SEM and AFM: Complementary Techniquesfor High Resolution Surface Investigations

TheWorldLeader In3D SurfaceMetrology

There are a wide range of analyti-cal techniques which may be usedfor materials characterizationdepending on the type of infor-mation needed. For high resolu-tion surface investigations, twocommonly used techniques areAtomic Force Microscopy (AFM)and Scanning Electron Micros-copy (SEM)–Figures 1 and 2,respectively. Each of thesetechniques resolves surfacestructure down to the nanometerscale. However, the image

formation mechanisms are quitedifferent, resulting in differenttypes of information about thesurface structure. The occurrenceof the SEM and AFM side-by-sideis becoming more common intoday’s analytical laboratories.This article will compare andcontrast the two techniques withrespect to specific types of surfacemeasurements, and demonstratehow these analytical techniquesprovide information which iscomplementary in nature.

History

The first SEM was constructed in1938 by von Ardenne by rasteringthe electron beam of a Transmis-sion Electron Microscope (TEM)to essentially form a ScanningTransmission Electron Microscope(STEM) (1, 2). In 1942, Zworkinet. al. developed the first SEM forbulk samples. This configurationcontains many of the basicprinciples of today’s SEMs (2, 3).Cambridge Scientific Instruments

Figure 2. Schematic of the primary components of atypical SEM

ByP. Russell,D. Batchelor,J. Thornton

Figure 1. Schematic of the major components of an AFMshowing the feedback loop for TappingModeTM operation.

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produced the first commercialinstrument in 1965. A number ofimprovements have occurred sincethis time, resulting in an increasein resolution from 50nm in 1942to ~0.7nm today. Besides thedevelopment of morphologicalimaging, the SEM has beendeveloped to detect signals whichare used to determine composi-tional information, such as X-rays,backscattered electrons,cathodoluminesce, Auger elec-trons, and specimen current.

The development of the AFM waspreceded by the development ofthe Scanning Tunneling Micro-scope (STM) in 1981 at IBMZurich Research Laboratory byBinnig and Rohrer (4). Its abilityto view the atomic lattice of asample surface earned the inven-tors the Nobel Prize in Physics in1986. Although the STM pro-vides subangstrom resolution in allthree dimensions, it is limited toconductive and semiconductivesamples. To image insulators aswell as conductors, the AtomicForce Microscope (AFM) wasdeveloped in 1986 (5), and thefirst commercial AFMs wereproduced in 1989 by DigitalInstruments.

AFM provides three-dimensionalsurface topography at nanometerlateral and subangstrom verticalresolution on insulators andconductors. From this beginning,the field of Scanning ProbeMicroscopy (SPM) was bornwhich consists of a family oftechniques that involves scanning asharp tip across the sample surfacewhile monitoring the tip-sampleinteraction to form a high resolu-tion image. Although the AFMhas become the most commonly

used form of SPM, many otherSPM techniques have beendeveloped which provide informa-tion on differences in friction,adhesion, elasticity, hardness,electric fields, magnetic fields,carrier concentration, temperaturedistribution, spreading resistance,and conductivity.

Imaging Mechanisms

Scanning Electron Microscopy

The operation of the SEMconsists of applying a voltagebetween a conductive sample andfilament, resulting in electronemission from the filament to thesample. This occurs in a vacuumenvironment ranging from 10-4 to10-10 Torr. The electrons areguided to the sample by a series ofelectromagnetic lenses in theelectron column. A schematic of atypical SEM is shown in Figure 2.The resolution and depth of fieldof the image are determined bythe beam current and the finalspot size, which are adjusted withone or more condenser lenses andthe final, probe-forming objectivelenses. The lenses are also used toshape the beam to minimize theeffects of spherical aberration,chromatic aberration, diffraction,and astigmatism.

