What is Light-Induced Damage? Pre-ablation regime optical and morphological changes in nanometer thick films and grating structures

Light-induced damage is an increasingly critical issue in the semiconductor manufacturing industry, where nanolithography is used to produce integrated circuits with ever-shrinking features. While most research in optics focuses on damage to optical components, the exposure of thin films and nanostructured materials to high-intensity light during chip fabrication poses new challenges. In this context, we investigate the effects of single femtosecond laser pulses on thin metallic films—such as gold, aluminum, and ruthenium—and on silicon grating structures, which are commonly present on wafers in nanolithography fabrication processes and alignment.
We find that even moderate light intensities, well below the threshold for ablation, can induce permanent optical and morphological changes. This so-called pre-ablation regime involves fluences that are insufficient to remove material but still cause significant structural transformation. Using techniques such as scanning electron microscopy, atomic force microscopy, and electron backscatter diffraction, we observe that a single ultrafast laser pulse is already sufficient to induce melting and resolidification, resulting in an altered grain structure.
These structural changes lead to various morphological changes—such as delamination, spallation, nanovolcano formation, top-level ablation, and grating line deformation—depending on the material and surface topography. Crucially, these processes are accompanied by subtle but measurable increases in optical reflectivity, resulting from the grain structure changes, induced by melting and resolidification.
These small light-induced reflectivity changes serve as reliable indicators of early-stage damage. As such, they offer valuable potential as in situ warning signals to prevent catastrophic damage, especially in high-precision environments like semiconductor manufacturing.