Conductive AFM

CAFM is a powerful current measuringtechnique for mapping variations inelectrical conductivity of materials(figure 3). CAFM can be applied tomaterials with medium conductivity (1pA to 1µA). The CAFM applicationmodule can be operated in eitherimaging or spectroscopy mode. In theimaging mode, images of the electrical current are obtained, while in thespectroscopy mode one can collectcurrent-voltage (I-V) or current-force (I-Z) spectra.

In the imaging mode, an electricallyconductive probe is scanned over thesample surface in contact mode as afeedback loop keeps the deflection ofthe cantilever constant while localheight of the sample is measured.During scanning, the user can apply a DC bias between the tip and thesample. A low-noise linear currentamplifier senses the resulting currentpassing through the sample as thetopography image is simultaneouslyobtained (see figure 2). The observedcurrent can be used as a measure forthe local conductivity or electricalintegrity of the sample under study.

In addition to the imaging mode,CAFM also measures local current-voltage (I-V) or current-force (I-Z) spectrausing the spectroscopy mode. In orderto obtain I-V spectra, the imaging scanis stopped and the tip is held in afixed location while the sample bias isramped up or down. The resultingcurrent through the sample is plottedversus the applied bias (figure 3a).User-selectable parameters include startand end voltage of the ramp, rampdirection, ramp rate, and delay timesbetween individual ramps. Thesoftware can either record a singlespectrum or average over multiplespectra. For some measurements, it isdesirable to limit the current passingthrough the sample. In this case, thesoftware provides the user with a“trigger” option, which stops thevoltage ramp as soon as the user-selected current value is reached. Inorder to obtain I-Z spectra, the samplebias is kept constant, while the scanneris moved in the Z-direction, similar tothe measurement of force-displacementcurves. The resulting current through the sample is plotted versus the Z-position of the scanner. Again,several parameters allow the user toperform and control specific I-Zramping experiments.

An I-Z spectrum is displayed in figure4. Here the CAFM module was usedto study the current on a conductivepolymer sample as a function of theapplied force, or cantilever deflection.The electrical current, sensed by theCAFM module, is monitored whileperforming a standard force-displacement ramping cycle. Thecurrent spectrum displays the details of the change in conductivity for thepoint contact as the probe makescontact with the sample and the forceis increased.

CAFM can be applied in manyresearch and manufacturing areas for the analysis of a wide range ofmaterials. It can also be used tolocalize and image electrical defectsin semiconductor and data storagedevices. In addition, CAFM can beapplied to characterize conductivepolymers and other materials withnonuniform conductivity, such assemimetals, semiconductor materials,conductive organic materials,nanotubes and others.

Figure 3a. TUNA Spectroscopy: Current-versus-Voltage (IV) plots with AFM tippositioned on weak spots of thin film (HfO2).(Averages from different locations) revealbackground leakage current dropped withannealing, and average current at a givenvoltage increased after annealing, withincrease annealing temperature.

Figure 3. TUNA images map the conductivity of HfO2 as deposited (left) and post-anneal at700C (middle) and 800C (right). These maps allows visualizing, and quantifying the size anddistribution of weak spots in the dielectric. Darker color means higher current through the tip,indicating higher conductivity, and thus the weaker spots in the dielectric. Weak spots emergeand proliferate upon annealing, more at the higher anneal temperature. 500nm scans courtesyJasmin Petry, IMEC, Belgium.

TUNA can be applied in manyresearch or manufacturing areas and on a wide range of materials. Itcan be used to study the thicknessuniformity or interface roughness of thindielectric films, such as gate oxides. It can also be applied to localize and image electrical defects insemiconductor or data storagedevices. In addition, TUNA can beused for the study of conductivepolymers or organics, and other low-conductive materials (such assemimetals, semiconductor materials,etc.). Some typical examples follow.