The electrons interact with thesample within a few nanometersto several microns of the surface,depending on beam parametersand sample type. Electrons areemitted from the sample primarilyas either backscattered electrons orsecondary electrons. Secondaryelectrons are the most commonsignal used for investigations ofsurface morphology. They areproduced as a result of interac-

Figure 3. SEM image of an integrated single crystalsilicon cantilever and tip which has an end radius of 2to 10nm. Tips for AFM are typically made of silicon orsilicon nitride. Bar=100µm.

Figure 4: TappingMode AFM image of 1.4Åmonoatomic steps on epitaxial silicon deposited on(100) Si. 1µm scan.

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tions between the beam electronsand weakly bound electrons in theconduction band of the sample.Some energy from the beamelectrons is transferred to theconduction band electrons in thesample, providing enough energyfor their escape from the samplesurface as secondary electrons.Secondary electrons are lowenergy electrons (<50eV), so onlythose formed within the first fewnanometers of the sample surfacehave enough energy to escape andbe detected. High energy beamelectrons which are scattered backout of the sample (backscatteredelectrons) can also form secondaryelectrons when they leave thesurface. Since these electronstravel farther into the sample thanthe secondary electrons, they canemerge from the sample at a muchlarger distance away from theimpact of the incident beamwhich makes their spatial distribu-tion larger. Once these electronsescape from the sample surface,they are typically detected by anEverhart-Thornley scintillator-photomultiplier detector. TheSEM image formed is the result ofthe intensity of the secondaryelectron emission from the sampleat each x,y data point during therastering of the electron beamacross the surface.

Atomic Force Microscopy

AFM consists of scanning a sharptip on the end of a flexiblecantilever across a sample surfacewhile maintaining a small,constant force. An integratedsilicon tip and cantilever can beseen in Figure 3. The tipstypically have an end radius of2nm to 20nm, depending on tiptype. The scanning motion is

conducted by a piezoelectrictube scanner which scans the tipin a raster pattern with respectto the sample (or scans to thesample with respect to the tip).The tip-sample interaction ismonitored by reflecting a laseroff the back of the cantileverinto a split photodiode detector.By detecting the difference inthe photodetector outputvoltages, changes in the cantile-ver deflection or oscillationamplitude are determined. Aschematic of this can be seen inFigure 1.

The two most commonly usedmodes of operation are contactmode AFM andTappingModeTM AFM, whichare conducted in air or liquidenvironments. Contact modeAFM consists of scanning theprobe across a sample surfacewhile monitoring the change incantilever deflection with thesplit photodiode detector. Afeedback loop maintains aconstant cantilever deflection byvertically moving the scanner tomaintain a constant photodetec-tor difference signal. Thedistance the scanner movesvertically at each x,y data pointis stored by the computer toform the topographic image ofthe sample surface. Thisfeedback loop maintains aconstant force during imaging,which typically ranges between0.1 to 100nN.

TappingMode AFM consists ofoscillating the cantilever at itsresonance frequency (typically~300kHz) and lightly “tapping”on the surface during scanning.The laser deflection method isused to detect the root-mean-

Figure 5: (a) SEM image of rugged polysilicon thinfilm. 100,000x, Bar=0.1µm; (b) TappingMode AFMimage of the same with roughness measurement.1µm scan.

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(b)

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square (RMS) amplitude ofcantilever oscillation. A feedbackloop maintains a constant oscilla-tion amplitude by moving thescanner vertically at every x,y datapoint. Recording this movementforms the topographical image.The advantage of TappingModewith respect to contact mode isthat it eliminates the lateral, shearforces present in contact mode.This enables TappingMode toimage soft, fragile, and adhesivesurfaces without damaging them,which can be a drawback ofcontact mode AFM.

Comparison ofTechniques

There are a number of differentways to compare and contrastthese two techniques with respectto each other. Although investi-gations that use both SEM andAFM to characterize a material arecommon, there are just a fewstudies that directly discuss thecomplementary nature of thetechniques (6-13). A comparisonof these techniques will beconducted with respect to 3factors: (1) Surface Structure, (2)Composition, and (3) Environ-ment. The comparisons arepresented for typical equipmentconfigurations and operatingprocedures.

