Chemistry of Ruthenium Diketonate Atomic Layer Deposition (ALD

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Chemistry of Ruthenium Diketonate Atomic Layer Deposition (ALD) Precursors on Metal Surfaces Xiangdong Qin and Francisco Zaera* Department of Chemistry, University of California, Riverside, California 92521, United States ABSTRACT: The thermal chemistry of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)ruthenium(III) (Ru(tmhd)3), a potential precursor for the chemical deposition of ruthenium -containing films, on Ni(110) single-crystal surfaces was characterized by using a combination of temperatureprogrammed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and reflection−absorption infrared spectroscopy (RAIRS). Additional characterization of the surface chemistry of the protonated ligand, Htmhd, was evaluated as well for reference. It was found that the molecularly adsorbed ruthenium compound reacts readily by approximately 310 K, loosing its ligands to both the gas phase and the surface as the central ion is reduced to its Ru0 metallic state. The diketonate ligand, now bonded to the nickel surface, starts to decompose at around 400 K, and generates gas-phase carbon monoxide and molecular hydrogen in TPD peaks at 435 K. More extensive decomposition is seen at 535 K, yielding 2,2-dimethyl-3oxopentanal, isobutene, ketene, and carbon monoxide, and also carbon dioxide and molecular hydrogen at slightly higher temperatures. The XPS data corroborate the early reduction of the metal center and the losses of carbon- and oxygen-containing adsorbates to the gas phase, and the RAIRS traces show similar chemistry followed by the Ru complex and the free ligand, both converting via an initial decarbonylation step and a subsequent loss of the terminal tert-butyl groups. The early decomposition of the ligand on the metal surface points to potential problems with the clean deposition of metal films using diketonate complexes, but the ease with which those ligands are displaced from the central ion suggests that there is a potential for low-temperature film deposition chemistry under specific circumstances.

1. INTRODUCTION Because of the need to deposit thin solid films conformally on surfaces with rough topographies in many practical applications, there is increasing interest in performing this task by chemical means.2,3 Chemical vapor deposition (CVD) methods, where appropriate inorganic compounds are used as precursors to provide the elements of the materials to be deposited, have been widely used over many decades4−7 but have received a recent boost with the development of a new version, atomic layer deposition (ALD), in which two self-limiting and complementary reactions are used sequentially and in alternating fashion to slowly build up solid films one monolayer at a time.2,8−10 By separating the deposition chemistry into two halves this way, ALD offers several advantages over other CVD processes, including the following:9,11 (1) the film thickness depends only on the number of cycles employed, not on the exposures used in each cycle (process control is simple and accurate); (2) more flexibility in the design of the operational deposition conditions, requiring lower temperatures than regular CVD; and (3) minimal or no interference from gasphase reactions during the deposition process because of the separation of the two half reactions in time. On the negative side, ALD, like other chemical processes, suffers from some important intrinsic limitations. In particular, the chemistry of the ALD precursors on the solid surfaces where they are converted often includes side steps that lead to © XXXX American Chemical Society

the incorporation of undesirable contaminants within the growing films.12−17 This is a particularly acute problem in the case of the ALD of late transition metals, for which the most viable precursors tend to be volatile metal−organic compounds containing large organic ligands.3,18−23 The selection of precursors in ALD has so far been made in an empirical fashion, mainly relying on knowledge extracted from inorganic studies in solution but without any systematic understanding of the key parameters controlling surface reactivity or impurity deposition. As a consequence, most of the ALD processes available to date still suffer from a number of unresolved chemical limitations leading to the degradation of the quality of the grown films. Addressing and solving these problems requires a good understanding of the surface chemistry involved, and in our laboratory we are dedicated to the enhancement of our knowledge on the mechanistic details of that chemistry to help with that endeavor.24−29 In this study, we have investigated the mechanistic details of the surface chemistry of tris(2,2,6,6-tetramethyl-3,5heptandionato)ruthenium(III) (Ru(tmhd)3), a well-known Special Issue: Prashant V. Kamat Festschrift Received: December 4, 2017 Revised: January 3, 2018

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DOI: 10.1021/acs.jpcc.7b11960 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C ruthenium ALD precursor22,30−34 and a good representative of ALD and CVD precursors based on diketonate ligands,20,21,35−37 on a metal substrate, specifically on a Ni(110) single crystal. We have used a multitechnique in situ approach based primarily on temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS), as we have done in the past,24−29 to which we have added reflection−absorption infrared spectroscopy (RAIRS) for the acquisition of vibrational information.38−41 There are many uses for metallic ruthenium and ruthenium oxide thin films. In microelectronics, Ru has been tested as a diffusion barrier material for copper interconnects; thin layers of Ru facilitate the direct deposition of Cu without the need for a seed layer and promote the growth of Cu films exposing (111) surfaces, a desirable property as that surface plane enhances its resistance to electromigration.42−45 In energy applications, Ru has been alloyed with Pt to improve the performance of fuel cells46−51 and can also be used to enhance the photogeneration of hydrogen via water splitting52−54 and other catalytic processes.55 ALD-deposited RuO2 films are being tested in supercapacitor manufacturing56−60 and as electrode materials in lithium-based batteries.61 Several metal−organic compounds have been investigated for Ru ALD,22,34,62 but none has shown clear superior properties. A better knowledge of the surface chemistry involved could help in the ultimate selection of Ru ALD precursors for specific uses, and our surface-science characterization of the conversion of Ru diketonates on metal substrates should contribute to that understanding.

