Considering a desktop scanning electron microscope? If so, then it is important to determine what type of electron source fits your needs, since it has a direct effect on the quality of your output.
The electron source — or cathode, filament or electron gun — is one of the most important modules of a desktop SEM. Its purpose is to provide a stable beam of electrons. There are two groups of electron sources used in SEM, varying in the amount of current they produce into a small beam size (spot), the stability of the beam, and the lifetime of the source.
In this blog, we inform you about a type of electron source that is being used in desktop SEM: the thermionic electron source. More specifically, we focus on (the differences of) two types thermionic electron sources: Tungsten and Cerium Hexaboride (CeB6).
What is a thermionic electron source?
When any solid material is heated, electrons will be emitted by thermionic emission. The emission becomes significant when the thermal energy of the electrons is sufficient to exceed the work function of the material. The cathode is made from a high melting point material with a relatively low work function in order to emit many electrons.
Cross-section view of an electron column with a schematic view of the source assembly
The electron beam that is projected onto your sample is created by the emitted electrons being accelerated from the high negative potential of the source to ground potential at the anode inside the electron column. This process can of course only happen inside the vacuum of an electron column and by using lenses to control the beam.
Before we analyze and compare the Tungsten and CeB6 source, it is useful to know which properties of an electron source are key when determining its performance. We focus on the most important properties:
Brightness is defined as the beam current per unit area per solid angle. The more current/electrons you have available in a small spot size, the better you can achieve high resolution (quality) images at high magnification.
The brightness increases linearly with the acceleration voltage. For example: every electron source is ten times as bright at 10kV as it is at 1kV.
The spot size of the electron beam can be made smaller to improve the resolution but at some point, the limitation is the signal-to-noise-ratio necessary to get a good quality image.
2. Source size
As mentioned before, a small spot size contributes to good image resolution and therefore high- quality images. The lenses (mainly facilitated by the condenser lens) inside the electron column are responsible for demagnifying the beam diameter (or spot size). You can imagine that a smaller physical size of your source leads to less (complex) demagnification.
3. Source temperature
The source temperature is the operational temperature which overcomes the work function in order to emit electrons. The operational temperature for thermionic sources lies between 1800 and 2800 Kelvin.
3a. Electron beam energy spread
The electron beam energy spread is the spread in electron energies leaving the source. Chromatic aberration becomes the dominant aberration at low acceleration voltage when the energy spread of the source is large. Chromatic aberration is an effect that causes a less focused beam due to electrons with slightly different energy leaving the source.
Lifetime represents the lifespan of an electron source before it breaks or needs to be replaced. Preferably, you want a source that is durable and for which you can accurately predict the moment of replacement.
Comparison: Tungsten vs. CeB6 source
We can now start our Tungsten and CeB6 comparison based on the most important properties of an electron source.
Tungsten filaments are widely used in scanning electron microscopy. Of all metals in pure form, Tungsten has the highest melting point, the lowest vapor pressure, the lowest thermal expansion and a very high tensile strength which are ideal properties for making an electron source.
But as you will notice in the comparison, Tungsten has some fundamental disadvantages compared to a Cerium Hexaboride (CeB6) electron source:
When we look at brightness, the Tungsten source provides 106 A/cm² sr. The lower work function of a CeB6 filament results in higher beam currents at lower cathode temperatures than Tungsten, which means greater brightness at all acceleration voltages. To concretize this: a CeB6 cathode provides ten times the brightness compared to Tungsten: 107 A/cm² sr. This gives the CeB6 source two advantages over a Tungsten source:
More current available in the same focused spot, which means a better signal-to-noise ratio at the same spot size.
At the same signal-to-noise ratio the CeB6 spot can be made smaller, which means that a better resolution can be achieved.
Image from TiO2 powder made with CeB6 system | Image from TiO2 powder made with Tungsten system
2. Source size
The source size is of Tungsten is elliptically shaped with a dimension ranging from 50µm to 100µm, depending on the source configurations and operating conditions. Compared to a CeB6 source, which has a dimension of <25µm, it means that considerable electron optic demagnification is required for a Tungsten source to achieve a small electron probe needed for good resolution in SEM.
3. Source temperature
The operational temperature of the Tungsten filament lies around 2800 Kelvin, where the CeB6 source has an operational temperature of 1800 Kelvin. The difference in temperature has a direct effect on the source.
3a. Electron beam energy spread
The higher temperature setting of the Tungsten source causes a larger energy spread than a CeB6 source. Typically the energy spread of a Tungsten source is about 2.5eV, where the CeB6 is about 1eV, resulting in better image quality — especially at lower acceleration voltages.
A Tungsten filament operates at white-hot temperatures which mean it gradually evaporates with time. Eventually, the Tungsten wire becomes thin and breaks which always happens during imaging. The breaking of the Tungsten wire can possibly contaminate the upper part of the electron column. This is why — when replacing the Tungsten filament — it is advised to replace or clean other source related parts inside the column as well.
The advantage of a CeB6 source: you can predict its lifetime ending as it slowly degrades in time. You will know when it is time to replace your CeB6 filament and can do so between operating sessions. You will not end in up in a situation where you have to terminate your analysis because of a broken filament — and more importantly: you do not have to worry about contamination of the column due to debris. Using a CeB6 source also minimizes the need to replace other source related parts along with your source.
The lifetime comparison for Tungsten and CeB6: the average lifetime of a Tungsten source is about 100 hours, depending on the vacuum. A CeB6 source typically provides more than fifteen times the service life: 1500+ hours.
Our recommendation to you
We highly recommend a CeB6 source because of its major benefits: worry-free and time-saving operation, less maintenance — and most important: high-quality output.
Against all the pros — an accurately predictable lifetime ending, a better signal-to-noise ratio, higher brightness, and a steady output of high-resolution images — there only seems to be one con: that CeB6 is more expensive than Tungsten. But CeB6 is actually lessexpensive in the long run: it lasts longer and minimizes the need to replace other source related parts. And that makes the investment in CeB6 — and desktop SEM itself — even more justifiable.
If you want to produce high-quality output quickly and worry-free, you are probably already looking into desktop SEM. Are you wondering what the size of your investment for a desktop SEM will be? If so, then we are happy to provide you with a quote. Simply request yours here:
Karl Kersten is head of the Application team at Phenom-World, world’s leading supplier of desktop scanning electron microscopes. He is passionate about the Phenom product and likes converting customer requirements into product or feature specifications so customers can achieve their goals.