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Electroplating of Amorphous Thin Films of Tungsten/ Nickel Alloys O. Younes,† L. Zhu,‡ Y. Rosenberg,§ Y. Shacham-Diamand,‡ and E. Gileadi*,† School of Chemistry, Faculty of Exact Sciences; Department of Physical Electronics, Faculty of Engineering; and Wolfson Applied Materials Research Center, Tel-Aviv University, Ramat-Aviv, Israel Received May 3, 2001. In Final Form: September 25, 2001 The electroplating of amorphous Ni/W alloys is described. The aqueous plating solution consists of NiSO4, Na2WO4, and Na3Cit at pH ) 8.0. The bath is operated at room temperature. By avoiding the use of NH4OH or any ammonium salt, it was possible to prepare alloys containing up to 50 a/o (76 w/o) W. XRD measurements revealed that amorphous alloys were obtained when the concentration of W in the alloy is 20-40 a/o. At lower concentrations of W the fcc substitutional solid solution Ni(1-x)Wx was formed. At higher concentration, an orthorhombic crystal structure corresponding to a 1/1 Ni/W alloy was observed. SEM and STM measurements supported the existence of the amorphous phase. The conditions under which amorphous alloys are expected to be formed preferentially are discussed. Thin films of the amorphous phase were prepared reproducibly at any tungsten concentration in the above range. Therefore, these alloys can be used for barrier or capping layers in the microelectronic industry for ULSI and MEMS applications.
Introduction Amorphous alloys present a separate class of materials, different from regular polycrystalline metals. Although both polycrystalline and amorphous thin films may have the same composition, they differ in many physical and electrical properties. Amorphous alloys do not have grain boundaries, thus they have better corrosion resistance and their permeability to diffusion of metal ions is lower. Amorphous alloys can be prepared by very fast cooling of the molten alloy, at rates that are on the order of 1 × 106 °C/s.1 Brenner2 was the first to develop a low-temperature method for plating amorphous Ni/P alloys from an electroless plating bath, employing NaH2PO2 as the reducing agent.2 Similar baths have been developed for amorphous Co/P plating.3 When a compound such as dimethylaminoborane is used as the reducing agent, similar amorphous alloys, which contain boron instead of phosphorus, are produced.4 Electroless plating baths for plating Co/W/P alloys that were “X-ray amorphous”, as determined by X-ray diffraction, but nanocrystalline, as determined by TEM, have been developed recently.5,6 The electroplating of W/Ni alloys has been studied for a long time.7-9 A similar system of Ni/Mo was discussed * Corresponding author. † School of Chemistry. ‡ Department of Physical Electronics. § Wolfson Applied Materials Research Center. (1) Klement, W. Jr.; Willness, R. H.; Duwez, P. Nature 1960, 187, 869. (2) Brenner, A.; Ridell, G. J. Res. NBS. 1947, 39, 385. (3) O_Sullivan, E. J.; Schrott, A. G.; Paunovic, M.; Sambucetti, C. J.; Marino, J. R.; Bailey, P. J.; Kaja, S.; Semkow, K. W. IBM J. Res. Dev. 1998, 42, No. 5, Electrochemical Microfabrication. (4) Graef, G.; Anderson, K.; Groza, J.; Palazoglu, A. Mater. Sci. Eng. 1996, B41, 257. (5) Okinaka, Y.; Osaka, T. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH Publ: New York, 1994; pp 57-116. (6) Shacham-Diamand, Y. J. Microelectronics Eng. 1997, 37/38, 77. (7) Brenner, A. Electrodeposition of Alloys; Academic Press: New York, 1963; Vol. 2. (8) Nielsen, M. L.; Holt, M. L. Trans. Electrochem. Soc. 1942, 82, 217. (9) Holt, M. L.; Vaaler, L. E. J. Electrochem. Soc. 1948, 94, 50.
