Effect of target power on the physical properties of Ti thin films prepared by DC magnetron sputtering with supported discharge

Fig. 4. shows the dependence of thickness/rate of deposition on target power for the films deposited in diode (at 3 Pa) and supported discharge (at 0.7 Pa) modes. The relationship between sputtering pressure and mean free path (λ) is governed by the relation [18]: λ<!-- ? -->=2.330×<!-- ? -->10−<!-- - -->18T/(Pδ<!-- d -->m2) $$ \begin{array}{} \lambda=2.330\times10^{-18}T/(P\delta^2_{m}) \end{array} $$ (1)

where T is the room temperature in [K], P is argon sputtering pressure in [Pa] and δ_m is the molecular diameter in [cm]. From the above equation, when sputtering pressure is high, the mean free path is short; the sputtered atoms undergo higher number of collisions resulting in loss of sputtered atoms due to scattering. Hence, lesser number of atoms arrives at the substrate. At lower pressure the mean free path is relatively high, so the number of collisions decreases and the number of atoms reaching the substrate is high. As a result it is possible to achieve higher thicknesses with decreasing pressure for a given target power [19].

Typical EDAX spectra for the Ti films deposited at an argon partial pressure of 3 Pa in diode mode and at 0.7 Pa in supported discharge mode at target power of 120 W have been shown in Fig. 5. From the spectra it is clear that the films deposited at lower pressures in supported discharge mode have better purity. The reason for obtaining relatively pure films is probably caused by lesser incorporation of background gases into the film at higher incident rate of the sputtered atoms as well as lesser interaction collisions with residual reactive gases in the chamber due to longer mean free path before reaching the substrate.

Structural variations of Ti films deposited in diode mode at a pressure of 3 Pa and in supported discharge mode at a pressure of 0.7 Pa at different powers are shown in Fig. 6a and Fig. 6b, respectively. From Fig. 6a, the films deposited at a power up to 100 W do not show any crystallinity. Only at a power of 120 W the films show crystallinity with preferred orientations of (1 0 0) and (0 0 2). In the films deposited at lower pressure of 0.7 Pa with supported discharge, the onset of crystallinity starts at the power of 80 W with a preferred orientation of (0 0 2), while no crystallinity has been observed in the films prepared at 60 W. Further increase in power to 100 W resulted in another preferred orientations of (1 0 0) along with (0 0 2). At a power of 120 W, the intensity of the peaks increased with the appearance of (1 0 1) peak [20]. Similar results were obtained by Chawla et al. [9] who prepared Ti thin films by DC magnetron sputtering at substrate temperature of 200 °C. Junga et al. [3] deposited Ti thin films with grid-attached magnetron sputtering in which the substrate bias of 150 V was applied. Fontana et al. [1] prepared Ti thin films by triode magnetron sputtering at substrate temperature of 100 °C and substrate bias of 300 V. Golan et al. [13] deposited Ti thin films using triode sputtering by biasing the substrate and at substrate temperature of 50 °C.

Both, the target power and working gas pressure have a great influence on crystallinity of the growing film. The increase in target power for a given pressure results in an increase of the plasma density and hence, higher Ar ion density at the target surface. As a result more titanium atoms get sputtered. This leads to an increase in film deposition rate. With increasing target power, the substrate will also be subjected to a more energetic electron/ion interaction. At low sputtering pressure (0.7 Pa), the mean free path of Ti atoms travelling through Ar gas is of the same order as the target-substrate distance. In this case, they can reach the growing surface without losing energy due to lesser number of gas collisions. In addition, ionization of Ti and Ar occurs due to electrons emitted by the thermionic filament. This results in higher energy of Ti atoms/ions reaching the substrate, which leads to improved crystallinity of the films. These effects might account for the observed changes in the crystallinity. Similar observations were made by other reserachers [21, 22]. However, as the pressure is increased, both the electron temperature and the electron density decrease. At higher argon pressure (3 Pa) it may not be possible because the plasma is confined very close to the target surface and the effect of plasma can be reduced at the substrate surface.

The qualitative analysis of particle size and relative intensity is shown in Table 1. The grain size increases as the target power increases. Movchan et al. [23] and Thornton [24] reported an increase in the crystal size with an increase in temperature. Our present studies indicate an increase in crystal size with increasing the target power. Hence, the effect of increasing target power and reduced gas pressure is similar to that of increasing the substrate temperature.

The SEM photographs of Ti thin films deposited at the pressures of 3 Pa and 0.7 Pa at different target powers (60 W to 120 W) are shown in Fig. 7 and Fig. 8. The film deposited in diode modes has smaller grains as compared to the films deposited at 0.7 Pa, in triode mode. A very smooth surface with larger grains has been observed in triode mode. The films deposited at both triode and diode modes show an increase in the grain size with an increase in target power. The microstructure of the Ti coating is directly influenced by kinetic energy (i.e. surface mobility) of the titanium atoms reaching the substrate. Hence, an important parameter to be considered is vapor flux arriving at the substrate. At the higher pressure (3 Pa), the amount of thermalized flux increases and fewer energetic titanium atoms reach the substrate. At the highest gas pressure, the finegrained structure is more porous, since the lowest kinetic energy is achieved under these conditions [25]. This can be confirmed by the SEM image shown in Fig. 8. At lower working pressure (0.7 Pa), the mean free path increases and the number of collisions is reduced before reaching the substrate. As a result, the sputtered Ti atoms grow into larger grains with different crystal orientations having fine rounded edges, which can be confirmed by AFM as shown Fig. 9d. Fig. 9 shows the Ti thin films deposited at different target powers with supported discharge. The film deposited at lower target power exhibits smooth surface without any pits and pinholes over the surface. Higher target power with a lower working pressure resulted in the large agglomerates with bigger grains. The AFM images show an increase in surface roughness values from 3 nm to 15 nm with an increase in target power.

The electrical resistivity of titanium thin films as a function of target power is shown in Fig. 10. The resistivity decreases as the target power increases from 60 W to 120 W for both diode and triode system. There is a large difference in resistivity of the films deposited with and without supported discharge. This can be explained by the difference in crystalline structure and on the basis of Matthiessen’s rule. The Ti thin film prepared in diode mode has an amorphous structure with smaller grains. According to Igasaki et al. [26], when the grain size is smaller than the mean free path of electrons, the scattering mechanism causes an increase in the resistivity. The influence of gaseous impurities incorporated during the deposition process as discussed in EDAX also raises the electrical resistivity. The Ti thin films prepared in supported discharge mode have larger grains and lower amount of incorporated impurities, which reduces their electrical resistivity.