Bright sources under the projection microscope: using an insulating crystal on a conductor as electron source

Abstract

The development of bright sources is allowing technological break-
 throughs, especially in the field of microscopy. This requires a very ad-
 vanced control and understanding of the emission mechanisms. For bright
 electron sources, a projection microscope with a field emission tip provides
 an interference image that corresponds to a holographic recording. Image
 reconstruction can be performed digitally to form a "real" image of the
 object. However, interference images can only be obtained with a bright
 source that is small: often, an ultra-thin tip of tungsten whose radius of
 curvature is of the order of 10nm. The contrast and ultimate resolution of
 this image-projecting microscope depend only on the size of the apparent
 source. Thus, a projection microscope can be used to characterize source
 brightness: for example, analyzing the interference contrast enables the
 size of the source to be estimated. Ultra-thin W tips are not the only
 way to obtain bright sources: field emission can also be achieved by ap-
 plying voltages leading to a weak macroscopic electric field (< 1V/μm)
 to insulating micron crystals deposited on conductors with a large ra-
 dius of curvature (> 10μm). Moreover, analyzing the holograms reveals
 the source size, and the brightness of these new emitters equals that of
 traditional field emission sources.