高次谐波体声波谐振器的有效机电耦合系数

高次谐波体声波谐振器HBAR (High-overtone Bulk Acoustic Resonator)由基底、压电薄膜和上下电极组成,系统地研究了它们的结构参数(厚度)和性能参数(特性阻抗)对HBAR的重要性能参数有效机电耦合系数K_eff2的影响。在谐振频率附近,通过将HBAR的分布参数等效电路简化为集总参数等效电路得到了它们之间的关系表达式,分析了K_eff2在所关心频率最近谐振点的变化情况。结果表明,保持压电薄膜厚度不变,连续增加基底厚度,K_eff2呈振荡(非单调)下降,当基底厚度达到一定值时K_eff2与厚度成反比下降;保持基底厚度不变,连续增加压电薄膜厚度,K_eff2的峰值随基底和压电层的特性阻抗之比增加快速下降,到达极小值后缓慢增加;选择低阻抗的熔融石英作为基底可以获得较大的K_eff2;与Al电极相比,Au电极选择适当厚度可以获得较高的K_eff2。上述揭示的一些规律为HBAR的优化设计提供了理论依据。 

Effective electromechanical coupling coefficient of high-overtone bulk acoustic resonator

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 K_eff2 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,K_eff2 of the nearest resonance point at the given frequency is analyzed.The results show that when the piezoelectric film thickness is fixed,K_eff2 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 K_eff2 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 K_eff2.Comparing with Al electrode,Au electrode can obtain higher K_eff2 when the appropriate electrode thickness is selected.The revealed laws above mentioned provide the theoretical basis for optimizing parameters of HBAR.