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 10(15) cm(-3) both for oxygen and carbon, and 10(14) 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 x 10(18) cm(-3) using conventional UHV/PECVD techniques, and not below 5 x 10(17) 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 x 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.