Technology: SAW Sensors

SAW (surface acoustic wave) sensors utilize the same basic low-cost device technology that is widely used for signal processing applications such as radio frequency (RF) and intermediate frequency (IF) filtering in cell phones and consumer electronics, adapted to measure chemical, biological, or physical properties of the device environment.

As described under SAW Technology, SAW devices respond to a radio frequency (RF) signal by launching an acoustic wave at the surface of the device. Since this wave is propagating in a piezoelectric substrate, it is not purely a mechanical wave, but rather is a combined electro-mechanical wave. Any interaction with the environment that alters the propagation of the wave can be measured. Physical modifications of the substrate material, such as softening or stiffening with changes in temperature, thermal expansion or contraction, and changes in substrate strain state induced by applied stress all result in modification of acoustic wave propagation speed. Chemically or biologically selective films placed on the surface of the device can also modify wave propagation when the films interact with target analytes. Any interaction that changes the mass loading, viscoelastic properties, or electrical conductivity of the film at the device surface can be measured. Since device operating frequency is set by (essentially fixed) electrode geometries and wave speed, as the wave speed changes, so does the device operating frequency. Similarly, for a given propagation distance, as wave speed changes, so does device delay. SAW sensors can be designed to take advantage of changes in frequency, delay, response amplitude, or other parameters.

Coded SAW sensors can be interrogated wirelessly using RF signals, and respond with a signal that encodes both the sensor's identity and the measurement(s), with no batteries required in the sensor. These rugged solid-state devices are capable of operating from very low (cryogenic) to high temperatures (up to 1000°C or more). SAW devices have no single even upset mechanisms, and have been demonstrated to be RadHard to over 10 MRad of ionizing radiation, with demonstrated lifetimes of several decades and in military applications. These devices can be embedded in materials for real-time wireless monitoring of specific parameters, for example in curing concrete, epoxies, or composite materials. SAW sensors can also be used on rotating parts, and in hard to reach locations, to monitor target parameters for use in structural or system health monitoring and prognostics. Selective coatings can be used to produce sensors specific for target biological or chemical analytes, to provide quantitative detection and/or measurement of analyte concentration in either vapor or liquid phase.