Double-walled carbon nanotubes (DWNTs) are potential candidates for new generation of on chip interconnections due to their nearly metallic behaviour. For such large scale integration purpose it is mandatory to characterize their electrical properties in a statistical way. We thus propose a new methodology for characterizing in one step, the electrical properties of a large population of nanotubes. The method enables to obtain histograms of the conductance and maximum current density of individual nanoobjects

Flahaut, Emmanuel

Seichepine, Florent

Vieu, Christophe

English

International audienceDouble-walled carbon nanotubes (DWNTs) are potential candidates for new generation of on chip interconnections due to their nearly metallic behaviour. For such large scale integration purpose it is mandatory to characterize their electrical properties in a statistical way. We thus propose a new methodology for characterizing in one step, the electrical properties of a large population of nanotubes. The method enables to obtain histograms of the conductance and maximum current density of individual nanoobjects

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A simple and versatile method for statistical analysis of the electrical properties of individual double walled carbon nanotubes

Scientific Publications of the University of Toulouse II Le Mirail

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Any correspondence concerning this service should be sent to the repository administrator: staff-oatao@inp-toulouse.fr                    To link to this article: DOI:10.1016/j.mee.2011.01.081  Official URL: http://dx.doi.org/10.1016/j.mee.2011.01.081       This is an author-deposited version published in: http://oatao.univ-toulouse.fr/  Eprints ID: 8692 To cite this version:  Seichepine, Florent and Flahaut, Emmanuel and Vieu, Christophe A simple and versatile method for statistical analysis of the electrical properties of individual double walled carbon nanotubes. (2011) Microelectronic Engineering, vol. 88 (n ° 7). pp. 1637-1639. ISSN 0167-9317 Open Archive Toulouse Archive Ouverte (OATAO)  OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.   A simple and versatile method for statistical analysis of the electrical propertiesof individual double walled carbon nanotubesFlorent Seichepine a,b,c,d,⇑, Emmanuel Flahaut a,c, Christophe Vieu b,daUniversité de Toulouse; UPS, INP; Institut Carnot Cirimat; 118, route de Narbonne, F-31062 Toulouse, cedex 9, FrancebUniversité de Toulouse, UPS, INSA, INP, ISAE, LAAS F-31059, FrancecCNRS, Institut Carnot Cirimat, F-31062 Toulouse, FrancedCNRS-LAAS, 7avenue du colonel Roche, F-31077 Toulouse, FranceKeywords:Carbon nanotubeInterconnectionLarge scale integrationCurrent densityNanomaterialNanoelectronica b s t r a c tDouble-walled carbon nanotubes (DWNTs) are potential candidates for new generation of on chip inter-connections due to their nearly metallic behaviour. For such large scale integration purpose it is manda-tory to characterize their electrical properties in a statistical way. We thus propose a new methodologyfor characterizing in one step, the electrical properties of a large population of nanotubes. The methodenables to obtain histograms of the conductance and maximum current density of individual nano-objects.1. IntroductionThe use of metallic carbon nanotubes bundles is investigated asan alternative to copper for the next generation of on chip intercon-nections. CNT bundles can outperform copper interconnects interms of propagation delay for local, intermediate and global wires[1]. The other advantage of those nano-objects in the microelec-tronics field is their ability to carry extremely high current densities(>109 [A cm2]) [2] (about 100 times more than copper). The realisa-tion of this new kind of CNT-based interconnections also requiresmajor innovations in the field of large scale processing. The chal-lenge is to find an efficient way to integrate dense bundles of ori-ented CNTs at wafer scale. Several techniques are currentlyinvestigated [3–7]. One of the problems to obtain devices with con-trolled behaviours is the large dispersion of conduction propertiesobserved in CNT samples. The number of walls, the chirality, the de-fect concentration, the surface properties, the length and the diam-eter