4. Conclusion
In this work, pulsed DC magnetron sputtering was used for the reactive deposition of titanium dioxide in the anatase phase. The pure anatase phase was detected by Raman spectroscopy on all prepared coatings, whereby the maximum substrate temperature was 220 °C and the lowest substrate temperature was measured in coating experiment No. 9 and amounts to 110 °C. Over a wide process window covering nine coating experiments, the coating remained in the pure anatase phase.
The layer thickness was determined using the calotte grinding method. For evaluation, five coating thickness measurements were averaged in each case and the deviation of the coating thickness measurement was between 0.01 µm and 0.10 µm. The lowest coating thickness was observed in Experiment No. 4 and measures 0.5 µm. Experiment No. 9 has the highest measured film thickness with 1.2 µm while Experiment No. 1 has a film thickness of 0.7 µm, followed by No. 2 at 0.8 µm, No. 3 at 1.1 µm, No. 5 at 0.8 µm, No. 6 at 1.0 µm, and No. 7 and No. 8 display a film thickness of 0.6 µm.
The design of the ideal proportion of oxygen in the oxygen/argon gas mixture reported in the literature for the deposition of the anatase phase during the reactive deposition of titanium dioxide, which is described as ranging from 14.6 % [20] to 34.4 % [23], was indirectly determined in this work by establishing the operating points in the hysteresis experiment. In the coating tests No. 1 to No. 9, the ratio was measured at a minimum of 13 % (coating test No. 9) and a maximum of 33 % (coating test No. 1). The main influencing factor of absolute process pressure, as described in the comparison of coating parameters in the literature from Table 1, was interpreted by attempting to determine the process pressures (Table 2). According to the evaluation of the real process pressure experiment, the pressure ranged between a minimum of 6.0 x 10-1 Pa (coating test No. 3) and a maximum of 2.0 Pa (coating test No. 7). Thus, the range of absolute process pressure of this work is within the promising absolute process pressure of a minimum of 2.7 x 10-1 Pa to 4.0 Pa summarized from the literature as shown in Table 1. The promising absolute process pressure for the deposition of titanium dioxide in the anatase phase according to Table 1 was confirmed using Raman spectroscopic investigations.
The Raman spectroscopic studies show that the pure anatase phase of titanium dioxide is present in all coatings. Five wavenumbers specific to the anatase phase were found in the coating tests No. 1 to No. 9. The characteristic wavenumbers found correspond – with minor deviations – to the six specific wavenumbers of anatase, namely 144 cm-1, 197 cm-1, 399 cm-1, 513 cm-1, 519 cm-1, and 639 cm-1 according to the calculations and observations of [26], whereby the two wavenumbers 513 cm-1 and 519 cm-1 are so close to each other that they overlap. The specific wavenumbers were found to be temperature-dependent [27]. Furthermore, different wavelengths of the laser used can lead to a slight deviation in the wavenumber.
The scanning electron microscopic observation of the coatings showed a columnar and homogeneous layer structure in all coating tests, whereby the columnar layer structure was favored due to the relatively low temperature of a maximum of 220 °C, the deposition pressure of minimum
6.0 x 10-1 Pa, and the non-use of an additional BIAS voltage.