4. EDITORIAL BOARD
Elton N. Kaufmann, (Editor-in-Chief) Ronald Gronsky
Argonne National Laboratory University of California at Berkeley
Argonne, IL Berkeley, CA
Reza Abbaschian Leonard Leibowitz
University of Florida at Gainesville Argonne National Laboratory
Gainesville, FL Argonne, IL
Peter A. Barnes Thomas Mason
Clemson University Spallation Neutron Source Project
Clemson, SC Oak Ridge, TN
Andrew B. Bocarsly Juan M. Sanchez
Princeton University University of Texas at Austin
Princeton, NJ Austin, TX
Chia-Ling Chien Alan C. Samuels, Developmental Editor
Johns Hopkins University Edgewood Chemical Biological Center
Baltimore, MD Aberdeen Proving Ground, MD
David Dollimore
University of Toledo EDITORIAL STAFF
Toledo, OH
VP, STM Books: Janet Bailey
Barney L. Doyle Executive Editor: Jacqueline I. Kroschwitz
Sandia National Laboratories Editor: Arza Seidel
Albuquerque, NM Director, Book Production
and Manufacturing:
Brent Fultz Camille P. Carter
Managing Editor: Shirley Thomas
California Institute of Technology
Assistant Managing Editor: Kristen Parrish
Pasadena, CA
Alan I. Goldman
Iowa State University
Ames, IA
5. CHARACTERIZATION
OF MATERIALS
VOLUMES 1 AND 2
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Library of Congress Cataloging in Publication Data is available.
Characterization of Materials, 2 volume set
Elton N. Kaufmann, editor-in-chief
ISBN: 0-471-26882-8 (acid-free paper)
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
7. CONTENTS, VOLUMES 1 AND 2
FOREWORD vii THERMAL ANALYSIS 337
PREFACE ix Thermal Analysis, Introduction 337
Thermal Analysis—Definitions, Codes of Practice,
CONTRIBUTORS xiii and Nomenclature 337
Thermogravimetric Analysis 344
COMMON CONCEPTS 1 Differential Thermal Analysis and Differential
Scanning Calorimetry 362
Common Concepts in Materials Characterization,
Combustion Calorimetry 373
Introduction 1
Thermal Diffusivity by the Laser Flash Technique 383
General Vacuum Techniques 1
Simultaneous Techniques Including Analysis
Mass and Density Measurements 24
of Gaseous Products 392
Thermometry 30
Symmetry in Crystallography 39
Particle Scattering 51 ELECTRICAL AND ELECTRONIC MEASUREMENTS 401
Sample Preparation for Metallography 63 Electrical and Electronic Measurement,
Introduction 401
Conductivity Measurement 401
COMPUTATION AND THEORETICAL METHODS 71
Hall Effect in Semiconductors 411
Computation and Theoretical Methods, Deep-Level Transient Spectroscopy 418
Introduction 71 Carrier Lifetime: Free Carrier Absorption,
Introduction to Computation 71 Photoconductivity, and Photoluminescence 427
Summary of Electronic Structure Methods 74 Capacitance-Voltage (C-V) Characterization
Prediction of Phase Diagrams 90 of Semiconductors 456
Simulation of Microstructural Evolution Characterization of pn Junctions 466
Using the Field Method 112 Electrical Measurements on Superconductors
Bonding in Metals 134 by Transport 472
Binary and Multicomponent Diffusion 145
Molecular-Dynamics Simulation of Surface MAGNETISM AND MAGNETIC MEASUREMENTS 491
Phenomena 156
Simulation of Chemical Vapor Magnetism and Magnetic Measurement,
Deposition Processes 166 Introduction 491
Magnetism in Alloys 180 Generation and Measurement of Magnetic Fields 495
Kinematic Diffraction of X Rays 206 Magnetic Moment and Magnetization 511
Dynamical Diffraction 224 Theory of Magnetic Phase Transitions 528
Computation of Diffuse Intensities in Alloys 252 Magnetometry 531
Thermomagnetic Analysis 540
Techniques to Measure Magnetic Domain
MECHANICAL TESTING 279 Structures 545
Magnetotransport in Metals and Alloys 559
Mechanical Testing, Introduction 279 Surface Magneto-Optic Kerr Effect 569
Tension Testing 279
High-Strain-Rate Testing of Materials 288
Fracture Toughness Testing Methods 302 ELECTROCHEMICAL TECHNIQUES 579
Hardness Testing 316 Electrochemical Techniques, Introduction 579
Tribological and Wear Testing 324 Cyclic Voltammetry 580
v
8. vi CONTENTS, VOLUMES 1 AND 2
Electrochemical Techniques for Corrosion X-Ray Diffraction Techniques for Liquid
Quantification 592 Surfaces and Monomolecular Layers 1027
Semiconductor Photoelectrochemistry 605
Scanning Electrochemical Microscopy 636 ELECTRON TECHNIQUES 1049
The Quartz Crystal Microbalance
in Electrochemistry 653 Electron Techniques, Introduction 1049
Scanning Electron Microscopy 1050
OPTICAL IMAGING AND SPECTROSCOPY 665 Transmission Electron Microscopy 1063
Scanning Transmission Electron Microscopy:
Optical Imaging and Spectroscopy, Introduction 665 Z-Contrast Imaging 1090
Optical Microscopy 667 Scanning Tunneling Microscopy 1111
Reflected-Light Optical Microscopy 674 Low-Energy Electron Diffraction 1120
Photoluminescence Spectroscopy 681 Energy-Dispersive Spectrometry 1135
Ultraviolet and Visible Absorption Spectroscopy 688 Auger Electron Spectroscopy 1157
Raman Spectroscopy of Solids 698
Ultraviolet Photoelectron Spectroscopy 722
ION-BEAM TECHNIQUES 1175
Ellipsometry 735
Impulsive Stimulated Thermal Scattering 744 Ion-Beam Techniques, Introduction 1175
High-Energy Ion-Beam Analysis 1176
RESONANCE METHODS 761 Elastic Ion Scattering for Composition
Analysis 1179
Resonance Methods, Introduction 761 Nuclear Reaction Analysis and Proton-Induced
Nuclear Magnetic Resonance Imaging 762 Gamma Ray Emission 1200
Nuclear Quadrupole Resonance 775 Particle-Induced X-Ray Emission 1210
Electron Paramagnetic Resonance Spectroscopy 792 Radiation Effects Microscopy 1223
Cyclotron Resonance 805 Trace Element Accelerator Mass
¨
Mossbauer Spectrometry 816 Spectrometry 1235
Introduction to Medium-Energy Ion Beam
X-RAY TECHNIQUES 835 Analysis 1258
X-Ray Techniques, Introduction 835 Medium-Energy Backscattering and
X-Ray Powder Diffraction 835 Forward-Recoil Spectrometry 1259
Single-Crystal X-Ray Structure Determination 850 Heavy-Ion Backscattering Spectrometry 1273
XAFS Spectroscopy 869
X-Ray and Neutron Diffuse Scattering NEUTRON TECHNIQUES 1285
Measurements 882
Resonant Scattering Techniques 905 Neutron Techniques, Introduction 1285
Magnetic X-Ray Scattering 917 Neutron Powder Diffraction 1285
X-Ray Microprobe for Fluorescence Single-Crystal Neutron Diffraction 1307
and Diffraction Analysis 939 Phonon Studies 1316
X-Ray Magnetic Circular Dichroism 953 Magnetic Neutron Scattering 1328
X-Ray Photoelectron Spectroscopy 970
Surface X-Ray Diffraction 1007 INDEX 1341
9. FOREWORD
Whatever standards may have been used for materials The successes that accompanied the new approach to
research in antiquity, when fabrication was regarded materials research and development stimulated an
more as an art than a science and tended to be shrouded entirely new spirit of invention. What had once been
in secrecy, an abrupt change occurred with the systematic dreams, such as the invention of the automobile and the
discovery of the chemical elements two centuries ago by airplane, were transformed into reality, in part through
Cavendish, Priestly, Lavoisier, and their numerous suc- the modification of old materials and in part by creation
cessors. This revolution was enhanced by the parallel of new ones. The growth in basic understanding of electro-
development of electrochemistry and eventually capped magnetic phenomena, coupled with the discovery that
by the consolidating work of Mendeleyev which led to the some materials possessed special electrical properties,
periodic chart of the elements. encouraged the development of new equipment for power
The age of materials science and technology had finally conversion and new methods of long-distance communica-
begun. This does not mean that empirical or trial and error tion with the use of wired or wireless systems. In brief, the
work was abandoned as unnecessary. But rather that a successes derived from the new approach to the develop-
new attitude had entered the field. The diligent fabricator ment of materials had the effect of stimulating attempts
of materials would welcome the development of new tools to achieve practical goals which had previously seemed
that could advance his or her work whether exploratory beyond reach. The technical base of society was being
or applied. For example, electrochemistry became an shaken to its foundations. And the end is not yet in sight.
