Kinetics of Adsorption of Methylcyclopentadienyl Manganese

Article pubs.acs.org/JPCC

Kinetics of Adsorption of Methylcyclopentadienyl Manganese Tricarbonyl on Copper Surfaces and Implications for the Atomic Layer Deposition of Thin Solid Films Menno Bouman and Francisco Zaera* Department of Chemistry, University of California, Riverside, California 92521, United States ABSTRACT: The kinetics of the thermal chemistry of methylcyclopentadienyl manganese tricarbonyl on copper surfaces was followed by infrared absorption spectroscopy analysis of the gas surrounding the substrate. Dissociative adsorption of the MeCpMn(CO)3 precursor was seen at low temperatures, below 400 K, and was determined to occur via the initial loss of a single carbon monoxide ligand. Full decarbonylation was observed starting at a surface temperature of approximately 510 K, and full decomposition of the Mn precursor was seen to be complete by 540 K. All these transitions were determined to occur at temperatures at least 100 K lower than those reported on silicon oxide surfaces. Comparison with reference spectra afforded the identification of the main hydrocarbon product as a mixture of methylcyclopentadiene isomers. Isothermal measurements of MeCpMn(CO)3 partial pressure versus reaction time highlighted a deviation from first-order kinetics, an effect associated with changes in surface reactivity. A self-limiting regime typical of atomic layer deposition processes was identified between 475 and 525 K by the incomplete conversion of the precursor, which displayed similar reaction rate and reached the same asymptotic partial pressure at all temperatures within that temperature bracket. Higher surface temperatures promoted total precursor consumption, at a rate that increased with increasing surface temperature; an apparent activation energy of approximately 40 kJ/mol was estimated in that high-temperature regime. The resulting chemical-vapor deposited multilayers were identified by X-ray photoelectron spectroscopy to consist of MnO.

1. INTRODUCTION The surface chemistry of metallorganic compounds has gained much interest in connection with their use for the deposition of thin solid films. Metal-based films can be grown by physical means, but requirements related to conformality on substrates with complex topographies have made chemical vapor deposition (CVD) a preferred method for this purpose.1,2 More recently, atomic layer deposition (ALD), where the chemical reactions are chosen to be self-limiting and complementary and are separated in time in order to control film growth at a monolayer level, has been incorporated in several industrial applications,3 in particular in the microelectronics industry.4−6 Many inorganic compounds have been explored for the CVD and ALD of a wide variety of materials,7−10 but the search still continues to achieve optimal film deposition. Late transition metals present a particular challenge, because their volatilization usually requires large organic ligands, and those can exhibit complex surface chemistry leading to decomposition and the deposition of impurities in the growing films.11 Understanding the relevant surface chemistry is crucial for the task of minimizing these undesirable side reactions. To address this need, our group 12−17 and a few others5,10,18−20 have focused on the study of the thermal chemistry of relevant CVD and ALD precursors on solid surfaces by using a combination of surface-sensitive techniques. © XXXX American Chemical Society

The challenge is to combine the information extracted with individual techniques in order to develop a complete mechanistic picture of that chemistry. One of the analytical techniques that has provided quite comprehensive information in this area has been infrared absorption spectroscopy (IR). Several research groups have reported interesting results on the surface reactivity of ALD precursors based on the use of IR.21−30 Here we report an example where IR was used to follow in situ the composition of the gas-phase compounds that form during the exposure of a solid surface to an ALD precursor under film-growth conditions. This approach has been used in the past for the characterization of a handful of ALD systems,31−34 but in our present study we have added time resolution to incorporate kinetic measurements to our mechanistic investigation. The specific system addressed is that of the deposition of manganese-containing films on copper surfaces using methylcyclopentadienyl manganese tricarbonyl, MeCpMn(CO)3, in work motivated by the interest in using manganese-based films as diffusion barriers for copper interconnects in the semiconductor industry.35−37 Cyclopentadienyl-based complexes are good candidate precursors for the deposition of metal films, and their surface chemistry Received: March 1, 2016 Revised: March 28, 2016

