The bright field image in Fig. 4(e) shows the film deposited with a substrate bias of − 500 V exhibits a different microstructure to the films deposited at lower substrate bias. In the dark field image (Fig. 4(f)), the microstructure near to the substrate appears similar to that in the film deposited at − 300 V with embedded nano-crystallites. Near the film surface, there are larger embedded crystals that form layered structures parallel to the substrate. The diffraction pattern (inset in Fig. 4(f)) shows that the higher bias has resulted in a less oriented film. The growth of the crystallites in this film may have resulted from increased substrate temperature, a consequence of the high bias conditions and/or increased dynamic annealing effects [24].
Fig. 4(g) and (h) shows bright field and dark field TEM images of the film deposited at 500 °C with a bias of − 25 V. This film also exhibited strong preferred orientation of the {110} Fe/{200} TiN planes (seen in the diffraction pattern, inset Fig. 4(h)), which lie parallel to the substrate. The dark field image (Fig. 4(h)) reveals an extended columnar microstructure. The more ordered, crystalline microstructure achieved with elevated substrate temperature results from increased ion mobility, leading to structural rearrangement. Combining the XRD and TEM from this film, the elevated temperature appears to have caused significant ordering of the BCC Fe phase. These results show that films deposited with a bias of − 25 V (irrespective of temperature) exhibit this ordered BCC Fe phase whilst the addition of bias results in preferred orientation of the FCC TiN phase.
The stress, hardness (H) and elastic recovery (wp) of the HSS-TiN films deposited at RT are plotted versus applied substrate bias in Fig. 5 and summarised in Table 1. Compressive stress increased from 1.3 GPa in the RT film deposited with a bias of − 25 V to 2.3 GPa in the RT film deposited with − 100 V bias and decreased again in films deposited with bias up to − 400 V (see Fig. 5(a)). This trend is typical and has been observed elsewhere, including in metallic films and TiN [25], [26] and [27]. The film deposited with − 500 V bias exhibited higher compressive stress than the film deposited at − 400 V, possibly due to the more crystalline microstructure of the former.