Sub-µm air-gap resonant MEMS mass sensors fabrication and electrical characterization for the detection of airborne particles

As excessive exposure to airborne particles results in adverse health effects, the development of a real-time aerosol monitoring system is highly relevant. Widely used systems are bulky, time consuming, labor intensive or expensive to maintain. Based on the airborne particle detection capacities of micro sensors, typical acoustic wave mass sensors such as the Surface Acoustic Wave (SAW), the Film Bulk Acoustic Resonator (FBAR), and the Quartz Crystal Microbalance (QCM) sensors show lack of mass sensitivity and uniformity. However, resonant MEMS sensors show good sensitivity and could be relevant alternatives to typical acoustic wave mass sensors for low mass measurements [1]. Among the various types of bulk mode MEMS resonant transduction methods, capacitively transduced MEMS resonators show sensitive and relatively uniform mass measurements [2]. Herein, we propose the thick oxide mask layer fabrication technique to realize high performance mass sensors. This fabrication technique was previously proposed for reducing resonator-to-electrode gaps to sub-μm [3]. In this study, we used the thick oxide mask layer technique to fabricate high aspect ratio sub-μm air-gap MEMS mass sensors. We obtained the mean gap size as ~868 nm for 40 μm thick resonators. Preliminary electrical characterization results showed that the resonance frequency of the resonators is 4.409 MHz, and the quality factor is estimated as 15303.