A uniformly distributed multipolar microwave plasma reactor using electron cyclotron resonance at 2.45 GHz (600 W) was used to deposit a-C:H thin films at RT. C2H2 was used as the precursor gas. Single crystal [100] Si and CR39 allelic resin substrates were RF biased to a negative voltage within the range between - 10 and - 200 V. The influence of the process parameters (gas flow and substrate bias) on the growth rate and hydrogen content have been investigated in detail. Optical parameters (optical gap E-T, index of refraction n and extinction coefficient k) were measured using spectroscopic ellipsometry. The resonant (6.385 MeV) nuclear reaction: H-1(N-15, alpha gamma )12C was used to determine the hydrogen content. For the C2H2 pressure range of 0.6 < P(C2H2) < 1.1 mtorr the optical parameters remain constant within the limits of experimental uncertainty. The sp(3) content is seen to vary monotonically as a function of pressure and to be reaching a maximum of approximately 40% for 0.6 < P(C2H2) < 0.7 mtorr. The variation of the substrate bias within the range from -10 to -190 V [at P(C2H2) = 0.6 mtorr] has no measurable impact neither on the deposition rate nor on the hydrogen content. The corresponding average values of E-T and n remain stable (E-T= 1.83 +/- 0.11 eV, n = 2.12 +/- 0.04). However, an increase in the bias is followed by a significant decrease of the extinction coefficient k and of the absorption tail width E-o. The observed evolution of k and Eo suggests that the sp(2) clustering mode may be related to the substrate bias.