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ELECTRON SPECIMEN INTERACTIONS SPECIMEN PREPARATION ELECTRON MICROSCOPES

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Figure 10 shows the interaction volume as a function of beam energy (Monte Carlo ... Figure 17 shows a typical configuration of an electron gun. 28 ... – PowerPoint PPT presentation

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Title: ELECTRON SPECIMEN INTERACTIONS SPECIMEN PREPARATION ELECTRON MICROSCOPES


1
ELECTRON -SPECIMEN INTERACTIONSSPECIMEN
PREPARATIONELECTRON MICROSCOPES
EML 5930 (27-750) Advanced Characterization and
Microstructural Analysis A. D. Rollett, P.N
Kalu Spring 2008
2
  • Electron Target Interactions
  • Electron Optical Imaging and chemical analysis
    are based upon
  • Detecting ,
  • Collecting, and
  • Processing a range of signals that are produced
    when an electron beam interacts with either bulk
    or foil samples.
  • Figures 9(a) and (b) show beams of high energy
    electrons
  • interacting with bulk and foil samples
    respectively.

3
  • Figure 9(a). A bulk specimen showing the
    spread of the beam and the activated volume
    giving rise to (i) backscattered, secondary and
    Auger electrons (ii) characteristic X-rays and
    (iii) electron current. Typical values given for
    the depth of electron penetration and the excited
    volume for a 30keV, 100nm diameter electron probe.

4
  • Figure 9(b). A foil specimen of thickness, t, (t
    lt 300nm) where in addition to (a) transmitted
    signals give (i) image contrast and electron
    diffraction and (ii) Kikuchi diffraction

5
  • Figure 10 shows the interaction volume as a
    function of beam energy (Monte Carlo
    calculations).
  • The penetration of an electron into a solid
    depends on the beam energy, which in turn depends
    on the accelerating voltage, usually between 20
    kV and 1,000 kV.

6
10 kV
20 kV
7
30 kV
  • Figure 10. Monte Carlo calculations of the
    interaction volume in iron as a function of beam
    energy (a) 10 kV (b) 20 kV, and 30 kV.

8
  • SPECIMEN PREPARATION METHODS
  • BULK SAMPLES
  • Specimens for OPTICAL or SEM examination must be
    prepared via a sequence of polishing stages,
    except for
  • (i) Non conducting specimens the specimen
    surface must be coated with gold or carbon.
    This procedure is not needed if Environmental
    Scanning Electron Microscope (ESEM) is used.
  • (ii) Fracture Evaluation or Failure Analysis -
    the specimen must be preserved and the fracture
    surface should be covered and padded (if
    possible) for protection against further damage.
    On no account should the fracture surface be
    subjected to any form of polishing.

9
  • POLISHING STAGES
  • Mechanical Grinding This is a form of Rough
    Polishing, which involves using different grades
    (or grits) of SiC abrasive papers/belts/cloths,
    with water as lubricant. A commonly employed
    grit sequence uses 120-, 240-, 320-400- and 600-
    mesh abrasive papers.
  • The initial grit size depends on the surface
    roughness and the depth of damage from
    sectioning. Sample orientation must be
    continuously changed to avoid comet tailing.

10
  • Mechanical Polishing After grinding to a 600-
    grit finish, the sample is polished to produce a
    flat, reasonably scratch-free surface with high
    reflectivity.
  • (i) Coarse polishing (30 to 3 ?m finish) - on
    special cloths smeared with diamond paste plus
    some lubricant.
  • (ii) Fine polishing (? 1 ?m finish) - use
    diamond paste plus lubricant or Silica or
    alumina plus water lubricant.
  • (3) Electropolishing or Electrolytic Polishing
    This is a process whereby a mechanically polished
    sample is made smooth and brighter by making the
    sample the anode in an electrolytic cell. This
    is only possible when the correct combination of
    bath temperature, voltage, current density and
    time is employed.

11
  • (4) Chemical Polishing This procedure can be
    used instead of (3). It is a method for
    obtaining a polished surface by immersion in or
    swabbing with suitable solution without need of
    an external electric current.
  • NOTE
  • Specimens for Electron Backscattered Diffraction
    (EBSD) analysis or Orientation Imaging Microscopy
    (OIM) may require special specimen preparation
    methods. This will be discussed latter.

12
  • SPECIMENS FOR TEM
  • Specimens are usually in one or two forms
  • Replica
  • Thin foil
  • REPLICAS
  • These are prepared by different techniques
  • Surface or Carbon replica
  • Extraction replica

13
  • SURFACE or CARBON REPLICA
  • This is produced from a specimen as prepared for
    light microscopy
  • i) The specimen surface is coated with a
    cellulose acetate film (e.g., collodion) to
    produce a negative impression of the surface
    features.
  • ii) The plastic film is stripped from the
    specimen surface and coated with a layer of
    carbon (about 10 to 20 nm thick). The
    carbon-coating process takes place in a vacuum
    evaporator unit.
  • iii) The plastic is then removed (dissolved) from
    the carbon replica by solvent.
  • iv) Before the replica is observed in the
    microscope, it is usually shadowed (sputtered)
    with carbon or a heavy element to enhance the
    topographical features of the surface.

