Physical Principles of Electron Microscopy: An Introduction to TEM ...

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142 Chapter 5 Figure 5-14. Red, green and blue phosphor dots in a color-TV screen, imaged using secondary electrons (on the left) and in CL mode (on the right) without wavelength filtering. The lower images show the individual grains of light-emitting phosphor imaged at higher magnification. From Reimer (1998), courtesy of J. Hersener, Th. Ricker, and Springer-Verlag. The light can be detected by a photomultiplier tube, sometimes preceded by a color filter or a wavelength-dispersive device (glass prism or diffraction grating) so that a limited range of photon wavelengths are recorded. Cathodoluminescence is more efficient at low temperatures, so the specimen is often cooled to below 20 K, using liquid helium as the refrigerant. Visible light is emitted when a primary electron, undergoing an inelastic collision, transfers a few eV of energy to an outer-shell (valence) electron, which then emits a photon while returning to its lowest-energy state. If the primary electron collides with an inner-shell electron, more energy must be transferred to excite the atomic electron to a vacant energy level (an outer orbit or orbital) and a photon of higher energy (hundreds or thousands of eV)
The Scanning Electron Microscope 143 may be emitted as a characteristic x-ray photon. The x-ray energy can be measured and used to identify the atomic number of the participating atom, as discussed in Chapter 6. If the characteristic x-ray signal is used to control the scanned-image intensity, the result is an elemental map showing the distribution of a particular chemical element within the SEM specimen. 5.6 SEM Operating Conditions The SEM operator is able to control several parameters of the SEM, such as the electron-accelerating voltage, the distance of the specimen below the objective lens, known as the working distance (WD), and sometimes the diameter of the aperture used in the objective lens to control spherical aberration. The choice of these variables in turn influences the performance obtained from the SEM. The accelerating voltage determines the kinetic energy E0 of the primary electrons, their penetration depth, and therefore the information depth of the BSE image. As we have seen, secondary electrons are generated within a very shallow escape depth below the specimen surface, therefore the SE image might be expected to be independent of the choice of E0. However, only part of the SE signal (the so-called SE1 component) comes from the generation of secondaries by primary electrons close to the surface. Other components (named SE2 and SE3, respectively) arise from the secondaries generated by backscattered electrons, as they exit the specimen, and from backscattered electrons that strike an internal surface of the specimen chamber; see Fig. 5-15a. As a result of these SE2 and SE3 components, secondary-electron images can show contrast from structure present well below the surface (but within the primary-electron penetration depth), and this structure results from changes in backscattering coefficient, for example due to local differences in atomic number. Such effects are more prominent if the primary-electron penetration depth is large, in other words at a higher accelerating voltage, making the sample appear more “transparent” in the SE image; see Fig. 5-16. Conversely, if the accelerating voltage is reduced below 1 kV, the penetration depth becomes very small (even compared to the secondary-electron escape depth) and only surface features are seen in the SE and BSE images. Because the primary beam spreads laterally as it penetrates the specimen and because backscattering occurs over a broad angular range, some SE2 electrons are generated relatively far from the entrance of the incident probe and reflect the properties of a large part of the primary-electron interaction volume. The SE3 component also depends on the amount of backscattering from this relatively large volume. In consequence, the spatial resolution of
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Physical Principles of Electron Mic
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Ray F. Egerton Department of Physic
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Contents Preface xi 1. An Introduct
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Contents ix 7. Recent Developments
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xii Preface textbooks, such as Will
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2 1.1 Limitations of the Human Eye
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4 Chapter 1 parallel beam of light
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6 Chapter 1 Figure 1-3. One of the
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8 Chapter 1 Figure 1-5. Light-micro
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10 Chapter 1 Figure 1-7. Scanning t
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12 Chapter 1 Figure 1-8. Early phot
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14 Chapter 1 Figure 1-10. JEOL tran
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16 Chapter 1 hydrated. But high-ene
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18 Chapter 1 Figure 1-14. Scanning
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20 Chapter 1 Figure 1-16. Photograp
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22 Chapter 1 visible-light photons
