Encoder Scale Calibration Using Multiple Sensors
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Solution Overview
Problem
Conventional encoder scales in lithographic apparatuses face calibration challenges due to fabrication inaccuracies, damage, and contamination, which affect position measurement accuracy and cannot be adequately addressed by initial calibration methods.
Innovation Solution
A method involving multiple encoder-type sensors spaced at fixed distances to create position signals, allowing for the creation of a calibration map that corrects for errors in the encoder scale, enabling straightness calibration and improving accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If an encoder scale is manufactured with high precision, then initial calibration accuracy is improved, but errors are still introduced during mounting and use that cannot be corrected by initial calibration
Solution Approach 1:
The patent performs a first calibration by the manufacturer after manufacturing to establish initial accuracy, then enables a second calibration after installation to correct errors introduced during mounting and use. This preliminary action followed by subsequent correction resolves the contradiction by addressing both manufacturing precision and installation-induced errors.
Solution Approach 2:
The system uses the lithographic apparatus itself to perform the second calibration of the encoder scale, rather than requiring external calibration equipment. The apparatus measures its own position errors and generates correction data, enabling self-calibration that accounts for mounting errors and environmental factors.
2Device complexity
If a single encoder-type sensor is used, then device complexity is reduced, but measurement accuracy is insufficient to detect and correct scale errors
Solution Approach 1:
The patent divides the measurement function into multiple sensors (first, second, and third encoder-type sensors) spaced at different distances from the encoder scale. Each sensor provides independent measurement data, and by comparing these segmented measurements, the system can detect and correct scale errors more accurately than with a single sensor.
Solution Approach 2:
The patent extends the measurement from a single point to multiple points along the direction of movement. By using sensors at different positions (different distances from the scale), the system adds spatial dimensionality to the measurement, enabling detection of scale errors that would be invisible to a single sensor.
3Loss of time
If calibration is performed only by the manufacturer, then initial setup time is reduced, but calibration cannot account for errors introduced after manufacturing
Solution Approach 1:
The manufacturer performs a first calibration before shipping the encoder scale, which reduces initial setup time. Then, after installation in the lithographic apparatus, a second calibration is performed to correct mounting errors. This two-stage approach balances time efficiency with measurement accuracy.
Solution Approach 2:
The system performs a second calibration after installation by measuring the actual position of the movable object and comparing it with the encoder scale readings. This feedback loop identifies errors introduced during mounting and generates correction data, ensuring long-term measurement accuracy without requiring excessive initial calibration time.
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 method enhances the accuracy of position measurement systems by accounting for errors introduced during installation and use, ensuring precise calibration of encoder scales within the lithographic apparatus.
Implementation Method 1
The scale marks are readable by the encoder-type sensor such that by movement of the sensor relatively to the encoder scale a signal representative of relative movement between the encoder-type sensor and the encoder scale in the first direction may be obtained
Data Source
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AI summary
A method for calibrating an encoder scale having an array of marks in a first direction, comprising the step of: moving the encoder scale in the first direction relative to a first encoder-type sensor, a second encoder-type sensor and a third encoder-type sensor, wherein the first encoder-type sensor and the second encoder-type sensor are fixedly spaced in the first direction at a first distance relative to each other, wherein the second encoder-type sensor and the third encoder-type sensor are fixedly spaced in the first direction at a second distance relative to each other.