is sealed and purged with dry air, purified by using a scrubber (Balston 75-62) for CO2 and water removal. Spectra are typically acquired by averaging the data from 1024 scans taken at a resolution of 4 cm−1, a process that takes about 10 min per experiment, and ratioed against spectra from the clean sample obtained in the same way but before gas dosing. Spectra were taken with both s- and p-polarized light to discriminate between gas-phase and adsorbed species;38,40,68,69 the reported traces correspond to the p/s ratio. A ∼1 cm in diameter, ∼1 mm thick Ni single-crystal disk with a polished (110) surface exposed was used as the substrate. Nickel is a good representative of the metals upon which many films are grown in practical applications (including the nitrides used as diffusion barriers in microelectronics),70 and the use of the (110) plane affords a direct comparison with knowledge acquired for other ALD systems.29,70−73 The Ni(110) disk was held in place and heated resistively via two tantalum wires spot-welded to its edge. Cooling of the crystal was done by immersing the two copper heating leads connected to the Ta wires, which are coupled to the crystal via UHV electrical feedthroughs, in a vertical liquid nitrogen reservoir; this arrangement afforded reaching temperatures below 100 K. A chromel/alumel thermocouple spot-welded to the edge of the crystal was used to monitor its temperature, which was controlled by using a homemade proportional− integral−derivative (PID) circuit. The ruthenium precursor, Ru(tmhd)3 was purchased from Strem Chemicals (99% total purity, 99.9% Ru purity) and purified in situ via pumping of the gas manifold while dosing, which was done by using a controlled leak valve. Because that compound is an orange solid with a relatively low vapor pressure (TMelting = 473−483 K, TDecomposition ≈ 523 K),33 it needed to be heated (together with the gas feeding line) to 383 K during dosing to attain dosing pressures inside of the UHV chamber of up to 2 × 10−6 Torr. The protonated ligand, 2,2,6,6-tetramethyl-3,5-heptanedione (Htmhd, Sigma-Aldrich, ≥98% purity), was purified in situ via multiple freeze−pump−thaw cycles and delivered using the same leak valve as that used with the Ru precursor. Because Htmhd is a liquid with a high vapor pressure, however, it could be delivered directly at room temperature, without any need for heating of the feeding line. All exposures are reported in Langmuirs (1 L = 1 × 10−6 Torr s), using the values of pressures measured with a nude ion gauge in the UHV chamber without correcting for any differences in ionization sensitivities.

2. EXPERIMENTAL DETAILS The experiments reported here were conducted in a surface analytical instrument described in detail elsewhere.63,64 The system is based on a two-tier ultrahigh vacuum (UHV) chamber turbopumped to a base pressure in the 10−10 Torr range. The main level of that chamber is equipped with a UTI quadrupole mass spectrometer, used for TPD experiments, a concentric hemispherical electron and ion energy analyzer (VG 100AX) and an Al Kα/Mg Kα dual-anode X-ray source, used for XPS data acquisition, and a Kratos rasterable ion gun, used for sample cleaning and, together with the energy analyzer, for low-energy ion scattering spectroscopy (LEISS). The XPS spectra were taken at a total resolution of approximately 1.0 eV. Atomic ratios were calculated from the XPS peak areas by using reported sensitivity factors.65 The TPD experiments were run by using a heating rate for the crystal of 10 K/s, and were controlled by a personal computer designed to follow the evolution of as many as 15 ions (amus) simultaneously in a single run. A long-travel manipulator is used to transfer the sample between this main analytical tier of the UHV chamber and a second level set up to carry out RAIRS experiments. The arrangement used for this technique is similar to that of another instrument available in our laboratory for catalytic studies.41,66,67 The IR beam from a Bruker Tensor 27 Fouriertransform infrared (FTIR) spectrometer is directed through a polarizer and a NaCl window into the UHV environment and focused at grazing incidence (∼85° from the surface normal) onto the sample by using a spherical mirror (ϕ = 3 in., f = 6 in.). The reflected beam is then collected by a second spherical mirror (after going through a second NaCl window) and focused onto a narrow-band mercury−cadmium−telluride (MCT) detector. The beam path outside of the UHV chamber

3. RESULTS The thermal chemistry of the Ru(tmhd)3 precursor on the Ni(110) single-crystal surface was first probed by TPD. Many experiments were carried out to acquire desorption data for a wide range of masses (amus), always recording traces for selected common reference amus in order to make sure that the data are compatible among the different runs. Typical traces obtained from these experiments, for a 10 L exposure at ∼100 K, are displayed in Figure 1. First, it should be indicated that the exposure chosen was sufficient to saturate the first monolayer and to condense one to two extra layers on top; the latter were seen to desorb molecularly at around 170 K (data not shown). Additional molecular desorption from the first monolayer is seen at approximately 310 K in the traces for most amus. Because of extensive cracking of the precursor as well as of the potential products from its decomposition in the ionizer of the mass spectrometer, it is in general difficult to associate peaks B


