The Field Emission Scanning Electron Microscope (FESEM) is an instrument which, just like the SEM, provides a wide variety of information from the sample surface, but with higher resolution and a much greater energy range. It works just like a conventional SEM: the sample surface is scanned with an electron beam while a monitor displays the information that interests us on the basis of the detectors available.

The biggest difference between a FESEM and a SEM lies in the electron generation system.  As a source of electrons, the FESEM uses a field emission gun that provides extremely focused high and low-energy electron beams, which greatly improves spatial resolution and enables work to be carried out at very low potentials (0.02–5 kV).  This helps to minimise the charging effect on non-conductive specimens and to avoid damage to electron beam sensitive samples.

Another highly remarkable feature of FESEM is its use of in-lens detectors.  These detectors, which are optimised to work at high resolution and very low acceleration potential, are fundamental for getting the maximum performance from the equipment.

The equipment in operation in the UPV Microscopy Service is the ZEISS ULTRA 55 model, incorporating the following detectors:

A Secondary Electron Detector (SE2), which provides an SEM topography image of the sample surface with a large depth of field.  It is best suited to obtain medium and low resolutions with high acceleration potentials. It is mainly used to browse the sample at low magnification in search of points of interest and to study samples with a large amount of topographic information.

A Secondary Electron In-Lens Detector, located inside the electron column, which works with low energy secondary electrons and provides images with a higher resolution.  It is very sensitive to the surface characteristics of the sample, which makes it very suitable for surface characterisation of any material.  It performs best at low acceleration potentials (<5 kV), hence it is highly recommended for working with electron beam sensitive samples and for minimising the charging effect on non-conductive samples.

A Backscattered Electron Detector (AsB) which is sensitive to the variation of atomic number in the elements present in the sample; therefore, it is used to observe changes in the chemical composition of the specimen. The AsB detector comprises four quadrants and permits selection of images with topographic and compositional contrast. Moreover, its high-angle working mode allows us to observe structural contrast in crystalline samples.

A Backscattered Electron In-lens Detector (EsB), independent of the secondary In-lens detector, which provides a pure backscattered signal with no secondary electron contamination and very low acceleration potential.  It gives a higher Z-contrast than any other backscattered detector, and it is the only one that can select the electrons according to their energy, enabling differentiation between elements which are only distinguished by a few atoms.  It is also capable of working at very low voltage (in the same range as the secondary in-lens detector), which makes it ideal for sensitive samples. The fact of working independently facilitates the simultaneous capture of signal from secondary and backscattered electrons.

An X-Ray Dispersive Energy Detector, EDS, (Oxford Instruments) which receives x-rays from each surface point the electron beam passes over.  As the x-rays scattered energy is a characteristic of each chemical element, it provides qualitative and quantitative analytical information about selected points, lines, or areas on the surface of the sample.  This technique is known as Microanalysis by EDS.