- Email: [email protected]
Molecular orbital study on the reaction mechanisms of electroless deposition processes
Electrochimica Acta 47 (2001) 47 – 53 www.elsevier.com/locate/electacta
Molecular orbital study on the reaction mechanisms of electroless deposition processes Takayuki Homma a,*, Isao Komatsu a, Amiko Tamaki a, Hiromi Nakai b, Tetsuya Osaka a a
Department of Applied Chemistry, School of Science and Engineering, Waseda Uni6ersity, Okubo, Shinjuku-ku, Tokyo 169 -8555, Japan b Department of Chemistry, School of Science and Engineering, Waseda Uni6ersity, Okubo, Shinjuku-ku, Tokyo 169 -8555, Japan Received 4 October 2000; received in revised form 4 April 2001
Abstract Molecular orbital study was applied to investigate electroless deposition processes, focusing upon reducibility of reductants as well as the effect of catalytic activity of the metal surfaces. Elementary reaction pathways of the reductants such as dimethylamine borane, hypophosphite ion, formaldehyde, and titanium trichloride were quantitatively examined in terms of the heat of reaction. It was indicated that the reaction via 5-coordinate intermediates was favorable for these species and that the value could be used for quantitative evaluation of the reducibility of the reductant species. The analysis of solvation effect on the reactions suggested that the reactions preferably proceed at the interfacial region, i.e. the surface of the metal, rather than in the ‘bulk’ solution region. The effect of catalytic activity of metal surfaces was investigated using a cluster-model surface. It was suggested that the elementary reactions were stabilized on the metal surfaces to enhance the electron emission from the reductants. On the other hand, difference in the stabilization effects for the reaction of hypophosphite ion was obtained due to the variation in the metal species; it was expected that the copper surface is not catalytically active for the reaction, which corresponds to the experimentally known results. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Electroless deposition; Molecular orbital study; Dimethylamine borane; Hypophosphite; Formaldehyde; Titanium trichloride; Catalytic activity of metal surfaces
1. Introduction Electroless deposition reactions are widely applied in fabrication processes for advanced functional devices such as microelectronic devices [1,2], featuring its capability to form uniform deposits on either wide or selected areas of various surfaces, which can be controlled in sub-micrometer order. Unlike conventional electrodeposition processes, in the case of the electroless deposition, electron for reductive deposition of metal species is supplied by catalytic oxidation reaction of reductants in electrolytes. Thus, unique characteristics of the electroless process can be summarized as: (i) ‘reducibility’ of reductants; and (ii) ‘catalytic activity’ of metal surface. However, these characteristics have * Corresponding author. Tel.: + 81-352-86-3209; fax: +81-352-052074. E-mail address: [email protected] (T. Homma).
not been well defined yet, and so current developments in electroless deposition process have met with difficulties such as many trial-and-error factors, complicated bath system, limitation of metal –reductant combinations. In order to overcome these difficulties and to maximize the capability of the electroless deposition process as a useful and universal tool for the electrochemical microsystem technologies (EMT) [3] and moreover, electrochemical ‘nano’-system technologies, precise analyses of deposition process is required. For the experimental approaches, a number of analyses have been carried out from electrochemical and microstructural viewpoints [4–7]. On the other hand, theoretical approaches such as molecular orbital (MO) calculation can provide quantitative evaluation of the reaction mechanisms at the elementary reaction level, such as the formation of intermediate species, which cannot be investigated by conventional approaches.
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 5 7 4 - 6
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
48
From these viewpoints, we have started to explore the reaction mechanisms of the electroless deposition processes using ab initio MO methods [8,9] to establish the methodology for such a novel approach. In this paper, we describe the MO investigation on the reaction process of various reductant species for electroless deposition, focusing upon their reducibility as well as the effect of the catalytic activity of the metal surfaces.
2. Method of calculation In the present work, GAUSSIAN 98 rev. a.7 was used, which was designed to perform ab initio (non-empirical) MO calculations [10]. In the case of the semi-empirical MO calculations such as the MOPAC package, property of the transition elements cannot always be processed properly. On the other hand, the approximate wave function of electrons can be obtained by using the Hartree–Fock (HF) method, which is the primary process of the ab initio MO calculation. However, the HF method occasionally provides an inadequate treatment of the correlation between the motions of the electrons within a molecular system, especially that arising between electrons of opposite spin. In order to obtain the energy taking the electron correlation effects into account, therefore, more precise calculation such as the configuration interaction (CI) method or perturbation method is required [11]. Based upon these issues, the calculation of the present work was carried out as follows: first, oxidation reaction pathways of the reductant molecules were assumed and the geometry of the species at each step was optimized at the HF level. Then the second-order Møller – Plesset perturbation (MP2) method was employed to the optimized geometry for the energy calculations, and the reaction pathways were quantitatively evaluated. In these calculations, selection of the basis sets is significant in order to obtain reliable results. Especially, those for the anions such as OH− require careful consideration. Table 1 shows the results of the MP2 calculation for the electron affinity of OH− using various basis sets. Although largely scattered values are Table 1 Experimental and calculated values for the electron affinity of OH− Basis sets
Electron affinity of OH− (kJ mol−1)
(a) 6-31G** (b) 6-31+G** (c) cc-pVDZ (d) AUG-cc-pVDZ (Experimental value)
−15.77 160.33 −45.14 183.55 176.56
Basis sets (b) and (d) consist of basically the same structure as those of (a) and (c), respectively, while diffuse s- and p-type functions are augmented to the (b) and (d) basis sets.
seen in the table by applying various basis sets, it is clear that values reasonably close to the experimental one (176.56 kJ mol − 1) can be obtained by using the 6-31+ G** and AUG-cc-pVDZ basis sets in which diffuse functions are argumented. In the present work therefore, these basis sets were used for the calculation of oxygen. For the metallic atoms such as palladium, nickel, and copper, the basis sets with effective core potential by Hay and Wadt [12] were used.
