Inhomogeneities in PECVD deposited a-Si:H films induced by a spacing between substrate and substrate holder

Homogeneous deposition of a-Si:H films on glass substrates is routinely done by plasma deposition. A major cause of non-uniformity is the presence of a spacing between substrate and (grounded) electrode to which it is mounted. We have observed a reduced deposition rate if a gap exists between the glass substrate and the metal electrode. The amount of deposition-rate reduction scales with the size of this gap. A reduction of 50% of the initial deposition rate is measured in the case of a gap of 2-mm thickness, for a 65 MHz SiH4/H2 plasma at 0.35 mbar. In addition, the material quality is affected. Filling the gap with a dielectric (Corning 7059 glass) leads to a smaller reduction, i.e., of 20%. These effects are explained by using the equivalent electrical circuit of the plasma reactor system: an extra capacitor representing the gap is added. For a conductive substrate no deposition-rate reduction is observed, which supports the electrical origin of the effect.