Abstract
Abstract. Carbon dioxide (CO2) is a gas that well represents air quality in indoor environments as well as being an important greenhouse gas. However, the reliable and affordable sensing of environmental CO2 at room temperature, with techniques other than optical spectroscopy, remains an unsolved problem to this day. One major challenge for solid state sensors is the realisation of adequate selectivity, especially towards changing humidity. The thin film bulk acoustic resonator (FBAR) is a MEMS (Microelectromechanical systems) device that can not only detect gas-induced mass changes but also changes in the acoustic velocity and density of its layers. This multi-sensing provides a suitable platform for selective gas sensing. In this work we present studies done on polyaminosiloxane- and ethyl cellulose-functionalised FBARs regarding CO2 sensitivity, selectivity towards humidity, and stability. We demonstrate how CO2 and humidity signals can be separated and that CO2 can be sensed with a resolution of 50 ppm between 400 and 1000 ppm. Using the Mason model, we show how the acoustic velocity and density of an absorption layer can be determined and how changes in those parameters affect the resonance frequency shift. The understanding of these results ultimately presents a tool to theoretically separate any number of gas analytes.
Highlights
The sensing of carbon dioxide (CO2) content in air is of interest for many different reasons
Despite the deviations, we got a rough estimation about the density and acoustic velocity of a thin layer of ethyl cellulose and could prove that both parameters increase with increasing humidity
Using ethyl cellulose as humidity sensitive material, we could confirm that these theoretical results are reasonable and can be used for real systems
Summary
The sensing of carbon dioxide (CO2) content in air is of interest for many different reasons. Contrary to NDIR and electrochemical sensors, it can be fabricated in very small dimensions and integrated in standardised CMOS (Complementary metal-oxidesemiconductor) micro-fabrication processes It is a robust device capable of being mass produced, which reduces fabrication costs. Amino groups bound in the form of amino-polysiloxanes have the ability to be deposited as thin solid layers All of those properties are the reason why they were considered as functional materials for CO2 detection in the following research. The approach that we introduce is interesting as it does not require any additional sensor set-up, such as preselection membranes (Kaneyasu et al, 2000) or auxiliary electrodes (Sadaoka, 2007), and theoretically can separate any number of gases with the right amount of FBARs. The approach is useful to determine unknown acoustic parameters of thin layers based on their gas sensitivity.
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