Retarding Lens Energy Filter for Cold Field Emission Electron Beam
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Solution Overview
Problem
Existing electron beam apparatuses face limitations in reducing chromatic aberration due to energy spread, which is typically addressed by expensive and complex monochromators that also reduce beam intensity.
Innovation Solution
A method and apparatus utilizing a cold field emission electron source with a retarding lens as a high-pass energy filter, controlled by a predetermined offset voltage, to reduce energy spread while maintaining a large fraction of the original beam intensity, and incorporating a beam limiting aperture to minimize unwanted energy spreading.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a monochromator is used to reduce energy spread, then chromatic aberration is reduced, but beam intensity is considerably reduced and device complexity increases
Solution Approach 1:
Instead of using a traditional monochromator that selects a narrow energy band from the center of the emission spectrum, this invention inverts the approach by using the asymmetric low-voltage emission spectrum to selectively transmit the low-energy portion of the beam through a retarding lens. This inverted energy selection method achieves chromatic aberration reduction while preserving beam intensity by utilizing rather than rejecting the asymmetric spectral characteristics.
Solution Approach 2:
The invention changes the energy distribution parameters by operating the field emission electron source at low voltage (less than 5 keV, particularly less than 1 keV) to generate an asymmetric energy spectrum. The retarding lens then applies a specific retarding voltage to filter the beam, transforming the energy distribution to achieve reduced energy spread while maintaining intensity through proper voltage parameter selection.
2Manufacturing precision
If a monochromator is used to reduce energy spread, then chromatic aberration is reduced, but device complexity and cost increase
Solution Approach 1:
This invention extracts only the essential energy filtering function from the complex monochromator system by using a simple retarding lens to select and transmit the low-energy portion of the asymmetric emission spectrum. This extraction approach achieves the necessary energy spread reduction for improved spatial resolution without requiring the complex mechanical and optical components of a traditional monochromator.
Solution Approach 2:
The invention replaces the expensive, complex monochromator with a simple, cost-effective retarding lens configuration that uses the inherent asymmetric spectrum of low-voltage field emission sources. This substitution uses inexpensive components to achieve the same functional outcome, making high-resolution electron microscopy more accessible and cost-effective.
3Manufacturing precision
If field emission electron sources are used, then energy spread is smaller than thermal sources, but further reduction of energy spread is desired to reduce chromatic aberrations
Solution Approach 1:
The invention performs preliminary energy filtering by using the retarding lens to selectively transmit the low-energy portion of the beam before the electrons enter the optical system. This preliminary action of energy selection at the source, combined with the asymmetric spectrum characteristics, achieves further energy spread reduction beyond what standard field emission sources provide, thereby reducing chromatic aberrations.
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 solution effectively reduces energy spread while preserving a significant portion of the electron beam intensity, achieving improved spatial resolution without the need for costly monochromators.
Implementation Method 1
generating a primary electron beam having an asymmetric energy distribution by means of an electron source; applying a first voltage to an electron emitter of the electron source
Implementation Method 2
high-pass energy filtering the primary electron beam using the retarding lens. The retarding lens reflects electrons of the primary electron beam having a substantially lower energy than a predetermined threshold energy
Implementation Method 3
applying a second voltage to a retarding lens, wherein the first voltage and the second voltage are controlled to have a voltage difference equal to a predetermined offset voltage
Data Source
Figure 1
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AI summary
An electron beam apparatus 1 and a method for providing an energy-filtered primary electron beam are described. Therein, a primary electron beam 14 having an asymmetric first energy distribution is generated by means of an electron source 10. The primary electron beam is high-pass energy filtered using a retarding lens 30.