Micromachined mass sensor based on film bulk acoustic resonator

In this paper, an Al-ZnO-Al film bulk acoustic resonator (FBAR) is described, which has a high mass sensitivity. And the series resonant frequency (f_s) and parallel resonant frequency (f_P) of the micromachined FBAR have been measured by using a network analyzer, which are 1.546 GHz and 1.590 GHz, respectively (close to the theoretical value simulated by mason equivalent circuit model). Moreover, the effective electromechanical coupling coefficient (K_eff²) and quality Q have been calculated to be 6.83% and 350, respectively. After the FBAR packaging, the mass loading was added on the FBAR backside surface by sputtering different thicknesses of ZnO layer. From the graph of the resonant frequency versus added ZnO thickness, the FBAR is measured to have a mass sensitivity S_m of 1116.55 cm² / g. The result is almost agreed with the theoretical value of 1166.86 cm² / g, which is about 80 times of the conventional quartz crystal microbalance (QCM).