Role of Energetic Ions in the Growth of fcc and ω Crystalline Phases in Ti Films Deposited by HiPIMS

Titanium (Ti), due to its excellent properties, is widely exploited in thin film technology that usually leads to the production of hcp (α-phase) Ti films. In this work, we show how to control the nucleation of fcc and hexagonal (ω-phase) crystalline phases in Ti films by adding a proper Ti ion flux to the film-forming species. To this aim, films with different thicknesses are grown by High Power Impulse Magnetron Sputtering (HiPIMS) and conventional Direct Current Magnetron Sputtering (DCMS) as a reference. Furthermore, HiPIMS depositions with different substrate bias voltage Us(0 V, -300 V and -500 V) are performed to investigate different ion energy ranges. Microstructure, morphology and residual stress of the deposited films, as well as the DCMS and HiPIMS plasma composition, are analysed with different characterization techniques. The DCMS samples, where the species are mostly atoms, exhibit the Ti α-phase only and show a tensile residual stress decreasing with thickness. As far as HiPIMS samples are concerned, films deposited in low energy ion conditions (Us=0 V) show the presence of the Ti fcc phase up to a maximum thickness of about 370 nm. Differently, films deposited under high energy conditions (Us= -300 V and -500 V) show the nucleation of the Ti ω-phase for thicknesses greater than 260 and 330 nm, respectively. A compressive-tensile-compressive (CTC) behavior is observed for residual stresses as thickness increases. The formation of these unusual Ti phases is discussed considering the different deposition conditions.