Electrodeposition of Copper Using Polyelectrolyte Membranes: Effective Ion Transport through Solid-Liquid Interface
Authors: Akamatsu Kensuke , Fujiwara Ryosuke , Takashima Yohei , Tsuruoka Takaaki , Sato Yuki , Murai Junya , Yanagimoto Hiroshi 
Published: May 2020

The widespread demand of coating, corrosion protection, metallurgy, and microelectronics industries for electrodeposition has been met by developing current successful, commercially-available deposition baths containing metallic salts at higher concentration, highly concentrated supporting electrolytes, and various kinds of organic additives, enabling higher deposition rate, greater current efficiency, and long-term stability for the baths. However, developing effective deposition systems for high-performance electrodeposition is challenging, because, (1) in product point of view, the additives sometimes and rather often cause mechanical and/or electrical problems for deposited metal circuit elements due to inclusion of such additives during deposition process, and, (2) in processing point of view, commercial electrodeposition setups cause huge amount of detrimental effluents that can be a serious environmental and cost problems. These issues may be a product of compromise that is particularly relevant for structure and properties of the deposited metals (film resistivity, surface roughness, mechanical and adhesion strength), and processing (costs and manufacturing steps including rinsing and detrimental effluents treatment). The current solution-based electrodeposition systems are generally incapable of significantly increasing reaction rate (deposition rate constant) form dilute electrolyte solution, because diffusion-limiting current that needs to be higher for industrial manufacturing depends on the concentration (and its gradient) of electrolyte solution, which can be described by Fick’s first law in a steady-state electrodeposition. Additionally, current electrodeposition systems are optimized for large area substrate and mass production manufacturing, thereby cannot be suitable for small-lot, on-demand, and multi-product microfabrication. These have inspired the use of polyelectrolyte membranes to encapsulate metallic ions within inside their channels to concentrate from electrolyte solution while maintaining efficient ion transport properties owing to high density anionic counterparts inside channels and the swelling nature of the membrane.We now show that when the polyelectrolyte membrane does not anchor itself to the cathode substrate, stable electrodeposition of metals on cathode surface is achieved, not as much like a solution-phase electrodeposition, resulting in a new type of solid-state electrodeposition (SED) system (Fig. 1).
The SED system consists of cathode electrode, polyelectrolyte membrane (PE), electrolyte solution, and anode electrode.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 (Fig.1).
In the current contribution, we describe transfer process of copper ions between electrodes where the additional PE-solution interfacial layer has been introduced to explain concentration behavior of PE layer for copper ions and electrodeposition characteristics in the present system. From detailed theoretical (both thermodynamic and kinetic) and experimental study regarding with the ion concentration process for PE membrane suggest that (1) the “ion penetration” process is slower than ion exchange (diffusion) process inside PE layer and is therefore rate-determining step for ion concentration, and (2) ion penetration rate increases exponentially as the concentration of copper ions in PE layer decreases.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 to optimize the deposition conditions to achieve higher deposition rate. This strategy can be suitable for small-lot, on-demand, and multi-product microfabrication.

Figure 1