Characterization of thin silicon oxide (SiO2) filmsOne of the most demanding steps inthe manufacturing of semiconductordevices is the gate dielectric. The thindielectrics films (often SiO2 or high-kdielectrics) are used as gate oxides infield effect transistors and as tunnelingoxides and dielectrics for memorycapacitors like dynamic randomaccess memory (DRAM) andelectrically erasable andprogrammable read only memory(EEPROM) devices. A structurally andelectrically homogeneous oxide is ofprimary importance in order to complywith the requirements for reliability andlong-term stability of the gate andtunneling oxides. Otherwise,degradation and breakdown lead toearly device failure. Even variation ofoxide thickness in the angstrom rangecan have a large impact on theelectrical behavior of the transistor andmemory devices. With decreasingoxide thickness this problem becomesmuch more severe. Surface andinterface roughness result in increasingoxide fields and enhance leakagecurrents and Fowler-Nordheimtunneling, leading to fast degradationand limiting the scaling of oxides for metal-oxide-semiconductor (MOS) devices.

Tunneling AFM

TUNA measures ultralow currents onlow-conductivity samples. As withCAFM, a DC bias is applied betweenthe sample and the conductive tip asthe tip is scanning the sample incontact mode. A linear currentamplifier with a range of 60 fA to120 pA senses the resulting currentpassing through the sample. In thisway, the sample’s topography andcurrent are measured simultaneously,enabling direct correlation of a samplelocation with its electrical properties.The noise level of the TUNA module(typ. 50 fA) allows one to performextremely sensitive current measure-ments. In addition, the TUNA modulealso allows local measurement ofcurrent-voltage spectra on the sample.

The TUNA technique is especiallyuseful for evaluation of thin dielectricfilms such as gate oxides (often SiO2)in transistors. In TUNA, the currenttunneling from the tip through thedielectric film strongly depends on filmthickness, leakage paths (possiblycaused by defects) and charge traps.All of these may significantly affect the properties and the integrity of thewhole film, thus compromising anentire device’s performance.

Figure 4. Force-displacement (top) and I-Zspectrum (bottom) measured simultaneouslywith the CAFM module.

Conventional measurementtechniques—macroscopic current-voltage (I-V) and capacitance-voltage(C-V) spectroscopy, emissionmicroscopy (EM), and transmissionelectron microscopy (TEM)—areinadequate for locating and measuringthe degree of oxide thinning. Thesemethods either lack the requiredresolution or measure only structural(not electrical) information. TUNA onthe other hand, provides the spatialresolution and sensitivity required tomap effective thickness variations ofthin dielectric films. A bias voltage,applied between sample and probe,gives rise to a tunneling current (hencethe name Tunneling AFM) that stronglydepends on the local electricalproperties, i.e., the effective electricalthickness of the oxide. The conductiveprobe, the oxide film underinvestigation, and the semiconductorsubstrate form a local MOS structure.Generally, the current (often Fowler-Nordheim) increasesexponentially with a linear change in film thickness, thus providing a very sensitive technique to monitorthickness variations.

Figure 5. TUNA current measurements takenon a 5nm thick SiO2 gate oxide at increasingsample bias voltages from left to right: (top)6V, 7V, (bottom) 8V, and 9V. 0.5µm scans.

Figure 8. Topography (left) and tunneling(right) current images of an 8.5nm thick tunneloxide (SiO2) measured at a sample bias of10V. 2µm scans, 200 fA current scale. Image courtesy A. Olbrich, Infineon, Munich, Germany.

Figure 7. Topography (left) andsimultaneously-obtained tunneling currentimage (right) at the transition from a field oxideto a 40nm thick gate oxide. At the transitionthe tunneling current is increased, whichindicates thinning of the silicon oxide. 1µmscans, 0.5 pA current scale. Image courtesyA. Olbrich, Infineon, Munich, Germany.

Figure 5 shows a series of TUNAcurrent images taken at different biasvoltages on the bare surface of theoxide film. One can clearly observethe increase in current upon increasingthe bias voltage. The current datataken at the highest voltage (9V) shows localized high-current spots inthe scanned area, indicating electricalbreakdown of the oxide film in these locations.

Specialized software also permits themeasurement of I-V spectra in a singlelocation on the sample surface. Figure6 shows a typical I-V spectrummeasured on the 5nm thick gateoxide, obtained by ramping the biasvoltage from 0V to 2V. The

spectrum shows the exponentialdependence of the current on theapplied bias voltage.

