Investigation of reactive HiPIMS + MF sputtering of TiO2 crystalline thin films

Abstract

A hybrid high-power impulse magnetron sputtering (HiPIMS)+mid-frequency (MF) magnetron sputtering system was used for the deposition of crystalline TiO2 thin films at a low substrate temperature. Ion velocity distribution functions (IVDFs) were measured in a pulsed magnetron discharge at substrate positions depending on the type of plasma excitation and on the working gas mixtures. Several different pulsed discharge configurations were used: (i) HiPIMS, (ii) pulsed bipolar MF with frequency 350kHz and (iii) both HiPIMS+MF connected in parallel to the magnetron cathode. The timing of HiPIMS excitation was set to period time T=10ms with “ON” time TON=100μs. Ti targets were sputtered in three different types of atmospheres: (i) inert pure Ar, (ii) a reactive mixture of Ar+O2 and (iii) a reactive mixture of Ar+O2+N2 with a constant gas pressure p=1Pa. All IVDFs were measured using a time-resolved retarding field energy analyzer (RFEA) located in the substrate position. TiO2 thin films were deposited under identical experimental conditions on silica (SiO2) glass substrate, glass with an indium tin oxide (ITO) electrode and polycarbonate polymer foil. The thin film properties are discussed with respect to the measured plasma parameters. It is shown that the combination of HiPIMS+MF excitation in a reactive atmosphere effectively reduces the delay between the edge of cathode voltage and current onset. The highest ion energy was reached in the case of an inert Ar atmosphere due to the highest ratio of fast Ti+ ions in the overall ion flux. All TiO2 thin films deposited in reactive atmospheres formed pure rutile phase regardless of the excitation mode; however, those films deposited by a HiPIMS+MF excitation mode exhibited the greatest ability to produce a photocurrent. Furthermore, HiPIMS+MF was the best setting for the deposition of crystalline TiO2 on polycarbonate foil because of the low heating flux on the substrate and suitable plasma parameters leading to the formation of the rutile phase.