Atomic Layer Deposition of Ruthenium on Ruthenium Surfaces: A

Article pubs.acs.org/JPCC

Atomic Layer Deposition of Ruthenium on Ruthenium Surfaces: A Theoretical Study Quan Manh Phung,† 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: Atomic layer deposition (ALD) of ruthenium using two ruthenium precursors, i.e., Ru(C5H5)2 (RuCp2) and Ru(C5H5)(C4H4N) (RuCpPy), is studied using density functional theory. By investigating the reaction mechanisms on bare ruthenium surfaces, i.e., (001), (101), and (100), and H-terminated surfaces, an atomistic insight in the Ru ALD is provided. The calculated results show that on the Ru surfaces both RuCp2 and RuCpPy can undergo dehydrogenation and ligand dissociation reactions. RuCpPy is more reactive than RuCp2. By forming a strong bond between N of Py and Ru of the surface, RuCpPy can easily chemisorb on the surfaces. The reactions of RuCp2 on the surfaces are less favorable as the adsorption is not strong enough. This could be a factor contributing to the higher growth-per-cycle of Ru using RuCpPy, as observed experimentally. By studying the adsorption on H-terminated Ru surfaces, we showed that H can prevent the adsorption of the precursors, thus inhibiting the growth of Ru. Our calculations indicate that the H content on the surface can have an impact on the growth-percycle. Finally, our simulations also demonstrate large impacts of the surface structure on the reaction mechanisms. Of the three surfaces, the (100) surface, which is the less stable and has a zigzag surface structure, is also the most reactive one.

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preadsorbed precursor to form the films.17 One of the main advantages of using PEALD relies in the fact that the thin films can be grown at much lower temperatures than using thermal ALD. This technique has also been shown to be useful to deposit low impurities metal nitrides, such as TiN and TaN.18 Recently, ALD of Ru has been the subject of numerous experimental studies.4,19−26 The main goal of those studies is to improve the nucleation, growth, and quality of the film by using different precursors, substrates, and reaction conditions. The Ru precursors can be classified into three main groups. The first and most widely studied and applied group of precursors consists of ruthenocenes and their derivatives, including RuCp 2 , 19 Ru(EtCp) 2 , 4 (MeCp)Ru(EtCp), 27 (EtCp)Ru(DMPD),28 and Ru(DMPD)2,29 with Me = methyl, Cp = cyclopentadienyl, Et = ethyl, and DMPD = dimethylpentadienyl. By replacing one or both cyclopentadienyl rings with pyrrolyl (Py), pyrrolyl-based precursors are obtained, e.g., (MeCp)Ru(Py)30−33 and Ru(Me2Py)2.34 The second group of precursors consists of β-diketonate precursors, e.g., Ru(thd)321 and Ru(od)3.25 These compounds are less well studied in ALD, as it was shown that using this type of precursors the impurity content of the thin film is slightly higher than with

INTRODUCTION Among the noble metals, ruthenium (Ru) is of particular interest for many applications, including nanotechnology, because it has attractive material properties. Ru has a low resistivity (7.1 μΩ·cm in the bulk), a high work function (4.7 eV), and a good physical and chemical stability. These properties make Ru a potential material for the electrode in dynamic random access memory (DRAM),1,2 for the gate electrode in metal-oxide-semiconductor field effect transistors (MOSFETs),3 and for a seed layer for metallization of interconnects.4,5 Most of the applications of Ru in nanotechnology require the deposition of Ru thin films on complex 3D nanostructures. Therefore, deposition techniques that can deliver growth control at the atomic level and conformal deposition are needed. Atomic layer deposition6 (ALD) is such a technique. ALD is based on a cyclic process of at least two consecutive self-limiting chemisorption reactions, the so-called reaction cycle. The basic principles of ALD can be found in many reviews.6−11 Because of its advantages, ALD has become an important method to deposit thin films used in microelectronics and nanotechnology, for example for the fabrication MOSFETs and memory devices. Applications in sensors, batteries, solar cells, etc. are also under investigation.10,12−16 Plasma-enhanced ALD (PEALD) is an energy-enhanced ALD method, making use of a plasma for the reaction with © 2015 American Chemical Society

Received: December 17, 2014 Revised: February 28, 2015 Published: March 4, 2015 6592

DOI: 10.1021/jp5125958 J. Phys. Chem. C 2015, 119, 6592−6603

Article

The Journal of Physical Chemistry C

Figure 1. (A) Growth curves for PEALD Ru films on TiN deposited at 330 °C from Ru(EtCp)2 (full symbols) and Ru(MeCp)Py (open symbols). (B) Temperature dependence of GPC using Ru(MeCp)Py. The Ru films were deposited on 10 nm physical vapor deposited TiN substrates in a 300 mm PEALD chamber at various temperatures using N2/H2 plasma. Thickness characterization was performed by X-ray reflectometry (A) and spectroscopic ellipsometry (B).