Surface Structure

Although both SEM and AFM aresimilar in lateral resolution, thereare situations in which onetechnique can provide a morecomplete representation of thesample surface, depending on theinformation desired. Oneprinciple difference is in how thetwo techniques process verticalchanges in topography. Below we

will discuss measurements ofdifferent vertical scales of topogra-phy, beginning with very smoothsurfaces and working up to veryrough surfaces to determine howthe surface topography affects theability of each technique toperform the measurement.

Atomically Smooth Surfaces

Atomically smooth surfaces canoccur either naturally, such as onmineral surfaces, or by processing,such as polishing and epitaxialgrowth on semiconductor, datastorage, and optical surfaces. ATappingMode AFM image of anepitaxial silicon surface is shownin Figure 4. Note that, unlikeSEM, the AFM can measure in allthree dimensions (x, y, and z) witha single scan. Since the AFM hasa vertical resolution of <0.5Å, itcan resolve the 1.4Å monoatomicsilicon steps on the surface as wellas calculate an RMS roughness of0.7Å (14). On a sample thissmooth, the SEM has difficultyresolving these features due to thesubtle variations in height.

Thin Films

On most thin films, the SEM andAFM produce a similar represen-tation of the sample surface. Acommon application of surfaceinvestigations of thin filmsconsists of determining changes inmorphology with variations ofdeposition parameters, such astemperature, pressure, time, etc.Figure 5 shows SEM and AFMimages of a polysilicon thin film atapproximately the same lateralmagnification. The two imagesshow similar surface structure,however, they differ in the othertypes of information that can be

(a)

(b)

(c)

Figure 6. (a) SEM image of partially GaP-covered Siafter chemical beam epitaxy deposition for 10 minutes.30,000x, Bar=1µm; (b) AFM image of the samesample as in figure 6a showing the presence ofnodules during the growth of GaP by chemical beamepitaxy. 10µm scan; (c) Cross-sectional measurementwith AFM across the image in Figure 6b showing 3nodules which have a height of approximately 70nm.(16)

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acquired on this sample. Thethree-dimensional nature of theAFM can be used to calculatechanges in roughness and surfacearea variations due to differencesin deposition parameters. For theSEM, a large area view of thevariations in surface structure canbe acquired all at once (such asseveral mm’s), whereas a 100µm x100µm area is typically the largestarea viewed by an AFM. Theseimages are an example of “rugged”polysilicon films which are used ascapacitors in memory devices. Bymaking these films rough, thesurface area is increased whichmakes it possible to hold morecharge without increasing thelateral dimensions of the capaci-tors on the chips. By adjustingthe deposition parameters andusing the AFM to analyze thesurface area of the films, thedeposition parameters needed toproduce a film with the maximumsurface area were determined (15).

Another example of the differencebetween the two techniques is ininterpreting subtle differences inheight. In the SEM image,changes in slope can result in anincrease in electron emission fromthe sample surface, producing ahigher intensity in the image.However, it can sometimes bedifficult to determine whether thefeature is sloping up or down. Forinstance, in the SEM image inFigure 6a it is very difficult todetermine whether the smallround structures are bumps orpits, even when tilting the samplestage in the SEM. The only otheroption would be to cleave thesample through one of thesefeatures and look at the sample incross-section, which would be

tedious and time consuming.Since the AFM data contains theheight information, determiningwhether a feature is a bump or pitis straightforward. As can be seenin Figures 6b and 6c, the featureson this sample are bumps. Thisinformation was used in the studyof the growth mechanisms of GaPon Si during chemical beamepitaxy deposition (16). Determi-nation of whether these featureswere small bumps or depressionswould have changed how thedeposition process was altered toproduce an epitaxial GaP film.

