Multiscale and Uniform Liquid Metal Thin‐Film Patterning Based on Soft Lithography for 3D Heterogeneous Integrated Soft Microsystems: Additive Stamping and Subtractive Reverse Stamping

Abstract

to healthcare. [5,6] Unlike conventional solid-state electronics, soft electronics can be lightweight, stretchable, and reconfigurable, with biocompatible characteristics for skin-mountable and wearable sensing electronics. [7,8] Thereby, flexible and stretchable characteristics are achieved by using either 2D or 3D compliant wave-like, solid metal patterns [9,10] or elastic conductors based on conductive nanomaterials embedded in a polymer matrix. [11,12] An alternative approach to realize all-soft microsystems is the use of intrinsically soft conductors, such as gallium-based liquid metal (eutectic gallium-indium alloy, EGaIn). EGaIn-based soft electronics benefits from its nontoxicity, mechanical stability (unlimited stretchability, but ultimately limited by the mechanical properties of the encasing material), thermal conductivity (κ = 26.6 W m −1 K −1 ), and electrical conductivity (σ = 3.4 × 10 6 S m −1 ). [13][14][15] The low melting temperature (M P < 15 °C) and negligible vapor pressure of EGaIn facilitate room-temperature and ambient pressure manufacturing processing. [13][14][15] Moreover, thanks to the formation of a thin oxide layer (t ≈ 1-3 nm) on the EGaIn surface under atmospheric oxygen level, EGaIn structures maintain their mechanical shapes, [16,17] allowing 2D/3D EGaIn patterns on a soft elastomeric substrate, such as poly(dimethylsiloxane) (PDMS).The moldable characteristic of EGaIn has enabled a broad range of patterning methods based on lithography-enabled stamping and stencil printing, injection, as well as additive and subtractive direct write/patterning processes, [18][19][20] as summarized in Table S1 in the Supporting Information. Thereby, printing using lithography-defined stencils [21][22][23][24] yields simple and high throughput EGaIn patterning on elastomeric substrates with small features of w (width) ≈200 µm/t (thickness) ≈50 µm using metal stencil films, [21] w ≈ 20 µm/t ≈ 2 µm using microfabricated metal stencil films, [22] and w ≈ 20 µm/t ≈ 10 µm using polymer stencil films. [23] Limitations of this approach are the relatively low resolution, rough EGaIn surface, and excessive EGaIn loss during the stencil lift-off process. SubtractiveThe use of intrinsically soft conductors, such as gallium-based liquid metal (eutectic gallium-indium alloy, EGaIn), has enabled bioinspired and skin-like soft electronics. Thereby, creating patterned, smooth, and uniform EGaIn thin films with high resolution and size scalability is one of the primary technical hurdles. Soft lithography using wetting/nonwetting surface modifications and 3D heterogeneous integration can address current EGaIn patterning challenges. This paper demonstrates multiscale and uniform EGaIn thin-film patterning by utilizing an additive stamping process for large-scale (mm-cm) soft electronics and a subtractive reverse stamping process for microscale (µm-mm) soft electronics. While EGaIn patterning based on stamping is regarded as the least reliable patterning technique, this paper highlights multiscale and uniform thin-fi...

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.…”

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

“…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 .…”