14
  • EXTRACTION REPLICA
  • i) The metallographic specimen is etched
    beforehand to put particles and carbides in
    relief.
  • ii) Carbon film is deposited on the surface of
    the etched specimen.
  • iii) The carbon film itself is not physically
    stripped from the specimen surface, but etched or
    floated away from the surface so that those
    particles attached to the deposited carbon film
    will be extracted from the specimen.
  • THIN FOIL PREPARATION
  • Electropolishing window technique disk
    method jet thinning
  • Ion Beam Thinning
  • Chemical Polishing

15
  • window technique

Figure 11. Typical stages in electropolishing
sheet material using the window technique. The
shaded regions a-d are lacquered.
16
  • disk method

Figure 12. (a) Anode current density I vs.
specimen potential E. Curve ABCD shows a
polishing plateau BC whereas curve EF does not.
(b) Schematic diagram illustrating
potentiostatically controlled electropolishing
17
  • Jet thinning

Figure 13. Schematic diagram illustrating the
action of an a.c. jet thinning technique due to
Bainbridge and Thorne.
18
  • Ion Beam Thinning

Figure 14. Schematic diagram of typical ion-beam
thinning equipment
19
  • Figure 15. Schematic diagrams showing
  • (a) the depth of penetration of argon ions as a
    function of the angle between the beam and
    specimen, and
  • (b) the variation of thinning rate with angle of
    beam incidence

20
Other Thin-Foil Preparation Techniques
  • Microtomy
  • Replicas
  • Single-stage replicas 1. Deposit carbon
    (100-200A) on surface of interest 2. Remove by
    etching the specimen to free the porous film
    and floating the film off the specimen 3.
    Shadow with a heavy element (gold or silver)
  • Two-stage replicas
  • Extraction replicas - useful for small particle
    ID and characterization

21
  • Artifacts in Thin Foils
  • Careless Handling Dislocations, Bend Contours,
    Cracks
  • Careless Washing Surface films/deposits
  • Hydrogen Contamination During electropolishing
    with acid electrolytes (Ti and Zr alloys)
  • Interstitial Contamination during
    electropolishing or storage (Nb and Ta)
  • Surface Oxidation even during examination in the
    microscope- can effect structure (stacking
    faults, extra reflections of Moire patterns if
    the oxide is crystalline) or apparent
    composition also, surface mottling common
    streaks in diffraction pattern

22
  • Artefacts in Thin Foils Continued
  • Electron or ion beam damage in the
    microscope--may produce dislocation loops
  • Ion Beam damage during ion beam thinning
  • Heating during ion beam thinning or examination
    might cause microstructural changes, e.g.,
    crystallization
  • Uneven polishing--surface etching or mottling
    redeposition of precipitates on the surface of
    the foil
  • Dislocations might move and/or annihilate when a
    thin foil is produced--especially in materials
    with low lattice resistance (Peierls stress)
    Likewise, dislocations might dissociate to form
    visible stacking faults due to local stresses--
    not representative of bulk behavior

23
  • Artefacts in Thin Foils Continued
  • Martensite or other displacive transformations
    might form spontaneously in thin regions of the
    foil even above Ms (Ti, Zr, and Fe alloys)
  • Deformation twinning might occur in bent foils
    but not in the bulk
  • Hot stage work might result in precipitates
    nucleating on the surface (not representative of
    the bulk)
  • Metastable b.c.c. Ti alloys can transform via
    twinning to produce planar features

24
SCANNING ELECTRON MICROSCOPY
25
  • The Instrument-
  • SEM is primarily used to study the surface, or
    near surface structure of bulk specimens.
  • The electron gun, condenser lenses and vacuum
    system, are similar in SEM and TEM.
  • A schematic diagram of the electron optical
    column for a two lens scanning electron
    microscope (SEM), is presented in Figure 16.

26
  • Figure 16. Schematic diagram of a scanning
    electron microscope.

27
  • The electron gun produces electrons, and
    accelerates them at the operating voltage.
  • The operating voltage ranges from 0 to 30 60 kV
    depending upon the type of instrument.
  • Figure 17 shows a typical configuration of an
    electron gun.

28
  • Figure 17. Gun assembly of a Scanning Electron
    Microscope.

29
  • The microscope is a probe forming system.
  • Two or three condenser lenses demagnify the
    electron beam until it hits the specimen surface
    as a focused spot or probe.
  • Electron probes of sizes down to 6 nm are
    attainable with conventional thermionic emission
    sources, although smaller probes 2 nm can be
    achieved using field emission sources.

