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

This paper presents a novel technique for the fabrication of a micro humidity sensor with suspending structures. A MEMS device is developed which uses thin-film platinum resistors as temperature sensing elements and a nitride/silicon microstructure suspended at a small distance above the surface of a glass substrate as the movable electrode of a capacitor. A mechanism is proposed for the measurement of the capacitance between the suspended wafer structure and the glass substrate for different values of relative humidity. The fundamental component of the micromachine-based humidity sensor is a nitride/silicon cantilever coated with a vapor-absorbent polymer film (polyimide). A variation in humidity causes moisture-dependent bending of the microcantilever, which changes the measured capacitance between the microcantilever and the substrate. The current experimental data show that the low stiffness of the microcantilever and the large electrode area on the microcantilever tip yield a high degree of sensitivity, i.e. 2.0 nF/% RH. To compensate for the temperature drift of the capacitance signals, the proposed humidity sensor is integrated with a micro resistance-type temperature detector comprised of platinum resistors. The experimental data indicate a low hysteresis value at high relative humidity (>65% RH). The relationship between the measured resistance/capacitance and relative humidity is fully explored and documented. The numerical and experimental samples all indicate a high degree of linearity (R-2 = 0.9989), a high stability (+/-0.76%) and a reasonable response time (1.10 s).