Metal-ion subplantation: A game changer for controlling nanostructure and phase formation during film growth by physical vapor deposition

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, an 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. Employing only gas-ion bombardment results in an inefficient energy and momentum transfer to the growing film surface. Also, for enhanced substrate biasing, the higher concentration of implanted noble gas atoms at interstitial lattice positions causes elevated compressive stress levels. High-power impulse magnetron sputtering (HiPIMS), however, provides controllable metal-ion ionization and, more importantly, enables the minimization of adverse gas-ion irradiation effects. The latter can be realized by the use of pulsed substrate bias applied synchronously with the metal-ion-rich portion of each HiPIMS pulse (metal-ion-synchronized HiPIMS), based on the results of time-resolved ion mass spectrometry analyses performed at the substrate position. In this way, both the metal-ion energy and the momentum can be precisely controlled for one to exploit the benefits of irradiation by metal-ions, which are also the film-forming species. Systematic studies performed in recent years using binary and ternary transition metal-based nitrides as model systems revealed new phenomena with accompanying unique and attractive film growth pathways. This Perspective paper focuses on the effects of low-mass metal-ion irradiation and their role for the nanostructure and phase control. We review basic findings and present original results from ion mass spectrometry studies and materials characterization for the effect of metal-ion subplantation. Key correlations are highlighted, which, if properly engaged, enable unprecedented control over film nanostructure and phase formation and, hence, the resulting properties. We show generalization from the findings to present a new concept for thin film growth in a hybrid HiPIMS/DCMS configuration with metal-ion-synchronized bias. Based on the results obtained for TM-based nitrides, there are no evident physical limitations preventing the extension of this deposition process concept for other materials systems or other metal-ion-based thin film growth techniques. Further exciting findings could, thus, be anticipated for the future.