EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Direct Laser Interference Patterning (DLIP) is a versatile technique which allows for the fabrication of nano- to micrometric scale periodic structures over large areas in a variety of materials. The sinusoidal shape of the intensity profile at the sample surface induces a periodic alternating pattern of molten and non-molten fringes, whose periodicity and width can be tuned by modifying the irradiation configuration and pulse fluence, respectively. However, few works have reported on the complex formation dynamics of the topography modulation within the fringes as a result of the laser-induced melting-solidification process.

In this work, we have employed DLIP to process crystalline Ge and Si wafers under different irradiation conditions, using interfering unpolarised UV excimer laser pulses (ArF, λ=193 nm,τ=20 ns). Topographical AFM measurements revealed a steep modulation profile of the surface across the molten fringes for specific irradiation conditions. This final state into which the molten material is frozen after single pulse irradiation provides strong evidence that thermocapillary waves are driving the motion of the molten material. The different thermophysical properties of Ge and Si influence directly the complex dynamics of the topography change and solidification processes, resulting in different final states. However, since both the laser pulse duration and thermocapillary waves occur in the ns time scale, separation of such physical phenomena becomes extremely challenging. Thus, in order to gain a deeper understanding of the mechanisms responsible for the formation of topography profiles observed, our experiments have been complemented by numerical simulations based on coupled physical phenomena.