September 1992 · NREL/CP-412-5007 Current Status, Research Needs, and Opportunities in Applications of Surface Processing to Transportation and Utilities Technologies Proceedings of a December 1991 Workshop A.W. Czanderna and A.R. Landgrebe, Editors • Executive Summary • Assessment of the Current Status in Surface Characterization • Assessment of the Current Status in Surface Modification • Discussion of Research Needs and Opportunities for Advancing Transportation and Utilities Technologies Through Applied Surface Science and Surface Processing National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393 A national laboratory of the U.S. Department of Energy Managed by Midwest Research Institute for the U.S. Department of Energy under contract No. DE-AC36-83CH10093 Prepared under task number DO140101 September 1992
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September 1992 NREL/CP-412-5007 Current Status, Research Needs, and Opportunities in Applications of Surface Processing to Transportation and Utilities Technologies Proceedings of a December 1991 Workshop A.W. Czanderna and A.R. Landgrebe, Editors Executive Summary Assessment of the Current Status in Surface Characterization Assessment of the Current Status in Surface Modification Discussion of Research Needs and Opportunities for Advancing Transportation and Utilities Technologies Through Applied Surface Science and Surface Processing National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado80401-3393 A national laboratory of the U.S. Department of Energy Managed by Midwest Research Institute for the U.S. Department of Energy under contract No. DE-AC36-83CH10093 Prepared under task number DO140101 September 1992 NOTICE ThisreportwaspreparedasanaccountofworksponsoredbyanagencyoftheUnitedStates government. Neither the United States government nor any agencythereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents thatitsusewouldnotinfringeprivatelyownedrights.Referencehereintoanyspecificcommercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof.The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Printed on paper containing at least 50% wastepaper, including 10% postconsumer waste SUMMARY Surface Processing is a subset of applied surface science for (1) preparing tailor-made surfaces for specific end-useapplications,(2)characterizing thesurfaces,and (3)developinga theoreticalframework.These _ proceedingsdocumenttheprincipaldiscussionsandconclusionsreachedataworkshopheldon December10-12,1991,andcosponsoredbytheU.S.DepartmentofEnergy(DOE),Officeof TransportationTechnologiesandOfficeof UtilityTechnologies;andtheNationalRenewableEnergy Laboratory (NREL) under contract toDOE.The proceedings document eight chapters about the current status of surface characterization with a principal focus on the composition, structure, bonding, and atomic-scale topography of surfaces.The proceedings alsodocument eleven chapters about the current status of surface modification and include asummary of techniques-electrochemical, plasma-aided, reactiveand nonreactivephysical ' vapordeposition,sol-gelcoatings,high-energyionimplantation,ion-assisted deposition,organizedmolecularassemblies,andsolar energy.Threebrief chaptersin theAppendices document basic research in surface science by the NationalScience Foundation, theAir Force Office of ScientificResearch,andtheDivisionofMaterialsSciences,Officeof EnergyResearch,DOEthat underpinssurface processing. The purposes of theworkshopweretobring together scientistsandengineersfromacademia,indUStry, andfederallyfundedlaboratoriestoformworkinggroupswithamixtureofexpertiseinsurface characterizationandsurfacemodification,andtoidentifyandprioritizetheresearchneedsand opportunities ofeach of 10different topical areas of applied surface science asthey relate toapplications in the transportation and utilities technologies.Each participant was invited to serve on one of 10 working groupsconcernedwithatopicinappliedsurfacescience.Thetopicswerecorrosionprotection,solid batteriesandfuelscells,lubricatingandwearsurfaces,polymer/metal(oxide)interfaces,thin-film multilayer solar collectors,accelerated life testingof deviceswithsolid/solid and solidlliquid interfaces, interfacialmicrochemicalcharacterization,conductingpolymers,photoelectrochemicalsystems,and modificationwithorganizedmolecularassemblies.Theparticipantsidentifiedgenericandspecific problemsat materials interfaces in the10 topicalareasand conceivedstimulating ideason research and development that focus materials and systems with targeted end-use applications.With these proceedings, researchersand technologistscan be exposed to the topicalareasand deliberationsof theworkshop. NOTICE This report wasprepared asanaccount of worksponsored by anagency of the United statesgovernmentNeither the United States government nor anylIgencythereof. nor any of their makes any wmanty. express or implied.or aSsumesany legal liability or responsibilityfortheaccuracy.completeness.or usefulnessof anyinformation.apparatus.product,or process disclosed.orrepresentsthatitsusewouldnotinfringeprivatelyownedrights.Referencehereintoanyspecificcommercial product,process.or service by tradename.trademark.manufacturer.orotherwisedoesnot necessarilyconstitute or implyits endorsement. recommendation.or favoringby theUnitedStates government or anyagency thereof.Theviews and opinions of authorsexpressedhereindonotnecessarily $tateor reflect those of theUnitedStates government or anyagencythereof. Printed in theUnitedStates of America Availablefrom: National Technical InformationService (NTIS).U.S.Department of Commerce 5285Port RoyalRoad,Springfield. Virginia22161 Price:MicroficheAOI.Printed Copy A19 Codesareusedforpricingall publications.The codeisdeterminedbythenilmber of pagesinthe publication.Information pertaining tothe pricing codescanbe foundin the cWTentissue of thefollowingpublicatiooswhicharegenerally availablein most libraries:EnergyResearchAbstracts(ERA);Government Repons Announcements and Index (GRAand I);ScientifIC and TechnicalAbstract Repons (STAR);and publicationNTIS-PR-360available fromNTISat theaboveaddress. Workshop Organizers A.W.Czanderna,Ph.D. MeasurementsandCharacterization Branch NationalRenewable Energy Laboratory A.R.Landgrebe,PltD . . Office of PropulsionSystems Officeof Transportation Technologies U.S.Department of Energy T.Vojnovich,Ph.D. Office of Transportation Materials Officeof Transportation Technologies U.S.Department of Energy Supporting Organizations NationalRenewable Energy Laboratory,Director's Office Officeof Transportation Technologies,U.S.DOE Office of UtilitiesTechnologies,U.S.DOE Tableof Contents Preface... .... ... ... .... ... .... ... ... ....... .... ... .... ... .... ... ....v ExecutiveSummary... ....... ... .. " ....... :....... ... ... .... ....... ... ..ES-l Part I.Current Statusof Surface Characterization and Surface Modification 1.CompositionalAnalysesof Surfacesand Thin Films by Electron andIon Spectroscopies.C.1.Powell... ... .... ... .... ....... ... .... ... ... .... .1-1 2.Characterization of Surfaces:Current Statusof Surface Structure Determination (January1991).C.B.Duke..........................................2-1 3'.ASummary of Critical IssuesforApplication of IR Spectroscopy toCharacterization of Surface Processing.D. L.Allara... .. .. ... ... .... ... .... ... .... ......3-1., 4.OpticalSecond HarmonicGeneration Studiesof Adsorption,Orientation,andOrder at the ElectrochemicalInterface.R.M.Com..............................4-1 5.Analysisof InterfacesUsingSecond HarmonicGeneration and Sum Frequency Generation.R.J.Anderson... ... .... ... .... ... .. ..... .... ... ... .... ..5-1 6.Scanning Probe Microscopy.D.A.Griggand P.E.Russell... .... ... ... .... ...6-1 7.LocalOrder and Bonding Primarily USing~ SandSEXAFS.R.A.Mayanovic...7-1 8.High Resolution Electron Energy LossSpectroscopic Characterization of Insulators forSiTechnology.M.Liehr and P.A.Thiry..............................8-1 9.Surface Modification Techniques:A Summary Report.K.Mittal....... .........9-1 10.ElectrochemicalDeposition Techniques.P.C.Searson10-1 11.Plasma-Aided Manufacturing.J.L.Shohet... ... .... ... .... ... .... ... .....Il.. 1 12.AtomisticInorganic Film Formation by Reactiveand Nonreactive PVDTechniques. D.M.Mattox... ... .... ... .... ... .... ... .... .. .... .... ... .... .....12-1 13.Particle Bombardment Effects on Thin-Film Deposition:AReview.D.M.Mattox...13-1 14.Sol-GelCoatings for Energy-Related Materials.A.1.Hurd14-1 15.High-Energy Ion Implantation of Materials.J.M.Williams15-1 16.Surface Treatmentswith IonBeamAssisted Depositions.G.K.Hubler. .. .. ..... ..16-1 iii Tableof Contents(Concluded) 17.ASummaryof CriticalIssuesforApplicationof OrganizedMolecular Assembliesat Surfaces.D.L.Allara... .... ...... .... ....... ... .... ... ..17-1 18.Statusof Surface Modification forMinimizingDirt Retention:Organized MolecularAssemblies.L.M.Speaker...................................18-1 19..Applicationsof Solar Energy toSurface Modification Processes.J.R.Pitts, E.Tracy,andY.Shinton... ..: .... ... .... ....... ... ..... .. .. ... ... ....19-1 Part II.Research NeedsandOpportunities forApplications to Transportation and Utilities Technologies 20.Panelon Corrosion Protection............................... . .........20-1 2t.Batteriesand FuelCells.......................... . ........ . .........21-1 22.Lubricatingand Wear Surfaces........................................22-1 23.Polymer-Metal(Oxide)Interfaces.................... ,..................23-1 24.TheImpact of Surface Processingon the Fabrication and Performance of Thin-Film,Multilayer Solar Collectors...................................24-1 25.Accelerated LifeTestingof Deviceswith SIS,SIL,andS/GInterfaces....... ... . ..25-1 26.InterfacialMicrochemicalCharacterization Needsin Surface Processing... ....... ..26-1 27.Conducting Polymers in Energyand Transportation Technologies -....... ... .... ..27-1 28.PhotoelectrochemicalSystems.........................................28-1 29.OrganizedMolecular Assemblies.......................... . .......... ..29-1 Part III.Appendices:FundamentalSupport UnderpinningSurface Processing At.Office of Basic EnergySciences (OBES).M. Kassner, LBL for BES... . ......... Al-1 A2.NationalScience Foundation (NSF).P.M.A.Sherwood,KansasState University, forNSF...... .... .......... ....... ... .... ... ... ....... ... .... ..A2-1 A3.TheAir ForceOffice of Scientific Research.L. Burgraff,AFOSR........... . ...A3-1 A4.Glossary of AcronymsandAbbreviations................................. A4-1 AS.Participantsat theWorkshop on Surface Processing.......................... AS-I I.IndexI-I iv PREFACE These proceedings document the principal discussions and conclusions reached at a workshop held December10-12,1991,andcosponsoredbytheU.S.DepartmentofEnergy(DOE),Officeof TransportationTechnologiesandOfficeof UtilityTechnologies;andtheNationalRenewableEnergy Laboratory(NREL)undercontracttoDOE.Thepurposesof theworkshopweretobringtogether scientists and engineers from academia, industry, and federally-funded laboratories to form working groups withamixtureof . expertiseinsurfacecharacterizationandsurfacemodification,andtoidentifyand prioritize. theresearchneedsandopportunitiesof