This project seeks to develop a new laser based technique for the generation of finite amplitude surface acoustic stress waves. The proposed technique would facilitate the understanding of the nonlinear elastic response of thin film materials at ultrasonic frequencies. This development would lead to quantitative measurement of various nonlinear elastic properties including; decohesion and fracture strength of thin film interfaces, using nonlinear elastic stress waves. The technique uses a laser source focused to line on a sample surface to generate plane surface acoustic stress waves through the thermoelastic effect. The laser generated stress waves are monitored using a path stabilized Michelson interferometer. By scanning the laser line source on the sample surface at the phase velocity of the surface wave, the stress wave amplitude is amplified continuously with distance. The technique would enable loading thin film materials with strain amplitudes of the order of 0.01 with strain rates greater than one million per second. The contact-free nature of the technique allows for accurate modeling of the processes of stress wave generation and propagation, leading to reliable extraction of thin film material properties.
If the project is successful, the proposed technique would enable the quantitative assessment of the mechanical reliability and stability of film based small scale structures including MEMS and NEMS devices. These properties are critical to their successful integration into more complex microelectronic devices. Furthermore, the project is integrated with various educational activities including, graduate course enrichment, graduate student training, and outreach activities to high school students.
