Atomic Layer Deposition of Ruthenium on a Titanium Nitride Surface

Article pubs.acs.org/JPCC

Atomic Layer Deposition of Ruthenium on a Titanium Nitride Surface: A Density Functional Theory Study Quan Manh Phung,† Steven Vancoillie,† Geoffrey Pourtois,‡,§ Johan Swerts,‡ Kristine Pierloot,† and Annelies Delabie*,‡,† †

Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium imec, Kapeldreef 75, B-3001 Leuven, Belgium § Department of Chemistry, PASMANT Research Group, University of Antwerp, B-2610 Wilrijk, Antwerp, Belgium ‡

S Supporting Information *

ABSTRACT: Because of its excellent properties in nanotechnology applications, atomic layer deposition of ruthenium (Ru) has been the subject of numerous experimental studies. Recently, two different Ru precursors were compared for plasma-enhanced atomic layer deposition (PEALD) of Ru, and their reactivity was found to be different. Inhibition was observed for bis(ethylcyclopentadienyl)ruthenium (Ru(EtCp)2), while nearly linear growth behavior was observed for (methylcyclopentadienyl-pyrrolyl)ruthenium (Ru(MeCp)Py). To understand this difference in reactivity, we investigate the adsorption of RuCp2 and RuCpPy (i.e., without substituents) on a TiN surface using calculations based on periodic boundary conditions density functional theory (DFT) combined with experiments based on Rutherford backscattering spectroscopy (RBS). The calculations demonstrate that the RuCpPy precursor chemisorbs on the TiN(100) surface while the RuCp2 precursor only physisorbs. We propose a reaction mechanism for the chemisorption of RuCpPy. The area density of the calculated RuCpPy surface species is compared with the experimental values from RBS. The impact of a H-plasma is also investigated. The DFT calculations and experimental results from RBS provide insight into the adsorption processes of the RuCpPy and RuCp2 precursors on the TiN(100) surface.

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INTRODUCTION

designing the most optimal precursors or by optimizing the reaction conditions or starting surface. Recently, ALD of ruthenium (Ru) has been the subject of numerous experimental studies13−19 because Ru displays good physical (thermal conductivity), chemical (stability), and electrical (low resistivity and high work function) properties. Thermal ALD of Ru with organometallic precursors (e.g., Ru(C5H5)2) and O2 has been extensively studied, focusing on its growth characteristics and on the impact of various experimental conditions. The following mechanisms were experimentally explored and proposed:15

Atomic layer deposition (ALD), a thin film deposition technique, has emerged as a powerful method for growing high quality thin films. ALD is based on a cyclic process of at least two consecutive self-limiting chemisorption reactions, the so-called reaction cycle. The basics of ALD with its applications can be found in many reviews.1−5 Two important properties, growth control at the atomic level and conformal deposition on complex nanostructures, have turned ALD into an increasingly important tool for applications in nanotechnology.5−10 Hundreds of ALD chemistries have been developed for metal oxides, nitrides, chalcogenides, and pure elements.11 Theoretical studies have also been performed to understand the mechanisms of ALD processes.12 Although the basic mechanism of ALD of oxides is extensively documented, the mechanisms of numerous ALD processes of metals are less well-understood.12 If one takes into account the complexity of the substrates (steps or defects on surfaces), the different types of precursors, and the experimental conditions (thermal or plasma-enhanced ALD), a study of the reaction mechanism becomes very complex. However, once the mechanism and the effect of the experimental conditions (surface, precursors, temperature, etc.) are understood, the performance of the ALD processes and the quality of the thin film can be improved by 1

© 2013 American Chemical Society

(A)RuO*x + RuCp2 → RuRu(Cp)* + 5CO2 + 5/2H 2O (1)

(B)RuCp * + yO2 → RuO*x + 5CO2 + 5/2H 2O

(2)

One of the most serious drawbacks of thermal Ru ALD using organometallic precursors concerns the difficulty in nucleation on metal nitrides surfaces, such as TiN or TaN. Thermal ALD can result in a nucleation delay and high surface roughness.16,20,21 Plasma-enhanced ALD (PEALD) can improve the Received: June 4, 2013 Revised: August 26, 2013 Published: August 28, 2013 19442

dx.doi.org/10.1021/jp405489w | J. Phys. Chem. C 2013, 117, 19442−19453

The Journal of Physical Chemistry C

Article

Table 1. Descriptions of the Parameters Used for the Generation of the C, H, N, Ti, and Ru Pseudopotentials cutoff radius (au)a rs

rp

rd

rf

C (2s 2p 3d 4f ) N (2s22p33d04f0) H (1s12p03d04f0)

1.56 1.48 1.33

1.56 1.48 1.33

1.56 1.48 0.37

1.56 1.48 1.33

no no no

no no no

Ti (4s24p03d24f0) Ru (5s15p04d74f0)

2.70 2.30

2.70 2.65

1.10 1.80

2.00 2.00

yes (0.9)b yes (1.9)b

yes yes

reference configuration 2

a

2

0

0

core correction

relativistic

Cutoff radii of s, p, d, and f channels. bCutoff radius of nonlinear core correction in au.

film nucleation and lead to smoother films. The PEALD process of Ru with two precursors, i.e. bis(ethylcyclopentadienyl)ruthenium (Ru(EtCp)2) and (methylcyclopentadienylpyrrolyl)ruthenium (Ru(MeCp)Py), was recently compared,21,22 and a markedly different reactivity was observed. Starting from a TiN surface, a major nucleation delay was observed for the Ru(EtCp)2 precursor in combination with H2/ N2 plasma, whereas better nucleation was observed with Ru(MeCp)Py. It was also observed that the precursors strongly influence the growth-per-cycle (GPC). Using Ru(MeCp)Py, a GPC of 0.038 nm/cycle was reported, whereas with Ru(EtCp)2, a much smaller GPC of 0.016 nm/cycle was observed. To the best of our knowledge, there is only a limited number of theoretical studies of ALD of Ru. Following our previous study of the dissociation energies of ruthenium precursors,23 this paper presents a study based on periodic boundary conditions density functional theory (PBC-DFT) to understand the difference in reactivity between the RuCp2 and RuCpPy precursors on the TiN substrates. The difference in computed adsorption energies may explain the difference in reactivity between the two precursors. The reaction mechanism of chemisorption of RuCpPy is investigated in detail. In order to support the theoretical results of the reaction on a stoichiometric TiN surface, we also performed experiments with only one ALD cycle on the TiN substrate. The Ru content chemisorbed after the first ALD cycle on different TiN substrates was measured. The experimental Ru content was compared with the theoretical results, thus supporting the calculated reaction mechanism.

