Abstract Resonating piezoelectric devices, such as aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducers (PMUTs), display superior performance to previous generations of resonating microelectromechanical systems (MEMS). However, the quality of the piezoelectric thin film can greatly impact operating characteristics, such as the resonant frequency. Several AlN PMUT devices fabricated on the same silicon wafer exhibited a range of resonance frequencies (400-600 kHz), indicating that there is nonuniformity across the processed wafer. AlN film nonuniformity is likely introduced during the reactive sputtering process. Two key parameters identified as influencing the resonance frequency include: (i) the membrane diameter and (ii) residual stress. This work focuses on the residual stress, and uses X-ray diffraction technique (XRD) to determine the in-plane biaxial residual stress values, as a function of die position on the wafer. Results show that there is a compressive stress gradient along the wafer, ranging from -357 MPa to -56 MPa. A plot of in-plane biaxial residual stress as a function of resonance frequency shows a relation between the measured stress and frequency. As resonating piezoelectric devices require well defined operating frequencies and bandwidths, this work demonstrates the importance of studying not only global stresses, but also local residual stresses.

中文翻译：

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残余应力对谐振压电器件的影响

摘要 谐振压电器件，例如氮化铝 (AlN) 压电微机械超声换能器 (PMUT)，表现出优于前几代谐振微机电系统 (MEMS) 的性能。然而，压电薄膜的质量会极大地影响工作特性，例如谐振频率。在同一硅晶片上制造的几个 AlN PMUT 器件表现出一系列共振频率 (400-600 kHz)，表明处理过的晶片存在不均匀性。在反应溅射过程中可能会引入 AlN 膜的不均匀性。确定影响共振频率的两个关键参数包括：(i) 膜直径和 (ii) 残余应力。这项工作侧重于残余应力，并使用 X 射线衍射技术 (XRD) 确定面内双轴残余应力值，作为晶片上芯片位置的函数。结果表明，沿晶片存在压应力梯度，范围从 -357 MPa 到 -56 MPa。作为共振频率函数的面内双轴残余应力图显示了测得的应力与频率之间的关系。由于谐振压电设备需要明确定义的工作频率和带宽，这项工作证明了不仅研究全局应力而且研究局部残余应力的重要性。作为共振频率函数的面内双轴残余应力图显示了测得的应力与频率之间的关系。由于谐振压电设备需要明确定义的工作频率和带宽，这项工作证明了不仅研究全局应力而且研究局部残余应力的重要性。作为共振频率函数的面内双轴残余应力图显示了测得的应力与频率之间的关系。由于谐振压电设备需要明确定义的工作频率和带宽，这项工作证明了不仅研究全局应力而且研究局部残余应力的重要性。