term paper on electron microscopy by udochukwu mark [2005]
DESCRIPTION
Microscopy, Materials Science and Engineering, MetallurgyTRANSCRIPT
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APPLICATIONS OF ELECTRON MICROSCOPY IN MATERIALS
AND METALLURGICAL ENGINEERING
A TERM PAPER
PRESENTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS
FOR THE POSTGRADUATE COURSE
MME 604
[ELECTON OPTICS AND MICROSCOPY]
BY
MARK, UDOCHUKWU 20044449298
SUBMITTED TO
ENGR. PROF. O. O. ONYEMAOBI [EXAMINER]
DEPARTMENT OF MATERIALS AND METALLURGICAL ENGNEERING
FEDERAL UNIVERSITY OF TECHNOLOGY, OWERRI
AUGUST 2005
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PREFACE
The electron microscope is an indispensable modern analytical and research
tool. Microscopy is employed in all branches of science to identify materials,
characterize unknown substances or study the properties of known materials.
This term paper surveys the applications of electron microscopy in the field
of materials and metallurgical engineering.
I hereby acknowledge my lecturer on Electron Optics and Microscopy
(MME 604), Engr. Prof. O. O. Onyemaobi. He has been sharpening my
research and writing skills since my undergraduate days. This is the third
term paper I will be submitting to him.
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TABLE OF CONTENTS
Title Page i
Preface ii
Table of Contents iii-v
CHAPTER ONE 1-12
1.0 Introduction 1
1.1 Materials and Metallurgical Engineering 1
1.2 Microscopes and Microscopy 3
1.2.1 Levels of Structure 4
1.2.2 Methods of Structural and Compositional Elucidation 6
1.2.3 Microscopy 8
CHAPTER TWO 13-24
2.0 Transmission Electron Microscopy 13
2.1 Interaction of Electrons with Solids 13
2.2 Transmission Electron Microscope (TEM) 14
2.3 TEM Modes and Applications 16
2.3.1 General 17
2.3.2 Surface Information & External Morphology 18
2.3.3 Contrast from an Imperfect Crystal 19
2.3.4 Precipitates and Second Phases 21
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2.3.5 Specialized Techniques of TEM 21
CHAPTER THREE 25-41
3.0 Scanning Electron Microscopy 25
3.1 Scanning Electron Microscope (SEM) 25
3.2 SEM Modes and Applications 27
3.2.1 The Reflective and Emissive Modes 28
3.2.2 Absorptive Mode 33
3.2.3 Conductive Mode 33
3.2.4 Luminescent Mode 35
3.2.5 X-Ray and Auger Modes 36
3.3 Exploiting the Versatility of SEM 40
CHAPTER FOUR 42-47
4.0 The STEM and Other Developments 42
4.1 Scanning Transmission Electron Microscope 42
4.2 Applications of the STEM 43
4.3 High Resolution STEM 44
4.4 The TEM at High Voltages 44
4.5 Analytical TEM 46
4.6 Energy Analyzing Microscopes 47
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CHAPTER FIVE 48-62
5.0 Special Techniques of Surface Microscopy and Analysis 48
5.1 Introduction 48
5.2 Acoustic and Thermal Wave Imaging 49
5.3 Field – Electron and Field – Ion Emission 51
5.3.1 Field – Electron Microscopy 52
5.3.2 Field-Ion Emission and Field-Ion Desorption
Microscopy and the Atom Probe 53
5.4 Photon - Induced Radiation 54
5.4.1 X-Ray Microscopy and Topography 55
5.4.2 Fluorescence Microscopy and Spectroscopy 56
5.5 Photo-Electron Emission 57
5.5.1 Photo-Electron Emission Microscopy 57
5.5.2 Photo-Electron Spectroscopy 58
5.6 Electron-Beam and Ion-Induced Radiation 58
5.7 Electron-Electron Interaction 60
5.8 Ion Spectroscopy 61
CHAPTER SIX 63-64
6.0 Conclusion 63
References 65-67
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CHAPTER ONE
2.0 INTRODUCTION
1.1 MATERIALS AND METALLURGICAL ENGINEERING
The title of this paper indicates an interest in the field of materials and
metallurgical engineering. It is therefore necessary to define these
disciplines as to give this treatise both direction and scope.
Metallurgy or metallurgical engineering is the science and technology of the
production, properties and uses of metals and their alloys. It is concerned
with every aspect of metals processing; their extraction from ores or
recycled components, their refining, shaping and manufacturing processes,
and the exploitation of their physical and mechanical properties for
application in every sector of industry8. Materials engineering is all about
metallurgical engineering except that it de-emphasizes metals and focuses on
non-metallic materials. It is therefore interested in the production and
properties of a wide range of materials, including electronic materials, glass
and ceramics, polymers and many other natural and man-made materials.
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The nucleation, growth and development of materials and metallurgical
engineering disciplines followed a unique trend. First, metallurgy was
recognized because of the extensive use of metals. With the diversification
of engineering materials beyond primary metals and their alloys, it became
necessary in the late 1960s to recognize as it were, the ‘metallurgy’ of non-
metallic materials. Hence, materials engineering was born in the early 1970s.
Finally, materials and metallurgical engineering fields have metamorphosed
into a composite discipline called Materials Science and Engineering. This
came in the 1990s as a result of the classification of engineering materials,
and the recognition of the fact that the same principles of science and
technology underlie their production, properties and uses. In this view,
metals are considered as a class of engineering materials, so that materials
science and engineering covers and includes the field of metallurgy.
This term paper therefore, may have been better titled, ‘Applications of
Electron Microscopy in Materials Science and Engineering’. Materials
science and engineering includes both the basic knowledge (the science) and
the applied knowledge (the engineering) of materials. The term engineering
materials is used to refer specifically to solid materials used to produce
technical products. Broadly speaking, they include metals and alloys,
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ceramics and glass, polymers, and composites. By this definition, liquids and
gases are ruled out. Again, solid-state physics and chemistry are implied in
the term materials science, so that the biological sciences are not
considered.15, 16, and 18
In conclusion, this term paper shall survey the applications of electron
microscopy in the physical sciences; specifically in physical metallurgy,
extractive metallurgy and mineral processing, polymer & textile technology,
corrosion studies, mechanical metallurgy, microelectronics and
nanotechnology, etc.
1.2 MICROSCOPES AND MICROSCOPY
Present-day materials science depends heavily on understanding how the
properties of a material relate to its composition and structure14. To
investigate the structure and composition of materials, analytical tools and
techniques are employed. The microscope ranks top among such analytical
tools, and microscopy is an indispensable analytical technique.
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1.2.1 LEVELS OF STRUCTURE
The internal structure of a material, simply called the structure, can be
studied at various levels of observation. The magnification and resolution of
the physical aid used are a measure of the level of the observation. The
higher is the magnification, the finer is the level. The details that are
disclosed at a certain level of observation are generally different from the
details disclosed at some other level1, 15. Depending on the level, we can
classify the structure of materials as: macrostructure, microstructure, sub-
microstructure, crystal structure, electronic structure, and nuclear structure.
Macrostructure of a material is examined with the naked (unaided) eye or
under a low magnification, e.g. a hand lens. Standard procedures of macro-
examination reveal flaws and segregation in a material.
Microstructure generally refers to the structure as observed under the
optical or light microscope. This microscope can magnify a structure up to
about 1500 times linear, without loss of resolution of details of the structure.
The limit of resolution of the human eye is about 0.3mm, that is, the eye can
distinguish two lines as separate lines, only when their distance of separation
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is more than 0.3mm. The optical microscope can resolve details up to a limit
of about 0.1µm (100nm or 0.0001mm).