intimate part of the armature of materials technology. The process of fabricating special materials for well
Fortunately, the physicist as well as the chemist were defined practical missions, such as the development of
able to offer new tools. Initially these included such mat- new inventions or improving old ones, has, and continues
ters as a vast improvement of the optical microscope, the to have, its counterpart in exploratory research that is
development of the analytic spectroscope, the discovery carried out primarily to expand the range of knowledge
of x-ray diffraction and the invention of the electron and properties of materials of various types. Such investi-
microscope. Moreover, many other items such as isotopic gations began in the field of mineralogy somewhat before
tracers, laser spectroscopes and magnetic resonance the age of modern chemistry and were stimulated by the
equipment eventually emerged and were found useful in fact that many common minerals display regular cleavage
their turn as the science of physics and the demands for planes and may exhibit unusual optical properties, such
better materials evolved. as different indices of refraction in different directions.
Quite apart from being used to re-evaluate the basis for Studies of this type became much broader and more sys-
the properties of materials that had long been useful, the tematic, however, once the variety of sophisticated
new approaches provided much more important dividends. exploratory tools provided by chemistry and physics
The ever-expanding knowledge of chemistry made it possi- became available. Although the groups of individuals
ble not only to improve upon those properties by varying involved in this work tended to live somewhat apart from
composition, structure and other factors in controlled the technologists, it was inevitable that some of their dis-
amounts, but revealed the existence of completely new coveries would eventually prove to be very useful. Many
materials that frequently turned out to be exceedingly use- examples can be given. In the 1870s a young investigator
ful. The mechanical properties of relatively inexpensive who was studying the electrical properties of a group of
steels were improved by the additions of silicon, an element poorly conducting metal sulfides, today classed among
which had been produced first as a chemist’s oddity. More the family of semiconductors, noted that his specimens
complex ferrosilicon alloys revolutionized the performance seemed to exhibit a different electrical conductivity when
of electric transformers. A hitherto all but unknown ele- the voltage was applied in opposite directions. Careful
ment, tungsten, provided a long-term solution in the search measurements at a later date demonstrated that specially
for a durable filament for the incandescent lamp. Even- prepared specimens of silicon displayed this rectifying
tually the chemists were to emerge with valuable families effect to an even more marked degree. Another investiga-
of organic polymers that replaced many natural materials. tor discovered a family of crystals that displayed surface
vii
10. viii FOREWORD
charges of opposite polarity when placed under unidirec- bearing on the properties of materials being explored for
tional pressure, so called piezoelectricity. Natural radioac- novel purposes. The semiconductor industry has been an
tivity was discovered in a specimen of a uranium mineral important beneficiary of this form of exploratory research
whose physical properties were under study. Supercon- since the operation of integrated circuits can be highly sen-
ductivity was discovered incidentally in a systematic study sitive to imperfections.
of the electrical conductivity of simple metals close to the In this connection, it should be added that the ever-
absolute zero of temperature. The possibility of creating a increasing search for special materials that possess new
light-emitting crystal diode was suggested once wave or superior properties under conditions in which the spon-
mechanics was developed and began to be applied to sors of exploratory research and development and the pro-
advance our understanding of the properties of materials spective beneficiaries of the technological advance have
further. Actually, achievement of the device proved to be parallel interests has made it possible for those engaged
more difficult than its conception. The materials involved in the exploratory research to share in the funds directed
had to be prepared with great care. toward applications. This has done much to enhance the
Among the many avenues explored for the sake of degree of partnership between the scientist and engineer
obtaining new basic knowledge is that related to the in advancing the field of materials research.
influence of imperfections on the properties of materials. Finally, it should be emphasized again that whenever
Some imperfections, such as those which give rise to materials research has played a decisive role in advancing
temperature-dependent electrical conductivity in semicon- some aspect of technology, the advance has frequently
ductors, salts and metals could be ascribed to thermal been aided by the introduction of an increasingly sophisti-
fluctuations. Others were linked to foreign atoms which cated set of characterization tools that are drawn from a
were added intentionally or occurred by accident. Still wide range of scientific disciplines. These tools usually
others were the result of deviations in the arrangement remain a part of the array of test equipment.
of atoms from that expected in ideal lattice structures.
As might be expected, discoveries in this area not only FREDERICK SEITZ
clarified mysteries associated with ancient aspects of President Emeritus, Rockefeller University
materials research, but provided tests that could have a Past President, National Academy of Sciences, USA
11. PREFACE
Materials research is an extraordinarily broad and diverse that is observed. When both tool and sample each contri-
field. It draws on the science, the technology, and the tools bute their own materials properties—e.g., electrolyte and
of a variety of scientific and engineering disciplines as it electrode, pin and disc, source and absorber, etc.—distinc-
pursues research objectives spanning the very fundamen- tions are blurred. Although these distinctions in principle
tal to the highly applied. Beyond the generic idea of a ought not to be taken too seriously, keeping them in mind
‘‘material’’ per se, perhaps the single unifying element will aid in efficiently accessing content of interest in these
that qualifies this collection of pursuits as a field of volumes.
research and study is the existence of a portfolio of charac- Frequently, the materials property sought is not what
terization methods that is widely applicable irrespective of is directly measured. Rather it is deduced from direct
discipline or ultimate materials application. Characteriza- observation of some other property or phenomenon that
tion of Materials specifically addresses that portfolio with acts as a signature of what is of interest. These relation-
which researchers and educators must have working ships take many forms. Thermal arrest, magnetic anomaly,
familiarity. diffraction spot intensity, relaxation rate and resistivity,
The immediate challenge to organizing the content for a to name only a few, might all serve as signatures of a phase
methodological reference work is determining how best to transition and be used as ‘‘spectator’’ properties to deter-
parse the field. By far the largest number of materials mine a critical temperature. Similarly, inferred properties
researchers are focused on particular classes of materials such as charge carrier mobility are deduced from basic
and also perhaps on their uses. Thus a comfortable choice electrical quantities and temperature-composition phase
would have been to commission chapters accordingly. diagrams are deduced from observed microstructures.
