Low Temperature ABC-Type Ru Atomic Layer Deposition through

Article pubs.acs.org/cm

Low Temperature ABC-Type Ru Atomic Layer Deposition through Consecutive Dissociative Chemisorption, Combustion, and Reduction Steps Junling Lu† and Jeffrey W. Elam* Energy Systems Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ABSTRACT: Thermal atomic layer deposition (ALD) of noble metals is frequently performed using molecular oxygen as the nonmetal precursor to effect combustion-type chemistry at relatively high temperatures of 300 °C. Bis(ethylcyclopentadienyl)ruthenium (Ru(EtCp)2) is one of the commonly used metal precursors for Ru ALD. Using Ru(EtCp)2 and oxygen as reactants, Ru ALD was achieved at near 300 °C. Here, we demonstrate that Ru ALD can proceed at as low as 150 °C by using successive exposures to oxygen and hydrogen as the coreactants. In situ quartz crystal microbalance (QCM) and quadrupole mass spectroscopy (QMS) measurements both suggest that this ABC-type ALD occurs through dissociative chemisorption, combustion, and reduction for the Ru(EtCp)2, oxygen, and hydrogen steps, respectively, in a similar manner to processes using ozone and hydrogen as coreactants reported previously. Moreover, we believe this molecular O2 and H2 based ABC-type ALD could be exploited for the ALD of other noble metals to decrease the deposition temperature and reduce oxygen impurities.

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INTRODUCTION Ultrathin conformal Ru films have been used as electrodes in a variety of microelectronics applications including positivechannel metal oxide semiconductor (PMOS) metal gates and dynamic random access memory (DRAM) capacitors, due to the high work function (4.7 eV), low bulk resistivity (7.1 μΩ· cm), good thermal stability, and, more importantly, oxygen diffusion barrier property of Ru.1−5 As the size of semiconductor devices decreases according to Moore’s Law, complicated three-dimensional (3-D) nanostructures will be needed to achieve high storage capacities.6−9 As a consequence, the requirements for conformality, thickness control, and low temperature of the Ru deposition process become more severe. Among the various thin film deposition techniques, atomic layer deposition (ALD) is considered to be the most promising for meeting these requirements.10−12 Ruthenium ALD has been widely explored using a variety of Ru precursors including metallocenes, β-diketonates, and their derivatives.13−21 In these studies, molecular oxygen is most often employed as the nonmetal precursor for thermal Ru ALD. As with similar noble metal ALD processes utilizing O2 (e.g., Pt, Rh, and Ir), the Ru ALD is thought to follow a combustiontype mechanism in which the metal organic precursor first reacts with surface oxygen to burn off a fraction of the ligands, and then the rest of the ligands are further combusted in the subsequent oxygen exposure step. The oxygen exposure also serves to replenish the surface oxygen through dissociation on the noble metal surface. Carbon dioxide and water are the major gaseous products formed during the two half reactions,13,22−25 while in some cases dehydrogenation can also occur.24,26 © 2015 American Chemical Society

This oxygen-based, combustion-type noble metal ALD typically requires relatively high temperature above 200 °C (near 300 °C in most cases), to burn off the organic ligands.13−20,25,27,28 The most well-studied Ru ALD precursor is bis(ethylcyclopentadienyl)ruthenium (Ru(EtCp)2).15,17,18,29 Kang et al. demonstrated Ru ALD at near 300 °C using Ru(EtCp)2 with oxygen;15 Park and co-workers further showed the growth temperature can be lowered to 225 °C using Ru(EtCp)2 with ozone.17 Achieving metallic ALD Ru films using oxygen requires careful control over the oxygen exposure time, flow rate, and concentrations to avoid forming RuO2 or even etching of the Ru through formation of the volatile RuO4.15,18,25,29 Thus, a thermal ALD process for metallic Ru films at low temperatures, without the high sensitivity to oxygen dosing conditions, is still missing. Hämäläinen and co-workers first reported that adding an additional reduction step to each IrO2 ALD cycle in an ABCtype process (Ir(acac)3 (acac = acetylacetonato)−O3−H2) can produce metallic Ir films at temperatures as low as 165 °C.30 According to the observation of no Hacac or any byproducts released during the Ir(acac)3 exposure by in situ quadrupole mass spectroscopy (QMS), the authors suggested that Ir(acac)3 adsorbed molecularly on the Ir metal surface during this ABCtype ALD process, similar to the case of AB-type Ir ALD with a sequence of Ir(acac)3-O3, where Ir(acac)3 was also molecularly adsorbed on the Ir oxide surface.22 This ABC-type ALD process using ozone and molecular hydrogen as coreactants was Received: March 3, 2015 Revised: June 23, 2015 Published: June 25, 2015 4950

