Optical Element Surface Shape Characterization via Polarization Interferometry
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Solution Overview
Problem
In microlithography, especially in EUV projection lenses, the surface shape characterization of optical elements is hindered by polarization-induced phase components and defects in diffractive structures, leading to inaccurate interferogram measurements and figure determination.
Innovation Solution
A method involving multiple interferogram measurements on calibration mirrors with different input polarizations to determine calibration corrections, which are then used to accurately characterize the surface shape of optical elements by reducing polarization-induced phase components and identifying structure defects in the diffractive element.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If interferometric measurement methods are used to test mirrors in EUV projection lenses, then measurement precision is improved, but polarization-induced phase components and diffractive structure defects introduce errors that worsen measurement accuracy
Solution Approach 1:
The measurement process is segmented into multiple independent interferogram measurements taken with different input polarizations (e.g., s-polarization and p-polarization). Each measurement captures phase information affected differently by polarization-induced errors and diffractive structure defects. By separating the total phase into multiple polarization-dependent components, the method enables identification and removal of systematic errors through comparison and analysis of the segmented measurements.
Solution Approach 2:
The input polarization parameter is deliberately changed between measurements to create a set of interferograms with different error characteristics. By varying the polarization state (e.g., switching between orthogonal linear polarizations), the method exploits the different responses of polarization-induced phase components and diffractive structure defects to polarization changes, enabling mathematical separation and correction of these error sources from the measured phase data.
2Measurement precision
If multiple interferogram measurements with different polarizations are performed, then accuracy of surface shape characterization is improved, but measurement time and process complexity increase
Solution Approach 1:
The method performs preliminary interferogram measurements with different polarizations to characterize and quantify polarization-induced phase components and diffractive structure defects before final surface shape determination. These preliminary measurements establish error profiles that can be used to correct subsequent measurements, reducing the need for repeated measurements and minimizing total measurement time while maintaining high accuracy.
Solution Approach 2:
The measurement process incorporates feedback by using results from preliminary polarization-varying measurements to inform and correct subsequent surface shape characterization. The phase information extracted from multiple polarization states provides feedback that enables identification of systematic errors, which are then compensated for in the final measurement, reducing the need for additional corrective measurements and optimizing measurement efficiency.
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 significantly enhances the accuracy of surface shape characterization by isolating and correcting for phase components caused by the diffractive element, resulting in a more precise determination of the optical element's figure and structure defects.
Implementation Method 1
a test wave that was generated by diffraction of electromagnetic radiation at a diffractive element
Implementation Method 2
carrying out at least one first interferogram measurement on the optical element in an interferometric test arrangement by way of superposing on a test wave... a reference wave
Data Source
AI summary
Methods for characterizing the surface shapes of optical elements include the following steps: carrying out, in an interferometric test arrangement, at least a first interferogram measurement on the optical element by superimposing a test wave, which has been generated by diffraction of electromagnetic radiation on a diffractive element and has been reflected at the optical element, carrying out at least one additional interferogram measurement on in each case one calibrating mirror for determining calibration corrections, and determining the deviation from the target shape of the optical element based on the first interferogram measurement carried out on the optical element and the determined calibration corrections. At least two interferogram measurements are carried out for the at least one calibrating mirror, which differ from one another with regard to the polarization state of the electromagnetic radiation.