TUNA can also be employed tomonitor weak spots andinhomogeneities of thin dielectric filmsor oxides. Structural defects on thesurface, as well as structural andelectrical inhomogeneities within theoxide (e.g., SiO2) and at the interfaceto the substrate (e.g., Si), can beinvestigated. Figure 7 shows thetopography and simultaneously-obtained tunneling current image (atconstant sample bias) at the transitionfrom a field oxide to a 40nm thickgate oxide. The topography imageshows (from left to right) the gateoxide, the interface region, and thefield oxide. At the edge between thetwo regions an increased Fowler-Nordheim tunneling current ismeasured. This indicates localstructural thinning of the gate oxideduring the fabrication of the fieldoxide, which serves as an insulatingarea between adjacent active regions.

In figure 8, topography and tunnelingcurrent images of an 8.5nm thicktunnel oxide for an EEPROM deviceare displayed. The tunnel oxide (SiO2)is enclosed by a thicker field oxide tothe left and right. The data wasobtained at a sample bias of 10V.Whereas the field oxide is too thick to

show any measurable current at thegiven sample bias, the tunnel oxideshows inhomogeneities in currentdensity, which indicates variations inthe effective oxide thickness.

Imaging of ultrathin aluminum oxide (Al2O3) films As an alternative to traditionalnonvolatile memory devices, magneticrandom access memories (MRAMs) arebeing investigated and developed.MRAMs operate based on thetunneling magneto-resistive (TMR)effect, and their essential part is ametal-insulator-metal (MIM) tunneljunction. Successful operation of these structures requires a chemicallyhomogeneous (free of impurities)insulating barrier, as well as minimalfluctuations of the barrier thickness.Therefore it is important to spatiallyresolve the tunnel barrier and relate itto the macroscopic tunnel magneto-resistance. While conventionaltransmission electron microscopy (TEM)and x-ray photoelectron spectroscopy(XPS) studies provide globalinformation on the atomicorganization, surface-interfacestructure, and chemical composition,these techniques give incompleteinformation on the tunnel barrier qualityat the atomic scale because theyaverage over depth and surface. Todate, TUNA is the only method that

Figure 9. Topography (left) and tunnelingcurrent (right) images taken on a 1.2nm thinaluminum oxide (Al2O3) film at a sample biasvoltage of 0.14V. 500nm scans, 5 pA currentrange. Data courtesy A. Olbrich, Infineon,Munich, Germany.

Figure 6. I-V spectrum taken on a 5nm thickgate oxide. The spectrum was recordedwhile ramping the bias from the start voltage(0V) to the end voltage (2V) forward, andthen back.

Figure 11. Tunneling current scans (left)measured on a magneto-resistive read/writehead covered with a thin diamond-likecarbon (DLC) film. Tunneling current image(right) of a similar head with defective DLCcoating. 20µm scans. Samples courtesy T.Ahmed, Seagate, Minneapolis, Minnesota.

data. Upon increasing the bias voltage from 1V to 5V, more dots appear in the TUNA current images. This canbe related to the size of the defectsand the depth of the defectsunderneath the top surface. Thisexample illustrates the possibility ofusing the TUNA technique to imageand localize subsurface electricaldefects, as well as to measure thedefect size and density.

Imaging of diamond-like carbon (DLC) films in data storageA different application for TUNA canbe found in the characterization of thindielectric films used in the data-storageindustry. A continuous effort is aimedat improving the performance andreliability of magneto-resistive (MR)read/write heads and disc media. Forprotection against corrosion and wear,the discs and heads are commonlycoated with a thin non-conductive DLCfilm. TUNA can be used to determinethe quality of these films. Whenapplying a bias to the disc or head,the tunneling current is an excellentindicator of leakage paths, smallinhomogeneities or defects (electricalcontaminants, short circuits, DLCthinning, etc.) in the DLC coating.

Figure 11 shows the tunneling currentimages of two MR-heads with uniform

allows characterization of the localelectrical properties of these films withvery high lateral resolution. Figure 9shows the topography and thetunneling current image of a 1.2nmthick Al2O3 layer, which lends itself asa very good insulator for MRAMs dueto its large band gap (about 8 eV).Local variations in effective electricalthickness result in elevated currents—up to several orders of magnitude. It isnoticeable that most often, areas withincreased tunneling current seem tocorrespond with topographicallyelevated features.

