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1、4.1.4. The TEM Operation 4.1.4.1. THE ILLUMINATION SYSTEM We will discuss the two different ways to use the illumination system: well refer to these as forming a parallel beam (it is almost never truly parallel) or a convergent beam. 4.1.4.1.A. TEM Operation Using a Parallel Beam Figure 4.1.4.1. Par
2、allel-beam operation in the TEM (A) using just the C1 and an underfocused C2 lens and (B) using the C1 and C2 lenses to image the source at the front focal plane of the upper objective lens. A B 4.1.4.1.B. Convergent-Beam (S)TEM Mode The convergent beam is a probe. We use such a probe when we want t
3、o localize the signals coming from the specimen, as in microanalysis or convergent-beam (also known as micro or nano) diffraction. Figure 4.1.4.2. Effect of the C2 aperture on the parallel nature of the beam: a smaller aperture creates a more parallel beam. Figure 4.1.4.3. A focused C2 lens illumina
4、tes a small area of the specimen with a nonparallel beam. Figure 4.1.4.4. Use of the objective polepiece as a third condenser lens (also called a condenser-objective, or C3, lens) gives the smallest possible probe and large convergence angles. The large u/v ratio gives the maximum demagnification of
5、 the image of the gun crossover. Figure 4.1.4.5. Effect of the CI lens strength on probe size: a stronger C1 lens (A) results in greater demagnification by any subsequent lens (C2 or C3), giving a smaller electron beam at the specimen. A weaker lens (B) gives a broader probe. 4.1.4.1.C. Translating
6、and Tilting the Beam Figure 4.1.4.6. The use of pre-specimen scan coils for (A) traversing the beam and (B) tilting the beam. Traversing moves the beam to a different area of the specimen but it stays parallel to the optic axis. Conversely, tilting the beam illuminates the same area of the specimen,
7、 but from a different angle 4.1.4.1.D. Alignment If you correctly align the illumination system, the gun crossover is on the optic axis and the electrons can then follow a straight line through the lenses and apertures until they hit the specimen. Gun Alignment Figure 4.1.1.5. (A) The tip of a tungs
8、ten hairpin filament and the distribution of electrons when the filament is (B) undersaturated and misaligned, (C) undersaturated and aligned, and (D) saturated. A B C D Figure 4.1.1.6. (A) An LaB6crystal and the electron distribution when the source is (B) undersaturated and aligned and (C) saturat
9、ed. ABC Alignment of the C2 Aperture Figure 4.1.4.7. If the C2 aperture is misaligned, underfocusing or over- focusing the C2 lens causes the image of the beam to sweep off axis (i.e., across the viewing screen) and to become distorted. Figure 4.1.4.8. If the C2 aperture is aligned, the image of the
10、 beam remains circular and expands or contracts about the optic axis as the lens is underfocused or overfocused. 4.1.4.1.E. Condenser Lens Defects Spherical Aberration This defect plays no role in limiting parallel-beam formation. Chromatic Aberration Remember this aberration depends on the energy s
11、pread of the electrons. Since the electrons in the illumination system have such a small energy spread, you can regard them as monochromatic and there is no detectable degeneration of the probe dimensions. Astigmatism Figure 4.1.4.9. The effect of astigmatism in the illumination system is to distort
12、 the image of the beam elliptically as the C2 lens isundeffocusedoroveffocused. Correction of this astigmatism results in an image that remains circular as the C2 lens is defocused. 4.1.4.1.F. Calibration The End! 4.1.4.2. THE OBJECTIVE LENS AND STAGE This combination is the heart of the TEM. We use
13、 the stage to clamp the specimen holder in the correct position so the objective lens can form images and diffraction patterns in a reproducible manner. There are two different types of holder: top-entry and side-entry Our discussion will emphasize the side-entry holder since this is becoming the st
14、andard, but top-entry holders require the same adjustment of the z-control or specimen height. We need to fix the height of the specimen on the optic axis. This will allow us to work at the same objective lens current and thus at a fixed objective lens magnification. 4.1.4.3. FORMING DIFFRACTION PAT
15、TERNS AND IMAGES: THE TEM IMAGING SYSTEM You know that the objective lens takes the electrons emerging from the exit surface of the specimen, disperses them to create a diffraction pattern (DP) in the back focal plane, and recombines them to form an image in the image plane (see Figure 2.3). Figure
16、2.3. A complete ray diagram for a finite object, symmetrically positioned around the optic axis. All rays emerging from a point in the object (distance u from the lens) that are gathered by the lens converge to a point in the image (distance v from the lens) and all parallel rays are focused in the
17、focal plane (distance f from the lens). The first operation that you need to master when using the TEM is viewing the diffraction pattern. In all the subsequent imaging, well use this pattern to select electrons that have suffered particular angles of scatter to form our images. 1. To see the diffra
