P. Verardi

Advances in SAW-based gas sensors

This paper deals with some advances in surface acoustic wave (SAW) sensor research relating to the use of both a new chemically interactive material (metal porphyrin) and new amplifiers for low-noise SAW oscillators, which are illustrated and discussed in some detail. The trend toward SAW matrixes, implemented on silicon, required for more sensitive chemical image systems, has led to the observation of coupling effects between adjacent SAW oscillators, even if the electronics were not implemented on Si. This effect must be considered at design level in order to achieve a higher resolution value in detecting either single chemical species or chemical images.

Analysis of the different contributions to the response of SAW gas sensors

Numerical estimations of the mass and elastic loadings are performed for different possible material combinations. In order to do this, the density and elastic constants of the film are changed consequently and one by one for each of the combination selected in accordance with Farnell-Adlers theory. In addition, the perturbation approach is applied to analyse the SAW gas response in analytical form. On considering, as an example, Pd coated SAW delay lines exposed to hydrogen, the relative changes in Pd parameters are measured, the mass and elastic loadings are estimated and the SAW hydrogen responses are theoretically predicted and experimentally verified. An increase of a factor 10 in the SAW gas response is demonstrated for Pd film deposited on different substrate materials and/or crystallographic orientations.

Analysis of acoustic waves in multilayers using compound matrices

A modification of the matrix formalism for studying acoustic wave propagation in anisotropic piezoelectric multilayers is developed. The modification is based upon the use of the compound matrices, well known from the theory of the matrices. Using the modified method, analytical expressions for the relationship between the real and imaginary parts of the determinant of the boundary conditions are derived. One of the advantages of the method is demonstrated for thick plates and thick film structures. On considering the dispersion curves for thick multilayers, the slow dispersion regions inherent for anisotropic materials are discovered. The regions are related with the cutoff velocities of the bulk waves in the film materials.

Real time characterization of elastic variations in palladium films produced by hydrogen adsorption

The magnitude and time variations of the SAW propagation velocity /spl Delta/v/v, of the mass and elastic loading, and of the fractional changes in the density /spl Delta//spl rho///spl rho/ and in the elastic modulii /spl Delta/c/sub ij//c/sub ij/ of Pd films are experimentally evaluated at room temperature and atmospheric pressure for 0.1, 0.5, 1% H/sub 2/, 1% CO, 1% N/sub 2/O and 1% NO gas mixtures in pure nitrogen. Two different thermally evaporated Pd films have been tested: one as evaporated, the other one after annealing at 423/spl deg/K and 10/sup -5/ Pa for 15 hours. The annealing is shown to change all set of the parameters. For not annealed films, hydrogen gives rise to large variations in the elastic constants (/spl sime/5-40%) and to an increase in the phase velocity; while for annealed films, it produces small variations in the elastic constants (/spl sime/1-2%) and a decrease in the phase velocity. The difference is attributed to weakly and strongly coupled oxygen preadsorbed in the film. The results obtained can allow an improvement of Pd-based sensors.

Effect of humidity on SAW devices

Adsorption of water vapors on single crystal and polycrystalline substrates is analyzed using the propagation of surface acoustic waves as an experimental tool. The perturbation in the SAW velocity (/spl Delta/v/v/sub 0/), the number N of adsorbed species as well as the changes in the density /spl Delta//spl rho///spl rho/ and in the elastic moduli /spl Delta/c/sub ij//c/sub ij/ of the sorbent materials, are deduced in terms of the normalized SAW displacements A/sub x/, A/sub y/, A/sub z/ and of the unperturbed SAW velocity v/sub 0/. Vapour adsorption on polished plates of single crystal quartz and polycrystalline films of Pd and Pd:Ni is experimentally investigated. Results of the measurements clearly show that humidity adsorption gives rise to effects whose amplitude is larger and whose time duration is longer in the films than in the plates. These effects are also dependent on the orientation of the quartz, allowing to monitor the value and the rate of the effect by proper selection of a substrate material.

K/sup +/ detection using shear horizontal acoustic modes

An acoustic delay line sensor for the detection of potassium ion concentration in water has been implemented and tested. The device uses the propagation of shear horizontal (SH) waves in a piezoelectric plate (Love waves) in order to minimize acoustic energy loss in the liquid. One of the plate surfaces is covered with a polyvinyl-chloride-valinomycin membrane, a well-known ligand for the detection of potassium ions. The devices has shown a fairly good linearity of the response in the analyzed pK/sup +/ range of K/sup +/ concentration between 1.49 and 4. The operation is reversible and the response is highly reproducible. Measurements have demonstrated a high selectivity of the device with respect to exposure to Na/sup +/ solutions.<<ETX>>

A study on elastic energy distribution in layered structures for application to surface acoustic wave devices

The relative fraction r of the elastic energy of an acoustic mode localized in a supported film is shown to play an important role in some applications of layered structures such as electroelastic effect, gas sensing and acoustic characterization. Computer simulation of the parameter r as a function of the film thickness normalized to the acoustic wavelength is performed for different possible material combinations in accordance with Farnell-Adlers scheme. Some general features of the parameter r are discussed. Measurements of acoustic attenuation and temperature coefficient of delay, are shown to correlate with the behavior of this parameter for Rayleigh and Sezawa modes and for both conditions of slow film on fast substrate and fast film on slow substrate. The analysis and experiments have been carried out on layered structures made of AlN and ZnO films on glass, fused quartz, and Si substrates

New properties of the SAW gas sensing

Novel attractive properties of SAW gas sensing are theoretically predicted and experimentally verified. It is demonstrated that the SAW response can be increased, decreased, reversed, cancelled, speeded up, aged down and selected without changing the sensitive layer, simply by properly selecting the acoustic substrate material and its crystallographic orientation. These new properties are shown to come from the elliptic polarization of the acoustic wave and from its dependence on the propagation conditions. An experimental verification of the theoretical predictions is carried out on SAW H/sub 2/ sensors using both Pd and Pd:Ni sensitive films, deposited on ATx-, STx- and yx-quartz substrates upon exposure to 1% H/sub 2/+N/sub 2/, 1% CO+N/sub 2/ and 1% N/sub 2/O+N/sub 2/ gas mixtures. These new peculiarities will make possible to develop a new generation of SAW sensors.