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.