Substructure or sub-microstructure refers to the structure obtained by
using a microscope with a much higher magnification and resolution than
the optical microscope. In an electron microscope, a magnification of 1, 000,
000 times linear is possible. By virtue of the smaller wavelength of electron
waves as compared to visible light, the resolving power also increases
correspondingly, so that much finer details show up in the electron
microscope. We can obtain a wealth of additional information on very fine
particles or on crystal imperfections such as dislocations. The electron
diffraction patterns obtained along with the photograph of the substructure
greatly aid in understanding the processes taking place in materials on such a
minute scale. NB: The term ‘microstructure’ is used quite often in technical
literature to mean both microstructure and sub-microstructure. This usage
shall be followed in this term paper.
Crystal structure tells us the details of the atomic arrangements within a
grain or crystal of a material. It is usually sufficient to describe the
arrangement of a few atoms within what is called a unit cell. The crystal
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consists of a very large number of unit cells forming regularly repeating
patterns in space i.e. three dimensions. The main technique employed for
determining the crystal structure is the x-ray diffraction.
Electronic structure (part of atomic structure) of a solid usually refers to
the arrangement of electrons in the shells and orbitals of individual atoms
that constitute the solid. Spectroscopic techniques are very useful in
determining the electronic structure.
Nuclear structure (another part of atomic structure) reveals the
arrangement of protons and neutrons in the atomic nucleus. It is studied by
nuclear spectroscopic techniques, such as nuclear magnetic resonance
(NMR) and MÖssbauer studies.
1.2.2 METHODS OF STRUCTURAL AND COMPOSITIONAL
ELUCIDATION
The science of materials uses diverse methods for the testing and analysis of
materials to obtain exhaustive and reliable information on the properties
depending on the composition, structure, and processing of the materials
being studied11. With the great diversity of instrumentation available today,
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the role of the traditional analytical laboratory has now been expanded to
include determinations of morphology, microstructure, crystallography and a
variety of physical properties. The term often associated with the
combination of analytical chemistry techniques and these other methods of
analysis is materials characterization.2, 14 Not only have the variety of
materials and the types of instrumentation available increased, but the
reasons for looking at these materials have also multiplied. The demand for
materials with unique properties grows on daily basis, and so also does the
complexity of the materials themselves and the methods needed to analyze
them.
The numerous methods, which may differ substantially from one another,
may be divided into two large groups as follows:
1. Methods for determining the structure and structural transformations
in materials. These in turn should be classed into:
Direct methods for examining and determining the structure of
materials; they are termed structural methods and include
macroscopic examination or macro-analysis, microscopic
examination (microanalysis), and x-ray examination.
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Indirect methods based on certain relationships existing
between the structure and properties of materials; they can
provide quite reliable data on the structural transformations
occurring in metals during their treatment or service condition
by measuring the variations of their physical properties e.g.
thermal analysis (for enthalpy changes), dilatometric analysis
(for linear and volumetric thermal expansion), and various
analytic techniques for electrical resistance, saturation
magnetization, and some chemical and mechanical properties.
2. Direct methods for determining the properties of materials as required
by certain operational conditions, in the first place their mechanical
properties, and also physical and chemical properties.
1.2.3 MICROSCOPY
Microscopic examination (microanalysis) is the study of the structure of
materials under microscope at large magnifications. Depending on the
magnification required the phases of a structure, their number, shape and
distribution may be studied by using visible light beam, electron beam, or
any other electromagnetic radiation that may result from the interaction of
electrons with the material under study.
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A. OPTICAL MICROSCOPY
The metallurgical microscope is an optical microscope designed for use in
the study of metals, and their alloys. It enables opaque objects to be seen
with a certain magnification in reflected light. A metallurgical microscope
comprises an optical system (lenses, prisms, and mirrors), illuminating
system (including a light source, lenses, light filters, diaphragms, and a
photographic camera), and mechanical system (stand, tube and stage –
where the micro-section or specimen is placed).
Different magnifications can be obtained by changing the combinations of
glass lenses and prisms. The useful magnification, however, cannot exceed
1500 because of light diffraction. With such a magnification it is possible to
detect elements of a structure not less than 0.2µm in size, which is in most
cases sufficient for determination of the majority of phases present in an
alloy.
Optical microscopy can be used to achieve the following:
Determination of phase composition and structure of alloys in
equilibrium, e.g. in castings and annealed components.
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Determination of non-equilibrium structures, e.g. structures resulting
from rapid cooling or quenching.
Determination of the method of metal treatment and the effect of such
processing methods on the structure, e.g. casting, plastic working,
welding, heat treatment, etc.
Quantitative metallography, e.g. determination of grain size, size of
inclusions, and phase distribution in a material.10
B. ELECTRON MICROSCOPY
In the electron microscope, electron beams and an electron-optical systems
consisting of electromagnetic and electrostatic lenses are used instead of
light beam and glass lenses. The wavelength of electrons is inversely
proportional to their momentum. Thus, it is possible to change (reduce) the
wavelength by varying (increasing) the velocity of electrons, by passing
them through an electric field of high intensity which accelerates them. It
follows that the lower the wavelength, the better the resolving power
achievable.
The resolving power feasible in an electron microscope is 100 000 times that
attainable in an optical microscope. However, because of various
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phenomena accompanying the passage of a stream of electrons, e.g.
spherical and chromatic aberrations, etc, the maximum useful resolving
power of an electron microscope is actually only 100 – 200 times greater
than that of optical microscopes. Thus, the maximum magnification that can
be realized in an electron microscope is 100 000 to 200 000 times linear.
According to the method in which the object is examined by means of
electron beams, the following basic types of electron microscope exist:
Transmission Electron Microscope (TEM) in which the stream of
electrons passes through the object, the image formed being the result
of different scattering of electrons by the object;
Scanning Electron Microscope (SEM) wherein the image is produced
from the secondary emission of electrons emitted by the surface being
scanned by a stream of primary electrons;
Scanning Transmission Electron Microscope (STEM) which marries
the SEM and the TEM, i.e. it is a hybrid of the two basic types of
electron microscopes.
This term paper shall be concerned with the application of electron
microscope techniques to the general study of solid matter in the physical
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sciences. The range of materials that is susceptible to these probing
techniques is very wide, encompassing metals, semiconductors, minerals,
fibres and amorphous structures.12
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CHAPTER TWO
2.0 TRANSMISSION ELECTRON MICROSCOPY
2.1 INTERACTION OF ELECTRONS WITH SOLIDS
Different kinds of electrons and electromagnetic waves are emitted from a
specimen irradiated with high-energy or high-speed electrons,9, 12, 17 the
different waves resulting from elastic or inelastic scattering processes. In
elastic scattering the path or trajectory of the moving electron is changed,
but its energy or velocity is not altered significantly. Inelastic scattering
occurs when the moving electron losses some of its kinetic energy as a result
of its interaction with the specimen.
The different signals are used in different microscopes and for different
imaging modes. Information on the crystal structure and on defects in the
specimen can be obtained by studying the elastically scattered electrons
whereas investigations of in-elastically scattered electrons and of other
waves leaving the specimen allow the determination of chemical
composition and topology of the specimen surface.
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2.2 TRANSMISSION ELECTRON MICROSCOPE (TEM)
The development of the TEM provided a powerful technique for
metallurgists or materials scientists / engineers to study the internal structure
of thin crystalline films or foils. The conventional or standard mode uses the
transmitted beam coming out of the specimen; hence the name –
transmission electron microscopy. The objects to be examined in a
transmission microscope must be transparent to electrons, i.e. their thickness
must be very small so that electron waves can pass through. They are usually
made in form of thin metallic films (100 - 2000Ǻ thick) or replicas (moulds)
of the surface of a metallic micro-section.
