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Aperture design for a dark-field wafer defect inspection system
Applied Optics
|February 24, 2022
Summary
A novel aperture design method improves wafer inspection by reducing haze signals, significantly lowering the detection limit for defects. This advancement enhances signal-to-noise ratio (SNR) for more accurate unpatterned wafer inspection.
Area of Science:
- Semiconductor manufacturing
- Optical metrology
- Surface science
Background:
- Dark-field defect inspection systems dominate unpatterned wafer inspection (70% market share).
- Current systems face limitations in detection limits due to haze signals obscuring defect signals.
- Existing signal-to-noise ratio (SNR) enhancement methods are constrained by system architecture and empirical aperture designs.
Purpose of the Study:
- To propose a novel aperture design method based on scattering field analysis to mitigate haze signal interference.
- To reduce the influence of wafer surface roughness-induced haze on particle defect signals.
- To enhance the SNR and improve defect detection sensitivity in unpatterned wafer inspection.
Main Methods:
- Designed apertures based on the ratio field of particle signal to haze, utilizing bidirectional reflectance distribution function (BRDF).
- Verified aperture designs using a scattering model developed with National Institute of Standards and Technology (NIST) tools.
- Optimized incident conditions (polarization, angle) to maximize SNR.
- Proposed three multiaperture combination schemes to balance scattering characteristics and improve contrast.
Main Results:
- The proposed aperture design significantly reduces the detection limit from 48 nm to 22 nm for specific conditions (p-polarized light, 72° incident angle, threshold 3).
- Multi-aperture schemes demonstrated superior performance over single-aperture designs, particularly the one-to-one scheme.
- The method effectively improves contrast by reducing haze signal influence on particle signals.
Conclusions:
- The novel aperture design method offers a significant improvement in wafer defect inspection sensitivity.
- Optimized apertures and multiaperture schemes are crucial for overcoming limitations imposed by haze signals.
- This approach enhances the capability of dark-field inspection systems for detecting smaller defects on unpatterned wafers.
