On the Cover · Fig. 1 – Nanoscale Characterization by Electron Microscopy—MgO cubes with...

On the Cover:Left to Right:

Fig. 1 – Nanoscale Characterization by Electron Microscopy—MgO cubes withsurfaces decorated by small gold particles. These particle ensembles are used inmodel studies of 3D chemical imaging at the nanoscale (electron tomography).

Fig. 2 – Carbon Nanotubes—Vascular smooth muscle cell growing on a cluster ofmultiwalled carbon nanotubes on a glass substrate.

Fig. 3 – Wet Nanomanufacturing—Molecular model of the artificial membrane poreviewed from top.

Fig. 4 – Friction Scaling—3D profile, obtained using the replica technique, of the tipsused in the friction measurements.

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Materials Science andEngineering Laboratory

NanometrologyFY 2004/2005 Projects

MSELClare AlloccaSenior Scientific Advisor to the Director, MSEL

Stephen FreimanDeputy Director, MSEL

NISTIR 7130

April 2005

National Institute ofStandards and TechnologyHratch G. SemerjianActing Director

TechnologyAdministrationPhillip J. BondUndersecretary ofCommerce for Technology

U.S. Departmentof CommerceCarlos M. GutierrezSecretary

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Certain commercial entities, equipment, or materials may be identified in this documentin order to describe an experimental procedure or concept adequately. Such identification

is not intended to imply recommendation or endorsement by the National Institute ofStandards and Technology, nor is it intended to imply that the entities, materials,

or equipment are necessarily the best available for the purpose.

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Table of Contents

Table of Contents

Executive Summary ............................................................................................... 1

Projects in Nanometrology

Nanomechanics

Quantitative Nanomechanical Properties ..................................................................... 2

Mechanical Metrology for Small-Scale Structures ........................................................ 4

Nanoindentation Methods and Standards ................................................................... 5

Nanotribology and Surface Properties ......................................................................... 6

Nanomechanics: Coupling Modeling with Experiments ................................................ 7

Friction Scaling Artifact .............................................................................................. 8

Metrology for Nanoscale Properties: Brillouin Light Scattering .................................. 10

Physical Properties of Thin Films and Nanostructures:Green’s-Function Methods ....................................................................................... 11

Combinatorial Adhesion and Mechanical Properties:Axisymmetric Adhesion Testing ................................................................................. 12

Combinatorial Adhesion and Mechanical Properties:Innovative Approaches to Peel Tests ......................................................................... 13

Nanocharacterization

Thermochemistry and Metrology of Interfacial Interactions ......................................... 14

Chemistry and Structure of Nanomaterials ................................................................ 15

Metrology for Nanoscale Properties: X-ray Methods ................................................. 16

Physical Properties of Thin Films and Nanostructures:Grain Size Effects on Actuator Fatigue ...................................................................... 17

Physical Properties of Thin Films and Nanostructures:Nanoporous Low-κ Dielectric Films ......................................................................... 18

Nanoscale Characterization by Electron Microscopy ................................................... 19

Gradient Reference Specimens for AdvancedScanned Probe Microscopy ...................................................................................... 20

Characterization of Counterion Association withPolyelectrolytes: Novel Flexible Template Behavior ................................................... 21

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Table of Contents

Nanofabrication and Processing

Grand Challenges in Nanomagnetics: High Coercivity FePtAlloys for Future Perpendicular Magnetic Data Storage ............................................. 22

On-Chip Interconnects:Extending Performance of Sub-100 nm Lines ........................................................... 24

Particle Metrology and Nanoassembly ....................................................................... 26

Nanostructure Fabrication Processes:Patterned Electrodeposition by Surfactant-Mediated Growth ..................................... 27

Nanostructure Fabrication Processes:Thin Film Stress Measurements ............................................................................... 28

Wet Nanomanufacturing ........................................................................................... 29

2nd Joint Workshop on Measurement Issues in Single-WallCarbon Nanotubes: Purity and Dispersion, Part II .................................................... 30

Extraordinary Transport Properties of Nanotube/Polymer Nanocomposites ................. 32

Applications of Carbon Nanotubes:Carbon Nanotubes and Nanotube Contacts ............................................................... 34

Applications of Carbon Nanotubes:Cell Viability in Contact with Carbon Nanotubes ......................................................... 35

Applications of Carbon Nanotubes:Electrochemical Characterization of In-Vivo Neuronal Probes ...................................... 36

Organizational Charts ................................................................................................ 37

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Executive Summary

Executive Summary

The burgeoning field of nanomaterials extendsacross the full range of traditional material classes,including all forms of metals, polymers, and ceramics.No previous materials technology has shown concurrentadvances in research and industry as does the fieldof nanomaterials in mechanical devices, electronic,magnetic, and optical components, quantum computing,biotechnology, and as-yet unanticipated exploitationsof as-yet undiscovered novel properties of assemblies.There is growing excitement surrounding the abilityof some molecules or particles to self-assemble atthe nanoscale to form new materials with unusualproperties. Nanometrology, i.e., the ability to conductmeasurements at these dimensions, to characterizethe materials, and to elucidate the structure and natureof these new and novel assemblies, is a requisite andfundamental cornerstone that must be establishedsecurely if this technology is to flourish.

NIST is uniquely positioned to lead the developmentof the measurement methods, instrumentation,standards, and reference materials that togetherwill form the metrological infrastructure essentialto the success of nanotechnology.

The MSEL Nanometrology Program incorporatesbasic measurement metrologies to determine materialproperties, process monitoring at the nanoscale,nanomanufacturing and fabrication techniques, andstructural characterization and analysis techniquessuch as advanced imaging and multiscale modeling.The Program comprises projects in the Ceramics,Materials Reliability, Metallurgy, and PolymersDivisions, and includes structural characterizationusing neutron scattering at the NIST Center forNeutron Research (NCNR). The projects cover awide range of measurement and characterizationmethods grouped into the areas of mechanical propertymeasurement, chemical and structural characterizationand imaging, fabrication and monitoring of nanoprocessesand events, and modeling of nanoscale properties.In each area, we work to advance basic measurementcapabilities and lead the intercomparison, standardization,and calibration of test methods. The newly completedAdvanced Measurement Laboratory at the NISTGaithersburg site provides an incomparableenvironment for accurate nanoscale metrology.

In the area of nanomechanics, we are developingand standardizing techniques for determiningnanoscale elastic properties (elastic moduli, Poisson’sratio, and internal stress), plastic deformation, density,

adhesion, friction, stiction, and tribological behavior.Work in nanoindentation, used extensively indetermining mechanical properties of thin films andnanostructures, focuses on developing traceablecalibration methodologies and standard test methods.We also use atomic force acoustic microscopy,surface acoustic wave spectroscopy, and Brillouinlight scattering to measure the mechanical propertiesof thin films. In addition, we are developing micro-and nano-scale structures and test methods to measurestrength and fracture behavior of interfaces andmaterials having very small volumes. Experimentalstudies are accompanied by efforts in the theory,modeling, and prediction of material properties andbehavior extending from nanoscale to macroscaledimensions. Modeling efforts include large-scale finiteelement methods, multiscale Green’s-function methods,classical atomistic simulations, and first principles,quantum mechanical calculations using densityfunctional theory.

Nanocharacterization utilizes neutron and x-raybeam lines at three facilities: the NCNR; the NationalSynchrotron Light Source at Brookhaven NationalLaboratory; and the Advanced Photon Source atArgonne National Laboratory. Innovative scatteringand spectroscopy methods are advancing our abilityto obtain a wide range of chemical and structuralinformation at the nanoscale, including chemical bondidentification and orientation, polyelectrolyte dynamics,and equilibrium structures. In collaboration with threeother NIST laboratories, we are developing electronmicroscopy and spectroscopy instrumentation forquantitative, 3D chemical imaging at the nanoscale.Other characterization projects include work ongradient reference specimens for the calibration ofadvanced scanning probe microscopy, and theapplication of carbon nanotubes as physical probesof cell membranes.

Efforts in the nanofabrication and processing includethe study of electrochemical and microfluidic methodsfor fabricating nanostructures, novel approaches tonanocalorimetry for the study of interfacial reactions,and in situ observations of nanoparticle and nanotubedispersion and alignment. A second joint NASA–NISTWorkshop was held in order to establish the measurementneeds for determination of purity and dispersion ofsingle-wall carbon nanotubes. The results of thisworkshop will be captured in “NIST RecommendedPractice Guides.”

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Nanomechanics

We are developing AFM-based metrology forrapid, nondestructive measurement of mechanicalproperties with true nanoscale spatial resolution.Atomic force acoustic microscopy methods enableelastic-modulus measurements at either a singlepoint or as a map of local property variations.Complementary information obtained withscanning electron microscopy provides insightinto structure-property relations and helps tointerpret nanoscale contact-mechanics behavior.The information obtained furthers our understandingof the nanomechanical properties of surfaces,thin films, and nanoscale structures.

Donna C. Hurley

New measurement solutions are required to addressthe rapidly burgeoning field of nanotechnology.

In particular, information about mechanical properties onthe nanoscale is needed. Knowledge of properties likeelastic modulus and interfacial quality (defects, strain,adhesion, etc.) is critical to successful development ofnew films and nanoscale assemblies. Such informationcould also assess integrity or reliability in applicationsfrom microelectronics to biotechnology. Existingmethods for mechanical-property measurements havedrawbacks: they are destructive, limited to specializedtest specimens, or not quantitative. Instrumented or“nano-” indentation (NI), a current industry workhorse,will have limited value as scales shrink well below1 µm, and softer materials are more frequently used.

To meet this need, we are developing tools thatexploit the spatial resolution of atomic force microscopy(AFM). Our approach, called atomic force acousticmicroscopy (AFAM), involves the vibrational resonanceof an AFM cantilever when its tip is in contact with asample. By comparing the cantilever’s contact-resonancefrequencies for an unknown material to those for areference sample with known properties, the indentationmodulus M of the unknown material can be determined.[For an isotropic material M = E/(1–ν2), where E isYoung’s modulus and ν is Poisson’s ratio.] The smalltip radius (~5–50 nm) means that we can obtain in-situelastic stiffness images with nanoscale spatial resolution.

In FY04, we extended our quantitative AFAMtechniques in a variety of ways. In one effort, theeffect of film thickness on measurement accuracywas investigated. We measured M for three nickel(Ni) films approximately 50, 200 and 800 nm thick.The values of M ranged from 220 GPa to 223 GPa,significantly lower than that expected for bulk

Quantitative Nanomechanical Properties

polycrystalline Ni. Scanning electron microscopy (SEM)revealed that the films were nanocrystalline (grain diameter< 30 nm). The observed reduction in M may be attributedto an increased volume fraction of grain boundaries in thenanocrystalline films. More importantly, the average valuesof M for all three films were the same within measurementuncertainty (~10 %). Thus the AFAM results were notinfluenced by the elastic properties of the silicon substrate,even for a 50 nm film. This behavior is due to the factthat the AFAM stress field extends less than 100 nm intothe sample and decreases rapidly with depth due to thesmall applied static loads (0.3–3 µN) and small radius ofcontact (5–25 nm). Our result contrasts sharply withnanoindentation, in which substrate properties must beincluded to accurately measure submicrometer films.

The elastic properties of the 800 nm Ni film were alsomeasured using NI, microtensile testing, and surfaceacoustic wave spectroscopy (SAWS). Both AFAM andNI measure the film’s out-of-plane indentation modulus.The results were in excellent agreement, validating AFAMas a quantitative method in spite of its relative newness.Microtensile testing values for the in-plane Young’smodulus of the film were not consistent with the AFAMand NI results if the film was assumed to be elasticallyisotropic. The apparently contradictory results werereconciled by use of a transversely anisotropic model forthe film’s elastic properties. This model is consistent withthe strong <111> film texture observed by x-ray diffraction.When analyzed with the same model, the SAWS resultsindicated that the film density was only slightly lower(< 5 %) than the bulk value. These results illustrate arelatively straightforward way to interpret mechanical-property measurements of thin films that is based on amore physically realistic model than the simple assumptionof elastic isotropy.

Another effort investigated the effects of relativehumidity (RH) on AFAM measurements. AFAM contact

Figure 1: a) AFM tapping-mode topography and b) AFAMrelative-stiffness images of Si with n-octyldimethylchlorosilane SAM.

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Nanomechanics

stiffness measurements may be affected by a variablewater layer between the tip and sample, at least in somecases. By refining our data analysis methods to includethe effects of such a layer, apparent correlations betweenthe measured values of M and the ambient RH wereeliminated. This year we also developed a RH-controlledAFM chamber in order to systematically studythese effects. Samples for the experiments wereprovided by the Polymers Division and consistedof patterned self-assembled monolayers (SAM) ofn-octyldimethylchloro-silane on Si substrates.Through controlled ultraviolet-ozone exposure, therelative hydrophobic/hydrophilic nature of the SAMwas adjusted in different sample regions. Figure 1shows an AFM topography image and an AFAMrelative-stiffness image for a SAM/Si sample. TheSAM stripes are virtually invisible in the topographyimage, even at very high resolution (10 nm full scaleheight). However, the AFAM image clearly reveals thehydrophobic SAM stripes. The regions covered by theSAM appear more compliant (lower contact stiffness)due to AFAM’s sensitivity to local variations in thetip-sample adhesion. We are currently performingfurther experiments on similar samples in order toquantify how humidity and adhesion effects can bedistinguished from true mechanical-property variations.

measurements were done at relatively low static loads;the load was then successively increased up to severalmicronewtons to try to break and/or plastically deformthe tip. High-magnification SEM images were obtainedbefore and after each AFAM measurement. As can beseen in Figure 2, R increased with use, indicating tipwear. Values of R measured from the SEM imageswere compared to the values obtained from AFAMdata using a Hertzian contact-mechanics model.The AFAM values of R were consistently smaller thanthe SEM values. Further data analysis and additionalexperiments are planned to clarify this issue. Theknowledge gained in this way will help us to refineour understanding of AFAM contact mechanics,beyond the Hertz approximation, in order to improvemeasurement accuracy and repeatability.

In related work, we are performing finite-elementstudies of the AFM cantilever. The finite-element meshis based on actual cantilever dimensions from SEMimages and includes elastic anisotropy. The predictedfree-space resonant frequencies are in excellentagreement with those observed experimentally.Work is underway to predict the vibrational behaviorin contact. These results will allow us to refineour data analysis models and thus improvemeasurement accuracy.

