A method to extract the contact resistance and bulk resistivity of vertically grown carbon nanofibers (CNFs) or similar one-dimensional nanostructures is described. Using a silicon-compatible process to fabricate a terrace test structure needed for the CNF length variation, the contact resistance is extracted by measuring in situ the resistances of individual CNFs with different lengths and within a small range of diameters using a nanoprober inside a scanning electron microscope. Accurate determination of contact resistances for various combinations of catalysts and underlayer metals can lead to eventual optimization of materials’ growth and device fabrication processes for CNF via interconnects

Khera, Vinit

Krishnan, Shoba

Li, Ken

Sun, Xuhui

Wilhite, Patrick

Yang, Cary Y.

English

Scholar Commons - Santa Clara University

Santa Clara UniversityScholar CommonsElectrical Engineering School of Engineering12-20-2010Extraction of contact resistance in carbon nanofibervia interconnects with varying lengthsKen LiPatrick WilhiteSanta Clara University, pwilhite@scu.eduVinit KheraShoba KrishnanSanta Clara University, skrishnan@scu.eduXuhui SunSee next page for additional authorsFollow this and additional works at: https://scholarcommons.scu.edu/elecCopyright © 2010 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 CitationK. Li, R. Wu, P. Wilhite, V. Khera, S. Krishnan, X. Sun, and C.Y. Yang, “Extraction of contact resistance in carbon nanofiber viainterconnects with varying lengths,” Applied Physics Letters 97, 253109 (3 pp) (2010). https://doi.org/10.1063/1.3527927AuthorsKen Li, Patrick Wilhite, Vinit Khera, Shoba Krishnan, Xuhui Sun, and Cary Y. YangThis article is available at Scholar Commons: https://scholarcommons.scu.edu/elec/23Extraction of contact resistance in carbon nanofiber via interconnectswith varying lengthsKe Li,1 Raymond Wu,1 Patrick Wilhite,1 Vinit Khera,1 Shoba Krishnan,1 Xuhui Sun,2,a andCary Y. Yang11Center for Nanostructures, Santa Clara University, Santa Clara, California 95053, USA2Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou 215123,People’s Republic of ChinaReceived 10 November 2010; accepted 23 November 2010; published online 20 December 2010A method to extract the contact resistance and bulk resistivity of vertically grown carbon nanofibersCNFs or similar one-dimensional nanostructures is described. Using a silicon-compatible processto fabricate a terrace test structure needed for the CNF length variation, the contact resistance isextracted by measuring in situ the resistances of individual CNFs with different lengths and withina small range of diameters using a nanoprober inside a scanning electron microscope. Accuratedetermination of contact resistances for various combinations of catalysts and underlayer metals canlead to eventual optimization of materials’ growth and device fabrication processes for CNF viainterconnects. © 2010 American Institute of Physics. doi:10.1063/1.3527927Copper interconnects used in current complementarymetal-oxide-semiconductor CMOS technology are rapidlyapproaching its scaling limit because of increasing resistivitydue to grain boundary scattering and surface scattering, aswell as reliability problems arising from electromigration.One-dimensional carbon nanostructures such as carbonnanotubes CNTs and CNFs provide an alternative to cop-per, due to their high current-carrying capacity1–3 and theirrobust thermal and mechanical properties.4–7 To better assessthe potential of these nanostructures for use in next-generation integrated circuits, their electrical properties needto be extensively characterized and well understood. One ofthe key properties in a vertical via interconnect is the contactresistance between the nanostructure and the metal under-layer. Minimizing contact resistance is paramount to the fab-rication of performance-driven interconnects since contactresistance tends to dominate the overall resistance of the de-vice in the nanoscale.Standard two-point measurements are generally unableto isolate the contact resistance from the bulk resistance ofthe device under test. Combinations of two-point and four-point measurements have been used to extract contact resis-tance and bulk resistivity of horizontal nanostructures.8,9While the electrical properties of CNT vias have beenreported,10,11 it is nontrivial to perform four-point measure-ments on such vertical one-dimensional nanostructures. Thusa method to extract the contact