Thermal and non-thermal ablation mechanisms in crystalline silicon by femtosecond laser pulses: classical approach of the carrier density two temperature model

Abstract

Laser micromachining has attracted considerable interest because of its wide range of applications across nearly all manufacturing sectors and mostly in semiconductors such as silicon. However, modern micro-manufacturing demands progressive product miniaturization, high accuracy, and high-precision material removal. For this purpose, a fundamental study of the interaction between ultrashort laser pulses and silicon will be valuable for studying ablation characteristics and ablation performance. The femtosecond laser pulse interaction with the silicon is divided into five parts: (a) the interaction of laser light with the carriers, (b) variation of the carrier density and carrier temperature, (c) energy exchange between carriers and lattice, (d) thermomechanical response of the material, and (e) ablation. The evolution of the carrier density and carrier-lattice energy coupling equation is solved simultaneously to determine the optimum value of the ablation width and ablation depth of femtosecond laser pulses on the silicon. The first time, 2D axial symmetry thermal and non-thermal ablation profiles were compared with the experimental result at fluence ranging from 0.75 to 9 J cm−2 at the wavelength of 515 nm and 180 fs laser on the silicon sample. A comparative study of damage thresholds from experiments and simulations is presented. The concordance between model calculations and experimental data demonstrates that fs laser ablation is thermal in nature in low fluence regime, whereas it is non- thermal in a high-fluence regime. Fundamental information such as the time evolution of the carrier density, carrier temperature evolution, and lattice temperature evolution can be obtained from the simulation results.