Characterization of Electrolessly Deposited Copper andNickel Nanofilms on Modified Si(100) Surface

Yan Zhang, S. S. Ang, and Andrew A. O. Tay

Department of Mechanical Engineering, National University of Singapore,10 Kent Ridge Crescent, Singapore 119260

Dan Xu, E. T. Kang,* and K. G. Neoh

Department of Chemical Engineering, National University of Singapore,10 Kent Ridge Crescent, Singapore 119260

Lim Poh Chong and A. C. H. Huan

Institute of Materials Research and Engineering, 3 Research Link, Singapore, 117602

Received January 20, 2003. In Final Form: May 16, 2003

Ultrathin Cu and Ni films, with thicknesses on the orders of 40 nm (nanofilms) and 200 nm, wereelectrolessly deposited on the hydrogen-terminated Si(100) surface modified by coupling with vinylimidazole(VIDz). Transmission electron microscopy (TEM) images and X-ray diffraction (XRD) patterns revealedthat the electrolessly deposited Cu films were nanostructured, with grain sizes smaller than 65 nm. Onthe other hand, the Ni films were partially crystalline, with grain sizes on the order of 10 nm or less. Theelectrolessly deposited Cu and Ni nanofilms had physical properties rather different from those of thethicker films. Atomic force microscopy (AFM) images revealed that the Cu and Ni nanofilms had a higherdensity of defects and smaller metal clusters in the surface region. The Cu and Ni nanofilms also exhibiteda substantially higher electrical resistivity. X-ray photoelectron spectroscopy (XPS) results suggested thatthe chemical composition and the state of the as-deposited Ni and Cu films were independent of the filmthickness. However, the as-deposited Ni and Cu nanofilms oxidized at a much faster rate than their200-nm-thick counterparts, when subjected to a direct current (DC) loading of about 1.0 × 109 A/m2 in air.The higher oxidation rate was attributed to the higher density of defects, higher electrical resistivity, andlarger surface area to bulk volume ratio of the nanofilms.

IntroductionWith the advances in the microelectronics industry,

copper is rapidly replacing the commonly used aluminumin the multilevel metallization process. Copper has a lowerresistance-capacitance (RC) delay and lower susceptibilityto electromigration than aluminum. Metallization ofvarious substrates via electroless plating has attractedincreasing attention due to its simplicity, low cost, lowprocessing temperature, and good step coverage.1,2 Thetechnique has promising applications in the submicronand nanolevel electronics.

Much effort has been devoted to the study of electrolessdeposition of copper on silicon substrates.3-6 A majordrawback of the process is the lack of adhesion of theelectrolessly deposited copper to the silicon substrate, sincethere are only limited chemical interactions between thedeposited copper and the silicon surface. In addition, thediffusion of copper into the silicon substrate is anotherproblem encountered in this approach. To retard copper

diffusion, a barrier layer, consisting of tantalum (Ta),tantalum nitride (TaN), or titanium nitride (TiN), issputter-deposited on the silicon surface prior to theelectroless deposition of copper.7,8

One of the most important advances in the modificationof silicon surfaces involves the formation of monolayersby the reaction of alkenes with the hydrogen-terminatedsilicon surface, using free radical initiation, thermalcoupling, Lewis acid-catalyzed hydrosilylation, and ultra-violet activation.9-19 A recent study has taken advantageof this technique to introduce a 4-vinylpyridine (4VP)monolayer on the hydrogen-terminated Si(100) surface(H-Si(100) surface) for use as the low-temperature diffu-

* To whom all correspondence should be addressed. Tele-phone: +65-6874-2189. Fax: +65-6779-1936.E-mail address:cheket@nus.edu.sg.

(1) Shacham-Diamand, Y.; Lopatin, S. Microelectron. Eng. 1997, 37/38, 77.