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24 Chapter 1 Typically, the STM hea
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Chapter 2 ELECTRON OPTICS Chapter 1
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Electron Optics 29 a c b object len
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Electron Optics 31 � a n 2 ~1.5
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Electron Optics 33 We can define im
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Electron Optics 35 electron source
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Electron Optics 37 symmetry and use
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Electron Optics 39 As we said earli
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Electron Optics 41 2.4 Focusing Pro
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Electron Optics 43 � = [e/(8mE0)
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Electron Optics 45 image plane. Whe
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Electron Optics 47 procedure that i
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Electron Optics 49 The basic physic
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Electron Optics 51 From Eq. (2.7),
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Electron Optics 53 y +V 1 +V 2 -V 2
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Electron Optics 55 point from the o
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58 Chapter 3 3.1 The Electron Gun T
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60 Chapter 3 The rate of electron e
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62 Chapter 3 electron emission curr
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64 Chapter 3 As seen from the right
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66 Chapter 3 � r r s r � (a) (b
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68 Chapter 3 Table 3-2. Speed v and
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70 Chapter 3 where G is a large amp
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72 Chapter 3 The second condenser (
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74 Chapter 3 Figure 3-9 summarizes
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76 Chapter 3 resolution (especially
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78 Chapter 3 The major advantage of
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80 Chapter 3 contrast (for a given
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82 Chapter 3 A more correct procedu
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84 Chapter 3 diffraction pattern (s
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86 Chapter 3 Nowadays, photographic
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88 Chapter 3 A related concept is d
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90 Chapter 3 molecules, imparting a
Page 100 and 101: 92 Chapter 3 Frequently, liquid nit
Page 102 and 103: 94 Chapter 4 -e -e -e -e +Ze Figure
Page 104 and 105: 96 Chapter 4 � (a) elastic z x m
Page 106 and 107: 98 Chapter 4 � b a (a) (b) Figure
Page 108 and 109: 100 Chapter 4 fraction of electrons
Page 110 and 111: 102 Chapter 4 whose composition or
Page 112 and 113: 104 Chapter 4 replica crystallites
Page 114 and 115: 106 Chapter 4 4.5 Diffraction Contr
Page 116 and 117: 108 Chapter 4 remain undiffracted (
Page 118 and 119: 110 Chapter 4 Close examination of
Page 120 and 121: 112 Chapter 4 Unfortunately, the va
Page 122 and 123: 114 Chapter 4 Crystals can also con
Page 124 and 125: 116 Chapter 4 A simple example of p
Page 126 and 127: 118 Chapter 4 High-magnification ph
Page 128 and 129: 120 Chapter 4 At this stage it is c
Page 130 and 131: 122 Chapter 4 All of the above meth
Page 132 and 133: 124 Chapter 4 The procedures outlin
Page 134 and 135: 126 Chapter 5 specimen scan coils o
Page 136 and 137: 128 Chapter 5 functions with m and
Page 138 and 139: 130 Chapter 5 inelastic collisions
Page 140 and 141: 132 Chapter 5 Figure 5-5. Dependenc
Page 142 and 143: 134 Chapter 5 Figure 5-7. (a) Secon
Page 144 and 145: 136 Chapter 5 Because each secondar
Page 146 and 147: 138 Chapter 5 Backscattered electro
Page 148 and 149: 140 Chapter 5 Whereas Ip remains co
Page 152 and 153: 144 Chapter 5 the SE2 and SE3 compo
Page 154 and 155: 146 Chapter 5 just-observable loss
Page 156 and 157: 148 Chapter 5 Section 4-10. Films o
Page 158 and 159: 150 Chapter 5 A backscattered-elect
Page 160 and 161: 152 Chapter 5 One example of this p
Page 162 and 163: Chapter 6 ANALYTICAL ELECTRON MICRO
Page 164 and 165: Analytical Electron Microscopy 157
Page 184 and 185: 178 Chapter 7 Figure 7-1. Modified
Page 186 and 187: 180 Chapter 7 7.2 Aberration Correc
Page 188 and 189: 182 Chapter 7 Besides decreasing th
Page 190 and 191: 184 Chapter 7 7.4 Electron Holograp
Page 192 and 193: 186 Chapter 7 There are now many al
Page 194 and 195: 188 Chapter 7 holography could ther
Page 196 and 197: Appendix MATHEMATICAL DERIVATIONS A
Page 198 and 199: Mathematical Derivations 193 A.2 Im
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REFERENCES Binnig, G., Rohrer, H.,
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INDEX Aberrations, 3, 28 axial, 44
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Index 199 EELS, 172 Electromagnetic
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Index 201 Principal plane, 32, 80 P
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