The deep ultraviolet spectroscopic properties of a next-generation photoresist

Significance of the Topic

As optical lithography advances toward shorter wavelengths, deep ultraviolet characterization of photoresists becomes critical. Ensuring adequate transparency and coating uniformity at wavelengths down to 157 nm directly impacts pattern fidelity and device scaling in semiconductor manufacturing.

Objectives and Overview of the Study

This application note reports on the measurement of specular reflectance properties of a next-generation chemically amplified photoresist. The main goals were to assess instrument performance in the 150–250 nm range, confirm reproducibility of deep UV measurements, and evaluate the homogeneity of a spin-coated resist film on silicon.

Methodology and Instrumentation Used

The resist was spin-coated onto 100 mm silicon wafers and analyzed in a nitrogen-purged glovebox to minimize atmospheric absorption. Absolute specular reflectance spectra were acquired between 150 nm and 250 nm using the following setup:

• Cary Deep UV spectrophotometer with extended sample compartment
• ‘VW’ Absolute Specular Reflectance Accessory configured for dual VW calibration
• Rear beam attenuator and reduced slit height to maintain optimal signal levels
• Nitrogen purging to extend the usable wavelength limit below 190 nm

Baseline corrections for 0 % and 100 % reflectance were applied automatically by the instrument software prior to sample measurement. Multiple scans were taken to evaluate both repeatability at a fixed position and uniformity across five locations on the wafer surface.

Key Findings and Discussion

Repeat scans without repositioning the sample demonstrated high reproducibility, indicating stable instrument performance. Reflectance spectra from five different spots on the wafer overlapped closely, suggesting a uniform resist thickness and coating homogeneity. Although the absolute film thickness was not measured, the consistent reflectance behavior implies that the material exhibits suitable transparency and surface quality for deep UV applications.

Benefits and Practical Applications of the Method

Deep UV specular reflectance measurement offers several advantages:

• Non-destructive evaluation of film uniformity and optical properties
• Rapid assessment of photoresist transparency at target lithography wavelengths
• Capability to operate without full vacuum by using nitrogen purging
• Direct comparison between reference and sample positions for improved accuracy

These features support quality control in resist development and process optimization for advanced microlithography.

Future Trends and Potential Applications

As lithography moves toward extreme ultraviolet (EUV) and beyond, characterization tools will need to cover even shorter wavelengths with higher precision. Potential directions include:

• Integration of higher-sensitivity detectors and improved optics for wavelengths below 140 nm
• Development of new resist materials—such as fluorinated polymers and silesquioxanes—with enhanced transparency and etch resistance
• Automated mapping of large substrates for full-wafer uniformity analysis
• Combining reflectance with complementary techniques (scatterometry, ellipsometry) to extract film thickness and refractive index simultaneously

Conclusion

This study demonstrates that the Cary Deep UV spectrophotometer with the VW absolute specular reflectance accessory, combined with nitrogen purging, can reliably measure deep ultraviolet reflectance properties of photoresist films. The method achieves high reproducibility and reveals coating uniformity, making it a valuable tool for evaluating next-generation resist materials intended for 157 nm and shorter wavelength lithography.

References

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6. Agilent Technologies, Cary UV At Work No. 082, 2011
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