An implantable humidity-to-frequency sensor in CMOS technology

This paper reports the development of a thin-film humidity sensor which is integrated as part of a standard CMOS chip. The sensor element is fabricated using the topmost polyimide layer and the top metal layer available in standard CMOS processes. The sensor has been made part of an integrated wireless device which converts its capacitance to an output frequency. Power transmission to, and data transmission from the device, use the same pair of inductively coupled coils. The humidity readout is transmitted by load shift keying. The chip was fabricated in a 0.6-μm CMOS technology and occupies an area of 4.8 mm². The sensor´s capacitance exhibits good linearity against relative humidity (RH) levels from 15-85%. The normalized sensitivity is 0.073% per %RH at 37°C. Power consumption is 1.19 mW. The device has two purposes: it can be a stand-alone wireless humidity sensor (the first wireless humidity sensor in standard CMOS technology reported to date) and it can be employed for evaluating the hermeticity of implantable biomedical micropackages.