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2003 | Book

Scanning Electron Microscopy and X-ray Microanalysis

Third Edition

Introduction Read first chapter

Authors: Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

Publisher: Springer US

Print ISBN: 978-1-4613-4969-3

Electronic ISBN: 978-1-4615-0215-9

Included in: Professional Book Archive

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About this book

In the decade since the publication of the second edition of Scanning Electron Microscopy and X-Ray Microanalysis, there has been a great expansion in the capabilities of the basic scanning electron microscope (SEM) and the x-ray spectrometers. The emergence of the variab- pressure/environmental SEM has enabled the observation of samples c- taining water or other liquids or vapor and has allowed for an entirely new class of dynamic experiments, that of direct observation of che- cal reactions in situ. Critical advances in electron detector technology and computer-aided analysis have enabled structural (crystallographic) analysis of specimens at the micrometer scale through electron backscatter diffr- tion (EBSD). Low-voltage operation below 5 kV has improved x-ray spatial resolution by more than an order of magnitude and provided an effective route to minimizing sample charging. High-resolution imaging has cont- ued to develop with a more thorough understanding of how secondary el- trons are generated. The ?eld emission gun SEM, with its high brightness, advanced electron optics, which minimizes lens aberrations to yield an - fective nanometer-scale beam, and “through-the-lens” detector to enhance the measurement of primary-beam-excited secondary electrons, has made high-resolution imaging the rule rather than the exception. Methods of x-ray analysis have evolved allowing for better measurement of specimens with complex morphology: multiple thin layers of different compositions, and rough specimens and particles. Digital mapping has transformed classic x-ray area scanning, a purely qualitative technique, into fully quantitative compositional mapping.

Table of Contents

Frontmatter

1. Introduction
Abstract
The scanning electron microscope (SEM) permits the observation and characterization of heterogeneous organic and inorganic materials on a nanometer (nm) to micrometer (μm) scale. The popularity of the SEM stems from its capability of obtaining three-dimensional-like images of the surfaces of a very wide range of materials. SEM images are used in a wide variety of media from scientific journals to popular magazines to the movies. Although the major use of the SEM is to obtain topographic images in the magnification range 10–10,000x, the SEM is much more versatile, as we shall now see.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

2. The SEM and Its Modes of Operation
Abstract
Obtaining a low-magnification (<1000x) image of a rough three-dimensional object is remarkably easy with an SEM. To obtain all the information the SEM can provide, however, requires an understanding of the major modes of microscopyand the electron beam parameters that affect them. We will discuss the following microscopy modes: resolution mode, high-current mode, depth-of-focus mode, and low-voltage mode. The electron beam diameter at the specimen limits the image resolution, and the amount of electron current in the final probe determines the intensity of the secondary and backscattered electron and x-ray signals. Unfortunately, the smaller the electron probe, the lower is the probe current available and the poorer is the visibility of image features. The angle of the conical beam impinging on the specimen governs the range of heights on the specimen that will simultaneously be in focus. The accelerating voltage (kilovolts) of the beam determines how faithful the image will be in representing the actual surface of the specimen. The operator must control these beam parameters to achieve optimum results in each microscopy mode. In this chapterwe will describe the electron beam optical column, the modes of microscopy, and the important relationship between electron probe current and electron probe diameter (spot size).
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

3. Electron Beam–Specimen Interactions
Abstract
In the introductory survey presented in Chapter 1, we learned that the SEM image is constructed by scanning a finely focused probe in a regular pattern (the scan raster) across the specimen surface. The spatial resolution achieved in this imaging process is ultimately limited by the size and shape of the focused probe that strikes the specimen. In Chapter 2 we learned how to control the critical parameters of the electron beam, energy, diameter, current, and divergence, through the use of electrical fields in the gun, magnetic fields in the lenses and stigmators, and beam-defining apertures. We saw how, depending on the type of electron source (tungsten hairpin, lanthanum hexaboride, thermal field emission, or cold field emission) and its inherent brightness (a constant dependent upon the beam energy that has been selected), it is possible to create focused beams with sizes ranging from nanometers to micrometers (three orders of magnitude) carrying currents ranging from picoamperes to microamperes (six orders of magnitude). This great flexibility in operational conditions permits the SEM microscopist/microanalyst to successfully attack a wide range of problems, provided that the proper strategy is employed. The strategy needed for selecting the proper operating conditions depends critically upon understanding (1) what happens when the beam reaches the specimen and (2) how the signals produced by the electron beam–specimen interactions are converted into images and/or spectra that convey useful information.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

4. Image Formation and Interpretation
Abstract
Scanning electron microscopy is a technique in which images form the major avenue of information to the user. A great measure of the enormous popularity of the SEM arises from the ease with which useful images can be obtained. Modern instruments incorporate many computer-controlled, automatic features that permit even a new user to rapidly obtain images that contain fine detail and features that are readily visible, even at scanning rates up to that of television (TV) display. Although such automatic “computer-aided” microscopy provides a powerful tool capable of solving many problems, there will always remain a class of problems for which the general optimized solution may not be sufficient. For these problems, the careful microscopist must be aware of the consequences of the choices for the beam parameters (Chapter 2), the range of electron–specimen interactions that sample the specimen properties (Chapter 3), and finally, the measurement of those electron signals with appropriate detectors, to be discussed in this chapter. With such a systematic approach, an advanced practice of SEM can be achieved that will significantly expand the range of application to include many difficult imaging problems.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

