Study On A Quartz Crystal Microbalance Under Lateral Field Excitation

The QCM sensors with the standard electrode geometry using thickness field excitation have been widely used in liquid phase for chemical sensing applications, in which the detection mechanism is based primarily on mechanical loading effects such as mass, density and viscoelasticity. However, the measurement of the electrical property changes is also important in a practical application, so the traditional QCM sensor is limited. The QCM under lateral field excitation employs different electrode structure, the two electrodes for exciting thickness shear mode vibrations on one side of the crystal and a conductive layer on the other side, which allows the electric field of the bulk acoustic wave to penetrate into the liquid.The operating principle of QCM and the relationship of resonant frequency shift of quartz and superficial mass shift are introduced in this paper. The reason for choosing AT-cut quartz can be considered from three aspects:vibrational mode, coupling coefficient and frequency-temperature coefficient. On the basis of theoretical analysis, a lateral field excited high-frequency QCM is proposed, which consists of a QCM chip, a flow cell and oscillating circuit. An inverted-mesa structure is designed and fabricated in this work, which not only can improve mass-frequency sensitivity, but also maintain necessary mechanical strength. The energy trapping of the LFE device is investigated, and an electrode configuration for energy-trapping LFE device is proposed. The spring needle contacting electrode based three-layer flow cell is developed in the research with easy assembling and repeatable use. The operating principle of oscillating circuit is analyzed and a oscillating circuit is designed for driving the achieved high frequency QCM sensor. Finally, a series of experiments are performed to investigate basic electrical characteristics of QCM and the sensor is applied to liquid detection and immunosensor. For the high frequency QCM chip used in this study, the Q value is larger than 20000 when it vibrates in air. The Q value drops to be about 1000 in pure water. In addition, the QCM is tested in NaCl solution and glycerol solution with different concentration. The LFE sensor has been demonstrated to be extremely sensitive to mechanical property changes and dentect changes in liquid electrical properties. Moreover, high-frequency QCM chip is adopted in this work, which leads to broader measuring scope and higher sensitivity. The immunoreaction between the antibody and the antigen confirms the feasibility of designing QCM system as immunosensor.