3. Results and discussion
3.1. Elementary reaction pathways Van den Meerakker proposed a general reaction pathways for the reductant species as follows [13]: RH R+ H
dehydrogenation
(1)
oxidation
(2)
H+ HH2
recombination
(3)
H+ OH− H2O+ e−
oxidation
(4)
R+ OH ROH + e −
−
In this mechanism, the reaction proceeds via 3-coordinate intermediate species by primary dehydrogenation reaction (1). On the other hand, it is also possible to consider alternative pathway via 5-coordinate species, by primary additions of OH− followed by dehydrogenation. Based upon these, we first investigated the elementary reaction process of the reductant species in the gas phase in order to understand their fundamental reaction mechanisms theoretically. Scheme 1 shows the two possible pathways, via 3-coordinate and 5-coordinate intermediates, for the oxidation reaction of dimethylamine borane [8]. Following these pathways, energy diagram of the reaction steps was calculated, as is shown in Fig. 1, in which the relative energy of each species is plotted with respect to the initial species NH(CH3)2BH3 (1). In Fig. 1, higher energy barrier appears at the intermediates species, and it is clear that the pathway via 5-coordinate species possesses lower ‘barrier height’ compared with that via 3-coordinate species in all of the steps. Therefore, it is expected that the reaction proceeds via 5-coordinate intermediates. The results of our previous study on the charge and spin population [8] indicated that the axial bondings in the 5-coordinate species are stabilized by the 3-center 3- or 4-electron bondings. The total reaction, from NH(CH3)2BH3 (1) to B(OH)− 4 (9), is exothermic and the heat of reaction is estimated to be 669 kJ mol − 1. Scheme 2 and Fig. 2 show the possible two pathways and corresponding energy diagrams for the oxidation reaction process of hypophosphite ion. In Fig. 2, the step 6 corresponds to the recombination of produced
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
49
Scheme 1. Reaction pathways for the oxidation process of dimethylamine borane (DMAB) via 3-coordinate intermediates (2a, 4a, 6a, and 8a) and 5-coordinate intermediates (2b, 4b, 6b, and 8b).
hydrogen radicals, and the total reaction is exothermic with the heat of reaction being 218 kJ mol − 1. As was indicated by our recent results [9], the pathway via 5-coordinate intermediates is also favorable for the reaction processes; the energy level of the 5-coordinate intermediate H2PO2(OH)− (3b) is ca. 200 kJ mol − 1 lower than that of the 3-coordinate HPO− 2 (3a). In Fig. 2, the energy levels of dianion-type intermediate species, H3PO23 − (2) and H2PO23 − (4) are also shown, which are formed by addition of OH− to − H2PO− 2 (1) and HPO2 (3a), respectively. It is expected that such species are quite unstable, thus immediate emission of one electron should occur to transform them to monoanionic species. Scheme 3 and Fig. 3 show two possible pathways and corresponding energy diagrams for the elementary reaction process of formaldehyde. In Fig. 3, step 6 also corresponds to the recombination of produced hydrogen radicals, and the total reaction is exothermic with a heat of reaction 142 kJ mol − 1. In the case of formaldehyde, it is also expected that the pathway via 5-coordinate intermediates is more preferable than that via the 3-coordinate species, although the difference in the energy barrier for the dianion formation is not so large compared with those for the hypophosphite ion. As described, the elementary reaction pathway of representative reductants for electroless deposition, such as dimethylamine borane, hypophosphite ion, and formaldehyde, has been evaluated. It is indicated with the results of the theoretical analysis that the reaction via 5-coordinate intermediates is favorable for these three species. The heat of reaction for each process, which indicates the thermodynamic energy of the reaction, is 669, 218, and 142 kJ mol − 1, respectively. The order of the values, dimethylamine borane\hypophosphite ion\formaldehyde, qualitatively corresponds to the empirically known difference in their ‘reducibility’ for the electroless deposition process. It is expected that the value could be used for quantitative evaluation of the ‘reducibility’ of the reductant species.
Fig. 1. Energy diagram for the oxidation reaction process of DMAB via 3-coordinate intermediates () and 5-coordinate intermediates ( ).
Scheme 2. Reaction pathways for the oxidation process of hypophosphite ion via 3-coordinate intermediates (3a) and 5-coordinate intermediates (3b).
Fig. 2. Energy diagram for the oxidation reaction process of hypophosphite ion via 3-coordinate intermediates () and 5-coordinate intermediates ( ).
50
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
water [14]) and its effect on the heat of reaction, Q, of following reaction: − − − H2PO− 2 + OH H2PO3 + 1/2H2 + e ,
Scheme 3. Reaction pathways for the oxidation process of formaldehyde via 3-coordinate intermediates (3a) and 5-coordinate intermediates (3b).
