An Investigation of Lateral Modes in FBAR Resonators

Area of Science:

• Acoustics
• Materials Science
• Electrical Engineering

Background:

• Thin Film Bulk Acoustic Resonators (FBARs) are critical components in modern electronics.
• Understanding lateral mode dispersion is essential for optimizing FBAR performance.
• Existing models often lack the precision to capture electrode-material interactions.

Purpose of the Study:

• To develop an analytical model for acoustic wave dispersion in piezoelectric membranes.
• To investigate the impact of electrodes on FBAR lateral mode dispersion.
• To provide a more accurate theoretical framework for FBAR analysis.

Main Methods:

• Solving 2D acoustic wave equations using first principles and constitutive equations for piezoelectric materials.
• Deriving analytical expressions for dispersion near the longitudinal resonant frequency (Fs).
• Introducing an electrical current flow model for acoustic waves within electroded piezoelectric materials.

Main Results:

• The acoustic eigenfunction is a dual wave (longitudinal and shear components).
• Analytical dispersion expressions are derived, offering advantages over numerical methods like FEM.
• Electrode interactions were found to shift the dispersion zero from Fp to Fs, aligning with experimental observations.
• An electrode-loss mechanism dependent on dispersion was identified, with higher dissipation for modes closer to Fs.

Conclusions:

• The developed model accurately describes FBAR lateral mode dispersion, accounting for electrode effects.
• The findings provide a more elegant and useful understanding of dispersion compared to existing methods.
• The study elucidates an electrode-loss mechanism crucial for understanding insertion loss in FBAR filters.