This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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2 Thin Solid Films 517 (2009) Contents lists available at ScienceDirect Thin Solid Films journal homepage: Mechanism of immersion deposition of Ni P films on Si(100) in an aqueous alkaline solution containing sodium hypophosphite H.F. Hsu, C.L. Tsai, C.W. Lee, H.Y. Wu Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC article info abstract Available online 13 March 2009 Keywords: Immersion deposition Oxidation Nickel Silicon Deposition mechanism The immersion deposition of Ni P films on Si(100) surface without prior activation by metallic catalytic was carried out in an aqueous alkaline solution containing sodium hypophosphite. The deposition mechanism was investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Two stages of deposition were observed when the Si substrate was immersed in the deposition solution at an appropriate ph value. In the first stage, crystalline Ni nanoparticles were formed through a galvanic displacement reaction, which accompanied the oxidation of Si substrate without involving the reducing agent, NaH 2 PO 2. Experimental results indicate that the oxidation states of Si 4+ and Si 3+ exist in the oxide layer. The amount of suboxide, Si 3+, increased with deposition time, and the oxide layer became activated. In the second stage, amorphous Ni P was deposited on this activation oxide layer in a process involving the reducing agent. The microscopic structure of the deposition film, observed by TEM crosssectional analysis, verifies the mechanism of deposition suggested in this study Elsevier B.V. All rights reserved. 1. Introduction The mass production of metallic structures on silicon surface in wafer scale s one of key technique in micro and nanoscale device applications [1,2]. The metallization of silicon surface is widely performed by the deposition of metal thin films in a vacuum, such as by evaporation or sputtering. However, wet processes, including electrodeposition, electroless and immersion depositions, are attractive because they are the simple, reasonable and efficient methods of depositing metal. Electroless and immersion depositions are simpler than electrodeposition because they do not require an electrical bias to be externally applied to a substrate during deposition. The former approach requires surface activation, which involves immersing the substrate in an aqueous solution containing metal ions (such as Pd 2+ ). Metal (Pd) islands are formed as deposition seeds, and a reducing agent is required in the deposition solution. The latter involves simply immersing the substrate in a solution containing appropriate metal ions. However, the rate of immersion deposition is less than that of conventional electroless deposition. The electroless deposition and immersion deposition of Ni films on silicon substrates have received considerable attention in recent years [3 9]. By using an acidic aqueous solution, Liu et al. [3] deposited Ni on a silicon surface with prior sensitization and activation treatment in a conventional Ni P electroless deposition bath. According to their results, Ni P was deposited in a very short deposition time (b2 s). Corresponding author. Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Rd., Taichung 402, Taiwan, ROC. Tel.: x300; fax: address: hfhsu@dragon.nchu.edu.tw (H.F. Hsu). However, the immersion deposition is limited in that the amount of nucleated Ni particles is low, explaining why several subsequent investigations attempted to improve the properties of the coatings. X. Zhang et al. [4,5] indicated that adding NH 4 F to the solution increase the density of nickel nuclei particles in the initial stage of immersion deposition. However, the coatings were not uniform. In the study of C. K. Lin et al. [9], pulsating electric current was used to increase the deposition rate. T. Osaka et al. [10] demonstrated the feasibility of using two-step deposition to form the Ni dots in the patterned silicon wafers, subsequently leading to the formation of high quality Ni thin films. In the first step, Ni nuclei were formed by immersing the silicon substrate into a deposition solution without a reducing agent. In the second step, the substrate was transferred into a deposition solution with a reducing agent. This study demonstrates the feasibility of adding sodium hypophosphite, a reducing agent, in an aqueous alkaline solution at an appropriate ph value to achieve two steps of Ni nucleation and Ni P film growth in one immersion process. Consequently, the deposition process is simplified, and a uniform film is obtained. The deposition mechanism is also investigated in detail by AFM, SEM, TEM and highenergy-resolution XPS analyses. 2. Experimental details N-type Si(100) substrates (P doped with a resistivity of Ω cm) were used in this study. The substrates were cleaned using standard processes, followed by dipping in a dilute HF solution. The substrates were then pretreated with a pretreatment solution (30% H 2 O 2 :36% HCl:H 2 O=1:1:4) at 80 C for 15 min to form a hydrophilic /$ see front matter 2009 Elsevier B.V. All rights reserved. doi: /j.tsf

