Fabrication of ZnO nanorods and Chitosan@ZnO nanorods on MEMS piezoresistive self-actuating silicon microcantilever for humidity sensing

This paper reports the fabrication method and humidity sensing performances of zinc oxide nanorods (ZONRs) and chitosan self-assembled monolayers (SAMs) modified ZONRs patterned piezoresistive silicon MEMS microcantilevers. The self-actuating and self-sensing microcantilevers were fabricated using standard silicon wafers and bulk-micromachining techniques; a p-diffused heating resistor was patterned at the clamped end of the microcantilever as the self-actuator. Moreover, the resonant frequency shift induced by the mass change of adsorbed water on the hybrid sensing nanostructure can be read-out directly by a p-diffused full Wheatstone bridge. Aligned high-surface-to-volume-ratio ZONRs were selectively deposited on the microcantilever as the sensing material using a simple low-temperature chemical-bath deposition (CBD) process based on a DC sputtering/annealing ZnO seed-layer. In addition, for comparison, a low-cost seed-layer method using aqueous chemical growth (ACG) method was also applied to deposit ZONRs. Furthermore, chitosan SAMs were self-assembled on the ZONRs, yielding a microcantilever-based gravimetric relative humidity (RH) sensor with good stability and repeatability (± 0.06%RH over 90 min), high sensitivity (ZONRs: 4.4 ppm/%RH; chitosan-SAMs@ZONRs: 16.9 ppm/%RH), and low hysteresis error (2.1%RH).