Off-Axis Mutually Coherent Dark-Field Imaging for Overlay Metrology
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Solution Overview
Problem
Overlay metrology systems face challenges in accurately measuring small alignment errors as feature sizes decrease and density increases, requiring more sensitive and precise measurements.
Innovation Solution
A metrology system utilizing incoherent illumination split into pairs of mutually coherent beams, directed through an objective lens to a metrology target with specific masks to pass single nonzero-order diffraction beams and block zero-order diffraction, generating dark-field images for precise overlay measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If incoherent illumination is used, then speckle and edge effects are suppressed, but measurement sensitivity is reduced
Solution Approach 1:
The incoherent illumination is segmented into multiple pairs of mutually coherent beams using a diffraction grating. Each pair illuminates the sample from different angles, and the coherent interference between beams in each pair provides the necessary measurement sensitivity while the incoherence between pairs suppresses speckle and edge effects.
Solution Approach 2:
The illumination coherence parameters are changed by using a diffraction grating to create multiple beam pairs from incoherent light. This transforms the illumination from purely incoherent to having partial coherence within beam pairs while maintaining incoherence between pairs, thereby achieving both speckle suppression and measurement sensitivity.
2Manufacturing precision
If feature size decreases and density increases, then manufacturing capability improves, but measurement accuracy deteriorates
Solution Approach 1:
The illumination is segmented into multiple beam pairs, each providing independent measurement information. This segmentation allows the system to maintain measurement accuracy for small, dense features by using the interference patterns from multiple angle pairs rather than relying on a single illumination direction.
Solution Approach 2:
The measurement is extended from a single illumination angle to multiple angles in different dimensions. By using multiple pairs of coherent beams incident from different angles, the system captures alignment information in multiple spatial dimensions, improving measurement accuracy for small and dense features.
3Measurement precision
If multiple pairs of mutually coherent beams are used, then measurement sensitivity increases, but device complexity increases
Solution Approach 1:
A diffraction grating is introduced as an intermediary element to automatically generate multiple pairs of mutually coherent beams from incoherent illumination. This intermediary component simplifies the overall system by replacing what would otherwise require complex coherent light sources and beam splitting optics, thereby reducing device complexity while maintaining high measurement sensitivity.
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 enhances measurement sensitivity and accuracy, suppressing speckle and edge effects, allowing for high-contrast imaging of periodic features and enabling control of process tools like lithography and etching.
Implementation Method 1
a diffraction grating to split the incoherent illumination into one or more pairs of mutually coherent illumination beams
Implementation Method 2
an objective lens to direct the one or more pairs of mutually coherent illumination beams to a metrology target on a sample
Implementation Method 3
the objective lens further collects sample light associated with diffraction of the one or more pairs of mutually coherent illumination beams
Implementation Method 4
one or more masks, wherein the one or more masks are configured in accordance with the metrology recipe to pass a single nonzero-order diffraction beam and block a zero-order diffraction beam
Data Source
AI summary
A metrology system is disclosed. The system includes an incoherent illumination source to generate incoherent illumination. The system includes a diffraction grating to split the incoherent illumination. The system includes an objective lens to direct one or more pairs of mutually coherent illumination beams to a metrology target on a sample and collect sample light associated with diffraction of pairs of mutually coherent illumination beams. The system includes a mask configured to pass a single nonzero-order diffraction beam and block a zero-order diffraction beam associated with each of the mutually coherent illumination beams. The system includes a detector configured to generate an image of the metrology target based on light passed by the mask. The system includes a controller communicatively coupled to the detector, including one or more processors configured to generate one or more metrology measurements of the sample based on the image.