Deposition of ultrapure hydrogenated amorphous silicon

We have succeeded in a drastic reduction of impurity contents in hydrogenated amorphous silicon (a-Si:H) films by a newly developed ultrahigh vacuum plasma-enhanced chemical vapor deposition (UHV/PECVD) system. High sensitivity secondary ion mass spectrometry shows that impurity contents in the films are as low as 1015 cm−3 both for oxygen and carbon, and 1014 cm−3 for nitrogen. These values represent the lowest concentrations of atmospheric contaminants for a-Si:H films reported so far. In particular, oxygen content has not been reduced below 1×1018 cm−3 using conventional UHV/PECVD techniques, and not below 5×1017 cm−3 even by high growth rate process of very high frequency plasma. The essential features of the present UHV/PECVD system are an extremely low outgassing rate of 8×10−9 Torr ℓ/s, extremely low partial pressure of contaminant gas species <10−12 Torr, and purification of feed gas SiH4 at “point of use.” The specific origins of impurities in the films are discussed: outgassing of the reactor wall, the purity of feed gas, and the plasma-wall interaction process.