Scanning Electron Microscopy and X-Ray Microanalysis

Scanning Electron

Microscopy and

X-Ray Microanalysis

Joseph I. Goldstein

Dale E. Newbury

Joseph R. Michael

Nicholas W.M. Ritchie

John Henry J. Scott

David C. Joy

Fourth Edition

Scanning Electron Microscopy and X-Ray Microanalysis

Joseph I. Goldstein

Dale E. Newbury

Joseph R. Michael

Nicholas W.M. Ritchie

John Henry J. Scott

David C. Joy

Scanning Electron

Microscopy and

X-Ray Microanalysis

Fourth Edition

ISBN 978-1-4939-6674-5 ISBN 978-1-4939-6676-9 (eBook)

https://doi.org/10.1007/978-1-4939-6676-9

Library of Congress Control Number: 2017943045

© Springer Science+Business Media LLC 2018

This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is con￾cerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on

microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, com￾puter software, or by similar or dissimilar methodology now known or hereafter developed.

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even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations

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Joseph I. Goldstein

University of Massachusetts

Amherst, MA, USA

Joseph R. Michael

Sandia National Laboratories

Albuquerque, NM, USA

John Henry J. Scott

National Institute of Standards and Technology

Gaithersburg, MD, USA

Dale E. Newbury

National Institute of Standards and Technology

Gaithersburg, MD, USA

Nicholas W.M. Ritchie

National Institute of Standards and Technology

Gaithersburg, MD, USA

David C. Joy

University of Tennessee

Knoxville, TN, USA

V

Preface

This is not your father’s, your mother’s, or your

grandparent’s Scanning Electron Microscopy and

X-Ray Microanalysis (SEMXM). But that is not to

say that there is no continuity or to deny a family

resemblance. SEMXM4 is the fourth in the series

of textbooks with this title, and continues a tradi￾tion that extends back to the “zero-th edition” in

1975 published under the title, “Practical Scanning

Electron Microscopy” (Plenum Press, New York).

However, the latest edition differs sharply from

its predecessors, which attempted an encyclope￾dic approach to the subject by providing extensive

details on how the SEM and its associated devices

actually work, for example, electron sources, lenses,

electron detectors, X-ray spectrometers, and so on.

In constructing this new edition, the authors have

chosen a different approach. Modern SEMs and the

associated X-ray spectrometry and crystallography

measurement functions operate under such exten￾sive computer control and automation that it is

actually difficult for the microscopist-microanalyst

to interact with the instrument except within care￾fully prescribed boundaries. Much of the flexibility

of parameter selection that early instruments pro￾vided has now been lost, as instrumental operation

functions have been folded into software control.

Thus, electron sources are merely turned “on,” with

the computer control optimizing the operation, or

for the thermally assisted field emission gun, the

electron source may be permanently “on.” The user

can certainly adjust the lenses to focus the image,

but this focusing action often involves complex

interactions of two or more lenses, which formerly

would have required individual adjustment. More￾over, the nature of the SEM field has fundamentally

changed. What was once a very specialized instru￾ment system that required a high level of training

and knowledge on the part of the user has become

much more of a routine tool. The SEM is now sim￾ply one of a considerable suite of instruments that

can be employed to solve problems in the physical

and biological sciences, in engineering, in technol￾ogy, in manufacturing and quality control, in fail￾ure analysis, in forensic science, and other fields.

The authors also recognize the profound changes

that have occurred in the manner in which peo￾ple obtain information. The units of SEMXM4,

whether referred to as chapters or modules, are

meant to be relatively self-contained. Our hope

is that a reader seeking specific information can

select a topic from the list and obtain a good

understanding of the topic from that module

alone. While each topic is supported by informa￾tion in other modules, we acknowledge the like￾lihood that not all users of SEMXM4 will “read

it all.” This approach inevitably leads to a degree

of overlap and repetition since similar informa￾tion may appear in two or more places, and this is

entirely intentional.

In recognition of these fundamental changes, the

authors have chosen to modify SEMXM4 exten￾sively to provide a guide on the actual use of the

instrument without overwhelming the reader with

the burden of details on the physics of the opera￾tion of the instrument and its various attachments.

