Deposition Mechanism of Ni on Si(100) Surfaces in Aqueous Alkaline

Immersion nickel deposition on blank silicon in aqueous solution containing ammonium fluoride. Xi Zhang , Zhong Chen , K.N. Tu. Thin Solid Films 2007 ...
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J. Phys. Chem. B 2004, 108, 9900-9904

Deposition Mechanism of Ni on Si(100) Surfaces in Aqueous Alkaline Solution Daisuke Niwa, Takayuki Homma, and Tetsuya Osaka* Department of Applied Chemistry, School of Science and Engineering, Waseda UniVersity, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan ReceiVed: October 18, 2003; In Final Form: April 1, 2004

The spontaneous deposition process of Ni on Si(100) surface in aqueous alkaline solution was investigated by electrochemical measurements and in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR). The open circuit potential (OCP) profile revealed that the deposition reaction was self-terminated as the potential steeply shifted in the positive direction after a certain immersion time. Auger depth profiles of specimens after the termination of the deposition reaction indicated that the surface was not completely covered by Ni. To clarify the deposition behavior, anodic reaction of Si in an aqueous alkaline solution containing no Ni ions was investigated by in situ ATR FTIR and electrochemical measurements. It was found that the deposition reaction was initiated by the formation of suboxide species of Si at the surface. This suboxide species served as “reductant” for Ni ions. Subsequently, the anodic reactions such as the oxidation of suboxide species and the anisotropic etching to 〈111〉 direction proceeded spontaneously. When 〈111〉 oriented microfacets were formed, the surface was passivated with silicon dioxide. Because of the silicon dioxide insulating properties, the electron generation by Si oxidation is inhibited, and then, the OCP sharply shifted in the positive direction. Apparently, the formation of 〈111〉 oriented microfacets at the surface led to the self-termination of the Ni deposition process. This study shows that the overall deposition reaction of Ni is significantly affected by the anodic reaction of Si, including the oxidation and the anisotropic etching.

1. Introduction Wafer-scale fabrication of metallic nanostructures on Si surface is one of the key technologies to develop various nanoscale devices such as nanotransistors,1-3 patterned recording media,4-6 and highly integrated sensors.7,8 Among various processes of metallization of Si surface, wet processes such as electrodeposition and electroless deposition are attractive for mass production of fine structures.9-12 We have investigated the metallization process of Si surface using an electrochemical method, specifically the electroless Ni deposition on Si(100) wafer in an aqueous alkaline solution because of the simplicity of operation and the ability to form fine structures. In our previous papers, we reported that Ni could be spontaneously deposited on Si(100) wafers by immersion in a pH-adjusted nickel sulfate solution containing no reducing agent.13-16 Furthermore, by pretreatment of a hydrogenterminated Si wafer surface with a solution containing HCl and H2O2, it was found that the uniformity of the Ni deposition was significantly improved and the deposition reaction was considerably accelerated, compared with the case of the hydrogenterminated surface.14-16 Following these results, we attempted to carry out position selective Ni deposition on a patterned Si substrate. The patterned deposition was performed in two steps; initially a Ni bath without reductant was used for “seeding”, and then a second Ni bath containing sodium hydrophosphite was used for bulk deposition. By this process, arrays of metallic Ni dots 80 nm in diameter were successfully formed. It was clear that the seeding step was the key process for fabricating metallic nanostructures with various functional properties. As a deposition mechanism for this seeding step, it was suggested * To whom correspondence should be addressed: e-mail osakatet@ waseda.jp.

