Structure and principle of mass-type piezoelectric chemical sensor

Structure and principle of mass-type piezoelectric chemical sensor

1. Bulk Acoustic Wave Chemical Sensor (QCM)

Fig. 1 Quartz wafer with electrodes on QCM Fig. 1 QCM in standard holder

QCM generally uses AT2-cut piezoelectric quartz crystals because their temperature coefficient is zero at room temperature. The QCM device is shown in Figure 1: There is a metal electrode on each surface of the thin piezoelectric crystal plate (Au or Pt is deposited on the upper and lower surfaces of the piezoelectric wafer) and is connected to the resonant circuit. When a resonant electric field is applied to the crystal plate, sound waves are generated. Thickness is the determining factor for frequency changes in this type of device. When there is a material change on the QCM surface (increase or decrease), it can cause a change in the thickness of the device, which in turn causes a change in the resonant frequency, as shown in Figure 2.

M is the mass of the quartz wafer, A is the surface area of ​​the wafer, ρq is the quartz density, f0 is the resonance frequency, ΔMs is the mass of the material deposited on the quartz surface, the vibration frequency of the AT2-cut piezoelectric quartz crystal is f0 = VqP/2l, and Vq is the propagation speed of the shear wave in quartz.

2 Surface Acoustic Wave Chemical Sensor (SAW)

At the end of the 19th century, British scientist Rayleigh discovered sound waves propagating on the surface of solids, namely surface acoustic waves [11] (some literature also calls them surface acoustic waves, or SAW for short). In recent decades, people have gained a deeper understanding of the basic properties of SAW. In 1965, American scientists White and Voltmer [12] invented the metal interdigital transducer (IDT) that can excite surface waves on the surface of piezoelectric crystal materials, making the application of SAW more and more extensive.

SAW sensor is the product of the combination of electronic technology and material science. It consists of SAW oscillator, sensitive interface film material and oscillation circuit. The core component of SAW sensor is SAW oscillator, which consists of piezoelectric material substrate and interdigital transducers with different functions deposited on the substrate. There are two types: delay line type (DL type) and resonator type (R type), as shown in Figure 3:

Fig. 3 Type of SAW sensors
The metal electrode material is evaporated or sputtered onto the piezoelectric substrate in a forked-finger shape, forming a component that generates surface acoustic waves. The surface acoustic waves propagating along the substrate are excited by the forked-finger electrodes. When the substrate or the special is modulated by the object being measured, the operating
frequency of the surface acoustic wave will change and be received by the receiving forked-finger electrodes (via the opposite mechanism), thus forming a frequency output sensor. In 1984, Wohlfjen et al. [13] first proposed the mass response relationship of the surface acoustic wave chemical sensor:

This equation has the same form as the Sauerbrey equation (4), reflecting the effect of mass change on frequency shift, that is, the oscillation frequency decreases as the mass of the adsorbed substance on the substrate or the film covered by the substrate increases. Since the fundamental frequency of QCM can only reach tens of MHz, while the fundamental frequency of SAW can reach GHz level, SAW chemical sensors are more sensitive than QCM chemical sensors, and their detection limit can theoretically reach fg.

From the above, it can be seen that the principle of mass-sensitive piezoelectric chemical sensors QCM and SAW is that when the sensitive element of the sensor interacts with the object being measured, it causes changes in the amplitude, frequency, wave velocity, etc. of the oscillator's own acoustic wave parameters, and the mass P concentration information of the measured quantity is obtained by measuring the frequency change.