Wearable Magnetic Field Sensors for Flexible Electronics

Abstract

Highly flexible bismuth Hall sensors on polymeric foils are fabricated, and the key optimization steps that are required to boost their sensitivity to the bulk value are identified. The sensor can be bent around the wrist or positioned on the finger to realize an interactive pointing device for wearable electronics. Furthermore, this technology is of great interest for the rapidly developing market of ­eMobility, for optimization of eMotors and magnetic bearings. The flourishing and eagerness of portable consumer electronics necessitates functional elements to be lightweight, flexible, and even wearable.1-7 Flexible devices advance from interconnects and individual sensing elements toward complex platforms consisting of communication and diagnostic components that fuel the vision of creating multifaceted, interactive,8 and wearable electronics.9 Next generation flexible appliances aim to become fully autonomous and will require ultra-thin and flexible navigation modules, body tracking, and relative position monitoring systems. Such devices fulfill the needs of soft robotics,10, 11 functional medical implants12, 13 as well as epidermal,9, 14 imperceptible,15, 16 and transient17, 18 electronics. Magnetic field sensors offer the possibility to sense and respond to external magnetic fields, which are considered as a vital feature when integrated into these novel kinds of electronic devices. Magnetosensorics is a versatile tool to assess mechanical movements not only in artificial devices like robotics but also in vivo. Foreseeable applications include real time monitoring of artificial joints or valves of the heart to diagnose early stages of dysfunctions. Key building blocks of navigation and position tracking devices are Hall effect sensors. Unfortunately, conventional semiconductor-based Hall sensors are about 400-μm-thick and rigid, limiting their direct applicability in flexible electronics. Here, we introduce a technology platform that allows us to fabricate highly flexible magnetic field sensors relying on the Hall effect. We combine inorganic functional nanomembranes with polymeric foils to achieve flexible sensing elements with a near bulk sensitivity of about −2.3 V (AT)−1. The flexible sensors fabricated on 100-μm-thick polyimide foils withstand severe mechanical deformations. We observe only a minor reduction in the sensor performance when bent into a radius of 6 mm, which is fully recovered in the flat state. We chose 100-μm-thick polyimide (PI) and 25-μm-thick polyether ether ketone (PEEK) foils to demonstrate the viability of our technology platform. Hall sensor elements are formed by depositing bismuth films via magnetron sputtering onto the PI and PEEK supports. These materials reveal exceptionally good mechanical, thermal, and chemical stability and are applied in state of the art consumer electronics. Bismuth-based Hall sensors19-22 bear several distinct advantages: i) Bi has the largest Hall coefficient of −(5–6) × 10−5 V cm (AT)−123 and thus the largest sensitivity among all (half-) metals,24 ii) the use of Bi allows surface depletion effects relevant for miniaturized semiconducting micro-Hall probes to be overcome; hence, Hall sensors with a sub-μm lateral size operating at room temperatures have been fabricated using Bi thin films,19, 20 and iii) Bi is readily processed using standard microelectronic facilities. Here, we deposit bismuth films onto electrical contacts, which are prepared on flexible foils (see Experimental Section for details). The resulting sensors can be bent on demand after their fabrication (Figure 1a). Adhering the flexible sensor to a finger (Figure 1b), creates an interactive pointing device and builds a component for wearable electronics (Figure 1c). By monitoring the sensor output, we visualize the relative position of the finger with respect to a permanent magnet (compare Figure 1d and e). Details on the conditioning electronics used here can be found in Figure S1 in the Supporting