Investigation of green femtosecond laser ablation of silicon nitride for different laser fluences and pulse spacing

The study investigates the effect of laser fluence and normalized pulse spacing at two different focal distances of 70 mm and 100 mm on the green femtosecond ablation of Si₃N₄, focusing on removal threshold and the morphology of the ablated craters. As the fluence increases, the ablation depth reaches a plateau. A considerable difference in ablation depth was observed for both focal lengths in the fluence range of 20 to 40 J/cm² compared to values above 40 J/cm². Surface ablation tests with varying normalized pulse spacings were conducted based on the results of the single-point experiments, showing that higher material removal was achieved at greater pulse overlaps when using high laser fluence. However, at low laser fluence, the difference in material removal was less pronounced. Additionally, the surface quality at different laser fluences and pulse spacings was compared, indicating considerably improved quality at higher pulse spacings and lower laser fluences.