Photomask Inspection Using Knife-Edge Interferometric Fringe Scanning
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Solution Overview
Problem
Conventional photomask inspection techniques, such as scanning electron microscopy, are labor-intensive, time-consuming, and expensive, making them inconvenient for wide application beyond in-process inspection, especially with the need for fast and accurate inspection of photomasks used in semiconductor fabrication.
Innovation Solution
The use of knife-edge interferometry for photomask inspection, involving an interferometer to generate an interferometric fringe pattern, scan the photomask, and detect geometric parameters like line-edge roughness (LER) through interferograms, allowing for rapid and convenient inspection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If scanning electron microscopy is used for photomask inspection, then measurement precision is improved, but productivity deteriorates due to labor-intensive and time-consuming processes
Solution Approach 1:
The patent replaces the mechanical scanning electron microscopy system with an optical interferometry system. The interferometer uses optical waves to scan the photomask and capture interferograms, which are then processed to extract geometric parameters. This substitution of mechanical scanning with optical interference enables faster inspection while maintaining precision through the wave-based measurement approach.
Solution Approach 2:
The patent changes the measurement parameter from physical contact or mechanical scanning (SEM) to optical interference patterns. By measuring the interference fringes formed when light waves interact with the photomask surface, the system extracts geometric parameters such as line-edge roughness and pattern dimensions. This parameter transformation enables rapid non-contact measurement while preserving measurement accuracy.
2Measurement precision
If conventional inspection techniques are used, then measurement precision is improved, but loss of time increases due to labor-intensive processes
Solution Approach 1:
The interferometer continuously scans the photomask surface and captures interferograms without interruption, enabling rapid data acquisition. The continuous optical scanning eliminates the stop-start nature of conventional inspection methods, maintaining measurement precision while significantly reducing the time required to inspect the entire photomask area.
Solution Approach 2:
The system creates an optical copy or interferogram of the photomask surface features rather than directly measuring the physical structure. This interferogram serves as a digital representation that can be rapidly processed to extract geometric parameters, eliminating the time-consuming physical measurement processes of conventional techniques while maintaining measurement accuracy.
3Measurement precision
If conventional inspection techniques are used, then measurement precision is improved, but device complexity increases due to expensive equipment and labor requirements
Solution Approach 1:
The interferometer is designed to perform multiple inspection functions using a single optical system. It can measure various geometric parameters including line-edge roughness, pattern width, and surface quality, as well as detect defects. This multi-functionality reduces the need for multiple specialized devices and labor-intensive processes, thereby reducing overall system complexity while maintaining comprehensive measurement precision.
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
Enables efficient and accurate detection of defects and geometric parameters in photomasks, reducing time and labor requirements while ensuring high-quality photomask production for semiconductor devices.
Implementation Method 1
superpositioning a reference incident spherical wave and an edge-diffracted wave
Implementation Method 2
edge-diffracted wave
Implementation Method 3
shaping a collimated beam using an objective lens of the interferometer to focus an incident beam on a focal point
Implementation Method 4
a photodetector configured to detect the beam generated by the laser
Data Source
AI summary
A method for inspecting a photomask includes scanning the photomask with an interferometric fringe pattern generated by an interferometer, generating an interferogram associated with the photomask in response to scanning the photomask using the interferometer, and detecting one or more geometric parameters of the photomask using the generated interferogram.