The chemical sensing system plays an important role in medical and environmental monitoring. Gases exhaled by humans include nitrogen, oxygen, water vapor, carbon dioxide and volatile organic compounds (VOCs). The VOCs are important and provide valuable information for non- invasive diagnosis. For instance, ethanol detection is beneficial for checking blood alcohol. In time blood alcohol level checking before checking can prevent a person from unsafe driving.
Due to the extremely low concentration of the target gases, a gas sensor with high sensitivity, selectivity and low detection limit is required. There is a high demand for low cost, fast, accurate and easy-to-use self-check diagnosis devices. With low cost and high portability, micro-electromechanical systems (MEMS) sensors have been extensively studied for chemical sensing, which provide a cheap self-diagnosis solution.
Capacitive Micromachined Ultrasonic Transducers (CMUTs) and Piezoelectric Micromachined Ultrasonic Transducer (PMUTs), which both work based on the mass-loading effect, are considered as the promising types of MEMS sensors for gas sensing. Since they are fabricated in a batch manner with the similar process of silicon-based integrated circuits, CMUTs and PMUTs are able to provide massive parallelism, easy integration with microelectronic circuits, and a higher quality factor.
In this research, studied the feasibility of using PMUTs and CMUTs fabricated by our lab for ethanol detection through simulation and experiments. Models for are built via COMSOL for PMUT and CMUT respectively. The simulation results of a single sensing element demonstrated that both CMUTs and PMUTs show great potential for gas sensors. The chemical experiments through frequency response measurement exhibit that both the PMUTs and CMUTs are effective for ethanol detection based on the mass-loading effect. When the gas analyte is attached to the sensing layer, a higher resonance frequency of the transducer induces a higher frequency shift, which means the higher resonance frequency of transducer, the higher sensitivity of a gas sensor is and the lower concentration of ethanol can be detected. Additionally, a CMUT array is also applied to ethanol detection. It provides a good preliminary study of the CMUTs functionalized with more sensing materials for chemical detection in future