Double-ζ numerical basis sets augmented with polarization functions26 were used. The basis sets were optimized with the PBE functional using the downhill simplex method proposed in the work of Anglada et al.27 The quality of the pseudopotentials and the basis sets was carefully assessed by regenerating the structural and energetic properties of various molecules computed with a full-electron potential (or ecp in Ru) and converged Gaussian basis sets. For the TiN surface, the basis sets of the top layer Ti and N atoms were augmented with diffuse functions to properly describe the long decay of the wave function on the surface.28 The radii of these diffuse functions were varied from 7.0 to 10.0 au and were chosen such that the total energy of the TiN surface was minimized. The N atoms were augmented with both 3s and 3p diffuse functions with cutoff radii of 9.0 and 8.5 au, respectively, whereas 4d diffuse functions with a cutoff radius of 9.0 au were used for the Ti atoms. To evaluate the electron density, a grid constructed by a plane wave with a cutoff energy of 350 Ry was used. Properties of the investigated systems were generated and analyzed using the partial density of states (DOS) and the electron density. For the geometry optimizations, we used the Broyden method with a force convergence threshold of 20 meV/Å. We used five images within the climbing image nudged elastic band (CI-NEB) formalism,29 which has been reported to give reliable minimum energy paths (MEPs) and saddle points.29,30 The images were relaxed until the maximum residual force reached was less than the threshold of 40 meV/Å. All stationary points of the MEP were checked by calculating the surface phonons. The surface phonons were obtained by making use of a finite difference approach, i.e., numerical calculations of the second derivative of the potential energy surface were carried out, using displacements of the atomic coordinates of 0.01 Å (harmonic level). To study the Gibbs free adsorption energy, we followed the procedure described in the work of Loffreda.31 The Gibbs free adsorption energy is calculated as

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EXPERIMENTAL AND THEORETICAL METHODS A theoretical investigation of the adsorption of RuCp2 and RuCpPy was performed using periodic boundary condition density functional theory (PBC-DFT) as implemented in the SIESTA code.24 The Perdew−Burke−Ernzerhof (PBE) functional and the van-der-Waals Dion−Rydberg−Schröeder− Langreth−Lundqvist functional (vdW-DF)25 were used, with the latter functional accounting for the presence of possible dispersion interactions that may strengthen the adsorption of the two complexes. With the vdW-DF, only single-point calculations at PBE geometries were performed. The core electrons of C, N, Ti, and Ru were replaced by Troullier and Martins type norm-conserving pseudopotentials, generated with the corresponding functionals. These pseudopotentials have the following reference configurations: 2s22p2 for C, 1s1 for H, 2s22p3 for N, 4s23d2 for Ti, and 4d75s1 for Ru. Core and relativistic corrections were included in the pseudopotentials of Ti and Ru. The cutoff radii of the pseudopotentials used are listed in Table 1.

ΔGads ≈

1 [ΔEads + ΔE0 + T (ΔSvib + ΔStrans,rot) − kT A

ln(P /P0)]

where A is the area of the (2 × 2) surface. ΔEads = Eads − Esur − Egas is the DFT adsorption energy, where Eads, Esur, and Egas are the energies of the adsorbed system (ads), the bare surface (sur), and the free gas molecule (gas), respectively. ΔE0 is the zero-point energy change ΔE0 = ZPEads − ZPEsur − ZPEgas. The contribution of the entropy to the Gibbs free energy contains both translational, rotational ΔStrans,rot and vibrational ΔSvib entropy changes. P is the experimental partial pressure of the precursor, P = 3.5 Torr. The detailed derivations of the 19443



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Table 2. Ru Content (atoms/nm2) after the First ALD Cycle Measured by Rutherford Backscattering Spectroscopy precursors Ru(EtCp)Py

starting surface stoichiometric TiN (∼50% stoichiometric TiN (∼50% stoichiometric TiN (∼50% stoichiometric TiN (∼50% Ti-rich TiN (∼5% N) Ti-rich TiN (∼15% N) Ti-rich TiN (∼15% N) Ti-rich TiN (∼15% N)

surface pretreatment

Ru content (atoms/nm2) ± 0.4

steady GPC (atoms/nm2)

N) N) N) N)

no (in situ) N2/H2 plasma (in situ) air exposure air exposure +873 K anneal N2 no (in situ) no (in situ) N2/H2 plasma (in situ) air exposure

4.2 4.0 3.0 1.7 1.9 2.0 2.3 1.4

2.0

Ru(MeCp)Py

stoichiometric TiN (∼50% N)

air exposure +873 K anneal N2

1.6

2.5

Ru(EtCp)2

stoichiometric TiN (∼50% N)

air exposure +873 K anneal N2

0.1

1.3

± 0.2 ± 0.1 ± 0.3

for Ti-rich TiN substrates, the measured Ru content was only 2 Ru/nm2, close to the steady growth-per-cycle value. A N2/H2 plasma treatment, similar to that used during PEALD of Ru, did not affect the Ru content. Next, the adsorption of Ru(EtCp)Py, Ru(MeCp)Py, and Ru(EtCp)2 was compared on ex situ annealed TiN surfaces. In agreement with our previous results, a much lower reactivity was found for the Ru(EtCp)2 precursor as compared to that for Ru(EtCp)Py and Ru(MeCp)Py. Almost no chemisorption occurred for Ru(EtCp)2: the Ru content was only 0.1 Ru/nm2. On the other hand, the Ru content was 1.6 and 1.7 Ru/nm2 for the Ru(EtCp)Py and Ru(MeCp)Py precursors, respectively, indicating only a minor inhibition as compared to the bulk growth-per-cycle values of 2.0 and 2.5 Ru/nm 2 . The substituents on the Cp ligand (Et versus Me) had only a small impact on the Ru content on the TiN surfaces.

expression of the Gibbs free adsorption energy can be found in ref 31. In all calculations, the counterpoise correction32,33 was used to account for basis set superposition errors (BSSEs) on the adsorption energy. For comparison with the theoretical calculations, we performed an experimental study of the Ru content after chemisorption of different Ru precursors on various TiN surfaces. Physical vapor deposition (PVD) at 573 K was used to deposit 20 nm TiN films on 300 mm Si substrates. The composition of the TiN was verified by X-ray photoelectron spectroscopy (XPS) depth profiling. The composition of TiN was varied from stoichiometric (50% N) to Ti-rich (15 or 5% N) by reducing the N2 flow in the PVD process. After TiN deposition, the TiN films were exposed to the different Ru precursors in a 300 mm PEALD reactor attached to the same platform as the PVD reactor. The Ru(EtCp)Py, Ru(MeCp)Py, and Ru(EtCp)2 precursors were contained in cross-flow type ampules heated to temperatures between 351 and 358 K in order to reach sufficient vapor pressure for saturated chemisorption reactions.34 N2 was used as carrier gas. The pressure in the chamber during the Ru precursor exposure was 3.5 Torr, and the wafer temperature was 603 K. Air exposure between the TiN deposition and the Ru precursor chemisorption was avoided. On a few samples, the effect of air exposure between the TiN deposition and the Ru chemisorption reaction was investigated, as well as the effect of ex situ anneals (N2, 873 K) and in situ plasma treatments (N2/H2 plasma, 603 K). The Ru content after the Ru precursor exposure was determined by Rutherford backscattering spectroscopy (RBS).

