Magnetic Focusing of an Electron Beam from a Point Field Emitter

This article presents a construction of a MEMS electron microcolumn with a field emitter in a form of a silicon tip covered with a carbon nanotube layer. Although it ensures high electron current, its focal spot and uniformity is far from optimal. Therefore, to improve the parameters of the electron beam authors proposed to use magnetic focusing. The perpendicular magnetic field is generated by a strong neodymium magnet placed below the cathode. It confines the electrons near the optical axis and prevents them from spreading. The tests proved that it is possible to obtain a small, homogeneous electron beam spot with a diameter lower than 1 mm. Magnetic focusing combined with electron optics significantly improved the quality of the beam, in comparison to the system with only electron optics. In addition, a satisfactory level of current was achieved at the anode. For UA = 2.5 kV, UG = 1.5 kV and UF = 1.4 kV, with additional magnetic focus, the current on the anode was as high as 16 µA, with the distance between the emission cathode and the anode equal to 9.6 mm.