Our guiding principle is that the microscopist￾microanalyst must understand which parameters

can be adjusted and what is an effective strategy to

select those parameters to solve a particular prob￾lem. The modern SEM is an extraordinarily flex￾ible tool, capable of operating over a wide range

of electron optical parameters and producing

images from electron detectors with different sig￾nal characteristics. Those users who restrict them￾selves to a single set of operating parameters may

be able to solve certain problems, but they may

never know what they are missing by not explor￾ing the range of parameter space available to them.

SEMXM4 seeks to provide sufficient understand￾ing of the technique for a user to become a com￾petent and efficient problem solver. That is not to

say that there are only a few things to learn. To

help the reader to approach the considerable body

of knowledge needed to operate at a high degree

of competency, a new feature of SEMXM-4 is the

summary checklist provided for each of the major

areas of operation: SEM imaging, elemental X-ray

microanalysis, and backscatter-diffraction crystal￾lography.

Readers familiar with earlier editions of SEMXM

will notice the absence of the extensive material

previously provided on specimen preparation.

Proper specimen preparation is a critical step in

solving most problems, but with the vast range of

applications to materials of diverse character, the

topic of specimen preparation itself has become

the subject of entire books, often devoted to just

one specialized area.

VI

Throughout their history, the authors of the

SEMXM textbooks have been closely associated

as lecturers with the Lehigh University Summer

Microscopy School. The opportunity to teach and

interact with each year’s students has provided a

very useful experience in understanding the com￾munity of users of the technique and its evolution

over time. We hope that these interactions have

improved our written presentation of the subject

as a benefit to newcomers as well as established

practitioners.

Finally, the author team sadly notes the passing in

2015 of Professor Joseph I.  Goldstein (University

of Massachusetts, Amherst) who was the “found￾ing father” of the Lehigh University Summer

Microscopy School in 1970, and who organized

and contributed so extensively to the microscopy

courses and to the SEMXM textbooks throughout

the ensuing 45 years. Joe provided the stimulus to

the production of SEMXM4 with his indefatigable

spirit, and his technical contributions are embed￾ded in the X-ray microanalysis sections.

Dale E. Newbury

Nicholas W.M. Ritchie

John Henry J. Scott

Gaithersburg, MD, USA

Joseph R. Michael

Albuquerque, NM, USA

David C. Joy

Knoxville, TN, USA

The original version of this book was revised. Index has been updated.

Preface

VII

Scanning Electron Microscopy and Associated

Techniques: Overview

Imaging Microscopic Features

The scanning electron microscope (SEM) is an

instrument that creates magnified images which

reveal microscopic-scale information on the size,

shape, composition, crystallography, and other

physical and chemical properties of a specimen.

The principle of the SEM was originally demon￾strated by Knoll (1935; Knoll and Theile 1939)

with the first true SEM being developed by von

Ardenne (1938). The modern commercial SEM

emerged from extensive development in the 1950s

and 1960s by Prof. Sir Charles Oatley and his

many students at the University of Cambridge

(Oatley 1972). The basic operating principle of

the SEM involves the creation of a finely focused

beam of energetic electrons by means of emission

from an electron source. The energy of the elec￾trons in this beam, E0

, is typically selected in the

range from E0=0.1 to 30  keV). After emission

from the source and acceleration to high energy,

the electron beam is modified by apertures, mag￾netic and/or electrostatic lenses, and electromag￾netic coils which act to successively reduce the

beam diameter and to scan the focused beam in a

raster (x-y) pattern to place it sequentially at a

series of closely spaced but discrete locations on

the specimen. At each one of these discrete loca￾tions in the scan pattern, the interaction of the

electron beam with the specimen produces two

outgoing electron products: (1) backscattered

electrons (BSEs), which are beam electrons that

emerge from the specimen with a large fraction of

their incident energy intact after experiencing

scattering and deflection by the electric fields of

the atoms in the sample; and (2) secondary elec￾trons (SEs), which are electrons that escape the

specimen surface after beam electrons have

ejected them from atoms in the sample. Even

though the beam electrons are typically at high

energy, these secondary electrons experience low

kinetic energy transfer and subsequently escape

the specimen surface with very low kinetic ener￾gies, in the range 0–50 eV, with the majority below

