Real-time spectroellipsometry investigation of the interaction of silane with a Pd thin film: Formation of palladium silicides

Abstract

We present the application of spectroscopic phase modulated ellipsometry to the study of both ultrafast and slow processes of the interaction of silane (SiH4) with thin films of palladium. Changes of the optical properties of thin films exposed to different SiH4 fluxes are monitored by in situ single wavelength ellipsometry in the case of high fluxes which lead to ultrafast processes and by in situ spectroscopic ellipsometry at low fluxes and slow kinetics. The study of the interaction of SiH4 with Pd at 250 °C reveals the complicated character of the process which depends on the flux of silane and leads to the formation of palladium disilicide, palladium hydrides, and an intrinsic porosity. A qualitative model of the process is proposed. The initial stage of the reaction at high fluxes of SiH4 is dominated by a grain boundary diffusion of SiH4 inside the Pd film, followed by the catalytic decomposition of SiH4 and a strong process of an intrinsic formation of porosity. This fast process (duration of 10 ms) proceeds through the formation of a metastable Pd-rich phase. The appearance of the intrinsic porosity enhances the diffusion of SiH4 inside the Pd film, which results in a drastic acceleration of the silicide formation. At low SiH4 fluxes the characteristic time of the reaction increases up to some tens of minutes and proceeds through the formation of both silicide and palladium hydride.