Anisotropic Illumination for EUV Mask Inspection
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Solution Overview
Problem
Current imaging technologies face challenges in achieving high-accuracy mask inspection of reflective masks using extreme ultraviolet (EUV) illumination light in a short time, as two-dimensional detectors are limited by interconnect area density and one-dimensional detectors require increased exposure time for accurate inspection.
Innovation Solution
An imaging apparatus with a critical illumination optical system providing greater magnification in the X-axis direction than the Y-axis, combined with a one-dimensional detector having a longer detection area in the X-axis direction, allows for efficient illumination and detection, enabling high-accuracy imaging and inspection in a short time by matching the illumination area with the detector's axis.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a two-dimensional detector is used for mask inspection, then imaging speed is improved, but detector density cannot be increased due to interconnect area requirements
Solution Approach 1:
The patent transitions from a two-dimensional detector to a one-dimensional detector configuration. By reducing the detector dimensionality and pairing it with an illumination optical system that provides different magnifications in different directions, the system achieves both high detector density and fast imaging speed. The one-dimensional detector can be made more dense while the anisotropic illumination system compensates by providing sufficient illumination coverage in the direction perpendicular to the detector's sensitive axis.
2Manufacturing precision
If a one-dimensional detector is used for mask inspection, then detector density is improved, but exposure time must be increased to secure fixed exposure amount
Solution Approach 1:
The illumination optical system is designed with different magnifications in different directions, creating an anisotropic illumination pattern. The illumination area is elongated in the direction parallel to the one-dimensional detector's sensitive axis, providing concentrated illumination exactly where the detector needs it. This local optimization of illumination distribution ensures sufficient exposure amount is achieved quickly without requiring increased overall exposure time.
3Device complexity
If conventional illumination and detection systems are used, then system simplicity is maintained, but imaging accuracy cannot be achieved in short time
Solution Approach 1:
The patent introduces asymmetry into the optical system by using an illumination optical system with different magnifications in the X and Y directions. This asymmetric illumination configuration, combined with a one-dimensional detector, creates a optimized imaging path that achieves high accuracy in short time. The asymmetric design allows the illumination area to be precisely matched to the detector's sensitive direction, improving imaging accuracy without requiring complex two-dimensional detector systems.
Data Source
AI summary
According to one embodiment, an imaging apparatus includes a light source for illumination, a stage on which an imaging object illuminated by illumination light from the light source is to be placed, a critical illumination optical system configured to supply the illumination light to the imaging object placed on the stage, and to have a greater magnification in a first axis direction than in a second axis direction, an imaging optical system configured to form an image of the imaging object placed on the stage and illuminated using the critical illumination optical system, and a detector configured to detect the image of the imaging object formed by the imaging optical system, and to have a detection area longer in the first axis direction than in the second axis direction.