5 eV in energy. At each beam location, these out￾going electron signals are measured using one or

more electron detectors, usually an Everhart–

Thornley “secondary electron” detector (which is

actually sensitive to both SEs and BSEs) and a

“dedicated backscattered electron detector” that is

insensitive to SEs. For each of these detectors, the

signal measured at each individual raster scan

location on the sample is digitized and recorded

into computer memory, and is subsequently used

to determine the gray level at the corresponding

X-Y location of a computer display screen, form￾ing a single picture element (or pixel). In a con￾ventional-vacuum SEM, the electron-optical

column and the specimen chamber must operate

under high vacuum conditions (<10−4

Pa) to min￾imize the unwanted scattering that beam elec￾trons as well as the BSEs and SEs would suffer by

encountering atoms and molecules of atmo￾spheric gasses. Insulating specimens that would

develop surface electrical charge because of

impact of the beam electrons must be given a con￾ductive coating that is properly grounded to pro￾vide an electrical discharge path. In the variable

pressure SEM (VPSEM), specimen chamber pres￾sures can range from 1  Pa to 2000  Pa (derived

from atmospheric gas or a supplied gas such as

water vapor), which provides automatic discharg￾ing of uncoated insulating specimens through the

ionized gas atoms and free electrons generated by

beam, BSE, and SE interactions. At the high end

of this VPSEM pressure range with modest speci￾men cooling (2–5 °C), water can be maintained in

a gas–liquid equilibrium, enabling direct exami￾nation of wet specimens.

SEM electron-optical parameters can be optimized

for different operational modes:

1. A small beam diameter can be selected for high

spatial resolution imaging, with extremely fine

scale detail revealed by possible imaging strate￾gies employing high beam energy, for example,

. Fig. 1a (E0=15 keV) and low beam energy,

. Fig. 1b (E0=0.8 keV), . Fig. 1c (E0=0.5 keV),

and . Fig. 1d (E0=0.3 keV). However, a nega￾tive consequence of choosing a small beam

size is that the beam current is reduced as the

inverse square of the beam diameter. Low beam

current means that visibility is compromised

for features that produce weak contrast.

2. A high beam current improves visibility of low

contrast objects (e.g., . Fig. 2). For any combi￾nation of beam current, pixel dwell time, and

detector efficiency there is always a threshold

contrast below which features of the speci￾men will not be visible. This threshold contrast

depends on the relative size and shape of the

feature of interest. The visibility of large objects

and extended linear objects persists when

small objects have dropped below the visibility

VIII

threshold. This threshold can only be lowered

by increasing beam current, pixel dwell time,

and/or detector efficiency. Selecting higher

beam current means a larger beam size, caus￾ing resolution to deteriorate. Thus, there is a

dynamic contest between resolution and visibil￾ity leading to inevitable limitations on feature

size and feature visibility that can be achieved.

3. The beam divergence angle can be minimized

to increase the depth-of-field (e.g., . Fig. 3).

With optimized selection of aperture size and

specimen-to-objective lens distance (work￾ing distance), it is generally possible to achieve

small beam convergence angles and therefore

effective focus along the beam axis that is at

least equal to the horizontal width of the image.

a

c

b

d

100 nm

YK EHT - 15.00 kV

WD - 1.7 mm

Signal A = InlLens

I Probe - 135 pA ESB Grid = 800 V Image Pixel Size - 1.184 nm

HV mag HFW WD 500 nm

100 000 x Helios

200nm

100nm

x500,000 0.30kV UED 10nm JEOL GBSH WD 2.0mm

SU8200 0.50kV-D 1.6mm X 200k SE+BSE(TU)

SU8200 0.50kV-D 1.6mm X 500k SE+BSE(TU)

800.00 V 1.49 µm 990.7 µm

Mag - 94.28 K X Width - 1.213 mm Date: 19 Oct 2015

Signal B = InlLens

..      Fig. 1 a High resolution SEM image taken at high

beam energy (E0=15 keV) of a finFET transistor (16-nm

technology) using an in-lens secondary electron detector.

