The quartz crystal resonator as detector of electrical loading: An analysis of sensing mechanisms

Quartz crystal resonators (QCR) are being investigated for the detection of electrical properties of an adjacent medium. Recent studies have suggested that the sensitivity can be improved by utilizing modified electrode configuration. In this study, an analysis of the electrical loading sensing mechanism and of the mass sensitiviy profile are performed for examples of modified-electrode QCRs. The total electric field which interacts with the load on the sensing surface is a superposition of the electric fringing field due to the asymmetric configuration of the electrodes and the field associated with the acoustic vibration along the unelectroded surface. However, the latter can be neglected for a quartz substrate. Changes in the QCR antiresonent frequency, a measure of the electrical loading, are shown to be very similar to the changes in the electrostatic capacitance. This makes the QCR a capacitance sensor. However, the use of a piezoelectric crystal resonator allows for direct measurements of this effect. It is also shown that as the conductivity of the loading increases the effective surface area of the electrode increases, hence the mass sensitivity profile.