Scanning Auger ElectronMicroscopy

Martin PruttonUniversity of York, York, UK

Mohamed M. El GomatiUniversity of York, York, UK

Scanning Auger Electron Microscopy

Scanning Auger ElectronMicroscopy

Martin PruttonUniversity of York, York, UK

Mohamed M. El GomatiUniversity of York, York, UK

Copyright � 2006 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester,West Sussex PO19 8SQ, England

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Library of Congress Cataloging-in-Publication Data

Scanning Auger electron microscopy / [edited by] Martin M. Prutton, MohamedM. El Gomati.

p. cm.Includes bibliographical references and index.ISBN-13: 978-0-470-86677-1 (cloth : acid-free paper)ISBN-10: 0-470-86677-2 (cloth : acid-free paper)1. Scanning Auger electron microscopy. I. Prutton, M. II. El Gomati,Mohamed M.

QH212.S24S31 2006502.8025–dc22 2005031938

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

ISBN-13 978-0-470-86677-1 (HB)ISBN-10 0-470-86677-2 (HB)

Typeset in 10/12pt Sabon by Thomson Press (India) Limited, New DelhiPrinted and bound in Spain by Grafos S.A., BarcelonaThis book is printed on acid-free paper responsibly manufactured from sustainable forestry inwhich at least two trees are planted for each one used for paper production.

To Aisha and in loving memory of Elsie who both believed thatMULSAM was a concept worth supporting. You did so withcare, encouragement and love. We could not have spent somuch time, energy and effort on it without your support.

Contents

List of Contributors ix

Preface xi

Acknowledgments xv

1. Introduction 1M.M. El Gomati and M. Prutton

2. The Auger Process 15J.A.D. Matthew

3. Instrumentation 45M.M. El Gomati and M. Prutton

4. The Spatial Resolution 125

M.M. El Gomati

5. Forming an Auger Image 165M.M. El Gomati and M. Prutton

6. Image Processing and Interpretation 201

M. Prutton

7. Quantification of Auger Images 245

M. Prutton

8. Applications: Materials Science 259

R.K. Wild

9. Applications: Semiconductor Manufacturing 295

C.F.H. Gondran

10. Concluding Remarks 341

M.M. El Gomati and M. Prutton

Author Index 351

Subject Index 359

viii CONTENTS

List of Contributors

M. M. El Gomati,Department of Electronics, University of York, Heslington, York, YO105DD, UK

C. F. H. GondranProcess Characterization Laboratory, ATDF Inc. [A subsidiary ofSEMATECH], 2706 Montopolis Drive, Austin, TX 78741, USA

J. A. D. MatthewDepartment of Physics, University of York, Heslington, York, YO105DD, UK

M. PruttonDepartment of Physics, University of York, Heslington, York, YO105DD, UK

R. K. WildInterface Analysis Centre, Oldbury House, 121 St Michaels Hill, BristolBS2 8BS, UK

Preface

Activity in surface science underwent an enormous explosion in the mid-1960s when ultra-high vacuum (UHV) technology became commerciallyavailable and so it became possible to clean a surface and maintainsufficient cleanliness for a time adequate to make a measurement ofsome kind. The activity started with measurements of work functionsand of the diffraction of low energy electrons but soon expanded to thedevelopment and use of many spectroscopies that yielded informationabout the electronic and vibrational states of atoms at or near the freesurface. Applications to the early stages of chemical reactions and to themetallurgical properties of materials were very prominent at this stage.Thus, the state of a nominally clean surface was of interest as were theeffects of various surface contaminants upon its properties and thesubsequent reaction with the components of a gaseous atmosphere ora beam of some other kind of atoms. Once the electron spectroscopieshad been applied to these problems there was considerable progress inthe development of the understanding of surface processes. Thesespectroscopies included X-ray photoelectron spectroscopies and Augerelectron spectroscopy that were sensitive to the number and kind ofatoms that were present right at the surface of the sample. However,practical surfaces were unlikely to consist of a clean, flat, arrangement ofatoms and the spatial resolution of these spectroscopies was so poor thatthe information so gleaned required the preparation of such perfectsurfaces or was an integration over many differently oriented crystal-lographic grains or even different materials. Nevertheless, enormousprogress was made in revealing the nature of the free surface of a solidand many surprising phenomena were discovered.