Alternatively, the objective and product of any measure- Characterization of Materials, being organized by techni-
ment,—i.e., a materials property—could easily form a logi- que, naturally places initial emphasis on the most directly
cal basis. Unfortunately, each of these approaches would measured properties, but authors have provided many
have required mention of several of the measurement application examples that illustrate the derivative proper-
methods in just about every chapter. Therefore, if only to ties a techniques may address.
reduce redundancy, we have chosen a less intuitive taxon- First among our objectives is to help the researcher dis-
omy by arranging the content according to the type of mea- criminate among alternative measurement modalities
surement ‘‘probe’’ upon which a method relies. Thus you that may apply to the property under study. The field of
will find chapters focused on application of electrons, possibilities is often very wide, and although excellent
ions, x rays, heat, light, etc., to a sample as the generic texts treating each possible method in great detail exist,
thread tying several methods together. Our field is too identifying the most appropriate method before delving
complex for this not to be an oversimplification, and indeed deeply into any one seems the most efficient approach.
some logical inconsistencies are inevitable. Characterization of Materials serves to sort the options at
We have tried to maintain the distinction between a the outset, with individual articles affording the research-
property and a method. This is easy and clear for methods er a description of the method sufficient to understand its
based on external independent probes such as electron applicability, limitations, and relationship to competing
beams, ion beams, neutrons, or x-rays. However many techniques, while directing the reader to more extensive
techniques rely on one and the same phenomenon for resources that fit specific measurement needs.
probe and property, as is the case for mechanical, electro- Whether one plans to perform such measurements one-
nic, and thermal methods. Many methods fall into both self or whether one simply needs to gain sufficient famil-
regimes. For example, light may be used to observe a iarity to effectively collaborate with experts in the
microstructure, but may also be used to measure an optical method, Characterization of Materials will be a useful
property. From the most general viewpoint, we recognize reference. Although our expert authors were given great
that the properties of the measuring device and those of latitude to adjust their presentations to the ‘‘personalities’’
the specimen under study are inextricably linked. It is of their specific methods, some uniformity and circum-
actually a joint property of the tool-plus-sample system scription of content was sought. Thus, you will find most
ix
12. x PREFACE
units organized in a similar fashion. First, an introduction be a necessary corollary to an experiment to understand
serves to succinctly describe for what properties the the result after the fact or to predict the result and thus
method is useful and what alternatives may exist. Under- help direct an experimental search in advance. More
lying physical principles of the method and practical than this, as equipment needs of many experimental stu-
aspects of its implementation follow. Most units will offer dies increase in complexity and cost, as the materials
examples of data and their analyses as well as warnings themselves become more complex and multicomponent in
about common problems of which one should be aware. nature, and as computational power continues to expand,
Preparation of samples and automation of the methods simulation of properties will in fact become the measure-
are also treated as appropriate. ment method of choice in many cases.
As implied above, the level of presentation of these Another unique chapter is the first, covering ‘‘common
volumes is intended to be intermediate between cursory concepts.’’ It collects some of the ubiquitous aspects of mea-
overview and detailed instruction. Readers will find that, surement methods that would have had to be described
in practice, the level of coverage is also very much dictated repeatedly and in more detail in later units. Readers
by the character of the technique described. Many are may refer back to this chapter as related topics arise
based on quite complex concepts and devices. Others are around specific methods, or they may use this chapter as
less so, but still, of course, demand a precision of under- a general tutorial. The Common Concepts chapter, how-
standing and execution. What is or is not included in a pre- ever, does not and should not eliminate all redundancies
sentation also depends on the technical background in the remaining chapters. Expositions within individual
assumed of the reader. This obviates the need to delve articles attempt to be somewhat self-contained and the
into concepts that are part of rather standard technical details as to how a common concept actually relates to a
curricula, while requiring inclusion of less common, more given method are bound to differ from one to the next.
specialized topics. Although Characterization of Materials is directed more
As much as possible, we have avoided extended discus- toward the research lab than the classroom, the focused
sion of the science and application of the materials proper- units in conjunction with chapters one and two can serve
ties themselves, which, although very interesting and as a useful educational tool.
clearly the motivation for research in first place, do not The content of Characterization of Materials had pre-
generally speak to efficacy of a method or its accomplish- viously appeared as Methods in Materials Research, a
ment. loose-leaf compilation amenable to updating. To retain
This is a materials-oriented volume, and as such, must the ability to keep content as up to date as possible, Char-
overlap fields such as physics, chemistry, and engineering. acterization of Materials is also being published on-line
There is no sharp delineation possible between a ‘‘physics’’ where several new and expanded topics will be added
property (e.g., the band structure of a solid) and the mate- over time.
rials consequences (e.g., conductivity, mobility, etc.) At the
other extreme, it is not at all clear where a materials prop-
erty such as toughness ends and an engineering property ACKNOWLEDGMENTS
associated with performance and life-cycle begins. The
very attempt to assign such concepts to only one disciplin- First we express our appreciation to the many expert
ary category serves no useful purpose. Suffice it to say, authors who have contributed to Characterization of
therefore, that Characterization of Materials has focused Materials. On the production side of the predecessor
its coverage on a core of materials topics while trying to publication, Methods in Materials Research, we are
remain inclusive at the boundaries of the field. pleased to acknowledge the work of a great many staff of
Processing and fabrication are also important aspect of the Current Protocols division of John Wiley & Sons, Inc.
materials research. Characterization of Materials does not We also thank the previous series editors, Dr. Virginia
deal with these methods per se because they are not Chanda and Dr. Alan Samuels. Republication in the
strictly measurement methods. However, here again no present on-line and hard-bound forms owes its continu-
clear line is found and in such methods as electrochemis- ing quality to staff of the Major Reference Works group of
try, tribology, mechanical testing, and even ion-beam irra- John Wiley & Sons, Inc., most notably Dr. Jacqueline
diation, where the processing can be the measurement, Kroschwitz and Dr. Arza Seidel.
these aspects are perforce included.
The second chapter is unique in that it collects methods
that are not, literally speaking, measurement methods; For the editors,
these articles do not follow the format found in subsequent
chapters. As theory or simulation or modeling methods, ELTON N. KAUFMANN
