ABSTRACT
To date, lanthanum hexaboride (LaB6) thermionic electron sources have not been able fully to capitalize on their inherent potential, resulting in an ambiguous position within the application area. Although they exhibit higher brightness compared with a tungsten filament source, they still fall short of the performance of Schottky electron sources. This study aims to explore the capabilities of the LaB6 electron source under different operating conditions to bridge the gap, ultimately to realize its untapped potential. Simulations in virtual source mode indicated enhanced beam brightness and a reduced beam half-angle with an increase the extraction voltage, promising up to tenfold times higher beam brightness compared with the crossover mode. The energy distribution measured using a prelens retarding field energy analyzer revealed an energy distribution of 0.55 eV and a high angular current density of 33 mA/sr in the virtual source mode. Therefore, the virtual source mode of LaB6 can provide a narrow energy distribution akin to that of a ZrO/W Schottky electron gun (1600 K) while having an angular current density over 2,000 times higher. In addition, the stability of the virtual source mode is ±0.022%, while that of the crossover mode is ±0.138%.
ABSTRACT
The energy distribution of an electron gun is one of the most important characteristics determining the performance of electron beam-based instruments, such as electron microscopes and electron energy loss spectroscopes. For accurate measurements of the energy distribution, this study presents a novel retarding field energy analyzer (RFEA) with the feature of an additional integrated pre-lens, which enables an adjustment of beam trajectory into the analyzer. The advantages of this analyzer are its compact size and simple electrode configuration. According to trajectory simulation theories, the optimum condition arises when the incident electron beam inside the RFEA is focused on the center of a retarding electrode. Comparing IV curves depending on whether the pre-lens working or not, it is confirmed that the use of the pre-lens dramatically improves the energy resolution and efficiency of the signal acquisition process. The pre-lens RFEA was applied to characterize a Schottky electron gun under various temperatures and extraction voltages as operational conditions. When the tip temperature was increased by 50 K, we were able to measure an energy distribution broadening of 13.8 meV with the proposed pre-lens RFEA. The relative standard deviation of energy distribution was 0.7% for each working condition.
