How Scanning Electron Microscope Works

Electron microscopes are critical to observing the structure of microorganisms and molecules for better understanding. Scanning electron microscopes is one of the two important types of electron microscopes. They have wide applications in chemistry and biology to examine specimens, especially through scanning tunneling microscope. If you are unfamiliar with the scanning electron microscope function, you might wonder, “How does a scanning electron microscope work?”

This article tells you how a scanning electron microscope works and what it is used for.

Table of Contents

What is a Scanning Electron Microscope?

A scanning electron microscope is a type of electron microscope that uses electrons to produce images of the specimen. These microscopes have a higher resolution and a large depth of field, making them ideal for molecular-level analysis. Scanning electron microscopes have a resolution of <1 nm up to several nanometers. The high resolution allows them to enlarge and view closely placed specimens.

The use of electrons in scanning electron microscopes is attributed to this high resolution, and visible light has a comparatively low wavelength. So, the resolution depends not only on the lenses used in the microscope.

History of a Scanning Electron Microscope

The history of a scanning electron microscope dates back to 1937 when Manfred von Ardenne established the first SEM to provide better results than transmission electron microscopes. He scanned a small raster using a beam of electrons. He improved resolution and chromatic aberrations in TEM to produce better results. Later, more researchers worked on the scanning electron microscope, leading to the production of the SEM in 1965 by Cambridge Scientific Instrument Company.

While you can find research articles on how scanning electron microscope works, we have explained the working of a scanning microscope precisely here:

How a Scanning Electron Microscope Works?

A scanning electron microscope uses an electron beam instead of visible light to produce high-resolution images with clear visibility in nanometers. The electron beam obtains signals about the sample’s composition through specialized detectors. Scanning electron microscopes typically have a backscattered or secondary electron detector (SED) for high-resolution imaging.

Here’s how a scanning electron microscope works:

The electron gun at the top of the column produces an electron beam at 1 to 30 keV.
Then, condenser lenses converge the beam’s diameter from 20 nm to 1 nm or less, and the objective lens focuses on the sample mounted on a stage in a vacuum.
The scan coils above the objective lens controls the electron beam’s position.
Then, the scanned beam lands on the sample and generates signals detected by detectors.
The interaction of the electrons and specimen depends on the accelerating voltage of the electrons and the sample’s density. Higher voltages have a higher depth of penetration.
The obtained images are shown on a computer screen attached to the microscope.

The sample and the electron beam interaction produces signals, including X-rays, secondary electrons, and backscattered electrons. Some materials also emit photons through a process called cathodoluminescence.

How Different Parts of a Scanning Electron Microscope Works?

The electron microscope produces high-resolution precise images of the sample using an electron beam, but it involves multiple components that enable the process. Here’s how different parts of a scanning electron microscope work:

Electron Source

The electron gun emits a coherent stream of electrons that looks like a continuous electron beam. This electron beam is the light source to illuminate the specimen after passing through the lenses. Electron microscopes have solid-state hexaboride crystals, field emission guns, or tungsten filament electron sources.

Cerium Hexaboride Crystals: Cerium hexaboride crystals have ten times the brightness of tungsten and offer a better signal-to-noise ratio with a lifetime of 1,500 hours.
Tungsten Filament: Tungsten filament electron sources, also known as thermionic filament, heat up until they emit electrons and have a lifetime of 100 hours.
Field Emission Gun: These electron sources generate high-resolution images by creating a powerful electric source using a vacuum design.

Lenses

Like a light microscope, electron microscopes also have lenses to further converge the light beam. However, scanning electron microscopes have electromagnetic lenses that narrow the electron beam. These lenses typically comprise copper wire. The off-axis electrons bend towards the optic axis, resulting in a beam crossover at a particular distance.

Besides the condenser lens, electron microscopes have an objective lens, the last lens in the column. It establishes the working distance by directing the beams toward the sample. Shorter working distances result in a smaller depth of field, higher resolution, magnification, and vice versa.

Coil

Scanning coils are unique to electron microscopes; you do not find them in optical microscopes. They are critical to producing the required images for the scanning electron microscope function. The coils deflect the beam across the X and Y axes to scan the image thoroughly. Their connection with the detectors enables real-time imaging.

Sample Chamber

The sample chamber contains the sample onto which the electrons fall to form an image. The chamber is evacuated and contains a translation stage, temperature-control stages, cameras, and tilt and rotation devices.

Detectors

Detectors pick the signals emitted by the sample in interaction with electrons. Depending on the sample density and accelerating voltage, different detectors distinguish x-rays, secondary electrons, and backscattered electrons.

Secondary Electron Detector: Secondary electron detectors (SED) produce image resolution regardless of the material. They generate images using the scattered electrons close to the sample surface; thus, they do not provide material composition details. An Everhart-Thornley (E-T) detector is a common secondary electron detector with a scintillator within a Faraday cage. The detector is above the sample to a side and attracts low-energy electrons, which are then converted to photons and yield signals.
Backscattered Electron Detector: Backscattered electron detectors (BSD) are among the most common choices; they detect high-energy elastically scattered electrons. While they reduce sample preparation requirements, BSDs do not produce high-resolution images like SEDs. At the same time, the specimen composition, like the atomic number, angle, and acceleration voltage, impacts the backscattered electrons. A high atomic number of elements produces a higher number of backscattered elements. So, these images can also help identify varying composition phases.
Energy Dispersive Spectroscopy (EDS): This type of detector detects x-rays emitted during the process to produce specimen images. Every element displays a unique energy difference between inner and outer shell electrons, so they can help identify the element. These detectors are accurate and fast, giving you high-quality images. Silicon drift detectors are widely used energy-dispersive spectroscopy detectors that absorb X-rays to detect signals.

Imaging Screen

Lastly, the greyscale images are displayed on the screen, which helps observers understand the details of the specimen.

How to Prepare a Sample for SEM?

Now that you know how a scanning microscope works, it’s also important to know how to prepare a sample. You can only obtain expected results by preparing the samples correctly.

Make sure that your sample is clean.
If your sample lacks conductive properties, coat it with a metal coating like platinum, gold, silver, or chromium. Also, materials like plastic require coating.
Use a fixative to maintain the structure of the sample.
Dehydrate your sample using alcohol where required, as water vapors can interfere with the process.
Place the sample in the chamber for imaging.

Advantages of Scanning Electron Microscope

Scanning electron microscopes have user-friendly software.
Modern scanning electron microscopes provide data in digital form.
It is a fast process that gives you images and data in less than five minutes.
SEMs require a low level of sample preparation.
These microscopes are used widely in various fields, including biomedical studies, nanosciences, industrial analysis, chemical research, microbiology, etc.
They help analyze and identify materials.

Limitations of Scanning Electron Microscope

Despite the unparalleled scanning electron microscope function and high resolution, the microscopes have some limitations:

SEMs are expensive to purchase and repair.
They need consistent voltage maintenance and cooling system access.
These microscopes are non-portable compared to optical microscopes.
Scanning electron microscopes need space free from electromagnetic elements and vibrations.

The Bottom Line

Scanning electron microscopes are critical to studying the characteristics of compounds and biological samples. However, a common question regarding the working of SEMs is how a scanning electron microscope works. SEMS uses electron beams that fall onto the specimen and are detected by the detector. This article explained the working of scanning electron microscopes in detail, along with the applications and limitations of the microscope.