Interferometric Mirror Surface Measurement Under Installation Deflection
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Solution Overview
Problem
The interferometric determination of mirror surfaces in microlithographic projection exposure apparatuses is challenged by differences in installation positions between the actual optical system and the testing environment, leading to optical aberrations due to gravitational effects and fixed installation positions, which cause deflections and deformations, especially in large mirrors with low stiffness.
Innovation Solution
The method involves generating a reference wave from a test wave reflected at a reference surface upstream of the test object, considering the specific installation positions in both systems, and adjusting the tilt angles and stiffnesses to minimize the effects of different orientations and deformations, while accounting for systematic errors through advanced measurements and adaptations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If mirrors with larger surface areas are used to increase image-side numerical aperture, then the imaging performance is improved, but the mirrors become more susceptible to gravitational deflection and installation position errors
Solution Approach 1:
The patent replaces mechanical support structures with an interferometric measurement system that uses optical fields (test wave and reference wave) to detect surface shape. This substitution allows for contactless, high-precision measurement that does not mechanically disturb the mirror, thereby maintaining surface shape stability while accommodating large mirror areas.
Solution Approach 2:
The patent introduces a reference wave as an intermediary element that mediates between the test wave and the measurement system. This reference wave, generated by a reference surface, serves as a stable comparison standard that enables precise detection of surface shape deviations without physically contacting or constraining the large mirror being measured.
2Device complexity
If interferometric measurement is performed with fixed installation positions, then the measurement setup is simplified, but gravitational effects cause deflection differences between test and measurement positions
Solution Approach 1:
The patent changes the measurement parameters by using wavelength-based optical measurement instead of mechanical measurement. This allows the system to measure surface shapes with nanometer precision while accounting for gravitational effects through the interference pattern analysis, thereby maintaining measurement accuracy without requiring identical installation positions.
Solution Approach 2:
The patent creates an optical copy of the reference surface through the reference wave, which can be compared against the test object's surface reflection. This optical copying allows the reference standard to be virtually present in the measurement without requiring physical repositioning, thereby simplifying the setup while maintaining measurement precision.
3Measurement precision
If tilt angles are adjusted to match installation positions, then systematic errors are reduced, but the alignment process becomes more complex
Solution Approach 1:
The patent implements feedback through the interferometric measurement process itself. The interference pattern between test and reference waves provides real-time information about misalignment and systematic errors, allowing for iterative adjustment of tilt angles until the measurement accuracy is optimized, thereby reducing systematic errors through measured feedback rather than theoretical calculation.
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 reduces error contributions from installation position differences, minimizing optical aberrations and ensuring accurate surface shape determination of test objects, particularly in EUV mirrors, by aligning tilt angles and stiffnesses to match both systems and correcting for systematic errors.
Implementation Method 1
The determination of the surface shape of the respective mirror or test object is based on an interferometric superposition of a test wave having a wavefront adapted to the target shape of the surface of the test object and a reference wave
Implementation Method 2
generate the reference wave required for interferometric measurement or superposition with the test wave reflected at the test object by being split from the test wave, which is achieved in reflection via a reference surface
Data Source
AI summary
Methods for interferometrically determining the surface shape of a test object. In one aspect, in the test arrangement (100) a test wave, generated from electromagnetic radiation and reflected from the test object (110), is overlaid with a reference wave, which is split in reflection from the test wave at a reference surface (121) of a reference element (120) before this test wave hits the test object. The test object is designed for installation in a predefined installation position in an optical system (700). The reference surface is designed based on a measurement previously carried out on the reference element in a predefined installation position in a measuring system (800). The surface shape of the test object in the test arrangement is determined taking account of both the installation position of the test object in the optical system (700) and the installation position of the reference element in the measuring system.