Q
(5)
is evaluated. Fig. 4 shows representative results obtained by plotting the change in Q as a function of the dielectric constant. As described above, the reaction is originally exothermic with a Q value of −218 kJ mol − 1. However, the reaction turns to endothermic with a Q value of 126 kJ mol − 1 by taking the effect of solvation, i.e. dielectric constant of 78.3, into account. On the other hand, the Q value sharply shifts to the exothermic direction in the lower dielectric constant region. As is indicated in Fig. 4, the dielectric constants of the outer and inner Helmholtz layers for the Bockris–Devanathan–Mu¨ ller model [15] is estimated to be ca. 32 and ca. 6, respectively. From these results, it is expected that the reaction preferably proceeds at the interfacial region, i.e. the surface of the metal, rather than at the ‘bulk’ solution region.
3.3. Catalytic acti6ity of metal surface Fig. 3. Energy diagram for the oxidation reaction process of formaldehyde via 3-coordinate intermediates () and 5-coordinate intermediates ( ).
In order to investigate the catalytic activity of the metal surface, we used metal cluster as a model surface. First we focused upon the palladium(111) surface consisting of 4–7 atoms with fixed geometry (PdPd distance of 0.275 nm). Fig. 5 shows possible geometries for adsorption of hypophosphite ion onto the Pd cluster surface, which is the initial step of the oxidation reaction of the reductant. The calculated stabilization energy for each geometry is also shown. Among them, it is expected that the most stable geometry is the adsorption of the oxygen-side of the hypophosphite ion at the hollow site of the Pd surface (Fig. 5(d)).
Fig. 4. Effect of dielectric constant of the solution on the heat of reaction of the oxidation reaction of the hypophosphite ion (H2PO− 2 − +OH− H2PO− 3 + 1/2H2 + e , Q).
3.2. Effect of sol6ation In order to evaluate the effect of solvation, we applied the self-consistent reaction field method with an isodensity surface polarized continuum model (SCRFIPCM), in which the solvation effect can be quantitatively examined in terms of the dielectric constant of the solution [9]. In this approach, dielectric constant is changed from 1 (non-polar solvent) to 78.3 (‘bulk’ pure
Fig. 5. Four kinds of adsorbed geometries and the stabilization energy of hypophosphite ion on the Pd4 cluster. (a) H-side to the Pd on-top site; (b) H-side to the Pd hollow site; (c) O-side to the Pd on-top site; and (d) O-side to the Pd hollow site.
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
Fig. 6. Effect of the size of Pd cluster on instability energy of adsorbed H2PO2(OH)2 − with respect to adsorbed H2PO− 2 .
Fig. 7. Energy diagram for the oxidation reaction process of hypophosphite ion via 5-coordinate intermediates on various metal surfaces; without the metal surface (), on copper surface ( ), on palladium surface (), and on nickel surface ( ).
Fig. 6 shows the effect of Pd cluster size on the instability of adsorbed H3PO23 − species on Pd cluster with respect to the H2PO− 2 species on Pd cluster. The instability becomes smaller with an increase in the cluster size due to the stabilization effect of the Pd surface, which could be one of the origins of the ‘catalytic activity’. Based upon these results, we attempted to evaluate the ‘catalytic activity’ of various metal surfaces. Fig. 7 shows the energy diagrams for the oxidation reaction process of hypophosphite ion shown in Scheme 2, on various metal surfaces. Here the energy is plotted with respect to those of the initial states, H2PO− 2 – metal cluster. In general, lower energy barrier is obtained by applying the effect of the metal surfaces (compare the plots with the dotted line to those with the solid lines).
51
Fig. 8. Energy diagram for the oxidation reaction process of titanium trichloride via 5-coordinate intermediates, assuming trivalent Ti () and tetravalent Ti ( ).
The reaction pathway with the nickel cluster shows the lowest energy, indicating that the reaction is catalytically enhanced at the nickel surface. The total reaction is also expected to be exothermic with palladium surface. On the other hand, the reaction with the copper cluster is not stabilized at step 5, suggesting that the copper surface is not catalytically active for the reaction. It is experimentally indicated that the copper surface is inactive for the oxidation reaction of hypophosphite ion [16]. One of the reasons for this could be theoretically explained by the results shown in Fig. 7. This is the significant contribution for understanding the electroless deposition processes and detailed investigation along this approach should be carried out successively.
3.4. Reaction mechanism of titanium trichloride reductant As described, the reaction mechanisms of conventional reductants such as dimethylamine borane, hypophosphite ion, and formaldehyde, are investigated by ab initio MO approach. While the electron emission in these species is due to the substitution of hydrogen to OH−, reductants based on different mechanism are also proposed. Among them, TiCl3 demonstrates unique characteristics such as hydrogen-evolution-free reaction and higher reducibility [17–20]. Scheme 4 shows the pathway for the oxidation reaction process of TiCl3 reductant, in which the reaction proceeds with the substitution of Cl to OH−. In this
Scheme 4. Reaction pathways for the oxidation process of titanium trichloride via 5-coordinate intermediates.
52
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
Fig. 9. Change in net charge and spin density in the reaction of TiCl3 at the palladium cluster surface.
