Interfacial Process for Electrochemical System Using Polyelectrolyte Membranes for High-Performance Electroplating
Authors: Akamatsu Kensuke , Fujiwara Ryosuke , Takashima Yohei , Tsuruoka Takaaki , Sato Yuki , Murai Junya , Yanagimoto Hiroshi 
Published: February 2020

Demand for the development of deposition strategies of well-defined metal thin films with good chemical and physical characteristics has been rapidly growing in the field of various electronics applications. The conventional plating bath for electro- or electroless copper deposition involves various chemical reagents such as deposition promoter (or inhibiter), brightening and smoothing agent, etc. These additives, typically organic molecules, sometimes cause mechanical problems due to inclusion of such additives during deposition process, and the discharge from the deposition bath can be a serious environmental issue. Therefore, the development of a facile process that enables additive-free deposition of metallic thin films as well as low environmental toxicity is an important challenge for the fabrication of next generation microelectronic elements. In this context, herein we report that metallic thin films can be deposited through polyelectrolyte thin layers placed on the cathode electrode by the diffusion of metallic ions from the interior of polyelectrolyte, which is essentially different to conventional solution-phase electrodeposition processes (Figure 1). As a proof of concept study, we present the successful electrochemical deposition of copper thin films at the interface between an electrode and ion-doped polyelectrolyte layers from the baths containing only metal salts (without any additives).
In the case for the present three-phase electrochemical system consisting of cathode electrode, polyelectrolyte membrane (PE), and electrolyte solution, PE works as an additional interfacial phase, in which metal ions are transferred through ion exchange reaction based on concentration gradient of metal ions bound with sulfonic acid functional groups in PE layer. Most important feature of the use of PE at the electrode surface is their ability for metal ions to be concentrated in inner ion transport channels with high density sulfonic anions inside the channels.
In this electrochemical system, reduction of metal ions to form metal atoms (thus metal films) occurs at electrode-PE interface, and ion exchange reaction occurs inside PE and also at the interface between PE and electrolyte solution. Specific characteristics for the present three-phase electrochemical deposition system can be described as follows. In the case that the deposition rate of metals is below maximum ion exchange rate for PE, (1) concentration of metal ions in PE phase remain unchanged due to higher ion exchange rate than deposition rate in this condition, thus enabling constant current and deposition rate during electroplating, and (2) ion transport number is nearly unity in PE phase because sulfonic anions in PE layer can form metal salts in this condition, i.e., there are mostly no protons to be reduced to form hydrogen bubble, where higher current efficiency for metal deposition can be guaranteed.
In the current contribution, the effect of cation concentration, temperature, and electrochemical deposition conditions on the current efficiency, deposition rate, growth process and morphology of the films has been investigated. Several electrochemical, microscopic and quantitative analysis revealed that the metallic films have been successfully deposited at the cathode-polyelectrolyte interface, the deposition rate of which are substantially determined by ion exchange rate of cations. This strategy offers an opportunity to translate solution-phase electrochemical deposition into an on-demand, high-throughput, cost-effective, and environmentally-friendly process for the fabrication of microelectronics circuit elements.

Figure 1