Sheath thickness in very-high-frequency plasma chemical vapor deposition of hydrogenated amorphous silicon

Abstract

When very-high-frequency (VHF) glow discharges are used to obtain homogeneous hydrogenated amorphous silicon films on glass substrates, an optimum combination of process pressure and excitation frequency is required. With frequencies in the range of 30 to 80 MHz pressures from 0.65 to 0.35 mbar are used. It was suggested that the sheath thickness plays an important role in this. We use two recently developed experimental methods to determine the sheath thickness, i.e., gap-induced inhomogeneity and in situ energy-resolved mass spectrometry. The gap-induced inhomogeneity method (GI method) is based on the observation of a reduced deposition rate, if a gap exists between the glass substrate and the metal substrate electrode. The reduction scales with gap thickness and, more importantly, with the sheath thickness. In situ energy-resolved mass spectrometry is employed to measure the ion-energy distributions (IEDs) at the grounded electrode. From the observed charge-exchange peaks in the IEDs in silane plasmas we are able to deduce the sheath thickness (IED method). Both methods are employed to determine the sheath thickness in VHF silane/hydrogen glow discharges. For the optimum frequency-pressure combinations it appears that all sheath thicknesses are about equal and amount to about 4 mm (GI method), while the IED method shows a decreasing sheath thickness with frequency. The total integrated IEDs of all ions, i.e., the total energy dose, is constant for these conditions, which correlates well with the fact that material properties are similar for the optimum frequency-pressure combinations.