Optical System Heating Segments for EUV Mirror Deformation Control

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Solution Overview

Problem

EUV mirrors in microlithographic projection exposure apparatuses experience thermal expansion and surface deformations due to radiation absorption, leading to optical aberrations, which existing methods like ultra-low expansion materials and active cooling struggle to address effectively, especially when dealing with varying radiation intensities.

Innovation Solution

An optical system with a heating device comprising multiple heating segments that apply electrical current to generate a continuous thermally induced deformation profile, ensuring a deformation amplitude of at least 1λ with an integral Fourier analysis of less than 10 mλ in a decadic spatial wavelength range, allowing for smoother gradients and reduced stray light effects.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If radiation-based heating is used to compensate for thermal expansion, then thermal deformation can be corrected, but spatial resolution is limited and stray light is coupled into the optical system

Engineering Contradiction:

Improvesurface deformation correctionVSAvoidspatial resolution

Core Design Contradiction:

Manufacturing precisionVSMeasurement precision

Solution Approach 1:

The patent replaces radiation-based heating with direct electrical contact heating through heating segments embedded in the mirror substrate. This substitution eliminates the limitations of radiation-based methods by enabling precise local heating without coupling stray light into the optical system, while achieving higher spatial resolution through direct electrical control of heating zones.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Measurement precision

If resistance heating is used to achieve higher spatial resolution, then heating precision improves, but deformation profiles show undesired steps that impair optical properties

Engineering Contradiction:

Improvespatial resolutionVSAvoiddeformation profile smoothness

Core Design Contradiction:

Measurement precisionVSManufacturing precision

Solution Approach 1:

The patent merges multiple heating segments into a coordinated system where adjacent heating segments are activated simultaneously with adjusted power levels. This combination creates smooth transition zones between heated regions, eliminating the step-like deformations that occur when heating segments operate independently, while maintaining high spatial resolution.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The patent implements dynamic control of heating segments through independent power adjustment for each segment. By dynamically varying the power distribution across adjacent heating segments, the system creates smooth deformation profiles without steps, while maintaining the ability to resolve fine spatial features through localized heating control.

Inventive Principle:

Principle #15Dynamics

3Stability of the object's composition

If cooling channels are used to avoid surface deformations, then thermal expansion is reduced, but device complexity increases

Engineering Contradiction:

Improvesurface deformation preventionVSAvoidcooling channel structure

Core Design Contradiction:

Stability of the object's compositionVSDevice complexity

Solution Approach 1:

Instead of using cooling channels to prevent thermal expansion, the patent applies the opposite approach by using heating segments to actively compensate for thermal deformation. The heating segments are positioned to create controlled thermal zones that counteract the natural thermal expansion caused by EUV radiation absorption, thereby preventing surface deformations without requiring complex cooling infrastructure.

Inventive Principle:

Principle #13The other way round (Inversion)

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 effectively avoids surface deformations and associated optical aberrations by providing higher spatial resolution and controlled temperature distribution, compensating for thermal deformations and aberrations, while minimizing stray light and enabling dynamic adaptation to illumination changes.

Implementation Method 1

a heating device for heating this optical element, wherein the heating device has a plurality of heating segments to which electrical current can be applied to generate heat

Methodology Applied

Scientific EffectJoule heating: Joule Heating

Implementation Method 2

as a result of absorption of the radiation emitted by the EUV light source, the EUV mirrors can heat up and undergo an associated thermal expansion or surface deformation

Methodology Applied

Scientific EffectAbsorption (EM radiation): Absorption (EM radiation)

Implementation Method 3

the heating segments can be used to set a continuous thermally induced deformation profile of the optical effective surface with a deformation amplitude of at least 1λ

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Data Source

PatentUS20250258444A1Optical system, and method for operating an optical system

Publication Date: 2025.08.14 CARL ZEISS SMT GMBH

AI summary

An optical system, such as in a microlithographic projection exposure apparatus, comprising at least one optical element and a heating device for heating the optical element. The heating device comprises a plurality of heating segments to which electric current can be applied in order to generate heat. A continuous thermally induced deformation profile of the optical active surface having a deformation amplitude of at least 1λ can be adjusted by the heating segments so that the integral of the Fourier decomposition over at least one decadic spatial wavelength range is less than 10 mλ.