Compensation Optical Unit for Interferometric Measurement Precision

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Solution Overview

Problem

Interferometric measurement systems face challenges in maintaining high accuracy due to changes in optical properties of components like computer-generated holograms (CGHs) caused by temperature changes and limitations in manufacturing accuracy of diffractive structures, especially for high numerical aperture test surfaces.

Innovation Solution

A compensation optical unit comprising a first optical element and a diffractive optical element, where at least 20% of the refractive power is allocated to the first element, reducing the refractive power of the diffractive element and minimizing manufacturing inaccuracies, and allowing for precise wavefront adaptation and error compensation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If the refractive power is concentrated in the diffractive optical element (CGH), then the wavefront can be adapted to the target shape, but manufacturing inaccuracies and electromagnetic effects increase, reducing measurement precision

Engineering Contradiction:

Improvemeasurement precisionVSAvoidmanufacturing precision of diffractive structures

Core Design Contradiction:

Measurement precisionVSManufacturing precision

Solution Approach 1:

The compensation optical unit is divided into two separate optical elements: a refractive optical element and a diffractive optical element. The refractive element handles the majority of wavefront adaptation (at least 20% of refractive power), while the diffractive element provides additional correction. This segmentation reduces the manufacturing complexity and electromagnetic effects on the diffractive structures, thereby improving measurement precision.

Inventive Principle:

Principle #1Segmentation

2Adaptability or versatility

If the CGH stripe density is increased to measure high numerical aperture surfaces, then the measurement capability improves, but electromagnetic effects increase, limiting accuracy

Engineering Contradiction:

Improvecapability to measure high numerical aperture surfacesVSAvoidaccuracy of diffractive fringe patterns

Core Design Contradiction:

Adaptability or versatilityVSMeasurement precision

Solution Approach 1:

By separating the optical compensation into refractive and diffractive elements, the diffractive element no longer needs to handle the full range of wavefront adaptations. This allows the use of lower stripe densities in the CGH while still achieving the required measurement capability for high numerical aperture surfaces, thereby reducing electromagnetic effects and improving accuracy.

Inventive Principle:

Principle #1Segmentation

3Measurement precision

If the refractive power of the compensation optical unit is increased to improve wavefront adaptation, then the measurement accuracy improves, but the sensitivity to temperature changes increases due to temperature dependence of refractive index

Engineering Contradiction:

Improvemeasurement accuracyVSAvoidstability of optical properties against temperature changes

Core Design Contradiction:

Measurement precisionVSStability of the object's composition

Solution Approach 1:

The invention changes the physical parameter distribution by allocating refractive power to a refractive optical element whose optical properties are less sensitive to temperature changes compared to diffractive structures. By having the refractive element provide at least 20% of the refractive power, the system achieves improved wavefront adaptation while reducing overall sensitivity to temperature variations, thereby maintaining measurement accuracy.

Inventive Principle:

Principle #35Parameter changes

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 configuration enhances the accuracy of measuring high numerical aperture test surfaces by reducing electromagnetic effects and compensating for temperature-induced deviations, leading to improved measurement precision.

Implementation Method 1

The second optical element is a diffractive optical element, which is configured to split the input wave into the measuring wave and a reference wave

Methodology Applied

Scientific EffectDiffraction: Diffraction

Implementation Method 2

a first optical element...configured to adapt a wavefront of the input wave at least in part to a target shape of the optical surface

Methodology Applied

Scientific EffectRefraction: Refraction

Data Source

PatentUS11199396B2Compensation optical system for an interferometric measuring system

Publication Date: 2021.12.14 CARL ZEISS SMT GMBH

AI summary

A compensation optical unit (30) for a measurement system (10) for determining a shape of an optical surface (12) of a test object (14) by interferometry generates a measuring wave (44), directed at the test object, with a wavefront that is at least partly adapted to a target shape of the optical surface from an input wave (18). The unit includes first (32) and second (34) optical elements disposed in a beam path of the input wave. The second optical element is a diffractive optical element configured to split the input wave into the measuring wave and a reference wave (42) following an interaction with the first optical element. At least 20% of a refractive power of the entire compensation optical unit is allotted to the first optical element, and this allotted refractive power has the same sign as the refractive power of the entire compensation optical unit.