Atomic Layer Deposition of Ruthenium Thin Films from (Ethylbenzyl

Article pubs.acs.org/cm

Atomic Layer Deposition of Ruthenium Thin Films from (Ethylbenzyl) (1-Ethyl-1,4-cyclohexadienyl) Ru: Process Characteristics, Surface Chemistry, and Film Properties Mihaela Popovici,† Benjamin Groven,†,‡,§ Kristof Marcoen¶,†,§ Quan Manh Phung,§ Shibesh Dutta,†,∥ Johan Swerts,† Johan Meersschaut,† Jaap A. van den Berg,⊥ Alexis Franquet,† Alain Moussa,† Kris Vanstreels,† Pieter Lagrain,† Hugo Bender,† Malgorzata Jurczak¶,† Sven Van Elshocht,† Annelies Delabie,†,§ and Christoph Adelmann*,† †

Imec, B-3001 Leuven, Belgium Faculteit Technische Natuurkunde, Technische Universiteit Eindhoven, 5600 MB Eindhoven, The Netherlands § Departement Chemie, KU Leuven, B-3001 Leuven, Belgium ∥ Departement Natuurkunde en Sterrenkunde, KU Leuven, B-3001 Leuven, Belgium ⊥ International Institute for Accelerator Applications, School of Applied Sciences, University of Huddersfield, Huddersfield HD1 3DH, United Kingdom ‡

ABSTRACT: The process characteristics, the surface chemistry, and the resulting film properties of Ru deposited by atomic l a y e r d e p o s i t io n f r o m ( e t h y l b e n z y l ) ( 1 -e t h y l - 1 , 4 cyclohexadienyl)Ru(0) (EBECHRu) and O2 are discussed. The surface chemistry was characterized by both combustion reactions as well as EBECHRu surface reactions by ligand release. The process behavior on TiN starting surfaces at 325 °C was strongly influenced by Ti(O,N)x segregation on the growing Ru surface with consequences for both the growth per cycle as well as the film properties. For optimized process conditions, the films showed high purity with low C and O concentrations of the order of 1020 at./cm3. Higher deposition temperature led to strong (001) fiber texture of the films on SiO2 starting surfaces. Annealing in forming gas improved the crystallinity and led to resistivity values as low as 11 μΩcm for Ru films with a thickness of 10 nm.

■

ALD, a negligible incubation period has been reported.20 However, for deep trench structures, the rapid surface recombination of radicals in the plasma generally leads to a degraded conformality. Therefore, thermal ALD is typically preferred over plasma-enhanced ALD when excellent conformality or step coverage is required.21 Thermal ALD of Ru has been mostly reported using Ru(II) precursors22 such as RuCp2,23,24 Ru(EtCp)2,25,26 RuCp(PrCp),27 or Ru(EtCp) (DMPD).21,28 These processes showed excellent step coverage but suffered from nucleation delays and growth inhibition on oxide dielectrics. This resulted in a rough island-like morphology, which is undesirable for applications as electrodes. In 2009, Eom et al. reported the ALD of metallic Ru thin films using (isopropylmethylbenzene) (cyclohexadiene) Ru(0) (IMBCHRu)29 with much reduced growth inhibition. This precursor belongs to a series of zerovalent precursors where

INTRODUCTION Ruthenium is a noble metal that has been widely investigated for microelectronic applications, for example, as an electrode in metal−insulator−metal capacitors,1−6 as a metal gate in metaloxide-semiconductor field-effect transistors,7−9 or as (part of) the metallization in damascene interconnects.10−12 Ru offers a high work function (4.7 eV), a low bulk resistivity (∼7 μΩcm), and high chemical stability. Moreover, Ru forms a conductive oxide, RuO2, with a high work function (>5 eV),13 which avoids the formation of insulating interfacial layers in contact with oxides. Microelectronic applications increasingly require the conformal deposition of thin films in the nanometer range onto or into 3D structures, and thus atomic layer deposition (ALD) has become the reference deposition technique because of its outstanding conformality and thickness control.14 However, the ALD of regular (noble) metal thin films as electrodes onto (oxide) dielectrics has been found to be challenging due to growth inhibition and island-like nucleation on oxide surfaces.15−19 In the case of Ru deposited by plasma-enhanced © 2017 American Chemical Society

Received: December 23, 2016 Revised: May 22, 2017 Published: May 23, 2017 4654

DOI: 10.1021/acs.chemmater.6b05437 Chem. Mater. 2017, 29, 4654−4666

Article

Chemistry of Materials

Figure 1. (a) Structure of the EBECHRu molecule. Saturation curves: Ru growth per cycle (GPC) as a function of (b) the EBECHRu and (c) O2 pulse length for process temperatures (225 °C, 325 °C) and starting surfaces (SiO2, TiN), as indicated. Panels d and e show scanning electron micrographs of the surface of 20 nm thick Ru films deposited on TiN at 325 °C using 1.0 s [(d), condition (1)] and 4.0 s [(e), condition (2)] O2 pulse lengths, respectively. Growth curves: (f) Ru GPC as a function of the number of ALD pulses for process temperatures (225 °C, 275 °C, and 325 °C) and starting surfaces (SiO2, TiN), as indicated. (g) Amount of Ru deposited by EBECHRu pulsing due to precursor decomposition at both 325 and 225 °C.

Ru is bonded to cyclohexadiene by a coordinate bond.30 The interest in (arene) (diene) Ru(0) π-type complexes stems from the observation of minimal carbon contamination in deposited films due to the fact that the ligands can be dissociated as free molecules by thermolysis without decomposition.31 In a subsequent study, Hong et al. found that (ethylbenzyl) (1ethyl-1,4-cyclohexadienyl) Ru(0) (EBECHRu) has an ALD behavior similar to IMBCHRu with an about 2-fold growth per cycle.32 Both reports highlight the short nucleation period both on SiO2 and TiN. This renders these ALD processes highly interesting for the ALD of Ru electrodes for microelectronics applications. In this article, we study in detail the ALD using (ethylbenzyl) (1-ethyl-1,4-cyclohexadienyl) Ru(0) (EBECHRu, C16H22Ru) in combination with molecular oxygen (O2) as coreagent on both SiO2 and TiN starting surfaces, in particular the process behavior, the surface chemistry, and the film contamination and properties. We show that the Ru growth behavior strongly depends on the starting surface and how this influences the film properties. More specifically, we demonstrate that the enhanced growth observed on TiN substrates can be linked to Ti(O,N)x surface segregation that lowers the surface free energy. This has important repercussions on the film properties, specifically on the crystalline grain structure with a suppression of the commonly observed (001) fiber texture33,34 on TiN starting surfaces with strong impact on the physical properties of the films.

■

Identical Ru ALD behavior (data not shown) was observed on ALD TiN (TDMAT, NH3), so the TiN deposition process did not have any apparent influence. The ALD process behavior was characterized by measuring the film thickness by X-ray reflectivity (XRR), the film mass, as well as the atomic Ru area density by Rutherford backscattering spectrometry (RBS). XRR was performed in a Bede MetrixL diffractometer from Jordan Valley using Cu Kα radiation. The wafer mass was measured using a Metryx Mentor mass metrology system. RBS was carried out using a 1.52 MeV He+ ion beam in a rotating random mode at a backscatter angle of 170°. The film density was determined from a combination of XRR and mass measurements. The reaction chemistry of the ALD process was studied by recording the gas phase reaction products using in situ quadrupole mass spectrometry (QMS). The experiments were performed using a Hiden Analytical HPR-20 system that was connected to the exhaust pump line of the ALD reactor. The gas species were sampled via a heated quartz capillary and ionized by electron impact (ionization energy of 70 eV). A secondary electron multiplier was used as the detector. The crystallinity of the films was determined by X-ray diffraction (XRD) using Cu Kα radiation in Bede MetrixL and Panalytical X’Pert diffractometers. Additional information about the crystallinity and texture was obtained by transmission electron microscopy (TEM) using a FEI Tecnai F30 electron microscope operating at 300 kV. The resistivity of the Ru films was determined by measuring the sheet resistance on full wafers with a KLA Tencor RS100 four-point prober in combination with the XRR film thickness. In the case of TiN starting surfaces, the Ru sheet resistance measurements were corrected for the TiN sheet resistance (resistivity ∼220 μΩcm). The surface chemistry and the film closure were assessed by time-offlight secondary-ion mass spectrometry (TOF-SIMS) using a TOFSIMS IV system from ION-TOF. The Ru layer closure was studied by measuring the secondary-ion surface yield of Si+ and Ti+ ions as a function of deposited Ru (measured by RBS). Chemical analysis of the films was performed by elastic recoil detection analysis (ERDA) using Cl4+ ions at 8 MeV and a scatter angle of 40.5°. Additional composition analysis was carried out by medium-energy ion scattering (MEIS) at the University of Huddersfield using 100 keV

