# Low-Temperature Diffusion of Au and Ag Nanolayers for Cu Bonding
## 2 Materials and Methods  
In this study, an e-beam evaporator (Evaporation System, SRN-200, Sorona Inc., San Luis Obispo, CA, USA) was used to deposit the Au and Ag, and sputter (Sputtering system, SRN-110, Sorona Inc.) was used to deposit Tl layer followed by Cu layer. Evaporated Au and Ag nanolayers tend to form porous films rather than thin films, which is expected to facilitate solid diffusion with the Cu thin film layer. An 8-inch silicon wafer (100) was thermally oxidized (Diffusion Furmaec, SJF-1000, DS GLOBAL, Seoul, Republic of Korea) to grow approximately 0.7 \(\upmu\)m of SiO\({}_{2}\) and then diced (Automatic Dicing Saw DAD 3350, DISCO Corporation, Tokyo, Japan) to 1 cm \(\times\) 1 cm. After the dicing step, Au or Ag nanolayers were deposited on the Cu surface to a thickness of approximately 12-15 nm. During our evaporation process, the deposition rate was so fast that it was very difficult to control the thickness to less than 10 nm. For this reason, we have deposited 12 nm Au and 15 nm Ag nanolayers on the Cu surface. To confirm how Au and Ag mutually diffuse with Cu, an XPS (X-ray photoelectron spectroscopy, Thermo Fisher Scientific Brno s.r.o, Nexsa, Brno, Czech Republic) analysis was utilized. The specimens were processed as follows: Ti was sputtered with a thickness of 50 nm as an adhesion layer on the Si/SiO\({}_{2}\) surface, after which 1 \(\upmu\)m of Cu film was sputtered. Afterwards, 12 nm of Au or 15 nm of Ag was deposited by e-beam evaporator, and the specimens were heated on a hot plate at temperatures of 25 \({}^{\circ}\)C, 100 \({}^{\circ}\)C, 150 \({}^{\circ}\)C, and 200 \({}^{\circ}\)C for one hour to evaluate the degree of mutual diffusion. The process flow for the diffusion test is presented in Figure 1. The film structure was analyzed by TEM \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Element** & **Au** & **Ag** & **Cu** \\ \hline Atomic Number & 79 & 47 & 29 \\ \hline Empirical Radius (pm) & 144 & 144 & 128 \\ \hline Young’s Modulus (GPa) & 79 & 83 & 110–128 \\ \hline Poisson’s Ratio & 0.4 & 0.37 & 0.34 \\ \hline Electrical Resistivity (n\(\Omega\cdot\)m at 20 \({}^{\circ}\)C) & 22.14 & 15.87 & 16.78 \\ \hline Density (g/cm\({}^{3}\)) & 19.3 & 10.49 & 8.96 \\ \hline CTE (mm/(m\(\cdot\)K) at 25 \({}^{\circ}\)C & 14.2 & 18.9 & 16.5 \\ \hline Melting Point (\({}^{\circ}\)C) & 1067.18 & 961.78 & 1084.62 \\ \hline Crystal Structure & FCC & FCC & FCC \\ \hline Electronegativity & 2.54 & 1.93 & 1.90 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material properties of Au, Ag, and Cu. Finally, Cu bonding was performed using Au or Ag as a passivation layer. In this study, W2W bonding (SB 8e, Suss Microtec, Garching, Germany) was employed. The process flow for the bonding is presented in Figure 2. As shown in Figure 2, the top wafer was fabricated with 1 cm \(\times\) 1 cm square Cu structures and bonded to the bottom wafer on a Cu blanket film. The bonding procedure was conducted at 180 \({}^{\circ}\)C under a pressure of 0.8 MPa for 30 min. Subsequently, an annealing step was performed at 200 \({}^{\circ}\)C under a pressure of 0.2 MPa for a duration of 60 min. Both the bonding temperature and annealing temperature were chosen to achieve Cu bonding at temperatures below 200 \({}^{\circ}\)C, facilitating low-temperature Cu bonding with the use of a metal passivation layer. The temperature was ramped up from room temperature to 180 \({}^{\circ}\)C at first, and then bonding and annealing processes were performed. Inside the bonding chamber, a base pressure of 5 \(\times\) 10\({}^{-5}\) mbar was maintained from the beginning to the end of the process. The effects of the annealing process are reported in the literature [18].