Measurement of thermal diffusivity of super lattice film by pico second pulse laser with variable wave length.

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We developed the equipment to measure the thermal diffusivity of thin film of 10 to 1000nm thickness. The surface of sample is heated by pulse laser. From the temperature decay of the irradiated surface after pulse heating, the thermal diffusivity of sample is derived. To avoid the effect of substrate under the thin film, only information of surface layer is required. This corresponds to initial time region less than 100 pico second of temperature decay response after laser pulse. The Ti- Sapphire short pulse laser of 80 femto-second half-width time was used as the pulse laser source. The temperature response was measured by the change of reflection coefficient of surface of sample. The thermal diffusivity of Mo film was measured. Thermal diffusivity of Mo film is significantly small compared with bulk sample. Meanwhile, we developed a simulation program which mimics Laser flash method based on the one dimensional lattice model with harmonic potentials. One dimensionally arrayed atoms, which interacts each other via harmonic potential, were produced and then the most left sided atom was forced to oscillate in a fixed period. After that, the movements of all the atoms were computed by the molecular dynamics method to clarify the heat pulse transfer in this system. In the present study, the harmonic potential of the atoms was assumed to be common and only the arrangements of the masses of atoms were changed. We examined two types of model, i.e., homogeneous (all masses are same) and super lattice systems (two kinds of masses were repeated periodically). Many simulation runs were performed under the same conditions and the results showed that the dumping of the heat pulse was reduced drastically in the super lattice system in which heavy atoms were dispersed periodically compared to the case of the homogeneous system.