Study of Scanning Parameters and Variable Focal Adjustment in Multi-Layer Laser Ablation of Silicon Nitride

This study investigated the effects of laser processing parameters on ablation performance during multi-layer femtosecond laser ablation of silicon nitride ceramic. Initial experiments were conducted to establish a baseline and identify challenges associated with the volume ablation. The results revealed significant thermal damage, including local melting, primarily due to excessive heat accumulation caused by laser beam defocusing intensity. Subsequently, a systematic parameter study was conducted to analyze the influence of the laser parameters on the ablation depth and surface roughness. The findings indicate that the lower scanning speeds and smaller hatch distances result in higher energy densities, leading to greater material removal but also increased surface roughness due to enhanced melt formation. The scanning direction was found to have a margin impact on the ablation depth but significantly influenced the surface roughness, with a one-directional mixed strategy yielding improved surface quality. Furthermore, as the number of scanning layers increased, the ablation depth also increased but remained lower than the expected values, showing an average decrease of 15% and 36%, respectively. Additionally, the focal position adjustments with a variable value were implemented to counteract the defocusing effect, demonstrating a consistent material removal. High-resolution SEM and FIB cross-sectional images revealed the presence of a recast layer, further confirming that excessive heat accumulation can degrade surface quality in multi-layer ablation.