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tert-butyl moieties in the original ligands. This dicarbonyl formation is accompanied by the production of isobutene via βhydride elimination from tert-butyl moieties, as indicated by the peak seen in the 56 amu trace. No isobutane is produced, however, as no significant signal is detected at around 530 K in the 58 amu trace. Other products detected in these TPD experiments include ketene (CH2CO, 41 amu) and CO2 (44 amu), which evolve at a slightly higher temperature than the rest of the desorbing products. The identity of the desorbing species was corroborated by contrasting their cracking patterns in the TPD experiments, determined by the relative intensities of the peaks for the different masses, against reported mass spectra. 1 The appropriate data for the identification of isobutene and ketene are shown in Figure 2. The identification of isobutene is quite

Figure 1. TPD data for Ru(tmhd)3 adsorbed at 100 K on a Ni(110) single-crystal surface. A 10 L exposure was used in these runs, sufficient to saturate the first monolayer and condense a few extra layers on top. Shown are desorption traces for selected masses (amus), used to identify the main desorbing products.

such as those seen here at 310 K to a specific species. Nevertheless, in this case we rely on the fact that signal was detected for high amus, above that of the protonated ligand (Htmhd, MW = 184 amu), to associate at least some of the intensity of such a peak with the Ru(tmhd)3 precursor; note, for instance, the peak seen at 310 K in the 200 amu trace. However, we also assume that other species may desorb in the same temperature range as well. In fact, we suggest later in this report that some Htmhd may be produced at this stage of the surface chemistry of the Ru precursor. Although not conclusive, the TPD data are consistent with that proposal. The remaining adsorbed species after molecular desorption at 310 K undergo stepwise decomposition and release a number of products resulting from their fragmentation. The first reaction detected in these TPD traces is the production of carbon monoxide at 435 K, as seen in the 28 amu trace. It is interesting to note that the decarbonylation steps required to produce such a molecule must lead to the creation of other organic fragments. However, those appear to, by and large, remain on the surface and decompose further, to produce other desorbing products, only at higher temperatures. The only other gas-phase product detected at this stage is molecular hydrogen (2 amu), an indication that some dehydrogenation steps take place on the surface at this temperature as well. The main feature in the H2 desorption trace starts at approximately 410 K and peaks at around 555 K. The 410 K threshold seen in these data provides a good indication of the temperature at which extensive surface conversion of the organic adsorbates occurs. Some of the products of this chemistry desorb into the gas phase, as indicated by the signals seen in many of the other traces. Of particular significance here are the peaks seen at around 530 K in the 85 and 127 amu traces, as those can be associated with a new dicarbonyl compound, 2,2-dimethyl-3-oxopentanal (t-But−CO−CH2− COH), a product of the elimination of one of the terminal

Figure 2. Relative intensities of the peaks seen for selected amus in our TPD data, contrasted against mass spectra for potential desorbing products.1 Two of the main desorbing products were identified this way, isobutene (left) and ketene (right).

straightforward, as that appears to be the predominant product in the TPD peak seen at 530 K, and because we have access to the signals from several of the main amus in the cracking pattern of that molecule. The detection of ketene is somewhat more tentative, as that species displays a simple mass spectrum with only a handful of peaks and at masses that overlap with other compounds. The main indication that such a molecule is produced in this case comes from the fact that the TPD traces for 14, 29, and 41 amu all peak at a slightly lower temperature (∼5 K) than the rest. Taking advantage of that subtle feature of the TPD data set, we proceeded to subtract the contributions from other species from the signals for the 14, 29, and 41 amu traces and then estimated the relative intensities of the residual signals; these are the values plotted in Figure 2. The formation of ketene is easy to understand in terms of fragmentation of the central diketonate moiety, stripped of both terminal tert-butyl groups: scission of one of the C−C bonds in that core fragment would lead to the formation of ketene (CH2CO) plus carbon monoxide (CO). Additional insights on the thermal chemistry of the ruthenium precursor on Ni(110) were extracted from TPD data from surfaces dosed with the protonated ligand by itself (Htmhd). The traces for some of the most relevant amus are reported in Figure 3, together with the corresponding data for Ru(tmhd)3. In general, it is interesting to note that the two sets, for Htmhd and Ru(tmhd)3, share many common features, suggesting that most of the thermal decomposition of the organic ligands seen in the latter case takes place on the surface of the nickel substrate, once the ligands have been displaced C


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Figure 4. C 1s and Ru 3d (left panel; overlapping signals) and O 1s (right) XPS for 30 L of Ru(tmhd)3 adsorbed on Ni(110) at 100 K as a function of subsequent annealing temperature.

