SiPM Pulse Discrimination for Low-Noise SEM Measurement
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Solution Overview
Problem
Charged particle beam devices, such as scanning electron microscopes, face challenges in achieving high-precision shape measurements due to the degradation of signal-to-noise ratio (S/N) caused by dark pulses from SiPMs, which are exacerbated by variations between SiPM elements and DC offset in detection circuits, limiting accurate detection of three-dimensional structures like holes and grooves.
Innovation Solution
A measurement device and signal processing method that includes a photoelectric conversion element and a signal processing unit capable of discriminating pulse amplitudes based on pre-acquired dark pulse amplitude distribution, setting threshold values and amplitude discrimination values to improve S/N ratio by removing dark pulses and performing photon counting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If SiPM elements are used for detecting reflected electrons to improve measurement capability for three-dimensional structures, then measurement capability for 3D structures is improved, but signal-to-noise ratio is degraded due to dark pulses
Solution Approach 1:
The patent applies preliminary action by acquiring the amplitude distribution of dark pulses in advance through calibration measurements before actual sample measurement. This pre-acquired distribution is stored and used during measurement to distinguish signal pulses from dark pulses, thereby resolving the noise problem while maintaining the ability to detect three-dimensional structures
Solution Approach 2:
The patent implements feedback by using the pre-acquired dark pulse amplitude distribution to dynamically determine discrimination thresholds during signal processing. The system continuously references the stored distribution to adaptively distinguish between dark pulses and genuine signal pulses, improving signal-to-noise ratio while preserving measurement capability
2Reliability
If threshold values are set to remove dark pulses, then signal-to-noise ratio is improved, but discrimination accuracy between signal pulses and dark pulses is reduced due to SiPM element variations and DC offset
Solution Approach 1:
The patent applies parameter changes by utilizing the amplitude distribution characteristics of dark pulses across different SiPM elements. Instead of using a fixed threshold, the system adjusts the discrimination threshold based on the statistical parameters (mean and standard deviation) of the pre-acquired dark pulse distribution, thereby adapting to element variations and DC offset while maintaining high signal-to-noise ratio
Solution Approach 2:
The patent introduces an intermediary approach by using the amplitude distribution statistics (mean and standard deviation) as intermediaries between the raw pulse signals and the final discrimination decision. This statistical intermediary layer accounts for SiPM element variations and DC offset, enabling accurate distinction between signal and dark pulses despite hardware imperfections
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 highly accurate measurements by stabilizing threshold and amplitude discrimination values, reducing the impact of SiPM variations and DC offset, thereby enhancing the quality of SEM images and improving signal processing in charged particle beam devices.
Implementation Method 1
a photoelectric conversion element such as a photodiode or SiPM capable of supplementing reflected electrons near a sample
Implementation Method 2
The SiPM outputs a current corresponding to the number of photons of incident light by combining a plurality of avalanche photodiodes (APDs) of a basic element in an array and applying a bias voltage to enter a geiger mode
Data Source
AI summary
A measurement device that comprises a photoelectric conversion element and a signal processing part that receives, from the photoelectric conversion element, detected pulses that include dark pulses and signal pulses that are outputted in accordance with inputted photons. The signal processing part performs amplitude discrimination on the detected pulses on the basis of a pre-acquired dark pulse amplitude distribution for the photoelectric conversion element.