Quality control at microscopic scales is becoming increasingly important in industrial applications and research. High-resolution images obtained with a scanning electron microscope (SEM) can provide insight and valuable information, making SEM an indispensable tool across various fields.
Its uses and applications aren't restricted within the confines of a laboratory. SEMs are being used in a variety of industrial, commercial, and research settings. From cutting-edge forensic applications to fabrication processes, the range of applications and uses for modern SEMs and their peripherals are more extensive than ever.
How does an SEM work?
A Scanning Electron Microscope (SEM) creates high-resolution, three-dimensional images by focusing electron beams. These visuals hold information about the topography, composition, and morphology of a specific sample.
Electromagnetic lenses are used to regulate the electrons' path. The condenser determines the size of the electron beam (which in turn defines the resolution). Meanwhile, the objective lens' primary function is to focus the beam on the sample. To raster the beam onto the sample, scanning coils is used. Apertures are frequently used in conjunction with lenses to adjust the size of the beam.
When samples interact with an electron beam, they emit several types of electrons. A Back Scattered Electron (BSE) detector is mounted above the sample to detect backscattered electrons. Images depict contrast information between areas with varying chemical compositions, as heavier elements (those with a high atomic number) appear brighter. A Secondary Electron (SE) detector is installed at an angle on the side of the electron chamber to improve the efficiency of detecting secondary electrons, which can provide more detailed surface information.
Electron Microscope vs. Optical Microscope
In contrast to the optical microscope, which utilizes visible light to magnify images, the electron microscope uses a beam of electrons and their wave-like properties to magnify an object's image. Conventional optical microscopes can magnify between 40 to 2000 times, but "super-resolution" light microscopes that can magnify living biological cells up to 20,000 times or more have already been designed and developed. On the other hand, the electron microscope can resolve elements that are over a million times smaller.
Electron microscopes (EMs) employ the same mechanism as optical microscopes, except that instead of photons, they apply a focused beam of electrons to capture images of the specimen and collect information about its structure and composition.
Currently, there are two major types of electron microscopes making the rounds in clinical and biomedical research settings: the transmission electron microscope (TEM) and the scanning electron microscope (SEM). In specific instances, the TEM and SEM can be combined in one instrument, the scanning transmission electron microscope (STEM):
- TEM: magnifies the specimen 50 to ~50 million times; the specimen appears flat.
- SEM: magnifies 5 to ~500,000 times; sharp images of surface properties and features
- STEM: magnifies 5 to ~50 million times; the specimen appears flat.
SEM Electron Sources
The electron beam is perhaps more important to the electron microscope more than anything else. However, not all electron sources are created alike. Understanding their basic functionality, benefits, and disadvantages is critical in choosing the best instrument for your experiments.
The three different types of electron sources are as follows:
- Thermionic filament
A Tungsten filament inside the microscope is heated up to the point that it emits electrons. Since this type of filament operates at white-hot temperatures, it gradually evaporates over time and eventually breaks, which can contaminate the upper part of the electron column. Depending on the vacuum, the average lifetime of a Tungsten source is approximately 100 hours.
- Cerium Hexaboride cathode (CeB6)
This electron source delivers ten times the brightness of Tungsten, resulting in a better signal-to-noise ratio and higher resolution. A CeB6 source typically has a service life that is more than fifteen times that of tungsten: 1500+ hours.
- Field emission gun (FEG)
A FEG generates a strong electrical field that pulls electrons away from their atoms. This electron source is often the preferred option in SEMs since it produces high-resolution images; but, it requires a vacuum design, which can be costly.
SEM Uses and Applications
As mentioned at the start, SEMs are used in various research, commercial, and industrial applications. Some of the top industries are as follows:
SEMs are used in criminal and other forensic investigations to uncover evidence and gain additional forensic insight. Since they enable the ability to examine a wide range of materials at high and low magnification without compromising the depth of focus, their use in forensic sciences permits the drawing of conclusions, identification of material origins, and contribution to a body of evidence in criminal and legal matters.
Specific uses include:
- Examination of jewelry
- Gunshot residue analysis
- Bullet marking comparison
- Print and handwriting analysis
- Examination of banknote authenticity
- Fiber and paint particle analysis
- Filament bulb analysis in traffic accidents
SEMs are increasingly being utilized in microchip production to acquire insight into the feasibility of novel production and fabrication methods. With the advent of smaller scales and materials, as well as the prospect of complex self-assembling polymers, The high resolution and three-dimensional capability of SEMs have become undoubtedly invaluable to microchip design and manufacturing.
SEMs are currently being utilized in materials science for research, quality control, and failure analysis purposes.
Investigations into nanotubes and nanofibers, high-temperature superconductors, mesoporous structures, and alloy strength all rely significantly on the use of SEMs for research and analysis in modern materials science.
As a matter of fact, every materials science industry at present, from electronics and energy usage to aerospace and chemistry, has been made possible by SEM technology.
Arguably, there’s no other instrument with the same scope of applications in the study of solid materials that compare with the SEM. Not only are SEMs relatively easy to operate with user-friendly intuitive interfaces, most of its related applications only require minimal sample preparation. Even more impressive is how rapid the length of time is to acquire needed data (usually less than 5 minutes per image). Finally, modern SEM units now allow you to generate data in digital formats at a time when everybody puts a high premium on portability.
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