Small-Target Optical Metrology With Zero-Order Side-Lobe Control

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Solution Overview

Problem

The presence of zero-order side lobes negatively impacts optical metrology measurements, particularly as the size of optical metrology targets is reduced, necessitating the development of systems and methods to mitigate their influence.

Innovation Solution

Utilizing pairs of mutually-coherent illumination beams with opposing azimuth incidence angles and common altitude incidence angles to generate dark-field images, where zero-order side lobes are blocked or algorithmically filtered, allowing for optical metrology measurements based on single non-zero diffraction orders.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Area of stationary object

If the size of optical metrology target is reduced to maximize device fabrication area, then the available area for creating devices increases, but the impact of zero-order side lobes on measurement accuracy worsens

Engineering Contradiction:

Improveavailable area for device fabricationVSAvoidmeasurement accuracy

Core Design Contradiction:

Area of stationary objectVSMeasurement precision

Solution Approach 1:

The patent extracts and removes zero-order side lobes from the measurement system using spatial filtering techniques. A filter is placed in the Fourier transform plane to selectively block zero-order diffraction components while preserving first-order diffraction signals, thereby eliminating the harmful side lobe interference that would otherwise degrade measurement accuracy on small targets

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent transforms the problem from spatial domain to frequency domain by performing a Fourier transform of the image data. This dimensional transformation allows separation of different diffraction orders in the frequency domain, enabling selective filtering of zero-order side lobes while maintaining first-order measurement signals for accurate overlay detection

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Length of moving object

If the pitch of periodic features is reduced to minimize target size, then the target footprint decreases, but the influence of peripheral features and side lobes on measurement increases

Engineering Contradiction:

Improvetarget footprintVSAvoidoverlay measurement precision

Core Design Contradiction:

Length of moving objectVSManufacturing precision

Solution Approach 1:

The patent converts the harmful zero-order side lobe interference into a beneficial measurement approach by using dark-field imaging geometry. The system is configured to collect only first-order diffraction signals while inherently rejecting zero-order light, thereby transforming what would be a harmful interference into a selective signal collection mechanism that improves measurement precision on compact targets

Inventive Principle:

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

3Measurement precision

If dark-field imaging with mutually-coherent oblique illumination is used to reduce zero-order side lobe impact, then side lobe interference decreases, but the complexity of the illumination system increases

Engineering Contradiction:

Improvemeasurement accuracyVSAvoidillumination system complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent implements a multi-functional illumination system where the same optical train serves both illumination and collection functions. The objective lens acts as both the illumination source for oblique illumination and the collection lens for detecting diffraction orders, eliminating the need for separate illumination and collection optics and thereby reducing overall system complexity while maintaining measurement precision

Inventive Principle:

Principle #6Universality (Multi-functionality)

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 enables accurate optical metrology measurements by reducing the impact of zero-order side lobes, enabling the reduction of optical metrology target size without sacrificing performance, and providing high image contrast and brightness.

Implementation Method 1

the presence of zero-order side lobes (e.g., side lobes associated with zero-order diffraction) may negatively impact an optical metrology measurement

Methodology Applied

Scientific EffectDiffraction: Diffraction

Implementation Method 2

dark-field imaging optical metrology with mutually-coherent oblique illumination

Methodology Applied

Scientific EffectInterference: Interference

Data Source

PatentUS20250257992A1Metrology measurements on small targets with control of zero-order side lobes

Publication Date: 2025.08.14 KLA CORP

AI summary

An optical metrology system may include illumination optics to direct pairs of mutually-coherent illumination beams to an optical metrology target, where the optical metrology target includes sets of periodic features having features with periodicity along different measurement directions. A pair of mutually-coherent illumination beams has opposing azimuth incidence angles and a common altitude incidence angle, where the azimuth incidence angles are rotated with respect to the measurement directions. The system may further generate dark-field images of the optical metrology target, where an image of a periodic structures is formed as a sinusoidal interference pattern generated by interference of a single non-zero diffraction order of light from each of the illumination beams within a pair of mutually-coherent illumination beams. A controller may generate optical metrology measurements along the measurement directions based on the images. The system may mitigate an impact of zero-order side lobes through blocking or image filtering.