Lithography Encoder Calibration via Interferometer Reference
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Solution Overview
Problem
The long-term stability of encoder-type position measurement devices in lithographic apparatuses is limited, requiring frequent calibration to maintain accuracy, which can lead to significant downtime and errors due to physical imperfections and changes in refractive index.
Innovation Solution
A method is introduced to update calibration data using a second independent position detection system, calculating differences in position measurements made during normal operation to correct for physical imperfections in the first position detection system, thereby improving accuracy and reducing downtime.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If encoder-type position measurement system is used to improve measurement precision, then accuracy is improved, but long-term stability deteriorates requiring frequent calibration
Solution Approach 1:
The patent combines two independent position detection systems (encoder and interferometer) into a unified measurement system. The encoder provides high-precision measurements during normal operation, while the interferometer serves as a stable reference for periodic calibration. This merging allows the system to leverage the strengths of both technologies simultaneously, maintaining high accuracy without sacrificing long-term stability.
Solution Approach 2:
The patent implements a feedback mechanism where the interferometer continuously monitors the encoder's drift over time. By comparing the encoder readings against the interferometer's stable reference measurements, the system generates correction factors that are fed back to compensate for the encoder's instability. This closed-loop feedback ensures long-term accuracy is maintained despite the encoder's inherent drift characteristics.
2Measurement precision
If frequent calibration is performed to maintain accuracy of first position detection system, then measurement precision is improved, but productivity deteriorates due to significant downtime
Solution Approach 1:
The patent enables continuous calibration by having the interferometer operate in parallel with the encoder during normal lithographic operations. Instead of stopping production for calibration, the interferometer continuously provides reference measurements that allow real-time drift compensation. This continuous calibration approach eliminates downtime while maintaining measurement precision.
Solution Approach 2:
The system performs preliminary drift characterization during manufacturing and operation, building up a database of drift patterns over time. This preliminary data collection allows the system to predict and pre-compensate for upcoming drift issues, maintaining accuracy without requiring interruptive calibration events.
3Measurement precision
If encoder is used to reduce the distance radiation beams travel, then refractive index effects are reduced improving accuracy, but long-term stability deteriorates
Solution Approach 1:
The interferometer acts as an intermediary reference system that mediates between the encoder's high-precision but unstable measurements and the need for long-term stability. By introducing this third, highly stable measurement system, the patent enables the encoder to operate at its full precision potential while the interferometer periodically corrects for drift, effectively decoupling the precision and stability requirements.
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 allows for continuous and accurate calibration of the first position detection system, reducing errors and downtime by leveraging the stability of the second position detection system, while minimizing random errors and ensuring systematic corrections.
Implementation Method 1
the position of a table, for example a substrate table, is measured using a laser interferometer. In this system, gratings are placed on the edge of the table and laser beams shined at the gratings. The reflection is measured by a sensor positioned next to the laser light source.
Implementation Method 2
A 2D grating target is positioned on the table or above the area at which the table is being used for processes which require accurate positional measurement of the table. Encoders comprising an emitter and a receiver are positioned at the other of the table and above the table. This has the benefit that the distance any radiation beams need to travel is reduced (so that any change in refractive of the medium through which the radiation beam passes has a smaller effect)
Data Source
AI summary
A method of updating calibration data of a first position detection system adapted to determine the position of an object, is presented. The first position detection system includes a target and a plurality of sensors one of which is mounted on an object and the calibration data including coefficients relating an apparent measured position to an actual position and which can be used to convert an apparent measured position to an actual position thereby to correct for physical imperfections in the first position detection system and enable determination of the actual position from the apparent measured position.