Lithographic Overlay Metrology: Separating Structural Asymmetry from Overlay Errors

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Solution Overview

Problem

Conventional dark-field image-based overlay measurements in lithographic processes are inaccurate due to the inability to distinguish between overlay errors and structural asymmetries in target structures, leading to misinterpretation of intensity asymmetry caused by factors other than overlay errors.

Innovation Solution

A method and apparatus that measure target asymmetry in lithographic processes by obtaining structural asymmetry characteristics independent of measurement radiation characteristics, allowing for the separation of overlay contributions from structural contributions, enabling more accurate overlay error determination.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If dark-field scatterometry is used to measure overlay on small target structures, then measurement speed and computational simplicity are improved, but measurement accuracy deteriorates due to inability to distinguish overlay errors from structural asymmetries

Engineering Contradiction:

Improvemeasurement speedVSAvoidoverlay measurement accuracy

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

The patent segments the overlay measurement process into two distinct phases: (1) obtaining a structural asymmetry characteristic that is independent of measurement radiation characteristics, and (2) determining the overlay contribution by combining this structural characteristic with intensity asymmetry measurements. This segmentation allows the system to separate structural asymmetry effects from overlay effects, thereby resolving the accuracy problem while maintaining the speed advantages of dark-field scatterometry.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent introduces a structural asymmetry characteristic as an intermediary parameter that mediates between the raw intensity asymmetry measurement and the final overlay determination. This intermediary captures the structural asymmetry component independently, allowing it to be subtracted or accounted for when calculating the true overlay error, thus improving measurement accuracy without sacrificing measurement speed.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Ease of operation

If intensity asymmetry measurements are used directly to determine overlay errors, then the measurement process remains simple and fast, but measurement accuracy deteriorates due to contamination from structural asymmetry contributions

Engineering Contradiction:

Improvemeasurement process simplicityVSAvoidoverlay error determination accuracy

Core Design Contradiction:

Ease of operationVSMeasurement precision

Solution Approach 1:

The measurement process is segmented into obtaining a structural asymmetry characteristic (independent of measurement conditions) and then using this characteristic to correct the intensity asymmetry measurements. This maintains operational simplicity by building upon the existing dark-field scatterometry workflow while adding a corrective step that eliminates structural asymmetry contamination.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent implements a feedback mechanism where the structural asymmetry characteristic, obtained from preliminary measurements, is fed back into the overlay determination process. This feedback allows the system to compensate for structural asymmetry effects in real-time, improving accuracy without significantly increasing operational complexity.

Inventive Principle:

Principle #23Feedback

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 provides more robust and accurate overlay measurements by isolating overlay errors from structural asymmetries, improving the precision of lithographic process control without requiring new target structures or hardware changes.

Implementation Method 1

These devices direct a beam of radiation onto a target structure and measure one or more properties of the scattered radiation

Methodology Applied

Scientific EffectScattering: Scattering

Implementation Method 2

Diffraction-based overlay using dark-field detection of the diffraction orders enables overlay measurements on smaller target structures

Methodology Applied

Scientific EffectDiffraction: Diffraction

Data Source

PatentUS9879988B2Metrology method and apparatus, computer program and lithographic system

Publication Date: 2018.01.30 ASML NETHERLANDS BV

AI summary

Disclosed is a method of measuring a parameter of a lithographic process, and associated computer program and apparatuses. The method comprises providing a plurality of target structures on a substrate, each target structure comprising a first structure and a second structure on different layers of the substrate. Each target structure is measured with measurement radiation to obtain a measurement of target asymmetry in the target structure, the target asymmetry comprising an overlay contribution due to misalignment of the first and second structures, and a structural contribution due to structural asymmetry in at least the first structure. A structural asymmetry characteristic relating to the structural asymmetry in at least the first structure of each target structure is obtained, the structural asymmetry characteristic being independent of at least one selected characteristic of the measurement radiation. The measurement of target asymmetry and the structural asymmetry characteristic is then used to determine the overlay contribution of the target asymmetry of each target structure.