Imaging of defects embedded insilicon oxide (SiO2) filmsAn important application of TUNA isthe localization and identification ofelectrical defects in thin dielectric films.In figure 10, a thin SiO2 film wasembedded with a controlled amount ofquantum dots, which can be viewedas small electrical defects. The smalldots could not be observed whenusing standard SPM topography, butclearly show up in the TUNA current

Figure 12. Tunneling current scans measuredon two magnetic disc media covered with a thin (left) and slightly thicker (right.)DLC film. 0.5µm scans, 20 pA current scale.Sample courtesy J. Leigh, Seagate, Fremont, California.

coating. The topography image showslittle detail of the MR head, whereasthe TUNA current data looks throughthe DLC film and shows the different(metallic) regions of the MR headrevealing defective coating. Thetunneling current images clearly showsweak spots in the coating of thedefective MR head. The application ofTUNA on magnetic disc media,covered with a thin DLC film, isillustrated in figure 12. Thetopographic and TUNA current dataare displayed for two samples, with adifferent DLC-coating thickness. The leftimage displays the data obtained on adisc with a thin DLC film. The TUNAcurrent data vary between about 0and 20 pA and show a strong spatialvariation correlating with the polishingscratches present on the discs. Theright image displays the data obtainedon a disc with a slightly thicker DLCcoating. The average tunneling current is muchlower, corresponding to the change infilm thickness. Also, the correlation tothe surface morphology is much lesspronounced. This example illustrateshow the TUNA technique cansuccessfully assist in optimizing thethickness, composition and processingconditions of the DLC films.

Figure 10. Sequence of TUNA imagesobtained on a SiO2 film with embeddeddefects. The sample bias voltage was from left to right: (top) 1V, 2V, (bottom) 3V, and 5V. 1µm scan, 1 pA current scale.Samples courtesy S. Madhukar, Motorola,Austin, Texas.

Imaging of thin ferroelectric andpiezoelectric filmsAnother important group of materialsfor applications in both MEMS andmicroelectronics are piezoelectric andferroelectric materials. Of particularinterest is BST (BaXSr1-XTiO3), whichserves as a high-epsilon dielectric involatile memory devices (DRAM) andPZT (PbZrXTi1-XO3), which may be usedin ferroelectric memory devices. Theseoxides are polycrystalline, and so farthe correlation between microscopicstructure and their electrical propertiesare not well understood. Again, TUNAcan be a useful technique to analyzelocal properties of these films.Enhanced current flow is noticeablealong the grain boundaries, whereasless current is observed on individualgrains. This behavior can, for example,explain the undesired leakage currentof the ferroelectric capacitorsfabricated with this type of film.

The TUNA data obtained on a 500nmthick BaTiO3 layer, is displayed infigure 13. A higher current is observedat some of the grain boundaries, aswell as along some longer lines. Oneof these crack-like lines is displayed inthe current image. The presence ofthese high-leakage current lines maybe explained by stress phenomena in the imaged film. This effect is onlyobserved in the TUNA current data.

TUNA and CAFM combined imagingof conductive polymersIn addition to the previously describedinorganic materials, TUNA has alsoproven to be very useful for organicmaterials like conductive polymers. The data displayed in figure 14 wasobtained using low spring constantprobes on a 100nm thick poly-phenylene vinylene layer (PPV) on topof a 200nm thick poly-aniline (PANI)layer. The PPV is a light-emittingpolymer, which is spin-cast on top of a conductive buffer layer (PANI) tofacilitate the transport of chargecarriers. This particular sample had thePPV layer partially peeled off to enablemeasurements of the PANI layerunderneath. The PPV part of thesample is slightly higher and visible onthe left in the topography image. Thecorresponding current image clearlyshows a uniformly lower conductivity of the PPV compared to theinhomogeneous PANI buffer layer.

A second conductive polymerexample, with higher spatial resolution,is shown in figure 15. The CAFMtechnique was used to map the spatialvariation of the conductivity in a thinpoly-analine film deposited on anIndium Tin Oxide substrate. Therelatively high conductivity of thissample required the use of the CAFMmodule, instead of the TUNA module.

