Interferometer Calibration for Tilt-Dependent Cyclic Error Compensation
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Solution Overview
Problem
Existing interferometer systems in lithographic apparatuses suffer from complex cyclic errors, particularly tilt-dependent non-linearities, which are not adequately addressed by conventional calibration methods, leading to significant machine downtime and reduced accuracy in overlay performance.
Innovation Solution
A method and system for calibrating optical measurement systems using a parametric model with superimposed complex-valued functions to capture tilt-dependent oscillation patterns, allowing for rapid recalibration by updating a subset of model parameters based on reduced data points.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional calibration methods are used to measure and compensate cyclic errors, then measurement accuracy is improved, but calibration time increases significantly causing machine downtime
Solution Approach 1:
The patent performs a comprehensive calibration measurement once to establish error model parameters, then uses these pre-determined parameters for rapid periodic compensation without requiring full recalibration. This preliminary action captures the cyclic error characteristics initially, enabling subsequent fast updates.
Solution Approach 2:
The patent changes the calibration approach from measuring all error parameters repeatedly to updating only a subset of parameters periodically. The error model parameters are determined once comprehensively, then selectively updated based on monitored changes, reducing calibration time while maintaining accuracy.
2Measurement precision
If comprehensive error measurement is performed to capture all cyclic error components, then measurement accuracy is improved, but the complexity of the calibration process increases
Solution Approach 1:
The patent segments the calibration process into two distinct phases: an initial comprehensive measurement phase to establish the complete error model parameters, and subsequent simplified update phases that only require monitoring changes in specific parameters. This segmentation reduces the complexity of periodic calibration while maintaining comprehensive error coverage.
Solution Approach 2:
The comprehensive error model parameter determination is performed as a preliminary action before normal operation. This one-time thorough measurement captures all cyclic error components, allowing the system to operate with simplified periodic updates that only track parameter changes rather than re-measuring everything.
3Measurement precision
If frequent recalibration is performed to maintain accuracy, then measurement precision is improved, but productivity decreases due to repeated machine downtime
Solution Approach 1:
The patent implements selective parameter updating where only changes in error model parameters trigger recalibration activities. By monitoring parameter stability and updating only when necessary, the system maintains overlay accuracy without requiring frequent full recalibrations, thus preserving machine productivity.
Solution Approach 2:
The system performs periodic monitoring of error model parameters and updates the calibration only when parameter changes exceed thresholds or at scheduled intervals. This periodic action with conditional updates maintains measurement precision while minimizing interruptions to machine productivity compared to continuous or frequent recalibration.
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
The method significantly reduces calibration time from hours to minutes, maintaining high accuracy and minimizing machine downtime, while effectively compensating for complex cyclic errors in interferometers.
Implementation Method 1
The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object
Implementation Method 2
The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam
Implementation Method 3
the measured beat frequency includes a Doppler shift equal to 2unp/l, where u is the relative speed of the measurement and reference objects
Data Source
Figure 1
Figure 2~3
Figure 4~5
AI summary
The present disclosure provides a method for calibrating an optical measurement system involving radiation interference, the method comprising the steps of: determining an error of the optical measurement system during a first measurement, wherein a target of the optical measurement system is tilted in one or more degrees of freedom; defining parameters of an error model function based on the first measurement; determining an error of the optical measurement system during at least one second measurement, wherein the at least one second measurement comprises fewer data points than the first measurement; and updating a subset of the parameters of the error model function to define an updated error model function based on the at least one second measurement.