Electron stimulated Auger electron spectroscopy was a good candi-date for a microscopy because the beam of incident electrons upon thesample could be focused onto the surface and could be scanned acrossthe surface. These two properties were critical in the conversion of aspectroscopy to a microscopy that could provide maps or images of thedistribution of a selected element in the surface of a sample. All that was

required was a scanning electron microscope that provided a UHVenvironment and was equipped with an electron energy analyser toselect a particular Auger electron energy from the distribution ofelectron energies emitted from the sample. This apparently simplestatement is quite deceptive. This book sets out to describe the con-siderations required for this development to be brought to reality andthen to illustrate the application of scanning Auger electron microscopyto the surfaces of semiconducting device structures and to systems inmaterials science. It is now a mature methodology that is used incommercial instruments developed in Europe, Japan and the USA.Naturally, the complexity of these instruments means that they areexpensive and so unlikely to be found in the corner of any laboratory!Notwithstanding this expense they have provided information crucial,for instance, to the successful production of integrated circuits and tothe oxidation of the superalloys, and so the reliability of the jet enginesof the aeroplanes that we all use.

The editors (and authors) of this book have been contributing to thedevelopment and the applications of this microscopy since the early1970s. They have built, developed and used three complete Augerelectron microscopes. The first was a UHV scanning electron micro-scope (HB200) from Vacuum Generators Ltd and funded by the R.W.Paul Instrument Fund of the Royal Society that we adapted by adding aconcentric hemispherical analyser for electron energy analysis. Thisinstrument proved, to our satisfaction, that Auger electron imagingwas feasible and delivered our first results. It also showed that analogdetection of the electron current leaving the analyser was not a practicalway forward and that field emission electron sources were a subject forresearch in their own right!

The second instrument was developed with electron counting techni-ques for measuring the current leaving the analyser and Schottky fieldemission sources in the gun for the electron column. This instrumentwas much more successful and we were able to use it for the study of thesurfaces of nickel superalloys and for the identification of artefacts thatcan occur at the sharp edges of structures on top of a surface. Attemptswere started at this stage to acquire quantitative images of the distribu-tion of a given element on a surface. This appeared feasible because ofsuccessful attempts to analyse the amount of material on a surface usingAuger electron spectroscopy carried out by ourselves and others (seeChapters 2 and 3).

The difficulties of this image quantification led us to consider thesimultaneous acquisition of other types of electrons and photons from

xii PREFACE

the sample in order to try to disentangle the structure of the surfaceregion from the yield of each kind of signal being detected. This led tothe invention and development of the third microscope, the multi-spectral Auger microscope (MULSAM), that was equipped with detec-tors for simultaneous measurement of images from an area of interestusing scanning electron microscopy, energetic back-scattered electrons,loss electrons and characteristic X-rays. This instrument has been usedfor a wide variety of applications, some of which are described in thisbook. It may be clear from this description that the design, building anddevelopment of these three instruments has been a huge task that hasoccupied the time of many people. We are very pleased that we are ableto acknowledge their various contributions in the subsequent section.

Spending all this time on instrument development has meant that theeditors felt it unwise to attempt to review other areas of the subject.Accordingly, we are very pleased that Jim Matthew, Bob Wild andCarolyn Gondran were able to contribute Chapters 2, 8 and 9 from theirareas of expertise – contributions that reflect the value of their work inthis field.

The material in this book is intended as a guide to the subject of Augerelectron microscopy and so it is hoped that it will be of interest toresearchers in this field as well as to others who wish to discover whatcan be achieved with this technique and what are its limitations. Thus itis hoped that it will be useful to analysts working with scanning Augerelectron microscopes, who are hard pressed to hurry up and measuremany samples and so have little time to work on other aspects of thebehaviour of their instrument or the problems that they may, perhapsunwittingly, encounter.

M. M. El Gomati

M. Prutton

PREFACE xiii

Acknowledgments

The editors (and authors) have many people to thank for the help theyhave been given in the building, development and use of the MULSAMinstrument at York. Building an entire electron microscope from scratchis a far from minor undertaking especially when it has to be a UHVinstrument with energy analysis and it is entirely computer controlled.There were times when we wished that we had not committed ourselvesto the job!

