Fabrication and characteristics of highly ⟨110⟩\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\langle {110} \rangle $$\end{document}-oriented nanotwinned Au films

Fine grained and nanotwinned Au has many excellent properties and is widely used in electronic devices. We have fabricated ⟨110⟩\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\langle {110} \rangle $$\end{document} preferred-oriented Au thin films by DC plating at 5 mA/cm2. Microstructure analysis of the films show a unique fine grain structure with a twin formation. Hardness tests performed on electroplated ⟨110⟩\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\langle {110} \rangle $$\end{document} Au films show a hardness 47% greater than random and untwinned Au. We then achieved direct bonding between two Au ⟨110⟩\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\langle {110} \rangle $$\end{document} surfaces operating at 200 °C for an hour in a vacuum oven. The highly-oriented ⟨110⟩\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\langle {110} \rangle $$\end{document} nanotwinned Au films could be an ideal material in many gold products.


Results and discussion
The substrate composition will affect the grain growth during film deposition, thus the orientation of the seed layer is essential in electroplating. Figure 1a shows the XRD pattern of the Si, Ti and Au seed layer before the electroplating process. The 111 peak is aligned with the normal direction of the substrate, indicating that the substrate possess a strong 111 -preferred orientation. EBSD analysis (Fig. 1b) of the 100 nm-thick Au seed layer shows that the Au surface consists of 111 grains.
The plan-view EBSD analysis of the as-deposited Au films shows the preferred orientation of the surface grains, as shown in Fig. 2a. The green color scheme represents 110 -oriented signals. Figure 2b shows the size distribution of the 110 Au grains deviated from the EBSD results. The average grain size is about 102 nm, which can be referred as fine-grain. Figure 2c is an enlarged plan-view EBSD result.
XRD tests were performed on as-deposited 110 Au films and randomly oriented Au films. Figure 3a is the XRD pattern of 110 films, showing a high Au (220) peak. This confirms that the columnar fine grains are 110 oriented. Randomly oriented Au films are also confirmed by XRD, as shown in Fig. 3b.
Surface roughness of the film was performed with AFM analysis. The root-mean-square (RMS) of the 100 nmthick Au seed layer was 3.7 nm, (Fig. 4a) and the RMS of the as-deposited 10 μm-thick 110 -Au film was 10.6 nm (Fig. 4b).   Fig. 5a. Under magnification, the twin planes display different directions and lengths that relate to different grain angles (Fig. 5b). The twin spacing is measured to be about 14.4 ± 1.47 nm. Furthermore, adjacent grains affected the diffraction pattern to be displayed as a polycrystalline image (Fig. 6).
The FIB cross-sectional image of as-deposited films show a columnar fine-grained structure as shown in Fig. 7a. A 2 μm-thick transition layer is present near the seed layer and the columnar fine-grain structure is measured to be 8 μm-thick. In the cross sectional TEM analysis, it is clear that the columnar fine grains have many straight boundaries and dislocations as shown in Fig. 7b. The red arrow indicates the 110 direction. It can be confirmed from the diffraction pattern that twin boundaries are present.
Hardness tests of the as-deposited Au thin films were performed by nanoindenter. Continuous harmonic contact stiffness measurement was used for all indentation experiments with the modulation frequency of 45 Hz and 2 nm harmonic displacement. The drift rate was 0.3 nm/s and strain rate was 0.05 s −1 . The nanoindentation tests are perfomed with an indentation depth of 500 nm. Load-displacement curves and hardness-displacement curves of 110 Au films and random Au films are shown in Figs. 8 and 9. A total of eight contact points were tested on both samples and its elastic modulus and hardness values are displayed in Table 1. The elastic modulus of the 200 Au films is 108 GPa and hardness is 1.75 GPa. Random Au has an elastic modulus of 67.2 GPa and a    (Fig. 10a). The original 100 nm-large fine grains undergoes crystallization and twin elimination during aging and turns into micrometer-scale large    (Fig. 10b). From the plan-view EBSD analysis, the grain orientation transforms into a 210 preferred texture (Fig. 10c).
Our direct bonding tests were perfomed at 200 °C for an hour in a vacuum environment. 110 preferred oriented Au films were bonded and the cross-section microstructure can be observed in Fig. 11. From the FIB polished cross-section of the Au bonding, we can observe no obvious voids at the bonding interface. The columnar fine grains crystalized into large grains during the thermo-compression bonding process. Further TEM analysis of the bonded structure was performed. From the TEM image as shown in Fig. 12a, we can observe slight grain growth across the interface and void formation and travel along the boundaries, as marked. Enlarged images of certain points of the original interface, presented in Fig. 12b, show successful grain growth and interface elimination. EDAX analysis of the interface (Fig. 12c) show that the bonding has no impurities.  www.nature.com/scientificreports/ Grain orientation and twin formation can be controlled during DC plating, by tweaking important parameters such as the applied current, stirring rate, bath temperature and electrolyte components. High currentsare avoided during deposition to prevent nonuniformity. Adding a magnetic stirring could add disturbance to the solution and increase film stress during deposition allowing twin formation while DC plating. Under these conditions, 110 oriented columnar fine-grained Au with nanotwins could be fabricated, and surface polishing on the 10 μm-thick Au film was not necessary to achieve low surface roughness for our bonding tests.
Nanotwinned 111 planes parallel to the substrate surface has been reported 12,17,19,26 . They are observed to exist in columnar fine grains prolonging several micrometers vertical to the substrate, but do not have a preferred direction from the plan view due to grains having other orientations. Nanotwins are unstable boundaries, and grains with nanotwins will undergo grain growth during thermal treatment. This leads to 110 grains transforming into 210 grains.
The columnar fine grains containing nanotwins has 47% greater hardness when compared to random Au. Nanotwins and fine grains could help improve the mechanical strength in metal films, this way gold could be produced to withstand external shocks resulting in deformation. www.nature.com/scientificreports/ Direct bonding of two films has been achieved. This process is highly dependent on surface diffusion, roughness, temperature, load pressure, and bonding time length. The surface diffusion rate could be calculated by Arrhenius equation 24,25 : where k B is 8.617 × 10 -5 eV/K and ambient temperature is 473 K. So k B T is about 0.04 eV. For the (100) surface, the active energy E a and the pre-factor D 0 is 0.64 eV and 0.0015 (cm 2 /s), respectively. For the (110) surface, E a and D 0 is 0.86 eV and 0.0063 (cm 2 /s), respectively. The self-diffusion coefficient of an Au adatom is 2.273 × 10 -6 cm 2 /s on the (111) surface, and 4.323 × 10 -12 cm 2 /s at 200 °C on the (110) surface.
Although the self-diffusion rate of the (110) surface is six orders lower than the (111) surface, nanotwins exist inside the grains of (110) which can help achieve direct bonding at low temperature and pressure when the RMS of Au film is 10.6 nm. Because its high resistance to oxidation, gold films used in direct bonding could be ideal for in electronic package.

conclusions
We have fabricated highly 110 -oriented nanotwinned Au films by DC plating with low surface roughness. The hardness is that of 1.76 GPa and 47% greater than the randomly oriented polycrytalline Au. After thermal treatment, the preferred orientation transformed from 110 into 210 . Bonding tests of two films could be achieved at 200 °C under a bonding pressure of 0.76 MPa for an hour in a 10 -3 torr vacuum chamber. The fine-grained and nanotwinned Au has many ideal properties that could be used in electronic devices and packaging processes.