What is a Acoustic-Wave Microsensors ?
The term acoustic-wave microsensor in its widest meaning can be used to indicate a number of significantly different devices. Their common characteristic is the fact that acoustic waves are involved in the operating principles.
Acoustic-wave microsensors can be grouped into the following three classes.
- Microfabricated, or miniaturized, sensors where acoustic waves, i.e. matter vibrations propagating in elastic media, are involved in the sense that they define the domain of the measurand quantity. Examples of this type of devices are accelerometers, microphones, and acoustic-emission pick-ups. The piezoelectric effect, though often used, is not necessarily required in this class of sensors.
- Microfabricated, or miniaturized, sensors that emit and receive acoustic waves in a surrounding medium along a distance which is typically longer than several wavelengths, in order to sense the properties of the medium and/or the presence and nature of internal discontinuities. This class of devices essentially includes ultrasound transducers, both singleelement and arrays, for acoustic inspection, monitoring, and imaging in air, solids, and liquids. The majority, though not the totality, of these devices base their functioning on the piezoelectric effect, mostly because of its reversibility and efficiency.
- Microfabricated, or miniaturized, sensors in which acoustic waves propagate and interact with a surrounding medium, in such a way that the degree of interaction or the properties of the medium can be sensed and measured from the characteristics of the acoustic or electro-acoustic field in the sensor itself.
The sensors of this latter kind essentially behave as acoustic waveguides which, depending on the configurations, can be made responsive to a wide range of physical quantities, like applied stress, force, pressure, temperature, added surface mass, density and viscosity of surrounding fluids. In addition, sensors can be made responsive to chemical and biological quantities by functionalizing their surface with a coating which, depending on its composition, is (bio)chemically active and works as a “receptor” for the analytes to be detected. The coating film has the role of a (bio)chemical-to-physical transducer element, as it converts signals from the (bio)chemical domain into variations of physical parameters, typically the equivalent mass, stiffness, or damping, that the acoustic sensor can detect and measure.
This class of acoustic-wave sensors makes an extensive use of the piezoelectric effect and comprises a number of device types that differ either in the nature of the acoustic waves involved or in configurations adopted.
In the following, the main characteristics of piezoelectric acoustic-wave microsensors belonging to the class 3 will be illustrated.
General Concepts Of Acoustic-Wave Microsensors
The basic for a generic acoustic-wave sensor is a traveling wave combined with a confinement structure to produce a principle of operationstanding wave whose frequency is determined jointly by the velocity of the traveling wave and the dimensions of the confinement structure. Consequently, there are two main effects that a measurand can have on an acoustic-wave microsensor: the wave velocity can be perturbed or the confinement dimensions can be changed. In addition, the measurand can also cause a certain degree of damping of the travelling wave.
An important distinction between sensor types can be made according to the nature of the acoustic waves and vibration modes involved in different devices. The devices usually have the same name as the wave dominant in the device.
In the case of a piezoelectric crystal resonator, the traveling wave is either a bulk acoustic wave (BAW) propagating through the interior of the substrate or a surface acoustic wave (SAW) propagating on the surface of the substrate.
In the bulk of an ideally infinite unbounded solid, two types of bulk acoustic waves (BAW) can propagate. They are the longitudinal waves, also called compressional/extensional waves, and the transverse waves, also called shear waves, which respectively identify vibrations where particle motion is parallel and perpendicular to the direction of wave propagation. Longitudinal waves have higher velocity than shear waves.
When a single plane boundary interface is present forming a semiinfinite solid, surface acoustic waves (SAW) can propagate along the boundary. Probably the most common type of SAWs are the Rayleigh waves, which are actually two-dimensional waves given by the combination of longitudinal and transverse waves and are confined at the surface down to a penetration depth of the order of the wavelength. Rayleigh waves are not suited for liquid applications because of radiation losses.
Shear horizontal (SH) particle displacement has only a very low penetration depth into a liquid, hence a device with pure or predominant SH modes can operate in liquids without significant radiation losses in the device. By contrast, waves with particle displacement perpendicular to the device surface can be radiated into a liquid and cause significant propagation losses, as in the case of Rayleigh waves. The only exception are devices with wave velocities in the device smaller than in the liquid.
Other surface waves with important applications in acoustic microsensors are Love waves (LW), where the acoustic wave is guided in a foreign layer and surface transverse waves (STW), where wave guiding is realized with so-called gratings.
Plate waves, also called Lamb waves, require two parallel boundary planes. The lowest anti-symmetric mode is the so-called flexural plate wave (FPW). Acoustic plate modes (APM), although generated at the device surface, belong to BAWs. Devices based on acoustic-wave microsensors are shortly described in the next section.