This cross section was prepared by inverted Ga FIB milling

from backside (Zeiss Auriga Cross beam; image courtesy

of John Notte, Carl Zeiss); Bar = 100 nm. b High resolu￾tion SEM image taken at low beam energy (E0=0.8 keV) of

zeolite (uncoated) using a through-the-lens SE detector

(image courtesy of Trevan Landin, FEI); Bar = 500 nm. c

Mesoporous silica nanosphere showing 5-nm-diameter

pores imaged with a landing energy of 0.5 keV (specimen

courtesy of T. Yokoi, Tokyo Institute of Technology; images

courtesy of A. Muto, Hitachi High Technologies); Upper image

Bar = 200 nm, Lower image Bar = 100 nm. d Si nanoparticle

imaged with a landing energy of 0.3 keV; Bar = 10 nm

(image courtesy V. Robertson, JEOL)

Scanning Electron Microscopy and Associated Techniques: Overview

IX

A negative consequence of using a small aper￾ture to reduce the convergence/divergence angle

is a reduction in beam current.

Vendor software supports collection, dynamic

processing, and interpretation of SEM images,

including extensive spatial measurements. Open

source software such as ImageJ-Fiji, which is

highlighted in this textbook, further extends these

digital image processing capabilities and provides

the user access to a large microscopy community

effort that supports advanced image processing.

General specimen property information that

can be obtained from SEM images:

1. Compositional microstructure (e.g., . Fig. 4).

Compositional variations of 1 unit differ￾ence in average atomic number (Z) can be

observed generally with BSE detection, with

even greater sensitivity (ΔZ=0.1) for low

(Z=6) and intermediate (Z=30) atomic num￾bers. The lateral spatial resolution is generally

limited to approximately 10–100 nm depend￾ing on the specimen composition and the

beam energy selected.

2. Topography (shape) (e.g., . Fig. 5). Topo￾graphic structure can be imaged with varia￾tions in local surface inclination as small as

a few degrees. The edges of structures can

be localized with a spatial resolution rang￾ing from the incident beam diameter (which

can be 1 nm or less, depending on the elec￾tron source) up to 10 nm or greater, depend￾ing on the material and the geometric nature

of the edge (vertical, rounded, tapered, re￾entrant, etc.).

3. Visualizing the third dimension (e.g., . Fig. 6).

Optimizing for a large depth-of-field permits

visualizing the three-dimensional structure

of a specimen. However, in conventional X-Y

image presentation, the resulting image is a

projection of the three dimensional informa￾tion onto a two dimensional plane, suffering

0.5 nA 20 nA

BSE MAG: 1000 x HV: 20.0 kV WD: 11.0 mm BSE MAG: 1000 x HV: 20.0 kV WD: 11.0 mm

20 mm 20 mm

..      Fig. 2 Effect of increasing beam current (at constant pixel dwell time) to improve visibility of low contrast features.

Al-Si eutectic alloy; E0=20 keV; semiconductor BSE detector (sum mode): (left) 0.5 nA; (right) 20 nA; Bar = 20 µm

BSE MAG: 750 x HV: 20.0 kV WD: 11.0mm

4

3

1

2

10 mm

..      Fig. 4 Atomic number contrast with backscattered

electrons. Raney nickel alloy, polished cross section;

E0=20 keV; semiconductor BSE (sum mode) detector. Note

that four different phases corresponding to different com￾positions can be discerned; Bar = 10 µm

HV WD mag det mode HFW

11.7 mm NIST FEG ESEM

4 mm

20.00 kV 8.0 mm 12 711 x ETD Custom

..      Fig. 3 Large the depth-of-focus; Sn spheres;

E0=20 keV; Everhart–Thornley(positive bias) detector;

Bar = 4 µm (Scott Wight, NIST)

Scanning Electron Microscopy and Associated Techniques: Overview

X

spatial distortion due to foreshortening. The

true three-dimensional nature of the speci￾men can be recovered by applying the tech￾niques of stereomicroscopy, which invokes

the natural human visual process for stereo

imaging by combining two independent views

of the same area made with small angular dif￾ferences.