are generally different from one tube to another. Thosecharacteristics are leading to very different conduction behaviourwhich can be detrimental for VLSI applications. To alleviate thisproblem one of the possibility relies on CNTs sorting in order to re-duce the dispersion of the conduction properties of the initial mate-rial [8]. The control of the properties may also be achieved by usingspecific catalysts to obtain only one desired CNT type [9]. In anycase, it seems very important to develop a reliable method in orderto statistically measure the conduction properties of a given batchof CNTs. Such amethod could be essential for investigating themostsuitable process for tuning the metallic properties of CNTs and forqualifying any CNT material for on chip interconnects.This paper describes a new methodology for one-step charac-terization of the electrical properties of a large population of car-bon nanotubes. This original method relies on the parallelintegration of many CNTs on a set of microelectrodes and the care-ful investigation of their electrical properties. The method enablesto obtain histograms of the electrical conductivity and maximumcurrent density of individual nano-objects.2. Microelectrode fabricationMetallic microelectrodes were fabricated on a 4 inch silicon wa-fer. A thermal oxide layer (300 nm) was grown to avoid any con-duction through the substrate and UV proximityphotolithography using AZ1529 photoresist was implemented forgenerating the patterns. The metal electrodes were obtained by alift-off process of a thin metallic film composed of Ti and Au (Ti10nm/Au100nm). Gold electrodes are known to decrease the contact resis-tance with CNTs [10]. A large number of electrodes were designedon the wafer (about 19,000). The electrodes are 125 lm long,doi:10.1016/j.mee.2011.01.081⇑ Corresponding author at: Université de Toulouse, UPS, INP, Institut CarnotCirimat; 118, route de Narbonne, F-31062 Toulouse, cedex 9, France.E-mail address: seichepine@gmail.com (F. Seichepine).10 lm wide and exhibit insulating gaps ranging from 1 to 50 lm.Different orientations and shapes of electrodes are designed onthe substrate (Fig. 1 is a SEM image of a typical one).3. CNT synthesis and depositionDWNTs used in this work are prepared in-house by CatalyticChemical Vapour Deposition (CCVD), by decomposition of CH4 at1000 °C (H2:CH4 atm.) [11]. After catalyst removal (HCl), the sam-ple contains approximately 80% of DWNTs, the rest being mainlySWNTs (15%) and Triple-Walled Nanotubes. The large proportionof metallic CNTs obtained in these conditions represents a hugeadvantage for interconnect applications [12]. The DWNTs werethen purified by air oxidation (500 °C, 30 min) to finally obtainhigh-purity samples [13]. A suspension in ethanol was preparedby mixing the obtained DWNT powder with high purity ethanolusing a sonication probe. Suspensions of different concentrationshave been prepared. The suspensions were then sprayed using amanual airbrush. The use of a spray coating technique allows avery homogeneous deposition of CNTs over the entire substrate.By controlling the concentration of the solution and the depositionparameters, we could adjust the CNT density on the substrate.4. Electrical characterizationThe aim of this study was to find an efficient way for testingsimultaneously the electrical properties of large assemblies ofCNTs. This is required when one wishes to investigate the influenceof synthesis conditions or chemical treatments applied to a CNTsample. The main parameters of interest for interconnects applica-tions are the maximum current density and the resistivity of CNTwires. By controlling the CNT concentration in the ethanol suspen-sion and the deposition parameters, we could obtain electrodesconnected with only few CNTs (between 1 and 10). The inset inthe Fig. 1a is a SEM picture of such a connection (after breakdown).A disrupted CNT bundle is visible between the gold electrodes.The electrical tests have been performed using an Agilent4142B Modular DC Source/Monitor used with a Suss MicrotecPA200 tip station. The experiment was driven by Metrics ICS soft-ware. The temperature of the substrate was carefully maintainedat 25 °C using the tip station and the tests have been performedunder ambient atmosphere. Depending on the length of thenano-object