Key Components of a Scanning Electron Microscope

A scanning electron microscope (SEM) is a sophisticated analytical instrument that far outperforms conventional light microscopy.

A compound microscope's standard array of magnifying lenses allows for sample magnification of up to 1000x using visible wavelengths of light in the 400 – 700 nanometer (nm) range. Such distance allows researchers and analysts to optically resolve specific spots in a specimen no more than 200nm apart. However, they’re unable to recognize topographical characteristics in closer proximity than this lower detection range at a reliable degree of certainty.

As the requirement for nanoscale material characterization and elemental topography assessments became more prominent worldwide, the wavelength range of traditional microscopy became an unfortunate constraint. As a result, SEM products were created, resulting in unique methods of sample imaging via electron scanning.

In this blog post, we will explore the key components required for an SEM to operate.

Electron Source

The electron source produces the steady stream of electrons necessary for an SEM to operate. These electrons generated through thermionic heating are accelerated to voltages ranging from 1 to 40 kV and then condensed into a narrow beam for imaging and in-depth analysis.

There are three standard types of electron sources, namely:

• Tungsten Filament
• Solid-state crystal (Cerium hexaboride or Lanthanum hexaboride)
• Field Emission Gun

Tungsten (W) Electron Filament

This electron source consists of a 100 µm-long, inverted V-shaped tungsten wire heated resistively to generate electrons. It’s the most basic kind of electron source.

Lanthanum hexaboride (LaB?) / Cerium hexaboride (CeB?)

The next type of electron source is a thermionic emission gun. It’s the most typical high-brightness source available. This solid-state crystal source is approximately 5-10 times brighter and has a much longer lifespan than tungsten.

Field Emission Gun (FEG)

The FEG electron source is a tungsten wire with a very sharp tip (less than 100 nm) that generates an electron beam via field electron emission. The small tip radius optimizes emission and focusing ability.

Lenses

SEMs, like optical microscopes, use lenses to produce sharp, detailed images. However, SEM lenses function differently. They're not made of glass, for starters. Instead, they're made of magnets capable of bending the electrons' paths. This way, the lenses focus and regulate the electron beam, ensuring that the electrons end up exactly where they need to go.

Scanning Coils

Scanning coils use fluctuating voltage to generate a magnetic field that allows them to manipulate the electron beam. The scanning coils can precisely move the beam back and forth over a designated section of an object. If researchers want to zoom in on an image, they simply need to direct the electron beam to scan a specific area of the sample.

After the beam is focused, scanning coils are used to deflect it in the X and Y axes, causing it to scan in a raster pattern across the sample's surface.

Sample Chamber

The sample chamber of an SEM holds the specimen under examination. Since the specimen must be kept perfectly still for the microscope to generate high-quality images, the sample chamber must be extremely sturdy and free from vibration. Indeed, SEMs are very sensitive to vibrations that they're frequently installed on a building’s ground floor.

An SEM's sample chamber does more than just hold a specimen still. They also manipulate the specimen, positioning it at various angles and repositioning it so that researchers don't have to remount the object repeatedly to take different images.

Detectors

These devices detect the different ways electron beams interact with the sample specimen. The various types of SEM detectors are as follows:

Everhart-Thornley Detectors (ETD)

The Everhart-Thornley Detector is the most common SEM detector (often seen as E-T or ETD). This type of detector gathers secondary electrons leading to information about sample topography. It also collects a combination of secondary and backscattered electrons with a broad-angle of collection. This allows the capturing of a signal from various angles between the sample and the detector.

Backscattered Electron Detectors (BSE)

In contrast to low energy SE that must be drawn to a detector, BSE detectors can utilize the higher energy, the relatively straight trajectory of the BSE, and particular proximity to samples.

This increases the solid angle of collection relative to ETD detectors. In effect, these detectors only detect higher energy BSE and do not capture lower energy SE. Dedicated solid-state BSE detectors are frequently mounted or inserted right below the objective lens and usually feature an annular (ring) structure that is further divided into different sections or arrays.

The notable difference between the two detector types are:

• Everhart-Thornley detectors detect secondary electrons, which are electrons that have become dislodged from a specimen's outer surface. These detectors can construct the most detailed images of an object's surface.
• Backscattered electron and X-ray detectors can provide researchers with information about the composition of a substance.

Vacuum Chamber

SEMs require a vacuum to function. The electron beam emitted by the electron gun would be constantly interfered with by air particles in the atmosphere if there's no vacuum.

These particles obstruct the electron beam’s path and get knocked out of the air straight into the specimen, distorting the specimen's surface.

Key Takeaways

The electron column, specimen chamber, and computer control system make up the main components of a scanning electron microscope. They are utilized to perform diverse functions of microscopy and microchemical analysis.

Secondary and backscattered electron detectors, energy-dispersive x-ray spectrometer (EDS), a low vacuum detector, an electron backscattered diffraction (EBSD) detector, and other devices also make up SEM instrumentation.

While some aren't required for basic imaging, they play an increasingly relevant role in more demanding microscopy applications.

Like other equipment, an SEM product is more than the sum of its parts. To utilize it to the fullest, one should learn how these components work together to produce astounding images of extremely tiny objects.