recently by Podlaha and Landolt in great detail.10-12 Deposition of partially amorphous alloys of W with Ni, Co, and Fe was reported by Donten.13 Careful examination of the alloys by different techniques proved that there was no oxygen in the bulk. Surprisingly, he reports that the tungsten content of the alloys was independent of pH and of the concentration of the iron-group metal in the plating bath, over a wide range. Yamasaki et al.14-17 studied the mechanical properties of Ni/W alloys before and after annealing at high temperatures and reported the existence of amorphous alloys when the tungsten content of the alloy exceeded about 20 a/o. The commonly used plating bath for Ni/W deposition consists of a solution of NiSO4, Na2WO4, and Na3Cit, in which the pH is adjusted with sulfuric acid and ammonium hydroxide to a value of 8.0. At this pH citric acid is predominantly in the form of the Cit3- ion. The tungsten content reported is usually in the range of 5-25 a/o. It was found difficult to increase the concentration of W in the alloy, even when the concentration of the WO42- ion in solution was in large excess, compared to Ni2+. It should be noted that the citrate ion, which is known to form complexes both with Ni18 and with the tungstate ion,19 is added as a ligand. Ammonium hydroxide or an ammonium salt, on the other hand, are added to increase the Faradaic (10) Podlaha, E. J.; Landolt, D. J. Electrochem. Soc. 1993, 140, L149. (11) Podlaha, E. J.; Landolt, D. J. Electrochem. Soc. 1996, 143, 885, 893. (12) Podlaha, E. J.; Landolt, D. J. Electrochem. Soc. 1997, 144, 1672. (13) Donten, M. J. Solid-State Electrochem. 1999, 3, 87. (14) Yamasaki, T.; Schneider, W.; Schlossmacher, P.; Erlich, K. Proceedings of the 2nd International Conference on Micro Materials 97; Berlin, 1997; p 654. (15) Yamasaki, T.; Schlossmacher, P.; Erlich, K.; Ogino, Y. Mater. Sci. Forum 1998, 269, 975. (16) Yamasaki, T.; Schlossmacher, P.; Erlich, K.; Ogino, Y. NanoStruct. Mater. 1998, 10, 375. (17) Yamasaki, T.; Tomohira, R.; Ogino, Y.; Schlossmacher, P.; Erlich, K. Plating Surf. Finishing 2000, May, 148. (18) Stability Constants of Metal-Ion Complexes; The Electrochemical Society; Alden Press: Oxford, 1971; Supplement No 1. (19) Cruywagen, J. J.; Kruger, L.; Rohwer, E. A. J. Chem. Soc., Dalton Trans. 1991, 1727.
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efficiency and sometimes to fine-tune the pH to a desired value, in the range of 7-9. The role of NH3 as a ligand has so far been ignored. In recent publications20,21 we have reported on a novel plating bath that does not contain ammonia. The working hypotheses in the above studies are that (a) the tungstate/ citrate complex forms a ternary complex with Ni in the bulk of the solution or on the surface; (b) tungsten can only be deposited (together with nickel) from this ternary complex, which has the general formula
[(Ni)p(WO4)q(Cit)m]2(p-q-1.5m)
(1)
in which all the stoichiometric coefficients are likely to assume the value of unity, leading to the simpler formula
[(Ni)(WO4)(Cit)]3-
(2)
and (c) nickel can also be deposited from its complexes with either citrate or with NH3. The complex formed from tungstate and citrate at pH ) 8 is somewhat problematic, since it should have a very high negative charge of -5. 2-
WO4
3-
+ Cit
5-
f [(WO4)(Cit)]
(3)
It is indeed found that this complex is partially protonated, to an extent that depends on the value of the pH. At pH ) 8, the predominant species contains one proton and should probably be written as
H+ + WO42- + Cit3- f [(HWO4)(Cit)]4-
(4)
The ternary complex could accordingly have the formula
[(Ni)(HWO4)(Cit)]2-
(5)
While it is interesting in its own right, the difference between the complexes shown in eqs 2 and 5 has no bearing on the relative concentrations of Ni and W in the alloy or its structure, and will hence not be discussed further below. The existence of a ternary complex containing nickel and tungsten can explain the observation that, although W is only deposited with Ni (by discharge of the ternary complex), a parallel route for deposition of Ni from its complexes (either with citrate or with NH3) exists, leading to high nickel content of the alloy, even when there is an excess of WO42- ions in solution. Removing ammonia should increase the concentration of W in the alloy. Comparison of the alloy composition obtained in identical solutions, except that one did not contain ammonia, confirmed these hypotheses. The concentration of W in the alloy was found to be higher in the absence of NH3 than in its presence. Under suitable conditions, an alloy having the orthorhombic structure corresponding to the composition NiW was formed, supporting the assumption that the stoichiometric coefficients in the ternary complex are indeed unity.20-22 In the present study we report on the formation of amorphous Ni/W alloys by electroplating at room tem(20) Younes, O.; Gileadi, E. Electrochem. Solid-State Lett. 2000, 3, 543. (21) Younes, O.; Shacham-Diamand, Y.; Gileadi, E. Proceedings of Advanced Metallization Conference (AMC 2000) 337, Edelstein, D., Dixit, G., Yasuda, Y., Ohba, T., Eds.; MRS 2000, San Diego. (22) JCPDS-International Center Diffraction Data, PDF card # 471172.