30
  • The fine beam of electrons is scanned across the
    specimen by deflector coils.
  • The low energy electrons or other radiation
    emitted from the surface are detected/collected
    and amplified to form a video signal, which
    modulates the brightness of a cathode-ray tube
    (c.r.t.) display.
  • The electron beam and the c.r.t. spot are both
    scanned in a similar way to a television
    receiver, that is in a rectangular set of
    straight lines known as raster.

31
  • Imaging Modes and Information-
  • The different signals produced when the electron
    beam interacts with the bulk sample in the SEM
    are used to create an image.
  • One of the main features of the SEM is that, in
    principle, any radiation emitted from the
    specimen or any measurable change in the specimen
    can be used to provide the signal to modulate the
    c.r.t., and thus provide the contrast.
  • The various methods used to collect these emitted
    electron signals are summarized in Figure 18.

32
  • Figure 18. Methods of detecting electrons in a
    scanning electron microscope (a) secondary
    electrons (b) backscattered electrons, solid
    state detector (c) backscattered electrons,
    scintillation counter, and
  • (d) absorbed electron current.

33
  • Table 3. Commonly Used Scanning Electron
    Microscopy Imaging Modes Together with Resolution
    Attainable

34
(a) Topographic Images
  • One of the principal uses of SEM is to study the
    surface features, or topography of a sample.
  • Although this form of image may be obtained using
    most signals, secondary or backscattered
    electrons are usually recommended.

35
Secondary Electrons
  • Secondary electrons are detected by a
    scintillator - photomultiplier system known as
    the Everhart-Thorny detector
  • The number of detected secondary electrons
    increase with surface tilt. About 20 to 40
    degrees specimen tilt towards the detector may be
    necessary.
  • Topographic images obtained with secondary
    electrons look remarkably like images of solid
    objects viewed with light.
  • We find these topographic images easy to
    interpret.

36
Backscattered Electrons
  • Topographical images may be obtained by using
    Backscattered electrons, which are detected with
  • (i) scintillator - light pipe - photomultiplier
    (e.g. Robinson detector) type detectors, known
    for their rapid response time, or
  • (ii) solid state detectors, with a disadvantage
    of relatively slow response time.

37
(b) Compositional Images
  • Backscattered electrons and X-rays from the
    specimen is also capable of yielding composition
    information.
  • Backscattered Electrons
  • The composition of samples are sensitive to
    backscattered coefficient (?), which varies
    monotonically with atomic number.
  • It has been shown that the coefficient can be
    expressed as

  • 13

38
  • The magnitude of the compositional or atomic
    number contrast from two phases of backscattered
    coefficients ?1 and ?2 is readily calculated as

  • 14
  • This method is not recommended for phases with
    similar atomic number, as the resolution may be
    very poor.

39
  • Quantitative compositional information for phases
    of low atomic number can be obtained by comparing
    the intensity of the backscattered signal from
    the phases with a standard element. Care must be
    taken.
  • Such information is not easily determined using
    X-ray method.

40
X-ray Microanalysis
  • The characteristic X-rays emitted by bombarding
    specimen with high-energy electrons may be
    identified from either wavelength or
    characteristic energy since

  • where h is the Plancks constant and c is the
    velocity of light.
  • Equation (15) forms the basis of techniques which
    use characteristic X-rays for compositional
    analysis.
  • There may be other spectroscopic and
    microanalytical techniques, X-ray microanalysis
    is excellent for routine chemical analysis of
    small volumes

(15)
41
  • The analytical techniques rely on the efficient
    detection and discrimination of X-rays.
  • Detection of the X-rays can be accomplished by
    using any of the following methods
  • Energy Dispersive Spectroscopy (EDS)
  • on scanning and transmission electron microscopes
  • Wavelength Dispersive Spectroscopy (WDS)
  • on electron microprobes
  • Electron Energy Loss Spectroscopy (EELS)

42
Figure 19(a). Wavelength dispersive crystal
spectrometer Collection of X-rays by
spectrometer generated X-rays have a range of
wavelengths, Dl, but only one is selectively
diffracted to the detector.
43
  • Figure 19(b). The constant take off angle and
    change of crystal position required to retain
    focussed condition.

44
  • Table 4. Crystals Used in Wavelength Dispersive
    Spectrometers

45
  • Figure 20. Gas-flow, argon/methane or xenon,
    proportional counter used in conjunction with a
    crystal spectrometer.

46
  • Figure 21. X-rays enter energy dispersive
    spectrometer through a thin Be window and
    produce electron-hole pairs within the
    semiconductor crystal. A typical energy spectrum
    obtained from a general area of a ferric
    stainless steel is shown.

47
(c) Crystallographic Information
  • There are three SEM-based techniques for
    obtaining crystallographic information from
    samples. These are
  • Kossel technique
  • Electron Channeling Diffraction, and
  • Electron Backscattered Diffraction technique

48
Figure 22. Electron channel pattern The
production of channel patterns
49
Figure 23 Electron channel pattern obtained
from a gallium phosphide crystal of (111) of
orientation.
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