4. Other properties which can be accessed by

SEM imaging: (1) crystal structure, including

grain boundaries, crystal defects, and crystal

deformation effects (e.g., . Fig. 8); (2) mag￾netic microstructure, including magnetic

domains and interfaces; (3) applied electri￾cal fields in engineered microstructures; (4)

electron-stimulated optical emission (cath￾odoluminescence), which is sensitive to low

energy electronic structure.

Measuring the Elemental

Composition

The beam interaction with the specimen produces

two types of X-ray photon emissions which com￾pose the X-ray spectrum: (1) characteristic X-rays,

whose specific energies provide a fingerprint that

is specific to each element, with the exception of H

and He, which do not emit X-rays; and (2) con￾tinuum X-rays, which occur at all photon energies

from the measurement threshold to E0

and form a

background beneath the characteristic X-rays.

This X-ray spectrum can be used to identify and

quantify the specific elements (excepting H and

He, which do not produce X-rays) present within

the beam-excited interaction volume, which has

dimensions ranging from approximately 100  nm

to 10  μm depending on composition and beam

energy, over a wide range of concentrations (C,

expressed in mass fraction):

“Major constituent”: 0.1<C≤1

“Minor constituent”: 0.01≤C≤0.1

“Trace constituent”: C<0.01

The X-ray spectrum is measured with the semi￾conductor energy dispersive X-ray spectrometer

(EDS), which can detect photons from a threshold

of approximately 40 eV to E0

(which can be as high

as 30  keV). Vendor software supports collection

and analysis of spectra, and these tools can be aug￾mented significantly with the open source software

National Institute of Standards and Technology

DTSA II for quantitative spectral processing and

simulation, discussed in this textbook.

Analytical software supports qualitative X-ray

microanalysis which involves assigning the char￾acteristic peaks recognized in the spectrum to

specific elements. Qualitative analysis presents

significant challenges because of mutual peak

interferences that can occur between certain

SE MAG: 500 X HV: 20.0 kV WD: 11.0 mm BSE MAG: 500 X HV: 20.0 kV WD: 11.0 mm

20 mm 20 mm

..      Fig. 5 Topographic contrast as viewed with different detectors: Everhart–Thornley (positive bias) and semiconductor

BSE (sum mode); silver crystals; E0=20 keV; Bar = 20 µm

SEM HV: 15.0 kV WD: 9.42 mm

View field: 439 mm Det: SE 100 mm

..      Fig. 6 Visualizing the third dimension. Anaglyph ste￾reo pair (red filter over left eye) of pollen grains on plant

fibers; E0=15 keV; coated with Au-Pd; Bar = 100 µm

Scanning Electron Microscopy and Associated Techniques: Overview

XI

combinations of elements, for example, Ti and

Ba; S, Mo, and Pb; and many others, especially

when the peaks of major constituents interfere

with the peaks of minor or trace constituents.

Operator knowledge of the physical rules govern￾ing X-ray generation and detection is needed to

perform a careful review of software-generated

peak identifications, and this careful review must

always be performed to achieve a robust mea￾surement result.

After a successful qualitative analysis has been

performed, quantitative analysis can proceed.

The characteristic intensity for each peak is auto￾matically determined by peak fitting procedures,

such as the multiple linear least squares method.

The intensity measured for each element is pro￾portional to the concentration of that element,

but that intensity is also modified by all other ele￾ments present in the interaction volume through

their influence on the electron scattering and

retardation (“atomic number” matrix effect, Z),

X-ray absorption within the specimen (“absorp￾tion” matrix effect, A), and X-ray generation

induced by absorption of X-rays (“secondary flu￾orescence” matrix effects, F, induced by charac￾teristic X-rays and c, induced by continuum

X-rays). The complex physics of these “ZAFc”

matrix corrections has been rendered into algo￾rithms by a combined theoretical and empirical

approach. The basis of quantitative electron￾excited X-ray microanalysis is the “k-ratio proto￾col”: measurement under identical conditions

(beam energy, known electron dose, and spec￾trometer performance) of the characteristic

intensities for all elements recognized in the

unknown spectrum against a suite of standards

containing those same elements, producing a set

of k-ratios, where

k I = I Unknown Standard / (1)

for each element in the unknown. Standards are

materials of known composition that are tested to

be homogeneous at the microscopic scale, and

preferably homogeneous at the nanoscale.