EXPERIMENTAL DETAILS

The Ru layers were deposited by ALD on 300 mm Si (100) wafers in a hot-wall, cross-flow-type ASM Pulsar 3000 reactor, connected to a Polygon 8300 platform. The susceptor temperatures were varied in the range between 225 and 325 °C. Both SiO2 and TiN were considered as starting surfaces. SiO2 (1.1 nm) was grown on the Si wafer by chemical oxidation in ozonated H2O (“imec clean”), whereas TiN (10 nm) was deposited by reactive sputtering of Ti in a N atmosphere. 4655


Article


°C indicated that much longer O2 pulses were required to reach saturation (≫6 s of O2). This suggests that the O2 reaction is kinetically limited in this temperature regime. An interesting observation is the decrease of the GPC for long O2 pulses on TiN starting surfaces at 325 °C. This decrease is accompanied by the observation of “blisters” due to partial film delamination for 4 s of O2 (Figure 1e). Moreover, these films had a low density (10.5 g/cm3) as compared to the smooth films obtained in the saturated regime between 0.3 and 1 s of O2 pulse length (12.4 g/cm3, consistent with the Ru bulk density). The film delamination can be attributed to large stresses in the film. A similar behavior was observed by Wang et al. and was attributed to incomplete consumption of subsurface oxygen by the subsequent Ru pulse at larger oxygen exposures.33 This incorporated oxygen leads to a distortion of the closed-packed Ru structure resulting in large stresses and delamination of the film. This is consistent with the surface chemistry discussed below as well as with the observed much larger O incorporation (4.6 at. % for 4s of O2 pulse length vs concentrations below the detection limit of 0.4 at. % for 1s of O2), as determined by ERDA. The O2 pulse length and the starting surface also had a strong effect on the adhesion energy, as determined by fourpoint bending. For a 0.4 s O2 pulse, for which the films approached the Ru bulk density, the adhesion energy was 1.6 ± 0.2 J/m2 on SiO2, much lower than the value of 14.0 ± 0.8 J/m2 on TiN. Longer O2 pulses resulted in an even lower adhesion energy (0.6 ± 0.2 J/m2 for 2s O2) for Ru on SiO2. Growth curves on both SiO2 and TiN starting surfaces at different temperatures (225 °C, 275 °C, and 325 °C) are shown in Figure 1f. Typically, negligible growth inhibition was observed, notably also for ALD on SiO2. The data also illustrate the 2-fold higher GPC for ALD on TiN at 325 °C (3.69 ± 0.05 Ru at./nm2) as compared to ALD on SiO2 at the same temperature (1.85 ± 0.02 Ru at./nm2). Note that the higher GPC persisted even for thick (closed) films up to several 10 nm. While the GPC on SiO2 was essentially independent of temperature in the studied range (1.91 ± 0.05 Ru at./nm2 for ALD at 225 °C), this was not true for ALD on TiN. At 225 °C, the GPC on TiN (1.73 ± 0.03 Ru at./nm2) was very similar to that of SiO2 and significantly lower than at 325 °C. This will be discussed in more detail below. The amount of Ru deposited by EBECHRu pulses only (pulse length of 5 s, without O2) due to thermal precursor decomposition is shown in Figure 1g. Here, a SiO2 starting surface was used. At 325 °C, a deposition of 0.03 Ru at./nm2 per EBECHRu pulse was observed, about 1.5% of the ALD GPC on SiO2 at the same temperature. Wafer mass measurements indicated that the added mass per Ru atom due to precursor decomposition was about a factor of 2 higher than that for ALD, which suggests the incorporation of high C concentrations in this CVD mode. At a lower deposition temperature of 225 °C, the amount of Ru deposited by thermal decomposition was about five times lower than that at 325 °C. These results show that thermal precursor composition did not contribute strongly to the ALD GPC in the studied temperature window. This further suggests that the slow increase of the ALD GPC with EBECHRu pulse length on SiO2 starting surfaces (Figure 1b) cannot be directly attributed to parasitic CVD due to thermal precursor decomposition. The unintentional contamination in the Ru films deposited on SiO2 at 325 °C using optimized pulse lengths was quantified by secondary-ion mass spectrometry (SIMS, Figure 2). Both

He+ ions, incident along the [−1−11] channeling direction and detected along the [001] blocking direction, resulting in a 125.3° scattering angle. Tilted top view scanning electron microscopy (SEM) with a FEI XSEM Nova 200 system and atomic force microscopy (AFM) with a Veeco Nanoscope IV in tapping mode were used to obtain information about morphology and roughness of the films. Four-point bending tests were used to quantify the adhesion strength of 15 nm thick Ru on SiO2 and TiN substrates. No thickness dependence of the adhesion energy was observed up to 30 nm of Ru. Auger spectra were recorded with a Themo Scientific VG350f Microlab tool at 10 kV beam energy and a tilt angle of 30°.

■

COMPUTATIONAL DETAILS The homolytic dissociation enthalpies of EBECHRu were computationally studied using density functional theory (DFT) and explicitly correlated coupled cluster theory (CCSD(T)F12b).35 The structures were first obtained by DFT using the PBE0 functional including dispersion correction36 D3 by applying the Turbomole 6.4 software.37 A def2-QZVPP basis set was used for Ru with the effective core potential ecp-28mwb and def2-TZVP for other atoms.38 In a next step, binding energies were computed using CCSD(T)-F12b for the PBE0 structures. The calculations were done with the Molpro 2012 software.39 Correlation-consistent basis sets were used: aug-ccpVTZ for C,40 cc-pVTZ for H,41 and aug-cc-pVQZ-PP for Ru.42 All valence electrons were treated as correlated. The final dissociation enthalpies were estimated by including all available corrections: basis set superposition errors, zero-point corrections, and thermal corrections taken from the PBE0 results.

■

RESULTS AND DISCUSSION The structure of the EBECHRu molecule optimized by DFT using PBE0 is shown in Figure 1a. Quantum chemical calculations found that (ethylbenzyl) (1-ethyl-1,4-cyclohexadienyl) Ru(0) was indeed the stable configuration with the ethyl moiety bonded to C(sp2) in the cyclohexadienyl ligand (Figure 1a). The relative position of the two ethyl groups in the ethylbenzyl and the ethyl-1,4-cyclohexadienyl ligands had no influence on the dissociation enthalpy of the molecule (ΔH < 0.1 kcal/mol). It can be expected that the different conformal isomers will not lead to a different ligand dissociation behavior and not to a different surface chemistry of the ALD process, although some differences in steric impact cannot be excluded. Saturation Behavior. The self-limiting behavior of the surface reactions was studied by varying both EBECHRu and O2 pulse lengths. Figure 1b and c show the resulting saturation curves, expressed in Ru area density deposited per ALD cycle, as determined by RBS. The equivalent nominal film thickness (assuming bulk Ru density) is also indicated. Both the deposition on SiO2 as well as TiN starting surfaces were studied. Figure 1b shows the variation of the growth per cycle (GPC) as a function of the EBECHRu pulse length on both SiO2 and TiN starting surfaces at a deposition temperature of 325 °C and an O2 pulse length of 0.4 s. On both starting surfaces, the GPC showed signs of saturation with the EBECHRu pulse time. An interesting observation, however, was that the saturating GPC was markedly higher (about 2×) on TiN than on SiO2. This will be discussed in detail below. The evolution of the GPC as a function of the O2 pulse is shown in Figure 1c. The GPC was found to saturate at 325 °C for sufficiently long O2 pulses, as expected for an ALD process. In keeping with the EBECHRu saturation curves, the saturated GPC at 325 °C was much larger on TiN than on SiO2. The variation of the GPC at a lower deposition temperature of 225 4656