Figure 1. The atom-percent (a/o) concentration of W in the alloy, as a function of the concentration of tungstate in solution: 0.1 M NiSO4, 0.6 M Na3Cit; pH ) 8.0; ω ) 2000 rpm, j ) 15 mA/cm2. (2) Excess NH4OH, (b) without NH4OH.
perature. These alloys are found when the concentration of W in the alloy is in the range of 20-40 a/o. Experimental Section Electroplating was conducted in solutions containing 0.1 M NiSO4, 0.6 M Na3Cit, and varying concentrations of Na2WO4, in the range of 0.01-0.5 M. The pH was adjusted to a value of 8.0, using H2SO4 and NaOH. A gold cylinder, 0.2 cm diameter and 1.0 cm length, rotated at 2000 rpm, served as the working electrode. The current density was constant at 15 mA/cm2 and the temperature was 25 °C. A Pt wire counter electrode and an Ag/AgCl reference electrode were used. The composition of the alloy was measured by an energy dispersive spectroscopy (EDS) probe (Link Corp.) attached to a Joel, Model 6400 SEM. X-ray diffraction (XRD) data were collected with Cu KR radiation on a Θ∠Θ Scintag powder diffractometer, equipped with a liquid nitrogen cooled germanium solid-state detector. Scanning tunneling microscopy images were obtained ex situ, with a Nanoscope II instrument.
Results The most important observation reported here is the critical effect of NH4OH on the concentration of W in the alloy, as shown in Figure 1. In the presence of excess ammonia, the concentration of W is below 12 a/o, over the whole range of concentration of Na2WO4, In its absence, the concentration of W in the alloy increased dramatically. Alloys having a tungsten content of 20-40% could be prepared routinely. The atom percent (a/o) of W in the alloy could be controlled in this range by varying the concentration of WO42- ion in solution, with all other parameters of the reaction kept constant. In this range of composition the alloy is found to be amorphous, as will be shown below. By increasing the ratio of concentrations of WO42-/Ni2+, it was possible to increase the tungsten content of the alloy to the range of 40-50%. In this range an orthorhombic NiW phase could be observed by XRD. This structure has been reported earlier in the literature, but it has not been produced by electroplating until recently.20 All further measurements in this work were performed without NH4OH in the plating bath, to allow deposition and investigation of amorphous Ni/W alloys. The effect of pH on the composition of the alloy in solutions containing ammonia has been discussed before.20,21 In Figure 2 we show that this effect persists even in the absence of ammonia. A broad maximum in tungsten content, with a peak at pH ) 7.5-8.0 is observed, decreasing sharply at higher pH and rather more mod-
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Figure 4. XRD patterns for alloys in the intermediate range of W concentration, all typical of the amorphous phase. Figure 2. The effect of pH on the concentration of W in the alloy: 0.1 M NiSO4; 0.4 M Na2WO4, 0.6 M Na3Cit; ω ) 2000 rpm, j ) 15 mA/cm2.
Figure 3. XRD patterns for alloys in the three regions, characterizing (1) fcc Ni phase; (2) amorphous phase, (3 and 4) orthorhombic NiW phase.
erately as the pH is decreased. Measurements were not extended below a pH value of 6.0, since in that region formation of several polytungstate ions is known to occur, making the interpretation of the data very difficult. The structure of the deposit was characterized by X-ray diffraction. In Figure 3 we show XRD patterns for four alloy compositions. Bragg peaks of curve 1, taken on an alloy containing 7 a/o W, correspond to the fcc phase, the Bragg peaks of which are shifted slightly to lower 2Θ angles, compared to pure Ni. Using the underlying Au substrate as an internal standard, we found a value of 0.3531 nm for the lattice parameter of our alloy. As a first approximation, one can regard the deposit as a substitutional solid solution, consisting of hard spheres having a radius that is the weighted average of the atomic radii of the two components:
R ) RNi (1 - XW) + RWXW
(6)
where RNi and RW are the atomic radii of the constituents, and XW represents the atom-fraction of W in the alloy. This approach leads to an increase of the lattice parameter by 3.5 × 10-4 nm per a/o W added to the Ni. The above-mentioned lattice parameter of 0.3531 nm corresponds to the formula Ni98W2. The remaining 5 a/o of W may be situated on the grain boundaries or in vacancies in the fcc Ni crystal. Curve 2 of Figure 3 is typical of the intermediate concentration of W, in the range of 20-40 a/o. It consists of an amorphous halo, without any sharp lines, testifying to the absence of a crystalline phase in this sample. Curves
Figure 5. Nearest neighbor distance, as a function of W content. Broken line calculated from eq 6. (b) nearest neighbor distance calculated from the position of the peak of the halo. Solid line: least-squares fit to points.
