Principles of Scanning Electron Microscopy

As the name implies, electron microscopes employ an electron beam for imaging. In Fig.1, you can see the various products that are possible as a result of the interaction between electrons and matter. All these different types of signals carry different useful information about the sample and it is the choice of the microscope’s operator which signal to capture.

When high energy electrons reach the sample, several electron and X-ray signals are generated. These include:

• Backscattered electrons (BSE) 

  These are high energy electrons, scattered out of the sample and only losing a small amount of energy. They originate from deep within the sample (a few microns below the surface) and interact strongly with the sample. They provide compositional information and lower resolution images. Backscattered electrons are reflected back after elastic interactions between the beam and the sample.

• Secondary electrons (SE)

  These come from within a few nanometers of the sample surface, with a lower energy compared to the backscattered electrons. They are very sensitive to surface structure and provide topographic information. Secondary electrons, originate from the atoms of the sample: they are a result of inelastic interactions between the electron beam and the sample.

• X-rays

  These characteristic X-rays are produced when electrons hit the sample surface. They give information about the elemental composition of the sample.

BSE come from deeper regions of the sample, 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. SE imaging can provide more detailed surface information.

This type of electrons originates from a broad region within the interaction volume. They are a result of elastic collisions of electrons with atoms, which results in a change in the electrons’ trajectory. Think of the electron-atom collision as the so-called “billiard-ball” model, where small particles (electrons) collide with larger particles (atoms). Larger atoms are much stronger scatterers of electrons than light atoms, and therefore produce a higher signal (Fig.2). The number of the backscattered electrons reaching the detector is proportional to their Z number. This dependence of the number of BSE on the atomic number helps us differentiate between different phases, providing imaging that carries information on the sample’s composition. Moreover, BSE images can also provide valuable information on crystallography, topography and the magnetic field of the sample.

The most common BSE detectors are solid state detectors which typically contain p-n junctions. The working principle is based on the generation of electron-hole pairs by the backscattered electrons which escape the sample and are absorbed by the detector. The amount of these pairs depends on the energy of the backscattered electrons. The p-n junction is connected to two electrodes, one of which attracts the electrons and the other the holes, thereby generating an electrical current, which also depends on the amount of the absorbed backscattered electrons.

The BSE detectors are placed above the sample, concentrically to the electron beam in a “doughnut” arrangement, in order to maximize the collection of the backscattered electrons and they consist of symmetrically divided parts. When all parts are enabled, the contrast of the image depicts the atomic number Z of the element. On the other hand, by enabling only specific quadrants of the detector, topographical information from the image can be retrieved.

Secondary electrons (SE) in contrast, secondary electrons originate from the surface or the near-surface regions of the sample. They are a result of inelastic interactions between the primary electron beam and the sample and have lower energy than the backscattered electrons. Secondary electrons are very useful for the inspection of the topography of the sample’s surface, as you can see in Fig. 4.

The Everhart-Thornley detector is the most frequently used device for the detection of SE. 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, to increase the efficiency of detecting secondary electrons.

These two types of electrons are the most used signals by SEM users for imaging. Not all SEM users require the same type of information, so the capability of having multiple detectors makes SEM a very versatile tool that can provide valuable solutions for many different applications. It can help you save valuable time, improve image resolution, and even automate your analyses.

Electrons are negatively charged particles and travel through the electron column at high energy and high speed. One way to deflect these particles is to let them travel through an electric field generated by two plates at potential +U and -U, as shown in Fig. 1a.

Under the influence of the electric field, the electron is deflected at an angle that depends on the electron energy, the electric field applied in between the plates, and the length of the plates.

The faster, or the more energetic the electron, the smaller the deflection angle. The higher the electric field and the longer the plates, the bigger the deflection angle. A device consisting of two plates at different potential is called a deflector.

To get an electrostatic lens, one could think of mirroring the effect of a deflector, such that the outer electrons traveling off the optical axis can be focused on the same point, as schematically shown in Fig. 1b.

Starting from the fact that electric fields can only start and end on electron charges, how can we get a lens effect as that depicted in Fig. 1b? The answer to this question lies in the fact that whenever there is a lens effect, the energy of the beam changes, meaning that the electrons either accelerate or decelerate. This can be done simply by having an aperture on different potential around the beam.

Electrostatic lenses consist of metallic plates connected to high voltage with an aperture that the electron beam travels through. Single-aperture lenses consist of a single metallic plate at high voltage and can often be found in electron sources.

The single-aperture lenses can either terminate an accelerating field or be followed by an accelerating field. In the first case, the lens is positive, meaning that the beam converges into a crossover, as shown in Fig. 2a, while in the second case, the lens is negative, meaning that the beam diverges, as shown in Fig. 2b.

A two-aperture lens consists of two metallic plates at different potential with aligned apertures. Fig. 2c shows an accelerating two-aperture lens, where the electric field in between the two plates points at the top plate.

The electrons that enter this lens will feel a strong field that pushes them closer to the optical axis. As they travel through the second plate, the electrons feel an opposite force that pushes them towards the aperture. As a total effect, this is a positive lens and the beam is focused in a plane below the second plate.

A three-aperture Einzel lens consists of three plates with aligned apertures, that can either have the same diameter, or a different one. Einzel lenses are commonly used in electron optics for the advantage of having an equal beam potential at the entrance and exit of the lens.

In Fig 2d, an accelerating Einzel lens is shown. The three electrodes generate three lenses: the first and the third are positive, where the electric field lines point towards the plates, and the second is negative. The total lens is positive, and the beam is focused on a plane below the third lens.

Magnetic lenses use the Lorentz force, that is proportional to the electron charge and velocity, to deflect electrons. Magnetic lenses consist of a metallic body (called the ferromagnetic circuit) that ends with two pole pieces.

The magnetic field is given by a coil positioned at the top of the ferromagnetic circuit, as shown in Fig. 3. The strength of the lens can be altered by varying the magnetic field B. This is done by modifying the geometry of the pole piece, namely the distance between the pole pieces, and the current flowing into the coils (excitation).

The electron column consists of the electron source, where the electrons are emitted, and a set of lenses. The electrons are condensed into a beam by the condenser lenses and then focused onto the sample surface by the final lens, also called the objective lens, as shown. The source tilt and the scanning of the beam at the sample is done by coils at the source and right above the final lens.

All SEMs — whether we’re talking desktop or floor models — have an electron column with electrostatic lenses and magnetic lenses.

References

• Introduction to charged particle optics, P. Kruit, Delft University of Technology
• Scanning electron microscopy, physics of image formation and microanalysis, L. Reimer, Springer