Scanning electron microscopy (SEM) is a powerful technique for the analysis of a wide range of materials at high resolutions. SEM imaging relies on the detection of electrons which have been scattered from the surface and the bulk of a sample material after exposure to an electron beam.

Beam diameter, accelerating voltage, and distance of the electron beam from the sample are some of the parameters that can be manipulated to achieve optimal image resolutions.

SEM imaging has a wide range of applications in research and industry for the characterization of naturally occurring and man-made materials at resolutions in the single digit nm-regime and even below – far exceeding conventional light microscopy.

In a scanning electron microscope, an electron gun directs a focused beam of electrons onto a specimen. The interactions of the electrons with the elements in the specimen reveal its chemical composition, structure, and shape ¹.

The electron beam scans the specimen using a raster pattern from top to bottom and side to side. This causes the electrons from the beam (primary electrons) to scatter at the surface of the specimen, resulting in secondary electrons, backscattered electrons, and characteristic X-rays.

Primary electrons which travel through the specimen without or low interaction with any of its elements are known as transmitted electrons. Key components of a scanning electron microscope include ¹:

• An electron source (or gun) comprising a thermionic gun, a field emission gun, or solid-state crystals
• A series of electrostatic, electromagnetic or magnetostatic lenses — including an objective lens — that focus the electron beam
• Scanning coils or electrostatic deflectors that deflect the electron beam in the X and Y directions for raster scanning
• A sample chamber
• A sample stage
• Secondary electron, backscattered electron, and X-ray detectors
• Infrastructure components, such as vacuum and cooling systems

In SEM imaging, data are collected over surface areas ranging from the multiple 10nm- regime up to the mm-regime. These data  are subsequently collated to form two-dimensional greyscale electron images.

Accelerated electrons used in SEM imaging carry a certain amount of energy which dissipates as the electrons interact with a sample. As the energy dissipates, secondary electrons, backscattered electrons, and photons (X-rays and other irradiations) are formed.

Secondary electron detection

Secondary electrons in SEM imaging result from inelastic collisions of primary electrons with a sample surface: Inelastic collisions cause primary electrons to dissipate energy, resulting in low-energy secondary electrons (50 eV or less) ².

Secondary electrons characterize the top few nanometers of a sample, revealing its topology and morphology ². Secondary electrons are also produced within a highly localized region surrounding the electron beam, which makes it possible to produce high-resolution SEM images. This type of electron is typically detected using an Everhart-Thornley detector, which consists of a scintillator placed inside a Faraday cage.

Backscattered electron detection

Backscattered electrons (BSEs) in SEM imaging result from the elastic scattering of primary electrons within a specimen sample. Although this scattering alters the direction of travel of primary electrons, it does not dissipate their energy. Consequently, backscattered electrons travel further into the sample, revealing its structure.

Because backscattered electrons travel deeper into the sample, BSE signals are highly correlated with the atomic numbers of elements, revealing important information about topography and structure. Thus, they reveal information about the distribution (but not the composition) of elements within the sample. Backscattered electrons are detected using solid-state detectors placed above the sample.

Characteristic X-ray detection

Characteristic X-rays in SEM imaging are generated from inelastic collisions of primary electrons with electrons inside the shells of the atoms making up the sample. After colliding with the electron beam’s primary electrons, electrons in the shell return to their original lower energy state, releasing the excess energy as X-ray radiation. This radiation is characteristic of each element in the sample. Thus, using energy-dispersive X-ray spectroscopy, it is possible to analyze the chemical composition of the sample.

SEM imaging has a broad range of research and commercial applications, providing rich information for analysis, including:

• Morphological information on shapes, forms, and structures of specimens
• Topological information about the distribution of features on the surface of a specimen
• Compositional information about the chemical makeup of specimens
• Structural information of the sample

The many applications of SEM imaging include:

• Materials science, where SEM imaging supports research into nanotubes, nanofibers, alloys, and other materials for aerospace, energy, electronics, and other industries.
• Semiconductor manufacturing, where SEMs enable the detailed development of microchips, nanochips, and other components. SEM imaging provides vital topographical information for reliability and performance testing.
• Geological characterization of rocks, soil, and minerals.
• Biochemical investigation of bacterial strains, viruses, vaccines, blood, tissues, and drugs.
• Forensic investigation of gunshot residues, particles, fibers, and other materials.

Additionally, an SEM can be used for transmission electron microscopy, whereby transmitted electrons can also be detected by fitting extra detectors, coils, and circuitry to the SEM. SEM imaging is arguably one of the most versatile techniques for the characterization of solid materials.

The PIONEER Two from Raith combines SEM imaging with electron beam lithography into a complete turnkey solution for scientists and engineers who require both functionalities in equal measure. It presents the ideal alternative to proprietary or third-party pattern generators.

Supplied with an integrated thermal-field emission gun, a high-precision laser interferometer-controlled stage, and various detectors (inlens SE, inlens EsB, AsB, EDX …) the PIONEER Two produces crisp SEM images for the chemical and structural analysis of millimeter-sized (up to 2-inch wafer-sized) samples.

The system incorporates Raith Nanosuite nanofabrication software for ease of use (including “Write and View” configuration) while also featuring the world’s smallest beam size found in an EBL system at 1.6 nm.

References

1. Swapp, S. Scanning Electron Microscopy (SEM). Geochemical Instrumentation and Analysis [online] Available at: https://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html
2. Warwick University. Scanning Electron Microscopy (SEM). Department of Physics [online] Available at: https://warwick.ac.uk/fac/sci/physics/current/postgraduate/regs/mpagswarwick/ex5/techniques/structural/sem3/