Standards can be as simple as pure elements—e.g.,

C, Al, Si, Ti, Cr, Fe, Ni, Cu, Ag, Au, etc.—but for

those elements that are not stable in a vacuum

(e.g., gaseous elements such as O) or which

degrade during electron bombardment (e.g., S, P,

and Ga), stable stoichiometric compounds can be

used instead, e.g., MgO for O; FeS2

for S; and GaP

for Ga and P.  The most accurate analysis is per￾formed with standards measured on the same

instrument as the unknown(s), ideally in the same

measurement campaign, although archived

standard spectra can be effectively used if a quality

measurement program is implemented to ensure

the constancy of measurement conditions, includ￾ing spectrometer performance parameters. With

such a standards-based measurement protocol and

ZAFc matrix corrections, the accuracy of the anal￾ysis can be expressed as a relative deviation from

expected value (RDEV):

RDEV( ) % % = ´ [ ] ( ) Measured-True / True 100 (2)

Based on extensive testing of homogeneous

binary and multiple component compositions,

the distribution of RDEV values for major con￾stituents is such that a range of ±5 % relative cap￾tures 95 % of all analyses. The use of stable, high

integrated count spectra (>1 million total counts

from threshold to E0) now possible with the sili￾con drift detector EDS (SDD-EDS), enables this

level of accuracy to be achieved for major and

minor constituents even when severe peak inter￾ference occurs and there is also a large concen￾tration ratio, for example, a major constituent

interfering with a minor constituent. Trace con￾stituents that do not suffer severe interference

can be measured to limits of detection as low as

C = 0.0002 (200 parts per million) with spectra

containing >10 million counts. For interference

situations, much higher count spectra (>100

million counts) are required.

An alternative “standardless analysis” protocol

uses libraries of standard spectra (“remote stan￾dards”) measured on a different SEM platform

with a similar EDS spectrometer, ideally over a

wide range of beam energy and detector parame￾ters (resolution). These library spectra are then

adjusted to the local measurement conditions

through comparison of one or more key spectra

(e.g., locally measured spectra of particular ele￾ments such as Si and Ni). Interpolation/extrapola￾tion is used to supply estimated spectral intensities

for elements not present in or at a beam energy not

represented in the library elemental suite. Testing

of the standardless analysis method has shown

that an RDEV range of ±25% relative is needed to

capture 95% of all analyses.

High throughput (>100 kHz) EDS enables col￾lection of X-ray intensity maps with gray scale rep￾resentation of different concentration levels (e.g.,

. Fig. 7a). Compositional mapping by spectrum

imaging (SI) collects a full EDS spectrum at each

pixel of an x-y array, and after applying the quanti￾tative analysis procedure at each pixel, images are

created for each element where the gray (or color)

level is assigned based on the measured concentra￾tion (e.g., . Fig. 7b).

Scanning Electron Microscopy and Associated Techniques: Overview

XII

..      Fig. 7 a EDS X-ray intensity maps for Al, Fe, and Ni and color overlay; Raney nickel alloy; E0=20 keV. b SEM/BSE (sum)

image and compositional maps corresponding to a

Ni Al Fe Ni

Al Fe

20µm

a

20µm

Ni Fe

wt%

BSE Al

b

BSE MAG: 1000 x HV: 20.0 kV WD: 11.0 mm

20 mm

0.001 0.01 0.1 1.0

0.1 1.0 10 100

Scanning Electron Microscopy and Associated Techniques: Overview

XIII

Measuring the Crystal Structure

An electron beam incident on a crystal can undergo

electron channeling in a shallow near-surface layer

which increases the initial beam penetration for

certain orientations of the beam relative to the

crystal planes. The additional penetration results in

a slight reduction in the electron backscattering

coefficient, which creates weak crystallographic

contrast (a few percent) in SEM images by which

differences in  local crystallographic orientation

can be directly observed: grain boundaries, defor￾mations bands, and so on (e.g., . Fig. 8).

