Scanning Electron Microscopy

How does Scanning Electron Microscopy work?

Beam production

To scan a sample the electron beam is directed across the sample in a raster pattern, while the reflected electrons are continuously detected. Specialized software combines the intensity acquired by the detector with the position of the beam to reconstruct a gray-scale image.

The beam is produced by voltage heating an electrode (the filament), which will then emit electrons. This process is called thermionic emission. Common filaments are made of tungsten or solid-state lanthanum hexaboride crystals. Alternatively, a field emitter gun can be used. In this case, a strong electric field is applied to the narrow tip of a filament. This reduces the potential barrier which keeps the electrons inside the material, and releases them into the vacuum by quantum tunneling. The small tip results in a more narrow beam than can be accomplished by thermionic emitters, which can reveal finer details in the sample.

Imaging: Backscattered Electrons

A Scanning Electron Microscope can both image and analyze samples. Backscattered electrons (BSE) are mainly used to create an image. The negatively charged electrons from the SEM beam interact with positive nuclei in the near-surface region. The nuclei attract the electrons, but do not capture them: electrons follow a ‘sling shot’ trajectory, based on the weight of the nucleus. The returning BSE’s are then observed with detectors. Heavier nuclei will interact more strongly with the beam electrons than lighter ones, and thus appear brighter on an image. BSE’s therefore also provide some information about the chemical composition of a surface.

Imaging: Secondary Electrons

Alternatively, secondary electrons can be measured. This type of electrons is generated by inelastic interactions of the primary electron beam with surface atoms of the sample. They therefore provide topographical information of the surface area, but contain no information about the composition of the sample. However, they do provide a very high resolution, and can reveal details of under 0.5 nanometer. Secondary electrons have lower energy and can be analyzed separately from BSE’s.

Imaging: Energy-dispersive x-rays

Apart from electrons, the interaction of the electron beam with the sample can produce energy-dispersive X-rays (EDX/EDS). This happens when the incoming electrons transfer energy to an atom in the sample, which sends an electron to a higher orbital. When the electron returns to its ground state, an X-ray photon is emitted with an energy that is specific to the element. Thus, an EDS spectrum contains information on the different elements in a sample, as well as their relative abundance.

Key signals and detectors

Secondary Electrons (SE)

Secondary electrons are low-energy electrons ejected from the sample atoms due to inelastic collisions with the primary electron beam. Because they originate very close to the surface (within a few nanometers), secondary electrons provide high-resolution topographical information. The secondary electron detector (Everhart-Thornley detector) collects these electrons, producing detailed images that reveal surface morphology, texture, and fine features. SE imaging is the most widely used mode in SEM because it emphasizes surface details with excellent contrast.

Backscattered Electrons (BSE)

Backscattered electrons are high-energy electrons from the primary beam that are elastically scattered by atomic nuclei in the sample. Unlike secondary electrons, BSE originate from deeper beneath the surface (tens to hundreds of nanometers) and are sensitive to the atomic number of the elements. Heavier elements scatter electrons more strongly, appearing brighter in BSE images. The BSE detector is often positioned above the sample to capture these electrons, allowing visualization of compositional contrast and highlighting phase differences, inclusions, or coatings.

Characteristic X-rays

When the electron beam ejects an inner-shell electron from an atom, higher-energy electrons drop into the vacancy, emitting characteristic X-rays specific to each element. These X-rays are detected by energy-dispersive X-ray spectroscopy (EDS) detectors or wavelength-dispersive spectroscopy (WDS) detectors, providing qualitative and quantitative elemental analysis. EDS is widely used for rapid compositional mapping and identifying unknown materials, while WDS offers higher spectral resolution for more precise elemental identification.

Other signals and detectors

Cathodoluminescence (CL): Detects photons emitted when excited electrons relax, used for optical and electronic property analysis in minerals and semiconductors.

Electron Backscatter Diffraction (EBSD): Analyzes crystallographic orientation and phase by detecting diffraction patterns of backscattered electrons.

By combining these signals and detectors, SEM provides comprehensive information about a sample, including surface topography, compositional differences, crystallography, and microstructural features, making it an incredibly versatile tool in materials science, biology, geology, and engineering.

Why use a Scanning Electron Microscope?

As electrons have a much shorter wavelength than visible light, they can resolve much smaller details in the sample. Thus, Scanning Electron Microscopy allows you to see the smallest structures, even up to atomic resolution. The absence of colour information can be compensated by being able to derive the identity of the atoms in the sample. The technique also allows you to see a large part of your sample at low magnification and zoom in to study details at high magnification. Through EDS spectra, the composition can be analyzed.

As the electron beam will penetrate the sample (with a depth depending on the acceleration of the electrons), a SEM can provide information about the surface region, rather than just the top layer. This makes Scanning electron microscopy is the preferred method for the study of surfaces.

Advantages and Limitations of Scanning Electron Microscopy (SEM)

Advantages

One of the primary strengths of scanning electron microscopy (SEM) is its high resolution, which far surpasses that of optical microscopy. Electrons have much shorter wavelengths than visible light, allowing SEM to reveal features on the nanometer scale. This enables detailed examination of surface morphology, microstructures, and fine textures that would be impossible to observe with a light microscope.

SEM also offers a large depth of field, giving images a three-dimensional appearance and allowing features at different heights on the sample surface to remain in focus simultaneously. This is particularly useful for studying complex or rough surfaces, fractured materials, and microfabricated structures.

Beyond imaging, SEM can provide elemental and chemical information through techniques such as energy-dispersive X-ray spectroscopy (EDS) or wavelength-dispersive spectroscopy (WDS). These capabilities allow researchers to not only observe the structure of a material but also determine its composition, detect impurities, and map the distribution of elements across the surface.

Another advantage of SEM is its versatility in sample types. Metals, ceramics, polymers, composites, and biological specimens can all be imaged, often with only minor sample preparation. Low-vacuum or environmental SEM modes even allow imaging of hydrated or partially insulating materials, expanding the range of possible investigations.

Finally, SEM imaging is relatively rapid and reproducible, making it suitable for both research and quality control in industrial applications. It can detect defects, wear patterns, coating integrity, and microstructural variations efficiently, providing valuable insight for material development, failure analysis, and process optimization.

Limitations

A scanning electron microscope operates under vacuum conditions, from the electron beam source to the sample chamber. As a result, samples must be compatible with a vacuum environment, which can limit the types of materials that can be analyzed. In some cases, low-vacuum or variable-pressure SEM modes can be used to accommodate less compatible samples.

Additionally, SEM requires samples to be electrically conductive to prevent charge accumulation under the electron beam. Non-conductive materials, such as polymers or ceramics, often need to be coated with a thin layer of conductive material—commonly gold, platinum, or carbon. While this enables imaging, the coating can introduce surface artifacts or obscure fine features.

Biological or other delicate samples are also vulnerable to damage from the high-energy electron beam. Extended exposure can lead to structural changes, dehydration, or other artifacts. Low-voltage imaging or cryo-SEM techniques can help mitigate beam damage, but they may limit resolution or require specialized equipment.

How much does a Scanning Electron Microscope cost?

ST Instruments offers a wide variety of solutions to make (sub-)nanometer details visible. SEM’s are available in a range of desktop SEMs to top-of-the-line systems with tungsten or field emission beam emitters, and prices are dependent on  the system configuration. Reach out to us to find out the pricing on your preferred instrument.