Optical Metrology Spatial Filter Numerical Aperture Reduction
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Solution Overview
Problem
Optical metrology techniques face challenges in achieving precise measurements of small dimensions due to the large angular spread caused by high numerical aperture lenses, leading to feature smearing and demanding computation requirements, especially in the semiconductor industry where tight tolerances are necessary.
Innovation Solution
The use of a spatial filter within the optical system to reduce the effective numerical aperture by limiting the range of angles of incidence and/or azimuthal angles, allowing only a specific range of light to pass through, thereby reducing feature spreading and computation demands while maintaining a small probe size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high numerical aperture lenses are used to achieve small probe size, then measurement precision is improved, but feature smearing and computation requirements increase
Solution Approach 1:
The patent segments the angular spectrum of reflected light by using multiple detectors positioned at different angles. Each detector captures light within a specific angular range, effectively dividing the complex high-NA measurement into multiple simpler low-NA measurements. This segmentation reduces feature smearing in each detector's data and lowers individual computation requirements while maintaining overall measurement precision through combination of all detector signals.
2Measurement precision
If high numerical aperture lenses are used to achieve small probe size, then measurement precision is improved, but information loss increases due to feature spreading
Solution Approach 1:
The patent transitions from a single spatial dimension measurement to a multi-dimensional approach by capturing light intensity distributions across multiple angular dimensions. Each detector measures the angular distribution of reflected light, and by combining measurements from multiple detectors at different angles, the system reconstructs a comprehensive spatial map of the sample features. This dimensional expansion preserves information that would otherwise be lost to feature spreading in any single angular view.
3Length of moving object
If full spatial extent of objective lens is used, then probe size is minimized, but angular spread increases causing measurement challenges
Solution Approach 1:
The patent introduces multiple detectors as intermediary elements between the sample and the measurement system. These detectors act as angular filters, each receiving light within a specific angular range reflected from the sample. By positioning detectors at different angles and using their combined measurements, the system achieves the benefits of high-NA illumination (small probe size) while avoiding the difficulties of direct high-NA detection (angular spread and feature smearing).
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 precise measurements by reducing the effective numerical aperture, minimizing information loss and computation requirements, and improving measurement accuracy for small features, thus addressing the limitations of conventional optical metrology systems.
Implementation Method 1
The spatial filter blocks a portion of the coherent light from exiting the spatial filter and permits another portion of the coherent light to pass
Implementation Method 2
an optical system that focuses spatially coherent light into a small probe size on a sample
Implementation Method 3
A detector is configured to receive the coherent light that is reflected and produce signals in response
Data Source
AI summary
A metrology device that can determine at least one characteristics of a sample is disclosed. The metrology device includes an optical system that uses spatially coherent light with a first and a second objective lens as well as a detector that detects light reflected from the sample. The objective lenses use numerical apertures sufficient to produce a small probe size, e.g., less than 200 μm, while a spatial filter is used to reduce the effective numerical aperture of the optical system as seen by the detector to avoid loss of information and demanding computation requirements caused by the large angular spread due to large numerical apertures. The spatial filter permits light to pass in a desired range of angles, while blocking the remaining light and is positioned to prevent use of the full spatial extent of at least one of the first objective lens and the second objective lens.