Photoacoustic guided-wave and bulk-wave characterization of the mechanical properties of thin film materials

Thin film materials show unique mechanical and thermal properties, and are widely used in semiconductor devices, solar cells, electro-optic coatings, and micro-electromechanical systems (MEMS). They are also used as multifunctional protective hard coatings for machine components such as roller bearings and gears. Thin film materials come in a variety of forms and can be fabricated using different processing techniques. As a consequence, thin film properties vary over a wide range depending on fabrication conditions and parameters. Characterization of thin films is therefore of paramount importance in order to identify their electronic, mechanical, and optical properties.; This dissertation deals with the mechanical characterization of thin film materials. Compared with commonly used mechanical testing methods, many of which are destructive or require specimen contact, photoacoustically generated high-frequency ultrasonic waves are sensitive to the elastic properties of layered materials and can be used for non-contact, nondestructive in-situ characterization of thin film structures. This dissertation discusses theoretical models and applications of laser generated ultrasound for the evaluation of the properties of several types of multi-layered thin film materials.; An axisymmetric two-dimensional photothermoelastic transfer matrix model and a simplified one-dimensional model are developed to model ultrafast laser generated ultrasound in multi-layer structures. The models are benchmarked against well-known theoretical and experimental photoacoustic results. The photothermoelastic models, which include thermal and optical effects into account, are then shown to be suitable for simulating guided and bulk-wave photoacoustic measurements on several multi-layered coatings and thin film structures.; In the experimental part of this work, photoacoustic guided-wave techniques and femtosecond bulk-wave techniques have been applied to systematically characterize diamond-like carbon hard coatings and ultra-thin freestanding Al/Si3 N4 membranes. The measured mechanical properties of these systems using photoacoustic methods are compared with other techniques including line-focus acoustic microscopy and nano-indentation tests and good quantitative agreement is found. The comparisons verify that photoacoustic techniques are effective and accurate in nondestructive characterization of thin film systems.; As an extension of this work, potential applications of ultrafast photoacoustic techniques for the mechanical characterization of nanometer-sized structures including superlattice thin films and quantum dot structures are also discussed, and preliminary results are demonstrated.