f. power modulation from gas mixtures of SiH4, NH3 and Ar. The flow ratio R = [NH3] / [SiH4] was varied from 1 to 6. When ns-SiNx: H thin films are exposed to the open air they undergo a spontaneous oxidation process. Dense films deposited at low flow ratios and continuous wave r.f. power are partially oxidized by O, as is denoted by the appearance of the SiO stretching band at 1070 cm(-1). Porous films deposited with square wave modulated r.f. power and with continuous wave r.f. power at high flow ratios are fully oxidized by moisture; the IR absorption bands related to N decrease and the IR absorption bands corresponding to H2O appear. TEM and HR-TEM characterization revealed that films deposited with continuous wave r.f. power present a columnar structure, while films deposited with square wave modulated r.f. power consist of a disordered arrangement of nanoparticles. SAED was used to probe the crystallinity of the nanoparticles. (C) 2003 Elsevier B.V. All rights reserved.