Chapter 2 Instrumentation for Analytical Electron Microscopy Lecture 7. Chapter 2 CHEM Fall L. Ma

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1 Chapter 2 Instrumentation for Analytical Electron Microscopy Lecture 7

2 Outline Electron Sources (Electron Guns) Thermionic: LaB 6 or W Field emission gun: cold or Schottky Lenses Focusing Aberration Probe size Detectors electron detectors X-ray detectors energy loss spectrometers Imaging Diffraction Bright field imaging (BF) Dark field imaging (DF) Phase contrast imaging (HRTEM) mode Scanning transmission electron microscopy (STEM) mode Holography mode (not covered) etc.

3 Illumination system: SEM and TEM are similar Objective lens and stage Observation /imaging system

4 Lens A lens is an optical component which is used to focus beams of radiation. Lenses for light are usually made of a glassy material, whereas nonuniform electromagnetic fields are used as lens for electrons. Magnetic field, B Current, I, in coil

5 Magnetic Lenses Electron ray paths A magnetic lens consists of a wire winding around a soft iron core The magnetic field in the lens deflects the electron path, resulting in a focusing of the beam Right hand rule of vector product to determine the force on the electron r F r d 2 r = m dt 2 = r r ( r ) ( r ) q E + v B = e v B r

6 B Paraxial trajectory governing equation Φ: potential Z r ө m e B dz d r B dz r d = Φ = = Φ + η η θ η B ev dt r d m r r r = 2 2 P.W. Hawkes, Electron optics and electron microscopy, 1989, Taylor & Francis Ltd., ISBN

7 Vz Br Upper pole-piece B Bz BFP f lower pole-piece Focusing action of magnetic lens Optical Axis

8 Summary: The magnetic lens produces a strong magnetic field B by passing a current through a set of windings (copper coil). In TEM, this field act as a convex lens, bringing off axis rays back to focus. For magnetic lens, the image is rotated to a degree depending on the strength of the lens. Focal length also can be changed by changing the strength of the current, and thereby of B. Convex lens A convex lens (for light) is thicker in the centre than at its periphery. All electromagnetic lenses used in electron microscopy act as if they were convex lenses. Image is rotated Convex lens

9 Summary 2: All electronmagnetic lenses act like thin convex lenses. So their thickness can be ignored. Equations for convex lens are applicable, like lens formula 1/f= 1/v+ 1/u Magnification: M=v/u

10 E-gun Schematic lens configuration of a TEM system

11 In electron microscope, double condenser (C1 and C2) system is used to adjust the illumination condition. The double condenser system or illumination system consists of two or more lenses and an aperture. It is used in both SEM and TEM. Its function is to control spot size (C1) and beam convergence and intensity (C2). Two or more lenses can act together and their ray diagrams can be constructed using the thin lens approximation for each of them. The diagram opposite shows the ray diagram for the double condenser system. The black dots represent the focal point of each lens.

12 First condenser lens C1 the first condenser lens is shown highlighted in the diagram. Its function is to create a demagnified image of the gun crossover, which acts as the object for the illumination system control the minimum spot size obtainable in the rest of the condenser system.

13 Second condenser lens C2 C2 is adjusted the focus to produce an image of gun crossover at the front focal plane of the upper objective polepiece, and then generate a broad parallel beam of electron incident on the specimen. C2 can affects the convergence of the beam at the specimen, and the diameter of the illuminated area of the specimen. Question: Sitting at the microscope you can only see the image of the specimen on the fluorescent screen. How would you know when the condenser lens (C2) is focused on the specimen? If you now go away from this condition how could you tell whether the beam was overfocused or underfocused?"

14 Condenser aperture The condenser aperture controls the fraction of the beam which is allowed to hit the specimen. It therefore helps to control the intensity of illumination, and in the SEM, the depth of field.

