Reaction of Methylcyclopentadienyl Manganese Tricarbonyl on Silicon

Sep 9, 2014 - (1-3) More recently, metalorganic compounds containing .... its 1–2 nm thick silicon oxide native film,(46) at low temperatures (190 K...
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Reaction of Methylcyclopentadienyl Manganese Tricarbonyl on Silicon Oxide Surfaces: Implications for Thin Film Atomic Layer Depositions Menno Bouman, Xiangdong Qin, Vananh Doan, Benjamin L. D. Groven, and Francisco Zaera* Department of Chemistry, University of California, Riverside, California 92521, United States ABSTRACT: The thermal chemistry of methylcyclopentadienyl manganese tricarbonyl (MeCpMn(CO)3) on silicon oxide surfaces was characterized by a combination of analytical techniques, including gas chromatography/mass spectrometry (GC−MS), temperature-programmed desorption (TPD), infrared absorption spectroscopy, and Xray photoelectron spectroscopy (XPS). The compound was found to be fairly stable, but to be able to dissociatively chemisorb on the surface via the loss of one or more carbonyl ligands followed by the oxidative addition of a surface silanol group. Further activation leads to the loss of the aromatic ligand, at temperatures above approximately 575 K. This takes place primarily via the addition of a hydrogen atom to the MeCp ligand to form methylcyclopentadiene, which is released to the gas phase, but a small competing channel starts at slightly lower temperatures that yields fulvene, and/or possibly benzene, after molecular rearrangement. The clean nature of the chemistry seen here for the methylcyclopentadienyl Mn complex makes it a good candidate as a precursor in the chemical deposition of metal thin films. However, the high stability and the high temperatures required for its decomposition do introduce some limitations. M bond.14,15 Occurrence of any of those reactions on surfaces could complicate the chemistry of ALD processes.12,16 Here we discuss results from a mechanistic study of the chemistry of methylcyclopentadienyl manganese tricarbonyl (MeCpMn(CO)3) on silica surfaces. Manganese-containing films have gained some attention recently in the microelectronic industry as potential diffusion barriers to prevent the electromigration of copper interconnects,17−20 and cyclopentadienyl Mn complexes have been identified as some of the best candidates for the ALD of such films.21−23 Previous surface-science studies from our laboratory have identified a complex series of steps leading to the growth of layered structures during the deposition of Mn on silicon wafers using MeCpMn(CO)3, including the formation of thin manganese silicate and manganese silicide zones,24,25 but that chemistry seems to be inherent of the Mn−Si system, and not specifically connected to the nature of the film deposition method used.26−30 Much less is known about the surface conversion of the Mn precursors themselves; this is the question addressed in the present study. In general, the surface chemistry of ALD processes with cyclopentadienyl complexes is expected to be simple, since they usually produce films with little if any carbon deposition.25,31−37 Our present study corroborates that hypothesis for the case of MeCpMn(CO)3. Activation of that precursor requires either external excitation sources such as photons or electrons,38 or relatively high temperatures. Decomposition on silicon oxide

1. INTRODUCTION Cyclopentadienyl ligands are quite ubiquitous in organometallic chemistry. Because of their great stability, they are often used to complete the coordination of metal centers, and in many reactions act only as spectators. For instance, cyclopentadienyl complexes containing other active functionalities are used extensively in organic synthesis, mainly as catalysts.1−3 More recently, metalorganic compounds containing cyclopentadienyl ligands have also been tested as precursors for the chemical deposition of solid thin films.4,5 The original use of such compounds in chemical vapor deposition (CVD) processes6,7 has been revisited recently in connection with atomic layer depositions (ALD), a variation where the chemical reactions are separated in time in order to obtain monolayer control on the growth rate of the films.8−10 The success of organometallic precursors in CVD and ALD applications often relies on the stability of their ligands.11,12 When growing thin films of metals or other solid materials, the decomposition of organic moieties is often a problem, because that can result in the incorporation of carbon, oxygen, and other undesirable impurities; this is one of the main reasons for the interest in using cyclopentadienyl ligands. However, virtually nothing is known about the chemistry of cyclopentadienyl complexes on solid surfaces.13 In solution, coordinated cyclopentadienyl ligands are usually stable but also known to undergo a number of reactions, including attacks by electrophilic or nucleophilic agents, metallacyclopentadienylations, ring C−H insertions, cycloadditions, couplings, activations of ring C−C bonds, and insertions into the Cp− © XXXX American Chemical Society

