Oscillating behavior of quality factor of a film bulk acoustic resonator in liquids

When a film bulk acoustic resonator (FBAR) is coupled to a thin liquid layer, the quality factor (Q) of the resonator varies in a damped oscillatory pattern versus the liquid thickness. This paper reports this behavior with an analytical modeling and experimental demonstration of it. The thin liquid layer with thickness comparable to the acoustic wavelength is realized by integrating a microfluidic channel to the FBAR. Q assumes its maxima (up to 150) and minima (as low as 50) when the channel thickness is an odd multiple of quarter-wavelength and a multiple of half-wavelength, respectively. We also present a bio-molecular detection test for aptamer immobilization and thrombin binding as a proof-of-concept sensing application.