Photomask Blank Defect Inspection via Focus Variation
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Solution Overview
Problem
Conventional defect inspection methods for photomask blanks, particularly for thin films with thicknesses up to 10 nm, struggle to accurately distinguish between pit and bump defects due to unclear positional relations of bright and dark portions in inspection signals, leading to incorrect identification and reduced yield in semiconductor manufacturing.
Innovation Solution
An optical defect inspecting method that involves selecting specific inspection treatment procedures and criteria based on the mode of the optical and thin films, utilizing variations in light intensity and positional relations of bright and dark portions in inspection images, and applying non-polarized or polarized light under different focus and defocus conditions to accurately determine the rugged shape of defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional defect inspection methods are used for thin films with thicknesses up to 10 nm, then the inspection process can be performed, but the ability to accurately distinguish between pit and bump defects deteriorates due to unclear positional relations of bright and dark portions in inspection signals
Solution Approach 1:
The patent applies focus variation inspection, which introduces the focus distance dimension to the inspection process. By capturing inspection images at multiple focus distances (in-focus and out-of-focus conditions), the method transforms the two-dimensional surface inspection problem into a three-dimensional volume inspection, enabling accurate discrimination of defect rugged shapes even for thin films with thicknesses up to 10 nm
Solution Approach 2:
The patent changes the focus distance parameter during inspection to differentiate defect types. By varying the focus distance and observing how bright and dark portions shift in inspection images, the method enables accurate determination of defect rugged shapes. This parameter change approach allows clear distinction between pit and bump defects that cannot be resolved under fixed focus conditions
2Measurement precision
If multiple inspection conditions are applied to determine defect rugged shape, then the discrimination accuracy between pit and bump defects improves, but the inspection time and process complexity increase
Solution Approach 1:
The patent applies partial action by selecting only the essential inspection conditions needed for defect discrimination. Instead of exhaustive multi-angle and multi-wavelength inspections, the method uses a focused approach with specific focus distances (in-focus and out-of-focus) and predetermined determination criteria, achieving accurate defect classification with minimal inspection steps
Solution Approach 2:
The patent applies preliminary action by pre-establishing determination criteria for defect rugged shapes before inspection. The system stores predetermined criteria that define the positional relations of bright and dark portions for different defect types, allowing rapid automated classification during inspection without requiring complex real-time analysis
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method enables reliable discrimination of pit and bump defects, ensuring the exclusion of fatal defects and improving the yield of photomask blanks by accurately determining the rugged surface shape of defects in thin films, thereby enhancing the quality and production efficiency of semiconductor devices.
Implementation Method 1
applying inspection light to a region including the defect while maintaining a distance between the defect and an objective lens of the inspecting optical system, based on the inspection treatment procedure designated in the step (A2), and collecting reflected light from the region irradiated with the inspection light
Data Source
Figure 1A~1B
Figure 2
Figure 3
AI summary
Disclosed is a method of inspecting a defect present at a surface portion of a photomask blank which includes an optical film, and a thin film. The method includes: selecting and designating an inspection treatment procedure and a criterion for determination of rugged shape of the defect which correspond to modes of the optical film and the thin film of the photomask blank; applying inspection light to a region including the defect while maintaining a distance between the defect and an objective lens of an inspecting optical system, based on the designated inspection treatment procedure, and collecting reflected light from the region irradiated with the inspection light, as a magnified image of the region, through the inspecting optical system; and determining the rugged shape of the defect, from light intensity distribution of the magnified image, based on the designated criterion for determination.