Structural properties of a-Si:H related to ion energy distributions in VHF silane deposition plasmas

We present measurements on typical silane-hydrogen RF/VHF deposition plasmas and the corresponding a-Si:H films deposited from these plasmas. A range of process settings was used, covering both the alpha and the gamma' regime of the discharge. Mass resolved ion energy distributions were measured at the grounded electrode to determine the ion flux at the growing surface. Although the main precursors are radicals, in the lower pressure alpha regime the flux of ions towards the surface can account for at least 10% of the observed growth rate. In the gamma' regime this contribution to the growth of the film is less. We measured internal stress, hydrogen concentration, hydrogen bonding configuration, and refractive index to determine the effects of the ion bombardment on the structure of the deposited a-Si:H films. Good structural properties, i.e. a refractive index of about 4.25 at 600 nm and a minimum number of SiH2 bonds, are found above a threshold energy of 5 eV per deposited atom. This observation is explained in terms of knock-on processes of the deposited atoms by ions and an increased mobility of the growth precursors at the surface. Both these processes promote the formation of a dense film. (C) 1998 Elsevier Science B.V. All rights reserved.