Stress reduction of cubic boron nitride films by oxygen addition

Cubic boron nitride (c-BN) films with significantly reduced residual stresses were successfully grown onto silicon substrates by means of controlled oxygen addition into the films. The deposition was based on radio-frequency magnetron sputtering of a hexagonal boron nitride (h-BN) target, and was accomplished in a reactive mode using gas mixtures of argon, nitrogen, and oxygen at 0.3 Pa pressure, 400 °C growth temperature, and − 250 V substrate bias. Results of systematic investigations are shown in this article with respect to the critical influences of oxygen concentration during deposition upon the stress, cubic phase fraction, as well as nanohardness of the deposited films. Under the specified growth conditions, the formation of c-BN was generally completely hindered for oxygen concentrations above 1.5 vol.% in the gas mixture. At concentrations below approximately 1 vol.%, the added oxygen exhibits however marginal influences on the c-BN fraction, but on the other side a strong impact on the stress of the deposited films. Cubic-phase dominated films (containing 70–80 vol.% c-BN) with their compressive stress three times reduced were thus produced through careful control of oxygen fraction in the gas mixture, showing an excellent nanohardness of almost 60 GPa. For such films, a post-deposition thermal treatment at 900 °C led to an additional drastic stress reduction resulting in a final residual stress that is almost 10 times lower than that of as-deposited c-BN films without intentional oxygen addition.