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THEORETICAL RESULTS Test Calculations on Titanium Nitride and Ruthenium Complexes. We first examined the accuracy of the generated pseudopotentials and basis sets. For Ti and N, the structural and electronic properties of TiN, e.g., the lattice constant, bulk modulus, band structure, and the density of states (DOS), were studied. With numerical basis sets and the PBE functional, the calculated lattice constant (4.240 Å) is in excellent agreement with the experimental value (4.238 Å35). The calculated bulk modulus (304 GPa) is 5.6% higher than the experimental value (288 GPa36). The errors are acceptable and comparable to plane wave calculations. More detailed results can be found in the Supporting Information. A test calculation of the TiN(100) surface was also done, and the obtained results were compared with a previous study of Marlo and Milman.37 The TiN(100) surface was chosen because this is the main surface occurring in the ALD experiments and because this surface is significantly more stable than other important surfaces (e.g., (110), (111)), as shown in the work of Marlo and Milman.37 From the calculated structure of TiN bulk, a four-layer slab of the TiN(100)-(2 × 2) surface was prepared (Figure 1). The simulation box is big enough to ensure that (i) the distance between the adsorbed RuCp2 (or RuCpPy) molecules is about 5 Å and (ii) the vacuum width between slabs is 15 Å to reduce the artificial electric field across the slab. A Monkhorst−Pack k-point mesh38 of 5 × 5 × 1 was chosen to yield converged results for this metallic surface. The results of the relaxation of the surface can be found in the Supporting Information. We just notice that the computationally cheaper four-layer model is adequate

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EXPERIMENTAL RESULTS The results of the RBS experiments are shown in Table 2. We started by investigating the Ru(EtCp)Py adsorption on an in situ prepared TiN surface, thus avoiding oxidation of the TiN top surface which usually occurs during air exposure. After Ru(EtCp)Py chemisorption on a stoichiometric TiN surface (50% N), the Ru content was found to be 4.2 ± 0.4 Ru atoms/ nm2, indicating a significant growth enhancement as compared to the steady growth-per-cycle of 2.0 Ru/nm2 (Table 2). The Ru content was independent of the Ru(EtCp)Py exposure time (0.3−9.5 s), confirming that the surface reaction was saturated. The Ru content was affected by air exposure of TiN as well as by annealing, most likely because of oxidation of the TiN top surface. Air exposure and annealing of the TiN films reduced the Ru content to values of 3.0 and 1.7 Ru/nm2, respectively. The composition of TiN also had an impact on the Ru content: 19444



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Figure 1. (a) Schematic illustration of the TiN slab used in this work. (b) Structural relaxation on the TiN(100) surface. T denotes the plane width and D corresponds to the distance between the atomic planes.

to capture the structural changes associated with the relaxation process. Our results are also consistent with those based on plane wave basis sets reported in the literature.37 To test the quality of the basis sets for Ru, C, N, and H, we studied the structure, the homolytic and heterolytic dissociation energy of two ruthenium complexes, ruthenocene (RuCp2) and cyclopentadienyl pyrrolyl ruthenium (RuCpPy) (see the Supporting Information). Our results are in good agreement with results obtained from our previous study23 using converged Gaussian basis sets and high-quality wave functionbased methods. Adsorption of RuCp2 and RuCpPy on the Titanium Nitride (100) Surface: The Adsorption Step. In order to find possible adsorption configurations of the RuCp2 and RuCpPy precursors on the TiN(100) surface, we started with a series of constrained geometry optimizations in which the distance of the precursors with respect to the surface was fixed. The Ru center of the precursors was initially located at four positions, either on top of a Ti or a N atom or on bridge sites between Ti−Ti or N−N. From these calculations, several energetically favorable adsorption sites and precursor orientations were identified. Finally, the precursor−surface system was further optimized without any constraints, starting from the most favorable adsorption configurations. For RuCp2, we found that there is no chemisorption on the surface. There is a strong repulsive force that drives RuCp2 away from the surface when constraining it too close. The adsorption distance (i.e., the Ru−surface distance) varies from 4.3 to 5.0 Å, depending on the adsorption location and orientation. The adsorption energy is on the order of only −1 kcal/mol. The large adsorption distance and negligible adsorption energy indicate that only a physisorption process can occur. This is also reflected by the electronic properties of the system, consisting of just a superposition of the DOS of RuCp2 and TiN. With the vdW-DF, inclusion of dispersion interaction corrections leads to a more negative adsorption energy as compared to the result obtained with PBE (about −8 to −10 kcal/mol). For RuCpPy, we found two possible adsorption configurations (configurations A1 and A2 in Figure 2), with a very similar adsorption energy. RuCpPy binds through the N atom of the pyrrolyl group (NPy), with a NPy−Ti distance of about 2.26 Å for both configurations. The adsorption energies

Figure 2. Top and side views of RuCpPy during reaction on the TiN surface. The reaction pathway is shown in Figure 6. The distances are indicated in Å.

computed with PBE are −10.4 kcal/mol (configuration A1) and −10.0 kcal/mol (configuration A2). Again, the use of the vdW-DF yields a more negative value for the adsorption energy (−22.4 and −22.0 kcal/mol for configurations A1 and A2, respectively). Even though there is a large difference in the adsorption energy predicted by the two functionals, both agree that the adsorption of RuCpPy is a chemisorption process resulting from the interaction between the NPy atom and the Ti atom of the surface (Tis), while RuCp2 is only weakly physisorbed. The chemisorption of RuCpPy on TiN results in a weakening of the Ru−Cp and Ru−Py interactions, as indicated by the changes of bond lengths (Table 3). We define the distance between Ru and the center of the Cp and Py rings as Table 3. Bond Lengths (Å) of RuCpPy during Reaction Computed with the PBE Functional configuration

19445

distance

free molecule

A1

A2

IM1

IM2

Ru−NPy Ru−Py Ru−Cp Ru−Ns NPy−Tis

2.219 1.842 1.832 − −

2.269 1.859 1.847 3.897 2.266

2.300 1.864 1.845 4.105 2.261

2.202 − 1.876 2.507 2.193

5.274 − 1.918 1.937 2.126



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the Fermi energy (Figure 3d), the DOS is populated with several electronic levels corresponding to the interaction between the 2pz orbital of NPy and the 3d orbitals of Ti. No major change in the three highest occupied levels is found (Figure 3c). The DOS of configuration A2 shows a similar pattern. The bonding between RuCpPy and the surface can also be understood by analyzing the differences in the electron density before and after adsorption (Figure 4). It is clear that the

the Ru−Cp and Ru−Py distances, respectively. The Ru−Py distance in the adsorbed molecule is about 1.86 Å, which is 0.02 Å longer than that in the isolated molecule (1.84 Å), while the Ru−Cp distance increases by 0.015 Å (from 1.832 Å to 1.847 Å). The Ru−NPy distance of the adsorbed RuCpPy is about 2.3 Å, i.e., 0.05 Å longer than that in the isolated RuCpPy (2.219 Å). The C−C and C−H bond lengths are hardly affected by the RuCpPy adsorption. We also note that in this configuration there is no Ru−surface bond yet, as indicated by a large Ru−Ns distance of about 4 Å and Ru−Tis distances of about 4.5 Å. An insight into the bonding between the NPy atom and the Ti atom of the surface can be obtained by analyzing the DOS of configuration A1. The energy levels of the isolated and the projected DOS (PDOS) of the chemisorbed RuCpPy are shown in Figure 3. The broadening of the DOS in the energy range of −7 to −3 eV (Figure 3b) is a consequence of the interaction between the 2p orbitals of the NPy atom and the pz orbitals of the Ti and N atoms of the surface. Near and below