The first and most important acknowledgment is to Oliver Heavenswho backed us from the paper study stage through all the trials andtribulations with humour, a deep interest and, most importantly,money. In spite of an absurdly heavy workload when he was head ofdepartment, he managed to come by at least once a week to see how wewere progressing and to bring along some champagne at times ofsuccess. Thank you very much Oliver.

The project started with the design and build of a fast concentrichemispherical analyser which was funded by the R.W. Paul Fund of TheRoyal Society. The Royal Society Assessors for this work were PeterDuncumb and Reg Garton who came not to criticise but to offer theirexperience and help. This was invaluable and an important contributorto the speed of progress. Peter Bassett worked on this developmentwhich successfully began the whole set of moves to build, develop anduse a scanning Auger electron microscope.

In fact, three scanning Auger electron microscopes were constructedand a number of postdoctoral fellows and research students wereinvolved. The first instrument was an analog realisation to which RayBrowning and the late Dave Peacock were crucial contributors. Theyquickly demonstrated that although the Auger electrons could bedetected in this way it was the wrong way to go about it. Dave andRay stayed on and were joined by Peter Kenny and Chris Walker (atdifferent times) to build a digital instrument. Peter Kenny worked for 6years on the software for the microscope (writing some 200 000 lines of

software which worked first time) with occasional inputs from MartinPrutton and Mohamed El Gomati and from Chris Walker and DavePeacock. This was an exciting and productive time helped by the factthat we were joined by Ron Roberts from the University of Newcastle,New South Wales, Australia, who brought many skills in experimentaltechniques and electronics to the project. Ron has made repeated visitsto work with us and has been an invaluable help.

The final instrument to be built was MULSAM. This is the mostcomplex instrument and took the efforts of many individuals. MattWenham contributed to the software with multivariate statistics, JohnGreenwood and Ian Barkshire contributed to the hardware and softwaredesigns and execution in many important ways. David Wilkinsonextended the use of an energy dispersive X-ray detector to worksimultaneously with the various electron detectors. Support for thiswork relied on help generously given by Cedric Powell of NIST andMartin Seah of NPL.

There were many contributions from D. Phil. students includingAhmed Assa’d, Taib Bakoush, Martin Crone, Barack Kola, JohnKudjoe, Dan Loveday, Phil Tenney, John Walton, Torquil Wells,Andy Gelsthorpe and Li Chen, all of whom helped to move the wholeproject forward.

Of course, a big instrumental build of this kind also required thetechnical assistance of many people. Jack Dee worked as a technicianlooking after all three microscopes and his help was indispensable. It is apleasure to be able to thank Colin Ovenden, David Coulthard and MickPeters for a wide range of skills that helped to keep the instrumentsrunning. Staff in the mechanical workshop made beautiful jobs of theconcentric hemispherical analyser, the specimen manipulator, the elec-tron column and many other smaller components. They included JohnEastwood, the late Lennie Jarvis, Leigh Crosby, Peter Durkin, BobEaston, Brent Wilkinson, Ian Wright and Pete Turner. Staff in theelectronics workshop made excellent high stability power supplies,beam scanning and stigmators as well as many smaller pieces ofelectronic hardware. They included the late Jim Scott, Steve Lawson,Simon Hart, Pete Turner and Bob Hide. The technical staff was asimportant to this project as the scientists.

Help with electron optical design was generously given in the earlystages of the project by Ken Smith and Eric Munro of the EngineeringDepartment, University of Cambridge. We would also like to thankTom Mulvey, Aston University, Birmingham, for his encouragementand advice from his wide experience of many different kinds of electron

xvi ACKNOWLEDGMENTS

optical devices. Don Whitehead of the then VSW Ltd was extremelygenerous with financial support for electron optics for which we aremost grateful.

Collaboration with colleagues in the semiconductor fabrication indus-try was very important in demonstrating just what could be learned withan Auger microscope. Funding for the project came from the AlveyProject of the Department of Trade and Industry and from the AdequatProject of the European Union for which we are most grateful. In theseprojects we worked with Chris Hill, Pete Pearson, Peter Augustus andKevin Stribley of Plessey, with Barry Lamb of Standard Telecommuni-cations Ltd and Geoff Spiller and Chris Tuppen of British Telecom. Weare most grateful to these collaborators and for the excellent samplesthat they made available to us.

