Spurious Resonance of the QCM Sensor: Load Analysis Based on Impedance Spectroscopy

Abstract

A research topic of equal importance to technological and application fields related to quartz crystal is the presence of unwanted responses known as spurious resonances. Spurious resonances are influenced by the surface finish of the quartz crystal, its diameter and thickness, and the mounting technique. In this paper, spurious resonances associated with fundamental resonance are studied by impedance spectroscopy to determine their evolution under load conditions. Investigation of the response of these spurious resonances provides new insights into the dissipation process at the QCM sensor surface. The significant increase of the motional resistance for spurious resonances at the transition from air to pure water is a specific situation revealed experimentally in this study. It has been shown experimentally that in the range between the air and water media, spurious resonances are much more attenuated than the fundamental resonance, thus providing support for investigating the dissipation process in detail. In this range, there are many applications in the field of chemical sensors or biosensors, such as VOC sensors, humidity sensors, or dew point sensors. The evolution of D factor with increasing medium viscosity is significantly different for spurious resonances compared to fundamental resonance, suggesting the usefulness of monitoring them in liquid media.

Keywords: AT-cut quartz crystal; QCM sensor; extended BVD electrical model; impedance spectroscopy; spurious resonances; virtual impedance analyzer.

Figure 1

(a) Extended BVD electric model of the QCM sensor, (b) half-bridge configuration for the virtual impedance analyzer.

Figure 2

Simulation of the extended BVD model of the QCM sensor in air: (a) Bode plot, (b) Nyquist plot.

Figure 3

Simulation of the extended BVD model of the QCM sensor in water: (a) Bode plot, (b) Nyquist plot.

Figure 4

Wide scanning range virtual impedance analyzer: (a) experimental setup, (b) half-bridge shield.

Figure 5

Bode plot of raw data for the QCM sensor in the air and the electrical parameters of the extended BVD model for the series resonance, respectively the first two spurious resonances.

Figure 6

Bode plot of raw data for the QCM sensor in the water and the electrical parameters of the extended BVD model for the series resonance, respectively the first two spurious resonances.

Figure 7

Bode plot of raw data for the QCM sensor in the 40% glycerin-water solution and the electrical parameters of the extended BVD model for the series resonance, respectively the first two spurious resonances.

Figure 8

Bode plot of raw data for the QCM sensor in the 80% glycerin-water solution and the electrical parameters of the extended BVD model for the series resonance, respectively the first two spurious resonances.

Figure 9

QCM sensor: (a) evolution of series resistance and series resonance frequency shift in air, water, and glycerol-water solution; (b) evolution of Q-factor and D-factor in air, water, and glycerol-water solution.