A hyperbolic two-step model based finite difference method for studying thermal deformation in a micro thin film heated by ultrashort-pulsed lasers.

摘要

Heat transport through micro thin films plays a very important role in microtechnology applications. Many microelectronic devices have metal thin films as their key components. Microscale heat transfer is also important for the thermal processing of materials, including laser micromachining, laser patterning, laser synthesis and laser surface hardening. Hence, studying the thermal behavior of thin films is essential for predicting the performance of a microelectronic device or for obtaining the desired microstructure. Recently, it has become very popular to use ultrashort-pulsed lasers in thermal processing, which lasers have pulse durations of the order of subpicoseconds to femtoseconds, and these kinds of lasers can limit the undesirable spread of the thermal process zone in the heated sample. However, ultrashort-pulsed lasers can induce ultrafast damage, which occurs after the heating pulse is over. Therefore, in order to apply ultrashort-pulsed lasers successfully, one must study the thermal deformation to prevent the thermal damage.; In the previous research, the parabolic two-step micro heat transport equations have been widely applied in microscale heat transfer. However, when the laser pulse duration is much shorter than the electron-lattice thermal relaxation time for the activation of ballistic behavior in the electron gas, the parabolic two-step model may lose accuracy, as pointed out the in the literature.; It has not been seen in the literature employing the hyperbolic two-step model for studying thermal deformation in a micro thin film exposed to ultrashort-pulsed lasers, which is important for enhancing our understanding of micro heat transfer in a micro thin film exposed to ultrashort-pulsed lasers. Hence, the purpose of this dissertation is to employ the hyperbolic two-step model with temperature-dependent thermal properties for obtaining temperature distribution in a thin film induced by ultrashort-pulsed lasers and to couple with the dynamic equations of motions in order to study thermal deformation in the thin film. To this end, we first develop an implicit finite difference scheme for solving the hyperbolic two-step model with temperature-dependent thermal properties. The scheme is shown to satisfy a discrete analogus of an energy estimate. We then apply it to studying thermal deformations in two-dimensional (2D) thin films exposed to ultrashort-pulsed lasers. In this method, staggered grids are designed, and the coupling effect between lattice temperature and strain rate, as well as the hot electron blast effect in momentum transfer, are considered. As such, this obtained method allows us to avoid non-physical oscillations in the solution.; To demonstrate the applicability of the method, we test three physical cases, (1) 1D double-layered thin film with perfectly contacted interface irradiated by ultrashort-pulsed lasers, (2) 2D single-layered thin film irradiated by ultrashort-pulsed lasers, and (3) 2D double-layered thin film with perfectly contacted interface irradiated by ultrashort-pulsed lasers. Results show that the method is promising and there are some differences between the hyperbolic two-step model and the parabolic model. Particularly, one may see the differences regarding the change in electron temperature (Delta Te/(DeltaTe)max) and the displacement (u) in x direction.