Carolyn Gondran would like to thank Mark Clark, Chris Sparks,Charlene Johnson but most especially Milt Godwin and Laurie Modreyfor their many, many helpful comments and suggestions, and MarilynRedmond and Bob Ruliffson from the SEMATECH library for assis-tance with reference material. She would also like to thank her family:Chris for moral support and her daughters for the endless distractions.

ACKNOWLEDGMENTS xvii

1IntroductionM. M. El Gomati and M. Prutton

The region near to the surface of a solid material can play important-roles in the properties of that solid. Should an atom or molecule arrive atsuch a surface, be it in vacuo, in air, in a liquid or in contact with thesurface of a different material, then the crystallographic structure, theatomic type, the electronic structure, the vibrations of surface atoms andthe bonding forces between the arrival and the surface may all affectwhat happens next. Thus, for example, the arrival may adhere to thesolid surface or be scattered off of it, or the arrival may react withthe surface forming a new compound locally. Should the temperature,the structure and the binding energies of the atoms in the surface haveappropriate values then the arrival may diffuse into the solid or evencause atoms in the solid to diffuse out to the surface. For these reasonssolid surfaces are important in many processes in a wide variety ofdifferent parts of science, including biology, chemistry, materials scienceand physics. Further, they are important in many areas of technologysuch as semiconductor device fabrication and characterisation, thedesign of catalysts to speed up chemical reactions, and the developmentof anti-corrosion layers on alloys and metals. The subject of surfacescience is thus very broad indeed, having scientific and commercialimplications in the effects that it has on large industries. Introductions tothe subject include books by Prutton1, Walls2, Woodruff and Delchar3

and Zangwill4. The whole area has been reviewed, for instance, byDuke5 and by Duke and Plummer6.

What is meant by the surface of a solid? The answer to this questiondepends upon what surface properties are under investigation and what

Scanning Auger Electron Microscopy Edited by M. Prutton and M. El Gomati

# 2006 John Wiley & Sons, Ltd

experimental techniques are being used for their measurement. Thetheoretical physicist may be interested in the wave functions of atoms inthe outermost layer of the solid. Most extremely, interest may be on thewave functions and their properties in the region in a vacuum outsidethe solid surface. The experimental scientist may be measuring theproperties of the topmost few atomic layers of the solid or the topmostfew hundred layers depending upon the methods being used. Mostexperimental methods involve the bombardment of the surface understudy by particles or photons and the detection of scattered particles orphotons. If visible photons are incident and reflected photons aredetected then the depth of the region of the solid being probed is ofthe order of the wavelength of the light being used – the informationdepth is of the order of many hundreds of nanometers. If energetic X-raysare incident and detected then this depth may be of the order of microns.If energetic X-rays are incident and photoelectrons are detected thisdepth can be as small as a fraction of a nanometer – only a few atomlayers are being probed. A similar information depth is obtained whenenergetic electrons are incident and Auger electrons are emitted from theatoms in the solid. In this book the surface is taken to be the regionof a solid within a depth of a few (<20) atomic layers from its freesurface.

This information depth depends upon the relative sizes of the depthpenetrated by the incident photons or particles – the penetration depthand the depth from which the stimulated particles or photons can arriveat the detector with properties unchanged – the escape depth. If thepenetration depth is small compared with the escape depth then it is thepenetration depth that determines the information depth. This isthe case, for example, in energy dispersive X-ray (EDX) detectionwhere electrons are focused onto a solid sample and may penetrate toa depth of the order of a micron and characteristic X-rays are emittedand detected. The X-rays may reach their detector unchanged from sucha relatively small depth so the information depth is the penetrationdepth. At the other extreme, in Auger electron spectroscopy (AES),energetic (say 10 keV) electrons may be focused onto the solid and lowenergy Auger electrons are detected. The ingoing electrons may pene-trate a micron or so but the Auger electrons have much lower kineticenergies and can only escape from the solid with their energiesunchanged if they originate from very near the surface. In this casethe escape depth determines the information depth and may be verysmall – about 0.5 nm depending upon the kinetic energy of the Augerelectrons. The information depths for X-ray photoelectron spectroscopyare very similar to those of Auger spectroscopy – particularly when the

2 INTRODUCTION

kinetic energies of the photoelectrons are below about 1 keV. Thissubject is dealt with more completely in Chapter 2.

