Nanotwinned films have been attracting considerable attention because of their outstanding mechanical properties [4–6], good conductivity [7, 8], satisfactory electromigration resistance and superior thermal stability [9–13], which make it feasible for them to be applied in a wide range of engineering fields. Therefore, further exploration to promote the design of nanotwinned materials has become a major focus of research. In previous studies, nanotwinned Cu had been successfully fabricated by electroplating under conditions of high current density [15], low temperature [16], rapid stirring rate [17], and pulsed time [19] to facilitate the nanotwin formation. Here, we fabricated highly (111)-oriented Cu with a high density of nanotwins by magnetron sputtering with negative substrate bias of 150 V to discuss the effect of bias on the microstructure and structural properties of the Cu films.
The microstructure zone model in evaporation was first reported by Movchan and Demchishin [27]. They found that the microstructure of an evaporated coating of Ti, W and ZrO could be represented as a function of T/\({\text{T}}_{m}\), where T and \({\text{T}}_{m}\) are the deposition and melting temperature, respectively, as three zones, each with its own characteristic structure and physical properties. The zone model was subsequently extended to magnetron sputtering by adding an axis to account for the effect of the Ar working pressure and Zone T between Zone 1 and Zone 2 by Thornton [28]. Zone T presents a fibrous texture without voided boundaries due to the increased adatom mobility from energetic particle bombardment. In the sputtering process, the major parameters that influence the film microstructure are the ratio of T/\({\text{T}}_{m}\)and particle bombardment. The cross-sectional FIB ion images in Fig. 1 (d) show that a dense columnar grain structure appeared in Zone T with ion bombardment. When the substrate bias voltage was applied during deposition, Ar ions were attracted to bombard the films, transferring the energy to the adatoms on the substrate. The strain energy became larger during the bombardment and tended to be relieved through nanotwinning. From the thermodynamic point of view, the sum of the interfacial energies including grain boundaries and twin boundaries will be reduced by twinning because the excess energy for coherent TBs is much smaller than that for high-angle GBs. In addition, stresses induced by the impingement of the Ar ions on the film surface are also crucial for the twin formation within the Cu films. Similar results were observed in Cu films fabricated by electroplating bombarded by 5 keV of Ar ions at low temperature [29], indicating that the thermal spike cascades induced by ion bombardment cause the generation and motivation of twinning partial dislocations.
The preferred orientation can be adjusted by substrate bias and realized by the biaxial compressive stress induced by ion bombardment, which increases the strain energy [30]. Meng et al. deposited zirconium nitride films by DC magnetron sputtering and found that the preferred orientation transformed from ZrN(200) to ZrN(111) with increasing substrate bias [31]. This result can be explained by the thermodynamic calculation of the surface and strain energies of different planes in the films, where the driving force for such evolution of preferred orientation resulted from the decreased strain energy. On the other hand, Clement et al. found that ion bombardment by Ar+ during growth has the greatest influence on the preferred orientation of the films. As bombardment becomes more energetic, the tendency for microcrystals in the film to grow with the c-axis along the surface normal increased [32]. In this study, the Cu columnar grains grew along the (111) direction, which can also be explained by the decreasing strain energy induced by the formation of nanotwin structures.
Cu bonding is of great interest due to its potential to replace solder joints in advanced packing technologies. Typical solder reflows usually occurred at temperatures of about 250°C, so lowering the direct bonding temperature down to 250°C has become a key challenge. Since the essential process in direct bonding is known to be surface diffusion, the (111) orientation of Cu is known to have the highest surface diffusion coefficient, which is 3 to 4 orders of magnitude more rapid than those of other planes, implying that the bonding temperature can be lowered further on Cu (111) than on the other planes [33]. Here, we expected that the highly (111)-oriented Cu film would facilitate low temperature direct bonding because of the high surface diffusivity and low surface roughness. The results of the biased Cu film revealed that the entire surface was colored in blue, indicating that over 90% of the film surface was covered by (111) Cu nanotwins. In contrast, the microstructure of the unbiased Cu film was random, without any preferred orientation. An oriented (111) texture of extremely high degree, up to 93.7%, provided rapid paths for diffusion and enabled direct bonding to be carried out at a fast pace.
To further investigate the effect of substrate bias on nanotwin formation in sputter deposited Cu films, we therefore examined the cross-sectional film structure in detail using TEM analysis. The bright field TEM and HRTEM images revealed epitaxial nanotwinned Cu films, with an average twin spacing of 19.4 nm. In addition, (111)-oriented planes across the densely-packed nanotwinned columnar grains were misaligned by 4.5°, as confirmed by the selected area electron diffraction (SAD) pattern in Fig. 3(b). Grain boundaries with misorientation of less than ~ 10° are generally considered to be low-angle grain boundaries (LAGB), the interfacial energies of which are markedly lower than those of general HAGBs. From the thermodynamics perspective, HAGBs are seen as the major driving force in grain growth because of their high energy relative to the energy of LAGBs and twin boundaries. In addition, coherent twin boundaries carry much lower stored energy than high-angle grain boundaries and hence have a greatly reduced driving force for the coarsening of twins [12]. Anderoglu et al. also reported the thermal stability of Cu thin film with a high density of nanoscale growth twins by annealing at up to 800°C [10]. That thin film exhibited better thermal stability than did monolithic nanocrystals with high-angle grain boundaries. Combining numerous twin boundaries and LAGBs, the Cu nanotwinned films in this study can be expected to exhibit better thermal stability and mechanical properties contributed by these stable structures.
On the other hand, twin spacing is another important factor that affects the mechanical properties of Cu nanotwins. Similar to the concept of nanograining, nanotwinning has a critical twin spacing for achieving the maximum mechanical strength. Proposed by Lu et al., the strength of nanotwinned Cu increased with decreasing twin spacing and eventually reached a maximum value at around 15 nm [15]. In this study, the indentation hardness of Cu film with twin spacing of about 19.4 nm was measured to be 2.3 GPa. In this case, the twin lamella thickness plays a dominant role in controlling film hardness [10]. It is known that twin and grain boundaries can act as strong obstacles to dislocation motion and that the twin lamella thickness plays a dominant role in controlling film hardness. In addition, when the applied substrate bias voltage was increased, more Ar ions were attracted to bombard the films and enhance the mobility of the atoms, thus increasing the film density and reducing the formation of voids. Therefore, the distinct increase in the hardness of the film is ascribed to the addition of the substrate bias, which facilitated the formation of the nanotwinned structure. In addition to the mechanical properties, the electrical properties were also affected by structural changes caused by the applied substrate bias. As shown in Fig. 8, the sheet resistivity of Cu films slightly increased with increases in the applied substrate bias from 0 to -150 V. That is, the increase in nanotwin density was accompanied by a decrease in the average grain size, and the additional grain boundaries and defects led to higher electrical resistivity. It is known that the electrical resistivity of coherent twin boundaries is about one order lower than that of the high angle grain boundaries. Therefore, despite the dramatic change in structure, the value of the sheet resistivity remained below 10 mΩ/sq.