(2) Li, J.; Kohl, P. A. J. Electrochem. Soc. 2002, 149, C631.(3) Magagnin, L.; Maboudian, R.; Carraro, C. Electrochem. Solid-

State Lett. 2001, 4, C5.(4) Ye, S.; Ichihara, T.; Uosaki, K. J. Electrochem. Soc. 2001, 148,

C421.(5) Gorostiza, P.; Kulandainathan, M. A.; Diaz, R.; Sanz, F.; Allongue,

P.; Morante, J. R. J. Electrochem. Soc. 2000, 147, 1026.(6) dosSantos, S. G.; Martins, L. F. O.; DAjello, P. C. T.; Pasa, A. A.;

Hasenack, C. M. Microelectron. Eng. 1997, 3, 59.

(7) Hsu, H. H.; Lin, K. H.; Lin, S. J.; Yeh, J. W. J. Electrochem. Soc.2001, 148, C47.

(8) O’Kelly, J. P.; Mongey, K. F.; Gobil, Y.; Torres, J.; Kelly, P. V.;Crean, G. M. Microelectron. Eng. 2000, 50, 473.

(9) Bent, S. F. J. Phys. Chem. B 2002, 106, 2830.(10) Zhang, W. C.; Strother, T.; Smith, L. M.; Hamers, R. J. J. Phys.

Chem. B 2002, 106, 2656.(11) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D

J. Am. Chem. Soc. 1995, 117, 3145.(12) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R. Y.; Pianetta, P.;

Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056.(13) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M.

Langmuir 1999, 15, 3831.(14) Burrows, V. A.; Chabal, Y. J.; Higashi, G. S.; Raghavacharik, K.;

Christman, S. B. Appl. Phys. Lett. 1988, 53, 998.(15) Effenberger F.; Gotz, G.; Bidlingmaier, B.; Wezstein, M. Angew.

Chem., Int. Ed. Engl. 1998, 37, 2462.(16) Vondrak, T.; Zhu, X. Y. Phys. Rev. Lett. 1999, 82, 1967.(17) Pusel, A.; Wetterauer, U.; Hess, P. Phys. Rev. Lett. 1998, 81,

645.(18) Buriak, J. M. Chem. Rev. 2002, 102, 1271.(19) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000,

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6802 Langmuir 2003, 19, 6802-6806

10.1021/la034087o CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 07/11/2003

sion barrier and adhesion promoter for the electrolesslydeposited metals.20 The 4VP functional layer on the siliconsurface also accounts for the electroless deposition of cop-per without the need for prior sensitization by a tincompound.

In this study, ultrathin Cu and Ni films with thicknesseson the orders of 40 nm (nanofilms) and 200 nm areelectrolessly deposited on the H-Si(100) surface modifiedby coupling with vinylimidazole (VIDz). Transmissionelectron microscopy (TEM), X-ray diffraction (XRD),atomic force microscopy (AFM), X-ray photoelectronspectroscopy (XPS), and four-point probe measurementare employed to characterize, respectively, the grain size,crystalline structure, surface topography, surface com-position, and electrical resistivity of the electrolesslydeposited metal films in the presence and absence ofthermal annealing. The effect of DC current loading onthe chemical state of the nanofilms is also investigated.

Experimental Section

Materials. Single-crystal, (100)-oriented silicon wafers, orSi(100) wafers, having a thickness of about 650 µm and a diameterof 200 mm, were obtained from Wacker Siltronic Singapore Pte.Ltd., Singapore. The as-received wafers were polished on oneside and were without any dopant. The silicon wafers were slicedinto rectangular strips of about 2 cm × 1 cm in size. The siliconsubstrate was cleaned with the “piranha” solution, a mixture of98 wt % concentrated sulfuric acid (70 vol %) and hydrogenperoxide (30 vol %). Vinylimidazole was purchased from theAldrich Chemical Co. and was purified by vacuum distillationbefore use.

