Scanning Electron Microscopy Primer Bob Hafner This primer is intended as background for the Introductory Scanning Electron Microscopy training offered by the University of Minnesota’s Characterization Facility (CharFac). The primer addresses concepts fundamental to any scanning electron microscope (SEM); it also, where possible, informs the reader concerning specifics of the facility’s four SEMs: JEOL 6500; JEOL 6700; Hitachi S-4700; and
Hitachi S-900. You must learn this material prior to the hands-on training and you will be required to pass a test on it in order to become an independent SEM user at CharFac. A good source for further information is: “Scanning Electron Microscopy and X-Ray Microanalysis” by Joseph Goldstein et al. Characterization Facility, University of Minnesota—Twin Cities 4/16/2007 1 mage. A look inside the black box [1] reveals a source (electron gun) of the ted • ndenser and
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• (x,y,z-
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• e maintained at high er The Big Picture
To the right is a picture of our Hitachi S-4700. The microscope column, specimen chamber, and vacuum system are on the left; the computer, monitor, and many of the instrument controls on the right. As an operator you will need to understand what is happening inside the “black box” (microscope column and specimen chamber) when an instrument control is manipulated to produce a change in the monitor i quite a bit of complexity; however, we can simplify at this point. We have: • electron beam which is accelera down the column; a series of lenses (co objective) which act to control the diameter of the beam as well as to focus the beam on the specimen; a series of apertures (micron-scal holes in metal film) which the beam passes through and which affect properties of that beam; controls for specimen position height) and orientation (tilt, rotation); an area of beam/specimen interaction that generates se types of signals that can be detected and processed to produce an image or spectra; and all of the abov vacuum levels (the value of the upp column being greater than the specimen chamber). Characterization Facility, University of Minnesota—Twin Cities 4/16/2007 2 we take a closer look at see

tor e also see a pair of led r rastering that focused beam across the specimen surface. The size of the he beam is rastered from left to n e at he red dot within each pixel on collected by the detector and subsequently processed to generate the ith the beam focused on the specimen surface, all we need to do to change magnification is to change ation =
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the lower column and specimen chamber, we the objective lens which focuses the electron beam on the specimen surface. A signal is generated from the specimen, acquired by the detector, and processed to produce an image or spectrum on the moni display. W deflector coils, control by the Scan Generator, which are responsible fo rastering pattern is under Magnification Control. T right and top to bottom. There is a one-to-one correspondence between the rastering pattern o the specimen and the rastering pattern used to produce the image on the monitor. The resolution we choose to imag will obviously affect the number of pixels per row as well as the number of rows that constitute the scanned area. T the specimen represents an area of beam--specimen interaction from which the signal is derived
(more on this later). The signal is image. That processing takes the intensity of the signal coming from a pixel on the specimen and converts it to a grayscale value of the corresponding monitor pixel. The monitor image is a two dimensional rastered pattern of grayscale values. W the size of the rastered area on the specimen. The size of the monitor raster pattern is constant.
Magnification