Abstract
The frequency tuning of a surface micromachined film bulk acoustic resonator (FBAR), which integrates the tunable series capacitor and the piezoelectric resonating layer, had been demonstrated in previous publications. In the proposed structure, a movable piezoelectric cantilever is suspended above a bottom electrode. The air gap between the cantilever and the bottom electrode functions both as a tunable capacitor and an acoustic isolation. However, in case of electrostatic actuation, continuous tuning is limited in the range before the pull-in of the cantilever and is very small in prior art. In this paper, an improved design is suggested. By changing the the effective area ratio between the tunable capacitor and the piezoelectric resonating film, a larger continuous tuning range of 24.5MHz (0.39%) at a resonating frequncy of about 6.3GHz is realized. I. INTRODUCTION Film bulk acoustic resonators (FBARs) are microma- chined versions of the conventional piezoelectric resonators such as quartz crystals, with the advantages of small size, high frequency capability, high quality factor and CMOS- compatibility. In order to compensate for drifts from different origins such as aging, temperature change and fabricational inhomogeneities, a small range of frequency tuning is desired. Frequency tuning of a surface micromachined FBAR by electrostatic actuation has been designed and demonstrated in earlier works (1), (2), in which a vertically movable piezoelectric layer with a top electrode is suspended above a bottom electrode, as shown in Fig. 1. The air gap between the piezoelectric layer and the bottom electrode serves both as a tunable capacitor and as an acoustic isolation. Frequency tuning is realized by applying a DC voltage between the top and bottom electrode to electrostatically displace the suspended piezoelectric layer and thus change the air gap capacitance and the frequency response of the device. This design however only displays a very small continuous tuning range because of the pull-in of the cantilever by electrostatic actuation. In earlier works (2), only 1.9MHz (∼0.03% )o f continuous tuning was achieved before pull-in takes place. It is thus necessary to revise the design to meet the demand of a larger (> 0.2%) continuous tuning range. II. THE PRINCIPLE OF TUNING The electrical characteristics of an FBAR near its funda- mental mode of resonance can be approximated by a lumped- element circuit as shown in Fig. 2(a), in which the series RF signal+DC voltage
Published Version
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