DOI: 10.1021/acs.chemmater.5b00818 Chem. Mater. 2015, 27, 4950−4956

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Chemistry of Materials

Figure 1. In situ QCM (a) and QMS (b) measurements of AB-type Ru ALD using the sequence Ru(EtCp)2−O2 on a Ru oxide film at 200 °C. In situ QCM (c) and QMS (d) measurements of AB-type Ru ALD using the sequence Ru(EtCp)2−H2 on a Ru metal film at 200 °C. The inset is the expanded view of mass gain in the first three ALD cycles. The sequential exposure steps are shown as columns in color at the bottom of the inset figure; the nitrogen purge step is in between the columns. downstream of the flow tube in a differentially pumped chamber separated from the reactor tube by a 35 μm orifice and evacuated using a 50 L/s turbomolecular pump. A quartz crystal microbalance (QCM) was mounted in a commercial QCM housing modified to allow a nitrogen purge that prevents growth on the back of the sensor. The QCM was installed in the middle of the reactor tube for in situ monitoring of the ALD. Ru ALD. Ru(EtCp)2 (Sigma-Aldrich, 98%) was used as the Ru precursor and was contained in a stainless steel bubbler heated to 80 °C to increase the vapor pressure.35 Ultrahigh purity nitrogen with a flow rate of 50 sccm passed through the bubbler and carried the Ru(EtCp)2 vapor to the reaction chamber. The precursor inlet lines were heated to 150 °C to prevent condensation of the Ru precursor. Ru ALD was first tested at 200 °C using Ru(EtCp)2 with either oxygen (99.999%) or hydrogen (99.99%) as coreactant; next, ABC-type Ru ALD was carried out using the sequence Ru(EtCp)2−O2−H2 at 200 °C. This ABC-type Ru ALD was further examined at 150 °C. In this work, the conventional (AB-type) ALD timing sequences are expressed as t1−t2−t3−t4, corresponding to the metal precursor exposure time, the metal precursor purge time, the coreactant exposure time, and the coreactant purge time, respectively, with all times in seconds (s). Similarly the ABC-type ALD timing sequences are expressed as t1−t2−t3−t4−t5−t6. In Situ QCM and QMS Measurements. In situ QCM and QMS were employed to investigate the growth mechanism of Ru ALD at 200 and 150 °C, respectively. In order to facilitate metallic Ru or Ru oxide growth on the QCM, we first deposited an ∼5 nm thick ALD Pt film using trimethyl(methylcyclopentadienyl)-platinum(IV) (MeCpPtMe3, Sigma-Aldrich, 98%) and O3 at 200 °C for 100 cycles (2−2−2−2);35 next a ∼4 nm Ru film was deposited on the Pt film using the sequence Ru(EtCp)2−O2−H2 (3−5−5−10−5−5) at 200 °C for 200 cycles. To investigate the Ru ALD at 200 °C using O2 as the

