Thermionic Cathode Gap Design for Uniform Electric Field
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Solution Overview
Problem
Cathodes used in electron microscopes and mask writers face limited usable time due to evaporation and chemical interaction between lanthanum hexaboride or cerium hexaboride materials and carbon coatings, leading to uneven electric field distribution and reduced writing precision.
Innovation Solution
A cathode design featuring a carbon covering layer with a controlled gap along the conical surface to prevent direct contact and maintain even electric field distribution, using materials like graphite or diamond-like carbon with a gap depth of 10 μm to 200 μm and width of 1 μm to 10 μm to suppress evaporation and ensure precise electron beam emission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If the cathode tip is covered with carbon to suppress evaporation, then the usable time is extended, but chemical interaction between cathode material and carbon causes partial evaporation at the edges
Solution Approach 1:
The cathode structure is segmented into distinct regions: a carbon-free electron emitting surface at the tip and a carbon-covered tapered portion. This segmentation prevents direct contact between carbon and the cathode material at the emitting surface, eliminating chemical interaction and material loss while maintaining the protective benefits of carbon coating on the body.
Solution Approach 2:
The carbon coating is extracted or removed from the electron emitting surface at the cathode tip, leaving only the tapered portion covered with carbon. This extraction eliminates the harmful chemical interaction at the emitting surface while preserving the evaporation suppression benefits of carbon coating on the cathode body.
2Loss of substance
If a gap is provided between the cathode and carbon covering to prevent chemical interaction, then material evaporation is suppressed, but uneven gap width causes uneven electric field distribution and beam displacement
Solution Approach 1:
The carbon coating is applied with locally different properties: a controlled gap is maintained at the electron emitting surface region to prevent chemical interaction, while the carbon coating directly contacts the cathode on the tapered portion. This local differentiation in coating configuration optimizes both material protection and electric field uniformity.
3Productivity
If the cathode operates at high temperature to emit electrons, then electron emission is sufficient, but the cathode material evaporates and lifetime is limited
Solution Approach 1:
Carbon serves as an intermediary protective layer on the cathode's tapered portion, acting as a barrier that suppresses evaporation of the cathode material at high operating temperatures. This allows the cathode to maintain high temperature operation for sufficient electron emission while the carbon layer protects against material loss.
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 cathode design extends the usable time by preventing chemical interaction and maintaining even electric field distribution, enhancing writing precision and electron beam convergence in mask writers and other electron microscopy applications.
Implementation Method 1
the cathode formed of the above-described material evaporates at 1650 K to 1900 K (Kelvin) which are operating temperatures
Implementation Method 2
evaporation or disappearance occurs partially in the cathode by chemical interaction between the cathode material (for example, the lanthanum hexaboride (LaB6), the cerium hexaboride (CeB6), or the like) and carbon
Implementation Method 3
A lanthanum hexaboride (LaB6), a cerium hexaboride (CeB6), a hafnium carbide (HfC), or the like which is crystallized or sintered is used as an electron source or emitter
Implementation Method 4
when the width of the gap formed in the tip of the cathode differs depending on its position, an electric field distribution in the cathode tip becomes uneven
Data Source
AI summary
A thermionic cathode of an embodiment includes a carbon coating applied to an outer surface of the side, the carbon coating comprising a contiguous extended portion surrounding the upper section and spaced apart from said upper section by a gap having 1 μm or more and 10 μm or less in width and having a difference of 1 μm or less in the width between a maximum value and a minimum value in a periphery of the electron emitting surface.