metallocenes.35 Besides metallocene-based and β-diketonate precursors, so-called zero valence precursors, in which Ru has a zero oxidation state, have been synthesized and studied. This type of precursors recently has gained more attention because it is more reactive than other types. Notable examples are (η6-1isopropyl-4-methylbenzene)(η 4 -cyclohexa-1,3-dien)ruthenium36 and (ethylbenzen)(cyclohexadiene)ruthenium.37 Besides the precursors, also the substrates and reaction conditions will have an impact on the Ru growth rate and the quality of the films. Substrates are mostly oxides, e.g., Al2O3,19 TiO2,19 ZrO2,27 SiO2,38 or nitrides, e.g., TiN39 and TaN.40 The reaction temperature can be varied in a large range, starting as low as 100 °C in PEALD1 up to 500 °C in thermal ALD.41 Based on many studies of Ru ALD, it is clear that both the precursors and reaction conditions, in particular the reaction temperature, can strongly affect the nucleation and thin film growth. Our recent results on PEALD of Ru also illustrate this30−32 (see Figure 1). We observed a markedly different reactivity of the bis(ethylcyclopentadienyl)ruthenium (Ru(EtCp)2) and (methylcyclopentadienylpyrrolyl)ruthenium (Ru(MeCp)Py) precursor. The precursor strongly influences both the inhibition period and the steady state (or bulk) growth-percycle (GPC). Using Ru(EtCp)2, an inhibition period of at least 120 cycles was observed, while with Ru(MeCp)Py, the inhibition period is negligible. Moreover, using Ru(MeCp)Py the bulk GPC (0.038 nm/cycle) is much higher than using Ru(EtCp)2 (0.016 nm/cycle). Finally, the GPC depends on the deposition temperature; that is, it increases as a function of temperature. In addition, for Ru(EtCp)2, the initial growth behavior as well as the growth-per-cycle (GPC) values vary strongly from report to report,30 indicating unknown or uncontrolled experimental factors. Thus, despite of the fact that Ru ALD has recently gained attention and there are many experimental studies, the surface chemistry of Ru ALD lacks atomistic insight. Because of this, the impacts of the precursor and experimental conditions are still not well understood. For ALD in general, many theoretical and experimental studies have been performed in order to understand the chemistry of ALD using different precursors, e.g., trimethylaluminum,42−45 HfCl4,46 ZrCl4,47,48 LaCp3,49

ZnEt2,50 etc. An overview of recent theoretical results on ALD can be found in a review of ref 51. In contrast, the number of studies on the surface chemistry of Ru ALD is rather limited, e.g., a study of the experimental reaction mechanism on thermal ALD using Ru(C5H5)2 of Aaltonen et al.,20 experimental studies on catalytic combustion reactions of (C 5 H 5 )Ru(CO)2(C2H5),52,53 a mechanism study of homoleptic precursors and oxygen,54 and our previous work on the reaction mechanism of two Ru precursors on a TiN substrate.55 In order to increase the insight of Ru PEALD in the steady state situation and as a continuation of our previous theoretical work,55 we report here the results of a theoretical study of the reaction mechanism of the two Ru precursors Ru(EtCp)2 and Ru(MeCp)Py on a number of different Ru surfaces. We have investigated the reaction mechanisms of these two precursors on the bare Ru surfaces. We investigate the impact of the crystal orientation, Ru(001), Ru(100), and Ru(101), on the chemisorption reaction mechanisms. The impact of H on the surface also is investigated, as after the H2 plasma step the surface might be (partially) terminated with H. We therefore study the adsorption of the precursors on hydrogen-terminated surfaces. Finally, we compare the chemisorption reaction mechanisms of the two precursors, and we investigate possible links between our atomistic insight in the reaction mechanisms of the two Ru precursors and our experimental observations, i.e., the difference of the GPC between the two precursors, and the temperature dependence of the GPC (Figure 1).30

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COMPUTATIONAL DETAILS Following our previous work,55 theoretical calculations were performed using periodic boundary conditions density functional theory (PBC-DFT) in the SIESTA code.56 We used the PBE and the van-der-Waals Dion-Rydberg-Schröeder-LangrethLundqvist (vdW-DF)57 functionals. The latter functional should yield better binding energy as it includes the van der Waals interaction that is not accurately described using the PBE functional. With vdW-DF, only single-point calculations at PBE geometries were performed. It should be noted that in our previous study,55 test calculations using PBE and vdW-DF showed that using PBE geometries is sufficient. In this work, we only report the vdW-DF energetic results. The core electrons of 6593


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Figure 2. Top and side views of the Ru(001), Ru(101), and Ru(100) surfaces. The red numbers in the top view denote possible adsorption locations.

mechanisms of Ru precursors on a TiN surface. In this work, test calculations of Ru bulk and different Ru surfaces also give good results as compared to experiments (see the Supporting Information). To summarize the computed lattice constants of bulk Ru are overestimated by less than 1.0% as expected in DFT calculations, the bulk modulus is overestimated by 2.4%, and the cohesive energy is underestimated by about 3%. For the simulation models, we simplify both the precursors and the surfaces. As the impact of substituents on the cyclopentadienyl rings on the growth of Ru is limited,30 our calculations involve simplified precursors, i.e., ruthenocene RuCp2 and (cyclopentadienyl)(pyrrolyl)ruthenium RuCpPy. For the Ru surfaces, it has been shown in many previous Ru ALD studies using different precursors and initial substrates33,64−68 that the Ru thin film is polycrystalline. Modeling a polycrystal using periodic boundary conditions is not possible. Instead, we simulate three different Ru surfaces, i.e., Ru(001), Ru(101), and Ru(100) (Figure 2). All three surfaces have been detected in X-ray diffraction results.33,64−68 Moreover, by studying different surfaces, we can investigate the impacts of surface orientation and stability on the adsorption of the precursors. All surfaces are simulated by using a slab model of 5 Ru layers with 2 fixed bottom layers. The simulation box contains 20 Å vacuum to reduce the artificial electric field across the slab. The relaxed structures of the slabs are shown in Figure 2. A discussion on the relaxations of the three Ru surfaces can be found in the Supporting Information. Before discussing the adsorption of the precursors on the surfaces, it is noteworthy to discuss some physical properties of the surfaces, such as surface density, surface stability, and adsorption locations (Table 1). A surface with a higher Ru surface density should provide more chemisorption sites for the Ru precursor. On the other hand, a surface with a higher surface energy is less stable; thus, its catalytic reactivity should be higher. A good surface for ALD should be a surface that has

