Figure 1 summarizes the measured parameters that are most frequently used for different types of applications. However, some measurement technique is required to extract this parameter, unlike resonant frequency, which can be estimated by the QCM oscillating frequency (if an oscillator circuit was used). If the attached film is not rigid but viscous, its viscoelasticity property can also be deducted from the Q-factor change.
Although other acoustic devices such as a SAW (Surface Acoustic Wave) device or FBAR (Film Bulk Acoustic Resonator) can be used to detect mass, the high Q-factor characteristic of QCM enables the detection of a subtle frequency shift in comparison with those devices even if the device structure and measurement setup are simple. QCM is capable to detect very slight mass changes, around a nanogram in real-time only using a simple measurement setup. Its capability to detect the mass makes the acoustic resonator a universal transducer. Since mass is the fundamental property of an analyte, it can be monitored using acoustic devices. The first application of quartz crystal was not a sensor but a resonator, mainly used in communications and oscillator circuits. For more than 40 years, the Quartz Crystal Microbalance (QCM) has been one of the choices amongst many acoustic sensors due to its stability and sensitivity.
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