successfully extended to three additional noble metals: Rh, Pt, and Pd.31,32 Very recently, Hämäläinen and co-workers published a review paper, detailing the growth conditions, surface chemistries, etc., of ALD of noble metals and their oxides.25 However, low temperature ABC-type Ru ALD has not yet been reported. Furthermore, using molecular oxygen instead of ozone with hydrogen as coreactants has not yet been tested in low temperature ABC-type noble metal ALD, even though molecular oxygen can offer a possibility for better conformality in high aspect ratio features than ozone, given that ozone decomposes rapidly on metal surfaces and can even etch the metal by creating volatile, high-oxidation-state oxides.25,31,33 Here we report that metallic Ru ALD can be achieved at temperatures as low as 150 °C with an ABC-type thermal ALD process utilizing Ru(EtCp)2, oxygen, and hydrogen. Using this process we measured growth rates of 0.18 and 0.08 Å/cycle at temperatures of 200 and 150 °C, respectively. Interestingly, in situ QCM and QMS measurements performed during Ru(EtCp)2−O2, Ru(EtCp)2−H2, and Ru(EtCp)2−O2−H2 ALD sequences at 200 °C revealed that Ru(EtCp)2 adsorbs molecularly on Ru oxide but undergoes dissociative chemisorption on Ru metal.

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EXPEREIMENTAL SECTION

ALD Reactor. ALD was performed at 200 and 150 °C in a custom viscous flow stainless tube reactor system, using ultrahigh purity N2 (UHP, 99.999%) carrier gas at a flow rate of 300 sccm and a pressure of 1 Torr.34 The ALD reactor was equipped with a quadrupole mass spectrometer (QMS, Stanford Research Systems RGA300) located 4951


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Chemistry of Materials coreactant, we first oxidized the Ru film by exposing to oxygen for 300 s, followed by 30 cycles of Ru ALD using the sequence Ru(EtCp)2−O2 (3−5−5−5). Similarly, to investigate Ru ALD at 200 °C using H2 as the coreactant, we first fully cleaned and reduced the Ru film by alternatively exposing it to O2 (300 s) and H2 (300 s) for three cycles and then executed 30 Ru ALD cycles using the sequence Ru(EtCp)2− H2 (3−5−5−5). Finally, the ABC-type Ru ALD was performed on the fully reduced Ru film surface using the sequence Ru(EtCp)2−O2−H2 (3−5−5−10−5−5). Throughout all of the processes, the in situ QCM and QMS signals were recorded simultaneously.

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RESULTS AND DISCUSSION We first investigated the possibility for Ru ALD using molecular oxygen as the coreactant at 200 °C using in situ QCM and QMS. As shown in Figure 1a, in situ QCM clearly demonstrated that AB-type Ru ALD using Ru(EtCp)2−O2 yielded virtually no growth at 200 °C, except for an initial mass gain of ∼70.4 ng/cm2 during the first cycle. These findings are consistent with previous reports of Ru ALD using Ru(EtCp)2−O2 that concluded higher temperatures of ∼300 °C are necessary for sustained growth.15,18,29 In addition, in situ QMS measurement (Figure 1b) detected only faint CO2 (m/z = 44) signals during the initial Ru ALD cycles but no H2O (m/ z = 18) signals whatsoever. Therefore, in situ QCM and QMS measurements indicate that no combustion reactions occurred using the ALD sequence Ru(EtCp)2−O2 beyond the first few cycles. Next we performed in situ QCM measurements using the ALD sequence Ru(EtCp)2−H2. Once again, we did not observe any appreciable growth beyond the initial mass gain of ∼100 ng/cm2 during the first Ru ALD cycle as shown in Figure 1c. The inset of Figure 1c reveals that the first cycle mass gain is due entirely to the Ru(EtCp)2 exposure, but the mass remains constant during the H2 exposure step suggesting that hydrogenation of the EtCp ligands does not occur. As expected, no gaseous CO2 (m/z = 44) or H2O (m/z = 18) reaction products were detected due to the absence of oxygen in this ALD sequence (Figure 1d). Next, we examined Ru ALD using the ABC-type ALD sequence Ru(EtCp)2−O2−H2 (3−5−5−10−5−5) at 200 °C. As shown in Figure 2a, in situ QCM demonstrated a linear growth versus Ru ALD cycles at a rate of ∼22.5 ng/cm2·cycle (0.18 Å/cycle assuming the bulk density of Ru, 12.2 g/cm3), which is significantly smaller than the 1.5−1.7 Å/cycle value obtained at 270 °C using oxygen as coreactant,15 and amounts to ∼0.08 monolayers (ML) of Ru per cycle. During the ABCtype Ru ALD, the net mass gains (Δm1, Δm2, and Δm0) after the individual Ru(EtCp)2, O2, and H2 exposures were 59.8, 81.0, and 22.5 ng/cm2, respectively (Figure 2b). These mass changes can be used to infer the growth chemistry as will be demonstrated below. Meanwhile, in situ QMS revealed that CO2 and H2O are formed during the O2 exposure step but not during the Ru(EtCp)2 exposure (Figure 2c). It is noteworthy that the H2 exposures produce H2O but practically no CO2. This suggests that the H2 serves to reduce the Ru surface oxide and allows the Ru(EtCp)2 to chemisorb on the Ru metal surface in the following cycle. The reduction of RuOx by H2 at 200 °C was further examined using in situ QCM and QMS, wherein a Ru film with a thickness of about 5 nm was sequentially exposed to O2 and H2 with N2 purge in between (5−10−5−5). As shown in Figure 3a, in situ QCM measurements clearly showed mass increases and decreases with an identical amount of 107.5 ng/ cm2 during the O2 and H2 exposure steps, respectively.

