DIRECT MEASUREMENT OF TEMPERATURE DISTRIBUTION IN SILICON BY ULTRASHORT LASER PULSES

摘要

Femtosecond (fs) laser has been demonstrated as a high precision micromachining tool for materials processing because of its advantages of reduced thermal damage to the concerned substrate compared to long-pulsed lasers. It is often believed that most of the absorbed laser energy is carried away by the ablated material, leaving negligible amounts of thermal energy dissipated into the bulk of the remaining material. In contrast, processing with the long pulse lasers leads to a substantial amount of thermal energy to remain in the bulk material, which causes thermal damage to the surface as well as inside the material. During fs laser processing of a silicon substrate, we measured in situ the temperature fields of the substrate using a technique of infrared thermography. It was observed that that a significant portion of laser power (two-thirds or more) was deposited into the silicon substrate instead of being reflected or carried away with the ablated material. This observation improves the common understanding of fs laser machining mechanisms. Simulation results using finite element analysis support the measured data.