Control of Ti1−xSixN nanostructure via tunable metal-ion momentum transfer during HIPIMS/DCMS co-deposition

Abstract

Ti1−xSixN (0≤x≤0.26) thin films are grown in mixed Ar/N2 discharges using hybrid high-power pulsed and dc magnetron co-sputtering (HIPIMS/DCMS). In the first set of experiments, the Si target is powered in HIPIMS mode and the Ti target in DCMS; the positions of the targets are then switched for the second set. In both cases, the Si concentration in co-sputtered films, deposited at Ts=500°C, is controlled by adjusting the average DCMS target power. A pulsed substrate bias of −60V is applied in synchronous with the HIPIMS pulse. Depending on the type of pulsed metal-ion irradiation incident at the growing film, Ti+/Ti2+vs. Si+/Si2+, completely different nanostructures are obtained. Ti+/Ti2+ irradiation during Ti-HIPIMS/Si-DCMS deposition leads to a phase-segregated nanocolumnar structure with TiN-rich grains encapsulated in a SiNz tissue phase, while Si+/Si2+ ion irradiation in the Si-HIPIMS/Ti-DCMS mode results in the formation of Ti1−xSixN solid solutions with x≤0.24. Film properties, including hardness, modulus of elasticity, and residual stress exhibit a dramatic dependence on the choice of target powered by HIPIMS. Ti-HIPIMS/Si-DCMS TiSiN nanocomposite films are superhard over a composition range of 0.04≤x≤0.26, which is significantly wider than previously reported. The hardness H of films with 0.13≤x≤0.26 is ~42GPa; however, the compressive stress is also high, ranging from −6.7 to −8.5GPa. Si-HIPIMS/Ti-DCMS films are softer at H~14GPa with 0.03≤x≤0.24, and essentially stress-free (σ~0.5GPa). Mass spectroscopy analyses at the substrate position reveal that the doubly-to-singly ionized metal-ion flux ratio during HIPIMS pulses is 0.05 for Si and 0.29 for Ti due to the difference between the second ionization potentials of Si and Ti vs. the first ionization potential of the sputtering gas. The average momentum transfer to the film growth surface per deposited atom per pulse 〈pd〉 is ~20× higher during Ti-HIPIMS/Si-DCMS, which results in significantly higher adatom mean-free paths (mfps) leading, in turn, to a phase-segregated nanocolumnar structure. In contrast, relatively low 〈pd〉 values during Si-HIPIMS/Ti-DCMS provide near-surface mixing with lower adatom mfps to form Ti1−xSixN solid solutions over a very wide composition range with x up to 0.24. Relaxed lattice constants decrease linearly, in agreement with ab-initio calculations for random Ti1−xSixN alloys, with increasing x.