Scanning electron microscopy (SEM) has become a powerful and versatile tool for material characterisation. This is especially so in recent years, due to the continuous shrinking of the dimension of materials used in various applications. 

What is SEM?
SEM stands for scanning electron microscope. As the name suggests, electron microscopes use electrons for imaging, in a similar way that light microscopes use visible light. The optimal resolution of an imaging instrument depends mainly on the wavelength of the medium. Since the wavelength of electrons is much smaller than the wavelength of light, the resolution of an electron microscope is superior to that of a light microscope. In fact, it is usually more than 1,000 times better.

There are two main types of electron microscopes:

  1. The transmission electron microscope (TEM), which detects electrons that pass through a very thin specimen;
  2. The scanning electron microscope (SEM), which uses the electrons that are reflected or knocked off the near-surface region of a sample to create an image.

How does SEM technology work?
A schematic representation of the technology of a SEM is shown in Figure 1 below. In this type of electron microscope, the electron beam scans the sample in a raster pattern. But first, electrons are generated at the top of the column by the electron source. These are emitted when their thermal energy overcomes the work function of the source material. They are then accelerated and attracted by the positively-charged anode. You can find a more detailed description of the different types of electron sources and their characteristics in this guide.

Figure 1: schematic representation of the basic SEM components

The entire electron column needs to be under vacuum. Like all the components of an electron microscope, the electron source is sealed inside a special chamber in order to preserve vacuum and protect it against contamination, vibrations or noise. Although the vacuum protects the electron source from being contaminated, it also allows the user to acquire a high-resolution image. In the absence of vacuum, other atoms and molecules can be present in the column. Their interaction with electrons causes the electron beam to deflect and reduces the image quality. Furthermore, high vacuum increases the collection efficiency of electrons by the detectors that are in the column.

How is the path of electrons controlled?
In a similar way to optical microscopes, lenses are used to control the path of the electrons. Because electrons cannot pass through glass, the lenses that are used here are electromagnetic. They simply consist of coils of wires inside metal pole pieces. When current passes through the coils, a magnetic field is generated. As electrons are very sensitive to magnetic fields, their path inside the microscope column can be controlled by these electromagnetic lenses - simply by adjusting the current that is applied to them. Generally, two types of electromagnetic lenses are used:

The condenser lens is the first lens that electrons meet as they travel towards the sample. This lens converges the beam before the electron beam cone opens again and is converged once more by the objective lens before hitting the sample. The condenser lens defines the size of the electron beam (which defines the resolution), while the main role of the objective lens is to focus the beam onto the sample.

The scanning electron microscope’s lens system also contains the scanning coils, which are used to raster the beam onto the sample. In many cases, apertures are combined with the lenses in order to control the size of the beam. These main components of a typical SEM instrument are shown in Figure 1.

What kind of electrons are there?
The interaction of electrons with a sample can result in the generation of many different types of electrons, photons or irradiations. In the case of SEM, the two types of electrons used for imaging are the backscattered (BSE) and the secondary electrons (SE).

Backscattered electrons belong to the primary electron beam and are reflected back after elastic interactions between the beam and the sample. On the other hand, secondary electrons originate from the atoms of the sample: they are a result of inelastic interactions between the electron beam and the sample.

BSE come from deeper regions of the sample (Figure 2), while SE originate from surface regions. Therefore, BSE and SE carry different types of information. BSE images show high sensitivity to differences in atomic number: the higher the atomic number, the brighter the material appears in the image.

Figure 2: Different types of signals used by a SEM and the area from which they originate

SE imaging can provide more detailed surface information — something you can see in Figure 3. In many microscopes, detection of the X-rays, which are generated from the electron-matter interaction, is also widely used to perform elemental analysis of the sample. Every material produces X-rays that have a specific energy; X-rays are the material’s fingerprint. So, by detecting the energies of X-rays that come out of a sample with an unknown composition, it is possible to identify all the different elements that it contains.

Figure 3: a) BSE and b) SE image of the FeO2 particle

How are electrons detected?
The types of electrons mentioned above are detected by different types of detectors. For the detection of BSE, solid state detectors are placed above the sample, concentrically to the electron beam, in order to maximise the BSE collection.

On the other hand, for the detection of SE, the Everhart-Thornley detector is mainly used. It consists of a scintillator inside a Faraday cage, which is positively charged and attracts the SE. The scintillator is then used to accelerate the electrons and convert them into light before reaching a photomultiplier for amplification. The SE detector is placed at the side of the electron chamber, at an angle, in order to increase the efficiency of detecting secondary electrons. These secondary electrons are used to form a 3D-image of the sample, which is shown on a PC monitor.

SEM: magic but meticulous
As you can see, there are different processes that the electrons must go through before an image can be shown on your monitor - Figure 4. You don’t have to wait for the electrons to finish their journey; the whole process is almost instantaneous, in the range of nanoseconds (10-9 seconds). Every “step” of an electron inside the column needs to be pre-calculated and controlled with precision in order to obtain a high-quality image. Scanning electron microscopes are continuously improved, and new applications are still arising, making them fascinating instruments with lots of undiscovered capabilities.

Figure 4: Backscattered electron image of Tungsten particles

Click here to take a closer look at our SEMs and you will realise that they have a multitude of interesting specifications worth investigating, like their advanced light and electron optical magnification, resolution and digital zoom.

Topics: Electrons, EDX/EDS Analysis, Scanning Electron Microscope