High-energy femtosecond pulsed laser micromachining of thin film deposited silicon in self-focused air medium

The ultrashort pulsed laser ablation of silicon wafers deposited with 1 μm thin film of SiC was performed in air medium using a 1 mJ, 120 fs, 800 nm Ti:sapphire laser, the objective being to determine the self-focusing capability of air to produce nanostructures. The effects of pulse energy, the number of pulses, pulse repetition rate and x–y translation speed on the size, shape, and thermal damage of holes and grooves were evaluated. In addition, a 200 ns pulsed, 1064 nm Nd:yttritium–aluminum–garnet laser was used to process grooves and the results were compared against those processed by the femtosecond pulses. Results show that holes as small as 1 μm were produced with femtosecond pulses using the self-action property of air. Despite the capability of air medium to produce tiny holes of a few micron diameters, the shape of the hole is highly asymmetrical due to the beam profile distortion. The critical laser power for nonlinear effects of air to become effective was calculated as 40 MW. However, both the beneficial and deleterious effects of air were not found when the peak power of laser beam was under 2 GW. A comparison of femtosecond pulsed laser ablation with nanosecond pulsed laser ablation demonstrated the advantages of small size, minimal thermal damage and cleanliness for ultrashort pulses.