Preparing a Celadonite Electron Source and Estimating Its Brightness

Abstract

The electron celadonite source described in this article performs well in a low-energy electron point-source projection microscope in long-range imaging. It presents major advantages compared to sharp metal tips. Its robustness affords a lifetime of months and it can be used under relatively high pressure. The celadonite crystal is deposited at the apex of a carbon fiber, maintained itself in a coaxial structure ensuring a spherical beam shape and easy mechanical positioning to align the source, the object and the electron-optical system axis. There is a single crystal deposition via generation of celadonite-containing water droplets with a micropipette. Scanning electron microscopy observation can be performed to verify the deposition. However, this adds steps and therefore increases the risk of damaging the source. Thus, after preparation, the source is usually inserted directly under vacuum in the projection microscope. A first high voltage supply provides the kick-off needed to start the electron emission. The field emission process involved is then measured: it has already been observed for dozens of electron sources prepared in this way. The brightness is underestimated through an overestimation of source size, intensity at one energy and cone angle measured in a projection system.

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

“…In the case of electron sources, the contrast cannot be measured directly because fringes are observed. To measure the source size for electrons, two methods are used: the last visible fringe [9], and the contrast in an interferometry experiment [14].…”

“…In the case of an ultra-thin metal tip, this is a common requirement. However, if the support of the source is too bulky, as with the so-called "celadonite source" [9], it is impossible to respect this distance without touching the extractor grid. To overcome this di culty, an electrostatic lens can be used to enlarge the fringe recording.…”

“…When this local field reaches some V /nm, electron emission takes place [6]. However, this source is not an ultra-thin tip; it is situated at the end of a carbon fiber of = 10µm diameter, and the approach is therefore limited to some tens of micrometers [9]. For this reason, a new point-projection microscope was realized, comprising a micro-manipulator, a double electrostatic Einzel lens, and a double micro-channel plate detector [10].…”

“…We will see in this article that a projection microscope is perfectly suited to characterizing sources and in particular their size, whatever the emitted particles [4,5]. An electron source consisting of an insulating crystal deposited on a carbon fiber, as described in several articles [6,7,8,9,10], was characterized with these methods and will be described here.…”

Low-macroscopic field emission structure: Using the shape of the conductor to identify a local difference in electric potential

“…This kind of structure appears to enhance electric fields sufficiently to produce field emission. Based on this principle, an electron source whose construction was detailed and presented in an article in the Journal of Visualized Experiments 11 made it possible to achieve electron field emission with brightness 12 of the order of B = 10 10 A.cm −2 .sr −1 , i.e. brightness equivalent to that of ultra-thin metal tips.…”