Light-induced optical and nanostructural changes of Si gratings

In nanolithography, optical diffraction from gratings etched into the scribe lanes of semiconductor devices is used for wafer alignment. As these gratings become increasingly smaller, achieving sufficiently strong diffraction signals requires higher light fluences, increasing the risk of optical damage. This study explores light-induced optical and structural changes in flat silicon and gratings etched in silicon when exposed to single femtosecond laser pulses. We find that the fluence thresholds for all observed damage mechanisms are 10-50% lower in gratings compared to flat silicon. Here, the fluence thresholds for the onset of optical change and crater formation are approximately 91 mJ/cm² and 816 mJ/cm², respectively. For certain gratings, these thresholds drop to below approximately 40 mJ/cm² and 400 mJ/cm². We attribute this to local field enhancements caused by the grating topography. Using near-field Rigorous Coupled-Wave Analysis (RCWA), we calculated absorbed power density profiles to establish a correlation between damage fluence thresholds and the local absorbed power density. While the method provides damage threshold estimates in a rather time-efficient manner, we found only a moderate correlation between the calculated and experimental threshold fluences. Careful analysis of our measurements shows that the observed deviations in relative pump-induced reflection changes in gratings, compared to flat silicon, are primarily due to grating line deformation, the onset of which can serve as an early warning for catastrophic damage. At high fluences, deformations become larger, even giving rise to grating line inversion, where lines become valleys and valleys become lines. Our findings offer insights into predicting and mitigating light-induced damage in silicon gratings, which is relevant to semiconductor device manufacturing.