Received: June 12, 2014

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pentadiene rather than 5-methyl-1,3-cyclopentadiene,39 but this is not a solid conclusion. The earlier GC feature, on the other hand, is believed to correspond to a mixture of fulvene or benzene; although the MS traces could not be unequivocally assigned to either molecule exclusively, the cracking patterns seen in our data are closer to that reported for the former.40 In any case, these were the only two peaks seen in the GC trace in our experiments, which indicates that the MeCpMn(CO)3 thermal decomposition inside the glass vials is clean, leading to the formation of only a couple of organic byproducts. It is also clear from the data in Figure 1 that the yields of the C6 products from MeCpMn(CO)3 decomposition increase with increasing temperature, with an initial significant jump taking place between 575 and 600 K. It was determined that the main product at high temperatures is MeCpH. This may not be obvious from the data in Figure 1, however, since the two GC peaks seen there are quite close and overlap to a significant extent. Moreover, the similarities of mass spectra of all the products detected complicate the data analysis further. Nevertheless, the relative increase in yield for MeCpH as compared to fulvene/benzene was corroborated by the increase in the contribution of the 79 amu signal to the GC traces seen at the higher reaction temperatures (Figure 2). Results from

surfaces was seen here starting at about 550 K, and to lead to the evolution of methylcyclopentadiene (and a small amount of fulvene or benzene) into the gas phase. Our evidence indicates the direct participation of surface silanol groups in this reaction, likely via their oxidative addition to the metal center followed by intramolecular migration of the coordinated hydrogen to protonate the cyclopentadienyl ligand. The implications of our results to ALD are briefly discussed.

2. RESULTS The thermal reactivity of MeCpMn(CO)3 was first tested inside glass vials by using a combined gas chromatography/ mass spectrometry (GC−MS) analytical approach. Preset quantities of the precursor, alone or mixed with water, were sealed inside the vials and annealed at a given temperature for a fixed time (30 min), after which an aliquot of the trapped gases was taken and its composition analyzed by GC−MS. The early times of the gas chromatography traces obtained with the pure Mn compound for a set of temperatures between 525 and 625 K are shown in the left panel of Figure 1. In addition to the

Figure 1. Left: Gas chromatograms (GC) obtained for the gas trapped inside glass vials filled with MeCpMn(CO)3 in an inert (N2) atmosphere, sealed, and heated to the indicated temperatures. Only the early elution times are shown. The two main features detected are highlighted and identified as a mixture of fulvene and toluene (t = 2.31 min) and methylcyclopentadiene (MeCpH, t = 2.37 min), respectively. Right: Mass spectra (MS) obtained in tandem for the gas eluted at the times of the peaks identified in the GC traces to corroborate the identity of the corresponding species. The increase in GC signals starting between 575 and 600 K marks the onset of the decomposition of the Mn complex.

Figure 2. Representative traces from the experiments reported in Figure 1 showing the contributions from the 78 and 79 amu components of the mass spectra to the total GC signal. The significantly larger signals seen at high temperatures are clearly due to an increase in detection of ions at 79 amu, indicating that the main product from MeCpMn(CO)3 thermal activation in the glass vials is MeCpH rather than fulvene or benzene.