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DOI: 10.1021/acs.jpcc.6b02197 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Torr) for about 10 min. A new and clean Cu substrate was used for each temperature investigated, and the experiments were repeated several (∼5) times in order to evaluate the reproducibility of the results. The spectra in Figure 2 were obtained by averaging 512 scans for better signal-to-noise ratio, but for the kinetic experiments, shorter infrared measurements (16 scans, 20 s acquisition time) were required. Ex situ X-ray photoelectron spectroscopy (XPS) data were acquired by using a Kratos analytical AXIS instrument equipped with a 165 mmmean-radius semihemispherical electron energy analyzer and a monochromatized Al anode. All binding energies (BE) are referenced to a value of 932.5 eV for the Cu 2p3/2 of metallic copper.45−47

and potential use in Mn ALD have been studied with other techniques in the past;34,37−44 here we complement those studies with a kinetic description of its reactivity on metal substrates. Our studies indicate that initial activation of that precursor starts with the removal of a single CO ligand at low temperatures, below 400 K, and that further reactivity is only possible above ∼510 K, at which point full decarbonylation takes place. An ALD window was identified between 475 and 525 K where the activated adsorption is self-limited; at higher temperatures Mn deposition is continuous, leading to multilayer growth. The IR data also indicate that the main organic byproduct from the MeCpMn(CO)3 uptake is methylcyclopentadiene, which starts evolving from the surface at temperatures above approximately 525 K. All these observations are put in context within the needs of Mn ALD in our discussion below.

3. RESULTS An initial evaluation of the chemistry of MeCpMn(CO)3 on copper surfaces was made by recording IR spectra as a function of temperature over the range from 475 to 675 K. The results are displayed in Figure 1. Some clear changes are observed

2. EXPERIMENTAL DETAILS The experiments reported here were performed in a homemade transmission infrared absorption spectroscopy cell described in a previous publication.25 The small reactor volume is defined by a 1 1/2 in. inside-diameter and 3/4 in. thick two-sided ultrahigh vacuum (UHV) stainless-steel Conflat flange flanked on both ends by NaCl windows. Two 1/4 in. stainless-steel access tubes located on the sides of the bottom half of the flange were used to pressurize the reactor for the ALD experiments and to evacuate the volume using a mechanical pump. A sample holder was added on a mini (1.33 in. diameter) Conflat flange placed on top, and it was equipped with feedthroughs for resistive heating and for thermocouple wires to read and control the surface temperature (using a proportional-integral-derivative controller and a variable transformer). The cell was placed at the focal point of the infrared beam of the sample compartment of the Fourier-transform infrared (FTIR) absorption spectrometer for data acquisition in transmission mode. The solid substrate consisted of a nickel wire coiled as in a light bulb and electro-coated with a copper film using the nickel substrate as the cathode, a copper anode, and a solution made out of copper sulfate (60 g/L) and sulfuric acid (20 g/L). The deposition was carried out for 120 s at a voltage of 1.5 V. By weighing the wire before and after deposition, the average copper film thickness was estimated to be ∼2 μm. The wire was attached to the electrical feedthroughs of the IR cell via copper barrel connectors. With this arrangement, the sample could be heated to temperatures of up to approximately 650 K. A chromel−alumel thermocouple, used to follow the temperature of the surface, was spotwelded to a small opening in the wire made by scraping the Cu film for better contact. The methylcyclopentadienyl manganese tricarbonyl (MeCpMn(CO)3) precursor was purchased from Strem Chemicals (97% minimum purity), and purified by a series of freeze− pump−thaw cycles in situ in the gas-handling manifold before use. Infrared absorption spectra were recorded with a Bruker Tensor 27 FTIR spectrometer equipped with a mercurycadmium-telluride (MCT) detector. For the temperaturedependence studies, infrared absorbance spectra were acquired by averaging 64 scans at a 4 cm−1 resolution (∼60 s acquisition time) after dosing the copper substrate with 0.1 Torr of the Mn precursor for about 2 min, and ratioed against background spectra taken using similar parameters. After each exposure, the cell was purged with 0.5 Torr of nitrogen gas for 2 min, followed by evacuation to the base pressure of the cell (∼0.01

Figure 1. Transmission infrared absorption (IR) spectra of the gas surrounding a copper-plated nickel wire after a 2 min exposure to 0.1 Torr or MeCpMn(CO)3 at several different temperatures. A transition is observed starting at approximately 510 K indicative of the onset of full decarbonylation and subsequent decomposition of the Mn precursor on the surface.

