Abstract
Thin-film bulk acoustic resonators (FBAR) can be used as mass sensors when the adsorbed mass is linear to the frequency shift caused by the adsorption. This is, however, only the case if the adsorbed layer is thin compared to the thickness of the resonator. In this paper, we investigate the adsorption of films with thicknesses of some nanometres up to few hundreds of nanometres. With this range, we cover films thicknesses being small compared to the resonator and thicknesses in the range of the resonator thickness. The adsorption of materials was simulated for materials with different mass densities and acoustic velocities. Thin films of platinum, aluminium oxide, tungsten and carbon nanotubes were deposited on the FBAR and the results were fitted to the model used in the simulations.
The acoustic velocity of the carbon nanotube films was much lower than the other materials investigated in this study. With this interesting property, carbon nanotube thin-films are a promising material for acoustic devices where materials with particularly low acoustic impedance are desired.
The paper shows that the FBAR can be a useful tool to characterise mechanical properties of thin films in situ in the micro- and nanoscale within a certain range of parameters.
Keywords: Acoustic properties, acoustic impedance, thin film bulk acoustic resonator, FBAR, shear mode, carbon nanotubes
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