© Copyright 1998 by the American Chemical Society
VOLUME 102, NUMBER 41, OCTOBER 8, 1998
LETTERS Nucleation of Trace Copper on the H-Si(111) Surface in Aqueous Fluoride Solutions Takayuki Homma*,† Department of Applied Chemistry, Waseda UniVersity, Okubo, Shinjuku, Tokyo 169-8555, Japan
Christopher P. Wade and Christopher E. D. Chidsey*,‡ Department of Chemistry, Stanford UniVersity, Stanford, California 94305-5080 ReceiVed: May 4, 1998; In Final Form: August 26, 1998
The initial nucleation process of trace amounts of copper metal on the H-Si(111)(1 × 1) surface in aqueous fluoride solution was investigated by ex-situ scanning tunneling microscopy. We observed that copper nucleates preferentially along step edges and appears to inhibit the step-flow etching of silicon. The presence of copper species in the fluoride solution also appears to inhibit the formation of terrace etch pits, which are known to be initiated by dissolved oxygen. It is suggested that silicon-dihydroxyl-cuprous ion complexes are formed at dihydroxyl kink sites, resulting in the preferential nucleation of Cu metal at step edges and the inhibition of step-flow etching. Copper ions in solution may act as catalysts for the disproportionation and electrocatalytic reduction of superoxide anion radical formed by the electrochemical reduction of dissolved oxygen at the silicon surface. Such reactions would reduce the rate of etch-pit initiation and may also decrease the rate of deposition of metallic copper on the silicon surface.
Introduction The chemical etching of silicon in aqueous fluoride solutions is one of the fundamental processes of semiconductor device manufacturing. Trace amounts of metal contaminants in the etching solutions can spontaneously deposit on silicon surfaces causing serious degradation of device yields.1-4 Among various metal species, copper has been extensively studied because it is a common contaminant with a tendency to deposit easily from aqueous fluoride solutions.5-9 Moreover, because copper is now expected to replace aluminum as the metallization of choice in microelectronics, its presence in the fabrication process naturally leads to heightened interest in its deleterious effects were it to contaminate the silicon etching solutions. In the earlier studies, * To whom correspondence should be addressed. † E-mail:
[email protected]. Fax: 81-3-3205-2074. ‡ E-mail:
[email protected]. Fax: 650-725-0259.
Si(100) surfaces etched in dilute aqueous HF were examined because this surface and the dilute HF etch are utilized for most commercial devices. Unfortunately, the Si(100) surface produced by HF etching is composed of atomically rough, complicated features that make careful analysis of the metal deposition mechanism very difficult. On the other hand, the H-Si(111)(1 × 1) surface has wide, atomically flat terraces separated by steps that are one atomic bilayer in height.10,11 This surface provides a model for several important features of the more complex (100) surface microstructure. The H-Si(111)(1 × 1) model surface is advantageous for an initial investigation of the mechanism of nucleation of metal deposits, especially the relationship between metal nucleation and the etching processes. In the present work, we investigate the initial nucleation mechanism of trace amounts of Cu on the H-Si(111) (1 × 1) surface immersed in aqueous fluoride solution and also examine
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Figure 1. STM images of H-Si(111) prepared in argon-sparged 40% NH4F for 15 min followed by (a) 10 s immersion in argon-sparged 40% NH4F; (b) 10 s immersion in argon-sparged 40% NH4F with 10 µM CuSO4; (c) 30 s immersion in argon-sparged 40% NH4F with 10 µM CuSO4. The vertical scale is from 0 nm (black) to 2 nm (white) for all images.
