Deposition of crystalline silicon thin films in electrically asymmetric capacitively coupled plasmas

Capacitively coupled radio frequency (CCRF) discharges in hydrogen diluted silane are typically used for the deposition of silicon thin films on large area substrates. In order to improve the properties of these films for specific applications such as photovoltaics, a detailed understanding of the plasma dynamics is required. Furthermore, advanced methods for the control of the plasma surface interaction, e.g. of the energy and the total flux of ions and the flux ratio of ions to neutral radicals, are highly desired. The focus of this work is on the application of the Electrical Asymmetry Effect (EAE) on silicon thin film deposition processes. Using the EAE in a geometrically symmetric parallel plate dual-frequency CCRF discharge, a DC self-bias develops and can be controlled by adjusting the phase angle between the driving frequencies of 13.56 MHz and 27.12 MHz, that are applied to the powered electrode. Accordingly, the mean sheath voltages and, thereby, the ion energy can be controlled separately from the ion flux, as the applied voltage amplitude and dissipated power remain approximately constant.constant. We study such a dual-frequency capacitive discharge in H₂/SiH₄ by Phase Resolved Optical Emission Spectroscopy (PROES) and Infrared Laser Absorption Spectroscopy (IRLAS). The electron dynamics visualized by PROES indicates that the plasma is operated in an intermediate regime between the capacitive α-mode and the resistive Ω-mode respectively. The total (time averaged) excitation profile depends only weakly on the phase angle. Accordingly, the dissociation rate is independent of the phase angle, as it is revealed by IRLAS measurement on SiH₄ feed gas. The resulting depositions on glass substrates have been analyzed by means of Raman Spectrometry and a profilometer. The Raman scattering spectrum proves that the deposited silicon layer will be more crystalline, if the DC self-bias is larger and, accordingly, the - on bombardment energy at the substrate is smaller. The film thickness is about the same for all phase angles and DC self-bias values, respectively, the generation and the flux of reactive species are about the same in all cases. Therefore, the EAE allows to identify the role of the ions in the deposition process and to improve the film crystallinity.