Mechanisms of network rearrangement and compaction in a-SiNx:H films exposed to synchrotron radiation

The network of hydrogenated silicon nitride (a-SiNx:H) films deposited by plasma-enhanced chemical vapor deposition is compacted when the film is irradiated by high-energy photons in the range from vacuum-ultraviolet to soft X-rays (hν>100 eV). In situ monitoring by spectroscopic ellipsometry of the change in the film’s optical response discriminated between the photon-stimulated structural change and the solely temperature-dependent change of the dielectric constant. To elucidate the compositional difference before and after irradiation, the dielectric function of a-SiNx:H was modelled by a mixture of its component dielectric functions, i.e., of Si3N4, crystalline Si, and voids, under the Bruggeman effective medium approximation. Irradiation decreased the nominal volumes of void and c-Si components from their initial values by 44% and 49%, respectively. The resulting total 9.8% reduction in thickness can be explained by the compaction model. The photolytic origin of the reaction was demonstrated by the quick commencement and termination of the changes in optical evolution in response to the supply and cessation of irradiation, as well as by the insensitivity of the change in the refractive index to the temperature of irradiation. The microscopic process which is primarily responsible for the rearrangement of the network is cross-linking between the H-terminated –SiHx and –NHy species followed by the release of H2. In contrast to this, the creation of dangling bonds is a minor process.