Article pubs.acs.org/JPCC
Catalytic Combustion Reactions During Atomic Layer Deposition of Ru Studied Using 18O2 Isotope Labeling N. Leick,† S. Agarwal,‡ A. J. M. Mackus,† S. E. Potts,† and W. M. M. Kessels*,† †
Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Department of Chemical and Biological Engineering, Colorado School of Mines, Golden, Colorado 80401, United States
‡
S Supporting Information *
ABSTRACT: The surface reaction products liberated during the atomic layer deposition (ALD) of Ru from (C5H5)Ru(CO)2(C2H5) and 18O2 were quantitatively analyzed using quadrupole mass spectrometry (QMS). The gas-phase reaction products during the Ru precursor pulse were CO2, CO, H2, and H2O, while during the O2 pulse primarily CO2 and CO were produced. Approximately 70% of the C atoms and ∼100% of the H atoms contained in the Ru precursor were released during the Ru pulse of the ALD process. From these observations, and on the basis of the surface science and catalysis literature, we conclude that the complex surface chemistry during Ru ALD can be described by catalytic combustion reactions. These reactions consist of the dissociative chemisorption of the Ru precursor’s ligands on the Ru surface during the metal pulse. These hydrocarbon ligands undergo dehydrogenation and combustion reactions on the catalytic metal surface in the presence of surface O formed due to the dissociation of O2 molecules in the previous O2 pulse. We postulate that the carbon-rich species produced due to the dehydrogenation of the Ru precursor’s hydrocarbon ligands self-limit precursor chemisorption by blocking adsorption sites. The use of the 18O2 isotope also unambiguously shows that a certain fraction of the precursor’s carbonyl ligands dissociate on the Ru surface.
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INTRODUCTION Thin films and nanostructures of Pt-group metals, such as Pt, Ru, Ir, and Pd, have been widely studied over the past decade for a variety of applications, such as power generation,1 electricity and information storage,2−4 sensing,5−7 and control of exhaust pollution.8−10 Atomic layer deposition (ALD) has emerged as a promising technique to deposit these metallic thin films and nanostructures with accurate thickness control onto demanding substrate topologies.11−13 In particular, Ru ALD is of interest in applications such as heterogeneous catalysis14,15 and memory devices.4,16−20 Similar to most other metal ALD processes, nucleation delays of 70−250 cycles have been reported during ALD of Ru on a variety of substrate surfaces.21−23 Novel Ru ALD precursors24−29 have been developed to reduce this long nucleation delay and to enhance the growth per cycle (GPC), which is typically 500 K.54−56 The results of these studies are summarized below, which are relevant to the pressure (∼1 Torr) and temperature regime (∼600 K) used for Ru ALD processes.54−61 In particular, these studies suggest that the surface oxygen coverage after the O2 pulse can significantly affect the surface reactions of organometallic Ru precursors. Figure 1 shows a summary of the literature on the Ru(0001)
CxHy* → CxHy−2* + H2 (gas), where * denotes surface species.70 Thus, based on previous literature, we expect that, depending on the surface O coverage prior to the Ru precursor half-cycle, different surface reactions could occur: combustionlike products would be dominant for a high surface O coverage, and dehydrogenation reaction products would be more likely for a low surface O coverage. In this article, we present our study of the surface reaction mechanisms during ALD of Ru from the heteroleptic precursor cyclopentadienylethyldi(carbonyl)ruthenium (CpRu(CO)2Et, see Figure 2a) and 18O2 using QMS. Isotope labeling enabled
Figure 1. O coverage as a function of O2 exposure in Langmuirs (1 L = 1 × 10−6 Torr·s) for a Ru(0001) surface at 600 K and the expected surface configurations of metastable O at coverages of 0.25, 0.5, 0.75, and 1 ML. Note, p refers to the primitive adlayer unit cell. The data have been compiled from refs 59, 62, and 63.
Figure 2. (a) Molecular structure of CpRu(CO)2Et [C5H5Ru(CO)2CH2CH3]. (b) Schematic overview of the ALD reactor. The top and bottom gate valves are used to isolate the chamber from the plasma source and pump, respectively, thereby trapping the surface reaction products within a known volume. A differentially pumped QMS is used to detect the gas-phase species. (c) Schematic of the pulsing sequence for the Ru ALD process in this study (2 s CpRu(CO)2Et and 18O2 pulses). The modified ALD process comprised of additional steps in which the top and bottom valves were closed for a 7 s period with 2 s CpRu(CO)2Et and 18O2 injection periods. After each half-cycle, the gas-phase products were pumped out of the reactor for 5 min, a duration that was sufficient to reach the QMS background signal intensity before each pulse.
