# Optimization of Additive and Current Conditions for Void-Free Filled Through-Silicon Via
## 3 Results and Discussion
### Cyclic Voltammetry (CV) Analysis  
The effects of additives during copper plating were confirmed by CV measurement.  
CV measurement is an electrochemical technique that measures the current developing in an electrochemical cell under conditions where the voltage is higher than that predicted by the Nernst equation [12]. CV is performed by changing the potential of a working electrode (Ewe) versus Ag/AgCl  
Figure 3: The cross-sectional optical microscope (OM) image before and after pretreatment in vacuum desiccator.  
Figure 2: Three-electrode system schematic.  
electrode through a cycle and measuring the resulting current. In the reduction section of the CV curve, the cathode peak potential value represents the voltage value needed for plating. If the cathode peak potential is shifted to a positive value by the additive, then the plating rate can be expected to increase. Conversely, if the cathode peak potential is shifted to a negative value, then the plating rate can be expected to decrease.  
CV measurements were performed between \(-0.6\) V and \(0.8\) V at a scan rate of \(20\) mV/s. Figure 4a shows the CV curves corresponding to the behavior of Cu ions in a VMS bath after the addition of PEG. The onset potential for Cu deposition started at \(-0.03\) V without PEG, and the cathode peak potential is observed at \(-0.192\) V. As the amount of PEG increased, the cathode peak potential tended to be negative and shifted toward the negative direction (from \(-0.18\) V to \(-0.4\) V). This means that an amount of shifted energy should be applied to allow the same current to flow, indicating that the reduction reaction shifted to a state where it is difficult to occur. In other words, as PEG concentration increases, plating is suppressed [13; 14; 15; 16; 17].  
When PEG concentration exceeded \(500\) ppm, no further negative shift was observed, indicating that the maximum concentration of PEG in the VMS bath is \(500\) ppm.  
Figure 4b shows the CV polarization curves for different SPS concentrations. When SPS was added to PEG with a concentration of \(500\) ppm, the cathode peak potential and onset potential shifted toward the positive direction. As SPS concentration increased from \(1\) ppm to \(5\) ppm, the cathode peak tended to present a positive shift (from \(-0.35\) V to \(-0.30\) V). This means that the reduction potential is reduced, indicating that the plating process speeds up as SPS concentration increases [14; 15; 16; 17; 18; 19].  
However, when SPS concentration exceeded \(5\) ppm, the peak shifted toward the negative direction. From this point of view, the suitable SPS concentration is expected to be \(5\) ppm.  
As observed in Figure 4c, similar to PEG, when JGB was added to the plating bath, both the cathode peak potential and initiation potential also shifted toward the negative direction [13; 16; 20]. It can be seen that, as JGB concentration increased from \(10\) ppm to \(50\) ppm, the cathode peak potential shifted from \(-0.35\) V to \(-0.37\) V. The peak potential values at JGB concentrations of \(30\) ppm and  
Figure 4: Result of cyclic voltammetry (CV): (**a**) CV curve according to the poly(ethylene glycol) (PEG) concentration in the virgin makeup solution (VMS) bath; (**b**) CV curve according to the bis-(3-sodiumsulopropyl disulfide) (SPS) concentration in a VMS bath with \(500\) ppm PEG; (**c**) CV curve according to Janus Green B (JGB) concentration in a VMS bath with \(500\) ppm PEG and \(3\) ppm SPS.  
50 ppm were not significantly different and were expected to be saturated at 30 ppm. The CV analysis further demonstrated that the suppressor and leveler suppressed the plating rate, while the accelerator accelerated the plating rate. Based on the CV analysis data, electrodeposition experiments were performed on the different additives.