they certainly serve to augment experiment. They may Editor-in-Chief
13. CONTRIBUTORS
Reza Abbaschian Peter A. Barnes
University of Florida at Gainesville Clemson University
Gainesville, FL Clemson, SC
Mechanical Testing, Introduction Electrical and Electronic Measurements, Introduction
˚ Capacitance-Voltage (C-V) Characterization of
John Agren
Semiconductors
Royal Institute of Technology, KTH
Stockholm, SWEDEN Jack Bass
Binary and Multicomponent Diffusion Michigan State University
Stephen D. Antolovich East Lansing, MI
Washington State University Magnetotransport in Metals and Alloys
Pullman, WA Bob Bastasz
Tension Testing Sandia National Laboratories
Samir J. Anz Livermore, CA
California Institute of Technology Particle Scattering
Pasadena, CA
Raymond G. Bayer
Semiconductor Photoelectrochemistry
Consultant
Georgia A. Arbuckle-Keil Vespal, NY
Rutgers University Tribological and Wear Testing
Camden, NJ
The Quartz Crystal Microbalance In Electrochemistry Goetz M. Bendele
SUNY Stony Brook
Ljubomir Arsov Stony Brook, NY
University of Kiril and Metodij X-Ray Powder Diffraction
Skopje, MACEDONIA
Ellipsometry Andrew B. Bocarsly
Princeton University
Albert G. Baca Princeton, NJ
Sandia National Laboratories Cyclic Voltammetry
Albuquerque, NM Electrochemical Techniques, Introduction
Characterization of pn Junctions
Mark B.H. Breese
Sam Bader
University of Surrey, Guildford
Argonne National Laboratory
Surrey, UNITED KINGDOM
Argonne, IL
Radiation Effects Microscopy
Surface Magneto-Optic Kerr Effect
James C. Banks Iain L. Campbell
Sandia National Laboratories University of Guelph
Albuquerque, NM Guelph, Ontario CANADA
Heavy-Ion Backscattering Spectrometry Particle-Induced X-Ray Emission
Charles J. Barbour Gerbrand Ceder
Sandia National Laboratory Massachusetts Institute of Technology
Albuquerque, NM Cambridge, MA
Elastic Ion Scattering for Composition Analysis Introduction to Computation
xi
14. xii CONTRIBUTORS
Robert Celotta Gareth R. Eaton
National Institute of Standards and University of Denver
Technology Gaithersburg, MD Denver, CO
Techniques to Measure Magnetic Domain Structures Electron Paramagnetic Resonance
Spectroscopy
Gary W. Chandler
University of Arizona Sandra S. Eaton
Tucson, AZ University of Denver
Scanning Electron Microscopy Denver, CO
Electron Paramagnetic Resonance
Haydn H. Chen Spectroscopy
University of Illinois
Urbana, IL Fereshteh Ebrahimi
Kinematic Diffraction of X Rays University of Florida
Gainesville, FL
Long-Qing Chen Fracture Toughness Testing Methods
Pennsylvania State University
University Park, PA Wolfgang Eckstein
Simulation of Microstructural Evolution Using the Max-Planck-Institut fur Plasmaphysik
Field Method Garching, GERMANY
Particle Scattering
Chia-Ling Chien
Johns Hopkins University Arnel M. Fajardo
Baltimore, MD California Institute of Technology
Magnetism and Magnetic Measurements, Introduction Pasadena, CA
Semiconductor Photoelectrochemistry
J.M.D. Coey
University of Dublin, Trinity College Kenneth D. Finkelstein
Dublin, IRELAND Cornell University
Generation and Measurement of Magnetic Fields Ithaca, NY
Resonant Scattering Technique
Richard G. Connell
University of Florida Simon Foner
Gainesville, FL Massachusetts Institute of Technology
Optical Microscopy Reflected-Light Cambridge, MA
Optical Microscopy Magnetometry
Brent Fultz
Didier de Fontaine
California Institute of Technology
University of California
Pasadena, CA
Berkeley, CA
Electron Techniques, Introduction
Prediction of Phase Diagrams
¨
Mossbauer Spectrometry
T.M. Devine Resonance Methods, Introduction
University of California Transmission Electron Microscopy
Berkeley, CA
Jozef Gembarovic
Raman Spectroscopy of Solids
Thermophysical Properties Research Laboratory
David Dollimore West Lafayette, IN
University of Toledo Thermal Diffusivity by the Laser
Toledo, OH Flash Technique
Mass and Density Measurements Thermal Analysis- Craig A. Gerken
Definitions, Codes of Practice, and Nomenclature University of Illinois
Thermometry Urbana, IL
Thermal Analysis, Introduction Low-Energy, Electron Diffraction
Barney L. Doyle Atul B. Gokhale
Sandia National Laboratory MetConsult, Inc.
Albuquerque, NM Roosevelt Island, NY
High-Energy Ion Beam Analysis Sample Preparation for Metallography
Ion-Beam Techniques, Introduction
Alan I. Goldman
Jeff G. Dunn Iowa State University
University of Toledo Ames, IA
Toledo, OH X-Ray Techniques, Introduction
Thermogravimetric Analysis Neutron Techniques, Introduction
15. CONTRIBUTORS xiii
John T. Grant Robert A. Jacobson
University of Dayton Iowa State University
Dayton, OH Ames, IA
Auger Electron Spectroscopy Single-Crystal X-Ray Structure Determination
George T. Gray Duane D. Johnson
Los Alamos National Laboratory University of Illinois
Los Alamos, NM Urbana, IL
High-Strain-Rate Testing of Materials Computation of Diffuse Intensities in Alloys
Magnetism in Alloys
Vytautas Grivickas
Michael H. Kelly
Vilnius University
National Institute of Standards and Technology
Vilnius, LITHUANIA
Gaithersburg, MD
Carrier Lifetime: Free Carrier Absorption,
Techniques to Measure Magnetic Domain Structures
Photoconductivity, and Photoluminescence
Elton N. Kaufmann
Robert P. Guertin
Argonne National Laboratory
Tufts University
Argonne, IL
Medford, MA
Common Concepts in Materials Characterization,
Magnetometry
Introduction
Gerard S. Harbison Janice Klansky
University of Nebraska Beuhler Ltd.
Lincoln, NE Lake Bluff, IL
Nuclear Quadrupole Resonance Hardness Testing
Steve Heald Chris R. Kleijn
Argonne National Laboratory Delft University of Technology
Argonne, IL Delft, THE NETHERLANDS
XAFS Spectroscopy Simulation of Chemical Vapor Deposition Processes
Bruno Herreros James A. Knapp
University of Southern California Sandia National Laboratories
Los Angeles, CA Albuquerque, NM
Nuclear Quadrupole Resonance Heavy-Ion Backscattering Spectrometry
Thomas Koetzle
John P. Hill
Brookhaven National Laboratory
Brookhaven National Laboratory
Upton, NY
Upton, NY
Single-Crystal Neutron Diffraction
Magnetic X-Ray Scattering
Ultraviolet Photoelectron Spectroscopy Junichiro Kono
Rice University
Kevin M. Horn Houston, TX
Sandia National Laboratories Cyclotron Resonance
Albuquerque, NM
Ion Beam Techniques, Introduction Phil Kuhns
Florida State University
Joseph P. Hornak Tallahassee, FL
Rochester Institute of Technology Generation and Measurement of Magnetic Fields
Rochester, NY Jonathan C. Lang
Nuclear Magnetic Resonance Imaging Argonne National Laboratory
James M. Howe Argonne, IL
University of Virginia X-Ray Magnetic Circular Dichroism
Charlottesville, VA David E. Laughlin
Transmission Electron Microscopy Carnegie Mellon University
Pittsburgh, PA
Gene E. Ice
Theory of Magnetic Phase Transitions
Oak Ridge National Laboratory
Oak Ridge, TN Leonard Leibowitz
X-Ray Microprobe for Fluorescence Argonne National Laboratory
and Diffraction Argonne, IL
Analysis X-Ray and Neutron Diffuse Scattering Differential Thermal Analysis and Differential Scanning
Measurements Calorimetry
16. xiv CONTRIBUTORS
Supaporn Lerdkanchanaporn Daniel T. Pierce
University of Toledo National Institute of Standards and Technology
Toledo, OH Gaithersburg, MD
Simultaneouse Techniques Including Analysis of Gaseous Techniques to Measure Magnetic
Products Domain Structures
Nathan S. Lewis Frank J. Pinski
California Institute of Technology University of Cincinnati
Pasadena, CA Cincinnati, OH
Semiconductor Photoelectrochemistry Magnetism in Alloys
Dusan Lexa Computation of Diffuse Intensities in Alloys
Argonne National Laboratory Branko N. Popov
Argonne, IL University of South Carolina
Differential Thermal Analysis and Differential Scanning Columbia, SC
Calorimetry Ellipsometry
Jan Linnros
Ziqiang Qiu
Royal Institute of Technology
University of California at Berkeley
Kista-Stockholm, SWEDEN
Berkeley, CA
Carrier Liftime: Free Carrier Absorption,
Surface Magneto-Optic Kerr Effect
Photoconductivity, and Photoluminescene
David C. Look Talat S. Rahman
Wright State University Kansas State University
Dayton, OH Manhattan, Kansas
Hall Effect in Semiconductors Molecular-Dynamics Simulation of Surface Phenomena
Jeffery W. Lynn T.A. Ramanarayanan
University of Maryland Exxon Research and Engineering Corp.