case, however, the electron is not supplied from the OH− since the chloride leaves as Cl−. Instead, electron is emitted by oxidation of titanium from trivalent Ti(III) to tetravalent Ti(IV). Therefore, the timing of the electron emission is a significant step to understand the overall reaction of TiCl3. Vaskelis also proposed a new reductant species utilizing the oxidation of center element, such as Co2 + – ethylendiamine complex [19]. Fig. 8 shows the energy diagrams for the oxidation reaction process of TiCl3, either in trivalent Ti(III) or tetravalent Ti(IV) state, in which the energy values are plotted with respect to those of the initial states, TiCl3. It is indicated that the trivalent species are more stable than the tetravalent species up to step 6, whereas the latter turns to be more stable at the final step in which the Ti(OH)4 is formed. Up to this step, the total heat of reaction is estimated to be 920 kJ mol − 1, suggesting high reducibility of this reductant. However, the energy difference between the trivalent and tetravalent species is quite small. Moreover, the effect of metal surface should also be taken into account. Therefore we carried out the spin density analysis on the TiCl3 on the palladium cluster. Fig. 9 shows the representative results of the analysisof the net charges and spin densities for TiX4 (X= Cl or OH) species and palladium cluster. At the initial stage, the charge and spin remains at the TiX4 part, whereas they shifts to the metal cluster part at the step 6 and finally they are transferred to the metal cluster at step 8. These results indicate that the electron emission,
i.e. the oxidation of titanium from trivalent to the tetravalent state takes place when all the Cl is replaced by OH− to form TiOH4 species. Fig. 10 shows the energy diagram for the oxidation process of TiCl3 via the 5-coordinate intermediate species. As is seen in the figure, the ‘energy barrier’ appearing at steps 3, 5, and 7 varnishes and the reaction is considerably stabilized at the palladium surface. From these results, it is indicated that the stabilization of the oxidation reaction to enhance the electron emission. In other words, oxidation of the reductant species is a significant issue to describe the catalytic activity of the metal surface to electroless deposition processes.
4. Conclusions In the present work, reaction mechanism of several reductants for electroless deposition such as dimethylamine borane, hypophosphite ion, formaldehyde, and titanium trichloride, was investigated using molecular orbital approach. It was indicated that the oxidation reaction for the reductants proceed via 5-coordinate intermediates and the calculated value of the heat of reaction could be used for quantitative evaluation of the reducibility of the reductants. It was also suggested that the reactions favorably proceed on the metal surfaces, while different effect or ‘catalytic activity’ was observed due to the variation in the metal species. For example, the calculated results indicated that copper surface was not catalytically active for the reaction of hypophosphite, which corresponds to the experimentally known results. As described, methodology for theoretical modeling and evaluation of electroless deposition reaction processes has been developed, and the elementary reaction process, as well as the reducibility of the reductant species and catalytic activity of the metal surfaces have been quantitatively evaluated for the first time. These results demonstrate the capability of such a new approach to achieve rigorous understanding of electroless deposition processes. For this approach, total modeling of the reaction mechanism and, moreover, designing of novel electroless deposition processs, such as new reductants in combination with new metals could be the future target, with further modification of the methodologies, especially in combination with experimental approaches.
Acknowledgements
Fig. 10. Effect of metal surface on the TiCl3 reaction.
This work was financially supported in part by Grant-in-Aid for Scientific Research, the Ministry of Education, Science, and Culture, and also by the Iwatani Naoji Foundation.
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
References [1] L.T. Romankiw, T. Osaka, Y. Yamazaki, C. Madore (Eds.), Electrochemical Technology Applications in Electronics. In: Electrochemical Society Proceedings, vol. PV 99-34, The Electrochemical Society, Pennington, NJ, 2000. [2] E.J. O’Sullivan, A.G. Schrott, M. Paunovic, C.J. Sambucetti, J.R. Marino, P.J. Bailey, S. Kaja, K.W. Semkow, IBM J. Res. Dev. 42 (1998) 607. [3] J.W. Schultze, V. Tsakova, Electrochim. Acta 44 (1999) 3605. [4] I. Ohno, Mater. Sci. Eng. A 146 (1991) 33. [5] G.O. Mallory, J.B. Hajdu, Electroless Plating: Fundamentals and Applications, American Electroplaters and Surface Finishers Society, Orland, 1990. [6] Y. Okinaka, T. Osaka, in: H. Gerischer, C.W. Tobias (Eds.), Advances in Electrochemical Science and Engineering, vol. 3, VCH, Weinheim, 1994, p. 55. [7] D. Tachev, D. Iorgov, S. Armyanov, J. Non-Cryst. Solids 270 (2000) 66. [8] T. Homma, H. Nakai, M. Onishi, T. Osaka, J. Phys. Chem. B 103 (1999) 1774. [9] H. Nakai, T. Homma, I. Komatsu, T. Osaka, J. Phys. Chem. B 105 (2001) 1701. [10] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery, R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
.
[11]
[12] [13] [14] [15] [16] [17] [18] [19] [20]
53
Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q.Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M. Head-Gordon, E.S. Replogle, J.A. Pople, GAUSSIAN 98, Gaussian, Inc., Pittsburgh, PA, 1998. A. Szabo, N. Ostlund, Modern Quantum Chemistry: Introduction to Advanced Electronic Structure Theory, MacMillan, New York, 1982. P. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299. J.E.A.M. van den Meerakker, J. Appl. Electrochem. 11 (1981) 397. The Chemical Society of Japan Kagaku-binran 1984. J.O’M. Bockris, M.A.V. Devanathan, K. Mu¨ ller, Proc. R. Soc. (London) A 274 (1963) 55. I. Ohno, O. Wakabayashi, S. Haruyama, J. Electrochem. Soc. 132 (1985) 2323. A. Senda, Y. Takano, T. Nakagawa, J. Surf. Finish. Soc. Jpn. 43 (1992) 967. A. Senda, Y. Takano, T. Nakagawa, J. Surf. Finish. Soc. Jpn. 44 (1993) 966. A. Senda, Y. Takano, T. Nakagawa, J. Surf. Finish. Soc. Jpn. 45 (1994) 923. A. Vaskelis, H.J. Norkus, G. Rozovskis, H.J. Vinkevicius, Trans. Inst. Metal Finish. 75 (1996) 1.
