Thermomechanical investigation of silicon wafer dynamics within the melting regime driven by picosecond laser pulses for surface structuring

TL;DR

This study uses multiphysics finite element modeling to understand how picosecond laser pulses induce surface ripples on silicon through thermomechanical effects, revealing the role of wave interference and penetration depth in pattern formation.

Contribution

It introduces a novel multiphysics simulation approach to analyze the thermomechanical dynamics and ripple formation mechanisms in silicon during ultrashort laser irradiation.

Findings

Surface ripples result from mechanical standing wave interference.

Penetration depth influences ripple stability and duration.

Sequential pulses amplify deformation via residual stresses.

Abstract

Laser-induced periodic surface structures (LIPSS) 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 double-pulse interactions. 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 scanning. The results show that periodic surface structures originate 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 (app. 2.5 micrometers) lead to extended melting and hydrodynamic smoothing. Simulations of sequential double-pulse irradiation confirm that residual stresses and strains from the first pulse amplify deformation during the second, enhancing ripple amplitude and uniformity. Controlled excitation of mechanical standing waves governed by PD, boundary geometry, and pulse sequencing - thus represents a fundamental mechanism for deterministic LIPSS formation on silicon.