Figure 3. Selected TPD data for 10 L of Htmhd adsorbed on a Ni(110) at 100 K. The corresponding traces for the Ru(tmhd)3 precursor are superimposed in faded colors for comparison.

from the Ru metal center. For instance, in both cases, the decomposition of the organic adsorbates seems to start with extensive decarbonylation and CO desorption (at around 425 K with Htmhd and at 435 K with Ru(tmhd)3). Also, isobutene (now reported using the 39 amu trace) is the main product here too, with a desorption feature peaking at 515 K (versus 530 K for the Ru complex). Ketene is detected as well, at approximately 510 K. On the other hand, there are some significant differences. For one, molecular desorption is here seen at about 205 K, a lower temperature than that with the Ru precursor because Htmhd is a lighter and more volatile compound. Also, the ligands from Ru(tmhd)3 yield less gas-phase products in absolute terms and decompose at higher temperatures than Htmhd. For instance, H2 production starts at about 300 K and peaks at 430 and 520 K with Htmhd, instead of at 410, 435, and 455 K with Ru(tmhd)3. This is possibly explained by a difference in saturation coverages between the two cases, as the Ru metal may block some adsorption sites (and possibly also affect the electronic properties of the nickel surface). It is also interesting that the fragmentation of the original diketonate skeleton at one of the t-But−CO bonds to produce t-But−CO−CH2− CO−H seen with Ru(tmhd)3 is not observed with Htmhd (no corresponding peak above 500 K is detected in the 127 amu trace). Finally, more extensive decomposition seems to take place in the first decarbonylation step (at 425 K) relative to the subsequent reactions (above 500 K) with Htmhd than with Ru(tmhd)3, suggesting that in the latter case activation of the ligand is partially inhibited at low temperatures. The TPD experiments were complemented with XPS data. Figures 4 and 5 report on the C 1s and Ru 3d (left panels; the signals from the two elements overlap)74 and O 1s (right) traces recorded after adsorption of 30 L of Ru(tmhd)3 (Figure 4) and Htmhd (Figure 5), respectively, on the Ni(110) surface, followed by annealing to the indicated temperatures. In general, three temperature regimes can be identified in the Ru(tmhd)3 data: (1) below 250 K, which corresponds to molecular adsorption; (2) between approximately 300 and 600 K, after molecular desorption, at which point a number of adsorbed

Figure 5. C 1s and Ru 3d (left panel) and O 1s (right) XPS for 30 L of Htmhd adsorbed on Ni(110) at 100 K as a function of subsequent annealing temperature.

intermediates may be present on the surface; and (3) above 600 K, corresponding to the final atomic species left behind on the Ni(110) surface. The equivalent ranges for the Htmhd protonated ligand are below 200 K, between 200 and 500 K, and above 500 K, respectively. In that case, a few more subtle changes are seen, in particular, an additional shift in the C 1s peak between 350 and 430 K, which is likely to reflect the decarbonylation step seen in TPD. It is also interesting to note that virtually no oxygen is left behind on the surface after hightemperature annealing of Htmhd, in contrast with the detectable amount O 1s signal still seen with Ru(tmhd)3 even after annealing at 825 K. A technical note: the two sets of XPS data were taken with different X-ray sources (Al Kα vs Mg Kα for the Ru(tmhd)3 and Htmhd cases, respectively), D


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Figure 6. Processed data resulting from analysis of the XPS in Figures 4 and 5. Shown are the peak areas (left panel), calculated atomic ratios (center), and peak positions (right) as a function of annealing temperature for Ru(tmhd)3 and Htmhd adsorbed on Ni(110).

molecular desorption. Further shifts are seen at 500 K, to 284.0 eV, and then above 600 K, until reaching a final value of 284.5 eV. The initial drop in BE value at 500 K must reflect the decarbonylation step as those carbons are the ones that exhibit the highest BEs (there is a ∼20% loss in signal intensity as well), and the return to higher BE values at higher temperatures is likely to correspond to the formation of carbidic carbon on the surface. Regarding the O 1s peak, its position transitions from 531.5 eV for the pure and condensed precursor to 532.8 eV for the adsorbed monolayer (before molecular desorption) and to 531.0 eV above 300 K. An additional shift (to 530.5 eV) is seen above 600 K, coincident with a significant reduction in signal and an increase in the C/O atomic ratio; most of the carbonyl and ketene groups remaining on the surface desorb by that temperature, and only a small amount of atomic oxygen is left behind as an impurity. The trends with Htmhd are similar to those seen with Ru(tmhd)3, although the absolute BE values are slightly different (the coadsorbed Ru is expected to block sites and induce some electronic changes on the surface). Also, the chemical transformations are detected at lower temperatures. All of these results are consistent with the TPD data. Finally, the thermal chemistry of these systems was characterized by RAIRS. The resulting temperature-dependent spectra are reported, in differential mode (that is, ratioed against the traces from the previous annealing temperature), in Figure 7. Two observations become evident upon glancing at that figure. First, the spectra for the adsorbed species is quite different from that recorded for the pure Ru(tmhd)3 powder (bottom, green trace). It should be mentioned that the initial adsorption in this case was carried out at 375 K to avoid condensation. That is a temperature higher than what was used in the TPD and XPS experiments but still lower than the threshold for the chemical reactions leading to ligand fragmentation. The differences seen in the RAIRS are therefore not likely due to the ligand decomposition seen above. Instead, this again points to the removal of ligands from the Ru precursor upon its activated adsorption on the Ni(110) surface. Second, the spectra recorded with Ru(tmhd)3 and Htmhd look quite similar, with most peaks centered at the same frequencies in both cases. The absolute intensities with Htmhd are higher, indicating higher surface coverages, and the chemical changes are seen at lower temperatures in that case as well (as also evident in the TPD and XPS data), but otherwise, it appears

