What is Scanning Electron Microscopy?

Scanning electron microscopes (SEMs) are powerful imaging tools that allow you to see extremely small details of materials at high magnifications with excellent clarity and depth of field. They differ enormously from conventional optical microscopes, such as the compound microscope, as they use electrons to create images rather than light. Simple microscopes allow you to observe a sample in its natural colour when placed under an objective lens in open air or water. The effect is like that of a magnifying glass, and the same limits apply. Magnification and depth of field are limited by the wavelength of light, which is physically set in a defined range. 
Scanning electron microscopy offers a much larger depth of field and higher magnifications than optical microscopy. This, combined with its ability to conduct chemical analyses using spectroscopic methods, makes it a very powerful research tool. SEMs provide a high degree of analytical capability and reveal surface details at nanoscale resolution. A single greyscale image from derived from scanning electron microscopy can often be enough to achieve critical objectives, for example, the ability to visualize microstructures in a materials sample. 
Figure 1. Optical vs. Scanning Electron Microscopes

The Principles of Scanning Electron Microscopy

SEMs produce images by directing a high-energy beam of electrons onto a sample and scanning it in a zig-zag pattern (raster scanning). This requires a suite of interconnected optical components, including condenser and objective lenses to focus the beam, and deflector coils to alter the beam’s path. 
Typically, three detectors are positioned at angles in the sample chamber, these are an X-ray detector, a back-scattered electron detector, and a secondary electron detector. Sample thickness is not an issue as none of these elements rely on transmission.
Magnets focus the electron beam to a point several nanometers in diameter. As the electron beam interacts with the surface of the sample, signals are produced and compiled by various imaging and analytical detectors. Thus, high-resolution nanoscale images are achieved along with precise measurements. Scanning electron microscopy may detect backscattered electrons (to reveal morphology and topography and give insight as to composition), or secondary electrons (to reveal surface topography).
Figure 2. Scanning Electron Microscope Column basic cross-sectional view.

Which SEM Microscope is Right for Which Application?

There are many types of SEMs, ranging from the more common type, which use a tungsten filament as an electron source, to the more specialized type which, with a field emission (FE) electron gun mounted, attains higher resolution and magnification. Choosing the ideal SEM instrument is dependent on many factors.
Below, we summarize some of the main considerations when selecting an SEM.

1. Microscope Magnification

Since electron wavelengths are up to 100,000 times smaller than the wavelengths of visible light, SEMs resolve details hundreds of thousands of times smaller than optical microscopes.
The field of view (FOV) in a microscope defines the size of the feature to be imaged. This value can range between millimeters, microns and nanometers. To define the FOV required to image samples, first the end goal must be decided. If the number of particles in a sample is what is of interest, having multiple particles per image is not an issue, so an SEM that provides a FOV of 100 times greater is enough. However, if a particle's structure is of interest, a closer FOV is needed to see the required detail. This is shown in Figures 3-5, which compare tabletop, tungsten, multipurpose FE and ultra-high resolution (UHR) FE SEM instruments.
Tabletop scanning electron microscopy can be very efficient for basic applications requiring magnification ranges up to 100,000X and some selectable settings. The relaxed vacuum requirements and small evacuated volume enable fast image production without extensive sample preparation.
Additionally, operating a tabletop SEM is simple enough to do it by the individual who requires the information instead of a dedicated scanning electron microscopist. As well as obtaining answers quickly, it is also beneficial to be able to carry out analysis straight away and for the user to be able to manage it in real-time response to observations.
For instances where higher magnification is needed, but space is also a limiting factor, conventional tungsten SEMs are an option to simplify specimen navigation, and advanced automation delivers crisp secondary and backscatter images in seconds. If a specimen is challenging to analyze, FE SEMs and UHR FE SEMs provide topographical and elemental information at magnifications of 10X up to 1,000,000X.
Figure 3: Materials SEM comparisons.
Figure 4: Biological SEM Comparisons.
Figure 5. SEM comparisons for imaging Titanium alloy cross section.

