A Low-Power Robust Humidity Sensor in a Standard CMOS Process

This paper presents a low-cost thermal-conductivity-based humidity sensor implemented using a 0.6-mum CMOS process, where suspended p-n junction diodes are used as the humidity-sensitive elements. The measurement method uses the difference between the thermal conductivities of air and water vapor at high temperatures by comparing the output voltages of two heated and thermally isolated diodes; one of which is exposed to the environment and has a humidity-dependent thermal conductance, while the other is sealed and has a fixed thermal conductance. Thermal isolation is obtained by a simple front-end bulk silicon etching process in a TMAH solution, while the diodes are protected by the electrochemical etch-stop technique. The suspended diodes are connected to an on-chip circuit using polysilicon interconnect layers in order to increase their thermal resistance to be able to heat them with less power. Due to the high electrical resistance and positive temperature coefficient of resistance of the polysilicon, temperature sensitivities of the diodes are reduced to -1.3 mV/K at a 100-muA bias level. The diodes and the readout circuit are monolithically integrated using a standard 0.6-mum CMOS process. Characterization results show that humidity sensitivity of the sensor is 14.3, 26, and 46.9 mV/%RH for 20degC, 30degC, and 40degC, respectively, with a nonlinearity less than 0.3%. Hysteresis of the sensor is less than 1%. The chip measures 1.65 mm times 1.90 mm, operates from a 5-V supply, and dissipates only 1.38-mW power.