Optimization of acoustic mirrors for solidly mounted BAW resonators

Stefan Marksteiner, J. Kaitila, G. Fattinger, R. Aigner
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引用次数: 90

Abstract

The overall performance of bulk acoustic wave (BAW) filters is dominated by the effective coupling coefficient and the quality factor of the constituting BAW resonators. Whereas the effective coupling coefficient and its dependency on the layer stack is quite accurately modeled with a simple one-dimensional acousto-electric model (e.g. Masonstransmission line model), the prediction and optimization of the resonators quality factor - particularly for solidly mounted resonators (SMR) - completely fails with this model: whereas a calculation of the acoustic reflectance of a standard quarter-wavelength mirror stack leads to theoretical Q-factors well above 10000, experimental SMR devices with this type of mirror show values of typically well below 1000. This discrepancy is commonly explained by either visco-elastic loss in the materials and/or laterally leaking waves leaving the active resonator area. However, we have found a new, far more important loss mechanism relating to shear waves generated in the device. These waves can be created by injection from the resonators border area as well as by reflection/refraction of longitudinal waves at non-perpendicular angle of incidence to a material interface. In this paper, a quantitative methodology for the optimization of the acoustic mirror layer stack will be proposed. The influence of the mirror structure on the trapping of both longitudinal and shear wave energy will be discussed based on this very simple approach. Trade-offs with respect to the other important device parameters, such as effective coupling coefficient, temperature coefficient of frequency (TCF) and purity of the electrical response, are analyzed. The usefulness of this approach for the optimization of resonator Q-values will be proven by experimental results demonstrating Q-factors of 1500 and higher.
固体安装BAW谐振器声反射镜的优化
体声波(BAW)滤波器的整体性能主要取决于构成体声波谐振腔的有效耦合系数和质量因子。虽然有效耦合系数及其对层叠的依赖关系可以用简单的一维声电模型(例如梅森传输线模型)精确地建模,但该模型完全无法预测和优化谐振器质量因子,特别是对于固体安装谐振器(SMR):然而,计算标准四分之一波长反射镜堆叠的声反射率导致理论q因子远高于10000,而具有这种类型反射镜的实验SMR设备显示的值通常远低于1000。这种差异通常可以解释为材料的粘弹性损失和/或离开有源谐振器区域的横向泄漏波。然而,我们发现了一种新的、更重要的与装置中产生的横波有关的损耗机制。这些波可以通过从谐振器边界区域注入产生,也可以通过以非垂直入射角与材料界面的纵波的反射/折射产生。本文提出了一种优化声反射层叠加的定量方法。基于这种非常简单的方法,我们将讨论镜面结构对纵波和横波能量捕获的影响。对其他重要器件参数,如有效耦合系数、频率温度系数(TCF)和电响应纯度进行了权衡分析。该方法对于优化谐振器q值的有用性将通过显示q因子为1500或更高的实验结果来证明。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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