We study the performance and reliability of carbon nanofiber (CNF) interconnects under high-current stress by examining CNF breakdown for four test configurations, suspended/supported with/without tungsten deposition. The use of W is to improve the CNF-electrode contact. The supported cases show a larger current density just before breakdown than the suspended ones, suggesting an effective heat dissipation to the substrate. The W-deposited contacts reduce the initial total resistance from megaohm range without W to kilo-ohms. High-current stress does not change the total resistance of the test structures with W unlike those without W deposition

Fabris, Drazen

Kitsuki, Hirohiko

Saito, Tsutomu

Suzuki, Makoto

Wilhite, Patrick

Yamada, Toshishige

Yang, Cary Y.

English

Scholar Commons - Santa Clara University

Santa Clara UniversityScholar CommonsElectrical Engineering School of Engineering9-8-2008Improved contact for thermal and electricaltransport in carbon nanofiber interconnectsTsutomu SaitoToshishige YamadaSanta Clara University, tyamada@scu.eduDrazen FabrisSanta Clara University, dfabris@scu.eduHirohiko KitsukiPatrick WilhiteSanta Clara University, pwilhite@scu.eduSee next page for additional authorsFollow this and additional works at: https://scholarcommons.scu.edu/elecCopyright © 2007 American Institute of Physics Publishing. Reprinted with permission.This Article is brought to you for free and open access by the School of Engineering at Scholar Commons. It has been accepted for inclusion inElectrical Engineering by an authorized administrator of Scholar Commons. For more information, please contact rscroggin@scu.edu.Recommended CitationT. Saito, T. Yamada, D. Fabris, H. Kitsuki, P. Wilhite, M. Suzuki, and C.Y. Yang, "Improved contact for thermal and electrical transportin carbon nanofiber interconnects," Applied Physics Letters 93, 102108 (3 pp) (2008). https://doi.org/10.1063/1.2979710AuthorsTsutomu Saito, Toshishige Yamada, Drazen Fabris, Hirohiko Kitsuki, Patrick Wilhite, Makoto Suzuki, andCary Y. YangThis article is available at Scholar Commons: https://scholarcommons.scu.edu/elec/35Improved contact for thermal and electrical transport in carbon nanofiberinterconnectsTsutomu Saito,a Toshishige Yamada, Drazen Fabris, Hirohiko Kitsuki, Patrick Wilhite,Makoto Suzuki, and Cary Y. YangCenter for Nanostructures, Santa Clara University, Santa Clara, California 95053, USAReceived 22 July 2008; accepted 19 August 2008; published online 10 September 2008We study the performance and reliability of carbon nanofiber CNF interconnects underhigh-current stress by examining CNF breakdown for four test configurations, suspended/supportedwith/without tungsten deposition. The use of W is to improve the CNF-electrode contact. Thesupported cases show a larger current density just before breakdown than the suspended ones,suggesting an effective heat dissipation to the substrate. The W-deposited contacts reduce the initialtotal resistance from megaohm range without W to kilo-ohms. High-current stress does not changethe total resistance of the test structures with W unlike those without W deposition. © 2008American Institute of Physics. DOI: 10.1063/1.2979710For carbon nanotubes1,2 CNTs or carbon nanofibers3,4CNFs to be implemented as next-generation interconnectmaterials,5 their performance and reliability properties mustbe studied and well understood. In recent studies of thermaland electrical transports in CNT Refs. 6–12 and CNF,13 itwas shown that their breakdown is closely related to Jouleheating. In our early work, it was observed that the maxi-mum current density Jmax is inversely proportional to theCNF length L for suspended CNFs, which was explainedwith a heat transport model.14 During the experiment, ther-mal and electrical transports between the CNF and metalelectrodes was improved by applying high dc to the systemcurrent stressing,15 and the extent to which current stress-ing affects the CNF-electrode contact was analyzed.14 Herewe report a systematic study of thermal and electrical trans-port measurements by comparing contacts with and withouttungsten deposition on CNF either supported on substrate orsuspended between a pair of electrodes, as shown in Fig. 1.These are 1 CNF suspended between two prepatterned pla-nar electrodes on a SiO2 /Si substrate, 2 CNF resting on aSiO2 /Si substrate between two prepatterned electrodes, 3CNF suspended between two prepatterned electrodes with Wdeposited on top of the CNF/electrode contact, and 4 CNFsupported between two prepatterned electrodes with W de-posited on top of the CNF/electrode contact.CNFs used in these measurements have a cup-shapedmorphology16 and were grown using plasma-enhancedchemical