When the intention is to study the topmost atomic layer in a solid theenvironment in which the sample is immersed becomes of criticalimportance. Even in a ‘vacuum’, in every second many atoms ormolecules in the ambient atmosphere will strike the surface. This ratedepends upon the pressure of the ambient gas – the lower the pressurethe lower the rate of impact. Since these events may change thesurface by knocking off atoms expected to be present, by sticking tothe surface or by reacting with the surface then the surface may bechanged from whatever state it was intended to be in – atomically clean,covered with specific atoms of a different kind or whatever the inves-tigator required. The kinetic theory associated with contamination fromthe ambient atmosphere is discussed in more detail in Chapter 3. Mostsurface science measurements are conducted in ultra-high vacuum(UHV)7 in which the total pressure is less than about 10�9 mbar. In suchpressures the arrival rate of molecules from the ambient gas can allowmeasurement times of several hours8 before the surface under study iscovered with a single layer (a monolayer) of contaminating molecules.

One question that usually needs to be answered about a surface is‘what is it composed of?’ The answer is revealed with those measure-ments that can be chemically specific and yet have sufficient sensitivityto detect the small amount of material in the topmost atomic layer. Thetechniques available to a surface analyst are summarised in Table 1.1where a rough guide to the sensitivity of each method is given.

Table 1.1 The commonly available methods to identify the elements in or near thesurface of a solid together with a rough estimate of their sensitivities in terms ofatomic monolayers. Most elemental materials contain about 1019 atoms m�2

RoughIncident ‘Particles’ sensitivity

Method Acronym ‘particles’ detected (monolayers)

Energy EDX Electrons Characteristic 200dispersive (5–30 keV) X-raysX-ray analysis (0.5–10 keV)

X-ray XPS Monochromatic Photoelectrons 0.1photoelectron X-rays (0.1–5 keV)spectroscopy (5 keV)

Auger AES Monochromatic Auger 0.1electron electrons electronsspectroscopy (5 keV) (0.1–3 keV)

Secondary SIMS Monochromatic Secondary 10 atomsion mass ions (Ar) ionsspectroscopy (mass 1–5000)

SCANNING AUGER ELECTRON MICROSCOPY 3

This book concentrates upon a form of scanning electron microscopyin which electrons are focused onto the surface of a solid sample andAuger electrons are emitted into an energy analyser in which theirkinetic energy is established. These electrons were first described as atheoretical possibility in 1923 by Rosseland9 and were identified byMeitner10,11 and independently by Auger12 from the results of cloudchamber experiments. Photographs of Auger and Meitner are repro-duced in Figure 1.1 and the story of Lise Meitner’s scientific struggles isdescribed by Sime13. All their work was directed at the explanation ofsharp spectral features in b-ray spectroscopy arising from internalconversion in g irradiation. The subject is described in the book byBurhop14. The use of Auger electrons in the analysis of surfaces was firstdescribed by Lander as early as 195314. As can be seen in the caption toFigure 1.2, the kinetic energy of an Auger electron is determined by thedifferences between the electronic energy levels in the atoms involved inthe process. This energy depends upon the element emitting Augerelectrons and is independent of the energy of the ionising beam ofelectrons. The intensity (the number of electrons detected) in a particular

Figure 1.1 Potraits of Pierre Auger and Lise Meitner. Both pictures are by courtesyof the American Institute of Physics, Emilio Segre Visual Archives. The picture ofMeitner is part of the Herzfeld Collection in those archives

4 INTRODUCTION

Figure 1.2 A one-electron energy level diagram for a mythical metallic sample. Thelabels on the right indicate the notation used for the energy levels in X-rayspectroscopy. The labels on the left correspond to the binding energies of theelectrons in each level measured from the vacuum level. (a) The process ofgeneration of an Auger electron is indicated. In this example the electron beam isincident with sufficient kinetic energy to ionise the K shell of the atoms involved. Theenergy of the ionised atom is reduced by one electron falling into the initial state holefrom a less tightly bound state and by a second electron being emitted from the sameor another less tightly bound state. The kinetic energy of the electron leaving theatom is approximately EK � EL1 � EL2;3. This energy is independent of the energy ofthe incident electron. This particular process would be described as the emission of aKL1L2;3 Auger electron. (b) For XPS the monochromatic X-ray beam with energy hnionises the 1s level and causes the emission of an electron with kinetic energyhn� E1s. E1s is the binding energy of the electron in the 1s (or K) state