The research described above involves eitherquantitative single-point measurements, or qualitativeimaging of relative stiffness. In FY04, we worked torealize our ultimate goal of quantitative nanomechanicalmapping. Critical to our success is a new frequencytracking circuit that can determine the contact-resonancefrequencies at each image pixel. The circuit is based ondigital signal processing architecture that enables rapiddata acquisition (typically < 30 min. for a 256 × 256image). We have begun to obtain resonance-frequencyimages for a variety of materials. Recent enhancementsto our AFM mean that we can acquire images from notonly the flexural modes, but also the torsional modes ofthe cantilever. By combining information from flexuraland torsional images, it may be possible to determinesimultaneously both Young’s modulus and Poisson’sratio for an isotropic material. Work in upcomingmonths will focus on issues related to quantitativeimage interpretation such as calibration procedures,cantilever selection, tip wear, and choice ofcontact-mechanics model. Each of these elementsplays a role in attaining our goal of truly quantitativenanomechanical imaging.

For More Information on this Topic

D.C. Hurley (Materials Reliability Division, NIST)

Figure 2: SEM images of AFM cantilever tip a) before use andb) after repeated AFAM contact experiments. The circled regionsindicate the tip wear that occurred through use.

AFAM contact-resonance frequencies depend notonly on elastic properties, but also on the value of thetip radius R. Thus knowledge of R and how it changesover time is essential for accurate measurements ofelastic properties. To address this issue, we performedAFAM experiments with different AFM cantileverson a sample with known elastic properties. The first

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Nanomechanics

Myriad industrial and biological systems arecomposed of small-scale structures for which themechanical behavior is not accurately known.Optimizing the performance and reliability ofthese systems requires either mechanical propertymeasurements on specimens of these structuresharvested from the appropriate phases orinterfaces of the system, or the ability to test thesestructures in situ. We are developing standardizedtesting configurations and methodologies forlocalized measurements of strength and fracturetoughness of materials and interfaces at themicro- to nanometer-length scale.

Edwin R. Fuller, Jr. and George D. Quinn

This project aims to: (1) measure mechanicalproperties of microstructures for myriad industrial

and biological systems that cannot be fabricated in bulksamples; (2) study small-scale phenomena that maybe controlled by surface effects, e.g., the influence ofsurface stresses on crack nucleation and extension; and(3) obtain quantitative mechanical property data of materialsand interfaces for designing small-scale structures andcomponents and for assessing their mechanical reliability.To address these goals, well-characterized testingconfigurations must be developed for small-scalemeasurements of strength and crack extension. We arepursuing four tasks: (1) configuration design and finiteelement analysis; (2) specimen fabrication; (3) mechanicaltesting and fracture analysis (fractography); and (4) lengthand force metrology. Work in the Ceramics Divisionthis year has focused on the first and third areas. Twocollaborations were established in the fabrication task:one with James A. Beall of the Quantum ElectricalMetrology Division (817) in NIST Boulder, and one withNorthwestern University. Work in the fourth task will

Mechanical Metrology for Small-Scale Structures

come in subsequent years, most likely in collaborationwith the Manufacturing Engineering Laboratory.Significant progress has been made in the design of acompressively loaded test configuration with a well-defined, tensile gage section. Such a specimen can beloaded using a depth-sensing nanoindenter as a universaltesting machine, thereby giving a record of both appliedload and load-point displacement. One of these specimens,fabricated by James Beall from a silicon wafer bydeep reactive ion etching, is shown in the right-side ofFigure 1. The configuration is similar to a theta specimen,except that the geometry is hexagonal. When a load(per unit thickness) is applied to the top beam, a uniformuniaxial tensile stress results in the middle gage section.Finite element analysis gives (horizontal) gage sectionstresses on the order of 1.25 GPa for 50 mN/µm of appliedload. For a 2 N applied load, these 100 mm-thickspecimens generate 500 MPa of tensile stress in the gagesection. The left-side of Figure 1 shows a reconstructedfailed specimen. The insert shows the fracture surfaceof the gage section, and the two [111] cleavage facetsthat were formed. Alternate geometries, including around theta specimen, are also being considered.

Figure 1: Prototype specimen design.

To extend this technique to a wide variety ofmaterials and systems, general fabrication proceduresneed to be developed. Towards this objective, focused-ion-beam (FIB) milling is being explored in collaborationwith Northwestern University. Figure 2 shows our firstattempt at producing a hexagonal theta specimen byFIBing. It has been scaled-down by about a factorof 10, and is fabricated from a lamellar directionallysolidified eutectic of Ni0.5Co0.5O and ZrO2.

Contributors and Collaborators

D. Xiang, D.T. Smith (Ceramics Division, NIST);A. Jillavenkatesa (Standards Services Division, NIST);J.A. Beall (Quantum Electrical Metrology Division,NIST); N. Alem, V.P. Dravid (Northwestern University)

Figure 2: Hexagonal theta specimen by FIBing.

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Nanomechanics

Nanoindentation has rapidly become the method ofchoice for quantitative determination of mechanicalproperties of thin films and small volumes of material,but standardization efforts have lagged farbehind the application of the technique in industrialresearch and development. Over 1000 commercialnanoindentation instruments from a variety ofmanufacturers are currently in use, with no traceableforce calibration and an extremely limited choice ofstandardized test methods. We work with standardsgroups and National Measurement Institutes aroundthe world to develop standard reference materials(SRMs), traceable calibration transducers, androbust, reliable methods for obtaining and analyzingnanoindentation data, so that nanoindentationresults can be used with confidence in product designand specification.

Douglas T. Smith

Nanoindentation is the most commonly used methodfor determining hardness and elastic properties of

small volumes of materials. In this technique, a diamondindenter is pushed into a specimen surface, and the forceon and displacement into the surface are recorded. Thetechnique is capable of providing information on the elasticand plastic deformation of a specimen for indentations asshallow as 5 nm to 10 nm, and requires minimal specimenpreparation effort. It is routinely used to measure themechanical properties of thin films. However, thereare only a very limited number of accepted test methodsavailable, and no means to traceably calibrate or verifythe performance of nanoindentation instruments.This leads to large interlaboratory variations in results,particularly for hard, high modulus materials, and preventsthe use of nanoindentation in thin film or coatingproduct specifications.

In many mechanical test methods, includingnanoindentation, a force is applied to a specimen, andsome displacement is measured. Traceable displacementmeasurement by interferometry is well established. Forcemeasurement is more problematic, however, because theSI unit for force is still based on an artifact kilogram mass.The Microforce Competence Program at NIST hasdeveloped a primary realization of force, traceable toelectronic and length SI units, for force calibration inthe range 1 mN to 10 nN. As part of this program,transfer force cells are being developed that will allowforce calibration, traceable to NIST, for commercialnanomechanical test equipment such as nanoindentationmachines and atomic force microscopes. One such

Figure 1: The error in the force applied to a specimen by acommercial nanoindentation instrument, as referenced to atraceably calibrated NIST force cell.

Nanoindentation Methods and Standards

transfer force cell was calibrated against the NIST primaryforce balance to an uncertainty less than 0.5 % for forcesin the range 0.05 mN to 5.0 mN. That cell was thenmounted, as if it were a specimen, in a popular commercialnanoindentation instrument that had been recentlycalibrated by its manufacturer using their recommendedforce calibration procedure. The figure shows the errorin the nanoindentation force recorded by the instrument,relative to the traceable force cell reading. For forcesabove 2 mN, the recorded force is within the 1 %uncertainty required by most draft standards fornanoindentation machines. However, for lower appliedforce, the error increases dramatically, and at 0.05 mN,the recorded force is almost 18 % below the actualapplied force.

We are also working with both ASTM (E28.06.11)and ISO (TC 164/SC 3) on a wide range of standardtest methods for nanoindentation, and an ISO documentfor the one method has now been approved (ISO14577). In addition, the Ceramics and MaterialsReliability Divisions at NIST are working with theBundesanstalt für Materialforschung und–prüfung(BAM) in Germany to develop joint thin film SRMs(CRMs, or Certified Reference Materials, in Europe)for use in nanoindentation machine verification.


D. Xiang, B. Hockey, G. Quinn (Ceramics Division,NIST); R. Machado (INMETRO, Brazil); J. Pratt(Manufacturing Engineering Laboratory, NIST);D. Hurley (Materials Reliability Division, NIST);U. Beck (BAM, Germany)

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Nanomechanics

Accurate adhesion and friction measurementsat the nanoscale are emerging as critical issuesin device industries and nanotechnology.Measurements characterizing nanometer-thickmolecular assemblies and surface textures tocontrol surface properties and to ensure reliabilityand durability are also needed. Working withdevice and magnetic storage industries and otheragencies, we have designed and built instrumentsto meet these needs. These instruments are housedin NIST’s new Advanced Measurement Laboratory.The facility features vibration isolation, class 1000clean room standards, and temperature control.

Stephen M. Hsu

The nanotribology and surface properties projectwas initiated as a part of the MSEL Nanotechnology

Initiative. The project aims to develop measurementcapability for adhesion, friction, and surface forcesat the nanoscale. We interact extensively with manyindustries and research centers in the U.S. and aroundthe world to promote advances in measurementscience and seek consensus standard harmonizationin three areas: nanocontacts, nanolubrication, andsurface texturing.

NanocontactsAdhesion and friction measurements at nanocontacts

require accurate detection of normal and lateral forcesat nanonewton levels. Scanning probe microscopesuse sensitive cantilevers and laser diodes to detect theseforces, but this approach introduces rotation of the tipsand crosstalk among the xyz planes. To understandthe influence of surface forces, many of which arefunctions of contact area, attention must be focusedon determining the real area of contact.

To resolve some of the instrumental challenges,we have engaged instrument makers, such asHysitron, Veeco, and others, to solicit their inputs andcollaborations. At the same time, we have establishedour own capability in tip fabrication, cantilever springconstant calibration, and tip characterization. Threenew instruments were successfully installed as a resultof these collaborations: the NIST–Hysitron multiscalefriction tester, the NIST nanoadhesion apparatus, andthe first prototype of an interferometer microscopefrom Veeco. We are continuing to work with ourpartners to develop next-generation instruments assuggested by the Nanometrology Grand ChallengesWorkshop (NIST, January 2004).

Nanotribology and Surface Properties

The overall objective of the nanocontacts activityis to develop the constitutive equation of adhesionand friction including the various componentsof surface forces. With our new adhesion andnanofriction apparatus, we are quantifying the effectsof plowing and electrostatic charge on measurements.We continue to work with our external academicpartners (UC Berkeley, UC Davis, and Ohio State U)under the Nanotechnology Extramural Initiative todevelop friction measurement via three approaches:friction measuring MEMS devices, AFM methods,and ultrahigh vacuum friction measurement. Theseefforts have been successful, and we have gainedconsiderable understanding of how meniscus forcesand electrostatic forces can exert significant effects onnanofriction measurements. A NIST special publicationsummarizing these findings is under preparation.

NanolubricationMolecular assemblies can be organized to impart

hydrophobicity, anti-adhesion, and friction controlcharacteristics on device surfaces. Supported by otheragencies and the magnetic storage industry, this activityfocuses on how to measure the effects of controlledcomposition and spacing on properties of nanometer-thick films. An ultrahigh vacuum scanning tunnelingmicroscope-atomic force microscope was installed inMay 2004 to provide imaging capability at the molecularlevel. A micro-tribometer was also developed to measurethe durability of these films. The synchrotron facilityoperated by the Division’s Characterization MethodsGroup at Brookhaven National Laboratory continues tobe vital in characterizing these complex molecular mixtures.

Surface TexturingSurface texture increasingly is being considered as

a tool to control surface energy, polarity, adhesion, andfriction. Supported by other agencies and industries,we are pioneering the use of specific surface featuressuch as dimples, triangles, and ellipses at micro-and nanoscale dimensions to supplement molecularassemblies to control surface properties of surfaces.An international cooperative study under the auspicesof International Energy Agency (IEA) is underway.


C. Ying, R. Gates, J. Chuang, J. Larson–Basse,L. Ives, D. Fischer (Ceramics Division, NIST); B. Bhushan(Ohio State U.); K. Komoupoulas (UC Berkeley);G. Liu (UC Davis); Y.T. Hsia (Seagate); J. Sengers(U. of Maryland); O. Warren (Hysitron); C. Su (Veeco)

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Nanomechanics

Knowledge of mechanical behavior is criticalwhen designing for device performance andreliability, even for “non-mechanical” systems.However, nanoscale mechanical behavior(including failure) is inherently difficult to measureaccurately, and existing modeling tools are onlyqualitative at best. We are developing modelingtechniques that provide quantitative predictionsand are validating these results experimentally.

Lyle E. Levine and Anne M. Chaka (838)

Mechanics at the nanoscale is inherently difficult tomodel accurately. Finite element modeling (FEM)

can effectively capture the elastic behavior of macroscopicstructures but includes no accurate failure criteriasince this depends upon atomic-scale behavior.Classical atomistic simulations can handle enough atoms(millions to billions) to model such events, but thesepotentials become inaccurate for large strains and theycannot effectively handle chemistry. Quantum-mechanics-based simulations using density functional theory (DFT)are extremely accurate and handle the chemistry exactly,but such simulations are so CPU intensive that theycan handle only a few hundred atoms. A combinationof all three modeling techniques is required toaccurately model device behavior at the nanoscale.

the predicted elastic displacement fields to generateboundary conditions and initial atom positions foran atomistic simulation using classical potentials.

The use of classical potentials in a large simulation cellallows the correct propagation of the long-range stressesto the critical regions where bond distortions are large orwhere chemistry effects need to be explored. In thesecritical regions, the techniques embed a DFT simulation.Iteratively, the critical region is relaxed using DFT, andthe classical cell is relaxed using a Monte-Carlo algorithm.The first application of this hybrid technique was todetermine the vacancy formation energy in aluminum asa function of distance (at a fixed angle) away from anedge dislocation. The simulation geometry is shown inFigure 2. The boundary conditions and initial atomicpositions were calculated from the known elasticdisplacement field of an edge dislocation.

Nanomechanics: Coupling Modeling with Experiments

Figure 1: FEM model of a rigid 100 nm diameter sphereindenting an Al sample to a depth of 10 nm.

Over the past year, we have developed techniquesto handle such multiscale modeling for quasistaticapplications. At the macroscale, FEM is used to simulatethe elastic behavior of a nanomechanical system. Figure 1shows an example in which an Al sample is being indentedby a rigid 100 nm diameter sphere. The indenter andthree of the sample quadrants have been removed tohighlight the resulting Von Mises stress distribution afterindenting 10 nm. The FEM mesh is fine enough to use

Figure 2: Diagram of a hybrid simulation for obtaining thevacancy formation energy at different positions relative to anedge dislocation. The large box represents the EAM cell inwhich the DFT region containing the vacancy was embedded.

After an initial dislocation nucleation event,nanoindentation progresses through the complexevolution of dislocation structures. For example, theraised lip around the indent is produced by largenumbers of dislocations exiting the surface. We areworking on modeling the early stages of this processusing 3D dislocation dynamics and assuming a randomdistribution of dislocation sources in the sample.

