Quartz Crystal Thickness Shear Mode Sensors
What is a Quartz Crystal Thickness Shear Mode Sensors ?
The oldest application of quartz crystal resonators (QCR) as sensors is the quartz crystal microbalance (QCM or QMB). These sensors typically consist of a thin AT-cut quartz plate with circular electrodes on both parallel main surfaces of the crystal. BAWs are generated by applying an electrical high-frequency (HF) signal to the electrodes. QCMs are operated as resonators in an almost pure thickness-shear mode, hence the sensors are also called TSM sensors.
The sensor resonant frequencies are inversely proportional to the crystal thickness. For the fundamental mode, resonance frequencies of 5 to 30 MHz are typical. For higher frequencies the crystals can be operated at overtones. Nowadays high-frequency QCRs with fundamental frequencies up to 150 MHz are available. The required crystal thickness down to 1 µm is prepared by chemical milling and, for mechanical stability reasons, the etching of the crystal is limited to the region of the electrode area, leading to inverted-mesa structures.
After their first use as frequency-reference elements in time-keeping applications in 1921 by W. Cady and as a microbalance in 1959 by G. Sauerbrey, quartz crystals have become probably the most common acoustic-wave sensors, finding application in the measurement of several other quantities and, in turn, opening the way to the development of newer and more specialized sensors. The typical configuration is as singleelement sensors, but multisensor arrays on the same crystal have been recently proposed.
The basic effect, common to the whole class of acoustic-wave microsensors, is the decrease in the resonant frequency caused by an added surface mass in the form of film. This gravimetric effect motivates the denomination of quartz-crystal microbalance and is exploited, for instance, in thin-film deposition monitors and in sorption gas and vapor sensors using a well-selected coating material as the chemically-active interface.
Within a certain range, the frequency shift Δf is sufficiently linear with the added loading mass Δm regardless of the film material properties, and the sensitivity Δf/Δm is proportional to f2. For higher loading, the sensor departs from the gravimetric regime and the frequency shift becomes a function of the mass as well as of the viscoelastic properties of the film.
TSM quartz sensors can also operate in liquid, due to the predominant thickness-shear mode. In this case, the frequency shift is a function of liquid density and viscosity, which makes it possible to use TSM quartz resonators as sensors for fluid properties. In addition, the mass sensitivity and in-liquid operation can be advantageously combined, and TSM sensors coated with (bio)chemically-active films can be used for in-solution (bio)chemical analysis, for instance in the chemical, biomedical and environmental fields.
Mass sensitivity and liquid density-viscosity sensitivity are two special cases of the more general sensitivity of all acoustic-wave microsensors to the so-called surface acoustic load impedance. Because of its importance and simplicity we further limit the discussion here to mass sensitivity and applicability of the devices in a liquid environment.