Ultrafast Heating Characteristics in Multi-Layer Metal Film Assembly Under Femtosecond Laser Pulses Irradiation

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In recent years, ultrashort pulsed laser micromachining of multi-layer metal film assembly had attracted great attention because the multi-layer configuration can be well applied for satisfaction of thermal, optical and electronic requirements in development of MEMs, photoelectric equipments and biochips (Liu, 2007). Generally, the thermal properties for metals are physically originated from the collision mechanisms for electron-electron and electron-phonon in the metal targets. For the multi-layer metal film assembly, the thermal properties, such as the electron-phonon coupling strength can actually vary significantly for different layers of the assembly, so the heating of muti-layer film assembly would take on various characteristics for different padding layer configurations. In this article, the ultrafast heating characteristics in multi-layer metal film assemblies irradiated by femtosecond laser pulse were investigated by numerical simulations. The effect of different padding layer configurations on the ultrafast thermalization characteristics for the multi-layer metal film assemblies are well discussed. The ultrafast heat transfer processes in the layered metal film systems after the femtosecond pulse excitation are described based on the two temperature model (TTM), in which the electron and phonon is considered at two different temperatures, and heat transfer is mainly due to the hot electron diffusion among the sub-electron system and the electron energy transfer to the local lattice characterized by the electron-phonon coupling strength. The thermal properties for the respective metal film layers and the optical surface reflectivity are all defined as temperature dependent parameters in order to well explore the ultrafast heating characrastics of the multi-layer metal assemblies. The coupling two temperature equations are calculated by the Finite Element Method (FEM) with respect to temperature dependent thermal and optical properties. The ultrafast two-dimension (2-D) temperature field evolutions for electron and phonon subsystems in the multi-layer metal film assemblies are obtained, which show that the electron and phonon temperature field distributions can be largely effected by adjusting padding layer configurations. The physical origins for the discrepant temperature field distributions in multi-layer film assemblies are analyzed in details. It indicates that electron-phonon coupling strength and phonon thermal capacity play key roles in determining the temperature field distributions of the multi-layer film assembly.