resistance in a via structure istimely and critically needed.Previously we reported contact resistance extraction forvia structures from current-voltage measurements on singleCNFs using conductive atomic force microscopyC-AFM.12 By analyzing the dependence of the measuredtotal resistance, as well as the contact resistance on CNFdiameter, the metal-CNF contact resistance was extracted.The key assumptions of that method were that all measureddevices had the same length, and that the diameter depen-dence of the contact resistance is predictable. In principle, amore straightforward approach would be to keep the diam-eter fixed and vary the length, as the contact resistance isclearly not a function of CNF length. In practice, however, itis difficult to control the CNF growth to yield a sufficientlylarge sample of test devices with the same or nearly the samediameter. Thus a certain diameter distribution within thesample becomes inevitable. Here we describe a method toextract contact resistance in a CNF via structure from themeasured total resistance as a function of CNF length withina small range of diameters. A silicon-compatible test struc-ture fabrication technique is used to yield the desired lengthvariation while ensuring that all measured devices have un-dergone the same fabrication processes.Small variations in CNF synthesis conditions can lead tovery different properties of the resulting material. To ensurehomogenous properties of all devices tested, all CNFs usedin the measurements are from the same growth batch, andtheir lengths are altered postsynthesis. To achieve this, a ter-raced substrate is designed and fabricated by chemicallyetching a Si wafer in potassium hydroxide KOH. To ensureproper step formation, silicon nitride Si3N4 is deposited onareas of the wafer as a mask, allowing only the area exposedto be etched. HF is then used to remove the nitride. Thisprocedure results in the formation of one smooth step withsharp edges verified by three-dimensional profilometry. Theprocess is then repeated several times allowing new areas ofthe substrate to be exposed along with the previously etchedsteps, ultimately resulting in the terrace structure, as shownschematically in Fig. 1. After the terraced structure is fabri-cated, the underlayer metal and the catalyst metal are depos-ited for CNF growth. Two different combinations of metalfor CNF growth are chosen: a 20 nm Ni catalyst on an 80 nmTi underlayer metal denoted by Ni/Ti and a 20 nm Ni cata-lyst on an 80 nm Cr Ni/Cr. Vertically aligned CNFs arethen grown using dc plasma-enhanced chemical vapor depo-sition at 750 °C.13 The entire wafer is then encapsulated inSiO2 formed by thermal decomposition of tetraethylorthosili-cate TEOS at 380 °C. This provides structural rigidity aswell as electrical isolation between CNFs. A chemical-mechanical planarization CMP process is carried out to cre-ate a flat surface and to expose the CNF tips. 40-nm-thick Ptelectrodes are then deposited on a 5 nm Ti buffer layer foraAuthor to whom correspondence should be addressed. Electronic mail:xhsun@suda.edu.cn.APPLIED PHYSICS LETTERS 97, 253109 20100003-6951/2010/9725/253109/3/$30.00 © 2010 American Institute of Physics97, 253109-1better adhesion. A scanning electron microscope SEM im-age of two adjacent terrace steps covered by CNFs is shownin Fig. 1.I-V measurements are performed in situ during SEM im-aging with a nanoprober consisting of piezoactuated manipu-lators coupled to tungsten probes. One probe rests on a con-tact pad while the other lands on individual CNFs, asillustrated in Fig. 2. Initially, the I-V curves for CNFs shownon-Ohmic characteristics nonlinearity, which are attrib-uted to tunneling across the interface between each probeand the CNF. Current-stressing in the order of 106 A /cm2 isused to lessen the nonlinearity and to improve the contacts.This process reduces Rtotal by up to an order of magnitude,but the result can lead to small variations in Rtotal for CNFswith similar dimensions.The measured resistance consists ofRtotal = Rpad + RCNF + RC + Rm + Rprobe-pad + Rprobe-CNF.1Rpad includes the pad resistance in series with all CNFs underthe pad and the contact resistance between them and the pad.Rm is the resistance of the underlayer metal between the twoprobes. Rprobe-pad or Rprobe-CNF is the sum of probe resistancesand contact resistances between the probes and pad or CNF,respectively. The sum of Rpad, Rm, and Rprobe-pad is estimatedby measuring the