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Basic principle and application of Scanning Electron Microscope (SEM), Tunneling Electron Microscope(TEM) and Atomic Force Microscope (AFM)

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  1. 1. Template for Microsoft PowerPointPRESENTED BY – ANAMIKA BANERJEE
  3. 3. Here comes your footer  Page 3 SCANNING ELECTRON MICROSCOPE (SEM) Von Ardenne first SEM in 1938 by rastering the electron beam. Zworykin et al. 1942, first SEM for bulk samples. 1965 first commercial SEM by Cambridge Scientific Instruments
  4. 4. CHARACTERISTIC INFORMATIONS  Topography- the surface features of an object or how it looks, its texture.  Morphology – the shape and size of the particles making up the object.  Composition - The elements and compounds that the object is composed of and the relative amount of them.  Crystallographic information – How the atoms are arranged in the object.
  5. 5. COMPONENTS OF SEM Here comes your footer  Page 5 a source (electron gun) of the electron beam which is accelerated down the column . a series of lenses which act to control the diameter of the beam as well as to focus the beam on the specimen; a series of apertures which the beam passes through and which affect properties of that beam; an area of beam/specimen interaction that generates several  types of signals that can be detectedand processed to produce an image or spectra; all of the above maintained at high vacuum levels
  6. 6. MENA3100 Electron beam-Sample interactions • The incident electron beam is scattered in the sample, both elastically and inelastically • This give rise to various signals that we can detect. • Interaction volume increases with increasing acceleration voltage and decreases with increasing atomic number Images: Smith College Northampton, Massachusetts
  7. 7. Electron Guns We want many electrons per time unit per area (high current density) and as small electron spot as possible • Thermionic Electron Gun(TEG): electrons are emitted when a solid is heated – W-wire, LaB6-crystal • Field Emission Guns (FEG): cold guns, a strong electric field is used to extract electrons With field emission guns we get a smaller spot and higher current densities compared to thermionic guns -Single crystal of W, etched to a thin tip Single crystal of LaB6 Tungsten wire Field emission tip
  8. 8. MENA3100 Detectors Secondary electron detector: (Everhart-Thornley) Backscattered electron detector: (Solid-State Detector) X-rays: Energy dispersive spectrometer (EDS) Image: Anders W. B. Skilbred, UiO
  9. 9. Vacuum • Chemical (corrosion!!) and thermal stability is necessary for a well-functioning filament (gun pressure) – A field emission gun requires ~ 10-10 Torr – LaB6: ~ 10-6 Torr • The signal electrons must travel from the sample to the detector (chamber pressure) – Vacuum requirements is dependant of the type of detector
  10. 10. HOW THE SEM WORKS? Here comes your footer  Page 10  The SEM uses electrons instead of light to form an image.  A beam of electrons is produced at the top of the microscope by heating of a metallic filament.  The electron beam follows a vertical path through the column of the microscope. It makes its way through electromagnetic lenses which focus and direct the beam down towards the sample.  Once it hits the sample, other electrons are ejected from the sample. Detectors collect the secondary or backscattered electrons, and convert them to a signal that is sent to a viewing screen similar to the one in an ordinary television, producing an image.
  11. 11. Here comes your footer  Page 11 Incoming electrons Secondary electrons Backscattered electrons Auger electrons X-rays Cathodo- luminescence Sample When the accelerated beam of electrons strike a specimen they penetrate inside it to depths of about 1 μm and interact both elastically and inelastically with the solid, forming a limiting interaction volume from which various types of radiation emerges -
  12. 12. Here comes your footer  Page 12 The most common imaging mode collects low- energy (<50 eV) Secondary electrons that are ejected from the k-shell of the specimen atoms by inelastic scattering interactions with beam electrons. Due to their low energy, these electrons originate within a few nanometers from the sample surface Backscattered electrons (BSE) consist of high-energy electrons originating in the electron beam, that are reflected or back-scattered out of the specimen interaction volume by elastic scattering interactions with specimen atoms. Since heavy elements (high atomic number) backscatter electrons more strongly than light elements (low atomic number), and thus appear brighter in the image. BSE- DETECTOR SE - DETECTOR
  13. 13. BSE v/s SE SE produces higher resolution images than BSE Resolution of 1 – 2 nm is possible Here comes your footer  Page 13 BSE SE
  14. 14. MENA3100 SUMMARY • Signals: – Secondary electrons (SE): mainly topography • Low energy electrons, high resolution • Surface signal dependent on curvature – Backscattered electrons (BSE): mainly chemistry • High energy electrons • “Bulk” signal dependent on atomic number – X-rays: chemistry • Longer recording times are needed
  15. 15. 156 electrons! Image Detector Electron gun 288 electrons!
