Ronchi lateral shearing interferometry with binary encoded sinusoidal transmittance grating

Area of Science:

• Optical engineering
• Metrology
• Wavefront sensing

Background:

• Double-grating Ronchi lateral shearing interferometry is a key wavefront detection method for lithography projection lenses.
• Traditional methods using Ronchi gratings create complex multi-wave interference, necessitating extensive phase-shifting for accurate phase retrieval.
• This complexity compromises real-time detection accuracy and increases susceptibility to phase-shifting errors.

Purpose of the Study:

• To investigate the use of sinusoidal transmission gratings to simplify interference patterns in lateral shearing interferometry.
• To analyze the performance of this new interferometry for low-NA optical systems.
• To develop methods for fabricating accurate sinusoidal gratings for improved wavefront detection.

Main Methods:

• Replaced the standard Ronchi grating with a sinusoidal transmission grating in the object plane.
• Analyzed the resulting interference fields using scalar diffraction theory for low-NA optical systems.
• Employed binary encoded methods to approximate target sinusoidal transmittance distributions, addressing fabrication challenges.

Main Results:

• Sinusoidal gratings generate only 0th and conjugate 1st diffraction orders, reducing interference to two coherent patterns.
• This simplification significantly reduces the required number of phase shifts for phase retrieval.
• The proposed method demonstrates superior real-time capability and enhanced resistance to phase-shifting errors and environmental noise.

Conclusions:

• Sinusoidal gratings offer a more efficient and robust approach to wavefront detection in lithography projection systems.
• The binary encoding technique provides a practical solution for fabricating the necessary gratings.
• This advanced interferometry method improves accuracy and real-time performance over conventional double-grating Ronchi systems.