Photomask Blank Defect Inspection via Focus Shift
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Solution Overview
Problem
Existing defect inspection methods for photomask blanks inaccurately distinguish between pit and bump defects, leading to erroneous determinations and reduced yield, as they fail to reliably discriminate the rugged shapes of defects, particularly in photomask blanks used for semiconductor device production.
Innovation Solution
A method involving an optical defect inspection system that applies inspection light at both focus and defocus conditions to collect and evaluate light intensity distributions, allowing for accurate re-determination of defect shapes by comparing light intensity variations in magnified images, thereby distinguishing between true pit and bump defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional defect inspection methods are used to detect defects on photomask blanks, then defects can be detected, but the rugged shapes of defects (pit vs. bump) cannot be accurately distinguished
Solution Approach 1:
The inspection system dynamically adjusts the focus position of the objective lens between focus and defocus states to capture different light intensity distribution patterns from the same defect, enabling accurate discrimination of defect rugged shapes
Solution Approach 2:
The system changes the optical parameter (focus position) to obtain different light intensity distribution patterns, comparing the patterns at focus and defocus conditions to determine whether a defect is a pit or bump type
2Reliability
If all detected defects are treated as fatal defects, then no fatal defects are missed, but yield is reduced due to false rejection of usable blanks
Solution Approach 1:
The system replaces subjective judgment or simple threshold-based classification with optical intensity distribution pattern analysis, using the physical principle of light scattering differences to objectively distinguish pit defects from bump defects
Solution Approach 2:
The light intensity distribution pattern serves as an intermediary that translates the physical shape of the defect into a measurable signal, allowing indirect but accurate determination of defect rugged shape without physical contact
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 approach ensures highly reliable discrimination of defect shapes, preventing false identification of bump defects as pit defects, thereby ensuring photomask blanks with fatal defects are excluded, and increasing yield by providing defect-free blanks at a lower cost.
Implementation Method 1
a step of collecting reflected light from a region irradiated with the inspection light, through the objective lens, as a first magnified image of the region
Data Source
Figure 1A~1F
Figure 2A~2C
Figure 3A~3B
AI summary
A method of inspecting a defect present at a surface portion of a photomask blank having at least one thin film formed on a substrate by use of the inspecting optical system. The method includes setting the distance between the defect and an objective lens of an inspecting optical system to a defocus distance, applying inspection light to the defect through the objective lens, collecting reflected light from the region irradiated with the inspection light, through the objective lens, as a magnified image, identifying a light intensity variation portion of the magnified image, and determining the rugged shape of the defect on the basis of a variation in light intensity of the light intensity variation portion of the magnified image.