Dark-Field Interferometric Microscopy for Lithography Metrology
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Solution Overview
Problem
Current metrology tools, such as digital holographic microscopy, face limitations due to coherent radiation, which results in increased optical crosstalk and speckles, reducing the accuracy of measurements in lithographic processes, especially in determining characteristics like overlay on substrates.
Innovation Solution
A dark-field interferometric microscope is developed, utilizing spatially incoherent object and reference radiation that are mutually pointwise coherent, with a filter arrangement to remove the zeroth order component and a detection system to capture interferometric images, improving measurement accuracy by reducing crosstalk and speckles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If coherent radiation is used in digital holographic microscopy, then interference patterns can be captured, but optical crosstalk and speckles increase, reducing measurement accuracy
Solution Approach 1:
The patent changes the coherence parameter of the radiation source from coherent to spatially incoherent while maintaining mutual pointwise coherence between object and reference radiation. This parameter change eliminates optical crosstalk and speckles while preserving the ability to capture interference patterns, thereby improving measurement accuracy
Solution Approach 2:
The patent introduces a beam splitting element that divides coherent input radiation into two separate incoherent beams (object radiation and reference radiation) that are mutually pointwise coherent. This intermediary mechanism allows the system to use incoherent radiation sources while maintaining the interference capability needed for holographic microscopy
2Object-generated harmful factors
If spatially incoherent radiation is used, then optical crosstalk and speckles are reduced, but achieving mutual coherence for interference becomes more difficult
Solution Approach 1:
The beam splitting element acts as an intermediary that takes coherent input radiation and creates two spatially incoherent beams that are mutually pointwise coherent. This simplifies the system by using a single coherent source rather than requiring two precisely matched coherent sources, reducing the complexity of coherence control
Solution Approach 2:
The patent changes the coherence parameters of the radiation beams through the beam splitting process. The input coherent radiation is transformed into two beams with specific coherence properties (spatially incoherent but mutually pointwise coherent), which automatically achieves the desired coherence relationship without complex control mechanisms
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 approach enhances measurement precision and reduces optical crosstalk, allowing for more accurate determination of substrate characteristics, such as overlay, with improved resolution and robustness in lithographic processes.
Implementation Method 1
detecting an interferometric image from interference of said filtered scattered radiation and reference radiation
Data Source
AI summary
Disclosed is a dark-field interferometric microscope and associated microscopy method. The microscope comprises an object branch being operable to propagate object radiation onto a sample and collect resultant scattered radiation from said sample and a reference branch being operable to propagate reference radiation. The object radiation and said reference radiation are mutually pointwise spatially coherent. A filter arrangement removes a zeroth order component from said scattered radiation to provide filtered scattered radiation; and a detection arrangement detects an interferometric image from interference of said filtered scattered radiation and reference radiation.