main peak seen for the molecular precursor, in particular at low temperatures, which is eluted at 13.35 min (outside the time range shown), an additional pair of peaks was observed at 2.31 and 2.37 min, with intensities that increase with increasing reaction temperature. The identity of the products corresponding to those peaks was established by mass spectrometry, by analyzing data such as those shown in the right panel of Figure 1. The mass spectra for the GC peak at 2.37 min could be easily assigned to methylcyclopentadiene (MeCpH), although it was not possible to establish which specific isomer of that molecule is formed (if only one): the relatively low intensity of the peak for 39 amu suggests the formation of 1-methyl-1,3-cyclo-

quantitation of the GC and MS signals from these experiments in terms of reaction conversions as a function of temperature are summarized in Figure 3. A small amount of fulvene/ benzene is seen starting at about 575 K, but the main product from MeCpMn(CO)3 is clearly methylcyclopentadiene. It is known that MeCpMn(CO)3 is quite stable, and that it starts to decompose thermally in the gas phase only after heating to temperatures above 675 K, presumably via an initial sequential loss of carbonyl ligands.41 Therefore, it was inferred that the lower decomposition temperatures observed here were due to interaction of the metalorganic Mn precursor with the surface of the glass vials. The silanol (Si−OH) groups that are almost always present in silicon oxide surfaces are well-known B

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Figure 3. Summary of the GC−MS data obtained for the decomposition of the Mn precursor as a function of reaction temperature in the form of total conversion (left) and relative composition (right). Again, the data emphasize the fact that the decomposition threshold for MeCpMn(CO)3 in these experiments using glass vials is approximately 575 K, and that above 600 K the production of MeCpH dominates the chemistry observed.

Figure 4. Temperature-programmed desorption (TPD) traces obtained from MeCpMn(CO)3 adsorbed on a SiO2/Si(100) wafer. Data are provided for the following, from top to bottom: 18 (H2O), 15 (CH3 moieties), 2 (H2), 28 (CO), and 134 ([MeCp−H]Mn moieties, from fragmentation of the molecular species) amu. After molecular desorption at 250 K and possible CO production between 250 and 300 K, no desorption is seen until reaching ∼540 K, at which point the production of hydrogen and water is accompanied by the evolution of a hydrocarbon that we assign to MeCpH. The precursor was activated via electron bombardment in the gas phase in these experiments to enhance its uptake on the surface.

to facilitate the activation of coordinated complexes42 and are generally assumed to act as nucleation sites in CVD and ALD processes.12,43−45 A couple of experiments were devised here to test this hypothesis. In the first, an excess of MeCpMn(CO)3 was dosed on the surface of a Si(100) wafer covered with its 1−2 nm thick silicon oxide native film,46 at low temperatures (190 K) to ensure that the initial adsorption is molecular and that the uptake goes slightly beyond monolayer chemisorption (to include a few condensed physisorbed layers), and the thermal surface chemistry was tested via temperature-programmed desorption (TPD) experiments under ultrahigh vacuum (UHV) conditions. The results are shown in Figure 4. Molecular desorption from condensed layers is clearly seen at 250 K, after which only a chemisorbed layer is left on the surface. The detection of any possible CO emission is difficult because of the overlap with the cracking pattern of the molecular species, but that appears to occur soon thereafter, below ∼300 K, as expected if decarbonylation is the first activation step in the Mn precursor. It should be noted that because the sticking coefficient of the Mn precursor is quite low (due to its high stability), gas-phase activation via electron bombardment was used in these experiments,38 and, consequently, the adsorbed monolayer is not expected to retain many CO ligands anyway. After the CO evolution, no additional reactivity is seen until reaching temperatures above 540 K, at which point two sets of peaks are detected, at 550 and 575 K. In addition to H 2 and water (possibly from disproportionation of surface OH groups), signal intensity is detected for 15 amu, a mass characteristic of hydrocarbons containing methyl moieties that we assign here to MeCpH. Formation of this compound is qualitatively consistent with the GC−MS data in Figures 1 and 2; the lower temperatures seen in the TPD traces are due to the nature of the kinetics of the UHV experiments.47 The second test of our hypothesis was carried out by using transmission infrared absorption spectroscopy. A fixed amount of MeCpMn(CO)3 in the gas phase was contained inside an IR

cell designed to also hold a pellet of silica powder so that the IR absorption data could probe both gas-phase and surface species. Typical IR traces obtained from experiments carried out as a function of surface temperature are shown, in differential mode, in Figure 5. The initial spectrum recorded at 475 K (bottom trace) shows only the two bands at 1960 and 2035 cm−1 characteristic of the C−O stretching modes of the carbonyls in molecular MeCpMn(CO)3.48 No significant changes are seen

