Thermomechanical Investigation of Silicon Wafer Dynamics Within the Melting Regime Driven by Picosecond Laser Pulses for Surface Structuring.

Laser-induced periodic surface structures (LIPSSs) on silicon, generated by ultrashort pulsed lasers, provide an efficient means to tailor surface functionality. This work presents a multiphysics finite element study on the thermomechanical dynamics of silicon wafers irradiated by picosecond laser pulses, focusing on the melting regime where thermomechanical and hydrodynamic effects dominate. To illustrate the sequential nature of laser scanning, single-pulse irradiation models are developed as thermomechanical analogues of sequential laser irradiations. By positioning the laser focus near reflective boundaries and corners of the target, these models reproduce the stress wave interference that would occur between successive pulses in laser scanning. The results show that periodic surface structures are enhanced from mechanical standing wave interference within the molten layer, forming ripples with near-wavelength periodicity. The penetration depth (PD) is identified as a key factor controlling the duration and stability of these ripples: shallow PDs (75-150 nm) yield distinct, persistent patterns, while deeper PDs (~2.5 μm) lead to extended melting and hydrodynamic smoothing. Simulations of sequential laser pulse irradiations confirm that residual stresses and strains from the first pulse amplify deformation during the second, enhancing ripple amplitude and uniformity. Thus, the role of controlled excitation of mechanical standing waves governed by PD, boundary geometry, and pulse sequencing, in deterministic LIPSSs formation on silicon is revealed.