resistance between any two pads on thesame terrace step, given byRpad-pad = 2Rpad + Rm + 2Rprobe-pad. 2Rtotal is generally in the k range, whereas the measuredRpad-pad varies from a few  to 100 . Therefore, Rpad,Rm, and Rprobe-pad are all negligible compared to Rtotal andneglected in our analysis. The contact resistance between thetungsten probe tip and the CNF Rprobe-CNF is dependent onthe pressure of the probe on the CNF. Even though this re-sistance can be minimized empirically,12 it is lumped into RCas the total contact resistance of the test device including theprobe tip/CNF and the CNF/underlayer contacts. The mea-sured resistance is then reduced toRtotal  RCNF + RC. 3I-V measurements are performed on CNFs on each ter-race step. Rtotal for each CNF is obtained as the reciprocalslope of the I-V curve at V=0. The measured CNFs at thesame step height are confined to a small area less than 5050 m2 due to variations that can result from CMP. Aftersufficient devices are measured to take into account statisti-cal variations, the sample is then cleaved at the area wherethe measurements are taken and a SEM side-view image isobtained for determining the exact length. Although the fab-rication of all devices used in this study was performed un-der the exact same conditions on the same wafer in order tominimize the inhomogeneity among individual CNFs, errorsmay still result from process variations and especially thediameter variation in device sampling. The method proposedhere uses a statistical approach, and the more devices it in-cludes, the more accurate it is.For a Ni/Ti CNF sample, 61 devices were measured indifferent terraces corresponding to 6 different lengths; forNi/Cr, 121 CNFs having 7 different lengths were measured.Based on the CNF diameter distributions shown in the insetsof Fig. 3, a 45 nm window is chosen for our extractionscheme, which results in 100–145 nm for Ni/Ti and 115–160nm for Ni/Cr. RC and  are determined from the intercept andslope, respectively, of a least-squares linear fit to the mea-sured Rtotal versus L /ACNF, where ACNF=DCNF2 /4 is theCNF cross-sectional area and Rtotal=L /ACNF+RC followsfrom Eq. 3. The data and results of the extraction areshown in Fig. 3. The extraction process is critically based onthe assumption that RC is nearly constant over the chosenrange of diameters. To test the validity of this assumption,we use our previously published model,12 as reproduced inFig. 4 for Ni/Ti, to estimate the variation of RC in this diam-eter range and determine it to be about 0.5 k for eithersample. In addition, based on this model, RC varies as1 /DCNF, while RCNF1 /DCNF2, further supporting the con-FIG. 1. Color online Top-right: schematic of partial terrace structureshowing its dimensions used to create different lengths. Top-left: terracestructure after growth, TEOS, CMP, and metal pad deposition. Bottom: SEMimage of two adjacent terrace steps with CNFs grown.FIG. 2. Color online Left: schematic of one step of the terrace structurewith probes on pad and exposed nanofibers. Right: SEM image of a landedprobe tip on a single CNF.FIG. 3. Color online Measured resistance vs length results for a Ni/Tiand b Ni/Cr. The lines are least-squares linear fit results for the best-fitsolid and upper and lower limits dashed, yielding contact resistance andresistivity. Insets are histograms showing the diameter distributions of themeasured CNFs.253109-2 Li et al. Appl. Phys. Lett. 97, 253109 2010stant RC assumption. Therefore, for the measured resistancevalues of a few k, such diameter variation has a little im-pact on our assumption of constant RC and the resulting ex-tracted values.To further examine our data and extraction scheme, inaddition to obtaining the best linear fit solid lines in Fig. 3,we compute the linear fits corresponding to the upperand lower limits for a two-standard-deviation variance ofL /ACNF. These linear fits are indicated by the dashed lines inFig. 