Iterative Reference Wave Refinement for Optical Surface Characterization
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Solution Overview
Problem
Existing methods for characterizing the surface form of optical elements in microlithographic projection exposure apparatuses, particularly mirrors, are inaccurate due to neglecting or insufficiently accounting for errors in the interferometric measurement setup, leading to incorrect characterization of the optical elements.
Innovation Solution
An iterative method where the reference wave is treated as a variable parameter, alternately determined alongside the optical element's figure in each iteration step, using forward and backward calculations to improve accuracy, and incorporating a grid transformation to refine the characterization process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional interferometric measurement methods are used to characterize the surface form of optical elements, then the measurement process can be completed, but the characterization accuracy is insufficient due to neglecting or insufficiently accounting for errors in the interferometric measurement setup
Solution Approach 1:
The patent applies feedback by using the measured surface form data to update and refine the reference wave model iteratively. The measured deviations from the nominal surface form are fed back into the reference wave definition, allowing the measurement system to compensate for its own errors and achieve higher accuracy in characterizing the optical element's surface form.
Solution Approach 2:
The measurement system performs self-calibration by using the measured data to improve its own reference wave model. The system serves itself by automatically updating the reference wave based on measured deviations, eliminating the need for external calibration procedures and enabling continuous improvement of measurement accuracy.
2Device complexity
If the reference wave is treated as a fixed nominal value, then the calculation process is simpler, but the characterization accuracy deteriorates due to unaccounted errors in the measurement setup
Solution Approach 1:
The patent transforms the static reference wave concept into a dynamic, adaptive model. Instead of using a fixed nominal reference wave, the system iteratively updates the reference wave based on measured deviations, allowing it to adapt to actual measurement conditions and compensate for setup errors, thereby improving characterization accuracy.
Solution Approach 2:
The patent changes the reference wave parameter from a fixed nominal value to a variable that is continuously refined based on measurement data. By allowing the reference wave parameters to change and update iteratively according to measured surface form deviations, the system achieves higher measurement precision while managing calculation complexity through structured iteration.
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 enables a more accurate and reliable characterization of the optical element's surface form by iteratively refining the reference wave and figure determination, resulting in improved stitching and interferogram processing, thus enhancing the precision of surface form characterization.
Implementation Method 1
carrying out a plurality of interferometric measurements, in each of which an interferogram is recorded between a test wave emanating from a portion of the optical element in each case and a reference wave
Data Source
AI summary
A method and apparatus for characterizing the surface form of an optical element, in particular a mirror or a lens element of a microlithographic projection exposure apparatus, includes: carrying out a plurality of interferometric measurements, in each of which an interferogram is recorded between a test wave emanating from a portion of the optical element in each case and a reference wave, the position of the optical element relative to the test wave being altered between these measurements, and calculating the figure of the optical element on the basis of these measurements. This calculation is carried out iteratively such that, in a plurality of iteration steps, the figure of the optical element is ascertained in each case by carrying out a forward calculation, each of these iteration steps being based in each case on a reference wave that was adapted based on the preceding iteration step.