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 degrees C at a deposition rate of 1 Angstrom/s contains 10(15) cm(-3) of O, 10(15) cm(-3) of C, and 10(14) cm(-3) of N impurities, while low defect hydrogenated microcrystalline silicon (mu c-Si:H) films deposited at 200 degrees C at a very low rate of 0.1 Angstrom/s include 10(16) cm(-3) of O, 10(15) cm(-3) of C and 10(16) 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 8x10(-9) Tor.Us, extremely low partial pressure of contaminant gas species <10(-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 mu c-Si:H. mu c-Si:H with a grain size of approximate to 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.