  16. 16. Here comes your footer  Page 16 The SEM image is a 2-D intensity map in the analog or digital domain. Each image pixel on the display corresponds to a point on the sample, which is proportional to the signal intensity captured by the detector at each specific point. Unlike optical TEM no true image exists in the SEM. It is not possible to place a film anywhere in the SEM and record an image. The image is generated and displayed electronically. The images in the SEM are formed by electronic synthesis, no optical transformation takes place, and no real of virtual optical images are produced in the SEM.
  17. 17. MAGNIFICATION IN THE SEM • No optical transformation is responsible for image magnification in the SEM. • Magnification in the SEM depends only on the excitation of the scan coils which determines the focus of the beam. • The magnification of the SEM image is changed by adjusting the length of the scan on the specimen (Lspec) for a constant length of scan on the monitor (Lmon), which gives the linear magnification of the image (M) • M = Lmon/Lspec Here comes your footer  Page 17
  18. 18. MENA3100 Some Comments on RESOLUTION • Best resolution that can be obtained when size of the electron spot is complimentary to the sample surface – The introduction of FEG has dramatically improved the resolution of SEM’s • The volume from which the signal electrons are formed defines the resolution – SE image has higher resolution than a BSE image • Scanning speed: – a weak signal requires slow speed to improve signal-to-noise ratio – when doing a slow scan drift in the electron beam can affect the accuracy of the analysis
  19. 19. SAMPLE REQUIREMENTS • Since the SEM is operated under high which means that liquids and materials containing water and other volatile components cannot be studied directly. Also fine powder samples need to be fixed firmly to a specimen holder substrate so that they will not contaminate the SEM specimen chamber. • Non-conductive materials need to be attached to a conductive specimen holder and coated with a thin conductive film by sputtering or evaporation. Typical coating materials are Au, Pt, Pd, their alloys, as well as carbon. • There are special types of SEM instruments such as VPSEM and ESEM that can operate at higher specimen chamber pressures thus allowing for non-conductive materials or even wet specimens to be studied. Here comes your footer  Page 19
  20. 20. MENA3100 WHY ESEM? • To image challenging samples such as: – insulating samples – vacuum-sensitive samples (e.g. biological samples) – irradiation-sensitive samples (e.g. thin organic films) – “wet” samples (oily, dirty, greasy) • To study and image chemical and physical processes in-situ such as: – mechanical stress-testing – oxidation of metals – hydration/dehydration
  22. 22. Here comes your footer  Page 22 TUNNELING ELECTRON MICROSCOPE (TEM) The first electron microscope was built 1932 by the German physicist Ernst Ruska, who was awarded the Nobel Prize in 1986 for its invention. the first commercial TEM in 1939. 1nm resolution Typical accel. volt. = 100-400 kV (some instruments - 1-3 MV)
  23. 23. BASIC PRINCIPLES The design of a transmission electron microscope (TEM) is analogous to that of an optical microscope. In a TEM high-energy (>100 kV) electrons are used instead of photons and electromagnetic lenses instead of glass lenses. The electron beam passes an electron- transparent sample and a magnified image is formed using a set of lenses. This image is projected onto a fluorescent screen or a CCD camera. Whereas the use of visible light limits the lateral resolution in an optical microscope to a few tenths of a micrometer, the much smaller wavelength of electrons allows for a resolution of 0.2 nm in a TEM.
