A Design Approach for High-Q FBARs with Double Frames

Film bulk acoustic resonator (FBAR) has attracted great attention due to its widespread applications for radio frequency front-end (RFFE) systems. To improve the performance of the wireless module, high-performance FBARs with large effective electrotechnical coupling coefficient $\left( {k_{eff}^2} \right)$, high quality factor (Q) and spurious-free are always highly demanded. However, the spurious modes near the main resonance and energy leakage at the edge of the device degrade the performance, and many methods have been developed to overcome these issues. The development of microfabrication technology, especially the advancement of the film transfer process-based fabrication method, makes it possible to manufacture optimized structures with framed electrodes on both faces of the suspended piezoelectric films. In this paper, FBARs operating at ~3.5 GHz are designed and optimized by the finite element method (FEM). New design that employs double-faces raised frames to suppress the lamb waves are demonstrated. The optimized structure leads to approximately 220% enhancement of the quality factor at anti-resonance frequency (f
a)
compared with traditional structure, a smoother impedance curve is obtained, at the cost of relatively slight decrement of $k_{eff}^2$.