eachof 10differenttopicalareasof appliedsurface science asthey relatetoapplicationsin the transportationand utilitiestechnologies. Surface Processingisasubset of appliedsurfacesciencefor(1)preparingtailor-madesurfaces forspecificend-useapplications,(2)characterizingthesurfaces,and(3)developingatheoretical framework.Participantsweresentcurrent-status,"plenary"summariesof thestateof theartinthe relevant methods for surface modification and surface characterization prior to the workshop.They were alsosentdocumentsthatindicatethecrucialimportanCeofsurfacepropertiesforapplicationsin conservation and renewable energy technologies,energydistribution,energy generation by utilities,and buildings, and by other industrial energy users.At the workshop, overview lectures on surface processing inthetransportationindustrybyDr.D.Schuetzleof theFordMotorCompanyandsurfaceprocessing opportunities in the utility industries by Dr. R.McConnell of NREL.The current status summary papers arecontained in Chapters1 through 8 for surface characterization and Chapters 9 through19for surface modification.. Eachparticipantwasinvitedtoserveononeof 10workinggroupsconcernedwithatopicin applied surface science.The topicswere corrosion protection,solid batteriesand fuelscells,lubricating and wear surfaces, polymer/metal(oxide) interfaces,thin-fIlm multilayer solar collectors,accelerated life -testing of deviceswithsolid/solid and solidlliquid interfaces, interfacialmicrochemicalcharacterization, conductingpolymers,photoelectrochemicalsystems,andmodificationwithorganizedmolecular assemblies. Theparticipants identified generic andspecific problems at materials interfaces in the10 topical areas and conceived stimulating ideas on interfacialR&Dthat focusmaterialsand systems with targeted end-use applications.With these proceedings, researchers and technologists can be exposed to the topical areasanddeliberationsof theworkshop.TheneedsandopportunitiesforsurfaceprocessingR&Dare identified anddiscussed in Chapters 20 through 29. - Basic research in surfacescience iswellfundedby severalfederalagenciesincludingtheDOE OfficeofEnergyResearch,theDivisionofMaterialsSciences(OERlDMS),theNationalScience Foundation (NSF),the Air ForceOffice of Scientific Research (AFOSR),theOffice of NavalResearch (ONR), and toa lesser extent, the Army Research Office.All these agencies were invited to summarize their activities for the participants.Relevant summaries of the activities of OERlDMS, NSF, and AFOSR are contained in AppendicesAlthrough A3,andrepresentativesfromOERlDMSand NSF participated in the workinggroups. Acronymsandabbreviations(AAA)usedthroughout thisdocument aregiven in aGlossaryon pages A4-1through A4-3.Acronyms and abbreviations are also defined in each chapter with their initial use, -buttablesof AAAweregenerallyeliminatedfromeachindividualchapter.Theparticipants' addressesand phonenumbersare given on pagesAS-IthroughAS-6. v Theeditorsexpresstheirdeepappreciationtoallparticipantsfortheiranalytical,creative,and incisive involvement.We especially thank the authors of Chapters1 through19 and the(co)chairmen of the working groups corresponding to Chapters 20 through 29 for their cooperation and effortS in producing and/orcoordinatingthepreparationof thewrittencopy.WealsogratefullyacknowledgeNRELstaff membersfortheirsignificantadministrative,coordinating,editing,andwordprocessingcontribu-tions-Julie Baxes and DoriNielsen,Conferences Group; Janet Fried, Pat Haefele,and Fran VanDerPol, WordProcessingGroup;Barbara Spitz,CorporateCommunicationsSection;andReginaWitherspoon, Measurementsand Characterization Branch;for their help before,during,and after theworkshop. vi EXECUTIVESUMMARY A.W.Czandema andA.R.Landgrebe The goalof surface processing is todevelopand use innovative methods of surface modification and characterization that will provide optimum performance and environmental protection for cost-effective operationallifetimesofsystems,materials,andcomponentsusedforspecificapplicationsinthe transportationandutilitiestechnologies. Chemical and/or physical effects at surfaces determine the performance and the lifetime of devices in avarietyof technologies.Thisfacthasbecomeapparent in the lasttwodecadesasnewtechniques haveallowed.us tocharacterizesurfacesat theatomic/molecular levelwith agreat degree of sensitivity. Theability tocontrol themodification of surfaces and tailor-make surfaces for specificapplications has alsoimproved significantly in thesame period.Major advances havealsobeen madein thetheoretical understanding of surface phenomena and theabilitytomodelsurface processes.Theseadvancesled to the possibility of developingmodel systems to predict surface properties.The study of these phenomena has spawned anew fieldof research that is commonly referred toassurface science. Basic surface science research is well funded by several federalagencies,including the National Science Foundation, the Office of Naval Research, the Air Force Office of Scientific Research, and DOE's Divisionof BasicEnergySciences,especiallybytheDivision of MaterialsSciences.Severalresearch centers have been created.The Center for Advanced Materials at Lawrence Berkeley Laboratory and the CenterforSurfaceandInterfaceResearch ' at.theUniversityofConnecticutaretypicalexamples. Furthermore, the Division of Materials Sciences has a long history of generously fundingsurface science researchat Lawrence Berkeley Laboratory,Ames Laboratory,ArgonneNational Laboratory,Oak Ridge NatiQnalLaboratory,andBrookhavenNationalLaboratories,buthasprovidedminimalfundingfor mission-orientedlaboratories(e.g.,SandiaNationalLaboratories,LosAlamosNationalLaboratory, LawrenceLivermoreLaboratory,andNationalRenewableEnergyLaboratory).Inaddition,federal researchlaboratories,suchastheNavalResearchLaboratory,andseveraluniversitieshavedeveloped strong capabilities in basic surfacescience research. Surface processing is a subfield of applied surface science for preparing tailor-made surfaces for specific end-useapplications,characterizing thesurfaces,and developingatheoreticalframework.The crucialimportanceof surfacepropertiesiswellknownforapplicationsinconservationandrenewable energytechnologies,energydistribution,energygenerationbyutilities,andbuildings,andbyother industrial energy users[1].The unique aspects of these applications areas provide surface scientists and engineers with unusual opportunities to carry out focused research and development on materials/systems withtargetedend-useapplications.Inthetransportationtechnologies,forexample,theefficiencies. performance.and lifetimes forelectrochemicalstorage and distribution,and surfaces for lubricatingand weardependverymuchonthesurfacepropertiesof theactivematerials.Theabilitytotailor-make surfaces for specific applications will have tremendous impacts on these technologies.Concurrently, being abletomonitorthechangesonthesesurfacesandcharacterizethechemicalandphysicalprocesses occurringon themwillallowustounderstand theunderlyingphenomena in microscopicdetailandto develop improvedmaterialsandsurfaces fortheseapplications. Applications of surface processing include a broad spectrum of energy technolOgies.All conserva-tion and renewableenergy technologieswillbenefit fromsurfaceprocessing research and development The National Materials Advisory Boards panel on corrosion estimated a potential savings of several billion dollars annually if surface processing technologies can reduce corrosion problems.Additional billions,can besavedif interfacereactionsthatdegradedeviceandcomponentperformancecanbeslowedor ES-1' essentially eliminated.The lifetime of our aging power generation facilitieswillalso be extended asnew materialsandsurfacesaredefinedbyrespondingtotheopportunitiesforresearchanddevelopment identified in thisdocument Information on the current status of surface characterization and surface modification is contained in Chapters1 through 8 and Chapters 9 through19, respectively.Progress in surface characterization and modification hasbeen spectacular since theearly1970s.Needsand opportunitiesin10 topicalareasof appliedsurfacesciencearecontained in Chapters20 through 29.. I.OVERVIEW OF SURFACE PROCESSING AND INTERFACIAL PHENOMENA WITH SPECIFIC APPLICATIONSIN TIlE TRANSPORTATIONANDUTILITIESTECHNOLOGIES Theessentialtechnicalknowledgeunderpinningtheneedfor asurfaceprocessingresearchand development is: Surface processing combines surface characterization and modification into an applied surface sciences subset of surfacescience. Essentialcomponentsand systems in theconservation and renewableenergy technologiesconsist of usingsurfacesor thin-filmmultilayers. Surfaces/interfaces are thermodynamically unstable and ultimately determine the lifetime of systems, materials,and components in conservationand renewableenergytechnologies. Therateofreactionatunstableinterfacesmustbeslowedtoachievethedesiredlifetimewhile maintainingperformance. Surface modification maybe successful in slowingreaction rates. Surfacecharacterization of compositional,bonding,structural,and topographicalchanges that often precededegradationareessentialformonitoringthereaction rate. A.SurfaceCharacterization Recentdevelopmentshaveprovidedthemeansforsurfacecharacterizationwithunprecedented resolution.The variety of techniquesallprovide someaspect of the information required,and usuallya combinationof techniquesisrequiredtoexploreaparticularsurfaceprocesstoobtainathorough understanding of the particular phenomenon.Some of these techniquesare labeled ex situ.meaning that the sample needs to be removed from its operating environment intoa specialized high vacuum chamber for analysis.In some applications, questions arise concerning the extent to which the surface is modified asitisremovedfromtheoperatingconditions,andcarefulcontrolof thetransferprocessbecomes essentialforproper interpretation of the data.Other techniquesarelabeled in situ,because the surfaces can be characterized in their operating environments.Usually,both typesof characterization techniques areessentialforathoroughunderstandingof aparticularsurfacephenomenon.Thepromisingsurface characterization techniques include microscopies for surface structure (high resolution electron microscopy, analytical electron microscopy, transmission electron microscopy, scanning tunneling microscopy, atomic force microscopy, low energy electron diffraction, and ion scattering spectroscopy), compositional surface analysis using ion (secondary ion mass spectrometry, secondary neutral mass spectrometry, surface analysis bylaserionization,ionscatteringspectroscopy,Rutherfordbackscatteringspectroscopy,andnuclear reaction analysis) and electron (X-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning ES-2 Augermicroscopy,ultravioletphotoelectronspectroscopy,andcore-electronenergylossspectroscopy) spectroscopies,vibrationalspectroscopiesforchemicalbonding(Fouriertransfonninfrared-reflection absorptionspectroscopy,Raman,highresolutionelectronenergylossspectroscopy,surfaceenhanced Raman spectroscopy, inelastic electron tunneling spectroscopy, and optical spectroscopic ellipsometry) and synchrotron radiationmethods(extended X-ray finestructurespectroscopy,surface extended X-ray fine structure,andX-rayabsorptionnearedgestructure)forlocalbondingandordering.Summariesof