Figure 4. Change of the electron density computed for the adsorbed RuCpPy in A1 and IM1. Electron density increase is shown in red; electron density decrease is shown in blue.

electron density increases in the region between NPy and Ti, while the electron density around the NPy atom decreases. This further illustrates that the main interaction contributing to the bonding between RuCpPy and the surface indeed occurs between the 2p orbital of NPy and 3d orbitals of Ti. Next, we studied the impact of the reaction conditions (temperature and pressure) on the adsorption of both RuCp2 and RuCpPy. In this study, we considered only a surface coverage with a RuCpPy/Tis ratio of 1/8. The experimental pressure is 3.5 Torr. The temperature dependence of the Gibbs free adsorption energy of RuCpPy and its contributions are shown in Figure 5. The change of translational and rotational entropies is positive and significant. They are responsible for the desorption of the molecules at high temperature. On the

Figure 3. Energy levels of isolated (red) and projected DOS of adsorbed RuCpPy (black) in A1 and IM1 computed with PBE. 19446



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ature increases. With the PBE functional, the computed adsorption energy of RuCpPy is −10.4 kcal/mol. One can see from Figure 5 that the PBE adsorption energy is too small and that RuCpPy can easily desorb at room temperature. With the vdW-DF and the corresponding adsorption energy of −22.6 kcal/mol, we expect that the molecule remains chemisorbed on the surface up to 670 K, i.e., higher than the ALD temperature of 600 K. In contrast, at such a high reaction temperature and with an adsorption energy of only about −10 kcal/mol (predicted with vdW-DF), RuCp2 can not remain adsorbed. ALD of RuCpPy on the Titanium Nitride (100) Surface: The Further Reactions. Once it is anchored on the surface, RuCpPy can react further (Figure 6). Interaction of the Ru atom with the surface further decreases the energy. A second configuration of RuCpPy, intermediate 1 (IM1), is displayed in Figure 2. In this configuration, the Ru−Ns distance is 2.057 Å, which is on the same order of magnitude as Ru−N distances reported in numerous ruthenium complexes, e.g., 2.05−2.06 Å in ruthenium(II) porphyrin complexes.39 At the same time, the bonding between Ru and Py is dramatically weakened; the Ru− Py interaction has changed from η5 with Py to η1 coordination with NPy. As compared to the distance found in the first adsorption configurations A1 and A2, the NPy−Ti distance is further reduced by 0.07 Å, indicating that the NPy−Ti bond becomes stronger. The Ru−Cp distance is 1.876 Å, slightly elongated (by 0.03 Å) as compared to that of the first adsorption configuration A1 (1.847 Å). The bonding between Ru and Cp is thus also further weakened. Because of the strong interaction between Ru and the surface, the adsorption energy of RuCpPy is strongly decreased (more negative). PBE predicts that the IM1 configuration is about 18.0 kcal/mol more stable than the first adsorption complex. The vdW-DF yields a slightly higher value (21.9 kcal/ mol) because the projection area of the IM1 configuration increases, which leads to an enhanced adsorbate−surface van der Waals interaction. In previous studies,37,40 it was found that various atoms and molecules, such as H, Au, and H2, are favorably adsorbed on the Ti atoms of a TiN surface. It therefore seems surprising that the

Figure 5. Evolution of the Gibbs free adsorption energy diagram (kcal mol−1 Å−2) of RuCpPy computed at constant pressure (3.5 Torr) as a function of temperature (K). The contribution of the zero-point energy change ΔE0 (not shown in the figure) for both RuCp2 and RuCpPy is negligible (less than 0.01 kcal mol−1 Å−2) because the geometry of the adsorbed molecule does not change significantly compared to that of the free molecule.

other hand, the vibrational entropy contribution is negative and its contribution favors the adsorption at high temperature. We observe that when a molecule is adsorbed, a set of low vibrational frequency modes appears between the molecule and the substrate. These frequencies strongly increase the vibrational partition function of the molecule−surface system, hence they increase the entropy of the molecule−surface system as compared to that of the isolated molecule plus surface. Thus, the vibrational entropy change TΔEvib is always negative and decreases when the temperature increases. Because the translational, rotational entropy and the pressure contributions are larger than the contribution from the vibrational entropy, the Gibbs free adsorption energy increases when the temper-

Figure 6. Reaction pathway computed for the dissociation of RuCpPy on the surface at the PBE (black line) and vdW-DF (red, dashed line) levels. The energies are expressed in kcal/mol. 19447



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investigated. The first option is that a H atom could be transferred from one of the other C of Py, or from Cp. Such a transfer is not thermally favored because it destroys the aromaticity of Py or Cp. For example, to detach one H of Cp and transfer it to NPy, an energy of about 50.3 kcal/mol is needed. The corresponding value for the H transfer within Py is about 58.8 kcal/mol. Therefore, these reactions are thermally prohibited. In the second scenario, two pyrrolyl radicals might form a stable dimer (C5H5)2 on the surface. Two isomers were considered (Figure 7). The first one adopts a bond between