Finally, connection to experimental measurementsrequires careful force calibration of the indenter andcalibrated atomic force microscopy measurements (AFM)of the indenter tip. These calibrations are mostly complete,and the AFM data will be used to generate a FEM meshfor the simulations. Bulk single-crystal copper samplesare being cut and polished using non-contact chemicalmethods to minimize the dislocation density.


S.M. Khan, G. Levi, L. Ma, F. Tavazza (MetallurgyDivision, NIST); B. Hockey, R. Machado, D. Smith,R. Wagner, D. Xiang (Ceramics Division, NIST)

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Nanomechanics

Nanofrictional forces are an important factorin the design of nanodevices and assemblies ofnanoparticles. Literature data on nanofriction,however, exhibit many discrepancies andhave a wide range of scatter. In a series ofwell-controlled experiments, we have discoveredthat a significant portion of this discrepancy mayactually be attributed to an unintended scratchingof the surface by the sharp tips used in makingthe measurements themselves. Deviation of tipshape from the “ideal” spherical shape is anothercontributor to the disagreements found among theliterature data. This work sheds important insightinto the nature of nanoscale friction measurement.

Z. Charles Ying and Stephen M. Hsu

Accurate measurement of nanofriction is emerging as one of the critical issues in nanotechnology.

Manipulation of molecules, clusters, and nanoparticlesacross surfaces to form a functional entity relies on theability to overcome the resistance of the particles tomotion across the surface. Further, reliable operationof micro- and nano-devices often depends on anaccurate estimate of lateral loading. Literature dataon nanofriction, however, have shown wide rangesof values depending on the length scale of the device.Silicon friction, e.g., shows an order of magnitudevariation from 0.03 to 0.25. Such results have led tospeculations that there might be an intrinsic scalingeffect such that different friction levels would occuracross the nano-, micro-, and macro-scales. If proventrue, well established friction laws would be invalidatedfor nanomaterials.

Given the broad technological significance of thisissue, we have undertaken a detailed investigation ofthe nature of this discrepancy. Friction measurementsbetween non-adhering surfaces were carried out in bothelastic and plastic deformation regions. Diamond tipsof various sizes were used to measure nanofrictionforces on well-characterized substrates such as singlecrystal silicon, silicon dioxide, and calcium fluoride.Experiments were performed using a specially designedmultiscale friction tester developed jointly by NIST andHysitron Incorporated based on a capacitance probeforce transducer. In each experiment, the environmentwas carefully controlled, and the tip penetration into thesubstrate during sliding was continuously measured.

The size and shape of each tip were characterizedusing a newly developed “replica technique.” Tipswere imprinted into a soft material, and the imprintswere scanned by AFM and digitally inverted to

Friction Scaling Artifact

Figure 1: 3D profiles, obtained using the replica technique, of thetips used in the friction measurements.

“reconstruct” the tip in three-dimensional details.Figure 1 shows images of two of the diamond tipsused in the experiments. The tip shown in Figure 1(a)is spherical in shape with a 1.2 µm tip radius whileFigure 1(b) shows an ellipsoidal shape with nominalradius of 4 µm. The detailed dimensional data arecrucial in determining the apparent contact areas andisolating the influence of surface forces.

Figure 2: Coefficient of friction between Si (100) surface and adiamond tip with radius R = 1.2 µm, as a function of mechanicalloading force.

Figure 2 shows the coefficient of friction (COF)between a Si (100) surface and a diamond tip with tipradius R = 1.2 µm.

9

Nanomechanics

path. Numerical integration of actual tip size and shapeover the sliding path is required. For the hypothesis of“elastic plowing,” the calculation shows that the frictionforce in this regime should be proportional to the projectedcross-sectional area in the direction of sliding. Figure 5confirms this hypothesis for all three tips.

Figure 3: Image of the silicon surface after frictionmeasurements, showing scratches at 2.5 mN and higher load.

Figure 4: Coefficient of friction between Si (100) surface andthree diamond tips used in the experiments, as a function of meancontact pressure.

Figure 5: Friction force as a function of projected cross-sectionarea in the direction of tip sliding.

Two distinct regions can be seen. Initially, the COFis constant up to a load of 2.0 mN. For loads above2.5 mN, the COF increases with load. This frictiontransition is contrary to Amoton’s law. Subsequentinvestigation revealed that this change in COF is causedby unintended plowing of the tip into the substrate.Atomic force microscopy, using a sharp tip on thesurface, suggests that the COF increase coincides withthe first appearance of plastic grooving. In the caseof silicon, a groove 5 Å deep and 2000 Å wide wasdetected on the surface at a load of 2.5 mN (Figure 3).

This observation was repeated for different tips anddifferent substrates. At higher loads, deeper and widergrooves were observed. These results suggested thatunintended scratching of the surfaces by the sharp tipsused in the friction measurements contributed to thediscrepancies among friction data in the literature.

The initiation of plastic grooving can be quantified interms of contact pressure. By determining the apparentcontact area using the digitally inverted tip method, thetransition contact pressures could be calculated for allof the observed cases. All the results corresponded to12 GPa which is approximately the hardness of silicon.

Elastic PlowingFigure 4 shows the friction data for three tips in the

elastic region (no plastic grooving). The data for the twospherical tips with radii R = 0.5 µm and 1.2 µm exhibit acommon value for the COF. However, a higher frictionvalue was observed for the 4 µm radius tip.

To explain this discrepancy, we tested the variouscontact mechanics models which require the accuratedetermination of the real contact areas. Considering a tipplowing through an elastic substrate, the real area ofcontact is the projected cross-section area of the tip inthe direction of sliding. This can be determined from thetip geometry and “penetration depths” across the sliding

This work shows clearly that nanoscale friction, inpractice, is a complex result of both elastic and inelasticstrains and deformations, and accurate nanofrictionmeasurement will require careful accounting of all theforces contributing to the phenomenon. The methodologyestablished in this work provides a substantial step towardsthe data and understanding needed by industry.


Z.C. Ying, S.M. Hsu (Ceramics Division, NIST)

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Nanomechanics

Brillouin light scattering is being employed toprovide information on acoustic and magneticwaves at gigahertz frequencies in thin-filmmaterials. Measurements and modeling in FY04have focused primarily on interactions of spinwaves in ferromagnetic films, elastic constants ofnanoporous low-κ dielectrics, and propagationof acoustic waves in nanopatterned polymers.

Ward Johnson, Sudook Kim, andColm Flannery

Technical Description

Brillouin light scattering (BLS) is an experimentaltechnique that measures the intensity of spectral

components of light that is inelastically scatteredby vibrational waves (phonons) or spin waves(magnons) in a material. Fabry–Perot interferometrictechniques are used to acquire accumulated spectrathrough repeated mechanical sweeping of theetalon spacing.

BLS is the only laboratory technique that iscurrently available for detecting magnons of finitewavenumber. Because of this capability and becauseof innovations in interferometric techniques, BLS hasbeen increasingly applied to problems in magneticsover the past couple of decades.

For characterization of phonons in thin films,BLS offers several advantages over other techniques.Since it detects phonons in the gigahertz range, elasticconstants can be determined in films of submicronthickness without the complication of vibrationalenergy penetrating significantly into the substrate.Modes localized in submicron structures can becharacterized. Spatial variations in elastic propertiesand vibrational patterns can be measured with a lateralresolution equal to the size of the laser focal spot onthe specimen (typically, ~50 micrometers).

In this division, techniques are being developedfor characterizing interactions of magnons in metallicferromagnetic thin films. Such interactions limitthe speed of magnetic-storage devices, spin-valvesensors, and other thin-film magnetic devices.

BLS measurements and modeling of vibrationalmodes are being pursued in several thin-film systems,including nanoporous low-dielectric-constant (low-κ)dielectrics, nanopatterned polymers, and arrays ofmolecular rotors.

Metrology for Nanoscale Properties: Brillouin Light Scattering

AccomplishmentsDuring FY04, we developed and applied methods

for measuring magnons that arise directly or indirectlyfrom microwave pumping of metallic ferromagneticfilms in a static magnetic field (Figure 1). BLSspectra were studied as a function of scatteringangle, microwave power, and laser power to provideinformation on magnon–magnon interactions inNi81Fe19. The results demonstrate, for the first time,detection of nonzero-wavevector magnons arisingfrom the decay of pumped uniform-precessionmagnons in metallic thin films.

Young’s modulus and Poisson’s ratio innanoporous low-κ dielectric films were measuredusing conventional backscattering techniques. Thisresearch is described elsewhere in this annual report.

Measurements and modeling of vibrational modes innanoimprinted polymers were pursued in collaborationwith the University of Akron and the NIST PolymersDivision. In addition to bulk and surface modes,low-frequency modes were detected and found tobe consistent with vibrational localization in theimprinted lines. This result suggests the possibilityof characterizing elastic properties on a scale oftens of nanometers.


P. Kabos (Radio-Frequency Technology Division,NIST); S. Russek (Magnetic Technology Division,NIST); A. Slavin (Oakland Univ.); J. Michl (Univ.of Colorado, Chemistry Dept.); R. Horansky (Univ. ofColorado, Physics Dept.); C. Soles (Polymers Division,NIST); R.H. Hartschuh, A. Kisliuk, and A. Sokolov(Univ. of Akron)

Figure 1: Scattering of light off waves in a thin film.

11

Nanomechanics

In the continuum model, we simulate free surfacesand interfaces by applying the usual boundary conditions.

AccomplishmentsWe have carried out a detailed analysis of SAW

measurements in a thin TiN film on Si, calculatedstresses in a wide range of solids, and modeled Geclusters in Si containing a free surface and a vacancy.The work is reported in six papers. A library of GFsis available at www.ctcms.nist.gov/gf, which containsdownloadable teaching and tutorial material onGFs and computer codes for calculation of GFs.A GF group of about 50 international users has beenformed at CTCMS.


D. Hurley, D.T. Read (Materials ReliabilityDivision, NIST); L. Bartlolo (Kent State University);A. Powell (M.I.T.)

Precise knowledge of atomistic configurationand strains in nanomaterials is needed for theircharacterization and development of new devices.We have developed computationally efficientGreen’s-function methods for calculation of latticedistortion, elastic strains and displacements ina variety of material systems. Our work is usefulfor interpretation of data obtained by AFAM,nanoindentation, and SAW experiments.

Vinod K. Tewary and Bo Yang

Technical Description

Green’s function (GF) provides a computationallyefficient tool for interpretation of data obtained by

nanoindentation and atomic force acoustic microscopy(AFAM) which is useful for elastic characterization ofnanomaterials. The dynamic GF is used for modelingpropagation of surface acoustic waves (SAWs) insolids and their phonon properties. The GF accountsfor interfaces and free surfaces in the solid and itselastic anisotropy, which play relatively important rolesin the elastic response of nanomaterials as comparedto ordinary solids. We use GF to calculate the elasticdisplacement fields, interaction energy between theembedded nanostructure and other defects and freesurfaces. The displacement field, due to a force atthe surface, is used for interpretation of AFAM andnanoindentation measurements. The interactionenergy of the nanostructures is an important factorin the stability and growth of the nanostructure.

We have developed GF methods both at the discreteatomistic and macro continuum scales. For a latticemodel containing N atoms, the GF is given by:

G = ΦΦΦΦΦ–1

where G and ΦΦΦΦΦ are 3N × 3N matrices of GF andforce constants respectively. The matrix inversionis carried out by using the discrete Fourier transformfor a perfect lattice or numerically by computersimulation for a model crystallite as in moleculardynamics.

In order to model an embedded nanostructurein a host solid, or a thin film on a substrate, wedefine an initial GF for the host and the final GF.The final GF is related to the initial GF through theDyson equation.

Physical Properties of Thin Films and Nanostructures:Green’s-Function Methods

Figure 1: Stress distribution in Si due to a surface load.

Figure 2: Interaction energy of a cluster of 147 Ge atomswith a vacancy and free surface in Si.

12

Nanomechanics

Axisymmetric adhesion tests promise a powerfulmeans to quantify adhesive performance withthe simultaneous ability to visualize debondingmechanisms. However, industrial adoption ofthese techniques has been slow, since traditionalaxisymmetric testing equipment does not allowrapid assessment of product performanceover a large parameter space. In response,we are developing instruments that permithigh-throughput (and high value) measurementsof adhesion across combinatorial libraries.

Aaron M. Forster and Seung-ho Moon

Adhesion is governed by surface interactions and the mechanical properties of the adhesive.

Axisymmetric adhesion (ASA) measurements, such asJKR[1] methods and “probe tack” tests, provide powerfulmeans for quantifying these contributions to adhesion,while providing visual insight into the mechanisms bywhich adhesives fail. However, current ASA instrumentsare geared towards serial testing of single specimens.This “one-at-a-time” paradigm is incompatible withindustry research, which increasingly requires methodsto rapidly and thoroughly measure large numbers ofadhesive formulations. Our objective is to produceASA measurement instruments that operate in ahigh-throughput manner, while retaining the high-valuedata these methods produce. These tools enable therapid identification of structure–property relationshipscritical for adhesive performance, thereby assistingindustry in developing new adhesive formulations.This report describes two of the high-throughput ASAinstruments that have been built to address these issues.

The JKR method employs a hemispherical lens thatis pressed into a specimen. Tracking the lens/specimencontact area versus load or lens displacement yieldsthe work of adhesion. Our instrument, the MultilensCombinatorial Adhesion Test (MCAT) employs an array

Combinatorial Adhesion and Mechanical Properties:Axisymmetric Adhesion Testing

Figure 1: a) Contact area map of MCAT lens array. Each circlerepresents data for a separate measurement. b) Contact area vs.displacement data for a single lens array element. The red line isthe fit to JKR theory, which yields the work of adhesion.

of microlenses to conduct multiple JKR-type adhesiontests in parallel. By simultaneously tracking thecontact radii of each element of the lens array, upto 400 measurements of the work of adhesion are collectedin the time required for a single traditional JKR test(see Figure 1). When MCAT is used in conjunctionwith gradient combinatorial specimen libraries, eachlens measures a different adhesive system. We recentlybenchmarked MCAT measurements of the work ofadhesion between glass and silicone against single lensJKR tests, with excellent agreement.

Figure 2: a) Schematic of CPT adhesion test instrument.b) Adhesion energy (W) of a model PSA vs. temperature. Inset:Image of probe/sample contact shows debonding of the PSA.