3 and 4 represent two samples with the highest concentration of W, reaching 40 and 50 a/o. XRD patterns corresponding to the orthorhombic NiW alloy22 are observed. It should be noted that the distribution of Bragg peak intensities in our case is quite different from that given in the literature.22 This is not surprising, in view of the differences in the methods of film formation and of the substrates used. Figure 4 shows the evolution of amorphous halo with increasing concentration of W in the alloy. The maximum of the halo is shifted slightly to lower 2Θ angles, as the concentration of W in the alloy increases. This shift indicates that the average nearest neighbor distance is expanding as the larger W atoms are incorporated into the amorphous structure. The nearest neighbor distance was calculated from the position of the maximum of the halo, according to the Debye equation,23 and plotted as a function of tungsten content in the alloy in Figure 5. For comparison, the atomic diameter calculated from eq 6 for a hypothetical substitutional solid solution of corresponding composition is also shown in this figure (dotted line). The fact that this is above that calculated for nearest neighbor distance (solid line) reflects the fact that, in the NiW system, the formation of substitutional solid solutions is not energetically favored in this range of concentrations. On the other hand, the similarity of the slopes of the two lines indicates that they may overlap by introducing a suitable scaling factor; i.e., the hard-spheres atomic model is a reasonable first-order approximationfor the description of the structure of the amorphous NiW phase. (23) Guinier, A. X-ray Diffraction; W. H. Freeman and Co.: San Francisco, 1963; eq 2.54, p 49 and eq 3.8, p 61.
Electroplating of Amorphous Thin Films
Figure 6. Mean coherent scattering distance as a function of tungsten content.
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across the sample, is also shown. Figure 8a corresponds to an alloy containing 10 a/o W, which, based on XRD analysis, has the fcc Ni-like structure. Figure 8b shows data taken on an amorphous alloy containing 27 a/o W. Even though these figures alone would not be sufficient to confirm the existence of an amorphous phase, a striking difference between the surface morphologies of the two structures can be observed. The cross section profile shown in Figure 8a exhibits a much higher degree of periodicity than that in Figure 8b, as expected for a crystalline structure, compared to an amorphous phase. Rather surprisingly, the cross section profile of the Ni73W27 alloy shown in Figure 8b does not show the completely random pattern expected for an amorphous phase,26 and periodic peaks, characteristic of a crystalline substance, can be detected. This indicates that, in the case shown here, there might be a mixture of crystalline and amorphous domains. This is consistent with the mechanism we proposed earlier,20 in which it was assumed that Ni can be deposited by two separate pathways, from its complex with citrate and from its ternary complex with tungstate and citrate. It appears that when both pathways occur at comparable rates, an amorphous deposit is formed. Discussion 1.The Effect of pH on Alloy Composition. The effect of pH on the composition of the alloy in solutions containing ammonia could be understood easily by considering the equilibrium of NH3 (which is a ligand for Ni2+) with NH4OH.
NH4+ + OH- r NH4OH f NH3 + H2O
Figure 7. Scanning electron microscope (SEM) image of a sample containing 24 a/o W.