Coupling of a Vinyl Monomer (VIDz) with the Hydrogen-Terminated Silicon Surface. The silicon strips were immersedin the 10 wt % hydrofluoric acid solution in Teflon vials for 25min to remove the native oxide layer, leaving behind a uniformhydrogen-terminated Si(100) surface (H-Si(100) surface).14 TheH-Si(100) strip was introduced into a Pyrex glass tube, contain-ing the liquid VIDz monomer prepurged with purified argon.The reaction mixture was purged with purified argon for another5 min. The glass tube with the reaction mixture was then sealedwith a ground glass stopper and subjected to UV irradiation ina Riko RH400-10W rotary photochemical reactor (manufacturedby Riko Denki Kogyo of Chiba, Japan). The reactor was equippedwith a 1000-W high-pressure Hg lamp and a constant-temper-ature bath. All UV-induced reactions were carried out at aconstant temperature of 28 °C for 30 min. After UV irradiation,the silicon strip was removed from the reaction mixture andwashed with copious amounts of water to remove the adsorbedVIDz residuals.

Electroless Plating of Metals on the Modified Si(100)Surface. The VIDz-coupled Si(100) strip was activated by the“Sn-free” one-step process for the subsequent electroless deposi-tion of copper or nickel. The activation and the electroless platingprocess had been described elsewhere.20,21

Surface Characterization. The chemical composition of theelectrolessly deposited metal surfaces was determined by X-rayphotoelectron spectroscopy. The conditions for XPS measure-ments were similar to those documented in the previous work.20

The surface topography of the metal films was investigated usinga Nanoscope IIIa atomic force microscope. All images werecollected in air using the tapping mode under a constant force(scan size, 1 µm; scan rate, 2 Hz).

Thermal Annealing and Electrical Resistivity Measure-ments of the Electrolessly Deposited Metal Films. The as-deposited metal films were thermally annealed in a furnace atdifferent temperatures under an argon pressure of about 1 Torr.The conductivities of the as-deposited and thermally annealedmetal films were measured using a four-point probe apparatus,

manufactured by Singaton Co., Gilroy, CA. The resistivity ofeach film was calculated from the measured voltage under anapplied current to the film, using the equations derived by Vander Pauw.22

XRD and TEM Measurements. X-ray diffraction profileswere measured using the Cu KR radiation (with wavelength )1.541 838 Å) at 40 kV and 40 mA. The XRD measurements werecarried out at an incident angle of 6° over a test area of about0.5 mm in diameter.23 The profiles were collected at a countingtime of 600 s. The resolution of the XRD patterns is about 0.04°.Transmission electron microscopic images were obtained fromthe Philips FE CM300 transmission electron microscope systemoperating at 300 kV. The grain size of the electrolessly depositedmetal films was determined directly from the dark field electronmicrographs.

Results and Discussion

Composition of the VIDz-Coupled H-Si(100) Sur-face. The introducing of an appropriate organic layer onthe silicon surface is the most critical step in facilitatingthe “Sn-free” electroless plating process. Figure 1 showsthe wide scan and N 1s core-level spectra of a VIDz-coupledH-Si(100) surface. The N 1s core-level spectrum consistsof two peak components, having binding energies (BE’s)at about 398.4 for the imino species (dN-) and at 400.4eV for the amino species (-N<) of the imidazole ring.22

The [dN-] to [-N<] molar ratio is about 1, in goodagreement with the theoretical molar ratio of the iminoand amino species in the imidazole ring. Previous studyhas indicated that the probing depth of the XPS techniquein an organic matrix is less than 7.5 nm.24 The persistenceof strong Si signals from the underlying silicon substratesuggests that the coupled VIDz exists as an ultrathin layer.Thus, the XPS results suggest that the UV-inducedcoupling of VIDz on the H-Si(100) surface proceeds viathe reaction of the Si radical site (dangling bond formedfrom UV cleavage of the Si-H bond) with the vinyl groupof VIDz, similar to the mechanism proposed for theformation of alkyl monolayers on the hydrogen-terminatedsilicon surfaces.19 Due to steric reasons, the surfacecoverage never exceeds one monolayer, even with the vinylmonomers that polymerize easily.19

Crystallography of the Electrolessly DepositedNanofilms. Parts a and b of Figure 2 show the respectiveTEM dark field images of the 200-nm-thick Cu and Ni

(20) Xu, D.; Kang, E. T.; Neoh, K. G.; Zhang, Y.; Tay, A. A. O.; Ang,S. S.; Lo, M. C. Y.; Vaidyanathan, K. J. Phys. Chem. B 2002, 106, 12508.