3. This procedure provides us with a 95% confidenceinterval for results between the two limits, and it takes intoaccount the somewhat random variations of measured totalresistance due to current-stressing, as mentioned previously.The extracted parameters from the best-fit best, upper up,and lower lo limit lines are given in Table I.The best-fit extracted contact resistance of Ni/Cr isfound to be 4.3 k, while the CNF resistivity is 4.610−4  cm. The corresponding extracted contact resis-tance and CNF resistivity values for Ni/Ti are 4.9 k and11.710−4  cm, respectively. In comparison, our previousstudy using diameter variation yielded a contact resistance of6.4 k and a resistivity of 7.310−4  cm for similarCNFs on Ni/Ti devices.12 The resulting resistivity of theCNFs is comparable to that measured from horizontalCNFs.14,15 In general, contact resistance occurs due to for-mation of an electrical barrier between the CNF and the un-derlayer metal, due to the presence of an oxide or carbideinterfacial layer or contaminants introduced during the syn-thesis and fabrication processes. It was shown that becauseof the relatively fast formation of Al2O3, using Al as anunderlayer metal yielded a very large contact resistance.13Here we find that for either underlayer metal, the CNF testdevice shows a much larger contact resistance than those formetal-metal contacts. X-ray photoelectron spectroscopy re-vealed that both the Ti and Cr underlayers had been oxidizedafter thermal annealing and CNF growth.13 The differencesin contact resistance for CNFs grown on Ni/Ti and Ni/Cr aremost likely due to dissimilar reaction kinetics in the earlystages of the two growth processes, resulting in differentinterfaces between the CNF and the underlayer metal. Inaddition, the difference in the CNF resistivity between theNi/Cr and Ni/Ti cases might be attributed to structural dif-ference of CNFs grown on the two dissimilar underlayermetals, and partly due to the small contact resistance varia-tion as a result of sampling CNFs with slightly different di-ameters. Nevertheless, in both cases, the contact resistancedominates the total resistance in the vertical CNF structure.Thus the terrace approach provides a platform to test variouscatalysts and underlayer metals, leading to the eventual op-timization of the growth process and minimizing the contactresistance in via interconnects.The similarities in results between the diametervariation12 and length-variation methods, as depicted in Fig.4, suggest that the diameter variation can be a convenientapproach to extract contact resistance in vertical one-dimensional nanostructures. However, because of the com-plex interface between the nanostructure and metal, whichcould result in equally complex transport mechanisms acrossit, the dependence of contact resistance on diameter is uncer-tain at best.12 Since contact resistance generally does notdepend on the length of the nanostructure, the method oflength-variation extraction should be considered more accu-rate and can be applied to any vertically aligned one-dimensional nanostructures.This work was supported by the United States ArmySpace and Missile Defense Command SMDC and carriesDistribution Statement A, approved for public release, distri-bution unlimited.1Z. Yao, C. L. Kane, and C. Dekker, Phys. Rev. Lett. 84, 2941 2000.2B. Q. Wei, R. Vajtai, and P. M. Ajayan, Appl. Phys. Lett. 79, 1172 2001.3A. Javey, P. Qi, Q. Wang, and H. Dai, Proc. Natl. Acad. Sci. U.S.A. 101,13408 2004.4Q. Ngo, T. Yamada, M. Suzuki, Y. Ominami, A. M. Cassell, J. Li, M.Meyyappan, and C. Y. Yang, IEEE Trans. Nanotechnol. 6, 688 2007.5M. 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Sun, P. Wilhite, R. Wu, K. Li, and C.Y. Yang, Appl. Phys. Lett. 94, 163113 2009.13X. Sun, K. Li, R. Wu, P. Wilhite, T. Saito, J. Gao, and C. Y. Yang,Nanotechnology 21, 045201 2010.14T. Saito, T. Yamada, D. Fabris, H. Kitsuki, P. Wilhite, M. Suzuki, and C.Y.Yang, Appl. Phys. Lett. 93, 102108 2008.15T. Yamada, T. Saito, M. Suzuki, P. Wilhite, X. Sun, N. Akhavantafti, D.Fabris, and C. Y. Yang, J. Appl. Phys. 107, 044304 2010.TABLE I. Extracted values of RC and  from the best, upper, and lower limitlinear fits for CNFs grown with a Ni catalyst on Cr and Ti underlayers.Sample Ni/Cr Ni/TiRcup k 5.1 5.9Rcbest k 4.3 4.9Rclo k 3.5 3.9up  cm 7.010−4 15.910−4best  cm 4.610−4 11.710−4lo  cm 2.210−4 7.510−4FIG. 4. Color online Comparison of various methods to extract contactresistance. The data represent the contact resistances extracted based ondiameter variation, using constant-RC and diameter-dependent models re-plotted from Ref. 12, and on length variation over a small range ofdiameters.253109-3 Li et al. Appl. Phys. Lett. 97, 253109 2010

Extraction of contact resistance in carbon nanofiber via interconnects with varying lengths

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Extraction of contact resistance in carbon nanofiber via interconnects with varying lengths

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