Article


particular not for m/z = 315, i.e., for the EBECHRu molecule. This is consistent with earlier measurements for different ALD processes using the same setup (QMS and reactor) and was explained by the presence of a ceramic absorber in the exhaust line that removes molecules with low vapor pressure from the exhaust gas.43,44 Figure 3a and d show the time dependence of the partial pressure for m/z = 44, i.e., for CO2, at a deposition temperature of 325 °C. In Figure 3a, the O2 pulse length was fixed at 0.5 s, and the EBECHRu pulse length was varied between 1 and 7 s, as shown in the figure. By contrast, in Figure 3d, the EBECHRu pulse length was fixed at 3 s, and the O2 pulse length was varied between 0.1 and 3 s. The presence of CO2 in the reaction products indicates that combustion reactions contributed to the surface chemistry. Other combustion products, such as CO or H2O may also have been formed but could not be identified due to large background pressures in the QMS and/or the reactor (CO is overlapping with N2). The majority of CO2 was formed during the exposure of the surface to O2, with little variation of the CO2 partial pressure with the O2 pulse length (Figure 3d), indicating that the combustion reactions were very rapid and essentially saturated after 0.1 s of O2 exposure. This is consistent with the saturation behavior in Figure 1c that shows a significant GPC already for 0.1 s of O2. The formation of CO2 was also observed during and after the exposure of the surface to EBECHRu. This requires the presence of an oxidant on the surface prior to EBECHRu exposure, i.e., after the O2 pulse. A similar behavior has been observed for several ALD processes45 using ozone,46,47 Oplasma,17,19 or O2.18,48 It has been ascribed to the presence of activated O moieties present on the surface after combustion reactions with the oxidizing reactant. The interaction of O2 molecules with Ru (001) surfaces has been studied before, and it has been found that it proceeds via dissociative surface reactions.49,50 Both the surface adsorption up to a full

the C and the O incorporation was limited to values of about 1020 at./cm3, i.e., of the order of 0.15 at. %. This demonstrates the high purity of the deposited Ru films.

Figure 2. O, C, and Ru SIMS profiles in ALD Ru deposited at 325 °C on SiO2. Impurity concentrations were of the order of 1020 at./cm3, i.e., about 0.15 at. %.

In Situ QMS Analysis of Ru ALD. The surface chemistry of Ru ALD from EBECHRu and O2 was investigated by in situ QMS measurements. SiO2 starting surfaces were used, and deposition temperatures of 225 and 325 °C were explored. The main reaction products identified in the gas phase were CO2 (m/z = 44) as well as different traces that can be linked to fragments of the ligands, with m/z = 91 (C7H7) showing the highest partial pressures. No Ru-containing molecules were found above the detection limits in the reaction gas, in

Figure 3. Real time partial pressure traces measured by QMS during Ru ALD on SiO2 at 325 °C. Panels a−c show data for variable EBECHRu pulse length (1 to 7 s, O2 pulse length 1 s), whereas d−f show data for variable O2 pulse length (0.1 to 3 s, EBECHRu pulse length 3 s). m/z = 44 corresponds to the CO2 partial pressure, whereas m/z = 79 (C6H7) and m/z = 91 (C7H7) correspond to fragments of both ethylbenzene and ethylcyclohexadiene. 4657


Article


Figure 4. Real time partial pressure traces measured by QMS during Ru ALD on SiO2 at 225 °C. Panels a−c show data for variable EBECHRu pulse length (1 to 7 s, O2 pulse length 1 s), whereas d−f show data for variable O2 pulse length (0.1 to 3 s, EBECHRu pulse length 3 s). m/z = 44 corresponds to the CO2 partial pressure, whereas m/z = 79 (C6H7) and m/z = 91 (C7H7) correspond to fragments of both ethylbenzene and ethylcyclohexadiene.

monolayer51,52 as well as the formation of subsurface oxygen50 have been reported. Hence, the formation of CO2 during EBECHRu exposure can be attributed to combustion reactions with O present on (or beneath) the surface forming pure (nearly O-free) Ru films. The amount of released CO2 during and after the EBECHRu pulse showed a clear increase with increasing O2 pulse length, occurring only for O2 pulse lengths of 1 s and longer. This indicates that the adsorption of active O on the surface occurs after the combustion of remaining ligands on the surface. In addition to CO2, also hydrocarbon reaction products were found in the gas phase. The time dependence of both m/z = 79 (C6H7) and m/z = 91 (C7H7) partial pressures is shown in Figure 3b,c and e,f, respectively. Both signals showed very similar temporal patterns and can be attributed to fragments of ethylbenzene or ethylcyclohexadiene.53 However, for ethylbenzene, the expected partial pressure ratio PP91/PP79 is 20.5, whereas it is 0.23 for ethylcyclohexadiene.53 The experimental ratio was about 12, which suggests that both ligands contributed (about equally) to the signals within the accuracy of the measurement. This is consistent with very similar calculated ligand dissociation enthalpies: 67.2 kcal/mol for ethylbenzene and 73.2 kcal/mol for ethylcyclohexadiene, which suggests rather similar dissociation thermodynamics. We remark, however, that catalytic dehydrogenation reactions of ethylcyclohexadiene, along the lines of what has been observed for the ALD of Pt,54 can potentially change the ratio of ethylbenzene and ethylcyclohexadiene during surface reactions as the latter can be converted into the former together with the formation of H2. The amount of released ethylbenzene and ethylcyclohexadiene during the surface reactions with EBECHRu showed a clear threshold behavior and increased strongly when the EBECHRu pulse length was increased from 2 to 3 s (see Figure 3b and c). For longer pulses, there was also a clear delay of 2 to 3 s between the beginning of the EBECHRu pulse and the

observation of ethylbenzene and/or ethylcyclohexadiene in the gas phase. Since such a delay was not observed for other reaction products using the same setup (see, e.g., Figure 3a and refs 43 and 55), it cannot be ascribed to the limited time response of the QMS system. This indicates that the surface reactions of EBECHRu by ligand release occurred after the activated oxygen had been exhausted by combustion processes. This is consistent with the observation that longer O2 pulses reduced the amount of released ethylbenzene and ethylcyclohexadiene for a fixed EBECHRu pulse length (see Figure 3e and f). The surface reactions of EBECHRu thus follow a distinct two-step process: combustion reactions with activated oxygen moieties present on the surface after O2 exposure, followed by surface reactions via ligand release. The second step has significantly slower reaction kinetics than the combustion reactions, which is consistent with the slow saturation of the GPC with EBECHRu pulse length in Figure 1b. It should be noted that the ligand release during the second step cannot be complete since significant combustion reactions were observed during a subsequent O2 pulse. Whether the surface reactions occurred via the preferential release of a specific ligand (i.e., via either exclusively ethylbenzene or exclusively ethylcyclohexadiene) cannot be answered conclusively based on the experimental results because of limitations in the accuracy of the fragmentation patterns and the possibility of dehydrogenation reactions.54 Additional quantum chemistry simulations will be required to fully understand the surface reactions of EBECHRu on Ru surfaces. Figure 4 shows the time dependence of the partial pressure for m/z = 44, m/z = 79, and m/z = 91 at a lower process temperature of 225 °C. The pulse lengths in this experiment were identical to those used at 325 °C shown in Figure 3. A marked observation at 225 °C was the absence of CO2 release during EBECHRu pulses (Figure 4a and d) except for the longest O2 pulse of 3 s. This indicates that the adsorption of active O on or near the Ru surface was strongly kinetically 4658


