  Ion energy distributions have been determined at the rf-bias electrode in an
inductively-coupled acetylene-argon plasma for various substrate bias voltages
and frequencies under conditions suitable for film deposition. These are
compared with those obtained at the grounded wall of a capacitively coupled
plasma. In the former, for pressures up to 25 mTorr, the IEDs exhibit bimodal
structures with peak separation values that follow the expected dependence on
voltage and frequency. At higher pressures, 120 mTorr, the bimodal structure is
replaced by a single peak. For all conditions, the dominant ion is Ar+ or ArH+
despite a set C2H2:Ar flow ratio of 2:1, and this can be attributed to the high
electron dissociation of the parent molecule. Diamond-like carbon films
indicate a peak hardness at an ion energy of around 90 eV and a very sharp fall
in hardness is noted beyond this value. This is similar to the observed
bombardment energy relationship for sp3 bond formation in hydrogen-free
tetragonal amorphous carbon or bias-sputtered films. However, due to the lack
of carbon-based ions, an alternative film growth mechanism must be considered,
possibly based on argon knock-on implantation of surface adsorbed carbon
species. The results have shown that the use of high frequency bias or bias
harmonics may lead to much narrower ion energy distributions.
