Low-Energy Plasma-Enhanced PECVD

The plasma in this case is generated from the ionization of a gas like Ar in a DC plasma where the large electron current emitted from a hot filament such as tantalum (Ta) is used as the plasma source. The plasma is maintained by a low DC voltage of typically 20-30 V. The underlying principle is that the large filament electron current ensures an intense plasma and the low voltage ensures a low ion energy. The plasma is oriented by the magnetic field by an auxiliary anode placed typically about 15 cm away from the plasma source region. As in ETP, the dissociation process takes place just above the substrate by introducing process gases such as SiH4, H2 etc flowing through a gas dispersal ring typically kept around 5 cm above the substrate. In addition to the inherent low ion energies due to small discharge voltages, additional reduction or increase of ion energies can be made by a floating potential to the substrate. Normally the ion energies are between 10-15 eV, however, the ion impact can be reduced by a bias voltage to the substrate. [23]. Such low energy allows the deposition of transition type (crystallinity (Vc) ~50%) nanocrystalline silicon films at the standard temperature of 230°C.