Emission stability in SEM thermionic electron sources: CeB6, LaB6 and W filaments

By Marijke Scotuzzi - Feb 1, 2018

Typically, desktop scanning electron microscopes (SEM) make use of thermionic sources, from which electrons are emitted when warming up the SEM filament. Although the working principles are the same, different thermionic sources show a different performance. Phenom SEMs are equipped with a CeB6 source because of its higher brightness and longer lifetime. A parameter that plays a crucial role is the emission current stability. How is the CeB6 source performing in terms of stability? What are the engineering smarts that enable the Phenom source to maximize a CeB6 source's potential? This blog answers these questions.

Working principle of thermionic sources in scanning electron microscopes

In an earlier blog on electron sourceswe discussed the performance of CeB6 and tungsten sources extensively. CeB6 and tungsten are both thermionic sources with a filament called a cathode, from which electrons are emitted. The emission starts when the electrons are provided with enough energy to cross the potential barrier, given by the work function of the cathode material, which can be either tungsten or CeB6.

The energy is provided by heating up the cathode, which in turn is done by letting current flow through it. A Wehnelt electrode that is negatively charged with respect to the cathode pushes the unwanted electrons back into the filament, effectively determining the size of the emitting area.

Below the cathode and the Wehnelt electrode, an anode provides a strong electric field, or a strong lens that makes the electron beam converge into a crossover between the Wehnelt and the anode. Figure 1 shows the schematics of the CeB6 source, consisting of a filament, a Wehnelt electrode and an anode. The filament is at high potential, as well as the Wehnelt, whereas the anode is grounded. The circuitry positioned in between the filament and the anode measures the emission current.

schematics-thermionic-source-ceb6-filament.jpg
Fig.1: Schematics of a thermionic source, consisting of a CeB6crystal (the filament), a Wehnelt electrode and the anode. In red, the trajectories of the electrons that are pushed back in the filament thanks to the Wehnelt voltage and the trajectories of the emitted electrons, forming the primary beam.

Comparison between thermionic electron sources: CeB6, LaB6 and W

CeB6 is not the only cathode for thermionic sources, LaB6 and tungsten are also used.

Tungsten cathodes are hair-pin filaments that are bent to reduce the size of the emitting surface. They are typically warmed up to a temperature of 2500-3000 K to achieve high current density, being the work function of tungsten 4.5 eV. At 2800 K, a practical value of current density is 3 A/cm2.

The lifetime of tungsten cathodes, which can vary between 40 and 200 hours, is limited by the evaporation of the cathode material, resulting in the wire breaking when it becomes too thin. To prevent too much oxidation, a vacuum of 10-3 Pa is kept at the source.

Hexaboride crystals (CeB6 and LaB6) cathodes are rods with a flat tip, and are typically heated up to 1400-2000 K, as the work function is lower than the tungsten (2.7 eV for LaB6 and 2.5 eV for CeB6). A low work function and low temperatures yield a higher current density than tungsten cathodes, in the range of 20-50 A/cm2.

Typically, hexaboride cathodes are 10 times brighter than tungsten cathodes, meaning they provide higher beam current in a smaller spot size at the sample. Also, the lifetime of hexaboride cathodes is higher, typically 10 times that of tungsten cathodes.

However, hexaboride cathodes need a vacuum of better than 10-4 Pa to prevent oxidation. The performance of hexaboride cathodes strongly depends on vacuum and temperature. Studies suggest that CeB6 cathodes are less likely to be affected by carbon contamination than LaB6 cathodes. Also, CeB6 cathodes have a lower evaporation rate at a working temperature of 1800 K compared to LaB6. Therefore, the shape of a CeB6 cathode tip lasts longer.

The following table summarizes the physical properties of the three thermionic sources:

  CeB6 LaB6 Tungsten
Work function [eV] 2.6 2.7 4.5
Temperature [K] 1800 1800 2800
Pressure [Pa] 10-4 10-4 10-3
Current density [A/cm2] 20-50 20-50 3 (@ 2800 K)
Lifetime [hours] 1500+ 1000+ 100


Emission current stability in the Phenom CeB6 electron source

The stability of the emission current is a key requirement for thermionic sources. During the operation of the microscope, the emission current is kept stable by adjusting the Wehnelt voltage in a constant control loop. The emission current is measured in the source, by a circuitry between the filament and the anode, as shown in Fig. 1. The Wehnelt voltage is then adjusted according to the read out of the emission.

It is of utmost importance that the current at the sample is kept constant, for given settings. An automated function measures the sample current as a function of the emission current. The emission current is adjusted by varying the voltage on the Wehnelt, thereby regulating the amount of electrons pushed back into the filament, for a constant filament temperature. The current at the sample can be measured indirectly from the signal of an image taken with the BSD detector on a reference material.

The optimal case, shown in blue in Figure 2, is when the current at the sample has a maximum at 62 µA. If the peak is before or after this value, it means that the temperature of the filament is either too low or too high and needs to be adjusted. The automated function sets the new temperature of the filament and measures the current at the sample for different emission currents again, until the peak falls at the ideal emission current of 62 µA.

current-sample-function-emission-current-filament-temperatures.jpg


Fig.2: Current at the sample (I spot) as function of the emission current for different filament temperatures.


Once the temperature is adjusted through the automated function, the voltage on the Wehnelt is set for an emission current of 40 µA, thus on the left of the peak of the blue curve. An emission of 40 µA is chosen as the optimum between resolution and beam current, which translates into image quality.

The current stability of thermionic sources is typically better than 1% RMS. Measurements on sample current stability of a CeB6 source in a Phenom microscope show that the fluctuation of the beam current is about 0.3 % in the first 5 hours from switching on the source and is 0.2 % from 5 hours up to 15 hours after switching on the source, as represented in Figure. 3. The drop in the first hour is measured to be approximately 10% and is caused by the stabilization of the temperature in the source unit.

Moreover, the vacuum stability at the source does not affect the emission current of the CeB6 source in a relevant way.

representation-measured-current-sample-15-hours.jpg

Fig. 3: Representation of the measured current at the sample over a period of 15 hours.
 

There’s much more to discover in Phenom desktop SEMs

So, the CeB6 source drives the highly reliable and durable character of a Phenom desktop SEM, but it’s hardly the sole contributor. If you take a closer look at Phenom scanning electron microscopes, you will realize that they have a multitude of interesting specifications worth investigating further, like their light and electron optical magnification, resolution, and digital zoom.

If you want to gain more insights into how Phenom SEMS are engineered and compare on specifications, I recommend looking into this desktop SEM comparison sheet.

Download your free copy below to discover how Phenom SEMs can vastly improve and speed up your analysis process:

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References

  • Introduction to charged particle optics, P. Kruit, Delft University of Technology

  • Scanning electron microscopy, Physics of image formation and microanalysis, L. Reimer, Springer

  • Cathodes for Electron Microscopes, CeBix and LaB6 Filaments Standard Tungsten Loop Filaments, Electron Microscopy Sciences


About the author

Marijke Scotuzzi is an Application Engineer at Thermo Fisher Scientific, the world leader in serving science. Marijke has a keen interest in microscopy and is driven by the performance and the versatility of the Phenom desktop SEM. She is dedicated to developing new applications and to improving the system capabilities, with the main focus on imaging techniques.

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