Spot size in scanning electron microscopy (SEM): why it matters!

Scanning electron microscopes have emerged as a very valuable characterization method in recent years, following the major technological developments and the continuous shrinking of material dimensions. SEMs are versatile tools that allow users to perform many different types of analyses on a wide range of materials and to achieve the best results, users should carefully select the main microscope settings. One of those settings is the spot size, i.e. the diameter of the probe at the sample. In this blog, I explain how to adjust the spot size in a SEM — and how to achieve the right balance between high-resolution imaging and a high beam current to get the results you’re looking for.

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How to adjust the spot size in a scanning electron microscope

The four major parameters that describe the beam properties in a SEM are shown in here. Here is the list:

1. The voltage at which the electrons are being accelerated as they travel through the electron column
2. The convergence angle of the electron-beam cone
3. The beam current that hits the sample
4. The diameter of the final beam spot onto the sample — the spot size.

In any modern scanning electron microscope, the user has the ability to control the size of the electron probe. This is mainly achieved by adjusting the condenser and the objective lenses of the system and by selecting different apertures.

Electrons are flowing through electromagnetic lenses (which simply consist of coils of wires inside metal pole pieces)  and the user is able to control the electron’s path by tuning the current that is applied to the lenses. Moreover, the spot size is dependent on the acceleration voltage (high accelerating voltages decrease the spot size), the working distance (the larger it is, the larger the spot size becomes), and the objective lens aperture (smaller apertures create spots of a smaller diameter).

However, the size of the final electron probe is a parameter that is far more complex to control and predict, as it depends on many (and interconnected) factors. The relation that describes the spot size has terms that depend on the Gaussian diameter of the gun, the diffraction effect of the final aperture, the chromatic aberration, and the spherical aberrations of the beam-forming lens.

If we take a look at Figure 1 again, it will seem trivial that in order to have a small probe and sufficient current on the sample, the user simply needs to increase the convergence angle of the probe. This will, however, increase the aberrations of the optical components in the microscope and therefore broaden the beam. It is therefore evident, that in order to perform an experiment with accuracy, it is important to understand how different parameters influence the characteristics of the electron beam and identify the trade-offs between them.

High-resolution imaging vs. high beam current

The major factor that affects resolution is spot size. To acquire a high-resolution image, the spot size should be kept as small as possible in order to be able to resolve and describe even the smaller features of the specimen sufficiently.

On the other hand, it is also important that the beam carries enough beam current for sufficient signal-to-noise ratio (S/N) and contrast resolution. Since reducing the spot size also decreases the beam current, users need to identify and select the settings that will best fit their goal.

In general, if high magnification images are needed, the spot size should be kept minimal. If the user only requires low magnification imaging, then it is recommended that the spot size is increased so that the images have more “electron juice” and look sharper.

In Figure 2, you can observe that images acquired at low magnification but with a larger spot size seem brighter and smoother. However, as the magnification increases, the user should switch to the smaller spot size, which gives better results when high-resolution imaging is required.

Also, broader spot sizes — and consequently higher beam currents — increase damage to the sample, something that should be taken into account, especially when beam-sensitive samples are to be imaged.