Thermal analysis of femtosecond laser processing for metal thin films

The temperature distributions in a metal film heated by femtosecond lasers are investigated in this paper. The time scale in which energy transfers from the electrons to the lattice is of the order of a picosecond for metals. Therefore, when the duration of femtosecond laser heating of metal films is of the order of or shorter than a picosecond, substantial nonequilibrium can occur between the electron and lattice temperatures and the metal lattices stay almost thermally undisturbed in this highly nonequilibrium regime. Assuming the system to be one dimensional and insulated at the front and rear surfaces, a parabolic two-temperature model is employed to investigate temperature distributions in metal films irradiated by a femtosecond laser. Using the Laplace transformation technique and the iterative method to solve the nonlinear model, the results reveal that the electron temperature distribution along x decreases with increasing absorption depth, coupling factor and electron specific heat. The high thermal conductivity leads to a decrease in electron temperature with time near the front surface of the film but an increase in electron temperature with time near the rear surface of the film.