Article

Chemistry of Materials limited at this temperature. This is consistent with the strong dependence of the amount of released CO2 on the O2 dose (pulse length), as shown in Figure 4d, which is in stark contrast to the observations at 325 °C (Figure 3d). It also explains the lower GPC for short O2 pulse lengths at 225 °C as compared to that at 325 °C (see Figure 1c). By contrast, the ligand release reactions (Figure 4b,c and e,f) were less affected by temperature. Absolute measured partial pressures at 225 °C were even higher than those at 325 °C, which may be attributed to more EBECHRu doses being available (at constant EBECHRu pulse length) for surface reactions by ligand release due to the absence of combustion reactions. The partial pressure ratio PP91/PP79 was similar to the value obtained at 325 °C, which suggests that the surface chemistry remained rather comparable. The observation that the GPC was almost identical at 225 and 325 °C further suggests that the contribution of the combustion reactions during EBECHRu pulses to the GPC were small and/or that the efficiency of ligand release and combustion reactions were rather similar. It is tempting to attribute the ligand release to the parasitic CVD component due to precursor decomposition, as shown in Figure 1g. Since the measured partial pressures can only be considered as qualitatively accurate, no definitive conclusion based on the relative CVD and ALD GPC can be drawn. However, the CVD GPC in Figure 1g shows a strong thermal activation, which is in contradiction to the QMS observations that the partial pressures of released ligands (their fragments) was higher at lower temperature for a given EBECHRu pulse length. Yet, the surface chemistry of EBECHRu decomposition on a bare Ru surface may differ from that during “steady state” CVD. For example, steric hindrance by residual ligands may reduce the thermal decomposition rate (i.e., the CVD GPC) and explain the signs of saturating behavior of the of EBECHRu adsorption. Effect of TiN Starting Surface on the ALD Behavior. The data in Figure 1 indicate a much higher GPC on TiN starting surfaces as compared to SiO2 for a deposition temperature of 325 °C. Since the difference persisted even for thick (closed) films of the order of 30 nm, it cannot be ascribed to nucleation effects on the starting surfaces. Nucleation effects may lead to growth inhibition or enhancement during the early stages of deposition but should result in a substrate-independent GPC once the film is closed.56 The observations have therefore to be explained by a persistent modification of the Ru surface. To identify the underlying root cause of the surface modification, the composition of the Ru surface was studied during the early stages of ALD on both TiN and SiO2 by ToFSIMS. Figure 5a shows the evolution of the Ru+ and Ti+ ToFSIMS surface yields as a function of the amount of deposited Ru at 225 °C (low GPC) and 325 °C (high GPC). For both temperatures, the Ti+ yield decreased initially, whereas Ru+ increased due to the nucleation of a Ru film on the TiN surface. However, the magnitude of the Ti+ yield decrease was small at 325 °C, and as much as 10% of the Ti+ surface yield of TiN was still observed even after the formation of a nominally 10 nm thick Ru film. By contrast, at 225 °C, the Ti+ yield decreased rapidly to 0.1% of the Ti+ surface yield, close to the Ti+ background level. This can be understood by rapid film closure, close to the expected behavior for the formation of a regular closed 2D film (see Figure 5a).

Figure 5. (a) Normalized Ru+ (closed symbols) and Ti+ (open symbols) yield from ToF-SIMS surface spectra for a series of films with different amounts of ALD Ru (determined by RBS) deposited at 225 °C (circles) and 325 °C (squares). (b) MEIS spectra of ∼5 nm thick Ru films deposited at 325 °C on TiN (blue symbols) and SiO2 (green symbols). ALD of Ru on TiN led to surface segregation of 9 × 1013 Ti at./cm2, whereas no surface Ti was observed after the ALD of Ru on SiO2, as expected (see the inset for a close-up).

The weak decrease of the Ti+ surface yield at 325 °C may at first sight be due to a lack of film closure due to island formation. However, XRR and SEM (not shown) indicated the formation of closed films even for 3−4 nm thick Ru films. The large Ti+ surface yield must thus stem from Ti segregation on the Ru surface during ALD. The Ti concentration on the Ru surface was quantified by MEIS. Figure 5b shows MEIS spectra of about 5 nm thick Ru films deposited at 325 °C on TiN and SiO2. Spectrum simulation (solid line) indicated the presence of 9 × 1013 Ti at./cm2 on the Ru surface. Note that the spectrum was consistent with a closed and regular 5 nm thick Ru film. This clearly demonstrates the segregation of a submonolayer of Ti [possibly in form of Ti(O,N)x] on the Ru surface. To unambiguously demonstrate the effect of segregated Ti(O,N)x on the GPC, Ru was deposited on TiN starting surfaces at 325 °C (see Figure 6a for a schematic of the experiment). The measured GPC was 0.59 Å (see Figure 6b), consistent with the data in Figure 1f. After the deposition of about 5 nm of Ru, the surface was etched by a CF4/CH2F2 dry 4659


Article


Figure 6. (a) Schematic of the experiment: Ru is deposited by ALD on a TiN starting surface, leading to Ti(O,N)x surface segregation. The surface Ti is then removed by dry etching, and the ALD of Ru is continued on the Ti-free surface. (b) Ru ALD growth curves (XRR film thickness vs number of ALD cycles) at 325 °C on TiN and SiO2 starting surfaces (cf. Figure 1f) as well as on Ru/TiN after the removal of Ti by dry etching. (c) XRD pattern of Ru on TiN both with (top curves) and without (bottom curves) etching. Patterns are shown both for as-deposited films as well as those after annealing in forming gas at 420 °C.

process, which removes surface Ti but does not etch Ru. After etching, the Ti surface concentration was below the detection limit of Auger spectrometry (not shown). Subsequently, the ALD was continued, and the GPC was determined again. It was found that the GPC after Ti removal was 0.26 Å (see Figure 6b), significantly lower than that initially and consistent with the GPC on SiO2 (0.25 Å) where no Ti(O,N)x segregation occurs. This clearly indicates that the enhancement of the Ru GPC on TiN can be linked to the presence of Ti on the surface that modifies the surface chemistry. During the ALD, Ti was incorporated at low concentrations into the Ru film. This is evidenced by the Ti SIMS profile in Figure 7a. A concentration of 4−5 × 1018 at./cm3 was found in ALD Ru on the TiN starting surface, well below the solubility limit of Ti in Ru of several at.%.57 A closer look at the evolution of the differential ALD Ru GPC on TiN with the number of cycles (Figure 7b) indicates that the GPC initially increased to ∼5 Ru at./nm2 per cycle followed by a decrease to ∼3 Ru at./ nm2 per cycle. The initial transient may be linked to the buildup of the segregated layer on the surface, whereas the progressing depletion of the surface Ti(O,N)x concentration (see the Ti+ TOF-SIMS surface yield in Figure 7b) led to subsequent reduction of the GPC enhancement toward the value of ∼2 Ru at./nm2 per cycle observed for ALD on SiO2. Note that the incorporation rate was low enough to ensure Ti(O,N)x segregation even for thick layers of the order of several 10 nm. The detailed effects of Ti on the Ru surface chemistry are not yet well understood. In situ QMS measurements are hampered by the fact that reactions not only on the TiN-coated wafer surface but also on the non-TiN-coated reactor walls contribute, and thus the reactant gas phase is still strongly determined by reactions on Ti-free Ru surfaces. Further experimental studies and quantum chemical calculations are therefore required to elucidate the detailed influence of Ti on

Figure 7. (a) Ti SIMS profile of ∼15 nm thick ALD Ru films deposited at 325 °C on a TiN starting surface. Ti incorporation in the mid 1018 at./cm3 range was observed. (b) Differential growth per cycle as well as Ti+ TOF-SIMS surface yield vs ALD cycle number.

the Ru ALD surface chemistry. The underlying physical reason for Ti(O,N)x segregation can, however, be understood by 4660


Article

Chemistry of Materials considering the relative surface energies of Ru and TiN or TiO2. Whereas the energy of the Ru (001) surface is as high as 3.9 J/m2,58 the surface energies of TiN and TiO2 are much lower [1.2 J/m2 and 0.9 J/m2 for TiN (100)59 and rutile TiO2 (110),60 respectively]. Hence, surface Ti(O,N)x lowers significantly the energy of the growing surface and provides thus a strong thermodynamic driving force for surface segregation.61,62 The segregation was found to be temperature-dependent, as it was absent within detection limits at lower temperatures of 225 °C (see Figure 5a). This might be due to temperature-dependent surface thermodynamics or kinetic limitations of the necessary atomic diffusion and exchange processes. In addition, a dependence on the O2 pulse length has been observed (Figure 8). Longer O2 pulses

Figure 8. Growth per cycle and Ti+ ToF-SIMS surface yield of ALD Ru on TiN as a function of the O2 pulse length. The deposition temperature was 325 °C.

