A novel diagnostic approach for studying silicon thin film growth

In the deposition of thin films, the material properties are formed through the interaction of gas phase species with the growing surface. The resulting surface kinetics and chemistry is determined by the reactivity of the different gas phase species as well as by the surface chemical nature and may involve physical and chemical mechanisms such as surface bombardment by energetic gas phase species and surface diffusion mechanisms. Therefore, to obtain a fundamental understanding of the growth process of thin films, these various aspects need to be studied by dedicated experiments, which are preferably carried out non-intrusively and in real-time during the deposition. The work in this thesis focuses on several of such dedicated experiments for the specific case that film growth takes place by reactive gas phase species, as for example created with a plasma or a hot filament. Three all-optical and real-time diagnostic techniques have been designed and implemented and they have been employed for the specific deposition system of hydrogenated amorphous silicon (a-Si:H), which has an exemplary role in the deposition of silicon-based films and other covalent systems. Two of the diagnostic techniques are based on the ultra-sensitive cavity ring-down spectroscopy (CRDS) method, which is an established technique for measuring low-density gas phase species: time-resolved CRDS for determining the surface reactivity of gas phase species and evanescent-wave CRDS for probing surface species. Real-time spectroscopic ellipsometry has been employed to investigate surface diffusion mechanisms while, in addition, a new ultrahigh vacuum reactor has been constructed to carry out these and other future dedicated experiments. The method of time-resolved cavity ring-down spectroscopy (t-CRDS) has been developed as a direct diagnostic to measure the surface reactivity of (low-density) gas phase species under regular thin film processing conditions. In t-CRDS, the surface reaction probability ß of a species is determined from time-resolved measurements of a time-modulated gas phase density in front of the substrate. For a-Si:H deposition in the so-called 'expanding thermal plasma', time-modulation of the gas phase densities was achieved by applying a pulsed rf power to the substrate. The probability for the Si radical to react at the a-Si:H surface was measured to be 0.95 <ßSi <1 at 200 °C, while ß of SiH3 was found independent of the substrate temperature in the range of 50–450 °C with an averaged value of ßSiH3 = 0.30±0.03. Combination of this data with the absolute gas phase densities from CRDS has revealed that SiH3 is the key growth precursor for a-Si:H film growth. These results together with measurements of the a-Si:H growth rate and the surface silicon hydride (SiHx) composition put strict constraints on the surface reactions of SiH3 that govern a-Si:H film growth. The results have therefore been used to discuss the growth mechanism of a-Si:H on the basis of SiH3 surface reactions proposed in the literature. The novel technique of evanescent-wave cavity ring-down spectroscopy (EW-CRDS) has been developed into a sensitive probe for in-situ and real-time detection of defect-related absorptions in thin films, such as due to dangling bonds. To this end, a monolithic folded optical cavity made of fused-silica has been carefully designed to provide sufficient sensitivity to detect surface and bulk dangling bonds during a-Si:H film growth. Preliminary ex-situ measurements showed a finesse of the folded cavity of ~165000 at 1200 nm, which easily satisfies the sensitivity requirements. Furthermore, as an initial study to explore the potential of EW-CRDS for probing surface species, the first C-H stretching overtones of three different chloroethylenes adsorbed on the fused silica surface of a folded cavity have been probed in an ex-situ study. This study has demonstrated that EW-CRDS is a generic diagnostic for measuring absolute surface number densities of surface species with sub-monolayer sensitivity, while EW-CRDS can also provide the orientation of the transition moments of the species on the surface. To carry out such dedicated experiments on surface species and to investigate the reactions of the SiH3 growth precursor at the a-Si:H surface, an ultrahigh vacuum setup has been designed and constructed that allows for surface science-like studies of thin film growth. Key features of the setup are (i) three radical sources to mimic the plasma deposition process by means of well-defined radical beams, (ii) full optical access to the substrate for the real-time optical diagnostics, and (iii) well-defined processing conditions. As a first step for studying the growth of a-Si:H in this new setup, the material properties of a-Si:H films deposited by hot wire CVD have been extensively characterized by real-time spectroscopic ellipsometry (RTSE) and several ex-situ film diagnostics. Insight into surface diffusion mechanisms during a-Si:H growth has been obtained by monitoring the surface roughness evolution as a function of film thickness with RTSE for substrate temperatures ranging from 70–450 °C. This study has yielded insight into the initial nucleation behavior of a-Si:H, the transition from surface smoothening to roughening, and the surface diffusion processes occurring on small and large lateral length scales. In addition, the description of the roughness evolution in terms of a generic nonlinear stochastic growth equation has been discussed as well as the consequences of the results for the atomistic surface reactions with regard to surface diffusing growth precursors and growth sites.