Light-induced optical and nanostructural changes of Si gratings

Area of Science:

• Materials Science
• Optics
• Semiconductor Manufacturing

Background:

• Optical diffraction from gratings is crucial for wafer alignment in nanolithography.
• Smaller gratings require higher laser fluences, increasing optical damage risk.
• Understanding laser-induced damage in silicon gratings is vital for semiconductor fabrication.

Purpose of the Study:

• To investigate light-induced optical and structural changes in silicon gratings under femtosecond laser pulses.
• To determine fluence thresholds for damage mechanisms in gratings versus flat silicon.
• To correlate laser damage thresholds with local absorbed power density and understand deformation effects.

Main Methods:

• Exposure of flat silicon and silicon gratings to single femtosecond laser pulses.
• Measurement of optical and structural changes, including crater formation and grating deformation.
• Near-field Rigorous Coupled-Wave Analysis (RCWA) to calculate absorbed power density profiles.

Main Results:

• Fluence thresholds for damage mechanisms were 10-50% lower in gratings than in flat silicon.
• Local field enhancements in gratings significantly reduce damage thresholds.
• Grating line deformation, including inversion, was observed and correlated with damage onset.

Conclusions:

• Grating topography lowers laser damage thresholds in silicon, necessitating careful fluence control.
• Grating line deformation serves as an early indicator of impending catastrophic laser-induced damage.
• Findings provide insights for mitigating laser damage in silicon gratings for semiconductor manufacturing.