Desktop SEM electron sources: why CeB6 is the right choice

By Karl Kersten - April 4, 2019

If you’re looking for a scanning electron microscope (SEM), you probably know by now that the electron source is one of the most important parts of the system. In a previous blog, we talked about the properties of three different electron sources: the Tungsten, CeB6 and FEG sourcesIn this blog, we’ll take a closer look at Tungsten and CeB6 electron sources.

Why Tungsten and CeB6 electron sources? 

Based solely on properties, a Field Emmission Gun (FEG) source is the most superb source for generating high-resolution images. But if we focus on what is important in the daily use of an SEM — generating high-quality images at a low price per image — a FEG source isn’t quite as attractive anymore, because it requires a vacuum design that often comes at a high price. So it’s important to know that a source's cost-efficiency is relative to its lifetime and maintenance of the SEM. That’s why you often see Tungsten and CeB6 sources appearing in the spec sheets of (desktop) SEM suppliers that offer compact SEM systems. 

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Figure 1: Tungsten vs. CeB6 electron source

An electron source is a consumable

Ideally, you would like to have an electron source emitting an electron beam throughout the lifetime of the SEM, which could be 10 years or even more. Unfortunately, an electron source is a consumable and will inevitably “wear out”. The source lifetime depends on two main factors: 1) the vacuum level in which the source operates and 2) the source temperature.

A poor vacuum with an overheated source will result in a very short lifetime. A very good vacuum with a source that’s too cold will lengthen the source lifetime, but will also generate images of lower quality. It is therefore important that the source is operated at the optimal conditions.

SEM vacuum systems: good to know

The design of the vacuum system of an SEM will vary. Tungsten systems often have a source vacuum that is affected by the operational state of the system. For example, every time the sample is loaded or unloaded, air will enter the column, affecting the lifetime of the source.

The source vacuum can also be affected due to the low sample vacuum when imaging non-conductive samples in their original state. A CeB6 system, like the Phenom, has a differential pumping system with pump apertures that preserve the high vacuum at the source. This means that loading and imaging samples at low vacuum does not affect source lifetime.

wk34_thermionic-emission.pngFigure 2: cross-section view of an electron column with a schematic view of the source assembly

SEM electron source: end of life

How do you tell when a source is at the end of its life? For a Tungsten source, you simply don’t see it coming. This means that the source can break during imaging or analysis — which is highly inconvenient. Debris from the source could also contaminate other parts of the electron column. This means that in addition to replacing the source, you may also need to clean, or even replace, other column parts.

Of course, you can replace the source before end of life, but then it is not being fully utilized and therefore needs replacing more often. If you have a large group of SEM users, you may not always have the chance to replace the source in time - unless the whole group is trained in replacing sources. But that can also introduce alignment mistakes after the source has been replaced.

A CeB6 source at its end of life will have a slowly degrading emission. This means that you see it coming:   a CeB6 source will not suddenly break, and so a replacement can be scheduled in time. Signs of an aging CeB6 source can be see when a user needs to increase contrast and brightness to a higher level than usual, or when a user needs to apply a higher spot size to get the “usual” image quality.

Electron source: lifetime

The average lifetime of a Tungsten source depends on its usage, but is about 150 hours. There are a few tricks to slightly extend the source lifetime. The first is to wait an extra five minutes before unloading a sample that you’re finished with. This will allow the source to cool down a little and as a result, less oxidation takes place. Of course, this trick decreases the sample throughput and causes annoying waiting times.

Another option is to under-saturate the source, which means that you run it at a lower emission. This will increase the lifetime, but decrease the imaging performance, especially at low kV’s.

The average source lifetime of a CeB6 source is about 2000 hours. The performance remains very good during its lifetime. Even when loading and unloading many samples, or using it with low sample vacuum.

Electron source: maintenance

A Tungsten source can be replaced by a user. This is a pre-condition that is necessary because the source sometimes needs to be replaced every week. Removing, mounting and aligning a source is easy. It just takes time. The time needed to replace a source depends on the status of the old source. If the old source did not break, then you will probably have the system up and running again within 20 minutes. If it did break, then more cleaning will be required to guarantee the same performance.

Replacing a CeB6 source takes more time, mainly because the source operates at a high vacuum. Therefore, the initial pumping time takes longer. The total time required to have the system back up and running is about one day, and that includes overnight pumping.

A CeB6 source can be replaced by a user too. But it could be a year or even longer before the source has to be replaced. So it won’t be a “routine” job like a Tungsten source replacement. A better way is to schedule replacement and have it done by a trained service engineer. This means that a user does not have to worry about keeping stock parts, which is especially useful when having multiple users. It’s like changing the oil in your car: you can do it yourself, but you would prefer to have a car that only needs the oil changing when it is in the garage for a long term regular checkup.

SEM electron source: recommendation

By considering all of the above pros and cons, we can conclude that the price per picture for a Tungsten and a CeB6 source is probably about the same. However, worry-free usage and operation — and the quality per picture — are clearly in favor of the CeB6 source.

If you would like to know more about the power of a desktop SEM system with a CeB6 electron source, you can download our desktop SEM comparison sheet. It contains information about all the Phenom SEMs with a CeB6 source, and helps you select the type of desktop SEM system that best fits your needs.

Download the free comparison sheet here:

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About the author

Karl Kersten is head of the Thermo Scientific Phenom Desktop SEM Application Team at Thermo Fisher Scientific. He is passionate about the Phenom Desktop SEM product and likes converting customer requirements into product or feature specifications so customers can achieve their goals.

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