This can be used both to investigate the internal defect structure of a
crystalline specimen using the instrument as a microscope, and to determine
a considerable degree of information about the crystallographic features of
the specimen using it as a diffraction instrument. Normally, when an
electron beam strikes the specimen, part of the beam is diffracted by the
crystal planes in the material, and the remainder will pass directly through
the specimen without being diffracted.
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In operating the instrument as a microscope, one has the choice of using
either the image formed by the direct or transmitted beam or the image
formed by the diffracted beam. The TEM is so constructed that either of the
images can be viewed on the fluorescent screen of the instrument, or be
photographed on a plate or film.
When the diffracted beam is intercepted [by a diaphragm in the optical path
of the TEM], while the transmitted beam is allowed to pass through the
aperture, the image formed is said to be a bright-field (BF) image.
Imperfections in the crystal normally appear as dark areas in a BF image.
These imperfections could be small inclusions of different transparency from
the matrix crystal and therefore visible in the image as a result of loss in
intensity of the beam where it passes through the more opaque particles. Of
more general interest, however, is the case where imperfections are faults in
the crystal lattice itself, e.g. dislocations. Because these imperfections cause
diffraction of the beam, they are visible in the image formed by a direct or
transmitted beam. In a bright-field image, dislocations normally appear as
dark lines.
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The alternate method of using the electron microscope [i.e. using it as a
diffractometer] is to place the aperture so that a diffracted ray is allowed to
pass, while the transmitted beam is cut off. The image of the specimen
formed in this case is known as the dark-field (DF) image. Here,
dislocations appear as white lines lying on a dark background. Also, a
diffraction pattern is obtained whose spots correspond to the planes of the
zone that has its axis parallel to the electron beam. The diffraction patterns
can yield information both about the nature of the crystal structure (bcc, fcc,
hcp, etc) and about the orientation of the crystals in a specimen.
Furthermore, the TEM has a diaphragm in its optical path that controls the
size of the area that is able to contribute to the diffraction pattern. As a
result, it is possible to obtain information about an area of specimen that has
a radius as small as 0.5µm. The diffraction patterns are therefore called
selected area diffraction patterns.
2.3 TEM MODES AND APPLICATIONS
The TEM can be operated in different modes. In the standard mode or
conventional transmission electron microscopy (CTEM) mode, the
microscope is operated to form images by bright field (BF), dark field (DF),
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or lattice image (phase) contrast. A lattice image or phase contrast is
formed by the interference of at least two beams in the image plane of the
objective lens. In the scanning mode, the TEM is used as a SEM i.e.
scanning transmission electron microscopy (STEM) mode. However, this
section shall be concerned with the conventional mode.
2.3.1 GENERAL
In most cases the TEM can be used to derive information of several different
kinds which extend right across the sciences concerned with elucidating
microstructure. The external surface of a body can be studied and
information obtained concerning the external morphology of the specimen
and also microscopic details of the surface roughness can be investigated.
Materials of interest here are fibres and small particles in which the natural
surface has a direct bearing on the properties and uses of the material.
However, even in materials in which surface properties are not important
much information about the constitution of the material can be achieved by
studying a prepared surface. The TEM may be used to cast a shadow of the
specimen (shadow microscopy), if it is sufficiently small, or to observe a
replica of the specimen surface. Information about the internal structure of a
material is obtained directly in transmission.
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2.3.2 SURFACE INFORMATION & EXTERNAL MORPHOLOGY
One of the first applications of the TEM was to study the size, shape and
dispersion (distribution) of small particles. Here, of course, transmission of
electrons is not essential and the microscope is used as a super optical
microscope of great magnifying power. Shadow micrographs of particles
and fibres show the shape of the object and, if the specimen preparation is
well controlled, a typical state of dispersion of the particles. Important
applications in this area are the study of particle shape and size distribution
from
Solutions such as colloidal preparations, soil fractions and
precipitates, e.g. in colloid and surface chemistry, mineral processing
and hydro-metallurgy, ceramics particulate materials processing;
‘Dry’ origins such as airborne dusts, paint pigments, powders and
fibres, e.g. in powder metallurgy, cement production, polymer and
textile processing, etc.
If the specimen has a characteristic shape, such as that possessed by certain
minerals and viruses, a study by electron microscopy can be an aid to
identification. An added bonus of crystal structure identification by electron
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diffraction is obtained if crystalline particles occur naturally as thin platelets
or are produced in thin form.
2.3.3 CONTRAST FROM AN IMPERFECT CRYSTAL
For a perfect single crystal, uniform intensity is expected in any particular
image. However, crystalline materials are not void of defects or
imperfections. A necessary requirement in microscopy is, of course, the
observation of changes in intensity or the presence of contrast as this is the
only way of detecting structural information. The presence of defects that
are of an effective size greater than the resolving power of the TEM can, in
principle, be detected by changes in contrast that result from differences in
electron scattering power between the defect and the surrounding perfect
lattice of atoms.
Perhaps the lattice defect most often studied in transmission electron
microscopy is the dislocation. This defect is important because it is related
to other properties of materials such as mechanical and electrical properties.
Contrast conditions in transmission electron microscopy are employed in the
study of line and planer defects. Information can be obtained about partial
dislocations and the interaction of dislocations in deformed material. The
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crystallographic nature and origin of the important line defects known as
dislocation loops can also be determined.
Dislocation loops can be formed during work hardening and deformation
processes, in quenched materials and as a result of irradiation damage. They
result from interaction mechanisms in long dislocation lines or from the
condensation of point defects. The nature of these loops i.e. whether they are
vacancy (formed from a condensation of vacancies) or interstitial (formed
from interstitial atoms) can be determined by fairly straight forward contrast
techniques. Transmission electron microscopy has, in recent years, played an
important role in characterizing the nature of irradiation damage in materials
used in nuclear reactor technology.
Dislocation densities can be computed from electron micrographs as long as
the crystal or foil thickness is known so that a true volume count can be
obtained. Slip planes or twinning planes can be characterized and the nature
of stacking faults can also be investigated by routine contrast analysis.
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2.3.4 PRECIPITATES AND SECOND PHASES
A major area of interest in materials science is the study of precipitation
phenomena in the solid state and the structure of multiphase materials. A
second phase often has a dramatic effect on the physical properties of a
material and is often at the size level too small to allow an examination by
optical techniques. Hence, electron microscopy is used to study these
precipitates and second phase particles. This could comprise, for example,
studies of precipitate identity, crystal structure, morphology, the kinetics of
precipitation, precipitate sizes and dispersions and interfacial effects such as
the problem of coherency and the characteristics of interfacial dislocations.
Interfacial dislocations also occur in composites where a composite structure
of several phases is created by solid state reactions or mechanical methods.
In the later class (composites formed by mechanical methods), of course, the
materials are often non-crystalline e.g. glass and carbon fiber material and
polymers.
2.3.5 SPECIALIZED TECHNIQUES OF TEM
Several relatively new and often specialized techniques of transmission
electron microscopy have been developed. They are techniques or modes of
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operation of the conventional TEM that yield useful information but have, a
rather narrow, application to a particular phenomenon or class of materials.