Figure 2. (a) In situ QCM measurements of ABC-type Ru ALD using the sequence Ru(EtCp)2−O2−H2 on a Ru metal film at 200 °C. (b) An expanded view of the in situ QCM measurements. (c) In situ QMS measurements of CO2 (m/z = 44) and H2O (m/z = 18) formation in the ABC-type Ru ALD at 200 °C. The sequential exposure steps are shown as columns in color at the bottom of (b) and (c), and the nitrogen purge step is in between the columns.

Meanwhile, H2O was only observed during the H2 exposure step by in situ QMS (Figure 3b). Together, the in situ QCM and QMS measurements demonstrate that the Ru oxidation− reduction process is reversible. Therefore, the reduction of RuOx by H2 can take place at 200 °C. Comparing the in situ QMS data for the two ALD sequences Ru(EtCp)2−O2 (Figure 1b) and Ru(EtCp)2−O2−H2 (Figure 2c), we find that combustion occurs during the O2 exposure of the ABC-type ALD process but not in the case where only O2 is 4952


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used as the coreactant. One explanation for this behavior is that the Ru(EtCp)2 adsorbs differently on Ru oxide compared to Ru metal at 200 °C. We hypothesize that the Ru precursor adsorbs “molecularly” on the RuOx surface during the Ru(EtCp)2−O2 ALD process (Figure 4a) but undergoes dissociative chemisorption on the metallic Ru surface during the ABC-type ALD process (Figure 4c). This interpretation differs slightly from the suggestion by Knapas and co-workers that Ir(acac)3 adsorbs “molecularly” on both IrOx during Ir(acac)3−O3 and Ir metal during Ir(acac)3−O3−H2 at 195 °C.22 Coincidentally, we have observed that trimethylaluminum (TMA) undergoes dissociative chemisorption to form AlCH3* (the asterisk designates a surface species) on noble metal surfaces (Pd, Pt, and Ir) in the first Al2O3 ALD cycle using TMA and water at 200 °C.36,37 Zaera et al. observed that bis[(N,N′-di-sec-butyl acetamidinate)Cu] adsorbs dissociatively on Ni(110) surfaces at temperatures between approximately 77 and 177 °C;38 Braun et al. reported that ferrocene can even adsorb dissociatively on Au(111) surfaces at temperatures as low as 80 K using low temperature scanning tunneling microscopy (STM).39 Such dissociative chemisorption of metal precursors on noble metal surfaces seems to be general due to the catalytic nature of noble metals. At 200 °C, molecular O2 can barely react with the “molecularly” adsorbed Ru(EtCp)2 (Figure 1a,b); thus, the EtCp ligands remain on the RuOx surface after the O2 exposures, which hinders subsequent Ru(EtCp)2 adsorption and prevents continuous growth as illustrated schematically in Figure 4a. Similarly, molecular H2 cannot hydrogenate the EtCp* species released during the dissociative chemisorption of Ru(EtCp)2, since no mass changes were observed during the H2 exposures (inset of Figure 1b). Consequently, the surface Ru(EtCp)* and EtCp* species act to poison the Ru surface toward additional ALD Ru(EtCp)2 adsorption and prevent the ALD growth in the following cycles (Figure 4b). In contrast, molecular O2 does react with the dissociated Ru(EtCp)* and EtCp* species to form the CO2 and H2O combustion products at 200 °C, as evidenced by in situ QMS (Figure 2c). As a consequence, Ru ALD is achieved using the ABC-type sequence Ru(EtCp)2−O2−H2. First, the Ru(EtCp)2 undergoes dissociative chemisorption on the existing Ru metal surface to

