Dynamic response of thin films on substrates subjected to femtosecond laser pulses

Femtosecond laser pulses can be used to precisely debond the interface between transparent films and substrates, thus providing a critical tool for interface characterization. This paper presents a model for the dynamic response of the film in the debonded region due to excitation from the laser pulse. The model illustrates that the timescale for transfer of energy to the film is typically much smaller than the inertial timescale associated with film bending; in such cases, the film deflection is governed entirely by the impulse delivered to the film and is insensitive to the details of the pressure generated by the laser pulse. Results for the maximum stress in the film and the dynamic energy release rate for debonding are used to generate regime maps that indicate failure modes (e.g. film fracture or interface debonding) as a function of pulse characteristics and film properties. The model is shown to exhibit good agreement with experiments, and hence provides a simple framework to design laser experiments to generate precisely defined interface flaws while avoiding film failures.