Schottky-emission electron gun, SE electron gun

An electron gun that utilizes the Schottky effect, in which thermoelectrons are easily emitted due to lowering potential barrier when a strong electric field is applied to a heated metal surface.
A single tungsten crystal coated with zirconium oxide (ZrO/W<100>) having a tip radius of a curvature of a few 100 nm is used as the cathode (emitter) of the electron gun. The cathode is operated at about1800 K. The coated ZrO greatly decreases the work function of the (100) plane of the tungsten emitter surface. The emitter requires an ultra-high vacuum of about 10^-7 Pa. The emitter has a lifetime of several years.

The diameter of the electron source (virtual source) is as small as 15 to 20 nm. The brightness of the Schottky-emission electron gun is approximately 10⁸ A/cm²sr, which is almost same as that of FEG. It is about three orders of magnitudes greater than that of the thermionic-emission gun. Thus, the Schottky-emission electron gun is used for high-resolution imaging.
Another feature of this gun is a long-time stable, large probe current because of less adsorption of residual gas molecules onto the emitter surface, owing to a high-temperature operation. Its large probe current of several 100 nA is about two orders of magnitudes greater than that for FEG. Thus, the Schottky electron gun is suitable for elemental analysis using an energy-dispersive X-ray spectrometer (EDS) and a wave-length dispersive X-ray spectrometer (WDS), and for crystal orientation analysis by electron backscatter diffraction (EBSD). Therefore, the Schottky-emission electron gun is an excellent gun used for multi-purpose applications.
Figure shows the basic structure of the Schottky-emission electron gun. The emitter is heated to about 1800 K by electrical heating. With a strong electric field generated by the extracting anode, electrons are emitted from the emitter tip of a W(100) plane. Then, the emitted electrons are accelerated by the voltage applied to the accelerating anode up to a predetermined energy. A negative charge is applied to the suppressor (electrode), so as to cut the unnecessary thermo-electrons emitted from the W(100) plane.

Fig. Basic structure of the Schottky-emission electron gun.

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