Polymer-based Flexible Wide Range Pressure Sensors with Hierarchical Micro Structure

Abstract

This paper reports the flexible piezoresistive pressure sensor equipped with multi-height microstructures, and their effects on sensor properties. Specifically, we have fabricated two different types of pressure sensors: 1) microdome structures via XeF2 isotropic etching and 2) micropyramid structures realized by using a KOH etched Si mold. Using the isotropic dry etch, we were able to realize the microdomes with a height distribution from 10μm to 20μm. This is because the etch rate differs from middle of the chip to the edge. For micropyramids, we simply implemented KOH etch to fabricate a set of structures with heights from 18μm to 60μm. In KOH etching, the height of micropyramids can be precisely controlled at lithography level. We have fabricated the polydimethylsiloxane (PDMS) substrate using silicon mold by adopting both silicon wet etching and silicon dry etching process, and implemented multi-wall carbon nanotube (MWCNT) as a conducting layer. By fabricating a selectively etched micropyramid flexible pressure sensor, performance evaluation of the sensor as a function of the number of multi-steps is performed. Our results show that the measurement sensitivity increases with the number of different heights of the microstructures. To improve the electrode stability, electroplating is performed to form a thin layer of Au on CNT electrodes. Such coating has dramatically improved the contact stability as well as the measurement sensitivity. The interdigital electrode design (IDE) is implemented for simpler electrical readout. For application, we have placed the sensor on an insole of shoe and tacked the walking motion of human. In addition, we have integrated our pressure sensor on a robotic gripper and measured the applied pressure upon grasping an object. The introduced multi-height microstructures and a detailed study on their effects on sensor performances can enable a next generation flexible sensor platform.