Photomask Inspection via Composite Image Limiter Processing
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Solution Overview
Problem
Existing photomask inspection methods face challenges in detecting defects near pattern edges due to diffraction effects, leading to reduced detection sensitivity and the generation of pseudo defects from low-luminance images, which complicates setting appropriate threshold levels for defect detection.
Innovation Solution
The method involves forming a composite image by optically synthesizing transmission and reflection images of a photomask, adjusting the illumination system to create a luminance difference between pattern and light-transmitting portions, and applying limiter processing to delete or convert low-luminance image portions, allowing for high-resolution defect detection without being affected by diffraction effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a transmission image and a reflection image are individually picked up to detect defects, then defect detection capability is improved, but the resolution of the reflection image significantly decreases due to diffraction effect at pattern edges
Solution Approach 1:
The patent combines the transmission image and reflection image into a composite image that is picked up by a single photodetector. This merging approach allows the system to utilize both imaging modes simultaneously, maintaining high detection capability while avoiding the resolution degradation that occurs when separately processing reflection images受 diffraction effects.
2Area of stationary object
If the field of view is divided into two regions to form transmission image and composite image, then detection coverage is improved, but device complexity increases
Solution Approach 1:
The patent employs a single photodetector that serves multiple functions: it detects both the transmission image and the composite image (formed by combining transmission and reflection images) through different optical paths. This multi-functional approach expands the effective detection coverage without proportionally increasing device complexity, as one detector performs what would otherwise require multiple detectors.
3Reliability
If threshold level is set high to avoid detecting low-luminance images as pseudo defects, then false positive rate decreases, but detection sensitivity deteriorates
Solution Approach 1:
The patent applies different processing approaches to different regions of the image: the composite image (which includes reflection components and suffers from diffraction effects at edges) is processed with special consideration for low-luminance regions, while the transmission image is processed separately. This allows the system to set appropriate detection thresholds for each region type, maintaining high sensitivity without being misled by pseudo-defects in low-luminance areas.
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 enables high-sensitivity defect detection near pattern edges and reduces the impact of diffraction effects, improving the accuracy of defect inspection by distinguishing between actual and pseudo defects, thereby enhancing the detection sensitivity and resolution.
Implementation Method 1
a transmitted illumination beam toward a back surface of a photomask to be inspected and a reflected illumination beam toward an element forming surface of the photomask
Implementation Method 2
an objective lens that forms a transmission image and a composite image of the photomask
Implementation Method 3
a photodetector that receives synthetic light of the reflected light and the transmitted light
Implementation Method 4
the diffraction effect on an edge portion of the pattern becomes more prominent
Data Source
AI summary
The field of view of an objective lens is divided into two areas, and a transmission image of a photomask and a composite image obtained by optically synthesizing a transmission image and a reflection image of the photomask are picked up in parallel. A drop image generated at an edge portion of a pattern portion in the composite image is deleted by limiter processing or masking processing, or is deleted by using primary-differentiated signals of a composite image signal and a transmission image signal.