that the Ni was deposited by galvanic displacement with the Si, and this reaction was accompanied by the formation of a silicon oxide.13 However, details have not been clarified. The present study was carried out to understand in detail the deposition mechanism of Ni on Si(100) in aqueous alkaline solution, by means of electrochemical analyses and in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR). 2. Experimental Section Wafers of n-type Si (100) (8-12 Ω cm) were cut into pieces measuring 10 mm × 40 mm, which were oxidized in O2 atmosphere at 950 °C (ca. 20 nm in thickness) to cover the entire surface with silicon dioxide. Subsequently, a specific area (7 mm in diameter) of the surface was etched with HF to expose the active area. A selected area on the back of the substrate was also etched to make electric contact. The wafers were then treated with SPM (96% H2SO4:30% H2O2 ) 4:1) at 120 °C followed by rinsing with 18 MΩ cm deionized (DI) water. The wafers were immersed in 1.0 wt % HF for 30 s to prepare a clean, hydrogen-terminated surface. The wafer surfaces were then mildly oxidized with HPM (36% HCl:30% H2O2:H2O ) 1:1:5) at 80 °C, followed by rinsing with DI water and was subsequently immersed at 80 °C into an aqueous alkaline solution containing 0.1 M NiSO4 and 0.3 M (NH4)2SO4 at pH 9.0 adjusted with NH4OH. The specimens were observed with a scanning electron microscope equipped with a field emission gun (FE-SEM; Hitachi, Ltd., S-4500). The amount of the Ni deposit was measured by using an inductively coupled Ar plasma spectrophotometer (ICP; Nippon Jarrell-Ash Co., Ltd. IRIS-AP). Prior to the ICP measurement, the deposited Ni on the surface was

10.1021/jp037159t CCC: $27.50 © 2004 American Chemical Society Published on Web 06/08/2004

Deposition Mechanism of Ni on Si(100) Surfaces

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Figure 1. Schematic illustration of experimental setup used for in situ ATR FTIR absorption and electrochemical measurements in solution.

Figure 3. FE-SEM images of specimens immersed in solution for the durations of (a) 15, (b) 40, (c) 50, (d) 60, (e) 150, and (f) 240 s. The first (initial) image is shown as the reference; it was obtained with the wafer before immersion. The numbers in this figure correspond to the immersion times shown in Figure 2. Figure 2. OCP profile and the amount of Ni deposit on the wafer immersed in alkaline NiSO4 solution (pH 9.0, 80 °C).

dissolved by a 10% HCl solution at 80 °C. An Auger electron spectrometer (AES; JEOL Ltd., JAMP-7100) with Ar ion gun depth profiling capability was used for the elemental analysis of the specimens. The AES depth profiling was performed at a SiO2 etch rate of 50 nm min-1. Electrochemical measurements of the wafers were carried out with a potentiostat (Hokuto Denko Co.,Ltd. HABF-501). An ohmic contact was achieved with a layer of Ga-In on the back of the Si sample. A platinum wire and Ag/AgCl electrode were used as a counter electrode and a reference electrode, respectively. All potential values reported refer to the Ag/AgCl reference. Infrared absorption spectra were measured by a FTIR spectrometer (Perkin-Elmer, Spectrum One) with HgCdTe (MCT) detector at the liquid N2 temperature. For the ATR measurement, an n-Si(100) prism (0.5 mm × 50 mm × 20 mm) with 45° bevels on each of the 20 mm sides was used. This prism performed internal reflection about 100 times. The resolution of the interferometer was set at 4 cm-1. The in situ ATR FTIR and the electrochemical measurements were carried out simultaneously by use of an experimental setup illustrated in Figure 1. Infrared absorption spectra of the Si surface during immersion in the alkaline solution exhibited strong, broad absorption bands resulting from the solution. The infrared spectrum of the surface with chemically grown oxide, which was recorded immediately after immersion in the solution, was used as the background reference. 3. Results and Discussion In our previous work, we have confirmed that Ni was deposited uniformly onto the Si(100) surface when the hydrogenterminated surface was mildly oxidized by the HPM solution prior to the Ni deposition. The pretreatment process using the HPM solution was employed for the preparation of all specimens used in the present investigation. The thickness of the chemical oxide was estimated to be 6 Å.16 Figure 2 shows changes in electrochemical open circuit potential (OCP) and the amount of Ni deposit with immersion time. The FE-SEM images at different stages of deposition are shown in Figure 3. The deposition reaction began to proceed as the OCP shifted in the negative direction at about 10-15 s [the region labeled a in Figure 2]. It is considered that the negative potential shift was caused by dissolution of the initially

formed suboxide layer.16 We have reported that the potential of this surface in region a was more negative than that observed with a hydrogen-terminated Si.16 Then, the OCP shifted positively between 15 and 40 s [a toward b] and reached a constant value of about -0.74 V at 40-50 s [b to c]. To interpret the change in OCP on the basis of the mixed potential theory, the following expression was proposed:17