C, N, Ti, and Ru were replaced by Troullier and Martins type norm-conserving pseudopotentials, generated with the corresponding functionals. In this work, we used optimized double-ζ numerical basis sets58 augmented with polarization functions.59 To describe the long decay of the wave function on the Ru surfaces,60 the basis sets of the top-layer Ru atoms were augmented with diffuse 6s functions. Varying the radii of the confinement potential of these diffuse functions from 7.0 to 10.0 Bohr leads to a converged total energy of the Ru surfaces at 8.0 Bohr. The quality of the pseudopotentials and of the basis sets has been carefully assessed in our previous work.55 To evaluate the electron density, we used a grid constructed by a plane wave with a cutoff energy of 350 Ry. To converge the SCF energy of the Ru surface, we used a k-point grid of 6 × 6 × 1 and a Fermi smearing temperature of 300 K. To minimize the atomic forces in structures, we used the Broyden method with a force convergence threshold of 20 meV/Å. The climbing image nudged elastic band (CI-NEB) formalism61 was used to find the transition state between two minima. Depending on the complexity of the reaction path, the number of images used in CI-NEB has been varied from 5 to 11 images. In some reactions, in which the convergence of CINEB is difficult, the number of images has been increased to 11. The images were relaxed until the maximum residual force was lower than the threshold of 40 meV/Å. In all calculations, the counterpoise correction62,63 was used to account for basis-set superposition errors (BSSE) on the total energy.

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RESULTS Models. We first discuss the models used in this work, thus justifying that the theoretical methods and the simulation models are adequate. For the theoretical method, we have shown in previous work55 that DFT can provide qualitative results of the reaction 6594


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a high number of chemisorption sites and that is reactive enough to dissociate the precursor. On a perfect Ru(001) surface, both the surface atoms and the atoms underneath form a hexagonal structure. This surface has a Ru density of about 15.6 atom/nm2. This is the most stable surface of Ru, with a calculated surface energy of 2.70 J/m2 (this work) and 2.65 J/ m2 using plane wave.69 The Ru(100) surface has a zigzag structure and thus has a higher Ru surface density (about 17

Table 1. Physical Properties of the Three Ru Surface: Surface Density, Surface Energy, and Adsorption Locations surface density (atom/nm2) surface energy (J/m2) adsorption location

Ru(001)

Ru(101)

Ru(100)

15.6 2.70 1,2,3,3′

14.9 2.99 1,1′,2,3,4

17.0 3.00 1,2,3,3′,4,5

Figure 3. Reaction mechanisms of RuCp2 on the Ru(001), Ru(101), and Ru(100) surfaces. The red lines indicate the most favorable reaction pathways. A = adsorption, DH = dehydrogenation, LD = ligand-dissociation. Energy values in kcal/mol. 6595


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adsorption energy is small, −6.1 kcal/mol on Ru(001) and −9.4 kcal/mol on Ru(101). On Ru(101), the adsorption is slightly stronger as the distances between C and Ru atoms of the surface (Rus) are about 2.3−2.4 Å, while on Ru(001), the C−Rus distances are larger than 2.6 Å. On both surfaces, the geometry of the molecule is not affected by the physisorption. Since the adsorption energy is still rather small, the molecule can easily desorb at temperatures of about 330 °C as used in experiments. To remain on the surface at such high temperature, a stronger bond between RuCp2 and the surface must be formed. It is well-known that the Ru surfaces are good catalytic surfaces that can catalyze both hydrogenation and dehydrogenation reactions.52,53 Our calculations indeed show that RuCp2 can be dehydrogenated with a breaking of a C−H bond and a formation of new C−Rus bonds (I2). This reaction is slightly exothermic by 3 kcal/mol and the reaction barrier (TS1) is not high (about 12.8 kcal/mol on both surfaces). As such, both RuCp(C5H4) and H become strongly chemisorbed at 3-fold hollow sites. To obtain more stable intermediates from I2, the precursor must be further adsorbed by forming stronger bonds with the surface, either Ruprec−Rus or C−Rus bonds. All of these reactions require either the breaking of the Ruprec−C bonds (pathway 1 or ligand dissociation pathway) or the C−H bond (pathway 2 or dehydrogenation pathway). In pathway 1 or ligand-dissociation pathway (I2 → I3 → I5), the Ruprec−Cp1 bond is completely broken. The distance between Ruprec and all C atoms of Cp1 becomes larger than 3.5 Å. This allows a stronger bonding between Ruprec and Rus (I3). Cp1 is also further adsorbed by forming strong bonds with the surface. As a result of the strong adsorptions of both Cp1 and RuCp2, the reaction is exothermic and I3 is more stable than I2 by 13.1 and 27.2 kcal/mol on Ru(001) and Ru(101), respectively. The reaction I2 → I3 is much more exothermic on Ru(101) than on Ru(001) because on Ru(101), RuCp2 is adsorbed at a 4-fold site, while on Ru(001), it is adsorbed at a 3-fold site. However, on both surfaces, the reaction barrier TS2 is rather large, 30.2 and 21.2 kcal/mol on Ru(001) and Ru(101), respectively. The lower reaction barrier on the (101) surface indicates that this surface is more reactive than the (001) surface, as expected. With a more modest energy barrier, Cp1 and RuCp2 can be further separated to form an even more stable structure (I5). On Ru(001), I5 is more stable than I3 by 9.3 kcal/mol because the adsorption of RuCp2 becomes stronger. The barrier between I3 and I5 is small, only 6.9 kcal/mol. On the (101) surface, the reaction I3 → I5 is slightly endothermic and the reaction barrier is higher (18.8 kcal/mol). When comparing between the two reactions I2 → I3 and I3 → I5, we see that the first step is the rate-determining step of the first pathway. As this step has a large reaction barrier, we do not expect this pathway to be favorable. In pathway 2 corresponding to the dehydrogenation process (I2 → I4), a C−H bond of the second Cp2 is broken (I4). There are significant differences between the Ru(001) and Ru(101) surfaces. On Ru(001), the H atom is chemisorbed at a 3-fold hollow site while Cp2 strongly binds with the surface by forming at least three C−Ru s bonds (2.00−2.30 Å). Consequently, the Ruprec−Cp2 bond is partially broken. Ruprec is now adsorbed on top of a Rus atom. The reaction is endothermic by 8.3 kcal/mol because the energy released by the formation of the C−Rus and H−Rus bonds does not suffice to compensate for the energy required to break the C−H and

