On the relationship between the plasma characteristics, the microstructure and the optical properties of reactively sputtered TiO2 thin films
A titanium target was reactively sputtered in an Ar/O2 atmosphere by (i) conventional direct current magnetron sputtering (DCMS), (ii) high-power impulse magnetron sputtering (HiPIMS), and (iii) bipolar HiPIMS (BPH) discharges for the deposition of titanium dioxide thin films without intentional heating and biasing. In the HiPIMS and BPH cases, the peak current density was set to either 0.32 A cm-2 or 0.86 A cm-2. The time-averaged power density delivered to the plasma was set to ≍1.2 W cm-2 in each case. For the BPH discharge, a positive pulse of +300 V was applied after the negative pulse to accelerate the positive ions toward the substrate. Energy-resolved mass spectrometry analysis has shown a low-energy peak for DCMS while a high energy tail extending in the range of several tens of eV was observed with HiPIMS. In the BPH discharge, the energy reached ≍300 eV for Ti+ ions, which is in agreement with the applied positive potential. According to the x-ray diffraction patterns, amorphous coatings were obtained with DCMS, while rutile TiO2 was obtained with both HiPIMS and BPH. However, in the BPH case, diffractograms were characterized by more intense high-angle diffraction peaks highlighting a modification of the growth process for these conditions. Rutherford backscattering spectrometry analysis has shown that a higher amount of argon atoms were incorporated into the TiO2 films which are found to be slightly understoichiometric for the BPH discharge. It was also found that the refractive index, n, varied as a function of the sputtering regime with the highest value obtained in the HiPIMS case (n = 2.73 at 550 nm). Finally, the experimentally determined optical data were compared to the ones extracted from NASCAM simulations which are found in good agreement except for the BPH case.