between 505 and 575 K, indicating a transition that suggests the triggering of some thermal chemistry on the copper surface. In order to analyze the data in more detail, the traces recorded at the two ends of our temperature range, 375 and 700 K, are contrasted in Figure 2 against reference spectra from the most likely gas-phase products, namely: (1) a methylcyclopentadiene dimer, (MeCp)2;48 (2) carbon monoxide, CO (acquired with our IR cell); (3, 4, and 5) 1-, 2-, and 5-methylcyclopentadiene, MeCpH;49,50 (6) fulvene;51 and (7) benzene.48 On the basis of those spectra, the Mn precursor after exposures at low temperatures can be easily assigned to the molecular complex. First, its C−H stretching region between approximately 2800 and 3200 cm−1 resembles those of the methylcyclopentadienyl dimer and MeCpPt(Me)3.31 In addition, the very intense peaks at 1960 and 2036 cm−1 are quite characteristic of this type of carbonyl complexes.52 Finally, our spectrum is similar to those reported for MeCpMn(CO)3 in the liquid and solid phases.53 In contrast, the IR trace for the gas resulting from exposing the copper-coated wire to MeCpMn(CO)3 at high temperB


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Figure 2. Comparison of the IR spectra recorded for the gas in experiments carried out at 375 (top trace) and 700 (third from top) K with reference data for (from top to bottom): methylcyclopentadiene dimer ((MeCp)2, second trace), carbon monoxide (CO, fourth), 1-, 2-, and 5-methylcyclopentadiene (1-, 2-, and 5-MeCpH, fifth, sixth, and seventh traces, respectively), fulvene (eighth), and benzene (bottom). Using these data, the low-temperature spectrum is assigned to molecular MeCpMn(CO)3, whereas the gas from the high-temperature experiments is identified as a mixture of CO and MeCpH.

Figure 3. Normalized partial pressures for MeCpMn(CO)3 (circles) and CO (squares) as a function of substrate temperature, extracted from IR spectra such as those in Figure 1. Data are provided for experiments with copper-plated nickel (solid symbols), and compared with previous results obtained with silicon oxide (top-half-filled symbols). Additional data from ex situ GC/MS analysis of the gas are also provided for the latter surface (bottom-half-filled symbols).34 The decomposition of the Mn precursor occurs at significantly lower temperatures on copper.

data are available for that case, from independent measurements using in situ IR absorption spectroscopy (top-half filled symbols in Figure 3) and ex situ GC/MS (bottom-filled symbols). The two sets are complementary and point to a starting of surface activation of the precursor at ∼600 K, approximately 100 K higher than the temperature determined here for the case of the copper metal surface. This suggests that nucleation may be rate limiting in ALD processes with MeCpMn(CO)3 on silicon oxide, and that once the film deposition is initiated, it can proceed more easily on the newly grown material. Nucleation may also occur on the surfaces of metals such as copper, but, that is less likely because of the lower surface tension of metal substrate. The results from exploratory experiments performed on nickel yielded similar kinetics on that metal (data not shown), suggesting the absence of any specific nucleation rate-limiting steps on either of the two (Cu or Ni) metals. In any case, if nucleation does happen on copper, our data indicate that it still takes place at much lower temperatures, and therefore should dominate over growth on the silica substrate. In order to get further insights into the thermal chemistry of MeCpMn(CO)3 decomposition promoted by copper surfaces, isothermal kinetic experiments were carried out as a function of the temperature of the substrate. Figure 4 displays representative data recorded for three temperatures by following both the consumption of the Mn precursor (left panel) and the evolution of the decomposition products, CO in particular (right). The traces show a continuous and smooth progress of the reaction over time in all cases. In addition, they highlight a few other noteworthy details of this chemical system. First, the conversion does not follow first-order kinetics, as it would be expected to for a simple unimolecular process. This deviation from exponential decays is likely to reflect changes on the surface as the reaction proceeds. In connection with that, it is also important to note that the conversion is not exhaustive at the lower temperatures, for instance at 500 K in Figure 4. This