The mean size of the region of coherent scattering, estimated from the full widths of the halo at half-maximum (fwhm) by the Scherrer equation,24 decreases with increasing concentration of W in the alloy, as shown in Figure 6. This parameter may also be considered to represent the effective size of the “nanocrystallites” in the amorphous phase. Its decrease implies a further increase in the amorphous character of the deposit with increasing concentration of W in the alloy. The variation of surface morphology of the “as-deposited” samples was studied by scanning electron microscopy (SEM) and scanning tunneling microscopy (STM) techniques. The SEM image of a sample containing 24 a/o W is shown in Figure 7. The deposit seems to be globular, with some smaller ellipsoid-shaped globules appearing on top of the larger globules. The borders of these smaller and bigger globules are circular or quasicircular, which is quite unlike the polygonal form of polycrystals, indicating the absence of grain boundaries. Compositional analysis of different globules by energy dispersive spectroscopy (EDS) did not show a significant heterogeneity of composition. These data are consistent with the features expected for polymorphous crystallization of amorphous alloys,25 supporting the XRD findings that the thin layer Ni/W deposit is indeed amorphous. Images obtained by STM are shown in Figure 8. For each image a cross section, showing the variation of height (24) Guinier, A. X-ray Diffraction; W. H. Freeman and Co.: San Francisco, 1963; eq 5.3, p 124. (25) Amorphous Metallic Alloys; Lubovsky, F. E., Ed.; Butherworths Publishers: Markham, ON, Canada, 1983; pp 151-159.
(7)
As the pH is increased, this equilibrium is shifted to the right, increasing the concentration of NH3 in solution. This will allow the formation of complexes of the form
Ni2+ + n(NH3) f [Ni(NH3)n]2+
(8)
where n can have values of 2-6, thus decreasing the concentration of Ni2+ ions available to form the ternary complex. A somewhat similar explanation applies here, except that the species involved is the complex of tungstate with citrate. It was pointed out above that this complex is in a protonated form at around pH ) 8.0. In Figure 9 we show the relative abundance of the complexes in a different state of protonation, as a function of pH, based on data of Cruywagen et al.19 The predominant species at pH ) 8.0 is the complex shown on the right-hand side of eq 4
H+ + WO42- + Cit3- f [(HWO4)(Cit)]4-
(4)
This complex is the precursor for the formation of the ternary complex with Ni2+
[(Ni)(HWO4)(Cit)]2-
(9)
As the pH is increased, the concentration of the complex shown in eq 4 decreases, because in its deprotonated form it is destabilized by the high negative charge of -5. As a result, the concentration of the ternary complex shown in eq 5 is also decreased, leading to a lower concentration of W in the alloy. (26) Burgler, D. E.; Schmidt, C. M.; Schaller, D. M.; Meisinger, F.; Schaub, T. M.; Baratoff, A.; Gunterroldt, H. J. Phys. Rev. 1999, B59, 10895.
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a
Younes et al.
b
Figure 8. Scanning-tunneling microscope (STM) images of two alloys: (a) 10 a/o W, (b) 27 a/o W. Cross sections, showing the variation of height across each sample, are also shown.
Figure 9. The relative abundance of the protonated [(H)n(WO4)(Cit)]-5+n complexes, as a function of pH. The abbreviated forms [1,1,1]4-, [1,1,2]3-, and [1,1,3]2- refer to values of n ) 1, 2, and 3 in the above complex, respectively.
It may be argued that the ternary complex can be formed with complexes of tungstate and citrate having different states of protonation. In Figure 10 we show the relative abundance of the complex denoted [1,1,1]4- and the sum of this complex and that denoted [1,1,2]3- in Figure 9. In the same figure we also show the effect of pH on the tungsten content of the alloy, as observed in two similar (but not identical) sets of experiments. The decrease in W with increasing pH in the range of pH ) 8.0-10.0 is clearly parallel to the decrease in the abundance of the complex [1,1,1].4- The much smaller effect observed in the range of pH ) 6.0-8.0 can be explained if it is assumed that W can be deposited both from the complex [1,1,1]4 and the
Figure 10. Relative abundance of the different protonated complexes as a function of pH, shown together with the dependence of the W content of the alloy on pH. 0.1 M NiSO4, 0.6 M Na3Cit, and (a) (b) 0.4 M Na2WO4 or (b) (2) 0.5 M Na2WO4.
complex [1,1,2]3-. The corresponding ternary complexes that should be considered as the probable precursors for the deposition of W (together with Ni) are
[(Ni)(HWO4)(Cit)]2- and [(Ni)(H2WO4)(Cit)]- (10) The correlation shown here between the abundance of the two tungstate/citrate complexes and the concentration of W in the alloy may be viewed as further evidence that W and Ni are deposited in a 1/1 ratio from the two complexes shown in eq 10. Nickel can, of course, be deposited also from its complexes with citrate.