The adhesion of viscoelastic materials, such as pressuresensitive adhesives (PSAs), is not described well byJKR-type tests. Probe tack tests provide quantitativeadhesion measurements for viscoelastic systems withsimultaneous visualization of debonding events. Wedeveloped a new combinatorial probe tack (CPT) testfor the measurement of PSA formulation performance(Figure 2). A key feature of the instrument, a gradienthot-stage, enables rapid assessment of temperature effects.Temperature is an important processing parameter andenvironmental factor for PSAs. Our CPT testing of PSAs(Figure 2) rapidly provides quantitative adhesion energydata and debonding images as a function of temperature.The device design significantly reduces experimentaluncertainties associated with fabrication of multiplesamples, and reduces the overall measurement timesignificantly, while maintaining high-quality, rich datasets.We are currently refining the CPT instrument and testingits use towards a variety of applications such as UV-curedPSAs and epoxy systems.

Reference1. K.L. Johnson, K. Kendall, and A.D. Roberts,

Proceedings of the Royal Society of London A:Materials 324, 301 (1971).

Contributors and CollaboratorsA. Chiche, C.M. Stafford, W.L. Wu, W. Zhang

(Polymers Division, NIST)

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Nanomechanics

Combinatorial Adhesion and Mechanical Properties:Innovative Approaches to Peel Tests

Figure 1: Master adhesion map delineating critical surfaceenergy conditions for adhesion vs. a reduced time-temperaturefunction, at .

as a function of substrate surface energy, annealing time,and annealing temperature (see Figure 1). This definesan operating window of temperature, time, and surfaceenergy to ensure proper adhesion of a thin polymer filmto the material of interest.

Peel testing is the primary tool industry uses togauge adhesive performance, but current peeltest methods involve “one-at-a-time” analysis ofsingle specimens. Our research aims to accelerateadhesive performance testing by developingdevices and measurement strategies that meldthe peel test construct with combinatorial andhigh-throughput (C&HT) methods.

Christopher M. Stafford and Martin Y.M. Chiang

As part of the mission of the NIST Combinatorial Methods Center (NCMC), we are developing

C&HT peel-test methods that enable rapid assessmentof adhesion within the large parameter space associatedwith adhesive formulation and processing. This reportdescribes two milestones we have met in this endeavor.

Our first achievement involves a NIST-developedexperiment design for measuring adhesion of a polymerlayer to an underlying substrate in a combinatorial manner.This test method utilizes the peel-test geometry toinvestigate the interfacial failure between a thin polymerfilm and a silicon substrate as a function of annealing timeand temperature, as well as the substrate surface energy.Combinatorial libraries are generated by using the existingNCMC gradient toolkits: a polymer film is coated ontoa substrate containing a surface energy gradient, andthis sample is subsequently exposed to an orthogonaltemperature gradient.[1] To further extend the parameterspace available in this study, we incorporated annealingtime as a third dimension. This requires fabricatingmultiple identical samples and successively annealing eachsample for longer times. By applying the Williams–Landel–Ferry (WLF) time-temperature superposition, we canconstruct a master curve (failure map) detailing thetransition from adhesion (bonded) to failure (debonded)

Figure 2: a) Force vs. distance curve for a typical peel test; andb) distribution curve of the force data.

Our second achievement addresses a challengein applying the peel test to combinatorial specimens:the lack of ample statistical information that is thefoundation of this type of measurement. For example,a conventional peel test conducted under constantconditions results in a fluctuating force to be averaged.Applying a continuous gradient of sample propertiesor test conditions in the peel direction implies that eachdata point (force) corresponds to a given test condition,thus prohibiting the average force to be calculatedfor a given condition. To address this issue, we havedeveloped a simple statistical treatment that allowsa relationship between the uncertainty of the forceand the domain size to be established. This treatmentultimately will dictate the number of data points requiredto obtain acceptable uncertainties in the measurement.

These studies demonstrate how combinatorialapproaches can be applied to characterize adhesionusing a peel test blueprint. In doing so, we havedesigned a new statistical tool to assist in defining thegradient step size (discrete gradients) or gradientsteepness (continuous gradients) that allows amplestatistical information to be obtained.

Reference1. For details, see A. Seghal, et al., Microscopy Today

11 (6), 26–29 (2003).

Contributors and CollaboratorsA. Chiche, R. Song, A. Karim (Polymers Division,

NIST); J. Filliben (Statistical Engineering Division, NIST)

14


Reactions occurring at interfaces during thehigh-temperature processing of complex electronicmaterials involve diverse chemical compositions.To optimize the design and control of theseinterfaces, we are developing a genericthermodynamic/kinetic model for interfacialinteractions. The model includes modificationof bulk reaction and mass transport parametersto accommodate nanoscale observations.

Lawrence P. Cook, Winnie Wong-Ng,and Igor Levin

State-of-the-art electronic devices contain complexmaterials with chemical constituents ranging widely

over the periodic table; furthermore, new materials arecontinually being added. Many electronic packagingmaterials consist of metals, ceramics, and semiconductors,with bi-phasic interfaces of several types. Processingtemperatures may extend up to 900 ºC where diffusionand reaction can be significant. Two principalproblems arise in the treatment of such interactions:1) the application of equilibrium thermodynamic datato the non-equilibrium growth of interfacial reactionzones, and 2) the transition from micro-scale (bulkdominated), to nanoscale (surface dominated) phenomena.To address these issues, we have instituted a combinedthermodynamic/kinetic approach to investigate the modelsystem Ag-Bi2O3-Nb2O5-O, as outlined in Figure 1.

Thermochemistry and Metrology of Interfacial Interactions

Progress includes determination of a preliminaryphase diagram at 850 ºC, and development of aprovisional thermodynamic model. Subsequentconstruction of a chemical potential diagram hasallowed us to predict alternative diffusion and reactionpaths. Observations on Ag/BiNb5O14 reaction couples(Figure 2) confirm the relatively high thermodynamicmobility of Ag. Thermogravimetric data on theinterfacial reaction require more than a simple parabolicrate law, due to the rapid initial spread of Ag alongreaction site surfaces. By comparison with particulateinterfacial systems, a two-step reaction model isproposed, for which rate constants can be obtained.

Data for the model system are of direct interestto the electronics community because Ag is animportant metallization component in many dielectricceramics. It is anticipated that continued workon Ag-Bi2O3-Nb2O5-O will lead to the goal of acomprehensive thermodynamic/kinetic model.The model will be tested, iteratively refined, andextended to other systems.


M.D. Vaudin, P.K. Schenck, T. Vanderah, M. Green(Ceramics Division, NIST); W. Luecke (MetallurgyDivision, NIST); C. Randall, M. Lanagan (Center forDielectric Studies, Pennsylvania State University)

Figure 2: Ag Lα X-ray map of Ag/BiNb5O14 reaction zone(≈ 80 µm wide) produced by annealing in air for 10 h at 850 ºC,with an applied uniaxial compression of ≈ 1 MPa.

Figure 1: Flow chart for development of interfacial model basedon Ag-Bi2O3-Nb2O5-O.

15


Chemistry and Structure of Nanomaterials

Successful nanoscale materials fabrication isempowered by a detailed knowledge of the chemistryand structure of surface bound molecules; e.g.,the optimization of self-assembled monolayers,molecular templates, micro-electro-mechanicalsystem lubricants, and functionalized nanotubes.Near-Edge X-ray Absorption Fine Structure(NEXAFS) spectroscopy is ideally suited to measurenon-destructively chemical bond concentration,rehybrization, and orientation with sub-monolayermolecular sensitivity in diverse nanoscale materials.Furthermore, NEXAFS can distinguish chemicalbonding in the light elements, measure the orientationof interfacial molecules, and separately measuresurface versus bulk chemistry simultaneously.

Daniel A. Fischer

Materials having low energy surfaces are used inmany applications, for example, in non-wetting

surfaces or fouling resistant marine coatings. We haveproduced a photo-responsive polymer surface bycombining the reversible photo-switching nature ofazobenzene with the self-assembly nature and lowsurface energy properties of semi-fluorinated segments,to create a fluoroazobenzene molecule surface.Upon UV exposure, this surface reorients betweenhydrophobic and less hydrophobic states, as shownin Figure 1 (left upper and lower panels). For suchsurfaces, one could imagine applications ranging fromlow cost surface patterning to polymer surfaces thatwould adsorb biological macromolecules on cue.

Figure 1: UV light reorients fluorobenzene semi-fluorinatedsegments (green) downwards, i.e., to a less hydrophobic state.

We have utilized NEXAFS to observe, verify, andquantify the reversible cis-trans molecular conformationtransformation from hydrophobic to less hydrophobicstates. The right panels of Figure 1 (upper and lower)show polarization dependent NEXAFS anisotropybehavior of the C–F and C–C peaks which reverse within situ UV light exposure highlighting the reorientationof the semi-fluorinated segments.

Figure 2: Carbon NEXAFS of oxidized/functionalized nanotubes.

Figure 3: Model of peroxide functionalization of nanotubes.

Application of NEXAFS spectroscopy to the study ofelectronic structure and chemical composition is illustratedin Figure 2 for various chemically functionalized, single-walled, carbon nanotubes. Upon peroxide functionalization,the C=C ring resonance is greatly diminished on extensivesidewall functionalization indicating loss of extendedconjugation and disruption of nanotube electronicstructure. The C= O peak intensity is greatest for peroxidechemistry. NEXAFS spectroscopy supports a modelof peroxide funtionalization, shown in Figure 3.


S. Samabasivan (Ceramics Division, NIST);X. Li, C.K. Ober (Cornell); A. Hexemer, E.J. Kramer(UCSB); S. Banerjee, T. Benny, S.S. Wong (SUSB);J.A. Misewich (BNL)

16

Metrology for Nanoscale Properties: X-ray Methods

Texture has a large influence on many propertiesof thin films. XRD was the primary method for thecharacterization of texture for many years. Recently,alternative local techniques have been developed tobring the measurements to the nanoscale. This year,we compared measurements taken with conventionalXRD and EBSD on two thin films: sputtered aluminumand electroplated copper. Both films had a thicknessof about 1 µm and were grown on [100] siliconwafers. The inverse pole figures for both samples andtechniques are shown in Figure 1. It is obvious thatthe agreement between XRD and EBSD in case ofaluminum film is very good, whereas it is rather poorfor copper film. This is a consequence of differentpenetration depths of electrons and x-rays; while thealuminum sample is homogeneous through the filmthickness, copper film is not. After sample preparation,electroplated copper is likely to undergo self-annealingat room temperature. This well-known phenomenonyields a difference in grain size and orientationbetween the surface and the bulk of the sample.


Adriana Lita, Jens Müller, Roy H. Geiss,Dave T. Read, M. Kopycinska–Müller, Robert Keller(NIST); Laurna Kaatz, Amitendra Chaudhuri(University of Denver)

Figure 1: EBSD (left) and XRD (right) inverse pole figures ofaluminum (top) and copper (bottom) thin films.

of the {321} lattice spacings (sin2ψ method) yielded thetotal residual stress at ambient temperature. The valuesobtained are about 0.7 GPa and higher, where the bulkof the stress is due to the intrinsic (growth) component.


Macroscopic properties of technologicallyinteresting materials originate from theirunderlying micro-structure. To design andunderstand improved materials, it is necessaryto characterize the microstructure and correlateits changes to the macroscopic properties ofinterest. We especially focus on x-ray diffractionstudies of biomedical, ferroelectric, optoelectronic,photovoltaic, semi-conducting, and other materialsrelevant to the health and microelectronicsindustries. In particular, studies of microstructuralproperties, such as strain and stress, crystallinedefects, and texture, complement the informationobtained by other techniques.

Thomas A. Siewert and Davor Balzar

In this report, we focus on two topics: the studies ofsuperconducting tungsten thin films and comparison

of the thin-film texture, as obtained by Electron BackScatter Diffraction (EBSD) and x-ray diffraction(XRD). Tungsten superconducting transition-edgesensors (TES) are used in different astrophysics andastronomy applications, and as single-photon detectorsfor the quantum computing experiments. Suchdetectors comprise thin tungsten films and operate attemperatures close to absolute zero and to the criticalsuperconducting transition temperature (Tc) oftungsten. Thin films, in general, contain both high-temperature α-W phase (with Tc = 15 mK) and β-W(A15 cubic structure with Tc = 1–4 K). Close to theTc, very small thermal changes, such as the absorptionof a single photon, increase the film electricalresistance. The result is the production of an electricaloutput signal that corresponds directly to the detectionof the absorbed photon energy. In general, residualstress can influence the Tc of the thin film both directlyand by inducing the β→α phase transformation. Thus,in order to control the Tc, it is necessary to measureand control both the phase composition and residualstresses. Residual stresses in films depend on thepreparation conditions, the thickness of the film, andthe difference in thermal expansion coefficient betweenthe film and the substrate. The total residual stressresults from both the intrinsic (growth) stress andthermal stress. We measured the stress by XRDat both ambient and cryogenic temperatures.The sample was held in a continuous flow cryostatthat was capable of achieving temperatures as lowas 8 K. The cryostat was mounted on a goniometerto enable the angle-dispersive XRD measurements.The shift of the α-W {110} Bragg reflection was usedto estimate stress at 8 K, and directional measurement

17


Several materials are now produced commerciallywith particle sizes on the order of 10–100 nm.The availability of ceramic materials in this sizeclassification is particularly important for theelectronic component industry. Nanopowderscan enable significant decreases in internal layerthickness (for multilayer devices), minimize thesize and prevalence of processing defects, andboost the overall performance of componentsranging from capacitors to resistive gas sensors.Quantifying these performance enhancementsand their dependence on microstructure is a keyobjective in the Materials Reliability Division.

Stephanie A. Hooker

Many multilayer ceramic devices can benefit fromthe use of ultra-fine powders, including passive

electronic components, gas sensors, and miniatureactuators. However, fabrication issues associatedwith processing these high-surface-area powdershave hindered their commercialization. Key challengeshave included nanopowder reproducibility, optimizationof processing slurries, and control of sinteringprofiles to produce the desired micro/nanostructurecharacteristics. Moreover, in many cases, predictingthe optimum microstructure features for maximumperformance is quite difficult, and extensiveexperimentation remains necessary.

One specific component that can benefit from theuse of nanopowders is the multilayer piezoelectricactuator. These devices generate precise, controlledmotion for translating optics, damping vibrations,redirecting air flow, and dispensing fluids. Eachcomponent consists of many active layers, thethickness of which determines the device’s powerrequirements. Layer thickness is typically on theorder of several hundred microns for conventionalactuators and is driven by the need for many individualpiezoelectric grains to span the active layer, therebyreducing the chance for electrical failure. Reducinggrain sizes from micrometer to submicrometer scalesenables dramatic decreases in layer thickness, reducingpower needs and increasing applicability.

However, at issue is the piezoelectric response and,in particular, the long-term fatigue resistance. Fatigue isthe change in polarization or displacement over time andis a recognized problem for state-of-the-art actuators.Because processing-induced defects tend to be on thescale of the starting powder, nanopowders can improvemechanical durability. However, a corresponding

Physical Properties of Thin Films and Nanostructures:Grain Size Effects on Actuator Fatigue

decrease in ferroelectric response is also anticipatedas grain size reduces. Fatigue is affected by boththe mechanical and electrical responses, making itsprediction quite difficult for fine-grained components.

