HIGH-SENSITIVITY CMOS HUMIDITY SENSORS WITH ON-CHIP ABSOLUTE CAPACITANCE MEASUREMENT SYSTEM

In this paper we report a new approach for an integrated humidity sensor fabricated using an industrial CMOS process with a measurement technique for evaluating on-chip floating capacitances. The sensor is based on a capacitive element covered with polyimide as humidity sensitive material. The circuit produces an output current which is precisely related to the absolute value of the capacitance and is insensitive to parasitic capacitances to ground, offsets and charge injection from switches, which enhances the sensitivity of the device. The response to relative humidity (r.h.) is highly linear and a maximum sensitivity of 0.9% (%r.h.)-1 is achieved, i.e. six times better than our previous CMOS humidity sensors (Sensors and Actuators A, 25-27 (1991) 509-512). The hysteresis is reduced to less than 1% r.h. and the temperature coefficient to less than 0.1% r.h. degrees-C-1. Cross-sensitivity to flow of humid air is below 2% r.h. for flow rates up to 15 m/s. Our measurement technique can be used for evaluating on-chip capacitive microsensors, whenever exact knowledge of the device capacitances is crucial.