Scalable Manufacturing of Soft and Stretchable Liquid-Metal-Based Circuits with Integrated Packaged Microelectronics

Abstract

Soft and stretchable electronics (SSEs) have recently drawn considerable attention as an alternative for conventional rigid electronic devices when physical interaction with biological tissue and delicate objects is needed, such as for wearable computing, implantable electronics, and soft robotics. Unlike their rigid counterparts, SSEs can allow large amounts of bending, stretching, and other modes of deformation (e.g., 40% or more strains) without losing functionality. Furthermore, SSEs could be designed to have a similar elasticity as that of natural biological tissues, and can be integrated into clothing or mounted on the skin without constraining natural body motion. An emerging approach for realizing SSEs is to create micro-scale traces of a liquid metal (LM), a metal that is liquid at the room temperature, embedded in a soft elastomeric matrix. The prevailing liquid metals used in these devices include the room temperature binary liquid metal alloy of gallium and indium (i.e., eutectic Gallium-Indium, EGaIn) and ternary liquid metal alloy of Gallium-Indium-Tin (Galinstan). These alloys provide high conductivity, non-toxicity, and processability (moldability and printability) at the micron-scale. Since LM alloys can flow inside channels, LM-based circuits can preserve their elastic properties and electrical conductivity even at large deformations (up to 800%). Hence, LM-based SSEs offer a unique combination of metallic electrical conductivity and extreme stretchability. Although this exciting and impactful promise of LM-based soft electronics has been well recognized, their commercialization has not yet been realized due to the lack of viable techniques for their high-throughput manufacturing. Commercially viable mass manufacturing of LM-based soft electronics require: (1) scalable, reproducible, and precise LM patterning techniques to create stretchable interconnects, analog sensors, passive circuit elements and antennas with consistent electrical performance, and (2) effective interfacing of packaged microelectronics (i.e., IC chips) with liquid gallium alloys to enable a wide variety of on-board sensing modalities and digital processing. To address these challenges, the overarching objective of this Ph.D. research is to develop, evaluate and characterize techniques for scalable, precise and reproducible manufacturing of LM-based soft and stretchable electronics with integrated microelectronic components.To address the overarching research objective, first, the electrical connection and the electromechanical coupling between liquid metal interconnects and the packaged microelectronic component pins were experimentally investigated. To create an effective electrical and mechanical connection between EGaIn and packaged microelectronics, a novel hydrochloric acid (HCl) vapor treatment was developed. This technique was combined with a novel selective wetting-based LM patterning approach for rapid prototyping of LM-based SSEs. The applicability of this approach in creating LM circuits was demonstrated through fabrication and evaluation of demo circuits. Reproducible and controllable LM deposition is crucial to obtain the desired performance. To this end, an experimental investigation was conducted on the LM dip-coating process to examine the influence of process and wetting layer parameters on the resulting LM pattern geometries and their reproducibility for dip-coating. A design of experiments approach was utilized for this analysis and a semi-empirical model between investigated parameters and output geometry was developed. The experimental data were used to calibrate and validate the proposed model. The thesis concludes with a discussion of future directions in LM circuit manufacturing.