A monitoring instrument with capacitive porous silicon humidity sensors

A monitoring instrument with novel porous silicon humidity sensors is presented. The sensors feature a humidity-sensitive capacitor with a porous silicon dielectric, as well as two on-chip integrated thermoresistors and a refresh resistor. The capacitance variation upon water uptake is here modelled with an effective medium approximation. This approximation can be used to understand how the water adsorption is influenced by the pore morphology. It for instance explains why the adsorption is, in general, nonlinear. Hence, here an electronic compensation concept is proposed. The complete instrument consists of hybrid integrated humidity and temperature sensors, a microcontroller interface and calibration software. A maximum of four humidity and temperature sensors can be read simultaneously. The concept shows the feasibility of simple relaxation oscillators for both temperature and humidity measurement with resolutions of 1 °C and 1%rh or better. These results can be interpreted as a first step towards monolithic integrated porous silicon humidity sensors.