3 H.F. Hsu et al. / Thin Solid Films 517 (2009) Fig. 1. SEM images of specimens immersed in the deposition solution for 2 min at ph (a) 8.0, (b) 8.4, (c) 8.6, and (d) 9.0. surface before immersion in the deposition solution [11]. The deposition solution was an aqueous alkaline solution that contained NiSO 4 (0.1 mol dm 3 ) for supplying nickel ions, NaH 2 PO 2 (0.15 mol dm 3 ) for serving as the reducing agent, Na 3 C 6 H 5 O 7 (0.2 mol dm 3 ) and (NH 4 )SO 4 (0.5 mol dm 3 ) for serving as complex and buffer agent. The ph value was adjusted at 8 9 with NH 4 OH and the temperature was controlled at 75 C. Topography of the specimens was observed using atomic force microscope (AFM) (Solver PRO-M model, NTMDT) operating in tapping mode and field emission scanning electron microscopy (FESEM) (JEM-6700F, JEOL) operating at 3 kv. The chemical reaction of the initial deposition stage was studied by using X-ray photoelectron spectroscope (XPS) (PHI Quantera, ULVAC-PHI) furnished with Al Kα radiation. The energy resolution was ev. The plane views, cross sections, and diffraction patterns of the deposition were analyzed by transmission electron microscopy (TEM) (JEM-2010, JEOL) operated at 200 kv to study the microstructure and crystallization of deposition. The micro-composition of the specimens was analyzed by energy dispersive X-ray spectrum (EDS), equipped in FESEM and TEM. 3. Results and discussion 3.1. Effects of ph on deposition behavior Fig. 1 shows SEM images of the specimens after immersion in the deposition solution for 2 min at various ph values. At ph=8.0, some precipitations were observed on the Si surface (Fig. 1(a)). However, the composition of the small amount of precipitate could not be determined by EDS analysis. At ph=8.4, Ni P particles, marked by an arrow in Fig. 1(b), were observed. Ni P continuous films formed at ph=8.6 and 9. The P content of Ni P depositions was 22, 19 and 15 at.% at ph values of 8.4, 8.6 and 9, respectively. These results those of correspond to the conventional electroless Ni P deposition, in which the P content decreases as the ph value increases. Table 1 shows the deposition behaviors of the silicon substrate immersed in solutions with various ph values. The onset time of Ni P deposition, which is the period required for Ni P to begin growth after immersion in the deposition solution were about 2 min, 1 min 10 s and 1 min at ph=8.4, 8.6 and 9, respectively. The onset time decreased as the ph increased. However, Liu et al. [3] reported that when the activated silicon substrate was immersed in the conventional electroless solution, Ni P precipitated out immediately. Therefore, above results indicate that the silicon surface became activated during the immersion and the activation rate increased with the ph of the deposition solution. A detail discussion is provided in Section Photographic analysis of the surface The mechanism of immersion deposition on the silicon surface without activation pretreatment was studied by immersing the specimens in the deposition solution at ph=8.6 for various deposition periods. Following pretreatment of the silicon substrate by immersing it in the pretreatment solution, some pin holes on the silicon surface were observed, as shown in Fig. 2(a). Following immersion of the substrate in the deposition solution for 30 s (Fig. 2(b)), the photograph of the silicon surface was nearly the same as that in Fig. 2(a); in addition, the roughness of these two specimens was 0.4 nm. However, after immersion for 1 min, many dots on the surface were observed to be higher, and the roughness of the surface increased to 4.8 nm (Fig. 2(c)). Moreover, the bright dots in the SEM plane view image (Fig. 3(a)) were significantly smaller than those by the AFM analysis because of the finite size of AFM tip apex. Few Ni P particles began to grow at 1 min 10 s immersion (Fig. 3(b)). Notably, a continuous Ni P film was formed after 1 min 30 s of deposition. Therefore, in this study, Table 1 The deposition behaviors of the silicon substrate immersed in the solutions with various ph values. 10 s 20 s 30 s 1 min 2 min 4 min 6 min ph=8 ph=8.4 ph=8.6 ph=9 No precipitation was observed by SEM. Some precipitations, not Ni P, were observed by SEM. The SEM images were the same as Fig. 1(a). Ni P particles were deposited on the substrate. Ni P film was formed on the substrate.

