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

In this paper we report a new approach for an integrated humidity sens or fabricated using an industrial CMOS process with a measurement tech nique for evaluating on-chip floating capacitances. The sensor is base d on a capacitive element covered with polyimide as humidity sensitive material. The circuit produces an output current which is precisely r elated to the absolute value of the capacitance and is insensitive to parasitic capacitances to ground, offsets and charge injection from sw itches, 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 C MOS humidity sensors (Sensors and Actuators A, 25-27 (1991) 509-512). The hysteresis is reduced to less than 1% r.h. and the temperature coe fficient 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 measur ement technique can be used for evaluating on-chip capacitive microsen sors, whenever exact knowledge of the device capacitances is crucial.