Influence of the process parameters on the properties of hydrogenated amorphous carbon thin films deposited using ECR plasma

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 film s at RT. C2H2 was used as the precursor gas. Single crystal [100] Si and CR 39 allelic resin substrates were RF biased to a negative voltage within the range between - 10 and - 200 V. The influence of the process parameters (g as flow and substrate bias) on the growth rate and hydrogen content have be en investigated in detail. Optical parameters (optical gap E-T, index of re fraction n and extinction coefficient k) were measured using spectroscopic ellipsometry. The resonant (6.385 MeV) nuclear reaction: H-1(N-15, alpha ga mma )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 w ithin 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 o f approximately 40% for 0.6 < P(C2H2) < 0.7 mtorr. The variation of the sub strate bias within the range from -10 to - 190 V [at P(C2H2) = 0.6 mtorr] h as 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 f ollowed by a significant decrease of the extinction coefficient k and of th e absorption tail width E-o. The observed evolution of k and Eo suggests th at the sp(2) clustering mode may be related to the substrate bias. (C) 2001 Elsevier Science B.V. All rights reserved.