which is why the O 1s XPS spectra show interference from the Ni L3M23M23 Auger peaks in Figure 5 but not in Figure 4. Quantitative analysis of the XPS data was performed in terms of both peak positions and peak areas. The data are shown in Figure 6. In terms of peak positions (Figure 6, right panel), the Ru 3d5/2 peak of the condensed precursor is centered at a binding energy (BE) of 281.7 eV, the same as that measured for the pure powder (data not shown). That value is consistent with a metal ion in the +3 oxidation state.74 The Ru 3d5/2 feature blue shifts to a new value of 282.2 eV upon annealing at 150 K or above, that is, after desorbing all condensed layers, reflecting an electronic effect of the Ni(110) substrate on the Ru central ion of the Ru(tmhd)3 complex. A more significant shift is then seen at around 300 K (and completed by 400 K), to a new value of 280.0 eV. The transition appears to take place concurrently with some molecular desorption from the monolayer, and indicates reduction of the metal ion to its metallic Ru0 state. We propose that the adsorbed complex that remains on the surface undergoes further conversion involving the loss of its ligands to either the gas phase or the Ni substrate and direct bonding of the Ru atom to the metal surface. This argument is supported by the changes seen in the XPS peak areas (Figure 6, left and center panels). In particular, it is seen that, although both Ru 3d5/2 and C 1s XPS peaks loose significant intensity, a reflection of the molecular desorption also seen in the TPD data, the C 1s signal is reduced by a larger fraction; therefore, the C/Ru atomic ratio decreases from a high value, approximately 56 (higher than expected from the stoichiometry of the precursor, 33, because of the shielding of the Ru signal by the organic ligands), to ∼25 ± 5. This suggests a loss, on average, of at least one (possibly two) out of the three ligands in each adsorbed Ru(tmhd)3 molecule to the gas phase. Neither the Ru 3d BE nor the C/Ru atomic ratio changes much upon annealing to higher temperatures, although the latter does vary in ways that suggest additional chemistry taking place at the surface. The C 1s and O 1s XPS peaks also undergo some changes throughout the annealing sequence. For the Ru(tmhd) 3 condensed layer (and its powder), the main feature in the C 1s trace is seen at 284.7 eV, a value typical of alkyl carbons, but a small shoulder is also observed, even if not fully differentiated, at higher BEs for the carbonyl carbons. The main signal shifts to 285.3 eV upon condensed multilayer desorption, and back down to approximately 284.6 eV right after monolayer E


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via β-hydride elimination, as discussed earlier. No new peaks that could be ascribed to new surface species were seen at any temperature (except perhaps the broad feature seen to disappear between 2900 and 2950 cm−1 after annealing the precursor layer to 670 K), most likely because of instrumental limitations but also because what may remain on the surface after high-temperature annealing are small C,O-containing dehydrogenated fragments, likely adsorbed as atoms or small clusters possibly oriented parallel to the surface plane (which makes them RAIRS-inactive). The similarities of the RAIRS traces for Ru(tmhd)3 and Htmhd further argue for the ligands being bonded to nickel (not ruthenium) surface atoms soon after the initial activation at or below 300 K.

4. DISCUSSION The TPD, XPS, and RAIRS data reported here provide useful information on the thermal chemistry of Ru(tmhd)3 (and Htmhd) on Ni(110) single-crystal surfaces, sufficient for delineation of the basic features of the reaction mechanism involved. The main features of that mechanism are summarized schematically in Figure 8. Several general characteristics can be identified readily from the data. First, there appear to be three main temperature regimes for the chemical transformations of the Ru(tmhd)3 precursor on the Ni(110) surface: (1) at around 300 K (room temperature), where molecular desorption takes place (together with some initial surface chemistry of the remaining adsorbates); (2) at about 435 K (162 °C), characterized by the detection of desorbing CO and H2; and (3) at around 530 K (257 °C), at which point a more extensive conversion takes place, resulting in the production of several gas-phase products. In addition, it is also worth pointing out that the spectroscopic results obtained with Ru(tmhd)3 show many similarities with those recorded with Htmhd, strongly suggesting that in the two cases the surface chemistry seen is that of the tmhd ligand adsorbed on the nickel metal. There are some small differences between the two cases because Htmhd appears to decompose at slightly earlier temperatures and to yield larger quantities of the main products, but those could be justified in terms of both the initial (lower) ligand coverages attainable with the Ru(tmhd)3 precursor and the potential electronic effect exerted by the coadsorbed Ru atoms on the electronic properties of the Ni surface. The first feature in the TPD data originating from the thermal activation of the Ru(tmhd)3 adsorbed monolayer is seen at 310 K. Because peaks are seen at this temperature for most of the masses recorded, including those for high amu values (up to 200 amu, the upper limit of our instrument; Figure 1), we conclude that much of the signal intensities in