led to a reduced amount of segregated Ti(O,N)x, as indicated by the lower Ti+ ToF-SIMS surface yield. This can be correlated to the reduction of the Ru GPC for longer O2 pulses (see Figure 1c). We remark that similar surface segregation phenomena have previously been reported in studies of the interactions of noble metal catalyst nanoparticles with supporting oxide substrates, in particular with TiO2.63−66 Apart from the GPC, the presence of Ti on the surface also affected the nucleation of the layer in the initial stages of ALD. Figure 9a shows the evolution of the rms surface roughness with film thickness on both TiN and SiO 2 . Sample morphologies for selected thicknesses and starting surfaces are shown in Figure 9b−d. For SiO2, a weak nearly linear increase of the rms roughness was observed, very similar to what was observed for the ALD of polycrystalline oxide films.67 Together with the fast film closure observed by TOF-SIMS (not show), this indicates that the ALD of Ru on SiO2 occurred in a 2D Frank−van der Merwe mode. By contrast, the rms roughness increased much faster at the initial stages of ALD on TiN, which can be ascribed to island nucleation (Volmer−Weber mode), followed by island coalescence around 4 nm, which led to a decrease of the roughening rate during further ALD. During this stage, the roughening rate was rather similar to that on SiO2. This indicates that the film closure on TiN was comparatively slow with respect to ALD on SiO2. Physical Properties of ALD Ru Films. Figure 10 shows a 2Θ−ω XRD pattern of 15 nm thick Ru films deposited on SiO2 at temperatures between 225 and 325 °C. All films were polycrystalline, and the patterns were consistent with the

Figure 9. (a) Thickness dependence of the rms roughness measured by AFM a for a series of ALD Ru films deposited at 325 °C on both SiO2 and TiN, as indicated. Panels b−d show the morphology of Ru films for thicknesses and starting surfaces, as indicated.

expected hcp crystal structure of Ru, irrespective of the deposition temperature. The crystallinity increased strongly with deposition temperature, in particular at 325 °C. The crystallinity of the films was also affected by the choice of the starting surface. Figure 11a shows 2Θ−ω XRD patterns of 20 nm thick Ru films deposited on both SiO2 and TiN at 325 °C. While the XRD pattern of the film on TiN was consistent with a random polycrystal, a strong preferential growth along the 4661


Article


the presence of a weak randomly oriented minority component in the film. It can be speculated that the random component stems from randomly oriented grains formed in early phases of the film nucleation on SiO2 that are, however, rapidly overgrown by faster growing (001) oriented grains. The substrate dependence of the film texture can be understood by the segregation of Ti(O,N)x on the Ru surface, as discussed above. The thermodynamic driving force for (001) texture is the lower surface energy of the Ru (001) surface with respect to Ru surfaces with different orientations.58 However, as discussed above, the presence of Ti(O,N)x reduces the energy of the growing surface and therefore counteracts the energetic advantage of the (001) surface. Moreover, if the density and/or structure of the Ti(O,N)x adsorbate layer does not depend strongly on the orientation of the underlying Ru grain, it eliminates the dependence of the surface energy on the Ru grain orientation altogether and therefore any driving force for texturing. Hence, it can be argued that the presence of segregated Ti(O,N)x reduces or removes the tendency of Ru films to texture. Note that the texture of films deposited on SiO2 at lower deposition temperatures, in particular at the lowest studied temperature of 225 °C, was much weaker or even absent. This suggests that the film microstructure at such deposition temperatures was dominated by kinetic processes rather than the thermodynamically most stable configuration. The effect of Ti(O,N)x segregation on the texture of Ru films was confirmed by the intermediate selective Ti removal after 5 nm of ALD on TiN, as discussed above for the effect on the GPC. Figure 6c shows a more pronounced (001) texturing (as indicated by the larger (002)/(101) peak intensity ratio) for the film after Ti removal and successive deposition compared to that for both the initial 5 nm thick film prior to Ti removal as well as a film of similar thickness deposited on TiN without

Figure 10. X-ray diffraction pattern of ∼15 nm thick ALD Ru films deposited on SiO2 at the indicated temperatures.

[001] direction was observed in the case of the SiO2 starting surfaces, consistent with Figure 10. On SiO2, only very weak (100) and (101) reflections were visible (Figure 11b), which indicates strong (001) texture with a small randomly oriented minority component. This behavior was confirmed by (002) rocking curves (Figure 11c) for both SiO2 and TiN. On TiN, the weak intensity enhancement around 2ω = 21° indicated incipient texturing of the Ru film. On SiO2, a high degree of mosaic (001) texturing was observed with an average (002) tilt misorientation of about ±3.5°, as deduced from the full-widthat-half-maximum of the rocking curve. The (101) pole figure of Ru/SiO2 in Figure 11d demonstrates (001) fiber texture with a random grain orientation in the plane of the film and confirms

Figure 11. Crystallinity of ALD Ru deposited at 325 °C. (a) 2Θ−ω XRD scan of ∼20 nm thick ALD Ru films on SiO2 and TiN starting surfaces, respectively. Panel b shows the 2Θ−ω XRD scan for Ru/SiO2 on a logarithmic intensity scale. (c) Wide-angle ω-scans (rocking curves) for the Ru (002) reflection (2Θ = 42.2°) indicating strong (001) texture for Ru/SiO2 but an almost random grain orientation (with a very weak (001) textured component) for Ru/TiN. (d) Ru (101) pole Figure (2Θ = 44.0°) for Ru/SiO2 confirming strong (001) fiber texture together with a weak component of randomly oriented grains. 4662


Article


resistivity for (001) textured films,68 opposite to that observed here, and therefore, the substrate dependence can only be ascribed to differences in film microstructure. For thinner films on TiN, the resistivity increased with decreasing thickness due to an increasing influence of surface and grain boundary scattering.69 However, on SiO2, the resistivity was almost independent of the film thickness down to about 6 nm. The sharp rise of the resistivity for thinner films below 5 nm can be tentatively attributed to an increasing discontinuity of the films on SiO2 since film closure was observed between 3 and 5 nm (data not shown). On both starting surfaces, post-deposition annealing at 420 °C reduced the resistivity due to recrystallization and grain growth. As a result, Ru films with a resistivity as low as 11 μΩcm were obtained for a 6 nm thick film.

intermediate Ti removal. Post-deposition annealing at 420 °C in forming gas reinforced this trend. This confirms the close relationship of film texturing and surface segregation. In addition to grain orientation, the underlying substrate also affected the average grain size. This is visible in the crosssectional TEM images in Figure 12, which indicate typical

■

CONCLUSIONS We have demonstrated the atomic layer deposition of Ru from EBECHRu and O2 in a temperature range between 225 and 325 °C. In particular, the influence of the underlying starting surface (SiO2 or TiN) was studied. Saturation of the surface reactions of O2 was found on both starting surfaces. Only signs of incipient saturation were however visible as a function of the EBECHRu pulse length on both starting surfaces. This may partially stem from a parasitic CVD component due to thermal decomposition, although measured steady state CVD deposition rates were low (about 1.5% of the ALD GPC at the same EBECHRu pulse length of 5 s). However, a much higher steady state GPC was observed at 325 °C on TiN. The ALD surface chemistry on SiO2 starting surfaces was studied using in situ mass spectrometry of the reaction products in the gas phase. EBECHRu surface reactions occurs initially by combustion reactions due to sorbed active O, followed by additional surface reactions via ligand release. Exposure to O2 in the subsequent half cycle leads to the combustion of remaining ligands and the sorption of active O on the surface. Detailed studies of the surface composition of ALD Ru deposited on TiN starting surfaces evidenced the segregation of a submonolayer of Ti(O,N)x on the growing Ru surface at a deposition temperature of 325 °C. This surface layer acted as a surfactant and had a large influence on the process behavior, leading to the 2-fold increase of the GPC with respect to Ru ALD on SiO2. These results indicate that surfactants can also be used to modify the surface chemistry and the growth behavior of ALD processes, introducing surfactants as an additional control parameter to design and optimize ALD processes. Similar surfactant effects have been observed for I on the CVD of Cu70−72 and Mn.72 Surfactant effects have also been reported for various systems in the molecular beam epitaxy of metallic films.73−75 The Ti(O,N)x surface layer also had an impact on the resulting film properties. Although all deposited films were polycrystalline, films on SiO2 [without a Ti(O,N)x surface layer] showed strong (001) fiber texture. By contrast, the fiber texture was suppressed on TiN starting surfaces, and an almost randomly oriented polycrystal was formed with grain sizes much smaller than those on SiO2. This can be explained by the lowering of the surface energy due to segregating Ti(O,N)x that reduced or even removed the thermodynamic driving forces for texture formation. Finally, the resistivity of the films was studied. Films on SiO2 had lower resistivities with values as low as 11 μΩcm for 6 nm thick films after post-deposition annealing at 420 °C. Such ALD Ru films with low resistivity

