Surface acoustic wave gas sensor with nanostructured silicon sensing element

Abstract

Surface acoustic wave (SAW) sensors have significant potential to monitor toxic gases, owing to their advantageous features such as fast response time, high stability, and remarkable sensitivity. Over the past few years, research has been focused on optimizing the design and fabrication of SAW sensors by adopting various materials as a sensing layer for enhanced sensitivity. Among the high gas sensitive materials available, zinc oxide (ZnO), palladium (Pd), tungsten oxide (WO3), Gallium nitride (GaN), and indium oxide (In₂O₃) have been widely adopted. However, the high gas recovery time with regard to its response time and poor detection stability make these materials adverse for gas sensing applications. This work presents a novel microelectromechanical system (MEMS) SAW gas sensor that employs an amorphous Silicon nanostructure (Si-nanostructure) as the sensing material. Due to the high surface-to-volume ratio of Si-nanostructure, it provides high sensitivity by adsorbing more gas molecules. The fabrication of the Si-nanostructure was carried out by a low-cost metal-assisted chemical (MACE) process. The formation and structural properties of these Si-nanostructure were characterized by Field-emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray (EDX) analysis. The fabricated SAW sensor with Si-nanostructure sensing material was then tested with Carbon dioxide (CO2) gas during the characterization process. The gas-sensing performance of the sensor was assessed in terms of its sensitivity performances, and then compared with previously reported SAW gas sensors. The developed Si nanostructure-based SAW sensor exhibits a frequency shift of 4.622 kHz at 2000 ppm, and a response and recovery time of 9 s and 9.5 s, respectively, when exposed to 700 ppm of CO2 gas. It demonstrates higher sensitivity and better response than previously reported devices in the literature, and has promising potential to be employed as a primary sensing layer for next-generation MEMS SAW gas sensors.