Figure 7. Differential RAIRS of Ru(tmhd)3 (solid traces) and Htmhd (faded traces) adsorbed on Ni(110) at 375 K after annealing to the indicated temperatures. Also shown is the spectrum for pure Ru(tmhd)3 (bottom, green trace) for reference.

that the species that form on the surface are the same with both compounds. The peak assignment in these spectra is fairly straightforward and has been made by relying on reported data for similar diketonate complexes75 and on typical group vibrational assignments.76 The spectrum at 375 K displays features at 1415, 1443, 1460, 1483, 1512, 1516, 1550, 1562, and 2962 cm−1; the first and last correspond to the asymmetric deformation and asymmetric C−H stretching of the methyl groups in the terminal tert-butyl moieties, the second, third, and fourth correspond to mixed modes involving the deformation of the central C−H bond, and the rest (the four peaks between 1510 and 1570 cm−1) correspond to CO and CC stretching motions. It is clear that the peaks associated with the carbonyl groups are the first ones to disappear; they are seen as positive peaks (meaning as decreasing in intensity compared with the previous, lower-temperature, data) by 430 K. Interestingly, this decarbonylation may occur in two stages, as the features at 1520, 1550, and 1562 cm−1 disappear first, by 430 K, whereas those at 1483 and 1512 cm−1 survive until annealing at 490 K (actually, they increase slightly in intensity in the 430 K trace, perhaps because of a geometrical reorientation of the adsorbed species toward a more perpendicular configuration). The methyl groups are then removed, starting at 490 K in both cases but more prominently at around 570 K in the case of Ru(tmhd)3; this correspond to the removal of the terminal tert-butyl groups from the diketonate ligand and the subsequent production of isobutene

Figure 8. Proposed mechanism for the thermal conversion of Ru(tmhd)3 on Ni(110) surfaces. F


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centered at 565 K, and between 1/4 and 1/3 of all molecular hydrogen produced desorbs above 600 K (Figure 1). Some species are retained on the surface after the last peak in the TPD data, but the nature of those species is not easy to establish. The long tail of the H2 TPD trace, which extends to ∼750 K (Figure 1), indicates stepwise dehydrogenation. The vibrational data also show that some of the methyl groups in the tert-butyl moieties survive to temperatures close to 570 K; witness the peaks seen at 1415, 1443, and 2962 cm−1 in the RAIRS trace for that temperature (Figure 7). Additional signal is seen in the C−H stretching region of the 670 K trace, indicating that more fragmented species may have formed at such high temperatures, likely CHx(ads) or CCHx(ads) adsorbed species.77,78 In the end, the XPS data indicate that some carbon (∼20% of the peak intensity at 250 K) and oxygen (∼15%) are left behind, most likely in the form of carbidic carbon and metal oxide oxygen (based on the corresponding XPS BEs; Figures 4 and 6). Interestingly, Htmhd leaves behind comparable amounts of carbon but virtually no oxygen. Perhaps the oxygen impurity in the first case is associated with the deposited ruthenium, in the form of RuOx. Unfortunately, the Ru XPS data are not sensitive enough to check this possibility. There are some general lessons, applicable to other similar CVD and ALD precursors, that may be extracted from this study. First, it is important to notice the temperature at which the diketonate ligands are initially activated. The particularly low temperature at which CO starts to desorb, below 400 K, is worrisome because that indicates early, extensive, and irreversible decomposition. The combination of a highly reactive surface (nickel) and a highly labile ligand (because of the terminal tert-butyl groups) chosen here can explain some of the high reactivity seen, but the problem may nevertheless be fairly general; in our past studies with bis(acetylacetonato)copper(II), Cu(acac)2, it was determined that ligand decomposition starts at 150 K on Ni(110) and at 200 K on Cu(110) (a much milder metal).72 The decomposition mechanism also shares some of the same steps in all of these cases, as a similar shorter dicarbonyl compound (resulting from the removal of one end group) was seen with the acac ligand as well, even though there the terminal methyl group is more stable (methyl removal from adsorbed acac has in fact been reported before).83,84 Also, with acac, acetone production was observed, which means that the internal C−CO bonds are vulnerable in that ligand as well (in the present case that became evident by the production of ketene). Overall, it would appear that, because of their high reactivity, diketonate-based metal complexes may not be useful for metal film deposition unless an oxidizing agent is used to remove the impurities deposited by ligand decomposition.32,85−95 This may be a reason why there are only a handful of examples in the literature where diketonate complexes are used together with reducing agents for these applications96−99 and why other Ru complexes have received more attention.34,100−117 On the other hand, diketonate complexes could be engineered to react more cleanly on surfaces,34 in which case they could provide an avenue for designing CVD and ALD processes at fairly low temperatures.96,118 This possibility may be worth exploring further.