Ru atom of RuCpPy is situated on top of the surface Ns atom instead of the Tis atom. This can be explained by analyzing the change of DOS and electron density. There is a major change in the DOS of the adsorbed molecule in the energy range from 0 to −7 eV (Figure 3), corresponding to the interaction between d orbitals of the Ru atom and d orbitals of Tis atoms. The electron density difference plot of configuration IM1 in Figure 4 also indicates a significant interaction between the 4d orbitals of Ru and the 3d orbitals of the three top-layer Tis atoms next to the Ns atom, whereas we find only a small interaction between the Ru atom and Ns. The interaction between NPy and the surface in the IM1 configuration is similar to the interaction in the first adsorption configuration A1. As can also be seen from Figure 3, the DOS is unaltered in the energy range from −8 to −10 eV, corresponding to the orbitals that localize on Cp and Py. These orbitals negligibly interact with the surface and hence have little contribution to the bonding between RuCpPy and the surface. In conclusion, the strong bond between RuCpPy and the TiN surface arises from the interaction between a 2p orbital of NPy and one of the Tis atoms and between the 4d orbitals of Ru and the 3d orbitals of three Tis atoms, in agreement with literature findings for other adsorption reactions on the TiN surface.37,40 The NPy−Tis interaction contributes about −10.4 kcal/mol with PBE or −22.4 kcal/mol with vdW-DF, whereas the Ru−Tis interaction contributes about −18.0 kcal/mol with PBE or −21.9 kcal/mol with vdWDF (Figure 6). The transition state (TS1) between the A1 and IM1 configurations was also identified (Figure 6). The activation energy is 26.5 kcal/mol calculated with the PBE functional. With the vdW-DF, the barrier is smaller by about 10 kcal/mol. This barrier reduction may be explained by the fact that the energy stabilization in IM1 compared to that in A1 is more pronounced with the vdW-DF than with the PBE functional. This kind of relationship between activation energies and reaction energies is well-described in the literature.41−43 The vdW-DF even predicts that the energy required to cross the barrier (16.4 kcal/mol) is smaller than that for desorption (22.4 kcal/mol). Because the activation energy is limited, it is possible that at the ALD temperature of 600 K the formation of the IM1 configuration can occur. In this configuration, the bond between Ru and pyrrolyl is already significantly weakened. In a next configuration, configuration IM2, the pyrrolyl fragment has dissociated from RuCp and has migrated to a neighboring Tis atom. The IM2 configuration is about 1−2 kcal/mol more stable than IM1. The energy barrier to be crossed to reach IM2 (TS2) is similar to that for TS1: 24.4 kcal/mol with the PBE functional and 17.5 kcal/mol with the vdW-DF. Both the Tis− NPy and the Ru−Tis bond distances are further reduced in this final reaction product. ALD of RuCpPy on the Titanium Nitride (100) Surface: The Desorption of Py, Cp, and RuCp. Desorption Mechanism of Py. In the final reaction product, there is still a strong interaction between the 2p orbitals of NPy and the 3d orbitals of the Ti atom on the surface. The desorption energy of the Py radical is high, i.e., 57.3 kcal/mol with PBE and 62.5 kcal/mol with vdW-DF (Figure 8), indicating that direct desorption of Py is difficult. To break the strong NPy−Ti bond, the NPy atom must bind to other atoms, such as H, N, etc., with an energy gain on the order of the desorption energy of Py. The remaining question then consists of identifying possible mechanisms to break the NPy−Ti bond. Several options were

Figure 7. Reaction energy between two pyrrolyl radicals to form a dimer at the PBE level.

two N atoms. Because an energy of 50.4 kcal/mol is needed to form this dimer from two radicals, this reaction is also thermally unfavored. The formation of a second isomer requires an energy of at least 22 kcal/mol. However, we believe that this reaction is also unfavored because the two radicals must first undergo rotation to form a suitable configuration to react and then H atoms would have to migrate from C to N. A third scenario presumes that H atoms that may react with the pyrrolyl could be present on the surface during the adsorption step. The adsorption of H atoms on the TiN (100) surface was previously studied by Marlo and Milman37 using RPBE and PW91 functionals with plane wave basis sets. They suggested that H adsorption occurs at Ti sites (global minimum). We found that the adsorption energy of H on Ti is very negative: −43.3 kcal/mol with PBE and −53.0 kcal/mol with vdW-DF. H could also be adsorbed on N atoms (local minimum) with an adsorption energy of −36.3 kcal/mol (PBE) and −36.7 kcal/mol (vdW-DF). It was also shown that after adsorption, H can easily diffuse on the surface.37 With the RPBE functional, the diffusion barrier between two Ti sites was found to be 16.8 kcal/mol while the diffusion barrier between Ti and N is slightly larger (17.5 kcal/mol).37 We obtain similar results with the PBE functional. The PBE activation energy for diffusion of H between Ti and N is 19.5 kcal/mol. Because the vdW-DF predicts a higher relative energy between the global 19448



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Figure 8. Reaction mechanism between pyrrolyl and hydrogen on TiN(100) computed at PBE level. Numbers in parentheses are computed with the vdW-DF. The energies are expressed in kcal/mol.

Figure 9. Geometry of adsorbed Ru(C5H5+n) molecules where n is the number of H atoms attached to the Cp ring.

mol with vdW-DF. The generated pyrrole would then be physisorbed on the surface with a NPy−Ti distance of 2.716 Å. The resulting adsorption energy is small, only −3.3 and −9.9 kcal/mol as predicted by PBE and vdW-DF, respectively. Hence, pyrrole can easily desorb because of its weak adsorption energy and low partial pressure, leaving a free surface site for other RuCpPy molecules. Moreover, pyrrole can not readsorb

minimum on top of Ti and the local minimum on top of N, we also calculate a higher diffusion barrier (26 kcal/mol). The mechanism of the reaction between Py and H atoms is described in Figure 8. At 600 K, H can easily diffuse on the surface and bind with NPy atoms of adsorbed Py molecules to form adsorbed pyrrole C5H5N. The reaction is slightly exothermic, by about 5 kcal/mol. The activation energy for this reaction is very small: 5.0 kcal/mol with PBE and 8.5 kcal/ 19449



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Table 4. Dissociation Energy of C5H5+n (E1) and Ru(C5H5+n) (E2) Computed with PBE and vdW-DF n PBE dRu−Ns (Å) dRu−C (Å) E1 (kcal/mol)a E2 (kcal/mol)b ΔE (kcal/mol)c vdW-DFd E1 (kcal/mol)a E2 (kcal/mol)b ΔE (kcal/mol)c

0(a)

1(b)

2

3

4

5

1.927

1.948

1.957

1.981

2.025

1.962

2.254−2.277 93.8 95.3 −

2.181−2.667 51.5 78.8 −42.4

2.151−2.858 60.8 78.2 −59.0

2.177−3.253 40.3 79.9 −62.3

2.108−3.790 47.8 78.4 −47.7

2.416−3.425 12.2 90.4 −61.4

85.4 94.6 −

44.8 78.1 −46.0

56.4 76.7 −64.2

38.5 78.2 −68.1

50.7 78.1 −54.7

7.5 88.8 −57.0

Dissociation energy of C5H5+n. bDissociation energy of Ru(C5H5+n). cΔE = EC5H5+n − EC5H4+n − EH. dSingle-point calculations at PBE optimized geometry. a

we consider only reactions of H atoms and focus on the stationary points on the potential energy surface. In Table 4, we summarize the results obtained with PBE and vdW-DF for the dissociation energies of C5H5+n and Ru(C5H5+n). The reaction process is rather simple. From the gas phase, highly reactive H atoms can adsorb at the surface or react with the C atoms of Cp. When H atoms adsorb at the surface, they can diffuse between adsorption centers and react either with each other to form adsorbed H237 or bind with Ru atoms. As discussed above, H can bind at a Ru center, giving rise to an insignificantly small reduction of the Ru−Cp bond strength. When H atoms bind with C of Cp, they gradually weaken the bond between Cp and Ru (Table 4). These reactions are barrierless and exothermic. Indeed, as can be seen from Table 4, the value ΔE = EC5H5+n − EC5H4+n − EH is always negative. When the first H atom binds with Cp, it destroys the aromaticity of the ring and strongly weakens both the Ru−Cp and Ru−surface bonds, reducing the binding energy between Ru and Cp (E1). Similarly, the binding energy between the Ru and the surface (E2) decreases by 16.5 kcal/mol even though the Ru−Ns distance increases by only 0.12 Å. When additional H atoms bind with Cp, the Ru−C distances increase and the binding energy of Ru−Cp further decreases, while the binding energy of the Ru−surface bond remains quasi-unaltered. When the fifth H atom is added, the Ru−C bonds are totally broken and C5H10 forms a complex with Ru by means of two weak Ru−H−C bonds. The Ru−H distance is about 1.88 Å, which is typical of numerous ruthenium−alkane complexes. The binding energy between Ru and C5H10 is 12.2 kcal/mol, implying that the ring is ready to be dissociated at the high-temperature conditions of ALD. This trend is confirmed by the results obtained with the vdW-DF.

