Abstract In this study, a high repetition rate laser ultrasonic system is developed for noncontact detection of silicon wafer crack. A high repetition rate pulse laser is used to generate two narrowband ultrasonic waves on a target silicon wafer, whereas a high-speed laser Doppler vibrometer is used to measure the corresponding ultrasonic responses. Nonlinear interaction occurs between the two generated ultrasonic waves due to crack formation, producing nonlinear ultrasonic modulation at the sum and difference of the two excitation frequencies. Outlier analysis is performed on the amplitudes of the nonlinear modulation components, and a fatigue crack is identified when the amplitudes of the nonlinear modulation components exceed a user specified threshold. The uniqueness of this study is as follows. (1) A noncontact, high-speed, and scanning laser ultrasonic system is developed using a high repetition rate laser for ultrasonic generation and a laser Doppler vibrometer for sensing as well as scanning the two laser beams simultaneously in a pitch-catch mode. (2) Two narrowband ultrasonic waves up to 1 MHz are simultaneously generated from a single laser pulse. (3) Micro cracks with widths of less than 10 µm are successfully detected by extracting the crack induced nonlinear ultrasonic modulation components. (4) The probability of detection of the proposed crack detection technique is evaluated. The performance of the proposed technique is experimentally validated using eight silicon wafer specimens with varying crack lengths. Results show that the proposed technique yields a 90% probability of detection in silicon wafers when the crack length is over 3.77 mm.

中文翻译：

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基于高重复频率脉冲激光诱导非线性超声调制的硅片裂纹检测

摘要 在这项研究中，开发了一种高重复频率的激光超声系统，用于硅片裂纹的非接触式检测。高重复频率脉冲激光器用于在目标硅晶片上产生两个窄带超声波，而高速激光多普勒测振仪用于测量相应的超声波响应。由于裂纹的形成，两个产生的超声波之间发生非线性相互作用，在两个激励频率的和和差处产生非线性超声调制。对非线性调制分量的幅度进行异常值分析，当非线性调制分量的幅度超过用户指定的阈值时，识别疲劳裂纹。本研究的独特之处如下。(1) 一种非接触式、高速、扫描激光超声系统是使用高重复频率的激光产生超声和激光多普勒测振仪进行传感和扫描，同时以一发一收模式扫描两个激光束。(2) 单个激光脉冲同时产生两个高达 1 MHz 的窄带超声波。(3) 通过提取裂纹引起的非线性超声调制分量，成功检测到宽度小于 10 µm 的微裂纹。(4) 评估所提出的裂纹检测技术的检测概率。所提出的技术的性能使用具有不同裂纹长度的八个硅晶片试样进行了实验验证。结果表明，当裂纹长度超过 3.77 毫米时，所提出的技术在硅晶片中产生 90% 的检测概率。