Spectrophotometric Spatial Profiling of Coated Optical Wafers

Significance of the Topic

Consistent and high-throughput spectroscopic characterization of optical thin film coatings is critical for quality assurance, process optimization, and cost reduction in coating manufacturing. Spatially resolved reflectance mapping across large wafers enables early detection of non-uniformities and supports tighter process control and higher yields.

Objectives and Overview of the Study

This study evaluates the performance of the Agilent Cary 7000 Universal Measurement Spectrophotometer (UMS) equipped with a Solids Autosampler for automated, angular-resolved reflectance mapping on 200 mm diameter coated optical wafers. The aim was to demonstrate accurate, unattended measurement across multiple points with high reproducibility and spatial resolution.

Methodology and Instrumentation

Prior to mapping, the viable radial measurement range was determined by monitoring transmission signals from 90 mm to 95 mm radius in 1 mm steps, establishing 94 mm as the maximum safe radius. The main mapping experiment used these conditions:

• Incident angle: 7° (near normal)
• Spectral range: centered at 1064 nm, interval 1 nm
• Spectral bandwidth: 4 nm
• Incident beam aperture: 3° horizontal × 1° vertical
• Polarizations: s- and p-polarized measurements
• Spatial pattern: 8 chords at 22.5° separation, 27 points per chord spaced 5 mm, plus points at 92 mm and 93 mm radii
• Detector: silicon photodiode with manual changeover
• Signal averaging: 1 s per measurement

The key instrumentation components were:

• Cary 7000 UMS spectrophotometer covering 250–2500 nm
• Universal Measurement Accessory (UMA) enabling independent sample and detector angling
• Solids Autosampler with two degrees of freedom (radial z and rotational Φ) for wafers up to 200 mm diameter
• 8-inch sample holder with minimal contact clamps to preserve coating integrity

Main Results and Discussion

The center point reflectance spectrum exhibited >99% reflectance over 950–1150 nm, confirming coating design performance. Mapping of reflectance at 1064 nm revealed a consistent decline of up to ~1% from center to edge in both s- and p-polarizations, indicating a centrosymmetric uniformity profile. Reproducibility tests at the wafer center over the 6.5 h experiment showed variability below 0.1%, underlining system stability. Occasional outliers correlated with detected surface contamination.

Benefits and Practical Applications

This automated mapping approach offers:

• Fully unattended operation reducing labor and cost per analysis
• High spatial resolution (beam patch ~5 mm × 1.5 mm, step size 5 mm)
• Angular-resolved data supporting comprehensive optical characterization
• Reproducibility better than 0.1% over extended runs
• Capability to identify production defects and contamination early

Future Trends and Opportunities

Advances may include tighter beam focusing for sub-millimeter resolution, integration of real-time data feedback for process control, expanded multi-wavelength transmission mapping, and inline measurement systems for rolling-to-roll coating processes.

Conclusion

The Agilent Cary 7000 UMS with Solids Autosampler successfully provided automated, angular-resolved reflectance mapping on large optical wafers, achieving high accuracy, reproducibility, and spatial coverage. This platform supports enhanced quality assurance and accelerated development of thin film coatings.

Reference

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