Optical properties of ta-C:H films deposited by ECR plasma using acetylene as precursor gas

A uniformly Distributed Multipolar Microwave plasma reactor using Electron Cyclotron Resonance at 2.45 GHz was used to deposit ta-C:H (DLC) films on silicon substrates at RT. The influence of the acetylene initial pressure (0.3 - 1.1 mTorr) and of the substrate bias (from -10 V to -190 V) on the DLC growth and structure has been investigated using spectroscopic ellipsometry. It is shown that the maximum sp³ content of about 40% (atomic) may be reached for the acetylene pressure of 0.6 - 0.7 mTorr. It is also shown that although the bias voltage has no measurable impact on the optical gap it has a significant influence on the extinction coefficient and the behaviour of the absorption tails. The results suggest that the sp² clustering mode and the sp² cluster size distribution are bias controlled.