# Electolytically Ionized Abrasive-Free CMP (EAF-CMP) for Copper
## 3 Results and Discussion  
### Comparison of AF-CMP and EAF-CMP  
The potentiodynamic polarization curve provides information on the oxidation and reduction reactions. In Figure 3, the corrosion current density (\(I_{Corr}\)) and corrosion rate (\(CR\)) were compared through polarization curves using a potentiostat to compare the electrolytically ionized CMP solution and general abrasive-free CMP solution. The prepared chemical solution consisted of 3 wt% H\({}_{2}\)O\({}_{2}\), 0.01 M citric acid and 0.1 wt% BTA in deionized water (DIW). It has been reported that \(I_{Corr}\) relates to the \(CR\) and material removal rate in CMP [36]. \(CR\) can be calculated as follows:  
\[CR(\text{mpy})=\frac{0.13\times I_{corr}\times E_{w}}{d}, \tag{3}\]  
where \(E_{w}\) and \(d\) are the equivalent weight of the corroding species and the density of the corroding species, respectively. \begin{table}
\begin{tabular}{c c} \hline
**Parameters** & **Values** \\ \hline Rotational speed of platen (rpm) & 80 \\ Rotational speed of head (rpm) & 80 \\ Wafer pressure (kPa) & 29.4 \\ Retaining ring pressure (kPa) & 39.2 \\ Slurry flow rate (mL/min) & 150 \\ Polishing time (s) & 60 \\ Wafer & 100 mm-diameter, 1.5 \(\mu\)m-thick Cu \\ Abrasive-free slurry & H\({}_{2}\)O\({}_{2}\) + Citric acid + BTA + Deionized water \\ Voltage (V) & 30 \\ \hline \end{tabular}
\end{table} 
Table 1: Experimental conditions. In Figure 3, the \(L_{Corr}\) of the non-ionized chemical solution was 3.35 \(\upmu\)A/cm\({}^{2}\) and the \(CR\) was 3.06 mils per year (mpy). If an AC voltage of 30 V was applied to the chemical solution through stainless steel electrodes, \(I_{Corr}\) increased to 4.56 \(\upmu\)A/cm\({}^{2}\), and the \(CR\) to 4.16 mpy. According to [PERSON] [35], CMP slurry ionization increased the number of -OH radicals in the CMP slurry, which improved the MRR in Cu CMP. In particular, H\({}_{2}\)O\({}_{2}\), used as an oxidizer in Cu CMP slurry, is known to increase the oxidizing capacity by activating the generation of -OH radicals during ionization [35]. Figure 4 indicates that the static etch rate and \(CR\) of EAF-CMP were larger than those of AF-CMP. The etch rate was obtained by dipping 20 mm \(\times\) 20 mm Cu wafers into chemical solutions for two minutes and measuring the thickness change of the Cu films. In the etching experiment, the chemical solution for EAF-CMP was electrolytically ionized by applying a 30 V voltage via electrodes to the chemical solution for the AF-CMP. The static etch rate was measured at five points (the center of the specimen and top, bottom, left, and right from the center of the specimen with a 5 mm span). The etch rates of AF-CMP and EAF-CMP were 14.5 nm/min and 35.1 nm/min, respectively. Figure 4: Etch rates and corrosion rates of AF-CMP and EAF-CMP. Figure 3: Potentiodynamic polarization curves of abrasive-free CMP solution; (**a**) no AC voltage and (**b**) AC 30 V.  
Figure 5 shows the scanning electron microscopy (SEM) images and EDS analysis results of the Cu surfaces after dipping them into chemical solutions. Copper reacted with electrolytically ionized CMP solutions appears to have had more surface oxidation reactions than the copper reacted with abrasive-free CMP solutions. Figure 6 shows the CMP results for AF-CMP and EAF-CMP. The detailed experimental conditions are listed in Table 1. The MRRs of AF-CMP and EAF-CMP were 236.9 nm/min and 303.9 nm/min, respectively. The within-wafer non-uniformity (WIWNU) of EAF-CMP was lower than that of AF-CMP. In AF-CMP, friction forces tended to decrease slightly over the processing time, with an average friction force of 5.509 kgf (Figure 7). In contrast, EAF-CMP showed a friction force similar to AF-CMP for the first 15 s of the process, but the friction force increased after 15 s. The increase in friction force during CMP may be due to the activation of the chemical reaction between the electrolytically ionized chemical solution and the Cu, which takes more than 15 s. The average frictional force in EAF-CMP was 7.946 kgf.