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 for a-C:H films contains two stages: when thickness is less than 50 Å, the film contains less sp3 sites and not continuous; and when thickness is up to 50 Å, the film contains more sp3 sites and become continuous. However, for ta-C films, it includes three stages. In the first stage of thickness lower than 20 Å, the film is not continuous, and also contains less sp3. In the second stage of thickness between 20 Å and 50 Å, the sp3 site abruptly shifts a higher value in 20 Å and then keeps stable. In the third stage of thickness over 50 Å, the sp3 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 Å (> 10 Å), and for a-C:H films is 50 Å. The implications of result on the mechanisms proposed for the film growth mode were also discussed.