15 Parallel beam with large aperture, i.e. large convergence. Parallel beam with small aperture, i.e. small convergence.

16 Objective lens and specimen stage Heart of TEM The objective lens forms an inverted initial image, which is subsequently magnified. In the back focal plane of the objective lens a diffraction pattern is formed. The objective aperture can be inserted here. The effect of inserting the aperture is shown on the next page. The objective lens would not usually provide a magnification of more than 50 and a TEM is routinely used to view regions of the specimen which are only a mm or so across. What does this imply about the diameter of a typical aperture if it were placed at the back focal plane as shown in this diagram? Diffraction (prior knowledge) Diffraction is an interference effect which leads to the scattering of strong beams of radiation in specific directions. Diffraction from crystals is described by the Bragg Law n λ = 2 d sin θ where n is an integer (the order of scattering), λ is the wavelength of the radiation, d is the spacing between the scattering entities (e.g. planes of atoms in the crystal) and θ is the angle of scattering. Electron and X-ray diffraction are both particularly powerful because their wavelengths are smaller than the typical spacings of atoms in crystals and strong, easily measurable, diffraction occurs.

17 Cross-sectional view of the Chapter gonionmeter 2 CHEM area of the Fall JEOL 120 CX microscope

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20 Objective aperture The objective aperture is placed in the back focal plane of the image. Its function is to select those electrons which will contribute to the image, and thereby affect the appearance of the image and improve the contrast of the final image. Assist to focus the diffraction image. Aperture removed Diffraction patterns are formed in BFP Contrast (prior knowledge) In microscopy, contrast is the difference in intensity between a feature of interest (Is) and its background (I0). Contrast is usually described as a fraction such as: (Is-I0)/Is. The term has also been extended to mean the physical mechanism by which a feature can be seen on its background (e.g. dislocation contrast in TEM). Aperture inserted In summary, the objective lens is to take electrons emerging from the exit surface of the specimen, disperse them to create a diffraction pattern (DP) in the BFP, and recombine them to form an image in the image plane

21 Objective aperture Change focal strength SAD aperture Intermediate lens The first intermediate lens magnifies the initial image that is formed by the objective lens. The lens can be focused on initial image formed by the objective lens, or Diffraction pattern formed in the back focal plane of the objective lens. This determines whether the viewing screen of Chapter the microscope 2 CHEM shows 793 a diffraction Fall pattern or an image.

22 TEM operation mode Setting and beam condition TEM operation mode: 1. Parallel beam Bright-field imaging mode Dark-field image mode HRTEM many beam condition SAD mode 2. Convergent beam STEM mode CBED mode 3. Holography mode ( optional) Beam Condition Image mode By inserting the aperture or tilting the beam, different types of images can be formed. The most common conditions are: (a) No aperture - the diffraction pattern is centered on the optical axis. (b). Aperture is centered on the optical axis. (c). Aperture displaced, selecting a diffracted beam. (d) Beam is tilted so that the diffracted beam is on the optical axis.

23 Bright field mode (BF) In the bright field (BF) mode of the TEM, an aperture is placed in the back focal plane (BFP) of the objective lens which allows only the direct beam to pass. In this case, the image results from a weakening of the direct beam by its interaction with the sample. Therefore, mass-thickness and diffraction contrast contribute to image formation: thick areas, areas in which heavy atoms are enriched, and crystalline areas appear with dark contrast. It should be mentioned that the interpretation of images is often impeded by the simultaneous occurrence of the contrastforming phenomena

24 Bright field mode (BF) In dark field (DF) images, the direct beam is blocked by the aperture while one or more diffracted beams are allowed to pass the objective aperture. Since diffracted beams have strongly interacted with the specimen, very useful information is present in DF images, e.g., about planar defects, stacking faults or particle size

25 Off axis DF image mode Aperture is displaced, a specific off-axis diffracted beam is selected to form an image. Also called dirty DF image. The off-axis beam suffer aberration and astigmatism, and DF image is hard to focus. The resolution cannot be enhanced. Dirty DF imaging mode Beam Condition