Figure 3. (a) In situ QCM measurements of mass variations for a Ru film during sequential exposures to O2 and H2 at 200 °C. (b) In situ QMS measurements of H2O (m/z = 18) formation during sequential exposures of the Ru film to O2 and H2 at 200 °C. The sequential exposure steps are shown as columns in color at the bottom; the nitrogen purge step is in between the columns.

Figure 4. Schematic models of AB-type Ru ALD using the sequences Ru(EtCp)2−O2 on a Ru oxide film (a) and Ru(EtCp)2−H2 on a Ru metal film (b) at 200 °C or lower temperatures, respectively. The Ru ALD growth stops after the first cycle due to the failure of EtCp ligand removal. 4953


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To explore the low-temperature limit for this ABC-type Ru ALD, we performed the sequence Ru(EtCp)2−O2−H2 (3−5− 5−10−5−5) at 150 °C. In situ QCM measurements again revealed a linear growth, but at a smaller rate of ∼10.5 ng/cm2· cycle (∼0.08 Å/cycle), as shown in Figure 6. During this process, the mass gains (Δm1, Δm2, and Δm0) after sequential exposures of Ru(EtCp)2, O2, and H2 were 29.1, 43.2, and 10.5 ng/cm2, respectively (Figure 2b). Thus, the step ratio R = 0.36 is again very close to the expected value of R = 0.35, based on

form Ru(EtCp)* and EtCp* (eq 1); next, the O2 combusts the EtCp* ligands releasing the CO2 and H2O gaseous products and forms Ru oxide (eq 2); finally, the H2 reduces the Ru oxide to Ru metal by releasing H2O vapor (eq 3) and regenerating new chemisorption sites for the next Ru(EtCp)2 exposure (also see the schematic model in Figure 5).

Figure 5. Schematic model of ABC-type Ru ALD using the sequence Ru(EtCp)2−O2−H2 on a Ru metal surface.

The detailed in situ QCM step traces in Figure 2b can be used to extract details about the surface chemistry if we assume the mechanism presented in eqs 1−3. For instance, the amount of oxygen chemisorbed on the surface in eq 2 can be calculated from the quantity (Δm2 − Δm0) = 58.5 ng/cm2 to be 20.3 O2 molecules per Ru(EtCp)2 precursor molecule. In addition, the predicted value for the step ratio R = Δm0/Δm1 in Figure 2b can be calculated from eqs 1−3 and the known atomic masses R = MRu/(MRuEtCp + MEtCp) = 0.35. This predicted value is very close to the measured value R = 0.37 obtained by in situ QCM in Figure 2b, providing firm support to our proposed mechanism. Ru(EtCp)2 (g) + 2* → RuEtCp* + EtCp*

(1)

RuEtCp* + EtCp* + 20.3O2 (g) → Ru* + 17.6O* + 14CO2 (g) + 9H 2O(g)

Ru* + 17.6O* + 17.6H 2(g) → Ru* + 17.6H 2O(g)

(2) (3) Figure 6. (a) In situ QCM measurements of ABC-type Ru ALD using the sequence Ru(EtCp)2−O2−H2 on a metallic Ru film at 150 °C. (b) Expanded view of the in situ QCM measurements. (c) In situ QMS measurements of CO2 (m/z = 44) and H2O (m/z = 18) formation in the ABC-type Ru ALD at 150 °C. The sequential exposure steps are shown as columns in color at the bottom of (b) and (c); the nitrogen purge step is in between the columns.