the effect of metal deposition on the step-flow etching and O2initiated pitting of H-Si(111). Experimental Section N-type silicon(111) wafers (Siltec, prime grade) were used for all experiments. The resistivity was 1-4 Ω‚cm, and the miscut angle was less than 0.5°. The wafers were immersed in a mixture of 4 volumes of 96% H2SO4 (J. T. Baker, CMOS electronic grade) to 1 volume of 30% aqueous H2O2 (J. T. Baker, CMOS electronic grade) for 10 min at 120 °C, followed by rinsing with 18 MΩ‚cm water from a Millipore four-bowl purification system. Then the wafers were immersed for 15 min in 40% aqueous NH4F (Kanto Corporation, Ultrapur grade; Cu < 0.01 ppb) that had been sparged with Ar for 30 min or longer. The wafers were held vertically in Teflon vials containing the solution. Immediately after emersion from the first NH4F solution, the wafers were immersed into 40% aqueous NH4F in separate vials with or without 10 µM CuSO4‚5H2O (Aldrich, 99.999%). These solutions were either sparged with argon prior to use or used as received in their air-saturated state. After emersion from the second NH4F solution, the wafers were transferred through air to a custom-built scanning tunneling microscope (STM) in a chamber filled with dry argon to prevent capillary condensation of aerated moisture in the tip-sample gap.12 Capillary condensation is thought to be responsible for the tip-induced oxidation of hydrogen-terminated silicon surfaces observed in air.12 Constant current-mode STM observations were carried out using a PtIr tip, which was electrochemically sharpened in a 6 M KCN, 2 M NaOH solution.13 The bias voltage and tunneling current were -1.5 V and 50 pA, respectively, for all of the images described below. Results and Discussion Figure 1a shows an STM image of the H-Si(111) surface prepared in Ar-sparged 40% NH4F solution for 15 min followed by another 10 s immersion in Ar-sparged 40% NH4F solution. This combination of immersions in Ar-sparged solutions with transfers through air leaves the surface nearly ideal, with the large, pit-free terraces seen previously on emersion from Arsparged 40% NH4F solutions.11 The double-immersion procedure is used simply for consistency with subsequent copperand oxygen-containing experiments, all of which follow an initial 15 min immersion in the copper-free, Ar-sparged 40% NH4F solution. The atomically flat terraces are approximately
50 nm wide separated by 0.31 nm high bilayer steps. Figure 1b shows the effect of including 10 µM CuSO4 dissolved in the second solution. After the 10 s immersion, numerous particles several nanometers in diameter are observed. The particles are almost exclusively located at step edges. After 30 s immersion in the same solution (Figure 1c), the average particle size has increased while the particle distribution and number density have remained approximately unchanged. Because X-ray photoelectron spectra (XPS) of similar surfaces indicate the presence of Cu, we infer that the particles in Figure 1b,c are electrochemically nucleated Cu metal. The spontaneous deposition of copper under open-circuit conditions has been reported many times5-9 and is believed to be the result of electroless deposition driven by electrons that accumulate in the Si conduction band during fluoride-enhanced Si dissolution. The appearance of the particles within 10 s of immersion and the relative constancy of their number density at 10 and 30 s suggest that nucleation occurs immediately after the immersion (within 5 s).14 The electrochemical reduction of the cupric ions in the solution to copper metal particles on the silicon surface is generally treated in the literature5-9 as
Cu2+(aq) + 2e-(Si) f Cu0(c)
(1)
However, it is known that the reduction of aqueous Cu2+ to crystalline Cu0 takes place by the initial reduction to aqueous Cu1+, which can subsequently be reduced again by one electron as it is incorporated into a growing nucleus of crystalline copper:15
Cu2+(aq) + e-(Si) f Cu1+(aq)
(2)
Cu1+(aq) + e-(Si) f Cu0(c)
(3)
The open-circuit potential of H-Si(111) in 40% NH4F solution has been measured to be -0.8 V vs NHE.11 This value is sufficiently negative of the redox potential of the Cu2+/Cu1+ couple (+0.159 V vs NHE16) that the one-electron reduction (eq 2) is expected to be rapid. The second step (eq 3) is also thermodynamically favorable (+0.521 V vs NHE16) and is expected to be rapid if a nucleus of copper is available. However, this step might be very slow on the H-Si(111) surface in the absence of preexisting copper nuclei or defect sites that can stabilize the copper nuclei. It would appear from Figure 2
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Figure 2. STM images of H-Si(111) prepared in argon-sparged 40% NH4F for 15 min followed by (a) 5 s immersion in air-saturated 40% NH4F; (b) 10 s immersion in air-saturated 40% NH4F; (c) 5 s immersion in air-saturated 40% NH4F with 10 µM CuSO4; (d) 10 s immersion in airsaturated 40% NH4F with 10 µM CuSO4. The vertical scale is fron 0 nm (black) to 2 nm (white) for all images.
