IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 8, AUGUST 2009 811

Low-Resistance Carbon NanotubeContact Plug to Silicon

Yang Chai, Zhiyong Xiao, and Philip C. H. Chan, Fellow, IEEE

Abstract—We demonstrated the integration of carbon nan-otubes (CNTs) as the contact plug to the Si MOSFET. A Ti silicidecontact layer was introduced between the CNTs and the Si. Theopen-ended CNT tip connected the metal one on the other end.We study the contact resistivity of the CNT contact plug usingthe cross-bridge Kelvin test structure, and compare with the Wcontact plug. The CNT contact plug also showed excellent thermalstability. The electrical and thermal behavior is closely related tothe intermediate layer between the CNT and the Si.

Index Terms—Carbon nanotube (CNT), contacts, interconnec-tion, plug.

I. INTRODUCTION

CONTACT plug, which is also called the metal zero inthe multilevel interconnect metallization system, has short

length and is directly contacted to the Si transistors. In thecurrent interconnect technology, chemical vapor deposition(CVD) W is used as the contact plug. As the feature size ofthe integrated circuits shrinks, the aspect-ratio of the contactholes is projected to be close to 2 : 1 [1]. It is difficult forCVD W to fill the high-aspect-ratio contact holes seamlessly.The increasing resistivity of the W due to the size effect willresult in unacceptably high resistance and lead to significantlyincreased resistance-capacitance time delay [2]. ElectroplatedRu and Cu with superfilling behavior and high conductivitywere investigated as an alternative for W contact plug [3], [4].However, the electroplating method leads to a more complexprocess and increase the production cost.

Carbon nanotube (CNT) has been investigated as a potentialmaterial for interconnect application due to excellent electri-cal properties, and extremely high electromigration–resistance[5]–[12]. Yang et al. [13] reported the directly grown carbonnanofiber on the active Si. However, the electrical measurementshowed that a Schottky barrier exists between the CNT and Si[13]. In this letter, we introduced a Ti silicide layer between theCNTs and the Si. We measured the current–voltage (I–V ) curveof the CNT contact plugs using the cross-bridge Kelvin test

Manuscript received April 7, 2009; revised May 23, 2009. First publishedJuly 10, 2009; current version published July 27, 2009. This work was sup-ported in part by the Croucher Foundation under Grant CAS-CF05/06.EG01and in part by the Research Grants Council of Hong Kong, China underCompetitive Earmarked Research Grant 611305. The review of this letter wasarranged by Editor M. Ostling.

The authors are with the Department of Electronic and Computer Engineer-ing, Hong Kong University of Science and Technology, Kowloon, Hong Kong(e-mail: eeychai@ee.ust.hk; eexiaozy@ust.hk; eepchan@ust.hk).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2009.2024778

structure. The results showed excellent electrical conductanceand thermal stability.

II. FABRICATION

The process flow for fabricating the CNT contact plug ismostly compatible with conventional W contact plug. We usedlightly p-type doped Si as substrate. P+ (boron) surface dopingwere achieved by ion implantation technique. A 1-μm oxidewas deposited by PECVD method as the interlayer dielectrics.The contact holes were defined by conventional photolithogra-phy, and formed by reactive ion etching. A Fe/Ti (5 nm/15 nm)layer was then e-beam evaporated on the contact holes after abuffered HF dip. Excessive Fe/Ti was lifted off with the residualresist, leaving only inside the contact holes. The Ti silicidewas then formed by two-step rapid thermal annealing (RTA)in nitrogen [14]. The wafers were first annealed for 60 s at600 ◦C to react the Ti and Si into TiSi2, then were annealedfor 60 s at 800 ◦C to form lower resistivity TiSi2. We grewthe CNTs using a PECVD system with Fe catalyst. We usedmethane as carbon source gas. The working pressure was keptat 8.5 torr. The growth process was kept for 2 min. A Au/Tilayer was evaporated and patterned by a lift-off technique toform the electrodes of the Kelvin structure.

III. RESULTS AND DISCUSSIONS

Fig. 1(a) and (b) shows the cross-sectional and top-viewscanning electron microscopy (SEM) image of a contact holefilled with CNTs. Aligned CNT were grown inside the contactholes on the catalyst particles, up to the top of the dielectriclayer. A CNT contact plug of 1.2 μm × 1.2 μm consists ofaround 150 CNTs. The average diameter of the multiwalledCNT is around 20 nm. The site density of the CNTs insidethe contact holes is around 1.1 × 1010 cm−2. To observe thedetailed microstructures of the CNT contact plugs at the two-side interfaces, we also prepared the transmission electronmicroscopy (TEM) samples. The substrate with CNT grown onthe Ti silicide surface was cut into two parts, which were gluedwith epoxy, and grinded, polished, and ion milled. Fig. 1(c)shows a cross-sectional TEM image of the CNT on the Tisilicide surface. It is clearly seen that the catalyst particle inmost of the CNTs is anchored on the Ti silicide, indicating thatthe growth mechanism is base-growth mode under these con-ditions. Fig. 1(d) shows the interface region between the CNTand the Ti silicide. The Fe catalyst particle and the Ti silicidepenetrate each other. The catalyst particle is encapsulated bythe multiple graphite shells. This structural feature is also in

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812 IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 8, AUGUST 2009

Fig. 1. (a) Cross-sectional and (b) top-view SEM images of CNT contact plug.TEM images of (c), (d) the bottom and (e) tip structure of the CNT grown onthe Ti silicide. (f) Schematic of the grown CNT in this letter.

