Interferometer Mode Hop Detection via Phase Shift Analysis
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Solution Overview
Problem
Existing interferometer systems in lithographic apparatuses face challenges in accurately determining the absolute position of movable objects due to susceptibility to mode hops in laser sources, which can occur from temperature, current variations, or external vibrations, affecting measurement accuracy.
Innovation Solution
An interferometer system is designed with a processing unit that determines mode hops by analyzing phase shifts in detector signals resulting from optical path length differences between first and second beams, allowing for the identification and mitigation of mode hops during position measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a separate zeroing sensor is provided to determine absolute position, then measurement accuracy is improved, but device complexity increases
Solution Approach 1:
The patent combines the zeroing sensor functionality with the existing interferometer system by using the same laser source and optical components. The zeroing sensor shares the laser source with the interferometer, merging multiple functions into a unified system that determines both relative displacement and absolute position without requiring completely separate sensing systems.
Solution Approach 2:
The laser source serves multiple functions: it provides the measurement beam for the interferometer to determine relative displacement, and simultaneously provides the beam for the zeroing sensor to determine absolute position. This multi-functional use of the laser source eliminates the need for separate light sources and reduces overall system complexity.
2Adaptability or versatility
If the measurement range is extended beyond the zeroing sensor's range, then versatility is improved, but measurement precision deteriorates
Solution Approach 1:
The patent uses the interferometer as an intermediary system that bridges the gap between the zeroing sensor's limited range and the need for extended measurement capability. The zeroing sensor provides absolute position within its limited range, while the interferometer continuously tracks relative displacement over much larger ranges. By combining these two measurements, the system achieves both extended versatility and maintained precision.
Solution Approach 2:
The patent replaces the need for a mechanically extended zeroing sensor range with an optical integration approach. Instead of physically extending the zeroing sensor's measurement capability, the system uses optical interference measurement to track displacements beyond the zeroing sensor's range, substituting mechanical limitations with optical measurement capabilities.
3Measurement precision
If mode hop detection is implemented, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent implements mode hop detection by monitoring the interferometer signal for characteristic discontinuities that indicate mode hops. When a mode hop is detected, the system provides feedback to correct or discard the affected measurement data. This feedback mechanism maintains measurement precision by identifying and compensating for mode hop errors without requiring complex additional hardware.
Solution Approach 2:
The interferometer system performs self-diagnosis for mode hops by analyzing its own measurement signal. The processing unit detects mode hops by examining the interferometer output for characteristic patterns, allowing the system to identify and correct its own measurement errors without external intervention or additional specialized sensors.
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 enhances the accuracy of position measurements by accounting for mode hops, ensuring reliable determination of absolute positions without the need for additional zeroing sensors and maintaining measurement precision despite external influences.
Implementation Method 1
an optical system arranged to split the radiation beam into a first beam along a first optical path and a second beam along a second optical path, wherein the optical system is arranged to recombine the first beam and the second beam to a recombined beam
Implementation Method 2
a detector to receive the recombined beam and to provide a detector signal on the basis of the received recombined beam
Data Source
AI summary
An interferometer system including: an optical system arranged to split a radiation beam from a laser source into a first beam along a first optical path and a second beam along a second optical path, and recombine the first beam and the second beam to a recombined beam, a detector to receive the recombined beam and to provide a detector signal based on the received recombined beam, and a processing unit, wherein a first optical path length of the first optical path and a second optical path length of the second optical path have an optical path length difference, and wherein the processing unit is arranged to determine a mode hop of the laser source on the basis of a phase shift in the detector signal.