4 4788 H.F. Hsu et al. / Thin Solid Films 517 (2009) Fig. 2. AFM images of specimens, which were pretreated using the solution of H 2 O 2 and HCl, immersed in the deposition solution for (a) 0 s, (b) 30 s, and (c) 1 min. the deposition process can be divided into two stages, Si-surface activation and Ni P deposition. In the former, the duration of immersion is less or equal to 1 min, which is the onset time of Ni P deposition. In the latter, immersion lasts more than 1 min Chemical reactions of the deposition Fig. 4 presents the Si 2p, Ni 2p and P 2p XPS spectra obtained from the specimens at various durations of immersion during the stage of Si-surface activation. The peak at a binding energy of about 99.3 ev corresponds to that of bulk Si. The other peak at a higher binding energy corresponds to the oxidation of the silicon substrate. The oxidation Si 2p peaks shifted from a binding energy of ev towards ev, indicating that when the silicon substrate was immersed in the deposition solution, the component of silicon oxide on the substrate surface varied gradually. During this stage of Sisurface activation, a small amount of Ni was also deposited on the silicon surface, as shown in Fig. 4(b). Three main peaks of the Ni 2p 3/2 spectra correspond to metallic Ni, NiO or Ni(OH) 2 and the multiple structure of Ni, respectively. Comparing our results with those of Oliveira et al. [12] reveals that this oxidation of Ni resembles that of Ni electrodes in a solution containing hypophosphite ions. Broad P 2p spectra were observed in Fig. 4(c) because of the adsorption of NaH 2 PO 2 or Na 2 HPO 3 [13] on the surface. The activation of Si surface was investigated by fitting the Si 2p spectra by doublet of a Voigt function with standard spin-orbit splitting and a branching ratio of the Si 2p 3/2 and Si 2p 1/2 peak constituents of 0.61 ev and 2, respectively after subtraction of the secondary electron background. Fig. 5 shows four deconvoluted components with Si 2p 3/2 peak maxima at 103.2, 102.4, and 99.3 ev, corresponding to silicon dioxide (Si 4+ ), silicon suboxide (Si 3+ ), hydride (Si H) and bulk silicon (Si 0 ) [14 17].Si H was formed at the silicon oxide/silicon interface when the silicon substrate was oxidized in an aqueous solution [14 16]. According to Table 2, increasing the immersion time leads to distributions with a larger proportion of the suboxidation state. Hattori et al.[16] demonstrated that different species of suboxides, Si 3+,Si 2+,andSi +, formed on the Si surface after immersion in various solutions. In this study, before their immersion in the deposition solution, silicon substrates were pretreated in a solution of H 2 O 2 and HCl. Oxide films were formed on the silicon surfaces, which were smooth with only some Fig. 3. SEM images of the specimens immersed in the deposition solution at ph 8.6 for (a) 60 s, (b) 1 min 10 s, (c) 1 min 20 s, and (d) 1 min 30 s.