atures shows an entirely new set of features. Most noticeable, there are two intense and broad peaks between 2050 and 2250 cm−1 associated with gas-phase carbon monoxide. This indicates that adsorption of the Mn precursor on the surface leads to its decarbonylation. A small amount of gas-phase CO is seen in all the spectra in Figure 1, but a significant increase in its partial pressure seems to occur only above 505 K. In addition, the shape of the features associated with the C−H stretching region, between approximately 2800 and 3200 cm−1, changes significantly, and develops appreciable intensity above 3000 cm−1, in the region associated with the C−H stretching vibrational modes of unsaturated carbons. That transition starts between 515 and 525 K, and is complete by 575 K. A quick comparison with the reference data rules out the presence of detectable amounts of either fulvene or benzene, and points toward the formation of MeCpH. The specific isomer of the MeCpH that forms is not easy to pin down using these data alone, but on the basis of the similarities seen in the skeletal deformation modes below 1000 cm−1 we suggest that there may primarily be a combination of 1- and 2-MeCpH. This interpretation is consistent with our previous ex situ studies on silica surfaces using gas chromatography/mass spectrometry (GC/MS),34 and also with the mechanism proposed by George and co-workers.40 The integrated intensities of the C−O stretching bands associated with the molecular Mn precursor (1900−2050 cm−1) and with the gas-phase CO (2050−2250 cm−1) in Figure 1 were used to quantify the extent of conversion of MeCpMn(CO)3 upon adsorption on the copper surface. Those data are summarized in Figure 3. It is clear that some reactivity is possible even at temperatures below 500 K. It is also seen that further conversion is promoted at higher temperatures, until full conversion of all the gas-phase Mn precursor is attained at 675 K. This reactivity on copper is higher than that previously seen on silicon oxide,34 the data of which are also provided in Figure 3 for comparison. Two sets of C


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the spectra in Figure 1 display some intensity in the features associated with gas-phase CO, even at 475 K (as mentioned above); it is only that a significant growth of those peaks is not seen until about 515−525 K. Consequently, we propose that dissociative adsorption via the loss of one single CO molecule is possible even at 475 K. A more detailed kinetic study of this reaction, focusing on the removal of MeCpMn(CO)3 from the gas phase and including more surface temperatures, is reported in Figure 5. A wider

Figure 4. MeCpMn(CO)3 (left) and CO (right) partial pressures (in arbitrary units) versus time measured during the conversion of the Mn precursor on the copper-plated nickel wire at three different temperatures. Conversion is fast and complete at high temperatures, but it stops at a set conversion in the case of 500 K, indicating the selflimiting nature of the dissociative adsorption.

suggests monolayer saturation behavior, as expected in ALD processes, leading to the stop of the uptake of the precursor on the surface (and therefore to its consumption from the gas phase). More extensive precursor decomposition with the implicit growth of multilayers on the surface is seen at higher temperatures, a typical behavior in CVD. It should be mentioned that the initial partial pressures of the MeCpMn(CO)3 compound is the same in all kinetic runs. The fact that this is not reflected in the data in the left panel of Figure 4 is due to the lack of the required time resolution in our experiments; the acquisition of the first data point in the hightemperature runs is completed only after a significant amount of the reactant has already disappeared from the gas phase. Luckily, the measurements on the accumulation of the CO product (Figure 4, right panel) better reflect the extent of the conversion, and the fact that this is complete at 600 and 700 K (but not at 500 K). The data in Figure 4 also provide some information about the stoichiometry of the decomposition steps during the dissociative adsorption of the Mn precursor. First, it appears that only about half of the total amount of MeCpMn(CO)3 in the gas is consumed at 500 K. If the uptake stops upon monolayer saturation on the Cu surface at that temperature, this suggests that the initial amount of the precursor provided when filling the IR cell in our experiments is equivalent to approximately two full monolayers. Crude back-of-the-envelope calculations based on the volume and pressure in the IR cell are consistent with this interpretation. On the basis of that premise, it is also interesting to note that the amount of free CO generated at 500 K is only about 1/6 of what is produced at higher temperatures. This implies that MeCpMn(CO) 3 adsorption on the Cu surface at 500 K does not lead to full decarbonylation. In fact, accounting for the fact that only half of the Mn precursors adsorb at 500 K but all do at higher temperatures, the 1/6 factor seen for CO suggests that only one of the three carbon monoxide ligands is initially released upon dissociative adsorption, and that what forms on the surface is a MeCpMn(CO)2(ads) moiety; the release of the other two CO molecules appears to require higher temperatures. Indeed, all of