because of the competitive adsorption of other RuCpPy molecules. Desorption of RuCp and Cp. The possibility that either RuCp or Cp could dissociate again from the surface already during the first ALD step was also investigated using the PBE functional. The Ru−Cp bond strength decreases only slightly upon reaction with the TiN surface. In configuration IM2, the Cp ring is parallel to the surface with a Ru−Cp distance of about 1.905 Å, which is slightly larger than the corresponding values of 1.832 Å for isolated RuCpPy, 1.847 Å in the A1 configuration, and 1.876 Å in the IM1 configuration. The PBE dissociation energy of the Cp radical on the surface (E1 in Figure 9.0(a)) is 93.8 kcal/mol, slightly lower than the homolytic dissociation energy of Cp from RuPy in isolated RuCpPy (126.7 kcal/mol). However, even though the surface facilitates the dissociation of the Cp radical, the dissociation energy is still high, indicating that Cp can not dissociate from Ru. The low-spin (doublet) RuCp radical will also not dissociate from the surface because an energy of 95.3 kcal/ mol (E2 in Figure 9.0(a)) is needed to break the strong Ru− surface bond. This indicates that the RuCp fragment is rather stable on the surface. We also considered the possibility that H atoms might facilitate the dissociation of Cp and RuCp radicals. Adsorbed H can bind with Ru (see Figure 9.1(a)) and weaken the Ru−Cp bond, but not significantly. The dissociation energy of the Cp radical (E1) decreases by only 2.5 kcal/mol, from 93.8 to 91.3 kcal/mol, as compared to that of adsorbed RuCp (Table 4). With an extra Tis−H−Ru bond, the dissociation of the RuCp radical becomes even more difficult. The binding energy of RuCp (E2) increases by more than 65 kcal/mol, going from 94 to 159 kcal/mol. On the other hand, the energy required to dissociate RuCp−H (E3) is smaller (83.3 kcal/mol). However, this dissociation energy remains very high. In conclusion, regardless of H atoms that might be present in the gas phase or on the surface, it is not possible to dissociate Cp or RuCp radicals. We therefore expect that RuCp remains chemisorbed on the surface after the adsorption step. ALD of RuCpPy on the Titanium Nitride (100) Surface: The Plasma Step. After the adsorption step, a nonreactive gas flow (e.g., N2) is applied to remove both the excess of precursor molecules and the reaction products remaining in the gas phase in the reactor. After that, a plasma of hydrogen composed of H atoms, H+ ions, and H2 as well as excited-state species is applied. Modeling the impact of this plasma is not a straightforward task, and simulating all possible events requires molecular dynamics techniques. To simplify the calculations,

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DISCUSSION

The DFT calculations described above provide insight in the adsorption processes of the RuCpPy and RuCp2 precursors on TiN surfaces. First, the calculations confirm the experimentally observed higher reactivity of the RuCpPy as compared to the RuCp2 precursor. RuCpPy can chemisorb on the TiN surface according to the reaction mechanism described in Figure 6. This can explain the good nucleation of Ru PEALD with RuCpPy precursors, as indicated by our current and previous experiments.22 The calculations also demonstrate that the RuCp2 precursor only physisorbs on the TiN(100) surface, as indicated by a large adsorption distance between RuCp2 and the TiN surface, a small adsorption energy, and little change in 19450



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the interaction between the Ru atom and the TiN substrate. The computed Ru−surface dissociation energies remain large, suggesting that the Ru atoms remain on the surface after the plasma treatment step.

the electron density during adsorption. This explains the high incubation time of Ru PEALD on TiN surfaces when using RuCp2.22 An island growth process of Ru could occur, starting from nucleation on defect sites at the TiN surface. Such processes are, however, outside the scope of the current theoretical investigation. The RuCpPy chemisorption reaction shown in Figure 6 starts with the formation of an initial adsorption complex (A1 and A2, see Figure 2). In this initial adsorption complex, there is no direct Ru−surface bonding yet, but the precursor binds through the NPy atom of the pyrrolyl ligand with the Tis atoms of the TiN surface. The bond strength between the Ru atom and the Cp or Py ligands is weakened in this initial complex as compared to the free precursor, enhancing the possibility of dissociation. Indeed, after the adsorption step, RuCpPy can react further on the TiN surface, resulting in the formation of a strong chemical bond between the Ru atom and three Ti atoms on the TiN surface (IM1) and finally in the dissociation of the precursor on the TiN surface into Py and RuCp fragments (IM2). Compared to that of the adsorption complex, the energies of IM1 and IM2 are strongly reduced, indicating that their formation is thermodynamically favorable and the calculated energy barriers are moderate. A comparison of the experimental and calculated Ru content can be made. On the basis of the van der Waals radius of all atoms involved,44 we can compute the projection area of the adsorbed RuCpPy species on the surface.45 From this, the Ru content on the TiN surface after the subsequent adsorption steps may be estimated. The calculation provides an upper limit for the Ru content, as it assumes a 100% efficient packing and no alkyl substituents on the Cp ring. The upper limits of the Ru contents corresponding to A1, IM1, and IM2 are 2.5, 2.1, and 1.9 Ru atoms/nm2, respectively. These values are in reasonable agreement with the experimental Ru content measured on Tirich TiN surfaces, which are 1.9−2.3 Ru/nm2 (Table 2). However, on the stoichiometric TiN surface, a much higher Ru content was experimentally observed: the Ru(EtCp)Py growth-per-cycle was enhanced by the stoichiometric TiN surface, resulting in a Ru content of 4.2 ± 0.4 Ru atoms/nm2 (Table 2). This high Ru content seems to suggest that further reaction of the surface species IM2 occurs, resulting in the desorption of the pyrrolyl ligands and leaving only RuCp fragments on the surface. Reaction of the surface with extra RuCpPy precursor molecules could then give rise to a higher Ru content. In its most energetically favorable configuration, the Cp ring is bonded in an η5 coordination to Ru, with the Cp ring oriented parallel to the surface. The maximum Ru content for this configuration corresponds to 3.3 Ru/nm2. A higher Ru content of 5.7 Ru/nm2 could be reached by rotation of the Cp rings in an η2 coordination to Ru (Figure 9.0(b)). Such a rotation is quite energy demanding (51.0 kcal/mol at the PBE level). However, this energy requirement might be compensated for by the formation of extra Ru−surface bonds, and the experimental Ru content of 4.2 ± 0.4 Ru atoms/nm2 for stoichiometric TiN suggests that at least part of the Cp are η2 coordinated to Ru. The calculations also indicate that desorption of pyrrole is favorable only in the presence of H atoms on the surface. Finally, preliminary calculations of the plasma step were also considered. We suggest that the Cp ring can dissociate from Ru in the plasma step by means of reaction with H atoms present in the plasma. Adding H atoms to the Cp ring gradually reduces the bond strength between Cp and Ru, with a weak impact on

