Letter pubs.acs.org/JPCL
From Structure to Function: Characterization of Cu(I) Adducts in Leveler Additives by DFT Calculations F. Simona, N. T. M. Hai, P. Broekmann, and M. Cascella* Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland S Supporting Information *
ABSTRACT: We present the first molecular model of the coordination complex formed by Cu(I) and imidazole-epichlorohydrin polymers. Our calculations show that the Cu(I) ion has linear coordination and the whole complex has neutral charge. Our model suggests salt couple pairing as the driving force for the formation of the surface-confined precipitation, which is crucial to obtain flat surfaces in industrial copper deposition processes, required for mass fabrication of state-of-the-art electronic and memory devices.
SECTION: Molecular Structure, Quantum Chemistry,General Theory
T
deactivated by the SPS (in contrast to the PAG-based chemistry). SPS acts as an antagonist in combination with the PAG only. Prototypical model systems leveler additives based on Cu(I) coordination are polymerizates of epichlorohydrin and imidazole11 (referred to as Imep hereafter).
he additive-assisted electrodeposition of copper is of vital importance for today’s mass fabrication of logic and memory devices.1,2 For the metallization of copper interconnects, an unconventional growth mode is required that is often referred to as superconformal growth or superf ill.1 The physical origin of such phenomenon is a copper deposition velocity at the feature bottom that exceeds the one at the planar wafer surface and the upper side-walls of those features. 1,2 This nonuniformity in the copper deposition rate relies on an inhomogeneous additive surface coverage inside and outside the features upon fill. Typically, a two-component additive package consisting of a suppressor polymer (polyalkylene glycols (PAGs)) and its specific antagonist (bis-(sodiumsulfopropyl)-disulfide (SPS)) regulates the superfill process. During the electrochemical process, SPS is reduced to mercaptopropyl-sulfonate (MPS), which is a chemical species that is meant to strongly interact with cuprous ions and the copper surface.3 Shape-evolution effects inside the feature during the fill4−7 combined with the particular additive transport/adsorption kinetics inside the feature8,9 have been identified as the origin for the required nonuniformity in the additive surface coverage. Both effects result in an excess concentration of the suppressor additives on the wafer surface and an accumulation of the antagonist at the feature bottom, thus keeping it active for the copper deposition. 3 One undesired side-effect of this approach, however, is the sustained acceleration of the copper deposition after the successful feature fill, thus leading to a characteristic bump formation over isolated trenches and vias. In order to avoid undesired longrange height corrugations on the wafer surface, a third additive component is typically added to the plating bath. These molecules, called levelers,10 can also be considered as suppressor ensembles for the copper deposition, which however, cannot be © 2011 American Chemical Society
Interestingly, state-of-the-art leveler additives as described in current patents11 reveal an increased suppressing activity, particularly in those regions of the wafer surface over the filled features, where it is indeed needed. The local selectivity of their suppressing action could recently be explained by assuming a Cu(I) coordination chemistry as a further driving force for such leveler film formation (unpublished results) besides interfacial anion/cation pairing effects.3 It could be experimentally proven that Cu(I)-thiolate, a byproduct of the copper deposition in the presence of those additives, is an essential constituent of the resulting leveler film (unpublished results). In this communication, we present a first structural model of the MPS-Cu(I)-Imep binding motif by means of density functional theory (DFT) calculations. We built our model using one Imep unit, one Cu(I) ion, and a single MPS molecule. This stoichiometry was suggested by experimental evidence that Cu(I) interacts with the polymer in the presence of not more than one MPS molecule at the time (see Supporting Information (SI)). First, we focused on a minimal in vacuo model, composed by a single Imep unit, one Received: October 27, 2011 Accepted: November 21, 2011 Published: November 21, 2011 3081
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Cu(I) ion, and a methyl-thiolate group that is intended to mimic the MPS unit. Despite the clear limitations of this initial model (i.e., neither solvation nor temperature effects are taken into account), this approach is computationally affordable, and already accurate enough to provide a good description of the basic chemistry of the system under consideration. Three different starting guess conformations were produced in silico using Molden software.12 For the first two conformations, the copper ion was placed within 5 Å from the imidazole ring, facing either of its sides. In the third structure, Cu(I) was placed far from the imidazole ring. The quantum problem was solved by DFT, using PBE,13,14 BLYP,15−17 B3LYP,15−19 and M062X20 approximations of the exchange-correlation (xc) functional, with the 6-311G(d,p) basis-set, and using the Gaussian09 software package.21 The structures of the methylthiolate−Cu(I)−Imep complex were optimized by DIIS methods22 with convergence criteria of 1.5 × 10−3 Bohr for the structure, and of 3 × 10−4 for the force. In our calculations, all initial geometries starting with copper proximal to the ring converged to similar global structures, characterized by a linear coordination of the copper ion, located between the thiolate sulfur and the hydroxyl oxygen atoms. Initial geometries with Cu(I) placed far from the imidazole ring did not converge to any coordination structure of the metal ion, and were not considered any further in our analysis.
