Overlay Shift Measurement Using Weighted Image Segmentation

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Solution Overview

Problem

Existing methods for determining the relative overlay shift of stacked layers are not accurate and efficient, requiring separation of pattern elements and absolute position determination, which leads to noise-induced errors.

Innovation Solution

A method involving the generation of weighted images to emphasize specific pattern elements, allowing for relative shift determination without separating pattern elements or determining absolute positions, combined with cross-correlation and iterative processes to enhance accuracy.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If pattern elements are separated and absolute positions are determined, then overlay shift can be measured, but measurement accuracy deteriorates due to noise-induced errors

Engineering Contradiction:

Improveoverlay shift measurement accuracyVSAvoidnoise-induced error

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The measurement image is segmented by creating weighted versions that emphasize different pattern elements. First and second weighted images are generated where each highlights a specific pattern element from different layers, allowing independent analysis without full separation that would amplify noise

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A reference image is created from design data to represent the ideal pattern positions. This reference copy is then correlated with the measurement image to determine overlay shift, avoiding direct measurement of absolute positions that are susceptible to noise

Inventive Principle:

Principle #26Copying

2Measurement precision

If pattern elements are separated and absolute positions are determined, then overlay shift can be measured, but measurement time increases

Engineering Contradiction:

Improveoverlay shift measurement capabilityVSAvoidmeasurement time

Core Design Contradiction:

Measurement precisionVSLoss of time

Solution Approach 1:

The mechanical process of separating and individually locating pattern elements is replaced by an optical correlation method. The evaluating device uses image correlation between reference and measurement images to directly determine overlay shift without the time-consuming separation and absolute positioning steps

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Measurement precision

If weighting is applied to emphasize specific pattern elements, then crosstalk between local maxima is prevented, but image processing complexity increases

Engineering Contradiction:

Improvecross-correlation accuracyVSAvoidimage processing complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

Different regions of the image are treated with different weighting factors. The weighting function assigns higher weights to areas containing specific pattern elements and lower weights to other areas, allowing localized enhancement without processing the entire image at full complexity

Inventive Principle:

Principle #3Local quality

4Loss of information

If absolute position determination is performed, then pattern element locations are known, but measurement accuracy decreases due to noise

Engineering Contradiction:

Improvepattern element position informationVSAvoidposition measurement accuracy

Core Design Contradiction:

Loss of informationVSMeasurement precision

Solution Approach 1:

Instead of directly measuring absolute positions which are noisy, a reference image serves as an intermediary. The reference image, derived from clean design data, mediates the measurement process by providing a noise-free comparison basis for determining relative overlay shift

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS8260033B2Method and apparatus for determining the relative overlay shift of stacked layers

Publication Date: 2012.09.04 CARL ZEISS SMT GMBH

AI summary

A method is provided for determining the relative overlay shift of stacked layers, said method comprising the steps of: a) providing a reference image including a reference pattern that comprises first and second pattern elements; b) providing a measurement image of a measurement pattern, which comprises a first pattern element formed by a first one of the layers and a second pattern element formed by a second one of the layers; c) weighting the reference or measurement image such that a weighted first image is generated, in which the first pattern element is emphasized relative to the second pattern element; d) determining the relative shift of the first pattern element on the basis of the weighted first image and of the measurement or reference image not weighted in step c); e) weighting the reference or measurement image such that a weighted second image is generated, in which the second pattern element is emphasized relative to the first pattern element; f) determining the relative shift of the second pattern element on the basis of the weighted second image and of the measurement or reference image not weighted in step e); g) determining the relative overlay shift on the basis of the relative shifts determined in steps d) and f).