Functions of the SEM subsystems Electronic column It consists of an electron gun and two or more electron lenses, which influence the path of electrons traveling down an evacuated tube. The base of the column is usually taken up with vacuum pumps that produce a vacuum of about 10-4 Pa Control console It consists of a computer system with a viewing screen and knobs and keyboard to control the electron beam
Electron gun The electron gun generates electrons and accelerates them to an energy in the range 0.1 30 KeV. Electromagnetic lenses Since the spot size from the source (tungsten hairpin gun) is too large to produce a sharp image, lenses are used to demagnify it. The beam emerges from the final lens with a spot size that could be also less than 10 nm. Deflection system The scanned image is formed point by point. The deflection system causes the beam to move to a series of discrete locations along a line and then along another line below the first, and so on, until a rectangular raster is generated on the specimen. Simultaneously, a similar raster is created on the viewing screen.
Electron detectors Contrast in an image arises when the signal collected from the beam-specimen interaction varies from one location to another. When the electron beam impinges on the specimen, many types of signals are generated and any of them can be displayed as an image. The electronics of the detector system converts the signals to point-by-point intensity changes on the viewing screen and produces an image. The two signals most often used to produce SEM images are secondary electrons (SE) and backscattered electrons (BSE). Sometimes X-ray analyses are performed simultaneously in order to have compositional information about the specimen.
Parameters characterizing a SEM micrograph Images can provide much more information about a specimen if we understand the conditions under where they are taken.the limiting sharpness and feature visibility of SEM micrographs are dependent upon four parameters d p electron probe size i p electron probe current а p electron probe convergence angle V 0 electron beam accelerating voltage (in kv) d p i p а p V 0 the spot size is defined as the diameter of the final beam at the surface of the specimen the probe current is the current that impinges upon the specimen and generates the various imaging signals the convergence angle is the half-angle of the cone of the electrons converging onto the specimen electron beam accelerating voltage is the accelerating voltage of the electron gun
Each of these four beam parameters dominates one of the four major SEM imaging mode d p i p а p V 0 resolution mode high-current mode depth-of-focus mode low voltage mode
Resolution mode For the highest resolution image d p must be as small as possible while at the same time containg sufficient current to exceed the visibility threshold for the contrast produced by the features of interest. Resolution refers to the finest details that can be observed. To image the finest details of the specimen surface, the probe diameter must be comparable with or smaller than the feature itself. High current mode For the best image visibility and quality, large beam currents i p are required. Unless the contrast between an image feature and the background is distinguishable above random signal fluctuactions (noise), details cannot be observed even if the spot size is small enough fot it to be easily resolved. Moreover, large beam currents are necessary for X-ray microanalysis because only a small fraction of beam interactions result in X-ray emission.
Resolution Small spot-size Large spot-size
Depth of- focus mode For the best depth of focus а p must be as small as possible. By making the beam convergence angle low, the beam diameter changes only a little over a long vertical distance and so feature on the surface at different heights will appear to be in focus at the same time. For large beam convergence angle details at different heights will be out of focus and no surface details can be seen. The large depth of focus of the SEM is one of the its greatest strengths. Low voltage mode At low accelerating voltages ( 5 kv), the beam interaction with the specimen is confined to regions very close to the surface. This provides an image which is rich in surface details compared to those obtained at higher accelerating voltages (10-15 kv).