PREPARATION OF CUBIC BORON-NITRIDE FILMS BY RADIO-FREQUENCY BIAS SPUTTERING

An attempt was made to prepare c-BN films by r.f. (13.56 MHZ) bias sputtering of a hexagonal boron nitride (h-BN) target in pure argon discharge. The self-bias voltage applied to the target (V(t)) and silicon substrate (V(s)) and pressure (p) were varied systematically. The c-BN phase was found in the films prepared only in pure argon discharge under conditions where p was higher than around 10 m Torr, and V(s) was above a threshold value which decreased from -250 V to -150 V as pressure increased from 10 to 30 mTorr. It is noteworthy that there was an upper limit of V(s) beyond which merely amorphous-like h-BN films were produced. Applying an appropriate V(s) resulted in the increase of the N/B composition ratio, c-BN content, and grain size of c-BN. The increase was accompanied by the effects of a decrease in deposition rate and an increase in substrate temperature up to 600-degrees-C. The c-BN films consisted of micrograins from 10 to 100 nm in size, containing stacking faults such as microtwins. Unfortunately, the films had such strong compressive stress that they peeled easily from the Si substrate, although an h-BN layer served effectively as a buffer to prevent the peeling.