Comp. by: SampathKumar Stage: Galleys Chapter No.: 25 Title Name: EOG Date:3/11/15 Time:17:39:22 Page Number: 1 1 S 2 SCANNING ELECTRON MICROSCOPY (SEM) 3 Ellery Frahm 4 Au1 Departments of Earth Sciences & Anthropology, Feinberg 5 Research Group, University of Minnesota-Twin Cities, 6 Minneapolis, MN, USA 7 Definition 8 Scanning electron microscopy . A form of microscopy in 9 which a focused beam of accelerated electrons is scanned 10 across the surface of a specimen, generating a number of 11 signals that yield information about its morphology, ele- 12 mental composition, and, when outfitted with appropriate 13 detectors, crystalline microstructure or other features. 14 SEM. Scanning electron microscopy or microscope. 15 This acronym is often used interchangeably to describe 16 the imaging/analytical technique and the instrument itself. 17 SEM in geoarchaeology 18 Introduction 19 SEM is a highly versatile imaging and microanalytical 20 technique that has been used throughout the archaeologi- 21 cal sciences for almost five decades (e.g., Pilcher, 1968; 22 Brothwell, 1969). Most instruments are equipped for two 23 primary functions: imaging (commonly at high magnifica- 24 tions) and providing compositional (i.e., elemental) infor- 25 mation. Instruments can also be outfitted with detectors 26 that offer additional information, such as the crystalline 27 microstructure and orientation of a specimen. Hence, 28 SEM has been frequently used in geoarchaeology when- 29 ever a researcher wishes to observe magnified images of 30 a specimen and/or establish its elemental composition on 31 a microscopic scale. Electron microprobe analysis 32 (EMPA; sometimes known as electron probe microanaly- 33 sis, EPMA) developed alongside SEM and is a closely 34 related technique for electron imaging and X-ray 35 microanalysis. These two techniques essentially exist on 36 an analytical continuum, and their capabilities consider- 37 ably, and increasingly with time, overlap. Figure 1 illus- 38 trates the principal systems of SEM and EMPA. 39 SEM enables high-magnification imaging of 40 a specimen, and the magnification range with SEM is 41 much greater than that with visible-light microscopy 42 (VLM), from as low as 5Â (equivalent to a hand lens) to 43 as high as 200,000Â (about two orders of magnitude 44 higher than a petrographic microscope) or even more. In 45 addition, SEM has a superior depth of field (about 46 300 times better than VLM), which means that the full 47 height of a specimen can appear focused (Figure 2). Most 48 SEMs can also quantitatively or semiquantitatively mea- 49 sure elemental composition based on the X-rays emitted 50 by a specimen under energetic electron bombardment. 51 SEM fundamentals 52 An “electron gun” atop the instrument (Figure 1) creates 53 a beam of electrons either via heating (thermionic emis- 54 sion) or electric fields (field emission) and accelerates 55 them toward a specimen. A column of apertures and elec- 56 tromagnetic lenses focus the electron beam onto the sur- 57 face of the specimen. To produce an image, the beam 58 quickly scans across the surface, very much like an old 59 CRT-based monitor or television. This process often 60 occurs under high vacuum to reduce scattering of the elec- 61 trons by air molecules and other undesirable effects. 62 A fairly recent development, though, is imaging at pres- 63 sures closer to atmosphere, generally called environmen- 64 tal SEM (ESEM), but a number of manufacturer-specific 65 terms have also been introduced (e.g., variable-pressure 66 SEM, low-vacuum SEM, wet SEM). The main advantage 67 is that additional air inside the chamber prevents an elec- 68 tric charge from building up on nonconductive specimens. 69 With the chamber under vacuum, it is necessary to coat 70 specimens with an ultrathin layer (about 100 Å thick) of A.S. Gilbert (ed.), Encyclopedia of Geoarchaeology, DOI 10.1007/978-1-4020-4409-0, © Springer Science+Business Media Dordrecht 2015