Design of a long working distance, large aperture microscope objective for semiconductor precision inspection

Area of Science:

• Optical Engineering
• Microscopy
• Semiconductor Metrology

Background:

• Microscope objectives with large numerical apertures typically suffer from short working distances, limiting their application in certain industrial inspection tasks.
• Achieving high resolution and maintaining image quality across multiple wavelength ranges, including visible and laser spectra, is a significant challenge in optical design.

Purpose of the Study:

• To design and evaluate a 50× microscope objective that overcomes the short working distance limitation associated with high numerical apertures.
• To achieve a flat-field, apochromatic performance across a broad visible wavelength range (436-656 nm).
• To ensure high focusing quality for both femtosecond (770-790 nm) and YAG (1059-1069 nm) laser applications.

Main Methods:

• Optical design using advanced simulation software to optimize lens elements for aberration correction.
• Performance evaluation through resolution measurements (line pairs per millimeter) and modulation transfer function (MTF) analysis.
• Tolerance analysis to assess the design's robustness for mass production.

Main Results:

• A 50× microscope objective with a numerical aperture of 0.75 and a working distance of 6.2 mm was successfully designed.
• The objective demonstrated a resolution of 2200 lp/mm and image quality near the diffraction limit.
• The design exhibited flat-field and apochromatic characteristics for visible light and maintained high focusing quality for specified laser wavelengths.

Conclusions:

• The developed microscope objective offers a practical solution for semiconductor defect detection by combining high resolution with a long working distance.
• The design's performance and manufacturability indicate its suitability for industrial applications requiring precise optical inspection.
• This work advances the capabilities of microscopy for demanding metrology tasks in the semiconductor industry.