the' current statusof themost important techniquesabovearecontained in Chapters1 through8. B.SurfaceModification Techniques The techniques commonly used to modify or protect a surface include coatings, physical treatment, and chemical modification.A variety of techniques are used to accomplish the treatment process in each of thethreecategories,asdiscussed below. Coatingstechnologiescan be subdivided intopolymer coatingsandmetaland ceramic coatings. Awidevarietyof organicpolymeriCcoatingscanbeapplied tosurfaces.Thepreparationof asurface prior totheapplicationof anycoatingiscritical,especiallywithorganiccoatings.The techniquesfor applicationincludebrushingandspraying,electrostaticsprayingandelectrochemicaldeposition,and coatingwithorganicsthat arethencured to producethe protective layer. Awidevarietyof high- and room-temperatureprocessesareused in thepreparation of metallic and ceramic(includingglassy)typesof coatings.The room-temperaturecoatingprocesses include: Electrochemicaland electroless deposition of metalliccoatings;pulsed electrodeposition of metallic glasses Ionplating,whichinvolvesthedepositionof vapor-phaseionsatelectricallychargedsurfacesto produce metallicandalloycoatings Chemicalvapordeposition,whichinvolveschemicalreactionsof vapor-phasespeciestoproduce metals,alloys,and other chemicalcompounds,includingceramics Evaporation and sputtering, which involve the deposition of materials and the combination of materials byphysicalvaportransport. Chemical vapor deposition, evaporation, and sputtering can also be conducted at high temperatures. In addition,techniquesspecifically conducted at elevated temperaturesincludesolar furnaceprocessing, laser-assisted sprayingandannealing,spray and detonation gun coating,and sol-gelcoating. Physicaltreatmentsmodifythesurfacesphysicallyusingmechanicaltechniques,which include eithershot peeningor shockhardening.Photon(solarfurnaceor lasers)orelectronbeamprocessing involves the rapid scanning of a surface with a high-power photon source or electron beam.In the latter cases,athinmoltensurfacelayerisproducedandrapidlysolidifiedbyheattransfer,thusproduCing metastablesurfacealloysand glassy alloy surface layers. Chemicalsurfacemodificationisusuallyaccomplishedwithoneof threeapproaches.These include: ES-3 (1)Chemicalreactionsatthesurface,in whichthesurfacecompositionischangedbyreactionwitha gaseousspecies,usuallyat anelevated temperature.Nitridingand carbidingareexamplesof such treatments,in which asurface phaseisproducedwhosethicknessisdetermined by diffusion. (2)Electrochemicalsurfacemodification,whichinvolvesanelectrochemicalreactiontoproducea modified surface. Typicalexamples include theelectrochemicalformationof polypyrrolefilmsand theattachment of Silane-typeligands. (3)Ion implantation,which involvestheimplantation of high-energyionsintothenear-surfaceregion. 'Thisproduces newmetastablesurface compositions,usuallywithout causingdimensionalchanges. Inadditiontotheabovewell-knowntechniques,therearechemicalapproachesinwhichthe adsorption of organic species in organized molecular assemblies improves the corrosion resistance.How theseinhibitorsactuallywork isnotveryclear,andthewholeeffort ismoreof anartthanascience. Understandingtheattachment chemistry of organized molecular assembliesontoinorganic surfacesand the state of organization of organized molecular assemblies could assist us in developing another powerful technique tomodify specific surface properties. n.NEEDSANDOPPORniNITIES FOR USINGSURFACE PROCESSING WITH SPECIFIC APPLICATIONS TO TRANSPORTATION ANDUTll.ITY 'tECHNOLOGIES A.Scope The scope of the research and development needs and opportunities in surface processing can be grouped quitenaturallyintothree areas: Surface ProcessingApplications forTransportation Technologies. Surface ProcessingApplications forUtility Technologies. GenericApplicationsof Surface Processing. Researchanddevelopmentinthesethreeareasdooverlap,asevidencedbythe10subjects considered at the workshop.These subjects and the principal technology applications areas are:corrosion protection, fuelcells and solid batteries, lubricating and wear surfaces, and photoelectrochemical systems forsurface proceSSingapplications for transportation technologies; polymer/metal(oxide) interface needs and thin-film,multilayer solar collectors for surface processingapplications for utility technologies;and accelerated life testing of systems, materials, and components with solid/solid, solidlliquid, and solid/gas interfaces, interfacial microchemical characterization needs, conducting polymers in energy technologies, and organized molecular assembliesforgeneric applications of surface processing. Otherstudyareascouldhavebeenincludedinwhichsurfacepropertiesareimportant,e.g., catalysis, coatings, adhesives, glass technologies, and low-energy surfaces such as diamond-like or silicon carbide films.Except for catalysis, these study areas are included as subsets of aspects of the10 subjects given previously. ES4 B.SURFACE PROCESSINGNEEDSANDOPPORTIJNITIESFORSURFACEPROCESSING APPLICATIONSFOR TRANSPORTATION TECHNOLOGIES,SURFACE PROCESSING APPLICATIONSFORUTILITYTECHNOLOGIES,ANDGENERICAPPLICATIONSOF SURFACE PROCESSING Thetechnicalapproachforusingsurfaceprocessingincludesidentifyingthedegradation mechanismsincandidatesystem,material,andcomponentconfigurationsprimarilyfromexperimental work that simulates conditions of use, and interpreting the data to deduce themechanisms of degradation. The Reliability Research Group at AT&T Bell Laboratories hassuccessfully used this approach formore than15yearS.As the degradation mechanisms are understood, the materials and/or use configurations will bemodifiedasneeded,or newmaterialsoptionswillbesoughtandrecommended.Theworkinany appliedscienceareamustbeweight:ec.t-towardfindingthelimitingfactorsforacost-effective,service lifetime, i.e., the fastest degradation processeswillbe studied firstand most intensively,and when those degradationproblemshavebeencorrected,researchwillbe focusedon thenextlimitingissue(s).The correlation between accelerated aging and lifetime prediction must necessarily be accomplished with results fromthe other studyareas.In each of the10 topicalareasof appliedsurface science listed below,key research issuesare indicated. 1.Corrosion Protection Therearemajorapplicationareasforcorrosionprotectioninthetransportationindustries, particularly the application of paint on metalsand plastics(seealsoII.B.4., below).Examples include galvanized steel, externalmirrors, headlamps, trim,hoodand door hem flanges,tailgateassemblies,and bumpers.Surface modifications may be achieved by using high-energy processes, zinc protective layers, sol-gelfllms,or alodineconversion coatings. Keyresearch issuesareto: (1)Improvegalvanizingalloyson steel. (2)Improveadhesion of protective layers(e.g., paint). (3)Improve theunderstandingof light metalalloy corrosion systems and reducethecorrosion rateof theprotectedmetal. (4)Identifycleaningprocessesthat precedesurfacemodification in (e). (5)Select surfacemodificationmethodsforachieving(b)and (c)(e.g.,anodic oxide films,phosphate conversion coatings,electrodeposition,and sol-gelcoatings). (6)Use in situ microchemical characterization of buried interfaces in light-alloy systems, electrochromic windows,andphotovoltaicmodulestoestablishchemistry,morphology,mechanicalstress,and mechanismsof degradation. (7)Establishcompatibility and retarded degradation after(5). (8)Measure accurately the interfacialadhesion bya physicalprocess. (9)Assesstherelativecontribution of chemicalversusphysicalbondingtosurfaceadhesion. ES-5 (10)Develop methodsfor homogeneousmodification of irregularshapes. (11)Perform researchand developmentonthosesurface processingmethodswithpotentialforuseon amanufacturingscalewithon-line processcontrol. Corrosion protection is also required in batteries and fuelcells, in mirrorsfor concentrating solar radiation (e.g., the Aglpolymethylmethacrylate interface); in photovoltaic cells (e.g., metallization, busline, andinterconnects),andinelectrochromicdevices.Theresearchneedsareconsideredbelowin SectionsII.B. 2.and II. B.S., respectively.Other corrosion problems considered to be oflower priority include the compatibility of working fluids in solar thermal systems, biofouling in ocean thermal systems, corrosion of ceramics,glasses,andsensors,and corrosion in fuelsystems. 2.Solid Batteriesand FuelCells Themajorapplication area forsolid batteriesand fuelcellsis in thetransportationindustry for theelectricvehicleprogram.Overlappingexistswithcorrosionprotection(e.g.,carbonandgraphite), polymer/metal(oxide) interfaces (e;g., lithium batteries), conducting polymers ( e . g ~ ,electronic and ionic), microchemicalcharacterization (surfacecompositionand microstructureof pores),andcatalysis. Because nogeneric examplesexist for all battery/fuel cell systems,modelsystems identified for study include polymer electrolyte membrane fuelcells,direct methanolfuelcells,zinc/air batteries,and lithium/polymerbatteries.Thekeyresearchissues,whicharerelatedtocatalysis,corrosion, electronic/ioniccontact,and hydrophobic/hydrophilic behavior at thesolidlboundaryinterface,areto: (1)Define or identifytheinterfacialfunction,desired properties,and specific problems. (2)Identifythesurfacemodification needed tosolve the problem. (3)Identifytheinsitucharacterizationneededtoelucidatereactionmechanismsanddegradation mechanisms. (4)Conduct durability testingon the modified fuelcellslbatteries. 3.Lubricatingand Wear Surfaces Themajorapplicationareasforlubricatingandwearsurfacesareinthetransportation(e.g., engines,fueldeliverysystems,valvetrains,andpowertransmissions),metalforming,andfoundry industries.The key research issues are to identify surface modifications (e.g., nitriding, ion implantation, plasma processing,phosphating,and sol-gelcoatings)to: (1)Reducewear in valveguides/stemsand piston ringllinerssystems. (2)Reducewear in power gearslbearingsand clutchmaterials. (3)Reducewear on cam rollersand journal bearings. (4)Reducewear on injector plungers,spray holes,and plunger tips. (5)Improve cylinder kitsand fuelpumps foralternate fuels. ES-6 (6)Reducewear on lightweightmaterials. (7)Improvethesurfacefinishon composites. (8)Reduce the wear.:on nonferrous formingtools, ferrousformingdies,and aluminum foundrypatterns. (9)Modelwear andadhesion between movingparts. 