SEM analysis is short for Scanning Electron Microscopy analysis. This spectroscopy technique is a form of high-resolution surface imaging that uses the principle of light microscopy. SEM analysis scans the sample being tested with a focused electron beam to produce a high-resolution image of its surface.

SEM analysis is fast, non-disruptive, and extremely accurate, making it one of the most commonly used high-resolution surface imaging techniques used today.  It is a crucial way to understand a sample surface’s topography and composition.

How Does SEM Analysis Work?

SEM uses a vacuum chamber to prevent any molecules and atoms already present in the microscope column from interacting with the electron beam and potentially distorting the image it produces. This ensures a high level of quality and accuracy of imaging.

Before testing can start, the sample being analyzed has to be treated so it will react to the vacuum chamber properly. All water must be removed from the sample so it won’t vaporize in the vacuum. Any non-metal sample has to be made conductive by coating it with a thin layer of gold foil material and then using a device called a “sputter coater” on it.

When the sample is prepared, the microscope can be activated. SEM microscopes work by attracting electrons down the instrument in several stages to focus and direct it. These stages include:

  1. The electron source is located at the top of the microscope. When the microscope is activated, this generates the electrons that will be used for the beam.
  2. The anode is located directly beneath the microscope. The anode is a metal plate that contains a positive charge. It attracts electrons down from the source, shaping them into the beginnings of the beam in the process.
  3. The condenser lens controls the size and number of electrons in the beam. Analysts adjust the condenser lens during imaging to define the resolution of the final image.
  4. Apertures, which also help control the size of the beam.
  5. Scanning coils, which deflect the electron beam across x and y axes to ensure it scans the entirety of the surface of the sample for an accurate image.
  6. The objective lens is the final lens the beam passes through before encountering the sample. This lens focuses the beam on a very small spot on the sample.

When the beam interacts with the sample, it generates two different types of electrons that are both used to produce the final image: backscattered electrons (BSEs) and secondary electrons (SEs).

BSEs are reflected up toward the beam’s origin when the primary electron beam interacts with the sample directly. These are recorded by a back-scattered electron detector located just beneath the objective lens. SEs are reflected throughout the vacuum chamber, where they are recorded by a secondary electron detector that is positively charged to attract them.

The BSE and SE collectors placed inside the vacuum sample collect their respective electrons and convert them into signals similar to the signals sent to television screens. These signals produce the final image.

BSEs come from deeper within the sample. The images they produce show up brighter or darker in specific, observable ways that equate to the presence of atomic numbers, and show compositional differences. This allows analysts to understand what elements the sample consists of. SEs come from the surface of the sample but are capable of being magnified further and processed into more detailed images. They help analysts see the surface topography of the sample in great detail.

What Are the Different Kinds of SEM Analysis?

There are several different forms of SEM analysis. Each uses the same basic principle, but the steps of the process and their specializations differ.

  • Conventional SEM places the sample into a high vacuum chamber and requires it to be specially coated before the analysis can be performed. Conventional SEM uses a tungsten filament in its electron light source and produces moderate magnifications of around 20,000 times.
  • Variable SEM (VPSEM) allows analysts to manually vary the pressure applied to the sample while it’s inside the high vacuum chamber. Unlike conventional SEM, variable SEM doesn’t require the sample to be specially coated before scanning. It can also be used to image wet and nonconductive samples. VPSEM instruments can produce moderately high magnifications of 50,000 times.
  • Field Emission SEM (FESEM) uses a field emission gun to generate a powerful electric field to pull electrons away from their atoms and generate high-resolution images. This form of SEM results in a brighter electron beam and smaller beam size than conventional SEM or VP SEM, which increases its useful range for magnification up to 500,000 times.

See a detailed comparison of the three types of SEM here. 

What is SEM Analysis Used For?

The high-resolution, detailed imagery SEM analysis provides is useful for a wide range of practical applications, including:

  • Local composition analysis: BSEs produced and recorded during SEM analysis help the SEM machine provide a mapping of the sample’s elemental composition.
  • Surface topography analysis: The SEs produced and recorded during SEM analysis provide a microscopically detailed mapping of the sample’s surface. This mapping is used for fractography analysis, in which experts closely examine fractures on a sample’s surface to determine their root cause. Fractography is highly useful for failure analysis in many industries.
  • Microstructure characterization: The surface analysis provided by SEM analysis allows analysts to see the microstructures present on a sample material’s surface and evaluate how they will affect the material’s interactions and uses.  In particular, electron backscatter diffraction (EBSD) can be used to look at the grain size and orientation.
  • Chemical analysis: Most SEMs are equipped with energy dispersive spectroscopy (EDS) detectors which can identify the chemistry of phases based on chemical analysis. This helps experts identify impurities and gradients within the sample.
  • Coating analysis: By studying the images produced by BSEs and SEs, analysts can use SEM analysis to understand both what a sample is coated with and how that coating may affect its function.

SEM analysis is very commonly used for quality assurance, material characterization, failure analysis, textural analysis, and topographical analysis in  business and scientific industries including:

  • Electronics manufacturing, including microchip assembly
  • Metals
  • Medical device manufacturing
  • Aerospace engineering
  • Chemical engineering
  • Rock and soil sampling
  • Medical science
  • Forensic analysis
  • and more

What Makes SEM Analysis So Effective?

SEM analysis has several distinct advantages over many other types of topographical analysis that have helped it become the gold standard for business and scientific surface analysis, including:

  • 3D imaging: Unlike optical magnification, SEM actually generates a 3D, topographical image of the sample’s surface, providing considerably more detail and information to analysts.
  • Breadth: Between the different forms of analysis, a huge variety of materials can be successfully characterized and analyzed using SEM.
  • Standardization: All imaging produced by SEM is calibrated to a traceable standard, making it a very easy technique to use for a wide variety of different analyses, from checking coating thickness to determining the grain size of a material and beyond
  • Resolution: Compared to conventional microscopy, SEM is considerably more powerful. The digital images it produces are also extremely high-resolution and accurate.
  • Chemical analysis: Unlike many other forms of spectroscopy, SEM can be relied upon to provide local elemental and chemical information on a sample’s structure at the same time as it produces topographical data.

If you think SEM analysis may be right for your needs, the experts at Secat are ready to help. Get in touch with our experienced SEM technicians today.