Scanning Electron Microscopy (SEM) is an extremely useful technique used to obtain high-resolution images and detailed information of a samples’ surface. The typical resolution of SEM instruments ranges from < 1 nanometer to several nanometers. SEM uses a focused beam of electrons to examine the surface of a sample and analyzes the different signals emitted in a detector.
The following is an illustration of how
an SEM instrument works:
1) Electron generation.
A high-voltage electron gun (typically 1 keV – 30 keV) generates and accelerates a beam of electrons, typically using thermionic or field emission.
- Thermionic emission involves heating a thin tungsten filament to a high temperature (approximately 2,500°C), causing electrons to be emitted.
- Field emission is an advanced technology used to capture the microstructure image of a material. It is usually performed in a high vacuum because gas molecules tend to disturb the electron beam and the emitted secondary and backscattered electrons used for imaging. Field emission produce electron beams with the highest brightness and coherence which allows for the highest resolutions to be obtained.
2) Electron beam focusing
The electron beam generated is then focused on the sample using a series of electromagnetic lenses. These lenses are similar to optical lenses, but instead of glass lenses, they use magnetic fields to bend the electron beam.
Condenser lenses are used for this and are carefully aligned to ensure that the beam is focused to a small spot on the sample surface.
3) Scanning
The focused electron beam is scanned across the sample surface in a raster pattern, similar to how a television screen is scanned.
This is achieved by using scanning coils that deflect the beam in the x and y directions.
4) Interaction of electron beam and the sample
As the electron beam interacts with the sample, it results in a variety of signals being emitted. These signals include:
- Secondary electrons: Low-energy electrons that are emitted from the sample as a result of the primary electron beam striking the surface.
- Backscattered electrons: High-energy electrons that are reflected back from the sample. These are high-energy electrons used to obtain high-resolution images that show the distribution of various elements that make up a sample.
- Characteristic X-rays: X-rays that are emitted when the primary electrons knock out inner-shell electrons from the atoms in the sample. The energy difference between two electron shells is equal to the characteristic X-ray energy. Characteristic X-rays that result from inelastic scattering allow for exact identification of elements present in the sample.
- Auger electrons: These are emitted at discrete energies that are characteristic of the elements present on the sample surface. The characteristic energies of the Auger electrons are such that only the electrons from the outer 0.5 to 5 nm of the sample can escape and be detected.
5) Detection of emitted signal
The emitted signals are detected using various detectors, such as:
- Secondary electron detector: Detects secondary electrons, which are used to create an image of the sample's surface topography.
- Backscattered electron detector: Detects elastically scattered electrons, which can provide information about the sample's composition.
- X-ray detector: Detects characteristic X-rays, which can be used to determine the elemental composition of the sample.
- Auger electron spectrometer: Detects Auger electrons, which can be used to analyze the surface composition of the sample.
6) Formation of image
The detected signals are processed by a computer to create an image. The brightness of each pixel in the image corresponds to the intensity of the detected signal at that point on the sample.
SEM images can be displayed in various modes, such as bright field, dark field, and elemental mapping.
7) Analysis of image
The SEM images can be analyzed to deduce the quantitative information about the sample, like size, shape, and distribution of features.
This can be done using image analysis software or by manually measuring features on the image.
Application of SEM
a) Materials Science
- Microstructure analysis: SEM is widely used to study the microstructure of materials like metals, ceramics and polymers. By examining the arrangement of grains, defects, and inclusions, researchers can gain insights into the material's properties and behavior.
- Failure analysis: When a material or component fails, SEM can be used to investigate the root cause of the failure. By examining the fracture surface, researchers can identify the type of fracture (e.g., brittle, ductile, fatigue) and identify potential defects or flaws that may have contributed to the failure.
b) Biology and Medicine
- Cell imaging: SEM can provide
high-resolution images of cells and their organelles, allowing researchers
to study their structure, function, and interactions. For example, SEM can
be used to examine the surface morphology of bacteria, the structure of
viruses, and the ultrastructure of tissues.
- Tissue analysis: SEM can be used to
analyze the structure and composition of tissues, such as skin, bone, and
muscle. This can be helpful for diagnosing diseases, studying the effects
of treatments, and understanding biological processes.
c) Nanotechnology
- Characterization of
nanomaterials: SEM is a valuable tool for characterizing nanomaterials,
such as nanoparticles, nanotubes, and nanowires. It can be used to measure
the size, shape, and distribution of nanomaterials, as well as to study
their surface morphology and internal structure.
d) Other Fields
- Forensic science: SEM can be used for various forensic applications, such as analyzing fingerprints, examining crime scene evidence, and identifying fibers or hairs.
- Semiconductor manufacturing: SEM is essential for quality control in the semiconductor industry, as it can be used to inspect the surface features of wafers and devices.
- Environmental science: SEM can be used to study the morphology and distribution of pollutants in the environment, such as particulate matter and microplastics.
Advantages of SEM
- High Resolution: SEM can provide images with extremely high resolution, allowing for detailed examination of even the smallest structures.
- Depth of Field: SEM has a large depth of field thus it can focus on a wide range of depths simultaneously, making it ideal for imaging rough or uneven surfaces.
- Three-Dimensional Imaging: SEM can create 3D images of samples, providing valuable insights into their structure and morphology.
- Versatile Applications: SEM can be used to study a wide range of materials and samples, from metals and ceramics to biological tissues and nanomaterials.
- Quantitative Analysis: SEM can be used to obtain quantitative information about samples, such as the size, shape, and distribution of features.
Disadvantages of SEM
- Sample Preparation: SEM requires careful
sample preparation, often involving coating the sample with a conductive
material to prevent charging. This can be time-consuming, expensive and may introduce
artifacts.
- Vacuum Environment: SEM operates in a vacuum
environment, which can limit the types of samples that can be examined.
Some samples may degrade or be altered under vacuum conditions.
- Cost: SEM equipment can be
expensive to purchase and maintain.
- Image Interpretation: Interpreting SEM images
can be challenging, especially for complex samples or when dealing with
unfamiliar materials.
- Limited Penetration Depth: SEM is primarily a
surface imaging technique, with limited penetration depth. This can be a
limitation for studying the internal structure of thick or opaque samples.
FURTHER READING
- Goldstein, D., & Newbury, D. (2012). Scanning electron microscopy: A beginner's guide. Springer Science & Business Media.
- Scott, J. F., & Joy, D. N. (2017). Scanning electron microscopy: Techniques and applications. Springer Science & Business Media.
- Scott, J. F., & Joy, D. N. (2019). Practical scanning electron microscopy. Springer Science & Business Media.
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