FEG vs. Tungsten source in a scanning electron microscope (SEM): what’s the difference?

By Kay Mam - March 28, 2019

After few years of operating a transmission electron microscope (TEM) in my postgraduate studies, in 2006 I started my career in electron microscopy as an SEM operator for a biological and medical research center in York (United Kingdom). Not knowing how to operate an SEM before, I found it relatively easy to switch from TEM to SEM.

My first SEM—equipped with a tungsten source—was getting too old and difficult to maintain. For that reason, it was replaced by two brand new SEMs; the first equipped with a tungsten source and the second with a Field Emission Gun (FEG). The tungsten system was considered the ‘workhorse’, and was used by many co-workers and researchers.

However, challenging specimens (such as nanoparticles and beam-sensitive specimens)could be difficult to image on a tungsten system due to the lack of resolution. Whereas with the FEG source, those difficult specimens were much easier to image. The FEG system allowed us to see things that we couldn’t resolve with a tungsten system. It was like exploring and discovering a completely new world. Ever since that day, I’ve been in love with the FEG source.

In this blog, I would like to make you enthusiastic too and explain why I prefer using an FEG source in an SEM system. You can learn what the main differences are between a tungsten thermionic emitter and a field emission source, and find out how an FEG source could enhance your research.

Curious how to select the right microscope for your processes and applications?
Our How to choose a SEM guide assists you in selecting an SEM that enables better an faster analysis

 

Thermionic emission sources vs. field emission sources

  • Thermionic emission sources (TEM)
    Typically, thermionic filaments are made of tungsten (W) in the form of a v-shaped wire. They are resistively heated to release electrons (hence the term thermionic) as they overcome the minimum energy needed to escape the material.

  • Field emission sources
    For a field emission source, a fine, sharp, single crystal tungsten tip is employed. An FEG emitter gives a more coherent beam and its brightness is much higher than the tungsten filament. Electrons are emitted from a smaller area of the FEG source, giving a source size of a few nanometers, compared to around 50 μm for the tungsten filament. This leads to greatly improved image quality with the FEG source. In addition, the lifetime of an FEG source is considerably longer than for a tungsten filament (roughly 10,000 hours vs 100-500 hours), although a better vacuum is required for the FEG, 10-8 Pa (10-10 torr), compared with 10-3 Pa (10-5 torr) for tungsten, as shown in Figure 1.

There are two types of FEG sources: Cold and Schottky FEGs

For a so-called cold emission source, heating of the filament is not required as it operates at room temperature. However, this type of filament is prone to contamination and requires more stringent vacuum conditions (10-8 Pa, 10-10 torr). Regular and rapid heating (‘flashing’) is required in order to remove contamination. The spread of electron energies is very small for a cold field emitter (0.3 eV) and the source size is around 5 nm.

Other field emission sources, known as thermal and Schottky sources, operate with lower field strengths. The Schottky source is also heated and dispenses zirconium dioxide onto the tungsten tip to further lower its work function. The Schottky source is slightly larger, 20–30 nm, with a small energy spread (about 1 eV).

 General properties of electron sourcesFigure 1. General properties of electron sources

 

It starts with sample preparation

When switching from tungsten to FEG emitter, it is worth mentioning that the specimen preparation becomes extremely critical in order to obtain high resolution and high magnification of any specimen.

In general, samples are generally mounted rigidly on a specimen holder or stub using a carbon ‘conductive’ adhesive. These carbon tabs are partially or non-conductive and can lead to charging artefacts. Hence, carbon tabs might be suitable for a tungsten system, but become inappropriate for an FEG system.

For high-resolution imaging on an FEG system, I always try to avoid using the carbon sticker. Specimens such as nanoparticles or fine powder should be prepared directly onto an aluminum pin stub for example.

For conventional imaging in the SEM, specimens must be electrically conductive, at least at the surface, and electrically grounded to prevent the accumulation of electrostatic charge (i.e. using silver paint, aluminum or copper tape). [copper tape?]

Non-conducting materials are usually coated with an ultra-thin coating of electrically conducting material, including gold, gold/palladium alloy, platinum, platinum/palladium, iridium, tungsten, and chromium. I recommend using the metals and thickness below for tungsten and FEG sources: 

Metals:

  • Au, Au/Pd (Tungsten source)
  • Pt, Pd/Pt, Ir, W (FEG source)

Thickness:

  • 5-10 nm for low magnification
  • 2-3 nm for high resolution, the thinner the better

Tungsten source vs. FEG source: imaging differences

FEG sources have an electron beam that is smaller in diameter, more coherent and with up to three orders of magnitude greater current density or brightness than could ever be achieved with a tungsten source.

The result of using an FEG source in scanning electron microscopy (SEM) is a significantly improved signal-to-noise ratio and spatial resolution, compared with thermionic devices.

Field emission sources are ideal for high resolution and low-voltage imaging in SEM. Therefore, focusing and working at higher magnification become easy for any operator.

Phenom Pharos Desktop SEM

In 2016, I joined the Phenom team that was working on building a desktop SEM with an FEG source. I had never expected that it was possible to build. The first time I tested the system, I was blown away by the image quality, resolution, ease of use and overall performance.

I fell deeply in love with the FEG source all over again—but this time on a desktop SEM.

If you would like to switch from a tungsten to FEG source, you might consider checking out the outstanding performance of the Phenom Pharos Desktop SEM . You can get a desktop FEG for the price of a tungsten floor model.  Learn more about the faster, higher-resolution desktop SEM in the specification sheet: Download the Phenom Pharos specification sheet

 


Topics: FEG

About the author

Kay Mam is an application engineer for the Thermo Scientific Phenom Desktop SEM product range at Thermo Fisher Scientific. In 2006 Kay started his career in electron microscopy as an SEM operator for a biological and medical research center in York (United Kingdom). With an FEG source, difficult specimens are easier to image. The FEG system allowed him to see new things, it was like exploring and discovering a completely new world. Ever since that day, Kay has been in love with the FEG source. In 2016, Kay joined the Phenom Desktop SEM Application Team, working on a desktop SEM with an FEG source.

Press Room | Privacy Policy | Terms of Use | Sitemap |