A Focused He+ Ion Beam with a High Angular Current Density

Abstract

Focusing characteristics of an ion beam from a He field ion source were investigated. For a W emitter as installed, a beam with an ion current of 0.7 pA was focused into 0.2 µm because the angular current density did not exceed 0.02 µA/sr. Improvement of angular current density was attempted by using the emission from a cleaned W (111) facet treated by field evaporation. From observation of field ion microscope images, it was found that the angular current density in the axial direction is dependent on the size of the (111) facet. For a large facet, a ringlike emission was obtained and the angular current density in the axial direction was not improved by adjustment of the extraction voltage. It is necessary to fabricate a facet of less than a few tens of atoms to achieve high current beam of axial direction. A beam with an angular current density of about 1 µA/sr was applicable to a focused ion beam after appropriate field evaporation.