4.PolymerlPolymer and PolymerlMetal(Oxide)Interfaces The principal applicationsareas for polymer/polymer and polymer/metal(oxide) interfaces are in theutilitiestechnologies(e.g.,photovoltaic,solarthermal,powertransmission,windows,andwind turbines), although significant applications exist in the transportation technologies.For practical purposes, polymer/metal interfaces is a misnomer except for polymers on gold.Even the noble metals (e.g.,silver, platinum, and iridium) have a monolayer of oxidized surface in practical use, and this oxidized monolayer or multilayers (e.g., aluminum, copper, and iron) will dictate the interface behavior.The interphase regime is betternomenclature as it indicatesthematerialin the region between twodifferent solid phases. The key research issuesare related to painted metalor plastics;adhesively bonded components; electronic devices including photovoltaic modules, sensors, and microelectronics; and metallized polymers such asmirrorsto reflect solar radiation.The issuesare to: (1)Select materials in the design stages for recyc1ability, compatibility, processibility, and performance. (2)Selecttheoptimalsurfacemodifications(e.g.,cleaning,vapordeposition,adhesionpromoters, plasma treatments,and ion implantation)for each specific materialssystem. (3)Understand polymer coating formation methods. (4)Elucidate interphase formationmechanisms. (5)Elucidateinterphase degradation mechanisms. (6)Characterizesurfacesand the interphase(morphology,composition,bonding,andstructure). (7)Develop in situ in-processmonitorsforlargescale production. (8)Develop nondestructive testingmethodsforassessingtheadhesionstrength. (9)Perform accelerated aging as screening tests-and then to establish degradation mechanisms and make lifetime predictions. (10)Establish tolerance in processingtreatmentsthat willstiIIyield.adequate quality of the interphase. S.Thin-Film, Multilayer Solar Devices Theprincipalapplicationareasforthin-film,multilayersolardevicesarefortheutilities technologies from photovoltaic, solar thermal, and solar buildings (i.e., electrochromic windows).Surface modificationmethods(e.g.;plasmadepositionandrapidthennalannealing)areagaincomponent- _or device-specific and must be chosen as part of the research program.Most of the key research issues have ES-7 been delineated in DOEJNREL 5-year program plansor initiativesrelated tothesetechnologies.These issuesare to: (1)Identify interfacemechanismsthat limit device lifetime or reducethe performance. (2)Improvethedurabilityof encapsulatedphotovoltaicdevicesbyloweringtherateof degradative interfacereactions(e.g.,metallizationcorrosion,surface-catalyzedpottantdegradation, environmentally tight edgeseals). (3)Improvethedurabilityof silvered polymersby eliminating polymer/silver interface corrosion. (4)Develop thin-film photovoltaic moduleswith improved junction properties to raise thevoltage and ,fIllfactors. (5)Improve the durability of electrical contacts in general, and specifically for cadmium telluride thin-filmmodules. (6)Developlow-energy,antisoilingsurfacesformirrors,photovoltaicmodules,andelectrochromic windows. (7)Improve photovoltaicandelectrochromicwindowmaterialstominimizeinterfacecharge-carrier trappingeffects. (8)Devisediffusionbarrierstominimize problems resultingfrominterdiffusion at interfaces. (9)Modifysurfacestoenhanceadhesion in multilayer stacks. (10)Modify semiconductor/metallization interfaces to obtain long-term stability, high performance, and corrosion resistance. (11)Develop deposition processesthat eliminate theneed forextractive patterningprocesses. (12)Identifysurfaceprocessedinterphasesthateliminatesmechanicalstressesbetweenthinfilm multilayers. (13)Conduct accelerated life testing,establish degradationmechanisms,andestimateservice lifetimes forphotovoltaic,solar thermal,and electrochromic windowdevices. (14)Improve glasssurfaceswith and without deposited thin filmsforstability. (15)Reducemetallization patterningcostsby usingasolar furnace. 6.Accelerated Ufe Testing The application areasforaccelerated life testingcertainly includeallutilitiesandtransportation technologies,buttheareasareasbroadasindustryitself.Surprisingly,thecurrentstatusisalmost universally dependent on the single point method of: ES-8 t.(realtime,outdoors)= kft (failurein accelerated test)(1) wherekisreallythought tobea constant. Thekeyresearch issuesareto: (1)Identify criticalacceleratingstresses including the maximumallowablestress before amechanism change occursand the rangeof thestressasit relatestoreality. (2)Expandtheuseof theSpearman rankcorrelation method. (3)Identifynewapproachestorelatinglifetimepredictionfromfailurein acceleratedtest,including definingwhat isafailure(e.g.,percent of performance loss,change in appearance). (4)Develop predictive models based on mathematical models,correlation methods,and data analysis, and comparethemwith resultsfrom large quantitiesof samplesstressed tofailure. (5)Understanddegradationmechanismsthatleadtofailure,dependingonthevariable(time, temperature,ultraviolet,relative humidity,water,etc.)and their synergism. (6)Quantify real-time. stresses versusaccelerated test stresses to establish the simplicity or complexity of kin Equation1. (7)Educate p e r s o ~ e land their managementson thelimits of accelerated life testing. (8)Define howstressesarecharacterized andlormeasured. (9)Develop and consolidate data bases, especially for solar exposures in hostile terrestrial environments. (10)Useextensive characterization of materialslinterfacestoexplain degradation mechanisms. 7.InterfacialMicrochemicalCharacterizationNeeds Asthesurfacecharacterizationcomponentof surfaceprocessing,theinterfacialmicrochemical characterization needs are applicable to all components of conservation and renewable energy technologies whereveraninterfaceispresent.Thelimitingstabilityofanysystemorcomponentisthe thermodynamicallyunstableinterface.Thecurrentstatusof presentmethodshasbeensummarized in Chapters1 through 8.The crucialkey research issuesare to: (1)Improve chemical specificity todetecting bonding,organicmaterials,and hydrogen. (2)Develop realtime techniquesfor use in on-line dynamic processingsteps. (3)Improve/develop new techniques (e.g., atomic force microscopy/scanning tunneling microscopy, X-ray optics, greater spatial resolution), especially for nondestructively studying realand buried interfaces, impuritiesat solid/solid interfaces,surfacecontaminants, interdiffusion,andinterfaceand thin-film stability(degradation). (4)Expandtheavailabilityof sophisticatedmeasurementsandanalysisexpertsthroughcollaboration, sharing,and moreefficient useamong universities,industry,andnationallaboratories. ES-9 (5)Improvedetectabilityof defects,microtopography,andmicrophaseformation. (6)Extendmoresurfacescience techniquestocharacterizingthesolidlliquid interface. (7)Imp'rovethemonolayersensitivityof Fouriertransforminfrared-reflectionabsorptionandoptical spectroscopic ellipsometry. 8.Conducting Polymers The principalapplication areas forconductingpolymersin the transportation technologiesarein fuelcells, solid batteries, sensors, variable transmission windows and reflectors, and fuel tank surfaces (to preventstaticchargebuildup).Intheutilitytechnologies,applications/areasincludesupercapacitors, electromechanical actuators,electromagnetic shielding, photon-based optoelectronic devices,sensors and displays,and thin-film,high-temperaturesuperconductor multilayer stacks.Thekey research issuesare to: (1)Understand theroleof the interface in both electronic and ionic conduction mechanisms. (2)Improveachievableelectricalconductivitywithout decreasing the ease of processability. (3)Obtain improved environmentalandthermalstability. (4)Minimize interfacialresistance between polymer electrolyte fIlmsand electrodematerials. (5)Develop processingprocedures forapplication tovariousmaterials. (6)Improvecycle lifeandcycle rate limitationsat the polymer electrolyte/electrode interface. (7)Optimizepolymersynthesisonsubstratesandsubsequentmodificationtoobtaindesired performance. (8)Develop optically transparent conducting polymers. (9)Improveadhesion between conducting polymersandvarioussubstrates. (10)Fabricate polymerswith improved physicalpropertiessuch astensilestrengthand flexibility. 9.PhotoelectrochemicalSystems Theprincipalapplicationareasforphotoelectrochemicalsystemscanbeintheutilityor transportation technologiesdependingon the design of the particular photoelectrochemicalsystem.The photoelectrochemical system may be constructed to yield electricity or a fuel,or to charge a battery.The key researchissuesareto: (1)Obtainmorefundamentalunderstandingof the interface issuestofacilitateidentifyingtheoptimal surfacemodification methods. (2)Optimize long... term stability and improved efficiency of the photoelectrochemical system by interface manipulation. ES-I0 (3)Identify improved characterization methods for interfaces arid bulk materials in photoelectrochemical systems. (4)Identify most cost-effective fabrication of photoelectrochemical system interfaces and structures (e.g., useinterphase protective layers,buried junctions,catalytic interfaces,or nanostructures). (5)Improvethesynthesis,detoxification,andstabilityof materialsused in thefabricationsteps. to.Organized Molecular Assemblies The opportunities fortechnologyapplicationsof organized molecular assembliesareplentifulin areas 1 through 6, and 9 above.The most important effort is to focus on using self-assembled monolayers in applied research areaswhile continuing fundamentalfeasibilitystudies.In the applied research areas, thekey research issuesare to: (I)Developthermallyandoxidativelystableself-assembledmonolayersfromorganicandinorganic molecules. (2)Directapplicationstotechnologicalsubstrates(e.g.,steels,aluminum,copper,ceramics,glass,and photostablepolymers). (3)Control interfacialpropertiesin fuelcellsat the gaslliquidlcatalystlmembrane interface. (4)Develop self-regeneratingself-assembledmonolayersforpassivatingcorrodible surfaces. (5)Identify(self-regenerating)self-assembled monolayersforfrictioncontrol. (6)Preparelowenergyopticalcomponentsurfaces(e.g.,mirrorsandphotovoltaicmodules)thatare resistant tosoilingand biofouling,and areeasily cleaned. (7)Employ self-assembled monolayers as self-cleaning, self-passivating materials in industrial processing steps. In theareasof fundamentalresearch,thekey research issuesareto: Develop methods for formingin-plane patternsand featuresin self-assembled monolayers. Use self-assembled monolayers with deposited overlayers as a model system to elucidate interactions. between organicfunctionalgroupsand inorganicspecies. Study self-assembled monolayers as model systems for complex phenomena, including but not limited towetting,adhesion,friction,wear,fouling,soiling,biofouling,corrosion,masstransfer,change transfer,and electron transfer adhesion. Useself-assembledmonolayerstounderstand phenomena in condensed-matter science,cooperative behaviors, phaseseparations,and the influences of defect frequencyandnature. Develop new types of self-assembled monolayers, focusingon enhanced stabilities andnew typesof surface-monolayer chemistry. ES-II Useself-assembledmonolayersasnuclearioncentersforthree-dimensionalprocesses,e.g., condensation of water,growth of crystals,andattachment of polymers. Useself-assembledmono layersasmodelsystemsfortwo-dimensionalorganizationofcomplex molecules,in relation tomolecular materials. Develop computational models for self-assembled monolayersand self-assembled monolayer-related phenomena. 