vapor deposition technique with Ni catalyst.17 TheCNFs were removed from the substrate and immersed inisopropyl alcohol. To separate CNFs individually, the solu-tion was sonicated for about 15 min before drop-cast onto asubstrate of prepatterned Au electrodes on an oxidized sili-con wafer. For configurations 3 and 4, W was depositedto improve the contact between the CNFs and metal elec-trodes using a focused ion beam system with source gasWCO6. W was deposited on each end of CNF such thatthe coverage area was approximately 1.21.2 m2, andthe CNF was sandwiched between the Au electrode anddeposited W. The ion beam condition had to be optimized soas not to damage the CNF structure, and the ion beam irra-diation to the bridged portion of CNF must be avoided.18Electrical measurement was then performed to examinetransport prior to breakdown from current stressing. To ob-tain the initial electrical characteristics, the current-voltageI-V curve was measured in a small voltage range a fewmillivolts around V=0. Then, a constant current was appliedaAuthor to whom correspondence should be addressed. Electronic mail:tsaito1@scu.edu.FIG. 1. Current-stressing experiments: a CNF suspended on gold elec-trodes; b CNF supported by SiO2 substrate; c CNF suspended over SiO2substrate with W deposited on contacts; and d CNF supported by SiO2substrate with W deposited on contacts. Upper figures show SEM image ofa CNF device at 75° tilted-angle view, and lower figures illustrate schematicof electrical measurement setup for current-stressing experiments.APPLIED PHYSICS LETTERS 93, 102108 20080003-6951/2008/9310/102108/3/$23.00 © 2008 American Institute of Physics93, 102108-1to the device for 3 min. Finally, the I-V curve was remea-sured under the same bias condition and the two results werecompared. The current-stressing cycle was repeated with anew larger stressing current until breakdown occurred. Herewe present results for 36 devices with CNFs ranging from100 to 200 nm in diameter and from 1.5 to 8.6 m inlength.To minimize contact resistance between the CNF andmetal electrodes for practical interconnect applications, con-figurations 3 and 4 with improved contact are studied. Inthese configurations, the region where the CNF is in contactwith the Au electrode is fully covered with deposited W.Contacts fabricated with this process have a typical resis-tance in the kilo-ohm range, which is one to three orders ofmagnitude smaller than those for configurations 1 and 2.The test structure is examined using scanning electronmicroscopy SEM to ensure that no structural damage wascreated by W deposition. By comparing the SEM imagesbefore and after W deposition, no difference is observedin the CNF. The I-V curve for configuration 1 shown inFig. 2a is nonlinear and noisy due to poor contact betweenCNF and electrodes. The resistance at V=0 is 87.6 k. Thisnonlinearity suggests tunneling transport between CNF andelectrodes.19 Fig. 2b shows the I-V behavior for configura-tion 3 with W contact. The resistance is 6.4 k, typical ofdevices with W deposition. Solid circles show measurementresults from forward voltage sweep, and open trianglesshow reverse voltage sweep. These two curves overlap quitewell and show no hysteresis. Moreover, the I-V curve is lin-ear and free from noise. Comparison of results from thesetwo configurations clearly shows that W deposition on thecontacts improves the contact resistance between CNF andelectrodes.Results for devices subject to current stressing are shownin Fig. 3. For configuration 1 solid circles, our previouswork demonstrated that breakdown always occurs at or nearthe middle of the nanofiber.14 Furthermore, the proposedthermal transport model predicts that the highest temperatureis reached in the middle of the CNF,14 while confirming thatthe maximum current density Jmax just before breakdown isinversely proportional to the length L for the suspended CNF,as illustrated by the straight line in Fig. 3. For configuration2 open circles, the CNF region contacted to the SiO2 /Sisubstrate can dissipate heat more effectively from the nanofi-ber to the substrate where the two are in contact. Thus thesupported CNFs in configuration 2 show significantlyhigher maximum current density in comparison to the sus-pended ones in configuration 1.For configuration 3, suspended CNF with W contactsolid triangles in Fig. 3, the maximum current density as afunction of 1 /L behaves the same as that for configuration1 regardless of the contact improvement. The fact that bothsuspended cases can be described by the same linear rela-tionship further confirms the validity of the thermal transportmodel,14 which assumes dissipation mainly through the con-tacts. This finding suggests