(21) Zhang, Y.; Tan, K. L.; Yang, G. H.; Kang E. T.; Neoh, K. G. J.Electrochem. Soc. 2001, 148, C574.

(22) Van der Pauw, L. J. Phillips. Res. Rep. 1958, 13, 1.

(23) Powder Diffraction Files; Joint Committee of Powder DiffractionStandards (JCPDS): PA, 1990.

(24) Tan, K. L.; Woon, L. L.; Kang, E. T.; Neoh K. G. Macromolecules1993, 26, 2832.

Figure 1. XPS (a) wide scan and (b) N 1s spectra of the VIDz-coupled H-Si(100) surface.

Electrolessly Deposited Cu and Ni Nanofilms Langmuir, Vol. 19, No. 17, 2003 6803

films electrolessly deposited on the VIDz-coupled Si(100)surfaces. The bright features in the images represent theindividual grains. The Cu film is crystalline and has anaverage grain size of about 65 nm. On the other hand, theimage of the Ni film reveals the presence of both theamorphous and the microcrystalline region, with grainsizes of only about 10 nm or below.

The XRD spectra of the electrolessly deposited Cu filmson the VIDz-coupled H-Si(100) surfaces before and afterthermal annealing at 300 °C show three crystalline copperpeaks with the X-ray diffraction angles (2θ) of 43.5, 50.4,and 74.0°, attributable to the orientation patterns ofCu(111), Cu(200), and Cu(220), respectively.7,23,25 The peakintensity (I) ratios and the full width at half-maximum(fwhm) of the Cu(111) peak are summarized in Table 1.The as-deposited copper film with a thickness of 40 nmis found to have a greater preference for the (111)orientation than its 200 nm thick counterpart. Both theI(200)/I(111) and I(220)/I(111) ratios increase with thethickness of the as-deposited copper films. On the otherhand, the fwhm (2θ) of the Cu(111) peak increases from0.88 ( 0.04 to 0.99 ( 0.04° when the film thicknessdecreases from 200 to 40 nm. This result suggests thatthe grain size of the as-deposited copper film decreaseswith the decrease in film thickness.25 Thermal annealingof the as-deposited copper film at 300 °C for 1 h gives rise

to an increase in grain size, which is confirmed by thedecrease in fwhm of the Cu(111) peak for both the 40- and200-nm-thick films after the thermal treatment. This effectof thermal annealing is in general agreement with thatreported by Nakahara et al.,25 who have carried out asystematic study on the low-temperature (<300 °C)annealing behavior of the electrolessly deposited Cu films.

On the other hand, the corresponding XRD patterns ofthe electrolessly deposited Ni films on the VIDz-coupledH-Si(100) surface show a broad peak between 2θ anglesof 40-50°. Previous work has shown that, in the absenceof thermal treatment and with dependence on the phos-phorus content, the electrolessly deposited Ni films canhave atomic structures ranging from the completely amor-phous state to the microcrystalline state with grain sizesof only 1-10 nm.26-28 XPS results show that the phos-phorus contents in the present 40- and 200-nm thick Nifilm are about 28 and 30 at. %, respectively. The TEMimages in Figure 2b reveal that the deposited Ni filmconsists of a mixture of amorphous and microcrystallineNi, having grain sizes of 10 nm or below. Thermal treat-ment above 300 °C can result in the recrystallization ofthe electrolessly deposited Ni films.29,30 The fwhm of thebroad peak at 2θ angles of 40° and 50° in the present Nifilms has decreased substantially after thermal annealingat 300 °C, again suggesting an increase in grain size.