College Park, MD Annandale, NJ
Magentic Neutron Scattering Electrochemical Techniques for Corrosion Quantification
Kosta Maglic M. Ramasubramanian
Institute of Nuclear Sciences ‘‘Vinca’’ University of South Carolina
Belgrade, YUGOSLAVIA Columbia, SC
Thermal Diffusivity by the Laser Flash Technique Ellipsometry
Floyd McDaniel S.S.A. Razee
University of North Texas University of Warwick
Denton, TX Coventry, UNITED KINGDOM
Trace Element Accelerator Mass Spectrometry Magnetism in Alloys
Michael E. McHenry
Carnegie Mellon University James L. Robertson
Pittsburgh, PA Oak Ridge National Laboratory
Magnetic Moment and Magnetization Oak Ridge, TN
Thermomagnetic Analysis X-Ray and Neutron Diffuse Scattering Measurements
Theory of Magnetic Phase Transitions Ian K. Robinson
Keith A. Nelson University of Illinois
Massachusetts Institute of Technology Urbana, IL
Cambridge, MA Surface X-Ray Diffraction
Impulsive Stimulated Thermal Scattering
John A. Rogers
Dale E. Newbury Bell Laboratories, Lucent Technologies
National Institute of Standards and Technology Murray Hill, NJ
Gaithersburg, MD Impulsive Stimulated Thermal Scattering
Energy-Dispersive Spectrometry
P.A.G. O’Hare William J. Royea
Darien, IL California Institute of Technology
Combustion Calorimetry Pasadena, CA
Semiconductor Photoelectrochemistry
Stephen J. Pennycook
Oak Ridge National Laboratory Larry Rubin
Oak Ridge, TN Massachusetts Institute of Technology
Scanning Transmission Electron Cambridge, MA
Microscopy: Z-Contrast Imaging Generation and Measurement of Magnetic Fields
17. CONTRIBUTORS xv
Miquel Salmeron Hugo Steinfink
Lawrence Berkeley National Laboratory University of Texas
Berkeley, CA Austin, TX
Scanning Tunneling Microscopy Symmetry in Crystallography
Alan C. Samuels Peter W. Stephens
Edgewood Chemical Biological Center SUNY Stony Brook
Aberdeen Proving Ground, MD Stony Brook, NY
Mass and Density Measurements X-Ray Powder Diffraction
Optical Imaging and Spectroscopy, Introduction
Thermometry Ray E. Taylor
Thermophysical Properties Research Laboratory
Juan M. Sanchez West Lafayette, IN 47906
University of Texas at Austin Thermal Diffusivity by the Laser Flash Technique
Austin, TX
Computational and Theoretical Methods, Introduction Chin-Che Tin
Auburn University
Hans J. Schneider-Muntau
Auburn, AL
Florida State University
Deep-Level Transient Spectroscopy
Tallahassee, FL
Generation and Measurement of Magnetic Fields Brian M. Tissue
Virginia Polytechnic Institute & State University
Christian Schott
Blacksburg, VA
Swiss Federal Institute of Technology
Ultraviolet and Visible Absorption Spectroscopy
Lausanne, SWITZERLAND
Generation and Measurement of Magnetic Fields James E. Toney
Justin Schwartz Applied Electro-Optics Corporation
Florida State University Bridgeville, PA
Tallahassee, FL Photoluminescene Spectroscopy
Electrical Measurements on Superconductors by John Unguris
Transport National Institute of Standards and Technology
Supapan Seraphin Gaithersburg, MD
University of Arizona Techniques to Measure Magnetic Domain Structures
Tucson, AZ David Vaknin
Scanning Electron Microscopy Iowa State University
Qun Shen Ames, IA
Cornell University X-Ray Diffraction Techniques for Liquid Surfaces and
Ithaca, NY Monomolecular Layers
Dynamical Diffraction Y
Mark van Schilfgaarde
Jack Singleton SRI International
Consultant Menlo Park, California
Monroeville, PA Summary of Electronic Structure Methods
General Vacuum Techniques
¨
Gyorgy Vizkelethy
Gabor A. Somorjai
Sandia National Laboratories
University of California & Lawrence Berkeley
Albuquerque, NM
National Laboratory
Nuclear Reaction Analysis and Proton-Induced Gamma
Berkeley, CA
Ray Emission
Low-Energy Electron Diffraction
Thomas Vogt
Cullie J. Sparks
Brookhaven National Laboratory
Oak Ridge National Laboratory
Upton, NY
Oak Ridge, TN
Neutron Powder Diffraction
X-Ray and Neutron Diffuse Scattering Measurements
Costas Stassis Yunzhi Wang
Iowa State University Ohio State University
Ames, IA Columbus, OH
Phonon Studies Simulation of Microstructural Evolution Using the Field
Method
Julie B. Staunton
University of Warwick Richard E. Watson
Coventry, UNITED KINGDOM Brookhaven National Laboratory
Computation of Diffuse Intensities in Alloys Upton, NY
Magnetism in Alloys Bonding in Metals
18. xvi CONTRIBUTORS
Huub Weijers Introduction To Medium-Energy Ion Beam Analysis
Florida State University Medium-Energy Backscattering and Forward-Recoil
Tallahassee, FL Spectrometry
Electrical Measurements on Superconductors
by Transport Stuart Wentworth
Auburn University
Jefferey Weimer Auburn University, AL
University of Alabama Conductivity Measurement
Huntsville, AL
X-Ray Photoelectron Spectroscopy David Wipf
Michael Weinert Mississippi State University
Brookhaven National Laboratory Mississippi State, MS
Upton, NY Scanning Electrochemical Microscopy
Bonding in Metals Gang Xiao
Robert A. Weller Brown University
Vanderbilt University Providence, RI
Nashville, TN Magnetism and Magnetic Measurements, Introduction
21. COMMON CONCEPTS
COMMON CONCEPTS IN MATERIALS As Characterization of Materials evolves, additional
CHARACTERIZATION, INTRODUCTION common concepts will be added. However, when it seems
more appropriate, such content will appear more closely
From a tutorial standpoint, one may view this chapter as tied to its primary topical chapter.
a good preparatory entrance to subsequent chapters of
Characterization of Materials. In an educational setting, ELTON N. KAUFMANN
the generally applicable topics of the units in this chapter
can play such a role, notwithstanding that they are each
quite independent without having been sequenced with
GENERAL VACUUM TECHNIQUES
any pedagogical thread in mind.
In practice, we expect that each unit of this chapter will
INTRODUCTION
be separately valuable to users of Characterization of
Materials as they choose to refer to it for concepts under-
In this unit we discuss the procedures and equipment used
lying many of those exposed in units covering specific mea-
to maintain a vacuum system at pressures in the range
surement methods.
from 10À3 to 10À11 torr. Total and partial pressure gauges
Of course, not every topic covered by a unit in this chap-
used in this range are also described.
ter will be relevant to every measurement method covered
Because there is a wide variety of equipment, we
in subsequent chapters. However, the concepts in this
describe each of the various components, including details
chapter are sufficiently common to appear repeatedly in
of their principles and technique of operation, as well as
the pursuit of materials research. It can be argued that
their recommended uses.
the units treating vacuum techniques, thermometry, and
SI units are not used in this unit. The American
sample preparation do not deal directly with the materials
Vacuum Society attempted their introduction many years
properties to be measured at all. Rather, they are crucial to
ago, but the more traditional units continue to dominate in
preparation and implementation of such a measurement.
this field in North America. Our usage will be consistent
It is interesting to note that the properties of materials
with that generally found in the current literature. The
nevertheless play absolutely crucial roles for each of these
following units will be used.
topics as they rely on materials performance to accomplish
Pressure is given in torr. 1 torr is equivalent to 133.32
their ends.
pascal (Pa).
Mass/density measurement does of course relate to a
Volume is given in liters (L), and time in seconds (s).
most basic materials property, but is itself more likely to
The flow of gas through a system, i.e., the ‘‘throughput’’
be an ancillary necessity of a measurement protocol than
(Q), is given in torr-L/s.
to be the end goal of a measurement (with the important
Pumping speed (S) and conductance (C) are given in
exceptions of properties related to porosity, defect density,
L/s.
etc.). In temperature and mass measurement, apprecia-
ting the role of standards and definitions is central to pro-
per use of these parameters.
