Download LxxB, Overview of Microscopy methods, part b

ELECTRON -SPECIMEN INTERACTIONS
SPECIMEN PREPARATION
ELECTRON MICROSCOPES
EML 5930 (27-750)
Advanced Characterization and Microstructural
Analysis
A. D. Rollett, P.N Kalu
Spring 2008
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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
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30keV, 100nm diameter electron probe.
Figure 9(b). A foil specimen of thickness, t, (t < 300nm) where in
addition to (a) transmitted signals give (i) image contrast and electron
diffraction and (ii) Kikuchi diffraction
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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
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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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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
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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.
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“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
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• window technique
Figure 11. Typical stages in electropolishing sheet material using
the window technique. The shaded regions a-d are
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lacquered.
• 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
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electropolishing
• 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 (Peierl’s 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 cathoderay 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
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(d) absorbed electron current.
Table 3. Commonly Used Scanning Electron Microscopy
Imaging Modes Together with Resolution Attainable
Mode
Information
Backscattered
Topographic
electrons
Crystallographic
Composition
Secondary
Topographic
electrons
Voltage
Magnetic and
Electric field
Absorbed
Topographic
specimen
composition
current
Typical Resolution
High
Resolution
10 nm
3nm
10 nm
100nm
500nm
3 nm
50 nm
100 nm
50 nm
20 nm
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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:
   0.254  0.016 Z  1.86 x10 4 Z 2  8.3x10 7 Z 3
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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:
1  2
C 
1
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• 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-ray Microanalysis
• The characteristic X-rays emitted by bombarding specimen
with high-energy electrons may be identified from either
wavelength or characteristic energy since:
E
hc

(15)
where h is the Planck’s 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
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• 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)
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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,
D, 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.
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