Effect of substrate bias on the plasma enhanced chemical vapor deposition of microcrystalline silicon thin films

The effects of substrate bias voltage on the growth rate and structure of silicon thin films, deposited from highly diluted 13.56 MHz SiH4/H2 discharges at elevated pressures (2.5 Torr) are presented. More precisely, the possibility to increase the deposition rate by exciting the substrate holder with positive or negative dc bias mixed with a low frequency voltage (20 kHz) was investigated. The results are compared to those obtained using grounded and floating substrate configurations. Significant effects were observed concerning both the deposition rate and the film crystallinity. Plasma diagnostics were implemented in order to understand if the changes are purely due to ion bombardment and to identify other plasma parameters that are affected as well. Namely, plasma electrical measurements were used to maintain constant power consumption and to monitor changes in the discharge current and impedance. In addition, spatially resolved emission spectroscopy was applied for recording changes in the discharge structure. The results are discussed and the main problems related to the application of bias together with the best configuration allowing fast deposition of μc-Si:H thin films are presented.