Sub-ablation-threshold light-induced modification of thin ruthenium layers detected using optical reflectance

In semiconductor device manufacturing, wafer materials may be exposed to intense light sources by optical metrology tools. The desired light fluence often needs to be maximized to levels just below the optical damage threshold of materials deposited on the wafer, such as ruthenium. We, therefore, investigate light-induced permanent structural changes to thin Ru films after exposure to single 400 nm wavelength femtosecond pulses in the fluence regime before catastrophic damage. For fluences below that where full-ablation occurs, small optical increases in the reflectivity of up to 4 % are observed in the aftermath with a weak probe beam. In this fluence regime, dark-field, scanning electron, and atomic force microscopy images reveal morphological changes such as top-level ablation, where only the top part of the ruthenium layer is ablated whereas the lower part still remains on the substrate, and nanovolcano formation. However, neither top-level ablation nor nanovolcano formation is responsible for the reflection increase. Instead, Electron Backscatter Diffraction reveals that in this low fluence regime where reflectivity increases, Ru grains melt and resolidify into larger grains, which is likely responsible for the observed reflectivity increases. This result is reminiscent of our earlier work on aluminum layers and it suggests that there may be more metals that display this behavior.