Mechanical properties of TiN films prepared in glow-arc transition region

Physical parameters of the electrical discharge process

Figure 1 shows the target voltage (U) – current density (J_a) curve of the Ti target. When J_a increases from 0.14 to 0.4 A cm⁻², the U first increases and then decreases, with an inflection point at 0.24 A cm⁻². The reason of the U change is that the bombardment of high-energy argon ions causes the target surface temperature to rise due to the increase in J_a. When J_a gradually increases to 0.24 A cm⁻², the target surface temperature rapidly attains the degree of thermionic emission due to the interaction of argon ions bombardment and Joule heating [7]. High flux of electrons, argon ions and Ti atoms increases the collision ionization rate, and the increase in ions and electrons leads to the improved conductivity. Thus, as J_a is >0.24 A cm⁻², the U gradually decreases to 306 V. OPEN IN VIEWER

The Ti target surface temperatures at different J_a are 397, 602, 1013, >1300 and >1300°C, respectively. Edison effect [8] indicated that thermionic emission phenomenon of metals tended to be significant for temperatures over 1000°C. The Ti target surface produces high flux of heat emission electrons as J_a increases to 0.24 A cm⁻² and the number of heat emission electrons increases rapidly with the increase in J_a from 0.32 to 0.4 A cm⁻², which is consistent with the discussion in Figure 1.

Structure and average deposition rate of TiN films

Figure 2 displays the XRD patterns of TiN films deposited at different J_a. All TiN films exhibit a NaCl type face centre cubic (fcc) structure (JCPDS 38-1420). All TiN films exhibit a strong (220) preferred orientation. According to different reports [9,10], TiN films often grew toward the (111) or (200) orientation, because the films generally grew along the direction of reducing the total energy on the system. However, according to I. Iordanova et al. [11], the development of the TiN (220) plane can be attributed to high flux of particles bombarding the growing films. In this paper, J_a is higher than the normal magnetron sputtering current density range (<0.1 A cm⁻²). This is the reason why the (220) orientation appears. Figure 2 shows the weaker diffraction peak intensities of (111), (200) and (311) except for J_a at 0.32 A cm⁻², which exhibits a mixture structure dominated by the (220) orientation. With the increase in J_a from 0.14 to 0.4 A cm⁻², the (220) peak intensity first increases and then decreases. The transformation of (220) peak intensity is due to the effect of particle energy. The high energy and flux of the deposited particles promote the growth of the (220) crystal plane due to the increase in J_a. When J_a >0.32 A cm⁻², the particles with excessive energy destroy the growth of the (220) peak. The average grain sizes of the films were estimated by the Scherrer formula [12-14], the grain size of TiN films increases from 63 nm to 96 nm as J_a is less than 0.24 A cm⁻². However, as J_a continues to increase to 0.4 A cm⁻², the average grain size decreases instead of increasing, which is 92 and 73 nm, respectively. The grain size of TiN films is mainly affected by deposition rate and substrate temperature [15]. In the GATD region, high deposition rate and low substrate temperature can form many nucleation sites and reduce the critical nucleation size, resulting in a smaller grain size. OPEN IN VIEWER

The top view SEM micrographs of TiN films deposited on single crystal silicon are presented in Figure 3. The top view micrographs of TiN films change from irregular shape granules (Figure 3(a)) to pyramidal granules (Figure 3(c)) as J_a increases from 0.14 to 0.24 A cm⁻². The AGD TiN films exhibit many micropores on its surface. However, the increase in J_a to 0.4 A cm⁻², it is evident that TiN films exhibit dense hilly granules (Figure 3(e)). In the GATD region, Ti particles exhibit high ionization and energy. Under high-energy ions bombardment, the diffusivity of particles is improved by transferring energy from ions to deposited particles on the substrate. In films growth stage, many nucleation sites can be formed, and a large number of nucleation sites and high-mobility can form a dense and smooth surface morphology, as shown in Figure 3(d,e). OPEN IN VIEWER

Figure 4 presents the cross-sectional SEM micrographs of TiN films. With the increase in J_a, the density of boundary pores between columnar grains increases gradually, as shown in Figure 4(a–c). Moreover, the growth of columnar grains in the AGD region is disordered. Compared with the micrographs (Figure 4(a–c)), it is evident that GATD TiN films exhibit regular dense columnar grains (Figure 4(d,e)), and the density of boundary pores significantly decreases as J_a increases to 0.4 A cm⁻². This is because the energy and diffusion ability of the AGD particles are lower, and it is easy to form a porous columnar morphology [6]. However, the GATD particles exhibit high ionization rate, diffusion ability and energy. Owing to the faster deposition rate of GATD particles, a large number of nucleation sites are formed. Nucleation sites and high mobility can form regular dense columnar crystal structure, resulting in the dense structure of GATD TiN films. OPEN IN VIEWER