The backscattered electrons exiting the speci￾men are subject to crystallographic diffraction

effects, producing small modulations in the intensi￾ties scattered to different angles that are superim￾posed on the overall angular distribution that an

amorphous target would produce. The resulting

“electron backscatter diffraction (EBSD)” pattern

provides extensive information on the local orienta￾tion, as shown in . Fig. 8b for a crystal of hematite.

EBSD pattern angular separations provide mea￾surements of the crystal plane spacing, while the

overall EBSD pattern reveals symmetry elements.

This crystallographic information combined with

elemental analysis information obtained simultane￾ously from the same specimen region can be used to

identify the crystal structure of an unknown.

Dual-Beam Platforms: Combined

Electron and Ion Beams

A “dual-beam” instrument combines a fully func￾tional SEM with a focused ion beam (FIB), typi￾cally gallium or argon. This combination provides

a flexible platform for in situ specimen modifica￾tion through precision ion beam milling and/or

ion beam mediated material deposition with

sequential or simultaneous electron beam tech￾nique characterization of the newly revealed

specimen surfaces. Precision material removal

enables detailed study of the third dimension of a

specimen with nanoscale resolution along the

depth axis. An example of ion beam milling of a

directionally solidified Al-Cu is shown in . Fig. 9,

as imaged with the SEM column on the dual￾beam instrument. Additionally, ion-beam

induced secondary electron emission provides

scanning ion microscopy (SIM) imaging to com￾plement SEM imaging. For imaging certain speci￾men properties, such as crystallographic

structure, SIM produces stronger contrast than

SEM.  There is also an important class of stand￾alone SIM instruments, such as the helium ion

microscope (HIM), that are optimized for high

resolution/high depth-of-field imaging perfor￾mance (e.g., the same area as viewed by HIM is

also shown in . Fig. 9).

a b

BSE MAG: 400 x HV: 20.0 kV WD: 11.0 mm

40 mm

..      Fig. 8 a Electron channeling contrast revealing grain boundaries in Ti-alloy (nominal composition: Ti-15Mo-3Nb-3Al￾0.2Si); E0=20 keV. b Electron backscatter diffraction (EBSD) pattern from hematite at E0=40 keV

Scanning Electron Microscopy and Associated Techniques: Overview

XIV

Modeling Electron and Ion

Interactions

An important component of modern Scanning

Electron Microscopy and X-ray Microanalysis

is modeling the interaction of beam electrons

and ions with the atoms of the specimen and its

environment. Such modeling supports image

interpretation, X-ray microanalysis of challeng￾ing specimens, electron crystallography methods,

and many other issues. Software tools for this pur￾pose, including Monte Carlo electron trajectory

simulation, are discussed within the text. These

tools are complemented by the extensive database

of Electron-Solid Interactions (e.g., electron scat￾tering and ionization cross sections, secondary

electron and backscattered electron coefficients,

etc.), developed by Prof. David Joy, can be found

in chapter 3 on SpringerLink: http://link.springer.

com/chapter/10.1007/978-1-4939-6676-9_3.

References

Knoll M (1935) Static potential and secondary emission of

bodies under electron radiation. Z Tech Physik 16:467

Knoll M, Theile R (1939) Scanning electron microscope for

determining the topography of surfaces and thin

layers. Z Physik 113:260

Oatley C (1972) The scanning electron microscope: part 1,

the instrument. Cambridge University Press, Cambridge

von Ardenne M (1938) The scanning electron microscope.

