Problem of catalyst ageing during the hot-wire chemical vapour deposition of thin silicon films

The filament in a hot-wire chemical vapour deposition (HWCVD) reactor is an important component. When tantalum (Ta) filaments are used for the deposition of thin silicon films, strong degradation takes place: there is a large amount of silicon not only at the surface but also in the bulk of the tantalum catalyst. Ta-Si phases form on the filament surface and in the bulk, which can lead to a porous structure of the catalyst filament. Filament contamination (silicide formation and thick silicon deposits (TSDs)) is the reason for the changes in filament resistance. It also reduces filament lifetime, which is a serious concern for HWCVD deposition technology. A cleaning procedure for the filament at high-temperatures in a vacuum (about 2000 °C) can neither remove the thick silicon deposits nor fully restore the filament surface properties. In order to decrease the silicon content in the tantalum catalyst and suppress TSD formations on the filament surface, we use radio-frequency alternating current (RF, 13.5 MHz) instead of direct current (DC) to heat the filament. The skin effect of the RF current reduces the formation of TSDs on the surface and silicon diffusion into the filament. We show that it is possible to clean the filament surface of TSDs by means of a high-frequency current. Combined RF + DC filament heating allows us to increase the lifetime of the catalyst (almost twofold) and to improve HWCVD process reproducibility without any deterioration in the quality of the deposited film.