EIS Study on Solid Electrolyte Deposition of Copper Film

Abstract

Copper plating is generally used for the fabrication of a printed circuit board of electronic device. The patterning is carried out by masking with insulator for the partial plating on the substrate. Solid electrolyte deposition (SED) method using solid electrolyte instead of liquid electrolyte is strong candidate for an alternative plating technique without masking because the copper is deposited only on the substrate contacted with solid electrolyte. Figure 1 shows the scheme of electrochemical cell for copper SED. The electrochemical cell is quite simple, namely, a thin solid electrolyte membrane is sandwiched by substrate and counter electrode. The substrate works as a cathode during copper deposition. Copper plate is used for the counter electrode because cupric ions are dissolved into the solid electrolyte membrane by the anodic reaction. Cation-exchange membrane is used for the solid electrolyte since the mass transfer of cupric ions is controlled by migration originating from the voltage between two electrodes. The solid electrolyte membrane is immersed in CuSO4solution to absorb cupric ion by ion-exchange for over 24 hours before the polarization. The copper is deposited on the substrate when the potentiostatic polarization is performed. Advantages of SED comparing with common electro-deposition are written as follows. ・Electrochemical cell is very simple. ・Post-processing is simple because of no masking. ・It’s environment-friendly because of a small quantity of liquid． ・The deposition rate is very fast because driving force of mass transfer of cupric ions is migration and the concentration of cupric ions is maintained constantly in the solid electrolyte membrane. The deposition rate is determined by three steps, which are anodic dissolution of copper counter electrode, the mass transfer of cupric ions in solid electrolyte membrane and the cathodic reaction on the substrate. Therefore, the measurements of 3D electrochemical impedance were carried out to discriminate above-mentioned three contributions to the deposition rate of copper by SED. Reference: 1) K. Akamatsu, Y. Fukumoto, T. Taniyama, T. Tsuruoka, H. Yanagimoto, H. Nawafune, Langumuir, 27, 11761-11766 (2011). Figure 1