Amorphous hydrogenated silicon (a-Si:H) films in the thickness range 0.1-4.5 nm were deposited on Si(100) surfaces at 350 K using the ion-beam-deposition method. The thermal stability of these a-Si:H films was studied by temperature programmed desorption spectroscopy. The films are stable up to 500 K, where a-Si:H starts to decompose via evolution of hydrogen (H-2) and silane (SiH4). Approximately 99% of the hydrogen initially bound to the Si network was detected in the hydrogen channel. The hydrogen evolution peaks, at similar to780 K caused by the decomposition of monohydride groups; the presence of SiH2 groups is indicated by hydrogen desorption below 700 K. The silane desorption states at 625. and 750 K reveal the existence of two different types of silyl (SiH3) groups. Etching of a-Si:H by impinging gas-phase H atoms was investigated in the temperature range from 150 to 700 K by in situ mass spectrometry. Silane was the sole etch product observed. The formation of silane proceeds via direct abstraction of silyl precursor groups by impinging hydrogen atoms, SiH3(a)+H(g)-->SiH4(g); the silyl abstraction probability increases by a factor of 6 with increasing substrate temperature between 150 and 525 K. However, the steady-state erosion rate is controlled by the supply of silyl groups by successive hydrogenation of the Si network with the formation of SiH2 as bottleneck of the silyl supply. (C) 2003 American Vacuum Society.