Molecular orbital study on the reaction mechanisms of electroless deposition processes Takayuki Homma a,*, Isao Komatsu a, Amiko Tamaki a, Hiromi Nakai b, Tetsuya Osaka a a
Department of Applied Chemistry, School of Science and Engineering, Waseda Uni6ersity, Okubo, Shinjuku-ku, Tokyo 169 -8555, Japan b Department of Chemistry, School of Science and Engineering, Waseda Uni6ersity, Okubo, Shinjuku-ku, Tokyo 169 -8555, Japan Received 4 October 2000; received in revised form 4 April 2001
Abstract Molecular orbital study was applied to investigate electroless deposition processes, focusing upon reducibility of reductants as well as the effect of catalytic activity of the metal surfaces. Elementary reaction pathways of the reductants such as dimethylamine borane, hypophosphite ion, formaldehyde, and titanium trichloride were quantitatively examined in terms of the heat of reaction. It was indicated that the reaction via 5-coordinate intermediates was favorable for these species and that the value could be used for quantitative evaluation of the reducibility of the reductant species. The analysis of solvation effect on the reactions suggested that the reactions preferably proceed at the interfacial region, i.e. the surface of the metal, rather than in the ‘bulk’ solution region. The effect of catalytic activity of metal surfaces was investigated using a cluster-model surface. It was suggested that the elementary reactions were stabilized on the metal surfaces to enhance the electron emission from the reductants. On the other hand, difference in the stabilization effects for the reaction of hypophosphite ion was obtained due to the variation in the metal species; it was expected that the copper surface is not catalytically active for the reaction, which corresponds to the experimentally known results. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Electroless deposition; Molecular orbital study; Dimethylamine borane; Hypophosphite; Formaldehyde; Titanium trichloride; Catalytic activity of metal surfaces
1. Introduction Electroless deposition reactions are widely applied in fabrication processes for advanced functional devices such as microelectronic devices [1,2], featuring its capability to form uniform deposits on either wide or selected areas of various surfaces, which can be controlled in sub-micrometer order. Unlike conventional electrodeposition processes, in the case of the electroless deposition, electron for reductive deposition of metal species is supplied by catalytic oxidation reaction of reductants in electrolytes. Thus, unique characteristics of the electroless process can be summarized as: (i) ‘reducibility’ of reductants; and (ii) ‘catalytic activity’ of metal surface. However, these characteristics have * Corresponding author. Tel.: + 81-352-86-3209; fax: +81-352-052074. E-mail address: [email protected] (T. Homma).
not been well defined yet, and so current developments in electroless deposition process have met with difficulties such as many trial-and-error factors, complicated bath system, limitation of metal –reductant combinations. In order to overcome these difficulties and to maximize the capability of the electroless deposition process as a useful and universal tool for the electrochemical microsystem technologies (EMT) [3] and moreover, electrochemical ‘nano’-system technologies, precise analyses of deposition process is required. For the experimental approaches, a number of analyses have been carried out from electrochemical and microstructural viewpoints [4–7]. On the other hand, theoretical approaches such as molecular orbital (MO) calculation can provide quantitative evaluation of the reaction mechanisms at the elementary reaction level, such as the formation of intermediate species, which cannot be investigated by conventional approaches.
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 5 7 4 - 6
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
48
From these viewpoints, we have started to explore the reaction mechanisms of the electroless deposition processes using ab initio MO methods [8,9] to establish the methodology for such a novel approach. In this paper, we describe the MO investigation on the reaction process of various reductant species for electroless deposition, focusing upon their reducibility as well as the effect of the catalytic activity of the metal surfaces.
2. Method of calculation In the present work, GAUSSIAN 98 rev. a.7 was used, which was designed to perform ab initio (non-empirical) MO calculations [10]. In the case of the semi-empirical MO calculations such as the MOPAC package, property of the transition elements cannot always be processed properly. On the other hand, the approximate wave function of electrons can be obtained by using the Hartree–Fock (HF) method, which is the primary process of the ab initio MO calculation. However, the HF method occasionally provides an inadequate treatment of the correlation between the motions of the electrons within a molecular system, especially that arising between electrons of opposite spin. In order to obtain the energy taking the electron correlation effects into account, therefore, more precise calculation such as the configuration interaction (CI) method or perturbation method is required [11]. Based upon these issues, the calculation of the present work was carried out as follows: first, oxidation reaction pathways of the reductant molecules were assumed and the geometry of the species at each step was optimized at the HF level. Then the second-order Møller – Plesset perturbation (MP2) method was employed to the optimized geometry for the energy calculations, and the reaction pathways were quantitatively evaluated. In these calculations, selection of the basis sets is significant in order to obtain reliable results. Especially, those for the anions such as OH− require careful consideration. Table 1 shows the results of the MP2 calculation for the electron affinity of OH− using various basis sets. Although largely scattered values are Table 1 Experimental and calculated values for the electron affinity of OH− Basis sets
Electron affinity of OH− (kJ mol−1)
(a) 6-31G** (b) 6-31+G** (c) cc-pVDZ (d) AUG-cc-pVDZ (Experimental value)
−15.77 160.33 −45.14 183.55 176.56
Basis sets (b) and (d) consist of basically the same structure as those of (a) and (c), respectively, while diffuse s- and p-type functions are augmented to the (b) and (d) basis sets.
seen in the table by applying various basis sets, it is clear that values reasonably close to the experimental one (176.56 kJ mol − 1) can be obtained by using the 6-31+ G** and AUG-cc-pVDZ basis sets in which diffuse functions are argumented. In the present work therefore, these basis sets were used for the calculation of oxygen. For the metallic atoms such as palladium, nickel, and copper, the basis sets with effective core potential by Hay and Wadt [12] were used.
