Fabrication of Copper Circuit Pattern By Using Solid-Electrodeposition Process
Authors: Yanagimoto Hiroshi , Mori Rentaro , Okamoto Kazuaki , Murai Junya 
Published: February 2020

Recently, automotive industry faces to revolution originated from the concept of Connected, Autonomous, Shared and Electric (CASE). For future society, it is noted that electric devices should be increasingly important components because a large number of the devices is indispensable to put CASE in practice.
In electric devices, printed circuit boards (PCBs) are particularly key materials, which require further high density wiring as well as cost reduction. The fine patterned PCBs is often fabricated by semi-additive process. However, it is not easy to achieve finer pattering, owing to limitation of photolithographic resolution as well as residual palladium and/or copper metal in etching area. Regarding with the fabrication cost, the drastic reduction cannot be expected without changing process flow because semi-additive process is one of the mature technologies. In addition to that, the exhaust water generated from developing, electroplating, etching and rinsing processes causes serious environmental issues on nature, almost all of which are related to masking and electro/electroless plating processes. Therefore, it is urgently necessary for CASE to develop simple process without masking and conventional plating.
We have developed novel electrodeposition process named solid electrodeposition (SED) which is characterized with metal ion electrophoresis through solid electrolyte membrane. The present SED apparatus is shown in Fig. 1. The head part composed of anode electrode, metal solution and the membrane is settled on the tip of press apparatus. The head part contacts with substrate under constant pressure and then a voltage is applied between anode electrode and cathode substrate. The metal deposition proceeds on solid-solid interface between membrane and substrate. It is important to note that cation exchangeable polyelectrolyte is used for membrane, i.e., metal ion can diffuse through ion-exchange reaction. When this system is accumulated into small probe system, this process enables electrodeposition of metals only by attaching the membrane surface into the substrate in a serial patterning manner. Moreover, SED has a merit of relatively high deposition rate, which is characterized with effective mass transfer mechanism of metal ion into membrane. We believe that SED can be highly potential process to dissolve not only the cost but also the environmental issues for current fabrication process of electronic elements.
In the current study, we report on the fabrication of PCBs by using SED process. Although SED process can apply for selective deposition, an area of which depends on contact area of membrane used for deposition, it is difficult to match membrane size with fine patterns. Therefore, the patterned seed layer is printed on the substrate before SED process. The deposited copper patterns completely reflect underlying seed patterns without widening metal lines. This is because copper predominantly deposits along height direction, presumably due to selective growth of copper at solid interface. In this contribution, we will report the characteristics of SED process and novel process to fabricate circuit patterns without using masking and rinsing process.

Figure 1