PRINCIPLES OF VACUUM TECHNOLOGY
It is hard to think of a materials property that does not
depend on the crystal structure of the materials in ques-
The most difficult step in designing and building a vacuum
tion. Whether the structure is a known part of the explana-
system is defining precisely the conditions required to ful-
tion of the value of another property or its determination is
fill the purpose at hand. Important factors to consider
itself the object of the measurement, a good grounding
include:
in essentials of crystallographic groups and syntax is a
common need in most measurement circumstances. A
1. The required system operating pressure and the
unit provided in this chapter serves that purpose well.
gaseous impurities that must be avoided;
Several chapters in Characterization of Materials deal
with impingement of projectiles of one kind or another 2. The frequency with which the system must be vented
on a sample, the reaction to which reflects properties of to the atmosphere, and the required recycling time;
interest in the target. Describing the scattering of the pro- 3. The kind of access to the vacuum system needed for
jectiles is necessary in all these cases. Many concepts in the insertion or removal of samples.
such a description are similar regardless of projectile
type, while the details differ greatly among ions, electrons, For systems operating at pressures of 10À6 to 10À7 torr,
neutrons, and photons. Although the particle scattering venting the system is the simplest way to gain access, but
unit in this chapter emphasizes the charged particle and for ultrahigh vacuum (UHV), e.g., below 10À8 torr, the
ions in particular, the concepts are somewhat portable. A pumpdown time can be very long, and system bakeout
good deal of generic scattering background is provided in would usually be required. A vacuum load-lock antecham-
the chapters covering neutrons, x rays, and electrons as ber for the introduction and removal of samples may be
projectiles as well. essential in such applications.
1
22. 2 COMMON CONCEPTS
Because it is difficult to address all of the above ques- rapidly. However, water will persist as the major outgas-
tions, a viable specification of system performance is often sing load. Every time a system is vented to air, the walls
neglected, and it is all too easy to assemble a more sophis- are exposed to moisture and one or more layers of water
ticated and expensive system than necessary, or, if bud- will adsorb virtually instantaneously. The amount
gets are low, to compromise on an inadequate system adsorbed will be greatest when the relative humidity is
that cannot easily be upgraded. high, increasing the time needed to reach base pressure.
Before any discussion of the specific components of a Water is bound by physical adsorption, a reversible pro-
vacuum system, it is instructive to consider the factors cess, but the binding energy of adsorption is so great
that govern the ultimate, or base, pressure. The pressure that the rate of desorption is slow at ambient temperature.
can be calculated from Physical adsorption involves van der Waal’s forces, which
are relatively weak. Physical adsorption should be distin-
Q guished from chemisorption, which typically involves the
P¼ ð1Þ
S formation of chemical-type bonding of a gas to an atomi-
cally clean surface—for example, oxygen on a stainless
where P is the pressure in torr, Q is the total flow, or
steel surface. Chemisorption of gas is irreversible under
throughput of gas, in torr-L/s, and S is the pumping speed
all conditions normally encountered in a vacuum system.
in L/s.
After the first few minutes of pumping, pressures are
The influx of gas, Q, can be a combination of a deliberate
almost always in the free molecular flow regime, and
influx of process gas from an exterior source and gas origi-
when a water molecule is desorbed, it experiences only col-
nating in the system itself. With no external source, the
lisions with the walls, rather than with other molecules.
base pressure achieved is frequently used as the principle
Consequently, as it leaves the system, it is readsorbed
indicator of system performance. The most important
many times, and on each occasion desorption is a slow pro-
internal sources of gas are outgassing from the walls and
cess.
permeation from the atmosphere, most frequently through
One way of accelerating the removal of adsorbed water
elastomer O-rings. There may also be leaks, but these can
is by purging at a pressure in the viscous flow region, using
readily be reduced to negligible levels by proper system
a dry gas such as nitrogen or argon. Under viscous flow
design and construction. Vacuum pumps also contribute
conditions, the desorbed water molecules rarely reach
to background pressure, and here again careful selection
the system walls, and readsorption is greatly reduced. A
and operation will minimize such problems.
second method is to heat the system above its normal
operating temperature.
The Problem of Outgassing
Any process that reduces the adsorption of water in a
Of the sources of gas described above, outgassing is often vacuum system will improve the rate of pumpdown. The
the most important. With a new system, the origin of out- simplest procedure is to vent a vacuum system with a
gassing may be in the manufacture of the materials used dry gas rather than with atmospheric air, and to minimize
in construction, in handling during construction, and in the time the system remains open following such a proce-
exposure of the system to the atmosphere. In general these dure. Dry air will work well, but it is usually more conve-
sources scale with the area of the system walls, so that it is nient to substitute nitrogen or argon.
wise to minimize the surface area and to avoid porous From Equation 1, it is evident that there are two
materials in construction. For example, aluminum is an approaches to achieving a lower ultimate pressure, and
excellent choice for use in vacuum systems, but anodized hence a low impurity level, in a system. The first is to
aluminum has a porous oxide layer that provides an inter- increase the effective pumping speed, and the second is
nal surface for gas adsorption many times greater than the to reduce the outgassing rate. There are severe limitations
apparent surface, making it much less suitable for use in to the first approach. In a typical system, most of one wall
vacuum. of the chamber will be occupied by the connection to the
The rate of outgassing in a new, unbaked system, fabri- high-vacuum pump; this limits the size of pump that can
cated from materials such as aluminum and stainless be used, imposing an upper limit on the achievable pum-
steel, is initially very high, on the order of 10À6 to ping speed. As already noted, the ultimate pressure
10À7 torr-L/s Á cm2 of surface area after one hour of expo- achieved in an unbaked system having this configuration
sure to vacuum (O’Hanlon, 1989). With continued pump- will rarely reach the mid-10À8 torr range. Even if one could
ing, the rate falls by one or two orders of magnitude mount a similar-sized pump on every side, the best to be
during the first 24 hr, but thereafter drops very slowly expected would be a 6-fold improvement, achieving a
over many months. Typically the main residual gas is base pressure barely into the 10À9 torr range, even after
water vapor. In a clean vacuum system, operating at ambi- very long exhaust times.
ent temperature and containing only a moderate number It is evident that, to routinely reach pressures in the
of O-rings, the lowest achievable pressure is usually 10À7 10À10 torr range in a realistic period of time, a reduction
to mid-10À8 torr. The limiting factor is generally residual in the rate of outgassing is necessary—e.g., by heating
outgassing, not the capability of the high-vacuum pump. the vacuum system. Baking an entire system to 4008C
The outgassing load is highest when a new system is for 16 hr can produce outgassing rates of 10À15 torr-L/
put into service, but with steady use the sins of construc- s Á cm2 (Alpert, 1959), a reduction of 108 from those found
tion are slowly erased, and on each subsequent evacuation, after 1 hr of pumping at ambient temperature. The mag-
the system will reach its typical base pressure more nitude of this reduction shows that as large a portion as
23. GENERAL VACUUM TECHNIQUES 3
possible of a system should be heated to obtain maximum Oil-Sealed Pumps
advantage.
The earliest roughing pumps used either a piston or liquid
to displace the gas. The first production methods for incan-
PRACTICAL ASPECTS OF VACUUM TECHNOLOGY descent lamps used such pumps, and the development of
the oil-sealed mechanical pump by Gaede, around 1907,
Vacuum Pumps was driven by the need to accelerate the pumping process.
