University of Massachusetts Lowell,
Keywords: quartz crystal microbalance, acoustic wave sensor, micropillars, Q-factor
Summary:Acoustic wave based sensors are piezoelectric effect-based sensing devices that has attracted a wide range of applications such as semiconductor fabrication, biological diagnostics and polymer characterization. Traditional acoustic wave-based sensing devices such as quartz crystal microbalance (QCM) rely on the thin films coated on the piezoelectric substrates as the sensing films for chemical and biological detection. Previous study demonstrates that significant sensitivity enhancement over traditional film based QCM (QCM-F) devices can be achieved by simply attaching a poly(methylmethacrylate) (PMMA) micropillar film onto a QCM substrate (QCM-P). However, one of the biggest challenges is to understand the resonance frequency shift and Q-factor of the QCM-P sensors. Particularly, Q-factor is a widely used parameter in acoustic wave based sensing system for quantifying the energy loss or damping effect. This research focuses on experimental and theoretical study of frequency shift and Q-factor of the new QCM-P devices for sensing applications. Our previous research shows Polymethyl methacrylate (PMMA) micropillars can be fabricated onto the surface of QCM by thermal nanoimprinting lithography (T-NIL) method. Due to the resonance phenomenon between micropillars and QCM substrate, the sensitivity improvement of QCM over bare QCM devices can be as high as eight-fold. The sensitivity or detection limit improvement of QCM-P sensors are primarily restricted by the high damping in the liquid environment or the low Q-factor which cannot be read by conventional frequency measurement systems. To understand the major contributors of the energy dissipation of QCM-P devices, an equivalent circuit concept combining mechanical vibration of beams and electrical impedance based models was developed. In the model, the vibration of beams was solved simultaneously with the liquid loading on the beam surface and the resultant force was integrated into the transmission line model to predict the load impedance of the electrical model. The predicted values for the resonance frequency and Q-factor of QCM-P devices compared with experimental results of actual QCM-P systems with different Micro-Pillar heights. It was confirmed that this equivalent-circuit model is a valuable tool in predicting the frequency response of QCM-P devices for different sensing applications.