3. Results and discussion
3.1. Elementary reaction pathways Van den Meerakker proposed a general reaction pathways for the reductant species as follows [13]: RH R+ H
dehydrogenation
(1)
oxidation
(2)
H+ HH2
recombination
(3)
H+ OH− H2O+ e−
oxidation
(4)
R+ OH ROH + e −
−
In this mechanism, the reaction proceeds via 3-coordinate intermediate species by primary dehydrogenation reaction (1). On the other hand, it is also possible to consider alternative pathway via 5-coordinate species, by primary additions of OH− followed by dehydrogenation. Based upon these, we first investigated the elementary reaction process of the reductant species in the gas phase in order to understand their fundamental reaction mechanisms theoretically. Scheme 1 shows the two possible pathways, via 3-coordinate and 5-coordinate intermediates, for the oxidation reaction of dimethylamine borane [8]. Following these pathways, energy diagram of the reaction steps was calculated, as is shown in Fig. 1, in which the relative energy of each species is plotted with respect to the initial species NH(CH3)2BH3 (1). In Fig. 1, higher energy barrier appears at the intermediates species, and it is clear that the pathway via 5-coordinate species possesses lower ‘barrier height’ compared with that via 3-coordinate species in all of the steps. Therefore, it is expected that the reaction proceeds via 5-coordinate intermediates. The results of our previous study on the charge and spin population [8] indicated that the axial bondings in the 5-coordinate species are stabilized by the 3-center 3- or 4-electron bondings. The total reaction, from NH(CH3)2BH3 (1) to B(OH)− 4 (9), is exothermic and the heat of reaction is estimated to be 669 kJ mol − 1. Scheme 2 and Fig. 2 show the possible two pathways and corresponding energy diagrams for the oxidation reaction process of hypophosphite ion. In Fig. 2, the step 6 corresponds to the recombination of produced
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
49
Scheme 1. Reaction pathways for the oxidation process of dimethylamine borane (DMAB) via 3-coordinate intermediates (2a, 4a, 6a, and 8a) and 5-coordinate intermediates (2b, 4b, 6b, and 8b).
hydrogen radicals, and the total reaction is exothermic with the heat of reaction being 218 kJ mol − 1. As was indicated by our recent results [9], the pathway via 5-coordinate intermediates is also favorable for the reaction processes; the energy level of the 5-coordinate intermediate H2PO2(OH)− (3b) is ca. 200 kJ mol − 1 lower than that of the 3-coordinate HPO− 2 (3a). In Fig. 2, the energy levels of dianion-type intermediate species, H3PO23 − (2) and H2PO23 − (4) are also shown, which are formed by addition of OH− to − H2PO− 2 (1) and HPO2 (3a), respectively. It is expected that such species are quite unstable, thus immediate emission of one electron should occur to transform them to monoanionic species. Scheme 3 and Fig. 3 show two possible pathways and corresponding energy diagrams for the elementary reaction process of formaldehyde. In Fig. 3, step 6 also corresponds to the recombination of produced hydrogen radicals, and the total reaction is exothermic with a heat of reaction 142 kJ mol − 1. In the case of formaldehyde, it is also expected that the pathway via 5-coordinate intermediates is more preferable than that via the 3-coordinate species, although the difference in the energy barrier for the dianion formation is not so large compared with those for the hypophosphite ion. As described, the elementary reaction pathway of representative reductants for electroless deposition, such as dimethylamine borane, hypophosphite ion, and formaldehyde, has been evaluated. It is indicated with the results of the theoretical analysis that the reaction via 5-coordinate intermediates is favorable for these three species. The heat of reaction for each process, which indicates the thermodynamic energy of the reaction, is 669, 218, and 142 kJ mol − 1, respectively. The order of the values, dimethylamine borane\hypophosphite ion\formaldehyde, qualitatively corresponds to the empirically known difference in their ‘reducibility’ for the electroless deposition process. It is expected that the value could be used for quantitative evaluation of the ‘reducibility’ of the reductant species.
Fig. 1. Energy diagram for the oxidation reaction process of DMAB via 3-coordinate intermediates () and 5-coordinate intermediates ( ).
Scheme 2. Reaction pathways for the oxidation process of hypophosphite ion via 3-coordinate intermediates (3a) and 5-coordinate intermediates (3b).
Fig. 2. Energy diagram for the oxidation reaction process of hypophosphite ion via 3-coordinate intermediates () and 5-coordinate intermediates ( ).
50
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
water [14]) and its effect on the heat of reaction, Q, of following reaction: − − − H2PO− 2 + OH H2PO3 + 1/2H2 + e ,
Scheme 3. Reaction pathways for the oxidation process of formaldehyde via 3-coordinate intermediates (3a) and 5-coordinate intermediates (3b).
Q
(5)
is evaluated. Fig. 4 shows representative results obtained by plotting the change in Q as a function of the dielectric constant. As described above, the reaction is originally exothermic with a Q value of −218 kJ mol − 1. However, the reaction turns to endothermic with a Q value of 126 kJ mol − 1 by taking the effect of solvation, i.e. dielectric constant of 78.3, into account. On the other hand, the Q value sharply shifts to the exothermic direction in the lower dielectric constant region. As is indicated in Fig. 4, the dielectric constants of the outer and inner Helmholtz layers for the Bockris–Devanathan–Mu¨ ller model [15] is estimated to be ca. 32 and ca. 6, respectively. From these results, it is expected that the reaction preferably proceeds at the interfacial region, i.e. the surface of the metal, rather than at the ‘bulk’ solution region.
