Microstructuring of tetrahedral amorphous carbon films (ta-C) using a 248 nm KRF excimer laser

Full text: The special properties of tetrahedral amorphous carbon (ta-C) films with low internal stress, such as extremely high hardness in the range of 60 GPa and low friction coefficient in the range of 0.15, result in increasing interest for using them in industrial applications. For some applications it may, however, be beneficial to modify the surface of those films by microstructuring, for example. A method for microstructuring of thin films of ta-C on different substrates will be presented. With this method the microstructures are fabricated using a KrF excimer laser LPX Pro 305 F by Coherent, Inc. with a wavelength of 248 nm. The laser micromachining station along with its specification concerning laser parameters, beam guidance, beam shaping and positioning of the samples will be presented. The microstructures were generated on ta-C films by using mask projection technique. Depending on the mask geometry and the structuring parameters the produced structures have different shapes, sizes and depths. Several selected structuring examples will be presented. For measuring the microstructures surface profilometry, scanning electron and optical confocal microscopy were used. It will be demonstrated that mask projection technique with a wavelength of 248 nm is suitable to produce microstructures with resolutions down to a few micrometers in ta-C films. Optimum structures can result in reduced friction coefficients of less than 0.05. The hardness of the structured films was found, however, to decrease simultaneously down to values in the range of 35 to 40 GPa. Thus it remains still very high so that microstructured ta-C films may have tremendous potential for tribological applications. Further investigations on the modified ta-C films were directed to the relationship between the microstructure and the mechanical properties, the results of which will also be presented. There were indications that the microstructures are not only the effect of material ablation but also of phase changes. Particularly, it was found that the structured regions were higher than neighboring unstructured regions, which can be accounted for by sp³ to sp² bonding transformation in the amorphous carbon films due to the microstructuring process. Further experimental results on this and on the microstructure of the transformed hard films along with calculations supporting the assumption will be the final main part of the presentation. (author)