SHEATH THICKNESS IN VERY-HIGH-FREQUENCY PLASMA CHEMICAL-VAPOR-DEPOSITION OF HYDROGENATED AMORPHOUS-SILICON

When very-high-frequency (VHF) glow discharges are used to obtain homo geneous hydrogenated amorphous silicon films on glass substrates, an o ptimum combination of process pressure and excitation frequency is req uired. 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 p lays an important role in this. We use two recently developed experime ntal methods to determine the sheath thickness, i.e., gap-induced inho mogeneity and in situ energy-resolved mass spectrometry. The gap-induc ed inhomogeneity method (GI method) is based on the observation of a r educed deposition rate, if a gap exists between the glass substrate an d the metal substrate electrode. The reduction scales with gap thickne ss and, more importantly, with the sheath thickness. In situ energy-re solved mass spectrometry is employed to measure the ion-energy distrib utions (IEDs) at the grounded electrode. From the observed charge-exch ange peaks in the IEDs in silane plasmas we are able to deduce the she ath thickness (IED method). Both methods are employed to determine the sheath thickness in VHF silane/hydrogen glow discharges. For the opti mum frequency-pressure combinations it appears that all sheath thickne sses are about equal and amount to about 4 mm (GI method), while the I ED method shows a decreasing sheath thickness with frequency. The tota l integrated IEDs of all ions, i.e., the total energy dose, is constan t for these conditions, which correlates well with the fact that mater ial properties are similar for the optimum frequency-pressure combinat ions. (C) 1997 American Vacuum Society.