Effect of laser intensity on the microstructural and mechanical properties of pulsed laser deposited diamond-like-carbon thin films

Diamond-like-carbon (DLC) thin films have been deposited at room temperature on Si substrates by ablation of a graphite target using a KrF excimer laser at intensities ranging from 0.9×108 W/cm2 to 6.0×109 W/cm2. The microstructure of the films was studied by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The macroscopic properties were evaluated by measurement of their optical constants using in-situ laser reflectometry and their hardness using the continuous stiffness measurement technique. Analysis of the XPS C 1s core level spectra of the DLC films shows that their sp3 hybridized carbon atom content increases with laser intensity up to a maximum value of about 60% obtained at 7.0×108 W/cm2. At higher laser intensities, the sp3 content appears to stabilize at about 53%. Such an evolution of the sp3 content can be understood in terms of the subsurface carbon ions implantation model which has been proposed for ion beam deposited films. On the other hand, Raman analysis indicates that an increase in laser intensity leads to the establishment of some long range order of the sp2 domains in the deposited layers. The extinction coefficient k of the deposited layers was found to be correlated to their sp3 content. Finally, it is shown that hardness values as high as 47 GPa can be obtained and that hardness is also correlated to the sp3 content of the films.