  24. 24. Condenser system :(lenses & apertures)for controlling illumination on specimen Objective lens system: image- forming lens - limits resolution; aperture - controls imaging conditions Projector lens system: magnifies image or diffraction pattern onto final screen Intermediate lens: transmitting or magnifying the enlarge image. INSTRUMENT COMPONENTS
  25. 25. SAMPLE Incoming electrons Secondary electrons Auger electrons X-rays Cathodo- luminescence Inelastically scattered electrons Inelastically scattered electronsunscattered electrons Backscattered electrons
  26. 26. IMAGING Image contrast is obtained by interaction of the electron beam with the sample. In the resulting TEM image denser areas and areas containing heavier elements appear darker due to scattering of the electrons in the sample. In addition, scattering from crystal planes introduces diffraction contrast. This contrast depends on the orientation of a crystalline area in the sample with respect to the electron beam. As a result, in a TEM image of a sample consisting of randomly oriented crystals each crystal will have its own grey-level. In this way one can distinguish between different materials, as well as image individual crystals an crystal defects. Because of the high resolution of the TEM, atomic arrangements in crystalline structures can be imaged in large detail High resolution TEM image of a multi- walled carbon nanowire. The wire consists of segments, bounded by inner segment boundaries (From 2008 Koninklijke Philips Electronics N.V.).
  27. 27. RESOLUTION IN TEM • In a TEM, a monochromatic beam of electrons is accelerated through a potential of 40 to 100 kilovolts (kV) and passed through a strong magnetic field that acts as a lens. The resolution of a modern TEM is about 0.2 nm. • More recently, advances in aberration corrector design have been able to reduce spherical aberrations and to achieve resolution below 0.5 Ångströms at magnifications above 50 million times. • rth = 0.61λ/β β= semi-collection angle of magnifying lens λ= electron wavelength 4/14/3 67.0 sCr  Best attained resolution ~0.07 nm Nature (2006) Cs = spherical aberration
  28. 28. Electron Energy Loss Spectroscopy (EELS) When travelling through the sample the electrons may lose energy due to (multiple) inelastic scattering events. The amount of energy that is transferred from the incident electron to the sample is dependent on the composition of the sample. Because the primary beam of electrons has one well-defined energy, the spectrum of the electrons that have passed the sample contains chemical information on the irradiated area. Quantification of the spectrum enables determination of (local) concentrations of elements. The fine structure in the EELS spectra provides information on the chemical binding of the atoms involved.
  29. 29. TOMOGRAPHY TEM tomography involves the acquisition of a large series of images at many tilt angles of the sample with respect to the electron beam. In analogy to the CT scanner used in medical diagnostics, the acquired tilt-series is reconstructed into a 3-D representation. This technique is especially useful in case of studies on 3-dimensionally shaped objects, such as layers in small pores and 3-D shapes of small objects The 3-dimensional reconstruction of the morphology of a part of a GaP-GaAs hetero-structured nanowire with 40 nm diameter.
  30. 30. SAMPLE PREPARATION IN TEM TEM foil specimens were prepared by mechanical dimpling down to 20 μm, followed by argon ion milling operating at an accelerating voltage of 5 kV and 10° incidence angle, with a liquid nitrogen cooling stage to avoid sample heating and microstructural changes associated with the annealing effect. For TEM observations, thin samples are required due to the important absorption of the electrons in the material. High acceleration voltage reduces the absorption effects but can cause radiation damage (estimated at 170 kV for Al). At these acceleration tensions, a maximum thickness of 60 nm is required for TEM
  31. 31. CONTRASTS: Electrons that go through a sample DIFFRACTION CONTRAST Formed by incident electrons that are scattered by the atoms of the specimen elastically. These electrons can then be collated using magnetic lenses to form a pattern of spots; each spot corresponds to a specific atomic spacing (a plane). This pattern can then yield information about the orientation, atomic arrangements and phases present in the area being examined. BRIGHT FIELD CONTRAST formed directly by occlusion and absorption of electrons in the sample. Thicker regions of the sample, or regions with a higher atomic number will appear dark, whilst regions with no sample in the beam path will appear bright hence the term "bright field".
