Diffractive Optical Element for Interferometric Surface Measurement
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Solution Overview
Problem
High-precision interferometric measurement of surfaces, particularly large and warm optical elements like EUV mirrors, faces challenges due to changes in optical properties of diffractive optical elements caused by temperature fluctuations, leading to reduced measurement accuracy.
Innovation Solution
A measuring arrangement that uses a diffractive optical element with superimposed structural patterns to generate both the test and reference waves, allowing for interferometric determination of surface shape without a separate beam splitter, and includes features like heat shields and athermal materials to minimize thermal errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a diffractive optical element is used to generate test and reference waves separately, then measurement precision is improved, but device complexity increases due to temperature sensitivity
Solution Approach 1:
The patent combines the generation of test waves and reference waves into a single diffractive optical element. The element simultaneously produces both wave types from a single input beam, eliminating the need for separate beam splitters and reducing the number of temperature-sensitive components in the optical path.
Solution Approach 2:
The diffractive optical element performs multiple functions: it acts as both a beam splitter and a wavefront shaper. By encoding multiple diffraction patterns into a single element, it generates the test wave with the desired wavefront adaptation and the reference wave with planar wavefront, making the system more compact and less sensitive to thermal variations.
2Adaptability or versatility
If large test objects are moved into test position, then measurement capability is improved, but heat input increases causing temperature variations
Solution Approach 1:
The patent extracts the reference wave generation from the main measurement path and creates a separate reference arm. This allows the reference wave to be generated and propagated independently, reducing the thermal coupling between the large test object and the sensitive optical elements.
Solution Approach 2:
The diffractive optical element acts as an intermediary that creates the reference wave from the same input beam used for the test wave. This ensures that both waves experience similar thermal conditions up to the point of separation, and the reference wave can serve as a stable reference despite temperature variations in the measurement path.
3Measurement precision
If multiple optical elements are used in beam path, then measurement accuracy is improved through calibration, but thermal errors increase due to more components
Solution Approach 1:
The patent merges the functions of multiple optical elements into a single diffractive optical element. By integrating beam splitting, wavefront shaping, and reference wave generation into one component, the system reduces the total number of optical elements and minimizes cumulative thermal errors while maintaining measurement accuracy.
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 solution enables highly accurate determination of surface shapes for large and warm test objects by compensating for thermal errors and maintaining measurement precision, even at elevated temperatures.
Implementation Method 1
the diffractive optical element is configured to generate, by diffraction in the first or higher order in magnitude, from the input wave, on the one hand, a test wave directed towards the test object
Implementation Method 2
a reflective optical element, which is arranged in the beam path of the reference wave and is designed to reflect the reference wave back
Implementation Method 3
detection device for capturing an interferogram. This interferogram is generated in a detection plane by superimposing the test wave after interaction with the test object and the reflected reference wave
Data Source
Figure 1
Figure 2
Figure 3a~3b
AI summary
The invention relates to a measuring arrangement (10) for interferometrically determining a shape of a surface (12) of a device under test (14). Said measuring arrangement (10) comprises a light source (16) for supplying an input wave (18) as well as a diffractive optical element (24). The diffractive optical element (24) is suitably designed to generate, from the input wave (18) by diffraction, a test wave (26) which is directed to the device under test (14) and which has a wavefront that is at least partially adjusted to an expected shape of the optical surface (12), and a reference wave (28). The measuring arrangement (10) further comprises a reflective optical element (30) for reflecting the reference wave (28) as well as a capturing device (36) for capturing an interferogram generated by superposing the test wave (26), upon its interaction with the device under test (14), and the reflected reference wave (28) following another diffraction of the two waves on the diffractive optical element (24) in a capturing plane (48). The invention further relates to a corresponding method for determining the surface shape of a device under test (14).