Applied Physics Lanthanum and Cerium Hexaboride (LaB6 and CeB6) Filaments / Cathodes

Description

The unique properties of hexaboride crystals provide stable electron-emitting media with work functions near 2.65 eV. The low work function yields higher currents at lower cathode temperatures than tungsten, which means greater brightness (or current at the beam focus) and longer cathode life. Typically, these cathodes exhibit 10 times the brightness and more than 10 times the service life of tungsten cathodes. In electron microscope applications, these characteristics translate to more beam current in a smaller spot at the sample, improved resolution, and less frequent cathode replacement. For applications with large beam spot sizes, where large total current and current density are required, large, flat crystal faces of LaB6 or CeB6 can be the cathodes of choice. This regime is unsuitable for point sources such as field emitters, which are unable to provide sufficient total current, and has been thought of as the realm of the dispenser cathode. However, LaB6 and CeB6 may be more suitable, being particularly robust and resistant to chemical poisoning. They have modest vacuum requirements and long shelf life, and need only be brought up to operating temperature to provide emission, eliminating the activation procedure required of dispenser cathodes. They can provide long-term, stable operation at current densities up to 50 A/cm2, and may be fabricated in a variety of shapes and with many different heating and mounting configurations. LaB6 and CeB6 are the materials of choice for high current cathodes in a variety of advanced and custom applications. The performance and lifetime of the hexaboride cathode are determined by several factors: vacuum level, cathode temperature, impurity level, crystal orientation, tip shape, and mount design. Vacuum requirements are more stringent for hexaboride emitters than for tungsten in order to minimize carbon contamination. In laboratory tests, CeB6 has proven to be more resistant to the negative impact of carbon contamination than LaB6, which gives it an edge in potential cathode lifetime. Excessive operating temperatures accelerate evaporation, thus decreasing the life of the cathode. Care must be taken to properly optimize cathode temperature to obtain the required emission without overheating the crystal. CeB6 has another advantage over LaB6 relating to lifetime: its evaporation rate at normal operating temperatures near 1800 K is lower than that of LaB6 . So long as care is taken to operate the cathode below 1850 K, CeB6 should maintain an optimum tip shape longer, and therefore last longer.

A comparison of electron emission characteristics of LaB6, CeB6 and tungsten at typical operating temperatures Crystal Growth Hexaboride crystals are grown and purified in an inert gas atmosphere to specified crystal orientations. Impurities in the crystal will reduce both brightness and lifetime of the emitter because impurities increase both work function and volatility. We grow and fabricate our own high quality, single-crystal materials using a well-defined process called Inert Gas Arc Float Zone Refining.’ An electric arc melts a pressed-powder stick of LaB6 or CeB6 in a controlled atmosphere of inert gas, allowing the liquid-phase zone to freeze onto a selected-orientation seed crystal as the arc is moved along the stick. The finished crystal assumes the desired orientation of the seed with less than 30 parts per million by weight metal impurities. Correct melt zone temperature and process speed minimize excessive boron evaporation to achieve the optimum ratio of metal to boron atoms in the grown crystal.

Hexaboride crystals are grown and purified in an inert gas atmosphere to specified crystal orientations.

Cathode Tip Design The design of the cathode tip is critical for maximum lifetime and optimum performance. Tip design must also match the specific application’s requirements for beam current, spot size, and brightness. For electron microscopy, a conical tip with a flat emitting surface at the apex has proven to be the optimum design. With the flat-tipped cone design, changes in both cone angle and flat diameter affect emission characteristics. In general, the small cone angle (60°) results in higher brightness, but a larger angle (90°) provides longer life and easier alignment. Small flat diameters also result in higher brightness plus a smaller source size, but larger flats provide longer lifetimes and more beam current. These trends allow us to tailor our cathodes to the requirements of practically all thermionic cathode applications. For example, SEM and most transmission electron microscope (TEM) applications are best served by a 90° cone angle and a 16 mm flat tip. This combination provides high brightness, a moderate source size, and very good lifetime. High resolution TEMs require a 60° cone and a 5 mm flat tip for very high brightness and a small source size. In applications requiring high total current in a large beam spot, a <310> oriented crystal in a “top hat” configuration may be preferred, providing a slightly lower work function and large emitting surface. We excel at developing specialized cathodes for custom applications and research purposes. Contact us for your custom cathode needs.

P/N	Manufacturer		Description
D80920-L	Amray all SEM models		LaB6 90/16
D80920			CeBix 90/16
D80921-L	Cameca EPMA SX50, SX100		LaB6 90/16 non-shunted
D80921			CeBix 90/16 non-shunted
D80922-20-L	Camscan all SEM models		LaB6 90/20 non-shunted
D80922-20			CeBix 90/20 non-shunted
D80922-16-L			LaB6 90/16 non-shunted
D80922-16			CeBix 90/16 non-shunted
D80923-L	Electronscan Philips ESEM 2020, E-3, XL		LaB6 90/16
D80923			CeBix 90/16
D80919-L	Etec Lithography		Etec LaB6 90/30 non-shunted
D80919			LaB6 90/30 non-shunted
D80924-L-S	Hitachi SEM		LaB6 90/16
D80924-S			CeBix 90/16
D80924-L-T	TEM		LaB6 90/16
D80924-T			CeBix 90/16
D80924-L-HT	High Res TEM, recommended for 200 kV+		LaB6 90/5
D80924-T			CeBix 60/5
D80925-L	ICT / Advantest SEM		LaB6 90/16 non-shunted
D80925			CeBix 90/16 non-shunted
D80926-L-S1	ISI / Topcon DS-130, DS-150, SS, IC and WB/CL series, SEM		LaB6 90/16 non-shunted
D80926-S1			CeBix 90/16 non-shunted
D80926-L-S2	DS-701, SM-501, ABT series, SX,-40, CCCD		LaB6 90/16
D80926-S2			CeBix 90/16
D80926-L-T	002B TEM		LaB6 60/5
D80926-T			CeBix 60/5
D80926-L-T1			LaB6 90/16
D80927-L	JEOL SEM, TEM, JAMP30, JXA 8600		LaB6 90/16
D80927			CeBix 90/16
D80927-L-HT	High Resolution TEM		LaB6 60/5
D80927-HT			CeBix 60/5
D80928-L	Leica / Leo (Cambridge SMVM base) 200, 400 Series SEM		LaB6 90/16
D80928			CeBix 90/16
D80928-L-S	360 SEM		LaB6 90/16
D80929-L-P	Leica Lithography (Philips base) EBPG, EBL, VB series		LaB6 90/16
D80929-P			CeBix 90/16
D80929-L-CM	(Cambridge MVM base) EBMF series 1-10.5, EBML		LaB6 90/20 non-shunted
D80929-CM			CeBix 90/20 non-shunted
D80929-L-20	(Cambridge SMVM base) EBMF constant voltage		LaB6 90/20
D80929-20			CeBix 90/20
D80929-L-20S			LaB6 60/40
D80929-20S			CeBix 60/40
D80930-L	Omicron LEED		LaB6 90/100
D80930			CeBix 90/100
D80931-L	Philips/FEI SEM, TEM, EBPG		LaB6 90/16
D80931			CeBix 90/16
D80933-L	Zeiss Leo SEM, TEM		LaB6 90/16
D80933			CeBix 90/16