Selective Band Gap to Suppress the Spurious Acoustic Mode in Film Bulk Acoustic Resonator Structures

Abstract

In this work, we investigate numerically the propagation of Lamb waves in a film bulk acoustic resonator (FBAR) structure formed by piezoelectric ZnO layer sandwiched between two Mo electrodes coupled with Bragg reflectors; the system is thus considered as a phononic-crystal (PnC) plate. The aim is to suppress the first-order symmetric Lamb wave mode considered as a spurious mode caused by the establishment of a lateral standing wave due to the reflection at the embedded lateral extremities of the structure; this spurious mode is superposing to the main longitudinal mode resonance of the FBAR. The finite element study, using harmonic and eigen-frequency analyses, is performed on the section of FBAR structure coupled with the PnC. In the presence of PnC, the simulation results show the evidence of a selective band gap where the parasitic mode is prohibited. The quality factor of the FBAR is enhanced by the introduction of the PnC. Indeed, the resonance and antiresonance frequencies passed from 1000 and 980 (without PnC) to 2350 and 1230 (with PnC), respectively. This is accompanied by a decrease in the electromechanical coupling coefficient from 10.60% to 6.61%.

Materials, Design, and Characteristics of Bulk Acoustic Wave Resonator: A Review

“…By adding the PnC structure, a bandgap can be formed on the dispersion curve, and the spurious mode in the bandgap is prohibited ( Figure 13 ). Generally, the position of the bandgap can be adjusted be close to the main resonance frequency by controlling the structural size of the PnC [ 93 ].…”

“… ( a ) Dispersion curve of acoustic waves in the FBAR; the formation of PnC directly bans the TE1 spurious mode generation at the antiresonance frequency [ 93 ]. ( b ) Comparison between the calculated input admittances of FBAR without PnC and with PnC.…”

“…( b ) Comparison between the calculated input admittances of FBAR without PnC and with PnC. The resonant frequency decreases due to the mass-loading effect, and the curve becomes smooth as the Q-value increases [ 93 ]. Reproduced with permission from [ 93 ].…”

Active area optimisation of film bulk acoustic resonator for improving performance parameters

“…Liu et al [6 ] reported that a transverse‐free fundamental resonance is obtained provided that period of grating and thickness is set so that the stop band covers the range of frequencies where the transverse mode resonances occur. This technique is effective to suppress the transverse resonances and improve the quality factor at the cost of reducing the effective electromechanical coupling coefficient [7 ].…”

Asymmetric Lamb Wave Propagation and Mode Isolation in Thin Plate With Spatiotemporal Periodic Stiffness

“…To circumvent these difficulties, filters and wave guides had previously been developed to isolate specific modes of the Lamb wave [6,7], as well as to confine the wave propagation to specific directions [8]. The strategies for mode isolation revolve mainly around the use of phononic crystals, which prevents the transmission of selective wave frequencies through the action of Bragg scattering [9][10][11][12][13][14][15][16][17][18][19][20][21]. To achieve unidirectional transmission, on the other hand, requires breaking either the spatial inversion symmetry or the time-reversal symmetry [22].…”