Direct Conductive Bonding of Silver Electrodes on Ultrathin Polymer Films

Integrating multiple flexible electronics using conductive bonding is essential for developing soft robotics and on-skin electronics. Increasing film thickness significantly reduces the flexibility of flexible electronics owing to increased bending stiffness. Therefore, direct electrode bonding is more suitable for fabricating flexible electronic systems than integration using a conductive adhesive layer. Atmospheric surface-activated bonding (SAB) is a room-temperature direct-bonding method between metals. However, atmospheric SAB of metal electrodes applicable to flexible devices with a root-mean-square roughness larger than that of silicon wafers is currently limited to using atmosphere-stable gold. This is because of the lack of a method for directly bonding unstable metal materials in ambient air without oxidation. To address this limitation, this study proposes a room-temperature direct conductive bonding in ambient air for a silver film fabricated on an ultrathin substrate using a water vapor plasma method. The 2 μm-thick evaporated silver film samples were directly bonded by analyzing the radical ration during water vapor plasma treatment and surface chemical state and physical state before and after treatment. Furthermore, direct bonding of dissimilar metals, silver and gold, was also achieved. The success of direct silver bonding was strongly correlated with the ratio of O radicals to OH radicals during plasma treatment, confirming bonding at a ratio of approximately 30% or less. The bonded silver thin-film samples exhibited high flexibility, with a minimum curvature radius of less than 0.5 mm at the bonding site and a resistance change rate of less than 6% after repetitive bending tests (1000 cycles).