In FY04, we investigated the fatigue behavior of14 microstructures obtained by controlled sinteringof 80-nm PZT-5A powders. The devices tested were1206-sized (0.30 cm × 0.15 cm, or 0.12´́ × 0.06´́ )chips, each with 10 active layers (50 µm thick).Characterization included dielectric, impedance, andferroelectric properties, as well as short- (< 1 millioncycles) and long-term (1–100 million cycles) degradationunder combined electrical and mechanical influences.

Figure 1: Effect of sintering temperature on ferroelectric fatigue.

The results clearly demonstrate the dependence offatigue on microstructure. Figure 1 compares changesin remanent (i.e., permanent) polarization over timewhen devices were cycled using a 35 Hz sinusoidalwave with an amplitude of +/–75 V. The averagedecrease in polarization for the finest-grained materials(sintered at 1175 °C) was less than 20 % after0.5 million cycles, compared to nearly 50 % forlarger-grained components.

In FY05, we will expand on this work and examinethe corresponding effects on displacement and domainreorientation for devices with even thinner internallayers. The resulting data will then be used to validatethe suitability of these components for biomedical andsmart structures applications, both of which demandhigh performance and long operational lifetimes.


R. Geiss (Materials Reliability Division, NIST);C. Kostelecky, D. Deininger, K. Womer (SynkeraTechnologies)

18

Nanoporous low-dielectric-constant (low-κ) filmspresent considerable implementation challenges forthe microelectronics industry and also raise newscientific questions. We apply optical methods tothese films to evaluate such critical properties asdensity/porosity, Young’s modulus, and Poisson’sratio. Our techniques allow measurement ofproperties that are otherwise difficult to determineand shed light on how the mechanical propertiesdepend on porosity.

Colm Flannery and Donna C. Hurley

Current miniaturization trends in microelectronicsrequire faster device switching. This need can be at

least partially met by lowering the resistance-capacitancefactor of the dielectric materials. One promising solutionis to introduce nanometer-sized pores that reduce thedielectric constant κ. Unfortunately, introduction ofporosity may lead to a drastic reduction in stiffness(Young’s modulus), adversely affecting the material’schances of surviving the fabrication process. In additionto predicting and ensuring process reliability, accuratevalues of a film’s mechanical properties are neededto model the mechanical behavior of the resultingmicroelectronics device. New tools are needed tocharacterize these relevant thin-film properties(e.g., Young’s modulus and Poisson’s ratio ν), aswell as to better define their dependence on porosity.

In FY04, we evaluated Poisson’s ratio ν inmethylsilsesquioxane (MSSQ) polymer films of varyingporosity with Brillouin light scattering. In this technique,a Fabry–Perot interferometer detects frequency-shiftedphotons scattered by ambient thermal phonons inthe material. The frequency shift of the photons ischaracteristic of the acoustic phonon modes in the film.Values of ν were determined from measurements of bothlongitudinal and surface acoustic wave modes. Figure 1reveals that ν decreases as porosity increases. The results

Physical Properties of Thin Films and Nanostructures:Nanoporous Low-κ Dielectric Films

Figure 1: Measured Poisson’s ratio vs. porosity for MSSQ films.

Figure 2: Predicted dependence of Poisson’s ratio on porosity.

of an existing finite-element model, shown in Figure 2,predict that ν will either decrease or increase dependingon the initial ν of the matrix material. In addition, ν tendstowards a constant value of 0.2 for all materials at highporosity. Our measurements are consistent with themodel. However, they show a much larger rate of decreasein ν, probably because a percolation threshold hadbeen reached. Quantitative results like these will provevaluable for modeling of structures involving low-κfilms. The results also yield insight into the porositydependence of ν, about which very little is known.

A second method to determine thin-film propertiesinvolves the frequency-dependent dispersion of laser-generated surface acoustic waves (SAWs). In FY04,efforts concentrated mainly on the development of astandard SAW data analysis procedure. With carefulsignal processing that minimizes sensitivity to noise andmaximizes the frequency range of the measured signals,we can extend the frequency range of our measurementsby 20 %. The result is a significant reduction in theuncertainty of the extracted film properties. The methodlends itself to automatation and has allowed us to inspectfilms less than 300 nm thick. (Previously, films less than500 nm thick were challenging.)

The improved measurement procedure, combinedwith new multilayer Green’s function analysis software,has allowed us to inspect multilayer structures.With these capabilities, we have extracted propertiesof thin (50–100 nm) capping layers of stiffer materialson top of more compliant functional films. In addition,the Young’s modulus, density, and thickness ofthe underlying dielectric films were evaluated.


S. Kim, V. Tewary (Materials Reliability Division,NIST); Y. Liu (International Sematech); J. Wetzel(Tokyo Electron America)


19


Electron microscopy is used to characterize thestructure and composition of materials at thenanometer scale to better understand and improvetheir properties. New measurement techniques inelectron microscopy are being developed andapplied to materials science research. The MSELElectron Microscopy Facility primarily serves theMetallurgy, Ceramics, and Polymers Divisions aswell as other NIST staff and outside collaborativeresearch efforts.

John E. Bonevich

Atomic-scale structure and compositional characterization of materials can lend crucial

insights to the control of their properties. For instance,direct observation of local structures by transmissionelectron microscopy (TEM) provides importantfeedback to the optimization of crystal growth andprocessing techniques. Various characteristics may beobserved such as crystal structure and orientation, grainsize and morphology, defects such as stacking faults,twins, grain boundaries and second phase particles.Structure, composition and internal defect structures,as well as the atomic structure of surfaces andinterfaces, can provide powerful knowledge forengineering and understanding materials.

The MSEL Electron Microscopy Facility consists oftwo transmission electron microscopes, three scanningelectron microscopes, a specimen preparation laboratory,and an image analysis/computational laboratory. TheJEM3010 TEM can resolve atomic structures and employsan energy selecting imaging filter (IF) and X-ray detector(EDS) for analytical characterization of thin foil specimens.The S-4700-II FE-SEM employs electron backscattereddiffraction/phase identification (EBSD) and EDS systemsto characterize the crystallographic texture andcomposition of materials.

Nanoscale Characterization by Electron Microscopy

Figure 1: MgO cubes with surfaces decorated by small goldparticles. These particle ensembles are used in model studies of3D chemical imaging at the nanoscale (electron tomography).

Figure 2: Confined Si single electron transistor device.

Highlights from the EM Facility for FY2004 include:■ A new, high-sensitivity EBSD CCD camera

which acquires in excess of 70 patterns/second,was installed on the FE-SEM.

■ 3D Chemical Imaging at the Nanoscale, a collaborativeproject with CSTL and PL on tomographiccharacterization of materials, was initiated (Figure 1).

■ Research collaboration with the SemiconductorElectronics and Electricity Divisions (EEEL) hascharacterized quantum effects in confined Si devices(Figure 2).

■ Sub-100 nm Ag interconnects formed bysuperconformal electrochemical deposition wascharacterized.


D. Josell, T. Moffat, L. Bendersky (MetallurgyDivision, NIST); I. Levin, B. Hockey (Ceramics Division,NIST); J.H.J. Scott (Surfaces & Microanalysis ScienceDivision, NIST); Z. Levine (Optoelectronics Division,NIST); E. Vogel (Semiconductor Electronics Division,NIST)

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Engineering of nanomaterials, biomaterials,and nano-electromechanical systems hinges ontechniques for imaging complex nano-structures.In this respect, new Scanned Probe Microscopy(SPM) methods promise nano-scale mapping ofchemical, mechanical, and electro-optical properties,but these techniques generally only offer qualitativeinformation. Through a suite of reference specimensfabricated with a combinatorial design, we aim tocalibrate image data from emerging SPM methods,thereby advancing these nanometrology tools.

Michael J. Fasolka and Duangrut Julthongpiput

Recent years have seen the development of a newgeneration of SPM techniques, which intend to

measure chemical, mechanical, and electro–opticalproperties on the nanoscale. However, contrast in newSPM images is difficult to quantify since probe fabricationcan be inconsistent and probe/sample interactions are notunderstood. Our research at the NIST CombinatorialMethods Center (NCMC) aims to provide a suite ofreference specimens for the quantification of nextgeneration SPM data. By design, our specimens will gaugethe quality of custom-made SPM probes, calibrate SPMimage contrast through “traditional” surface measurements(e.g., spectroscopy, contact angle) and provide informationfor understanding complex probe/sample interactions. Ourspecimens are produced with bench-top microfabricationroutes and combinatorial gradient methods developedby the NCMC. Here, combinatorial methods are keysince they enable the fabrication of specimens that varyproperties that govern SPM image contrast in a systematic,independent manner. Moreover, as opposed to traditionalreference specimens, combinatorial samples provide notone, but a multitude of calibration conditions.

Figure 1 illustrates principles of our specimen designthrough a specific case useful for quantifying chemicallysensitive SPM techniques such as friction-force SPM,

Gradient Reference Specimens for AdvancedScanned Probe Microscopy

Figure 1: Schematic illustration of our gradient referencespecimen for chemically sensitive SPM techniques.

Figure 2: Preliminary calibration curve relating friction force SPMimage contrast to differences in surface energy (γ), as determinedfrom a single gradient reference specimen. The minimum contrastpoint (red arrow) illuminates the sensitivity of the probe.

or Chemical Force Microscopy, which employs acustom-made probe. The crux of this specimen isa “gradient micropattern” (∇ -µp): a series of micron-scale lines that continuously change in their chemicalproperties (e.g., surface energy) compared to a constantmatrix. Two “calibration fields” adjacent to the ∇ -µpdirectly reflect the chemistry of the lines and the matrix.Thus, traditional measurements (e.g., contact angle)along the calibration fields (1) gauge local chemicaldifferences in the ∇ -µp and thereby (2) calibratecontrast in SPM images acquired along the ∇ -µp.

Figure 2 demonstrates use of this specimen forcalibrating friction force SPM image contrast. Wefabricate this specimen via microcontact printing of achlorosilane self-assembled monolayer (SAM) on a SiO2matrix. The chemical gradient is achieved via a gradedUV-ozonolysis of the SAM. The plot abscissa gives thedifference in friction force (contrast) between the linesand matrix for SPM images collected along the ∇ -µp.The ordinate expresses the corresponding surface energy(γ) data (from contact angle measurements) collectedalong the calibration fields. Thus, from a single specimenwe create a comprehensive calibration curve that relatesSPM friction force to differences in surface energy.Moreover, the plot neatly illuminates the smallestγ difference sensed by the probe (red arrow), which isuseful for gauging the quality of custom-made probes.

Currently, we are refining this reference specimendesign, and we are developing similar designs for otheradvanced SPM methods.


K. Beers (Polymers Division, NIST);D. Hurley (Materials Reliability Division, NIST);T. Nguyen (Materials and Construction ResearchDivision, BFRL); S. Magonov (Veeco/DigitalInstruments)

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Polyelectrolytes differ in chain dynamics andequilibrium structure from neutral polymers due tolong-range electrostatic interactions. This manifestsinto strongly interacting solutions for both syntheticand biopolymers, as observed by associativebehavior and multi-mode relaxations. Originsof these relaxations require new experimentalmethods probing the local structure and dynamics.

Vivek M. Prabhu and Eric J. Amis

Polymeric templates are found throughout naturemost commonly in the nucleic acid base-pairing

of DNA. This special hydrogen bond templatehas currently been explored for design of novelnanostructured materials. New routes to self-assembledor template-assisted assembly using externally suppliedfields such as electric, flow, magnetic, and patternedsubstrates must overcome Brownian motion as well asstructural correlations between and among molecules.For polyelectrolytes, such as DNA, the long-rangedelectrostatic interactions also contribute to the solutionstructure in addition to the local and directional hydrogenbond associations. The distribution and dynamics ofthe counterion species about a polyelectrolyte serve asa starting point to understand the interactions governingcontrolled assembly and complement the often-studiedpolymer structure point-of-view.

We have established, using neutron scatteringexperiments performed at the NIST Center forNeutron Research (NCNR), the remarkable ability forcounterions to essentially conform to linear flexiblepolyelectrolytes. This was accomplished by directmeasure of the counterion partial static and dynamicstructure factor highlighting the coupled polymer andcounterion association for model synthetic materialsshown below.

Characterization of Counterion Association withPolyelectrolytes: Novel Flexible Template Behavior

An example of the counterion correlations is shownby the SANS structural peak in Figure 1. The peakillustrates the counterion correlations mediated bythe chain. By tuning the range of the electrostaticinteractions with added NaCl, an “invisible” salt,the influence is two-fold: (1) the electrostatics arescreened, and the solution returns to neutral-likebehavior; and (2) at higher salt concentrations, theNa+ displace the visible h-TMA+ counterions as shownin the accompanying schematic. Although the polymertemplate is no longer observed, its influence is stillobserved with a “visible” salt.

The counterion dynamics are length scaledependent and slow down near the correlation peak.This behavior illustrates that the polyion-counterionmotions are coupled at the nanoscale. Hence, therole of counterions also serves as a design criteria inassembling structures. Unique labeling has provideda new viewpoint of charged polymer solution structureand dynamics. This project aims to characterize therole of charged polymer topology with branchedmaterials, networks, and gels on the coupled dynamicsto assess the parameters governing template-assistedassembly. Modern simulation and theoretical insightwill assist in understanding these challenges fornanoscale assembly.


D. Bossev, N. Rosov (NIST Center for NeutronResearch)

Small-angle neutron scattering (SANS) was used tocharacterize the equilibrium structure, while neutronspin echo (NSE) spectroscopy was used to examine thedynamics occurring at length scales between (60 and 3)nm and time scales between 45 ps to 100 ns.

Figure 1: “Visible” organic counterions associate with the negativelycharged chain at low salt and are displaced by added “invisible”sodium ions for fixed polymer concentration of 24.9 gL–1.

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The magnetic data storage industry is seekingto sustain the 30-year trend of exponentiallyincreasing storage density. This must be achievedat the same time as maintaining the stability of therecorded data. The Metallurgy Division at NISTis collaborating with Seagate Technology todevelop the processes required to producepatterned media of high-magnetic coercivitythat will meet this challenge.

Jonathan J. Mallett, Thomas P. Moffat, andWilliam F. Egelhoff, Jr.

Conventional magnetic data storage media are basedon granular films in which many fine grains are

used to define each magnetic bit. The minimumnumber of grains per bit is limited by the irregularityof the shape and spacing of the grains, which producesirregular boundaries between bits. This irregularityresults in noise in the read signal and can only beconstrained at an acceptable level by maintaining afixed minimum number of grains per bit. The naturalapproach to increasing data storage density has beento reduce the grain size, while maintaining the samenumber of grains per bit. The limit to this approachoccurs when the magnetic energy barrier to the reversalof magnetization of each grain approaches the energy ofrandom thermal fluctuations. At this point, the mediumis no longer stable against spontaneous magnetizationreversal and consequential data loss.

