Growth, microstructure and mechanical properties of boron nitride: carbon (BN:C) have been studied in films prepared by reactive r.f. diode sputter deposition as well as by reactive unbalanced d.c. magnetron (UBM) sputter deposition of low resistivity boron carbide (B4C) targets. Due to the fäet that d.c. glow discharges offers a better control of the important growth parameters such as particle flux and energy distribution in both neutral and ionized states at the substrate as compared to r.f. methods, fundamental growth and plasma studies were in some detail performed in the UBM sputtering system.

Films were prepared with three characteristic phase compositions; cubic c-BN:C, hexagonal (turbostratic-like) h-BN:C and a mixtures of these on Si (001) substrates. The same phase compositions were obtained in both deposition techniques. While keeping the B/N ratio close to unity, the formation of BN:C phases were mainly correlated to the energy and flux of impinging ions towards the negatively d.c. biased substrate. A c-BN:C phase formation was typically found in mixed c-BN:C and h-BN:C films prepared at high ion-to-neutral flux ratios (UBM: J_i/J_n ~24), within a narrow range in ion energies (UBM: 85 eV < E_i < 135 eV) and at low process temperatures <250 °C. However, almost single phase c-BN:C (~80 % of sp³-bonded B-N) was obtained at deposition conditions of high ion flux (UBM: Ji/Jn ~24) and low ion energy (UBM: E_i = 110 eV) whereas films deposited at much lower ion energy or at a lower ion fluxes contained exclusively h-BN:C. Also, c-BN:C films revealed a BN:C film phase evolution sequence from an initial amorphous BN:C layer followed by a highly oriented h-BN:C layer with the c-axis parallel to the film surface, to a c-BN:C layer exhibiting a (110)­preferred orientation. Furthermore, as-deposited films contained 5 - 20 at% of C that was found to decrease with increasing volume fraction of c-BN:C and also with increasing J/Jn. Chemical sputtering of volatile CN species is suggested for the reduction of C in BN:C films. Nevertheless, the C in c-BN:C films is mainly present as C-C and B-C bonds.

Unnoticed the presence of C, cubic phase BN:C films are shown to be formed at deposition conditions not included in the present models of c-BN growth. For example, in terms of momentum transfer, values of 200 - 250 (eV emu)^% have been reported to be required for the synthesis of c-BN whereas in this study an order of magnitude higher values were found. The results thus point at an extended deposition window for cubic phase formation, with potential for using further reduced ion energies such that intrinsic compressive stress in BN can be minimized.

Finally, the mechanical properties of randomly- and highly-oriented h-BN:C as well as of c-BN:C films were studied and compared to the uncoated Si substrate. Although all coated substrates showed an increased stiffness, the maybe most interesting properties were found in the highly oriented h-BN:C films with the c-axis parallel to the film surface. These films showed a predominantly elastic behaviour exhibiting an elastic recovery as high as 82% indicating high hardness values. The improved hardness and elasticity may be explained by the apparent microstructure with buckling of the hexagonal basal planes. This structure is proposed to correspond to a three dimensional strongly covalently bonded network of BN:C, similar to what has been reported for CN_x films.