Overlay Target Layout Using SRAFs for Accurate Placement

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Solution Overview

Problem

Current semiconductor circuit metrology tools face challenges in accurately measuring overlay errors due to pattern placement errors (PPE) caused by optical aberrations and differences in line widths between segmented target features, which are exacerbated by the larger dimensions of overlay targets compared to circuit features.

Innovation Solution

The introduction of sub-resolution assist features (SRAFs) interleaved with target features in the lithographic mask, which are below the resolution limit of the exposure system, reduces amplitude variations in the aerial image and minimizes dimensional differences between target feature segments, thereby improving the accuracy of optical overlay error metrology.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If overlay targets are made with larger dimensions to enable optical metrology measurement, then measurement capability is improved, but pattern placement errors increase due to optical aberrations and line width differences

Engineering Contradiction:

Improveoverlay error measurement capabilityVSAvoidpattern placement accuracy

Core Design Contradiction:

Measurement precisionVSManufacturing precision

Solution Approach 1:

Sub-resolution assist features are introduced as intermediary elements between the target features and the optical metrology system. These SRAFs mediate the optical field distribution to reduce amplitude variations in the aerial image, thereby reducing pattern placement errors while maintaining the target's measurability by optical tools

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The assist features are strategically placed only in specific locations where they are needed to correct local amplitude variations in the aerial image. This localized approach addresses the PPE issue at critical regions without unnecessarily complicating the entire target structure

Inventive Principle:

Principle #3Local quality

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

The application of SRAFs reduces typical dimensional differences between extreme segments from about 1 nm to 0.1 nm, enhancing the precision of optical overlay error measurement and addressing the PPE issue.

Implementation Method 1

a projection system having a predefined resolution limit to project optical radiation onto the semiconductor substrate through at least one mask

Methodology Applied

Scientific EffectOptical projection: Light

Implementation Method 2

assist features interleaved with the target features and having at least one assist feature dimension that is less than the predefined resolution limit

Methodology Applied

Scientific EffectOptical absorption: Absorption (EM radiation)

Data Source

PatentUS12140859B2Overlay target design for improved target placement accuracy

Publication Date: 2024.11.12 KLA CORP

AI summary

A method for semiconductor metrology includes depositing a first film layer on a semiconductor substrate and a second film layer overlying the first film layer. The first and second film layers are patterned to create an overlay target having a specified geometrical form by using a projection system having a predefined resolution limit to project optical radiation onto the semiconductor substrate through at least one mask. The mask contains target features having target feature dimensions no less than the predefined resolution limit in an arrangement corresponding to the specified geometrical form of the overlay target and assist features interleaved with the target features and having at least one assist feature dimension that is less than the predefined resolution limit.