1. Field of the Invention
This invention relates to the field of chemical, biological agent and radiation sensors, and in particular, to such sensors embodied in surface acoustic wave devices.
2. Description of Related Art
Surface acoustic wave (SAW) devices are electronic components, embodied as filters, resonators, and delay lines exploiting surface acoustic waves in piezoelectric crystals. An interdigital transducer (IDT) converts an electronic signal into a surface acoustic wave which, propagating 100,000 times slower than electromagnetic waves, can deliver significant signal delays in the device. Signal filtering is performed by apodized IDT during electric to SAW signal conversion, or back conversion by another IDT. This is referred to as transversal filtering.
SAW devices can be used to detect intrinsic properties, and as such, are used as chemical, biological agent and radiation sensors and analyzers. As noted, a basic SAW device is a quartz crystal designed to support high-frequency acoustic oscillations. These oscillations are quite sensitive to surface effects. If the surface of a SAW device is provided with a coating which is attractive to a desired chemical or class of chemicals, for example, then such chemicals will tend to be adsorbed by the coating when in the presence of the SAW device. The difference between the fundamental frequency of the coated SAW device prior to adsorption of chemicals and after the adsorption of chemicals can be a reliable and accurate measure of the presence or concentration of the adsorbed chemical, although not both simultaneously. SAW devices with coatings which exhibit a preference for absorbing particular kinds of radiation, for example photo-conductivity or heating under the effects of ultra violet or infrared light, can be used as detectors for such radiation. The difference between the fundamental frequency before and after exposure to the radiation can produce a measurable shift in the fundamental frequency of the coated SAW device which can be a reliable and accurate measure of the presence or concentration of radiation, although not both simultaneously. SAW devices with coatings which adsorb and desorb (that is, release) chemicals or thermal energy, for example, as concentrations of the chemical or thermal energy rise and fall, can be used as continuous detectors.
Development of new chemical, biological agent and radiation sensors, and/or new methods of improving the sensitivity and detection limit of existing systems, are important areas of current research. The conventional method of measuring gas concentration using a SAW sensor is by measuring changes in only the fundamental frequency. For practical considerations, the majority of SAW gas sensors operate at frequencies below 300 MHz. The sensitivity of SAW devices is a function of the square of the operating frequency. Thus, although potentially more sensitive than other gas sensing methods, such as the use of infrared absorption spectra, these practical limitations restrict their actual sensitivity to many gases. In addition, the identification of a particular species from interfering gases by measuring a single parameter, that is frequency, is impossible. Attempts to solve this problem have conventionally been using an array of several sensors. However, in many cases false identification remains a problem.
Most publications and patents on SAW chemical sensors relate to the development of new polymer or conductive coatings for sensing particular gases of interest, or the use of sensor arrays to detect a particular gas of interest among interfering gases. However, their sensitivity relative to other methods such as those described above remains poor. U.S. Pat. No. 5,076,094 describes a method for identifying and quantifying absorbed chemical species by measuring changes in both the velocity and attenuation of the acoustic wave traveling through a thin film into which the chemical species is absorbed.