Figure 5. Infrared (IR) absorption spectra for gas-phase MeCpMn(CO)3 in the presence of a pellet of silica powder. The traces were obtained while heating the powder to the indicated temperatures and, except for the initial trace recorded at 475 K (bottom, green), are displayed in differential mode to highlight the changes. No significant reaction is seen up to 575 K, above which total decomposition of the Mn complex is evidenced by the disappearance of the 1960 and 2035 cm−1 peaks characteristic of its carbonyls. The participation of surface silanol groups in the reaction is indicated by the concomitant reduction of the IR signal intensity between approximately 3500 and 3800 cm−1. C

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upon heating up to 575 K, but beyond that temperature the IR signature of the coordinated carbonyls disappear, indicating full decomposition of the precursor. In addition, a negative broad feature is also seen between approximately 3500 and 3800 cm−1 easily assignable to the O−H stretching mode of surface silanol groups.49 It is clear that the activation of the Mn complex involves those OH surface species. In fact, in GC−MS reactivity experiments carried out with stainless steel rather than glass vials, where there are virtually no surface OH groups, the thermal reactivity of MeCpMn(CO)3 was shown to be greatly diminished: at 625 K, only 5% conversion was observed in the stainless steel vial, compared to 45% with the glass vial (Figure 6). Unfortunately, attempts to passivate the silanol groups of

Figure 7. Transmission IR absorption spectra from a silica powder pellet after three sequential 2 min exposures to 200 mTorr of D2O at 390 K. Data are provided for the silica material in its original form, as purchased (bottom trace, green), and after sequential exposures to deuterated water (second, blue), air (third, purple), deuterated water again (fourth, red), and a 1 mTorr vacuum environment (top, brown). The alternation between the strong sharp peaks seen at 3745 (O−H stretching mode) and 2760 (O−D) cm−1 attest to the ease with which the silanol surface groups undergo isotope exchange, a fact that limits the usefulness of isotope labeling experiments for the elucidation of the reaction mechanism of the decomposition of our Mn metalorganic complex.

Figure 6. GC traces from GC−MS experiments with MeCpMn(CO)3 heated to 600 K in glass (bottom, red trace) and stainless steel (top, blue) vials. The much lower conversion seen in the latter case (5% with stainless steel versus 45% with glass) supports the idea of the participation of silanol groups in the conversion of the Mn precursor.

the glass vials via silylation with hexamethyldisilazane (HMDS) or octadecyltrichlorosilane (ODTS) were not successful: both significant conversion of the Mn precursor and extensive deuterium scrambling between heavy water and the surface silanol groups (see below) were always observed with the glass vials, even after such treatments. The surface silanol groups in silica are known to easily exchange protons with water.50 This feature could in principle afford their isotope labeling in protonation reaction studies such as ours. Unfortunately, the OH groups associated with the chemistry of the metalorganic Mn complex are mostly the isolated groups identified by the sharp peak seen in the IR spectra at 3745 cm−1, which is due to the O−H stretching mode (at 2760 cm−1 for O−D), and the isotope scrambling of those is too fast for the solid to be able to retain the deuterium labeling at the temperatures used in our experiments (Figure 7). Nevertheless, partial deuterium addition, to produce MeCpD, was seen in GC−MS experiments with mixtures of MeCpMn(CO)3 and D2O, as indicated by the development of an additional peak at 81 amu in the appropriate mass spectra (Figure 8). The possibility of H-D exchange between MeCpH and D2O in the gas phase, without intervention of the surface, was ruled out by the fact that virtually no MeCpD was detected in experiments with mixtures of MeCpMn(CO)3 and D2O heated inside a stainless-steel vial (data not shown). It should be noted that no enhancements in decomposition yields were seen here upon water (or deuterated water) addition to the gas mixture. In general, the atmosphere under which chemical vapor depositions are carried out can affect the