An important development in the study of defects in crystals at high
resolution is the application of dark field techniques and the use of
‘weak beam’ conditions. Application to stacking fault energy
determination from the separation of partial dislocations bounding the
fault is an obvious example for this technique and so is the study of
stress fields at the interface of different phases.6, 12
The fringes observed in the images of stacking faults are often termed
α-fringes. Other types of defect or deformation can introduce fringes
into the electron micrograph image and the origin of the fringe pattern
is often complicated, and there is need to investigate it. Fringe
systems are dependent on their imaging conditions and often behave
differently in bright and dark field settings. It is clear therefore that
the observation of a fringe system or boundary in the image of a
crystalline material does not imply the existence of a simple stacking
fault, grain boundary or wedge. In other words, it is not every fringe
that is an α-fringe -- originating from the relative displacement of
identical parts of the crystal.
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Special techniques of the TEM have shown that anti-phase boundaries
(A.P.B’s) in such systems as ordered Cu3Au and domain boundaries
in ferroelectric and some anti-ferromagnetic crystals are a source of
such fringes.
Specialized techniques of the TEM can be used to reveal phase
contrast. Two important examples of phase contrast microscopy are in
the imaging of structural detail approaching atomic dimensions and
magnetic structure. Magnetic contrast is revealed by defocusing the
objective lens and taking the distribution of electron intensity at a
distance above or below the specimen as the object. At large
defocusing distances recognizable interference patterns occur and it is
possible to use these patterns to investigate the detailed magnetic
structure inside domain boundaries. Lorentz microscopy, as this
technique has come to be known, enjoys applications in the
elucidation of domain structures at high resolution in such technically
important materials as those used in magnetic memory and logic areas
in computer technology. One of these applications is in research into
the magnetic properties of materials supporting a type of domain
known as the magnetic ‘bubble’ domain. These cylindrical domains
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are mobile and can be used to represent information in a binary code,
e.g. the presence of bubble signifies unity and its absence zero.
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CHAPTER THREE
3.0 SCANNING ELECTRON MICROSCOPY
3.1 SCANNING ELECTRON MICROSCOPE (SEM)
The scanning electron microscope (SEM) can be considered as an
instrument that greatly extends the usefulness of the optical microscope for
studying specimens that require higher magnifications and greater depths of
field than can be attained optically. The SEM is capable of greatly extending
the limited magnification range of the optical microscope beyond 1500x to
over 50 000x. Functionally, it should be the natural successor to the optical
microscope, but historically, the TEM came earlier.
A SEM has a lower resolution than a TEM, but its advantage is that the
structure of the surface of an object can be examined directly, i.e. without
making replicas or thin foils. Scanning electron microscopy has provided us
with many new data and extended our knowledge of peculiarities of the fine
structure of materials, the structure of ageing alloys, and the structures of
isothermal transformations in super-cooled austenite, etc. In addition, it is
possible to obtain useful images of specimens that have a great deal of
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surface relief such as are found on deeply etched specimens or on fracture
surfaces. The depth of field of the SEM can be as great as 300 times that of
the optical microscope. This feature makes the SEM especially valuable for
analyzing fractures.11, 17
On the other hand, at low magnifications, that is, below 300 to 400x, the
image formed by the SEM is normally inferior to that of an optical
microscope. Thus, the optical and scanning microscopes can be viewed as
complementing each other. The optical microscope is the superior
instrument at low magnifications with relatively flat surfaces and the
scanning microscope is superior at higher magnifications and with surfaces
having a strong relief.
In the SEM, the image is developed as in a television set. The specimen
surface is scanned by a pointed electron beam over an area known as the
raster. The interaction of this sharply pointed beam with the specimen
surface causes several types of energetic emissions, including back-scattered
or reflected electrons, secondary emitted electrons, Auger electron (a special
form of secondary electrons), continuous and characteristic x-rays, etc. Most
of these emissions/radiations [when collected by a detector and focused] can
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furnish useful information about the nature of the specimen at the spot under
the beam. In the standard mode of the SEM, one normally uses the
secondary electrons to develop an image. The reason for this is that the
secondary electron signal comes primarily from the area directly under the
beam and thus furnishes an image with a very high resolution or one in
which the detail is better revealed.
The typical SEM uses 1000 line scans to form a 10 x 10cm image. A CRT
(cathode ray tube) screen with a long persistence phosphor is used so that the
image will last long enough for the eye to be able to see a complete picture
without problems of fading. The complete scanning process is repeated
every thirtieth of a second, which conforms well to the one-twenty-fourth of
a second frame time of a motion picture. To obtain a permanent
photographic record of the image, on the other hand, a cathode ray tube with
a short persistence phosphor is used. This avoids overlapping of images
from adjacent lines.17
3.2 SEM MODES AND APPLICATIONS
The field of application of scanning microscopy is wide indeed, and the
requirements for suitable specimens are much less stringent than for
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transmission microscopy. With the SEM, virtually anything that does not
decompose or collapse in the beam or the vacuum of the instrument can be
examined using emissive effects. The SEM may utilize any of a number of
different types of signal [reflected or backscattered electrons, secondary
emitted electrons, light photons (cathodoluminescence), x-ray photons,
Auger electrons, transmitted electrons, conducted specimen currents, and
absorbed specimen currents] to produce an image from a specimen. In each
case the microscope will be employed in a particular operating mode.
3.2.1 THE REFLECTIVEAND EMISSIVE MODES
These modes are closely related. The reflective mode uses reflected or
backscattered electrons while the emissive mode makes use of secondary
emitted electrons. They can be used to reveal a lot of information as
discussed below.
A. TOPOGRAPHIC AND ATOMIC NUMBER CONTRAST
The characteristics of both reflected and secondary electrons are sensitive to
variations in atomic number (hence composition) and topography. However
the reflective mode is much more efficient in detecting atomic number
contrast while the emissive mode is used when topographical information is
required.
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The majority of observations undertaken with the SEM exploit the
topographic contrast provided by the emissive mode. Hence, this is the
conventional application of scanning microscopy. Any aspect of materials
science that is concerned with surfaces is likely to benefit from the emissive
mode of scanning microscopy. Particular examples in metallurgy are
precipitate morphology and fractography. Micrographs of fractured surfaces
for example, can reveal the presence or not of intergranular and
transgranular cracking. Also, fracture planes may be identified if
characteristic angles can be observed.
The advantages of scanning microscopy in the study of fibres, textile and
polymers were recognized at an early stage. On account of the fragility and
non-planar nature of these materials, other methods of observation are much
more difficult. The types of problem associated with textile research include
the determination of the size distribution of constituent fibres and the study
of the deleterious effects of washing, wearing and dyeing processes on
fabrics. An added bonus of the SEM is that the large specimen chamber
allows the possibility of dynamic experiments in stretching and fracture
studies.
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B. ELECTRIC AND MAGNETIC FIELD CONTRAST
Important contrast effects in the emissive mode arise from the presence of
external electric or magnetic fields above the specimen surface. In both
cases the trajectories of the secondary electrons are altered by the electric
fields and clearly observed contrast may be obtained. Interesting results of
the application of field contrast techniques are found in the study of
magnetic domains.
C. VOLTAGE (POTENTIAL) CONTRAST
This is another contrast effect observable in the emissive mode and one
which is particularly valuable in the study of semiconducting materials. The
best results seem to be obtained for untilted specimens and a low
accelerating voltage for the microscope. The lower the voltage at a particular
specimen area the brighter will be the corresponding area in the CRT image.
The most widespread use of voltage contrast is found in the examination of
semiconducting materials and devices e.g. the p-n junction. If a p-n junction
with reverse bias is mounted in the microscope the p and n type material will
be at different voltages and thus the junction region is revealed. The location
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of the junction in this simple way is often a prelude to the performance of
more complicated experiments in the conductive mode. Apart from the
simple p-n diode, voltage contrast can be obtained in transistor and
integrated microcircuits. Here the advantage of the SEM is that it will
provide information about the device under actual operating conditions and
allow its performance to be checked. The frequency dependent
characteristics of these devices are studied using stroboscopic techniques.
