Brightness Electron Beams Research Articles

Factors affecting high resolution fixed-beam transmission electron microscopy

An experimental and theoretical characterization of a fixed-beam transmission electron microscope with a field emission gun has been made with regard to the factors of electron beam brightness, spatial and temporal coherence of the incident electrons, objective lens current fluctuation, mechanical stability, and specimen contamination. It has been found that mechanical stability and temporal coherence are the primary factors that prevent the contrast transfer function from extending to 2.0 A in our microscope. Different amorphous thin films have also been used in order to compare their suitability for testing the imaging capability of the microscope at atomic resolution.

The brightness of electron beams

An expression is derived for the brightness or directional beam intensity of an electron beam, for a general velocity distribution at the emitting cathode. The result is then specialized to the case of the brightness on the axis in the direction of the axis for a cylindrically symmetric system. In this case the Langmuir law B/J=eV/πkT is shown to hold for emission in the Schottky region even though the velocity distribution is not Maxwellian; and an analogous expression B/J=eV/πd, where d is a function only of the applied field, is shown to hold for field-emitted beams. It is shown that detailed ray tracing is necessary to evaluate the brightness for points not on the axis and directions not in the axial direction.

High Brightness Electron Gun Using a Field Emission Cathode

A field emission cathode electron gun with two stages of acceleration has been built in order to measure the electron beam brightness that can be produced in practice from a tungsten field emission cathode. The gun is similar to that reported by A. Crewe except that the accelerating electrodes are plane, rather than shaped, apertures, and the cathode is located by a gimbal mechanism which allows the cathode to be tilted over an arc of 70° in any direction and positioned laterally. The gun electrodes have been precisely machined with the apertures round within 0.25 micron and aligned with respect to each other to better than 10 micron. The second accelerating electrode is followed by scan plates, a test grid, and an electron detector which together allow the probe size to be measured in the usual scanning electron microscope mode.

Focussing, Current Density and Brightness of Electron Beams from Electron Guns with Various Bias Voltages —A Study with Oxide-Cored Cathode—

By the use of oxide-cored cathode, which gives a very small electron source (J. Phys. Soc. Jap. 14, 180 (1959)), as well as the usual tungsten hair-pin filament, basic properties of electron beams from electron guns with various Wehnelt bias voltages were studied. It is proved that the image of a tip of cathode is formed on a plane at a certain bias voltage. Near this voltage, the brightness becomes maximum, agreeing with the ideal value. The decrease of the brightness at other valtages is attributed to an instrumental error due to the finite size of apertures used in the measurement, but is not due to the chromatic aberration as concluded by Haine and Einstein. Space charge limitation also makes the brightness lower near the cut-off voltage. Hollow beam does not appear for oxide-cored cathode. This is caused by electrons from the side surface of a filament.