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].