Figure 8. Mass spectra obtained for the t = 2.37 min (methylcyclopentadiene) GC peak of GC−MS experiments using glass vials and MeCpMn(CO)3 alone (bottom, blue) or mixed with either normal (center, green) or deuterated (top, red) water. The development of an intense new peak at 81 amu in the case involving D2O attests to the incorporation of deuterium from the deuterated water in the MeCp ring, likely via previous exchange with the surface silanol groups of the glass.

reactivity of the metal precursor. For example, film deposition using this MeCpMn(CO)3 precursor appears to be easier with oxidizing versus reducing atmospheres.51 Therefore, the fact that no significant influence in reactivity was seen here by the presence of water indicates that neither H−D exchange within the surface silanol groups nor steps involving reaction with water are involved in the reaction rate-limiting step of the Mn precursor conversion. Finally, ex situ X-ray photoelectron spectroscopy was used to characterize the surface resulting from exposure of the silicon oxide surfaces to MeCpMn(CO)3. Representative data are shown in Figure 9. Significant amounts of Mn are deposited on D

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particular interest in identifying the features relevant to thin film deposition by chemical means, via CVD or ALD processes. It was found that, in spite of the intrinsic stability of cyclopentadienyl metal complexes, it is possible to activate this Mn compound and to dissociatively adsorb it on silicon oxide surfaces. Establishing the mechanistic details of the chemistry of these heterogeneous systems at a molecular level is much more difficult than when considering reactions in homogeneous phases, because surface atoms amount to only a minor fraction of the total bulk of solids, and because the surface-sensitive techniques available to investigate the chemistry of such film deposition processes are limited. Nevertheless, some clear mechanistic conclusions can be drawn from the results reported here, as discussed below. It is not evident from the data available from these studies how the MeCpMn(CO)3 complex reacts initially to thermally adsorb on the silica surface. Most likely, that happens via the loss of one or more carbonyl ligands: virtually no C 1s XPS signal assignable to CO was seen in the XPS data in Figure 9, and little if any CO desorption was seen in the TPD experiments reported in Figure 4 (and what may have seen desorbs below ∼300 K). It should also be indicated that it was previously found by us that the uptake of this precursor on surfaces can be enhanced by its preactivation in the gas phase using electron bombardment, a process that leads mainly to extensive decarbonylation.38 The loss of at least one CO ligand is likely to be required for the compound to coordinate to the surface, which the data strongly suggest occurs via the oxidative addition of a silanol group to the Mn complex to create a Si−O−Mn(H)(CO)n(MeCp) surface species (Figures 5, 6, and 8). This type of oxidative additions is not common,53,54 but the equivalent addition of water or alcohols is known in homogeneous phase, and it is enhanced in transition metal precursor with their metal in a low oxidation state and also stabilized by coordination to unsaturated organic π-acceptors, as is the case here.55 Additions to silanol groups are also commonly used to anchor metalorganic catalysts such as metallocenes to high-surface-area silica supports.42 On the other hand, most reports on this type of reactivity have shown that the resulting hydride/hydroxide(siloxide) complex is typically unstable and easily undergo further conversion,55 often via hydride addition to one of the metal ligands.56 In fact, it has been reported that addition of many metallocenes (but not manganocene) to the OH groups of silica results in the spitting of CpH.57 In our case, the species resulting from dissociative adsorption of MeCpMn(CO)3 on Si−OH sites seems to retain the methylcyclopentadienyl ligand intact, at least at low temperatures. Conversion of this organic ligand was only detected above 540 K in the TPD traces (Figure 4), and to a significant degree only above 575 K in the GC−MS experiments (Figures 1 to 3). Thermal activation at higher temperatures does lead to further decomposition, but even then the chemistry is relatively simple: a small amount of either fulvene or benzene was seen to evolve from the surface starting at about 575 K, but the main product by far is methylcyclopentadiene, which accounts for ∼85% of all of the organic molecules released to the gas phase at 625 K (Figure 3). We suggest that this conversion may require a reductive elimination step,58 or perhaps a migratory insertion of the newly formed hydride ligand to the coordinated aromatic ring.59 Addition of nucleophilic groups, including hydrides, to coordinated cyclopentadienyl ligands (to produce

