Inverting Mirror Optical Resonator for EUV Position Measurement

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Solution Overview

Problem

In extreme ultraviolet (EUV) microlithography, the frequency-based position determination of mirrors in projection exposure apparatuses faces challenges due to parasitic movements such as tiltings and lateral displacements, which affect the coupling efficiency of optical resonators, leading to inaccuracies in position measurement.

Innovation Solution

A measuring assembly using an optical resonator with a stationary first resonator mirror and a movable measurement target, where the second resonator mirror is an inverting mirror that reflects the measurement beam back on itself, ensuring that lateral displacements and tiltings do not impact the measurement result, thereby increasing the system's insensitivity to parasitic movements.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a conventional optical resonator is used for frequency-based position measurement, then the measurement principle can be implemented, but parasitic movements (tiltings and lateral displacements) affect coupling efficiency and cause measurement inaccuracies

Engineering Contradiction:

Improveposition measurement accuracyVSAvoidcoupling efficiency stability

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent applies retroreflectors to invert the measurement beam path, causing the beam to return along the same path it took to reach the measurement target. This inversion ensures that lateral displacements and tiltings of the measurement target do not affect the beam's return path to the resonator, thereby maintaining stable coupling efficiency and reliable position measurements even in the presence of parasitic movements

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The patent converts the harmful effect of parasitic movements into a beneficial configuration by using retroreflectors. The retroreflectors are positioned such that any lateral displacement or tilting of the measurement target is compensated by the beam path inversion, transforming what would be measurement errors into a self-correcting measurement system that maintains high coupling efficiency

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

2Measurement precision

If the measurement target is moved to measure position changes, then position determination is enabled, but lateral displacements and tiltings occur that impact measurement accuracy

Engineering Contradiction:

Improveposition determination accuracyVSAvoidparasitic movements

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

Retroreflectors are introduced to invert the measurement beam path. When the measurement target undergoes lateral displacements or tiltings, the inverted beam path ensures that the beam returns to the resonator along the original path, effectively canceling out the effects of parasitic movements and maintaining measurement precision

Inventive Principle:

Principle #13The other way round (Inversion)

3Measurement precision

If the resonator length changes due to position changes, then position information is encoded in frequency, but coupling efficiency varies due to parasitic movements

Engineering Contradiction:

Improvefrequency-based position encodingVSAvoidcoupling efficiency loss

Core Design Contradiction:

Measurement precisionVSLoss of energy

Solution Approach 1:

The patent converts the energy loss due to varying coupling efficiency into a stable measurement system. By using retroreflectors to invert the beam path, the system ensures that parasitic movements do not cause beam loss or coupling efficiency variations, thereby maintaining consistent energy coupling and enabling reliable frequency-based position encoding

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

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 allows for highly accurate position measurement by eliminating the effects of parasitic movements, enabling stable and precise control of the measurement target even in scenarios where stable control is not possible, thus improving the accuracy of position determination in EUV microlithography.

Implementation Method 1

the second resonator mirror is formed by an inverting mirror, which reflects back on itself a measurement beam coming from the measurement target

Methodology Applied

Scientific EffectReflection: Reflection

Implementation Method 2

a resonator 152 in the form of a Fabry-Perot resonator comprises two resonator mirrors 154 and 155

Methodology Applied

Scientific EffectOptical resonance: Resonance

Data Source

PatentUS11274914B2Measuring assembly for the frequency-based determination of the position of a component

Publication Date: 2022.03.15 CARL ZEISS SMT GMBH

AI summary

A measuring assembly for the frequency-based determination of the position of a component, in particular in an optical system for microlithography, includes at least one optical resonator, which has a stationary first resonator mirror, a movable measurement target assigned to the component, and a stationary second resonator mirror. The second resonator mirror is formed by an inverting mirror (130, 330, 430, 530), which reflects back on itself a measurement beam coming from the measurement target.