Design and evaluation of an antiparallel coupled resonator for chemical sensor applications

In this paper, a new type of quartz crystal resonator in which the electrodes are located on one side has been developed for chemical sensing. The resonator has two electrodes for exciting thickness shear mode (TSM) vibrations on one side of the crystal and a conductive layer on the other side. These electrodes are capacitively coupled with the electric fields in opposite directions, forming an antiparallel coupled resonator (ACR). The resonant characteristics of the ACR were evaluated as a function of gap width between the two electrodes used to excite the TSM. The conductance value was observed to increase with decreasing gap width. We also discovered that the gap should be parallel with the crystallographic x-axis to obtain the highest sensitivity. The frequency response to a viscous loading was almost same as that of a standard quartz crystal microbalance (QCM). The ACR sensor is an attractive alternative to a QCM chemical sensor because it can be easily integrated into packaging and film coatings.