USE OF GENETIC ALGORITHMS FOR THE SIMULTANEOUS ESTIMATION OF ELECTRON-PHONON COUPLING FACTOR AND INTERFACIAL THERMAL RESISTANCE OF METALLIC THIN FILMS

摘要

With the rapid development of the micro-electronic devices and the application of the ultra-fast laser micro-machining technique, the study of the non-equilibrium energy transfer between the electrons and phonons in metallic nano-films has become a question of great concern. On the other hand, the interfacial thermal resistance between the film and substrate is commonly considered as a major factor of restraining the overall performance of the electric devices. In this paper, we proposed a genetic algorithm (GA) method to simultaneously determine the electron-phonon coupling factor and the interfacial thermal resistance of the metallic thin films. The GA method is shown to be a great searching tool for the best fitting of the experimental data. It is the first time to use GA method to study the ultra-fast electron-phonon coupling and interfacial thermal resistance. Combining the GA method with the femtosecond transient thermoreflectance (TTR) measurements, the transient energy transfer in metals can be quantitatively studied. In the experiments, the time resolution of femtosecond can be guaranteed by a pump-probe technique. An intense laser beam is focused on the rear surface to heat the film sample. And another weak laser beam is focused at the very spot of the front surface to monitor the change of the reflectivity which is proportional to the change of the electron temperature. By controlling the optical path delay between these two laser beams precisely, the whole electron temperature profile can be scanned. The micro parabolic two-step (PTS) model is used to calculate the change of the electron temperature varied with time. In this theoretical model, the electron-phonon coupling factor and the interfacial thermal resistance between the film sample and the substrate are chosen as the objective parameters. The GA method is applied to search for the best combination of these two parameters which can minimize the difference between the experimental and calculated electron temperature profiles. The measured results are close to the reported values. It demonstrates that the GA method is a useful tool for the thermal characterization of metallic thin films.