Dark Field Inspection Ring Illumination Speckle Reduction
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Solution Overview
Problem
Conventional dark field inspection systems face limitations in defect detection sensitivity due to speckle noise caused by surface roughness, which obfuscates small particles and is not effectively addressed by existing methods such as edge contrast modes or laser dark field systems with strong spatial coherence.
Innovation Solution
A dark field inspection system utilizing multiple beam shaping paths to generate ring illumination at oblique angles, reducing speckle noise by providing statistically independent noise sources and enhancing signal-to-noise ratio, with high numerical aperture objective lenses and digital image processing filters to improve optical resolution and defect detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single angle of illumination is used in laser dark field inspection, then the system structure is simple, but strong spatial coherence results in large roughness induced fluctuations (speckle) that reduce sensitivity to real defects
Solution Approach 1:
The single illumination source is segmented into multiple beam shaping paths (at least two), each illuminating the wafer at a different oblique angle. This segmentation divides the coherent illumination into multiple paths that reduce spatial coherence and minimize speckle effects while maintaining defect detection sensitivity.
Solution Approach 2:
The illumination approach transitions from a single angle (one-dimensional) to multiple angles in three-dimensional space. By introducing angular diversity across different oblique angles, the system reduces spatial coherence without requiring complex mechanical movement, effectively adding dimensional complexity to the illumination geometry.
2Device complexity
If conventional edge contrast mode with broadband light source is used, then the system is simple to implement, but the low brightness source results in lower illumination level at the imaging sensor
Solution Approach 1:
The system uses multiple beam shaping paths that can be activated in a periodic or sequential manner, or simultaneously to create a composite illumination pattern. This periodic or simultaneous action across multiple paths increases the effective illumination intensity at the sensor compared to conventional single-path broadband systems.
3Measurement precision
If conventional laser dark field with oblique light incidence is used, then the system provides good defect detection, but the relatively large line width (on the order of 1 um) limits the resolution
Solution Approach 1:
The system changes the illumination parameters by using multiple oblique angles instead of a single angle. This parameter change allows for a narrower effective line width (improved resolution) while maintaining defect detection capability through the combined information from multiple illumination angles.
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 system significantly improves defect detection sensitivity by minimizing speckle noise, allowing for clearer identification of small particles and reducing the need for extensive time-delay integration stages, thereby enhancing optical resolution and throughput.
Implementation Method 1
The surface roughness of a wafer typically results in small fluctuations in detected scattered light at an imaging sensor in a dark field inspection system. These fluctuations, called speckle, can be characterized as a noise floor.
Implementation Method 2
Each beam shaping path can advantageously illuminate the wafer at an oblique angle. In one embodiment, the oblique angles can be between 60-85 degrees with respect to the surface normal.
Implementation Method 3
an objective lens for capturing scattered light from the wafer. Because the wafer illumination occurs at oblique angles, the objective lens can have a high NA (numerical aperture) (e.g. at least 0.5), thereby improving optical resolution.
Implementation Method 4
the objective lens can have a high NA (numerical aperture) (e.g. at least 0.5), thereby improving optical resolution
Implementation Method 5
Each cylindrical lens can be tilted and rotated with respect to a light beam from its corresponding light source. Each cylindrical lens can have a cylindrical axis positioned parallel to the illumination line.
Data Source
AI summary
A dark field inspection system that minimizes the speckle noise due to sample surface roughness can include a plurality of beam shaping paths for generating a composite, focused illumination line on a wafer. Each beam shaping path can illuminate the wafer at an oblique angle. The plurality of beam shaping paths can form a ring illumination. This ring illumination can reduce the speckle effect, thereby improving SNR. An objective lens can capture scattered light from the wafer and an imaging sensor can receive an output of the objective lens. Because the wafer illumination occurs at oblique angles, the objective lens can have a high NA, thereby improving optical resolution of the imaging sensor, and the resulting signal level.