that defects at the steps are responsible for the formation of the copper nuclei. The fact that the copper nuclei in both parts b and c of Figure 1 remain at steps suggests that step-flow etching is inhibited by the presence of copper at the step edges. Infrared absorption spectroscopy has been used to measure the rate of step-flow etching of deuterium-labeled samples in copper-free, Ar-sparged 40% NH4F solutions.17 The step-flow etching rate has been found to be approximately 5 nm s-1 in the direction perpendicular to the step. The presence of copper nuclei almost exclusively at steps even after 30 s in Figure 1c suggests that the rate of step-flow etching has been significantly retarded. Because Wade and Chidsey have previously observed that the presence of dissolved O2 in the etch solution leads to the formation of etch pits and a corresponding increase in the total step perimeter on the H-Si(111) surface,11 we next examined the effects of dissolved O2 on the deposition of copper from fluoride solutions. We start by reviewing the effect of dissolved oxygen in the absence of copper. Parts a and b of Figure 2 show STM images of H-Si(111) immersed into Cu-free, airsaturated NH4F solution for 5 and 10 s, respectively. As expected,11 immersing the crystal into air-saturated NH4F solution results in the formation of numerous, largely triangular pits one bilayer deep and from 1 to 10 nm in width. It was proposed that, at the negative open-circuit potential of H-Si(111) in the fluoride solution, dissolved oxygen is reduced to
superoxide anion radical, O2-•, which initiates etch-pit formation on terraces by abstraction of the passivating hydrogen atom from a surface silicon atom.11 Parts c and d of Figure 2 show STM images of H-Si(111) immersed for 5 and 10 s, respectively, into an air-saturated NH4F solution containing 10 µM CuSO4. A comparison of Figure 2c,d with Figure 2a,b shows that terrace pits are smaller in size and are less numerous in the presence of copper. Moreover, the average size of particles on the surface is noticeably smaller than the size of the particles obtained in copper-containing, but Ar-sparged, solution (Figure 1b,c). The images in Figures 2c,d suggest that pit growth and possibly pit initiation are inhibited by the presence of copper species in solution and that the presence of oxygen inhibits the growth of copper particles. The most obvious inference from Figure 2 is that the decreased rate of pit growth in copper-containing, air-saturated solutions is due to the inhibition of step-flow etching by copper inferred earlier from Figure 1. Thus, once a pit is initiated owing to oxygen reduction to superoxide and subsequent hydrogenatom abstraction from a terrace site, copper acts to limit how large that pit will become. An explanation for the apparent reduction in pit density in solutions containing both copper and dissolved oxygen comes from pulse radiolysis studies in which it was concluded that copper ions can act as efficient catalysts for the disproportionation of the superoxide anion radicals.18,19
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Such catalysis should decrease the concentration of superoxide and thus decrease the resulting rate of pit initiation. The reactions of the copper ions with superoxide have been established by pulse radiolysis18,19 as shown below:
Cu2+ + O2-• f Cu1+ + O2
(4)
Cu1+ + O2-• + 2H2O f Cu2+ + H2O2 + 2OH-
(5)
In our case, yet another mechanism exists to decrease the concentration of superoxide. The copper ions can act as electrocatalysts for the electrochemical reduction of superoxide, which is otherwise expected to be a relatively slow electrontransfer event because of a requirement for prior protonation of the superoxide. The catalyzed reduction consists of the transfer of one electron from the conduction band of the silicon to Cu2+ to form Cu1+ (eq 2), followed by transfer of that electron from Cu1+ to O2-• with the abstraction of two protons from water molecules to form hydrogen peroxide (eq 5). Note that just as this last mechanism reduces the superoxide concentration and thus can decrease the rate of pit initiation, it also serves to decrease the concentration of Cu1+. With a decreased concentration of Cu1+, one expects a decrease in the rate of copper deposition. This oxygen-induced decrease in the Cu1+ concentration may be the reason that the particles are noticeably smaller in air-saturated, copper-containing solutions (Figure 2c,d) than they are in Ar-sparged, copper-containing solutions (Figure 1b,c). Preferential deposition at defect sites such as step edges and pits has been reported for many electrochemically, as well as evaporatively deposited, metal layers.20-23 The preferential deposition at such sites can be generally explained in terms of the higher local surface free energy or “activity” of these sites for deposition.24 On the other hand, unlike bare metal surfaces or bare semiconductor surfaces in an ultrahigh vacuum chamber, these silicon surfaces in aqueous solution are passivated by a monolayer of hydrogen. Thus even the steps on these surfaces are expected to have relatively low surface free energies and would not be expected to be particularly active sites for deposition. One possible insight into the preferential nucleation of copper at steps comes from known polynuclear complexes of Cu2+ with bridging hydroxide ligands, which form spontaneously in solution.25 We suggest that dihydroxyl groups present at kink sites on the steps on H-Si(111) may bind Cu1+, according to
The required dihydroxyl group is a possible intermediate during etching at kink sites on the (111) surface.26 Alternatively, anionic forms of such multivalent surface binding sites may play the key role in localizing copper nucleation at steps. If such sites do bind Cu1+, they could provide the coordination site at which copper is eventually reduced to Cu0. The presence of copper at step edges presumably provides a lower activation barrier for the reaction of other Cu1+ ions, resulting in preferential deposition of Cu clusters at step edges. An alternate mode of reaction may be reduction in solution to mononuclear Cu0 followed by insertion into a sterically accessible H-Si bond at a kink site.
