Optical Substrate Inspection via Doppler Burst Frequency Analysis
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Solution Overview
Problem
Existing substrate inspection techniques face challenges in reliably detecting and classifying defects, particularly due to signal saturation, spurious signals, and the need for complex and costly signal measurement and processing systems.
Innovation Solution
The method involves creating a measurement volume at the intersection of two optical beams crossing at a non-zero angle, generating interference fringes. This setup allows for the detection of defects through a Doppler burst signal, which is analyzed for frequency content to discriminate and classify defects without requiring extensive hardware modifications or complex signal processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If intensity measurements are used for defect detection, then the detection system can operate with simple hardware, but the signal-to-noise ratio deteriorates and defect detection reliability decreases
Solution Approach 1:
The patent changes the measurement parameter from intensity to frequency. By measuring the frequency of light scattered by defects rather than intensity, the system achieves better signal-to-noise ratio and defect detection reliability while maintaining relatively simple hardware configuration. The frequency measurement allows discrimination of defect signals from background noise through spectral analysis.
2Reliability
If dark field techniques are implemented to improve signal-to-noise ratio, then defect detection contrast improves, but device complexity and cost increase significantly
Solution Approach 1:
The patent implements dark field detection by measuring the frequency parameter of scattered light rather than intensity. This approach achieves the signal-to-noise ratio improvement characteristic of dark field techniques while avoiding the need for complex optical arrangements with multiple light sources and detectors. The frequency measurement inherently provides background suppression.
3Reliability
If multiple light sources and detectors are multiplied to reduce parasitic measurements, then signal-to-noise ratio improves, but device complexity and cost increase
Solution Approach 1:
The patent achieves signal-to-noise ratio improvement by measuring the frequency parameter of scattered light. This single-parameter measurement approach eliminates the need for multiple light sources and detectors, as the frequency analysis can distinguish defect signals from parasitic measurements using spectral characteristics rather than requiring redundant detection channels.
4Measurement precision
If significant data processing is implemented to extract defect signals, then measurement accuracy improves, but processing complexity and resource requirements increase
Solution Approach 1:
The patent improves measurement precision by measuring the frequency parameter of scattered light. The frequency measurement inherently provides better signal discrimination, and the data processing required is limited to spectral analysis to identify the Doppler frequency component. This is less complex than the significant data processing required by intensity-based methods to extract defect signals from noise.
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 the reliability of defect detection by improving the signal-to-noise ratio and allowing for the identification and classification of defects, including small or weakly scattering ones, with reduced reliance on costly equipment and processing resources.
Implementation Method 1
The measurement volume is created from two (or more) optical beams coming from the same source and crossing at a non-zero angle, generating interference fringes in this measurement volume.
Implementation Method 2
When a defect or a particle crosses the interference fringes, their presence is reflected on the detector by a measurement signal, for example an electrical signal, constituting a Doppler burst.
Data Source
Figure 1~3
Figure 2
Figure 4a~5d
AI summary
The present invention relates to a method (100, 200) for inspecting a substrate (2) and to an inspection system implementing such a method, the method comprising steps of: creating a measurement volume at the intersection between two light beams (4, 5) originating from the same light source (20) and containing interference fringes, said substrate (2) moving relative to said measurement volume; acquiring (102, 202) a measurement signal representative of the light diffused by the substrate (2); computing (108, 208) at least one expected modulation frequency of an expected signal representative of the passage of a fault (3) in the measurement volume; determining (114, 214) characteristic values representative of a frequency content of the measurement signal in a vicinity, so as to form a validated signal representative of the presence of faults (3); and analysing said validated signal in order to locate and/or identify faults (3).