11.Other Areasof Impact In addition totheexamples mentioned above,a program in thesurfaceproceSSingtechnologies willgreatlyimpactthemicroelectronic,semiconductor,corrosionprotection,andcaiaIystindustries. Technologies dealing with ultrathin films, protective and decorative surface coatings, variable conducting masks, and novel bimetallic alloy catalyst systems rely tremendously on the research progress on a surface processing technology.A DOE-supported technology base program will assist U.S. industry substantially in a wide variety of fields and will allow us to maintain and improve our competitive posture in the global" marketplace. Ill.EPILOGUE In usingsurface processing,twopotentialmajor technicalbarriersneed tobe overcome.These are: Securing stable materials that are compatible with each other, Le., long-term interface stability, without compromising the performance of the systems, materials, and components for specific applications to transportation and utility technologies. Developingcombinations of materials that meet the criteria forperformance,durability,economics, and user acceptance. Acrucialneedandtimelyopportunityexisttouseandextendthescientificandengineering knowledge base in surface processing and applied surface sciences to provide solutions to selected priority problemsinthetransportationandutilitytechnologies.Currentcomponentsandsystemsrepresent approachestaken in the1980sthat havebeenadequatefortheirpurposes,but need tobe enhancedfor futureneeds. Theemphasisof thesurfaceprocessingresearchanddevelopmentprogramneedstobeon formulating,preparing,characterizing,developing,testingandqualifyingwhereapplicable,modifying, demonstrating, and understandingadvanced systems,materials,and components for specific applications to transportationand utility technologies that willmeet lifetime,cost,and performance requirementsfor the1990sand beyond. To ensure future success in the related transportation and utility technologies, surface processing workneedstobe initiatedassoonaspossibleandaggressivelypursuedtoreachtheconfidencelevel needed for selecting, guaranteeing, and mass-producing future components/systems for long-term use.The research and development need is cross-cutting or interdisciplinary in nature [2]and requires involvement of industry,academia,and federallyfundedlaboratories. ES-12 For DOE, asignificant cross-cutting applied research program in surface processingtechnologies willprovideseveralbenefitstoallconservation and renewableenergy technologies,Le.,it will: Createacriticalmassof researchersandequipmentthateachProgramOfficeof conservationand renewableenergy technologiescan drawupon tosolvespecificend-useproblems. Provideamultidisciplinaryapproach,producestrongsynergistic effects,andallowvariousavenues tobeexplored tosolve aspecific problem. MakeavailabletotheemergingrenewableenergytechnologyindustriesintheUnitedStateswell-equippedfacilitieswithtalentedexpertstoassistinsolvingspecificproblemsandenhancingthe competitive position of U.S.industry in theglobalmarketplace. The need for a critical mass of researchers with multidisciplinary backgrounds, assisted by state-of-the-art instrumentation, is essential to the success of surface processing research and development.Surface characterization and modification technologies encompass techniques typically developed by physicists and chemists;these techniquesarebeing used and i m p r o v ~upon by physicistsand chemists,as wellasby materialsscientistsandelectrical,chemical,andother engineers.Theproblemsthat need solvingalso cover a wide spectrum of disciplines.The synergistic effect expected fromamultidisciplinary approach willmakesurface processing research and development unique and allow it to meet its goalof providing ~genuinetechnology-based,cross-cutting,researchsupportforallconservationandrenewableenergy technologies.Theabilityof surfaceprocessingresearchanddevelopmenttoimpact othermajorDOE effortsmakesit veryattractive. For U.S.industry, in general,aworld-class leadership in surface processing is an equallystrong impetus for the planning of an research and development program.As indicated in the preceding sections, a variety of technologies can benefit from this type of generic research, and advances in surface processing willallowinterested industrialconcernstobepartners in thiseffortandtousetheknowledgeandthe equipment. Althoughmanydifferentassessmentswillbemadeaboutthisworkshop,positiveresultscan already be stated.Another opportunity hasbeen provided for increased communication among scientists andengineersincludingthoseinvolvedwithbasicandappliedproblems.Industrial,academic,and federallyfundedmembersof thescientificandtechnologicalcommunitieshavebeeninvolvedinthe planningand have been madeawareof thedifficultiesinsettingpriorities,even in afocusedsubset of surfacescience.Theirdedicatedeffortinestablishingtheprioritiesandidentifyingthemostrelevant research issues in surfaceprocessingfor10 typicalareasof applied surfacesciencehasresulted in this documentThe tasknow is for allof ustocarry out the most important recommendations. REFERENCES 1.A.W.Czandema and R.Gottschall, Eds.,"Basic Research Needsand Opportunities on Interfaces in Solar Energy Materials,"Special Issue,Mat Sci. Eng., g1-168(1982). 2.A.W. Czandema, P. Call, and A. Zunger, Materials Sciences, in T.S. Jayadev and D. Roessner, Eds., "BasicResearchNeedsandPrioritiesinSolarEnergy,Vol.n,TechnologyCrosscutsforDOE," SERI/fR-351-358, January1980.(AvailableNREL,1617 ColeBlvd.,Golden,CO80401.) 3.See listingof workshopand panelreports in AppendixAI. ES-13 ABSTRACT 1.COMPOSmONAL ANALYSESOFSURFACESAND1HIN FILMS BY ELECTRONANDIONSPECTROSCOPIES C.1.Powell Surface and MicroanalysisScienceDivision NationalInstituteof StandardsandTechnology Gaithersburg,MD20899 An overview is given of the current capabilities, advantages, and limitations of the six techniques in common use for compositional analyses of surfaces and thin films:Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), sputtered-neutral mass spectrometry(SNMS),ion-scatteringspectroscopy(ISS),andRutherfordbackscatteringspectroscopy (RBS).Information isalsogivenon effortstoimprovethesetechniquesandonthepotentialof other methods fordeterminingsurface compositions duringprocessing. I.INTRODUcnON Surface and interface properties are crucial in many different energy and as is clear fromthe present workshop.While particular properties such ascorrosion- or wear-resistance, reactivity,andelectricalpropertiesareneededforspecificapplications,thesedependonthechemical composition at thesurfacesor interfacesof interest. Six techniques are frequently used for determining the composition of the outermost several atomic layers of a material:AES, XPS, SIMS, ISS, SNMS, and RBS.Measurements with the first four of these techniquesare generally made in an ultrahigh vacuum (UHV) environment; SNMScan be performed in UHVdependingonthemethodof ionizationofthesputteredneutrals.Certainsurfaceprocessing operations(e.g.,heating,filmdeposition,sputtering)canbecarriedout inUHV,butoften thematerial of interest has to be transferred to a separate chamber for other types of processing (e.g., interactions with liquids or high-pressure gases).H surface layers of the specimenmaterialare removed by some method (e.g.,sputtering),thefirstfivesurface-analysistechniqueslistedabovecanbeusedtodeterminethe instantaneoussurface composition asa functionof time or of distance fromtheoriginalsurface.In this way,thecompositionsof filmswiththicknessesuptoabout1 J.IIIlcan bedetermined togetherwiththe local compositions in the vicinity of interfaces.The sixth technique. RBS, is an essentially nondestructive method formeasuring .near-surfaceand thin-filmcompositions in a non-UHVenvironment. Thisreport is in twoparts.Fmt, theabovesixtechniquesare compared andcontrasted,based on their capabilities, advantages,and limitations-.This section of the report is drawn froma recent book chapter [1].Second, information is given on futureprospects for technique improvements to make them more usefulforapplications in surface processing.Mention is alsomade of other techniques that would beusefulfortheseapplications. ll.CURRENT STA11JSOFMAJOR 1ECHNIQUES FORSURFACEANALYSIS Information on the major features of the six techniques is summarize9 in Tables1-4.Tables5-10 giveinformationontherelativestrengthsandweaknessesof thetechniques.Thesesummariesare intendedonlyasa semiquantitativeguide.The information pertainstocommerciallyavailablesurface analysissystems, in their normal configurations,asopposed to custom-made instruments that optimizea 1-1 particular parameter.In many cases, trade-offs can and often must be made among the various parameters, e.g.,spatialresolution,accuracy and precision of analysis,sensitivity,sampledamage,and cost. A.PrincipalFeaturesof Techniques Table1 givesanoverview of theprincipalfeaturesof the techniques:One practicalconcern in theapplicationof thesetechniquesisthepotentialforspecimendamagebytheincidentbeam.Such damage can take the form of removal of specimen material (e.g.,by sputtering or desorption), of changes in specimen composition (e.g.,decomposition or polymerization),or of atomicdisplacements.XPSand RBSarerelativelynondestructive,butsomematerialsmaybedamagedduringaprolongedsurface analysis.Decomposition is not common in XPS, but prolonged X-ray exposures may damage adsorbates, organic compounds, and some ionic solids [2,3].AES is essentially nondestructive for metals, alloys, and many semiconductors, but electron beams will rapidly damage organic materials, adsorbates, glasses, and somecompounds[3,4].Electronbeameffectsaregenerallyproportionaltotheelectronbeamdose (primarycurrent densitytimesbombardment time).An electronbeam-induced temperature increaseis mostdetrimentalforthinfilmsonsubstrateswithlowthermalconductivity.Electron-stimulated desorption is often encountered in studies of ionic compounds.In extreme cases, e.g., for alkali halides, thenumber of desorbed atomsor moleculesper incident electron is greater thanone[4].Chargingof insulatorsissometimesdifficulttoavoidandcanevenleadtoinducedelectromigration.Ingeneral, electron beam effectson fragilespecimens can be minimized with alow primary electron beamcurrent density,short measuremept times,or scanningthe incident beam to different regions of the surface. SIMS,SNMS,and ISSareall intrinsically destructive (i.e., ion bombardment removes speCimen materialfromtheexposedsurface,and it is notpossible toremeasurethecomposition of apreviously sputtered surface).A trade-off must then be made between operating in the"static" mode for true surface analysis and the data acquisition time.The "static"mode requires very low erosion rates [5]and thus low current densities,whichalsomay requireoptimization between thecurrent densityand detectionlimits (i.e., signal-to-noise ratio).A low incident currentdensity for AES,SIMS, SNMS, andISSalsomeans that the lateral resolution will be degraded.Ion beams are also used for sputter depth profiling (SDP)to obtaincompositionalinformationasafunctionof depth.Ion bombardmentcandamagespecimensin manywaysincluding,forexample,surface roughening,knock-oneffects,atomic-scale cascademixing, preferential sputtering, amorphization and structural changes, decomposition, implantation, chemical state changes,enhanced diffusion,enhanced adsorption,and redeposition[6]. The sampled depth (or information depth) for SIMS and SNMS is listed in Table 1 for the "static" mode;in the dynamic mode (high erosion rates)[5],asoften needed for thin-fum analysis,thesampled depth may range from1 to 5nm or more,depending on the erosion rate, incident beam angle and beam energy usedIn RBS, surface monolayers can be studied on single crystals usingchanneling techniques. ThesampleddepthdependsontheescapedepthoftheejectedelectronsinXPSandAES,on neutralization probabilities in ISS,and on erosion ratesin SIMS and SMMS.The American Society for Testing and Materials (AS1M) definition for information depth gives the depth from the exposed surface from which aspecified percentage of the detected signaloriginates.This depth should be distinguished fromtwo other terms,the detection limit (or detectability)and thesensitivity.The detection limit for a particular techniqueisthe minimum quantityof materialthat can be detected under specified operating conditionsandisoftenexpressedasafractionof anatomiclayer;e.g.,it isoftenassumedthatthe detected material is present as an identifiable phase on a substrate of another material.The sensitivity is the slope of a calibration curve in which the measured signal strength for agiven element under specified conditions is plotted versuselementalconcentration. 