26 Off axis DF image mode Aperture is displaced, a specific off-axis diffracted beam is selected to form an image. Also called dirty DF image. The off-axis beam suffer aberration and astigmatism, and DF image is hard to focus. The resolution cannot be enhanced. Beam Condition

27 Centered DF image mode Aperture is centered on the optical axis. The incident beam is tilted so that the diffracted beam remains on axis. Typical magnification ranges is 25k~100k

28 Objective aperture Change focal strength SAD aperture SAD mode (selected area diffraction) By inserting an aperture in a plane conjugate with the specimen. I.e. in one of the image plane. Usually, the diameter of aperture is 10 µm. The magnification is x25, so a minimum selected area is ~ 0.4 µm. In order to make the diffraction pattern visible, the intermediate lens is refocused on the BFP, and the DP is passed to the intermediate lens Imaging Mode: The OL is focused on the specimen and forms an intermediate image. The intermediate lens magnifies this image further and passes it to the projector lenses for display.

29 Example ZrO2 micro-crystalline SAD BF DF Electron diffraction pattern: the spots indicate the presence of single microcrystals. The apertures (red circles) are localized around the direct beam for recording the bright field (BF) image and around a few diffracted beams for the dark field (DF) image. The intense direct beam is blocked by a metal rod (black shadow on the left center) to avoid overexposure. TEM BF and DF images of microcrystalline ZrO2. In the BF image (left), some crystals appear with dark contrast since they are oriented (almost) parallel to a zone axis (Bragg contrast). In addition, thickness contrast occurs: areas close to the edge are thinner and thus appear brighter (lower right side) than those far of the edge (upper left side). In the DF image (right), some of the microcrystals appear with bright contrast, namely such which diffract into the aperture

30 HRTEM: many beam condition To obtain lattice images, a larger objective aperture has to be selected that allows many beams including the direct beam to pass. The image is formed by the interference of the diffracted beams with the direct beam (phase contrast). If the point resolution of the microscope is sufficiently high and a suitable sample oriented along a zone axis, then highresolution TEM (HRTEM) images are obtained. In many cases, the atomic structure of a specimen can directly be investigated by HRTEM. Prior Knowledge The incident parallel electron beam, ideally a plane wave, interacts elastically while passing through the specimen, and the resulting modulations of its phase and amplitude are present in the electron wave leaving the specimen. The wave here, the object exit wave o(r), thus contains the information about the object structure. Unfortunately, the objective lens is not an ideal but has aberrations (astigmatism, spherical Cs and chromatic Cc aberration) that reduce image quality. The intensity distribution of the exit wave function is described by the contrast transfer function (CTF).

31 HRTEM Example ZrNb6W10O47 particle F. Krumeich, G. Lietke, W. Mader, Acta Crystallogr. B52 (1996) 917

32 STEM image Nano-probe is used. A Bright Field (BF) detector is placed in a conjugate plane to the back focal plane to intercept the direct beam while a concentric Annular Dark Field (ADF) detector intercepts the diffracted electrons. The signals from either detector are amplified and modulate the STEM CRT. The specimen (Au islands on a C film) gives complementary ADF and BF images as can be seen by clicking the button opposite. Image lenses are not used, so lens aberrations are avoided. Z-contrast image, good for biology samples, particles but not very good for crystalline samples. If stop the beam from scanning, a CBED pattern is obtained.

33 Example HRSTEM image: WO3 segregations in a niobium tungsten oxide HRTEM HRSTEM The positions of the metal atoms are recognizable in the HRTEM image recorded close to Scherzer defocus as dark dots while they always appear with bright contrast in the HAADF-STEM (Z contrast) image. The different brightness of dots in the Z contrast image indicate varying occupancies of the corresponding metal position by Nb and W (Krumeich, Nesper, J. Solid State Chem. 179 (2006) ).