The grain size at which this limit occurs dependson the magnetocrystalline anisotropy energy of the

Grand Challenges in Nanomagnetics: High Coercivity FePtAlloys for Future Perpendicular Magnetic Data Storage

magnetic material. FePt in its L10 phase has asufficiently large anisotropy energy to allowmagnetically stable grains of 5 nm diameter (Figure 1).Furthermore, FePt deposited by a variety of meanstypically forms grains approaching this size.Unfortunately, the annealing treatment required totransform the as-deposited A1-structured FePt tothe desirable L10 phase is widely found to resultin an increase in grain size to 100 nm. Currentefforts at NIST focus on developing a method ofelectrochemically depositing FePt into a regularpatterned template, circumventing the problem of graingrowth. Simultaneously, the imposed regularity of themagnetic cells opens the possibility of addressing singlecells (i.e., of using one “grain” per bit). The estimatedachievable density from such an approach is 7 Tbitsper square inch, which is 100 times higher than currentstorage densities. Electrodeposition is an obviouschoice for deposition into high aspect ratio templates,since it does not suffer from the shadowing effectthat characterizes vacuum deposition techniques.

Figure 2: The double cell, designed to minimize the concentrationof Fe3+ and dissolved O2 in the FePt plating solution.

Figure 1: The dependence of storage density on magnetocrystallineanisotropy energy. The most promising materials are FePtand CoPt.

Electrodeposition of FePt from aqueous solutionspresents many challenges. The double cell shown inFigure 2 has been developed to address the problem ofthe instability of Fe2+ solutions, which readily oxidizeto produce insoluble Fe(OH)3. The plating solution,containing FeCl2 and PtCl4 (left) is separated from theanode by a cation selective membrane, preventing theoxidation of Fe2+ to Fe3+ that would otherwise occurat the anode. Fe3+ formation by oxidation with air isalso minimized by blanketing the cell in nitrogen, andresidual Fe3+ is reduced to Fe2+ by an auxiliaryplatinum grid electrode.

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The alloy composition of 300 nm thick filmsdeposited from this solution was found to dependmainly on the applied potential and to be insensitiveto the concentration of the solution components.A thermodynamic regular solution model was usedto describe the dependence of film composition onapplied potential. The comparison of theory toexperimental data can be seen in Figure 3.

The deposited films were found to be remarkablysmooth, with RMS roughness values less than 5 nmfor micron-thick deposits. This was surprising giventhe large platinum overpotential (supersaturation),which usually results in growth instabilities andconsequent roughening.

The control of crystal orientation is an essentialconsideration for high-density recording applications.A transition in the recording industry is currently inprogress to perpendicular media, in which the bits aremagnetized perpendicular to the plane of the medium.Maximum advantage can be derived from L10 FePt as aperpendicular medium when it is correctly aligned withits magnetic easy-axis perpendicular to the substrateplane. A careful choice of substrate and annealingparameters is required to recrystallize the as-depositedrandom fcc alloy to appropriately oriented L10.

Figure 3: The theoretical and experimental dependence ofcomposition on the applied potential.

FePt electrodeposited onto a Cu (001) substratehas recently shown great promise. X-ray diffractionrevealed the transition to L10 upon annealing, and X-raypole figures indicated favorable orientations. Figure 4shows FePt L10 (001) and (110) pole figures. The(001) figure indicates perpendicular orientation of themagnetic easy-axis (the c-axis), while the (110) figureindicates an in-plane texture. The in-plane texture is anadded advantage, as it results in a narrower switchingfield distribution.

Copper additions to the alloy have been found tolower the A1/L10 phase transition temperature by upto 90 oC. It may be speculated that interdiffusion ofcopper from the substrate during the anneal allowed therecrystallization to proceed from the interface with thesubstrate. It is likely that this would allow the film toreplicate the orientation of the substrate.

Figure 4: X-ray pole figures showing favorably oriented FePt L10 .

Figure 5: Magnetic hysteresis measurements showing a 10 kOecoercivity.

Magnetic hysteresis measurements performedusing a Kerr magnetometer are shown in Figure 5.The magnetic coercivity of 10 kOe is comparable tofigures quoted in the literature for vacuum-depositedFePt. It is believed to be the highest value reportedfor an electrodeposited film.

Current efforts focus on controlling theinterdiffusion between substrate and film to minimizeloss of magnetization in the alloy, and on movingfrom planar films to through-mask deposited arraysof FePt pillars.


W.F. Egelhoff, Jr. (Metallurgy Division, NIST);E.B. Svedberg (Seagate Research, Pittsburgh)

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Conductors in on-chip metallizations are nowreaching dimensions so small that defectiveseed layers are impacting manufacturing yields,and electron scattering on surfaces and grainboundaries is reducing electrical transport inthe buried wires. Our goal is to provide toolsto overcome these barriers. Recent efforts havequantified sources of the increased resistivity inwires made of silver, the most conductive of allmetals, demonstrated seedless processing routesfor copper wires, and improved understandingand modeling of the superfill fabrication process.

Daniel Josell and Thomas P. Moffat

The steady reduction of transistor dimensions inintegrated circuits has been accompanied by similar

size reductions of the on-chip interconnects that carryelectrical signals, pushing the industrial technology forcopper seed deposition close to its limit for defect-freesidewall coverage. Defects in seed layers, whicharise from limitations in existing sputter technology,lead to voiding during electrodeposition of thecopper metallization.

Additionally, with the dimensions of the copperwires in these metallizations now approaching theintrinsic mean-free-path length of the conductionelectrons, scattering on the wire surfaces has begunto significantly reduce electron transport and, thus,the associated electrical conductance of the wires.With grains in these conductors similarly sized, areduction is also to be expected from grain boundaryscattering. While the penalty for both effects isincreased power dissipation and reduced clock speed,the appropriate approach for mitigation requiresquantitative determination of the relative sizes ofthe contributions.

On-Chip Interconnects:Extending Performance of Sub-100 nm Lines

Technical DetailsA tri-layer titanium, palladium and silver seed

(Figure 1) was shown to yield smooth, conductivesurfaces for the electrodeposition of silver wires forthe electrical properties study. The poor seed coveragethat is visible toward the bottom of the smallesttrench (Figure 1c) is a technical challenge noted in theInternational Technology Roadmap for Semiconductors.Such seed defects motivated the “seedless” rutheniumbarrier-based process detailed in last year’s report andcontinued by this year’s demonstration of seedlesscopper superfill in trenches with ruthenium or iridiumbarriers (Figure 2) deposited by perfectly conformalatomic layer deposition (ALD).

Figure 1: Ti/Pd/Ag seed layer in sub-100 nm deep trenches.

Figure 2: Trenches containing copper that was electrodepositeddirectly on an ALD iridium barrier (thin bright layer).

Figure 3: Cross-section views of silver wires fabricated byelectrodeposition in the seeded trenches followed by removal of themetal from the field (transmission electron microscope).

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Figure 4: Electrical resistivities of 300 nm tall silver wirescompared to predictions that account for surface scattering withvarying amounts of grain boundary scattering. Resistivities forwires less than 100 nm wide are impacted by defects.

For the electrical studies, silver wires (Figure 3) werefabricated by silver electrodeposition on the tri-layer seeds(Figure 1), using a superfill process developed in theMetallurgy Division, followed by removal of the metalin the field adjacent to the wires through chemical–mechanical planarization and ion polishing.

The wires were studied in a standard four-point probegeometry that permitted measurement of wire resistances.Resistivities, obtained from the resistances using measuredwire dimensions, increased significantly with decreasingwire width (Figure 4). To assess the origin of the resistivityincrease, the Fuchs–Sondheimer analysis for diffusescattering of electrons on surfaces was extended topermit analysis in the presence of both specular anddiffuse scattering. The resulting equations, along witha previously published equation for grain boundaryscattering, permitted quantitative evaluation of theexperimental data. Significantly, modeling of the datashowed that the increase of resistivity with decreaseof wire size arises as much from scattering on grainboundaries as from scattering on the wire surfaces.This indicates that efforts to mitigate size effects mustincrease grain size as well as surface specularity ifthey expect to be more than modestly successful.

The continuing industrial need for predictivesimulation of feature filling spurred experiment andmodeling of the superfill process itself. Consumptionof adsorbed accelerator, a detrimental deviation fromthe surface segregation behavior that is responsible forthe superfill process, was measured. Inclusion of suchconsumption in our Curvature Enhanced AcceleratorCoverage (CEAC) model and computer code have madethe filling predictions even more accurate.

This research has continued to impact industry,indicated by requests for our superfill code from Intel,

Applied Materials, ST Microelectronics, and ATMI;an invited article on superfill in the IBM Journal ofResearch and Development; and invited presentationsgiven at Cookson–Enthone, in addition to publicationsand presentations at conferences.

Selected Project Publicationsfor FY2004

T.P. Moffat, D. Wheeler, M. Edelstein and D. Josell,“Superconformal Film Growth: Mechanism andQuantification,” IBM J. Res. and Dev., in press.

D. Josell, C. Burkhard, Y. Li, Y.-W. Cheng, R.R. Keller,C.A. Witt, D. Kelley, J.E. Bonevich, B.C. Baker,T.P. Moffat, “Electrical Properties of SuperfilledSub-Micrometer Silver Metallizations,” J. Appl. Phys.96 (1), 759–768, (2004).

D. Wheeler, T.P. Moffat, G.B. McFadden, S. Corielland D. Josell, “Influence of Catalytic Surfactanton Roughness Evolution During Film Growth,”J. Electrochem. Soc. 151 (8), C538–C544, (2004).

T.P. Moffat, D. Wheeler, and D. Josell,“Electrodeposition of Copper in the SPS-PEG-Cl AdditiveSystem: I. Kinetic Measurements: Influence of SPS,”J. Electrochem. Soc. 151 (4), C262–C271, (2004).

W.J. Evans, D.G. Giarikos, D. Josell, and J.W. Ziller,“Synthesis and Structure of Polymeric Networks ofSilver Hexafluoroacetylacetonate Complexes of THF,Toluene, and Vinyltrimethylsilane,” Inorg. Chem. 42,8255–8261, (2003).

For More Information on this TopicD. Josell, T.P. Moffat (Metallurgy Division, NIST);

G. McFadden (Mathematical and Computational SciencesDivision, NIST); R.R. Keller, Y.-W. Cheng (MaterialsReliability Division, NIST); C. Witt (Cookson–Enthone);C. Burkhard, Y. Li (Clarkson University); D. Wheeler(University of Maryland); T.K. Aaltonen, M. Ritala,M. Leskelä (University of Helsinki, Finland)

Figure 5: Simulations of superconformal feature filling nowaccount for consumption (incorporation) of adsorbed catalyst.

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As technology migrates toward smaller physicaldimensions, new analytical approaches arerequired to characterize material properties andto investigate critical issues. Our primary focus isthe application of metrology and the developmentof new methods and standards for measuring thephysical and surface properties of nanostructuredparticle systems. Applications include functionalmaterials and devices for catalysis, powergeneration, and microelectronic, pharmaceutical,and biotechnology industries.

Vince Hackley

Nanocrystalline oxides of alkaline-earth cationsproduced by a supercritical drying process exhibit

unique and highly reactive surface chemistries. As aresult, these materials have been studied extensively asdestructive adsorbents, catalysts, and bioactive agents.Investigations were concluded regarding the role ofcation size on the evolution of microstructure in thesematerials and on the dispersion properties in aqueousNaCl solution. A series of small-angle neutronscattering (SANS) experiments were performed atthe NIST reactor in collaboration with researchersat Kansas State University. These results indicate acomplex picture for structure formation during thedrying and annealing processes, with string-like gelmorphology giving way to fractally rough particulateassemblies of compacted nanocrystals.

Particle Metrology and Nanoassembly

Figure 1: Maximum entropy fit to SANS data for annealedMgO aerogel.

Data were analyzed using the Unified Model ofBeaucage and the maximum entropy method (seeFigure 1). The scale of the finest structural featuresincreases with increasing cation size for the annealedproduct. Analysis indicates the absence of mass fractalstructures but the presence of surface fractal-likeobjects. A broad correlation peak in the data forheat-treated Mg and Ca oxides gives evidence forsome local ordering of the nanocrystals. This datawill help provide a more complete understanding ofthe structural development in this complex andtechnologically important system.

Figure 2: Schematic diagram of capillary flow cell.

A capillary flow-cell (Figure 2) was developed andcommissioned for the ultra-small-angle x-ray scattering(USAXS) instrument on the UNICAT beam line at theAdvanced Photon Source. This new capability willpermit in situ investigations of complex multiphaseparticulate systems under controlled flow conditions.Initial applications include the dispersion of single-wallcarbon nanotubes in collaboration with Rice Universityand depletion effects in binary colloidal suspensions.


A. Allen, L. Lum (Ceramics Division, NIST);D. Ho (NIST Center for Neutron Research);K. Klabunde (Kansas State University); M. Pasquali(Rice University); P. Jemian (University of Illinois)

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Novel structures and devices can be formedthrough template electrodeposition. The ultimatepattern resolution is determined by the size andpacking of molecules that comprise the templateand their ability to inhibit or catalyze variouselectrochemical reactions. In the past year, theeffect of molecular functionality on the metaldeposition process has been explored.

Thomas P. Moffat and Michael J. Fasolka

Over the last decade, electrochemical processing has been undergoing a renaissance with the

fabrication of new materials and novel microstructures.The development of new measurement techniques andmetrological tools for studying controlled growth atfine length scales is the primary objective of this effort.

2. The role of molecular functionality of the surfactantand robust design rules; and

3. The anisotropy induced in the electrocrystallizationreaction rate for a given surfactant.

As an example, an alkanethiol monolayer film caneither block or accelerate metal deposition dependingon the molecular functionality of its terminal group.A dynamic range spanning several orders of magnitudeis possible depending on the system in question (Figures1 and 2). Based on these measurements, selective metaldeposition may be obtained by patterning a substratewith the appropriate molecule.

Nanostructure Fabrication Processes:Patterned Electrodeposition by Surfactant-Mediated Growth

Figure 1: Influence of small changes in molecular structure,i.e., SO3

– vs. CH3, on the rate of copper deposition.

Figure 3: Spatially patterned monolayer films used to controlnucleation and anisotropic growth of copper crystals.

Through contact printing, higher resolution patterningis possible, along with some interesting opportunities forhigh-throughput combinatorial research. By varying thesurface chemistry, growth of isotropic or faceted crystalshas been shown to be possible (Figure 3).