SCANNING AUGER ELECTRON MICROSCOPY 5

Auger peak in the electron spectrum does depend upon the energy of theionizing beam. Indeed, using the example in Figure 1.2(a), if this energyis lower than the binding energy of the initial state electron (EK in thatexample) then no Auger electrons can be emitted based upon that initialstate. If the energy is above this threshold then the intensity is deter-mined, in part, by the ionisation cross-section for that initial state. Thedependence of Auger intensities upon the energy of the incident electronbeam is covered in more detail in Chapter 2. An example of some Augerpeaks from a contaminated silver sample is shown in Figure 1.3 andexamples of spectra from many elements, compounds and alloys can befound in the handbooks of spectra published by Japan Electron Optics(JEOL Ltd)16 and by Physical Electronics Inc. (PHI)17 and on the web athttp://www.lasurface.com. The whole subject of surface analysis hasbeen reviewed in books by Prutton1, Vickerman18, Venables19 andBriggs and Grant20.

X-ray photoelectron spectroscopy is similar to Auger electronspectroscopy in that the incident radiation has sufficient energy toionise core levels in the atoms of the surface. Photoelectrons areemitted with an energy corresponding to the difference between thatof the incident X-ray photons and the binding energy of electrons inthe level ionised. Thus, in Figure 1.2(b), the ionization was of a K stateand so the energy of the photoelectron emitted would be hn� EK.Again, for a fixed X-ray beam energy the kinetic energy of thephotoelectrons is characteristic of the emitting element. Photoelectronspectroscopy has played an important part in the history of the studyof wave particle duality. The effect was described by Innes21 in 1907and developed into a spectroscopy later by Siegbahn22 and others.Siegbahn gave the name ‘Electron Spectroscopy for Chemical Analysis’(ESCA) for the use of both Auger and photoelectron spectroscopiesexcited by X-ray photons.

It is technically simple to scan an electron or ion beam across a surfacewith a deflecting electric or magnetic field acting on the incident beam tocause it to remain focused on the sample but to be displaced across thesurface in some desired sequence of movements. This makes scanningmicroscopy possible. The beam can be scanned across the surface of thesample in a series of steps. Whilst the beam is static the ions or electronsemitted from the sample can be detected and mass or kinetic energyanalysed and the number of scattered ions with a particular mass (forSIMS) or the number of Auger electrons with a particular kinetic energy(for AES) counted and stored. The beam is then moved to the nextposition on the sample and the process repeated. This step, analyse,

6 INTRODUCTION

0

1000

00

2000

00

3000

00

4000

00

5000

00

5010

015

020

025

030

035

040

045

050

055

0

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erg

y (e

V)

st nuoC

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Figure

1.3

Asectionfrom

anAuger

spectrum

from

asilver

sample.Theincidentelectronbeam

hadenergyof5keV

andabeam

currentof

10nA.Theenergyanalyserwasaconcentric

hem

isphericaltypein

theYork

MULSAM

instrument(see

Chapter3).

Thepeaksare

superim

poseduponaslowly

changingbackgrounddueto

thesecondary

electronsleavingthesample

count and store cycle is usually carried out in either a square orrectangular array of points on the surface – a so-called digital raster.Either after or during this process an image of the surface can bedisplayed on a computer monitor or other device in which the positionof the incident beam on the sample has a simple mapping onto thescreen of the display and the counts detected at that position on thesample determine the intensity of the corresponding point on thedisplay. Thus, places on the sample with a large number of atomsemitting electrons of the chosen kinetic energy appear brighter on thedisplay than places with few of the same kind of atoms. The displaythus provides a chemical map of the distribution of that element in thesurface. It is a spectroscopic image in the sense that the variations inthe intensity of a feature in an energy spectrum have been mappedfrom place to place on a surface. The first to report the use of Augerelectrons to demonstrate scanning Auger microscopy were Harris23

and McDonald24. A diagram comparing the components of a scanningelectron microscope and a scanning Auger microscope is shown inFigure 1.4. Scanning Auger microscopy should strictly be defined asscanning Auger electron microscopy (abbreviated as SAEM) butcommon usage is now to refer to it as scanning Auger microscopy(SAM) which is conveniently shorter and easier to pronounce, at leastin English! Clearly it is electrons that are being scanned not anindividual whose name is Auger!