The Ru* species in eqs 2 and 3 represent the ∼0.08 ML Ru deposited by dissociation of the Ru(EtCp)2 precursor during the current ALD cycle. In contrast, the 17.6 O* in eq 2 is assumed to be uniformly spread over the entire Ru surface (i.e., 1.0 ML Ru). As a consequence, the surface composition would be about Ru:O = 1:1.4 following the O2 exposures at 200 °C. 4954


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Chemistry of Materials eqs 1−3. Although the growth rate at 150 °C was about half of the one at 200 °C, the growth mechanism is hardly changed. For instance, QCM shows that the amount of oxygen chemisorbed on the surface (Δm2 − Δm0) = 32.7 ng/cm2 implying a ratio of 28.5 O2 molecules per Ru(EtCp)2 as compared to 20.3 at 200 °C. Consequently, eqs 2 and 3 are modified only slightly for the ABC-type Ru ALD at 150 °C to yield eqs 4 and 5, respectively:

Department of Energy, Office of Science, Office of Basic Energy Sciences.

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(1) Wen, H.-C.; Lysaght, P.; Alshareef, H. N.; Huffman, C.; Harris, H. R.; Choi, K.; Senzaki, Y.; Luan, H.; Majhi, P.; Lee, B. H.; Campin, M. J.; Foran, B.; Lian, G. D.; Kwong, D. L. Thermal response of Ru electrodes in contact with SiO2 and Hf-based high-k gate dielectrics. J. Appl. Phys. 2005, 98, 043520. (2) 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 (Invited Paper). Microelectron. Eng. 2009, 86, 1789. (3) Bandaru, J.; Sands, T.; Tsakalakos, L. Simple Ru electrode scheme for ferroelectric (Pb,La)(Zr,Ti)O-3 capacitors directly on silicon. J. Appl. Phys. 1998, 84, 1121. (4) Josell, D.; Wheeler, D.; Witt, C.; Moffat, T. P. Seedless superfill: Copper electrodeposition in trenches with ruthenium barriers. Electrochem. Solid-State Lett. 2003, 6, C143. (5) Kawano, K.; Kosuge, H.; Oshima, N.; Funakubo, H. Conformability of ruthenium dioxide films prepared on substrates with capacitor holes by MOCVD and modification by annealing. Electrochem. Solid-State Lett. 2006, 9, C175. (6) Kil, D. S.; Lee, J. M.; Roh, J. S. Low-temperature ALD growth of SrTiO3 thin films from Sr beta-diketonates and Ti alkoxide precursors using oxygen remote plasma as an oxidation source. Chem. Vap. Deposition 2002, 8, 195. (7) Vehkamaki, M.; Hatanpaa, T.; Hanninen, T.; Ritala, M.; Leskela, M. Growth of SrTiO3 and BaTiO3 thin films by atomic layer deposition. Electrochem. Solid-State Lett. 1999, 2, 504. (8) Sandhage, K. H.; Allan, S. M.; Dickerson, M. B.; Gaddis, C. S.; Shian, S.; Weatherspoon, M. R.; Cai, Y.; Ahmad, G.; Haluska, M. S.; Snyder, R. L.; Unocic, R. R.; Zalar, F. M.; Zhang, Y. S.; Rapp, R. A.; Hildebrand, M.; Palenik, B. P. Merging biological self-assembly with synthetic chemical tailoring: The potential for 3-D genetically engineered micro/nano-devices (3-D GEMS). Int. J. Appl. Ceram. Technol. 2005, 2, 317. (9) Kim, S. K.; Lee, S. W.; Han, J. H.; Lee, B.; Han, S.; Hwang, C. S. Capacitors with an Equivalent Oxide Thickness of < 0.5 nm for Nanoscale Electronic Semiconductor Memory. Adv. Funct. Mater. 2010, 20, 2989. (10) Suntola, T.; Hyvarinen, J. Atomic layer epitaxy. Annu. Rev. Mater. Sci. 1985, 15, 177. (11) Knez, M.; Nielsch, K.; Niinisto, L. Synthesis and surface engineering of complex nanostructures by atomic layer deposition. Adv. Mater. 2007, 19, 3425. (12) Leskela, M.; Ritala, M. Atomic layer deposition chemistry: Recent developments and future challenges. Angew. Chem., Int. Ed. 2003, 42, 5548. (13) Aaltonen, T.; Rahtu, A.; Ritala, M.; Leskela, M. Reaction mechanism studies on atomic layer deposition of ruthenium and platinum. Electrochem. Solid-State Lett. 2003, 6, C130. (14) Aaltonen, T.; Alen, P.; Ritala, M.; Leskela, M. Ruthenium thin films grown by atomic layer deposition. Chem. Vap. Deposition 2003, 9, 45. (15) Kwon, O. K.; Kim, J. H.; Park, H. S.; Kang, S. W. Atomic layer deposition of ruthenium thin films for copper glue layer. J. Electrochem. Soc. 2004, 151, G109. (16) Lee, D. J.; Yim, S. S.; Kim, K. S.; Kim, S. H.; Kim, K. B. Formation of ru nanotubes by atomic layer deposition onto an anodized aluminum oxide template. Electrochem. Solid-State Lett. 2008, 11, K61.

