The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals that derive from electron-sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure, and orientation of materials making up the sample. Areas ranging from approximately 1 cm to 5 microns in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20X to approximately 30,000X, the spatial resolution of 50 to 100 nm).
The SEM is also capable of performing analyses of selected point locations on the sample; this approach is especially useful in qualitatively or semi-quantitatively determining chemical compositions (using EDS), crystalline structure, and crystal orientations (using EBSD). The design and function of the SEM are very similar to the EPMA and considerable overlap in capabilities exists between the two instruments.
How does SEM work?
The main SEM components include:
Source of electrons
Column down which electrons travel with electromagnetic lenses
Electron detector
Sample chamber
Computer and display to view the images
Electrons are produced at the top of the column, accelerated down, and passed through a combination of lenses and apertures to produce a focused beam of electrons that hits the surface of the sample. The sample is mounted on a stage in the chamber area and, unless the microscope is designed to operate at low vacuums, both the column and the chamber are evacuated by a combination of pumps. The level of the vacuum will depend on the design of the microscope.
The position of the electron beam on the sample is controlled by scan coils situated above the objective lens. These coils allow the beam to be scanned over the surface of the sample. This beam scanning enables information about a defined area on the sample to be collected. As a result of the electron-sample interaction, a number of signals are produced. These signals are then detected by appropriate detectors.
Sample-Electron Interaction
SEM produces images by scanning the sample with a high-energy beam of electrons. As the electrons interact with the sample, they produce secondary electrons, backscattered electrons, and characteristic X-rays. These signals are collected by one or more detectors to form images which are then displayed on the computer screen. When the electron beam hits the surface of the sample, it penetrates the sample to a depth of a few microns, depending on the accelerating voltage and the density of the sample. Many signals, like secondary electrons and X-rays, are produced as a result of this interaction inside the sample.
The maximum resolution obtained in an SEM depends on multiple factors, like the electron spot size and interaction volume of the electron beam with the sample. While it cannot provide atomic resolution, some SEMs can achieve resolution below 1 nm. Typically, modern full-sized SEMs provide a resolution between 1-20 nm whereas desktop systems can provide a resolution of 20 nm or more.
Where does SEM help?
The SEM is routinely used to generate high-resolution images of shapes of objects (SEI) and to show spatial variations in chemical compositions:
Acquiring elemental maps or spot chemical analyses using EDS,
Discrimination of phases based on mean atomic number (commonly related to relative density) using BSE
Compositional maps based on differences in trace element "activitors" (typically transition metal and Rare Earth elements) using CL.
The SEM is also widely used to identify phases based on qualitative chemical analysis and/or crystalline structure. Precise measurement of very small features and objects down to 50 nm in size is also accomplished using the SEM. Back Scattered Electron images (BSE) can be used for rapid discrimination of phases in multiphase samples. SEMs equipped with diffracted backscattered electron detectors (EBSD) can be used to examine micro fabric and crystallographic orientation in many materials.
Material Science - In modern material science, investigations into nanotubes and nanofibers, high-temperature superconductors, mesoporous architectures, and alloy strength, all rely heavily on the use of SEMs for research and investigation.
Nanowires for Gas Sensing - Researchers are exploring new ways nanowires can be used as gas sensors by improving existing fabrication methods and developing new ones.
Semiconductor Inspection - Reliable performance of semiconductors requires accurate topographical information. The high-resolution three-dimensional images produced by SEMs offers a speedy, accurate measurement of the composition of the semiconductor.
Microchip Assembly - As the Internet of Things (IoT) becomes more prevalent in the day-to-day lives of consumers and manufacturers, SEMs will continue to play an important role in the design of low cost, low power chipsets for non-traditional computers and networked devices.
Forensic Investigations - Criminal and other forensic investigations utilize SEMs to uncover evidence and gain further forensic insight. Uses include:
Analysis of gunshot residue
Jewellery examination
bullet marking comparison
Handwriting and print analysis
Examination of banknote authenticity
Paint particle and fiber analysis
Filament bulb analysis in traffic incidents
Biological Science - SEMs can be used on anything from insects and animal tissue to bacteria and viruses. Uses include:
Measuring the effect of climate change on species
Identifying new bacteria and virulent strains
Vaccination testing
Uncovering new species
Work within the field of genetics
Soil and Rock Sampling - Backscattered electron imaging can be used to identify compositional differences, while the composition of elements can be provided by microanalysis. Valid uses include:
Identification of tools and early human artifacts
Soil quality measurement for farming and agriculture
Dating historic ruins
Forensic evidence is soil quality, toxins, etc.
Medical Science - SEMs are used in medical science to compare blood and tissue samples in determining the cause of illness and measuring the effects of treatments on patients (while contributing to the design of new treatments). Common uses include:
Identifying diseases and viruses
Testing new vaccinations and medicines
Comparing tissue samples between patients in a control and test group
Testing samples over the lifespan of a patient
Art - Micrographs produced by SEMs have been used to create digital artworks. High-resolution three-dimensional images of various materials create a range of diverse landscapes, image subjects are both alien and familiar.
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