STTR Phase I: High-Frequency Laser Ultrasonic Inspection System for In-Situ Characterization of Nanoscale Structures

该小企业技术转让（STTR）第一阶段研究提案将开发一种基于高频激光的超声系统，适用于各种微米和纳米级薄膜和涂层的表征。该系统将包括一个强度调制的连续波激光器，用于激发极窄带宽的表面声波，和一种新型的超外差锁定干涉仪。这种组合允许比使用常规脉冲激光源可以实现的信噪比更高的信噪比，同时允许系统成本的大幅降低。由干涉仪检测到的高频（GHz）位移信号将被光学降频到固定的中频，从而允许使用低频检测电子器件。该干涉仪还使用了一种新颖的正交检测方法，并能够在半导体工业中使用的光学平坦薄膜和环境屏障涂层和沉积在未抛光的光学粗糙基底上的磨损保护涂层上进行高灵敏度测量。拟议的激光超声波系统将有广泛的商业应用，包括半导体行业的铜厚度测量，环境和热障涂层的无损检测，以及磨损保护涂层的检测。与现有系统相比，该系统提供了两个关键优点：由于该系统使用低成本光纤耦合激发激光器而不是通常采用的飞秒和皮秒脉冲激光器的事实，预计该系统的成本将大幅降低，并且该系统是紧凑的，其中产生和检测激光器都是光纤耦合的，使得其对于原位测量是有吸引力的。此外，该检测系统是鲁棒的，具有宽松的对准约束，并且可以在光学粗糙表面之外操作。

This Small Business Technology Transfer (STTR) Phase I research proposal will develop a high frequency laser based ultrasonic system suitable for the characterization of a wide range of micro- and nano-scale thin films and coatings. The system will incorporate an intensity-modulated continuous wave laser for the excitation of extremely narrow bandwidth surface acoustic waves, and a novel superheterodyne lock-in interferometer. This combination allows for higher signal-to-noise ratio than can be achieved using conventional pulsed laser sources, while at the same time allowing for a substantial reduction in system cost. The high frequency (GHz) displacement signal detected by the interferometer will be frequency downshifted optically to a fixed intermediate frequency, thereby allowing for the use of low frequency detection electronics. The interferometer also uses a novel quadrature detection approach, and is capable of making high sensitivity measurements on both optically flat thin films used in the semiconductor industry and environmental barrier coatings and wear protective coatings deposited on unpolished, optically rough substrates. The proposed laser based ultrasonic system will have a broad range of commercial applications including copper thickness measurement for the semiconductor industry, non-destructive testing of environmental and thermal barrier coatings, and inspection of wear protective coatings. The system offers to two key advantages over existing systems: the cost of the system is expected to be substantially lowered due to the fact that the system uses a low cost fiber coupled excitation laser rather than the femto- and picosecond pulsed lasers typically employed, and the system is compact, with both the generation and detection lasers fiber coupled, making it attractive for in-situ measurements. Furthermore, the detection system is robust, has relaxed alignment constraints, and can operate off of optically rough surfaces.