A surface conductance based fully integrated standard CMOS humidity sensor without post-processing

This work demonstrates a novel surface conductance based humidity sensor that can be fully integrated in standard CMOS process without any post-processing. The proposed humidity sensing mechanism is based on the surface conductance of SiO₂ between the two metals which is exponentially related to relative humidity. As SiO₂ can be provided by any standard CMOS process, this is distinct from the previous works that use post-processing to improve the functionality. To avoid the exponential output with relative humidity, a log-converter composed of an op-amp with a diode in feedback path is proposed. To reduce very high surface resistance of 10¹⁴ Ω to 10¹⁸Ω, the surface resistor was implemented as a comb-structure and made it large enough so that the surface current is bigger than the reverse leakage of the parasitic diode. The proposed sensor was fabricated in 0.25um standard CMOS process. A total of 8 chips were tested from 30%RH to 100%RH in a regulated temperature of 23°C, and the results show ±2%RH chip-to-chip error after 2-point calibration.