D. Josell, J. Mallett, W.F. Egelhoff (MetallurgyDivision, NIST); M. Walker, L. Richter (CSTL, NIST)

Figure 2: Copper deposition on an alknethiol derivatized goldsubstrate only occurs in regions bearing –SO3

– terminatedmolecules.

The three key metrology issues addressed are:1. The rate differentiation accessible via surfactant

mediated metal film growth;

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The microelectronics community useselectrodeposition to produce solderable surfacefinishes, magnetic recording media, and copperinterconnections in printed circuit boards andintegrated circuits. These films tend to developsizable mechanical stresses that can lead to loss ofadhesion and the generation of bulk and surfacedefects. This project focuses on the measurementof these stresses which should enable thedevelopment of effective mitigation strategies.

Gery R. Stafford and Ole Kongstein

Electrodeposited films tend to develop sizablemechanical stresses during deposition due to the

nucleation and growth process or solution additives andalloying elements. We have established an optical benchdedicated to the in situ measurement of growth andresidual stress during electrodeposition using the wafercurvature method. In one approach, a substrate ofborosilicate glass is evaporated with 250 nm of gold.The force exerted on the cantilever by the electrodepositcauses curvature of the cantilever, which is monitoredduring electrodeposition. Forces on the order of0.03 N/m can be resolved during film deposition.

negative deposition potentials. As the deposit thickens,the average film stress decreases and becomescompressive in deposits formed at small depositionoverpotentials. In the physical vapor deposition literature,this compressive stress has been attributed to thenon-equilibrium concentration of mobile ad-atoms onthe surface that are driven into the grain boundaries.

Nanostructure Fabrication Processes:Thin Film Stress Measurements

Figure 1: Average film stress during copper deposition.

Figure 1 shows the average in-plane stress associatedwith the deposition of copper from an additive-free sulfateelectrolyte. The rapid rise in tensile stress (within thefirst 20 nm) and its dependence on deposition potentialare consistent with nuclei coalescence and grain boundaryformation. The highest tensile stresses are associatedwith high nucleation densities which are obtained at more

Figure 2: Force /width exerted on the cantilever during theelectrochemical processing of Sn on Cu.

We have also examined the force exerted onto a coppercantilever electrode during the deposition of matte tin (Sn),Figure 2. This is a particularly important system sincethe growth of tin whiskers, known to produce electricalshort circuits and device failure, has been attributed toboth the residual stress in the electrodeposit as well asthat generated from intermetallic formation at the Sn-Cuinterface. The force curves show the following features:a tensile to compressive transition during deposition, asignificant tensile relaxation when plating is discontinued,the development of tensile stress while the deposit is atopen circuit, and a residual tensile stress after the depositis electrochemically dissolved. The two latter featuresare attributed to the formation of the Cu6Sn5 intermetallic(IMC) at the Sn-Cu interface. A tensile stress is generatedsince the IMC has approximately 6 % smaller volume thana rule of mixture combination of the Sn and Cu reactants.Calculating the IMC thickness from the difference in Sndeposited and stripped, a nominal stress in the intermetallicwas estimated to be 1.2 GPa after only one hour. Futurework will focus on determining to what degree thisincreases the compressive stress in the Sn electrodeposit.


W. Boettinger, U. Bertocci (Metallurgy Division, NIST)

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Wet nanomanufacturing is the generation ofnovel nanostructures and the control of chemicalreactions (and signals) in a fluid environment;this requires effective techniques and efficientmaterial measurements. Therefore, we aim todevelop sophisticated fluid-handling devices fordirecting material assembly, controlling materialtransport, and measuring material properties.

Steven D. Hudson

Flow control is essential for fluid measurement andmanufacturing applications. The type and strength of

flow are often crucial for producing the desired structureand properties when assembling advanced materials. Usingrecent advances in microfluidic technology, we havedeveloped a miniature tool without moving parts thatmimics the function of a four-roll mill, a rheological andprocessing instrument.[1] Several microchannels convergeto the measurement zone, where micro-particle-imagevelocimetry was used to map the flow field. By adjustingthe relative flow rates, the full range of planar linear flowscould be produced. This includes simple shear, whichis inaccessible in the four-roll mill.

Wet Nanomanufacturing

Figure 1: Carbon nanotube clusters suspended in the microfluidictrap in extensional flow in the horizontal direction. The cluster atright has fractured from the main one. Also, an individual tubethat has broken from the cluster is visible at the far left (arrow).Individual tubes have also been trapped and examined in flow.

Most importantly, this new device works as a trap,so that particles and other objects can be examined inflow. Using this and related devices, clusters of carbonnanotubes suspended in fluids have been examined(Figure 1). In addition, extension and alignment ofindividual wormlike micelles has been measured as afunction of flow strength and duration.

To achieve certain flow characteristics for particularmeasurement applications, other microchannel geometrieshave been and are being developed. Specifically, adevice for measuring cell and tissue viability, andmechanical response, is being assembled. Also, a newinstrument system for measuring interfacial tensionhas been developed.

Although microchannels are effective for theseimportant functions, reaching a smaller scale efficiently,requires a new approach. Miniaturization towardsnanochannels is usually not practical, because itnecessitates very high pressures. At these smallerscales, membranes mediated by pores (as found inliving systems) are likely to be efficient means tocontrol the transport of material in metrological andnanomanufacturing processes.

Figure 2: Molecular model of the artificial membrane poreviewed from: a) the side, b) top, and c) in cross section.

One such membrane pore (Figure 2) assembledby supramolecular chemistry has been characterizedrecently.[2] Determination of Fourier components(amplitude and phase) by transmission electronmicroscopy demonstrated that the supramolecularassemblies are tubular, with an inner diameter of 1.4 nm.Proton conduction through these channels has beenmeasured to be comparable to the transmembraneprotein channel gramacidin. Directed assembly ofother artificial transmembrane channels will be explored.

References1. Hudson, et al., Applied Physics Letters 85, (2004).2. Percec, et al., Nature, in press (2004).


F. Phelan Jr., K. Migler, P. Start, P. Stone, J. Taboas,E. Hobbie, E. Amis, J. Cabral, K. Beers, J. Douglas,J. Pathak (Polymers Division, NIST); J. Kasianowicz(Biotechnology Division, NIST); V. Percec, D. Discher(University of Pennsylvania); R. Tuan (NIH)

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Second Joint Workshop on Measurement Issues in Single-WallCarbon Nanotubes: Purity and Dispersion, Part II

PresentationsThere were two Keynote Presentations. The first

was given by Dr. Robert Haddon, from the University ofCalifornia at Riverside, who presented the latest findingsregarding the use of Near IR Spectroscopy (NIR) as aprimary means of assigning a quantitative value to thepurity of a SWCNT-containing sample. Significantprogress has been made in the use of this technique sinceit was discussed at the first workshop. There was generalagreement that NIR may be a viable technique to quantifypurity, but there are several questions remaining, includingaffects of tube diameter and type of processing.

The second Keynote Address was given byMr. James Von Ehr, the founder and president of ZyvexCorporation, who focused on the potential commercialapplications for carbon nanotubes in a number of differentareas. He called attention to the need for measurementtechniques that would lead to improved qualificationprocedures for nanotubes from a given supplier.

The remainder of the first day was devoted to anumber of plenary presentations on the topic of puritymeasurement, followed by breakout sessions whose goalwas to identify the critical measurement issues relative tothe determination of nanotube purity. A poster sessionwith 25 posters was held in the evening to allow workshopparticipants to present their latest findings and to holdinformal discussions with their colleagues.

The primary topic on the second day of the workshopwas the measurement of nanotube dispersion. There werealso several presentations on the characterization of isolatednanotubes. The end of the second day consisted ofbreakout sessions devoted to dispersion measurements.

The following sections provide some details of theimportant issues relative to measurement of purity anddispersion as discussed during the meeting.

PurityThe purity of single-wall carbon nanotubes was

loosely defined by the attendees at the first workshopas the quantity of SWCNTs relative to other carbon-likematerials present (amorphous, graphitic, and C60carbons) as well as metal impurities. The point wasmade however that not all producers or users employedthis definition in a way that allows for a quantitativeassessment of the quality of a sample. A number ofmeasurement techniques used for purity determinationwere discussed with the general consensus that nosingle technique can describe the quality of a sample


The 2nd Joint Workshop on Measurement Issues inSingle-Wall Carbon Nanotubes, organized by theNational Aeronautics and Space Administration,Lyndon B. Johnson Space Center (NASA-JSC) andthe National Institute of Standards and Technology(NIST), was held January 26–28, 2005, at NISTin Gaithersburg, MD. In attendance were over80 participants, representing private corporations,universities, and government laboratories. Theprimary purpose of the workshop was to bringtogether technical and business leaders in thefield of single-wall carbon nanotubes (SWCNTs)to discuss measurement priorities and aid in thedevelopment of measurement protocols. The primaryoutput of the meeting will be “NIST RecommendedPractice Guides,” authored by workshop participantsand edited and published by NIST, for use byscientists and engineers involved in R&D,processing, and the production of nanotubecontaining products. These “Practice Guides”will contain measurement protocols that will helpharmonize sample preparation, measurementprocedures, data analysis, and reporting among thenanotube manufacturers, researchers and end users.

It was decided to revisit the topics of purity and dispersion, which were the subjects of the first

NASA/NIST workshop held in May 2003. While progressin these measurements has been made, improvements arestill needed to accurately measure and describe the qualityof nanotube-containing materials and the dispersionof nanotubes in liquids or polymers, both of which areconsidered crucial for the continued growth of applicationsincorporating SWCNTs. The organizers recognized thatthere remains considerable confusion and ambiguityregarding which techniques to use for a particular purposeand the relative accuracy which one should expect toachieve. Significant differences in both methodology andinterpretation continue to exist from one measurementlaboratory to another. For this reason, comparisonand specification of the quality of SWCNT materials isextremely difficult, as noted in Nature, 16th Dec. 2004.

To address these challenges, the organizing committeeinvited 23 speakers and developed an agenda thatencouraged active participation from attendees. Breakoutsessions addressing both workshop topics were held tofoster open discussion and to invite consensus regardingbest techniques and measurement methods. A half-dayopen discussion led to a consensus on those measurementsfor which protocols were most urgently needed.

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of nanotubes. After much discussion it was agreedthat the most extensively utilized techniques arethermogravimetric analysis (TGA), scanning electronmicroscopy (SEM), transmission electron microscopy(TEM), Raman spectroscopy, and near infraredspectroscopy. The need for rapid, inexpensivemeasurement methods was emphasized.

Both TEM and SEM are used extensively forqualitative analysis of a sample containing SWCNTs.There was general accord that a TEM image demonstratingthe existence of a significant quantity of SWCNTs isan important measure of the quality of the material.For quantitative estimation, a combination of TGA,Raman, NIR, and ICP methods was recommended.

DispersionDispersion was divided into two categories,

macrodispersion, defined as the distribution of nanotubebundles, and nanodispersion, the splitting of the bundlesinto individual tubes. In macrodispersion the primary issueis the agglomeration of SWCNTs in solvents or polymers,while in nanodispersion the focus is on eliminatingSWCNT ropes. The question of dispersion stabilityover time was also viewed as important.

It was agreed that it is critical to have a consistentsample preparation for all the dispersion characterizationmethods. The technique of choice to determine thedegree of macrodispersion appears to be optical

microscopy. However, there is lack of agreementon what constitutes good versus poor dispersion.It was suggested that Raman mapping techniques,SEM, and scanned probe microscopy (SPM) may beuseful complements to optical microscopy. Small anglescattering (neutron and X-ray) was put forward as apossible fundamental method to quantify dispersion.

ConclusionsThe final session of the workshop was devoted to

presentations of the synopses of each of the breakoutsessions on the topics of purity and dispersion, anda general discussion and debate regarding priorityneeds for measurements. There was consensus thatprotocols for measurement techniques would bevaluable even if they are incomplete.

On the topic of purity, the strengths, limitations,and research needs for a number of measurementtechniques were discussed. A matrix showing techniqueson one axis and the item measured on the other axiswas used to reach a consensus for the most criticaltechniques to be addressed in the “Practice Guide.” Itwas eventually agreed that chapters would be written onfour primary procedures, TGA, TEM/SEM/SPM, Ramanspectroscopy, and NIR, and two secondary techniques,inductively coupled plasma (ICP) and x-ray fluorescence.Volunteer authors were found for each of the chapters.

The attendees agreed that the topic of dispersionneeded a separate “Practice Guide” from that of purity.The chapters for this “Practice Guide” were dividedinto the topics of macro- and nano-dispersion, andauthors were found to write them. There was also ageneral agreement that one of the needs was an agreedupon terminology, which will be included in the “PracticeGuide.” The chapter on macrodispersion would beprimarily devoted to microscopy procedures, while thaton nanodispersion would include mention of a widevariety of techniques including small-angle-neutronand x-ray scattering, atomic force microscopy, etc.

A discussion was held regarding the need forreference materials. While there was consensusthat such materials would be useful, it was felt thatproduction of such materials should be delayed untilthe protocols for the measurement procedures aremore firmly established. A number of participants alsocommented that inter-laboratory tests could be quiteuseful in establishing laboratory-to-laboratorycorrelations on measurements.

Finally, the question was raised regarding theneed for future workshops of this kind. There was ageneral sense that future workshops would be valuable,but there was not consensus on the particular topic.However, nanotube dimension measurements anddetermination of chirality both received strong support.


Figure 1: TEM image of a SWCNT rope observed at NIST.The presence of impurities, mixtures of single and multi-wallednanotubes, defects, and tube bundling, coupled with the ultra-smalldimensions, continue to complicate the characterization of theseunique materials.

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Extraordinary Transport Properties of Nanotube/PolymerNanocomposites

Figure 1: Optical microscopy image of 1 % by volume MWNT/PPnanocomposite (obtained using a 100x objective) demonstratesgood dispersion of the MWNT and reveals a polydispersity innanotube length and shape. The MWNT volume fraction in thisfigure equals φ = 0.01, which is close to the geometrical percolationconcentration where the CNT network first forms and where theconductivity and stiffness of the nanocomposite increases byorders of magnitude (see Figure 2).

the polymer matrix creates additional contributions tonanocomposite viscoelasticity that can have a radicaleffect on the processing characteristics of these materials.

In Figure 2, we characterize the large changesin viscoelasticity and conductivity for whichpolymer composites containing CNT are well known.Simultaneous measurements of σ and the shear moduli(G´, G´́ ) characterize the elastic and viscous propertiesof our composites. G´ can be thought of as a measureof “stiffness” and G´́ provides a measurement ofviscous resistance to deformation. The ratio (G´/G´́ )or “loss tangent” (tan δ ), is a measure of the composite“firmness,” and we compare this basic quantity to σ.We observe that both (G´/G´́ ) and σ increase withφ and that this variation becomes rapid for MWNTvolume fraction φ in the range from 0.0025 to 0.01.We see that adding MWNT to the PP matrix increasesthe conductivity by an impressive seven orders ofmagnitude as a percolating network structure forms.G´ and G´́ become frequency independent as φ isvaried through the “gelation concentration,” φc ≈ 0.01.

