Nanorobotic technologiesfor nanoelectronic devices.

NNANOMATERIALS HAVE BECOMEone of the most important researchdirections in nanotechnology. Theextraordinary physical properties ofthese materials are the reasons for theirseveral potential applications withunachieved performance. This articlefocuses on nanomaterials referring tostructures with nanometer sizes in atleast one dimension: monolayered sheetsof carbon, so-called graphene [1], is thebest-known example for two-dimen-sional nanostructures, while nanotubesand nanowires [2] made of different

materials, such as Ag, Au, BN, C, Si,TiO2, and ZnO, are fascinating one-dimensional structures in which carbonnanotubes (CNTs) [3] are the mostprominent example.

These nanomaterials have directapplications as conductive layers andleads; and their large surface-to-volumeratio provides extraordinary electrome-chanical or electrochemical interactionsfor applications as transducers. In partic-ular, these nanomaterials may improveand downscale existing nanoelectronicdevices and facilitate the exploration of

novel actuator and sensor technologieson the nanoscale.

With current fabrication techniques,the exact geometric and crystallographicproperties of nanomaterials, and thustheir physical characteristics, are notcompletely controllable. Nonetheless,chemical vapor deposition (CVD)-basedtechniques especially may become com-pletely compatible with standard micro-fabrication techniques in the near futureand seem to be the best approach to real-izing the direct synthesis of carbon-based nanomaterials in future devices.

Digital Object Identifier 10.1109/MNANO.2011.2181735

Date of publication: 23 March 2012

SERGEJ FATIKOW, VOLKMAR EICHHORN, AND MALTE BARTENWERFER

NANOTUBES COURTESY OF WIKIMEDIACOMMONS/MICHAEL STR €OCK.

NANOROBOT COURTESY OFCAN STOCK PHOTO/PAUL FLEET

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For other materials and components,a variety of fabrication techniques isavailable: vapor–liquid–solid growth,chemical vapor and laser deposition, aswell as conventional lithography areonly a few. Despite the fact that all ofthese techniques are well known and thegeneral quality of the fabricated nano-materials is high and reproducible, theystill suffer from slight tolerances in size,composition, and purity. To optimizethe fabrication techniques and allow theassembly of prototypic devices, reliablehandling and characterization of thesenanomaterials is required. For this rea-son, the micronanointegration of thesenanomaterials into existing microsys-tems remains a challenge.

Automated nanorobotic systems areone of the most promising enablingtechnologies for this challenge [4],[5]. A novel nanorobotic approach isdescribed in this article, providing aversatile tool for the handling and char-acterization of individual nanomaterials.Furthermore, three application scenariosare described, presenting the electricalcharacterization of as-grown CNTs aswell as manipulation and mechanicalcharacterization of Si-nanowires andgraphene flakes, both coming directlyfrom their fabrication process withoutany further treatment.

GRAPHENE AND CNTs ININTEGRATED CIRCUITSEspecially in microchip technology, theongoing miniaturization of the transis-tor’s feature size in microelectronicdevices leads to the increase in currentdensity and power dissipation per inte-grated circuit (IC) unit area. Electricalinterconnects for layered chip designsmade of standard copper (Cu) willbecome a major hurdle in future micro-processing units and application-specificICs. The International TechnologyRoadmap for Semiconductors showsthat the Cu resistivity rises as a functionof line width and aspect ratio. The sizeof nanostructured interconnects reachesthe same dimension as the mean freepath of Cu, and there is a significantcontribution to the increase in resistivityfrom both grain boundary and interfaceelectron scattering. For interconnect

line widths below 32 nm, the resistivitycaused by these two effects exceeds thebulk resistivity of Cu. In addition, theincreasing power and current density ledto electromigration defects in copperinterconnects. Novel materials are re-quired for both horizontal and verticalinterconnects (VIAs).