18、ction pattern you have to adjust the imaging system lenses so that the back focal plane of the objective lens acts as the object plane for the intermediate lens. Then the diffraction pattern is projected onto the viewing screen, as shown in Figure 4.1.4.12A. 2. If you want to look at an image instea
19、d, you readjust the intermediate lens so that its object plane is the image plane of the objective lens. Then an image is projected onto the viewing screen, as shown in Figure 4.1.4.12B. Figure 4.1.4.12. The two basic operations of the TEM imaging system involve (A) projecting the diffraction patter
20、n on the viewing screen and (B) projecting the image onto the screen. In each case the intermediate lens selects either the back focal plane or the image plane of the objective lens as its object. 4.1.4.3.A. Selected-Area Diffraction Such a pattern is not very useful because the specimen will often
21、be buckled. As you can see from Figure 4.1.4.12A, the diffraction pattern contains electrons from the whole area of the specimenthat we illuminate with the beam. Furthermore, the direct beam is often so intense that it will damage the viewing screen. So we perform a basic TEM operation both to selec
22、t a specific area of the specimen to contribute to the diffraction pattern and to reduce the intensity of the pattern falling on the screen. The first option involves using C2 and/or C3 to converge the beam at the specimen. We use this approach to form CBED patterns. Converging the beam destroys any
23、 parallelism, and spots in the pattern are not sharply defined but spread into disks. If we wish to obtain a diffraction patternwith a parallel beam of electrons, the standard way is to use a selecting aperture. This operation is called selected-area diffraction, or SAD, and was invented by Le-Poole
24、 (1947). Now, we cant insert an aperture at the specimen plane, because the specimen is already there! If we insert an aperture in a plane conjugate with the specimen, i.e., in one of the image planes, then it creates a virtual aperture at the plane of the specimen. The conjugate plane that we choos
25、e is the image of the objective lens, as shown in Figure 4.1.4.13. We insert the SAD aperture into the image plane of the objective lens and center the aperture on the optic axis in the middle of the viewing screen. You can see the image of this aperture on the viewing screen. It must be focused by
26、adjusting the intermediate lens so it is conjugate with (i.e., exactly in the plane of) the image of the specimen that we focused with the objective lens. Then any electron that hits the specimen outside the area defined by the virtual aperture will hit the real diaphragm when it travels on to the i
27、mage plane. It will thus be excluded from contributing to the diffraction pattern that is projected onto the viewing screen. Figure 4.1.4.13. Ray diagram showing SAD pattern formation: the insertion of an aperture in the image plane results in the creation of a virtual aperture in the plane of the s
28、pecimen. Only electrons falling inside the dimensions of the virtual aperture at the specimen will be allowed through into the imaging system. All other electrons will hit the SAD diaphragm. In practice, we cant make apertures smaller than about 10 m, and the demagnification back to the plane of the
29、 specimen is only about 25, which gives a minimum selected area of 0.4 m-which isnt as small as wed like. The SAD pattern is often displayed on the viewing screen at a fixed magnification. By analogy with the hand-held camera we define a distance called the “camera length“ (L). This distance corresp
30、onds to the distance of the “film“ from the diffraction pattern. We choose the value of L such that thespacingsin the diffraction pattern are easily discernible on the screen and on the photographic plate. This magnification can be changed by adjusting the intermediate lenses. It is a basic principl
31、e of TEM operation that when you want to look at the diffraction pattern (i.e., the back focal plane of the objective lens), you put an SAD aperture into the image plane of the objective lens. The End! 4.1.4.3.B. Bright-Field and Dark-Field Imaging When the SAD pattern is projected onto the viewing
32、screen, we can use the pattern to perform the two most basic imaging operations in the TEM. No matter what kind of specimen youre are looking at, the SAD pattern will contain: a bright central spot which contains the direct electrons and some scattered electrons. When we form images in the TEM, we e
33、ither form an image using the central spot, or we use some or all of the scattered electrons. The way we choose which electrons form the image is to insert an aperture into the back focal plane of the objective lens, thus blocking out most of the diffraction pattern except that which is visible thro
