Development of ultra-clean plasma deposition process

We have developed a new type of ultra-high vacuum plasma-enhanced chemical vapor deposition (UHV/PECVD) system. According to high sensitivity secondary ion mass spectrometry, device quality hydrogenated amorphous silicon (a-Si:H) films deposited at 250°C at a deposition rate of 1 angstrom/s contains 1015 cm-3 of O, 1015 cm-3 of C, and 1014 cm-3 of N impurities, while low defect hydrogenated microcrystalline silicon (μc-Si:H) films deposited at 200°C at a very low rate of 0.1 angstrom/s include 1016 cm-3 of O, 1015 cm-3 of C and 1016 cm-3 of N. These are the lowest concentrations of atmospheric contaminants for these kinds of materials observed so far. 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 -12 Torr, and purification of feed gas SiH4 at 'point of use'. These efforts are quite important not only for clarifying the microscopic mechanism of photo-induced degradation in a-Si:H, but also for enlarging the crystalline grain size in μc-Si:H. μc-Si:H with a grain size of ≈ 1000 angstrom as determined by Scherrer's formula can be obtained at the higher rate of 1.5 angstrom/s by utilizing a VHF (Very High Frequency) plasma. The specific origins of impurities in the films are also discussed.