tested, an adapted gap is chosen. In our case, 5 lmgaps between electrodes were appropriate for connecting effi-ciently the devices.The electrical characteristics were obtainedby sweeping the voltage from 0 to 60 V and measuring the cur-rent flowing through the CNTs bridging the gaps of the microelec-trodes. However, in order to decrease the resistance contact andavoid the influence of adsorbed species on CNTs, a heat treatmentwas systematically performed before each measurement. A sweepof voltage from 0 to 4 V in about 5 s was applied and repeatedfour times, which induced a heating by joule effect on the CNT-metal contacts. This treatment allowed us to obtain more linearI(V) characteristics [14]. A typical I(V) curve is shown in Fig. 2.The voltage sweep between the two electrodes leads to the suc-cessive destruction of individual CNT [15]. Even if this operationis performed simultaneously on several CNTs, it is however possi-ble to extract the electrical properties of single nano objectsthanks to the data analysis proposed and detailed below. Thesequential breakdown of CNTs leads to a progressive increase inthe global resistance of the system. This setup can be used tocharacterise very rapidly a large amount of individual CNTs ona large number of connections.5. Data analysisThe I(V) traces exhibit several very precise steps (Fig. 2). As aconsequence, the R(V) curves present a step by step increase inthe resistance as the voltage increases. The values of the resistanceregistered on the plateaux are very stable. The hypothesis is madethat the sharp resistance increase corresponds to the destruction ofa single conduction path. SEM observations of such connections,before and after the electrical stimulation, support this hypothesis(see inset of the Fig 1a). It has been reported, that breakdown oc-curred when the temperature reaches about 600 °C on the CNT.In our case, due to the low thermal conductivity of silicon dioxidecompared to the gold electrode, the maximal temperature is prob-ably localised at the centre of the CNT [16]. The final value of theresistance corresponds to the resistance level of an unconnectedmicroelectrode gap. We have also verified many times that thenumber of steps, observed in the I/V curve, corresponds to thenumber of CNTs which could be observed by SEM.Fig. 1. (a) SEM image of one of the test chip. In the inset, a CNT before and after the breakdown. (b) Schematic view of the principle of our experiments.Fig. 2. Typical examples of the I(V) (black line) and R(V) (dashed line) curvesobtained by our protocol . The I(V) is compared with the calculated model (greyline).6. CalculationsTo find out the electrical parameters of single conduction path-ways, data processing was necessary. For such a purpose, weapproximate our system as a range of perfect resistances arrangedin parallel. Some valuable parameters can be directly extractedfrom our measurements. Each local maximum gives the Imax,totand the Vmax,tot which are respectively the current and the voltagebetween the electrodes, just before a conduction pathways is dis-rupted. We can also calculate the global resistance of the system(Rtot). By using those measurements, we are able to isolate theproperties of each conduction pathways. First, we define R1, R2,. . ., Rn as the resistance of each conduction pathways. We can thendefine Rtot as follows:RtotðVÞ ¼11R1þ 1R2þ 1R3þ 1R4þ :::þ 1Rnð1Þwhere n is the total number of conduction pathways. Rtot is increas-ing step by step during the voltage sweep. In fact, one after another,conduction pathways are destroyed and the Ri (with i = n to 1) fac-tors disappears. Rtot can be described as a series of constants. Wedefine Ra, Rb and so on, byRa¼11R1þ 1R2þ 1R3þ 1R4þ :::þ 1Rn;Rb¼11R1þ 1R2þ 1R3þ 1R4þ :::þ 1Rnÿ1ð2ÞThen we can calculate each factor by the following equation:1Raÿ1Rb¼1R1þ :::þ1Rnÿ ð1R1þ :::þ1Rnÿ 1Þ ¼1Rn: ð3ÞWith the same calculation repeated n times, we can obtain theindividual resistance of each conduction path. Finally, through thesimple relation Vmax,tot = Ri Imax,tot for each resistance, we can cal-culate the maximum current Imax supported by each path.The average diameter of the CNTs or CNT bundle has been mea-sured by AFM. We assumed that the effective length of the CNTs isthe length of the gap between the electrodes