Research Of Ultrafast Dynamics In Copper Thin Films By Femtosecond Laser

Cu films are widely used in MEMS field and magnetic multi-layers films, so it is very important to investigate the ultrafast dynamics process of the material. Because of the ultrashort pulse width and ultrahigh energy density, femtosecond laser provides methods for studying dynamics of materials. The ultrafast dynamics of the Cu films is investigated by the femtosecond laser pump-probe measurement technique.Cu films with different parameters were deposited by magnetron sputtering technique, and a part of them were annealed at 300℃, 500℃and 700℃. The surface morphology of Cu films was investigated by scanning electron microscope (SEM) and atom force microscope (AFM). The results show that the surface of Cu film annealed at 300℃is affected hardly and the grains don't grow significantly. Islands structure and crystal were observed in the surface of the films annealed at higher temperature and the level of crystal was higher.The transient reflectivity changes of Cu films were measured by femtosecond laser pump-probe measurement technique in order to investigate the influence of annealing process,multi-layers and thickness. The results show that the attenuation time decreases with the increasing anneal temperature. The influence of Co and Au on theΔR/R curves of Cu films is different. The maximum ofΔR/R curve for Cu/Au composite film is greater than that for Cu single film. The Co layer has the effect of absorbing heat and protecting the surface layer. The thickness of films don't change the shape ofΔR/R curve, only the peak and equilibrium values are influenced. The energy transport between electron and substrate was investigated. The influence of substrate onΔR/R curves is greatest when the film's thickness is on the order of light penetration depth and the laser energy is high. It is easier for silicon than K9 glass to transfer heat from electron system in Cu film to the substrate, so the electron temperature in the film decreases. The peak and equilibrium values ofΔR/R curve for Cu/Si are lower than that for Cu/glass.The ultrafast dynamics of Cu films was simulated using two-temperature model. The results show that the electron temperature and lattice temperature are inconsistent in the initial moment of laser heating. The ballistic transport of electron has important effect on the maximum of electron temperature, but has little influence on the equilibrium temperature and attenuation time. When the depth increases, the maximum of electron temperature decreases. The electron relaxation time of experimental result is significantly shorter than that of simulation result by comparing the experiment and simulation. Because of the ballistic motion, hot electron diffusion and energy transport between the Cu film and substrate, the cooling process of electron system in the Cu film is speeded up.