that the reduction in contact elec-trical resistance from configurations 1 to 3 due to depos-ited W does not seem to alter the overall transport of Jouleheat for suspended CNFs. For configuration 4, supportedCNF with W contact open triangles, the maximum currentdensity is significantly and consistently higher compared tothe suspended cases of configurations 1 and 3. These re-sults demonstrate that for both supported CNF cases, heatdissipation in the CNF segments in contact with the substratedominates the transport and leads to higher maximum cur-rent density.For W-deposited cases, current stressing has little effecton the total resistance. Figure 4 shows the total resistancemeasured at the end of each current-stressing cycle as a func-tion of stress current with and without W deposition. WithoutW Fig. 4a, the total resistance decreases as higher stress-ing current is applied. Initially, each device has a differentCNF resistance due to variations in diameter and/or length aswell as contact resistance between the CNF and electrodes.For that reason, the initial total resistance varies between 0.1and 10 M. After successive current stressing cycles, theresistance variation starts to diminish, reaching a total resis-tance of several kilo-ohms just before breakdown. On theFIG. 2. Color online a Typical I-V characteristics with configuration 1measured before high-current stress. b Typical I-V characteristics withconfiguration 4 measured before high-current stress.FIG. 3. Color online Maximum current density as a function of reciprocalCNF length for 36 devices. Solid circles indicate the results for configura-tion 1, open circles for configuration 2, solid triangles for configuration3, and open triangles for configuration 4. The solid line shows a linear fitfor suspended CNFs, as predicted by our heat transport model Ref. 14.FIG. 4. Color online a Resistance of CNF devices without W depositionas a function of annealing current after each annealing cycle. b Resistanceof CNF devices with W deposition as a function of annealing current aftereach annealing cycle.102108-2 Saito et al. Appl. Phys. Lett. 93, 102108 2008other hand, devices with W contact Fig. 4b show verylittle effect from current stressing, with total resistance de-creasing by at most 20%. Our results suggest that currentstressing mainly improves the contact, and if the contact isalready electrically optimum at the outset, there is a littleroom for further improvement from current stressing. Alsoour results infer that CNF bulk resistance is not affected bycurrent stressing except at breakdown. At each current-stressing cycle, while Joule heat is generated within thenanofiber structure, the bulk CNF resistance remains virtu-ally unchanged, consistent with the metallic characteristicsof CNF.Suspended and supported CNFs with and without Wdeposition at the CNF/electrode contacts are studied experi-mentally. For suspended CNFs, the maximum current densityJmax is inversely proportional to its length with and withoutthe presence of W. For supported CNFs, heat dissipation tothe substrate is significant, and Jmax is significantly higherthan that for suspended CNFs. Current stressing is shown toimprove mainly the properties of the CNF/electrode contact.Initial resistance of W-deposited contact is typically a fewkilo-ohms and remains about the same throughout currentstressing until breakdown. This study provides the experi-mental basis for further elucidating the electrical and thermaltransport and breakdown mechanisms in CNF interconnects.We are grateful to Kevin Mcilwrath and Xiao FengZhang of Hitachi High-Technologies America for their ex-pert technical support. This work was supported by theUnited States Army Space and Missile Defense CommandSMDC and carries Distribution Statement A, approved forpublic release, distribution unlimited.1J. Li, Q. Ye, A. M. Cassell, H. T. Ng, R. Stevens, J. Han, and M.Meyyappan, Appl. Phys. Lett. 82, 2491 2003.2M. Nihei, A. Kawabata, D. Kondo, M. Horibe, S. Sato, and Y. Awano, Jpn.J. Appl. Phys., Part 1 44, 1626 2005.3A. V. Melechko, V. I. Merkulov, T. E. McKnight, M. A. Guillorn, K. L.Klein, D. H. Lowndes, and M. L. Simpson, J. Appl. Phys. 97, 0413012005.4Q. Ngo, A. M. Cassell, A. J. Austin, J. Li, S. Krishnan, M. Meyyappan,and C. Y. Yang, IEEE Electron Device Lett. 27, 221 2006.5W. Hoenlein, F. Kreupl, G. S. Duesberg, A. P. Graham, M. 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Improved contact for thermal and electrical transport in carbon nanofiber interconnects

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Improved contact for thermal and electrical transport in carbon nanofiber interconnects

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