Figure 9. Mn 2p (left), Si 2p (second from left), C 1s (second from right), and O 1s (right) X-ray photoelectron spectra (XPS) from silicon oxide surfaces exposed to MeCpMn(CO)3 at 625 K. The bottom two sets of traces correspond to a glass slide (bottom, green) and a SiO2/Si(100) wafer piece (center, blue) treated in sealed glass vials, whereas the top data (red) were obtained with a similar Si wafer but treated in the IR cell. The deposition of thick films of (oxidized) Mn is evidenced by the reduction in Si 2p signal intensities and by the strong Mn 2p peaks. Additional evidence was obtained for the presence of carbon containing species on the surface, possibly including a small amount of adsorbed carbonyls, and for the formation of a form of Mn oxide.

glass at 625 K, as evidenced by the total masking of the Si 2p XPS signal in experiments with a glass slide (second-from-left panel, bottom, green, trace), and by its replacement with strong features in the Mn 2p XPS trace, which shows a broad double peak for the Mn 2p3/2 centered around 640.7 and 641.5 eV (left panel). The first of those features is likely to be from Mn still complexed with the MeCp ligand, whereas the second is typically associated with oxidized Mn, possibly Mn2+.24,52 Indeed, a strong peak also grows at 529.0 eV in the O 1s XPS (right panel), indicative of the formation of either a metal oxide (MnOx) or a metal silicate.29 Finally, an intense C 1s peak is seen at 284.6 eV (second-from-right panel), possibly associated with MeCp but also likely to come from adventitious contamination acquired during sample transfer. An additional shoulder is detected in the C 1s XPS at 288.0 eV, which could originate from residual carbonyl ligands. The deposition of Mn films on the inner surface of the glass vials was easy to detect by visual inspection: the walls become opaque, and eventually coated with a metallic film. Mn deposition occurs to a much lesser extent on Si(100) wafers, presumably because of the lower concentration of OH groups on the surfaces of those substrates. The XPS spectra in that case are dominated by the Si 2p and O 1s signals from the SiO2/Si(100) substrate (Figure 9, middle, blue, traces), even though some Mn and C are evident as well. Treatment of the wafer under more stringent conditions (which were accomplished here by performing the experiments in the IR cell) leads to more extensive Mn deposition, and to the growth of Mncontaining films with characteristics similar to those seen with the glass (Figure 9, top, red, traces).

3. DISCUSSION The focus of this work has been the characterization of the surface chemistry of MeCpMn(CO)3 on silica surfaces, with a E

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first by the formation of a small amount of fulvene (which may isomerize to benzene), and at higher temperatures by the almost quantitative conversion of the methylcyclopentadienyl ligand to methylcyclopentadiene, which desorbs into the gas phase. TPD data obtained with silicon wafers in a UHV environment qualitatively corroborate the temperature range of these reactions. The involvement of the surface silanol groups present in glass, silica powders, or silicon wafers in the activation of the Mn metalorganic complex was evidenced by a depletion of the IR absorption signal from the O−H stretching mode of those groups, and supported by the observation that much less conversion is possible if stainless steel containers are used instead. The formation of MeCpD in isotope labeling experiments with D2O, which can readily undergo isotope scrambling with silanol surface groups, point to the OH surface groups as the source for the protonation of the methylcyclopentadienyl ligand. Finally, XPS data (and visual inspection of the vials) attested to the extensive deposition of Mn containing films on the walls of the glass vials. The clean chemistry seen here makes this Mn cyclopentadienyl precursor a good candidate for ALD applications as long as high temperatures can be used.

