THE TOTAL ENERGY FLUX: IS IT THE SOLE PARAMETER GOVERNING THE PHASE FORMATION OF TRANSITION METAL OXIDE THIN FILMS ?

In the first part of this study, the total energy flux transferred to the substrate was varied in a broad range by modifying systematically the working conditions. A titanium target was sputtered in an Ar/O2 reactive atmosphere (P= 0.6 Pa) either in DC Magnetron Sputtering (DCMS) or in the High Power Impulse Magnetron Sputtering (HiPIMS) mode of operation. Unbalanced (UB) and Balanced (B) magnetron cathodes were utilized and the time-averaged power delivered to the plasma was set to either 400 or 800W. The total energy flux was measured in situ thanks to a heat flux sensor located at the substrate position. The normalized energy flux (φnorm) was calculated by taking into account the number of Ti atoms deposited per unit time and the discharge regimes were compared accordingly. Regardless of the sputtering method, the film phase constitution evolves from phase pure anatase to rutile rich anatase/rutile phase mixtures as φnorm is increased. φnorm is the highest as the HiPIMS discharge is ignited with an UB magnetron. A surprising result is that the energy flux related to the B-DCMS discharge at 800W is higher than the one measured for the UB-DCMS, at the same power. The enhanced magnetic field confinement for the balanced cathode intensifies the target ion bombardment, which, in turn, promotes the heating of the target surface. Consequently the contribution of the IR photon flux emitted by the hot surface to the total energy flux is significantly augmented. In fine, we show that the phase constitution of TiO2 films depends on the total energy flux supplied during growth.
In the second part, the phase formation in zirconia (ZrO2) thin films synthesized by DC-MS has been investigated. A 2-inch Zr target is sputtered in an Ar/O2 ambient, in the poisoned mode or in the transition zone with the help of voltage feedback control loop. The films grown in the poisoned regime at 200 mA, 1.3 mTorr always exhibits the low-temperature monoclinic-phase. While working in the transition zone, the phase constitution is dramatically modified and the XRD spectra only exhibit peaks from the high-temperature cubic crystals. Elemental analysis shows that films grown in the transition zone are oxygen deficient. DFT based calculations data remarkably agree with these experimental observations and thus highlight that the formation of the cubic phase is solely due to the incorporation of O vacancies.
However, it should be noticed that a slight modification in these optimized working conditions would alter the phase constitution, i.e. the monoclinic phase appears if i) the film thickness is increased, ii) the energy of the depositing species is increased (by lowering the deposition pressure).