The observed currents varied between0 and 200 pA. The CAFM imageshows the high conductivity of thelarge areas covered with PANI, and a slightly lower conductivity for thesmaller, isolated spots covered withPANI. The substrate appears as apoorly conductive region. The lowspring constant probe employedallows scanning at very low contactforces, hence minimizing thedeformation or wear of these relativelysoft samples.

Figure 13. Topography (left) and tunneling current (right) imagesof a thin BaTiO3 ferroelectric film. 2µm scan, 2 pA current scale.Sample courtesy H. Ruda, University of Toronto, Canada.

Figure 14. Topography (left) and TUNA current (right) imagesof a 100nm PPV layer on a conductive PANI layer. The samplebias voltage was -6V. 50µm scan. Sample courtesy C. Zhang,Uniax, Goleta, California.

Figure 15. Topography (top) and CAFMcurrent (bottom) images of a poly-analine film on an indium tin oxidesubstrate. 2µm x 1µm scan, 200 pA currentscale. Sample courtesy S. Rane, University ofChicago, Illinois.

Scanning CapacitanceMicroscopy

The continuing miniaturization ofsemiconductor devices has created a serious challenge for traditionalmaterials analysis techniques, such as secondary ion mass spectrometry(SIMS), spreading resistance profiling(SRP), and capacitance voltage (C-V)measurements. While the accuracy,reliability, and improved capabilities of these instruments provide the basisfor current materials characterizationdata, their one-dimensional limitation,inability to measure sub-0.1-mmfeatures, and limited characterizationrepertoire have increased the value of scanning probe techniques. SCM was one of the first SPMtechniques to find its way into theadvanced semiconductor analysisworld. SCM instruments can show thecarrier concentration profiles in twodimensions in actual semiconductordevices, as well as the relationship of these profiles to critical devicestructures. This capability makes theSCM module useful in thedevelopment, manufacturing, testing, and failure analysis ofsemiconductor devices.

In SCM, the metalized probe forms ametal-insulator-semiconductor (MIS)capacitor with the semiconductorsample. An AC bias applied betweenthe scanning contact AFM tip and the

sample generates capacitancevariations, which are monitored usinga gigahertz resonant capacitancesensor. This system has been shown to be sensitive to variations smallerthan attofarads (<10–18 F). Thecapacitance variation (dC/dV) is a measure of the local carrierconcentration density and type (n-typeor p-type), and can therefore be usedfor high-resolution two-dimensionalcarrier profiling.

2-D carrier profiling in semiconductordevice structuresOne of the most important applicationsof SCM is two-dimensional carrierprofiling of semiconductor devicestructures. Both silicon and compoundsemiconductors are of great interest.Two-dimensional dopant profiling is a high priority on the InternationalTechnology Roadmap forSemiconductors, and it is expected tobecome an enabling technology fornext-generation device manufacturing.SCM provides the spatial resolution(about 10–20nm), and dynamic range(1015–1020 atoms/cm3) to answerthese needs.

Figure 16 shows the topography (left)and SCM dC/dV (right) imagesobtained simultaneously on a cross-

sectioned transistor from a Pentium IIprocessor. The topography shows thegate region with the two spacers(bright areas), but shows no details ofthe carrier profiles. The SCM dC/dVimage shows the differently dopedareas of the transistor: source, drain(both high- and low-dose implants arevisible) and gate. The SCM imagecan be used to extract valuableinformation such as the effective gatelength, junction depth, or details onthe lateral and vertical extension of the doped regions. With the exception of SSRM, this crucialinformation is not accessible throughany other technique.

In addition to imaging, SCM can alsobe used to measure dC/dV versus Vcurves in selected positions on thesample. In figure 17, the DC samplebias is ramped between two user-selected values, and the SCM sensoroutput (dC/dV) is monitored andplotted. The top curve was measuredon an n-type sample, while the bottomcurve was measured on a p-typesample. As expected, different typesresult in a different sign of the dC/dVsignal, and different dopant levelsresult in a different intensity of thedC/dV signal.

Figure 16. Topography (left) and SCM dC/dV (right) of a cross-sectioned transistor of a Pentium II processor. 1.25µm scan.

