Birefringent Encoder Head for Lithography
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Solution Overview
Problem
Conventional encoder heads for lithographic exposure systems are structurally complex, require multiple optical components, and face challenges in achieving precise positioning due to the need for multiple corner-cube prisms, which limits the size of the measurement beam and increases the number of grating lines or grooves required for accurate measurement.
Innovation Solution
The use of a single, substantially geometrically perfect optically-isotropic cuboid combined with a birefringent optical wedge in the encoder head allows for imperfect retroreflection of the measurement beam, enabling it to interact with the diffraction grating at the same location on both passes, reducing the number of optical components and simplifying the structure while maintaining high precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple corner-cube prisms are used in the encoder head, then retroreflection capability is improved, but device complexity increases and measurement beam size is limited
Solution Approach 1:
The patent combines multiple corner-cube prism functions into a single integrated optical component. The encoder head uses one optically-isotropic cuboid and one birefringent optical wedge that work together to provide the retroreflection capability previously requiring multiple separate prisms. This merging reduces the number of optical components while maintaining the necessary retroreflection functionality for precise wafer alignment measurement.
Solution Approach 2:
The patent employs composite optical materials with different properties - specifically combining an optically-isotropic cuboid with a birefringent optical wedge. The isotropic material provides geometric precision and the birefringent material provides polarization-dependent optical path control. This composite approach enables the simplified single-component design to achieve the complex optical functions previously requiring multiple homogeneous prisms.
2Reliability
If multiple corner-cube prisms are used in the encoder head, then retroreflection is achieved, but the size of the measurement beam is reduced
Solution Approach 1:
By merging the retroreflection functions into a single integrated optical system comprising one isotropic cuboid and one birefringent wedge, the patent eliminates the beam-splitting and spatial separation effects caused by multiple discrete prisms. This allows the measurement beam to maintain a larger cross-sectional area throughout the optical path, improving light efficiency and measurement sensitivity.
3Reliability
If multiple corner-cube prisms are used in the encoder head, then retroreflection capability is maintained, but the number of grating lines required increases
Solution Approach 1:
The birefringent optical wedge introduces polarization-dependent optical path differences that enhance the measurement signal. This allows the diffraction grating to use fewer lines while achieving the same measurement precision, because the birefringent material amplifies the phase modulation effect on the measurement beam.
4Device complexity
If a single optically-isotropic cuboid combined with a birefringent optical wedge is used, then device complexity is reduced, but achieving precise positioning becomes more challenging
Solution Approach 1:
The combination of optically-isotropic and birefringent materials creates complementary effects: the isotropic cuboid provides stable geometric reference and the birefringent wedge provides polarization-dependent phase modulation. This composite approach actually enhances measurement precision by creating more robust optical interference patterns for position detection, while using fewer components.
Solution Approach 2:
The birefringent optical wedge changes the optical path length differently for different polarization components of the light beam. This parameter change in the optical path creates enhanced interference patterns that improve positioning accuracy, compensating for the simplified structure.
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 configuration results in a compact, cyclic non-linear error-free encoder head with improved sensitivity and light efficiency, allowing for larger measurement beams and reduced geometric errors, thereby enhancing the accuracy and efficiency of wafer alignment in lithographic exposure systems.
Implementation Method 1
a birefringent optical wedge in the encoder head allows for imperfect retroreflection of the measurement beam
Implementation Method 2
an optical surface disposed to redirect a portion of said input beam of light, incident thereon at a chosen location, towards said combination. The retroreflected beam of light is redirected to interact with the optical surface at the very same chosen location
Implementation Method 3
a wafer-stage carrying a diffraction grating... retroreflect said initial measurement beam of light, that has impinged onto a location of the diffraction grating and then diffracted at the location
Data Source
AI summary
An encoder head configured for use with a lithographic exposure tool. The head is devoid of the multiplicity of optical corner-cubes and includes, instead, a single, geometrically substantially perfect cuboid of optically-isotropic material complemented, in operation, with prismatic elements made of optically anisotropic material to form a contraption that, as a unit, splits a single beam of light delivered to the contraption into four measurement (sub-)beams of light (two in xz-plane, two in yz-plane) and causes each of these sub-beams to interact with the wafer-stage diffraction grating at the same location upon the second pass by the grating as upon the first pass by the grating, thereby solving problems of (i) structural complexity of a conventional encoder head for use in an exposure tool, (ii) burdensome alignment of the multitude of optical prisms in the process of forming such encoder head, and (iii) cyclic non-linear errors associated with measurements involving conventional corner-cubes-based encoder heads while, at the same time, reducing the geometrical footprint of the encoder head.