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CONCLUSIONS Using density functional theory with the PBE functional and vdW-DF, we have performed a comparative study of the adsorption energies of two ruthenium precursors, RuCp2 and RuCpPy, on the TiN(100) surface. RuCpPy can be chemisorbed on the surface with an adsorption energy of about −10 kcal/mol (PBE) and −22 kcal/mol (vdW-DF), while RuCp2 is only physisorbed with a weak adsorption energy (−10 kcal/mol with vdW-DF). This explains the experimentally observed high incubation rate of Ru(EtCp)2 as compared to that of Ru(EtCp)Py and Ru(MeCp)Py. The chemisorption reaction mechanism of RuCpPy on the TiN surface is calculated (Figure 6). RuCpPy first forms an initial complex on the surface (A1 and A2) in which the NPy atom of the pyrrolyl binds to a Ti atom of the surface. Only during the further reaction do bonds between the Ru atom of the precursor and three Ti atoms of the TiN surface form, resulting in a more stable configuration (IM1). The bond between Ru and the pyrrolyl ligand is weakened, and finally, the precursor dissociates in pyrrolyl and RuCp fragments at the TiN surface (IM2). The calculated projection area of these RuCpPy surface species is in good agreement with the experimentally observed Ru content on Ti-rich TiN surfaces (1.9−2.3 Ru/nm2). However, it cannot explain the growth enhancement to 4.2 ± 0.4 Ru atoms/nm2 on stoichiometric TiN surfaces. The large Ru content observed on stoichiometric TiN implies the desorption of pyrrole, which is possible only in the presence of H atoms on the TiN surface. Once formed, pyrrole can easily desorb from the TiN surface, providing extra adsorption centers for other RuCpPy molecules. The reactions could continue until the surface is covered with RuCp, allowing a surface coverage varying between 3.3 and 5.7 Ru/nm2, which is closer to the experimental value. The impact of the hydrogen plasma was investigated by considering the reaction of the Py and RuCp surface species with H atoms. We showed that hydrogen atoms can destroy the aromaticity of the Cp ring, favoring desorption of the Cp ring from Ru.

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ASSOCIATED CONTENT

S Supporting Information *

Test calculations of TiN and ruthenium complexes. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This investigation has been supported by grants from the Flemish Science Foundation (FWO). 19451



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Article

Enhanced Atomic Layer Deposition. Electrochem. Solid-State Lett. 2012, 1, P19−P21. (23) Phung, Q. M.; Vancoillie, S.; Delabie, A.; Pourtois, G.; Pierloot, K. Ruthenocene and Cyclopentadienyl Pyrrolyl Ruthenium as Precursors for Ruthenium Atomic Layer Deposition: A Comparative Study of Dissociation Enthalpies. Theor. Chem. Acc. 2012, 131, 1238. (24) Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordejón, P.; Sánchez-Portal, D. The SIESTA Method for Ab Initio Order-N Materials Simulation. J. Phys.: Condens. Matter 2002, 14, 2745−2779. (25) Dion, M.; Rydberg, H.; Schröder, E.; Langreth, D. C.; Lundqvist, B. I. Van der Waals Density Functional for General Geometries. Phys. Rev. Lett. 2004, 92, 246401. (26) Junquera, J.; Paz, O.; Sánchez-Portal, D.; Artacho, E. Numerical Atomic Orbitals for Linear-Scaling Calculations. Phys. Rev. B 2001, 64, 235111. (27) Anglada, E.; Soler, J. M.; Junquera, J.; Artacho, E. Systematic Generation of Finite-Range Atomic Basis Sets for Linear-Scaling Calculations. Phys. Rev. B 2002, 66, 205101. (28) García-Gil, S.; García, A.; Lorente, N.; Ordejón, P. Optimal Strictly Localized Basis Sets for Noble Metal Surfaces. Phys. Rev. B 2009, 79, 075441. (29) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904. (30) Klimeš, J.; Bowler, D. R.; Michaelides, A. A Critical Assessment of Theoretical Methods for Finding Reaction Pathways and Transition States of Surface Processes. J. Phys.: Condens. Matter 2010, 22, 074203. (31) Loffreda, D. Theoretical Insight of Adsorption Thermodynamics of Multifunctional Molecules on Metal Surfaces. Surf. Sci. 2006, 600, 2103−2112. (32) Boys, S.; Bernardi, F. The Calculation of Small Molecular Interactions by the Differences of Separate Total Energies. Some Procedures with Reduced Errors. Mol. Phys. 1970, 19, 553−566. (33) Simon, S.; Duran, M.; Dannenberg, J. J. How Does Basis Set Superposition Error Change the Potential Surfaces for HydrogenBonded Dimers? J. Chem. Phys. 1996, 105, 11024−11031. (34) Swerts, J.; Armini, S.; Carbonell, L.; Delabie, A.; Franquet, A.; Mertens, S.; Popovici, M.; Schaekers, M.; Witters, T.; Tökei, Z.; et al. Scalability of Plasma Enhanced Atomic Layer Deposited Ruthenium Films for Interconnect Applications. J. Vac. Sci. Technol., A 2012, 30, 01A103. (35) Schönberg, N. An X-Ray Investigation on Ternary Phases in the Ta-Me-N Systems (Me = Ti, Cr, Mn, Fe, Co, Ni). Acta Chem. Scand. 1954, 8, 213. (36) Gubanov, V. A.; Ivanovsky, A. L.; Zhukov, V. P. Electronic Structure of Refractory Carbides and Nitrides; Cambridge University Press: Cambridge, U.K., 1994. (37) Marlo, M.; Milman, V. Density-Functional Study of Bulk and Surface Properties of Titanium Nitride Using Different ExchangeCorrelation Functionals. Phys. Rev. B 2000, 62, 2899−2907. (38) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (39) Ariel, S.; Dolphin, D.; Domazetis, G.; James, B.; Leung, T.; Rettig, S.; Trotter, J.; Williams, G. Preparation and Characterization of Some Ruthenium(II) Porphyrins Containing Tertiary Phosphine Axial Ligands, Including the Crystal-Structure of (Octaethylporphinato)bis(triphenylphosphine)ruthenium(II). Can. J. Chem. 1984, 62, 755−762. (40) Hernández, N.; Sanz, J. A First Principles Density Functional Study of Au Deposition on TiN (001) Surface. Int. J. Mol. Sci. 2001, 2, 263−270. (41) Michaelides, A.; Liu, Z. P.; Zhang, C. J.; Alavi, A.; King, D. A.; Hu, P. Identification of General Linear Relationships between Activation Energies and Enthalpy Changes for Dissociation Reactions at Surfaces. J. Am. Chem. Soc. 2003, 125, 3704−3705. (42) Bligaard, T.; Nørskov, J.; Dahl, S.; Matthiesen, J.; Christensen, C.; Sehested, J. The Brønsted−Evans−Polanyi Relation and the Volcano Curve in Heterogeneous Catalysis. J. Catal. 2004, 224, 206− 217.