Figure 12. Cross-sectional transmission electrographs of about 10 nm thick Ru films deposited on (a) SiO2 and (b) TiN starting surfaces.

lateral grain sizes of 20−25 nm for a 10 nm thick Ru film on SiO2. By contrast, much smaller grains with lateral sizes of about 10 nm were observed for a film of similar thickness (12 nm) deposited on a 10 nm thick TiN layer. The much higher surface roughness visible in the TEM image of the film on TiN is consistent with the AFM data in Figure 9. Finally, Figure 13 shows the resistivity of Ru films deposited at 325 °C on both SiO2 and TiN. The resistivity of films thicker than ∼15 nm deposited on SiO2 was about 16 μΩcm, slightly lower than that for the films on TiN (19 μΩcm). The dependence of the resistivity on the substrate can be linked to the difference in grain sizes (Figure 12). Note that the anisotropy of the Ru resistivity should lead to an increased

Figure 13. Resistivity of ALD Ru thin films deposited at 325 °C on TiN and SiO2 starting surfaces, both as deposited and after annealing at 420 °C for 20 min in forming gas. 4663


Article


for Sub-20 nm Technology Node in Dynamic Random Access Memory. Appl. Phys. Lett. 2014, 104, 082908. (7) Suh, Y.-S.; Lazar, H.; Chen, B.; Lee, J.-H.; Misra, V. Electrical Characteristics of HfO2 Dielectrics with Ru Metal Gate Electrodes. J. Electrochem. Soc. 2005, 152, F138−F141. (8) Li, Z.; Schram, T.; Pantisano, L.; Conard, T.; Van Elshocht, S.; Deweerd, W.; De Gendt, S.; De Meyer, K.; Stesmans, A.; Shamuilia, S.; Afanas’ev, V. V.; Akheyar, A.; Brunco, D. P.; Yamada, N.; Lehnen, P. Flatband Voltage Shift of Ruthenium Gated Stacks and its Link with the Formation of a Thin Ruthenium Oxide Layer at the Ruthenium/ Dielectric Interface. J. Appl. Phys. 2007, 101, 034503. (9) Rangan, S.; Bersch, E.; Bartynski, R. A.; Garfunkel, E.; Vescovo, E. Band Offsets of a Ruthenium Gate on Ultrathin High-κ Oxide Films on Silicon. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 75106. (10) Yang, C.; Cohen, S.; Shaw, T.; Wang, P.-C.; Nogami, T.; Edelstein, D. Characterization of “Ultrathin-Cu”/Ru(Ta)/TaN Liner Stack for Copper Interconnects. IEEE Electron Device Lett. 2010, 31, 722−724. (11) Armini, S.; El-Mekki, Z.; Swerts, J.; Nagar, M.; Demuynck, S. Direct Copper Electrochemical Deposition on Ru-Based Substrates for Advanced Interconnects Target 30 nm and 1/2 Pitch Lines: From Coupon to Full-Wafer Experiments. J. Electrochem. Soc. 2013, 160, D89−D94. (12) Adelmann, C.; Wen, L. G.; Peter, A. P.; Siew, Y. K.; Croes, K.; Swerts, J.; Popovici, M.; Sankaran, K.; Pourtois, G.; Van Elshocht, S.; Bömmels, J.; Tő kei, Z. Alternative Metals for Advanced Interconnects. Proc. IEEE Int. Interconnect Technol. Conf. 2014, 173−176. (13) Ť apajna, M.; Rosová, A.; Dobročka, E.; Štrbík, V.; Gaži, Š.; Fröhlich, K.; Benko, P.; Harmatha, L.; Manke, C.; Baumann, P. K. Work Function Thermal Stability of RuO2-Rich Ru−Si−O p-Channel Metal-Oxide-Semiconductor Field-Effect Transistor Gate Electrodes. J. Appl. Phys. 2008, 103, 073702. (14) Leskelä, M.; Ritala, M. Atomic Layer Deposition Chemistry: Recent Developments and Future Challenges. Angew. Chem., Int. Ed. 2003, 42, 5548−5554. (15) Elam, J. W.; Zinovev, A.; Han, C. Y.; Wang, H. H.; Welp, U.; Hryn, J. N.; Pellin, M. J. Atomic Layer Deposition of Palladium Films on Al2O3 Surfaces. Thin Solid Films 2006, 515, 1664−1673. (16) Christensen, S. T.; Elam, J. W.; Lee, B.; Feng, Z.; Bedzyk, M. J.; Hersam, M. C. Nanoscale Structure and Morphology of Atomic Layer Deposition Platinum on SrTiO3 (001). Chem. Mater. 2009, 21, 516− 521. (17) Baker, L.; Cavanagh, A. S.; Seghete, D.; George, S. M.; Mackus, A. J. M.; Kessels, W. M. M.; Liu, Z. Y.; Wagner, F. T. Nucleation and Growth of Pt Atomic Layer Deposition on Al2O3 Substrates Using (Methylcyclopentadienyl)-Trimethyl Platinum and O2 Plasma. J. Appl. Phys. 2011, 109, 084333. (18) Mackus, A. J. M.; Verheijen, M. A.; Leick, N.; Bol, A. A.; Kessels, W. M. M. Influence of Oxygen Exposure on the Nucleation of Platinum Atomic Layer Deposition: Consequences for Film Growth, Nanopatterning, and Nanoparticle Synthesis. Chem. Mater. 2013, 25, 1905−1911. (19) Weber, M. J.; Mackus, A. J. M.; Verheijen, M. A.; Longo, V.; Bol, A. A.; Kessels, W. M. M. Atomic Layer Deposition of High-Purity Palladium Films from Pd(hfac)2 and H2 and O2 Plasmas. J. Phys. Chem. C 2014, 118, 8702−8711. (20) Swerts, J.; Delabie, A.; Salimullah, M. M.; Popovici, M. I.; Kim, M.-S.; Schaekers, M.; Van Elshocht, S. Impact of the Plasma Ambient and the Ruthenium Precursor on the Growth of Ruthenium Films by Plasma Enhanced Atomic Layer Deposition. ECS Solid State Lett. 2012, 1, P19−P21. (21) Kim, S. K.; Lee, S. Y.; Lee, S. W.; Hwang, G. W.; Hwang, C. S.; Lee, J. W.; Jeong, J. Atomic Layer Deposition of Ru Thin Films Using 2,4-(Dimethylpentadienyl) (ethylcyclopentadienyl)Ru by a Liquid Injection System. J. Electrochem. Soc. 2007, 154, D95−D101. (22) Hämäläinen, J.; Ritala, M.; Leskelä, M. Atomic Layer Deposition of Noble Metals and Their Oxides. Chem. Mater. 2014, 26, 786−801.

have already shown great promise for applications in advanced interconnects with metallization schemes beyond Cu.76,77

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Annelies Delabie: 0000-0001-9739-7419 Christoph Adelmann: 0000-0002-4831-3159 Present Addresses

K.M.: Vrije Universiteit Brussel, Department of Materials and Chemistry, Research Group SURF, B-1050 Brussels, Belgium. M.J.: ASM Belgium, B-3001 Leuven, Belgium. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS Vasile Paraschiv from SC ETCH-TECH Solutions is kindly acknowledged for the selective Ti etch experiments. ABBREVIATIONS AFM, atomic force microscopy; ALD, atomic layer deposition; DFT, density functional theory; EBECHRu, (ethylbenzyl) (1ethyl-1,4-cyclohexadienyl) Ru(0); ERDA, elastic recoil detection analysis; GPC, growth per cycle; QMS, quadrupole mass spectrometry; RBS, Rutherford backscattering spectrometry; SEM, scanning electron microscopy; SIMS, secondary ion mass spectrometry; TEM, transmission electron microscopy; ToFSIMS, time-of-flight secondary ion mass spectrometry; XRD, Xray diffraction; XRR, X-ray reflectivity

