Study of Lamb acoustic waveguide modes in unsupported polyimide thin films using real-time impulsive stimulated thermal scattering

An extension of the impulsive stimulated thermal scattering method that allows for one-laser-shot characterization of thin film thermal and viscoelastic properties is described. The technique is demonstrated through an extensive study of free-standing thin (∼1 μm) polyimide films. An exact, analytical solution for excitation of the waveguide acoustic modes is given and used to extract the viscoelastic properties of the films. It is also shown how the technique can be used to detect film delaminations by comparing results from free standing films to results from films tightly bound to silicon substrates.