Modeling of laser assisted direct imprinting process based on pulsed laser heating and elastodynamic theory

In this study, the mechanism of LADI is investigated theoretically and experimentally. First of all, a numerical simulation of the pulsed laser heating is developed. The simulation is based on the laser-material interaction and a ID thermal analysis with both solid and a molten liquid under consideration. For given laser fluence, pulse duration (30ns, KrF 248nm excimer laser), and material properties, the complete history of temperature and melting depth during the laser heating is obtained. Secondly, since the surface melting is happening with a short period of time, the imprinting process is modeled based as a quick releasing of pre-loaded strain in the quartz mold and silicon sample. Such a transient behavior can be characterized by elastodynamics and wave motion. Based on the analysis, the imprinting velocity of the quartz surface is equal to the fast released pressure divided by acoustic impedance of quartz. By conjunction of the pulsed laser heating simulation and the surface movement equation, the relationship between the applied pre-loaded pressure and the imprinting depth can be quantitative determined provided the laser characteristics and material properties are given. It is found that at lower contact pressure, the imprinting depth is almost linear proportional to the contact pressure, while at higher contact pressure is most determined by the laser fluence. This new theoretical model not only provides a good insight to the fundamental mechanism of LADI but also a useful tool for quantitative control or optimization of LADI processes.