3.3. Catalytic acti6ity of metal surface Fig. 3. Energy diagram for the oxidation reaction process of formaldehyde via 3-coordinate intermediates () and 5-coordinate intermediates ( ).
In order to investigate the catalytic activity of the metal surface, we used metal cluster as a model surface. First we focused upon the palladium(111) surface consisting of 4–7 atoms with fixed geometry (PdPd distance of 0.275 nm). Fig. 5 shows possible geometries for adsorption of hypophosphite ion onto the Pd cluster surface, which is the initial step of the oxidation reaction of the reductant. The calculated stabilization energy for each geometry is also shown. Among them, it is expected that the most stable geometry is the adsorption of the oxygen-side of the hypophosphite ion at the hollow site of the Pd surface (Fig. 5(d)).
Fig. 4. Effect of dielectric constant of the solution on the heat of reaction of the oxidation reaction of the hypophosphite ion (H2PO− 2 − +OH− H2PO− 3 + 1/2H2 + e , Q).
3.2. Effect of sol6ation In order to evaluate the effect of solvation, we applied the self-consistent reaction field method with an isodensity surface polarized continuum model (SCRFIPCM), in which the solvation effect can be quantitatively examined in terms of the dielectric constant of the solution [9]. In this approach, dielectric constant is changed from 1 (non-polar solvent) to 78.3 (‘bulk’ pure
Fig. 5. Four kinds of adsorbed geometries and the stabilization energy of hypophosphite ion on the Pd4 cluster. (a) H-side to the Pd on-top site; (b) H-side to the Pd hollow site; (c) O-side to the Pd on-top site; and (d) O-side to the Pd hollow site.
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
Fig. 6. Effect of the size of Pd cluster on instability energy of adsorbed H2PO2(OH)2 − with respect to adsorbed H2PO− 2 .
Fig. 7. Energy diagram for the oxidation reaction process of hypophosphite ion via 5-coordinate intermediates on various metal surfaces; without the metal surface (), on copper surface ( ), on palladium surface (), and on nickel surface ( ).
Fig. 6 shows the effect of Pd cluster size on the instability of adsorbed H3PO23 − species on Pd cluster with respect to the H2PO− 2 species on Pd cluster. The instability becomes smaller with an increase in the cluster size due to the stabilization effect of the Pd surface, which could be one of the origins of the ‘catalytic activity’. Based upon these results, we attempted to evaluate the ‘catalytic activity’ of various metal surfaces. Fig. 7 shows the energy diagrams for the oxidation reaction process of hypophosphite ion shown in Scheme 2, on various metal surfaces. Here the energy is plotted with respect to those of the initial states, H2PO− 2 – metal cluster. In general, lower energy barrier is obtained by applying the effect of the metal surfaces (compare the plots with the dotted line to those with the solid lines).
51
Fig. 8. Energy diagram for the oxidation reaction process of titanium trichloride via 5-coordinate intermediates, assuming trivalent Ti () and tetravalent Ti ( ).
The reaction pathway with the nickel cluster shows the lowest energy, indicating that the reaction is catalytically enhanced at the nickel surface. The total reaction is also expected to be exothermic with palladium surface. On the other hand, the reaction with the copper cluster is not stabilized at step 5, suggesting that the copper surface is not catalytically active for the reaction. It is experimentally indicated that the copper surface is inactive for the oxidation reaction of hypophosphite ion [16]. One of the reasons for this could be theoretically explained by the results shown in Fig. 7. This is the significant contribution for understanding the electroless deposition processes and detailed investigation along this approach should be carried out successively.
3.4. Reaction mechanism of titanium trichloride reductant As described, the reaction mechanisms of conventional reductants such as dimethylamine borane, hypophosphite ion, and formaldehyde, are investigated by ab initio MO approach. While the electron emission in these species is due to the substitution of hydrogen to OH−, reductants based on different mechanism are also proposed. Among them, TiCl3 demonstrates unique characteristics such as hydrogen-evolution-free reaction and higher reducibility [17–20]. Scheme 4 shows the pathway for the oxidation reaction process of TiCl3 reductant, in which the reaction proceeds with the substitution of Cl to OH−. In this
Scheme 4. Reaction pathways for the oxidation process of titanium trichloride via 5-coordinate intermediates.
52
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
Fig. 9. Change in net charge and spin density in the reaction of TiCl3 at the palladium cluster surface.
