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Title: The operation of LaB6 cathode in the virtual source mode
Author: Bakush, Sherif Mohamed
ISNI:       0000 0004 2749 4645
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2011
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This thesis describes the design of a novel electron cathode as utilised in electron microscopes and electron beam lithography columns. The Lanthanum Hexaboride (LaB6) cathode is a high brightness thermionic electron cathode with lower operating temperature and increased lifetime compared to those obtained by the conventional thermionic tungsten (W) cathodes. This cathode also has several similarities to the zirconium on tungsten Zr/O/W (100) Schottky field emission cathode (SFE) including similar operating temperature of 1800K and similar work function. One important difference is that the LaB6 does not require as stringent vacuum requirements, as the SFE or indeed other field emission based cathodes. LaB6 cathodes normally operate with a stable beam current at vacuum regimes of 1O-7mbar. The significant difference between these two cathodes is their respective brightness being more than xl00 apart. The reason for this is their different mode of operation. As a result, the SFE cathode has a virtual source size of order 20-30 nrn, whilst the LaB6 cathode has a real crossover source of 5-20/-lm in size, depending on the physical cathode size used. The underlying objective of this research was to operate a thermionic LaB6 cathode in a virtual source mode, similar to a SFE cathode. The expectation is to increase this cathode's brightness, whilst benefiting from its less stringent vacuum operational requirements. The LaB6 cathode operating in the virtual source mode (VS-LaB6) is capable of filling the brightness gap between the thermionic class of cathodes and those operated in the virtual source mode. It also possesses the benefit of a simpler design and operational requirements of a new class of higher brightness electron beam columns with a moderate vacuum. Experiments were performed to demonstrate the theoretical principle underlying the operation of a LaB6 in the virtual source mode. Data obtained using the VS- LaB6 cathode showed a performance improvement in brightness of nearly a single order of magnitude when operated in an electron microscope over the conventional LaB6 cathode. The V - LaB6 cathode was also successfully installed on a modern Carl Zeiss Evo 15 Scanning Electron Microscope (SEM), which saw a considerable improvement in resolution.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available