The electron source is one of the most critical components of a scanning electron microscope (SEM) because it determines the quality, brightness, and size of the electron beam used for imaging. The characteristics of the electron beam will influence the overall imaging capabilities of the SEM. Two common types of electron sources used in SEMs are thermionic and field emission guns. Each relies on different mechanisms to produce the initial stream of electrons that is accelerated and focused onto the sample surfacing using electromagnetic lenses.
Choosing the right electron source for your intended application is often the first step when evaluating SEM instrumentation. It is important to have a solid understanding of how each type of electron source works and how it affects a microscope’s capabilities. This blog aims to provide a brief overview of the key differences between thermionic and field emission electron sources to help you select your ideal SEM system.
Thermionic vs Field Emission
SEM images of the thermionic electron source of the Phenom XL (left) and the field-emission gun of the Phenom Pharos (right) showing the tip material from where electrons are emitted.
Thermionic emission refers to the process in which electrons are emitted from the surface of a material when it is heated. Thermionic electron guns contain a tungsten wire filament or a solid-state hexaboride crystal such as CeB6 or LaB6 that is fabricated into a sharpened tip. Electrons are emitted by heating the source material until its electrons gain enough energy to overcome the work function and escape into the column.
Field emission of electrons occurs when electrons are emitted from a material’s surface due to the presence of a strong electric field. Field emission guns utilize an extremely sharp metal tip made of single-crystal tungsten. An electric field is applied by an anode and the field is concentrated at the tip’s surface causing tunneling and emission of electrons.
SEM Features: Thermionic vs Field Emission Electron Sources
1. Resolution
In terms of the resolution capability of the electron sources, the field emission gun is superior. A field emission gun has a smaller tip emitting diameter and the electrons emitted possess a narrower energy spread, which both improves its ability to be focused into a small probe and map out fine structures in the specimen surface.
Phenom Pharos – 200,000xPhenom XL – 200,000xThe above images were captured from the same alumina thin film sample using SEMs equipped with different types of electron sources. The improved resolution demonstrated by the Phenom Pharos (left) compared to the Phenom XL (right) can be attributed to the Pharos’ field emission gun in comparison to the XL’s CeB6-based thermionic electron source.
For routine SEM analysis, such as quality control inspections that are performed in manufacturing environments, an SEM with a thermionic emitter can provide adequate imaging capabilities at a lower cost. On the other hand, for advanced applications that demand nanostructural analyses or low-kV imaging, opting for an SEM with a high-brightness field emission gun is the best option.
2. Low-voltage Imaging
Because field emission sources are brighter, they can efficiently emit electrons even at low accelerating voltages. This gives FE-SEMs the unique ability to support low-kV imaging, which generally refers to acquiring SEM images at an accelerating voltage of less than 5 kV. Low voltage imaging provides distinct benefits, namely increased surface sensitivity, reduced beam damage, and improved imaging quality of non-conductive samples. These can be particularly advantageous when imaging biological or insulating samples or when analyzing nanomaterials and surface coatings.
Phenom Pharos – 4 kVPhenom XL – 5 kVThe above images were captured from the same PTFE sample at low kV. The improved image quality (improved resolution, less damage) demonstrated by the Phenom Pharos (left) compared to the Phenom XL (right) can be attributed to the Pharos’ field emission gun in comparison to the XL’s CeB6-based thermionic electron source.
3. STEM-in-SEM
Another advantage of having an SEM with a field emission source is the ability to conduct STEM (scanning transmission electron microscopy) imaging. So called STEM-in-SEM mode involves acquiring high-resolution images of a sample’s interior structure by analyzing the transmitted electrons that are emitted beneath the sample. The technique is suitable for imaging electron-transparent samples (typically < 100 nm in thickness) such as tissue sections or nanoparticles.
BF-STEM image of multi-walled carbon nanotubes at 150,000x acquired on a Phenom Pharos Desktop SEM operated in STEM-in-SEM mode.
Some of the key benefits of STEM-in-SEM is improved resolution (< 1 nm can be achieved using the Phenom Pharos in STEM-in-SEM mode) and the ability to distinguish between low- and high-angle electron scattering, which can be leveraged to create bright field (BF) and dark field (DF) images.
4. Cost
One major benefit of thermionic emitters compared to field emission guns is the cost. SEMs equipped with field emission guns require higher vacuum levels and more complex design, leading to a higher purchase price. Overall, the cost of ownership for a field emission SEM will be higher compared to an SEM with a thermionic emission source. While FE-SEMs come at a higher price point, their superior resolution and performance can provide immense value for demanding applications.
Key Take-aways
The choice between selecting an SEM with a thermionic and field emission source depends primarily on the specific requirements of the intended SEM application(s). Some key differences to keep in mind that may impact this decision are resolution capability, the option to do low-kV imaging and STEM-in-SEM, and the overall cost. In general, high-end SEMs often will have a field emission source for their superior resolution but they do offer a wider range of flexibility in terms of the types of samples that can be analyzed while thermionic sources offer a lower cost of ownership while still being sufficient for most routine applications.
