A Design Approach for High-Q FBARs With a Dual-Step Frame

This paper presents a new approach for the design of film bulk acoustic resonators (FBARs) with high-quality factor Q_a at antiresonant frequency f_a. Lamb waves are among the main contributors to the loss of energy at f_a and the deterioration of Q_a. The targeted approach is an optimization process of a dual-step frame structure to alleviate the problem with Lamb waves at f_a. Such a frame is a combination of two neighboring steps that have different acoustic impedances reflecting the two Lamb modes that carry the largest amount of energy. The dimensions of these steps are computed using the diffraction grating method. To do so, the dispersion curves of Lamb waves and their associated power flows are calculated using a global matrix method and the acoustic Poynting's Theorem. This results in an enhancement in Q_a of nearly 82% for an FBAR with a dual-step frame in comparison with an FBAR with no frame, as well as a smoother electrical response, at frequencies above fa but below the resonant frequency f_r spurious resonances are moderately increased.

I. Introduction

Bulk acoustic wave (BAW) resonators, implemented as thin film bulk acoustic resonator (FBAR) and solidly mounted resonator (SMR) devices, have been the key technology for RF filtering in the gigahertz frequency range in mobile telecommunication systems due to their low cost, small size, and better performance compared to other types of technology such as ceramic and surface acoustic wave filters [1], [2]. However, the evolution of these systems has posed a higher demand on the resonator performance. That is, the BAW resonator must have higher quality factor at resonant frequency and antiresonant frequency to achieve better filter selectivity and lower insertion loss. This is quite challenging as the losses increase with the increase of the operational frequency [3]. In the resonator, there are several loss mechanisms accounting for the degradation of values, such as ohmic loss, acoustic attenuation, dielectric loss, and loss due to nonplanarity of the layers. Besides them, there are other losses due to spurious modes and lateral acoustic leakage, both caused by the inevitable propagation of Lamb waves in the resonator. Some types of these losses depend on the quality of materials and fabrication process, while the rest depend on the resonator geometry [1], [4], [5].