atom/nm2). This surface is less stable with a higher surface energy of about 3.00 J/m2 (2.98 J/m2 using plane wave69). Ru(101) is a nearly flat surface. As compared to the Ru(001) surface, it has a similar Ru surface density (14.9 atom/nm2) but higher surface energy of 2.99 J/m2. To conclude, of the three surfaces studied, the Ru(100) surface has the highest Ru surface density and, together with the Ru(101) surface, has the highest surface energy. The structure of the top layer of the three surfaces and the different adsorption possibilities for an atom (or a molecule) are shown in Figure 2. On the Ru(001) surface, an atom can be adsorbed on-top of a Ru atom (1), at a bridge site between two Ru atoms (2), or at a 3-fold hollow site between three Ru atoms (3) and (3′). On Ru(101), beside top sites (1) and (1′) and bridge sites (2), an atom can also be adsorbed at either a 3fold (3) or 4-fold (4) site. For the Ru(100) surface, this surface is composed of zigzag lines along the [001] direction and long channels long the [120] direction. An atom can be adsorbed at several sites, e.g., top site (1), 2-fold bridge site (2), 3-fold hollow site (3) and (3′), and “fivefold” top site (5). On the other hand, a molecule can be oriented either along the channel ([120] direction) or along the zigzag line ([001] direction). All surfaces described above are bare surfaces. After the H2 plasma step, the surface could be partially terminated with H. Simulating reactions on such a surface is not straightforward because the content and the distribution of H on the surface (both temperature dependent) are unknown. To circumvent this problem, we study adsorption on surfaces terminated by one monolayer (ML) of H (1 hydrogen atom per 1 Ru surface atom). As the desorption temperature of atomic H is quite low (about 70 °C on Ru(100),70,71 we do not expect that the surface is nearly fully H-terminated in the ALD experiment. Still, this model is useful to investigate the impact of the H content on the adsorption of the precursors. Reaction Mechanisms of RuCp2. The reaction mechanisms of RuCp2 on the Ru(001), Ru(101), and Ru(100) surfaces are shown in Figure 3. The reaction pathways we proposed are mainly based on (i) the reaction mechanisms of the same precursors on a TiN surface presented in our previous work55 and (ii) the reaction mechanisms of a Ru precursor experimentally suggested in the works of Leick et al.52,53 The red lines in Figure 3 indicate the most favorable reaction pathways, i.e., the pathways leading to considerable exothermic reactions and small reaction barriers. When there are several competitive reaction pathways, the molecule always prefers the pathway having more negative adsorption energy and lower reaction barriers. [In this work, we define adsorption as the binding of a precursor on a surface without any reaction barrier and without bond breaking in the precursors. The adsorption energy is ΔEads =Esur+prec − (Esur + Eprec) < 0 where Esur is the energy of the bare surface, Eprec is the energy of the isolated molecule, and Esur+prec is the energy of the adsorbed system.] The principle of least action72 could be applied to choose the favorable pathway. For the purpose of convenience, we denote the Cp rings of RuCp2 as Cp1 and Cp2, Ru atoms on the surface as Rus, and the Ru atom of the precursor as Ruprec. Reaction Mechanisms on Ru(001) and Ru(101). We first discuss the reaction mechanisms on the Ru(001) and Ru(101) surfaces because they are rather similar. The reaction starts with the physisorption (van der Waals interaction) of the molecule on the surface. Of all possible adsorption configurations, the molecule tends to adsorb so that Cp1 stays parallel to the surface (I1). The molecule is only physisorbed as the 6596


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Figure 4. Reaction mechanisms of RuCpPy on the Ru(001), Ru(101), and Ru(100) surfaces. The red lines indicate the most favorable reaction pathways. A = adsorption, DH = dehydrogenation, LD = ligand-dissociation. Energy values in kcal/mol.

