Kinetic and in Situ Analysis for Ion Transport Process in Copper Electrodeposition Using Polyelectrolyte Membranes
Authors: Akamatsu Kensuke , Yamada Shunsuke , Takashima Yohei , Tsuruoka Takaaki 
Published: September 2023

Electrodeposition of metals has been one of the most important techniques for fabrication of microelectronics and semiconductor elements in printed circuit boards and electronics device manufacturing. From the industrial perspective, the management of electrodeposition bath is crucial to maintain product quality, productivity and long-term stability of the bath, the subject of which includes bath temperature, concentration of metal salts, supporting electrolytes, and various additives. Although these parameters have been optimized to achieve desirable deposition rate, depending on the product type and morphology, some environmental issues regarding with generating mist, sludges, and waste effluent during electrodeposition process are becoming apparent. Additionally, current electrodeposition process has been developed for substrates with large surface area and for large scale production of electronic components and could not be suitable for recent demand of small-lot, site-selective and multiproduct device microfabrication. Therefore, high-performance deposition system, allowing site-selective electrodeposition of metals without generation of large amount of effluents, needs to be developed to impact above environmental and cost perspectives. Recently, we reported a novel method for electrodeposition of metals using polyelectrolyte films attached on cathode substrate (solid electrodeposition: SED) [1]. The SED process enables one to deposit metals directly on the cathode surface from metal ions inside solid polyelectrolyte membranes and has desirable advantages including higher deposition rate, site-selectivity, and deposition using lower concentration electrolyte solution without additives. In this SED process, thin polyelectrolyte membrane sandwiched between electrodes effectively concentrates metal ions through interfacial penetration, which increases the conductance between the electrodes and realizes solid-state electrodeposition that produces no mist, sludge, or even waste effluent.
In this study, we present mechanistic analysis of the SED process, including transport kinetics and equilibrium reaction of copper ions at solution/membrane interface (Figure A). We show that both detailed experimental results and theoretical calculations reveal that ion transfer in the present electrodeposition system is a penetration-controlled process at the solution/polyelectrolyte interface, providing an inherently different ion-transport mechanism compared to that of conventional diffusion-controlled electrodeposition. Several parameters including penetration rate constant, electrode geometry, and distance between polyelectrolyte and anode are varied to investigate the ion-transport process in the SED system. When the anode is slightly apart from polyelectrolyte film to introduce electrolyte layer but is closer as far as possible to it, greater current density for copper electrodeposition is achieved at steady state than that for the systemwhere the membrane is sandwiched by electrodes (Figure B). This can be due to maximum ion penetration rate and minimum resistivity of electrolyte layer, providing better deposition setup for high performance electrodeposition. Additionally, we also performed in situ analysis of concentration of copper ions inside polyelectrolyte film via visible absorption spectroscopy using a miscofiber during electrodeposition. As a result, concentration of copper in the polyelectrolyte film decreased initially and then became constant (steady state) as the deposition proceeded, a fashion of which was quite similar with that of current-time curves simultaneously acquired during constant-voltage electrodeposition.
The developed deposition setup demonstrates stable electrodeposition of copper using aqueous copper sulfate solution with no supporting electrolytes and/or additives at deposition rate of few micrometers per minute. As a site-selective deposition tool, the SED process can be used to fabricate microelectronic and even semiconductor circuit elements in a manner that resembles stamping deposition, and is suitable for small-lot, on-demand, and multi-product microfabrication.
Reference:
[1] K. Akamatsu, S. Nakano, K. Kimura, Y. Takashima, T. Tsuruoka, H. Nawafune, Y. Sato, J. Murai, H. Yanagimoto, ACS Appl. Mater. Interfaces, 13 (2021) 13896-13906.

Figure 1