The operation of most vacuum systems can be divided into
two regimes. The first involves pumping the system from Applications. The modern versions of this pump are the
atmosphere to a pressure at which a high-vacuum pump most economic and convenient for achieving pressures as
can be brought into operation. This is traditionally known low as the 10À4 torr range. The pumps are widely used
as the rough vacuum regime and the pumps used are com- as a backing pump for both diffusion and turbomolecular
monly referred to as roughing pumps. Clearly, a system pumps; in this application the backstreaming of mechani-
that operates at an ultimate pressure within the capability cal pump oil is intercepted by the high vacuum pump, and
of the roughing pump will require no additional pumps. a foreline trap is not required.
Once the system has been roughed down, a high-
vacuum pump must be used to achieve lower pressures. Operating Principles. The oil-sealed pump is a positive-
If the high-vacuum pump is the type known as a transfer displacement pump, of either the vane or piston type, with
pump, such as a diffusion or turbomolecular pump, it will a compression ratio of the order of 105:1 (Dobrowolski,
require the continuous support of the roughing pump in 1979). It is available as a single or two-stage pump, capable
order to maintain the pressure at the exit of the high- of reaching base pressures in the 10À2 and 10À4 torr range,
vacuum pump at a tolerable level (in this phase of the respectively. The pump uses oil to maintain sealing, and to
pumping operation the function of the roughing pump provide lubrication and heat transfer, particularly at the
has changed, and it is frequently referred to as a backing contact between the sliding vanes and the pump wall.
or forepump). Transfer pumps have the advantage that Oil also serves to fill the significant dead space leading to
their capacity for continuous pumping of gas, within their the exhaust valve, essentially functioning as a hydraulic
operating pressure range, is limited only by their reliabi- valve lifter and permitting the very high compression
lity. They do not accumulate gas, an important considera- ratio.
tion where hazardous gases are involved. Note that the The speed of such pumps is often quoted as the ‘‘free-air
reliability of transfer pumping systems depends upon the displacement,’’ which is simply the volume swept by the
satisfactory performance of two separate pumps. A second pump rotor. In a typical two-stage pump this speed is sus-
class of pumps, known collectively as capture pumps, tained down to $1 Â 10À1 torr; below this pressure the
require no further support from a roughing pump once speed decreases, reaching zero in the 10À5 torr range. If
they have started to pump. Examples of this class are cryo- a pump is to sustain pressures near the bottom of its range,
genic pumps and sputter-ion pumps. These types of pump the required pump size must be determined from pub-
have the advantage that the vacuum system is isolated lished pumping-speed performance data. It should be
from the atmosphere, so that system operation depends noted that mechanical pumps have relatively small pump-
upon the reliability of only one pump. Their disadvantage ing speed, at least when compared with typical high-
is that they can provide only limited storage of pumped vacuum pumps. A typical laboratory-sized pump, powered
gas, and as that limit is reached, pumping will deteriorate. by a 1/3 hp motor, may have a speed of $3.5 cubic feet per
The effect of such a limitation is quite different for the two minute (cfm), or rather less than 2 L/s, as compared to the
examples cited. A cryogenic pump can be totally regene- smallest turbomolecular pump, which has a rated speed of
rated by a brief purging at ambient temperature, but a 50 L/s.
sputter-ion pump requires replacement of its internal com-
ponents. One aspect of the cryopump that should not be
overlooked is that hazardous gases are stored, unchanged, Avoiding Oil Contamination from an Oil-Sealed Mechani-
within the pump, so that an unexpected failure of the cal Pump. The versatility and reliability of the oil-sealed
pump can release these accumulated gases, requiring pro- mechanical pump carries with it a serious penalty. When
vision for their automatic safe dispersal in such an emer- used improperly, contamination of the vacuum system is
gency. inevitable. These pumps are probably the most prevalent
source of oil contamination in vacuum systems. The pro-
blem arises when thay are untrapped and pump a system
Roughing Pumps
down to its ultimate pressure, often in the free molecular
Two classes of roughing pumps are in use. The first type, flow regime. In this regime, oil molecules flow freely into
the oil-sealed mechanical pump, is by far the most com- the vacuum chamber. The problem can readily be avoided
mon, but because of the enormous concern in the semi- by careful control of the pumping procedures, but possible
conductor industry about oil contamination, a second system or operator malfunction, leading to contamination,
type, the so-called ‘‘dry’’ pump, is now frequently used. must be considered. For many years, it was common prac-
In this context, ‘‘dry’’ implies the absence of volatile orga- tice to leave a system in the standby condition evacuated
nics in the part of the pump that communicates with the only by an untrapped mechanical pump, making contami-
vacuum system. nation inevitable.
24. 4 COMMON CONCEPTS
Mechanical pump oil has a vapor pressure, at room tem- flowing from the system side of the trap to the pump
perature, in the low 10À5 torr range when first installed, (D.J. Santeler, pers. comm.). The foreline is isolated from
but this rapidly deteriorates up to two orders of magnitude the rest of the system and the gas flow is continued
as the pump is operated (Holland, 1971). A pump operates throughout the heating cycle, until the trap has cooled
at temperatures of 608C, or higher, so the oil vapor pres- back to ambient temperature. An adsorbent foreline trap
sure far exceeds 10À3 torr, and evaporation results in a must be optically dense, so the oil molecules have no
substantial flux of oil into the roughing line. When a sys- path past the adsorbent; commercial traps do not always
tem at atmospheric pressure is connected to the mechani- fulfill this basic requirement. Where regeneration of the
cal pump, the initial gas flow from the vacuum chamber is foreline trap has been totally neglected, acceptable perfor-
in the viscous flow regime, and oil molecules are driven mance may still be achieved simply because a diffusion
back to the pump by collisions with the gas being ex- pump or turbomolecular pump serves as the true ‘‘trap,’’
hausted (Holland, 1971; Lewin, 1985). Provided the rough- intercepting the oil from the forepump.
ing process is terminated while the gas flow is still in the Oil contamination can also result from improperly tur-
viscous flow regime, no significant contamination of the ning a pump off. If it is stopped and left under vacuum, oil
vacuum chamber will occur. The condition for viscous frequently leaks slowly across the exhaust valve into the
flow is given by the equation pump. When it is partially filled with oil, a hydraulic
lock may prevent the pump from starting. Continued leak-
PD ! 0:5 ð2Þ age will drive oil into the vacuum system itself; an inter-
esting procedure for recovery from such a catastrophe
where P is the pressure in torr and D is the internal dia- has been described (Hoffman, 1979).
meter of the roughing line in centimeters. Whenever the pump is stopped, either deliberately or by
Termination of the roughing process in the viscous flow power failure or other failure, automatic controls that first
region is entirely practical when the high-vacuum pump is isolate it from the vacuum system, and then vent it to
either a turbomolecular or modern diffusion pump (see atmospheric pressure, should be used.
precautions discussed under Diffusion Pumps and Turbo- Most gases exhausted from a system, including oxygen
molecular Pumps, below). Once these pumps are in opera- and nitrogen, are readily removed from the pump oil, but
tion, they function as an effective barrier against oil some can liquify under maximum compression just before
migration into the system from the forepump. Hoffman the exhaust valve opens. Such liquids mix with the oil and
(1979) has described the use of a continuous gas purge are more difficult to remove. They include water and sol-
on the foreline of a diffusion-pumped system as a means vents frequently used to clean system components. When
of avoiding backstreaming from the forepump. pumping large volumes of air from a vacuum chamber,
particularly during periods of high humidity (or whenever
Foreline Traps. A foreline trap is a second approach to solvent residues are present), it is advantageous to use a
preventing oil backstreaming. If a liquid nitrogenÀcooled gas-ballast feature commonly fitted to two-stage and also
trap is always in place between a forepump and the to some single-stage pumps. This feature admits air du-
vacuum chamber, cleanliness is assured. But the operative ring the final stage of compression, raising the pressure
word is ‘‘always.’’ If the trap warms to ambient tempera- and forcing the exhaust valve to open before the partial
ture, oil from the trap will migrate upstream, and this is pressure of water has reached saturation. The ballast fea-
much more serious if it occurs while the line is evacuated. ture minimizes pump contamination and reduces pump-
A different class of trap uses an adsorbent for oil. Typical down time for a chamber exposed to humid air, although
adsorbents are activated alumina, molecular sieve (a syn- at the cost of about ten-times-poorer base pressure.
