High-DQE Backside-Illuminated Electron Sensor for Low Backscattering
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Solution Overview
Problem
Current electron detectors in electron microscopy suffer from poor spatial resolution and sensitivity at electron energies between 40 keV and 120 keV due to increased scattering and backscattering, leading to degraded image quality and high noise, especially in low-energy scenarios, and existing detectors are limited by pixel size and cost in high-energy applications.
Innovation Solution
A monolithic active pixel sensor with back side illumination and substrate removal is developed, allowing electrons to enter from the epitaxial silicon layer, minimizing backscattering and enabling high detective quantum efficiency (DQE) across a wide energy range by using a thin, high-pixel-density design.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional MAPS detectors are used for low-energy electrons (40-120 keV), then detector thickness can be maintained, but backscattering increases and spatial resolution degrades
Solution Approach 1:
The patent inverts the conventional detector orientation by removing the substrate and using backside illumination. Electrons enter through the epitaxial silicon layer rather than the substrate side, which eliminates backscattering from substrate materials and improves spatial resolution while maintaining reliable detection across the full energy range.
Solution Approach 2:
The patent extracts and removes the substrate layer from the detector structure. This removal eliminates the source of backscattering electrons that would otherwise degrade image quality, allowing the detector to maintain high performance at low electron energies without the harmful backscattering effects.
2Object-affected harmful factors
If substrate is removed for backside illumination, then backscattering is reduced, but mechanical strength and handling difficulty increase
Solution Approach 1:
The patent applies local quality by selectively removing substrate material only from regions where it would cause backscattering, while retaining substrate in peripheral areas to provide mechanical support. This localized approach maintains structural integrity while eliminating the harmful backscattering effects in the active sensing region.
3Measurement precision
If thin epi layer is used to reduce backscattering, then spatial resolution improves, but detection efficiency at high energy decreases
Solution Approach 1:
By inverting the detector orientation to use backside illumination, the patent allows the thin epi layer to function effectively without compromising high-energy detection. Electrons enter through the thin epi layer which minimizes scattering, yet the overall detector structure maintains sufficient interaction volume for efficient detection across the full energy range.
4Ease of manufacture
If conventional front-side illumination is used, then manufacturing is simpler, but pixel size must be larger reducing field of view
Solution Approach 1:
The patent inverts the illumination approach to backside illumination, which allows for smaller pixel sizes and higher pixel density. This inversion enables a larger field of view while maintaining manufacturability through adapted fabrication processes that form the sensor circuitry in the epi layer before substrate removal.
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 sensor achieves high-DQE imaging with a large field of view and practical manufacturing costs, matching the performance of conventional detectors at higher energies while maintaining resolution and reducing backscattering, thus addressing the limitations of existing technologies.
Implementation Method 1
A monolithic active pixel sensor is formed in an epitaxial silicon layer... each incident electron produces many secondary electrons in a cloud trail as they traverse and scatter through the detector
Implementation Method 2
Some fraction of incident electrons can undergo scattering at large angles either within the epi layer or material under the epi layer-which is called backscattering-and as a result deposit secondary charge in locations far from their point of entry
Data Source
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AI summary
A detector is provided for forming images by detecting electrons in an electron microscope at energies in the range of 3 keV to 300 keV, more specifically in the range of 40 keV to 120 keV with very high spatial resolution and sensitivity. The detector is formed by bonding a handling wafer to the front side of a planarized monolithic active pixel sensor (MAPS), partially or completely removing the substrate layer on the back side and selectively removing the handling material from the front side to leave a periphery of handling material in the non-image forming area. The detector may be mounted in an electron microscope for back side illumination. The detector provides high resolution images at low-energies due to back side illumination and at higher energies due to a decreased epitaxial layer thickness and the absence of any backscattering substrate material.