Tungsten Cathode Orientation for High Angular Current Density
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Solution Overview
Problem
Conventional ZrO/W electron sources are limited to an angular current density of about 1.0 mA/sr, and increasing this density leads to vacuum deterioration due to high extraction voltage and undesired dump current, which causes instability and electric discharge.
Innovation Solution
Adjusting the angle of the tungsten single crystal cathode to 22.5±10° relative to the (100) direction allows electrons to be emitted almost parallel to the cathode axis from the boundary region between the (100) and (110) planes, reducing dump current and enabling operation at higher angular current densities without increasing extraction voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the angular current density is increased to improve throughput, then the extraction voltage must be increased, but this causes vacuum deterioration and electric discharge
Solution Approach 1:
The invention changes the crystallographic orientation parameter of the cathode from conventional <100> to <110>, which fundamentally alters the electron emission characteristics. This parameter change enables high angular current density (1.0 mA/sr or more) to be achieved without requiring high extraction voltage, thereby maintaining vacuum stability and preventing electric discharge while improving throughput
Solution Approach 2:
The invention uses a composite structure consisting of tungsten single crystal cathode with specific <110> orientation combined with ZrO coat layer. This composite material system provides both the mechanical strength of tungsten and the low work function of ZrO, enabling stable electron emission at high angular current density without the harmful effects of high extraction voltage
2Productivity
If the curvature radius of the cathode tip is increased to allow higher angular current density, then the (100) crystal face grows larger, but this increases dump current and causes vacuum deterioration
Solution Approach 1:
The invention changes the crystallographic orientation parameter from <100> to <110>, which fundamentally alters the electron emission pattern. This parameter change directs electrons almost parallel to the cathode axis, eliminating the lateral dump current that occurs with <100> orientation, even when the cathode tip curvature radius is increased
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 configuration allows for stable operation at high angular current densities of 1.0 mA/sr or more without vacuum deterioration, enhancing the performance and reliability of electron beam apparatuses like scanning electron microscopes and wafer inspection systems.
Implementation Method 1
The ZrO coat layer reduces the work function of the tungsten single crystal in the (100) plane from 4.5 eV to about 2.8 eV
Implementation Method 2
Because conventional cathodes are used generally at a temperature of about 1800K, as the filament is heated under application of current
Implementation Method 3
as the filament is heated under application of current, the ZrO coat layer on the cathode surface vaporizes
Implementation Method 4
zirconium and oxygen are supplied to the cathode surface from the supply source continuously by diffusion, resulting in preservation of the ZrO coat layer
Implementation Method 5
When an electric field is applied to the cathode in that shape in vacuum apparatus, the electrostatic force as driving force causes mass migration toward the tip along the surface
Data Source
AI summary
Provided are an electron source which allows a high-angle current density operation even at a low extraction voltage, and reduces excess current that causes vacuum deterioration; and an electronic device using the electron source. The electron source has a cathode composed of single-crystal tungsten, and a diffusion source provided in the intermediate portion of the cathode. In the cathode, the angle between the axial direction of the cathode and <100> orientation of the cathode is adjusted so that electrons to be emitted from the vicinity of the boundary between (100) surface and (110) surface formed on the tip of the cathode, are emitted substantially parallel to the axis of the cathode. The electronic device is provided with the electron source. The electron source has a cathode composed of single-crystal tungsten, wherein the angle between the axial direction of the cathode and <100> orientation of the cathode is adjusted so that electrons to be emitted from the vicinity of the boundary between (100) surface and (110) surface formed on the tip of the cathode, are emitted substantially parallel to the axis of the cathode; and a diffusion source provided in the intermediate portion of the cathode.