Theoretical fundamentals. Z Physik 109:553

mag HV WD HFW curr

3.9 mm 51.2 µm

20 µm

Field of view

5.00 um

Dwell Time Mag (4x5 Polaroid)

2,540.00 X

Detector

PrimaryETDetector

50.0 us

Blankar Current

0.7 9A

Image Size 1024x1024

Working Dist

50.00 um

5 000 x 15.00 kV 86 pA 12.1 mm

..      Fig. 9 Directionally-solidified Al-Cu eutectic alloy after ion beam milling in a dual-beam instrument, as imaged by the

SEM column (left image); same region imaged in the HIM (right image)

Scanning Electron Microscopy and Associated Techniques: Overview

XV

Contents

1 Electron Beam—Specimen Interactions: Interaction Volume ....................................................... 1

1.1 What Happens When the Beam Electrons Encounter Specimen Atoms? .......................................................... 2

1.2 Inelastic Scattering (Energy Loss) Limits Beam Electron

Travel in the Specimen ......................................................................................................................................................... 2

1.3 Elastic Scattering: Beam Electrons Change Direction of Flight ............................................................................ 4

1.3.1 How Frequently Does Elastic Scattering Occur? ............................................................................................................ 4

1.4 Simulating the Effects of Elastic Scattering: Monte Carlo Calculations ............................................................. 5

1.4.1 What Do Individual Monte Carlo Trajectories Look Like? ........................................................................................... 6

1.4.2 Monte Carlo Simulation To Visualize the Electron Interaction Volume .................................................................. 6

1.4.3 Using the Monte Carlo Electron Trajectory Simulation to Study

the Interaction Volume .......................................................................................................................................................... 8

1.5 A Range Equation To Estimate the Size of the Interaction Volume ................................................................... 12

References ................................................................................................................................................................................ 14

2 Backscattered Electrons ................................................................................................................................ 15

2.1 Origin .......................................................................................................................................................................................... 16

2.1.1 The Numerical Measure of Backscattered Electrons ..................................................................................................... 16

2.2 Critical Properties of Backscattered Electrons ............................................................................................................ 16

2.2.1 BSE Response to Specimen Composition (η vs. Atomic Number, Z) ....................................................................... 16

2.2.2 BSE Response to Specimen Inclination (η vs. Surface Tilt, θ) ..................................................................................... 20

2.2.3 Angular Distribution of Backscattering ............................................................................................................................ 22

2.2.4 Spatial Distribution of Backscattering ............................................................................................................................... 23

2.2.5 Energy Distribution of Backscattered Electrons............................................................................................................. 27

2.3 Summary .................................................................................................................................................................................... 27

References .................................................................................................................................................................................. 28

3 Secondary Electrons ....................................................................................................................................... 29

3.1 Origin .......................................................................................................................................................................................... 30

3.2 Energy Distribution ............................................................................................................................................................... 30

3.3 Escape Depth of Secondary Electrons ............................................................................................................................ 30

3.4 Secondary Electron Yield Versus Atomic Number ..................................................................................................... 30

3.5 Secondary Electron Yield Versus Specimen Tilt .......................................................................................................... 34

3.6 Angular Distribution of Secondary Electrons .............................................................................................................. 34

3.7 Secondary Electron Yield Versus Beam Energy ........................................................................................................... 35

3.8 Spatial Characteristics of Secondary Electrons ........................................................................................................... 35

References .................................................................................................................................................................................. 37

4 X-Rays .................................................................................................................................................................... 39

4.1 Overview .................................................................................................................................................................................... 40

4.2 Characteristic X-Rays ............................................................................................................................................................. 40

4.2.1 Origin ........................................................................................................................................................................................... 40

4.2.2 Fluorescence Yield ................................................................................................................................................................... 41

4.2.3 X-Ray Families ........................................................................................................................................................................... 42

4.2.4 X-Ray Nomenclature ............................................................................................................................................................... 43

4.2.5 X-Ray Weights of Lines ........................................................................................................................................................... 44

4.2.6 Characteristic X-Ray Intensity .............................................................................................................................................. 44

4.3 X-Ray Continuum (bremsstrahlung) ................................................................................................................................. 47

4.3.1 X-Ray Continuum Intensity ................................................................................................................................................... 49

4.3.2 The Electron-Excited X-Ray Spectrum, As-Generated .................................................................................................. 49

4.3.3 Range of X-ray Production .................................................................................................................................................... 50

4.3.4 Monte Carlo Simulation of X-Ray Generation ................................................................................................................. 51

4.3.5 X-ray Depth Distribution Function, ϕ(ρz) ......................................................................................................................... 53