In order to manufacture MWNT nanocompositesinto usable shapes, we must understand how thenetwork structure acts to influence their processingbehavior. The linear rheological and electrical transportproperties (Figure 2) are strongly altered by flow, as

There has been intense interest in composites ofpolymers and carbon nanotubes (CNT) becauseof the large transport property (conductivity,elasticity, viscosity, thermal conductivity) changesexhibited by these additives for relative lowCNT concentrations (≈ 1 % volume fraction).NIST’s experience in the area of dielectric andrheological measurement, in conjunction withexpertise in modeling, puts it in a unique positionto lead the development of new processingconcepts required by industry to utilize thisimportant new class of materials.

Kalman B. Migler and Jack F. Douglas

The combination of extended shape, rigidity anddeformability allows carbon nanotubes (CNT) to

be mechanically dispersed in polymer matrices in theform of disordered network structures exhibiting agel-like rheology. Our measurements on representativenetwork-forming multi-wall carbon nanotube (MWNT)dispersions in polypropylene (PP) indicate that thesematerials exhibit extraordinary flow-induced propertychanges. Specifically, electrical conductivity σ andsteady shear viscosity η both decrease strongly withincreasing shear rate γ& , and these nanocompositesexhibit impressively large and negative normal stressdifferences, a rarely reported phenomenon insoft condensed matter. We illustrate the practicalimplications of these non-linear transport propertiesby showing that MWNTs eliminate die swell in ournanocomposites, an effect crucial for their processing.

The strong interest in CNT “nanocomposites”stems from their ability to affect thermal, electrical andrheological properties for relatively small concentrationsof this type of additive. These additives have foundmanufacturing applications in electrostatic painting,protective coatings for electronic components, andflammability reduction. Utilization of CNT formore complex applications, however, requires anunderstanding of how processing conditions (mixing,molding, extrusion) influence nanocomposite properties.

Despite the high elastic modulus of CNT, their smallcross-sectional dimensions and large aspect ratio allowsthem to bend substantially in response to inter-tubeinteractions under processing conditions. This bendingleads to the formation of a disordered “web-like” structure(see Figure 1) of substantial mechanical integrity.The presence of a nanotube network interpenetrating

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Figure 3 indicates. Notably, both the conductivityand the viscosity η(γ& ) exhibit a strong thinning.The viscosity decreases over the full range of shearexplored here, whereas the conductivity shows aplateau region at low shear. Moreover, a positivenormal force ∆N is observed in our nanocomposite forφ < φc, where the matrix dominates the rheologicalresponse (Figure 3), but ∆N becomes large andnegative for φ ≥ φc, compensating the large ∆Nexhibited by the matrix polymer. (A negative ∆N innanotube dispersions was reported by Lin–Gibson,et al.) This has significant processing consequences.

Figure 2: Characterization of conductivity and viscolelasticityof MWNT/PP nanocomposites (φ = 0.025; T = 200 °C).Inset: Shear modulus as a function of frequency for a rangeof nanotube concentrations.

Figure 3: Normal stress measurements showing slightly positivenormal stress for pure PP and increasingly negative normalstress as the MWNT fraction increases. Inset: Conductivity andviscosity as a function of shear rate for (φ = 0.025; T = 200 °C).

Figure 4: Comparison of PP extrudate with (A) and without (B)added nanotubes. The red dashed lines correspond to the die size.

Since the extrusion of the nanocomposite is a basicprocessing operation for which normal forces areknown to be important, we extruded a nanocompositesample (φ = 0.025) and found that the cross-sectionactually shrinks upon extrusion (Figure 4). Thisstriking effect is contrasted with the extrusion of purePP where a nearly 6-fold increase in cross-sectionalarea is observed. Evidently, the CNT change thequalitative nature of the polymer flow.

The suppression of die swell of extruded polymersby adding a relatively small amount of MWNT (φ ≈ 0.01)offers a powerful tool for controlling dimensionalcharacteristics and surface distortion in manufacturingcomposites. Our observations of strongly non-linearrheology under flow (shear thinning and large, negativenormal stresses) imply that these fluids should exhibitother “anomalous” flow characteristics (e.g., dropletdistortion and thread break-up) that are quite unlikeNewtonian fluids. Understanding these flowcharacteristics is crucial for their processing.


S. Kharchenko, J. Obrzut, E. Hobbie (PolymersDivision, NIST)

S. Lin–Gibson, J.A. Pathak, E.A. Grulke, H. Wang,and E.K. Hobbie, “Elastic Flow Instability in NanotubeSuspensions,” Physical Review Letters 92, 048302-(1-4)(2004).

S.B. Kharchenko, J.F. Douglas, J. Obrzut,E.A. Grulke, and K.B. Migler, “Extraordinary FlowCharacteristics of Nanotube-Filled Polymer Materials,”Nature Materials, in press.

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Applications of Carbon Nanotubes:Carbon Nanotubes and Nanotube Contacts

A critical requirement for developing carbonnanotube based electronics is determining themechanical and electrical characteristics ofthe connection between the carbon nanotubeand its electrode. On this scale (nanometers),the properties of both the nanotube andthe contact are not scaleable from largermeasurements. To address this issue we areexploring a technique commonly used tomanipulate the nanotubes and attach themto substrates inside the scanning electronmicroscope. We are subsequently developingthe techniques to measure the small electricalcurrents and test the small mechanical strengthassociated with the contacts and nanotubes.

Paul Rice

We are developing techniques to measure themechanical and electrical properties of carbon

nanotube-based devices and also to image thenanometer-scale structure within the nanotube andtheir electrical contact. Shown in Figure 1 is atest device that we have fabricated to measure theelectrical characteristics of an individual multiwalledcarbon nanotube. The nanotube was attached,or welded, to the device in the scanning electronmicroscope (SEM) using an accepted technique calledelectron beam deposition (EBD). In this example,a single tube extends from one electrode to another.

Understanding the mechanical properties of the weldrequires visualizing its structure on the atomic scale if

possible. Using high-resolution microscopy techniquesavailable within the division, we have seen someinteresting new structures of the weld. Previously,it was assumed that the weld was a uniform materialcomposed of amorphous carbon. However, we haveseen, using atomic force microscopy (AFM) as wellas transmission electron microscopy (TEM), spuriousdeposits of carbon far outside the weld. Figure 2shows a nanotube welded to a chromium film.The residual carbon is seen as a surface texture thatcovers almost everything including the nanotube.

Figure 1: SEM image of carbon nanotube test device forelectrical measurements. The nanotube extends across a 2 µmgap. A second accidental tube is hanging on the side and is notconnected to the electrodes.

Figure 2: AFM image of a multiwalled carbon nanotube weldedto a chromium film. The orange peel texture is residual carbonleft behind from the weld process.

This work is beginning to shed some light on theproperties and structure of these very small devices.A paper has been submitted entitled, “Electron BeamDeposition Welding: A Practical Application to BuildingCarbon Nanotube Devices,” by P. Rice, R.H. Geiss,S.E. Russek, A. Hartman and D. Finch, to the Journalof Vacuum Science and Technology B. Further work isplanned to measure contact resistance within the weldand resistance changes due to distortions caused by theweld process.

Demonstrating the need for better standards fornanotube device characterization, a new standardscommittee was recently formed by IEEE, on which NISTcurrently serves as a contributing member. Furthermore,presentations have been given to the American VacuumSociety, NASA Johnson Space Center, and the engineeringdepartment of the University of Colorado.


R. Geiss (Materials Reliability Division, NIST);S.E. Russek, P. Kabos (EEEL, NIST); D. Finch,A.B. Hartman (University of Colorado)


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With the recent excitement about carbonnanotubes and their biological applications,preliminary studies are underway to determine thebiocompatibility of nanotubes with cell culturesusing our vascular smooth-muscle cell line. Theresponse of these cells will help us understand theeffects of cell proliferation or degradation dueto direct contact with nanotube mats or clusters.

Paul Rice and Tammy Oreskovic

Carbon nanotubes (CNTs) have great potential fornanometer scale electronics, superior strength

composites, biological implants, and many otherapplications where a well-organized nanometer scalematerial is needed. One possible application is usingnanotubes as scaffold materials to support the growthof transplanted cells and establish proper orientation togrow artificial tissue. NASA has recently demonstratedgrowing retinal cells on nanotube mats. Other possibleapplications include sensors and probes for miniaturebiological devices, and electrodes for detectingbiomolecules in solutions. However, many issuesstill remain in assessing the biocompatibility of thesenanomaterial biosystems for in vivo applications.

In FY04, we explored the viability and proliferationof cells in contact with carbon nanotubes and

dispersion techniques for compatible surfactants incellular environments. In preliminary experiments,smooth muscle cells were exposed to both entangledmats and suspensions of multi-walled CNTs by use ofsurfactants for dispersion (Figure 1). Results of theproliferation assay indicate that there is a reduction ingrowth and metabolic activity for cells grown directlyon the nanotubes as compared to the controls. Theseresults indicate that cell attachment may not be asstrong in regions of high CNT density. However, ifthe nanotubes are coated with a biologically compatiblesurfactant and suspended in media, there appearsto be little effect when compared to the controls.Figure 2 shows preliminary results of an Alamarblue proliferation assay.

In FY05, we will continue to explore theinteractions of structure and the effects of clumpingof CNTs on cell growth and orientation. Other possibledirections may include surface modification to promotecell adhesion and growth, tomography, and sizedistribution of CNTs.


Dr. N. Varaksa contributed the work on thesurfactant chemistry; Dr. E. Gruelky (University ofKentucky) supplied carbon nanotubes.

Applications of Carbon Nanotubes:Cell Viability in Contact with Carbon Nanotubes

Figure 1: Vascular smooth muscle cell growing on a clusterof multiwalled carbon nanotubes on a glass substrate.

Figure 2: Alamar blue assay, test for cell growth andproliferation. As the cells grow and divide they metabolizethe Alamar blue compound, changing the medium in which thecells are growing from a blue color to clear.

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Instruments for in-vivo recording of neural activityare critical for ongoing and future research aimedat Alzheimer’s, Parkinson’s, epilepsy, stroke, andspinal cord injuries. In addition, the developmentof a human-brain-machine interface (HBMI) relieson improved recording and processing of neuronalsignals. Future recording electrodes must besmall, stable, biocompatible, and robust. Morerugged test procedures are also needed forevaluating and qualifying new electrode materials.

Stephanie A. Hooker and Dudley Finch

New tools are needed to better image and measuresignal transfer inside live cells. One conventional

approach to electrical measurements in cellular materialsinvolves a two-electrode glass micropipette that clampsthe cell membrane. The discovery of this device,referred to as a patch clamp, in 1972 had widespreadimpact, leading to a Nobel Prize in 1991. However, thepatch clamp is too large for many of today’s desiredmeasurements in neurobiology, due to the extent ofphysical damage caused during its insertion and use.

Recently, silicon-based MEMS technology has beenexplored as a method to produce smaller probes forneuronal research. These devices typically consist of asilicon shank (several hundred micrometers long) withan array of microelectrode recording sites located alongits surface. In this manner, multiple neurons can bestudied simultaneously with a single insertion point.

While these devices show considerable promise,many biologists continue to employ individual metallicwires as their primary recording electrodes. Thesewires are similar to those used inside the patch clampsbut are inserted directly into the brain. Polymer coatingsare often applied to increase impedance, enable betterhandling, and improve biocompatibility. The wires aretypically tungsten or stainless steel with diameters onthe order of 50–100 microns. To date, few studies haveattempted to optimize the wire material, the coating,or the insertion technique (applied force, speed, etc.).

In collaboration with the University of Colorado’sHealth Sciences Center and the Departments ofChemistry, Electrical Engineering, and MechanicalEngineering at the University of Colorado at Boulder,we are developing measurements to help improve andoptimize in-vivo probes. In FY04, we completedpreliminary studies of wires (25 µm in diameter) coatedwith thin (~ 60 nm) layers of aluminum oxide, Al2O3.Atomic layer deposition (ALD) was used to precisely

Applications of Carbon Nanotubes:Electrochemical Characterization of In-Vivo Neuronal Probes

control coating thickness, and seven different wires werecompared to determine metal effects on performance.

Figure 1: Electrochemical impedance of ALD-coated microwiresin Ringer’s solution as a function of applied voltage at 100 Hz.

A two-electrode configuration was used for testingin a Ringer’s buffer solution. Several proceduralvariables were evaluated, including the type and sizeof the reference wire, the solution temperature and pH,and the electrode placement. Impedance was measuredas a function of frequency, voltage (Figure 1), and time.The latter indicates the stability and reliability duringextended recordings, a key requirement for long-termstudies. Stability measurements will continue duringthe upcoming months in order to ensure that the ALDcoating is not degraded either chemically (by thesolution) or electrically (by the measurement itself).

However, it should be noted that 25 µm wires are stilltoo large for subcellular studies. As a result, we planto evaluate carbon nanotube (CNT) probes in FY05.CNT-based AFM probes have already been developedby other researchers in the Materials Reliability Divisionfor studying microelectronic films. Moreover, dielectriccoatings have recently been applied to individual tubes(via ALD) to further enhance signal transfer. If successfulelectrochemical measurements can be demonstrated,CNT based in-vivo probes could have a significant impacton neuroscience due to their small size (0.1 µm or less)and anticipated long-term stability.


D. Restrepo, A. Sharp (University of ColoradoHealth Sciences Center); R. Artale, K. Gall, S. George,R. Zane (University of Colorado at Boulder)


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Organizational Charts

Organizational Charts

Materials Science and Engineering Laboratory

PolymersE.J. Amis, Chief

C.R. Snyder, Deputy

MetallurgyC.A. Handwerker, Chief

F.W. Gayle, Deputy

CeramicsD.L. Kaiser, Chief

R.G. Munro, Deputy

Materials ReliabilityT.A. Siewert, Acting Chief

NIST Center forNeutron Research

P.D. Gallagher, Director

DirectorDeputy Director

National Institute of Standards and Technology

DirectorBoulder Laboratories

Baldrige National QualityProgram

Director forAdministration

& Chief FinancialOfficer

TechnologyServices

AdvancedTechnologyProgram

ManufacturingExtension

Partnership

ManufacturingEngineeringLaboratory

Chemical Scienceand Technology

Laboratory

Materials Scienceand Engineering

Laboratory

Electronics andElectrical Engineering

Laboratory

InformationTechnologyLaboratory

PhysicsLaboratory

Building andFire ResearchLaboratory

L.E. Smith, DirectorS.W. Freiman, Deputy Director

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