Lanthanum and Cerium Hexaboride (LaB6 and CeB6) Cathodes

Building the World's Best Cathodes

Lanthanum hexaboride (LaB₆) and cerium hexaboride (CeB₆) cathodes are ideal for many small spot size applications such as SEM, TEM, surface analysis and metrology, and for high current applications such as microwave tubes, lithography, electron‐beam welders, X‐ray sources and free electron lasers. Applied Physics Technologies has decades of experience in research, development, and manufacturing of LaB₆ and CeB₆ cathodes.

We can provide the cathodes you need for replacement, OEM, and custom applications. 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 minimise 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 optimise 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, CeB₆should 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 minimise excessive boron evaporation to achieve the optimum ratio of metal to boron atoms in the grown crystal.

Crystal Orientation

Crystal orientation can be selected to match the cathode design or application. For electron microscopy, the orientation is most desirable due to its brightness and crystal plane symmetry about the optical axis. As the cathode ages, the plane symmetry ensures an even evaporation rate relative to the axis, maintaining a centered, flat emitting surface (See figure). Also, the emission patterns from the symmetric crystal planes will remain consistent as they become more exposed by evaporation, contributing to a brighter beam spot.

The "tophat" design cathode provides a large‐area crystal face electron source for maximum total current.

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, aoriented crystal in a "top hat" configuration may be preferred, providing a slightly lower work function and large emitting surface. We excel at developing specialised cathodes for custom applications and research purposes. Contact us for your custom cathode needs.

The cathode's mount design has a significant impact on performance.

The design must be simple, durable and precise. It must resist any movement of the crystal, despite the high operating temperatures, yet be easy to install and align. We feel we employ the best mount design in the industry, the Mini Vogel Mount.

In 1988, FEI of Hillsboro, Oregon introduced the Mini Vogel Mount (MVM) to provide the benefits of the original Vogel mount in a smaller, simpler, and more elegant design. Twin posts are rigidly fixed in a thick ceramic base, and bent towards the centre in an inverted "V". The posts are made of a molybdenum‐rhenium alloy that maintains a high modulus of elasticity even at high temperatures. The posts are spread slightly during assembly to allow placement of small pyrolytic graphite blocks between the crystal and posts. The blocks act as resistive heaters, and help thermally isolate the hot crystal from the highly conductive posts. When the compressive force of the posts is released, the crystal is held with strength and precision. The clamping force of the posts will remain near 5,000 psi for the life of the cathode.

The structure of the MVM is amazingly robust, sustaining reasonable impact without deviating from structural specifications. Because the graphite pads shield evaporation of the crystal in the direction of the clamping force, the emitter crystal can be fully utilised without degradation of the mount. Structural failure of the MVM is not a concern when the cathode is operated within the correct temperature and pressure range. Typically, the beam stability of the Mini Vogel Mount cathode exceeds the specifications of the system in which it runs.

Kimball Physics Lanthanum Hexaboride (LaB₆) Cathodes

Featuring

• Extended Life ‐ Thousands of Hours in Clean Vacuum. Guaranteed Life (Measured in Surface Loss). Guaranteed Against Mounting Structure Failure.
• Exceptional Stability ‐Thermal/Chemical/Electrical. Precision Machined Carbon Mounting. Highover‐Temperature Tolerance.
• High Brightness/Low Energy Spread ‐ Oriented Single Crystal. Best‐Quality/High‐Purity Material.
• Accurate Microflats ‐Superior Optics/Controlled Source Size Standard Diameter Available.

For Use in

• Scanning Electron Microscopes
• Transmission Electron Microscopes
• Electronlithography Systems
• X‐Ray Sources

The new improved Kimball Physics Model ES‐423E (Extended Life) LaB₆ Cathode is a high performance, resistively heated, thermionic electron source. The improvement stability of the LaB6 crystal, results in less exposure of LaB₆, which reduces Wehnelt aperture contamination.

Its lifetime is in excess of 6 months with continuous operation. Continuous operation at the full operating temperature improves the thermal stability of the gun and hence beam current stability. It is no longer necessary to wait hours for stable beam conditions in order to perform quantitative EEL or EDX measurements.

Specifications

The emitter is machined to 15 /‐2µm diameter (standard), microflat alignment to the instrument base can be provided (on request), oriented‐single‐crystal, surface.

Mounted on the end of a single‐piece, stress‐free, carbon heater rod, held in place by a carbon ferrule. A high degree of axial symmetry provides great mechanical stability. The rod has a 100µm slot along the axis, which allows the heating current to go up one side and down the other.

In SEM type instruments, its lifetime is up to 3000 ‐ 4000 hours In TEM instruments the lifetime can be even longer. Mounting structure will last more than 10,000 hours. Chemical reactivity and mechanical drift problems have been eliminated.

Instrument Conditions

While the cathodes can be fitted to virtually any instrument, there are several requirements needed for achieving quality results.

The most important requirement is for clean vacuum, with partial pressures of oxidising gases being kept below 10^‐7 torr in the electron gun. The condition of the cathode itself, along with that of the Wehnelt aperture, may often be used to verify the partial pressures in the gun.

The cathode must be accurately and correctly positioned behind the Wehnelt aperture. There must be controllable bias resistors which can reach adequately high values; some instruments are limited by values which are to low.