1-2 Table1.Summary of Features of SIMS,SNMS, ISS,RBS,AES,and XPS, CategorySIMSSNMSISSRBSAESXPS Input"particle"0.5-20 keVions0.5-20 keV ions0.5-3.0 keV ions0.5-3.0 MeV ions2-20 keVPhotons(X-rays) electrons Damage toMinimaltoMinimal toMinimalfor, Minimal ' exceptMinimal toMinimal sample by inputextensiveextensivedefocused beamfororganicsandextensive for(X-radiation) particledepending ontomoderatepolymersfocusede-beam beam parameters Output particlesSecondary ionsSecondaryInput ionInput ionsElectronsElectrons neutrals MeasuredMass/charge ofMass/ChargeofEnergy ofEnergy of ionEnergy of ejectedEnergy of ejected quantityofsputtered surfacepostionizedscattered ionsbackscatteredAuger electronscore-leveland output particleionssurfacespeciesafter binaryfromelasticAuger electrons collisionscollisions Sampled depth1-2monolayers1-2monolayersSurfaceMany J.UIlinto2-20 monolayers2-20 monolayers ("static")("static")monolayerthesolid B.Data Collection Table2summarizesfactorsimportantindatacollectionincludingvacuumrequirements,data acquisitionrates,signal-to-noise(SIN)ratio,turnaroundtime,anddepthprofilingcompatibility. Measurements must be made with a pressure in the vacuum chamber of 10-4 Pa or better so that collisions betweenelectronsorionsandresidualgasmoleculesareminimized.Residualgasesmay reactwith specimens being analyzed, and pressures of 10-7 Pa or lower are generally required tominimize reactions. Another factor is electron-stimulated adsorption, which can lead to oxidation or carburization [4] .Average data acquisition timesaregiven in Table2,although the actualtimes mayrangefromseconds tohours, dependingontheinformationdesired.TheSINratiogovernsthelimitof detection;therangesgiven should be comparedwith thedetection limitsin Table3. For SDP, XPS is clearly inferior, because the ion beam etching and surface analysis must be done alternately as a result of the relatively long data acquisition time and the relatively large X-ray beam size, even for current "small-spot" XPSsystems.Otherwise,a depth profIle is obtained with an inferior depth resolution.However,XPS(aswellasAES)canbeusednondestructivelytoobtaindepthprofilesfor shallowdepths($50A)bydetectionof ejectedelectronsatdifferenttakeoffangles.Thistechnique makes use of the variation of electron escape depth with takeoff angle; it is especially powerful when there areelectronswithwidelydifferentenergiesandthusescapedepthsforthesameelement.In custom-designed RBSsystems,near-surface nondestructive SDP may be obtained usingchanneling and grazing-exit detector techniques.SIMSand AEScombined with ion erosion are the two. most widely used SDP techniques. In recent years,the analog instrumentation for recording spectral intensities has been replaced by digitaldata systems.Thesesystemsareextremelypowerfulfordata acquisitionandmanipulation. C.Featuresof theAnalyticalMethods Important features of SIMS, SNMS, ISS, RBS, AES, and XPSare summarized in Table 3.Each technique is able todirectly identify elements with atomic numbers of 3 and higher.The elements H and Hecanbedetected bySIMSandSNMS,byforwardscatteringin ISS,orbynuclearreactionanalysis (NRA)withRBSequipmentHydrogenhasbeendetected in somematerialsbyAESandXPSfroma detailedanalysisof thespectrallineshapes of other elements.Themass-sensitivetechniquescandetect isotopes,butthiscapabilityislimitedinISSandRBSbypoorpeakresolutionformid- tohigh-Z elements.SIMSandSNMShavethelowestdetectionlimits,butSIMSsuffersfromwidesensitivity variationswith Z. Noneof theionspectroscopiesprovidesthedetailedchemicalinformationobtainedwiththe electron spectroscopies, especially XPS.Small chemical shifts of elemental lines in XPScan be used to identifydifferentchemicalstates of anelement;suchidentificationsareextremelyvaluable(e.g.,in identifying surface reactionsor processes) . . Auger lines alsoappear in XPS data and typicallyalsoshow chemicalshifts.ThedifferenceinXPSandAESpeakpositionsforanelement,theso-calledAuger parameter,often hasachemicalshift.Not only isthelatter shift a usefuldiagnostic,but it may bethe easiest to measure reliably on nonconductingspecimens.AES,aspracticed with electron excitation, has usuallybeendonewithanalyzersof lowenergyresolution.Thus,thechemicalinformation(shiftsof elemental lines and changes of peak shapes) obtained from the technique has been limitedIn some cases, chemical species can be identified by AES data alone, but the supporting data are much less than for XPS. StaticSIMSisalsoveryuseful,althoughitcanbedifficulttoextractthisinformationduetothe complicating signals from sputtered cluster ions and molecular fragments.Recent experiments using SIMS with high mass resolution have been particularly valuable in identifyingorganicmaterialsonsurfaces. 1-4 Table 2.FactorsImportant forData Collection in S u r f ~ eAnalysisby SIMS.SNMS.ISS.RBS.AES,andXPS FactorSIMSSNMSISSRBSAESXPS Vacuum(Pa)10"' to10-8 104 to10-8 10""to10-8 < 104 I 0""to10-8 104 to10-8 Acquisition timeSecondsSecondsMinutesMinutesSecondsMinutes Signal/noise ratioup toHf106 I ~toHt10 toI ~I ~toI ~102 toI ~ Turnaround time Fast+Fast+Fast+FastFast+Fast+ SOPMost naturalforSIMSISNMSEasilydonewithAutomaticallyEasily donewithLaboriouslydone because sample erosion is requiredbombardingdoneandnon- anauxiliaryionwith an auxiliary beamdestructi vel yguniongun Other depthNANANAYes"Yes"Yes" profiling methods Althoughdatacanbecollectedatpressuresashighas10-2 Pa,contaminationof thespecimenbytheresidualgasesisaseriousconcern depending on the reactivityof the surface.Note:IPascal= 7.5x10-3 Torr.. + Fast with instruments equipped with rapid-sample-introduction or load-lock facility.Turnaround is slow if samples are loaded onto a carousel anda flangehastobe reboIledtothevacuumsystem. Does not include artifactsin the sample resultingfromion bombardment,whicharethe sameforallanalyticaltechniquesexceptRBS . ..Seetext. Table3.Summaryof AnalyticalFeaturesof,SIMS,SNMS,ISS,RBS,AES,and XPS FeatureSIMSSNMSISSRBSAESXPS ElementsnotNoneNone H,HeH,HeH,HeH,He directly detected Detection of H,YesYes0< 190(H)*WithNRALineshapesof other elementsin a He< 4r (D)compound IsotopesYesYesLowZLowZNoNo Detection limit 10-6to10-9 10-610-2 to104 10-1 to104 10-2 to10-3 10-2 to10-3 (at.fraction) Variation of10" to10'10 to100- 1 ~- 103 - 20- 20 detection limit(LitoU)(LitoU) withZ ChemicalYesYesYes,but only forNoYesYes information andeight elements value/usefulnessSomeSomeNAConsiderableOutstanding Other informationPolymersandorganics,rnIZ to10,000Shadowing ofSurfaces ofPlasmon lossChemicalshifts, and beyond for SIMS;spectra areadsorbateover- epitaxial filmsstructure,oxidation states, matrix-dependent,but much lesssolayers on singlechemicalshifts,valenceband for SNMScrystals,lineshape changesstructure Information onYesYesRarelyNoFrequentlyYes compounds OrganicsamplesYesYesNoNoDamagelikelyYes, SurfacestructureSomeSomeAdsorbatesonEpitaxialfilmsAdsorbateson single-crystal single-crystalsubstrates,epitaxialfilms substrates Peak interference+RareSomeNoneNone+OccasionalOccasional ElementalExcellentVerygoodGood-low Z;Good-low Z;VerygoodVerygood specificity * *Poor-high ZPoor-high Z Lateral(x-y)0.1fJIIl0.2-5~0.1mm1 mm20-100nm20-100~ resolution Table 3.Summary of AnalyticalFeatures of SIMS,SNMS,ISS,RBS,AES,and XPS(Concluded) FeatureSIMS Lateral imagingYes and valueVerygood ChargingYes problems QuantificationVary by up to105 sensitivity factors.in different matrices Standard5%-100% deviation(for repeated measurements) 9is ionscatteringangle. SNMS No NA Yes Yes forgiven instrument 5%-10% Yes Poor Yes ISS Yes,but small data base 5% No NA No RBS Complete 2% AES Yes Outstanding Yes Yes 1%-5% XPS Yes Good Yes Yes 1% I +Extent towhich there may beaccidental overlaps of spectral peaks due to tWoor more elements.For RBS,signals at energies corresponding tocertain elements can overlap signals due toother elementsat different depths. Extenttowhich two or more elements can be distinguished due to finiteinstrumental resolution. It ispossiblenowtomeasurefemtomolesof smallorganicmolecules(molecular weight- 300)andto obtainstructuralandmolecularweightinformationforpolymers.Onecandetectsmallmoleculeson polymer surfacesor one polymer onanother. Each technique can provide different additional information about a specimen material.For example. informationaboutelectronicstructurecanbeobtainedfromXPSandAES.Structuralinformationfor adsorbed molecules or thin epitaxial films can be obtained from the angular distributions of photoelectrons orAuger electrons.Theenergy-lossstructuresinthevicinityof theelasticpeak duetoexcitationsof valence and core electrons in electron-excited AES can be analyzed to give electronic and local structural information. Peak interferences can occur in AES and XPS but are usually only an annoyance (e.g., for compounds with a large number of elements).An advantage of XPS is that accidentaloverlaps of photoelectron and Augerpeakscanbeidentified andovercomebychangingtheX-rayenergy.High-resolutionmass spectrometers can separate peaks with nearly identical mass/charge ratios in SIMS, but the broader peaks obtainedwithmanySNMSinstrumentscanleadtosomeambiguityinpeakidentificationformore complex specimens.Both ISSand RBSare severely limited in elemental identification whenanalyzing sampleswith several high-Z elements . . The highest lateral resolution is attained at present w i ~AES.The indicated AES resolution in Table3 is achieved,however,only with robust specimens that are not damaged by thehigh bombardingcurrent density.In addition,onlymajorconstituentsof thesurface region(withatomicconcentrations- 10%) can be detected with such lateral resolutions [5].Very good lateral resolution,about 0.1J.lIll,is achieved in SIMS, although then with relatively high erosion rates of-the surface (Le., "dynamic"SIMS).TIme-of-flight SIMS, however,enables greater efficiency in massanalysiscompared to dispersiveanalyzers,and correspondingly smaller erosion rates can be employed for a given lateral resolution.Considerable effort hasbeen made in recent yearsto improve thespatialresolution of XPS.Although'thelateralresolution withXPSisnowpoorer thanthat in AESor SIMS,it should be rememberedthat X-rayexcitationis generally much less damaging toa surface than electron or ion bombardment and that the chemical state information in- XPScan bevery useful. High spatial resolution is clearly important if the specimen is inhomogeneous.Practical materials can havedifferent typesof