A thermal conductivity based humidity sensor in a standard CMOS process

This paper presents a low-cost, thermal conductivity based humidity sensor implemented using a 0.6 μm CMOS process where suspended p-n junction diodes are used as the humidity sensitive elements. In this method, the difference between the thermal conductivities of air and water vapor at high temperatures is used. Humidity sensing idea is to compare 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 electrochemical etch-stop technique. Electrical connections to the suspended diodes are obtained with polysilicon interconnect layers in order to increase the thermal resistance so that the diodes are heated to the desired temperature levels with less power. Due to the high electrical resistance of the polysilicon, temperature sensitivities of the diodes reduced to -1.3 mV/K at 100 μA bias level. The diodes are connected to the readout circuit monolithically using the standard CMOS fabrication. Characterization results show that the humidity sensitivity of the sensor is 14.3 mV/%RH, 26 mV/%RH, and 46.9 mV/%RH for 20°C, 30°C, and 40°C, respectively, with a nonlinearity less than 0.3%. Hysteresis of the sensor is less than 1%. The chip operates from a 5 V supply and dissipates only 1.38 mW power.