Topography of the Electrolessly Deposited Cu andNi Nanofilms. Parts a and b of Figure 3 show therespective AFM images and the horizontal cross-sectioncurves of the as-deposited Cu films, having thicknessesof 40 and 200 nm, on the VIDz-coupled Si(100) surface.The corresponding AFM images and horizontal cross-section curves of the two copper films after thermaltreatment at 300 °C for 1 h are shown in Figure 3c,d. Thehorizontal cross-section curve from the AFM imagesreveals the size of the particles and their coalescencein the surface region. The surface of the as-deposited40-nm-thick Cu film is composed of the coalescences ofsmall Cu clusters, having cluster sizes around 25-40 nm,and some defects, shown as the dark areas in the topo-graphic images. The presence of the defects on the Cusurface probably arises from the epitaxy of the cop-per crystallites and the hydrogen bubble formation inthe electroless plating process. In comparison with the40-nm-thick copper film, the surface of the as-deposited200-nm-thick copper film, shown in Figure 3b, is composed

(25) Mark, C. Y.; Nakahara, S.; Okinaka, Y.; Trop, H. S.; Taylor, J.A. J. Electrochem. Soc. 1993, 140, 2363.

(26) Riedel, W. Electroless Nickel Plating; ASM International: MetalsPark, OH, 1991; Chapters 5 and 6.

(27) Goldstein, A. W.; Rostoker, W.; Schossberger, F.; Gutzeit, G. J.Electrochem. Soc. 1957, 104, 104.

(28) Agarwala, R. C.; Ray, S. Z. Metall. 1992, 83, 203.(29) Tzeng, S. S.; Chang, F. Y. Thin Solid Films 2001, 388, 143.(30) Watanabe, T.; Kato, J.; Matsuo, S.; Wakita, H.; Umesaki, N.

X-ray Spectrom. 2001, 30, 15.

Figure 2. TEM dark field micrograph of the electrolesslydeposited (a) 200-nm-tick Cu film and (b) 200-nm-thick Ni film.

Table 1. X-ray Diffraction Peak Intensity Ratios and thefwhm of (111) Peak of the Electrolessly Deposited Cu

Films

Cu filmsI(200)/I(111)

I(220)/I(111)

fwhm (111)(deg)

40 nmas-deposited 0.36 0.16 0.99 ( 0.04after thermal treatmenta 0.39 0.18 0.87 ( 0.04

200 nmas-deposited 0.41 0.20 0.88 ( 0.04after thermal treatmenta 0.44 0.22 0.79 ( 0.04

a Thermal treatment was carried out at 300 °C under Ar pressureof 1 Torr.

6804 Langmuir, Vol. 19, No. 17, 2003 Zhang et al.

of the coalescences of larger Cu clusters, having clustersizes around 40-80 nm. Also, the surface roughnessincreases from 5.8 to 7.7 nm with the increase in filmthickness from 40 to 200 nm. Comparison of Figure 3c toFigure 3a, and of Figure 3d to Figure 3b, reveals thatthermal treatment of the electrolessly deposited Cu filmscan give rise to a decrease in surface defects and anincrease in size of the Cu coalescences and clusters.