inhomogeneitiesaswellasfiniteroughnessandperhapscomplex morphologies. High spatialresolution is then desirableforestablishing the existence of multiple surface phasesand,if so, their spatial arrangementEven if a detailed compositional map of the surface is not needed, multiple high-resolutionanalysescanberequiredatdifferentsurfaceregionstoensurethatrepresentativeand statistically adequate information is acquired[5]. Althoughmost surfaceanalyses performed are qualitative, there is increasing interest in quantitative measurements, particularly with the availability of data systems for processing acquired spectraPhysical models have been developed for relating observed spectral intensities to elemental concentrations, but only in thecase of RBSis themodelwellestablished and theneeded parameters readily available[5].Most quantitative analyses with AES and XPS are made with the use of elemental sensitivity factors, but matrix effects,mainlythevariationof attenuation lengthsforAugerelectronsor photoelectronsfromagiven element from one matrix to another, can cause errors of about 20%-50%[7].Matrix effects in SIMScan besubstantial(uptoafactorof lOS),but usefulquantitativeanalysescan be obtainedwithappropriate standards[5,8].MatrixeffectsinSNMSaremuchreducedcomparedtoSIMS,butmorestudiesare needed of matrix effects on the energy and angular distributions of sputtered neutrals.In ISS,variations of elementalsignals in different matricesof up toan order of magnitudecan occur;these variationsare 1-8 dueinlargeparttochangingionneutralizationprobabilities.Standardmaterials,withcompositions approximatingthoseof thespecimenstobeanalyzed,providearelativelystraightforward' meansof mininiizing matrix and instrumental uncertainties.It is not generally easy to prepare standards with similar properties (distribution of surface phases, roughness, etc.) tothe specimens, but ion implantation hasbeen shown tobe a useful method forgenerating a reference signal in SIMS[8].The high surfacesensitivity of ISSissuch that care hasto be exercised in avoidingsurface impuritiessuch asadsorbedgases. D.Versatility,Ease of Use,andSupporting Data Table 4shows qualitative comparisons of selected feat.uresof theanalytical techniques.The symbols give arough guide tothe various featurestosupplement the information given in Tables1-3. Successful use of a particular technique depends on the availability of needed reference dataTable4 givesan indication of theextent towhich neededdata areavailable[8-11]. Information is alsogiven in Table 4on therelative use of thesix techniques in recentyearsand the effectivetakeoff year,that is,theyear in which publicationsstartedtogrowfollowingthe introduction of commercial equipment [9]. E.Summary of Advantagesand Limitations Specificadvantagesand limitationsof SIMS,SNMS,ISS,RBS,AES,andXPSaresummarizedin Tables 5-10.Thesetables,togetherwiththeinformationinTables1-4,giveanoverviewofthese techniquesforsurfaceanalysis.Moredetailsconcerning instrumentalcapabilities can be foundin other reviewarticles[5,9,12-19].Otherreviews[20-22]giveinformationon theapplicationsof thesurface analysistechniquestomany typesof materials problems. Table 4.Comparisons of Selected Features of SIMS, SNMS, ISS, RBS,AES,and XPS.The number of plus symbols indicatesavalue frompoor (+)tovery good (IIIII). FeatureSIMSSNMSISSRBSAESXPS Versatility+++++++++++++++++++++ Ease of use+++ (dynamic)+++++++++++++++ IIIII +(static) Thin-filmSDPIIIIIIIIII +++ IIII IIIIII ++ Simplicity of data ++++ IIII IIII IIII III+++++++ interpretation Accuracyof quantita- +++++++ IIIII ++++++++ tiveanalysis Availability of++++ (dynamic)+++++++II III11111 referencedata+(static) Approximate percentage242353333 of use1985-1990 Effective takeoff year197319841974196719691969 1-9 Table5.Major Advantagesand Limitationsof SIMSfor Surface AnalysisandSputter Depth Profiling(SDP) . Advantages is sensitive tooutermost 1-2monolayers(staticmode) can detect10 ppm or less can acquire SDP data as surface iseroded duringanalysis can detect isotopes can detect HandD can acquire data rapidly has lateralimaging capability Limitations requiresdestruction of sample for the analysis is quantitativewith difficulty,at best hasvarying elementalsensitivity has complexspectra maycause chemicalstate changes from ion bombardment Table6.Major Advantagesand Limitationsof SNMSfor Surface Analysisand Sputter Depth Profiling Advantages is sensitive tooutermost1-2monolayers(static mode) can detectto ppm or less can acquire SDP data assurface is eroded duringanalysis can detect isotopes can detect HandD is quantitativewithmodest useof standards(a major improvement over SIMS) can acquire data rapidly Limitations requiressample destruction has poor lateralresolution may cause chemicalstate changes fromion bombardment I-to Table7.MajorAdvantagesandLimitationsof ISSfor SurfaceAnalysisandSputter Depth Profiling Advantages issensitivetofirstmonolayer can detect10-2 to104 monolayer isgood forSDP . is usefulforstudyingordered surfacesandadsorbates isexcellent forsegregation and/or altered layer studies can detect isotopes32S_34S) hassimplespectra provideschemicalinformation fora fewsurfacecompounds canbequantitativewith standards Limitations haspoor lateral resolution haspoorspecificity forbigh-Zelements* isdestructivealthough erosion rate can be lessthan a monolayerlh requiresbetterof ion neutralization generallyprovidesnochemical information *For example,with incident 4Heionsanda scatteringangleof 138, the followingpairs of elements can be resolved:Na and Mg,FeandCu,andBaandTa.Betterresolutioncanbe obtained for bigh-Z elements with incident neon or argon ions. 1-11 Table8.MajorAdvantagesand Limitationsof RBSfor Surface AnalysisandSputter Depth Profiling Advantages ' is quantitativewithout standards can detect atomicfractionsof 10-1 to104 dependingon Z directlymeasuresdepth distributionsbelow the surface without sample destruction hasrapid data acquisition is especiallysensitive to high-Zelements has modest vacuum requirementsexcept forsurface studies c ~be customized tostudysurfacemonolayers can resolvesome low-Z isotopes can be combinedwith nuclear reaction an21ysisfor detectionof H,He,and other low-Z elements Limitations has poor lateralresolution providesnochemical information cannot resolvedifferent elementsof the same mass is limited fordetecting low-Z elements requireslateraluniformityof sample is limited by overlappingsignalsduetomassand depth is limited tospecimenswith onlya fewelements Table9.Major Advantagesand Limitations of AESfor Surface AnalysisandSputter Depth Profiling Advantages is sensitiveto2-20 monolayers can detect ca.10-3 atomicfraction is outstanding forSDP hasa sensitivity rangewithinafact9r of 20 hassuperblateralresolution giveschemical information forsome elements can acquiredata rapidly Limitations mayalter surfacecomposition may have severecharging problems fornonconducting specimens is of limited value fororganicmaterials hasa slow rateof element mapping 1-12 Table10.MajorAdvantagesand Limitationsof XPSfor Surface Analysis andSputter Depth Profiling Advantages issensitiveto2-20 monolayers can detect10-3 atomicfraction is especiallyusefulforchemicalshiftsfromthesame element in different compounds is least destructiveof alltechniques hasasensitivity rangewithin afactor of 20 has minimalsample charging Limitations hasmoderate lateralresolution is slower forSDP than other methods m.FUTURE PROSPECTS .While AES. xps. SIMS. SNMS. ISS.and RBShave proven to be very useful for solving a w i ~ e range of materials and surface processing problems. they each have significant limitations. as summarized in Tables5-10.A brief summary willnow be given of recent efforts to overcome or minimizesome of the major limitations of these techniques.Information will also be given on other techniquesfor surface and thin-filmanalyses in surface processingapplications. A.Recent Improvements to Current Surface AnalysisTechniques 1.Spatial Resolution Strongeffortsarebeingmadetoimprovethelateralresolutionof AES.XPS.andSIMS,in particular.ForAES.field-emissioncathodesand parallel-detection schemesarebeingused toimprove lateralresolutionwithoutreducingdata-acquisitiontimes.Different typesof X-rayfocusingopticsare being developed to utilize the very intense photon beams at new synchrotron light sources for XPSwith lateralresolutionsin the 0.1-1.0 J.Ullrange.Liquid-metalion sourcesand time-of-flightmassanalyzers have recently led to improved lateral resolution for SIMS.Depth resolution in sputter-depth profiling can be optimized by careful attention to the factorsthat can degrade measured profiles[6].Measurements of Auger-electron or X-ray photoelectron intensities as a function of takeoff angle can be analyzed togive depthprofilesforshallowdepths(uptoaboutthreetimestherelevantelectronattenuationlength) nondestructively. although statistical fluctuations in the data can lead to nonunique solutions. and angular anisotropiesdue tospecimen crystallinity may cause additionalcomplications[23]. 2.Quantitative Analyses Despite theconceptualsimplicity of thesurfaceanalysistechniques,therearemanysourcesof systematic errorsand artifacts[24].Seah. however.hasreported calibration schemes for the energy and ' intensity scales of AES and XPS instruments [25].Improved techniques. based on a valid physical model of electron transport,havebeen developedforbackground correction in xps. and theseshouldalsobe applicable to AES[26].The same techniques have also been used to obtain depth distributions (again for 1-13 shallow depths)nondestructively[27].Amajor problem in quantitativeAESand XPSconcerns theso-called matrix correction for elemental sensitivity factors[7]; recent calculations of electron inelastic mean freepaths(lMFPs)haveled tothe development of a predictive IMFP formula for50-2000 eVelectrons that can be applied tovariousmaterials[28].Pruttonandcoworkers[29]have developedanadvanced scanningAugermicroscopeandimprovedalgorithmsforthecorrectionof topographyeffects,beam-current changes,and backscattering effects.Both resonant and nonresonant laser postionization schemes are beingdeveloped forquantitativeanalysesby SNMS[30]. 