High Aspect Ratio Structures

Semiconductor processingcommonly requires measurementsof high aspect ratio structures suchas trenches and via holes. In aSEM, these structures are typicallymeasured in cross section bycleaving the wafer and imagingthe sample on end to obtain thedimensions of the structure. Acommon example of this is seen inFigure 7a. In contrast, the AFMimage of a trench or via is madeby scanning over the samplesurface. The ability of the AFMto measure these structuresnondestructively makes it possiblefor the wafer to be returned to theproduction line after the measure-ment is acquired. An AFM imageof vias in photoresist is shown inFigure 7b. To image some higheraspect ratio structures, the propertip shape is needed for the AFMto scan narrow openings and steepsidewalls. Although the SEMmeasurement is destructive to thesample, the ability to image theundercuts of these lines is a usefulapplication that AFMs are nottypically designed to perform.

Figure 7. (a) Cross-sectional SEM image ofpolysilicon lines which shows undercutting due toreactive ion etching. Scale bar=100nm; (b) Cross-sectional measurement of developed and incompletelydeveloped vias in photoresist acquired byTappingMode AFM. In order to image the high aspectratio structures on the sample, a silicon tip machinedwith a focused ion beam was used to scan the vias.6.2µm scan.

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(b)

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Rough Surfaces

One of the key advantages of theSEM with respect to other typesof microscopy is its large depth offield. This ability makes itpossible to image very roughsurfaces with millimeters ofvertical information within asingle image. A SEM image ofnon-woven polyethylene oxidefibers can be seen in Figure 8a.The depth of field and small beamsize makes it possible to image thefibers far below the top layer. Thisability also makes it possible tomeasure very rough surfaces overlarger lateral areas as well. Al-though the AFM can measurevertical surface variations below0.5Å, its ability to measure a tallstructure comes from how far thescanner can move vertically.Standard scanners typically have 5to 6µm of vertical range, however,in some configurations the verticalrange approaches 10µm or larger.For scanning areas that haveheights of greater than 5 to 10µm’sof variation, the SEM would bebetter suited for the analysis.

Another example of a complexthree-dimensional surface struc-ture which shows how the SEMand AFM can complement eachother can be seen in Figure 8b.The convoluted three dimensionalY

2O

3 oxide crystal shown growing

out of a relatively flat Y2O

3 thin

film on a Si substrate is easilyimaged in the SEM (Figure 8b).Although the AFM would haveprobelms imaging the obtuseangels and enclosed areas of thissurface, the roughness of the Y

2O

3

film can be measured whereas inthe SEM image the surfaceroughness is not evident. There-fore, the two techniques together

give a more complete "picture" ofthe sample.

Composition

Both SEM and SPM providecompositional informationthrough a variety of techniques.SEM is the only one of the twotechniques which provideselemental analysis, however, bothSEM and AFM are associated withtechniques which can providecompositional informationthrough analyzing materials andphysical properties of the sample.Some of the most common ofthese methods are describedbelow.

SEM

Along with the secondary electronemission which is used to form amorphological image of thesurface in the SEM, a number ofother signals are emitted as a resultof the electron beam impingingon the surface, as shown in Figure9. Each of these signals carriesinformation about the samplewhich provide clues to its compo-sition.

Two of the most commonly usedsignals for investigating composi-tion are x-rays and backscatteredelectrons. X-ray signals arecommonly used to provideelemental analysis by the attach-ment of an Energy-DispersiveSpectrometer (EDS) or Wave-length-Dispersive Spectrometer(WDS) to the SEM system. X-ray emission results from inelasticscattering between the beamelectrons and the electrons of thesample atoms. This interactionresults in the ejection of an innershell electron from the atom,

Figure 8. (a) SEM image of a non-woven textilesample of polyethylene oxide fibers. The large depthof field of the SEM makes it possible to image fiberswhich are 10’s of µm’s below the upper layer of fibers.Bar=10µm; (b) SEM image of Y2O3 crystal. Bar=1µm.

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(b)

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creating a vacancy that is filled byan outer shell electron. This jumpfrom an outer to inner shell resultsin a change in energy that pro-duces either a x-ray or Augerelectron. The emitted x-ray hasenergy equal to this change. Thex-rays are then detected by either alithium-drifted silicon detector foran EDS system, or a gas propor-tional counter detector for a WDSsystem. A typical x-ray spectrumcollected with an EDS system isshown in Figure 10.

