A review of metal-ion-flux-controlled growth of metastable TiAlN by HIPIMS/DCMS co-sputtering

We review results on the growth of metastable Ti1−xAlxN alloy films by hybrid high-power pulsed and dc magnetron co-sputtering (HIPIMS/DCMS) using the time domain to apply substrate bias either in synchronous with the entire HIPIMS pulse or just the metal-rich portion of the pulse in mixed Ar/N2 discharges. Depending upon which elemental target, Ti or Al, is powered by HIPIMS, distinctly different film-growth kinetic pathways are observed due to charge and mass differences in the metal-ion fluxes incident at the growth surface. Al+ ion irradiation during Al–HIPIMS/Ti–DCMS at 500°C, with a negative substrate bias Vs=60V synchronized to the HIPIMS pulse (thus suppressing Ar+ ion irradiation due to DCMS), leads to single-phase NaCl-structure Ti1−xAlxN films (x≤0.60) with high hardness (>30GPa with x>0.55) and low stress (0.2–0.8GPa compressive). Ar+ ion bombardment can be further suppressed in favor of predominantly Al+ ion irradiation by synchronizing the substrate bias to only the metal-ion-rich portion of the Al–HIPIMS pulse. In distinct contrast, Ti–HIPIMS/Al–DCMS Ti1−xAlxN layers grown with Ti+/Ti2+ metal ion irradiation and the same HIPIMS-synchronized Vs value, are two-phase mixtures, NaCl-structure Ti1−xAlxN plus wurtzite AlN, exhibiting low hardness (≃18GPa) with high compressive stresses, up to −3.5GPa. In both cases, film properties are controlled by the average metal-ion momentum per deposited atom transferred to the film surface. During Ti–HIPIMS, the growing film is subjected to an intense flux of doubly-ionized Ti2+, while Al2+ irradiation is insignificant during Al–HIPIMS. This asymmetry is decisive since the critical limit for precipitation of w-AlN, 135 [eV-amu]1/2, is easily exceeded during Ti–HIPIMS, even with no intentional bias. The high Ti2+ ion flux is primarily due to the second ionization potential (IP2) of Ti being lower than the first IP (IP1) of Ar. New results involving the HIPIMS growth of metastable Ti1−xAlxN alloy films from segmented TiAl targets are consistent with the above conclusions. •Ti1−xAlxN alloys with high hardness and low residual stress are demonstrated.•Hybrid HIPIMS/DCMS approach with opposing metal targets is used.•Film growth pathways depend upon which metal target is powered by HIPIMS.•Al–HIPIMS/Ti–DCMS alloys have a much higher solid-solubility limit, xmax=0.64.•Low mass and single charge of the Al+ ion permit tuning properties of metastable cubic Ti0.38Al0.62N by adjusting Vs.