∆E(OCP) ) kT/e∆jC/jA

(1)

where jC and jA are the anodic and cathodic exchange current densities. Equation 1 shows that the rapid change in potential from region a to b is due to an increase in cathodic current caused by the reduction of Ni. It is thought that the increase in cathodic current is caused by the catalytic activity of this metal. Indeed, Ni was seen to be deposited rapidly onto the Si surface in regions a to b, where the nucleation is assumed to be dominant. In this plateau region, a rapid increase in the amount of Ni deposit was observed by FE-SEM, suggesting that the deposition process of Ni changed from the “nucleation” stage to the “growth” stage. After this plateau region, the OCP gradually shifted in the positive direction up to the immersion time of 150 s [point e], and eventually no increase in the amount of Ni deposit was observed as the OCP shifted further beyond point e. At this point, the potential was equal to -0.70 V, which corresponds to the standard electrode potential of Ni estimated from the Pourbaix diagram under this measurement condition. The corresponding SEM images presented in Figure 3 show that the surface morphology of the deposit did not change beyond point e, indicating that the deposition reaction proceeded up to about 135 s and then terminated spontaneously. The AES depth profile was measured to acquire detailed information on the specimens after termination of the deposition. The profile of the specimen at 150 s immersion [point e in Figure 2] indicates the formation of silicon dioxide at the interface between the deposited Ni and the Si substrate (Figure 4).15 The profile of the specimen at 240 s immersion [point f] also showed similar features. The thickness of the deposits remained almost constant (about 250 nm) after immersion times longer than 150 s [point e], which was estimated from the crosssectional SEM images. It is thought that this value is equal to the sum of the thickness of the Ni deposit and that of the silicon dioxide layer. The thickness of Ni deposit of 120 µg cm-2 shown

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Figure 4. AES depth profile of a specimen with Ni deposit after immersion for 150 s. Depth profiling was performed at the SiO2 etch rate of 50 nm min-1 with Ar ion bombardment. Figure 6. ATR FTIR spectra of Si(100) surface during immersion in solution without Ni ions at pH 8.5 and 80 °C. The immersion time for each spectrum is (a) 1, (b) 4, (c) 6, (d) 10, (e) 20, (f) 30, (g) 40, (h) 50, and (i) 80 min.

Figure 5. OCP profiles for the wafers immersed in solution without Ni ion at (a) pH 9.0 and 80 °C, and (b) pH 8.5 and 80 °C.

in Figure 2 corresponds to a densely packed Ni layer of 135 nm, which was estimated from its density of 8.85 g cm-3. In addition, it should be noted that Si was detected at the initial stages of the sputtering, indicating that the surface is not completely covered with Ni. In other words, the Si surface was partially exposed even after the deposition reaction was terminated. Therefore, it is likely that the Ni was deposited simultaneously with the formation of silicon dioxide, and the self-termination was caused by the anodic reaction of Si leading to the surface passivation by an insulating layer of silicon dioxide, rather than by complete coverage of the entire surface with Ni. To clarify the deposition mechanism, further details of the anodic reaction process of Si were investigated by OCP and FTIR measurements in the Ni-free solution. First, the OCP of the wafer in the Ni-free solution was examined. A solution containing 0.3 M (NH4)2SO4, whose pH was adjusted to 9.0 with NH4OH, was used at 80 °C for this experiment. As shown in Figure 5, the initial potential shifted in the negative direction at 15-40 s, and then it continuously changed in the positive direction up to 350 s. The potential shift during the period of 40-350 s is due to a decrease in anodic current, which is thought to be caused by the oxidation and passivation of the surface. In addition, an abrupt shift in OCP was observed at 350-400 s. This positive potential shift continued to reach about -0.70 V, which is a behavior similar to that shown in Figure 2. However, the time it took for the abrupt potential shift to begin was longer here than that in the profile shown in Figure 2. The OCP profiles in the alkaline solution with and without Ni ions indicate that the deposition reaction is governed by the anodic reaction of Si. Thus the anode reaction process was further examined with an attention focused upon the change of the chemical species such as Si-H to the Si-H(O3) suboxide at the surface. The change in chemical structure of the wafer surface in the solution containing no Ni ions was monitored by in situ ATR FTIR and electrochemical measurements. The measurements were carried out simultaneously at 80 °C with the setup shown in Figure 1. Since the reaction at pH of 9.0 was too fast for the

