Thermally induced delamination of amorphous hydrogenated carbon coatings monitored by positron beam analysis

Abstract

In this work we have studied the adhesion of PVD amorphous hydrogenated carbon (a-C:H) thin coatings (<200 nm) on crystalline silicon by means of positron beam analysis (PBA). PBA allows the study of the a-C:H layer and the interface by monitoring changes on the Doppler broadening parameters S and W. Depending on the deposition condition, using PBA some samples were found to be delaminated after deposition. Annealing experiments were performed on samples that did not show signs of delamination after deposition. Some of these samples were distinguished by the fact that nitrogen was used in the manufacturing process. Samples were annealed for 30 min in vacuum up to 600 °C in order to remove gases absorbed at the interface. PBA experiments were performed after each annealing step. For the sample without N2 there are no S–W changes either in the coating or at the interface until 600 °C is reached. On the other hand, for the sample containing N2, there is a gradual change in these parameters with temperature. In this coating, after annealing to 500 °C, the increase in W is related to release of H2 and consequent restructuring of the layer. The decrease in the value of S between 500 and 600 °C indicates the formation of graphitic crystallites. There is open volume at the coating/substrate interface since the S parameter gradually increases while W remains constant until 400 °C. This behaviour in S–W is related to the removal of physisorbed hydrogen. At approximately 400 °C the H bonded to C at tetrahedral sites is also released which leads to an internal restructuring of the ‘lattice’, hence a decrease in the value of S is observed. Finally, at 600 °C both coatings delaminate, as the interface parameters tend towards the same values as those of the sample which had delaminated after deposition.