■

REFERENCES

(1) Kittl, J. A.; Opsomer, K.; Popovici, M.; Menou, N.; Kaczer, B.; Wang, X. P.; Adelmann, C.; Pawlak, M. A.; Tomida, K.; Rothschild, A.; Govoreanu, B.; Degraeve, R.; Schaekers, M.; Zahid, M.; Delabie, A.; Meersschaut, J.; Polspoel, W.; Clima, S.; Pourtois, G.; Knaepen, W.; Detavernier, C.; Afanas’ev, V. V.; Blomberg, T.; Pierreux, D.; Swerts, J.; Fischer, P.; Maes, J. W.; Manger, D.; Vandervorst, W.; Conard, T.; Franquet, A.; Favia, P.; Bender, H.; Brijs, B.; Van Elshocht, S.; Jurczak, M.; Van Houdt, J.; Wouters, D. J. High-K Dielectrics for Future Generation Memory Devices. Microelectron. Eng. 2009, 86, 1789− 1795. (2) Ahn, J.-H.; Kim, J.-Y.; Kang, S.-W.; Kim, J.-H.; Roh, J.-S. Increment of Dielectric Properties of SrTiO3 Thin Films by SrO Interlayer on Ru Bottom Electrodes. Appl. Phys. Lett. 2007, 91, 062910. (3) Kim, S. K.; Choi, G.-J.; Lee, S. Y.; Seo, M.; Lee, S. W.; Han, J. H.; Ahn, H.-S.; Han, S.; Hwang, C. S. Al-Doped TiO2 Films with Ultralow Leakage Currents for Next Generation DRAM Capacitors. Adv. Mater. 2008, 20, 1429−1435. (4) Zhao, C.; Pawlak, M.; Popovici, M. I.; Schaekers, M.; Sleeckx, E.; Vancoille, E.; Wouters, D.; Tő kei, Z.; Kittl, J. Atomic Layer Deposition of Ru and RuO2 for MIMCAP Applications. ECS Trans. 2009, 25, 377−387. (5) Pawlak, M. A.; Kaczer, B.; Kim, M.-S.; Popovici, M. I.; Swerts, J.; Wang, W.-C.; Opsomer, K.; Favia, P.; Tomida, K.; Belmonte, A.; Govoreanu, B.; Vrancken, C.; Demeurisse, C.; Bender, H.; Afanas’ev, V. V.; Debusschere, I.; Altimime, L.; Kittl, J. A. Impact of Bottom Electrode and SrxTiyOz Film Formation on Physical and Electrical Properties of Metal-Insulator-Metal Capacitors. Appl. Phys. Lett. 2011, 98, 182902. (6) Popovici, M. I.; Swerts, J.; Redolfi, A.; Kaczer, B.; Aoulaiche, M.; Radu, I.; Clima, S.; Everaert, J.-L.; Van Elshocht, S.; Jurczak, M. Low Leakage Ru-Strontium Titanate-Ru Metal-Insulator-Metal Capacitors 4664


Article


(42) Peterson, K. A.; Figgen, D.; Dolg, M.; Stoll, H. EnergyConsistent Relativistic Pseudopotentials and Correlation Consistent Basis Sets for the 4d Elements Y−Pd. J. Chem. Phys. 2007, 126, 124101. (43) Han, J. H.; Nyns, L.; Delabie, A.; Franquet, A.; Van Elshocht, S.; Adelmann, C. Reaction Chemistry during the Atomic Layer Deposition of Sc2O3 and Gd2O3 from Sc(MeCp)3, Gd(iPrCp)3, and H2O. Chem. Mater. 2014, 26, 1404−1412. (44) Han, J. H.; Ungur, E.; Franquet, A.; Opsomer, K.; Conard, T.; Moussa, A.; De Gendt, S.; Van Elshocht, S.; Adelmann, C. Atomic Layer Deposition of Tantalum Oxide and Tantalum Silicate from TaCl5, SiCl4, and O3: Growth Behaviour and Film Characteristics. J. Mater. Chem. C 2013, 1, 5981. (45) Knapas, K.; Ritala, M. In Situ Studies on Reaction Mechanisms in Atomic Layer Deposition. Crit. Rev. Solid State Mater. Sci. 2013, 38, 167−202. (46) Knapas, K.; Ritala, M. In Situ Reaction Mechanism Studies on Atomic Layer Deposition of ZrO2 from (CpMe)2Zr(OMe)Me and Water or Ozone. Chem. Mater. 2008, 20, 5698−5705. (47) Kaipio, M.; Blanquart, T.; Tomczak, Y.; Niinistö, J.; Gavagnin, M.; Longo, V.; Wanzenböck, H. D.; Pallem, V. R.; Dussarrat, C.; Puukilainen, E.; Ritala, M.; Leskelä, M. Atomic Layer Deposition, Characterization, and Growth Mechanistic Studies of TiO2 Thin Films. Langmuir 2014, 30, 7395−7404. (48) Knapas, K.; Ritala, M. In Situ Reaction Mechanism Studies on Atomic Layer Deposition of Ir and IrO2 from Ir(acac)3. Chem. Mater. 2011, 23, 2766−2771. (49) Madey, T. E.; Engelhardt, H. A.; Menzel, D. Adsorption of Oxygen and Oxidation of CO on the Ruthenium (001) Surface. Surf. Sci. 1975, 48, 304−328. (50) Böttcher, A.; Niehus, H. Formation of Subsurface Oxygen at Ru(0001). J. Chem. Phys. 1999, 110, 3186−3195. (51) Stampfl, C.; Schwegmann, S.; Over, H.; Scheffler, M.; Ertl, G. Structure and Stability of a High-Coverage (1 × 1) Oxygen Phase on Ru(0001). Phys. Rev. Lett. 1996, 77, 3371−3374. (52) Böttcher, A.; Niehus, H. Oxygen Adsorbed on Oxidized Ru(0001). Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60, 14396− 14404. (53) NIST Mass Spec Data Center. Mass Spectra. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD. (54) Mackus, A. J. M.; Leick, N.; Baker, L.; Kessels, W. M. M. Catalytic Combustion and Dehydrogenation Reactions during Atomic Layer Deposition of Platinum. Chem. Mater. 2012, 24, 1752−1761. (55) Han, J. H.; Delabie, A.; Franquet, A.; Conard, T.; Van Elshocht, S.; Adelmann, C. Ozone-Based Atomic Layer Deposition of Gd2O3 from Tris(isopropyl-Cyclopentadienyl)gadolinium: Growth Characteristics and Surface Chemistry. Chem. Vap. Deposition 2015, 21, 352− 359. (56) Puurunen, R. L.; Vandervorst, W. Island Growth as a Growth Mode in Atomic Layer Deposition: A Phenomenological Model. J. Appl. Phys. 2004, 96, 7686−7695. (57) Murray, J. L. The Ru- Ti (Ruthenium−Titanium) System. Bull. Alloy Phase Diagrams 1982, 3, 216−221. (58) Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollár, J. The Surface Energy of Metals. Surf. Sci. 1998, 411, 186−202. (59) Marlo, M.; Milman, V. Density-Functional Study of Bulk and Surface Properties of Titanium Nitride Using Different ExchangeCorrelation Functionals. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 2899. (60) Ramamoorthy, M.; Vanderbilt, D.; King-Smith, R. D. FirstPrinciples Calculations of the Energetics of Stoichiometric TiO2 Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 16721−16727. (61) Abraham, F. F.; Brundle, C. R. Surface Segregation in Binary Solid Solutions: A Theoretical and Experimental Perspective. J. Vac. Sci. Technol. 1981, 18, 506.