case, however, the electron is not supplied from the OH− since the chloride leaves as Cl−. Instead, electron is emitted by oxidation of titanium from trivalent Ti(III) to tetravalent Ti(IV). Therefore, the timing of the electron emission is a significant step to understand the overall reaction of TiCl3. Vaskelis also proposed a new reductant species utilizing the oxidation of center element, such as Co2 + – ethylendiamine complex [19]. Fig. 8 shows the energy diagrams for the oxidation reaction process of TiCl3, either in trivalent Ti(III) or tetravalent Ti(IV) state, in which the energy values are plotted with respect to those of the initial states, TiCl3. It is indicated that the trivalent species are more stable than the tetravalent species up to step 6, whereas the latter turns to be more stable at the final step in which the Ti(OH)4 is formed. Up to this step, the total heat of reaction is estimated to be 920 kJ mol − 1, suggesting high reducibility of this reductant. However, the energy difference between the trivalent and tetravalent species is quite small. Moreover, the effect of metal surface should also be taken into account. Therefore we carried out the spin density analysis on the TiCl3 on the palladium cluster. Fig. 9 shows the representative results of the analysisof the net charges and spin densities for TiX4 (X= Cl or OH) species and palladium cluster. At the initial stage, the charge and spin remains at the TiX4 part, whereas they shifts to the metal cluster part at the step 6 and finally they are transferred to the metal cluster at step 8. These results indicate that the electron emission,
i.e. the oxidation of titanium from trivalent to the tetravalent state takes place when all the Cl is replaced by OH− to form TiOH4 species. Fig. 10 shows the energy diagram for the oxidation process of TiCl3 via the 5-coordinate intermediate species. As is seen in the figure, the ‘energy barrier’ appearing at steps 3, 5, and 7 varnishes and the reaction is considerably stabilized at the palladium surface. From these results, it is indicated that the stabilization of the oxidation reaction to enhance the electron emission. In other words, oxidation of the reductant species is a significant issue to describe the catalytic activity of the metal surface to electroless deposition processes.
4. Conclusions In the present work, reaction mechanism of several reductants for electroless deposition such as dimethylamine borane, hypophosphite ion, formaldehyde, and titanium trichloride, was investigated using molecular orbital approach. It was indicated that the oxidation reaction for the reductants proceed via 5-coordinate intermediates and the calculated value of the heat of reaction could be used for quantitative evaluation of the reducibility of the reductants. It was also suggested that the reactions favorably proceed on the metal surfaces, while different effect or ‘catalytic activity’ was observed due to the variation in the metal species. For example, the calculated results indicated that copper surface was not catalytically active for the reaction of hypophosphite, which corresponds to the experimentally known results. As described, methodology for theoretical modeling and evaluation of electroless deposition reaction processes has been developed, and the elementary reaction process, as well as the reducibility of the reductant species and catalytic activity of the metal surfaces have been quantitatively evaluated for the first time. These results demonstrate the capability of such a new approach to achieve rigorous understanding of electroless deposition processes. For this approach, total modeling of the reaction mechanism and, moreover, designing of novel electroless deposition processs, such as new reductants in combination with new metals could be the future target, with further modification of the methodologies, especially in combination with experimental approaches.
Acknowledgements
Fig. 10. Effect of metal surface on the TiCl3 reaction.
This work was financially supported in part by Grant-in-Aid for Scientific Research, the Ministry of Education, Science, and Culture, and also by the Iwatani Naoji Foundation.
T. Homma et al. / Electrochimica Acta 47 (2001) 47–53
References [1] L.T. Romankiw, T. Osaka, Y. Yamazaki, C. Madore (Eds.), Electrochemical Technology Applications in Electronics. In: Electrochemical Society Proceedings, vol. PV 99-34, The Electrochemical Society, Pennington, NJ, 2000. [2] E.J. O’Sullivan, A.G. Schrott, M. Paunovic, C.J. Sambucetti, J.R. Marino, P.J. Bailey, S. Kaja, K.W. Semkow, IBM J. Res. Dev. 42 (1998) 607. [3] J.W. Schultze, V. Tsakova, Electrochim. Acta 44 (1999) 3605. [4] I. Ohno, Mater. Sci. Eng. A 146 (1991) 33. [5] G.O. Mallory, J.B. Hajdu, Electroless Plating: Fundamentals and Applications, American Electroplaters and Surface Finishers Society, Orland, 1990. [6] Y. Okinaka, T. Osaka, in: H. Gerischer, C.W. Tobias (Eds.), Advances in Electrochemical Science and Engineering, vol. 3, VCH, Weinheim, 1994, p. 55. [7] D. Tachev, D. Iorgov, S. Armyanov, J. Non-Cryst. Solids 270 (2000) 66. [8] T. Homma, H. Nakai, M. Onishi, T. Osaka, J. Phys. Chem. B 103 (1999) 1774. [9] H. Nakai, T. Homma, I. Komatsu, T. Osaka, J. Phys. Chem. B 105 (2001) 1701. [10] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery, R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
.
[11]
[12] [13] [14] [15] [16] [17] [18] [19] [20]
53
Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q.Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M. Head-Gordon, E.S. Replogle, J.A. Pople, GAUSSIAN 98, Gaussian, Inc., Pittsburgh, PA, 1998. A. Szabo, N. Ostlund, Modern Quantum Chemistry: Introduction to Advanced Electronic Structure Theory, MacMillan, New York, 1982. P. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299. J.E.A.M. van den Meerakker, J. Appl. Electrochem. 11 (1981) 397. The Chemical Society of Japan Kagaku-binran 1984. J.O’M. Bockris, M.A.V. Devanathan, K. Mu¨ ller, Proc. R. Soc. (London) A 274 (1963) 55. I. Ohno, O. Wakabayashi, S. Haruyama, J. Electrochem. Soc. 132 (1985) 2323. A. Senda, Y. Takano, T. Nakagawa, J. Surf. Finish. Soc. Jpn. 43 (1992) 967. A. Senda, Y. Takano, T. Nakagawa, J. Surf. Finish. Soc. Jpn. 44 (1993) 966. A. Senda, Y. Takano, T. Nakagawa, J. Surf. Finish. Soc. Jpn. 45 (1994) 923. A. Vaskelis, H.J. Norkus, G. Rozovskis, H.J. Vinkevicius, Trans. Inst. Metal Finish. 75 (1996) 1.