agreement with other reported results about CNT growth onsilicide layer [15], [16]. In this configuration, the bottom ofthe CNT forms a fairly good bonding with the Ti silicide layer.Fig. 1(e) shows that the tip structure of the as-grown CNT isopen-ended. The multiple graphite shells of the CNT are notcapped with the fullerenelike structure. The feature of the tipstructure of the CNT is closely related to the PECVD growthprocess. The plasma was abruptly switched off to end theCNT growth process, and a large hydrogen stream (200 sccm)was introduced to dilute the carbon reaction. Other researchgroup has also reported the open-ended CNTs using similar“abrupt termination process” [17]. The open-ended multiwalledCNT provides multiple channel transport. Not only outermostshell but also inner shells contribute the conductance [8], [10].Fig. 1(f) shows the schematic of the structure of the grown CNT.The silicide-rooted and open-ended features of the CNT help toform good electrical contact at the both ends of the contact plug.The current carriers pass through the Ti silicide, to the multiplegraphite shells and to the top metal electrode.

We measured the I–V curves of the CNT contact plug on theTi silicide and directly on the Si using the Kelvin test structure(Fig. 2). The contact holes are 1.2 μm × 1.2 μm with p-typeSi substrate. The linear I–V curve of CNT contact plug on theTi silicide suggests that the contact is Ohmic. The electricalresistance of the CNT contact plug is extracted from the linearlyfitted line of the I–V curve. The average contact resistanceof 1.2 μm × 1.2 μm CNT plug is around 128 Ω. The I–V

Fig. 2. I–V curves of CNT contact plug Ti silicide/Si and direct on Si.(Upper inset) Photograph of the four-point Kelvin test structure. (Lower inset)Resistance distribution of 1.2 μm × 1.2 μm CNT contact plug on Ti silicide/Si.

curve of the CNT contact plug directly on the Si is nonlinear.Yang et al. [13] explained that a Schottky barrier exists betweenthe CNT and Si. However, the current level is two orders inmagnitude lower than CNT plug on Ti silicide. This suggeststhe tunneling is a likely transport mechanism [18]. Inset ofFig. 2 is the statistical distribution of the electrical resistancesfor CNT contact plugs on Ti silicide. The contact resistanceis significantly reduced with the use of the Ti silicide contactlayer. The silicide has metallike resistivity and forms an Ohmiccontact to the Si. The work function of silicide is close tothe Fermi level of the Si. This enables the current carriers totransport between Si and CNT. We can evaluate the mean freepath of the CNT λCNT according to the CNT contact plugresistance Rcontact, the imperfect CNT-metal or CNT-silicideresistance Rc, the resistance of a single CNT RCNT, the numberof the CNTs inside the contact holes nCNT and the contactheight H [10]

Rcontact =Rc + RCNT

nCNT(1)

RCNT =H

λCNT• h

4e2• 2 • 1

nshell(2)

where h is the Planck constant, e is the fundamental charge,and nshell is the number of the graphite shells that contribute tothe conduction. Assuming the Rc is as low as 0.5 kΩ, and thenshell is 5, we calculated the λCNT to be around 140 nm, whichis larger than the contact height predicted for the 32-nm tech-nology node. This result suggests that the CNT is a promisingballistic conductor for future contact plug applications.

The relationship between the contact resistance Rcontact

and the actual contact resistivity ρcontact is determined byρcontact = RcontactA, where A is the contact area. The cal-culated contact resistivity for CNT contact plug and the Wcontact plug are shown in Table I. The contact resistivity ofCNT contact plug on Ti silicide is smaller than the FIB W plug,but higher than the conventional W plug. The contact resistivityof the CNT plug is possibly further decreased by removing theimperfect contact resistance or increasing the CNT density. Thesamples were annealed at 450 ◦C for 1 h. The contact resistivityof the CNT contact plug exhibits a slight increase, while thecontact resistivity of the W plug increases dramatically after

CHAI et al.: LOW-RESISTANCE CARBON NANOTUBE CONTACT PLUG TO SILICON 813

TABLE IRESISTIVITY FOR CONTACT PLUGS

Fig. 3. SIMS depth profile for CNT contact plug on Ti silicide.

the annealing process [19]. The result suggests the CNT contactplug has better thermal stability.

The CNT contact plug on the Ti silicide showed Ohmiccontact behavior, lower electrical resistance, and better thermalstability compared with W contact plug. These features arebelieved to be related to not only the properties of the CNTbut also the nature of the interface between the Ti silicide andCNT. We characterized the transition region between the Siand the CNT by the second ion mass spectroscopy (SIMS) inFig. 3. A Ti silicide signal appears in the transition region, andis much stronger than the Ti signal. This suggests the Ti layerwas mostly converted into the Ti silicide by the RTA process.The Ti silicide has metallic behavior, and its work function isclose to the Fermi level of the Si. These features help to form thebetter electrical contact between the Si and CNT. The Schottky-type contact reported in [13] did not occur in our work.

IV. CONCLUSION

We demonstrated a method to use silicide-rooted and open-ended CNT contact plug to the Si. The electrical measurementusing four-point cross-bridge Kelvin structure showed Ohmicbehavior and lower electrical resistance. The CNT contact plugalso exhibited better thermal stability. These unique featuresare closely related to the properties of the interface regionbetween the CNT and Si. The contact between CNT and Si wasimproved by introducing the Ti silicide contact layer.

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