Figure 5. Kinetic data similar to those in Figure 4 but for a larger set of temperatures (and following the partial pressure of MeCpMn(CO)3 only). Adsorption is detected at temperatures as low as 400 K, and shows similar self-limiting kinetics within the 475−525 K temperature range. At 540 K and above, conversion is complete, as expected from multilayer deposition by CVD, and faster at higher temperatures.

temperature range was covered in this case, going from 400 to 700 K. It is interesting to note that the uptake of the Mn precursor on the copper surface takes place even at the lowest temperature used in our isothermal kinetic measurements, 400 K. This adsorption is likely to follow the dissociative step involving one single carbonyl ligand discussed before. Another key observation from Figure 5 is the fact that the decrease in MeCpMn(CO)3 partial pressure is limited below ∼525 K, and that it reaches a fixed asymptotic value after long-time exposures at those temperatures. In fact, the limiting pressure is the same all throughout the temperature range between 475 and 525 K (as is the Mn precursor consumption rate). Only at 540 K and above is it possible to consume all of the gas-phase Mn complex. Our interpretation of these observations is that the uptake is self-limiting up to 525 K, reaching monolayer saturation after dissociative adsorption and full decarbonylation, as expected for ALD, but that it switches to a steady-state regime at 540 K and above, in a CVD mode. The preceding conclusion is further evidenced by the initial kinetic data extracted from the runs in Figure 5. To better illustrate the transition, the initial reaction rates Ri, calculated from the slopes of the kinetic traces around t = 0 s, are plotted as a function of reaction temperature in Arrhenius form in Figure 6. At least two distinct temperature ranges can be identified in that plot. Above 525 K, the rate of reaction clearly increases with increasing surface temperature, with an apparent activation energy of close to 40 kJ/mol. On the other hand, the D


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shows representative data for the Cu 2p (left frame) and Mn 2p (right) signals corresponding to surfaces exposed to the precursor at two temperatures, 575 and 775 K. The results clearly show Mn deposition on an underlying copper film, more at the higher temperature (for which the Mn 2p peaks are more intense and the Cu 2p are attenuated more). The metallic character of the copper in both samples is manifested by both the Cu 2p3/2 binding energy value of 932.5 eV (left frame) and the L2M23V Auger signal at 647.8 eV, which corresponds to a kinetic energy of 839.4 eV and an Auger parameter of 1851.1 eV (with the associated L3VV Auger peak).45−47 In terms of the oxidation state of the manganese, the Mn 2p3/2 peak position at 641.2 eV is typical of MnO. This is in fact the prevalent species reported in other studies on Mn ALD.34,39,40,54 In our case, the incorporated oxygen is likely to originate from background water.

4. DISCUSSION The chemistry of cyclopentadienyl-based manganese compounds in connection with ALD processes has been studied in the past already, by us as well as by others.34,40,55 The general consensus that has emerged from that work is that these precursors are activated on surfaces via initial decarbonylation steps that lead to dissociative adsorption of Mn ions coordinated to the cyclopentadienyl ligands. The latter have proven to be quite stable, remaining intact until their elimination from the surface via protonation to form the corresponding cyclopentadienes (which desorb from the surface into the gas phase). Because there are virtually no side reactions during the thermal activation of the precursor, the surface chemistry is quite simple and clean, a fact that makes these compounds good promising candidates for CVD and ALD applications; they lead to minimal deposition of impurities in the growing films. On the other hand, the great stability of such complexes means that high temperatures may be needed for the film growth, a disadvantage. We have recently shown that this limitation may be overcome by using electron excitation of the precursor in the gas phase,41 but this idea needs to be explored further before its implementation. In any case, the conclusions listed here in terms of the surface chemistry of mixed cyclopentadienyl-carbonyl complexes are likely to be general, since they match what is known of their chemistry in liquid phase56−58 and also apply to similar compounds with other metals.32,59,60 Our present study with MeCpMn(CO)3 and copper surfaces based on the analysis of the composition of the gas phase using infrared absorption spectroscopy adds a few pieces of information to our understanding of the surface chemistry of this type of ALD precursors. First, our data clearly indicate that dissociative adsorption of the precursor leads to the generation of gas-phase carbon monoxide and methylcyclopentadiene (MeCpH), the products expected from the clean chemistry discussed above. However, our careful isothermal kinetic measurements indicate that these steps do not happen simultaneously. The picture generated by our infrared absorption data is that the initial adsorption of the MeCpMn(CO)3 precursor is dissociative and leads to the ejection of one single CO molecule and the formation of a MeCpMn(CO)2(ads) surface intermediate. This dissociative uptake seems to be virtually unactivated, and it appears to already occur at 400 K or even below. The uptake is self-limiting, and stops at monolayer saturation, as required in ALD processes.

