SEM – Scanning Electron Microscopy

Introduction to Scanning Electron Microscopy (SEM) 

Scanning electron microscopy (SEM) is a widely used technique for observing the surface morphology of materials. SEM is employed to analyze surface fractures, surface contaminants, microstructures, crystalline structures, and spatial variations in chemical compositions of material samples. Scanning electron microscopy analysis, is a widely used method for analyzing organic and inorganic materials on a nanometer to micrometer scale across various disciplines worldwide. An SEM image produced from the intensity of back-scattered electrons and the beam position shows the distribution of different elements in the test sample. Heavier elements reflect more electrons and appear brighter in the image, so back-scattered electrons can show contrasts in chemical composition.

SEM provides information about the sample elements’ composition through dispersive X-ray spectroscopy, which enables the chemical analysis and mapping of the tested sample. In this procedure, sample preparation is performed by coating the conductive material with a thin layer to enhance the electrical conductivity. MaTestLab is one of the best testing service providers, with the best network of testing laboratories in the USA to conduct Scanning Electron Microscopy tests for our clients. 

Principle and Methodology of SEM

SEM uses a focused beam of electrons, which is reflected or knocks off the surface or near-surface of the sample to generate high-resolution images. The electron beam interacts with atoms at different depths within the sample to produce different signals, including secondary electrons, back-scattered electrons, and characteristic X-rays; each of these signals are detected by different detectors installed in a scanning electron microscope. Different Scanning Electron Microscopy images can be produced based on the type of detector being used, and these images can be used to see what the sample looks like on the nanoscale. Secondary and backscattered electron detectors are used to capture these interactions, which in turn visualize the morphological and topographical information about the sample. Backscattered electron images are also used for the rapid discrimination of the phases in multiphase samples.

Instrumentation

SEM is comprised of various components, such as an electron source, which is a tungsten filament, or a gun that emits a field. A beam of electrons is emitted, which then works as a probe for the formation of images. Electromagnetic lenses are used for the manipulation of the electron beam. The sample stage holding the specimen is responsible for precise image formation and manipulation. Several signals generated by the interaction of electrons with the sample surface are detected by a detector. Electroinc signals generated are responsible for controlling the process. 

Uses of Scanning Electron Microscopy

SEM is successfully used to identify cracks, imperfections, and contaminants on the surface of coated products, analyze particle size and shape in cosmetic formulations, assess nanoparticles in coatings and paints, topographically analyze semiconductor wafers, fail analysis of integrated circuit boards and analyze gunshot residue for forensic investigation.

Scanning Electron Microscopy is also used to structurally analyze new species of microscopic organisms, such as bacteria and viruses, and to test new vaccinations and medicines.

Strengths of Scanning Electron Microscopy (SEM) Technique

This easy-to-operate technique offers easy sample preparation steps as the sample does not need to be thin. In this technique with user-friendly interfaces, data acquisition is rapid and in the digital form where high resolution (up to 15 nanometers) and three-dimensional (3D) images are obtained.

Limitations of Scanning Electron Microscopy (SEM) Technique

This technique needs a solid sample that fits into the microscope chamber. Electrically insulating samples need the application of an electrically conductive coating. The EDS detector on SEM fails to detect very light elements (H, He, and Li), and many instruments cannot detect elements with atomic numbers less than 11. SEM instruments need to be placed in an area that is free of any possible magnetic, electric, or vibration interferences.

Related Techniques

Transmission electron microscopy (TEM) and atomic force microscopy (AFM) provide similar data about a sample. In the testing labs, an energy-dispersive X-ray Spectroscopy (EDS) detector within SEM detects the characteristic X-rays and provides qualitative and quantitative elemental analysis of the sample.

FIB-SEM is another variant of SEM, which provides 2D elemental mapping and 3D reconstruction of samples. Another variant, environmental SEM (ESEM), is available for analyzing samples containing water or other volatile substances. An ESEM uses a scanned electron beam and electromagnetic lenses to focus and direct the beam onto the specimen surface like a traditional SEM does.