Figure 17. dC/dV versus V curves measured with SCM ontwo different locations of a sample: n-type, 2x1017 atoms/cm3

(top curve) and p-type, 3x1019 atoms/cm3 (bottom curve).

Figure 18 illustrates topography (left),amplitude (middle), and capacitance(right) of top metal and dielectricetched layer of a Silicon DRAM cell.Figure 19 illustrates capacitance (left) and topography (right) of an ion-implanted Silicon structureconsisting of an n-type implant into a p-type substrate.

SCM is also a very valuable tool forfailure analysis on semiconductordevices. The SCM technique allowsvisualization of whether a particularimplantation is present, whether it isthe correct type (n or p), the expecteddimensions (both in-depth and lateral),and other characteristics. A typicalfailure analysis example is shown infigure 20. A cross-sectional surfacewas made through a number ofdevices, including one known to havebad electrical device characteristics.The SCM image of a good device isshown in the top image; the SCMimage of the corresponding area ofthe bad device is shown in the bottomimage. The bottom image indicatesthat the two implanted regions (i.e. thebright areas: n-type implant and n-type

well implant) are touching, whereasthey should be separated from eachother. This “short circuit“ resulted in avery high leakage current of thisparticular device.

Ferroelectric filmsFerroelectric thin films are veryattractive for their possible applicationsin nonvolatile memories andmicroelectromechanical devices(MEMS). The decrease in size (downto tens of nanometers) of these devicesrequires an appropriate description ofthe material properties and processesin ferroelectric films. For example, it isfundamental to investigate whetherferroelectric structures with nanometerdimensions still exhibit ferroelectric andpiezoelectric properties, and to studyhow these properties are affected bythe overall size. SCM is a possibletechnique for these studies. Forexample, SCM provides a method formeasuring the sign of the C-V slope forthe ferroelectric sample and, hence,the domain or polarization state in thethin film. SCM has been used to imageand manipulate the domain structure ina thin Pb1.0(Nb0.04Zr0.28Ti0.68)O3(PNZT) ferroelectric film. On an area of25x25µm, the tip was scanned with aDC sample bias voltage of -12V andsubsequently smaller areas were writtenwith DC sample bias voltages ofopposite polarity. A small AC biasvoltage was used to image thepolarized regions, i.e., to study themagnitude and sign of the slope of theC-V curve at zero DC bias. Figure 21shows the SCM image on the rightand the corresponding AFMtopography image on the left. The darkand light contrast regions indicateoppositely polarized regions, wherethe dC/dV signal is of high strengthbut opposite sign.

In addition, SCM can also measure thepolarization (or C-V) curves of smallferroelectric capacitors, or even of

Figure 19. Capacitance (SCM) dC/dVphase (left) and height/topography (right)images of an ion-implanted Silicon structureconsisting of an n-type implant (dark areas)into a p-type substrate. An additional(shallower) p-type implant can be observed.The structure has been cross-sectioned bystandard polishing techniques.

Figure 20. SCM image of a good (top) andfailed (bottom) silicon device. The bottomimage displays a short circuit between the twodoped regions. 8µm x 2µm scan.

Figure 18. Height or topography (left), Capacitance (SCM) dC/dV amplitude (middle), andCapacitance (SCM) dC/dV phase (right) images of a conventional Silicon DRAM cell. Top metaland dielectric layers were etched to expose Silicon. The SCM dC/dV phase image differentiates p-type (bright color) and n-type (dark color) doped areas. npn and pnp transistors in DRAM cell arethen clearly visualized, allowing extraction of critical parameters, e.g., effective gate length, andvisualizing defects. The SCM dC/dV amplitude image shows relative magnitude of dopantconcentration: bright color means larger depletion capacitance, and therefore, lower doped areas(e.g., well implant areas).

Figure 21. Topographic (left) and SCM dC/dV image (right) ofa PNZT ferroelectric film. Dark and bright areas correspond tooppositely polarized regions. The different polarization regionswere written using the SCM at different scan sizes and samplebias voltages prior to this SCM scan. 25µm scan. Samplecourtesy Ch. Ganpul and M. Ramesh, University of Maryland.