D. ELECTRON CHANNELLING PATTERNS (ECP’s)
Electron channeling effects are one of the growth points of scanning
microscopy because of the valuable crystallographic information they yield.
Although the reflective mode is often used to detect channeling patterns they
can also be seen in the emissive and absorptive modes. Moreover a
photographic film suitably placed in the vicinity of the specimen will record
a ‘channeling type’ pattern if the scan generator is switched off. The term
ECP (Electron Channeling Pattern) arises because the channeling effect
essentially causes a dependence of the backscattered signal on the angle
mode by the incident beam to the lattice and so produces contrast.
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Channeling patterns have very useful properties relating to the fact that they
depend upon the crystallography of the specimen. The angular width of any
band is 2θ and so depends not only on crystal properties i.e. interplanar or‘d’
spacings but also upon the accelerating voltage which controls the electron
wavelength. If V is known accurately then measurement of 2θ yield the‘d’
spacings and hence the lattice constants. Lateral movement of the specimen
causes no change in the ECP but a tilt or rotation will produce change, a
property shared with Kikuchi patterns seen in the conventional TEM. Hence,
ECPs are sometimes referred to as pseudo-kikuchi lines. By progressive
tilting of the specimen in various directions an ECP map corresponding to
the stereographic triangle can be constructed, and consequently the
crystallographic orientation of the specimen can be determined. Where the
pattern contains a low index pole it may be solved by inspection.
Apart from instrumental factors, the quality of ECP’s depends upon the
perfection of the crystalline sample and this property has been exploited in
various applications of the technique e.g. studies of in situ deformation and
effects of radiation damage. Under certain circumstances grain contrast can
be obtained from polished polycrystalline specimens, a situation which
yields direct information about the size distribution of constituent
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crystallites. The microscope conditions required to obtain grain contrast are
similar to those suitable for the observation of ECP’s. Selected area
channeling patterns can be used to determine the crystallographic orientation
of individual grains. And the density of lattice defects at particular specimen
locations can be evaluated.
3.2.2 ABSORPTIVE MODE
Absorbed currents will flow if an electrical lead is connected between an
illuminated specimen and earth. This specimen absorbed current is the
difference between the primary current (incident electron beam) and the sum
of secondary and reflected currents/beams. The information gained from the
absorptive mode is largely complementary to that provided by the reflective
and emissive modes operation. However compositional variations are
enhanced at the expense of surface topography. The absorptive mode should
not be confused with the widely used conductive mode which exploits
induced conductivity in semiconducting materials.
3.2.3 CONDUCTIVE MODE
Electron beam induced conductivity has proved of great value in the
investigation of semiconducting materials and is often used in conjunction
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with voltage contrast obtained in the emissive mode. The basis of the
technique is the production of electron-hole pairs by the beam, i.e. the
excitation of electrons from the valence to the conduction band which
thereby leave the holes. The process of charge separation with its resulting
effect in an external circuit is often known as charge collection and the
current produced as the charge collection current. Generally speaking those
areas in which efficient charge collection occurs appear correspondingly
bright in the image.
Two types of problem can be tackled with the conductive mode of scanning
microscopy. In the first, the variation in charge collection current is used to
probe structural features of the specimens. In the second, the behaviour of
semiconductor devices such as diodes and field effect transistors (FETs) can
be examined under various working conditions. In this way the microscope
is used as a diagnostic tool to detect possible faults and breakdowns in
devices. Certain electrical measurements can also be made.
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3.2.4 LUMINESCENT MODE
Cathodoluminescence (CL) is a phenomenon which occurs in a great variety
of materials ranging from biological specimens to semiconductors and
minerals. CL signals can only be obtained if:
The material under examination is transparent to the radiation being
collected and total internal reflection does not constitute a barrier to
escaping radiation;
The dwell time of the probe at any point on the sample is greater than
the relaxation time for the luminescent process otherwise blurring of
the image will occur.
The fact that cathodoluminescent intensity is strongly sensitive to impurities
and irregularities in a sample is of considerable benefit to the user of the
SEM in the luminescent mode. By taking sequential micrographs in the
emissive and luminescent modes the catholuminescent centres within a
sample can be identified with features of the physical structure such as
damage, defects or polytypic bands. By allowing some of the reflected
primaries to reach the collector, a composite image can be obtained which
gives luminescent as well as topographical information.
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The link between impurities and luminescent centres has been widely
utilized in the study of semiconducting materials. A case is the investigation
of the relevance of dopant concentration in the manufacture of
semiconductors. A good example is the investigation of czochralski-grown
crystals of laser quality GaAs heavily doped with Te atoms.12
3.2.5 X-RAY AND AUGER MODES
The SEM can be easily converted in to an instrument capable of chemically
microanalyzing specimens. X-rays and Auger electrons can be analyzed to
reveal information by x-ray microanalysis and Auger electron spectroscopy.
a) The electron probe microanalyzer uses the characteristic peaks of
the x-ray spectrum resulting from the bombardment of the specimen
by the beam electrons. An electron probe microanalyzer is thus
basically an SEM equipped with x-ray detectors. Two basic types of
detectors are used. In the energy- dispersive (ED) x-ray
spectrometer, a solid-state detector develops a histogram showing
the relative frequency of the x-ray photons as a function of their
energy. In the wavelength – dispersive (WD) method a crystal
spectrometer is used to disperse the emerging x-rays in such a way
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that only photons of selected wavelength (those fulfilling the Bragg
law) reach a counter. There is a tendency for reasons of speed and
convenience to favour the use of ED techniques in SEM
applications.
Various types of output signal are possible in the x-ray mode in
either ED or WD systems. An x-ray image can be thrown on to the
video CRT by choosing a particular x-ray wavelength or energy. The
location of the element possessing this characteristic wavelength
will be revealed as bright contrast image. A modification allows
similar information to show on a line scan. Alternatively the electron
probe may be focused on a spot to provide a point analysis. The
whole range of x-rays can be collected and analyzed to allow
identification of the region in question. In addition to identification
of elements determination of the concentration of an alloy
component at a point is also possible.
The electron microprobe is a useful instrument for the identification
of the various phases in a metal specimen, including the non metallic
inclusions found in almost all commercial metals. Another area
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where this instrument has proven valuable is in diffusion studies
where information about composition gradients is required. It can
also be used to prove whether or not \a metal alloy has a
homogenous composition.
b) There are difficulties associated with the detection of low atomic
number elements for which the x – rays have low energies and long
wavelengths. Although the crystal spectrometer can cope, another
technique used for the analysis of low atomic number materials (Z
<11) is that of Auger Spectroscopy. When an Auger electron is
ejected from an atom, it leaves with a fixed amount of kinetic
energy.
A significant feature of the Auger reaction is that it involves three
electrons: the electron knocked out of an inner shell, the outer – shell
electron that jumps into the inner – shell hole, and the ejected outer
– shell electron (the Auger electron). It is common practice to
describe a particular Auger reaction with a three – letter symbol
identifying the three basic types of electron energy levels involved
in the transition, e.g. KLL, MNN, and LMM.
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One cannot perform an Auger analysis for an element having less
than three electrons because three electrons are needed for an Auger
transition. This eliminates hydrogen and helium from consideration.
An Auger analysis is normally based on measurement of the
strengths of the Auger peaks in a plot of the back scattered electron
energy per unit energy interval (ε) versus the energy of the electrons
(E).
The Auger electron spectrum is displayed using a suitable detection
system and thus compositional analysis can be done. As with x –
rays a particular Auger electron energy can be chosen to display a
distribution map on the video CRT.