34 Some concepts for TEM operation Eucentric Plane (Eucentric height) Minimum contrast focus Astigmation The lists below are optional Alignment Beam tilt and shift Pivot point

35 Eucentric Plane ( Eucentric height) This plane is normal to the optics axis and contains the axis of the specimen When specimen is located at this plane, the image is in focus, and the objective lens current is an optimum value. The position of this plane within the objective lens is known as the eucentric height. A point on the optic axis does not move laterally when you tilt it around the holder axis If you tilt the specimen normal to the holder axis or rotate it off axis, then the point you are examining almost invariably moves out of the eucentric plane. The first thing you must always do when inserting your specimen into the TEM is to ensure that it is in the eucentric plane. To do this, you tilt the specimen and adjust the height of the specimen holder until the image of the specimen remains stationary. In summary, you tilt the specimen around the holder axis without having large apparent movements of the point of interest on the specimen. This is called eucentric tilting and is achieved by bringing the point of interest to the same height as the a tilt axis itself : the eucentric height In all TEM operation mode, specimen must be in eucentric height

36 Minimum contrast focus In microscopy there are three important focus values: Gaussian (0) (prior knowledge), Scherzer (prior knowledge), and minimum contrast focus. The minimum contrast has no fixed definition but in practice is the only one that can be recognized on the microscope Minimum contrast is roughly equal to 0.4 x Scherzer. Using the image of a hole in an amorphous carbon film under the parallel beam as an the example, at minimum contrast focus, there is no fringe and the image contrast is minimized. ( as shown in (b) ). a b c Dark Fresnel fringe: over focused Featureless in background and no fringe around the hole: minimum contrast focus Bright Fresnel fringe: over focused

37 Stigmation Astigmatism is an aberration which is present in all electromagnetic lenses. It is caused by asymmetry of the lens field which can result from inherent asymmetries or from asymmetrical charges on regions close to the beam, e.g. the specimen. Condenser stigmation: Astigmatism is corrected when the focused beam remains as circular as possible when going through beam focus (Intensity). Image stigmation: Three factors can cause image astigmatism: 1.Asymmetry of the objective lens. 2). Dirt on or charging of the objective aperture. 3). The specimen itself. The influence of the specimen on the observed astigmatism can be considerable, particularly in cases where an insulating specimen collects charge, either as a whole or locally. Magnetic specimens also cause strong astigmatism. Astigmatism correction using specimen or FFT ( fast Fourier transformation ) diffragram (discuss in high resolution TEM section)

38 Astigmatism is most easily observed on the screen when viewing Fresnel fringes. These fringes result from diffraction phenomena that occur at sharp edges of a specimen when the objective lens is slightly underfocused or overfocused. When the image is underfocused (objective lens weaker than focus), the Fresnel fringe appears as a bright line round the edge of the detail selected. If the detail is a hole, the line will appear on the inside. When the image is overfocused (objective lens stronger than focus), the Fresnel fringe appears as a dark line but otherwise has the same characteristics as in the underfocused condition. With a perfectly symmetrical objective lens field, the fringes will be of uniform width. With an asymmetrical (astigmatic) objective lens, the fringes will also be asymmetrical and, close to focus, parts of the hole will have a bright fringe and other parts (at 90 to the bright fringe) a dark fringe associated with it. For more information about astigmatism in electron lenses, reference is made to the many text books on electron optics, for example: Transmission Electron Microscopy (2ed.), L. Reimer (1989), Springer-Verlag, Berlin. Experimental High Resolution Electron Microscopy (2ed.), J. C. H. Spence, Oxford University Press, New York, Oxford.

39 Overfocus and astigmatism With an asymmetrical (astigmatic) objective lens, the fringes will also be asymmetrical and, close to focus, parts of the hole will have a bright fringe and other parts (at 90 to the bright fringe) a dark fringe associated with it Overfocus and no stigmatism With a perfectly symmetrical objective lens field, the fringes will be of uniform width

40 Overfocus and astigmatism Overfocus and No astigmatism

41 Over- focus and astigmatism In-focus and no stigmatism ( minimum contrast focus)

42 In focus and astigmatism In-focus and no stigmatism

43 Diffraction stigmation astigmatism No astigmatism