CNTs are considered as a potentialsolution for improving the performanceof on-chip interconnects in terms ofspeed, power dissipation, and reliabilitybecause of their unique properties suchas large electron mean free path and cur-rent-carrying capacity [6]. CNTs attractmajor research interest in their applic-ability as very-large-scale integrationinterconnects for future generations inchip technology [7]. At the same time,graphene as a two-dimensional materialwith excellent electronic properties isa promising material and may not re-place but enhance the silicon-basedCMOS era [8]. Figure 1 illustrates the

vision of a carbon nanoelectronic-en-hanced IC design.

NANOWIRES IN SENSORAPPLICATIONSOne-dimensional objects, in general, aswell as their application as nanowires,in particular, offer a specific advantage.Their ratio of surface to volume is excep-tionally high, and for this reason, they arepredestinated to act as transducers, par-ticularly, in sensors. This is due to thelarge ratio results in high sensitivity, highperformance, low energy consumption,and low detection limits and noise. Thus,nanowire-based sensors are very promis-ing in sensor applications for gases, ex-plosives, and light [9]. Furthermore, anadditional functionalization of nanowiresopens the entire field of label-free biolog-ical sensors and extends the possibleapplications to an unlimited domain ofall kinds of chemical, biological, and bio/chemical sensors [10], [11].

CuInterconnect

Single-Walled CNT (SWCNT)Bundle (VIA)

3-DIntegrated

CircuitDesign

Graphene FET

FIGURE 1 Illustration of a carbon nanoelectronic-enhanced IC design. Bundles of single-walled car-bon nanotubes are used as VIAs, and the channel of the FET is made of graphene.

Carbon-based nanomaterials such asgraphene and bundles of SWCNTs are

promising candidates to replace andoutperform classical materials in

integrated circuits.

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Besides this large variety of fields ofapplication, nanowires can act as trans-ducers in different ways. The moststraightforward principle is the detec-tion of conductivity change, and a morecomplex approach is the detecting ofchanges in the field-effect transistor(FET) characteristics. Both approachesbenefit from the fact that the electricalbehavior changes depending on theexposure to certain gases. Other possi-ble detection principles use the fact thatthe mass of a nanowire changes if mole-cules get attached to it. This can be usedto detect shifts in the resonance fre-quency of nanowires. However, for thedevelopment of all kinds of nanowire-based sensors, a key challenge is still thewell-directed placement of the nanowireitself onto an underlying substrate.

NANOROBOTIC SYSTEMS INSIDETHE DUAL-BEAM SEM/FIBA nanorobotic atomic force microscopy(AFM) system has been designed insidea high-resolution scanning electronmicroscope (HR-SEM), enabling thesimultaneous in situ operation of SEM,AFM, focused ion beam (FIB), and agas injection system (GIS) on the samesample spot [12].

Figure 2 shows the core system thatis mounted onto the standard SEMstage of a Tescan Lyra high-resolutionSEM. The core system features a coarsepositioning sample stage that is com-prised of SmarAct linear positioners andis currently capable of carrying up totwo samples. It provides fast coarse posi-tioning and easy switching of samplelocations across a range of up to 2 cm.

Because the linear positioners of thecoarse stage are equipped with opticalencoders, it permits the storage of sam-ple locations and the access to previouslystored locations on the sample with arepositioning accuracy of less than 50nm. This coarse positioning stage iscomplemented by a physik instrumente-Hera scanning stage that carries an AFMprobe for AFM operation but can alsocarry other end effectors, such as micro-grippers, to enable micro- and nano-manipulation. The scanning stage isequipped with capacitive positioningsensors and facilitates hysteresis andcreep-free, closed-loop AFM operationwith subnanometer precision in theworking range of 100 lm lateral and 50lm normal to the sample surface.

