Magnetic field superimposed cold field emission gun under extreme-high vacuum

Abstract

A magnetic-field superimposed cold field emission gun (M-FEG) was developed for a 1.2-MV holography transmission electron microscope (TEM). Differential pumping using three nonevaporative getter pumps reduced the pressure of the gun to an extreme-high vacuum of 3 × 10−10 Pa and provided a stable probe current. The 90% decrease time, at which the current falls to 90% of the initial value, was 900 min. The current variation over the course of 8 h was 5.2%. The superimposed magnetic field provided large probe currents ranging from 1 to 170 nA for total currents ranging from 1 to 300 μA. The reduced pressure caused the angular current density to be twice that of a conventional field emission gun operating at 10−8 Pa. This increase can be explained by the difference between current densities emitted from the clean and adsorbed emitter surfaces. These results show that the developed M-FEG can provide the 1.2-MV TEM with a bright and stable emission current.