5 H.F. Hsu et al. / Thin Solid Films 517 (2009) Table 2 Relative intensities of the oxidation states for silicon surface immersed in deposition solutions for various periods. I Si 3þ Immersion time I Si 3þ + I Si 4þ I Si 3þ + I Si 4þ I Si 3þ + I Si 4þ 30 s s min increased because of a high oxidation rate of the silicon substrate, which was also responsible for the higher roughness of silicon surface (Fig. 2(c)) and a larger proportion of suboxide (Si 3+ ) than previously I Si 4þ I Si 0 Fig. 4. XPS spectra of (a) Si 2p, (b) Ni 2p, and (c) P 2p on the specimens at various durations of immersion. The MS in the (b) means the multiple structures. pin holes, indicating that the oxidation rate was slow. Thus, most of the suboxide was localized at the interface of silicon oxide/silicon [11]. When the specimens were immersed in the deposition solution for 1 min, the absolute intensity of oxidation states (Si 4+ and Si 3+ )clearly Fig. 5. Photoemission spectra (solid curve) and peak analysis of Si 2p on the specimens at various durations of immersion. Fig. 6. Schematic diagram of the mechanism for the Ni nanoparticles and N P deposition on Si from the aqueous alkaline solution with sodium hypophosphite.

6 4790 H.F. Hsu et al. / Thin Solid Films 517 (2009) (Table 2) [17]. Under such a high-oxidation condition, a uniform distribution of Si 3+ in the oxide film was obtained [11]. Aninactiveoxide layer, formed by pretreatment, was transformed into an active oxide layer to become a catalytic surface Reaction mechanism and microscopic structure of deposition film The reaction mechanism of conventional electroless Ni P deposition can be expressed as follows [18]. The reaction solution contains hypophosphite ions as the reducing agent. Hypophosphite ions participate in two surface reactions oxidation and reduction. The former forms the phosphate. Hypophosphite absorbs on the catalytic surface and then reacts with water (1), reducing the Ni 2+ ions (2): H 2 PO 2 +H 2 O cat: H 2 PO 3 +2e +2H + ð1þ Ni 2+ +2e cat: Ni ð2þ The latter forms alloyed P. Hypophosphite reacts with Ni 2+ ions in the solution to produce the Ni P alloy(3), and it is directly reduced (4): Ni 2+ +H 2 PO 2 +2H + +3e cat: Ni P +2H 2 O ð3þ H 2 PO 2 +2H + +e cat: P+2H 2 O ð4þ Therefore, in conventional electroless Ni deposition, Ni and P are deposited simultaneously. However, in Fig. 4(c), P 2p peaks at ev and ev, which correspond to alloyed P and P, were not observed. Hence, P did not participate in the reaction during the stage of Si-surface activation. Additionally, the reactions of conventional electroless Ni P deposition, as described above, must proceed on a catalytic surface. Therefore, we can infer that the Si surface was not catalyzed completely during the stage of Si-surface activation. The suggested deposition mechanism is the precipitation of Ni through a galvanic displacement reaction that does not involve the reducing agent, NaH 2 PO 2. The galvanic displacement reaction is a spontaneous reaction, indicating a more active metal displaces a least active metal from solution. The reaction for Ni deposition can be expressed as [19] 2Ni 2+ +SiY2Ni 0 +Si 4+ During this reaction, the Si substrate oxidized. Fig. 6 illustrates the mechanism of deposition. Following immersion of the specimen for 1 min, the silicon surface was transformed into a catalytic surface, as discussed in Section 3.3. When the catalytic surface formed, hypophosphite began to participate in the oxidation and reduction reactions, followed by deposition of Ni P. Additionally, the silicon surface is oxidized in aqueous alkaline solution: Si+ 2OH YSiO 2 +H 2 +2e The oxidation rate increases with the ph value, explaining why the onset time of deposition decreased as the ph value increased, as shown in Section 3.1. ð5þ Fig. 7. (a) Cross-sectional TEM micrograph of the specimen immersed in the deposition solution for 1 min 30 s at ph 8.6. (b) The SAED analysis of (a). (c) The dark field image of the Ni (111) diffraction spot, marked by the bold arrow in (b), and the area corresponds to the big square in (a). (d) The atomic image of the interface of silicon substrate and oxide layer, and the area corresponds to the small square in (a).