The nanorobotic system provides aversatile nanoworkstation that facili-tates reliable manipulation, characteri-zation, and processing of individualnanostructures. It is designed to per-form AFM-, scanning tunneling micro-scopy, and HR-SEM-based imaging,as well as electrical and mechanicalcharacterization of nanomaterials, andto machine nanomaterials by AFMcutting and FIB milling. Furthermore,this nanorobotic approach is an ena-bling technology for the flexible andsystematic prototyping of nanomate-rial-enhanced devices.

ELECTRICAL CHARACTERIZATIONOF CNT BUNDLESThe electrical characterization of nano-tubes and kinds of nanowires is chal-lenging because it is difficult tomechanically and electrically contactthese nanostructures. In addition, theinfluence of contact resistance betweenthe nanowire and electrode structureinterferes with the resulting conductivityin the case of two-point measurements.

To overcome this problem and elimi-nate the influence of contact resistance,the so-called four-point measurementsare used [13]. The device under test iscontacted by four individual electrodeswith equidistant pitches. An electricalcurrent is driven by the two outer elec-trodes, and the resulting voltage dropis measured by the two inner elec-trodes. Because this is a currentless

Scanning Stage

SecondaryElectron Detector

AFM Probe

SEM FIB

BackscatteredElectron Detector

Sample Stage

FIGURE 2 Core of the nanorobotic AFM setup inside the SEM/FIB system. A fine positioning scan-ning stage carries end effectors, such as AFM probes, while a coarse positioning stage facilitatesquick sample relocation and exchange.

The nanorobotic approach is an enablingtechnology for flexible and systematic

micronanointegration and thus the prototypingof nanomaterial-enhanced devices.

16 | IEEE NANOTECHNOLOGY MAGAZINE | MARCH 2012

measurement of the voltage drop, thecontact resistance does not influence theoverall measurement. The idea of four-point measurements is also valid for quasi-ballistic conductors such as SWCNTs[14]. The novel four-point probe (4PP)design that was presented in [15] canbe used to perform nondestructiveelectrical characterization of as-grownSWCNT bundles. According to Fig-ure 3, SWCNT bundles can be directlycontacted and their characteristic I–Vcurves can be recorded.

MANIPULATION OFGRAPHENE FLAKESThe nanorobotic AFM setup can notonly be used as an imaging device at thenanoscale but also as a nanorobotic toolfor the manipulation and mechanicalcharacterization of graphene flakes. Forexample, exfoliated graphene flakes canbe separated, manipulated, and placedonto special testing or device structures,facilitating their characterization ordevice prototyping. Figure 4(a) showsthe AFM-based nanomanipulation of anexfoliated graphene flake. Furthermore,CVD-grown graphene suspended ontransmission electron microscope gridscan be used to perform mechanicalcharacterization. For this purpose, thetip of the AFM probe deflects andindents the graphene membrane moni-toring force and displacement data[compare with Figure 4(b)].

Previous studies demonstrated thatelastic constants can be measured inatomically perfect nanoscale materials

based on simple mechanical models[16]. Such experimentally determinedvalues for the intrinsic strength can serveas a benchmark for structural and mech-anical applications. The presented nano-robotic AFM system facilitates the insitu visual observation of such character-ization processes.

MANIPULATION ANDCHARACTERIZATIONOF NANOWIRESDealing with the high surface-to-massratio is still one of the major challengesfor the manipulation of nanoscaleobjects. The vanishing gravity forceand strong adhesive force led to anuncontrollable sticking behavior forthese tiny objects. Especially, for the

handling of wires where along theplacement itself a certain orientationof the wire is desired, new challengesfor the manipulation strategy arise.One of the most promising ap-proaches for the well-directed han-dling and transfer of nanowires isadhesive bond handling (see Figure 5)[17]. A very sharp tungsten tiptouches the nanowire to be handled.Subsequently, the tungsten tip andnanowire are welded together by anadditional gas led in the SEM chamberand activated by the electron beam.With the same technique, whichestablishes a reliable electrical andmechanical junction, the nanowireis welded to an electrode structure.The separation of the nanowire and

10 μm 2 μm

(a) (b)

FIGURE 4 SEM images of AFM-based manipulation and mechanical characterization of suspendedgraphene. (a) AFM-based nanomanipulation of a freestanding graphene flake. (b) Mechanical charac-terization of suspended graphene layers by nanoindentation.

