Thickness dependence of the structure of diamond-like carbon films by Raman spectroscopy

The thickness dependence on Structure of Diamond-like carbon films of a-C:H deposited by ECR-CVD and ta-C by FCVA has been studied by visible and UV Raman spectroscopy. The results show that the evolution of structure as a function of the thickness fora-C:H films contains two stages: when thickness is less than 50 angstrom, the film contains less sp(3) sites and not continuous; and when thickness is up to 50 angstrom. the film contains more sp(3) sites and become continuous. However, for ta-C films, it includes three stages. In the first stage of thickness lower than 20 angstrom, the film is not continuous, and also contains less sp(3). In the second stage of thickness between 20 angstrom and 50 angstrom, the sp(3) site abruptly shifts a higher value in 20 angstrom and then keeps stable. In the third stage of thickness over 50 angstrom, the sp(3) site has a little increase and then almost not changed. Thus, the fundamental limitation thickness in using DLC as an ultrathin overcoat for ta-C films is 20 angstrom (> 10 angstrom), and for a-C:H films is 50 A. The implications of result on the mechanisms proposed for the film growth mode were also discussed. (C) 2009 Elsevier B.V. All rights reserved.