Figure 7. (a) Peak area intensities for Si monohydride [SiH] and SiH(O3) [suboxide] as a function of immersion time. (b) OCP profile monitored together with IR measurement in the alkaline solution without Ni ion at pH 8.5 and 80 °C.

analysis, the pH was lowered to 8.5. As a preliminary examination, the changing behavior of the OCP was measured in the solution at pH 8.5 (Figure 5). The result showed that the profile was basically the same as that at pH 9.0, although the reaction rate was lower at pH 8.5. Thus, it is apparent that the deposition mechanism did not change when the pH was decreased to 8.5. Figure 6 shows a series of Si-H stretch vibration spectra of the specimen immersed in the Ni-free solution. The initial surface was covered with a thin layer of chemical oxide. Weak and broad peaks were detected at about 2100 cm-1, which can be assigned to Si hydride species, such as monohydride (Si-H), dihydride (Si-H2), and trihydride (Si-H3).18 The main peak observed at 2074 cm-1 is attributed to the surface Si monohydride (Si-H) species located at step edges of the Si(111) surface.19 On the other hand, a large peak appeared at 2250 cm-1 after the immersion. This peak can be assigned to intermediate suboxide species [Si-H(O3)].20-22 In addition, other peaks for species such as Si-OH and silicon dioxide were observed in the ex situ reflection absorption infrared spectra (IRRAS) of the specimens after the immersion. In this study, we focused our attention on changes in the peak for Si-H and the suboxide species. The time dependence of integrated peak area of Si hydride (Si-H) and intermediate oxidation (suboxide) species obtained from the in situ ATR FTIR spectra and that of the OCP profile

Deposition Mechanism of Ni on Si(100) Surfaces in the solution without Ni ions are shown in Figure 7. The OCP profile of the silicon prism was monitored simultaneously with the ATR FTIR measurement. The peak for Si-H species appeared in the region labeled A, indicating that the slight shift of OCP at the initial stage is caused by the dissolution of a very thin layer of the oxide on the wafer surface formed by the HPM pretreatment. The peak intensity for the Si-H decreased as the suboxide peak appeared and the OCP shifted negatively in region B. At this stage, the surface was covered mainly by the suboxide species, which were active for the anodic reaction in the electroless deposition of the metallic species, and this suboxide covered surface was present up to region C. The intensity of the suboxide peak decreased rapidly beyond region C, and a positive shift in OCP was observed in regions C-D. It is suggested that the suboxide-covered surface was further oxidized to the dioxide state, which was apparently inactive for the reduction of the metallic species. On the other hand, the amount of the Si-H species was almost constant at the surface in regions C-D. Thus, the surface became rough during the immersion, because the etching reaction seemed to occur together with the formation of the suboxide species. As can be seen in Figure 6, the monohydride Si-H species located at the step edges become dominant at the Si(111) surface during the immersion, although a clean Si(100) surface should have a dihydride- (Si-H2-) terminated surface. It is generally known that Si(100) surfaces are anisotropically etched toward 〈111〉 orientation in an aqueous alkaline solution. For example, the etching rates of (100) and (111) silicon surfaces in 30% KOH solution at 70 °C have been estimated to be 0.797 and 0.005 µm min-1, respectively.23 Such an orientation-dependent behavior in etching was also observed in solutions of NaOH and tetramethylammonium hydroxide (TMAH).23,24 Furthermore, Niwano et al.25 suggested that the suboxide at the surface enhances the formation of 〈111〉 microfacets in an aqueous NH4F solution and that the formation of the suboxide and its removal (etching) always proceeds at the Si surfaces during the immersion. Therefore, it appears that the formation of the suboxide species was accompanied by the etching reaction during the immersion and that both the formation of silicon dioxide by further oxidation of the suboxide species and the anisotropic etching toward the 〈111〉 direction proceeded on the suboxidecovered surface in regions C-D. In addition, a potential shift was observed as the peak intensity of the suboxide species was attenuated in region D. In this region, the suboxide species appears to be further oxidized at the interface between the deposited silicon dioxide and the Si surface. The peak intensity of the Si-H species decreased rapidly as the OCP shifted in the positive direction in region E. Once the 〈111〉 microfacets were formed, the etch rate decreased, and then the suboxide species could not be formed. Eventually, the surface was passivated by further oxidation of the suboxide species, resulting in the potential shift toward more positive values. Beyond region E, both intensities decreased with time as the potential continued to shift in the positive direction, indicating that the further growth of silicon dioxide passivated the surface. To prove the formation of the 〈111〉 oriented microfacets, we investigated the reaction of Si using a micropatterned substrate. A thermally grown silicon dioxide (50 nm in thickness) was used as a patterned resist mask. The pattern had arrays of dotlike openings. The diameter and the pitch of the dots were 1.5 and 3 µm, respectively.

