An Introduction To Sem Operational Principles And Geologic-Books Pdf

An Introduction to SEM Operational Principles and Geologic
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2 Huang et al, information generated by the primary electron beam. A more detailed description of a detection mechanism. related to imaging contrast is elaborated in the follow. ing section, SEM CONTRAST MECHANISMS, Several types of electrons are generated as the result. of the energetic bombardment of the specimen by the. primary beam Figure 2 All of these electrons carry. distinct structural information about the sample and. differ from one another in origin energy and traveling. direction For example type I secondary electrons, SE1 emit with a high angle at a close proximity from. the impact point and therefore carry high resolution. Figure 1 Schematic diagram of the GEMINI electron surface sensitive topographic information of the. column a and the electron beam path in the column b sample Type II secondary electrons SE2 however. are generated from a deeper and wider volume than, the SE1 and reflect at a lower angle therefore carrying. intrinsically lower resolution topographical informa. are cut off by an aperture The focusing strength of tion Similarly singly scattered backscattered electrons. the condenser lens and the size of the aperture deter BSE1 tend to emit at a high angle and are closely. mine the beam current in the final spot A final objec linked to compositional contrast while multiply scat. tive lens assembly then focuses the beam down to the tered BSE BSE2 take off at a lower angle and are used. desirable spot size Depending on manufacturer de to characterize composition and crystalline structures. sign the lenses used in the column can be magnetic or of a sample Photons can also be generated from the. electrostatic or a combination of the two types In the excitation by the primary electron beam For example. case of the GEMINI column the final objective lens x rays with a continuous spectrum in energy bremsst. is a combination of magnetic and electrostatic lenses rahlung are generated as a result of the deceleration. The main advantage of such a design is that there is of the electrons Additionally characteristic x ray lines. virtually no magnetic field on the specimen that could. distort the imaging if the specimen is magnetic, The earliest work describing the concept of SEM is.
that of Knoll in 1935 Goldstein et al 2003 In his work. the focused electron beam was generated to image a. metal plate with a transmission signal The first SEM. used to examine thick specimens was described by, Zworykin at the RCA Laboratories in 1942 Goldstein. et al 2003 Everhart and Thornley improved on, Zworykin s original design by employing a scintillator. to convert the electrons to light which was then trans. mitted by a light pipe directly to a photomultiplier. The Everhart Thornley detector greatly improves the. signal to noise ratio of SEM imaging The first commer. cial instrument was built in 1963 by Pease at Cambridge. Scientific Instruments Goldstein et al 2003, Since the first commercial instruments many ad. vances have been made including development of, a high brightness electron gun allowing more elec Figure 2 Profile view of a typical specimen electron interac. trons to be focused into a smaller beam spot an im tion volume Note that secondary electrons SE originate. proved electron optics design that can dramatically much shallower in the sample than backscattered electrons. enhance imaging resolution and a large portfolio of BSE and therefore provide more surface driven and less. detectors that capture and separate different types of composition influenced information. 13835 ch01 ptg01 hr p001 006 indd 2 23 05 13 1 33 PM. An Introduction to SEM Operational Principles and Geologic Applications 3. are generated from electron excitation within speci polish the surface of a sample by sputtering away ma. men atoms that interact with the primary electron terial thus providing an extremely smooth surface. beam The latter form of x ray is particularly useful in for SEM investigation The image was acquired with. chemical analysis with SEM an Everhart Thornley type secondary electron detec. Photons with wavelengths in the ultraviolet vis tor The SE2 image contrast reveals both topographi. ible and infrared light regions of the electromagnetic cal and compositional information due to the greater. spectrum can also be excited by the primary electrons sample interaction depth of SE2 electrons High SE. This phenomenon is called cathodoluminescence CL yield is scaled as lighter shades of gray and low SE. The generation of CL is commonly associated with the yield is scaled as dark shades of gray in SEM images. presence of certain trace impurities in minerals and Pores and fractures appear dark in the SE2 image re. therefore is also widely used in geological applica flecting lower secondary electron yield from negative. tions A few examples are illustrated next to demon depressions than from elsewhere on the sample sur. strate several common contrast mechanisms useful for face Therefore porosity information can be readily. the geological study of shales characterized with the SE2 electrons. The same cross section was imaged with BSE1, with the Energy selective backscatter EsB detec.
SEM Image Examples tor Figure 4b The image contrast grayscale varia. tion reflects compositional variations mean atomic. Figure 3 shows an SE1 image of a shale sample At number of the sample For example the midgray. low landing energy less than 1 keV the secondary represents silica matrix the darker level represents. electrons collected with the in lens detector originate organic matter The brighter gray level reflects higher. from the very surface of the sample The SE1 signal is density carbonate phases and the brightest gray level. commonly used to image surface details at the highest represents pyrite Note the greater compositional con. resolution at the expense of compositional informa trast provided by the BS1 image Figure 4b over the. tion On the right side of Figure 3 the SE2 image of SE2 image Figure 4a Although SE2 and BS1 images. the same area as the SE1 image on the left shows the are capable of providing compositional information. effects of higher landing energy and deeper specimen auxiliary techniques such as energy dispersive x. interaction including more compositional and less ray spectrometry EDS are required to characterize. topographical information elemental composition, Figure 4a shows an SE2 image of a cross section of Compared to BSE1 for which the contrast is modu. a shale sample that has been polished by argon ion lated by mean atomic number differences BSE2 yield. milling a sample preparation technique that consists depends strongly on crystalline structures such as. of using one or more beams of argon ions to gently grain orientations Figure 5 shows a BSE2 image of. Figure 3 A shale sample imaged using SE1 signal left and SE2 signal right Surface specific information such as pore space. and surface roughness is evident in the SE1 image The SE2 image has more compositional influence displaying organic. matter OM bodies that are not evident in the SE1 image. 13835 ch01 ptg01 hr p001 006 indd 3 23 05 13 1 33 PM. 4 Huang et al, Figure 4 SE2 a and BSE1 b image of a cross section of a shale rock Note that the contrast between carbonate ca and. silica si grains is much higher in BSE1 the topographical information is greater in the SE2 image OM associated nanopores. are not visible in BSE1, Figure 5 A BSE2 image of gold Au nanoparticles showing. crystallographic contrast, gold Au nanoparticles Contrast corresponding to. different grains is revealed in the image despite all the Figure 6 a A BSE and CL image of a polished shale. chemically identical grains Therefore BSE2 electrons sample Orange hued quartz grains reflect low grade. can be used to image crystallographic contrast in poly metamorphic origin slate blue hued quartz grains. indicate higher grade metamorphism phyllite schist. crystalline materials, b Detail of a large quartz grain in center of image arrow.
