SIZE EFFECTS ON NONEQUILIBRIUM LASER-HEATING OF METAL-FILMS

Picosecond and sub-picosecond lasers have become important tools in th e fabrication and study of microstructures. When the laser pulse durat ion becomes comparable with or less than the characteristic time of en ergy exchange among microscopic energy carriers, the excited carriers are no longer in thermal equilibrium with the other carriers, creating a nonequilibrium heating situation. The presence of interfaces in met als provides additional scattering processes for electrons, which in t urn affects the nonequilibrium heating process. This work studies size effects, due to both surface scattering and grain-boundary scattering , on the thermal conductivity and the energy exchange between electron s and the material lattice. A simple formula is established to predict the influence of film thickness, grain size, interface scattering par ameters, and the electron and lattice temperatures on the effective th ermal conductivity of metal thin films. Predictions of the analysis ag ree with the available experimental data. A three-energy-level model i s developed to characterize the energy exchange between electrons and the lattice. This study shows that the size effect reduces the effecti ve thermal conductivity and increases the electron-phonon energy excha nge rate. The results are useful for improving processing quality, int erpreting diagnostic results, and preventing thermal damage of thin fi lms during short-pulse laser heating.