ELECTRON SPECIMEN INTERACTIONS SPECIMEN PREPARATION ELECTRON MICROSCOPES

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ELECTRON -SPECIMEN INTERACTIONSSPECIMEN
PREPARATIONELECTRON MICROSCOPES
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• 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.

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• 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.

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• 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 (ii) Kikuchi diffraction and (iii)
  plasmon losses.

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• 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.

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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.

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• 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.

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• POLISHING STAGES
• (1) 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.

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• (2) 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.

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• (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.

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• THIN FOILS
• Electropolishing window technique disk
  method jet thinning
• Ion Beam Thinning
• Chemical Polishing

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• window technique

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

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

Figure 14. Schematic diagram of typical ion-beam
thinning equipment
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• 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

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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

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• 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

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• 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

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• 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

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SCANNING ELECTRON MICROSCOPY
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• 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.

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

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• 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.

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

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• 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.

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• 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.

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• 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.

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• 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.

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

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(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.

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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.

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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.

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(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

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• 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.

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• 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.

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X-rays

• The characteristic X-rays emitted by a specific
  element may be identified from either wavelength
  or characteristic energy since
• 
  15
• where h is Plancks constant and c is the
  velocity of light.
• Equation 15 forms the basis of techniques which
  use characteristic X-rays for compositional
  analysis.

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• The analytical techniques rely on the efficient
  detection and discrimination of X-rays.
• The detection may essentially be carried out by
  any of the following detectors
• Wavelength Dispersive (WDX)
• Gas Flow Proportional Counter
• Energy Dispersive Spectrometer (EDX or EDS)
• Electron Energy Loss Spectrometer (EELS), or

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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.
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• Figure 19(b). The constant take off angle and
  change of crystal position required to retain
  focussed condition.

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• Table 4. Crystals Used in Wavelength Dispersive
  Spectrometers

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

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• 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.

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(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

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