Photolithography-Based Patterning of Liquid Metal Interconnects for Monolithically Integrated Stretchable Circuits

Abstract

We demonstrate a new patterning technique for gallium-based liquid metals on flat substrates, which can provide both high pattern resolution (∼20 μm) and alignment precision as required for highly integrated circuits. In a very similar manner as in the patterning of solid metal films by photolithography and lift-off processes, the liquid metal layer painted over the whole substrate area can be selectively removed by dissolving the underlying photoresist layer, leaving behind robust liquid patterns as defined by the photolithography. This quick and simple method makes it possible to integrate fine-scale interconnects with preformed devices precisely, which is indispensable for realizing monolithically integrated stretchable circuits. As a way for constructing stretchable integrated circuits, we propose a hybrid configuration composed of rigid device regions and liquid interconnects, which is constructed on a rigid substrate first but highly stretchable after being transferred onto an elastomeric substrate. This new method can be useful in various applications requiring both high-resolution and precisely aligned patterning of gallium-based liquid metals.

Untethered soft robotics

“…Integrating soft actuation, sensing and circuitry with microelectronics and other miniaturized rigid hardware introduces unique challenges in fabrication and interfacing. Some groups have worked to improve integration by unifying the fabrication of actuators and sensors 128 , while others have sought to introduce transitions in material stiffness that reduce the stress concentrations caused by impedance mismatches between stiff and soft components [129][130][131] . Further progress can also be accomplished through advancements in functionally graded materials and digital multimaterial 3D printing 42 , as well as improvements in soft-matter functionality that decrease reliance on rigid hardware.…”

Nanofabrication for all-soft and high-density electronic devices based on liquid metal

“…1b, c), the stamped EGaIn uniformly spreads across the Au film and fills concave nano/micropatterns up to the designed PMMA thickness. Next, the remaining EGaIn structures are covered with a soft elastomer (e.g., poly(dimethylsiloxane), PDMS), and the fabricated devices are released from the Si carrier wafer by dissolving the sacrificial PAA layer in water for >6 h. Finally, the parylene-C barrier layer is etched using an oxygen plasma in a reactive-ion etching (RIE) system 27 , and the back side of the soft electronic device is sealed with a soft elastomer. It should be noted that optical lithography with a positive-tone photoresist can be utilized as well for the microstructure fabrication.…”

“…This work presents a nanofabrication strategy for submicronscale, all-soft electronic devices, and transducers based on EGaIn. Figure 6 shows a comparison of resolution and film thickness of published liquid metal patterning technologies, including lithography-enabled [26][27][28][29][30][31][32] , microfluidic injection 36 , and additive 37 and subtractive 40 patterning processes and highlights the resolution and film thickness range demonstrated using the nanofabrication strategy proposed in this work. Additive direct write and injection approaches enable simple, fast, and large-area EGaIn patterning.…”

“…Being a liquid-phase conductor with a brittle oxide layer on the surface, the shape of EGaIn-filled microchannels can be easily changed in response to applied mechanical forces, with a new oxide layer being formed instantaneously on the EGaIn surface after deformation, thus making it shape reconfigurable 22 . The moldable characteristics of EGaIn have resulted in the development of a broad range of patterning methods based on lithography-enabled stamping and stencil printing [26][27][28][29][30][31][32][33] , microfluidic injection [34][35][36] , as well as additive [37][38][39] and subtractive [40][41][42][43][44][45] patterning processes. However, creating fine and uniform EGaIn thin-film patterns using current EGaIn patterning technologies remains a major technical challenge because of the high surface tension of EGaIn (γ = 624 mN m −1 ) 23 .…”

“…Considering the size of a single biological cell, such as platelets with a diameter of 2-3 μm, mechanotransducers should be manufactured with submicronscale features and soft, biomimetic properties 1,48,49 . Existing fabrication technologies, including the transfer printing of compliant solid metal patterns [13][14][15] , nanoprinting 17,18 , direct printing of nanomaterials [19][20][21] , and EGaIn patterning [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] , are currently not suitable to fabricate such soft and stretchable electronic devices with submicron-scale resolution.…”