Gigahertz vibration measurement and analysis of FBARs with square and pentagon electrodes

Film bulk acoustic wave resonator (FBAR) technology has been widely used in 5G/6G communications due to its high quality-factor (Q). However, with the increase of center frequency and Q value, many unwanted vibration modes will appear when the resonator is working, which is easy to produce large spurious modes. To investigate the impact of the acoustic boundary on the dispersion characteristics and spurious modes of FBAR devices under high-frequency vibrations, it is necessary to use ultra-high frequency femtosecond pulse laser interferometer to characterize the vibration phenomenon. In this study, the vibration modes of FBAR devices near the resonance frequency of up to 2.6 GHz is measured. The measured results closely match the theoretical model established by finite element simulation. Based on these measurements, an algorithm is developed to calculate FBAR 2D-DFT mode diagrams and dispersion curves. It is found that non-parallel boundaries suppress lateral spurious modes, which extend the path of acoustic waves. This finding verifies the significant application potential of this measurement system in ultra-high-frequency thin-film acoustic wave devices.