Figure 6. Arrhenius plot of the initial rates of reaction versus temperature for the uptake of MeCpMn(CO)3 on the Cu-plated Ni wire, extracted from the data in Figure 5. The results for the selflimiting regime between 475 and 525 K are displayed in a expanded format in the inset. The reaction rate is approximately constant in that temperature range, but it increases at higher temperatures.

rates below 525 K do not vary much in value. This is particularly true in the 475−525 K bracket, as highlighted in the inset of Figure 6. This is the temperature range that we associate with ALD self-limited dissociative adsorption. It would appear that the decarbonylation steps are virtually unactivated but also self-limiting, and that only above 525 K, when additional chemistry leads to the removal of the methylcyclopentadienyl ligand, it is possible to promote further reaction and multilayer deposition. Finally, the films grown by exposures of the Cu substrates to MeCpMn(CO)3 were briefly analyzed ex situ by XPS. Figure 7

Figure 7. Cu 2p (left panel) and Mn 2p (right) X-ray photoelectron spectra (XPS) from Cu-plated Ni surfaces exposed to the MeCpMn(CO)3 precursor at 575 (blue traces) and 775 (red) K. The data are consistent with the deposition of a MnO film on the Cu substrate, with thickness that increases with deposition temperature. E


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on mixed surfaces film growth would occur preferentially at the metal sites. For instance, if metallic Mn were to be deposited on silicon oxide, an initial nucleation at OH surface sites would likely be followed by further deposition on the developing metal surface, a sequence that would lead to three-dimensional particle growth. Conversely, if manganese oxides are to be grown on metal substrates, the initial stage may be the formation of a flat monolayer, after which additional deposition would need to proceed on the newly formed oxide film and would therefore occur at lower rates. These differential deposition rates, due to differences in the nature of the substrate, need to be considered when designing ALD processes. Finally, our isothermal kinetic measurements afforded the identification of the temperature window where MeCpMn(CO)3 dissociative adsorption is self-limiting and viable for ALD. This is manifested by two observations deriving from our data, namely: (1) the similar rates of Mn precursor uptake, implying a virtually nonactivated process, and (2) the consumption of only some of the precursor available from the gas phase, the same amount over the ALD temperature window. The data in Figure 6 suggest that this occurs in the range from approximately 475 to 525 K. It was also seen that dissociative adsorption is possible at lower temperatures. However, uptake at those temperatures does most likely not lead to any further chemistry, because all copper surface sites are blocked and unavailable for reactivity, and therefore may not be conducive to ALD film growth. On the other end, exposure of the copper surface to the Mn precursor at temperatures of ∼540 K or above leads to more extensive decomposition and to the growth of multilayers. The process in this high-temperature regime is activated, and more typical of CVD.