cyclopentadienes in the case of the hydrides) is known.60,61 Interestingly, it has been recently shown that water addition of photoactivated CpMn(CO)3 leads to the production of CpH via a water-coordinated CpMn(CO)2(H2O) intermediate.62 Similar chemistry was also reported with methanol.63 It may be worth briefly discussing the implications of the chemistry described here to the use of cyclopentadienyl metal complexes for CVD or ALD applications. Precursors for such processes need to be volatile and thermally stable in order to be amenable to their easy handling and delivering to the surface, 5,16 and cyclopentadienyl compounds such as MeCpMn(CO)3 nicely fulfill that requirement. On the other hand, they are also required to display sufficient surface reactivity to afford the deposition of films at reasonably low temperatures. Many cyclopentadienyl-based candidates fail this test, either because of the difficulties encountered in their initial activation, or because of the stability of the Cp ligands in the resulting adsorbed species. In the first case, this shortcoming may be mitigated, at least in part, by the choice of auxiliary ligands, or perhaps by using nonthermal precursor activation steps such as the gas-phase electron bombardment cited above; the latter approach has not been used much to date, and deserves further exploration.38,64 That still leaves the problem of removing the chemisorbed aromatic rings from the surface, for which a suitable strong protonation agent may be needed as the ALD coreactant. Lastly, CVD and ALD precursors should follow a reasonably simple and complete surface chemistry during the ligand-removal steps, otherwise the adsorbed byproducts may leave undesirable impurities in the growing films.12 The present study indicates that cyclopentadienyls are good ligands for the fulfillment of this requirement. Perhaps the major concern is that cyclopentadienyl removal from the surface may require temperatures higher than those acceptable in many situations; many microelectronic processes, for instance, are better performed at as low a temperature as possible, certainly below the 575 K needed with the MeCpMn(CO)3 precursor. On the other hand, these steps may be facilitated by the use of appropriate coreactants. Certainly, this limitation needs to be addressed and balanced against the superior surface chemistry of cyclopentadienyls compared to other types of ligands. As cited above, methylcyclopentadienyl manganese tricarbonyl is a promising precursor for the atomic layer deposition (ALD) of Mn containing films, at least in processes that can stand high temperatures.1−3 Because of the particular research interests in our group, we have framed our discussion within that context. However, this Mn metalorganic compound is also a good representative of cyclopentadienyl complexes in general. Therefore, the surface chemistry unveiled by our studies may also be helpful in other field. For instance, it may provide some useful general understanding of the stability of metallocene metathesis and polymerization catalysts.1−3 We also like to emphasize the importance of using modern surface-science methodology, as was done here, to carry out mechanistic studies with this type of systems, to help expand the limited information available to date on the chemistry of metalorganic compounds with solid surfaces.12,16,65

