Film uniformity and substrate-to-electrode attachment in large-area VHF glow-discharge deposition of a-Si:H [solar cells]

The authors have discovered one of the major causes of thickness nonuniformities in large-area deposition of hydrogenated amorphous silicon, a-Si:H, on glass. The influence of a gap between the substrate and the grounded electrode on the deposition rate is presented. At a gap of 0.85 mm and at an RF excitation frequency of 40 MHz, the reduction of the deposition rate is 15%, whereas at 60 MHz it amounts to 25%. The deposition-rate reduction is described in terms of a reduced voltage across the sheath at the substrate electrode. The observed effects stress the importance of proper substrate-to-electrode attachment when the VHF glow-discharge technique is used for deposition of solar cells on large areas.