34、ugh the aperture. If the direct beam is selected as shown in Figure 4.1.4.14A, we call the resultant image a bright-field (BF) image, and if we select scattered electrons of any form, we call it a dark-field (DF) image, as shown in Figure 4.1.4.14B. Figure 4.1.4.14. Ray diagrams showing how the obje
35、ctive lens/aperture are used in combination to produce (A) a BF image formed from the direct beam, (B) a displaced- aperture DF image formed with a specific off-axis scattered beam, and (C) a CDF image where the incident beam is tilted so that the scattered beam remains on axis. The area selected by
36、 the objective aperture, as seen on the viewing screen, is shown below each ray diagram. 4.1.4.3.C. Centered DF Operation If you look at Figure 4.1.4.14B, the electrons that are selected by the aperture travel off the optic axis, since we displace the aperture to select the scattered electrons. Thes
37、e off-axis electrons suffer aberrations and astigmatism and the DF image is difficult to focus, since it will move on the screen as you adjust the objective lens strength. To avoid this you have to adjust the beam tilt potentiometers above the objective lens so that the incident beam hits the specim
38、en at an angle equal and opposite to the scattering angle. In this way the scattered electrons will now travel down the optic axis, as shown in Figure 4.1.4.14C. This operation is called centered dark-field (CDF) imaging and is the way to do DF imaging in the TEM, if you want to record the best, foc
39、used image. 4.1.4.4. FORMING DIFFRACTION PATTERNS AND IMAGES: THE STEM IMAGING SYSTEM If you want to use a fine probe to form STEM images, then the objective lens optics are a little more complex than in TEM. The key feature to remember is that the scanning beam must not change direction as the beam
40、 is scanned. The beam has to scan parallel to the optic axis at all times so that it mimics the parallel beam in a TEM even though its scanning. Figure 4.1.4.15. Scanning the convergent probe for STEM image formation using two pairs of scan coils between the C2 lens (usually switched off) and the up
41、per objective polepiece. The probe remains parallel to the optic axis as it scans. 4.1.4.4.A. Bright-Field STEM Images Figure 4.1.4.16. The creation of a stationary (convergent-beam) diffraction pattern in the back focal plane of the objective lens is a necessary prerequisite for STEM imaging. Note
42、that electrons scattered through 2at different points in the specimen are focused at the same point in the focal plane. Figure 4.1.4.17. The principle of forming a scanning image, showing how the same scan coils in the microscope control the beam-scan on the specimen and the beam-scan on the CRT. Th
43、us no lenses are required to form the image. The End! 4.1.4.4.B. Dark-Field STEM Images The approach is analogous to that of TEM. We form a DF image by selecting any of the scattered electrons, rather than the direct electrons. The End! 4.1.4.4.C. Annular DF Images Figure 4.1.4.18. STEM image format
44、ion: A BF detector is placed in a conjugate plane to the back focal plane to intercept the direct beam (A) and a concentric annular DF detector intercepts the diffracted electrons (B). The signals from either detector are amplified and modulate the STEM CRT. The specimen (Au islands on a C film) giv
45、es complementary ADF (C) and BF (D) images. The End! 4.1.4.4.D. Magnification in STEM The End! 4.1.4.5. ALIGNMENT AND STIGMATION 4.1.4.5.A. Lens Rotation Centers Figure 4.1.4.19. When the objective lens center of rotation is misaligned, the image appears to rotate about a point away from the center
46、of the viewing screen when the lens is wobbled about focus. The End! 4.1.4.5.B. Correction of Astigmatism in the Imaging Lenses Figure 4.1.4.20. The image of a hole in an amorphous carbon film illuminated with a parallel beam showing that (A) with the beamunderfocused, a brightFresnelfringe is visib
47、le; (B) with the beam overfocused a dark fringe is visible; (C) at exact focus there is no fringe; and (D) residual astigmatism distorts the fringe. The End! 4.1.4.6. CALIBRATION OF THE IMAGING SYSTEM 4.1.4.6.A. Magnification Calibration Figure 4.1.4.21. (A) An image of a diffraction grating replica
48、 in which the actual spacing of the grating is known. (B) The TEM magnification can thus be calibrated, relating specific magnification settings to be assigned specific magnifications. A B The End! 4.1.4.6.B. Camera-Length Calibration Figure 4.1.4.22. The relationship between the spacing R of diffra
49、ction maxima and the camera length, L. Increased magnification corresponds to effectively increasing L, although in practice this is accomplished with lenses. =22tan L R From the Bragg equation we know that /d = 2 sin 2, and so we can write: LRd= dKdLR/= Thus to calibrate the magnification of the diffraction pattern we need to record patterns from a specimen with known crystal spacing (d), such as a thin film of a polycrystalline Au or Al. We know the lattice parameter of the specimen, we can measure the ring radius R on the photographic film for any plane that is diffractin