Crystallinity of Sputtered TiO2 Thin Films: a Study of the Heating and Plasma Influence on the Growth

Abstract: AbstractHigh-Power Impulse Magnetron Sputtering (HIPIMS) of TiO2 thin films enhances their photocatalyticproperties compared to the films prepared by standard sputtering methods, especially when no substrateheating is employed. With no heating, the coatings present an amorphous structure that benefits from theionization of the sputtered flux granted by HiPIMS, with respect to their photocatalytic performance.However, these properties can be significantly improved by promoting the crystallization of the films andheating is crucial in attaining such structures. The experiments indicate that HiPIMS alone is incapable ofsuch, regardless of the deposition conditions, but it proves to be beneficial when some minimal temperatureis provided, by improving the crystalline quality. Therefore, the plasma generated by HiPIMS seems tosignificantly influence the film growth.Aiming to explore the contributions of both temperature and ions during the growth, different chambergeometries were tested: a long target-to-substrate distance was compared with an identical chamber setupbut with a significantly smaller target-to-substrate distance. As such, the latter configuration will result inthe full immersion of the substrate in the plasma, while having the opposite scenario for the other setup.Consequently, due to the different growth conditions, films grown inside and outside the plasma glow werecarefully compared, with a specific focus on their crystallinity. The long-distance setup resulted inamorphous films, while a short distance yields crystalline films. This difference can be attributed mostly tothe additional energy provided to the growth in the form of e.g. heating, which promotes crystallization.Several reports on the deposition of crystalline TiO2 claim room temperature conditions but utilize smalltarget-to-substrate distance systems, unintentionally heating the films.In this study, the characterization of film crystallinity grown in a long-distance setup using Grazing IncidentX-ray Diffraction (GIXRD) to identify the resulting structure. Subsequent investigations employ GrazingIncidence Wide Angle X-ray Scattering (GIWAXS) for in situ crystallization kinetics studies, aiming toidentify optimal growth conditions for room temperature depositions. These conditions are then replicatedin a short target-to-substrate distance setup, with an examination of various pressures to understand particlethermalization's growth impact. Lastly, a roll-to-roll setup with cooling mechanisms explores roomtemperature deposition on polymer substrates. The analysis reveals that crystalline TiO2 growth benefitsfrom HiPIMS and a 10% duty cycle in the oxide mode. According to the GIXRD findings, rutile starts toform when the pressure is low, but anatase appears as the pressure increases. Temperature experimentsshow significant substrate heating in HiPIMS (399°C) and Pulsed-DC (387°C) during static deposition,leveling at 154°C with substrate rotation. Attempts to deposit crystalline TiO2 films at room temperatureusing a roll-to-roll configuration prove unsuccessful, emphasizing the need for both kinetic energy and heatto achieve desired crystalline properties. Nevertheless, an anatase phase was produced at 154 °C, whichcould be advantageous for various polymer substrates with a melting point within that temperature range.