XANES and XRR study on phase evolution of TiO2 films developed using HiPIMS

Abstract

High power impulse magnetron sputtering (HiPIMS) is one of the recent advanced techniques for the development of high quality TiO2 thin films. Due to the formation of high energetic ions in the discharge, HiPIMS can produce various crystalline phases of TiO2 (anatase and rutile) with low roughness as compared to conventional direct current magnetron sputtering (DCMS). This work emphasizes on the effect of oxygen partial pressure in the evolution of phases and oxidation states of TiO2 films deposited using HiPIMS along with a comparative study against DCMS grown films under similar oxygen environment. TiO2 thin films were prepared with DCMS and HiPIMS, as a function of oxygen content in the sputter gas flow. Thickness, density and roughness were calculated using XRR fitting and analysis. It was observed that HiPIMS deposited films were smoother and denser than DCMS grown films. Crystallite sizes of HiPIMS deposited films were significantly lower than DCMS. Phase growth analyses were performed using X-ray diffraction (XRD), Raman and XANES (X-ray absorption near-edge spectroscopy of the Ti L2,3 and O K-edges) spectroscopy. In HiPIMS grown films, the increase in O2 partial pressure changed the phase from rutile to mixed and then to anatase phase whereas only rutile and mixed phases were detected in DCMS grown film under similar oxygen environment. HiPIMS developed films were smooth, uniform free from cracks as observed in FESEM and Atomic Force microscope (AFM) morphological images. Lognormal crystallite size distribution values decreased with increasing O2 partial pressure. Crystal field splitting values of (Δo) between ∼1.9 and 2.2 eV indicate crystalline phase formation by XANES Ti L2,3-edge analysis. These findings point to better crystalline growth TiO2 thin films as compared to DC-MS. HiPIMS grown films shows phase transformation of rutile into mix phase and anatase with increase in oxygen partial pressure.