## 2 Materials and Methods  
The process flow of a two-step plasma treatment on a Cu surface prior to Cu-to-Cu bonding is schematically illustrated in Figure 1. Both sputtered and electroplated Cu thin films were prepared on 700-nm-thick SiO\({}_{2}\) in a 200-mm Si wafer. For the sputtered Cu samples, a 50-nm-thick Ti thin film was deposited in a conventional sputter (Sorona, Inc., PyungTak, Korea, SRA-110) under 5 mtorr working pressure and 2500 W power with 80 sccm Ar gas flow. Subsequently, a \(\sim\)1-\(\upmu\)m-thick Cu thin film was deposited under 5 mtorr working pressure and 2500 W power with 80 sccm Ar gas flow. For the electroplated samples, 100-nm-thick Cu as a seed layer was deposited on a Ti-deposited wafer followed by \(\sim\)1-\(\upmu\)m-thick electroplated Cu that was done in a manual electroplater (Sungwon Forming, Ansan, Korea, SW-PM2-R2Q1) with 2 ASD (amps/square decimeter) current density and 6.48A current. Both the sputtered and electroplated samples were then subjected to two-step plasma treatment in a direct current (DC) sputter chamber. First, Ar plasma treatment was carried out for 30 s at an Ar gas flow rate of 150 sccm, radio frequency (RF) power of 100 W, and pressure of 7.5 mTorr. The samples were then treated by N\({}_{2}\) plasma for 300 s at a N\({}_{2}\) gas flow rate of 45 sccm, RF power of 200 W, and pressure of 7.5 mTorr. The process conditions for each sample are summarized in Table 1. After the two-step plasma treatment, Cu-to-Cu thermocompression bonding was performed at 300 \({}^{\circ}\)C under 7000 mbar for 1 h in a wafer bonder (SUSS Microtec, SB 8e), and the post-annealing process was undertaken at 200 \({}^{\circ}\)C under 200 mbar for 1 h. The bonding process temperature and pressure profile is shown in Figure 2.  
The plasma-treated samples were analyzed structurally by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Inc., Waltham, MA, USA, K-Alpha), X-ray diffraction (XRD, Malvern PANalytical, Ltd., Malvern, UK, Empyrean, X'pert PRO-MPD), transmission electron microscopy (TEM, Talos F200X), and atomic force microscopy (AFM, Veeco Dimension 3100). The sheet resistance was measured at 20 points per sample by semiconductor parameter analyzer (SPA, Agilent, Santa Clara, CA, USA, 4155C)  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Substrate** & **Cu Deposition** & \begin{tabular}{c} **Ar Plasma** \\ **Condition** \\ \end{tabular} & \begin{tabular}{c} **N\({}_{2}\) Plasma** \\ **Condition** \\ \end{tabular} \\ \hline \multirow{2}{*}{\begin{tabular}{c} Sputtered \\ Cu sample \\ \end{tabular} } & Si/SiO\({}_{2}\)(700 nm)/ & \begin{tabular}{c} Flow rate Ar 808 ccm, \\ RF 2500 W, \\ Pressure 5 mTorr \\ \end{tabular} & Flow rate 150 sccm, & Flow rate 45 sccm, \\  & & & & RF 200 W, \\  & & & & Pressure 7.5 mTorr, \\ \hline \multirow{2}{*}{\begin{tabular}{c} Electroplated \\ Cu sample \\ \end{tabular} } & Si/SiO\({}_{2}\)(700 nm)/ & \begin{tabular}{c} Current density 2 ASD \\ Freq. 40 Hz, \\ Speed 40 rpm \\ \end{tabular} & \
\begin{tabular}{c} Pressure 7.5 mTorr \\ Time 30 s \\ \end{tabular} &
\begin{tabular}{c} Pressure 7.5 mTorr \\ Time 300 s \\ \end{tabular} \\  & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Process condition for sputtered and electroplated Cu samples.  
Figure 1: Fabrication process flow.  
Figure 2: Bonding process profile.  
using a 4-point probe method. For the bonded wafers, the bonding quality of the Cu bonded interface was confirmed by scanning acoustic tomography (SAT, Hitachi Power Solution, Ltd., Hitachi-Shi, Japan, Hitachi FineSAT III).