RuEtCp* + EtCp* + 28.5O2 (g) → Ru* + 20O* + 14CO2 (g) + 9H 2O(g)

(4)

Ru* + 20O* + 20H 2(g) → Ru* + 20H 2O(g)

(5)

According to eq 4 and the Ru growth rate, the surface composition is determined to be about Ru:O = 1:0.7 at 150 °C, which is considerably lower than the one formed at 200 °C as we expected. Although the Ru growth rate of ∼0.08 Å/cycle at 150 °C might be too low for microelectronic applications, this could be advantageous for the ALD synthesis of nanostructured catalysts since it would allow more precise control over the composition of multimetallic nanoparticles.35 The different growth rates at 200 and 150 °C might reflect different packing geometries, structures, and densities for the Ru(EtCp)* and EtCp* species on the Ru metal surfaces at these two temperatures. Alternatively, one or more of the surface reactions might be slower at the lower growth temperature such that the Ru ALD is kinetically limited. Additional in situ studies including saturation measurements for each of the three precursor exposures could help answer these questions.

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CONCLUSIONS In this work, we have explored the low temperature limit for Ru ALD using in situ QCM and QMS measurements, and we found that Ru ALD can be achieved at as low as 150 °C using the ABC-type ALD sequence Ru(EtCp)2−O2−H2. Furthermore, by comparing the two ALD sequences of Ru(EtCp)2−O2 and Ru(EtCp)2−O2−H2 at 200 °C, we discovered that Ru(EtCp)2 likely adsorbs “molecularly” on the Ru oxide surface but dissociatively chemisorbs on the Ru metal surface to form RuEtCp* and EtCp*. Consequently, this ABC-type ALD sequence proceeds via dissociative chemisorption, combustion, and reduction for the Ru(EtCp)2, O2, and H2 steps, respectively. We believe this ABC-type ALD using molecular O2 and H2 coreactants might be extended to other noble metal ALD processes to decrease the deposition temperature for broader applications and reduce oxygen impurites, like the cases using ozone and H2 as coreactants achieved by other groups.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(J.W.E.) E-mail: [email protected]. Present Address †

(J.L.) Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported as part of the Institute for Atom-efficient Chemical Transformations (IACT), an Energy Frontier Research Center funded by the U.S. 4955


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Low Temperature ABC-Type Ru Atomic Layer Deposition through

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