q       Flim Bulk Acoustic-Wave Resonator (FBAR)

Piezoelectric thin films convert electrical energy into mechanical energy and vice versa. Film Bulk Acoustic Resonator (FBAR) consists of a piezoelectric thin film sandwiched by two metal layers. A resonance condition occurs if the thickness of piezoelectric thin film (d) is equal to an integer multiple of a half of the wavelength (lres). The fundamental resonant frequency (Fres=1/lres) is then inversely proportional to the thickness of the piezoelectric material used, and is equal to Va/2d where Va is an acoustic velocity at the resonant frequency (Fig. 1).
















A bulk-micromachined FBAR with Thickness Field Excitation (Error! Reference source not found.) uses a z-directed electric field to generate z-propagating longitudinal or compressive wave. In an LFE-FBAR, the applied electric field is in y-direction, and the shear acoustic wave (excited by the lateral electric field) propagates in z-direction, as illustrated in Error! Reference source not found..




Both TFE-FBAR and LFE-FBAR can equivalent be represented with the equivalent circuit, shown in Fig. , called Butterworth-Van Dyke (BVD) circuit.  The resonator is modeled by a constant “clamped” capacitance Co in parallel with an acoustic or “motional” arm that consists of motional capacitance Cm, motional inductance Lm, and motional resistance Rm. The Co is the electrical capacitance between the two electrodes through which the electric field is applied.  The motional components (Cm, Lm and Rm) model electromechanical response of a piezoelectric material.












The resonant frequency of a TFE FBAR can be tuned by removing a portion of the resonator support layer as shown in Fig. . The removal of the support layer reduces the mass loading effect of the resonator, and increases the resonant frequency.













Both of the resonant frequencies are measured to vary as the mass loading changes. When the 0.9mm thick Si3N4 support layer is removed by Reactive Ion Etching from the FBAR backside, the resonant frequency of the FBAR increases from 1.1 to 1.5 GHz, around 40% shift. In Fig. 6, we show the resonant frequency dependence on the Si3N4 support layer thickness measured on three FBARs. A similar experiment has been performed on an FBAR with parylene as its support layer, and the results are shown in Fig. 7. As can be seen by comparing Figs. 6 and 7, parylene has a much smaller mass loading effect on the frequency shift than Si3N4. This is due to the fact that Si3N4 has a larger mass density and much higher Young’s Modulus than parylene, and affects the FBAR resonant frequency much more strongly.
































Electromechanical coupling constant kt2 is related to the difference of the two resonant frequencies as




kt2 increases as the Si3N4 is removed from the FBAR backside. The change of kt2 as a function of the mass loading by the Si3N4 support layer are plotted in Fig. 8, respectively.


An air-backed, Al/ZnO/Al film-bulk-acoustic resonator (FBAR) that is free-standing by itself has been fabricated. Unlike a conventional FBAR structure, the newly fabricated resonator doesn’t employ any supporting layer below or above it, but the whole resonator body (consisting of ZnO piezoelectric layer sandwiched by two aluminum layers) suspends by itself in the air. Some SEM pictures of the fabricated FBAR devices with various shapes are shown in Fig. 9. Figure 10 shows SEM picture of the Al bridges that act as leading electrodes and also to hold the FBAR (which free stands in air). Picture of an FBAR viewed from its backside is shown in Fig. 11.



Fig. 10 SEM picture of the 0.3 mm thick Al bridge.


Fig. 11 Backside view of an air-backed FBAR.





























S11 measurements of an FBAR with its underneath Si3N4 layer at 0.9mm, 0.6mm, 0.3mm, 0mm and in the case of the ZnO being patterned from the wafer backside are shown in Fig. 12.





















The quality factor Qs at the series resonant frequency in the free-standing air-backed FBAR is calculated to be 1,322. It is about six times larger than that in the FBAR with 0.9mm thick Si3N4 as a support diaphragm. The quality factor Qp at the parallel resonant frequency in the case (e) is calculated to be 513, three times that in the case (a). Figure of Merit (FOM) of an FBAR is defined as the product of Q and kt2. Insertion loss in an FBAR-based filter is inversely proportional to the FOM of the FBARs. In the free-standing air-backed FBAR, we have increased kt2 twice and the quality factor six times, and filters made out of this kind of resonator are expected to have larger bandwidth and much lower insertion loss compared to that made by an FBAR built on a support diaphragm