Abstract:
High-overtone bulk acoustic resonator,HBAR is composed of substrate,piezoelectric film and the upper and lower electrodes,the influence of their structure parameter,thickness and performance parameter,characteristic impedance on effective electromechanical coupling coefficient
Keff2 of HBAR is studied systematically.A concise expression is obtained by a lumped parameter equivalent circuit instead of distributed parameter equivalent circuit near the resonant frequency,
Keff2 of the nearest resonance point at the given frequency is analyzed.The results show that when the piezoelectric film thickness is fixed,
Keff2 is found rapidly declined with continuous increasing of the substrate thickness by oscillation(nonmonotonous) way,and decreases inversely proportion to the thickness when the substrate thickness reaches a certain value.When the substrate thickness is fixed with continuous increase of the piezoelectric film thickness,the maximum of
Keff2 decreases rapidly before reaching the minimum value,and later increases slowly with the ratio of the characteristic impedance of the substrate to the piezoelectric layer increasing.Fused silica with low impedance is appropriate as the substrate of HBAR to get a larger
Keff2.Comparing with Al electrode,Au electrode can obtain higher
Keff2 when the appropriate electrode thickness is selected.The revealed laws above mentioned provide the theoretical basis for optimizing parameters of HBAR.