Aberration-corrected and energy-filtered low energy electron microscope with monochromatic dual beam illumination

Abstract

One embodiment relates to an apparatus for correcting aberrations introduced when an electron lens forms an image of a specimen and simultaneously forming an electron image using electrons with a narrow range of electron energies from an electron beam with a wide range of energies. A first electron beam source is configured to generate a lower energy electron beam, and a second electron beam source is configured to generate a higher energy electron beam. The higher energy beam is passed through a monochromator comprising an energy-dispersive beam separator, an electron mirror and a knife-edge plate that removes both the high and low energy tail from the propagating beam. Both the lower and higher energy electron beams are deflected by an energy-dispersive beam separator towards the specimen and form overlapping illuminating electron beams. An objective lens accelerates the electrons emitted or scattered by the sample. The electron beam leaving the specimen is deflected towards a first electron mirror by an energy-dispersive beam separator, which introduces an angular dispersion that disperses the electron beam according to its energy. A knife-edge plate, located between the beam separator and first electron mirror, is inserted that removes all of the beam with energy larger and smaller than a selected energy and filters the beam according to energy. One or more electron lenses focus the electron beam at the reflection surface of the first electron mirror so that after the reflection and another deflection by the same energy-dispersive beam separator the electron beam dispersion is removed. The dispersion-free and energy-filtered electron beam is then reflected in a second electron mirror which corrects one or more aberrations of the objective lens. After the second reflection, electrons are deflected by the magnetic beam separator towards the projection optics which forms a magnified, aberration-corrected, energy-filtered image on a viewing screen.