Backscattered electrons are theresult of beam electrons beingscattered back out of the sample.In this case, the incident beamelectrons undergo a number ofscattering events within thespecimen in which very littleenergy is lost, allowing theseelectrons to go much deeper intothe sample than secondaryelectrons and still emerge from thesample surface to be detected.The percentage of beam electronsthat become backscatteredelectrons has been found to bedependent on the atomic numberof the material, which makes it auseful signal for analyzing thematerial composition. Once theseelectrons escape from the surfacethey are detected by either theEverhart-Thornley detector or asolid state detector. An exampleof a backscattered image of a PbSnalloy is shown in Figure 11.

AFM/SPM

Although an AFM does notprovide elemental analysis, it cansupply compositional informationby differentiating materials basedon physical properties, such asstiffness, elasticity, compliance,friction, adhesion, magnetic and

electrostatic fields, carrier concen-tration, temperature distribution,spreading resistance, and conduc-tivity. Many of these techniquesconsist of looking simultaneouslyat another signal while performingstandard AFM imaging. One ofthe most common techniques formapping differences in materialsproperties is PhaseImagingTM.PhaseImaging is conducted duringTappingMode AFM operation bymonitoring the phase lag betweenthe oscillating drive signal used todrive the cantilever and theoscillating detection signal fromthe photodiode detector. Thissignal will indicate differences inviscoelasticity and/or adhesionacross the imaged area. Thistechnique is commonly applied tomapping the distribution ofpolymers in a heterogeneoussystem, or mapping the distribu-tion of filler, such as silica orcarbon black, in a polymer matrix.An example of PhaseImaging on apolyethylene film is shown inFigure 12. Other ways to getsimilar information are by ForceModulation AFM, which mapsdifferences in elasticity across thesample surface, and Lateral ForceMicroscopy (LFM), which mapsdifferences in friction across thesample surface.

There are also techniques that canbe used to investigate long rangeforces across the imaged area.Magnetic Force Microscopy(MFM) and Electric ForceMicroscopy (EFM) map themagnetic and electrostatic fieldgradients, respectively, whichextend from the sample surface.These techniques are performedby using either a magnetic orconductive probe to map theattractive and repulsive forces

Figure 9. Signals emitted from a sample surfaceafter interaction with an electron beam.

Figure 10. EDS X-ray spectrum of an AlGaN thinfilm on SiC substrate showing the presence of N,Ga, and Al.

Figure 11. Backscattered SEM image of an PbSnalloy showing contrast based on the atomic numberof the two components. The brighter areas are Pb-rich. 5,000x, Scale bar=1µm.

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between the tip and the sample.MFM is commonly used to detectthe domain structure of magneticbits written on magnetic media, toevaluate the performance ofmagnetic heads, and to investigatethe magnetic structure of experi-mental materials. This is con-ducted by a routine calledLiftModeTM in which a Tapping-Mode topographic image and amagnetic image are acquired overthe same area. LiftMode consistsof first collecting a line scan inTappingMode of the surfacemorphology. The tip is then liftedabove the surface and a secondscan is made over the same lineusing the saved topographic scanto maintain a constant tip-sampleseparation. The long-rangemagnetic forces shift the reso-nance frequency of the oscillatingcantilever, which is detected toproduce the magnetic image. Anexample of bits written on atextured hard disk is shown inFigure 13.

Although the AFM is applied sothat it is nondestructive to thesample surface, it can be used tostudy differences in mechanicalproperties by performingnanoindention to investigatehardness differences betweenmaterials. This technique uses adiamond tip mounted on a stiff,stainless steel cantilever. ATappingMode AFM image iscollected with the probe todetermine the area of interest forindentation, the nanoindention isthen made at a specified force, andan image is then collected of theindented area. An example ofcomparing the difference betweendiamond like carbon films on ahard disk is shown in Figure 14.In this example, the two filmsdemonstrate a difference in

Figure 12. Phase image of two components which areused to form a polyethelene (PE) film. The phaseimage (right) clearly shows the distribution of the twopolymers due to differences in stiffness which is notevident from the topographic image (left). 2µm scan.

