Humidity-to-Frequency Sensor in CMOS Technology With Wireless Readout

A remotely powered microsystem for humidity measurement is presented. The core of the active transponder consists of a humidity-to-frequency chip fabricated in a standard complementary metal-oxide-semiconductor (CMOS) technology, which contains both the humidity sensor and the readout circuit. The sensor is constructed using the polyimide passivation layer and the top metal layer available in the technology used. Power and data transmissions, to and from the chip, respectively, use the same pair of inductively coupled coils with a 13.56 MHz carrier. The humidity readout is transmitted by load shift keying to the external circuit. The chip is fabricated with commercial 0.6-μm CMOS technology and occupies an area of 4.8 mm². The sensor´s capacitance exhibited good linearity against relative humidity (RH) levels from 15% to 85%. The normalized sensitivity is 0.073% per %RH at 35°C. The circuit level calibration limited spread from process and mismatch variations to about 10%. The chip has a total power consumption of 1.39 mW. The device has two purposes; either as a stand-alone wireless humidity sensor or to evaluate the hermeticity of packages, such as in biomedical implants.