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Figure 8. FE-SEM images of patterned wafers immersed in solution without Ni ions at pH 9.0 for (a) 150, (b) 300, and (c) 400 s.

Figure 8 shows SEM images of the specimens at different immersion times in the Ni-free solution at pH 9.0 and 80 °C. The 〈111〉 oriented facets clearly formed after the 400 s immersion, which corresponds to the region where the OCP sharply shifted in the positive direction (see Figure 4). These results show that the origin of the potential shift in region E in Figure 7 is caused by the formation of 〈111〉 oriented microfacets, whose surface is significantly low in etch rate in the alkaline solution. Therefore, the suboxide species could not be continuously generated after the 〈111〉 faceting, and consequently the suboxide was further oxidized to silicon dioxide, resulting in the passivation of the surface. This mechanism accounts for the sharp shift in OCP in the positive direction around region E in Figure 7. On the basis of these results, it is clear that the anodic reaction of Si in the aqueous alkaline solution is greatly affected by the anisotropic etching to form the 〈111〉 oriented microfacets. Indeed, the reaction was accelerated by an increase in pH (see Figure 5). The rate of the anodic reaction strongly depended on the time to the faceting, because the anisotropic etching reaction is presumed to provide the driving force for the electron donation in this reaction. Finally, the following mechanism is proposed for the spontaneous deposition of Ni on Si (100) surface in the aqueous alkaline solution. The Ni deposition is initiated by the formation of suboxide at the Si surface, which is active for the deposition reaction. The reaction of Ni deposition is spontaneously driven by the donation of electrons from the oxidation reaction of the suboxide species as well as by the anisotropic etching reaction of the Si. Once 〈111〉 oriented microfacets are formed, the surface is passivated by the formation of silicon dioxide, which eventually stops the supply of the electrons necessary for the deposition of Ni and terminates the deposition reaction. 4. Conclusion The deposition process of Ni on Si(100) surface was investigated by electrochemical measurements, AES depth profiling, and in situ ATR FTIR. The results revealed that the deposition reaction is initiated by the formation of suboxidecovered surface as the OCP shifts negatively. The suboxidecovered surface is active for the anodic reaction of Si to enhance the electroless deposition of Ni. The anodic reaction is accompanied by the anisotropic etching to 〈111〉 oriented microfacets and by the further oxidation of suboxide to silicon dioxide. When the OCP shifts sharply in the positive direction, the surface is passivated by the silicon dioxide, which spontaneously terminates the deposition reaction. The understanding of such a metal deposition process is useful for fabricating metallic nanostructures with functional properties onto Si wafer surfaces. Acknowledgment. This work was financially supported by a Grant-in-Aid for COE Research (Establishment of Molecular Nano-Engineering by Utilizing Nanostructure Arrays and its Development into Micro-Systems), and 21st Century COE Program (Center for Practical Nano-Chemistry) from The

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