Electroplating of Amorphous Thin Films
2. The Conditions under Which Formation of Amorphous Alloys Is Favored. It was shown above that an amorphous Ni/W alloy can be formed by electrodeposition at room temperature. Although the plating of amorphous Ni/W alloys has been reported before, the range of composition over which the alloy is formed (2040 a/o W) shown in the present work is wider. On the basis of the recent work of Yamasaki et al.14-17 and our own findings,20,21 it can be concluded that the crystal structure of the alloy is primarily determined by its tungsten content. The composition of the bath, the plating temperature, and the current density applied influence the structure only indirectly, in their effect on the concentration of W in the alloy. Thus, by excluding ammonia from the plating bath, the tungsten content of the alloy was increased, leading to the formation of amorphous alloys, even when plating at room temperature. The important question to consider in the present context is, whether the formation of an amorphous alloy during electroplating is an anomalous process, and if so, what are the conditions under which such phases are likely to be formed? On one hand, the crystalline phase is thermodynamically more stable than the amorphous phase. On the other hand, at sufficiently high overpotentials, almost all metal atoms reaching the surface are immediately discharged (this is the equivalent of a sticking coefficient of unity during gas-phase adsorption), leading to randomly oriented, amorphous structures. If the rate of deposition is high, compared to the exchange rate, further layers of atoms will be formed before each layer has had a chance to assume its most stable crystalline form by a dissolution/deposition exchange with the corresponding species in the solution. Following this argument, it would seem that every alloy, and indeed most metals in the pure form, should first be deposited in an amorphous form, as long as the rate of deposition is high compared to the exchange rate, i.e., as long as they are being deposited at a high overpotential. This may not be observed experimentally, because in most cases transformation to the more stable crystalline phase occurs rapidly at room temperature. There should be a characteristic temperature for each metal and alloy, below which its amorphous form is kinetically stable for long enough to be detected. This transition temperature is difficult to reach for pure metals (since a suitable electrolyte in liquid form may not be available), but it may be expected that alloy deposition at low temperatures would lead more often to the formation of amorphous deposits than at room temperatures. The role of P in stabilizing the amorphous Ni/P phase is believed to be one of inhibiting the rate of surface rearrangement of the alloy to the thermodynamically most stable phase. In the present work, amorphous deposition may be caused by the fact that there are two different crystal phases, i.e., fcc Ni-like and orthorhombic NiW,
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that could be formed concurrently. Thus, long-range order is hindered by intimate mixing of the two different crystal phases. This can explain the fact that the amorphous phase is formed at intermediate concentrations of W in the alloy. A simple calculation shows that, for the two phases to be formed in a ratio between 1/4 and 4, the average concentration of W in the deposit should range from ca. 44 a/o to 17 a/o, which is roughly where the amorphous structure is observed. Conclusion The mechanism of deposition of Ni/W alloys was studied. It was concluded that the precursor for this process is a ternary complex of WO42- with Cit3- and Ni2+, containing one or two protons, as shown in eq 9. Nickel itself can also be deposited from its complex with citrate in a parallel route. Hydrogen evolution always occurs as a side reaction in this electroplating process, under all experimental conditions tested in our laboratory. The strongest evidence for this is derived from the fact that, under suitable conditions, an alloy composition of Ni/W ) 1/1 can be obtained. Furthermore, there is a good correlation between the pH dependence of the abundance of the ternary complexes shown in eq 9 and the concentration of W in the alloy. It has been demonstrated that amorphous Ni/W alloys could be produced reproducibly from aqueous baths in the absence of ammonia. The concentration of W in the amorphous phase can be anywhere between 20 a/o and 40 a/o of W. At a W content below 10 a/o, an fcc Ni-like phase is formed. The exact tungsten content at which the transition between these two phases occurs has not been determined. Fine-tuning of the solution chemistry can lead to the formation of a tungsten-rich orthorhombic NiW phase. The crystalline structure of the deposited alloy depends on its W content. This can be controlled primarily by the composition of the plating solution, paying special attention to its pH and, in a more subtle way, to the plating conditions, such as current density, rate of mass transport, and temperature.20 While alloys of all three crystal structures may be of practical importance, the amorphous alloy is of particular current interest, as a candidate for use as a barrier layer or capping layer in Cu metallization for ultra-large-scale integration (ULSI) for integrated circuits or MEMS applications. Other applications, taking advantage of the increased corrosion and abrasion resistance of Ni/W alloys, compared to Ni itself, may find important applications in the automotive and aviation industry. Acknowledgment. Financial support for this work by the Ministry of Science, Fine Arts and Sports is gratefully acknowledged. LA010660X
