Interferometric Surface Measurement Diffractive Optical Element
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Solution Overview
Problem
Existing interferometry methods for measuring non-spherical surfaces, such as those in microlithographic optical elements, suffer from measurement inaccuracies due to isolated flaws in the diffraction structure of computer-generated holograms (CGHs), leading to incorrect intensity values in the interference pattern.
Innovation Solution
A measurement apparatus and method using a diffractive optical element and a reference element with a low thermal expansion material, where the wavefronts of the test and reference waves are configured to be similar or identical, reducing the impact of structural flaws and allowing for more accurate surface shape determination by minimizing phase errors in both waves.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a computer-generated hologram (CGH) is used to generate test and reference waves for interferometric measurement, then the measurement capability for non-spherical surfaces is enabled, but isolated flaws in the diffraction structure cause errors in the interference pattern leading to measurement inaccuracies
Solution Approach 1:
The patent changes the physical parameter of the reference element by using low thermal expansion material with a mean coefficient of thermal expansion ≤200×10⁻⁶ K⁻1. This parameter change ensures the reference element maintains dimensional stability under varying thermal conditions, preventing thermal deformation from affecting measurement accuracy and complementing the flaw mitigation strategy
Solution Approach 2:
The patent converts the harmful effect of CGH flaws into a beneficial situation by making both test and reference waves subject to similar phase errors. By configuring similar wavefronts and using identical CGH regions for both waves, the flaws affect both waves uniformly, and their differences cancel out in the interference pattern, transforming the potential source of error into a robust measurement approach
2Adaptability or versatility
If the test wave and reference wave have different wavefront configurations, then the measurement setup can accommodate various surface types, but structural flaws in the CGH cause selective interfering reflections that falsify the interference pattern
Solution Approach 1:
The patent changes the wavefront configuration parameter by adapting both the test wave and reference wave to have similar or identical wavefronts. This is achieved by configuring the reference element with a reference shape that matches the target shape, ensuring both waves experience similar phase transformations and reducing the impact of selective interfering reflections from CGH flaws
Solution Approach 2:
The patent applies homogeneity by making the test and reference waves as similar as possible in their wavefront characteristics. By using the same CGH region for both waves and configuring the reference element to match the target shape, the waves become homogeneous in their interaction with the CGH structure, causing flaws to affect both waves uniformly rather than selectively
3Device complexity
If conventional reference elements are used without low thermal expansion material, then the device complexity is reduced, but thermal expansion causes deformation of the reference shape leading to measurement errors
Solution Approach 1:
The patent changes the material parameter of the reference element by selecting low thermal expansion material with a mean coefficient of thermal expansion ≤200×10⁻⁶ K⁻1 in the temperature range from 5°C to 35°C. This parameter change ensures the reference element maintains its reference shape stability under varying thermal conditions, preventing thermal deformation from introducing measurement errors
Solution Approach 2:
The patent directly addresses thermal expansion by selecting materials with low thermal expansion coefficients. The reference element is made from material whose thermal expansion behavior is controlled and predictable, ensuring that temperature variations do not cause significant dimensional changes that would affect the reference shape and subsequent measurements
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 enhances the accuracy of surface measurements by uniformly affecting both test and reference waves, reducing interference pattern falsification and enabling precise adaptation of the test object surface to a reference shape, even in high-pressure environments.
Implementation Method 1
a diffractive optical element for generating a test wave from measurement radiation
Implementation Method 2
the reference element comprising a low thermal expansion material with a mean coefficient of thermal expansion with an absolute value of no more than 200×10−6 K−1 in the temperature range from 5° C. to 35° C.
Implementation Method 3
by superimposing the test wave reflected by the surface with a reference wave, it is possible to determine deviations from the target shape very precisely
Data Source
AI summary
A measurement apparatus (10) for interferometrically measuring a shape of a surface (12) of a test object (14) in relation to a reference shape includes a diffractive optical element (30) generating a test wave (32) from measurement radiation (22), whereas a wavefront of the test wave is adapted to a target shape of the surface of the test object and the target shape is configured as a first non-spherical surface, and a reference element (38) with a reference surface (40) having the reference shape, the reference shape being configured as a further non-spherical surface and the reference element including a low thermal expansion material with a mean coefficient of thermal expansion having an absolute value of no more than 200×10−6 K−1 in the temperature range from 5° C. to 35° C.