While this contribution is most concerned with geological applications, it is important to note that these applications are a very small subset of the scientific and industrial applications that exist for this instrumentation. Most SEM's are comparatively easy to operate, with user-friendly "intuitive" interfaces.
Many applications require minimal sample preparation. Modern SEMs generate data in digital formats, which are highly portable. 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 outgas at low pressures rocks saturated with hydrocarbons, "wet" samples such as coal, organic materials or swelling clays, 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. 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. Goldstein, J. Reimer, L. Springer, p. Scanning electron microscopy, or SEM, produces detailed, magnified images of an object by scanning its surface to create a high resolution image.
SEM does this using a focused beam of electrons. The resulting images show information about what the object is made of and its physical features. The instrument which obtains this information about composition and topography is a scanning electron microscope. As a practical and useful tool, SEM has a broad range of applications, across several industries and sectors.
It can analyse both man-made and naturally occurring materials. View Scanning Electron Microscopy products from Scimed. Scanning electron microscopy works by scanning a sample with electron beams. An electron gun fires these beams, which then accelerate down the column of the scanning electron microscope. During this action, the electron beams pass through a series of lenses and apertures, which act to focus it. This occurs under vacuum conditions , which prevents and molecules or atoms already present in the microscope column from interacting with the electron beam.
The electron beams scan the sample in a raster pattern, scanning the surface area in lines from side to side, top to bottom. The electrons interact with atoms on the surface of the sample. This interaction creates signals in the form of secondary electrons, backscattered electrons and rays that are characteristic of the sample. Detectors in the microscope pick up these signals and create high-resolution images displayed on a computer screen.
The condenser lens controls the size of the beam, and determines the number of electrons in the beam. The size of the beam will define the resolution of the image. The scanning coils deflect the beam along x and y axes, to ensure it scans in a raster fashion over the surface of the sample.
The objective lens is the last lens in the sequence of lenses that create the electron beam. As the lens closest to the sample, it focuses the beam to a very small spot on the sample. Electrons cannot pass through glass, so SEM lenses are electromagnetic. They are made up of a coil of wires inside metal poles. When a current passes through these coils they generate a magnetic field. Electrons are highly sensitive to these magnetic fields, which therefore enables the lenses in the microscope to control them.
When electrons from the microscope interact with a sample, this can generate different kinds of other electrons, photons and irradiations. The two types of electrons essential for imaging are backscattered electrons BSEs and secondary electrons SEs. BSEs and SEs contain different types of information. BSEs originate from deeper areas of the sample, whereas SEs come from surface regions. Images from BSEs display high sensitivity to differences in atomic numbers, which will show up as brighter or darker.
The scanning electron microscope requires different types of detectors for backscattered and secondary electrons. Typically, for SEs, this will be an Everhart-Thornley detector. This consists of a scintillator inside a Faraday cage. This detector is positively charged to attract SEs. This enables you to view small objects such as cells, but there are limits to the magnification you can achieve, and therefore the materials and substances you can analyse. Electron microscopy is different because instead of using a beam of light, you use a beam of electrons.
Electron microscopes can overcome the limitations of optical microscopes, because they use a shorter wavelength, which creates a better resolution of image. With the right amount of light, the human eye can distinguish two points 0.
An optical microscope can magnify this resolution, so that the eye can see points that are closer together than 0. The maximum magnification power of an optical microscope is around x. The electron microscope was developed when the wavelength became the limiting factor in light microscopes. Electrons have much shorter wavelengths, enabling better resolution.
As dimensions are shrinking for materials and devices, many structures can no longer be characterized by light microscopy. For example, to determine the integrity of a nanofiber layer for filtration, as shown here, electron microscopy is required to characterize the sample. The main SEM components include:. 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 which 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 rastering or scanning, as the name of the microscope suggests, enables information about a defined area on the sample to be collected.
0コメント