10 μm

(a) (b)

FIGURE 3 SEM images showing a typical nanorobotic characterization sequence approaching andcontacting the 4PP and SWCNT bundle. The 4PP has an electrode pitch of 3 lm.

10 μm

FIGURE 5 SEM image (recolored) of a proto-typic nanowire-based gas sensor. An Si-nano-wire (green) is placed between two chromiumelectrodes (golden) and is connected by anelectron beam-induced deposition of tungsten(magenta).

MARCH 2012 | IEEE NANOTECHNOLOGY MAGAZINE | 17

tungsten tip is achieved by millingwith the FIB.

CONCLUSIONSThe presented nanorobotic AFM/SEM/FIB system provides an ena-bling technology for the processing,manipulation, and characterization ofindividual nanostructures and, espe-cially, for the flexible prototyping ofnanomaterial-based devices. As anexample, AFM-based electrical charac-terization of CNT bundles and themanipulation and characterization ofgraphene flakes and silicon nanowireshave been presented.

Future research activities will focus onthe automation of the presented process-ing, manipulation, and characterizationsequences. For this reason, the futuredevelopment of nanocomponent-baseddevices will benefit from a flexible inte-grated nanorobotic tool, which combineshigh-throughput prototyping and in situcharacterization. Furthermore, this uniquecombination of various analysis techniquesfacilitates the optimization of the fabrica-tion methods itself to be incorporated inconventional CMOS fabrication.

ACKNOWLEDGMENTSThis work was supported by the Euro-pean Commission in frame of the projectsFIBLYS, Hydromel, and NanoHand.The authors acknowledge all AMiR teammembers for their contribution to thisreview. Special thanks to Prof. PeterBøggild at DTU Nanotech in Denmarkfor the great collaboration on microgrip-per- and 4PP-based experiments.

ABOUT THE AUTHORSSergej Fatikow (fatikow@uni-oldenburg.de) studied computer science and

electrical engineering at Ufa AviationTechnical University in Russia, wherehe received his doctoral degree in 1988with a dissertation on ‘‘Fuzzy Controlof Complex Nonlinear Systems.’’ He isa full professor in the Department ofComputing Science and head of theDivision for Microrobotics and ControlEngineering (AMiR) at the Universityof Oldenburg, Germany. He is also thehead of Technology Cluster Auto-mated Nanohandling at the ResearchInstitute for Information Technology(OFFIS) in Germany. He is a found-ing chair of the International Confer-ence on Manipulation, Manufacturing,and Measurement on the Nanoscale(3M-NANO).

Volkmar Eichhorn (volkmar.eichhorn@uni-oldenburg.de) receivedhis diploma in physics and his doctoraldegree in natural sciences from theUniversity of Oldenburg in 2005 and2011, respectively. Since April 2005,he has been employed as a researchassociate at the Division Microroboticsand Control Engineering (AMiR), Uni-versity of Oldenburg. He is the head ofthe group ‘‘Handling and Characteriza-tion of Nanoscale Objects’’ and was thetechnical leader of European researchprojects: NANORAC, NanoHand,FIBLYS, and NanoBits.

Malte Bartenwerfer (m.bartenwerfer@uni-oldenburg.de) received his di-ploma in physics from the Universityof Oldenburg in 2009. Since 2010, hehas been employed as a research asso-ciate at the Division Microroboticsand Control Engineering (AMiR),University of Oldenburg, and con-ducts research on the handling andapplication of CNTs, nanowires, andnanoobjects. His main focus lies on

the applications in sensors and elec-tronic devices.

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The nanorobotic system provides a versatilenanoworkstation that facilitates reliable

manipulation, characterization, and structuringof individual nanostructures.

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