(23) Aaltonen, T.; Alén, P.; Ritala, M.; Leskelä, M. Ruthenium Thin Films Grown by Atomic Layer Deposition. Chem. Vap. Deposition 2003, 9, 45−49. (24) Park, K. J.; Terry, D. B.; Stewart, S. M.; Parsons, G. N. In Situ Auger Electron Spectroscopy Study of Atomic Layer Deposition: Growth Initiation and Interface Formation Reactions during Ruthenium ALD on Si−H, SiO2, and HfO2 Surfaces. Langmuir 2007, 23, 6106−6112. (25) Kim, W.-H.; Park, S.-J.; Kim, D.; Kim, H. Atomic Layer Deposition of Ruthenium and Ruthenium-Oxide Thin Films by Using a Ru(EtCp)2 Precursor and Oxygen Gas. J. Korean Phys. Soc. 2009, 55, 32. (26) Yim, S.-S.; Lee, D.-J.; Kim, K.-S.; Kim, S.-H.; Yoon, T.-S.; Kim, K.-B. Nucleation Kinetics of Ru on Silicon Oxide and Silicon Nitride Surfaces Deposited by Atomic Layer Deposition. J. Appl. Phys. 2008, 103, 113509. (27) Kang, S. Y.; Lim, H. J.; Hwang, C. S.; Kim, H. J. Metallorganic Chemical Vapor Deposition of Ru Films Using CyclopentadienylPropylcyclopentadienylruthenium(II) and Oxygen. J. Electrochem. Soc. 2002, 149, C317. (28) Kim, S. K.; Han, J. H.; Kim, G. H.; Hwang, C. S. Investigation on the Growth Initiation of Ru Thin Films by Atomic Layer Deposition. Chem. Mater. 2010, 22, 2850−2856. (29) Eom, T.-K.; Sari, W.; Choi, K.-J.; Shin, W.-C.; Kim, J. H.; Lee, D.-J.; Kim, K.-B.; Sohn, H.; Kim, S.-H. Low Temperature Atomic Layer Deposition of Ruthenium Thin Films Using Isopropylmethylbenzene-Cyclohexadiene-Ruthenium and O2. Electrochem. Solid-State Lett. 2009, 12, D85. (30) Kim, S.-H. Low Temperature Atomic Layer Deposition of Ru Thin Films with Enhanced Nucleation Using Various Ru(0) Metallorganic Precursors and Molecular O2. ECS Trans. 2011, 41, 19−23. (31) Schneider, A.; Popovska, N.; Jipa, I.; Atakan, B.; Siddiqi, M. A.; Siddiqui, R.; Zenneck, U. Minimizing the Carbon Content of Thin Ruthenium Films by MOCVD Precursor Complex Design and Process Control. Chem. Vap. Deposition 2007, 13, 389−395. (32) Hong, T. E.; Choi, S.-H.; Yeo, S.; Park, J.-Y.; Kim, S.-H.; Cheon, T.; Kim, H.; Kim, M.-K.; Kim, H. Atomic Layer Deposition of Ru Thin Films Using a Ru(0) Metallorganic Precursor and O2. ECS J. Solid State Sci. Technol. 2013, 2, P47−P53. (33) Wang, H.; Gordon, R. G.; Alvis, R.; Ulfig, R. M. Atomic Layer Deposition of Ruthenium Thin Films from an Amidinate Precursor. Chem. Vap. Deposition 2009, 15, 312−319. (34) Methaapanon, R.; Geyer, S. M.; Brennan, S.; Bent, S. F. Size Dependent Effects in Nucleation of Ru and Ru Oxide Thin Films by Atomic Layer Deposition Measured by Synchrotron Radiation X-Ray Diffraction. Chem. Mater. 2013, 25, 3458−3463. (35) Knizia, G.; Adler, T. B.; Werner, H.-J. Simplified CCSD(T)-F12 Methods: Theory and Benchmarks. J. Chem. Phys. 2009, 130, 054104. (36) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456−1465. (37) Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. Electronic Structure Calculations on Workstation Computers: The Program System Turbomole. Chem. Phys. Lett. 1989, 162, 165−169. (38) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (39) Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M. Molpro: A General-Purpose Quantum Chemistry Program Package. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 242−253. (40) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. Electron Affinities of the First-row Atoms Revisited. Systematic Basis Sets and Wave Functions. J. Chem. Phys. 1992, 96, 6796−6806. (41) Dunning, T. H., Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. 4665


Article

Chemistry of Materials (62) Dowben, P. A.; Miller, A. Surface Segregation Phenomena, 1st ed.; CRC Press: Boca Raton, FL, 1990. (63) Tauster, S. J.; Fung, S. C.; Garten, R. L. Strong Metal-Support Interactions. Group 8 Noble Metals Supported on Titanium Dioxide. J. Am. Chem. Soc. 1978, 100, 170−175. (64) Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Strong Interactions in Supported-Metal Catalysts. Science 1981, 211, 1121− 1125. (65) Tauster, S. J. Strong Metal-Support Interactions. Acc. Chem. Res. 1987, 20, 389−394. (66) Hadjiivanov, K. I.; Klissurski, D. G. Surface Chemistry of Titania (Anatase) and Titania-Supported Catalysts. Chem. Soc. Rev. 1996, 25, 61−69. (67) Premkumar, P. A.; Delabie, A.; Rodriguez, L. N.; Moussa, A.; Adelmann, C. Roughness Evolution during the Atomic Layer Deposition of Metal Oxides. J. Vac. Sci. Technol., A 2013, 31, 061501. (68) Volkenshtejn, N. V.; Startsev, V. E.; Cherepanov, V. I.; Azhazha, V. M.; Kovtun, G. P.; Elenskij, V. A. Electric resistance anisotropy of ruthenium and rhenium single crystals at 2−300 K. Phys. Met. Met. 1978, 45, 54−65. (69) Mayadas, A. F.; Shatzkes, M. Electrical-Resistivity Model for Polycrystalline Films: The Case of Arbitrary Reflection at External Surfaces. Phys. Rev. B 1970, 1, 1382−1389. (70) Shim, K.-C.; Lee, H.-B.; Kwon, O.-K.; Park, H.-S.; Koh, W.; Kang, S.-W. Bottom-up Filling of Submicrometer Features in CatalystEnhanced Chemical Vapor Deposition of Copper. J. Electrochem. Soc. 2002, 149, G109−G113. (71) Josell, D.; Wheeler, D.; Moffat, T. P. Superconformal Deposition by Surfactant-Catalyzed Chemical Vapor Deposition. Electrochem. Solid-State Lett. 2002, 5, C44−C47. (72) Au, Y.; Lin, Y.; Gordon, R. G. Filling Narrow Trenches by Iodine-Catalyzed CVD of Copper and Manganese on Manganese Nitride Barrier/Adhesion Layers. J. Electrochem. Soc. 2011, 158, D248−D253. (73) van der Vegt, H. A.; van Pinxteren, H. M.; Lohmeier, M.; Vlieg, E.; Thornton, J. M. C. Surfactant-Induced Layer-by-Layer Growth of Ag on Ag(111). Phys. Rev. Lett. 1992, 68, 3335−3338. (74) Esch, S.; Hohage, M.; Michely, T.; Comsa, G. Origin of Oxygen Induced Layer-by-Layer Growth in Homoepitaxy on Pt(111). Phys. Rev. Lett. 1994, 72, 518−521. (75) Camarero, J.; Ferrón, J.; Cros, V.; Gómez, L.; Vázquez de Parga, A. L.; Gallego, J. M.; Prieto, J. E.; de Miguel, J. J.; Miranda, R. Atomistic Mechanism of Surfactant-Assisted Epitaxial Growth. Phys. Rev. Lett. 1998, 81, 850−853. (76) Wen, L. G.; Adelmann, C.; Pedreira, O. V.; Dutta, S.; Popovici, M.; Briggs, B.; Heylen, N.; Vanstreels, K.; Wilson, C. J.; Van Elshocht, S.; Croes, K.; Bömmels, J.; Tő kei, Z. Ruthenium Metallization for Advanced Interconnects. Proc. IEEE Int. Interconnect Technol. Conf. 2016, 34−36. (77) Wen, L. G.; Roussel, P.; Pedreira, O. V.; Briggs, B.; Groven, B.; Dutta, S.; Popovici, M. I.; Heylen, N.; Ciofi, I.; Vanstreels, K.; Østerberg, F. W.; Hansen, O.; Petersen, D. H.; Opsomer, K.; Detavernier, C.; Wilson, C. J.; Elshocht, S. V.; Croes, K.; Bömmels, J.; Tő kei, Z.; Adelmann, C. Atomic Layer Deposition of Ruthenium with TiN Interface for Sub-10 nm Advanced Interconnects beyond Copper. ACS Appl. Mater. Interfaces 2016, 8, 26119−26125.

4666


Atomic Layer Deposition of Ruthenium Thin Films from (Ethylbenzyl

Recommend Documents