Characterisation of the interface region in stepwise bias-graded layers of DLC films by a high-resolution depth profiling method

The stepwise graded layer concept solves the problem of high internal compressive stresses in diamond-like carbon (DLC) films by adjusting a graded constitution by stepwise increasing the ion energy, i.e., the bias voltage, during sputter deposition. In order to optimize this concept, the detailed characterisation of the interface zones between the layers sputtered with different bias voltage plays a key role. The small-angle cross-section method (SACS) has been developed as a special nanoindentation (NI) technique to perform a high-resolution depth-profiling of the mechanical properties on the nanometer scale in multilayer or nanolaminated composite systems and especially to characterise their interfacial regions. Using improved area correction functions, by varying the maximum load, and by separating the instrumental broadening from the measured hardness profiles, it was possible to significantly improve the sensitivity and the resolution of the SACS. This allowed for the first time to investigate the dependence of the expansions of the interface zones between the graded layers on measuring and processing parameters. By using SACS, depth profiles of hardness and elastic modulus in dependence of applied load and layer thickness ratio have been measured.