Figure 1. Ball-and-stick representation of the optimized in vacuo structures of Imep−Cu(I)−MPS minimal model, obtained using B3LYP (right panel) and M062X (left panel) xc functionals. The angle θ (see also Table 1) formed between the plane defined by imidazole ring (in transparent blue), and the one defined by copper and the two nitrogen of histidine (transparent red), are outlined.
QUICKSTEP,30 within the CP2K package.31 Introduction of DCACP correction on the aromatic moiety only is sufficient to obtain a qualitatively similar orientation of the imidazole ring as that found using the M062X xc (Table 1). Voronoi analysis of the electron density on the imidazole ring in the structures of the leveler with or without the coordination of Cu(I) reports only negligible polarization on the C atom proximal to the metal ion. This confirms that there is no direct coordination of Cu(I) by imidazole, and that they interact mainly through dispersion forces. In a second set of calculations, the solvation properties of the Imep−Cu(I)−MPS monomer complex were investigated. In this case, the extended MPS fragment was used. Starting from the optimized geometry obtained in vacuo, the Imep−Cu(I)− MPS molecular moieity was built and solvated by 76 water molecules, and inserted in a periodic box of 14 × 14 × 14 Å3. The electronic problem was solved using PBE xc and DCACP potentials. The setup of the calculation were the same as those used for in vacuo calculations. The system was thermalized from 0.1 to 300 K with an annealing procedure in 5 ps, and kept at the target temperature with a velocity rescaling thermostat32 for 20 ps. After that 60 ps of Born−Oppenheimer molecular dynamics in the microcanonical NVE ensemble were produced and used for our analysis. Before the beginning of our simulations we determined the protonation state of the hydroxyl group by estimating its pK a shift upon Cu(I) binding by Linearized Poisson−Boltzmann calculations, using the software APBS33 as in ref 34. This quantity was calculated considering a free energy cycle among four different states: (i) MPS-Cu and Imep species separated or (ii) directly interacting, considering for each state the hydroxyl group protonated and unprotonated (Figure 3, SI). The free energy was calculated using continuum electrostatics methods, which offer a satisfying compromise between accuracy and computational efficiency. The atomic approximate point charges required for the calculation were determined using RESP procedure.35 Our calculations predict pKa shift of less than 1 pH unit with respect to reference pKa of propan-2-ol. Therefore, at the experimental strongly acidic conditions (pH ≈ 2) the hydroxyl group is not ionized. The solvated structure maintains the same geometry as in the gas phase model, with identical Cu(I) coordination and stacked orientation of the imidazole ring (Table 1). In particular the coordination distances remained unaffected. Figure 2 (right panel) reports the two radial distribution functions (rdf) of
Table 1. Optimized Structural Parameters of the in Vacuo and Solvated Imep−Cu(I)−MPS Models for the Different xc Functionals Useda XC
S−Cu
O−Cu
S−Cu−O
Cu−C
ΘHis
M062X PBE B3LYP BLYP PBEDCACP BLYPDCACP Water PBEDCACP
2.16 2.1 2.12 2.12 2.09 2.13 2.16
1.98 1.99 1.93 1.94 2.05 1.97 1.95
170 158 167 168 174 175 168
2.82 2.07 3.09 3.09 2.78 2.95 2.9
98 134 159 159 118 118 125
a
Distances are reported in Å, angle and dihedral values are in degrees. ΘHis is the angle between the plane of imidazole ring, and those defined by the copper atom and the two histidines, as depicted in Figure 1.
