Littrow Angle Diffraction Grating Position Measurement
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Solution Overview
Problem
Conventional lithographic apparatus positioning systems face challenges in accurately measuring out-of-plane positions due to the risk of contamination and maintenance issues associated with external reference mirrors, which can lead to focus drifts and increased costs.
Innovation Solution
A positioning system utilizing a diffraction grating and interferometer configuration, where the measurement beam is directed orthogonally to the movement direction and oriented at a Littrow angle, eliminating the need for an external reference mirror and enhancing accuracy and reducing maintenance requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an external reference mirror is used in the positioning system, then the position measurement can be performed, but the risk of contamination increases and maintenance requirements increase
Solution Approach 1:
The patent extracts the reference function from an external reference mirror and integrates it into the diffraction grating itself. The diffraction grating serves dual purposes: as the measurement target and as the reference element, eliminating the need for a separate external reference mirror that is susceptible to contamination.
Solution Approach 2:
The patent merges the reference mirror function with the diffraction grating by orienting the grating at a Littrow angle. This causes the measurement beam to reflect off the grating and return along the same path, making the grating itself serve as the reference, thereby combining multiple functions into a single component.
2Measurement precision
If an external reference mirror is used in the positioning system, then the position measurement can be performed, but maintenance requirements increase and costs increase
Solution Approach 1:
The patent removes the external reference mirror from the system and extracts its reference function into the diffraction grating itself, eliminating the component that requires periodic cleaning and replacement.
Solution Approach 2:
The diffraction grating serves its own reference function through the Littrow configuration, making the system self-sufficient. The grating provides both the measurement target and the reference, eliminating the need for external maintenance of separate reference mirrors.
3Measurement precision
If a 45 degrees mirror is used on the substrate table to position interferometers in the same plane, then out of plane position measurement is enabled, but the system complexity increases
Solution Approach 1:
The patent removes the 45-degree mirror from the system and extracts the out-of-plane measurement capability directly into the diffraction grating configuration. The Littrow-angle oriented grating enables vertical position measurement without requiring additional mirrors or complex optical paths.
Solution Approach 2:
Instead of using a 45-degree mirror to redirect the beam for vertical measurement, the patent inverts the approach by orienting the diffraction grating itself at the Littrow angle to the vertical axis. This causes the measurement beam to interact with the grating in a way that directly provides out-of-plane position information through the diffraction pattern.
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 provides more accurate position measurement with reduced maintenance needs, minimizing contamination risks and allowing for a more compact system design, thereby improving the reliability and efficiency of the lithographic apparatus.
Implementation Method 1
an interferometer, wherein the interferometer is configured to direct a measurement beam to the diffraction grating
Implementation Method 2
a diffraction grating, wherein the diffraction grating is oriented relative to the interferometer such that the measurement beam is substantially at a Littrow angle of the diffraction grating so that a diffracted beam to be received by the interferometer
Data Source
AI summary
The invention provides a position measurement system to measure a position of an object in a movement direction relative to a reference, said position measurement system comprising: —a diffraction grating, and —an interferometer, wherein the interferometer is configured to direct a measurement beam to the diffraction grating in a measuring direction that is orthogonal to the movement direction of the object, and wherein the diffraction grating is oriented relative to the interferometer such that the measurement beam is substantially at a Littrow angle of the diffraction grating so that a diffracted beam to be received by the interferometer is substantially parallel to the measuring direction.