Surface Acoustic Waves (SAW) find their application in a broad range of devices such as RF-filters, oscillators and more recent in micro-fluidics for both actuation and sensing. The application of Surface Acoustic Waves for sensing purposes at micro scale is nontrivial due to the high phase velocity involved. Lamb waves [2-4] are interesting wave types because the wave can propagate with an infinite number of unique phase velocities for both the anti-symmetric and symmetric wave modes. For sensing purposes resonators are usually preferred above other type of configuration such as delay lines because of the larger quality factor at resonance. In order to achieve acoustic resonance discontinuities are placed along the propagation direction of the Lamb wave. A strong reflection occurs if the period of the discontinuities are equal to half of the wave length λ0. This is called the Bragg condition. Under this condition the reflected waves are in phase and interfere constructively and form a standing wave. Even for weak reflectors with a small reflection coefficient the total reflection may be close to unity if the number of reflectors is large. Lamb waves can be actuated by means of Interdigitated electrodes (IDT) placed on top of thin film piezoelectric substrate taking advantage of the indirect piezoelectric effect to form the wave. To this purpose Lead Zirconate Titanate Pb(ZrxTii−x)O3 close to the morphological boundary will be used due to the large piezoelectric coefficients .
Lamb waves typically propagate in thin membranes where the wave length is much larger than the membrane thickness λ0 >> t. Consequently, the reflection behavior is not as large when electrodes or grooves, with a relatively small height h, are used compared to the reflections resulting from SAWs with a smaller wave length such as Rayleigh waves. This it the result of the larger wave length which is typically much larger than the reflector height λ0/h < 1%. In order to achieve a higher reflection coefficient and a smaller amount of electrodes reflections resulting from the edge of a free-standing membrane are proposed . The main goal of this assignment is to investigate experimentally the resonance behavior of a Lamb wave device as function of the type of wave reflections and the resonator dimensions as indicated in Figure 1 such that the highest quality factor is obtained for sensing purposes.
The tasks that have to be carried out in this project are:
- literature study in order to understand the wave mechanics involved,
- experiments both electric characterization and studying the wave modes with the aid of the vibrometer
More information can be obtained from Yves Janssens (email@example.com) and Niels Tas (firstname.lastname@example.org)
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