Scanning Electron Microscope: a Deep Dive into its Advantages and Drawbacks

A scanning electron microscope is an essential magnification tool that makes use of focused electron beams to gather a wealth of data. The high-resolution, three-dimensional images captured by SEMs provide compositional, topographical, and morphological information about a specific specimen. Given that an SEM falls under the category of electron microscopes, you can expect that it has a far greater capacity (aka resolving power) of showing us different amounts of detail than what light microscopes can provide.

Not only does it make what we can already see more prominent, but it also increases the amount and quality of the detail that’s visible. This depth of detail is the key difference between magnification and resolution, which is within the capacity of an SEM, making it an invaluable and handy resource in various scientific and industrial applications.

SEM Properties

SEMs are one of the three types of electron microscopes (EM). They feature the same basic principles that govern light microscopes. However, instead of utilizing photons to magnify a sample, they focus beams of energetic electrons. Generally, an SEM requires a vacuum and cooling system, reliable power supply, and a vibration-free space within an area that keeps the instrument away from ambient electric and magnetic fields. It also consists of the following components: ● Thermionic Gunrn● Electron Sourcern● Computerrn● One or more detectors ● Secondary Electron Detector (SED) ● Backscatter Detectorrn● Sample chamber and stagern● Field Emission Gunrn● Vacuum chamberrn● Diffracted Backscatter Detector (EBSD) ● X-ray Detector (EDS) ● Electromagnetic and/or Electrostatic Lenses

How Imaging Works in SEMs

An SEM produces detailed surface information by tracing a sample in a raster pattern with an electron beam. The process starts by generating a beam of energetic electrons onto a series of electromagnetic lenses and down the column through an electron gun. These lenses are solenoids, which are tubes wrapped in a coil. These coils are adjusted to focus the incident electron beam onto the sample; such adjustments cause voltage fluctuations, increasing or decreasing the rate at which the electrons come into contact with the surface of the specimen. The SEM operator can also adjust the beam to control magnification and determine the surface area to be scanned using a computer. The beam is then focused onto the stage, which contains a solid sample. Before being placed in the vacuum chamber, most samples require a bit of pretreatment. Sputter coating for non-conductive samples and dehydration for most biological specimens are the two most common preparatory techniques utilized prior to SEM analysis. Furthermore, the operator needs to make sure that all samples are able to withstand the low pressure inside the vacuum chamber. The acceleration rate of incident electrons (interacting electrons), which carry considerable amounts of kinetic energy before being focused onto the sample, determines the interaction between the incident electrons and the sample’s surface. When incident electrons interact with the sample, energetic electrons are released from the sample's surface. The resulting scatter patterns by the interaction provides information about the sample's size, shape, texture, and composition. End users utilize a variety of detectors to attract different forms of scattered electrons, such as secondary and backscattered electrons, as well as x-rays.

SEM Advantages

The advantages of a scanning electron microscope include its broad range of applications, detailed three-dimensional and topographical imaging, and the diverse information obtained by different detectors. With proper training, SEMs are also simple enough to operate, and advances in computer technology and associated software have made the operation even more user-friendly. This device is incredibly fast, frequently completing secondary electron imaging (SEI), backscattered electron imaging (BSE), and energy-dispersive X-ray spectroscopy (EDS) analyses in less than five minutes. Also, technological advancements in modern SEMs enable the generation of data in digital form. Although all samples must be prepared before being loaded in the vacuum chamber, most SEM samples only require minimal preparation steps.

SEM Areas for Improvement

rnOn the other hand, the size and cost of an SEM are two of its major drawbacks. SEMs are costly, huge, and must be housed in a space that's free of any potential electric, magnetic, or vibration interference. Maintaining a constant voltage, currents to electromagnetic coils, and the circulation of cool water are all part of an SEM's maintenance process. Also, you need to undergo special training to prepare samples and eventually operate an SEM. Artifacts can arise during the preparation of samples. However, the negative impact can be mitigated by knowledgeable and experienced researchers who can identify artifacts from actual data while possessing excellent preparation skills. However, there’s no foolproof method for removing or identifying all potential artifacts at present. Furthermore, SEMs are confined to solid, inorganic samples that are small enough to fit inside a vacuum chamber capable of handling moderate vacuum pressure. Finally, due to the electrons that scatter from beneath the sample surface, SEMs pose a minor risk of radiation exposure. The sample chamber is designed to prevent any electrical and magnetic interference, removing any possibility of radiation escaping the chamber. Even though the risk is low, SEM operators and researchers should take safety precautions all the time.

Key Takeaway

Despite the fact that SEMs are enormous, pricey pieces of equipment, they remain popular among researchers due to their diverse applications and capabilities, including the high-resolution, three-dimensional, detailed images they create.