thetic zeolite), a proprietary ceramic (Micromaze foreline
traps; Kurt J. Lesker Co.), and metal wool. The metal Oil-Free (‘‘Dry’’) Pumps
wool traps have much less capacity than the other types, Many different types of oil-free pumps are available. We
and unless there is evidence of their efficacy, they are will emphasize those that are most useful in analytical
best avoided. Published data show that activated alumina and diagnostic applications.
can trap 99% of the backstreaming oil molecules (Fulker,
1968). However, one must know when such traps should Diaphragm Pumps
be reactivated. Unequivocal determination requires inser- Applications: Diaphragm pumps are increasingly used
tion of an oil-detection device, such as a mass spectro- where the absence of oil is an imperative, for example, as
meter, on the foreline. The saturation time of a trap the forepump for compound turbomolecular pumps that
depends upon the rate of oil influx, which in turn depends incorporate a molecular drag stage. The combination ren-
upon the vapor pressure of oil in the pump and the conduc- ders oil contamination very unlikely. Most diaphragm
tance of the line between pump and trap. The only safe pro- pumps have relatively small pumping speeds. They are
cedure is frequent reactivation of traps on a conservative adequate once the system pressure reaches the operating
schedule. Reactivation may be done by venting the system, range of a turbomolecular pump, usually well below
replacing the adsorbent with a new charge, or by baking 10À2 torr, but not for rapidly roughing down a large
the adsorbent in a stream of dry air or inert gas to a tem- volume. Pumps are available with speeds up to several
perature of $3008C for several hours. Some traps can be liters per second, and base pressures from a few torr to
regenerated by heating in situ, but only using a stream as low as 10À3 torr, lower ultimate pressures being asso-
of inert gas, at a pressure in the viscous flow region, ciated with the lower-speed pumps.
25. GENERAL VACUUM TECHNIQUES 5
Operating Principles: Four diaphragm modules are been used in the compound turbomolecular pump as an
often arranged in three separate pumping stages, with integral backing stage. This will be discussed in detail
the lowest-pressure stage served by two modules in tan- under Turbomolecular Pumps.
dem to boost the capacity. Single modules are adequate
Operating Principles: The pump uses one or more
for subsequent stages, since the gas has already been com-
drums rotating at speeds as high as 90,000 rpm inside sta-
pressed to a smaller volume. Each module uses a flexible
tionary, coaxial housings. The clearance between drum
diaphragm of Viton or other elastomer, as well as inlet
and housing is $0.3 mm. Gas is dragged in the direction
and outlet valves. In some pumps the modules can be
of rotation by momentum transfer to the pump exit along
arranged to provide four stages of pumping, providing a
helical grooves machined in the housing. The bearings of
lower base pressure, but at lower pumping speed because
these devices are similar to those in turbomolecular pumps
only a single module is employed for the first stage. The
(see discussion of Turbomolecular Pumps, below). An
major required maintenance in such pumps is replacement
internal motor avoids difficulties inherent in a high-speed
of the diaphragm after 10,000 to 15,000 hr of operation.
vacuum seal. A typical pump uses two or more separate
stages, arranged in series, providing a compression ratio
Scroll Pumps
as high as 1:107 for air, but typically less than 1:103 for
Applications: Scroll pumps (Coffin, 1982; Hablanian,
hydrogen. It must be supported by a backing pump, often
1997) are used in some refrigeration systems, where the
of the diaphragm type, that can maintain the forepressure
limited number of moving parts is reputed to provide
below a critical value, typically 10 to 30 torr, depending
high reliability. The most recent versions introduced for
upon the particular design. The much lower compression
general vacuum applications have the advantages of dia-
ratio for hydrogen, a characteristic shared by all turbo-
phragm pumps, but with higher pumping speed. Published
molecular pumps, will increase its percentage in a vacuum
speeds on the order of 10 L/s and base pressures below
chamber, a factor to consider in rare cases where the pre-
10À2 torr make this an appealing combination. Speeds
sence of hydrogen affects the application.
decline rapidly at pressures below $2 Â 10À2 torr.
Sorption Pumps
Operating Principles: Scroll pumps use two enmeshed
Applications: Sorption pumps were introduced for
spiral components, one fixed and the other orbiting. Suc-
roughing down ultrahigh vacuum systems prior to turning
cessive crescent-shaped segments of gas are trapped
on a sputter-ion pump (Welch, 1991). The pumping speed
between the two scrolls and compressed from the inlet
of a typical sorption pump is similar to that of a small oil-
(vacuum side) toward the exit, where they are vented to
sealed mechanical pump, but they are rather awkward in
the atmosphere. A sophisticated and expensive version of
application. This is of little concern in a vacuum system
this pump has long been used for processes where leak-
likely to run many months before venting to the atmo-
tight operation and noncontamination are essential, for
sphere. Occasional inconvenience is a small price for the
example, in the nuclear industry for pumping radioactive
ultimate in contamination-free operation.
gases. An excellent description of the characteristics of this
design has been given by Coffin (1982). In this version, Operating Principles: A typical sorption pump is a
extremely close tolerances (10 mm) between the two scrolls cannister containing $3 lb of a molecular sieve material
minimize leakage between the high- and low-pressure that is cooled to liquid nitrogen temperature. Under these
ends of the scrolls. The more recent pump designs, which conditions the molecular sieve can adsorb $7.6 Â 104 torr-
substitute Teflon-like seals for the close tolerances, have liter of most atmospheric gases; exceptions are helium and
made the pump an affordable option for general oil-free hydrogen, which are not significantly adsorbed, and neon,
applications. The life of the seals is reported to be in the which is adsorbed to a limited extent. Together, these
same range as that of the diaphragm in a diaphragm gases, if not pumped, would leave a residual pressure in
pump. the 10À2 torr range. This is too high to guarantee the trou-
ble-free start of a sputter-ion pump, but the problem is
Screw Compressor. Although not yet widely used, readily avoided. For example, a sorption pump connected
pumps based on the principle of the screw compressor, such to a vacuum chamber of $100 L volume exhausts air to a
as that used in supercharging some high-performance cars, pressure in the viscous flow region, say 5 torr, and then is
appear to offer some interesting advantages: i.e., pumping valved off. The nonadsorbing gases are swept into the
speeds in excess of 10 L/s, direct discharge to the atmo- pump along with the adsorbed gases; the pump now con-
sphere, and ultimate pressures in the 10À3 torr range. If tains a fraction (760–5)/760 or 99.3% of the nonadsorbable
such pumps demonstrate high reliability in diverse appli- gases originally present, leaving hydrogen, helium, and
cations, they constitute the closest alternative, in a single- neon in the low 10À4 torr range in the vacuum chamber.
unit ‘‘dry’’ pump, to the oil-sealed mechanical pump. A second sorption pump on the vacuum chamber will
then readily achieve a base pressure below 5 Â 10À4 torr,
Molecular Drag Pump quite adequate to start even a recalcitrant ion pump.
Applications: The molecular drag pump is useful for
applications requiring pressures in the 1 to 10À7 torr range
High-Vacuum Pumps
and freedom from organic contamination. Over this range
the pump permits a far higher throughput of gas, com- Four types of high-vacuum pumps are in general use:
pared to a standard turbomolecular pump. It has also diffusion, turbomolecular, cryosorption, and sputter-ion.
