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When a film bulk acoustic resonator (FBAR) is coupled to a liquid layer with thickness comparable to the acoustic wavelength, the Q factor varies in a damped oscillatory pattern with the liquid thickness. This letter reports an analytical modeling and experimental demonstration of this behavior by integrating microfluidic channels to MEMS-based FBARs. It is found that Q assumes its maxima and minima when the channel thickness is an odd multiple of quarter-wavelength and a multiple of half-wave-length, respectively. The microfluidics integrated FBARs achieve a 10 × improvement of Q over fully immersed FBARs, showing the potential of use as high-resolution sensors involving liquids.