Acoustic wave devices

G.S. Kino, Acoustic waves devices, imaging and analog signal processing (Prentice-Hall, Englewood Cliffs, New Jersey, 1987). 

Lithium Niobate. Lithium niobate [12031 -64-9], LiNbO, is normally formed by reaction of lithium hydroxide and niobium oxide. The salt has important uses in switches for optical fiber communication systems and is the material of choice in many electrooptic appHcations including waveguide modulators and sound acoustic wave devices. Crystals of lithium niobate ate usually grown by the Czochralski method foUowed by infiltration of wafers by metal vapor to adjust the index of refraction. 

High-frequency acoustic wave devices (piezoelectric). 

The major piezoelectric applications are sensors (pickups, keyboards, microphones, etc.), electromechanical transducers (actuators, vibrators, etc ), signal devices, and surface acoustic wave devices (resonators, traps, filters, etc ). Typical materials are ZnO, AIN, PbTiOg, LiTaOg, and Pb(Zr.Ti)03 (PZT). 

Kino, G. S., Acoustic Waves Devices, Imaging and Analog Signal Processing, Prentice-HaU, Englewood Cliffs, NJ, 1987. 

Several other approaches for detecting nucleic acids are reported in the literature, based, for example, on the light-addressable potentiometric sensor (LAPS) (Kung et al. 1990) or on acoustic wave devices (Su et al. 1996). 

Slip is not always a purely dissipative process, and some energy can be stored at the solid-liquid interface. In the case that storage and dissipation at the interface are independent processes, a two-parameter slip model can be used. This can occur for a surface oscillating in the shear direction. Such a situation involves bulk-mode acoustic wave devices operating in liquid, which is where our interest in hydrodynamic couphng effects stems from. This type of sensor, an example of which is the transverse-shear mode acoustic wave device, the oft-quoted quartz crystal microbalance (QCM), measures changes in acoustic properties, such as resonant frequency and dissipation, in response to perturbations at the surface-liquid interface of the device. 

Figure 4. Theoretical trends for —(storage) and dissipation as the inner slip is varied between no slip (0) and strong slip (1) for a coated transverse shear acoustic wave device in water. The thickness of the film is 5 nm. The solid line displays the decrease in storage, and the dashed line shows the change in dissipation.

Ohno (54) used ac polarization data and Ricco and Martin (55) used an acoustic wave device for in situ determination and monitoring of the rate of deposition. Various empirical rate equations were derived for electroless deposition of copper (15,33). 

Acoustic wave devices have been used, primarily as electronic bandpass filters, for more than 60 years, but the first reported use as a chemical sensor appeared in 1979 [16, p7],... 

Table 4.2 Mass sensitivities of five types of acoustic wave devices... 

The materials listed in Table 5.1 are also not complete but the selection represents the most relevant composites which are of commercial interest. In addition to the performance issue, there is a strong tendency in research and development to reduce the costs. For that reason, a lot of research is devoted to the niobates as possible replacement for the tantalates, because niobium is cheaper than tantalum. In addition, compensated materials with very high values of the permittivity are currently under development. Recently, for the compound Ag(Nbi a Taa )03 with 0.35< x <0.65, er values of 450 were achieved for potential use as filters (to replace the surface-acoustic-wave devices) and planar antennas in mobile phones [19],... 

First, the underlying principles upon which bulk acoustic wave (BAW) devices operate are described. When a voltage is applied to a piezoelectric crystal, several fundamental wave modes are obtained, namely, longitudinal, lateral and torsional, as well as various harmonics. Depending on the way in which the crystal is cut, one of these principal modes will predominate. In practice, the high-frequency thickness shear mode is often chosen since it is the most sensitive to mass changes. Figure 3.4 schematically illustrates the structure of a bulk acoustic wave device, i.e. the quartz crystal microbalance. 

Figure 3.6 Schematic layout of a single-acoustic-aperture surface acoustic wave device...

Zinc oxide has been investigated already in 1912. With the beginning of the semiconductor age after the invention of the transistor [1], systematic investigations of ZnO as a compound semiconductor were performed. In 1960, the good piezoelectric properties of zinc oxide were discovered [2], which led to the first electronic application of zinc oxide as a thin layer for surface acoustic wave devices [3]. 

First sputtering processes for ZnO deposition were developed in the late 1960s for manufacturing surface acoustic wave devices [2]. The piezoelectric properties of ZnO films are crucial for that application and major efforts were made to develop ZnO sputtering processes which enabled c-axis oriented growth, high resistivity and unique termination of the ZnO crystallites [3,4]. 

Philippe Bergonzo and Richard B. Jackman, Diamond-Based Radiation and Photon Detectors Hiroshi Kawarada, Diamond Field Effect Transistors Using H-Terminated Surfaces Shinichi Shikata and Hideald Nakahata, Diamond Surface Acoustic Wave Device... 

Piezoelectric acoustic wave devices also respond to small changes in mass at surfaces immersed in (viscous) liquids [9]. The resonance frequency of AT-cut quartz resonators immersed in liquids depends on the liquid density and viscosity. The transverse shear wave which penetrates into the viscoelastic deposit and into the liquid is damped due to energy dissipation associated with the viscosity of the medium (film or liquid) at the acoustic frequencies. 

Mason [46] first observed that the viscoelastic properties of a fluid in contact with quartz crystals can affect the resonant properties. However, Mason s work had been forgotten and for a long time there have not been studies of piezoelectric acoustic wave devices in contact with liquids until Nomura and Okuhara [15] found an empirical expression that described the changes in the quartz resonant frequency as a function of the liquid density, its viscosity and the conductivity in which the crystal was immersed. Shortly after the empirical observations of Nomura were described in terms of physical models by Kanazawa [1] and Bruckenstein [2] who derived the equation that describes the changes in resonant frequency of a loss-less quartz crystal in contact with an infinite, non conductive and perfectly Newtonian fluid ... 

Datta, S. Surface Acoustic Wave Devices, Prentice-Hall Englewood Cliffs, NJ (1986). 

Absorption of a solute liquid or vapor into a polymer film can profoundly affect the viscoelastic behavior of the polymer. The magnitude of this effect depends on the nature of the solute/polymer interactions and on the amount of solute absorbed. The solute/polymer interactions can range fttun simple dispersion to hydrogen-bonding and other specific interactions. The extent of absorption can be described by the partition coefficient, AT, which quantifies the thermcxlynamic distribution of the solute between two phases (K = coiKentration in polymer divided by die concentration in the liquid or vapor phase in contact with the polymer). It has long been known that acoustic wave devices can be used to probe solubility and partition coefficients (53,67). Due to the relevance of these topics to chemical sensors, more comprehensive discussions of these interaction mechanisms and the significance of the partition coefficient are included in Chapter 5. 

Because acoustic wave devices are sensitive and respond rapidly, they are ideally suited for real-time monitoring of chemical and physical systems. As discussed in the introduction to this chapter, thin films represent a growing industrial and technological concern for a variety of applications. The use of acoustic devices to characterize the physical properties of these films has been dealt with in the previous sections. Here we describe how these devices can be used to monitor film formation or dissolution processes, or to observe and characterize film properties as a function of time (similar to the monitoring of diffusion in polymers described in Section 4.2.2). 

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