5. Special Topics in Scanning Electron Microscopy
Abstract
From the basic description in Chapter 4, the SEM image formation process can be summarized as a geometric mapping of information collected when the beam is sequentially addressed to an x–y pattern of specific locations on the specimen. When we are interested in studying the fine-scale details of a specimen, we must understand the factors that influence SEM image resolution. We can define the limit of resolution as the minimum spacing at which two features of the specimen can be recognized as distinct and separate. Such a definition may seem straightforward, but actually applying it to a real situation becomes complicated because we must consider issues beyond the obvious problem of adjusting the beam diameter to the scale of the features of interest. The visibility of a feature must be established before we can consider any issues concerning the spatial scale. For a feature to be visible above the surrounding general background we must first satisfy the conditions contained within the threshold equation (4.26). For a specified beam current, pixel dwell time, and detector efficiency, the threshold equation defines the threshold contrast, the minimum level of contrast (C = ΔS/S max) that the feature must produce relative to the background to be visible in an image presented to the viewer with appropriate image processing.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

6. Generation of X-Rays in the SEM Specimen
Abstract
The electron beam generates x-ray photons in the beam–specimen interaction volume beneath the specimen surface. X-ray photons emerging from the specimen have energies specific to the elements in the specimen; these are the characteristic x-rays that provide the SEM’s analytical capabilities (see Fig. 6.1). Other photons have no relationship to specimen elements and constitute the continuum background of the spectrum. The x-rays we analyze in the SEM usually have energies between 0.1and 20 keV. Our task in this chapter is to understand the physical basis for the features in an x-ray spectrum like that shown in Fig. 6.1.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

7. X-Ray Spectral Measurement: EDS and WDS
Abstract
Chemical analysis in the scanning electron microscope and electron microprobe is performed by measuring the energy and intensity distribution of the x-ray signal generated by a focused electron beam. The subject of x-ray production has already been introduced in Chapter 6, which describes the mechanisms for both characteristic and continuum x-ray production. This chapter is concerned with the methods for detecting and measuring these x-rays as well as converting them into a useful form for qualitative and quantitative analysis.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

8. Qualitative X-Ray Analysis
Abstract
The first stage in the analysis of an unknown is the identification of the elements present, a process referred to as qualitative analysis. Qualitative analysis is itself a powerful tool in microanalysis. When a simple understanding of the nature of a chemical microstructure is sought, often the elemental identification accompanied by a broad classification of concentration into categories (such as “major,” “minor,” or “trace”) is sufficient to solve many problems. Note that the terms “major,” “minor,” and “trace” as applied to the concentration of constituents of a sample are not strictly defined and are therefore somewhat subjective. Each analytical method tends to define “trace” differently based upon its degree of fractional sensitivity. Because the energy dispersive x-ray spectrometer (EDS) limit of detection in bulk materials is about 0.1 wt%, the following arbitrary working definitions will be used in this text:
  • • Major: more than 10 wt% (C > 0.1 mass fraction)
  • • Minor: 1–10 wt% (0.01 ≤ C ≤ 0.1)
  • • Trace: less than 1 wt% (C < 0.01)
Note that the definition of trace has no bottom limit. Generally, wavelength-dispersive x-ray spectrometry (WDS) can achieve limits of detection of 100 ppm in favorable cases, with 10 ppm in ideal situations where there are no peak interferences and negligible matrix absorption (see Chapter 9, Section 9.8.4).
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

9. Quantitative X-Ray Analysis: The Basics
Abstract
As discussed in Chapters 6–8, the x-rays emitted from a specimen bombarded with the finely focused electron beam of the scanning electron microscope (SEM) can be used to identify which elements are present (qualitative analysis). Using flat-polished samples and a proper experimental setup and data reduction procedure, one can use the measured x-rays from the identified elements to quantitatively analyze chemical composition with an accuracy and precision approaching 1%. We term the analysis of flatpolished samples “conventional microprobe analysis.” The beam energy for conventional microprobe analysis is usually E 0 ≥ 10 keV.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

10. Special Topics in Electron Beam X-Ray Microanalysis
Abstract
Chapter 9 presented the procedures for performing quantitative electron probe x-ray microanalysis for the casef an ideal specimen. The ideal specimen surface is flat and highly polished to reduce surface roughness to a negligible level so that electron and x-ray interactions are unaffected by geometric effects. Such a highly polished surface has a short-range surface topography (sampled at distances less than 1 μm) that is reduced to an amplitude of a few nanometers and the long-range topography (sampled at distances greater than 100 μm) that is reduced to 100 nm or less. These ideal specimens satisfy three “zeroth” assumptions that underlie the conventional EPMA technique:
1.
The only reason that the x-ray intensities measured on the unknown differ from those measured on the standards is that the compositions of specimen and standard are different. Specifically, no other factors such as surface roughness, size, shape, and thickness, which can be generally grouped together as “geometric” factors, act to affect the intensities measured on the unknown.
 
