Low-Energy Electron Microscope of Novel Design

Abstract

The pioneering work of Bauer and Telieps stimulated us to construct a low-energy electron microscope intended mainly for imaging solid surfaces with reflected and diffracted electrons in the 10 to 100 eV range, as well as with photoelectrons. Several novel design features were incorporated.Figure 1 shows the layout of the instrument. The primary-beam column consists essentially of the electron gun, the primary-beam lens, the energy-focusing deflector and the magnet. al lenses are electrostatic. The electron gun comprises the cathode and two acceleration electrodes, the first of which is shaped semi-spherical about the cathode tip and is connected to a variable voltage supply, and the second is the tubular vacuum housing. This constitutes an immersion lens with variable focal length. The cathode is operated in the diode mode. Its virtual diameter is assumed to be dv = dtip √2.7 kT/eVa because in thermionic emission the energy distribution drops to half the peak value at 2.7 kT. The final acceleration voltage is Va = 15 KV. The variable immersion lens together with the primary-beam lens provides a magnification range of about 1 to about 70 without a crossover from the cathode to the backfocal plane of the emission lens. For example, a pointed W cathode with a tip diameter of 1μm imaged with 15× magnification yields a lμm diameter image.