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Langmuir 2003, 19, 6802-6806
Characterization of Electrolessly Deposited Copper and Nickel 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, were electrolessly 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 revealed that the electrolessly deposited Cu films were nanostructured, with grain sizes smaller than 65 nm. On the other hand, the Ni films were partially crystalline, with grain sizes on the order of 10 nm or less. The electrolessly deposited Cu and Ni nanofilms had physical properties rather different from those of the thicker films. Atomic force microscopy (AFM) images revealed that the Cu and Ni nanofilms had a higher density of defects and smaller metal clusters in the surface region. The Cu and Ni nanofilms also exhibited a substantially higher electrical resistivity. X-ray photoelectron spectroscopy (XPS) results suggested that the chemical composition and the state of the as-deposited Ni and Cu films were independent of the film thickness. However, the as-deposited Ni and Cu nanofilms oxidized at a much faster rate than their 200-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, and larger surface area to bulk volume ratio of the nanofilms.
Introduction With the advances in the microelectronics industry, copper is rapidly replacing the commonly used aluminum in the multilevel metallization process. Copper has a lower resistance-capacitance (RC) delay and lower susceptibility to electromigration than aluminum. Metallization of various substrates via electroless plating has attracted increasing attention due to its simplicity, low cost, low processing temperature, and good step coverage.1,2 The technique has promising applications in the submicron and nanolevel electronics. Much effort has been devoted to the study of electroless deposition of copper on silicon substrates.3-6 A major drawback of the process is the lack of adhesion of the electrolessly deposited copper to the silicon substrate, since there are only limited chemical interactions between the deposited copper and the silicon surface. In addition, the diffusion of copper into the silicon substrate is another problem encountered in this approach. To retard copper * To whom all correspondence should be addressed. Telephone: +65-6874-2189. Fax: +65-6779-1936.E-mail address:
[email protected]. (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. SolidState 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.
diffusion, a barrier layer, consisting of tantalum (Ta), tantalum nitride (TaN), or titanium nitride (TiN), is sputter-deposited on the silicon surface prior to the electroless deposition of copper.7,8 One of the most important advances in the modification of silicon surfaces involves the formation of monolayers by the reaction of alkenes with the hydrogen-terminated silicon surface, using free radical initiation, thermal coupling, Lewis acid-catalyzed hydrosilylation, and ultraviolet activation.9-19 A recent study has taken advantage of 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(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.; Go¨tz, 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, 16, 5688.
10.1021/la034087o CCC: $25.00 © 2003 American Chemical Society Published on Web 07/11/2003
Electrolessly Deposited Cu and Ni Nanofilms
sion barrier and adhesion promoter for the electrolessly deposited metals.20 The 4VP functional layer on the silicon surface also accounts for the electroless deposition of copper without the need for prior sensitization by a tin compound. In this study, ultrathin Cu and Ni films with thicknesses on the orders of 40 nm (nanofilms) and 200 nm are electrolessly deposited on the H-Si(100) surface modified by coupling with vinylimidazole (VIDz). Transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and four-point probe measurement are employed to characterize, respectively, the grain size, crystalline structure, surface topography, surface composition, and electrical resistivity of the electrolessly deposited metal films in the presence and absence of thermal annealing. The effect of DC current loading on the chemical state of the nanofilms is also investigated. Experimental Section Materials. Single-crystal, (100)-oriented silicon wafers, or Si(100) wafers, having a thickness of about 650 µm and a diameter of 200 mm, were obtained from Wacker Siltronic Singapore Pte. Ltd., Singapore. The as-received wafers were polished on one side and were without any dopant. The silicon wafers were sliced into rectangular strips of about 2 cm × 1 cm in size. The silicon substrate was cleaned with the “piranha” solution, a mixture of 98 wt % concentrated sulfuric acid (70 vol %) and hydrogen peroxide (30 vol %). Vinylimidazole was purchased from the Aldrich Chemical Co. and was purified by vacuum distillation before use. Coupling of a Vinyl Monomer (VIDz) with the HydrogenTerminated Silicon Surface. The silicon strips were immersed in the 10 wt % hydrofluoric acid solution in Teflon vials for 25 min to remove the native oxide layer, leaving behind a uniform hydrogen-terminated Si(100) surface (H-Si(100) surface).14 The H-Si(100) strip was introduced into a Pyrex glass tube, containing the liquid VIDz monomer prepurged with purified argon. The reaction mixture was purged with purified argon for another 5 min. The glass tube with the reaction mixture was then sealed with a ground glass stopper and subjected to UV irradiation in a Riko RH400-10W rotary photochemical reactor (manufactured by Riko Denki Kogyo of Chiba, Japan). The reactor was equipped with a 1000-W high-pressure Hg lamp and a constant-temperature bath. All UV-induced reactions were carried out at a constant temperature of 28 °C for 30 min. After UV irradiation, the silicon strip was removed from the reaction mixture and washed with copious amounts of water to remove the adsorbed VIDz 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 deposition of copper or nickel. The activation and the electroless plating process had been described elsewhere.20,21 Surface Characterization. The chemical composition of the electrolessly deposited metal surfaces was determined by X-ray photoelectron spectroscopy. The conditions for XPS measurements were similar to those documented in the previous work.20 The surface topography of the metal films was investigated using a Nanoscope IIIa atomic force microscope. All images were collected in air using the tapping mode under a constant force (scan size, 1 µm; scan rate, 2 Hz). Thermal Annealing and Electrical Resistivity Measurements of the Electrolessly Deposited Metal Films. The asdeposited metal films were thermally annealed in a furnace at different temperatures under an argon pressure of about 1 Torr. The conductivities of the as-deposited and thermally annealed metal films were measured using a four-point probe apparatus, (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.
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Figure 1. XPS (a) wide scan and (b) N 1s spectra of the VIDzcoupled H-Si(100) surface. manufactured by Singaton Co., Gilroy, CA. The resistivity of each film was calculated from the measured voltage under an applied current to the film, using the equations derived by Van der Pauw.22 XRD and TEM Measurements. X-ray diffraction profiles were measured using the Cu KR radiation (with wavelength ) 1.541 838 Å) at 40 kV and 40 mA. The XRD measurements were carried out at an incident angle of 6° over a test area of about 0.5 mm in diameter.23 The profiles were collected at a counting time of 600 s. The resolution of the XRD patterns is about 0.04°. Transmission electron microscopic images were obtained from the Philips FE CM300 transmission electron microscope system operating at 300 kV. The grain size of the electrolessly deposited metal films was determined directly from the dark field electron micrographs.
Results and Discussion Composition of the VIDz-Coupled H-Si(100) Surface. The introducing of an appropriate organic layer on the silicon surface is the most critical step in facilitating the “Sn-free” electroless plating process. Figure 1 shows the wide scan and N 1s core-level spectra of a VIDz-coupled H-Si(100) surface. The N 1s core-level spectrum consists of two peak components, having binding energies (BE’s) at about 398.4 for the imino species (dN-) and at 400.4 eV for the amino species (-N
