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The resolution of the scanning electron microscope: exploration in depth

In the world of science and technology, microscopes play a crucial role in the study of nano-sized samples. Among them, the scanning electron microscope (SEM) is particularly renowned for its ability to deliver high-resolution images. Let’s take a look at the specific features that influence this resolution.

General SEM operation

The basic operating principle of a SEM is to scan the surface of a sample with a beam of electrons. This beam, generated by an electron source, is focused and controlled by magnetic lenses. It then interacts with the sample, producing secondary signals such as backscattered electrons and secondary electrons. These signals are captured and processed to form a high-quality, highly accurate three-dimensional image of the sample surface.

Factors influencing SEM resolution

To understand the key elements of SEM resolution, several parameters need to be taken into account:

  • Electron beam size: the finer the beam, the better the resolution of the image obtained. To achieve this, we generally aim to use electron sources with small diameters.
  • Electron energy: The energy associated with the electrons, defined by their acceleration voltage, also has an influence on image resolution: a high-energy electron beam will deliver better resolution than a lower-energy beam.
  • Interaction with the sample: the way the electrons interact with the sample also has an impact on image quality. Depending on the type of sample and its components (material, density, conductivity), the interactions can be more or less complex and influence the final image resolution.

Spatial resolution and depth resolution

There are two types of SEM resolution:

Spatial resolution: this concerns the microscope’s ability to distinguish between two adjacent points located in the same plane. It mainly depends on the size of the electron beam and is expressed in nanometers (nm).
Depth resolution: refers to the microscope’s ability to distinguish between different planes, i.e. to recognize different strata according to their depth. This resolution is often poorer than spatial resolution, as it depends directly on electron energy and interactions with the sample.

Improving resolution with lenses

We recommend the use of high-quality magnetic lenses to enhance SEM resolution. These lenses act on the electron beam by adjusting its size, orientation and trajectory until it reaches the sample. The more precise these adjustments are, the better the resolution obtained. That’s why it’s important to ensure that the mechanical and software alignment of the column (lenses, condensers, diaphragms) is carried out regularly, for optimum resolution.

Limits to SEM resolution

Although the scanning electron microscope is an extremely powerful tool for visualizing nanometric structures, there are certain limits to its resolution. These include:

  • The diffraction limit: Like light waves, electrons are subject to diffraction, which means that their trajectories are altered when they pass through a small aperture (e.g. a diaphragm).
  • Magnetic lens aberrations: imperfections in magnetic lenses can distort the electron beam and cause focusing errors. This can degrade spatial resolution, although digital corrections are often used to minimize this effect.
  • Sample preparation: A poorly prepared sample can result in a poor image, even with a high-quality SEM. The sample surface must be clean and conductive to enable optimum interaction with the electron beam.
  • The inherent limitations of electrons: Electrons are negatively charged particles, which means they can interact with each other and create certain disturbances. So, even under the best possible conditions, the resolution of a SEM will always be limited by these interactions.
  • The acoustic and electromagnetic environment: this generates disturbances on the beam and the electron column that often degrade the expected resolution.

The importance of contrast

In addition to resolution, another crucial factor in obtaining quality images with a scanning electron microscope is contrast. Contrast represents the difference in intensity between two adjacent areas of the image, and also depends on several parameters:

  • The type of signal detected (secondary or backscattered electrons)
  • Interactions between the electron beam and the sample
  • The density and thickness of the materials making up the sample
  • The quality of the detection chain

Various techniques can be used to improve contrast and facilitate image interpretation. We recommend the use of specific detectors, digital processing or even the addition of coatings or grids on the sample.

Typical SEM resolution values

Modern scanning electron microscopes generally offer resolutions between a few nanometers (2-5 nm) and a few tenths of a nanometer (0.1-0.4 nm). However, it should be borne in mind that these values are highly dependent on the specific characteristics of each instrument and the conditions of use (including sample preparation, acoustic and electromagnetic environment).

In conclusion, the resolution of a scanning electron microscope is influenced by many factors, some of which are specific to the device used (beam size, magnetic lenses, etc.) and others by the characteristics of the sample under study. Although some inherent limitations remain, SEMs continue to deliver extraordinarily precise images, enabling scientists to explore the nanoscale world with great finesse.
If you’re looking for quality equipment, EMS is the partner you can trust. We sell and maintain reconditioned Thermofisher electron microscopes for research and characterization of advanced materials.

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