Role of Tin+ and Aln+ ion irradiation (n = 1, 2) during Ti1-xAlxN alloy film growth in a hybrid HIPIMS/magnetron mode

Abstract

Metastable Ti1-xAlxN (0.4≤x≤0.76) films are grown using a hybrid approach in which high-power pulsed magnetron sputtering (HIPIMS) is combined with dc magnetron sputtering (DCMS). Elemental Al and Ti metal targets are co-sputtered with one operated in HIPIMS mode and the other target in DCMS; the positions of the targets are then switched for the next set of experiments. In both cases, the AlN concentration in the co-sputtered films, deposited at Ts=500°C with R=1.5–5.3Å/s, is controlled by adjusting the average DCMS target power. Resulting films are analyzed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, elastic recoil detection analysis, and nanoindentation. Mass spectroscopy is used to determine ion energy distribution functions at the substrate. The distinctly different flux distributions obtained from targets driven in HIPIMS vs. DCMS modes allow the effects of Aln+ and Tin+ (n=1, 2) ion irradiation on film growth kinetics, and resulting properties, to be investigated separately. Bombardment with Aln+ ions (primarily Al+ in the Al-HIPIMS/Ti-DCMS configuration) during film growth leads to NaCl-structure Ti1-xAlxN (0.53≤x≤0.60) films which exhibit high hardness (>30GPa) with low stress (0.2–0.7GPa tensile). In contrast, films with corresponding AlN concentrations grown under Tin+ metal ion irradiation (with a significant Ti2+ component) in the Ti-HIPIMS/Al-DCMS mode have much lower hardness, 18–19GPa, and high compressive stress ranging up to 2.7GPa. The surprisingly large variation in mechanical properties results from the fact that the kinetic AlN solubility limit xmax in Ti1-xAlxN depends strongly on, in addition to Ts and R, the target power configuration during growth and hence the composition of the ion flux. AlN with xmax~64mol% can be accommodated in the NaCl structure under Aln+ ion flux, compared with ~40mol% for growth with Tin+ flux. The strong asymmetry in film growth reaction paths is due primarily to the fact that the doubly-ionized metal ion flux is approximately two orders of magnitude higher from the Ti target, than from Al, powered with HIPIMS. This asymmetry becomes decisive upon application of a moderate substrate bias voltage, −60V, applied synchronously with HIPIMS pulses, during growth.