surface O coverage, θO, as a function of O2 exposure.60 It has been reported that O chemisorption extends over four different ordered overlayer phases on Ru(0001): p(2 × 2)-O at θO = 0.25 monolayer (ML), p(2 × 1)-O at θO = 0.5 ML, p(2 × 2)3O at θO = 0.75 ML, and p(1 × 1)-O at θO = 1 ML, where p refers to the primitive adlayer phase.60−63 According to these studies, Ru also has the ability to adsorb atomic O beyond 1 ML, where atomic O dissolves into the subsurface region. 55,59,60,64,65 At coverages beyond θ O = 3 ML, RuO2(110) microregions begin to form, which can coexist with p(1 × 1)-O domains on Ru(0001).57,58,66 In fact, RuO2 has been identified as the active phase in the oxidation of CO at high pressures and temperatures.56,67,68 It is also known from catalysis that dehydrogenation reactions occur on catalytic metal surfaces in which C−H bonds are broken, and the atomic H from the hydrocarbon is transferred onto the metal surface. Eventually, surface H atoms recombine and desorb as H2, leaving carbon-rich surface dehydrogenation products on the metal surface.40,41,69 This overall reaction can be written as
us to differentiate between the 16O in the Ru precursor’s carbonyl ligands from the 18O delivered in the O2 half-reaction cycle. In addition, we were also able to distinguish between C18O and C2H4 in the QMS since both species can be released during the surface reactions of the Ru precursor. To quantify the gas-phase concentration of the surface reaction products, we have carefully calibrated the QMS ion current for different mass-to-charge (m/z) ratios. On the basis of this analysis, we conclude that the Ru surface catalyzes the dissociation of adsorbed specieshydrocarbons in the Ru-precursor half-cycle and 18O2 in the subsequent half-cyclethereby facilitating the combustion of the fragmented precursor ligands. Such 21321
dx.doi.org/10.1021/jp4060457 | J. Phys. Chem. C 2013, 117, 21320−21330
The Journal of Physical Chemistry C
Article
properties of the Ru films. Specifically, in the modified cycles, a 5 min evacuation step separated the Ru and the 18O2 pulses, during which all of the remaining gas-phase species were pumped out of the chamber. This was necessary to establish a constant baseline for the QMS ion current at any given m/z value. In addition, the top and bottom gate valves of the vacuum chamber were closed for 7 s (see Figure 2c) during which time we introduced 2 s CpRu(CO)2Et and 18O2 pulses. Since the gate valves were closed, the reaction products were trapped within the chamber. The QMS signal at a given m/z was thus proportional to the mole fraction of the corresponding neutral species in the reaction chamber. During ALD, the gas-phase concentration of the surface reaction products can be significantly lower than that of the excess precursor in the chamber. In the QMS, the precursor is dissociatively ionized by the electrons in the ionizer leading to ionic fragments whose m/z values can overlap with those of the reaction products. To select the relevant m/z values that result from fragmentation of the reaction products, and not the parent precursor molecule, we first performed a mass scan over the range of m/z = 1−60 with a low time resolution during a reference measurement (CpRu(CO)2Et pulsing only) and during the ALD process (data shown in Supporting Information).78 On the basis of the difference in intensity in these mass spectra (see Figure S.1, Supporting Information), no signal from reaction products could be detected above the noise level beyond m/z = 48. By comparing the fragmentation patterns of the measured m/z values with those published in the NIST database,79 we could assign the different peaks to specific parent molecules produced during surface reactions. We also checked for potential artifacts in the signal intensity due to the limited resolution of the QMS and, if present, took this into consideration in the assignment of the m/z values to particular gas-phase species. For example, the QMS signal at a given m/z value has an asymmetric shape with a tail toward the lower m/z value.80 Thus, when the peak intensity at a certain m/z is very high compared to the intensity at the neighboring lower m/z, it can result in an erroneous determination of the QMS signal intensity at this lower m/z value. In such cases, we carefully analyzed the peak shape to ascertain the true origin of the signal. Following the identification of the relevant m/z values to track different surface reaction products, we recorded the data in a multiplexing mode where the QMS signal intensity for the relevant m/z values was tracked simultaneously during the ALD cycles. To perform a quantitative analysis of the QMS data, it was necessary to calibrate the QMS signal intensities. The QMS signal S for any parent molecule XY is related to its gas-phase number density nXY as81
catalytically assisted oxidation reactions of hydrocarbons are generally referred to as catalytic combustion reactions in the field of heterogeneous catalysis:71−73 herein, we show that they also play a significant role under the experimental conditions for Ru ALD.
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EXPERIMENTAL DETAILS ALD Reactor and Film Deposition. The Ru films were deposited at 325 °C onto polished 150 mm diameter Si(001) wafers in an Oxford Instruments FlexAL ALD reactor74 (see reactor schematic in Figure 2b) suited for thermal and plasmaassisted ALD. The deposition chamber was evacuated by a turbomolecular pump, which provided a base pressure of ∼10−6 Torr. The CpRu(CO)2Et (SAFC Hitech, purity >99.999%) precursor was kept at 90 °C and was vapor-drawn into the chamber through a stainless steel tube heated to 110 °C and pulsed by an ALD valve. In this chamber, 18O2 (Linde Gas, purity >99.8%) was delivered via a mass flow controller into the alumina tube of the inductively coupled plasma source that was not operated in the experiments described here. The pressure in the chamber increased to ∼30 and ∼120 mTorr during Ru and 18O2 pulsing, respectively. The chamber walls were heated to 120 °C to avoid precursor condensation. The reactor surfaces and the Si wafer were conditioned prior to the QMS study by depositing at least 30 nm of Ru. Since almost no film deposition was detected at substrate temperatures
