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 Preface Table of Contents CHAPTER ONE 1.0 1.1 1.2 1.2.1 1.2.2 1.2.3 Introduction Materials and Metallurgical Engineering Microscopes and Microscopy Levels of Structure Methods of Structural and Compositional Elucidation Microscopy i ii iii-v 1-12 1 1 3 4 6 8 13-24 13 13 14 16 17 18 19 21

CHAPTER TWO 2.0 2.1 2.2 2.3 2.3.1 2.3.2 2.3.3 2.3.4 Transmission Electron Microscopy Interaction of Electrons with Solids Transmission Electron Microscope (TEM) TEM Modes and Applications General Surface Information & External Morphology Contrast from an Imperfect Crystal Precipitates and Second Phases iii Markudo [2005]

2.3.5

Specialized Techniques of TEM

21 25-41 25 25 27 28 33 33 35 36 40 42-47 42 42 43 44 44 46 47

CHAPTER THREE 3.0 3.1 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.3 Scanning Electron Microscopy Scanning Electron Microscope (SEM) SEM Modes and Applications The Reflective and Emissive Modes Absorptive Mode Conductive Mode Luminescent Mode X-Ray and Auger Modes Exploiting the Versatility of SEM

CHAPTER FOUR 4.0 4.1 4.2 4.3 4.4 4.5 4.6 The STEM and Other Developments Scanning Transmission Electron Microscope Applications of the STEM High Resolution STEM The TEM at High Voltages Analytical TEM Energy Analyzing Microscopes

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CHAPTER FIVE 5.0 5.1 5.2 5.3 5.3.1 5.3.2 Special Techniques of Surface Microscopy and Analysis Introduction Acoustic and Thermal Wave Imaging Field – Electron and Field – Ion Emission Field – Electron Microscopy Field-Ion Emission and Field-Ion Desorption Microscopy and the Atom Probe 5.4 5.4.1 5.4.2 5.5 5.5.1 5.5.2 5.6 5.7 5.8 Photon - Induced Radiation X-Ray Microscopy and Topography Fluorescence Microscopy and Spectroscopy Photo-Electron Emission Photo-Electron Emission Microscopy Photo-Electron Spectroscopy Electron-Beam and Ion-Induced Radiation Electron-Electron Interaction Ion Spectroscopy

48-62 48 48 49 51 52

53 54 55 56 57 57 58 58 60 61 63-64 63 65-67

CHAPTER SIX 6.0 Conclusion References v 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 nonmetallic 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, submicrostructure, 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 macroexamination 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 100nm, which is about is about a tenth of that of the electron probe x – ray microanalyzer. 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) (ii) (iii) (iv) structural and topographical, chemical or compositional, crystallographic, 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 ionaccelerating 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 secondary electrons, Auger electrons and

Electron-beam-induced

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. Lowenergy 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, ionmicroprobe mass analysis (IMMA) and statistical and dynamic secondaryion 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, solidstate 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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1.

ASM Metals Handbook, vol. 9, 1985: Metallography and Microstructures. American Society for Metals, Metals Park, Ohio.

2.

ASM Metals Handbook, vol. 10, 1986: Materials Characterization. American Society for Metals, Metals Park, Ohio.

3.

Barret, C. & Massalki, T. B. 1992: Structure of Metals – Crystallographic Methods, Principles, and Data. 3rd Revised Edition. Pergamon Press, England. pp. 418 – 419, 430.

4.

Bethge, H. & Heydenreich, J (eds.), 1987. Electron Microscopy in Solid – State Physics. VEB Deutscher, Berlin. pp. 287 – 526.

5.

Bousfield, Brian. 1992: Surface Preparation and Microscopy of Materials. (For Buchler Europe Ltd, Coventry, UK). John Willey & Sons, England. pp. 1-3, 229-230

6.

Buseck, P; Cowley, J & Eyring, L. (eds). 1988: High-Resolution Transmission Electron Microscopy and Associated

Techniques. Oxford Uni. Press, NY. pp v-vii (preface). 7. Colliex, C & Mory, C. 1984: Quantitative Aspects of Scanning Transmission Electron Microscopy, in Chapman, J. N and
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Craven, A.J (eds.): Proceedings of the Twenty-fifth Scottish Universities Summer School in Physics, Glassgow, August (1983). A NATO Advanced Study Institute. Redwood Burn Ltd, UK. pp. 149-150. 8. Cottrell, A. H. 1975: An Introduction to Metallurgy, Second Edition, Edward Arnold (publishers), London. pp. 1-8 9. Exner, H. E. 1983: Qualitative and Quantitative Surface Microscopy. Chapter 10A of Physical Metallurgy – Third, revised and enlarged edition, Part 1, 1992 Reprint. Cahn, R.W & Haseen, P (eds.), Elsevier Sc. Pub. B.V. pp. 581-637. 10. Feder, R, McGowan, R. Wm & Shinozaki, D.M (eds.), 1986: Examining the Submicron World – A NATO Advanced Science Institute Series. Plenum Press, NY. pp. preface, 133 – 136 11. Geller, A. Yu & Rakhshtadt, A. G. 1977: Science of Materials; Methods of Analysis, Laboratory Exercises and Problems. (Translated from the Russian by V. Afanasyev). Mir Publishers, Moscow. pp. 15 – 103.

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12.

Grundy, P. J. & Jones, G. A. 1976: Electron Microscopy in the Study of Materials. (The Structures and properties of Solids Series 7), Edward Arnold, London.

13.

Guy, A. G. 1976: Essentials of Materials Science. McGraw – Hill, Tokyo. pp. 34 – 48, 57.

14.

Lifshin, E. 1986: Investigation and Characterization of Materials. In Bever, M. B (ed), Encyclopedia of Materials Science and Engineering, vol.3, Pergamon Press / The MIT press. pp. 2389 – 2398.

15.

Raghavan, V. 1990: Materials Science and Engineering – A First Course, third edition. Prentice – Hall of India, New Delhi. pp. 1-7

16.

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