T. Nandy, R. Coutu
Marquette University, WI, USA,
Keywords: microelectromechanical systems (MEMS), photoacoustic (PA), piezoelectric (PZT), infrared (IR), light emitting diode (LED)
Summary:The gases, which are not part of combined nitrogen, oxygen and argon (total 99.934% of air), are called trace gases. There are lots of hazardous trace gases such as carbon monoxide, carbon dioxide, ammonia, nitrogen dioxide etc. Small amount of them are very harmful to human beings. So, their detection in both household and residential environment is very necessary . Recent microelectromechanical (MEMS) gas sensors are mainly on metal oxide semiconductors. But these electrochemical sensors have some severe issues regarding materials and operating conditions. These sensors sense gases via oxidation reaction which leads to the degradation of metal oxide thin film. Hence, life time and sensitivity both decreases. These sensors also need higher operating temperature . Recent developed optical methods can surpass these problems; but they are big in size and expensive. Moreover, multiple gas sensing with single device is always challenging [3-5]. So, our goal is to develop a miniaturized optical multi-gas sensing system using cost-effective MEMS batch fabrication process. In this research, we will introduce new micromachined piezoelectric (PZT)/silicon-on-insulator (SOI) membrane-based photoacoustic (PA) detection system for multiple hazardous gas sensing application with high accuracy and low cost. We will fabricate thin and deformable micro-machined SOI membrane and deposit PZT layer on it. When piezoelectric material is strained via applied mechanical pressure or force, it produces electric charges through property changes . Therefore, we will use this PZT/SOI membrane to detect the acoustic vibration created via light-gas interaction inside the PA chamber. Gases will absorb the incident light and it will create pressure following photoacoustic phenomena which will deflect the PZT/SOI membrane. This deflection will be measured through voltage measurement. Here, Multi-wavelength infrared (IR) light emitting diode (LED) will be used for detecting multiple gases with single test fixture. As particular gas will absorb only in its absorption line, the system will be very selective towards target gas. In this work, we have analyzed photoacoustic theory and pressure change inside gas chamber. We have determined the pressure changes using different modulated frequencies for different trace gases. Based on these, we have designed our photoacoustic chamber. Our whole experimental set-up is a miniaturized photoacoustic chamber based on light source and detector according to the absorption spectra of target gases (shown in Figure-1). Primarily, 3D printed chambers have been made for prototype system. The fabrication process has been developed for making MEMS PZT/SOI membrane (Figure-2). Both circular and square shaped membranes have very small thickness (~5-10 μm) for smooth vibration. We have analyzed the static motion where stress, strain and displacement simulation have been performed. This modeling and simulation on displacement has given an approximation on the sensitivity towards target gases. Next, the PZT/SOI membrane will be fabricated for preparing the whole system. We will deposit a metal layer on PZT film which will be used for electric potential measurement. Besides, we will also detect the deflection through white light interferometer. These results will give sensitivity as well as total concentration of gas.