Scanning Electron Microscope(SEM)
SEM
Scanning Electron Microscope(SEM)
Scanning electron microscope is a one of the particle analytical method. SEM is one of the type of electron microscope. Here we using the single electron ray which is kinetically ejected from the electron gun. The ejected single electron powerfully hits the specimen which is known as sample and placed in the holder, the secondary electron rays is produced by the sample and secondary electrons are commonly detected by an Everhart-Thornley detector. the secondary electrons are contains the different kinds of properties and the detector has counted the number of rays are scattered by the sample. the detected signal intensity is depends on the specimen topography.
SEM can achieve 1 nm
Principal of SEM
Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons (that produce SEM images), back-scattered electrons (BSE), diffracted back-scattered electrons (EBSD that are used to determine crystal structures and orientations of minerals), photons (characteristic X-rays that are used for elemental analysis and continuum X-rays), visible light (cathodoluminescence--CL), and heat. Secondary electrons and back-scattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and back-scattered electrons are most valuable for illustrating contrasts in composition in multi-phase samples (i.e. for rapid phase discrimination). X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete orbitals (shells) of atoms in the sample. As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength (that is related to the difference in energy levels of electrons in different shells for a given element). Thus, characteristic X-rays are produced for each element in a mineral that is "excited" by the electron beam. SEM analysis is considered to be "non-destructive"; that is, x-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same materials repeatedly.
Accelerating voltage (kV or keV) is the voltage difference between the filament and the anode which accelerates the electron beam towards the anode. The accelerating voltage (kV or High Tension) of a typical SEM ranges from 0 to 30kV.
Samples must be solid and they must fit into the microscope chamber. Maximum size in horizontal dimensions is usually on the order of 10 cm, vertical dimensions are generally much more limited and rarely exceed 40 mm. For most instruments samples must be stable in a vacuum on the order of 10-5 - 10-6 torr. Samples likely to out gas at low pressures (rocks saturated with hydrocarbons, "wet" samples such as coal, organic materials or swelling clay, and samples likely to decrepitate at low pressure) are unsuitable for examination in conventional SEM's. However, "low vacuum" and "environmental" SEMs also exist, and many of these types of samples can be successfully examined in these specialized instruments. EDS detectors on SEM's cannot detect very light elements (H, He, and Li), and many instruments cannot detect elements with atomic numbers less than 11 (Na). Most SEMs use a solid state x-ray detector (EDS), and while these detectors are very fast and easy to utilize, they have relatively poor energy resolution and sensitivity to elements present in low abundances when compared to wavelength dispersive x-ray detectors (WDS) on most electron probe microanalyzers (EPMA). An electrically conductive coating must be applied to electrically insulating samples for study in conventional SEM's, unless the instrument is capable of operation in a low vacuum mode.
Sample preparation can be minimal or elaborate for SEM analysis, depending on the nature of the samples and the data required. Minimal preparation includes acquisition of a sample that will fit into the SEM chamber and some accommodation to prevent charge build-up on electrically insulating samples. Most electrically insulating samples are coated with a thin layer of conducting material, commonly carbon, gold, or some other metal or alloy. The choice of material for conductive coatings depends on the data to be acquired: carbon is most desirable if elemental analysis is a priority, while metal coatings are most effective for high resolution electron imaging applications. Alternatively, an electrically insulating sample can be examined without a conductive coating in an instrument capable of "low vacuum" operation.
SEM analysis is a powerful tool for analyse the high resolution of surface topography of the specimen by the monochromatic electron.
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SEM produces only Black and white images of surface topography because of since Electron Microscope using an electron beam to read the specimen, there's no color information recorded. However, the images it produces contain only two colors – red and green.
Components of SEM
EHT
EHT is nothing but Electron High Tension. value of EHT is displayed in the data zone, and in the SEM Control window. The accelerating voltage ranges from 200V to 30 kV. Use 10 kV for imaging, 15 or 20 kV for EDS at the beginning, then tune EHT when needed for different applications.
Probe current
The SEM electron column. The electron column consists of the electron source, where the electrons are emitted, and a set of lenses. The electrons are condensed into a beam by the condenser lenses and then focused onto the sample surface by the final lens.
The volume inside the specimen in which interactions occur while being struck with an electron beam. Atomic number of the material being examined; higher atomic number materials absorb or stop more electrons and so have a smaller interaction volume.
Cababilities of SEM
- SEM Analysis with EDS – qualitative and semi-quantitative results
- Magnification – from 5x to 300,000x
- Sample Size – up to 200 mm (7.87 in.) in diameter and 80 mm (3.14 in.) in height
- Materials Analyzed – solid inorganic materials including metals and polymers
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Application
- SEMs can be as essential research tool in fields such as life science, biology, gemology, medical and forensic science, metallurgy
- SEMs have practical industrial and technological applications such as semiconductor inspection, production line of miniscule products and assembly of microchips for computers.
- It can detect and analyze surface fractures, provide information in microstructures, examine surface contaminations, reveal spatial variations in chemical compositions, provide qualitative chemical analyses and identify crystalline structures.
Advantages
- easy to operate with the proper training and advances in computer technology and associated software make operation user-friendly.
- which is works too fast, often completing SEI, BSE and EDS analyses in less than five minutes.
- SEM samples require minimal preparation actions.
Disadvantages
- SEMs are expensive, large and must be housed in an area free of any possible electric, magnetic or vibration interference.
- There is no absolute way to eliminate or identify all potential artifacts.
- SEMs are limited to solid, inorganic samples small enough to fit inside the vacuum chamber that can handle moderate vacuum pressure.
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