The relative orientation of the Cu(I) ion with respect to the imidazole ring is strongly affected by the different xc functionals used. Table 1 reports the value of the dihedral angle ΘHis, defined by the plane of the imidazole ring, and the one containing the copper and the two nitrogen atoms (Figure 1). Both BLYP and B3LYP predict the copper ion to stay close to the plane of the imidazole, while M062X and partially PBE predict a conformation where the aromatic ring faces its π orbitals to the metal (Table 1). This discrepancy is reflected in a different distance between Cu(I) and the carbon atom connecting the two nitrogen atoms (Table 1). These differences can be attributed to the well-known poor description of dispersion interactions in DFT calculation,23−25 unless explicit calibration of the xc functional (like in M062X), or explicit corrections to the atomic potential are used. In order to verify that, we have repeated the geometry optimization calculations for both PBE and BLYP functionals using dispersion-corrected atomic centered potentials (DCACPs),26,27 GTH28,29 pseudopotentials, and DZVP30 basis set. Calculations were conducted using the program 3082
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ACKNOWLEDGMENTS
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REFERENCES
This work was funded by the Swiss National Science Foundation Grant No. PP02_118930. Peter Broekmann acknowledges the financial support by BASF SE (Electronic Materials)
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Figure 2. Left panel: structure of the Imep−Cu(I)−MPS complex in water. The water molecule H-bonded to the alcoholic group is highlighted in licorice. Right panel: Cu(I)−Owat (red line) and OImep− Owat (black line) rdf's.
water oxygen (Owat) around the Cu(I) cation and the Imephydroxyl oxygen OImep. The first peak of the Cu(I)−Owat rdf is located at 3.9 Å, therefore the Cu(I) ion is not solvated by the water. The OH group of the Imep is instead involved as a donor in a strong and stable H-bond with a water molecule, as shown by the first peak in the OImep−Owat located at 2.4 Å. The MPS tail remains in an extended conformation, with the sulfonic group of MPS fully solvated by water and without interacting with the imidazole ring, although the simulated time is too short for a thorough exploration of the whole conformational space. The geometry of the adducts allows the formation of salt contacts between different Imep units. Therefore, our calculations provide a very straightforward molecular mechanism for the experimental evidence of the formation of soluble precipitates upon Imep−Cu(I)−MPS interaction. Within our model, a two-step mechanism toward the formation of this kind of leveler ensemble can be suggested. The first step is the Cu(I) coordination by the OH-group of the Imep. The Cu(I) species can be considered as the carrier for the MPS that introduces a negative charge into the resulting ensemble via the sulfonate group. The pristine hydrophilic, polycationic Imep polymer is therefore transformed into a neutral species with distinct groups carrying opposite electric charges. Intra- and interchain anion/cation pairing initiated by the primary Cu(I) coordination can be therefore identified as the main driving force for the surface-confined precipitation of these suppressor ensembles. These precipitates are key for the deceleration of the copper deposition by the formation of an effective diffusional barrier for the copper ions at the wafer surface in the industrial copper plating process. From a mechanistic point of view such “intrinsic” anion/ cation pairing can be considered as a promising alternative route to the “interfacial” anion/cation pairing between a positively charged polymer and an anion-modified surface on which the classical leveler approach is based on.
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ASSOCIATED CONTENT
S Supporting Information *
Additionals pictures and tables. This material is available free of charge via the Internet http://pubs.acs.org .
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AUTHOR INFORMATION
Corresponding Author *E-mail:
[email protected]. 3083
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