A scanning electron microscope (SEM) creates an image by scanning a focused beam of electrons across a surface. The electrons in the beam interact with the sample and generate a variety of signals that can be utilized to deduce surface topography and composition. Without the use of any extra lenses, the human eye can detect two points that are 0.2 mm apart in suitable light. This distance is referred to as the eye’s resolving power or resolution. This distance can be magnified with lens, allowing the eye to see points that are even closer together than 0.2 mm. here we will discuss about scanning electron microscopy technique in detail.
The scanning electron microscope (SEM) generates a variety of signals at the surface of solid specimens using a focused beam of high-energy electrons. The signals generated by electron-sample interactions provide information on the sample’s exterior morphology (texture), chemical composition, crystalline structure and orientation of the components that make up the sample. In most cases, data is collected across a specific area of the sample’s surface and a 2-dimensional picture is created to show spatial variations in these qualities.
Using traditional SEM techniques, areas spanning in width from 1 cm to 5 microns can be scanned in a scanning mode. The SEM may also do studies of specific point locations on the material; this method is particularly effective for detecting chemical compositions, crystalline structure, and crystal orientations qualitatively or semi-quantitatively.
Principle of scanning electron microscope
In a SEM, accelerated electrons carry a lot of kinetic energy, which is dissipated as a variety of signals caused by electron-sample interactions as the incident electrons decelerate in the solid sample. Secondary electrons (which form SEM images), backscattered electrons, diffracted backscattered electrons, photons, visible light and heat are all examples of these signals. Secondary electrons and backscattered electrons are both often employed to image samples: secondary electrons are best for displaying morphology and topography on samples, while backscattered electrons are best for highlighting compositional contrasts in multiphase samples.
Inelastic collisions of incoming electrons with electrons in discrete shells of atoms in a sample produce X-rays. When excited electrons return to lower energy states, they produce X-rays with a specific wavelength (equal to the difference in energy levels). As a result, each element in a mineral that is stimulated by the electron beam produces distinct X-rays. SEM analysis is deemed “non-destructive,” meaning that the x-rays created by electron interactions do not cause the sample to lose volume, allowing the same materials to be analyzed multiple times.
What is scanning electron microscopy?
The instrumentation of scanning electron microscopy is given as:
- Electron gun
- Display devices
At least one detector is always present in a SEM and most contain multiple detectors. The detectors that an instrument may accommodate have a significant impact on its unique capabilities.
Electrons are generated at the top of the column, rushed down the column, and passed through a series of lenses and apertures to produce a focused beam of electrons that strikes the sample’s surface. The sample is then placed on a stage in the chamber region, and both the column and the chamber are evacuated using a combination of pumps, unless the microscope is intended to function at low vacuums. The vacuum level will be determined by the microscope’s design.
Scanning coils above the objective lens control the position of the electron beam on the sample. These coils allow the beam to be scanned across the sample’s surface. The information about a designated area on the sample can be obtained using this beam scanning technique. A variety of signals are produced as a result of the electron-sample interaction. The appropriate detectors then detect these signals.
SEM images are created by scanning a material with a high-energy electron beam. Secondary electrons, backscattered electrons, and distinctive X-rays are produced as the electrons interact with the sample. One or more detectors capture these signals to create images, which are then shown on the computer screen. Depending on the accelerating voltage and the density of the sample, the electron beam penetrates the sample to a depth of a few microns when it hits the surface. This interaction inside the sample produces a variety of signals, including secondary electrons and X-rays.
The greatest resolution obtained in a SEM is determined by a number of parameters, including the size of the electron spot and the electron beam’s interaction volume with the material. Some SEMs can attain resolutions of less than 1 nm, yet they cannot provide atomic resolution. Modern full-sized SEMs typically have a resolution of 1-20 nm, whereas desktop systems can have a resolution of 20 nm or more.
Scanning electron microscope vs. optical microscope
Many structures can no longer be described by light microscopy as materials and gadgets reduce in size. To determine the integrity of a nano-fiber layer for filtering, scanning electron microscopy is required to analyze the sample. The type of beam supplied to the sample is the major distinction between a Scanning Electron Microscope (SEM) and an Optical Microscope (OM). A beam of light is used in optical microscopy to allow the observer to examine the effects of light as it interacts with the material. Scanning electron microscopy, on the other hand, examines the sample with a beam of electrons, allowing the observer to assess the effects of electrons as they interact with the material.
Optical microscopy is good for general inspection while scanning electron microscopy provides very specific topographical and compositional information to the user. There are three types of detectors in a scanning electron microscope: a Secondary Electron Detector (SED), a Back-Scattered Electron Detector (BSED), and an Energy Dispersive Spectrum Detector (EDS).
Because secondary electrons interact primarily with the sample surface and have a large reflection angle, the SED provides detailed topographical information to the user, whereas the BSED provides both basic topographical and basic compositional information to the user because back-scattered electrons penetrate deeper into the material and have a smaller reflection angle. In the compositional data, low-atomic number materials seem dark in the SEM, while high-atomic number materials seem light, but the BSED cannot offer the actual chemical composition.
Scanning electron microscope uses
- SEM is used very commonly for creating high resolution images of objects to show spatial variations in chemical compositions.
- It discriminates the phases based on mean atomic number by using back scattered electrons.
- It is used to identify the phases, based on chemical analysis or crystalline structure.
- Very precise measurements of a small object even down to 50 nm can also be taken by SEM.
- Back scattered electrons are used for discriminating phases of multiple samples.
- Crystallographic orientations can be examined by the SEM equipped with diffracted back scattered electrons (DBSE).
Limitations of scanning electron microscope
- The sample which is to be examined, must be in solid form and must fit in the chamber.
- Horizontally, maximum size is of the order of 10 cm and vertically may exceed 40 mm.
- Sample must be stable under the pressure of 10-5 to 10-6 in vacuum.
- SEM cannot detect lighter elements like H, He and Li.
- To analyze electrically insulating material with a typical SEM, an electrically conductive coating must be added unless the instrument can operate in a low vacuum mode.
Scanning electron microscopy vs. transmission electron microscopy
The major distinction between SEM and TEM is that SEM makes a picture by detecting reflected or knocked-off electrons, whereas TEM makes an image by detecting transmitted electrons. SEM VSTEM images are shown below:
SEM vs TEM
- The light source in SEM is scattered electrons, which come out of the surface of certain material while TEM is based on the transmitted electrons.
- SEM needs the sample to be coated with a heavy metal like gold but in TEM sample must be of small order as electrons can’t penetrate otherwise.
- SEM can provide 3D image while TEM can give only 2D picture of sample.
- SEM produces detailed pictures of surfaces and focuses on the surface and composition of the sample (morphology). TEM, on the other hand, is used to examine thin specimens such as tissue segments, molecules, and so on. The sample’s internal composition, shape, crystallization, and other properties can all be seen via TEM.
- Magnifying power of SEM is 50,000X and TEM has of the order of 2 million.
- At an instant, SEM can take a large amount of sample under observation while TEM only takes small amount of samples.
- SEM collects secondary or backscattered electrons generated by the interaction of an electron beam with a metal-coated object and displays the image on a computer screen. TEM, on the other hand, uses transmitted electrons that impact a fluorescent screen, creating a shadow image of the specimen with its various components exhibited in varying degrees of darkness according to their density.