Structure, phase evolution, and mechanical properties of DC, pulsed DC, and high power impulse magnetron sputtered Ta-N films

The microstructural development of reactively sputter-deposited tantalum nitride thin films is investigated as a function of the N-2-to-total-pressure ratio (p(N2)/p(T)) and the operating mode of the metallic tantalum cathode; by direct current magnetron sputtering (DCMS), pulsed DCMS, and high power impulse magnetron sputtering (HiPIMS). For all sputtering modes investigated, the phase evolution of the Ta-N films strongly depends on the nitrogen partial pressure, p(N2), used when keeping the total pressure, p(T), constant at 0.3 or 0.6 Pa. The major crystalline phases identified, with increasing the p(N2)/p(T)-ratio, are alpha-Ta, orthorhombic o-Ta4N, hexagonal close packed (hcp) gamma-Ta2N, face-centred cubic delta-TaN, and hcp epsilon-TaN. Their development is basically determined by the N-2 partial pressure present. The deposition rate decreases slowest with increasing p(N2) for the HiPIMS mode and fastest for the DCMS mode. The coatings with a dominating gamma-Ta2N phase (which could be obtained for pN(2) between 0.08 and 0.15 Pa) exhibit the highest hardness with 38.2 +/- 4.6 GPa (HiPIMS), 38.0 +/- 2.3 GPa (pulsed DCMS), and 40.0 +/- 3.8 GPa (DCMS) among all samples studied. Higher p(N2) lead to the formation of delta- and epsilon-TaN phases and a reduction in hardness. Especially, the epsilon-TaN rich coatings are comparably soft with only 20 GPa. Based on our results we can conclude that coatings dominated by Ta-rich o-Ta4N or gamma-Ta2N phases exhibit the most favourable mechanical properties, according to which the most important p(N2)-range is between 0.04 and 0.15 Pa, as for higher N-2 partial pressures the films undergo a transformation from gamma-Ta2N (plus epsilon-Ta4N) into mixed delta- and epsilon-TaN.