QCM Coupled Resonating Systems Under Vacuum: Sensitivity and Characteristics

We present the underlying physics in the resonance frequency characteristics of a bare quartz crystal microbalance (QCM) and a QCM-micro pillar based coupled resonator sensor (QCM-CRS) when they were subjected to pressure changes in a vacuum chamber. The QCMs experienced an increase in resonance frequency with decrease in pressure due to an inverse mass loading effect introduced on the QCM surface. However, when the pressure (below atmospheric pressure) dominated the mass loading, the resonance frequency of QCMs decreased. A sensitivity increase of 11 times to the pressure changes was observed for the case of QCM-CRS made of resonating micropillars attached around the central region of the electrode. In addition, the resonance frequency shift was linear for pressure changes between 31 and 71 kPa, demonstrating the possibility of employing the QCM-CRS as a vacuum detector element. The proposed sensor element is envisioned to have wide industrial applications ranging from detecting vacuum in chambers used in coating systems to crack detection in closed chambers.