In summary, Auger electron spectroscopy is useful for determining
the compositions of surface layers to a depth of about 2nm for
elements above He. It also has a spatial resolution > 100nm, which
is about is about a tenth of that of the electron probe x – ray micro-
analyzer. This makes this technique well suited to the studies of
grain boundaries in metals and alloys, especially with specimens
susceptible to brittle grain boundary fractures. It is also useful for
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surface segregation studies as in the solving of stress – corrosion
problem. 12, 17
3.3 EXPLOITING THE VERSATILITY OF SEM
As far as applications are concerned the SEM is an instrument endowed with
considerable versatility. Unfortunately, the instrument is sometimes under –
utilized by relying heavily on the conventional mode (topographic contrast
of the emissive mode). When tackling a particular problem therefore it is
prudent to consider what extra information might be gained in other
accessible modes. The information obtained with scanning microscopy may
be broadly categorized into four, viz;
(i) structural and topographical,
(ii) chemical or compositional,
(iii) crystallographic,
(iv) electrical and magnetic .
As a result of recent developments the emissive and reflective modes render
possible the direct observation of magnetic domain structures in a great
range of materials. Moreover the method has certain advantages over more
traditional techniques e.g. it can reveal the internal domain structure. Apart
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from the domain patterns themselves the probe may be used to investigate
the field distributions from recording tapes and heads. As far as electrical
properties are concerned the emissive mode signals is sensitive to and
distinguish between surface field and surface voltage. Variations in these
quantities are therefore made visible in circuits and devices.
Success in the use of all the modes of the SEM came as a result of
improvement in signal processing, signal processing increases the signal to
noise ratio, thus improving contrast generally, and discriminates between
signals from different sources. 12
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CHAPTER FOUR
4.0 THE STEM AND OTHER DEVELOPMENTS
4.1 SCANNING TRANSMISSION ELECTRON MICROSCOPE
The disadvantage of the SEM is its comparatively poor resolution which in
ordinary imaging mode nowhere approaches that of the conventional TEM.
The attraction of marrying the resolution of the TEM with the versatility and
signal processing of the SEM is obvious and this explains the growing
interest in scanning transmission microscopy. The scanning transmission
electron microscope (STEM) is so named because it combines features of
the two basic types of electron microscopes.
Essentially, the STEM consists of a series of lenses which focuses a probe
on to the specimen which is them scanned in the usual way. Unlike the
normal SEM modes however, the specimen is made sufficiently thin to
allow the transmission of electrons. After transmission these are detected
and the signals amplified and displayed for analysis. 12
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4.2 APPLICATIONS OF THE STEM
Since its early development in the late 1960’s by a group in Chicago, the
dedicated STEM has established itself as a powerful instrument for high
resolution imaging, as clearly evidenced by the visualization of individual
atoms and molecules. Another group in Arizona emphasized the impact of
the STEM for the structural analysis of crystalline objects. They pointed out
its capabilities for delivering much localized structural information, because
it provides a convergent beam electron diffraction pattern from each point on
the specimen. 7
STEM instrument have proved to be the most efficient category of analytical
electron microscopes, pushing the limits of sensitivity of the identification,
by use of electron energy loss spectroscopy (EELS), to typically ten atoms.
[EELS is applied mainly to ceramic specimens since it is the only technique
for the determination of the distribution of light elements. Ion implantation
is sometimes employed to introduce light – element dopants into
semiconductors. EELS could be used to measure local dopant
concentrations, if the latter exceed ~ 0.1%. Light elements occur in certain
metal specimens in the form of nitrides or carbide precipitates; these
materials have also been analyzed by the EELS technique]. When equipped
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with different analytical devices such as an energy dispersive x - ray detector
(EDX), a cathodoluminescence detector and an Auger detector, the STEM
constitutes an extremely powerful tool for microanalysis and its impact into
the microelectronic age has been noted. All aspects of its performance rely
on the “nanoprobe”.7
The future trend of the STEM is that of eclipsing the conventional TEM and
becoming an all-embracing multipurpose electron microscope.12
4.3 HIGH RESOLUTION STEM
Certain research groups and commercial manufacturers have pursued the
goal of designing a high resolution STEM, i.e. a device which will match the
resolution of a good conventional TEM. Incorporated with an energy
analyzer, this instrument and technique has been used to observe single
atoms of heavy elements. There is also the possibility of imaging unstrained
biological molecules and obtaining better resolved images of
crystallographic defects.12
4.4 THE TEM AT HIGH VOLTAGES
A high voltage electron microscope (HVEM) can be defined as one working
at and above 500kV. Most high voltage instrument in operation at the
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present time work at voltages up to 1MV, and some are designed to use
electron beams with energy up to 3MeV or even 10MeV.
The motivation for going to high voltages is to achieve increased
transparency. Increased penetration of the electrons should enable thicker
specimens to be observed in transmission. Also, the decrease in electron
wavelengths should lead to better resolution. The possibility of using thicker
specimens is extremely important in some materials science applications
where it is clear that many defects structures and dynamic processes in very
thin sections are not typical of the bulk material.
High voltage electron microscopy has found applications in many problems
of materials science. The major interest areas are:
The utilization of thick specimens for studies of defects and ‘difficult’
materials and for structure-related, in situ dynamic experiments,
Experimentation with ‘environmental cells’ to reproduce for example
surface chemical reactions with the intention of studying the kinetics
and products of the reaction in microscopic detail and,
To take advantage of features peculiar to the HVEM such as the
energetic electron beam for radiation damage studies.
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The terminology ‘difficult specimens’ applies to materials from which it is
difficult to make good TEM specimens suitable for ordinary voltage
(100kV) microscopy. Also, the term ‘environmental cells’ implies that the
specimens are kept in their normal environment whilst under observation.
A wide application to corrosion metallurgy, catalysis studies and surface
physics and chemistry abound in the use of environment cells for high
voltage electron microscopy. With the design of efficient environmental
cells the life sciences will also benefit. The design of successful and
economic nuclear power reactors and piles depends on knowledge of the
radiation damage sustained by the materials making up the fabric of the
reactor itself. High voltage electron microscopy is exploited in the
experimental study of such effects.12
4.5 ANALYTICAL TEM
We have already considered the incorporation of x-ray microanalysis
facilities into the SEM. Similar techniques can also be combined with the
conventional TEM. One of such analytical transmission electron
microscopes is the EMMA- 4. This instrument is said to probably have the
highest spectral resolution of any commercial machine available.
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The great advantage of an analytical microscope is that it allows a
correlation of chemical composition with microscopic detail and diffraction
data on a very fine scale. For this reason it is ideally suited to a whole range
of biological and materials science problems where small concentrations of a
minority element or precipitation are concerned.
4.6 ENERGY ANALYSING MICROSCOPES
The discussion of energy analysis occupies a place in the rapidly growing
field of electron spectroscopy. It is becoming a specialized research tool
fitted to the new STEM class of microscopes. Energy analyzers have also
been fitted to the conventional TEM in order to obtain information from
inelastically scattered electrons which otherwise would be lost.
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CHAPTER FIVE
5.0 SPECIAL TECHNIQUES OF SURFACE MICROSCOPY AND
ANALYSIS
5.1 INTRODUCTION
In addition to light microscopy and transmission and scanning electron
microscopy used routinely in all fields of materials research, development
and control, microstructures can be investigated by several more exotic
image techniques. While some of these, such as photoemission or field-ion
microscopy, are of high interest for various advanced studies of material
surfaces, others are still in the stage of experimentation, have been
substituted by other techniques or are more useful in other fields of
application like biology or mineralogy.
