Sensors for Wireless Body Monitoring Applications

Abstract

Body monitoring systems have recently drawn great attention to modern electronic consumers due to their various health−care and security applications. However, most of the existing monitoring systems need wire connections that prevent free body movements. Complementary metal−oxide−semiconductor (CMOS) technology based wireless sensor systems need integration of different components that make the device volume and production cost high. In adition, their dependency on on−sensor power source limits the continuous monitoring capability. In the thesis, to demonstrate the feasibility of low cost and simple body monitoring systems, we propose a near−infrared (NIR) photodetector (PD) and a humidity sensor (HS) using low−temperature thin−film processes suitable for large−area electronics application.

For NIR detection, a novel lateral metal−semiconductor−metal (MSM) PD architecture is proposed using low−temperature nanocrystalline silicon (nc−Si) as a NIR absorption layer and organic polyimide (PI) as a blocking layer. Experimental results show that addition of PI layer reduces the dark current (ID) up to 103−105 times compared with the PDs without PI layer. Fabricated devices exhibit a low ID of ~10−10 A, a response time of <1.5 ms, and an external quantum efficiency (EQE) of 35−15% for the 740−850 nm wavelengths of light under 100−150 V biasing conditions. Unlike the standard p−i−n PD, our high−performance lateral PD does not require doped p+ and n+ layers. Thus, the reported device is compatible with industry standard amorphous silicon (a−Si) thin−film transistor (TFT) fabrication process, which makes it promising for large−area full hand biometric imagers suitable for various non−invasive body monitoring applications.

For humidity detection, a 30 mm diameter passive LC (p−LC) HS is formed by joining an octagonal planer inductor and a moisture sensitive interdigital zinc oxide (ZnO) capacitor in series. A PCB reader coil is also designed, which is able to sense the HS from <25 mm distance. The HS reads 30−90% of relative humidity (RH) by interrogating change of the resonance frequency (fR) of the reader−sensor system. The reading resolution is ±2.38%RH and the sensitivity is 53.33−93.33 kHz/1%RH for the above 45% RH measurements. Experimental results show that the proposed HS is operational in a range of 0−75 oC as long as recalibration is performed for a temperature drift of above ±3 oC, which makes it suitable for various promising applications operated at different temperatures. Above all, the presented results are promising for the continuous body monitoring applications to observe the humidity wirelessly without any power source on the sensor.