Recent in-situ studies of the evolution of surfaces and interfaces of thin films by spectroscopic phase modulated ellipsometry

Application of spectroscopic phase modulated ellipsometry (PME) to study both ultrafast and slow processes of interaction of silane (SiH4) with thin film Pd, and to the investigation of the growth kinetics of a-Si:H films deposited by RF glow discharge under UV light irradiation are presented. As compared to other ellipsometric techniques like rotating analyzer ellipsometry (RAE), the phase modulation uses a high frequency of about 50 kHz provided by a photoelastic modulator. Thus, PME allows one to reach 1-5 ms time resolution which permits faster realtime measurements than RAE. This remarkable feature of PME makes it particularly suitable for in-situ applications. Changes of optical properties of Pd thin films exposed to SiH4 at different 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 small fluxes and slow kinetics. The study reveals a complicated character of the process which depends on initial flux of silane and leads to formation of Pd disilicide, Pd hydride 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 grain boundary diffusion of SiH₄ inside the Pd film, followed by the catalytic decomposition of SiH₄ and a strong process of intrinsic porosity formation. The fast process (duration of tenths ms) proceeds through the formation of a metastable Pd-rich phase followed by a formation of Pd2Si. At low SiH₄ fluxes the characteristic time of the reaction increases to tenths min and it proceeds through formation of both silicide and Palladium hybride. A study of growth kinetics of a-Si:H films irradiated by an XeCl excimer laser shows that the UV light accelerates the initial stage of the deposition, slightly increases the deposition rate, and induces an increase of the film porosity and small changes of optical properties.