Paradigm shift in magnetron sputtering: from gas-ion to metal-ion-controlled irradiation of the growing film

Up until recently, thin film growth by magnetron sputtering relied on enhancing adatom mobility in the surface region by gas ion irradiation to obtain dense layers at low deposition temperatures. However, inherently low degree of ionization in the sputtered material flux during direct current magnetron sputtering (DCMS), owing to relatively low plasma densities involved, prevented systematic exploration of the effects of metal-ion irradiation on the film nanostructure, phase content, and physical properties. The situation changed recently with the development of high power pulsed magnetron sputtering (HiPIMS), in which pulsed substrate bias is applied in synchronous to the metal-ion-rich portion of each pulse [1]. Careful choice of sputtering conditions allows exploitation of gas rarefaction effects such that the charge state, energy, and momentum of metal ions incident at the growing film surface can be controlled. In contrast to gas-ions, a fraction of which are trapped at interstitial sites, metal-ions are primarily incorporated at lattice sites resulting in much lower compressive stresses. In addition, the closer mass match with the film-forming species results in more efficient momentum transfer and provides the recoil density and energy necessary to eliminate film porosity at low growth temperatures.

In the first part of the talk the results of time-resolved mass spectrometry analyses performed at the substrate position during HiPIMS and HiPIMS/DCMS co-sputtering of transition-metal (TM) targets in Ar and Ar/N2 atmospheres are reviewed. Knowledge of the temporal evolution of metal- and gas-ion fluxes is essential for precise control of the incident metal-ion energy and minimizing the role of gas-ion irradiation. Several examples of novel film-growth pathways are described in the second part of the presentation: (i) nanostructured N-doped bcc-CrN0.05 films combining properties of both metals and ceramics, (ii) fully-dense, hard, and stress-free Ti0.39Al0.61N, (iii) single-phase cubic Ti1-xSixN with the highest reported SiN concentrations, (iv) unprecedented AlN supersaturation in single-phase NaCl-structure V1-xAlxN, (v) a dramatic increase in the hardness, due to selective heavy-metal-ion bombardment during growth, of dense Ti0.92Ta0.08N and Ti0.41Al0.51Ta0.08N films deposited with no external heating, and (vi) simultaneous increase in both hardness and toughness of Zr1-xTaxBy layers deposited with synchronized Ta+ irradiation.

Finally, Ti1-xTaxN alloys grown with no external heating are shown to produce high-quality Cu diffusion barriers and provide excellent corrosion protection for stainless-steel substrates.

[1] G. Greczynski, I. Petrov, J.E. Greene, and L. Hultman, JVSTA 37 (2019) 060801.