Surface Acoustic Wave Response Research Articles

Software for a Multilayered Surface Acoustic Wave Device Based on the Finite Element Method and Coupling-of-Modes Theory

A software package that provides an interactive and graphical environment for the design and modeling of multilayered surface acoustic wave (SAW) devices is described. The software is based on the finite element method (FEM) and the coupling-of-modes theory (COM). FEM is used to calculate the dispersion diagram of an infinite periodic array and the capacitance of one pair of interdigital transducers (IDTs). COM parameters are then determined using these calculated data. As a result of COM calculations, a 6-port circuit representation is obtained for basic device elements such as IDTs, grating or IDT reflectors and delay lines. The frequency response of SAW devices composed of these elements can be systematically evaluated by cascading the 6-port circuits. As an example, a high frequency SAW filter with a ZnO/diamond/Si structure is designed.

Examination of mass and modulus contributions to thickness shear mode and surface acoustic wave vapour sensor responses using partition coefficients

New data are reported on vapour sorption by four different polymer thin films on thickness shear mode (TSM) and surface acoustic wave (SAW) devices. The polymers examined were poly(isobutylene), PIB, poly(epichlorohydrin), PECH, poly(butadiene), PBD, and poly(dimethylsiloxane), PDMS. The test vapours for the TSM sensor experiments were benzene, toluene, hexane, cyclohexane, chloroform, dichloromethane, carbon tetrachloride and butan-2-one, tested at several concentrations from P/P0=ca. 0.03 to 0.64. Measurements on the TSM devices using oscillators were complemented with measurements by impedance analysis. Acoustic wave sensor data were converted to apparent partition coefficients calculated assuming the response was due to mass-loading only. Values derived from SAW and TSM measurements were compared with one another, and with other partition coefficient data determined by non-acoustic wave techniques. It was found that TSM devices coated with PIB, PECH, and PBD (film thickness <1 µm) behaved as gravimetric sensors at low vapour concentrations. The PDMS-coated TSM sensor displayed significant losses and unusual behaviour suggestive of other effects. SAW sensors with PIB and PECH films gave responses indicating that modulus effects were contributing to the response, in addition to mass-loading effects. This enhancement appeared to be at least a factor of two, suggesting the mass-loading accounted for ca. 50% of the SAW response. The responses of PBD and PDMS-coated SAW devices were not enhanced relative to the expected mass-loading responses. Differences in behaviour among the test polymers on SAW devices are discussed in terms of the differing viscoelastic properties of these materials.

Dynamics and Response of Polymer-Coated Surface Acoustic Wave Devices: Effect of Viscoelastic Properties and Film Resonance

The response of polymer-coated surface acoustic wave (SAW) devices to temperature changes and polymer vapor absorption is examined. A perturbational approach is used to relate velocity and attenuation responses to film translational and strain modes generated by the SAW. Two distinct regimes of film behavior arise, causing different SAW responses. For glassy films, displacement is nearly uniform across the film thickness, varying only in the direction of propagation. A model developed to predict velocity and attenuation in this regime (model 1), reduces to the familiar Tiersten (Wohltjen) equation for purely elastic films. For elastomeric (rubbery) films, inertial effects cause a phase lag to occur across the film for shear displacements. A model to account for these cross-film displacement gradients (model 2) predicts a characteristic resonant response when the film phase shift reaches n[pi]/2, where n is an odd integer. These model predictions are compared with measured responses from polyisobutylene-coated SAW devices as temperature is varied and during exposure to high vapor concentrations. 48 refs., 15 figs., 6 tabs.

Temperature effect and chemical response of surface acoustic wave (SAW) single-delay-line chemosensors

The effects of temperature and chemical vapour absorption on the frequency shift of coated and uncoated SAW single-delay-line oscillators are reported. It is found that the temperature-induced frequency-shift function depends on the nature of the coating of the delay-line surface. It changes from the flat parabolic temperature dependence of an uncoated ST-quartz delay line near room temperature, to a steep, almost linear, curve for polymer-coated delay lines in the same temperature range. The slope of the curves depends on the nature of the polymers. The dual-delay-line concept, where an uncoated SAW delay line is used as a reference element to compensate for the ambient temperature variations, is therefore not universal and should be implemented with great caution. The response of an array of four different SAW delay-line sensors to various chemical vapours is represented by pie charts, visualizing chemical pattern variability that can be used for pattern recognition. The frequency shifts are correlated with calculated theoretical partition-coefficient values, based on thermodynamic parameters and regular solution theory. It is shown that this approach may be used to predict the interaction between the polymer and a specific target chemical vapour. A simple method for a preliminary design of a SAW chemosensor is thus suggested.

