Patent Number: 058959190
Section: summary

The invention relates to a gun lens for generating a particle beam according to the preamble to claim 1. The gun lens has a cathode, an extraction electrode, an anode and a condenser lens. The invention also relates to a particle beam device with such a gun lens. Gun lenses with the most varied designs are known in the art. A. Delong, J. Chmelik, V. Kolarik, J. Komurka, J. Ocadlik, "A new design of field emission electron gun with a magnetic lens", Optik No. 3, 1989, pages 104 to 108, shows the field emission sources used in the art with a partially superimposed magnetic field. Such field emission sources are used for example in particle beam devices, particularly electron beam microscopes. In the past it was assumed that the smallest spherical and chromatic aberrations were obtained if the electrons were first of all accelerated and only decelerated to the final energy shortly before reaching the specimen. Such devices have proved worthwhile in practice. Tests were also carried out in which the particles already have their final energy in the gun. However, with these guns relatively high aberration coefficients have to be accepted, and in many applications these can no longer be tolerated. The object of the invention, therefore, is to provide a gun lens according to the preamble to claim 1 and a particle beam device according to the preamble to claim 14 for generating a particle beam with low aberration coefficients and high spatial resolution. This object is achieved according to the invention by the combination of the characterising features of claims 1 and 14 respectively, in that a deceleration field is generated between the extraction electrode and the anode and the condenser lens causes a magnetic field which is superimposed on both the cathode, the extraction electrode and the anode. The superimposed magnetic field forces the generated particle beam on trajectories around the optical axis, so that in particular spherical and chromatic aberrations are reduced. Further embodiments of the invention are the subject matter of the subordinate claims. In a preferred embodiment the anode is at ground potential. This substantially simplifies the construction of a particle beam device, since all subsequent arrangements can also be at ground potential.