Correlation between stress profiles of cubic boron nitride thin films and the phase sequence revealed from infrared data

Abstract.

Cubic boron nitride thin films have been ion-beam-assisted deposited on silicon cantilever structures and subsequently back-etched in order to study the stress evolution and finally the growth mechanisms. After each sputtering step, the film stress, the remaining thickness, and the IR data were examined. In this way, the layered sequence of cBN on top of a hBN base layer, influencing the development of the intrinsic film stress, could be studied in detail. The observed stress distribution can be divided into three regions. First, a non-cubic base layer with a constant stress value is formed, followed by a linear increase in the stress after cBN nucleation as a result of the coalescence of cBN nanocrystals. Finally, the stress reaches a second plateau characteristic of the cBN top layer. In addition, the layered sequence was verified by the evolution of the FTIR spectra. Furthermore, the fraction of the sp²-bonded material of the cBN top layer was determined from the IR data. For various deposition conditions, a linear relationship between the stress of the nanocrystalline cBN top layer and the amount of sp³-bonded material was observed. From this, it can be concluded that stress relaxation occurs at the sp²-bonded grain boundary material. No evidence for stress relaxation after cBN nucleation was found.