Multiple-frequency SAW devices for chemical sensing and materials characterization

We have designed, tested, and demonstrated the use of two types of multiple-frequency surface acoustic wave (SAW) device for chemical detection and characterization of thin-film properties. These devices, both of which utilize ST-cut quartz substrates, allow the frequency dependence of SAW sensor perturbations to be examined, aiding in the elucidation of the operative interaction mechanism(s). A four-frequency device with nominal SAW center frequencies of 16, 40, 100, and 250 MHz is designed in such a way that all four sets of transducers probe a common region of a thin film deposited in the wave path. A seven-frequency SAW device is designed and fabricated with nominal Rayleigh-wave center frequencies ranging from 25 to 200 MHz in √2 multiples; each pair of transducers probes a distinct region of a thin film on the active area of the device. For the seven-frequency device, transducer periodicity, finger length, and center-to-center transducer separation all scale with the reciprocal of the center frequency. Measurements of the SAW response to the vacuum deposition of a thin nickel film show the expected frequency dependence of mass sensitivity and the acoustoelectric effect. Coating multifrequency devices with a thin polymer film allows the frequency-dependent changes in the viscoelastic properties of the polymer to be studied as a function of temperature and of the concentration of various organic solvents, which in turn provides a means for selective chemical detection of these compounds. By measuring changes in both wave velocity and attenuation at multiple frequencies, extrinsic perturbations such as temperature and pressure changes are readily differentiated from one another and from changes in surface mass and other perturbations.

Synthesis of new microsensor coatings and their response to test vapors 2,4,6-trisubstituted-1,3,5-triazine derivatives

Novel 1,3,5-triazine derivatives were spray-coated onto surface acoustic wave (SAW) devices and exposed to vapors of dimethyl methylphosphonate (DMMP), chloroethyl ethyl sulfide (CEES) and water. Changes in chemiresistor and SAW responses were monitored and recorded by computer-controlled data-acquisition techniques. All the derivatives tested showed little or no chemiresistor or SAW responses to water vapor. The largest reversible chemiresistor response to DMMP vapor was observed with the dicarboxylic acid derivative. The largest SAW response to DMMP was with the dithione, and the dichloro-octylthio derivative showed the largest response to CEES.

Surface acoustic wave response to changes in viscoelastic film properties

Changes in polymer viscoelastic properties have been monitored using polymer-coated surface acoustic wave (SAW) devices. Glass transitions induced by temperature changes and absorption of gas phase species have been observed. The changes in wave propagation velocity and attenuation which accompany these transitions are explained using a Maxwell model to describe the viscoelastic properties of the film.

Surface acoustic-wave piezoelectric crystal aerosol mass microbalance

The feasibility of applying surface acoustic-wave (SAW) piezoelectric crystal technology to the measurement of subnanogram levels of particles and parts-per-million levels of sulfur dioxide has been demonstrated. Improvements necessary for implementing the development of a practical instrument for the real-time measurement of very low aerosol mass and ambient gases have been identified. Mass sensitivity comparisons of a 158-MHz SAW piezoelectric microbalance and a conventional 10-MHz quartz-crystal microbalance (QCM) showed that the SAW crystal was 266 times more sensitive. This is in good agreement with the theoretical value of 250. Frequency stability of a single SAW resonator was 6 parts in 108 over 1 min. Response to temperature change is found to be very linear over the range +30 to −30 °C. Response to pressure change is found to exhibit an aging effect whereby after one complete cycle of pressure variation from 1 atm to 70 mm Hg and back to 1 atm, the SAW response to pressure change decreases by an order of magnitude. A strong response to 15 ppm SO2 has been demonstrated on a chemically coated SAW crystal. With improvements, SO2 measurements at the ppb level should be achievable.

I n s i t u compensation of surface acoustic wave filter response

An experimental methodology is demonstrated for compensating the response of surface acoustic wave (SAW) bandpass filters. For a filter on (YZ) LiNbO3, we present measurements of changes in the SAW impulse and frequency response produced by in situ perturbations to the transducer’s apodization. Many transducer electrodes are perturbed in their length to determine filter sensitivity to electrode length variation. A methodology that is adaptive in these length variables is applied to determine the optimal apodization for the compensation of frequency domain sidelobes initially at the −26 dB level. They are deterministically reduced to the −39 dB level.

Sputtering and chemical vapour deposition of piezoelectric ZnO, AlN and K 3Li 2Nb 5O 15 films for optical waveguides and surface acoustic wave devices

Epitaxial and c-axis-oriented ZnO thin films with surface flatness were successfully grown on various kinds of substrate by r.f. planar magnetron sputtering, ZnOH 2H 2OO 2 isothermal chemical vapour deposition (CVD) and plasma-enhanced organometallic CVD methods. c-axis-oriented AlN films were also prepared on glass and silicon substrates at substrate temperatures as low as 20–460°C by reactive r.f. planar magnetron sputtering. Furthermore, ferroelectric K 3Li 2Nb 5O 15(KLN) crystal films were grown with good epitaxy on K 2BiNb 5O 15 or sapphire substrates by sputtering. The quality of the ZnO, AlN and KLN films was evaluated from X-ray diffraction, scanning electron microscopy and reflection electron diffraction measurements. These films are excellently transparent and are of extremely good quality. The epitaxial CVD and c-axis-oriented sputtered ZnO films have guided optical wave losses as low as 0.7 dB cm −1 and 2.1 dB cm −1 respectively. All the films are piezoelectrically active and show surface acoustic wave response.