7 H.F. Hsu et al. / Thin Solid Films 517 (2009) Fig. 7 shows the results of the TEM cross-section analysis of the Ni P film, formed by deposition for 1 min 30 s. The selected area electron diffraction (SAED) analysis (Fig. 7(b)) shows a broadened diffraction ring of Ni(111), indicating that the structure of Ni P film is almost amorphous. From the dark field image (Fig. 7(c)), crystalline Ni nanoparticles localized on top of the Ni P film was observed. The Si:O ratio of the interlayer between the silicon substrate and Ni P film was 1:1.825, based on EDS analysis, thus proving that oxide layer contains the suboxide. A faceted silicon surface was formed (Fig. 7(d)) by the anisotropic etching of silicon for immersion in an aqueous alkaline solution [8]. Additionally, crystalline Ni nanoparticles were formed during the stage of Si-surface activation. After the active oxide layer was produced between the Ni and the silicon substrate, the Ni 2+ ions diffuse through the boundary of Ni nanoparticles and Ni P was deposited on top of the active oxide layer, causing the Ni nanoparticles to be localized on top of the Ni P film. 4. Conclusions Immersion deposition of Ni P on Si(100) surface without prior activation by metallic catalyer was carried out in an aqueous alkaline solution that contains sodium hypophosphite. Two stages of deposition were observed when Si substrate was immersed in the deposition bath. In the initial stage of deposition, called the stage of Si-surface activation, crystalline Ni nanoparticles were formed through the galvanic displacement reaction and the Si substrate was oxidized without involving the reducing agent, sodium hypophosphite. The oxide layer exhibited two oxidation states, Si 4+ and Si 3+. The amount of suboxide, Si 3+,increased with deposition time and until the oxide layer became activated. Immediately, amorphous Ni P was deposited on the activation oxide layer by a mechanism involving the reducing agent. The crystalline Ni nanoparticles were localized on the surface of amorphous Ni P layer because the activated oxide layer acted as a catalytic surface in the reduction of Ni 2+ ions. Acknowledgments The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC E References [1] C.H. Ting, M. Paunovic, J. Electrochem. Soc. 136 (1989) 456. [2] L. Guo, E. Leobandung, S.Y. Chou, Science 275 (1997) 649. [3] W.L. Liu, S.H. Hsueh, T.K. Tsau, W.J. Chen, S.S. Wu, Thin Solid Films 510 (2006) 102. [4] X. Zhang, F. Ren, M.S. Goorsky, K.N. Tu, Surf. Coat. Technol. 201 (2006) [5] X. Zhang, Z. Chen, K.N. Tu, Thin Solid Films 515 (2007) [6] N. Takano, N. Hosoda, T. Yamada, T. Osaka, J. Electrochem. Soc. 146 (1999) [7] D. Niwa, N. Takano, T. Yamada, T. Osaka, Electrochim. Acta 48 (2003) [8] D. Niwa, T. Homma, T. Osaka, J. Phys. Chem., B 108 (2004) [9] C.K. Lin, H.T. Hsu, C.T. Chen, T.J. Yang, Thin Solid Films 516 (2007) 355. [10] N. Takano, N. Hosoda, T. Yamada, T. Osaka, Electrochim. Acta 44 (1999) [11] T. Hattori, K. Takase, H. Yamagishi, R. Sugino, Y. Nara, T. Ito, Jpn. J. Appl. Phys. 28 (1989) L296. [12] M.C. Oliveira, A.M. Botelho do Rego, J. Alloys Compd. 425 (2006) 64. [13] [13] E. Fluck, D. Weber, Pure Appl. Chem. 44 (1074) 373. [14] H. Ibach, H. Wagner, D. Brunchmann, Solid State Commun. 42 (1982) 457. [15] E.M. Oellig, R. Butz, H. Wagner, H. Ibach, Solid State Commun. 51 (1984) 7. [16] Y.J. Chabal, Phys. Rev., B 29 (1984) [17] F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmon, G. Hollinger, Phys. Rev. B 38 (1988) [18] M.E. Touhami, E. Chassaing, M. Cherkaoui, Electrochim. Acta 48 (2003) [19] C.H. Ting, M. Paunovic, J. Electrochem. Soc. 136 (1989) 456.