Ruprec−Cp2 bonds. The reaction barrier (TS3) is 20.1 kcal/mol. On the other hand, the reaction I2 → I4 on Ru(101) is exothermic by 6.9 kcal/mol and has a much smaller reaction barrier of 7.5 kcal/mol. Its exothermic character arises from the fact that in I4 the Ruprec atom maintains bonds with both the Cp

rings. No extra energy is hence required to break these bonds. The reaction needs much less energy to pass the barrier TS4, which indicates the higher reactivity of the Ru(101) surface. By comparing the reaction mechanisms of RuCp2 on the two Ru surfaces, we can conclude that the Ru(101) surface is more 6597


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form I2. From I2, the precursor prefers passing the TS1 barrier of 19.7 kcal/mol (I2 → I3). Based on the principle of least action, the reverse reaction I2 → I1 → I0 → I′1 → I′3 should be less favorable even though it has a comparative reaction barrier (of about 17.2 kcal/mol). Thus, the reaction along the [120] direction should be more favorable than along the [001] direction. Reaction Mechanisms of RuCpPy. The reaction mechanisms of RuCpPy on the Ru(001), Ru(101), and Ru(100) surfaces are shown in Figure 4. The red lines in Figure 4 underline the most favorable reaction pathways. Reaction Mechanisms on Ru(001) and Ru(101). Similar to the reactions of RuCp2, we calculated for RuCpPy the physisorption on the surface, followed by the formation of a bond between C and Rus by a dehydrogenation reaction. However, this pathway is not the most favorable pathway for RuCpPy. A more stable adsorption configuration (J1) can be formed, where the precursor is chemisorbed with formation of a N−Rus bond (of about 2.1 Å). The adsorption energy of J1 is very negative, about −30 kcal/mol on both Ru(001) and Ru(101). We again investigated two paths starting from J1. In the ligand-dissociation pathway (J1 → J2 → J3 or pathway 1), RuCpPy reaches a very stable intermediate J2 on both surfaces. In this stable intermediate, the Ruprec is strongly chemisorbed at the 3-fold hollow site and the Py ring is adsorbed parallel to the surface. The Ruprec−Py bond is significantly weaker because the interaction between Ruprec and Py changes from η5 to η1, with bonding through a σ interaction with the N atom of Py. The reaction is exothermic by −22.2 kcal/mol on Ru(001) and −25.0 kcal/mol on Ru(101). The reaction barrier TS1 is only 12.9 kcal/mol on Ru(001) and 16.9 kcal/mol on Ru(101). Finally, the Py fragment can fully dissociate from RuCp (J2 → J3). On Ru(001), J2 and J3 have a similar stability and the reaction barrier is about 14.6 kcal/mol. On Ru(101), the reaction is slightly endothermic by 5.3 kcal/mol and the reaction barrier is 13.9 kcal/mol. Because of their similar stability, both J2 and J3 can occur on Ru(001) and Ru(101). In the dehydrogenation pathway (J1 → J4 or pathway 2), a C−H bond of the Cp ligand of RuCpPy is broken. The geometry and stability of J4 on Ru(001) is significantly different from J4 on Ru(101). On the Ru(001) surface, RuCpPy still maintains its gas phase geometry, i.e., the Cp and Py rings are parallel to each other. On the other hand, on the (101) surface, the structure of RuCpPy is highly distorted and the Py ring becomes strongly adsorbed on the surface. Consequently, on Ru(001), J4 is less stable than J1 by 2.5 kcal/mol while on Ru(101) J1 and J4 have a similar stability. On both surfaces, the reaction barriers TS2 are similar, about 20 kcal/mol. The comparison of the two pathways suggests that the ligand-dissociation pathway (pathway 1) is more favorable than the dehydrogenation pathway (pathway 2) because the reaction J1 → J2 is strongly exothermic by at least −20 kcal/mol. Moreover, TS1 is lower than TS2 by about 7.6 kcal/mol on Ru(001) and 2.7 kcal/mol on Ru(101). We predict that the reaction product of RuCpPy on the two surfaces should be J2 and J3. Reaction Mechanisms on Ru(100). On Ru(100), RuCpPy is initially also strongly chemisorbed with the formation of a N−Rus bond with a distance of about 2.1 Å. The adsorption energy of RuCpPy is again very negative, i.e., −36.2 kcal/mol (J1) along the [120] direction and −29.0 kcal/ mol (J′1) along the [001] direction.