Another complimentary technique is the detection, displays multiple generations of growth in CL this distinc. of CL in which certain materials will emit photons in tion in zoning is not visible in SEM. the form of visible light as a result of interactions be. tween specimen electrons and primary beam electrons. Figure 6 shows an example of the effects of CL on a individual quartz grains Cathodoluminescence can. shale sample The image was acquired with a dedi also be used to differentiate between generations of. cated CL detector Variations in CL emission caused quartz growth that are not distinguishable in SEM due. by mineral impurities could indicate provenance of to identical mean atomic number. 13835 ch01 ptg01 hr p001 006 indd 4 23 05 13 1 33 PM. An Introduction to SEM Operational Principles and Geologic Applications 5. X RAY MICROANALYSIS To take advantage of the element specific informa. tion caused by x ray excitation EDS or EDX can be, Both the bremsstrahlung continuum and characteris performed As the electron beam scans the sample. tics x rays are produced by electron beam sample in surface pixel by pixel a full x ray spectrum can be. teractions The characteristic x rays originate from the acquired from each pixel Elemental distribution can. inner shell ionization process where an electron from therefore be mapped using the relative peak intensity. the electron beam interacts with the tightly bound to build an image of the scanned area This can then. inner shell electrons ejecting an atomic electron and be interpreted to estimate mineral phase composition. leaving a vacancy in that shell The atom relaxes to Figure 8 shows a secondary electron image of a shale. its ground state through a limited set of transitions sample with EDS derived mineral segmentation of the. of outer shell electrons to fill the inner shell vacancy same region. The energies of the electrons in the shells are sharply. defined with values characteristic of the binding en. ergies of the electrons therefore the characteristic x FOCUSED ION BEAM APPLICATIONS. rays are elemental specific creating a powerful tool for. elemental analysis in SEM A typical x ray spectrum is Focused ion beam FIB systems also find a growing. shown in Figure 7 which was acquired from a shale number of applications in geology Goldstein et al. sample In the spectrum the broad continuous back 2003 In a typical FIB SEM system an extraction field. ground is the bremsstrahlung the sharp peaks corre is applied to a gallium Ga liquid metal ion source. spond to the elemental lines associated with specific to field emit Ga ions and form a Ga beam Due to the. intershell transitions higher atomic mass the Ga beam not only can be used. Figure 7 An example of an x ray spectrum acquired from a shale sample Individual peaks indicate an elevated concentration. of a given element C5carbon O5oxygen Mg5magnesium Al5aluminum Si5silicon Ca5calcium. Figure 8 A secondary electron image, of a shale sample with an EDS derived. mineral segmentation overlay In the, segmented region blue 5 carbonate. green 5 clay minerals yellow 5 quartz, pink 5 feldspar white 5 pyrite and.
gray 5 organic matter, 13835 ch01 ptg01 hr p001 006 indd 5 23 05 13 1 33 PM. 6 Huang et al, a serial fashion to acquire a stack of two dimensional. images from which a 3 D image volume can be, c onstructed from the data set This technique is. particularly useful in revealing the 3D distribution of. mineral types organic matter porosity and the like in. shale and other rock samples, CONCLUSIONS, Figure 9 At left a schematic diagram of operation of an. FIB SEM system The FIB sputters away a thin layer of the Scanning electron microscopy provides different. sample at a time while the electron beam detector system modes and techniques for acquiring high quality im. captures an image of each newly exposed surface At right ages of shale and other rock samples The images in. a picture of a commercial FIB SEM CrossBeam system this chapter demonstrate their fine resolution and their. Auriga applicability for the characterization of shale reservoirs. to generate electron and ion images but also may be REFERENCE CITED. used to mill samples to remove material Figure 9a, shows the schematic diagram of an FIB SEM system Goldstein J D E Newbury D C Joy C E Lyman P Echlin.
where a cross section of the sample is milled by a Ga E Lifshin L Sawyer and J R Michael 2003 Scanning. FIB beam and is imaged simultaneously by the SEM electron microscopy and X ray microanalysis 3d ed. This milling and imaging process can be automated in New York Springer 695 p. 13835 ch01 ptg01 hr p001 006 indd 6 23 05 13 1 33 PM. An Introduction to SEM Operational Principles and Geologic Applications 5 To take advantage of the element specific informa tion caused by x ray excitation EDS or EDX can be performed As the electron beam scans the sample surface pixel by pixel a full x ray spectrum can be acquired from each pixel Elemental distribution can

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