Figure 13. Magnetic Force Microscopy (MFM) imageof overwritten tracks on a textured hard disk. Thetopography (left) was imaged using TappingMode; themagnetic force image of the same area (right) wascaptured with LiftMode (lift height 35 nm) by mappingshifts in cantilever resonant frequency. 25µm scan.(17)

hardness from indentations madeat the same forces, producingdifferent sized indents. Scratchingand wear testing may also beconducted with this configurationto investigate adhesion anddelamination of films under asmall applied force.

Environment

One of the primary differencesbetween these two types ofmicroscopy is the environment inwhich they are performed, i.e.,SEM is conducted in a vacuumenvironment, and AFM isconducted in an ambient or fluidenvironment. There are severalissues which make environmentan important issue. First, there isa frequent need in fields such asbiology and biomaterials to studyhydrated samples. These twotechniques compensate for thisneed by different means: anenvironmental chamber for aSEM, and a fluid cell for theAFM. Second, the SEM isrequired to work in a vacuumenvironment due to the nature ofthe technique which brings up theissues of vacuum compatibility ofthe sample, the conductivity ofthe surface, and vacuum mainte-nance. To image poorly conduc-tive surfaces without samplecharging may require conductivecoatings or staining, which mayalter or obscure the features ofinterest; or it may require lowvoltage operation, or an environ-mental chamber, which maysacrifice resolution.

For SEM, hydrated samples areaddressed by placing a specimenin an environmental chamber witheither an electron transparentwindow or a small aperture for thebeam to enter the chamber. The

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chamber is typically flushed withan inert gas saturated with watervapor. Common applications areto either investigate hydratedsurfaces to preserve their surfacestructure when hydrated, or toreduce charging on insulatingsamples. An example of imagingof a pesticide film on skin can beseen in Figure 15. For theelectron beam to interact with thesurface in this configuration, itmust go though an environmentof gas and water vapor. Onedrawback of this configuration isthat it will result in an increase inscattering of the electron beam onthe way to and from the surface,which may result in the sacrificeof image quality and resolution.

One of the primary attractions tothe AFM is its ability to imageinsulating surfaces at high resolu-tion in fluid. Imaging samples ina hydrated state with an AFM iscommonly performed by enclos-ing the sample and probe in afluid environment, as shown inFigure 16. Since AFM is notbased on conductivity, the imageand scanning mechanism is notdisturbed by the presence of thefluid. Common applications forAFM investigations in fluid are inthe biological sciences, biomateri-als, crystal growth, force interac-tion studies, and for investigatingprocesses in situ (Figures 17, 18).The resolution of the image willbe determined by the radius of thetip, the applied force, and thenoise floor of the instrument.Because of these factors, thisconfiguration allows the study ofhydrated specimens at a lateralresolution of 1 to 5nm and avertical resolution down to 0.5Åwithout sample damage, as seen inthe image of the GroES chaperon(Figure 17). With the appropriate

Figure 14: Indentations on two different diamond-likecarbon thin films using three different forces (23, 34,and 45µN) with four indents made at each force tocompare differences in hardness. 500nm scans.

accessories, AFM can also be usedin varied gaseous environmentsand at elevated temperature. Thelatter is particularly important forresearch and development ofpolymers.

Further Discussion

One thing to keep in mind whencomparing these two techniques isthat although SEM and AFMappear very different, they actuallyshare a number of similarities.Both techniques raster a probeacross the surface to detect someinteraction with the surface toform an image. Both have alateral resolution which is similarin scale (although under certainconditions AFM is superior). Andboth techniques have imageartifacts that the operator istrained to identify. The SEM hashad a much longer time to matureas a technique and to develop anunderstanding of how to identifyand avoid artifacts, but the rapidadoption and implementation ofAFM has resulted in a similarunderstanding of artifacts. Thisarticle has avoided discussing suchartifacts unless they are relevant tothe comparison. Furthermore, byusing two techniques which arecomplementary, one techniquewill often compensate for theimaging artifact of the othertechnique.

