Gerald G. Magera

Defined emission area and custom thermal electron sources

The authors report on electron emission defining and stability techniques use for specialized thermionic cathodes. Primarily lanthanum hexaboride and cerium hexaboride have been used for cathode materials but we also use hafnium carbide for cases where background atmospheres preclude the use of hexaborides. A common form of emission suppression is to embed an oriented single crystal in graphite to suppress side emission and to help shape the electric field. Single planar discs 50 μm in diameter have been tested for use as high brightness, stable, and long life thermal sources. Line sources have also been developed with linewidth/lengths to 10/500 μm. Emission tests performed have shown that long-term drift and short-term instabilities can originate from boride and carbon interactions respectively. Improved mounting techniques are shown to yield emission with short-term beam current stability <0.05%.

Using HfC(210) and HfC(110) as high brightness electron sources

We report on high-brightness, thermal-field emission cathodes using (210) and (110) oriented hafnium carbide. HfC(210) cathodes are shown to deliver very high angular intensities to >50 mA/sr and remain stable. Modeling and experimental data are shown which highlights the utility of these sources for SEM and other high-brightness uses.

HfC(310) high brightness sources for advanced imaging applications

We report using a Philips XL40 SEM to demonstrate the performance of HfC(310) emitters operating in extended Schottky mode. Higher brightness and smaller spot sizes were obtained over commercial Schottky emitters operating under identical conditions.

Hafnium carbide CFE, TFE, and schottky electron sources

We report on CFE and TFE cathodes using (310) oriented hafnium carbide. These have been operated in UHV and temperatures from 300 K to 1900 K. Emission data show dramatic increases while operating at elevated temperatures due to work function lowering. Artificial faceting using FIB techniques shows promise for this cathode operating in Schottky emission mode.

Hafnium carbide thermal sources in O 2 , CO and CO 2 environments

We have tested thermionic cathodes using (210) and (310) oriented hafnium carbide. These have been operated in UHV, and pressures of oxygen, CO₂ and CO to 5 × 10^-5 Torr. Emission data show slight initial improvements due to work function lowering. Relatively stable operation at 1900 K in oxygen or CO₂ at 1 × 10^-6 Torr is reported.

11.3: Electron emission from hafnium carbide

HfC was evaluated as a cold field emission source. Single crystal HfC was produced and fabricated into cold field emitters, then angular intensity and reduced brightness were determined from experimental I(V) data. Energy distribution data were in agreement with a theoretical model. The reduced brightness, energy distribution, and emission stability are compared to commercially available sources which show that HfC produced a higher brightness and a lower energy spread than a W cold field source or a ZrO/W Schottky emitter. HfC maintains its emission level for one hour in moderate UHV condition; a dramatic improvement over the stability of W. This combined with a durability that allows for frequent flash cleaning without degradation of the emitter end form make HfC a highly promising source. We compared stability and noise to emission from a tungsten tip at the same angular intensity. By increasing the emitter temperature slightly, stability is improved while maintaining a low energy spread.

P1–34: Stability improvements on Vogel mounted thermal sources

We have produced thermionic cathodes using (100) oriented lanthanum and cerium hexaboride as the emission sources. These cathodes use the Vogel mount system for mechanical alignment and heating. While this mount is superior for mechanical stability we show methods to improve emission stability which are mount related.

6.6: Emission imaging of a single crystal LaB 6 cathode surface

LaB6 cathode is still the emitter of choice in a variable-shape beam (VSB) electron beam lithography tool. In commercial LaB6 cathodes, the (100) crystalline plane is used as the emissive surface. Typical size of emitter is ∼70 µm DIA. Though generally stable, this crystalline plane is sensitive to residual atmosphere, and it may have microscopic defects (inclusions, dislocations, etc.) which appear and evolve over the time. Routine initial microscopic inspection of LaB6 cathodes gives us an initial optical/SEM image of surface, which may change during cathode life. With a simple technique, we have obtained LaB6 cathode emission images, which showed features unavailable to optical and electron microscopy. This technique can be used for LaB6 quality evaluation during standard cathode test runs.

Electron emission from hafnium carbide

Past research has studied single-crystal transition metal carbides operating in thermionic or field emission modes.1 In particular, hafnium carbide cathodes have properties making them attractive candidates for stable emission sources in moderate to good vacuum applications. The use of HfC or ZrC with a (310) orientation provides a relatively low work function emitting surface (3.4 eV) that has a very low evaporation rate, is resistant to ion bombardment and sputtering2, has a high melting point (∼4200 K), and has a very low surface mobility. As field emission sources they can operate at high current densities and using the mini Vogel mount can withstand many thousands of flash heating cycles. The robustness of this material has been demonstrated in field emission, photo-field, and thermionic studies.