A concerted reduction and insertion reaction to form this second possible surface complex might alternatively be operative. The formation of either of the above types of copper surface complexes could block and stabilize kinks at step edges, resulting in the inhibition of step-flow etching for a significant period of time.14 Conclusion We have investigated the initial nucleation mechanism of trace amounts of Cu on Si immersed in aqueous fluoride solution using atomically flat H-Si(111)(1 × 1) surfaces. By using ex situ STM, preferential nucleation of Cu along step edges and inhibition of step-flow etching were observed. The rate of etchpit initiation by dissolved oxygen also appears to be reduced by the presence of copper. These observations provide mechanistic insight into trace metal contamination and etching processes, both of which are key issues for Si surface cleaning. The formation of silicon-dihydroxyl-cuprous ion complexes at step edges is proposed to explain the preferential nucleation of Cu at step edges and the inhibition of step-flow etching. Acknowledgment. We thank Professor T. Daniel P. Stack, Stanford University, and Dr. Yutaka Okinaka, Waseda University, for their valuable advice and comments. We also thank Renee Mo, Stanford University, for the XPS measurements and Scott Craig, Kanto Corporation, for providing the Ultrapur grade chemicals. T.H. acknowledges Iketani Science and Technology Foundation, The Kurata Foundation, and The Foundation of Ando Laboratory for financial support. C.P.W. acknowledges a Fellowship from Elf Atochem. This work was supported by the NSF and the Semiconductor Research Corporation through the Center for Environmentally Benign Semiconductor Manufacturing and by Stanford University. The XPS work has benefited from the facilities and equipment made available to Stanford University by the NSF-MRSEC program through the Center for Materials Research at Stanford University. References and Notes (1) Kern, W. J. Electrochem. Soc. 1990, 137, 1887. (2) Hiraiwa, A.; Itoga, T. IEEE Trans. Semicond. Manufact. 1994, 7, 60. (3) Norga, G. J.; Kimerling, L. C. J. Electron. Mater. 1995, 24, 397. (4) Ajioka, T.; Shibata, M.; Mizokami, Y. IEICE Trans. Electron., E79C, 1996, 337. (5) Morinaga, H.; Suyama, M.; Nose, M.; Verhaverbeke, S.; Ohmi, T. IEICE Trans. Electron., E79-C, 1996, 343. (6) Chyan, O. M. R.; Chen, J. J.; Chien, H. Y.; Sees, J.; Hall, L. J. Electrochem. Soc. 1996, 143, 92. (7) Jeon, J. S.; Raghavan, S.; Parks, H. G.; Lowell, J. K.; Ali, I. J. Electrochem. Soc. 1996, 143, 2870. (8) Norga, G. J.; Platero, M.; Black, K. A.; Reddy, A. J.; Michel, J.; Kimerling, L. C. J. Electrochem. Soc. 1997, 144, 2801. (9) Bertagna, V.; Rouelle, F.; Chemla, M. Z. Naturforsch., Section A-A, J. Phys. Sci. 1997, 52, 465. (10) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 12, 656. (11) Wade, C. P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1679. (12) Wade, C. P.; Dunbar, W. L.; Chidsey, C. E. D. Submitted to Anal. Chem. (13) Heben, M. J.; Dovek, M. M.; Lewis, N. S.; Penner, R. M.; Quate, C. F. J. Microsc. 1988, 152, 651. (14) Homma, T.; Wade, C. P.; Chidsey, C. E. D. in preparation.
Letters (15) Bockris, J. O’M.; Razumney, G. A. Fundamental Aspects of Electrocrystallyzation; Plenum Press: New York 1967; p 36. (16) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solutions; Marcel Dekker: New York, 1985. (17) Luo, H.; Chidsey, C. E. D. Appl. Phys. Lett. 1998, 72, 477. (18) von Piechowski, M.; Nauser, T.; Hoigne, J.; Buhler, R. E. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 762. (19) Ravani, J.; Klug-Roth, D.; Lilie, J. J. Phys. Chem. 1973, 77, 1169. (20) Nichols, R. J.; Kolb, D. M.; Behm, R. J. J. Electroanal. Chem. 1991, 313, 109.
J. Phys. Chem. B, Vol. 102, No. 41, 1998 7923 (21) Hendricks, S. A.; Kim, K. T.; Bard, A. J. J. Electrochem. Soc. 1992, 139, 2818. (22) Potzschke, R. T.; Gervasi, C. A.; Vinzelberg, S.; Staikov, G.; Lorenz, W. J. Electrochim. Acta 1995, 40, 1469. (23) Francis, G. M.; Goldby, I. M.; Kuipers, L.; Vonissendorff, B.; Palmer, R. E. J. Chem. Soc., Dalton Trans. 1996, 665. (24) Williams, E. D.; Bartelt, N. C. Science 1991, 251, 393. (25) Ardizzoia, G. A.; Angaroni, M.; Lamonica, G.; Cariati, F.; Moret, M.; Masciocchi, N. J. Chem. Soc., Chem. Commun. 1990, 1021. (26) Cerofolini, G. F.; Meda, L. Appl. Surf. Sci. 1995, 89, 351.
