Field Emission Scanning Electron Microscope

FESEM is the abbreviation of Field Emission Scanning Electron Microscope. With the advancement of technology used in Scanning Electron Microscopy (SEM), a type of electron microscope, higher resolution images bacame available with the advent of Field Emission Microscopy in 1936 by Erwin Muller, resulting in the emergence of a new technique named as Field Emission Scanning Electron Microscopy (FE-SEM). It works with electrons instead of light. The electrons are liberated by a field emission source. The object is scanned by electrons according to a zig-zag pattern. It is used to visualize very small topographic details on the surface or entire fractioned objects. Researchers in biology, chemistry and physics apply this technique to observe structures that may be as small as 1 nanometer.

Working principle:

The method of operation of these microscopes is similar to that of conventional scanning electron microscope (SEM). An electron beam focused by electromagnetic lenses scans the surface of specimen, where the reflected/interacted electrons create an image of the sample surface and topography.

Difference between SEM and FE-SEM

In general FESEM follows the same principle as the one in SEM, while the biggest difference between SEM and FESEM is the electron generation system.

FESEMs use a field emission gun (FEG) as electron source. In FEGs a potential gradient is applied to emit the electron beam, while in SEM thermionic emission is used.

How does field emission work in field emissions scanning electron microscopy FESEM

Field emission in FE-SEM is performed by FEGs through applying low voltages on an electron source, usually a single tungsten filament with a pointed sharp tip(as shown in figure), which concentrates low-energy and high-energy electrons at a low electrical potential (about 0.02 to 5 kV) and increased spatial resolution.

This method prevents contaminating the sample surface since does not require thermal energy to overcome the surface potential.

Sample preparation

As sample preparation in SEM, surface of the FE-SEM samples should be conductive to reduce charging effects of the samples and improve image quality. In the case of insulating specimen, coating its surface with a thin film of conductive material with a minimum thickness (0.5 to 3 nm),  containing fine grain size smaller than the probe diameter, improves image contrast in low-density materials without affecting the sample appearance.

FESEM versus SEM benefits and limitations

Thermionic emission of the electrons result in the substrate contamination, not occurring in field emission electron sources

In the SEM method, a resolution of 3-7 nm is achievable, while in the FE-SEM the resolution is 1.5 nm or better.

The conductive layer coated on a sample to be subsequently imaged by FE-SEM must be very thin and uniform and have finar grain size compared to what is required for SEM imaging.

Better contrast in low density materials imaging through FE SEM technique is possible.

In FESEM, the electron source requires higher vacuum environment (higher than 10-⁹ torr) during operation to ensure electron stability and prevent contamination of the cathode.

FE-SEM electron sources suffer low beam current stability.

FESEM Applications:

①Measurement of sample microscopic features.

② Study of surface morphology.

③ Characterization of coatings.

④ Integrated circuit evaluation.

⑤Fine structure analysis.

⑥Study of microstructures.

⑦Fracture and structural defects analysis.

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