PMT Anode Saturation Extension via Dynode Switching
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Solution Overview
Problem
Current inspection systems for semiconductor wafers face limitations in detection range due to anode saturation of photomultiplier tubes (PMTs) and require complex gain adjustments or additional detectors, which increase cost and complexity.
Innovation Solution
The system addresses anode saturation by switching to an intermediate dynode current once the anode current reaches a threshold, allowing for extended detection range without additional detectors or complex electronics, maintaining signal linearity and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a single detector is used to inspect both unpatterned and patterned wafers, then device versatility is improved, but the detector becomes saturated and measurement precision deteriorates
Solution Approach 1:
The system dynamically switches between anode current and intermediate dynode current based on the inspection mode. For unpatterned wafers, the high-gain anode current path is used to detect low-level scattered light from defects. For patterned wafers, the system switches to the intermediate dynode current path with lower gain to handle the high-level scattered light from pattern features, preventing saturation and maintaining measurement precision across both application types.
2Measurement precision
If additional detectors are added to extend detection range, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
A single PMT detector is made multi-functional by utilizing multiple current paths within the same device. The anode current path provides high gain for detecting low light signals, while the intermediate dynode current path provides lower gain for handling high light signals. This multi-functionality allows one detector to perform the role of multiple detectors with different gain characteristics, extending the detection range without increasing device complexity or cost.
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 significantly extends the detection range of inspection systems while maintaining sensitivity and avoiding anode saturation, reducing costs and complexity by using a single PMT detector with dynamic switching between anode and dynode currents.
Implementation Method 1
a photomultiplier tube (PMT) detector... detecting light scattered from the specimen
Implementation Method 2
The amount of scattered light detected by the system generally depends on the optical characteristics of the spot under inspection
Data Source
AI summary
Inspection systems, circuits and methods are provided to enhance defect detection by addressing anode saturation as a limiting factor of the measurement detection range of a photomultiplier tube (PMT) detector. In accordance with one embodiment of the invention, a method for inspecting a specimen includes directing light to the specimen and detecting light scattered from the specimen. The step of detecting may include monitoring an anode current of the PMT detector, and detecting features, defects or light scattering properties of the specimen using the anode current until the anode current reaches a predetermined threshold. Thereafter, the method may use a dynode current of the PMT for detecting the features, defects or light scattering properties of the specimen.