Enhanced angular current intensity from Schottky emitters

Abstract

Even though the Schottky emitter is a high-brightness source of choice for electron beam systems, its angular current intensity is substantially lower than that of thermionic cathodes, rendering the emitter impractical for applications that require high beam current. In this study, two strategies were attempted to enhance its angular intensity, and their experimental results are reported. The first scheme is to employ a higher extraction field for increasing the brightness. However, the tip shape transformation was found to induce undesirably elevated emission from the facet edges at high fields. The second scheme exploits the fact that the angular intensity is proportional to the square of the electron gun focal length [Fujita, S. & Shimoyama, H. (2005) Theory of cathode trajectory characterization by canonical mapping transformation. J. Electron Microsc. 54, 331-343], which can be increased by scaling-up the emitter tip radius. A high angular current intensity (J(Omega) approximately 1.5 mA sr(-1)) was obtained from a scaled-up emitter. Preliminary performance tests were conducted on an electron probe-forming column by substituting the new emitter for the original tungsten filament gun. The beam current up to a few microamperes was achieved with submicron spatial resolution.

Computer modeling of the Schottky electron source

“…Previous studies [1][2][3][4][5][6][7] have shown that the SE source reaches a quasi equilibrium state resulting in one of four polyhedral shapes that involve various combinations of enlargement or contraction of the four {112} and {110} crystal planes adjacent to the central (100) facet. The SE source in this context is a point electron source consisting of a W(100) oriented single crystal with a ZrO 2 over layer and operated at 1800 K. In order to obtain the optimum source performance for a given electron optical application, it is necessary to know how the emission parameters vary with emitter geometry and evolve over time.…”