All-soft electronic devices and integrated microsystems enabled by liquid metal

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Kim, Mingu
Brand, Oliver
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The objective of this thesis is to explore all-soft electronic devices and integrated microsystems enabled by gallium-based liquid metal (eutectic gallium-indium alloy, EGaIn) to realize fully-integrated wearable and skin-mountable electronics. Lightweight, flexible, and stretchable wearable electronics have gained significant attention for various sensing applications ranging from entertainment to healthcare, but the mechanical mismatch between soft biological skins and conventional rigid and bulky electronic materials often limits the ultimate usability and leads to hard-soft material interface failure. To circumvent this limitation, the use of conducting liquid, such as EGaIn, has great potential because of its low melting temperature, and excellent electrical and mechanical properties. However, EGaIn patterning challenges, particularly regarding minimum feature sizes, size-scalability, uniformity, and residue-free surfaces, have limited its use for the demonstration of high-density electronic devices. These technical challenges have motivated the development of novel EGaIn patterning and integration technologies to develop all-soft microelectronic devices and fully-integrated soft microsystems. This research particularly focuses on lithography-enabled thin-film patterning techniques for EGaIn structures with dimensions ranging from the nanometer scale to the centimeter scale. Soft-lithography-based EGaIn thin-film patterning techniques utilizing subtractive reverse stamping and additive stamping processes is investigated. Microscale EGaIn thin-film patterning using subtractive reverse stamping yields high-resolution (>2 μm), size-scalable (μm to 1-2 mm), uniform, and residue-free EGaIn thin-film structures. Uniform, large-area (mm to cm) EGaIn thin-film patterning is achieved using a complementary additive stamping technique. While micrometer-scale EGaIn thin-film patterning is demonstrated using the subtractive reverse stamping technique, scaling this process down to submicron features is difficult because of the high surface tension of EGaIn. To overcome this limitation, a novel hybrid fabrication technique using electron-beam lithography in combination with soft lithography is investigated. Using this technique, for the first time, submicron-scale EGaIn thin-film patterning with feature sizes as small as 180 nm is demonstrated. The developed multiscale EGaIn patterning techniques in combination with a vertical integration approach based on EGaIn-filled soft vias overcome the current limitation in EGaIn fabrication and integration. Combining the scalable fabrication based on soft lithography and vertical integration techniques, 3D-integrated, soft functional microsystems are demonstrated for physical strain and pressure sensing and chemical environmental sensing applications. The developed 3D-integrated, physical and chemical microsystems improve over a single sensor by enabling high-density integration, multifunctional sensing capabilities, as well as mechanical flexibility and stretchability. In addition, all-soft and liquid-phase supercapacitors based on oxygen functionalized CNT-integrated EGaIn electrodes are demonstrated for powering soft electronic devices. The supercapacitors represent the first demonstration of an all-soft platform based on EGaIn for high-performance and wearable energy storage.
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