reactive than the Ru(001) surface. On Ru(001), the reaction should be limited to the formation of I2 as both subsequent steps are unfavorable (kinetically in the pathway 1 and thermodynamically in the pathway 2). Moreover, the reverse reaction I2 → I0, i.e., the desorption can occur, as the backward reaction barrier is relatively low (about 16.1 kcal/mol). We therefore conclude that the growth of the thin film on the Ru(001) surface using RuCp2 should be slow. On the other hand, on Ru(101), pathway 2, in which two C−H bonds are broken, is favorable. The rather stable intermediate I4 should be the most dominant one among all intermediates. Reaction Mechanisms on Ru(100). As shown above, the Ru(100) surface exhibits some interesting properties, such as more adsorption sites are available and a molecule can be adsorbed along or over the zigzag channels. The reaction mechanisms on this surface are substantially different from the other two surfaces. On the Ru(100) surface, the precursors chemisorb and react by ligand-dissociation reactions (see below) instead of by dehydrogenation reactions. We noticed that on this surface RuCp2 might also be physisorbed and dehydrogenated, by reactions similar as described above for the Ru(001) and Ru(101) surfaces. However, the dehydrogenation reaction always requires passing a reaction barrier while on this surface no barriers are observed for the ligand dissociation. Thus, only the more favorable ligand-dissociation reactions are described below. Two ligand-dissociation reaction pathways along two directions ([120] and [001]) were computed and are shown separately (right and left in Figure 3). The most favorable reaction occurs along the [120] direction. RuCp2 is first physisorbed with an adsorption energy of −7.5 kcal/mol. The distance between Ruprec and Rus is large (of about 5 Å). As I1 is a metastable intermediate, the precursor starts to dissociate on the surface to form a more stable adsorption intermediate (I2) without a reaction barrier. In I2, Ruprec is at a 5-fold adsorption site and most of the Ruprec−C bonds are broken (changing from Ru−Cp η5 to η2 interaction). Both the Cp rings are strongly chemisorbed with C−Rus bond lengths smaller than 2.3 Å. I2 is about 9.7 kcal/mol more stable than I1. In the next step, Ruprec and Cp2 are completely dissociated (I2 → I3). Cp2 slips along the [120] direction, the last Ruprec−C bond is broken and Cp2 stays parallel to the surface. Ruprec moves along the opposite direction and the bond between Ruprec and Cp1 is restored. RuCp1 is chemisorbed at a 5-fold site. This reaction is strongly exothermic by about −33 kcal/ mol. Moreover, the reaction barrier TS1 is not too high (19.7 kcal/mol), indicating that formation of I3 is possible. Since the interactions between Cp2, RuCp1 and the surface are stable, we expect I3 to be the end-point of the reaction pathway along the [120] direction. Along the [001] direction, RuCp2 is also first physisorbed with an adsorption energy of −8.7 kcal/mol (I′1). From I′1, two strongly exothermic reactions I′1 → I′2 (ΔE = −39.6 kcal/mol) and I′1 → I′3 (ΔE = −51.5 kcal/mol) can occur. Both reactions have small energetic barriers (about 12 kcal/mol). I′2 is equivalent to I2, the Ruprec−Cp bonds are nearly broken (changing from η5 to η2 interaction) and I′3 is equivalent to I3, in which the precursor is fully dissociated into two separated fragments RuCp2 and Cp1. In both I′2 and I′3, Ruprec is strongly adsorbed on the surface with a 5-fold bond. As the precursor always favors the pathway having the most negative adsorption energy and because RuCp2 is adsorbed more strongly in I2 than in I′1 by 8.5 kcal/mol, RuCp2 tends to 6598


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On the bare surfaces, RuCpPy is chemisorbed by forming a N−Rus bond of about 2.1 Å (J1). The adsorption energy varies from −29.4 to −36.2 kcal/mol. On the H-terminated surfaces, the N−Rus bond is systematically longer as compared to the bare surface, 2.2 Å on Ru(100)/H and Ru(101)/H, and up to 2.7 Å on Ru(001)/H. Consequently, the adsorption energy on the H-terminated surfaces is more positive than on the bare surfaces, −26.4, −19.1, and −10.3 kcal/mol on Ru(100)/H, Ru(101)/H, and Ru(001)/H, respectively. On the bare surfaces, all reactions J1 → J2 are exothermic by at least −22 kcal/mol, while on the H-terminated surfaces, the reaction is exothermic by only −6 kcal/mol on the Ru(001)/H and Ru(101)/H surfaces and is even endothermic on the Ru(100)/ H surface. By comparing between the adsorption energy of RuCp2 and RuCpPy on the bare and the H-terminated surfaces, it is clear that the H layer strongly influences the adsorption of the precursors. It can prevent the adsorption of not only the precursors but also other adsorbates, such as H, Cp, Py, etc. This indicates that an increased concentration of H on the surfaces leads to a worsened adsorption of the precursors.

Along the [120] direction, we investigated two mechanisms, i.e., a partial ligand dissociation (J1 → J2) and a complete dissociation of the ligand (J1 → J3). In J2, both Cp and Py are partly dissociated from Ruprec and the resulting Ruprec is adsorbed at a 4-fold site. The reaction J1 → J2 is slightly endothermic; an energy of about 20 kcal/mol is needed to pass the reaction barrier (TS1). As indicated above, the analogous reaction I1 → I2 of RuCp2 on Ru(100) (Figure 3) was strongly exothermic and barrierless. The difference can be explained by the fact that RuCpPy is already strongly chemisorbed in J1 and a further adsorption of Cp, Py, and Ruprec in J2 is not sufficient to compensate the energy required to dissociate both the Ruprec−Cp and Ruprec−Py bonds. In contrast, the full ligand dissociation reaction (J1 → J3) is strongly exothermic by −33.7 kcal/mol as only the Ruprec−Py bond is broken. J3 is the most stable intermediate as it has a 5-fold bond between Ruprec and the surface. This reaction is also easier because it has a small reaction barrier of about 12.5 kcal/mol. Along the [001] directions, we also investigated two similar ligand-dissociation reactions, J′1 → J′2 and J′1 → J′3. Reactions along this direction are less favorable than along the [120] direction. Going from J′1 to J′2 and J′3 requires a larger energy than from J1 to J2 and J3, about 29.7 kcal/mol (TS3) and 17.4 kcal/mol (TS4). The reaction energy of J′1 → J′2 and J′1 → J′3 is 17.0 and 25.1 kcal/mol, respectively. Adsorption of RuCp 2 and RuCpPy on the HTerminated Ru Surfaces. As introduced above, the Hterminated Ru surfaces are also investigated as H atoms can be present on the surface after the H2 plasma step. The adsorption energy of RuCp2 and RuCpPy on the bare and the Hterminated surface is presented in Table 2, whereas a

