Size‐Scalable and High‐Density Liquid‐Metal‐Based Soft Electronic Passive Components and Circuits Using Soft Lithography

Abstract

The use of conducting liquids with high electrical conductivity, such as eutectic gallium–indium (EGaIn), has great potential in electronics applications requiring stretchability and deformability beyond conventional flexible electronics relying on solid conductors. An advanced liquid metal thin‐line patterning process based on soft lithography and a compatible vertical integration technique are presented that enable size‐scalable and high‐density EGaIn‐based, soft microelectronic components and circuits. The advanced liquid metal thin‐line patterning process based on poly(dimethylsiloxane) (PDMS) substrates and soft lithography techniques allows for simultaneous patterning of uniform and residue‐free EGaIn lines with line width from single micrometers to several millimeters at room temperature and under ambient pressure. Using this fabrication technique, passive electronic components and circuits are investigated under elastic deformations using numerical and experimental approaches. In addition, soft through‐PDMS vias with high aspect ratio are demonstrated for multilayer interconnections in 2.5D and 3D integration approaches. To highlight the system‐level potential of the patterning technique, a chemical sensor based on an integrated LC resonance circuit with a microfluidic‐tunable interdigitated capacitor and a planar spiral inductor is fabricated and characterized. Finally, to show the flexibility and stretchability of the resulting electronics, circuits with embedded light emitting diodes (LEDs) are investigated under bending, twisting, and stretching deformations.

Soft Electronics Manufacturing Using Microcontact Printing

“…The height variations of the printed features along their length are characterized using white-light interferometry (Zygo NewView 7300). Overall, the average EGaIn heights, which were greater than 1 µm, are larger than those seen by Gozen et al [49] and considerably larger than those seen in Kim et al [50,51] For the Si stamps, in general, a larger line width results in a smaller height. Since the oxide skin thickness does not exceed only a few nanometers (e.g., between ≈0.5-2.5 nm [8] ), the observed variations likely arise from the variations in EGaIn volume rather than changes in the oxide layer thickness.…”

“…The unwanted residue formation was one of the limitations of other EGaIn fabrication approaches [49][50][51] and has been addressed in our approach. The unwanted residue formation was one of the limitations of other EGaIn fabrication approaches [49][50][51] and has been addressed in our approach.…”

“…[5,6] Specifically, the binary alloy of eutectic gallium-indium (EGaIn: 75 wt% Ga and 25 wt% of In) and the ternary alloy of gallium-indium-tin (Galinstan) are both liquid at room temperature: they flow freely with negligible resistance (viscosity of EGaIn is about 1.9 mPa s [7] ) and accommodate changes in channel shapes, thereby maintaining electrical circuit functionality without failure even at large strains (>100%). It is noted that although some liquid metal fabrication approaches, e.g., [48][49][50][51] have been referred to as microcontact printing, those processes do not follow the stamp-based µCP approach that is used in this paper, and therefore are fundamentally different from the µCP technique presented here.µCP has emerged as a high resolution, inherently parallel and highly scalable, and experimentally simple method for microfabrication of a diverse set of materials on various substrates. These alloys are of low toxicity [9] and have very low vapor pressure, [8] making them ideal for wearable computing and health care applications.…”

“…[35][36][37][38] Although all of the aforementioned techniques have been demonstrated for fabricating various liquidmetal-based soft and stretchable electronics, in their current state, none of them fully provide the desired combination of precision, reproducibility, dimensional/geometric capability, and especially, scalability required to realize commercially applicable soft and stretchable electronics based on gallium-based liquid metals. It is noted that although some liquid metal fabrication approaches, e.g., [48][49][50][51] have been referred to as microcontact printing, those processes do not follow the stamp-based µCP approach that is used in this paper, and therefore are fundamentally different from the µCP technique presented here. Another promising approach is microcontact printing (µCP), which has not been explored for fabrication of liquid metal circuits previously.…”

Analysis and Transformations of Room‐Temperature Liquid Metal Interfaces – A Closer Look through Interfacial Tension

“…Since the discovery of liquid metals (LM), metals in their liquid state at or near room temperature, have attracted increasing attention in the scientific community. Applications based on these liquid metals are ranging from soft electronics, environmentally responsive electronics, soft sensors,, pumps and actuators, (self)‐propulsion of LMs, energy harvesters,, heat transfer systems,, bioimaging, catalysisand media for running chemical reactions, e.g., for generation of atomically thin metal oxides dual‐trans printing, microchannels,, masked deposition, lithography,, dielectrophoresis, and handwriting .…”