高次谐波体声波谐振器的机械品质因数

机械品质因数Q_M是高次谐波体声波谐振器(High-overtone Bulk Acoustic Resonator,HBAR)一个关键的特性参数。首次较系统地研究了Q_M随构成HBAR的3个组成部分(基底、压电薄膜和电极)的结构参数(厚度)和性能参数(特性阻抗与机械衰减因子)的变化规律。在谐振频率附近,将HBAR的分布参数等效电路简化为集总参数等效电路,首次用解析表达式给出它们的变化规律,分析了Q_M在给定频率最近谐振点的变化情况。结果表明,固定压电层厚度,Q_M随基底厚度的连续增加略呈振荡(非单调)上升,当基底厚度很大时趋于基底材料的机械品质因数;固定基底厚度,Q_M随压电层厚度的连续增加呈波浪式下降;选择低损耗的蓝宝石或YAG作为基底可以获得较大的Q_M值;电极的损耗必须考虑,它会降低Q_M值;与Au电极相比,具有较低损耗的A1电极选择适当厚度可以获得较高的Q_M值;此外,Q_M随频率的增加呈下降趋势。上述的结果为HBAR的优化设计提供了相应的理论依据。根据我们对K_eff²的研究,Q_M与K_eff²的变化规律往往是相悖的,因此在设计HBAR时要在这两者作适当的权衡。

Mechanical quality factor of high-overtone bulk acoustic resonator

Mechanical quality factor Q_M is a key characteristic parameter of High-overtone bulk acoustic resonator (HBAR). The effects of structure parameter (thickness) and performance parameters (characteristic impedance and mechanical attenuation factor) of substrate, piezoelectric film and electrode composing a HBAR on Q_M are carried out firstly. The relationships between Q_M and these parameters are obtained by a lumped parameter equivalent circuit instead of distributed parameter equivalent circuit near the resonance frequency, and the analytical expressions of Q_M are given first. The results show that Q_M increases non-monotonically with the continuous increase of the substrate thickness for HBAR with certain piezoelectric film thickness, and it approaches to the mechanical quality factor of the substrate as the substrate thickness is large. Q_M decreases wavily with the continuous increase of the piezoelectric film thickness for HBAR with certain substrate thickness. Sapphire and YAG with low mechanical loss are appropriate as the substrate to get a larger Q_M. The electrode loss must be considered since it can reduce Q_M. Compared with Au electrode, A1 electrode with lower loss can obtain higher Q_M when the appropriate electrode thickness is selected. In addition, Q_M decreases with the increase of frequency. These results provide the theoretical basis for optimizing the parameters of HBAR and show that trade-offs between Q_M and K_eff2 must be made in the design because their changes are often inconsistent.