5. EXPERIMENTAL SECTION For the gas chromatography/mass spectrometry (GC−MS) experiments, a small, predetermined amount of the liquid reactant(s), MeCpMn(CO)3 (0.2 μL, Strem Chemicals, 97% minimum purity) by itself or with either regular (deionized) or deuterated (Sigma-Aldrich, 99.9 atom % D) water (0.2 μL), was/were introduced under nitrogen atmosphere into a small (10 mL) vial, made out of either glass or stainless steel, and sealed with a Teflon/silicon septum and heated to the desired temperature for approximately 30 min. Gas aliquots (2 μL) were taken using a small syringe and injected directly into the port of the GC−MS instrument (Agilent 7890/Waters GCT). Most experiments were performed with glass vials, to test reactivity with the OH groups of their inner silica surface, but a few contrasting experiments were also carried out with a homemade stainless-steel vial to test the role of those surface species in the chemical conversion. Previous examples of the use of this approach have been published already.66,67 The temperature-programmed desorption (TPD) experiments were carried out in a two-tier ultrahigh vacuum (UHV) instrument described in detail before.68,69 The silicon wafer (Si-Tech, covered with a native ∼1−2 nm thick SiO2 layer, cut into ∼1 × 1 cm2 squares) was first cooled down to 190 K and dosed with 1000 L (1 L = 1 × 106 Torr s) of the MeCpMn(CO)3 precursor while keeping the ion gauge (used for pressure measurements) on in order to facilitate its activation in the gas phase.38 The surface was then heated at a rate of approximately 5 K/s while following the desorption of the products with a UTI quadrupole mass spectrometer multiplexed by using a personal computer to be able to detect up to 15 masses simultaneously.47,70,71 Transmission infrared absorption spectroscopy data were acquired by using a setup similar to that described elsewhere.66,72,73 A small cell made out of a 5-way stainless steel cross (∼7 cu in.) capped with opposing NaCl windows was equipped with inlet and outlet ports for gas dosing and for pumping (with a mechanical pump, to base pressures in the mTorr range), and with a sample holder capable of resistive heating and of cooling of the solid to cryogenic temperatures. This cell was placed inside the sample compartment of a Bruker Tensor 27 Fourier transform infrared (FTIR) spectrometer equipped with a mercury−cadmium−telluride (MCT) detector. Infrared absorbance spectra were acquired by averaging 1024 scans at 4 cm−1 resolution and ratioing those against similar background spectra taken prior to the gas exposure or the other surface treatments. The substrate used for the IR data in Figures 5 and 7 was a commercial aerosil fumed silica powder (Sigma-Aldrich, 200 m2/g, 0.2−0.3 μm average particle size), which was made into a pellet,

4. CONCLUSIONS The chemistry of MeCpMn(CO)3 on silicon oxide surfaces was studied with the aid of GC−MS, TPD, infrared-absorption spectroscopy, and XPS. Thermal decomposition of that compound inside glass vials was seen to start at about 575 K, F

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pressed onto a nickel mesh, and dried by heating to 575 K for 30 min. The temperature was measured using a type K chromel−alumel thermocouple sportwelded to the Ni mesh. For the MeCpMn(CO)3 adsorption studies (Figure 5), the solid was set to the indicated temperatures and exposed to approximately 0.1 Torr of the precursor for 2 min while acquiring the IR spectra, after which the cell was purged with 0.5 Torr of dry N2 for 2 min and pumped for another 8 min before carrying out any additional experiments. In the case of the H2O/D2O isotope exchange experiments (Figure 7), the powder was heated to 390 K and then exposed sequentially to three cycles of 200 mTorr of the water vapor and N2 purging before data acquisition. X-ray photoelectron spectroscopy (XPS) data were acquired ex situ using a Kratos AXIS analytical instrument equipped with a 165 mm mean radius semihemispherical electron energy analyzer and a 120element delay line detector. A monochromatized Al-anode X-ray gun was used as the excitation source, and an electron flood source was employed as needed to compensate for sample charging. Data are provided for three samples, after CVD of MeCpMn(CO)3 on both a glass slide and a Si(100) wafer piece (1 × 1 cm2) placed inside a glass vial (estimated Mn film = 2 μm), and on a similar Si(100) wafer sample treated inside the IR transmission cell. The samples in the vial were prepared by adding 0.25 μL of MeCpMn(CO)3 under a N2 atmosphere in a glovebox and heating gradually to 625 K. The last sample was exposed to 100 mTorr of MeCpMn(CO)3 at 625 K for ∼1 min.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 (951) 827-5498. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial assistance was provided by the U.S. Department of Energy, Materials Science Division, under Grant No. DE-FG0203ER46599. The XPS data were acquired with an instrument purchased with funds from NSF, Contract DMR-0958796.

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