High-rate deposition of amorphous silicon films by microwave-excited high-density plasma

In this study, the deposition of amorphous silicon (a-Si) thin films using a microwave-excited high-density plasma system is described. We investigate the effects of plasma excitation gas species (argon or helium), total gas pressure, silane (SiH₄) flow rate, and substrate stage temperature, estimating the resultant films from cross-sectional morphology, photoconductivity, and dark conductivity measured without lightinduced degradation. It is confirmed that high-quality a-Si films (photosensitivity = 1.29 × 10⁶) can be formed in the plasma excitation gas helium at a pressure of 13.3 Pa by relatively high rate (1.1 nm/s) deposition. At the same time, we measure the plasma emission derived from various radicals such as Si and SiH radicals in order to discuss the mechanism of radical generation in the plasma. The result of the measurement implies that when argon is used as plasma excitation gas, metastable states of argon markedly dissociate silane, which produces low-quality a-Si films. On the other hand, it seems that electrons dissociate silane mainly, which produces high-quality a-Si films, in helium.