Further decarbonylation of the MeCpMn(CO)2(ads) intermediate starts between 505 and 525 K, according to the spectra in Figure 1. Close observation of that figure also shows that the increase in the intensity of the CO IR peaks precedes that of those associated with MeCpH, the other main product from the decomposition of the adsorbed Mn precursor; the latter starts to grow at 525 K. It could be inferred that full decarbonylation of the adsorbed species is required to open up surface sites so the organic ligand can be displaced onto copper surface sites and undergo further chemistry. Accordingly, our proposed model for this reaction is that the Mn ALD precursor decomposes on the Cu surface in three stages: (1) an initial monodecarbonylation step occurring at low temperatures and leading to the buildup of a saturated monolayer of a MeCpMn(CO)2(ads) intermediate; (2) the full decarbonylation of that intermediate between 505 and 525 K, leaving MeCpMn(ads) behind; and (3) the conversion and elimination of the cyclopentadienyl ring above 525 K, at which point continuous steady-state Mn deposition is possible in CVD mode. To complete the mechanistic picture of the thermal chemistry of MeCpMn(CO)3 on copper surfaces, the mechanism by which the methylcyclopentadienyl ligand is eliminated from the surface needs to be discussed. One thing that we know from the IR spectra in this work is that the majority of those ligands are eliminated as methylcyclopentadiene. We have reported similar chemistry on silicon oxide surfaces, but in that case the detection of MeCpH, the product from protonation of the organic ligand, was determined to occur via reaction with surface silanol groups.34 It is difficult to conceive the same type of mechanism on metal substrates, as is the case here; the source of the needed hydrogen/proton is not as straightforward to identify. One possibility is for a small fraction of the methylcyclopentadienyl ligands to undergo dehydrogenation. Small amounts of fulvene and benzene were in fact detected in our previous study,34 but those would have to be produced in stoichiometric quantities to account for all the methylcyclopentadiene detected, and no evidence of that can be seen in the IR data in Figures 1 and 2. More likely, the surface intermediates may undergo further dehydrogenation until yielding carbon on the surface. If so, small amounts of carbon impurities may be incorporated in the growing films in ALD with the Mn-cyclopentadienyl precursors unless their decomposition can be prevented and the ligand protonation promoted in the second half of the ALD cycle. Fortunately, this is what seems to occur in ALD with water, in which case surface Mn−OH groups are created in the second half-cycle that act as adsorption sites for the Mn compound and as the protonation agents for the organic ligand.40 It is important to highlight that this still leaves the issue of the initiation of the ALD process on metals in applications in microelectronics, where Mn is to be used as a diffusion barrier for copper, unresolved.61−63 More research is needed to evaluate that aspect of this ALD chemistry. Another important conclusion from the work reported here is that MeCpMn(CO)3 decomposition on copper surfaces starts at temperatures below 500 K. This value is significantly lower than that observed when the deposition is carried out on silicon oxide surfaces. The difference has some significant consequences for ALD processes. For one, it indicates that the deposition of Mn-based films on metals, at least on copper (and perhaps nickel), can and probably should be carried out at lower temperatures than on oxides. In addition, it suggests that

5. CONCLUSIONS The kinetics of the adsorption of MeCpMn(CO)3 on copper surfaces was studied as a function of substrate temperature by analyzing the composition of the gas phase. This work has highlighted a change from molecular MeCpMn(CO)3 to a mixture of carbon monoxide and methylcyclopentadienes. A sequence of transitions was identified, which was interpreted as the result of a stepwise decomposition mechanism consisting of three steps, namely: (1) an initial monodecarbonylation at low temperatures leading to the dissociative adsorption of the Mn precursor to produce a MeCpMn(CO)2(ads) intermediate, (2) full decarbonylation of that surface species to yield MeCpMn(ads) around ∼510 K, and (3) elimination of the cyclopentadienyl ligands as MeCpH above 525 K. This sequence can be visualized by the following reaction mechanism: MeCpMn(CO)3 (g) + Cu(s) → MeCpMn(CO)2 (ads) + CO(g)

T ≤ 400 K

MeCpMn(CO)2 (ads) → MeCpMn(ads) + 2CO(g)

T ≥ 510 K

MeCpMn(ads) + H(ads) → Mn(ads) + MeCpH(g)

T ≥ 525 K

The formation of significant amounts of methylcyclopentadiene dimers, fulvene, or benzene was ruled out via comparison F


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with their IR spectra. Some decomposition was detected at temperatures as low as 475 K, more than 100 K lower than the threshold previously identified for the activation of the same compound on silicon oxide. Isothermal kinetic measurements pointed to a bracket of temperatures, between 475 and 525 K, where the disappearance of the reactant shows the same time evolution. In that regime, the initial rate of adsorption and decomposition is approximately constant, and the partial pressure of the precursor reaches a nonzero asymptotic value, presumably due to the unreacted compounds left behind once a monolayer of adsorbates is saturated on the copper surface. The rate of deposition then increases at temperatures above 540 K, and the consumption of the gas-phase reactant reaches completion because of the viability of deposition of multiple layers in CVD mode. The resulting film consists mainly of MnO.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]; Phone: 1 (951) 827-5498. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was funded by a grant from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering (MSE) Division, under Award Number DE-FG02-03ER46599. The XPS instrument used in this research was acquired with funds from the U.S. National Science Foundation, Grant DMR-0958796. The XPS data were acquired by Dr. Ilkeun Lee.

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Kinetics of Adsorption of Methylcyclopentadienyl Manganese

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