2. SEM Resolution

The word resolution indicates the smallest observable element in an image. For the human eye, that is about 0.2 millimeters (mm). Scanning electron microscopy typically offers resolutions between 0.5 and 4 nanometers (nm), providing the opportunity for particle diameters and geometries to be studied in great detail. There are many contributing factors that can affect the maximum resolution obtained in an SEM, like the electron spot size and interaction volume of the electron beam with the sample.
Scanning electron microscopy’s high resolution is attributed to the fact that the wavelength of the electrons becomes shorter because the accelerating voltage of the electrons used in the SEM is as high as several kV to several tens kV, and to the characteristic difference of the electromagnetic lenses used to converge the electron beams. By utilizing several images together with software, the size distribution of particles may be determined and a concentration versus particle diameter may also be calculated.
SEM images are stored in an image file (e.g., JPEG, TIFF) with a user-defined number of pixels. An SEM will scan small areas with an electron beam, meaning the surface portions will become a pixel of the final image. More pixels result in a longer processing time; however, a long analysis process can negatively affect the sample.
Tabletop or benchtop SEMs can generate an electron beam at the specimen surface with spot size of 8nm, and a price range similar to that of a high-end optical device, thus they are slowly revolutionizing the industry, realigning production standards to a new level of miniaturization.
Tungsten SEM is suitable for analysis of larger structures (hundreds of nm). The lower kV allows for a smaller X-ray signal depth within the sample and, thus higher X-ray spatial resolution.
If ultra-high X-ray spatial resolution is needed to resolve ~50nm layers, then an FE SEM is the best option, since FE emitters maintain a tiny spot size even at low kV. Table 1 compares some relevant parameters between thermionic tungsten emitters and Schottky FE emitters.
Parameters Thermionic Tungsten Schottky Field Emission
Brightness (A cm⁻²sr⁻¹) 10⁵ 10⁷-10⁸
Energy spread (eV) 1-3 0.5-0.6
Lifetime ~150 h ~3 years or longer
Table 1: A comparison of parameters between thermionic tungsten and Schottky field emission emitters.
Figure 6. JEOL SEM Microscopes range from Benchtop SEM to Ultrahigh Resolution Field Emission SEM.

3. Microscope Applications

The applications of scanning electron microscopy are extremely varied. They are routinely used in a wide range of applications and industries, including electronics, chemicals, additive manufacturing, forensics, batteries, failure analysis, semiconductor, botany, marine biology, medical, and pharmaceuticals and are used in research, quality control and product inspection. For materials science research, investigations into nanotubes and nanofibers, high-temperature superconductors, mesoporous architectures and alloy strength rely heavily on SEMs for research and investigation. Many material science industries, from aerospace and chemistry to electronics and energy usage, have been made possible with the help of scanning electron microscopy.
    Scanning electron microscopy can play a pivotal role in pharmaceutical applications, assisting with rapid and efficient characterizations of new drug treatments, and providing insights into their interactions with human cells and their applications in complex therapies.
    Due to the nearly unrestricted field of view (FOV) of Field Emission (FE) and Ultra-high resolution (UHR) FE SEMs, high-resolution imaging and high current analyses can be achieved without sacrificing performance. These SEMs are suited to imaging and analyzing magnetic light element materials and nanostructures.

    4. Microscope Users

    As electron microscopy technology has evolved, the user experience has been improved to suit any operator, from the novice to the most advanced user. Today’s compact SEM microscope designs have made it feasible to incorporate a variety of analysis capabilities into a single instrument, creating essentially a self-contained nano-laboratory that allows not only imaging but also chemical analysis, 3D imaging and analysis, metrology, and particle analysis.
    When considering what type of SEM to buy or use, it is worth considering the number of individuals who will be utilizing the system, how much training they have, and how much time it might take to train them. All JEOL SEMs are simple to use at some level, and many automated features help streamline workflows, but training is recommended to optimize performance.

    History of SEM Microscopes

    Electron microscopy was invented in Germany in 1931 by Max Knoll and Ernst Ruska [1] to overcome the inherent resolution limit of visible-light microscopes (about 200 nm). M. Knoll and Manfred von Ardenne were also two pioneers in this field. JEOL introduced its first commercial SEM in 1966 and has produced the highest number of SEMs in the world since that date. However, 16 years earlier, JEOL produced its first Transmission Electron Microscope[2]. See the JEOL timeline and milestones here.

    JEOL Scanning Electron Microscopes

    JEOL has played a leading role in the development and evolution of scanning electron microscopes since the early 1960s. JEOL provides valuable applications support, comprehensive training, and award-winning service for the long lifetime of our instruments.
    JEOL innovations in resolution and functionality enable the microscopist to better image and characterize a new generation of nanomaterials, capture biological details, analyze forensic evidence in detail, direct write fine nanopatterns, and pinpoint elusive quality problems.
    What SEM is right for your applications? Let JEOL applications experts help you make the right decision.
    View applications and our full list of SEM Products.
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