Large area deposition of amorphous and microcrystalline silicon by very high frequency plasma

Two aspects of VHF plasma deposition in a large area reactor are investigated: 1) Experiments and model show that voltage inhomogeneities become serious at high frequency but can be reduced by suitable RF connection configuration. These voltage inhomogeneities are the main factor limiting the excitation frequency in order to satisfy film thickness uniformity requirements; 2) the effect of the frequency on the plasma properties has been studied between 13.56 MHz and 70 MHz. The results show that an increase of the electron density with the frequency leads to a more efficient dissociation of silane. This increase of the gas phase reactivity of the plasma is largely responsible for the higher deposition rates observed at high frequency. The choice of the excitation frequency for a given application is a compromise between the gain of higher deposition rate and the frequency limit imposed by homogeneity requirements.