Figure 22. Height/topography (left) and capacitance (SCM)dC/dV phase images (right) of FerroElectric thin film on top ofplatinum electrode. Height image shows granular structure of thinfilm with 20-100nm sized grains. The SCM dC/dV phase imageshows individual grains’ polarization states; it is obtained byapplying a small amplitude AC voltage between tip and samplethat modulates the capacitance at the same frequency (DC voltageis kept at 0V not to change the polarization state by the presenceof AFM tip). Ferroelectric polarization state is determined by chargemeasurement performed in SCM. The resolution of the SCMtechnique allows one to observe variations within single grains.

single ferroelectric grains. This isimpossible with conventional probingtechniques. Figure 22 illustratestopography (left) and capacitance(right) of FerroElectric thin film on top ofplatinum electrode. Whereas figure 23shows approximately the same areasas in above images, but now with a5V DC applied between tip andsample. The DC voltage changes thepolarization such that all grains havethe same polarization state. And lastly,figure 24 shows hysteresis typical of aferroelectric domain on a single grainwith localized capacitancespectroscopy versus applied DC bias (V).

Scanning Spreading ResistanceMicroscopy

Like SCM, SSRM is often used tomeasure dopant profiling insemiconductors, but it does so byquantifying electrical conductivity orresistivity. SSRM provides two-dimensional information on theelectrical conductivity or resistivity ofthe sample under study. Veeco has

Figure 23. Approximately the same areas asin above images, but now with a 5V DCapplied between tip and sample. The DCvoltage changes the polarization such that allgrains have the same polarization state.

Figure 24. Localized capacitancespectroscopy (exclusive to SCM) dC/dVversus applied DC bias (V) on a single grainshows the hysteresis typical of a ferroelectricdomain. The data can be integrated once toobtain a Capacitance-versus-DC bias curve,and twice to obtain a relative Polarization-versus-DC bias curve.

developed and patented this techniquein collaboration with IMEC, Belgium.In SSRM, an electrically conductiveprobe is used to measure the sample’slocal resistivity. When the probe isscanned in contact mode over regionswith different resistivity r, the electricalresistance R formed by the probe-sample contact will varyproportionally. If the contact isassumed to be circular and of Ohmicnature, the relation between R and r isgiven by the basic spreadingresistance formula: R = ρ / 4r,whereby r is the radius of the contact.Since the resistance can vary overseveral orders of magnitude, alogarithmic current amplifier is used forSSRM. The logamp has a currentrange of seven orders of magnitudefrom 10 pA up to 0.1 mA. A majorapplication of SSRM is themeasurement of the two-dimensionaldistribution of electrical carriers insidesemiconductor structures.

2-D carrier profiling in semiconductordevice structuresWhile the probe is scanned across the cross-section of the semiconductordevice, the electrical resistance ismeasured between the conductive tipand a large current collecting backcontact. When the applied forceexceeds a certain threshold force, the measured resistance is dominatedby the spreading resistance. On Sistructures, high forces (typically, a fewµN) are required in order to penetratethe native oxide and to establish astable electrical contact. Sincestandard AFM probes deform at thesehigh forces, doped diamond ordiamond-coated silicon probes areemployed. The extreme hardness, highYoung’s modulus, and the electricalconductivity obtained through dopingmake diamond particularly suitable foruse as SSRM tip coating material.

In figure 25, analysis has beenperformed on a Si DMOS transistorstructure. The transistor structure wascross-sectioned to expose thedifferently doped regions, and thenpolished using standard polishingtechniques. The topographical image(left) shows quite clearly the Al-contact(black region), the underlying oxide(brown) and the polysilicon andunderlying gate oxide. The SSRMresistance image (right) shows theelectrically active regions. The different

colors reflect different levels ofresistivity: dark indicates highlyconductive regions, and brightindicates low conductivity. Clearlyobservable are the highly doped n+-substrate (black); the lower doped n-epilayer (dark brown); the p+-body(which appears as highly resistive), then+-implant (black), the metal and theoxide regions, as well as the highlyconductive polysilicon material.Junction positions correspond to thesharp transition between the variouscolor levels.

