Mask Inspection Drift Correction Using Photomask Feedback Motion
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Solution Overview
Problem
Thermal deformation of the projection lens in mask inspection devices causes image drift, leading to suboptimal image quality due to shifted image lines, which conventional methods fail to correct effectively.
Innovation Solution
A method involving a positioning system that moves the photomask relative to the projection lens during exposure, comparing image structures with reference structures to perform corrective movements, and using a stitching method to create a coherent image from time sequences, while incorporating additional degrees of freedom for precise alignment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the photomask is moved continuously during exposure to capture an image line, then the image line coverage is improved, but the ability to detect and correct drift becomes insufficient
Solution Approach 1:
The system compares the captured image line with a reference structure to detect drift, and uses this feedback information to actuate the positioning system for corrective movement, thereby maintaining imaging precision despite continuous scanning
Solution Approach 2:
The patent replaces conventional mechanical drift correction methods with a computational approach that uses image comparison and variance analysis to detect and correct drift through positioning system actuation
2Reliability
If heat is supplied to the projection lens during operation, then the imaging capability is maintained, but thermal expansion causes image drift
Solution Approach 1:
The system continuously monitors image line variance caused by thermal drift and uses feedback control to actuate the positioning system, compensating for thermal expansion effects in real-time
Solution Approach 2:
The patent changes the operational parameters of the positioning system in response to thermal conditions, adjusting corrective movements based on detected variance to maintain precision despite thermal expansion
3Area of stationary object
If the field of view on the photomask is kept small relative to the photomask area, then the scanning coverage is improved, but the image quality deteriorates due to drift
Solution Approach 1:
The system uses feedback from image variance comparison to continuously correct positioning errors, enabling small field-of-view scanning to cover the entire photomask while maintaining image line alignment through active drift compensation
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 effectively corrects thermal deformation-induced image drift, ensuring high-quality imaging by maintaining image line alignment and overlap, thereby producing a flawless image of the photomask.
Implementation Method 1
a photomask (17) illuminated with EUV radiation being imaged onto an image sensor (24) of an EUV camera (23) by a projection lens (22)
Implementation Method 2
Heat is supplied to the projection lens of the mask inspection device during operation, which may be accompanied by thermal expansion of components of the projection lens
Data Source
AI summary
A method for operating a mask inspection device, in which a photomask illuminated with EUV radiation is imaged onto an image sensor of an EUV camera by a projection lens. The photomask is borne by a positioning system, the positioning system being designed to change the position of the photomask relative to the projection lens. The photomask is moved relative to the projection lens during an exposure process of the EUV camera so that an image line is captured. An image structure included in the image line is compared with a reference structure and a variance between the image structure and the reference structure results in the positioning system being actuated in order to perform a corrective movement of the photomask. The invention also relates to a mask inspection device, a control system and a computer program product.