Synthesis and characterization of TiBx (1.2 ≤ x ≤ 2.8) thin films grown by DC magnetron co-sputtering from TiB2 and Ti targets

Titanium boride, TiBx, thin films were grown by direct current magnetron co-sputtering from a compound TiB2 target and a Ti target at an Ar pressure of 2.2 mTorr (0.3 Pa) and substrate temperature of 450 °C. While keeping the power of the TiB2 target constant at 250 W, and by varying the power on the Ti target, PTi, between 0 and 100 W, the B/Ti ratio in the film could be continuously and controllably varied from 1.2 to 2.8, with close-to-stoichiometric diboride films achieved for PTi = 50 W. This was done without altering the deposition pressure, which is otherwise the main modulator for the composition of magnetron sputtered TiBx diboride thin films. The film structure and properties of the as-deposited films were compared to those after vacuum-annealing for 2 h at 1100 °C. As-deposited films consisted of small (≲50 nm) randomly oriented TiB2 crystallites, interspersed in an amorphous, sometimes porous tissue phase. Upon annealing, some of the tissue phase crystallized, but the diboride average grain size did not change noticeably. The near-stoichiometric film had the lowest resistivity, 122 μΩcm, after annealing. Although this film had growth-induced porosity, an interconnected network of elongated crystallites provides a path for conduction. All films exhibited high hardness, in the 25–30 GPa range, where the highest value of ~32 GPa was obtained for the most Ti-rich film after annealing. This film had the highest density and was nano-crystalline, where dislocation propagation is interrupted by the off-stoichiometric grain boundaries.