regardless of their ori-entation. Then the maximum current density and the resistivity ofeach conduction pathways could be calculated. It must be pointedout however, that those figures do not correspond to the intrinsicproperties of individual CNT, but rather describe the CNTs withtheir environment at the metal/CNT interface. This is not a limita-tion because for interconnect applications, these contacts will bealso present. Moreover, the use of an average diameter for all theconnections allows fast computing, but induces a quite largeapproximation when the distribution is large. To validate ourhypothesis we simulated the response to a voltage sweep of a sys-tem of resistances using our extracted values. The comparison be-tween the measured data and the response of the equivalentcircuit gives us an indication on the validity of our calculationsand hypothesis. An example can be seen in Fig. 1b (comparison be-tween the black (measurement) and grey (equivalent circuit) plots.This protocol has been used to test several connections with asemi automatic procedure. This technique is very efficient for elec-trodes connected by one to about twenty CNTs. The systematiccomparison between the original I(V) data and the correspondingsimulated I(V) curve show that our assumptions and model de-scribes the experiment with an accuracy better than 5%. Finally,histograms of individual electrical characteristics can be extracted.Fig. 3 shows, as an example, the distribution of maximum currentdensity capable to flow through DWNTs which have been previ-ously chemically oxidized using HNO3. In this case, we can see aquite large distribution of maximum current density, with an aver-age value below previous reports of the literature. This observationcan be explained by the possible damage that occurred on the CNTsstructure during the oxidation process.7. Conclusion and perspectivesWe have proposed an electrical method for measuring withhigh throughput the electrical characteristics of any CNT suspen-sion. Without any advanced lithography steps we can record mas-sive measurements of individual nanotubes. The method enablesto obtain clean statistical data over a large population of CNTsand permits a much more accurate comparison between differentpopulations of CNTs than previous proposals. Indeed rather thancomparing average values as usually done, we are able to comparehistograms of electrical parameters.References[1] A. Naeemi, R. Sarvari, J.D. Meindl, Electron Device Letters, IEEE 84 (2005).[2] B. Wei, R. Spolenak, P. Kohler-Redlich, M. Ruhle, E. Arzt, Applied Physics Letters74 (1999) 3149.[3] Q. Cao, J.A. Rogers, Nano Research 1 (2008) 259.[4] L.A. Nagahara, I. Amlani, J. Lewenstein, R.K. Tsui, Applied Physics Letters 80(2002) 3826.[5] S. Lyonnais, L. Goux-Capes, C. Escudï¿½, D. Cote, A. Filoramo, J.-P. Bourgoin,Small 4 (2004) 442.[6] M. Hiramatsu, T. Deguchi, H. Nagao, M. Hori, Japanese Journal of AppliedPhysics 46 (2007) L303.[7] J.C. Coiffic, M. Fayolle, P. Faucherand, M. Levis, H.L. Poche, J. Dijon, S.Maitrejean, Physica Status Solidi (a) 205 (2008) 1399.[8] M.C. Hersam, Nature Nanotechnology 3 (2008) 387.[9] R. Jasti, J. Bhattacharjee, J.B. Neaton, C.R. Bertozzi, Journal of the AmericanChemical Society 130 (2008) 17646.[10] G. Close, H.-S. Wong, Nanotechnology IEEE Transactions 7 (2008) 596.[11] E. Flahaut, R. Bacsa, A. Peigney, C. Laurent, Chemical Communications (2003)1442.[12] P. Puech, A. Ghandour, A. Sapelkin, C. Tinguely, E. Flahaut, D.J. Dunstan, W.Bacsa, Physical Review B 78 (2008) 045413.[13] S. Osswald, E. Flahaut, H. Ye, Y. Gogotsi, Chemical Physics Letters 402 (2005)422.[14] L. Dong, S. Youkey, J. Bush, J. Jiao, V.M. Dubin, R.V. Chebiam, Journal of AppliedPhysics 101 (2007) 024320.[15] A. Javey, J. Guo, M. Paulsson, Q. Wang, D. Mann, M. Lundstrom, H. Dai, PhysicalReview Letters 92 (2004) 106804.[16] E. Pop, Nanotechnology 1 (2008) 295202.Fig. 3. Example of an histogram of maximum current densities measured on adispersion of HNO3 oxidized DWNTs.

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Physica Status Solidi (a) 205

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A simple and versatile method for statistical analysis of the electrical properties of individual double walled carbon nanotubes

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