those can be ascribed to molecular desorption of the full Ru precursor. Nevertheless, there are reasons to believe that other products are made and desorb from the surface in the same temperature range. Specifically, the XPS data show that the C/ Ru atomic ratio decreases significantly, indicating the loss of a large fraction of the organic ligands to the gas phase (somewhere between one and two ligands per ruthenium atom on average; Figure 6). We suggest that some ligands may be protonated at this stage, desorbing as Htmhd. Moreover, it appears that the reminding ligands may be displaced from the original Ru complex to the nickel substrate. Certainly, the RAIRS traces recorded by 375 K for the adsorbed (and thermally activated) Ru(tmhd)3 are quite different from those of the pure compound (Figure 7), a fact that clearly indicates that the molecular structure is not preserved. Finally, the ruthenium ion of the original complex is reduced to its metallic electronic state by 400 K, pointing to a possible direct bonding of that atom to the nickel metal surface. The next stage of decomposition of the species adsorbed on Ni(110) after Ru(tmhd)3 adsorption starts at around 400 K and peaks at 435 K in the TPD spectra (Figure 1). This chemistry is characterized by the gas-phase detection of CO and H2. The production of CO in particular is interesting, as that requires the scission of two C−C bonds. It is not clear what the sequence of events in this conversion is, but one requirement is the elimination of a terminal tert-butyl group from the diketonate original ligand, which could then easily adsorb on the Ni(110) surface and undergo β-hydride elimination to produce isobutene, as seen on many other surfaces.77−79 Indeed, the latter reaction has been reported to take place at temperatures as low as 180 K on Ni(100)80,81 and would explain the simultaneous detection of H2 with CO in the TPD of this system. Given that the alkyl terminal groups have proven labile in other diketonate,72 amidinate,70,73 and iminopyrrolidinate82 compounds, we suggest that it is the external C−C bond that breaks first here as well. Regardless, it is quite surprising that the internal C−C bond breaks at all, because the core diketonate moiety has two double bonds and a certain degree of π-electron delocalization that should make it fairly stable. Another aspect of this chemistry is the fact that the decarbonylation step is only partial; some signal remains in the O 1s XPS data afterward that only goes away upon heating to higher temperatures (>500 K; Figure 6), and the carbonyl stretching feature at 1512 cm−1 in the RAIRS data survives until 490 K. We estimate that the CO loss in the 435 K TPD peak corresponds to approximately half of all of the carbonyl groups available on the surface at that stage. The bulk of the conversion of the organic adsorbed species in this Ru(tmhd)3/Ni(110) system starts at around 460 K and peaks at 530 K (Figure 1). Several products desorb into the gas phase in TPD experiments at this point, including 2,2-dimethyl3-oxopentanal, isobutene, ketene, carbon monoxide, carbon dioxide, and hydrogen. The formation of the first two is easy to rationalize as the result of one t-But−CO bond scission step followed by a hydrogen atom transfer from the tert-butyl species to the diketonate fragment (possibly via β-H elimination followed by a reductive elimination reaction). Ketene production is also easily explained as the result of fragmentation of the central diketonate moiety once the two tert-butyl groups have been stripped out of the original ligand (the other byproduct being CO). CO2 and H2 seem to be products of more extensive decomposition, some of it happening at higher temperatures; the CO2 TPD peak is

5. CONCLUSIONS The surface chemistry of the Ru(tmhd)3 diketonate complex used for ruthenium metal film depositions, as well as that of the free Htmhd protonated ligand, was characterized by a G


