Direct Copper Patterning at High Deposition Rate By Using Environmentally Compatible Electroplating Process
Authors: Yanagimoto Hiroshi , Mori Rentaro , Okamoto Kazuaki , Kondo Haruki , Kuroda Keiji , Murai Junya 
Published: May 2020

Electroplating has been an important process to produce electronic devices, automotive parts, decorative materials and so on. In contrast to that, plating is endowed with environmental issues originated from exhaust fluids and acid mists. Therefore, it is limited to construct new production line in spite of necessity of plating. Moreover, it is difficult to improve productivity drastically due to completely established plating technology in 21th century. To break down demerits of conventional electroplating, we focus on the fundamental issue regarding with the plating process that can be performed in electrolyte bath. Recently, we have developed novel electroplating process named solid electrodeposition (SED), which is characterized by metal ion electrophoresis through solid electrolyte membrane1). This process has unique deposition behavior that the metal deposition on substrate proceeds only on contact area with the membrane, like a stamping process. The SED has a merit of reducing amount of exhaust fluids as well as direct patterned deposition reflecting the membrane shape. Therefore, we believe that SED is highly promising process to metallize the substrate instead of the conventional electroplating. In this study, we report on copper deposition mechanism and direct fine pattering process by applying SED.
We evaluate the limitation of copper deposition rate under controlled process parameters. As a result of that, the maximum deposition rate reaches over 0.63A/dm2 (14μm/min), which should be relatively higher than conventional one. The impressive value is originated from metal transfer mechanism into membrane as well as the device configuration. The metal deposition proceeds on solid-solid interface between membrane and substrate so that no diffusion layer exist near the substrate2). Moreover, the distance between anode and cathode can be shorten because the membrane preserves electrolyte solution. The copper film deposited from electrolyte solution without additives exhibits relatively smooth and minute surface morphology. This may be because membrane contacted on substrate restricts abnormal Cu growth due to contact pressure.
SED has a merit for direct patterned deposition reflecting membrane shape, but it is not easy to fabricate fine patterns without masking. Therefore, it is limited to apply high-end parts such as printed circuit boards and sensor devices. To overcome this problem, we have developed direct fine patterning process by combining SED with dry coating and etching technologies. Figure 1 shows schematic diagram and photographs of each step. Firstly, the metal thin layer is formed on substrate by means of sputtering (electrode layer). Secondary, silver nanoparticles are printed as fine circuit patterns and sintered at optimal temperature (seed layer). Thirdly, copper was selectively deposited on seed layer by means of SED, which utilizes the potential difference between electrode and seed layers. The SED proceeds through isotropic growth along vertical direction because the deposition occurs on the surface of seed layer contacted with the membrane. Therefore, SED can produce relative thick and fine patterns following line width of seed layers. Finally, the bear electrode layer is etched away by plasma exposure. We believe that this approach is highly promising process to fabricate circuit pattern in terms of cost (simple step) and environmental (no exhaust fluids) point of view.
1. H. Yanagimoto, R. Mori, K. Okamoto and J. Murai, F01-994, 236nd ECS Meeting.
2. Y. Narui, Y. Hoshi, I. Shitanda, M. Itagaki, H. Yanagimoto, M. Hiraoka and H. Iisaka, E01-920, 232nd ECS Meeting.

Figure 1