  32. 32. LIMITATIONS OF THE TEM • Sampling---0.3mm3 of materials: The higher the resolution the smaller the analyzed volume becomes. Drawing conclusions from a single observation or even single sample is dangerous and can lead to completely false interpretations • Interpreting transmission images---2D images of 3D specimens, viewed in transmission, no depth-sensitivity. • Electron beam damage and safety---particularly in polymer and ceramics: The high energy of the electron beam utilized in electron microscopy causes damage by ionization, radiolysis, and heating • Specimen preparation---”thin” below 100nm
  33. 33. ADVANCES IN TEM CRYO-TEM Using dedicated equipment, it is possible to freeze 0.1 μm thick water films and study these films at -170˚C in the TEM. This enables imaging of the natural shape of organic bilayer structures. Also, agglomeration processes in a dispersion can be studied. In addition, the application of cryogenic conditions facilitates studies of beam-sensitive samples. HAADF Another way of obtaining compositional as well as structural information using TEM is High Angle Annular Dark Field (HAADF) imaging. For this application a dedicated detector is used that only collects electrons that are elastically scattered over large angles by the specimen. The intensity that is detected is dependent on the average atomic number Z ENERGY FILTERED TEM (EFTEM) A special filter on the TEM allows for selection of a very narrow window of energies in the EELS spectrum. Using the corresponding electrons for imaging, EFTEM is performed. As a result, a qualitative elemental map is obtained. EFTEM is the only chemical analysis procedure in the TEM that does not use a scanning beam. As a consequence, it is much faster.
  34. 34. SEM V/S TEM • in SEM is based on scattered electrons • The scattered electrons in SEM produced the image of the sample after the microscope collects and counts the scattered electrons. • SEM focuses on the sample’s surface and its composition. • SEM shows the sample bit by bit • SEM provides a three- dimensional image • SEM only offers 2 million as a maximum level of magnification. • SEM has 0.4 nanometers. • TEM is based on transmitted electrons • In TEM, electrons are directly pointed toward the sample. • TEM seeks to see what is inside or beyond the surface. • TEM shows the sample as a whole. • TEM delivers a two-dimensional picture. • TEM has up to a 50 million magnification • The resolution of TEM is 0.5 angstroms
  35. 35. SUMMARY Each microscope works is very different from another. SEM scans the surface of the sample by releasing electrons and making the electrons bounce or scatter upon impact. The machine collects the scattered electrons and produces an image. The image is visualized on a television-like screen. On the other hand, TEM processes the sample by directing an electron beam through the sample. Images are also a point of difference between two tools. SEM images are three-dimensional and are accurate representations while TEM pictures are two-dimensional and might require a little bit of interpretation. In terms of resolution and magnification, TEM gains more advantages compared to SEM. The result is seen using a fluorescent screen
  37. 37. BRIEF HISTORY OF AFM Atomic force microscopy (AFM) to investigate the electrically non-conductive materials, like proteins. In 1986, Binnig and Quate demonstrated for the first time the ideas of AFM, which used an ultra-small probe tip at the end of a cantilever (Phys. Rev. Letters, 1986, Vol. 56, p 930). In 1987, Wickramsinghe et al. developed an AFM setup with a vibrating cantilever technique (J. Appl. Phys. 1987, Vol. 61, p 4723), which used the light-lever mechanism.
  39. 39. COMPARISON BETWEEN AFM AND ELECTRONIC MICROSCOPES • Optical and electron microscopes can easily generate two dimensional images of a sample surface, with a magnification as large as 1000X for an optical microscope, and a few hundreds thousands ~100,000X for an electron microscope. • However, these microscopes cannot measure the vertical dimension (z- direction) of the sample, the height (e.g. particles) or depth (e.g. holes, pits) of the surface features. • AFM, which uses a sharp tip to probe the surface features by raster scanning, can image the surface topography with extremely high magnifications, up to 1,000,000X, comparable or even better than electronic microscopes. • measurement of an AFM is made in three dimensions, the horizontal X-Y plane and the vertical Z dimension. Resolution (magnification) at Z-direction is normally higher than X-Y plane.
  40. 40. ATOMIC INTERACTION AT DIFFERENT TIP-SAMPLE DISTANCES Repulsion: • At very small tip-sample distances (a few angstroms) a very strong repulsive force appears between the tip and sample atoms. Its origin is the so-called exchange interactions due to the overlap of the electronic orbitals at atomic distances. When this repulsive force is predominant, the tip and sample are considered to be in “contact”. Attraction (Van der Waals): • A polarization interaction between atoms: An instantaneous polarization of an atom induces a polarization in nearby atoms – and therefore an attractive interaction.