Spectroscopic imaging of this kind involves the acquisition of a greatdeal of data. Imagine, for example, the characterization of the surfacechemical composition of an area of a sample containing five elementswhose concentrations vary from place to place. At each position on thesample at which a measurement is made the heights or areas of at leastfive peaks in a spectrum must be estimated. The simplest approachmight be to make estimates of the peak heights by subtracting themeasured background counts in the spectrum with energy above orbelow each peak from the counts in the spectrum at the peak energy.Thus, for five elements, a minimum of 10 measurements must be made.Consider that a set of five images are to be formed, one for each element,each with 256 by 256 picture points (pixels) for adequate image qualityfor presentation or subsequent analysis. This means that at least 655360measurements are required to derive the five images. If it is assumed that32-bit precision is needed in each measurement in order to haveadequate dynamic range in the possible counts and to allow subsequentnumerical processing then this image set requires at least 2.5 Mbytes ofstorage space. If some experimental parameter is to be varied – say

8 INTRODUCTION

1

2

3

4

5 6

7 8

9

Conventionalvacuum

(a)

10

1

2

3

4

5 6

7 8 9

Ultrahighvacuum

(b)

11

1210

13

Figure 1.4 A comparison of the principal components of (a) an SEM and (b) aSAM. Item 1 is an electron source; items 2 and 3 are electron lenses and 4 is a set ofpower supplies all of which are used to form a focused electron beam on the samplesurface, item 7. Items 5 and 6 are components to scan the focused beam across thesurface of the sample. Item 8 detects secondary electrons in (a) and electrons ofselected kinetic energy emerging from an energy analyser 11 in (b). Items 9 and 10amplify and condition the signal from 8 to control the brightness of a display 10 orprovide input to a control computer 13. Item 12 represents the electronics providingcontrol of the potentials in the electron energy analyser in (b)

SCANNING AUGER ELECTRON MICROSCOPY 9

the sample temperature or exposure to some gas reacting with thesurface – then many such sets of data may be required as the surfacechanges. The implications for data storage are obvious and some formof data compression may be essential for efficient use of availableresources. Some considerations of the hardware and software require-ments for these instruments are described in Chapter 3.

A more important issue arises from the time that must be taken toacquire such a set of images. Most electron energy analysers aresequential devices. The potentials are set on each electrode of ananalyser to determine the kinetic energies of electrons that lie in asmall band of energies about the energy of the feature to be measured.Counting can then begin and is allowed to continue for the dwell time ofmeasurement. After this time the analyser has to be set for a new energyor the beam exciting the electrons is moved on to the next position onthe sample surface. Using the case of Auger electron emission and theequation for the yield of Auger electrons proposed by Bishop andRiviere25 (see Chapter 2), the example described in the previous para-graph can be pursued. Consider, for example, an incident electron beamof 10 nA striking a monolayer of oxygen atoms absorbed upon a siliconsurface. Auger electrons with 505 eV kinetic energy are emitted from theoxygen atoms that are present with a density of about 1015 cm�2. Thecross-section for this process is about 10�21 cm2. If, say, 1% ofthe Auger electrons emitted enter the energy analyser and are detectedthen the current collected is about 10�16 A or about 600 electrons s�1.Using Poisson statistics for the counting of electrons and a counting timeof t s then the signal to noise ratio in a single measurement will be(600t)1/2. Thus, for example, if a measurement is made for 17 ms thenabout 10 electrons will be detected and the signal to noise ratio will beabout 3:1. If this dwell time is chosen for each of the 10 energies in theexample above and for each position of an incident electron beam onthe sample then the 655360 measurements must take at least 3 h. Lesstime can be taken only by modifying the energy analyser to acquireseveral energy channels simultaneously or to collect a greater fraction ofthe total emission from the sample, accepting a lower number of pointsin the image, increasing the current in the beam reaching the sample orallowing a further reduction in the signal to noise ratio. The saving intime so gained scales with the square root of the number of electronscounted in each pixel and at each energy and so it is difficult to makelarge reductions in the total data acquisition time unless radical changesare made to the energy analyser.

10 INTRODUCTION