Parts a and b of Figure 4 show the respective AFMimages and horizontal cross-section curves of the as-deposited Ni films, with a thickness of 40 and 200 nm, onthe VIDz-coupled Si(100) surface. The corresponding AFMimages and horizontal cross-section curves of the Ni filmafter thermal treatment at 300 °C for 1 h are shown inFigure 4c,d. The topography of the as-deposited 40-nm-thick Ni film shows Ni clusters side by side. The as-deposited 200-nm-thick Ni film shows larger coalescencesof the Ni clusters than those of its 40-nm-thick counterpart.However, the Ra value of the as-deposited 200-nm-thickNi film is only 1.9 nm, which is lower than the Ra valueof 3.0 nm for the 40-nm-thick Ni film. Since the electro-lessly deposited Ni films are predominantly amorphousin nature (as shown by the TEM and XRD results), thegrowth of Ni atoms will be layer by layer without anysignificant epitaxy. Therefore, with the increase in platingtime, the Ni clusters increase in size. The edges of the Niclusters will eventually merge with one another and resultin the elimination of the boundaries among the isolatedNi clusters. As a result, the surface roughness decreases.Comparisons of Figure 4c to Figure 4a and Figure 4d toFigure 4b suggest that thermal annealing of the Ni filmsalso gives rise to an increase in coalescences of the Niclusters. The absence of a significant increase in surfaceroughness of the two nickel films upon thermal annealingis probably associated with the predominantly amorphousnature of the films.

Resistivity of the Electrolessly Deposited Cu andNi Nanofilms. The electrical resistivity (Rs) of theelectrolessly deposited metal films is of great importancefor metals used as interconnects in microelectronics.Figure 5 shows the electrical resistivity of the electrolesslydeposited Cu and Ni films of different thicknesses as afunction of the annealing temperature. It can be observedthat the resistivity of the as-deposited Cu and Ni filmsdecrease with the increase in film thickness. The resistivityof the as-deposited 40-nm-thick Cu film, having an Rsvalve of 41.7 µΩ‚cm, is 10 times higher than that of theas-deposited 200-nm-thick Cu film and 25 times higherthan that of the conventional bulk Cu film. The latter hasan Rs value of 1.6 µΩ‚cm.31 On the other hand, the as-deposited 40-nm-thick Ni nanofilm has an electricalresistivity of about 1200 µΩ‚cm. This value is almost 5

Figure 3. AFM images of the electrolessly deposited (a) 40-nm-thick and (b) 200-nm-thick Cu films. The correspondingAFM images of the films after thermal annealing at 300 °C areshown in c and d.

Figure 4. AFM images of the as-deposited (a) 40-nm-thickand (b) 200-nm-thick Ni films. The corresponding AFM imagesof the films after thermal annealing at 300 °C are shown in cand d.

Figure 5. Electrical resistivity of the electrolessly depositedCu and Ni films of different film thickness as a function ofannealing temperature.

Electrolessly Deposited Cu and Ni Nanofilms Langmuir, Vol. 19, No. 17, 2003 6805

times that of the 100-nm-thick Ni film and is much higherthan that of the conventional bulk Ni film. The latter hasan Rs value of about 6.9 µΩ‚cm.31 The high resistivity ofthe electrolessly deposited Cu and Ni nanofilms is probablyattributable to the high degree of structural defects andthe small grain size of the crystallites. The smaller grainsize of the Cu crystallites will result in a higher proportionof grain boundaries in the Cu film, which, in turn, willgive rise to a higher resistivity of the Cu film.32,33

The data in Figure 5 suggest that the effect of thethermal annealing on the resistivity of the electrolesslydeposited Cu and Ni film is also thickness-dependent. Theresistivity of the electrolessly deposited Cu and Ni filmwith a thickness of 40 nm decreases drastically with thethermal annealing temperature. On the other hand, thedecrease in resistivity of the 100- and 200 nm-thick metalfilms upon thermal annealing is rather limited. Sincethermal treatment can greatly reduce the amount of thedefects in the metal films, as shown by the XRD and AFMresults, the larger amount of defects in the as-deposited40-nm-thick nanofilm will give rise to a more markeddecrease in resistivity after the thermal treatment.