3.ReferenceData Efficientuseofanycharacterizationtechniqueonawiderangeofmaterialsrequiresthe availability and accessibility of needed reference data.For AES, XPS,and SIMS, handbooks have been publishedwithspectraldataneededformostsolidelementsandforalimitednumberof compounds. NIST has recently announced the availability of an XPS data base containing over 13,000 entries for core-electron bindingenergies,doublet splittings,andkineticenergiesof X-ray-excitedAugerelectronsthat appear in typical XPSmeasurements (with AI. or Mg characteristic X-rays)[11].The American Vacuum Society is in the process of creating a data base of AESand XPSspectra that will be published in a new journal,SurfaceScienceSpectra,beginningin1992.An evaluatedset of ionsputteringyielddatafor elemental solids bombarded by ions and with energies relevant for SDP and SIMS will be published soon [31]. B.Other Techniques Oneof thelimitationsof AESand,toalesserextent,XPSisthelimitedamountof chemical informationtheyprovide.Othertechniquessuchashigh-resolutionelectronenergy-lossspectroscopy (HREELS),infraredspectroscopy,orRamanspectroscopy,cangiveusefulsupplementalinformation, particularly formoleculesadsorbed on surfaces[32]. Amajor limitationof AES,XPS,SIMS,andISSisthattheyaregenerallyrestrictedtoUHV environments;in onemethod of SNMS,thespecimen is exposed toan argon plasma.Somescientists, nevertheless, have used efficient differential pumping arrangements to analyze solids or liquids with XPS inlocalenvironmentswherethepressurewasin thevicinityof 10-2 Pa..It alsomightbepossibleto performXPSat highergaspressureswithsynchrotronsourcesof X -raysandin placeswherethegas atoms or molecules near the specimen act as a "converter" [33].Total-reflection X-ray fluorescence offers amoregeneralmeansof analysisat atmosphericor higher pressuresbut not,however,with thesurface sensitivity of thetechniquesconsidered here. Electronmicroscopyisahighlydevelopedtechniqueformaterialscharacterization[34-37]. Modern Scanning transmission electron microscopes equipped with electron energy-loss spectrometers can be used to probe chemical changes in the vicinity of solid-solid interfaces with a lateral resolution of about 0.4 nm.Atom-probe field ion microscopy can also be used to characterize solid-solid interfaces and the compositionof smallgrainswithcomparablespatialresolution[38].Forbothformsof microscopy, specialspecimen preparation techniquesareneeded. High resolution scanning electron microscopes have been developed to study surfaces and thin-film nucleation andgrowth phenomena in aUHV environment [39].Elementalanalysiswith submonolayer sensitivityhasbeenattainedbymeasuringcharacteristicX-rayemission at near-grazingtakeoff angles [40].Recent efforts to improve the spatial resolution in electron microscopy and various forms of electron spectroscopy havebeen summarized in the report of arecent workshop[41]. 1-14 Scanningtunnelingmicroscopyandatomicforcemicroscopycan beusedtoobtain topographic information and other surface properties.These techniques cannot be used, in general,for compositional analysesof "unknown"surfaces. IV.SUMMARY AES, XPS, SIMS,SNMS,ISS,and RBSare powerful techniques for theanalysisof surfaces and thinfllms.Acarefulselectionof oneormoretechniquesisneededtoensurethataproblemcanbe solved,given thecapabilitiesand limitationsof each method.Complementary techniquesmayoftenbe needed,however,toprovidemorespecific chemical information,higher spatialresolution,or theability to monitor chemical changes at solid-fluid interfaces.The report of an earlier workshop analyzes different approaches forcharacterizingthemicrostructureand microchemistryof interfaces[42]. ACKNOWLEDGMENTS The author is greatly indebted to Drs. D.M. Hercules and A. W. Czanderna for their contributions tothebook chapter[1]onwhichthisreport isbased. REFERENCES 1.C.1.Powell, D.M.Hercules,and A.W.Czanderna, in A. W.Czandema and D.M.Hercules, Eds., IonSpectroscopies forSurfaceAnalysis,Plenum Press,NewYork,NY,1991, pp.417-437. 2.R.G.Copperthwaite,Surf.InterfaceAnal., b 7 (1980). 3.1.Cazaux,Appl.Surf.Science, ~457(1985). 4.C.G.Pantanoand T. E.Madey,Appl. Surf.Science,L 115(1981); T.E. Madey, in D.C.Joy,Ed., AnalyticalElectron Microscopy-1987,San FranciscoPress,San Francisco,CA,1987. 5.H.W.WernerandR.P.H.Garten,Rep.Prog.Phys., , 221(1984). .6.S. Hofmann, in D.Briggs and M.M.Seah, Eds., Practical Surface Analysis, 2nd Edit, Vol.1,Auger and X-RayPhotoelectron Spectroscopy,Wiley,NewYork,NY,1990, pp.143-199. 7.A.I.Zagorenkoand V.I.Zaporozchenko,Surf.InterfaceAnal., 1$ 438(1989). 8.J.T.Grant,P. Williams,J.Fme, and C.J.Powell,Surf.Interface Anal., 1146 (1988). 9.S.Hofmann,Surf. InterfaceAnal., 2,3 (1986). 10.M.P.Seah,Surf.Interface Anal., 2,85(1986). 11.C.1.Powell,Surf. Interface Anal., 1L 308(1991). 12.A.W.Czanderna,Solar Energy Materials, ~349(1981). 13.C.J.Powell, in H. Windawiand F.Ho,Eds., Applied Electron Spectroscopy forSurfaceAnalysis, Wiley,NewYork,NY,1982,pp.19-36. 1-15 14.M.P.Seah and D.Briggs, in D.Briggsand M.P.Seah, Eds., Practical Surface Analysis,2nd Edit, Vol.I, Auger and X-Ray Photoelectron Spectroscopy,Wiley,New York,NY,1990,pp.1-18. 15. H. W.Werner, in H.Oechsner, Ed., 1bin Film and Depth Profile Analysis, Springer Verlag,Berlin, Germany,1984, p.5. 16.A.Benninghoven, F.G.Riidenauer,and H. W. Werner,Secondary IonMassSpectrometry,Wiley, New York,NY,1987, pp.1022-1047. 17.J.R.BirdandJ.S.Williams,inJ.R.BirdandJ.S.Williams,Eds.,IonBeamsforMaterials Analysis,Academic Press,New York,NY,1989, pp. 515-537. 18.I. V.Bletso, D. M. Hercules, D. van Leyen, and A.Benninghoven, Macromolecules, 1Q, 407 (1987). 19.H.Hantsche,Scanning, 11257 (1989). 20.C.J.Powell,Appl.Surf.Science, .1143 (1978). 21.C.J.Powell,Aust J.Phys., li. 769(1982). 22.D.Briggs,Surf.Science,189/190.801(1987). 23.H.E.Bishop,Surf.InterfaceAnal.,.!L197(1991);W.H.Gries,Surf.Interface Anal., .!L803 (1991). 24.C.J.Powelland M. P.Seah,J.Vac.Sci.Tech.,A8.735(1990). 25.M. P. Seah, J.Vac. Sci. Tech.,A 9. 1227 (1991); M. P. Seah and G.C.Smith, Surf. Interface Anal., ll, 751(1990). 26.S. Tougaard, Surf. Interface Anal., .11453 (1988); J. Electron Spectrosc., g243 (1990); H.Bender, Surf.InterfaceAnal., .!L 584 (1991). 27.S.Tougaard and H. S.Hansen,Surf.InterfaceAnal., H. 730 (1989). 28.S.Tanuma,C.J.Powell,andD.R.Penn,Surf.InterfaceAnal.,.11577(1988);S.Tanuma, C.J.Powell,andD.R.Penn,Surf. InterfaceAnal., .!L 911(1991). 29.I. R.Barkshire,M.M. ElGomati,J.C.Greenwood,P.G.Kenny,M.Prutton,and R.H .. Roberts, Surf.InterfaceAnal.,.!L203(1991);I.R.Barkshire,J.C.Greenwood,P.G.Kenny,M.Prutton, R.H.Roberts,andM.M.ElGomati,Surf.InterfaceAnal.,.!L209(1991);I.R.Barkshire, M.Prutton,and D.K. Skinner,Surf.Interface Anal., .!L 213(1991). 30.D.G.Welkie,S.Daiser,and C.H.Becker, Vacuum, ., 1665(1990). 31.G.P.Chambersand J.Fine to be published. 32.J.T. Yates, Jr.,and T. E.Madey, Eds., Vibrational Spectroscopy of Molecules on Surfaces, Plenum Press,New York,NY,1987. 1-16 33.C.1.Powell, 1.Vac.Sci.Tech.,li. 549 (1978). 34.A.Howie and U.Valdre, Eds.,Surface and Interface Characterization by Electron Optical Methods, Plenum Press,NewYork,NY,1988. 35.G.W.Lorimer,Ed.,AnalyticalElectronMicroscopy,TheInstituteofMetals,London,United Kingdom,1988. 36.D.C.Joy,A.D.Romig,and J.I. Goldstein,Principles of Analytical Electron Microscopy,Plenum Press,NewYork,NY,1986. 37.ProceedingsofTIlirdConferenceonFrontiersofElectronMicroscopyinMaterialsScience, Ultramicroscopy,37(1-4),1991. 38.T.T.Tsong,Atom-Probe Field Ion Microscopy:Field Ion Emission and Surfacesand Interfacesat AtomicResolution,Cambridge University Press,NewYork,NY,1990. 39.T.Doust,F. L.Metcalfe,and J.A.Venables,Ultramicroscopy, 11 116 (1989). 40.H.Daimon, C.Chung,S.Ino,and Y.Watanabe,Surf.Science,235.142(1990). 41.Proceedings of Workshop on Electron-Beam-Induced Spectroscopieswith High Spatial Resolution, Ultramicroscopy,28(1-4),1989. 42.J.Silcox,P.H.Holloway,K.R.Lawless,D.Lichtman,R.G.Meisenheimer,L.E.Murr,and C.1.Powell,MaterialsSci.Eng., a149 (1982). 1-17 2.CHARACTERIZATIONOFSURFACES:CURRENT STATUS OFSURFACESTRUCTURE DETERMINATION I.INTRODUCTION (January1991) C.B.Duke XeroxWebster Research Center 800 Phillips Road,0114-38D Webster,NewYork14580 Thefirstpurposeof thispaperistoprovideabriefoverviewof thecurrentstatusofthe determination of theatomicstructure of solid surfaces.We organize thissurvey by classifyingsurfaces bytype(cleanversusoverlayergeometries;metallic,semiconducting,andinsulatingsubstrates)and presenting an aggregate view of the results for each type.We further consider only surfaces whose atomic geometry is known quantitatively.which usually restrictsour attention toepitaxialoverlayerstructures. Finally,wecite the review literaturealmost exclusively, using original sources primarily for illustrative examples.Coverage is restricted tosolid-gas(i.e.,vacuum)surfaces.asopposedtosolid-solid or solid liquid interfaces. The second purpose of this paper is to indicate some of the current key issues in surface structure. predictionandmeasurementForexample,it hasbeenproposedthatthecleavagesurfacesof both zincblende [1]and wurtzite [2] crystals exhibit essentially universal structures if all distances are measured in units of the bulk lattice constantMoreover, these nearly universal relaxed surface structures revert to theirbulkformontheadsorptionofSborBi[3,4].Indeed,theissueofadsorption-induced reconstructions is a topic of great interest in studies of surface chemical reactions, including catalysis [5]. In thecaseof compound semiconductors,agood exampleisAIonGaAs(1lO),whichformsepitaxial AlAs"layerbylayer"onannealing[3].Importantissuesconcerningthesereactionsinvolvetheir spontaneity (as opposed to activation), their dependence on the initial surface structure, and the extent to which theydisrupt the initialsurface structure. The paper is organized into two context-setting sections on theoretical concepts and experimental methodsin thepredictionandmeasurement,respectively,of surfacestructures.Thesearefollowedby synopsesof availableresultson cleanandadsorbate-coveredsurfaces.Thereferencesaredesignedto affordan entre into the pertinent literature. n.CONCEPTSIN TIlE TIlEORY OF SURFACESTRUC1URE Metallic surface structuresare determined primarily by theself-consistent electrostatic responSe of surfaceioncorestothemetallicelectronsspillingout into thevacuum.Recent reviewshavebeen given by Hamann [6]and Inglesfield [7,8].For close-packed surfaces (e.g., fcc1 J.1IIl),volumefractionsof the particulate in the rangeof 0%-30%have been repOrted. Autocatalytic depositionof nickelhasalsobeenusedtofabricatecompositeNilpolytetrafluoro-ethylene(PIFE) filmsusing- l-JIDlPTFEparticles[108,109].Indeed,NiIPTFEcompositesareused commercially in a number of aerospace,electronics,andautomotiveapplicationsduetotheir enhanced wear resistanceand relatively low costComposite N i l C r 3 ~filmshave been fabricated by autocatalytic deposition of nickelfroman electrolyte containingC r 3 ~particles2-3JIDlin diameter[110]. 10-6 In addition to research intocompositeswith relatively large particles(>1 1JIIl),therehasbeen some work directed toward composites with submicron particles.Fpr example, Auf Alz03 composites have been electrodepositedusing50-nmalumina particles[11 Il. REFERENCES 1.ElectroplatingEngineeringHandbook,LJ.Durney,Ed.,4thEdit.,VanNostrandReinhold,New York,NY,1984. 2.. Metal Finishing Handbook, 58th Guidebook Issue, Metals and Plastics Publications Inc., Hackensack, NJ,1990. 3.G.A.Kralik,Platingand Surf.Fin., 1116 (1984). 4.D.P.Seraphim, IBM J.Res.Dev., ~37(1982). 5.DJ. Auerbach,C.R.Brundle,and D.C.Miller,IBM J.Res.Dev., 1b 669(1988). 6.T.R.Baum,J.Electrochem.Soc.,137.252(1990). 7.M.Paunovic,J.Electrochem.Soc.,127, 441C(1980). 8.I.Kiflawiand MSchlesinger, J.Electrochem.Soc.,128,872(1983). 9.C.E.Baumgartner,EJ. Lamby,andK..A.Kollmansberger,J.Electrochem.Soc., 137.1091(1990). 10.C.H.Tingand M.Paunovic,J.Electrochem.Soc.,136,456 (1989). 11.C.H.Ting,M.Paunovic, P.L.Pai,andG.Chill,J.Electrochem.Soc.,136, 462(1989). 12.W.A.Fairweather, Trans.Inst.Met.Fin., M. 15(1986). 13.H.O.Ali and I.R.A.Christie,Gold Bulletin, 11.188(1984). 14.J.E.WilliamsandC.Davison,J.Electrochem.Soc.,137.3260 (1990). 15.G.Stremsdoerfer,Y.Wang,P.Clechet,and J.R.Martin,1.Electrocbem.Soc.,137.3317 (1990). 16.G.Stremsdoerfer,C. Calais,J.R.Martin, P.Clechet, and D.Nguyen,J.Electrochem.Soc.,137.835 (19
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