Scanning Electron Microscope


What is electron microscopy? Electron microscopes use a focused electron beam to visualize a sample, much like an optical microscope uses visible light. Since the wavelength of electrons is much smaller than that of light, the resolution of SEMs is superior to that of a light microscope, even down to the nanometer range.
All forms of electron microscopy use electromagnetic lenses instead of visual optics. The condenser lenses reduce the diameter of the beam, while the objective lens focuses the electron beam on the sample to create the image. The electrons in the beam are accelerated: a higher acceleration voltage results in a higher resolution, but it may damage fragile samples like biological samples or very thin materials, like graphene. By using different techniques, our microscopes can combine low voltage with high resolution.
There are two main types of electron microscopy. The first is transmission electron microscopy (TEM), in which the electrons pass through a thin sample, where they are scattered by the interactions with the atoms in the sample. The second is Scanning Electron Microscopy (SEM): here, an electrons beam is used to scan the surface of a sample by detecting reflected electrons, and in some cases photons.

Table Of Contents

How does Scanning Electron Microscopy work?

Beam production

 

To scan a sample the electron beam is directed across the sample in a raster pattern, while the reflected electrons are continuously detected. Specialized software combines the intensity acquired by the detector with the position of the beam to reconstruct a gray-scale image.

The beam is produced by voltage heating an electrode (the filament), which will then emit electrons. This process is called thermionic emission. Common filaments are made of tungsten or solid-state lanthanum hexaboride crystals. Alternatively, a field emitter gun can be used. In this case, a strong electric field is applied to the narrow tip of a filament. This reduces the potential barrier which keeps the electrons inside the material, and releases them into the vacuum by quantum tunneling. The small tip results in a more narrow beam than can be accomplished by thermionic emitters, which can reveal finer details in the sample.

Imaging: Backscattered Electrons

 

A Scanning Electron Microscope can both image and analyze samples. Backscattered electrons (BSE) are mainly used to create an image. The negatively charged electrons from the SEM beam interact with positive nuclei in the near-surface region. The nuclei attract the electrons, but do not capture them: electrons follow a ‘sling shot’ trajectory, based on the weight of the nucleus. The returning BSE’s are then observed with detectors. Heavier nuclei will interact more strongly with the beam electrons than lighter ones, and thus appear brighter on an image. BSE’s therefore also provide some information about the chemical composition of a surface.

 

Imaging: Secondary Electrons

 

Alternatively, secondary electrons can be measured. This type of electrons is generated by inelastic interactions of the primary electron beam with surface atoms of the sample. They therefore provide topographical information of the surface area, but contain no information about the composition of the sample. However, they do provide a very high resolution, and can reveal details of under 0.5 nanometer. Secondary electrons have lower energy and can be analyzed separately from BSE’s.

 

Imaging: Energy-dispersive x-rays

 

Apart from electrons, the interaction of the electron beam with the sample can produce energy-dispersive X-rays (EDX/EDS). This happens when the incoming electrons transfer energy to an atom in the sample, which sends an electron to a higher orbital. When the electron returns to its ground state, an X-ray photon is emitted with an energy that is specific to the element. Thus, an EDS spectrum contains information on the different elements in a sample, as well as their relative abundance.

Britannica - Scanning Electron Microscope

Why use a Scanning Electron Microscope?

As electrons have a much shorter wavelength than visible light, they can resolve much smaller details in the sample. Thus, Scanning Electron Microscopy allows you to see the smallest structures, even up to atomic resolution. The absence of colour information can be compensated by being able to derive the identity of the atoms in the sample. The technique also allows you to see a large part of your sample at low magnification and zoom in to study details at high magnification. Through EDS spectra, the composition can be analyzed.

 

As the electron beam will penetrate the sample (with a depth depending on the acceleration of the electrons), a SEM can provide information about the surface region, rather than just the top layer. This makes Scanning electron microscopy is the preferred method for the study of surfaces.

Strengths and Limitations of Scanning Electron Microscopy (SEM)

Limitations

A scanning electron microscope works in a vacuum from beam production to sample. The samples must be compatible with these circumstances. If necessary, low vacuum conditions may be used. Samples also need to be conducting, although strong insulators can be coated with a conductors such as gold. However, such sample preparation might produce artifacts. Especially biological samples can be damaged by a high velocity electron beam. Again, low-voltage conditions are often possible.

 

Strengths

As mentioned before, SEM has a higher resolution than light microscopy, due to the shorter wavelength of the electrons. Furthermore, the depth of field is much wider in an scanning electron microscope than in a light microscope. Also, SEM can provide information about the chemical composition of a sample.

How much does a Scanning Electron Microscope cost?

ST Instruments offers a wide variety of solutions to make (sub-)nanometer details visible. SEM’s are available in a range of desktop SEMs to top-of-the-line systems with different beam emitters, and prices are completely dependent on  the system configuration.

Applications for Scanning Electron Microscopy

SEM / TEM asbestos fiber analysis

In the past, asbestos was commonly used as building material because of its strong mechanical qualities and thermal insulating properties. Research demonstrated that asbestos can cause serious health issues and for that reason, this mineral was banned from construction sites in the mid-nineties. Many building materials are tested for asbestos to ensure a safe working environment. SEM’s and TEM’s equipped with EDS are deployed to analyse and detect the microscopic asbestos fibres.

SEM nanoparticle analysis

Nanoparticle analysis is a common field of interest in many disciplines such as material science, life science and semiconductor industry. With a Scaning Electron Micoroscope it is possible to obtain high-resolution images of individual nanoparticles and observe the surface. In addition, particle distribution characterization and counting are available.

Sample preparation

Many samples are cut and polished before SEM imaging. For optimal surface or cross-section analysis an ion-miller is beneficial. An ion-beam ejects atoms from the surface of a sample without mechanical stress. This leaves a perfectly polished surface, which is ideal for many damage-sensitive materials.

 

SEM image of asbestos fibers

SEM visualization of gold nanoparticles

Ion milling

The elemental composition of these particle is confirmed by EDX