Surface Composition of Electrolessly DepositedCopper and Nickel Films. The Cu 2p core-level and Cu(LMM) spectra of the electrolessly deposited 40-nm-thickCu film before and after being subjected to a direct currentdensity of about 1.0 × 109 A/m2 for 10 min are shown inFigure 6a,b, respectively. The Cu 2p core-level and Cu(LMM) spectra of the as-deposited 200-nm-thick Cu filmbefore and after being subjected to a DC current densityof about 1.0 × 109 A/m2 for 10 and 30 min are shown inFigure 6c-e, respectively. The Cu 2p spectrum of the as-deposited 40-nm-thick Cu film has a spin-orbit splitdoublet, having BE’s at about 932.7 (2p3/2) and 952.5 eV(2p1/ 2).34 The Cu (LMM) spectrum of the as-deposited Cufilm shows two distinct peak components, having BE’s at571.3 and 573.3 eV. Since the Cu 2p core-level spectrumof the Cu+ and Cu0 species has the same binding energy,the Auger parameters, defined as the sum of the BE of Cu2p3/2 and the kinetic energy (KE) of Cu (LMM), are usuallyused to differentiate the two Cu species.35 The KE of theCu (LMM) can be derived from the equation KE ) (1486.6eV) - BE. Therefore, the as-deposited Cu film has twoAugerparametersof1848eVfor theCuatoms (Cu0 species)and 1846 eV for the Cu+ species.35 Comparison of Figure6c to Figure 6a suggests that the as-deposited 200-nm-thick Cu film has a similar XPS spectrum and, thus,chemical composition in the surface region, as that of theas-deposited 40-nm-thick Cu film. This result suggeststhat the surface composition of the as-deposited Cu filmsis independent of the film thickness. However, after beingsubjected to the DC current in air, the 40-nm-thick copperfilm oxidizes more readily than the 200-nm-thick film.The XPS spectra in Figure 6b reveal that the surface ofthe 40-nm-thick Cu film is almost completely oxidizedafter being subjected to the DC current in air for only 10min. On the other hand, the 200-nm-thick Cu film is ratherstable under the same current density for an even longerperiod of time. This phenomenon is consistent with thefact that the Cu nanofilm has a higher electrical resistivityand a larger surface area to bulk volume ratio. XPS resultsalso reveal that the electrolessly deposited Ni films exhibitoxidation behavior similar to that of the electrolesslydeposited Cu films.

Conclusions

Nanostructured Cu and Ni films of 40-200 nm inthickness could be electrolessly deposited on the hydrogen-terminated Si(100) surface modified by coupling withvinylimidazole. The grain size of the Cu crystallitesdecreases with the film thickness. The Ni films, on theother hand, were only partially crystalline. The chemicalcomposition and the state of the metal films wereindependent of the film thickness within the thicknessrange studied. However, the density of structure defects,and thus the electrical resistivity, increased with thedecrease in metal film thickness. As a result, the tendencyfor surface oxidation under DC loading in air also increasedwith the decrease in film thickness. Thermal annealingof the metal nanofilms at a mild temperature of 300 °Csignificantly reduced the structural defects, improved theelectrical conductivity, and increased the crystalline grainsize.

LA034087O

(31) Catalogue of Goodfellow Cambridge Limited; Goodfellow: Lon-don, U.K., 2002; pp 85 and 147.

(32) Gleiter, H. Prog. Mater. Sci. 1989, 33, 223.

(33) Pekala, K.; Pekala, M. Nanostruct. Mater. 1995, 6, 819.(34) Flechon, J.; Machizaud, F. J. Chim. Phys. Phys.-Chim. Biol.

1972, 69, 1105.(35) Moulder, J. F.; Stickle, W. F.; Bomben, K. D. In X-ray Photo-

electron Spectroscopy; Chastian, J., Ed.; Perkin-Elmer: Eden Prairie,MN, 1992; pp 84, 86, and 203.

Figure 6. XPS Cu 2p core-level and Cu (LMM) spectra of theas-deposited 40-nm-thick Cu film (a) before and (b) after beingsubjected to a DC current for 10 min and the as-deposited 200-nm-thick Cu film (c) before and after being subjected to a DCcurrent for (d) 10 min and (e) 30 min. (The current density wasabout 1.0 × 109 A/m2.)

6806 Langmuir, Vol. 19, No. 17, 2003 Zhang et al.