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DISCUSSION Impact of the Precursor (RuCp2 and RuCpPy) in Ru ALD. Our current study provides an atomistic insight in the reaction mechanisms of the RuCp2 and RuCpPy precursors on different Ru surfaces. In this section, we will now investigate possible links between the chemisorption reaction mechanisms of the RuCp2 and RuCpPy precursors and the experimental observations, i.e., the difference of the GPC between the two precursors and the temperature dependence of the GPC (Figure 1). As can be seen in Figure 1, the GPC values using different precursors for ALD at 330 °C are significantly different. Using Ru(MeCp)Py, the GPC is 0.038 nm/cycle, much higher than for Ru(EtCp)2 (0.016 nm/cycle). By comparing the most favorable reaction mechanisms of RuCp2 and RuCpPy on the bare Ru surfaces (the red lines in Figures 3 and 4), we indeed find considerable differences in the chemisorption reaction mechanisms of the two precursors. In every surface reaction, the adsorption step plays a crucial role. This is the key step that decides whether a film can grow or not. It is clear that adsorption of RuCpPy is much more favorable than adsorption of RuCp2. On all Ru surfaces, the first adsorption step of RuCp2 is only physisorption with adsorption energies varying from −6.1 to −9.4 kcal/mol. Such low adsorption energies are not sufficient to keep the adsorbate on the surface at the high temperatures in ALD experiments. However, due to the high catalytic reactivity of the Ru surfaces, the precursor can be either dehydrogenated or it can dissociate, leading to more stable intermediates. The relative energies of these intermediates vary between −9.5 and −17.2 kcal/mol. Thus, through dehydrogenation reactions, RuCp2 can remain on the surface. On the other hand, the initial adsorption of RuCpPy on all the three surfaces is chemisorption through formation of a strong bond between N of the Py ligand and the surface. The adsorption energies vary from −29.4 to −36.2 kcal/mol. Because RuCpPy adsorbs much more strongly than RuCp2 on all Ru surfaces, more RuCpPy molecules can stay adsorbed on the Ru surfaces at the ALD temperature. Beside the first adsorption step, the further chemisorption reactions will also affect the GPC in ALD. The calculated results show that both RuCp2 and RuCpPy undergo ligand dissociation reactions, ending with the formation of two stable

Table 2. Calculated Adsorption Energy (in kcal/mol) of RuCp2 and RuCpPy on H-Terminated Ru(001), Ru(100), and Ru(101) Surfaces Ru(001) Hterminated

Ru(101) bare

I1 I2

−9.9 39.8

−6.1 −9.5

J1 J2

−10.3 −16.5

−29.4 −51.6

Hterminated RuCp2 −5.3 26.0 RuCpPy −19.1 −25.6

Ru(100) bare

Hterminated

bare

−9.4 −12.2

−12.9 0.1

−7.5 −17.2

−30.0 −55.0

−26.4 −7.15

−36.2 −69.9

comparison of the adsorption steps is shown in Figure 5. With the presence of H on the surface, the adsorption of both RuCp2 and RuCpPy is strongly inhibited, i.e., the adsorption energy is (much) less negative in many cases as compared to the bare surfaces. RuCp2 is first physisorbed (I1) with an adsorption energy between −5.3 and −12.9 kcal/mol. This is mainly because of the van der Waals interaction between RuCp2 and the layer of H. All the reactions I1 → I2 on the H-terminated surfaces are strongly endothermic by at least 13 kcal/mol. On the Ru(001)/ H and Ru(101)/H surfaces, I2 corresponds to a C−H bond dissociation, and on Ru(100)/H, I2 corresponds to Ruprec−Cp bonds dissociation. Moreover, the relative energies of I2 with respect to the starting configuration I0 on all H-terminated surfaces are positive (from 0.1 up to 40 kcal/mol), indicating that I2 is even less stable than the nonadsorbed configuration I0. 6599


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Figure 5. Side views of RuCp2 and RuCpPy on bare and H-terminated surfaces. The numbers in parentheses correspond to the adsorption energy (in kcal/mol) of equivalent geometry on the bare surfaces. DH = dehydrogenation, LD = ligand dissociation.