However, one should be wary ofcombined systems in which anAFM is placed inside the SEMchamber. One of the trueadvantages of the AFM is itsability to perform high resolutionmeasurements outside of avacuum environment. Placing itinside a vacuum environmentreduces its flexibility and increasesits operating time. Combined

Figure 15. Environmental SEM image of a pesticidefilm on skin. A hydrated environment was needed inorder to maintain the integrity of the pesticide layerand to reduce charging. Bar=1mm.

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Figure 16. Fluid cell for an AFM which allows imagingin an enclosed, liquid environment.

Figure 17. Image of two GroES molecules positionedside-by-side in physiologic fluid, demonstrating 10Ålateral resolution and 1Å vertical resolution. Theentire molecule measures 84Å across, and a distinct45Å heptameric “crown” structure protrudes 8Å abovethe remaining GroES surface and surrounds a centraldepression. 18nm scan. Image courtesy of Z. Shao,University of Virginia. (18)

AFM/SEM systems often havereduced capabilities and typicallycompromise the performance ofboth instruments.

Summary

SEM and AFM are complemen-tary techniques that provide amore complete representation of asurface when used together than ifeach were the only techniqueavailable. These techniquesoverlap in their capabilities toprovide nanometer scale lateralinformation. However, theydeviate in the fact that the AFMcan provide measurements in allthree dimensions, includingheight information with a verticalresolution of <0.5Å, whereas theSEM has the ability to image veryrough samples due to its largedepth of field and large lateralfield of view.

The SEM can provide elementalanalysis using X-ray detection,whereas the AFM can providecompositional information basedon physical properties. The factthat the two techniques operate indifferent environments can be astrength when used together sincethe AFM does not encountervacuum issues (difficult samplepreparation, sample modification,etc.) and may image samples in anenclosed fluid or other environ-ment. The vacuum environmentof the SEM makes it possible toconduct a number of techniquesthat require vacuum, such as X-rayanalysis. By having both tech-niques side-by-side in an analyticalfacility, the overall scope ofanalytical capabilities is broad-ened, adding to the flexibility ofthe facility.

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O-ring Seal

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Figure 18. Living human vascular endothelial cells imaged in culture media byTappingMode AFM. These images, collected at 30 minute intervals, reveal themovement of living cells which were incubated with 200ng/ml vascular endotheliagrowth factor (VEGF). With TappingMode, the nucleus as well as othersubmembraneous structures are visible. These cells appear flatter and moreelongated compared to control (untreated) cells. 65µm scans. Sample courtesy ofGeorges Primbs, Miravant Inc.

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16. Kelliher, J.T., Thornton, J.,Dietz, N., Lucovsky, G.,Bachmann, K.J., “Low tempera-ture chemical beam epitaxy of

gallium phosphide/siliconheterostrucutres, Materials Scienceand Engineering, B22 (1993) 97.

17. Babcock, K., “Magnetic ForceMicroscopy: High resolutionimaging for data storage,” DataStorage, September 1994.

18. Mou, J., Czajkowsky, D.M.,Sheng, S., Ho, R., Shao, Z.,“High resolution surface structureof E. coli GroeS Oligomer byAtomic Force Microscopy,” FEBSLetters 381 (1996) 161.

TappingMode, LiftMode, and PhaseImaging areregistered trademarks of Veeco Instruments, Inc. AN46 2/01

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Distributors World Wide

To Contact the Author:

Phil Russell or Dale BatchelorAnalytical Instrumentation FacilityCollege of EngineeringNorth Carolina UniversityBox 7531Raleigh, NC 27695-7531Email: prussell@ncsu.edu,

dale_batchelor@ncsu.edu

John ThorntonDigital Instruments/Veeco MetrologyEmail: john@di.com