Piezoelectric substrates

Surface Acoustic Waves (SA Ws). The basic idea of this technique is to use the dependence of the frequency and propagation of surface acoustic waves on mass loading in a film. The porous film has to be deposited on a piezoelectric substrate (quartz), which is then placed into a physisorption setup to condense nitrogen at 77 K. Adsorption and condensation of N2 result in a shift of the oscillation frequency, and thus measurements of the oscillation frequency as a function of N2 partial pressure provide an adsorption-desorption isotherm.30 Although the technique has proven to provide a concise characterization of porous films,29,30 the requirement for the deposition directly onto the SAW piezoelectric substrate represents a certain restriction. 

In the previous section we considered the conditions under which mechanical resonances would occur in a TSM resonator. In considering only the mechanical properties of the crystal, however, we neglected consideration of how these resonances would actually be excited or detected. The device uses a piezoelectric substrate material in which the electric field generated between electrodes couples to mechanical displacement. This allows electrical excitation and detection of mechanical resonances. In constructing a practical sensor, changes in resonant frequency of the device are measured electrically. The electrical characteristics of the resonator can be described in terms of an equivalent-circuit model that describes the impedance (ratio of applied voltage to current) or admittance (reciprocal of impedance) over a range of frequencies near resonance. 

The previous example illustrates that the acoustoelectric response can be very significant with SAW devices, particularly those using strongly piezoelectric substrates such as LiNbOs. In fact, the acoustoelectric response can be much greater than the mass loading response in certain instances. 

Surface mass changes can result from sorptive interactions (i.e., adsorption or absorption) or chemical reactions between analyte and coating, and can be used for sensing applications in bodi liquid and gas phases. Although the absolute mass sensitivity of the uncoated sensor depends on the nature of the piezoelectric substrate, device dimensions, frequoicy of operation, and the acoustic mode that is utilized, a linear dependence is predicted in all cases. This allows a very general description of the working relationship between mass-loading and frequency shift, A/ , for AW devices to be written ... 

An example of a one-port device is the bulk resonator shown in Figure 6.1, which has a single, planar electrode on each side of a slab of piezoelectric material (these two electrodes together comprise a single port). Most often, the material takes the form of a disk and the electrodes are circular, covering less than the entire surface of the disk. Connection to an external circuit is typically made via a coaxial cable, with one of the two electrodes connected to the shield and the other to the center conductor. This device is known as a resonator because an external circuit (see Section 6.3.3.2) excites the piezoelectric substrate in such a way that a standing wave is set up in the crystal, which thus resonates. 

While a thin transduction layer below and/or above the input and output transducers must be piezoelectric, there is no such restriction upon the balance of the substrate. A piezoelectric thin film (such as crystallographically oriented polycrystalline ZnO or AIN) can be deposited on a non-piezoelectric substrate to provide a medium for AW excitation and detection. Thus, (non-piezoelectric) silicon wafers often serve as the substrate for SAW or FPW devices, with piezoelectric transduction provided by a layer of ZnO. Note also that this transduction layer need not extend laterally past the regions in which the IDTs are defined. 

The excitation and detection of surface acoustic waves, flexural plate waves, and other plate waves on piezoelectric substrates is most readily accomplished by use of an interdigital transducer (IDT) first reported by White and Voltmer [6]. The comb-like structure of the IDT, illustrated in Figure 6.4, is typically made from a lithographically patterned thin film that has been deposited onto the surface of a piezoelectric substrate or thin film. The metal film used to make the IDT must be thick enough to offer low electrical resistance and thin enough so that it does not present an excessive mechanical load to the AW. Typical IDTs are made... 

Valpey-Fisher 75 South St. Hopkinton, MA 01748 Piezoelectric substrates... 

Crystal Technology 1035 East Meadow Circle Palo Alto. CA 94303 Piezoelectric substrates... 

P. R. Hoffman Materials Processing 321 Cherry St. Carlisle, PA 17013 Piezoelectric substrates... 

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]. 

A new Pt(II) polyyne polymer, P15, prepared from the reaction of cfs-[Pt(PPh3)2Cl2] with l,4-diethynyl-2,5-dihexadecyloxybenzene using the extended one pot polymerization route, was tested for its sensing properties and showed fast and reproducible response to relative humidity variations and methanol vapor in surface acoustic-wave (SAW) sensors.46 A SAW sensor was fabricated from polymer P15 as a sensitive membrane, and the polymer was deposited as thin film on the surface of SAW delay lines implemented on three different piezoelectric substrates. High sensitivity and reproducibility were recorded for such devices. The acoustic characterization of the polymer film was also studied with the aid of theoretical results obtained by the perturbation theory. 

Viscosity effects can be additionally minimised by applying a shear transverse wave (STW) resonator, because in these devices the oscillation takes place exclusively within the piezoelectric substrate. Therefore, they are almost uninfluenced by viscosity and additionally lead to highly increased sensitivities as a consequence of the higher fundamental frequency. 

There are few works involved in the detection of aliphatic hydrocarbons, due to their very week interactions between the analytes and Pcs. Urbanczyk and coworkers employed SAW technique to detect trichloroethylene [15]. The acoustic waveguide was fabricated on the y-cut of the LiNb03 piezoelectric substrate. The changes in the physical properties of the CuPc layer placed on a piezoelectric crystal surface can be recorded as a change in differential frequency in a dual delay-line oscillator system, under the exposure of the vapors of the VOCs. The sensitivity is normally quoted as differential response, that is, Afp/ppm of gas, and the greatest sensitivity (approximately 0.1 Hz/ppm) was obtained for trichloroethylene. 

Subdividing the body to be computed into finite elements results in a mesh composed of numerous single elements. A set of linear differential equation represents the complete finite element mesh of the modeled piezoelectric substrate. 

Hasegawa K. and Koshiba M., Finite-element solution of Rayleigh-wave scattering from reflective gratings on a piezoelectric substrate, IEEE Trans. Ultrason. Ferroelectrics Freq. Contr., 37, 99-105, 1990. 

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