## 3 Results
### Characterization of the Coatings  
Figure 3 displays the top-view morphology of the coatings on the etched samples obtained at different bias values at two magnifications. A cluster structure with spherical morphology is observed on the surface of all films. At a higher magnification (Figure 3, right), each cluster consists of smaller grains with irregular shapes. Such surface morphology can be attributed to the self-shadowing effect during deposition (zone I of the [PERSON] model [29]) since the ratio \(\rm T/T_{m}\) is around or less than 0.3 (where \(\rm T_{m}\) is the melting point  
Figure 2: Minimal and maximal temperature values measured at different biases of the substrates during a rotating cycle. of the coating material and T is the substrate temperature). At such temperatures, adatom surface diffusion is too modest to permit migration into the shaded areas. Values for thickness were taken straight from the SEM film cross-sections as shown in Figure 4. The cross-sections of \(-25\), \(-50\), and \(-75\) V biased samples showed rough surfaces with visible structural details while that of \(-100\) V indicated a smoother fractured surface and less visible details in it. This corresponds to the diffraction analysis which is shown in Figure 6. Film thicknesses ranged from 1.7 \(\pm\) 0.05 to 2.6 \(\pm\) 0.08 \(\upmu\)m. With the increasing bias value, the coating thicknesses decreased while the films exhibited increased crystallization with increasing substrate temperature according to structural characterization. A columnar grain structure extending through the whole coating thickness is visible for the lower-biased films. Figure 3: Representative microstructure images (left for low magnification and right for high) of the samples biased at (**a**) \(-25\) V, (**b**) \(-50\) V, (**c**) \(-75\) V, and (**d**) \(-100\) V.  
Figure 5 illustrates copper mapping by energy dispersive X-ray spectrometry (SEM-EDS) on the surface of the coatings. All examined films show a homogenous distribution of copper. Integrated area spectra were carried out at different places on each coating to confirm the chemical compositions. The results are tabulated with the weight percentage of the constituent elements in Table 1. As Table 1 shows, Cu concentration drops as the bias voltage increases while the Ti content slightly changes. Raising the bias value to the substrates, the ion bombardment increases, and the ion sputtering process in plasma intensifies. Copper and titanium are reported to have sputtering yields in oxygen that differ by more than ten times [30]. Due to its higher sputtering yield than Ti, copper may experience stronger \"self-sputtering\" during reverse sputtering at high substrate biasing [31] that etches the coated surface. Consequently, at \(-\)100 V bias, the amount of copper is at its lowest and the coating's thickness decreases (Figure 4). Figure 4: Representative cross-section images of the coatings on the samples biased at (**a**) \(-\)25 V, (**b**) \(-\)50 V, (**c**) \(-\)75 V, and (**d**) \(-\)100 V.  
Figure 5: Representative SEM-EDS elemental mapping of Cu within the coatings on the samples biased at (**a**) \(-\)25 V, (**b**) \(-\)50 V, (**c**) \(-\)75 V, and (**d**) \(-\)100 V.  
Figure 6 shows X-ray diffractograms for the substrate and films deposited at different bias values. By increasing the bias values and, therefore, decreasing the T\({}_{\rm s}\), the films crystallize in all cases but with different TiO\({}_{2}\) polymorphs. Without altering the preferred orientation of TiO\({}_{2}\) polymorphs, the increase in bias only changed the oxide phase. At a low bias value (\(-25\) V), only anatase diffraction peaks corresponding to (101), (200), and (211) are visible; at a high bias value (\(-100\) V), only (110), (101), and (111) rutile maxima occur while the samples biased at \(-50\) and \(-75\) V indicate a biphasic titanium dioxide structure.