The average deposition rates of TiN films were determined by thickness measurements which were the average value of 5 positions per sample. The average deposition rate of TiN films deposited on Si is illustrated in Figure 5. The average deposition rate increases from 42.4 nm min⁻¹ to 87.2 nm min⁻¹ as J_a increases from 0.14 to 0.24 A cm⁻², and it could be related to the increase in the U and the plasma density. Nevertheless, the average deposition rate is almost unchanged as J_a increases from 0.24 to 0.32 A cm⁻². This can be attributed to the competition of the following two factors. On the one hand, the decrease in U in the GATD region reduces the energy of argon ions, which is likely to decrease the sputtering yield of Ti atoms. On the other hand, the increase in J_a enhances the density of the plasma, which is beneficial to improve the deposition rate. As J_a >0.32 A cm⁻², a large number of electrons spontaneously escape from the Ti target surface and drag out high flux of Ti atoms. As a result, the average deposition rate of TiN film increases to 110.4 nm min⁻¹. OPEN IN VIEWER

The above results exhibit that there is a strong correlation between the plasma properties generated in different J_a and the structure of TiN films. I. Petrov et al. [16] showed that varying the ratio of incident ions to neutral particles (J_i/J_n) during deposition could be a useful tool for controlling structure and properties of films. In fact, the intrinsic effect of varying the incident ion to neutral flux ratio is to vary the ionization rate of particles. In this paper, the variation of J_i/J_n ratio is indirectly characterized by the ratio of bias current to thickness of films (J_i/J_film). As J_a increases from 0.14 to 0.4 A cm⁻², J_i/J_film is 0.23, 0.25, 0.28, 0.37 and 0.48, respectively. J_i/J_film of the AGD region varies from 0.23 to 0.28. However, the value of J_i/J_film increases rapidly from 0.37 to 0.48 as J_a increases in the GATD region. Owing to the bias current is positively correlated with the ions and electrons, it means that the ionization rate of particles in the GATD region is significantly higher than that in the AGD region. Under the acceleration of bias electric field, the particles with high ionization rate exhibit high diffusion ability and many nucleation sites. This can be the reason why the GATD TiN films exhibit denser columnar structure, smoother surface as compared to the AGD films.

Mechanical properties

Figure 6 shows the adhesion of the films evaluated by scratch test. The scratch track images indicate that the delaminated areas of the films increase (Figure 6(a–c)) as the J_a increases from 0.14 to 0.24 A cm⁻² and rapidly decrease (Figure 6(d,e)) as the further increase in the J_a from 0.32 to 0.4 A cm⁻². To more accurately describe the adhesion of TiN films, the Lc1 (the critical load for the first edge chipping) and the Lc2 (the critical load for the first delamination with the track) are used to display the failure events of TiN films. When J_a increases from 0.14 to 0.24 A cm⁻², the Lc1 of TiN films decreases from 18 N to 10 N and the Lc2 decreases from 25 to 10 N. However, J_a >0.32 A cm⁻², the Lc1 increases from 17 N to 33 N and the Lc2 increases rapidly from 27 N to 55 N, as shown in Figure 6(d,e). This shows the adhesion of TiN films is improved in the GATD region. Considering the effect of the thickness on bonding strength [17], as films (Figure 4(c,d)) are similar in thickness, but the bonding strength is very different, as shown in Figure 6(c,d). Therefore, it shows the effect of the thickness on bonding strength is not obvious. When J_a is <0.24 A cm⁻², low-energy neutral atoms form loose porous films due to weak diffusion ability. Hence, the films have a small bonding strength. As J_a is >0.24 A cm⁻², the increase in the particles ionization rate enhances the diffusion ability, which forms dense films. Meanwhile, the bombardment of target positive ions also plays an important role in improving the adhesion of TiN films. OPEN IN VIEWER

Figure 7 is the relationship between the hardness of TiN film and J_a. In order to ensure the accuracy, each sample was measured 5 times and averaged. As J_a increases from 0.14 to 0.24 A cm⁻², a gradual increase in the hardness from 1006 HV to 1224 HV is observed. However, the hardness of TiN films increases rapidly to 1958 HV as J_a increases to 0.32 A cm⁻². The rapid increase in the hardness can be mainly attributed to the formation of regular dense columnar grains in films as shown in Figure 4(d). TiN films prepared at 0.4 A cm⁻² exhibit the maximum hardness of 2388 HV, which can be explained as follows. First, TiN films of 0.4 A cm⁻² exhibit denser structures as compared to those of 0.32 A cm⁻² (Figure 4(d,e)). Moreover, the decrease in the average grain size probably contributes to the increase in the films hardness, according to the Hall-Petch effect [18,19]. OPEN IN VIEWER