Instruments capable of analyzing the chemical nature and the electronic state
of surface atoms have been developed at a rapid rate in the last few years,
utilizing all kinds of interaction with incident photons, electrons and ions.
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5.2 ACOUSTIC AND THERMAL WAVE IMAGING
The acoustic microscope is a rather new development. A
piezoelectric crystal is attached to the sample, which emits acoustic
signals and after reflection at the surface, transforms them back to an
electric signal. This signal writes the image on a CRT. The
information furnished by the acoustic microscope is different from
that furnished by optical microscopes and scanning electron
microscopes in that it reveals sub-surface defects like grain
boundaries. The most recent development is the scanning acoustic
microscopy (SAM).
Macroscopic and microscopic features on the surface or close to it can
be imaged using the dependence of the photo-acoustic effect on local
variations of the thermal properties of a material (density, specific
heat, and conductivity). This new technique, not only offers sensitive
detection of minor as well as more substantial disruptions of the
lattice structure (as for example, foreign atoms in concentrations
below 10-3, vacancies, compositional changes, mechanical defects) but
also a means for nondestructive depth profiling.
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In a thermal – wave microscope, an electron beam (or a laser beam)
is focused and scanned across the surface of a sample. Periodic
surface heating results as the beam intensity is modulated in the range
of 10Hz – 10MHz. Thus, thermal waves are produced which interact
with features. Reflected and scattered waves are detected by
monitoring local surface temperature by means of gas microphone
(scanning photoacoustic microscopy, SPAM), by measuring the
deflection of a laser beam transversing through a liquid or gas layer
adjacent to the heated surface (optical beam deflection) or by
detecting the infra-red radiation emitted from the sample surface. The
spatial resolution is determined by the spot size of the incident beam,
the thermal wavelength, and thermal conductivity ranging for metals
[i.e. thermal conductors] from a few µm at high modulation frequency
(1MHz) to a few mm at 100Hz. For thermal insulators, resolution is
approximately one order of magnitude better. Since the depth of
penetration into the material is proportional to the wavelength, the
bulk of a sample can be reached at low frequencies, and
thermoacoustic signals can be detected by ultrasonic transducers
attached to the sample. This technique allows three-dimensional
information to be obtained simply by changing the frequency and has
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been termed thermoacoustic probe. Usually, thermal wave imaging
systems are attached to scanning electron microscopes using
excitation by the electron beam.
Applications of both the thermal-wave microscope and the
thermoacoustic probe have been mostly restricted to the investigation
of microelectronic components where most of the features of interest
lie within 10 µm of the surface. However, owing to the fact that the
thermal waves are more sensitive to local variations in lattice structure
than photons (optical or x-ray) and have a better resolution than
acoustic and x-ray imaging, there are numerous potential applications
for other materials, e.g. for detection of planes and grains in alloys or
composites without special contrasting or in-situ investigation during
dynamic studies.
5.3 FIELD – ELECTRON AND FIELD – ION EMISSION
Very high gradients of electric fields at the surface of a metal cause emission
of electrons and ions. This is the basis of field-electron and field-ion
microscopy.
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5.3.1 FIELD – ELECTRON MICROSCOPY
Field-electron microscopes (FEMs) are non-commercially made laboratory
equipment in which an etched single-crystal tip is heated in high vacuum.
The emitted electrons are accelerated by an anode and produce an image on
a fluorescent screen. The intensity of electrons emitted (field emission
current) depends on the voltage and the work of emission; the lattice
structure and local geometric structure of surfaces can be studied with high
resolution down to a few nanometers.
The crystallographic structure of clean surfaces and (if by chance a grain
boundary was located in the tip) the structure and the movement of grain
boundaries as well as changes of the tip material during heating have been
studied; by measuring the energy distribution of the field electrons the
electronic structure of the single-crystal tip can be investigated. Adsorption
of gas from the vacuum chamber or of evaporated substances (metal or
oxides) changes the image drastically, which has been used for studying the
sites of adsorption, the migration of adsorbed species along grain boundaries
and the formation of compounds.
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5.3.2 FIELD-ION EMISSION AND FIELD-ION DESORPTION
MICROSCOPY AND THE ATOM PROBE
Compared with field-electron microscopy, much higher resolutions, down to
atomic dimensions (< 0.15 nm), are achieved in field ion microscopy (FIM).
Noble gas atoms (usually Helium) are ionized at the cooled surface of a
pointed metal tip. The ions are accelerated by a high voltage and hit a
channel plate converter which produces and multiplies secondary electrons
which are emitted radially to a fluorescent screen. In this way, a high
resolution image of the tip is obtained showing individual atoms and their
arrangement.
Terrace steps ionize most strongly and, therefore, appear bright. Lattice
defects cut by the tip surface, such as dislocations, stacking faults, grain
boundaries and anti-phase boundaries in ordered structures are revealed.
Vacancies and interstitials can be observed and their movement studied by
taking photographs after certain time intervals. Moiré simulation can be used
to provide a simple and direct means of visualizing the physical
interpretation of field-ion micrographs.9, 11
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If the field –ion microscope is combined with a time-of-flight (TOF) mass
spectrometer, the chemical nature of atoms pulled off the tip surface by a
very large high voltage impulse can be identified. The atom passes a hole in
the screen and hits a detector, and from TOF the specific mass is calculated.
By positioning the tip with respect to the aperture hole, it is possible to focus
each individual atom (FIM atom-probe). The same physical principle allows
to analyze the chemical composition of the entire tip in the field desorption
microscope (FDS). The image is formed by the desorbed atoms by activating
the screen with a pulsed potential. Successive layers of the tip can be
analyzed in this way (field evaporation). Using this technique the
morphology, crystallography and chemistry of special alloys and particles in
statunascends can be analyzed.
Furthermore, in-situ studies of radiation damage, adsorption and desorption,
nucleation and all other investigations mentioned above for field-electron
microscopy can be carried out by field-ion microscopy and the atom probe.
5.4 PHOTON - INDUCED RADIATION
A variety of kinds of radiation can be produced by photons hitting the
surface of a material. X-ray diffraction, which shall not be considered in this
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work, is used most widely in the study of metals. Here, a short account is
given of two other methods capable of producing surface images.
5.4.1 X-RAY MICROSCOPY AND TOPOGRAPHY
Soft x-ray microscopy was developed early but was then overshadowed by
the rapid growth of electron microscopy. More recently, with synchrotron
radiation available, high resolution scanning x-ray microscopy has proved to
possess positive features (high contrast especially) in materials
investigations.
For studying defects in the surface of single crystals, x-ray topography is a
useful technique. The penetration depth of 5µm and a lateral resolution of
>1µm restricts application to relatively perfect crystals (defect density <
105/cm2) but owing to its high selectivity for different types of defects and
their location (sub-grain boundaries, stacking faults, structure of
ferromagnetic domains, dislocations) x-ray topography has become a
standard technique for monitoring crystal quality, especially in the
semiconductor industry. A classical study is the investigation of the internal
magnetic structure of non-transparent ferromagnetic crystals. It is not
possible to magnify the image directly, owing to the lack of x-ray lenses.
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High-resolution film and photographic magnification has been widely used,
typical exposure times ranging from 10 min. to 2 hours with a 1kW x-ray
source. More recently, digital image storage and accumulation have become
available, providing better resolution and higher speed.
5.4.2 FLOURESCENCE MICROSCOPY AND SPECTROSCOPY
If a fluorescing substance is irradiated by photons (x-rays or light of short
wavelength, usually ultraviolet), some of the energy is re-emitted as light of
longer wavelength which is typical for the substance. This effect is called
fluorescence and is used in mineralogy for identification purposes and, in
biology after suitable staining with fluorescent substances. Very few phases
in metallic alloys are fluorescent; therefore this technique is rarely used.
