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

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182 Chapter 7 Besides decreasing the probe size and improving the STEM image resolution, aberration correction allows an electron probe to have a larger convergence semi-angle without its diameter being increased by spherical aberration. In turn, this means higher probe current and larger signals from the ADF or XEDS detector, or the energy-loss spectrometer, allowing the possibility of elemental analysis with higher sensitivity. In fact, the current density in an aberration-corrected probe can exceed 10 6 A/cm 2 , high enough to cause radiation damage in many inorganic (as well as organic) materials. Because radiation damage cannot be completely avoided, this effect sets the ultimate practical limit to TEM spatial resolution and elemental sensitivity. Aberration correction can also be applied to a conventional TEM, by adding a multipole corrector to the imaging system to correct for spherical aberration of the objective lens. But to optimize the instrument for analytical microscopy, a second Cs-corrector must be added to the illumination system to decrease the probe size and maximize the probe current. Because the aberration of an objective lens depends on its focus setting, the currents in each multipole must be readjusted for every new specimen. In practice, this adjustment is achieved through computer control, based on the appearance of a “Ronchigram” (essentially an in-line Gabor hologram; see Section 7.4) recorded by a CCD camera placed after the specimen. 7.3 Electron-Beam Monochromators The electron beam used in a TEM or SEM has an energy spread �E in the range 0.3 eV to 1.5 eV, depending on the type of electron source, as summarized in Table 3.1. Because of chromatic aberration (which remains or is even increased after using multipoles to correct spherical aberration), this energy spread degrades the spatial resolution in both scanning and fixedbeam microscopy. It also broadens the peaks in an energy-loss spectrum, reducing the amount of fine structure that can be observed. One solution to this problem is to use a monochromator in the illumination system. This device is essentially an energy filter, consisting of an electron spectrometer operated at a fixed excitation, followed by an energy-selecting slit that restricts the velocity range of the beam. The aim is to reduce �E to below 0.2 eV, so with E0 = 200 keV the spectrometer must have an energy resolution and stability of 1 part in 10 6 . If the spectrometer were a magnetic prism, as discussed in Section 6.9, this requirement would place severe demands on the stability of the spectrometer and high-voltage power supplies. An easier option is to incorporate an energy filter into the electron gun, where the electrons have not yet acquired their final kinetic energy and the fractional energy resolution can be lower. Figure 7-1 shows
Recent Developments 183 one type of gun monochromator, in this case a Wien filter in which a magnetic field Bx and an electric field Ey are applied perpendicular to the beam, such that they introduce equal and opposite forces on the electron. The net force is zero when (�e)vBx + (�e)Ey = 0 (7.1) Equation (7.1) is satisfied only for a single electron speed v; faster or slower electrons are deflected to one side and removed by the energy-selecting slit. A convenient feature of the Wien filter is that the electrons allowed through it travel in a straight line, unlike a magnetic prism, which changes the direction of the optic axis. Monochromators are expected to be useful for revealing additional fine structure in an energy-loss spectrum, as illustrated in Fig. 7-4. This fine structure can sometimes be used as a “fingerprint” to identify the chemical nature of very small volumes of material. In addition, good energy resolution is important for resolving energy losses below 5 eV, in order to measure the energy gap (between valence and conduction bands) in a semiconductor or insulator and to investigate energy levels within the gap, which may be associated with localized defects such as dislocations. Figure 7-4. Part of the electron energy-loss spectrum recorded from a vanadium pentoxide sample using several FEI microscopes. The CM20 has a thermionic electron source, giving an energy spread of about 1.5 eV. The TF20 has a field-emission source and when fitted with a gun monochromator, it achieves an energy resolution below 0.2 eV, revealing additional fine structure just above the vanadium L-ionization threshold (� 515 eV). Although the energy resolution obtained by x-ray absorption spectroscopy (using synchrotron radiation) is even better, the TEM offers greatly superior spatial resolution. From D.S. Su et al., Micron 34 (2003) 235�238, courtesy of Elsevier Ltd. and F. Hofer, Technical University of Graz.
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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
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92 Chapter 3 Frequently, liquid nit
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94 Chapter 4 -e -e -e -e +Ze Figure
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96 Chapter 4 � (a) elastic z x m
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98 Chapter 4 � b a (a) (b) Figure
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100 Chapter 4 fraction of electrons
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102 Chapter 4 whose composition or
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104 Chapter 4 replica crystallites
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106 Chapter 4 4.5 Diffraction Contr
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108 Chapter 4 remain undiffracted (
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110 Chapter 4 Close examination of
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112 Chapter 4 Unfortunately, the va
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114 Chapter 4 Crystals can also con
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116 Chapter 4 A simple example of p
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118 Chapter 4 High-magnification ph
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120 Chapter 4 At this stage it is c
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122 Chapter 4 All of the above meth
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124 Chapter 4 The procedures outlin
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126 Chapter 5 specimen scan coils o
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128 Chapter 5 functions with m and
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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
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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
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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
Page 200 and 201: REFERENCES Binnig, G., Rohrer, H.,
Page 202 and 203: INDEX Aberrations, 3, 28 axial, 44
Page 204 and 205: Index 199 EELS, 172 Electromagnetic
Page 206 and 207: Index 201 Principal plane, 32, 80 P
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