Calculation of the electron-optical characteristics of electron beams transmitted into vacuum from a sharp tip-thin foil junction

The electron-optical characteristics of a novel electron source consisting of a sharp tip-thin foil tunnel junction are calculated, taking into account the tunnel junction, electron transport through the freestanding metal foil, and transmission across the opposing vacuum emission surface. A tunable high-pass energy filter is obtained, via adjustment of the tunnel bias voltage, enabling monochromatization of the electron beam. The dependence of the vacuum emission current, energy spread, reduced brightness, and virtual source size on the tunnel bias voltage are evaluated for a constant tunnel junction current of 10 nA and a foil thickness of 5 nm. Because the dimensions of the tunnel junction are comparable to the electron wavelength, diffraction plays an important role. As a result, the reduced brightness and vacuum emission current are related via the expression B = I-emission(2 me/h(2)). First, the source may be operated at a tunnel bias voltage for which the energy spread approaches the value for a room-temperature field-emission source (0.2 eV), with a vacuum emission current of 1 nA and a reduced brightness of 7 x 10(8) A m(-2) sr(-1) V-1. By careful adjustment of the tunnel bias voltage to the foil work function value it is possible, in principle, to contain 50% of the beam current within an energy spread of 100 meV at a total vacuum emission current of 0.1 nA and a reduced brightness of 7 x 10(7) A m(-2) sr(-1) V-1. The virtual sourer size in this case is approximately 1.4 nm. The energy spread may be decreased even further, down to the room-temperature thermionic limit, at the expense of vacuum emission current and, consequently, reduced brightness. (C) 1998 American Institute of Physics.