2.
The specimen is homogeneous over the full extent of the interaction volume excited by the primary electron beam and sampled by the primary and secondary x-rays. Because x-rays of different excitation energies are generated with different distributions within the interaction volume, it is critical that the specimen has a uniform composition over the full region. If a thin surface layer of different composition than the underlying bulk material is present, this discontinuity is not properly considered in the conventional matrix correction analysis procedure.
 
3.
The specimen is stable under the electron beam. That is, the interaction volume is not modified through loss of one or more atomic or molecular species by the electron beam over the time period necessary to collect the x-ray spectrum (EDS) or peak intensities (WDS). Biological and polymer specimens are likely to alter composition under electron bombardment.
 
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

11. Specimen Preparation of Hard Materials: Metals, Ceramics, Rocks, Minerals, Microelectronic and Packaged Devices, Particles, and Fibers
Abstract
This chapter outlines a variety of sample preparation procedures for imaging and x-ray analysis of hard materials in the SEM. Several special and relatively new techniques, such as the use of focused ion beams for preparation of cross sections of various materials, are also discussed.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

12. Specimen Preparation of Polymer Materials
Abstract
This chapter outlines a variety of sample preparation procedures for performing scanning electron microscopy and x-ray microanalysis of polymer materials. Typical specimen preparation methods will be described and shown in a range of applications that are encountered every day in academic and industrial laboratories. The reader new to the study of polymer materials is directed to Chapter 12 of the Enhancements section on the accompanying CD, which provides background material for this chapter. The topics found on the CD are an introduction to polymer materials, E12.1, and polymer morphology, E12.2, and a description of some typical processes used to produce materials from polymers, E12.3. The book by Sawyer and Grubb (1996) is comprehensive, providing details of specimen preparation methods, dozens of practical examples, and references that describe both the structure and the microscopy of polymers.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

13. Ambient-Temperature Specimen Preparation of Biological Material
Abstract
We need only to consider our own bodies as an example to realize the complexity of biological material. Our body is three-dimensional and composed primarily of light elements, most of which are organized into a mixture of periodic and aperiodic structures. These structures range from simple molecules and macromolecules to complex heteropolymers, all bathed in an aqueous solution of ions and electrolytes. We are thermodynamically unstable, live at ambient temperatures and pressures, and are sensitive to ionizing radiation.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

14. Low-Temperature Specimen Preparation
Abstract
Low-temperature specimen preparation is not to be considered solely in the context of hydrated biological systems, although much of what will be discussed here will be directed toward these types of samples. Low temperatures are an essential prerequiste for studying naturally frozen materials such as snow and ice and frozen foods such as ice cream (Fig. 14.1). The technology is central to the preparation of aqueous systems such as paints, suspensions, emulsions, solutions, soils, clays, muds, and cements; of any nonaqueous liquid systems such as oils and organic liquids; and even of gases, vapors, and volatile materials. In addition, as has been shown in Chapter 12, low temperatures may play an important part in the preparation of plastics, polymers, and elastomers. The common physical parameter of these diverse samples is that they all can be solidified, provided the temperature is below their melting point. Once solidified they can then be further manipulated for subsequent examination and analysis in electron beam instruments.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

15. Procedures for Elimination of Charging in Nonconducting Specimens
Abstract
The specimen can be thought of as an electrical junction into which flows the beam current i B. The phenomena of backscattering of the beam electrons and secondary electron emission represent currents flowing out of the junction, i BSE and i SE, respectively. For a copper target with an incident beam energy of 20 keV, η is approximately 0.3 and δ is approximately 0.1, which accounts for 0.4, or 40%, of the beam current. The remaining beam current must flow from the specimen to ground to avoid the accumulation of charge in the junction (Thevenin’s current theorem). The balance of the currents is then given by
$$ \eqalign{ & \sum {{i_{in}}} = \sum {{i_{out}}} \cr & {\rm{ }}{i_B} = {i_{BSE}} + {i_{SE}} + {i_{SC}}, \cr} $$
(15.1)
where i SC is the specimen (or absorbed) current. For the example of copper, i SC = 0.6i B. Thus, even with a conducting specimen such as a metal, an electrical connection must be established to conduct this substantial current from the specimen to ground (typically the specimen stage is wellgrounded). Because all materials (except superconductors) have the property of electrical resistivity ρ, the specimen has a resistance R ( R = ρ l / A, where l is the length of the specimen and A is the cross section). The passage of the specimen current i SC through this resistance will cause a potential drop across the specimen, V = i SC R. For a metal, ρ is typically of the order of 10-6 ohm-cm, so that a specimen 1 cm thick with a crosssectional area of 1 cm2 will have a resistance of 10-6 ohm, and a beam current of 1 nA (10-9 A) will cause a negligible potential of about 10-15 V to develop across the specimen.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer, Joseph R. Michael

Backmatter

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Version: 0.1251.0