FEG source: what you need to know

By Karl Kersten - Nov 26, 2018

Until very recently, we have not seen a high kilovolt(kV) imaging desktop scanning electron microscope (SEM) with a Field Emission Gun (FEG) source in it. Why not? And why can it be useful to have a FEG source in a desktop SEM? This article provides some answers.

Why FEG in desktop SEM

Desktop SEM is a relatively young but strongly established product category. Many applications do not require SEM systems with ultra-high resolutions below 1nm, and 10nm is often more than sufficient. However, due to shrinking features and increasing requirements, desktop SEM users would sometimes like to have a system with better resolution, but with the same speed and ease of use of a desktop SEM.

SEM system design

Many challenges arise when designing a new SEM platform. Normally specifications and requirements for a new platform are set higher in order to obtain better performance from the system. Another challenge, especially for desktop SEM, is the requirement that a new platform should not affect the ease of use. A person in a lab must be able to work with a desktop SEM without having SEM experience. So what are the challenges when the requirement has been set to step up from a CeB6 source column design to an FEG source column design? To answer this, we will first take a small step back to explain the differences between SEM sources.

The SEM source

One of the main components in a SEM microscope is the electron source. As explained in previous blogs, we can distinguish three different types of sources; Tungsten (W), Cerium Hexaboride (CeB6) and the Field Emission Gun (FEG). Based solely on properties like brightness and filament size, a FEG is the most superb source for generating high-resolution images.

FEG in a desktop SEM

So why, until August 2018, have we not seen a high kV FEG source in a Phenom desktop SEM? As mentioned before, the answer lies mainly in the mechanical design challenges:

  • Operating a FEG source requires a high vacuum system. Typically a FEG source operates at 10-9 or 10-10 mBar vacuum level. A stronger pumping system is unfortunately not enough to reach such a high vacuum. Water molecules tend to stick to the side walls of the electron column and cannot be pumped away. The only way to remove these water molecules is to start heating the column. For a desktop SEM this is a very complex process due to the small column size as you do not want to overheat your lenses or detectors.

  • Improved system stability is needed to accommodate higher magnification and better resolution. This increases the need for stable electronics and mechanics.

The electronics for controlling a FEG source are different from a Tungsten or CeB6 source, but the mechanical sensitivity of the system also requires a very robust design.

The Thermo Scientific™ Phenom Pharos overcomes all these design challenges and was launched in August 2018 to much acclaim. If you like to learn more about it, you can view our customer videos online and download the spec sheet here to see its remarkable capabilities.

Download the Phenom Pharos SEM specification sheet


About the author

Karl Kersten is head of the Application team at Thermo Fisher Scientific, the world leader in serving science. He is passionate about the Thermo Fisher Scientific product and likes converting customer requirements into product or feature specifications so customers can achieve their goals.

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