Cold field emitter

#The present invention relates to electron emitters, and in particular to cold field electron emitters.

#Electron emitters are used to generate electrons that are directed into beams for electron microscopy and other applications. Electron microscopy includes scanning electron microscopy, transmission electron microscopy, and scanning transmission electron microscopy, as well as analytical variations of these techniques. An ideal electron source produces a beam of electrons that can be focused to an extremely small spot with sufficient current to provide rapid, consistent data collection. Such an electron source is typically characterized by low energy spread among the emitted electrons, high brightness, and long-term stability.

#To be freed from a solid surface, an electron must overcome an energy barrier. The height of this energy barrier is referred to as the “work function” of the material. Thermionic electron emitters are heated by a filament to provide the electrons with sufficient thermal energy to overcome the energy barrier and leave the surface. Schottky electron emitters use a combination of coating materials that lower the work function, heat to provide thermal energy, and an electric field to free the electrons. Cold field electron emitters, on the other hand, use an electric field to provide the conditions for electrons to tunnel through the energy barrier, rather than providing the electrons with the sufficient thermal energy to pass over the barrier.

#For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

#FIG. 1 shows a cold field electron source embodying the present invention.

#FIG. 2 is an enlarged, top view of the emitter of the cold field emitter of FIG. 1.

#FIG. 1 shows a cold field electron source 100 embodying the invention. Cold field emitter 100 includes a filament 102 that supports and heats an emitter 104 having a tip 106 from which the electrons are emitted. Tip 106 has as small a radius as can be practically constructed, preferably less than 500 nm, more preferably less than 200 nm, and most preferably less than, or about, 100 nm. A heating current can be supplied to filament 102 through electrodes 110 that penetrate a base 112. Emitter 104 is heated as explained below as part of its preparation for operation, but is not typically heated during operation.

#Emitter 104 preferably comprises a base material of a single crystal of a metal oriented typically with the <111>, <100>, <110>, or <310> crystal direction aligned along the emitter axis. Emitter 104 typically comprises tungsten, tantalum, rhenium, molybdenum, iridium, other similar metals or alloys of these metals. A preferred emitter base material is conductive, non-magnetic, has a low work function, and can be heated to a high temperature without significantly increasing the tip radius. The emitter base material is coated with a coating material.

#A preferred coating material does not readily adsorb background gases, such as oxygen, nitrogen, water vapor, and carbon containing gases. A preferred coating also has a low sputtering coefficient, that is, when hit by positive ions, relatively few atoms are released from the emitter. A preferred coating also increases the angular confinement of the electron beam. These desirable coating characteristics reduce short and long term current instabilities. A preferred coating may also reduce the work function of the emitter, although reducing the work function is not as important for a cold field emitter as for a Schottky emitter. The coating material can be grown on the emitter by causing a reaction between the material composing the emitter and another material introduced into the vacuum chamber, such as oxygen, nitrogen, or carbon. The coating can also be deposited onto the emitter. The coating could include, for example, compounds, such as oxide, nitrides and carbon compounds, of the emitter base material or of, for example, zirconium, titanium, hafnium, yttrium, niobium, vanadium, thorium, scandium, beryllium or lanthanum. The coating may include multiple species, such as a combination of carbon and oxygen on a tungsten emitter.