Multiple-frequency SAW devices for chemical sensing and materials characterization

Abstract

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.