Figure 26 illustrates an SSRM imageof a cross-sectioned capacitorfeaturing a metal think film area withdielectric between them. SSRMimages are often complementary to

SCM images, as SCM images giveno signal on dielectrics and metals,whereas SSRM shows a big contrastbetween the two.

In a second example (figure 27),SSRM imaging was performed on asilicon MOSFET with a gate length of0.25µm. The image (left) shows thedifferently doped regions – source,drain, and gate – as highly conductiveareas (dark), dielectric and substrateas low-conductivity areas, as well asthe intermediately conductive area.The source and drain junctions areobserved as thin, bright lines of lowconductivity. A section made throughthe source region of the transistor isalso shown in figure 27 (right). Thesection shows from left to right: the

Figure 25. Topography (left) and resistance (right) scans of across-sectioned Si DMOS transistor. 12µm scan courtesyIMEC, Belgium.

Figure 26. SSRM height and resistance images of cross-sectioned capacitor. The capacitor has metal thin film area withdielectric between them. If there were leakage paths betweenthe different electrical contacts, the resistance image wouldshow them (none found in this image).

Figure 27. SSRM resistance scan of a cross-sectioned 0.25µm silicon MOSFET transistor.2µm scan courtesy IMEC, Belgium.

Summary

The Digital Instruments MultiMode,Dimension 3100/5000 and CP-IISPMs set the standard for ultrahigh-resolution topographical profiling andmapping of sample hardness,elasticity, friction, adhesion, andmagnetic or electrical field strength.Now, with the addition of add-onapplication modules for CAFM,TUNA, SCM, and SSRM, theseindustry-leading instruments can beenhanced to map nanometer-scaleresolution on a wide range ofadditional properties on variousmaterials including low- and mid-strength electrical currents, resistance,and capacitance. These enablingtechnologies, which are already beingused successfully in both industry andscience, promise to play ever-increasing roles in future processoptimization and scientific research.

Figure 28. Topography (left) and SSRM resistance (right) scanof an InP-based heterostructure. 7µm scans. Sample courtesy ofM. Geva, Lucent Technologies, Breinigsville, PA.

dielectric top layer, source implant (p-type), junction peak, well (n-type),and substrate. The junction depth caneasily be extracted from this section as the distance between the junctionpeak and the dielectric top layer, andis found to be 184nm.

In addition to silicon, compoundsemiconductors are also of greatinterest. LEDs, photodetectors, anddiode lasers are only a few of themany devices fabricated from III-V andII-VI semiconductors. Knowledge of thetwo-dimensional distribution ofconductivity (the electrical-activedopant atoms in particular) isimportant for process development andmonitoring. For compoundsemiconductors, sample preparation isminimal; simple cleaving provides thebest surface on complex samples, andallows imaging of critical deviceproperties after only minutes of samplepreparation. Metal and metal-coatedsilicon tips prove to be sufficiently rigidfor stable and reproducible SSRMmeasurements. For good signal-to-noise ratio, a relatively high biasvoltage (several volts) in combinationwith a medium probe pressure (sub-µN) is required. An example of thehigh-resolution carrier density profilingavailable with SSRM can be seen inthe cross-sectioned InP-basedheterostructure shown in figure 28. The resistance image shows the

Figure 29. Topography (left) and SSRM resistance (right) scanof a granular metal film. 500nm scans.

different regions of the heterostructure:alternating Zn-doped p-type and S-doped n-type layers with differentthickness values. The image revealsthe two-dimensional nature of thelayers toward the mesa area. Thisexample demonstrates SSRManalytical power in two-dimensionalimaging of carrier distributions incompound semiconductor structures,and in particular, for the analysis ofInP hetero-structures, an area that isgaining high interest.

Conductivity mapping ofnonsemiconductor materialsSSRM can also be used for studyingthe electrical properties of nonsemi-conductor materials. This includesapplications for metals, semimetals,conductive polymers, and otherintermediately conductive materials.For optimum performance, differentmaterials often require different probematerial and force set points.

In the example in figure 29, a Cu film is imaged. The granular structureof the film is clearly observed in thetopographic data. The SSRMresistance data show that the resistivityof the grains is higher towards theedges, as compared to the center ofthe grains. Note that the averagegrain size is 30nm, and the spatialresolution is on the order of 5nm.