# Investigation of the Two-Way Injection Slurry-Supply Method for the Cu CMP Process
## 3 Results and Discussion  
Figure 2 shows the MRRs between the general and the two-way injection method in a slurry-supply system. As shown in Figure 2a, the H\({}_{2}\)O\({}_{2}\) nozzle was placed close to the head, and the slurry nozzle was placed far away from the head due to the mechanism of Cu CMP [4]. This is because, in Cu CMP, after the oxide film on the surface of Cu is formed by the oxidant, the wafer is polished with the abrasive. When H\({}_{2}\)O\({}_{2}\) is first introduced by positioning the H\({}_{2}\)O\({}_{2}\) nozzle close to the head, the oxidation layer is formed by the wafer, and the wafer is then polished by the introduced slurry. These results show that, when using the two-way injection, the MRR was similar to that of the general injection method (indicated by'reference' in the figure). Even when the flow rates of H\({}_{2}\)O\({}_{2}\) and slurry changed in the two-way injection, the MRRs showed similar results. Contrary to the previous experiment, an evaluation was conducted to determine whether the nozzle position affected the MRRs in the two-way injection. Table 1 shows the removal rate numerically for each condition in Figure 2.  
Figure 2: (**a**) Position of the two way injections slurry–H\({}_{2}\)O\({}_{2}\) (**b**) MRR of the reference and the two-way injection with position; slurry–H\({}_{2}\)O\({}_{2}\). As shown in Figure 3a, the slurry nozzle was close to the head and the H\({}_{2}\)O\({}_{2}\) nozzle was placed farther away. The results of the MRRs when the injecting positions of slurry and H\({}_{2}\)O\({}_{2}\) were reversed are plotted in Figure 3b. The MRRs for Cu remained nearly the same despite the change in the supply position. This means that, even if the slurry and H\({}_{2}\)O\({}_{2}\) were injected without mixing, it did not affect the MRR in Cu CMP. Table 2 shows the removal rate numerically for each condition in Figure 3.  
In the CMP process, the platen to which the CMP pad was attached rotated, and the slurry was injected onto the pad flows along the grooves in the pad [15]. Therefore, the slurry and H\({}_{2}\)O\({}_{2}\) were injected from the two nozzles, but since the positions of the two nozzles were not far apart, two fluids flowed into the wafer at almost the same time as the pad was rotating, and there was no difference in polishing performance. Many studies on slurry injection have been conducted in the field of CMP process. [PERSON] et al. argued that the MRR decreases as the slurry-injection position goes from the center to the edge of the pad [16]. This is because an increasing amount of slurry is thrown off the pad due to the centrifugal force. [PERSON] et al. observed that, the higher the rotational speed was, the more minimal the effect on the slurry-injection position between center of the pad and edge of the wafer carrier [17]. In the two-way slurry-injection method, the slurry and H\({}_{2}\)O\({}_{2}\) were \begin{table}
\begin{tabular}{c c c} \hline \hline
**Flow Rate (mL/min)** & **MRR (Å/min)** & **pH** \\ \hline Reference & 3105.4 & 9.78 \\
100/50 & 2969.99 & 9.76 \\
75/75 & 3010.31 & 9.72 \\
50/100 & 3313.02 & 9.76 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: MRR and pH of reference and the two-way injection with position; slurry–H\({}_{2}\)O\({}_{2}\). Figure 3: (**a**) Position of the two way injection;H\({}_{2}\)O\({}_{2}\)-slurry (**b**) MRR of the reference and the two-way injection with position; H\({}_{2}\)O\({}_{2}\)-slurry. injected near the center of the pad, and the solutions were set to fall into the same circular groove. Therefore, it is possible to minimize the case where the two fluids fall out of the pad due to centrifugal force, and there is no effect on the injection position near the center due to the high rotational speed. As described above, the flow rate of the slurry and H\({}_{2}\)O\({}_{2}\) was controlled to a total of 150 mL/min, even during the two-way injection, which is the flow rate used in the general method.