REFERENCES

(1) George, S. M. Atomic Layer Deposition: An Overview. Chem. Rev. 2010, 110, 111−131. (2) George, S. M.; Ott, A. W.; Klaus, J. W. Surface Chemistry for Atomic Layer Growth. J. Phys. Chem. 1996, 100, 13121−13131. (3) Puurunen, R. L. Surface Chemistry of Atomic Layer Deposition: A Case Study for the Trimethylaluminum/Water Process. J. Appl. Phys. 2005, 97, 121301−52. (4) Suntola, T. Atomic Layer Epitaxy. Thin Solid Films 1992, 216, 84−89. (5) Leskelä, M.; Ritala, M. Atomic Layer Deposition (ALD): From Precursors to Thin Film Structures. Thin Solid Films 2002, 409, 138− 146. (6) Kim, H. Atomic Layer Deposition of Metal and Nitride Thin Films: Current Research Efforts and Applications for Semiconductor Device Processing. J. Vac. Sci. Technol. B 2003, 21, 2231−2261. (7) Kim, H.; Lee, H.; Maeng, W. Applications of Atomic Layer Deposition to Nanofabrication and Emerging Nanodevices. Thin Solid Films 2009, 517, 2563−2580. (8) Knez, M.; Nielsch, K.; Niinistö, L. Synthesis and Surface Engineering of Complex Nanostructures by Atomic Layer Deposition. Adv. Mater. 2007, 19, 3425−3438. (9) Leskelä, M.; Ritala, M. Atomic Layer Deposition Chemistry: Recent Developments and Future Challenges. Angew. Chem., Int. Ed. 2003, 42, 5548−5554. (10) Niinistö, L.; Nieminen, M.; Päiväsaari, J.; Niinistö, J.; Putkonen, M.; Nieminen, M. Advanced Electronic and Optoelectronic Materials by Atomic Layer Deposition: An Overview With Special Emphasis on Recent Progress in Processing of High-K Dielectrics and Other Oxide Materials. Phys. Status Solidi A 2004, 201, 1443−1452. (11) Miikkulainen, V.; Leskelä, M.; Ritala, M.; Puurunen, R. L. Crystallinity of Inorganic Films Grown by Atomic Layer Deposition: Overview and General Trends. J. Appl. Phys. 2013, 113, 021301. (12) Elliott, S. D. Atomic-Scale Simulation of ALD Chemistry. Semicond. Sci. Technol. 2012, 27, 074008+. (13) Kwon, O.-K.; Kim, J.-H.; Park, H.-S.; Kang, S.-W. Atomic Layer Deposition of Ruthenium Thin Films for Copper Glue Layer. J. Electrochem. Soc. 2004, 151, G109−G112. (14) Aaltonen, T.; Alén, P.; Ritala, M.; Leskelä, M. Ruthenium Thin Films Grown by Atomic Layer Deposition. Chem. Vap. Deposition 2003, 9, 45−49. (15) Aaltonen, T.; Rahtu, A.; Ritala, M.; Leskelä, M. Reaction Mechanism Studies on Atomic Layer Deposition of Ruthenium and Platinum. Electrochem. Solid-State Lett. 2003, 6, C130−C133. (16) Kwon, O.-K.; Kwon, S.-H.; Park, H.-S.; Kang, S.-W. PlasmaEnhanced Atomic Layer Deposition of Ruthenium Thin Films. Electrochem. Solid-State Lett. 2004, 7, C46−C48. (17) Kwon, O.-K.; Kwon, S.-H.; Park, H.-S.; Kang, S.-W. PEALD of a Ruthenium Adhesion Layer for Copper Interconnects. J. Electrochem. Soc. 2004, 151, C753−C756. (18) Min, Y.-S.; Bae, E.; Jeong, K.; Cho, Y.; Lee, J.-H.; Choi, W.; Park, G.-S. Ruthenium Oxide Nanotube Arrays Fabricated by Atomic Layer Deposition Using a Carbon Nanotube Template. Adv. Mater. 2003, 15, 1019−1022. (19) Lashdaf, M.; Hatanpäa,̈ T.; Krause, A.; Lahtinen, J.; Lindblad, M.; Tiitta, M. Deposition of Palladium and Ruthenium β-Diketonates on Alumina and Silica Supports in Gas and Liquid Phase. Appl. Catal., A 2003, 241, 51−63. (20) Park, S.-J.; Kim, W.-H.; Lee, H.-B.-R.; Maeng, W. J.; Kim, H. Thermal and Plasma Enhanced Atomic Layer Deposition Ruthenium and Electrical Characterization as a Metal Electrode. Microelectron. Eng. 2008, 85, 39−44. (21) Swerts, J.; Salimullah, M. M.; Popovici, M.; Kim, M. S.; Pawlak, M. A.; Delabie, A.; Schaekers, M.; Tomida, K.; Kaczer, B.; Opsomer, K.; et al. Plasma Enhanced Atomic Layer Deposited Ruthenium for MIMCAP Applications. ECS Trans. 2011, 41, 41−51. (22) Swerts, J.; Delabie, A.; Salimullah, M.; Popovici, M.; Kim, M.-S.; Schaekers, M.; Elshocht, S. V. Impact of the Plasma Ambient and the Ruthenium Precursor on the Growth of Ruthenium Films by Plasma 19452



Article

(43) Inderwildi, O. R.; Lebiedz, D.; Warnatz, J. Linear Relationship Between Activation Energies and Reaction Energies for CoverageDependent Dissociation Reactions on Rhodium Surfaces. Phys. Chem. Chem. Phys. 2005, 7, 2552−2553. (44) Webster, C. E.; Drago, R. S.; Zerner, M. C. Molecular Dimensions for Adsorptives. J. Am. Chem. Soc. 1998, 120, 5509−5516. (45) Taverner, B. C. Improved Algorithm for Accurate Computation of Molecular Solid Angles. J. Comput. Chem. 1996, 17, 1612−1623.

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Atomic Layer Deposition of Ruthenium on a Titanium Nitride Surface

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