Trace Chemical Vapor Detection Using SAW Delay Line Oscillators

The resonant frequencies of SAW delay line oscillators are shown to be very sensitive to the presence of thin organic films that are deposited onto the delay line surface. Theory suggests that the sensi- tivity of the SAW device to mass loading depends on the square of the resonant frequency. This dependence has been studied experimentally using devices resonating at 31, 52, and 112 MHz that were coated with organic films of precisely controlled composition and thickness. The mass sensitivity of SAW oscillators can be exploited to make very sen- sitive chemical sensors if an appropriate sorptive coating is applied to the device. Results obtained from a delay line oscillator operating at 290 MHz (the highest frequency SAW chemical sensor reported to date) suggest that the rapid detection of organic vapors at concentra- tions substantially below 100 parts per billion (by volume) is readily achievable.