surfaces, i.e. nonflat surfaces. For the polycrystalline Ru thin films (as deposited by ALD), we could expect that Ru ALD is possible if the Ru(101) and Ru(100) surface orientations exist. In addition, the results suggest that deposition can occur on other nonflat structures, i.e., at defects sites and grain boundaries. In contrast, RuCpPy can strongly chemisorb on all surfaces. Hence, the growth of Ru using RuCpPy is not limited by the Ru surface orientation. In addition, the surface termination plays a further role. Our calculations indicate that the adsorption of both precursors is very limited. It is clear that H atoms prevent the adsorption of both precursor molecules and their fragments, i.e., H, Cp, or Py. The presence of H on the surface might therefore inhibit the growth of Ru. Thus, the experimentally observed differences in GPC for different precursors and temperatures might also be related to differences in Ru surface orientation and termination. The GPC of the Ru ALD process indeed depends on temperature. In most ALD processes, it is expected that the GPC has a small temperature dependence within the ALD temperature window. However, the GPC of Ru increases quite significantly with the temperature, which is also observed in other studies.66,73 Based on the current atomistic insight, we propose the following possible explanations for the raise of the GPC with temperature: • The adsorption and chemisorption reactions of the precursors become more efficient with temperature and/or more ligands are being released. However, from our current investigation, the final impact on the GPC is difficult to predict,

separated fragments RuCp and Cp (or Py), except for the reaction of RuCp2 on the Ru(001) and Ru(101) surfaces, where only the dehydrogenation is favorable. The stability of the end products of the ligand dissociation reactions is quite different for the two precursors; that is, RuCpPy can form much more stable reaction products than RuCp2. The whole adsorption reaction of RuCpPy is therefore strongly exothermic by at least −50 kcal/mol, up to about −70 kcal/mol on Ru(100). The energy released by the reactions of RuCp2 is only about −20 kcal/mol on Ru(101), up to −50 kcal/mol on Ru(100). Thus, with more released reaction energy, we expect that the reactions of RuCpPy are more efficient than of RuCp2. To conclude, RuCpPy is more reactive than RuCp2 on the bare Ru surfaces. This is in line with the higher growth-per-cycle of RuCpPy observed experimentally. It should be noted that, in order to fully explain a GPC value of an ALD process, the desorption reactions of the ligands from the surface should also be considered. However, an investigation of these ligand desorption reactions is extensive and beyond the scope of the current paper. Other Factors Affecting the Ru ALD GPC. Our calculations furthermore indicate that the Ru precursor chemisorption reactions are strongly dependent on the Ru surface structure, i.e., both the Ru surface orientation and the surface termination (by H). First, the surface reactions of RuCp2 are strongly dependent on the surface orientation of the Ru crystal. Adsorption of RuCp2 on the flat Ru(001) surface is very limited; thus, the growth on flat surfaces should be slow. RuCp2 can be only adsorbed on the Ru(101) and Ru(100) 6600


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The Journal of Physical Chemistry C Notes

as the ligand desorption reactions have not yet been investigated. • With increasing temperature, more reactive sites can become available on the surface for precursor adsorption, resulting in a higher GPC. After the H2 plasma pulse, the surface can be (partly) “poisoned” with H atoms at low temperature, hindering the Ru precursor chemisorption reactions. Increasing the temperature can increase desorption of these “poisonous” H atoms, thus creating more adsorption sites for the precursors, as such increasing the GPC. • Our calculations furthermore indicated that the Ru surface orientation affects the chemisorption reactions of Ru precursors, in particular for RuCp2. Therefore, changes in the orientations of the crystal grains in the polycrystalline Ru thin film might also affect the GPC and its temperature dependence.

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). The computational resources and services used in this work were provided by the VSC (Flemish Supercomputer Center), funded by the Hercules Foundation and the Flemish Government−department EWI. All calculations were performed at the Tier-2 computing facilities of KU Leuven.

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CONCLUSIONS In this work, we have performed a comparative study on the reaction mechanisms of two Ru precursors RuCp2 and RuCpPy on different Ru surfaces, i.e., bare surfaces Ru(001), Ru(101), and Ru(100) and H-terminated surfaces. The study provides an atomistic insight in the chemisorption reactions of the two precursors, and points out considerable differences in the chemisorption reaction mechanisms of the two precursors. On the bare Ru surfaces, RuCpPy is more reactive than RuCp2. During the first adsorption step, RuCp2 is only physisorbed while RuCpPy is strongly chemisorbed. Both RuCp2 and RuCpPy undergo ligand dissociation reactions, ending with the formation of two stable separated fragments RuCp and Cp (or Py), except for the reaction of RuCp2 on the Ru(001) and Ru(101) surfaces, where only the dehydrogenation is favorable. The reaction of RuCpPy should be more favorable than RuCp2 because RuCpPy has much more stable intermediates along the reaction pathways. This can be in line with experimentally observed higher GPC value for RuCpPy. In addition, our study provides additional insights in the Ru ALD that are not yet experimentally investigated or understood. The calculated results demonstrate a large impact of the Ru surface orientation and termination on the chemisorption reaction mechanisms. Of all surfaces, the flat and most stable surface Ru(001) is the least reactive, while the zigzag Ru(100) surface is the most reactive due to its lower stability and special surface structures. This proves that the best surface for ALD should be a surface having a high surface energy and complex topology, i.e., surface with more defective sites for the adsorption and reaction. In addition, the study on the adsorption of the precursors on the H-terminated shows that H atoms can poison the surface, inhibiting the adsorption of the precursors. Thus, experimentally observed differences in GPC for different precursors and temperatures can also be related to differences in Ru surface orientation and termination. This could also explain the variation in GPC values reported in the literature.30

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ASSOCIATED CONTENT

S Supporting Information *

Structure and electronic properties of the Ru bulk and Ru surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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Atomic Layer Deposition of Ruthenium on Ruthenium Surfaces: A

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