Extremely small amounts of fluorescent nonmetallic phases can be detected
in this way.
For chemical analysis, x-ray fluorescence has been widely used in the last
five decades and has become a standard technique in materials science and
technology. The average composition of large areas (approximately 10cm2)
and relatively thick surface layers (about 100µm) is obtained by analyzing
the x-ray spectra excited by a high-intensity x-ray beam which has a wide
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wavelength distribution in order to assess all elements in a wide range of
atomic numbers (9- 92). Concentrations from some ppm to 100% can be
evaluated with a relative accuracy up to 0.2%.
5.5 PHOTO-ELECTRON EMISSION
Electrons excited by photons are used for high-resolution imaging and for
chemical analysis of surface and thin films.
5.5.1 PHOTO-ELECTRON EMISSION MICROSCOPY
In photo-electron emission microscopes (PEEM), a high-intensity beam of
UV light is focused by means of quartz lenses and mirrors on a small area of
surface which activates emission of relatively slow electrons. The instrument
has been applied for studies in materials research, providing much
interesting information in all kinds of high-quality metallographic work.
Owing to the very small depth of information (10nm), the excellent phase
separation and the possibility for in-situ heating, photo-electron microscopy
is excellently capable for quantitative kinetic studies of changes in
microstructural geometry. It has been used to reveal the bonding sequence
(grain-boundary movement and annihilation) during diffusion-bonding of
steel under load of temperatures up to 10000C.
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5.5.2 PHOTO-ELECTRON SPECTROSCOPY
The kinetic energy of photo-electrons leaving the surface can be analyzed by
a spectrometer. From the electrum energy spectrum, the chemical
composition is obtained by calculating the binding energy of the emitted
electrons. This technique is usually called ESCA (i.e. electron spectroscopy
for chemical analysis); more precisely, XPS (x-ray-induced photoelectron
spectroscopy) and UPS (ultraviolet light-excited photoelectron
spectroscopy) are differentiated.
Typical applications of ESCA are the exact characterization of oxide layers
formed on metal surfaces (allowing not only to specify composition of the
oxidation products but also the electronic state of the metal atoms in the
oxides), and investigations of catalytic reactions (chemical changes of
catalysts as well as of adsorbed species).
5.6 ELCTRON-BEAM AND ION-INDUCED RADIATION
X-rays are excited when an electron-beam hits a surface. Also, an incident
beam used for surface microscopy can also create electron-hole pairs.
During recombination of these pairs, some materials emit long-wave
radiation known as cathodoluminescence (CL) which can be exploited in the
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SEM using suitable accessories. Biological and mineralogical applications
abound. These phenomena are exploited in (i) x-ray mapping, (ii) Energy –
dispersive x-ray spectroscopy (EDS), (iii) wavelength-dispersive x-ray
spectroscopy (WDS), cathodoluminescence mapping; as discussed in
sections 3.2.4 and 3.2.5 of this term paper.
Similarly as in x-ray fluorescence and electron beam-induced x-ray
spectroscopy, x-rays activated by charged particles (ions, mostly protons)
can be registered and analyzed with respect to intensity as a function of
energy. Particle-induced x-ray emission (PIXE) or ion-induced x-ray
emission (IIXE) was first introduced in nuclear physics where ion-
accelerating facilities were available; now this method has spread since
small accelerators are not much more expensive than other instruments
described so far. By using these techniques, fast chemical analysis of
elements with atomic numbers higher than 14 can be achieved with very
high sensitivity. The main fields of application have been in air pollution
and in biology; however, several interesting studies including the detection
of impurity traces in oxide layers, implantation and oxidation mechanisms of
steel have been reported.9
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5.7 ELECTRON-ELECTRON INTERACTION
Electron-beam-induced secondary electrons, Auger electrons and
backscattered electrons are most successfully used for surface imaging in the
SEM. In Auger-electron spectroscopy (AES), an electron detector and an
electron spectrometer are used to register the number of electrons N(E) as a
function of energy E and the differentiated signal dN(E)/dE is plotted and
analyzed. In scanning Auger Microscopy (SAM), the electron beam is
scanned, an image can be formed by activating a CRT modulated by the
signal intensity of the Auger electrons in the same way as in x-ray mapping.
AES and SAM techniques have been applied in a variety of problems
including studies of contamination, inhomogeneity, diffusion, and profile
analysis of thin layers, segregation in grain boundaries and oxide layers and
many other topics of scientific and technological importance.
Electron diffraction is another outcome of electron-electron interaction.
Electron diffraction methods investigate the stray angle distribution of a
monochromatic electron-beam scattered back from surface atoms. Low-
energy electron diffraction (LEED) uses primary energies between 10 and
500eV (corresponding to wavelengths of 0.4 – 0.05nm) and yields
information on the structure and electronic bonding states of surface atoms.
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Reflection high-energy electron diffraction (RHEED) can be applied
similarly to x-ray diffraction, and is more sensitive to contamination and
deformation of the surface. Electron diffraction is advantageously used in
combination with other techniques of surface analysis, adding information
which cannot be obtained otherwise.
5.8 ION SPECTROSCOPY
Ions can leave the surface owing to excitation by photons, electrons, or ions,
or by scattering. The extremely high sensitivity of ion detectors can be used
for analyzing the chemical composition of surfaces down to minute scale.
Ion-scattering spectroscopy (ISS) also called ion-reflection spectroscopy and
Rutherford backscattering (RSS) is in competition with AES, and it seems it
is superseded at present in metallurgical applications.
Secondary-ion mass spectroscopy (SIMS) and its subcategories, ion-
microprobe mass analysis (IMMA) and statistical and dynamic secondary-
ion mass spectroscopy (SSIMS and DSIMS), rate among the most powerful
analytical instruments, revealing qualitative data on chemical traces in
surfaces with a sensitivity of better than 1ppm or 10-15g. SIMS and IMMA
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have been utilized to a large extent in the fields of mineralogy and
semiconductor technology but a number of applications in physical
metallurgy have been reported as well.9
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CHAPTER SIX
6.0 CONCLUSION
Materials scientists and engineers, metallurgists, physical chemists, solid-
state physicist, and geophysical scientists all owe a lot of thanks to Henry
Sorby for opening up a new world to the microscopist. In the second half of
the nineteenth century (and particularly in 1886) he looked at metals as
never before and revealed grains and structures by employing ‘higher
powers’. Utilizing reflected light techniques the microscope was to be an
industrial tool as industry required materials to function in many different
ways; the microscope was the key – the fingerprint of metallography. It is
now more than just a research tool; it is used for quality control of
components to ensure there are no weak links in the chain of manufacture of
articles.5, 15
Every item we touch or see has at some time been investigated by
microscopist: nearly every object has a microstructure and this is what the
microscopist is looking for. The condition of this structure tells us all: why
the bolt failed, why the bridge collapsed, why the paint peeled off, etc.
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Microscopy has become a vital science in the quality control of materials
and components, without which our confidence in traveling on airplanes, in
motor cars, on trains, etc. would be greatly reduced. New engineering
materials have brought a new dimension in our microscopic studies since we
are required not just to observe the material grain structure but also the
relationship of one composite with another. There is therefore a great
demand upon the materials technologist to make the maximum use of all
microscopic techniques and exploit all applications of microscopy (optical
and electron-optical) in materials science and engineering.
Thank God that modern technological development in electronics and
computers have improved the speed and quality of results obtained in
microscopy. University and industrial laboratories now routinely undertake
types of analysis that would formerly have required the effort for a PhD
thesis.
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