Most desktop scanning electron microscopy systems (SEMs) and SEM systems, in general, utilize a standard tungsten filament that’s readily available in many microscopy consumable suppliers. While it’s adequate for many applications and has been the industry workhorse for a while, there are situations wherein a CeB6 electron source is the better choice.
Before proceeding, we need to note why Tungsten is the go-to choice as an electron source. Of all metals in pure form, Tungsten has the:
- Lowest vapor pressure
- Highest melting point
- Lowest thermal expansion and,
- Very high tensile strength
Although it has all these properties, Tungsten has fundamental disadvantages compared to a Cerium Hexaboride (CeB6) electron source in terms of the following:
When it comes to brightness, Tungsten supplies 106 A/cm2 sr. The lower work mechanism of a CeB6 filament yields higher beam currents at lower cathode temperatures, resulting in enhanced brightness at all acceleration voltages.
Tungsten gradually evaporates over time since it operates at white-hot temperatures. Here, the CeB6 source has the advantage since you can predict the end of its service life as it slowly degrades.
Overall, the average lifespan of Tungsten is about 100 hours depending on the vacuum, while a CeB6 source lasts 15 times longer, at approximately 1500+ hours.
Tungsten – elliptical in shape, with dimensions ranging from 50µm to 100µm
CeB6 – <25µm in dimension
These figures show that Tungsten requires a significant electron optic demagnification to achieve a small electron probe required by an SEM to have a high-quality resolution.
To recap, the advantages of a CeB6 source are:
- a more accurately predictable service life-ending
- higher brightness, and
- a steady output of high-resolution images
Its only disadvantage is the upfront cost. CeB6 filaments cost around $900 compared to $50-100 per Tungsten filament. However, note that the former lasts 15 times longer, making it less expensive in the long run. For these reasons, a CeB6 electron source is the more sensible investment between the two.