Entirely Flexible On‐Site Conditioned Magnetic Sensorics

Abstract

The first entirely flexible integrated magnetic field sensor system is realized consisting of a flexible giant magnetoresistive bridge on-site conditioned using high-performance IGZO-based readout electronics. The system outperforms commercial fully integrated rigid magnetic sensors by at least one order of magnitude, whereas all components stay fully functional when bend to a radius of 5 mm. Flexible electronics1-4 naturally conform to static or dynamically reconfigurable complex 3D shaped surfaces5, 6 offering intimate yet durable contact with biological as well as synthetic tissues. The inherent feature of this new concept of electronics of being soft and compliant enables a plethora of new applications. Especially the fields of medical7 and consumer electronics8 benefit from the variety of already available flexible devices including electrode arrays,9, 10 solar cells,11 diagnostic devices,12-15 displays,16 memory elements,17, 18 and various types of sensors.19-24 At the moment, postprocessing, e.g., signal amplification, or multiplexing of the data acquired by entirely flexible and even imperceptible devices,5, 25-28 such as pressure,11, 29, 30 temperature,26, 30 encephalography,7, 27 or magnetic field22, 24, 25 sensors, is done using external rigid electronics. The connection between the flexible and the rigid parts of the measurement system is established using cables. This measurement scheme—although commonly applied to condition flexible sensors—limits the unobtrusiveness, narrows the system bandwidth, and possesses strong disadvantages in terms of signal-to-noise ratio (SNR). In particular, the poor noise performance is due to the simultaneous amplification of the signals and the noise, which is either picked up upon the signal transmission using long cables, or produced by power supplies and electronic circuits itself. To enhance the responsiveness and sensitivity of an acquisition system, the output of a sensory system should be amplified directly at the sensor location. This so called frontend sensor signal conditioning is a standard approach in conventional rigid microelectronics but is not yet established for flexible electronics.26, 31 Here, we demonstrate a fully integrated yet entirely flexible magnetosensory system, which can be fabricated over large areas (Figure 1a–d). The complete system (Figure 1e–f) is integrated on a single 50 μm thick polyimide foil and consists of a differential giant magnetoresistive (GMR) sensing element arranged in a Wheatstone bridge configuration (Figure 1g), an operational amplifier with differential high impedance input and single ended output, based on 16 indium–gallium–zinc–oxide (IGZO) thin-film transistors (TFTs)2 (Figure 1e and Figure 2a, inset), and a high current output amplifier TFT operated as class A power amplifier with an open drain output to provide maximum adaptability to different loads (Figures 1e and 2b, inset). The differential and power amplifier readout circuitry remains fully operational and reveals remarkable amplification of 48.6 dB also at high frequencies with a unity gain frequency of about 200 kHz resulting in a gain-bandwidth product of 54 MHz even while bent to a radius of 5 mm. The readout circuitry is designed to operate in the digital or analog regime and is able to convert a low voltage differential signal (LVDS) such as switching square shaped signals into larger voltage single-ended responses for data transmission or control operations. Furthermore, the analog differential signaling promotes an efficient rejection of common mode noises (achieved by an internal common mode rejection feedback) leading to an extremely low noise floor of −124 dBm Hz−1. The presented work demonstrates for the first time that IGZO semiconductor devices enable the realization of entirely flexible low-noise electronics, suitable for sensor readout circuits. To achieve a high as possible SNR, the flexible GMR elements were arranged in a Wheatstone bridge configuration