MEMS-based humidity sensors with integrated temperature sensors for signal drift compensation

Abstract

Humidity sensing is essential in such fields as environmental control, process monitoring and biomedical analysis. A microfabricated humidity sensor is developed which uses a nitride/silicon microstructure suspended at a small distance above the surface of a glass substrate as the movable electrode of a capacitor and thin-film platinum resistors as temperature sensing elements. The suspending structure is coated with polyimide - a vapor-absorbent polymer film. Moisture-dependent bending of the micro-cantilever is caused by the variation of humidity. The measured capacitance between the micro suspending structure and the substrate is changed. The humidity sensor is integrated with a platinum resistor as a micro temperature detector for compensation of the capacitance signal drift caused by temperature. A low hysteresis value is indicated at high relative humidity. Because of the large electrode area on the microcantilever tip and the low stiffness of the microcantilever, a high degree of sensitivity is also attained. The relative humidity and calibrated capacitance/resistance is documented and both of the simulated and experimental samples indicate a high stability and a high degree of linearity. The average time constant of the proposed microcantilever-based humidity sensors is 1.10 sec in the relative humidity range of 20 %R.H. to 40 %R.H.