Bulk acoustic wave piezoelectric sensors

A schematic diagram of a bulk acoustic wave (BAW) chemical sensor is composed of a BAW piezoelectric resonator with one or both surfaces covered by a membrane (CIM) (fig. 14). 

Surface acoustic waves (SAW), which are sensitive to surface changes, are especially sensitive to mass loading and theoretically orders of magnitude more sensitive than bulk acoustic waves [43]. Adsorption of gas onto the device surface causes a perturbation in the propagation velocity of the surface acoustic wave, this effect can be used to observe very small changes in mass density of 10 g/cm (the film has to be deposited on a piezoelectric substrate). SAW device can be useful as sensors for vapour or solution species and as monitors for thin film properties such as diffusivity. They can be used for example as a mass sensor or microbalance to determine the adsorption isotherms of small thin film samples (only 0.2 cm of sample are required in the cell) [42]. 

Piezoelectric bulk acoustic wave (BAW) chemical vapor sensor Glucose sensor... 

Bulk acoustic wave (BAW) biosensors employ either longitudinal or shear waves, although the latter is often preferred to reduce acoustic radiation in the medium of interest. They are the oldest and the simplest acoustic wave devices. BAW devices consist of a parallel electrode placed on both sides of the thin piece of crystal. BAW sensor can technically employ any piezoelectric element, and typically quartz is used, as it is an inexpensive material readily available in nature and easily synthesizable in abundant quantities. In addition, thin disks of quartz are more stable at high temperatures than other piezoelectric elements. When an alternating electric field is applied, it results in a potential difference... 

Piezoelectric-based or acoustic wave (AW) sensors such as surface acoustic wave (SAW), quartz crystal microbalance (QCM) or bulk acoustic wave (BAW), and cantilever-based devices create a specific class of gas sensors widely used in various applications (Ippolito et al. 2009 Korotcenkov 2011) (see Fig. 13.1). Virtually all acoustic wave-based devices use a piezoelectric material to generate the acoustic wave which propagates along the surface in SAW devices or throughout the bulk of the structure in BAW devices. Piezoelectricity involves the ability of certain crystals to couple mechanical strain to electrical polarization and will only occur in crystals that lack a center of inversion symmetry (Ballantine et al. 1996). 

The brothers Jacques and Pierre Curie are credited with the discovery of piezoelectricity in a number of hemiedric crystals (Curie and Curie, 1880). Today, piezoelectrics are utiUzed in acousto-electronic devices and sensors based on bulk and surface acoustic waves, piezomechanical sensors to monitor pressure, power, and acceleration, as actuators for micropositioning devices, band pass filters with low insertion losses, as electro-optic devices for optical memories, displays for high-definition televisions, and possibly as transparent piezoelectric speaker membranes as well as miniaturized piezoelectric transformers and motors. As the classic piezoelectric material is a-quartz, the basic relationships are detailed below using it as a model structure. Further details on the piezoelectric properties of quartz, and of its history, discovery and utilization, are available elsewhere (Ballato, 2009). 

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. There are two main types of piezoelectric sensors ... 

QCM sensors (or BAW sensors) concern probing alterations to the bulk properties of the piezoelectric crystal. If an acoustic wave is made to travel along the surface of the substrate, then a surface acoustic wave (SAW) sensor can be produced. 

Zinc oxide (ZnO) is a cheap but important material with many unique properties and potential applications [123-125]. Firstly, it is a gas/pressure-sensing ceramic and can be used as non-linear varistors and sensors [126]. Secondly, piezoelectric ZnO can find applications in surface and bulk acoustic wave devices [127-129]. Thirdly, it is a semiconducting oxide with a wide band gap of 3.37 eV at room temperature and a large exciton energy of 60 meV,... 

A chemical microsensor can be defined as an extremely small device that detects components in gases or Hquids (52—55). Ideally, such a sensor generates a response which either varies with the nature or concentration of the material or is reversible for repeated cycles of exposure. Of the many types of microsensors that have been described (56), three are the most prominent the chemiresistor, the bulk-wave piezoelectric quartz crystal sensor, and the surface acoustic wave (saw) device (57). 

Chemical sensors for gas molecules may, in principle, monitor physisorp-tion, chemisorption, surface defects, grain boundaries or bulk defect reactions [40]. Several chemical sensors are available mass-sensitive sensors, conducting polymers and semiconductors. Mass-sensitive sensors include quartz resonators, piezoelectric sensors or surface acoustic wave sensors [41-43]. The basis is a quartz resonator coated with a sensing membrane which works as a chemical sensor. 

TSM resonators possess lower mass sensitivity than most acoustic wave sensors that employ other mechanisms since they typically operate between only 5 and 30 MHz. Thinner devices can be used to increase frequencies that result in increased mass sensitivity, and they are more fragile and harder to manufacture and handle [5]. Recent research on this topic has been on creating high-frequency TSM resonators with piezoelectric Aims and bulk silicon micromachining techniques. 

In these relatively new sensors, an acoustic wave is propagated by an externally applied alternative current between two electrodes or interdigited electrodes deposited on a piezoelectric substrate such as quartz. There are several subclasses of piezoelectric transducers (bulk wave, surface acoustic wave, or... 

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