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

Abstract

The microstructural development of reactively sputter-deposited tantalum nitride thin films is investigated as a function of the N2-to-total-pressure ratio (pN2/pT) 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, pN2, used when keeping the total pressure, pT, constant at 0.3 or 0.6 Pa. The major crystalline phases identified, with increasing the pN2/pT-ratio, are α-Ta, orthorhombic o-Ta4N, hexagonal close packed (hcp) γ-Ta2N, face-centred cubic δ-TaN, and hcp ε-TaN. Their development is basically determined by the N2 partial pressure present. The deposition rate decreases slowest with increasing pN2 for the HiPIMS mode and fastest for the DCMS mode. The coatings with a dominating γ-Ta2N phase (which could be obtained for pN2 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 pN2 lead to the formation of δ- and ε-TaN phases and a reduction in hardness. Especially, the ε-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 γ-Ta2N phases exhibit the most favourable mechanical properties, according to which the most important pN2-range is between 0.04 and 0.15 Pa, as for higher N2 partial pressures the films undergo a transformation from γ-Ta2N (plus o-Ta4N) into mixed δ- and ε-TaN.