Direct Conductive\nBonding of Silver Electrodes on\nUltrathin Polymer Films

Integrating multiple flexible electronics using conductive\nbonding\nis essential for developing soft robotics and on-skin electronics.\nIncreasing film thickness significantly reduces the flexibility of\nflexible electronics owing to increased bending stiffness. Therefore,\ndirect electrode bonding is more suitable for fabricating flexible\nelectronic systems than integration using a conductive adhesive layer.\nAtmospheric surface-activated bonding (SAB) is a room-temperature\ndirect-bonding method between metals. However, atmospheric SAB of\nmetal electrodes applicable to flexible devices with a root-mean-square\nroughness larger than that of silicon wafers is currently limited\nto using atmosphere-stable gold. This is because of the lack of a\nmethod for directly bonding unstable metal materials in ambient air\nwithout oxidation. To address this limitation, this study proposes\na room-temperature direct conductive bonding in ambient air for a\nsilver film fabricated on an ultrathin substrate using a water vapor\nplasma method. The 2 μm-thick evaporated silver film samples\nwere directly bonded by analyzing the radical ration during water\nvapor plasma treatment and surface chemical state and physical state\nbefore and after treatment. Furthermore, direct bonding of dissimilar\nmetals, silver and gold, was also achieved. The success of direct\nsilver bonding was strongly correlated with the ratio of O radicals\nto OH radicals during plasma treatment, confirming bonding at a ratio\nof approximately 30% or less. The bonded silver thin-film samples\nexhibited high flexibility, with a minimum curvature radius of less\nthan 0.5 mm at the bonding site and a resistance change rate of less\nthan 6% after repetitive bending tests (1000 cycles).