Optimizing repetition rate and fluence for improved groove depth in femtosecond laser processing of silicon

Femtosecond laser processing allows controlled modification of the optical, electrical, and chemical properties of silicon microstructures, expanding their functional range. However, the fabrication of high–aspect ratio features remain difficult due to incomplete understanding of the interaction between laser parameters and material response. Herein, we processed silicon using femtosecond lasers at different fluences (2.32, 5.03, and 10.06 J/cm 2 ) and repetition rates (1, 10, and 50 kHz). The optimal condition of 10 kHz and 2.32 J/cm 2 enabled significant structural improvement, multiple scans increased groove depth from 4 μm to 18 μm relative to a single scan, while simultaneously enhancing edge sharpness and suppressing recast layer formation. Transient reflectivity and plasma emission analyses revealed that material removal at a laser fluence of 2.32 J/cm 2 proceeds via a non-thermal phase transition, whereas higher fluences trigger pronounced phase explosion. Morphological evolution of the processed surfaces further suggests that at 10 kHz, cumulative thermal effects enhance ablation efficiency without inducing the plasma shielding observed at higher repetition rates. Under the proposed parameter conditions, repeated scanning increases the contact angle from 126° to 136°, demonstrating the effectiveness of this approach in tailoring surface functionality. This study provides a systematic understanding of silicon structuring dynamics and establishes a practical strategy for fabricating high-quality, application-relevant structures.