Strain-free, single-phase metastable Ti0.38Al0.62N alloys with high hardness: metal-ion energy vs. momentum effects during film growth by hybrid high-power pulsed/dc magnetron cosputtering

A hybrid deposition process consisting of reactive high-power pulsed and dc magnetron cosputtering (HIPIMS and DCMS) from Ti and Al targets is used to grow Ti1−xAlxN alloys, with x ~ 0.6, on Si(001) at 500 °C. Two series of films are deposited in which the energy and momentum of metal ions incident at the growing film are individually varied. In both sets of experiments, a negative bias Vs ranging from 20 to 280 V is applied to the substrate in synchronous, as determined by in-situ mass spectrometry, with the metal-ion-rich part of the HIPIMS pulse. Ion momentum is varied by switching the HIPIMS and dc power supplies to change the mass m and average charge of the primary metal ion. Al-HIPIMS/Ti-DCMS layers grown under Al+ (mAl = 26.98 amu) bombardment with 20 ≤ Vs ≤ 160 V are single-phase NaCl-structure alloys, while films deposited with Vs > 160 V are two-phase, cubic plus wurtzite. The corresponding critical average metal-ion momentum transfer per deposited atom for phase separation is 〈pd⁎〉 ≥ 135 [eV-amu]1/2. In distinct contrast, layers deposited in the Ti-HIPIMS/Al-DCMS configuration with Ti+/Ti2+ (mTi = 47.88 amu) ion irradiation are two-phase even with the lowest bias, Vs = 20 V, for which 〈pd⁎〉 > 135 [eV-amu]1/2. Precipitation of wurtzite-structure AlN is primarily determined by the average metal-ion momentum transfer to the growing film, rather than by the deposited metal-ion energy. Ti-HIPIMS/Al-DCMS layers grown with Vs = 20 V are two-phase with compressive stress σ = − 2 GPa which increases to − 6.2 GPa at Vs = 120 V; hardness H values range from 17.5 to 27 GPa and are directly correlated with σ. However, for Al-HIPIMS/Ti-DCMS, the relatively low mass and single charge of the Al+ ion permits tuning properties of metastable cubic Ti0.38Al0.62 N by adjusting Vs to vary, for example, the hardness from 12 to 31 GPa while maintaining σ ~ 0.