Precision Measurement of Rayleigh Wave Velocity Perturbation

We have developed a technique for imaging variation in the surface characteristics of a sample by measuring the local perturbation of the Rayleigh wave velocity. A 50 MHz acoustic microscope operated out-of-focus is excited with a very short tone burst so that the on-axis longitudinal and off-axis Rayleigh reflection pulses are temporally separated. The relative phase between these two signals is measured using a synchronous detection scheme. This technique has a potential sensitivity of 10 ppm. We have demonstrated experimentally that we can detect a 240 Å thick film of indium deposited on glass which corresponds to a velocity perturbation of 0.18%