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Influence of oxidation in the formation and performance of laser-induced periodic surface structures
(2022)
The fabrication of laser-induced periodic surface structures (LIPSS) has been reported for metallic, semiconductor and dielectric materials when irradiated with linearly polarized ultrashort laser pulses. Different sets of processing parameters have been identified and extensively studied, allowing the use of LIPSS for applications in medicine, tribology and surface wetting to name a few. One important parameter that is often not considered is the laser-induced oxidation that is produced especially when oxidation prone materials are irradiated in air environment. This surface oxidation can potentially influence the final structures
size, shape and orientation and modify their functionality in some specific applications.
In this work, we first study the influence of the laser-induced oxidation layer on the formation of LIPSS by characterizing the response of the oxidation prone
hard-coating material chromium nitride. Moreover, we employ complementary surface and depth-profiling analytic techniques to reveal morphological, chemical and structural features of different types of surface structures and LIPSS produced on titanium-based substrates. Finite-difference time-domain (FDTD) calculations finally explain how this superficial oxidation layer promotes the formation of a new type of low spatial frequency LIPSS. Also, we measure the depth-extent of the different oxides formed and demonstrate its beneficial influence for friction reduction and wear resistance applications.
Silicon is the material responsible for most of the technological developments during the past century, making it one of the most studied materials along different disciplines. However, there are still unturned stones regarding its superficial re-solidification after femtosecond laser-induced local melting. In this presentation, we report irradiation experiments with single femtosecond pulses (790 nm, 30 fs) with a spatially Gaussian distribution on two different types of silicon with orientations <111> and <100>. The surface modifications were studied in detail via different techniques, including optical microscopy, atomic force microscopy, spectroscopic imaging ellipsometry, energy dispersive X-ray spectroscopy and high-resolution transmission electron microscopy. We quantitatively estimate the resulting radial amorphous layer depth profiles with maximum thicknesses around some tenths of nanometers for fluences in between the melting and ablation thresholds. In particular, spectroscopic imaging ellipsometry (SIE) allowed fast data acquisition using multiple wavelengths to provide experimental measurements for calculating the nanometric radial amorphous layer thickness profiles with micrometric lateral resolution based on a thin-film layer model. SIE proved to be capable of detecting and measuring nanometric structural and chemical modifications (oxidation) on the studied laser spots. The accuracy of the SIE-based calculations is verified experimentally by characterizing an in-depth material lamella via high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (STEM-EDX). For completeness, we present a mathematical modelling for the melt layer thickness considering different optical absorption processes including one photon absorption, two photon absorption and free-carrier absorption, highlighting the relevance of the latter one in the femtosecond laser-induced melting of silicon.