  41. 41. Different modes of tip-sample interaction when in contact Friction: The cantilever bends laterally due to a friction force between the tip and the sample surfaces. Adhesion: • Adhesion can be defined as “the free energy change to separate unit areas of two media from contact to infinity in vacuum or in a third medium”. • In general, care has to be taken with the term adhesion, since it is also used to define a force - the adhesion force. In addition to the intrinsic adhesion between tip and sample, there is another one from the capillary neck condensing between the tip and water meniscus --- interference from the huminity.
  42. 42. Electromagnetic interactions between tip and sample • Electrostatic interaction: Caused by both the localized charges and the polarization of the substrate due to the potential difference between the tip and the sample. It has been used to study the electrostatic properties of samples such as charges on insulator surfaces or ferroelectric domains. • Magnetic interaction: Caused by magnetic dipoles both on the tip and the sample.
  43. 43. AFM IMAGING MODES Contact mode (left): the deflection of cantilever is kept constant. Non-contact mode (right): the tip is oscillated at the resonance frequency and the amplitude of the oscillation is kept constant. Tapping mode: somewhere between the contact and non- contact mode.
  44. 44. CONTACT MODE Contact mode AFM consists of raster-scanning the probe (or sample) while monitoring the change in cantilever deflection with the split photodiode detector. A feedback loop maintains a constant cantilever deflection by vertically moving the scanner to maintain a constant photo-detector difference signal. The distance scanner moves vertically at each x, y data point is stored by the computer to form the topographic image of the sample surface. This feedback loop maintains a constant force during imaging.
  45. 45. Two contact scanning modes: Constant Height and Constant Force Constant-force mode Constant-height mode
  46. 46. Constant-force scan v/s constant-height scan Constant-force • Advantages: – Large vertical range – Constant force (can be optimized to the minimum) • Disadvantages: – Requires feedback control – Slow response Constant-height • Advantages: – Simple structure (no feedback control) – Fast response • Disadvantages: – Limited vertical range (cantilever bending and detector dynamic range) – Varied force
  47. 47. Tapping Mode AFM consists of oscillating the cantilever at its resonance frequency (typically ~300kHz) and lightly “tapping” the tip on the surface during scanning. A feedback loop maintains a constant oscillation amplitude by moving the scanner vertically at every x,y data point. Recording this movement forms the topographical image. The advantage of Tapping Mode over contact mode is that it eliminates the lateral, shear forces present in contact mode. This enables Tapping Mode to image soft, fragile, and adhesive surfaces without damaging them, which can be a drawback of contact mode AFM.
  48. 48. Comparison between the three scanning modes: damage to the sample  Contact mode imaging (left) is heavily influenced by frictional and adhesive forces, and can damage samples and distort image data.  Non-contact imaging (center) generally provides low resolution and can also be hampered by the contaminant (e.g., water) layer which can interfere with oscillation.  Tapping Mode imaging (right) takes advantages of the two above. It eliminates frictional forces by intermittently contacting the surface and oscillating with sufficient amplitude to prevent the tip from being trapped by adhesive meniscus forces from the contaminant layer.
  49. 49. Imaging by contact and non-contact (tapping) mode
  50. 50. AFM IN LIQUID ENVIRONMENT One extraordinary feature of AFM is to work in liquid environment. A key point for liquid AFM is a transparent solid (usually glass) surface, which, together with the solid sample surface, retains the liquid environment whilst maintains stable optical paths for the laser beams. An optional O- ring can be used to form a sealed liquid cell. Otherwise, the system can also work in an “open cell” fashion.
  51. 51. Advanced imaging techniques of AFM Contact-Mode scanning • Lateral force microscope (LFM) --- measures lateral deflections, shows surface friction. • Force modulation microscope (FMM) --- detecting surface stiffness or elasticity; Tapping-Mode scanning • Phase mode imaging --- detecting surface structure or elasticity property.
  52. 52. AFM v/s SEM Compared with Scanning Electron Microscope, AFM provides extraordinary topographic contrast direct height measurements and un-obscured views of surface features (no coating is necessary). SEM is conducted in a vacuum environment, and AFM is conducted in an ambient or fluid environment Si covered with GaP SEM IMAGE AFM IMAGE
  53. 53. AFM v/s TEM • Compared with two dimensional Transmission Electron Microscopes, three dimensional AFM images are obtained. • No expensive sample preparation in AFM is required as compared to TEM and yield far more complete information than the two dimensional profiles available from cross- sectioned samples.
  54. 54. SUMMARY