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(3) Hatanpäa,̈ T.; Ritala, M.; Leskelä, M. Precursors as Enablers of ALD Technology: Contributions from University of Helsinki. Coord. Chem. Rev. 2013, 257, 3297−3322. (4) Jensen, K. F. Organometallic Chemical Vapor Deposition of Compound Semiconductors: A Chemical Perspective. In Materials Chemistry: An Emerging Discipline; Interrante, L. V., Caspar, L. A., Ellis, A. B., Eds.; Advances in Chemistry Series; American Chemical Society: Washington, DC, 1995; Vol. 245, pp 297−423. (5) Creighton, J. R.; Ho, P. Introduction to Chemical Vapor Deposition (CVD). In Chemical Vapor Deposition; Park, J.-H., Sudarshan, T. S., Eds.; Surface Engineering Series; ASM International: Materials Park, OH, 2001; Vol. 2, pp 1−22. (6) Crowell, J. E. Chemical Methods of Thin Film Deposition: Chemical Vapor Deposition, Atomic Layer Deposition, and Related Technologies. J. Vac. Sci. Technol., A 2003, 21, S88−S95. (7) Fahlman, B. D. Recent Advances in Chemical Vapor Deposition. Curr. Org. Chem. 2006, 10, 1021−1033. (8) Lim, B. S.; Rahtu, A.; Gordon, R. G. Atomic Layer Deposition of Transition Metals. Nat. Mater. 2003, 2, 749−754. (9) Kim, H. Atomic Layer Deposition of Metal and Nitride Thin Films: Current Research Efforts and Applications for Semiconductor Device Processing. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 2003, 21, 2231−2261. (10) Ritala, M. Atomic Layer Deposition. In High-k Gate Dielectrics; Houssa, M., Ed.; Institute of Physics: Bristol; Philadelphia, 2004; pp 17−64. (11) Leskelä, M.; Ritala, M. Atomic Layer Deposition Chemistry: Recent Developments and Future Challenges. Angew. Chem., Int. Ed. 2003, 42, 5548−5554. (12) Wu, Y. Y.; Kohn, A.; Eizenberg, M. Structures of Ultrathin Atomic-Layer-Deposited Tanx Films. J. Appl. Phys. 2004, 95, 6167− 6174. (13) Van der Straten, O.; Zhu, Y.; Dunn, K.; Eisenbraun, E. T.; Kaloyeros, A. E. Atomic Layer Deposition of Tantalum Nitride for Ultrathin Liner Applications in Advanced Copper Metalization Schemes. J. Mater. Res. 2004, 19, 447−453. (14) Dai, M.; Kwon, J.; Halls, M. D.; Gordon, R. G.; Chabal, Y. J. Surface and Interface Processes During Atomic Layer Deposition of Copper on Silicon Oxide. Langmuir 2010, 26, 3911−3917. (15) Potts, S. E.; Kessels, W. M. M. Energy-Enhanced Atomic Layer Deposition for More Process and Precursor Versatility. Coord. Chem. Rev. 2013, 257, 3254−3270. (16) Cha, S. W.; Cho, G. Y.; Lee, Y.; Park, T.; Kim, Y.; Lee, J.-m. Effects of Carbon Contaminations on Y2O3-Stabilized ZrO2 Thin Film Electrolyte Prepared by Atomic Layer Deposition for Thin Film Solid Oxide Fuel Cells. CIRP Ann. 2016, 65, 515−518. (17) Kim, C. S.; Jeon, H.-G.; Jung, Y.; Choi, M.; O, B.; Kim, K.-H. Observation of Surface Contamination Layer by X-Ray Reflectometry (XRR) Analyses. Surf. Interface Anal. 2017, 49, 522−526. (18) Puurunen, R. L. Surface Chemistry of Atomic Layer Deposition: A Case Study for the Trimethylaluminum/Water Process. J. Appl. Phys. 2005, 97, 121301. (19) Knisley, T. J.; Kalutarage, L. C.; Winter, C. H. Precursors and Chemistry for the Atomic Layer Deposition of Metallic First Row Transition Metal Films. Coord. Chem. Rev. 2013, 257, 3222−3231. (20) Bernal Ramos, K.; Saly, M. J.; Chabal, Y. J. Precursor Design and Reaction Mechanisms for the Atomic Layer Deposition of Metal Films. Coord. Chem. Rev. 2013, 257, 3271−3281. (21) Gordon, R. G. ALD Precursors and Reaction Mechanisms. In Atomic Layer Deposition for Semiconductors; Hwang, S. C., Ed.; Springer US: Boston, MA, 2014; pp 15−46. (22) Hämäläinen, J.; Ritala, M.; Leskelä, M. Atomic Layer Deposition of Noble Metals and Their Oxides. Chem. Mater. 2014, 26, 786−801. (23) Johnson, R. W.; Hultqvist, A.; Bent, S. F. A Brief Review of Atomic Layer Deposition: From Fundamentals to Applications. Mater. Today 2014, 17, 236−246. (24) Xu, M.; Tiznado, H.; Kang, B.-C.; Bouman, M.; Lee, I.; Zaera, F. Mechanistic Details of Atomic Layer Deposition (ALD) Processes. J. Korean Phys. Soc. 2007, 51, 1063−1068.

combination of surface-sensitive techniques. TPD studies indicated early initial decomposition, simultaneously during the molecular desorption of a fraction of the monolayer of the precursor that can be deposited at lower temperatures. The first steps take place at around 310 K (37 °C) and appear to involve the removal of the ligands, some to the gas phase and some to the substrate, and the direct bonding of the Ru atom to the underlying metal surface; the latter step is accompanied by reduction of the Ru3+ ion to its Ru0 metallic state. The fragmentation of the adsorbed ligands starts below 400 K (127 °C), and initially yields CO and H2 in the gas phase. The most likely reactions leading to this result are the elimination of at least one of the terminal tert-butyl groups, which quite possibly undergo immediate β-hydride elimination to isobutene, and further fragmentation of the inner moiety at the other C−CO bond. More extensive fragmentation starts at 460 K (187 °C) and peaks at 530 K (257 °C), to produce 2,2-dimethyl-3oxopentanal and isobutene via scission of an external C−CO bond, and ketene and carbon monoxide from fragmentation of the inner diketonate moiety. Additional carbon dioxide and molecular hydrogen are produced at higher temperatures as the remaining adsorbates decompose further, and some carbidic carbon and oxidic oxygen are left on the surface. This sequence of thermal chemical steps may exhibit some general features common to other diketonate precursors, in particular those made with the common acac ligand. They all seem to be quite reactive, decomposing on metal surfaces at low temperatures soon after being displaced from the original metal−organic complex. The lability of the external alkyl groups, the tert-butyls in this case but even the more stable methyls in acac, is a handicap when considering CVD and ALD precursors to be used with reducing agents, as the fragmentation products once the original ligand breaks apart on the surface are more difficult to remove cleanly from the surface by protonation or hydrogenation reactions. On the other hand, the ease with which the diketonate ligands can be displaced from the central metal ion suggests that, under the proper conditions, this type of precursors could be used in lowtemperature film growth processes. That possibility needs to be explored further.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Francisco Zaera: 0000-0002-0128-7221 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this project was provided by a grant from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering (MSE) Division, under Award No. DE-SC0001839.

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REFERENCES

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