Method to Reduce the Contact Resistivity between Galinstan and a Copper Electrode for Electrical Connection in Flexible Devices

This study demonstrates a method to mount electronic components using gallium-based liquid metals (LMs) with reduced contact resistivity between the LM and a copper (Cu) electrode. Gallium-based LMs have low volume resistivity and low melting points, and they are used as electronic components such as interconnects and sensors of stretchable electronic devices. However, the high contact resistivity of the oxide layer on the surface of the Ga-based LMs becomes a problem when the Ga-based LMs are used in contact with rigid electronic components. To overcome this problem, we studied herein the effect of the oxide layer on contact resistivity via the contact methods of the Ga-based LM (galinstan) and the Cu film. Through the placement of galinstan after the placement of the Cu film and application of vacuum to reduce the effect of the oxide layer, the contact resistivity was reduced to 0.59 × 10-7 ωm2, which was 90% lower than that in the case where the Cu film was placed on galinstan on which the oxide layer grew (5.7 × 10-7 ωm2) (day 1). Additionally, it was found that the contact resistivity decreased in the same order (10-8 ωm2) over time regardless of the methods in which galinstan was applied (day 103). Furthermore, alloy formation on the Cu film surface was confirmed via elemental analysis. Finally, the mounting method using galinstan was demonstrated, which enabled the change in contact resistance to be maintained as low as 7.2% during 100% stretching deformation repeated 100 times (day 1 and day 130). Our results show that low and stable contact resistance with a high stretch tolerance can be achieved via the mounting method using galinstan based on our contact methods. This mounting method, therefore, expands the range of materials suitable for use as substrates and provides new opportunities for the development of stretchable electronics.