EUV Lens Imaging Quality Measurement Using Diffractive Test Structure
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Solution Overview
Problem
Existing methods for measuring wavefront aberrations in EUV lenses face difficulties in achieving high measurement accuracy due to the high magnification scale required in semiconductor industry applications, particularly with interferometric methods, which are not effective for EUV lithography optical units.
Innovation Solution
A measuring system utilizing a diffractive test structure that radiates measurement light in the EUV wavelength range, varying image-determining parameters, and an evaluation device to determine the imaging quality of the lens from an image stack, ensuring high spatial resolution and accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If interferometric measuring methods are used for EUV lithography optical units, then measurement accuracy can be achieved for systems with reduction factor of 4:1, but the method becomes ineffective for systems with high magnification scale
Solution Approach 1:
The patent changes the fundamental measurement approach from interferometric methods to non-interferometric phase retrieval methods. This parameter change in the measurement technique enables adaptation to high magnification scales while maintaining measurement accuracy, resolving the contradiction between measurement precision and adaptability to different magnification scales
Solution Approach 2:
The patent replaces the interferometric measurement system with a phase retrieval system that uses phase diversity techniques. This substitution introduces a known phase shift between reference and test wavefronts, enabling accurate wavefront measurement for high magnification EUV lenses without relying on interferometric methods that are ineffective for such systems
2Ease of manufacture
If a non-optimized wavefront source is used, then the measurement system is simpler to implement, but artificial wavefront aberrations are introduced due to the source topography
Solution Approach 1:
The patent applies preliminary optimization to the wavefront source design by shaping it to produce a spherical wavefront with maximum deviation of 0.1 nm or less from the target shape. This preliminary action ensures that the source does not introduce artificial aberrations, maintaining measurement precision while the overall system remains implementable
Solution Approach 2:
The patent applies different quality requirements to different parts of the measurement system. The wavefront source requires high precision (0.1 nm deviation) to avoid introducing artificial aberrations, while other components can have relaxed tolerances. This local quality approach maintains measurement accuracy without requiring the entire system to be overly complex
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 precise measurement of EUV lens imaging quality with high accuracy, overcoming the limitations of existing methods by using a diffractive test structure to generate a well-defined wavefront and phase retrieval methods for evaluation.
Implementation Method 1
the measurement light is radiated onto the lens with a wavefront having the shape of a conic section in cross section
Implementation Method 2
phase retrieval methods for evaluation
Data Source
AI summary
A measuring system (10) for measuring an imaging quality of an EUV lens (30) includes a diffractive test structure (26), a measurement light radiating device (16) which is configured to radiate measurement light (21) in the EUV wavelength range onto the test structure, a variation device (28) for varying at least one image-determining parameter of an imaging of the test structure that is effected by a lens, a detector (14) for recording an image stack including a plurality of images generated with different image-determining parameters being set, and an evaluation device (15) which is configured to determine the imaging quality of the lens from the image stack.