Modulated Illumination Source for Optical Metrology

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Solution Overview

Problem

Optical metrology systems face challenges with coherence artifacts such as interference fringes and speckle due to the long coherence length of laser-based illumination sources, leading to noise and instability, and require efficient time-sequencing of multi-wavelength illumination sources for accurate metrology.

Innovation Solution

The implementation of a modulatable illumination source with a modulation control system that modulates the drive current at a selected frequency to generate illumination with specific coherence features, reducing coherence artifacts and noise, and uses multiple illumination sources of different wavelengths with interleaved pulse trains for time-sequencing control.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Stability of the object's composition

If laser-based illumination sources are used to provide coherent light, then the light source stability and intensity are improved, but coherence artifacts such as interference fringes and speckle are generated

Engineering Contradiction:

Improvelight source stabilityVSAvoidcoherence artifacts

Core Design Contradiction:

Stability of the object's compositionVSObject-generated harmful factors

Solution Approach 1:

The patent applies periodic action by modulating the laser illumination source at a specific frequency to introduce temporal variations in the light output. This periodic modulation breaks the coherence of the laser light, thereby suppressing coherence artifacts such as interference fringes and speckle while maintaining the stability and intensity benefits of laser-based illumination.

Inventive Principle:

Principle #19Periodic action

2Adaptability or versatility

If optical-mechanical devices such as shutters and Pockels cells are used for time-sequencing multiple illumination sources, then time-sequencing control is achieved, but system stability and repeatability are reduced

Engineering Contradiction:

Improvetime-sequencing controlVSAvoidsystem stability

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The patent replaces optical-mechanical time-sequencing devices with electronic modulation control. By using electronic modulation to time-sequence multiple illumination sources, the system achieves reliable and repeatable control without the instability and noise introduced by mechanical components such as shutters and Pockels cells.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Adaptability or versatility

If multiple illumination sources of different wavelengths are used for comprehensive metrology, then measurement capability is improved, but system complexity increases

Engineering Contradiction:

Improvemeasurement capabilityVSAvoidsystem complexity

Core Design Contradiction:

Adaptability or versatilityVSDevice complexity

Solution Approach 1:

The patent merges multiple illumination sources of different wavelengths into a single integrated system with a unified modulation control mechanism. This allows comprehensive metrology capabilities using multiple wavelengths while avoiding the complexity of separate control systems for each source, as all sources are modulated and time-sequenced through a common electronic control architecture.

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 approach suppresses coherence artifacts and noise, enhancing the precision and accuracy of optical metrology measurements while simplifying the system by eliminating the need for optical-mechanical components, thereby improving metrology throughput.

Implementation Method 1

a modulation control system communicatively coupled to the modulatable illumination source, wherein the modulation control system is configured to modulate a drive current of the modulatable illumination source at a selected modulation frequency suitable for generating illumination having a selected coherence feature

Methodology Applied

Scientific EffectElectrical modulation of light source: Phase Modulation

Implementation Method 2

a detector configured to detect at least a portion of illumination emanating from a surface of the sample

Methodology Applied

Scientific EffectLight detection: Photoelectric Effect

Implementation Method 3

In optical metrology settings where coherent light sources are implemented, the production of coherent artifacts, such as interference fringes resulting from duplicate images

Methodology Applied

Scientific EffectCoherent light generation: Coherent Light

Data Source

PatentUS11913874B2Optical metrology tool equipped with modulated illumination sources

Publication Date: 2024.02.27 KLA CORP

AI summary

The system includes a modulatable illumination source configured to illuminate a surface of a sample disposed on a sample stage, a detector configured to detect illumination emanating from a surface of the sample, illumination optics configured to direct illumination from the modulatable illumination source to the surface of the sample, collection optics configured to direct illumination from the surface of the sample to the detector, and a modulation control system communicatively coupled to the modulatable illumination source, wherein the modulation control system is configured to modulate a drive current of the modulatable illumination source at a selected modulation frequency suitable for generating illumination having a selected coherence feature length. In addition, the present invention includes the time-sequential interleaving of outputs of multiple light sources to generate periodic pulse trains for use in multi-wavelength time-sequential optical metrology.