The first surface acoustic wave (SAW) sensor that functions via changes in conductivity of a thin surface film is reported. A lead phthalocyanine (PbPc) thin film is deposited on the acoustic propagation path of a LiNbO 3 SAW delay line, which serves as the feedback element of an oscillator circuit. Reaction with strongly oxidizing gases, in particular NO 2 , increases the conductivity of the PbPc film. Acousto-electric coupling of the traveling electric potential wave associated with the SAW to charge carriers in the PbPc film slows the acoustic wave velocity, altering the oscillation frequency of the circuit. This sensor is about 1000 times more sensitive, in terms of the number of NO 2 molecules that can be detected (10 16 molecules/cm 3 of PbPc film), than an identical SAW sensor functioning via mass loading would be. Sensitivity to a few ppm of NO 2 in N 2 has been demonstrated.

Surface acoustic wave gas sensor based on film conductivity changes

This paper is concerned with the theory of excitation of surface acoustic waves on a nonpiezoelectric material, by using an interdigital electrode structure overlaid with a piezoelectric film The impedance and capacity of such transducers have been worked out The results are given in terms of Δv/v, the perturbation in acoustic wave velocity when a perfect conductor is placed in the position of the transducer A computer program which calculates surface wave propagation on a layered medium consisting of a film on a substrate, with arbitrary elastic and piezoelectric properties for both media, has been used to determine Δv/v, and hence the coupling coefficients and radiation resistance of the interdigital transducer As a function of piezoelectric film thickness, two peaks in coupling are observed The results for the interdigital couplers are in quantitative agreement with experiments employing single‐crystal materials Data for experiments using sputtered piezoelectric films also give qualitative agreement with the theory If high‐quality crystalline layers are used, the theory suggests that interdigital couplers on nonpiezoelectric delay lines with properties comparable to or better than the present piezoelectric delay lines may be achievable

Theory of interdigital couplers on nonpiezoelectric substrates

This book gives the fundamental principles and device design techniques for surface acoustic wave filters. It covers the devices in widespread use today: bandpass and pulse compression filters, correlators and non-linear convolvers and resonators. The newest technologies for low bandpass filters are fully covered such as unidirectional transducers, resonators in impedance element filters, resonators in double-mode surface acoustic wave filters and transverse-coupled resonators using waveguides.The book covers the theory of acoustic wave physics, the piezoelectric effect, electrostatics at a surface, effective permittivity, piezoelectric SAW excitation and reception, and the SAW element factor. These are the main requirements for developing quasi-static theory, which gives a basis for the non-reflective transducers in transversal bandpass filters and interdigital pulse compression filters. It is also needed for the reflective transducers used in the newer devices. It is a thorough revision of a classic on surface acoustic wave filters first published in 1985 and still in print. It uniquely combines easy-to-understand principles with practical design techniques for all the devices in widespread use today. It includes complete coverage of all the latest devices which are key to mobile phones, TVs and radar systems; and, a new foreword by Sir Eric Albert Ash.

Surface Acoustic Wave Filters: With Applications to Electronic Communications and Signal Processing

A matrix formalism is outlined for studying surface wave propagation in layered structures utilizing materials of arbitrary anisotropy, piezoelectricity, and conductivity. The analytical formulation reduces the equations describing the system to a first‐order matrix differential equation which is readily solved. The conceptual advantages and reduction of programming effort and computing time resulting from this approach are illustrated using as examples an infinite plate, a multilayer plate, a layered half‐space, and a free half‐space.

Propagation of acoustic surface waves in multilayers: A matrix description

A Rayleigh acoustic wave traveling on the surface of a semi-infinite piezoelectric medium may be amplified by interaction with drifting carriers in an adjacent semiconductor. The gain and frequency response of this interaction is determined here by using a normal mode expansion of the Rayleigh wave piezoelectric fields. The configuration which uses a thin semiconductor film supported by a semi-infinite dielectric and separated from the piezoelectric by a small air gap is described in detail and the results are expressed in a form that clearly shows the effect of piezoelectric, air gap, and semiconductor parameters. Comparisons with experimental data for the lithium niobate-silicon film configuration show that acoustic gains on the order of 100 dB/cm can be obtained in the frequency range above 0.1 GHz.

T.M. Reeder

Papers

A normal mode theory for the Rayleigh wave amplifier