The characterization of optical components using the Cary Deep UV spectrophotometer and ‘VW’ specular reflectance accessory

Significance of the topic

Deep ultraviolet spectrophotometry extends the measurable UV range below 190 nm without resorting to complex vacuum systems. This capability is essential for the precise characterization of advanced optical coatings used in microlithography, semiconductor metrology, and other deep UV applications.

Objectives and overview of the study

The main aim was to compare the specular reflectance properties of three magnesium fluoride coated optical components produced by different methods. Measurements were performed between 140 nm and 250 nm to reveal coating-dependent performance and demonstrate the Cary Deep UV spectrophotometer’s ability to acquire high-quality spectra down to 140 nm.

Instrumentation

• Cary Deep UV Spectrophotometer equipped with dual 1200 lines/mm gratings blazed at 150 nm
• ‘VW’ Absolute Specular Reflectance Accessory featuring a dual VW configuration with toroidal and movable spherical mirrors
• Magnesium fluoride coated optics and deuterium lamp with solar-blind detector
• Nitrogen-purged glovebox maintaining O2 and moisture below 1 ppm

Methodology

Samples were mounted in the specular reflectance accessory inside a nitrogen-filled glovebox. Zero SRA calibration was performed to establish 0 % and 100 % reflectance baselines. Duplicate scans were acquired for each component using 5 nm spectral bandwidth, double beam mode, and reduced slit height. Comparative measurements were enabled by placing a reference mirror in the rear beam path and the sample in the front path.

Main results and discussion

• Mirrors 1 and 2, from the same manufacturer, exhibited contrasting reflectance peaks at 164 nm and 225 nm, indicating different coating thickness or deposition processes.
• Mirror 3 showed a smoother, more uniform reflectance profile across the measured range, suggesting a more homogeneous coating.
• High reproducibility was observed between duplicate scans, illustrating the accessory’s accuracy and stability.
• The spectrophotometer successfully recorded spectra down to 140 nm, confirming its suitability for deep UV metrology without vacuum conditions.

Benefits and practical applications

• Enables rapid quality control of UV optical coatings in industrial and research laboratories.
• Supports development of next-generation microlithography optics by providing precise reflectance data in the deep UV region.
• Simplifies DUV metrology by eliminating vacuum-based instrumentation complexity and cost.

Future trends and potential applications

• Integration of in situ monitoring to track coating deposition in real time.
• Development of extended-range detectors and light sources pushing measurements below 140 nm.
• Application of AI-driven data analysis to optimize coating processes and predict performance.
• Expansion into other materials and multimodal optical characterization combining reflectance, transmittance, and ellipsometry.

Conclusion

The study demonstrates that a Cary Deep UV spectrophotometer equipped with a VW specular reflectance accessory and operated under nitrogen can reliably measure absolute specular reflectance of MgF2-coated optics down to 140 nm. Significant differences between coating methods were resolved, highlighting the system’s utility for both research and industrial quality assurance in deep UV optics.

References

1. Perkampus H-H. Encyclopedia of Spectroscopy; VCH: 1995.
2. Strong J. Procedures in Experimental Physics, 1st Ed.; Prentice-Hall: New York, 1938; p 376.
3. Agilent Technologies, Inc. Cary Deep UV Spectrophotometer, ‘VW’ Absolute Specular Reflectance Accessory and Extended Sample Compartment; Part Numbers 00 100783 00, 00 100438 00, 00 100466 00.
4. Agilent Technologies, Inc. Cary WinUV Scan Software Help; Version 2.0.
5. Hind AR. R&D at the deep end. Chemistry in Australia 2001, 68(4), 12.