Lithography Overlay Target With Overlapping Gratings

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Solution Overview

Problem

Conventional metrology techniques for overlay measurement in lithographic processes face challenges in achieving high accuracy and efficient use of space, particularly when using small targets surrounded by product features, due to edge effects and the assumption of linearity with limited offsets.

Innovation Solution

A target design featuring overlapping gratings with varying pitches, allowing for increased accuracy and reduced space usage by incorporating multiple bias values and continuous bias variations, enabling more robust and accurate overlay measurements.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of operation

If conventional scatterometry with large targets (40 μm by 40 μm) is used, then measurement simplicity is improved, but space usage increases and targets cannot be positioned amongst product features

Engineering Contradiction:

Improvemeasurement simplicityVSAvoidtarget size

Core Design Contradiction:

Ease of operationVSArea of stationary object

Solution Approach 1:

The target is divided into multiple sub-targets (first sub-target, second sub-target, etc.), each formed by overlapping gratings with different pitches. This segmentation allows the overall target to fit within smaller spaces while maintaining measurement capability through multiple pitch variations.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent introduces pitch variation as an additional dimension beyond the physical target size. By using gratings with different pitches (first pitch, second pitch, third pitch, fourth pitch) in overlapping configurations, the measurement system gains another degree of freedom that compensates for the reduced physical dimensions.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Device complexity

If linear assumptions are used in overlay measurement, then calculation simplicity is improved, but measurement accuracy deteriorates due to non-linear relationships

Engineering Contradiction:

Improvecalculation simplicityVSAvoidoverlay measurement accuracy

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent changes the pitch parameter of the gratings to create non-linear relationships. By using multiple pitches (first pitch, second pitch, third pitch, fourth pitch) in the overlapping grating structure, the system introduces controlled non-linearity that enables more accurate overlay measurements while maintaining manageable calculation complexity through the structured approach.

Inventive Principle:

Principle #35Parameter changes

3Device complexity

If single-pitch gratings are used, then target design simplicity is improved, but measurement robustness deteriorates due to edge effects and limited information

Engineering Contradiction:

Improvetarget design simplicityVSAvoidmeasurement robustness

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The target uses a composite structure of multiple gratings with different pitches (first pitch, second pitch, third pitch, fourth pitch) overlaid on each other. This composite grating structure provides multiple measurement signals from a single target configuration, improving robustness against edge effects and providing redundant information for more reliable overlay determination.

Inventive Principle:

Principle #40Composite materials

Solution Approach 2:

Multiple gratings with different pitches are merged into overlapping sub-targets. The first sub-target combines gratings with first and second pitches, while the second sub-target combines gratings with third and fourth pitches. This merging creates a unified target structure that provides enhanced measurement capability and robustness.

Inventive Principle:

Principle #5Merging (Combining)

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 design enhances measurement accuracy and reduces the area required for metrology targets, improving the robustness and efficiency of overlay measurements by utilizing multiple bias values and continuous bias variations, which can correct and calibrate single bias target measurements.

Implementation Method 1

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

Methodology Applied

Scientific EffectDiffraction: Diffraction

Implementation Method 2

Typically such targets are measured using dark field scatterometry in which the zeroth order of diffraction (corresponding to a specular reflection) is blocked

Methodology Applied

Scientific EffectReflection: Reflection

Data Source

PatentUS12019377B2Target for measuring a parameter of a lithographic process

Publication Date: 2024.06.25 ASML NETHERLANDS BV

AI summary

A target for determining a performance parameter of a lithographic process, the target comprising a first sub-target formed by at least two overlapping gratings, wherein the underlying grating of the first sub-target has a first pitch and the top lying grating of the first sub-target has a second pitch, at least a second sub-target formed by at least two overlapping gratings, wherein the underlying grating of the second sub-target has a third pitch and the top lying grating of the second sub-target has a fourth pitch.