Surface Chemistry of Pentakis(dimethylamido) - American Chemical

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Surface Chemistry of Pentakis(dimethylamido)tantalum on Ta Surfaces Taeseung Kim and Francisco Zaera* Department of Chemistry, University of California, Riverside, California 92521, United States ABSTRACT: The surface chemistry of pentakis(dimethylamido)tantalum (PDMAT), a precursor commonly used for atomic layer deposition (ALD), has been investigated on a Ta surface under ultrahigh vacuum (UHV) conditions. Results from both temperature-programmed desorption (TPD) and isothermal kinetic data obtained using an effusive molecular beam arrangement identified H2, CH4, C2H4, HCN, HN(CH3)2, and H2CdNCH3 as the main desorption species. Several reactions involving the amido ligands were observed on the Ta surface starting at about 450 K. Those include not only the expected hydrogenation to dimethylamine and β-hydride elimination to N-methyl methyleneimine but also decomposition to methane and hydrogen cyanide and a carbon
carbon forming step to yield ethylene. Addition of ammonia to the reaction mixture leads to an enhancement in hydrogenation to the amine at the expense of methane formation, but only at temperatures above approximately 550 K. Isotope labeling was used to establish that hydrogenation of the amido groups involves hydrogen atoms from the ammonia but methane formation occurs via intramolecular hydrogen transfer instead.

1. INTRODUCTION Because of their thermal stability and low resistivity, tantalum nitride (TaN) films are widely used as diffusion barriers for copper interconnects in microelectronics applications.1
3 As the dimension of those interconnects shrinks and the topography of the devices upon which the interconnects are deposited becomes rougher and with higher aspect ratios, the physical film deposition methods used nowadays are becoming less practical, and chemical vapor deposition (CVD) procedures, atomic layer deposition (ALD) in particular, are becoming more viable to get good step coverages.4,5 Unfortunately, the ALD growth of TaN films has some drawbacks6,7 because that process needs the prior deposition of a Ta seed layer to improve adhesion and also requires a strong reducing agent. In addition, it is not easy to control the stoichiometry of the final nitride film, a property that affects the barrier behavior against Cu diffusion.8 Lastly, as with other ALD processes, the potential deposition of carbon impurities often results in films with high resistivity.9,10 All these problems may be addressed once a better understanding of the surface chemistry involved is available. To date, however, that chemistry has not been studied in sufficient detail.11 The focus of this manuscript is the chemistry of pentakis(dimethylamido)tantalum (PDMAT), Ta[N(CH3)2]5, on tantalum surfaces. PDMAT is one of the most common precursors used for TaN deposition.9,12
15 The surface chemistry of PDMAT is also closely related to that of other similar precursors such as tetrakis(dimethylamido)titanium (TDMAT), Ti[N(CH3)2]4,16 and tetrakis(dimethylamido)hafnium (TDMAH), Hf[N(CH3)2]4,17
20 which contain the same dimethylamido ligands, and even to that of tetrakis(methylethylamido)titanium (TEMAT), Ti[N(CH3)(C2H5)]4,20
22 tetrakis(methylethylamido)zirconium (TEMAZ), Zr[N(CH3)(C2H5)]4,23
25 and tetrakisr 2011 American Chemical Society

(methylethylamido)hafnium (TEMAH), Hf[N(CH3)(C2H5)]4,20,24 which have closely related amido groups. There have been a few reports on the molecular aspects of the surface chemistry of those compounds already from studies using surface-science tools.11,20,22,26
28 However, many of the details of the mechanism of the relevant surface reaction remain poorly understood still. For example, it is assumed that these metal organic compounds adsorb dissociatively and lose some of their amido ligands to the surface, but this has yet to be definitely established. There is also limited knowledge on how the remaining amido ligands are eliminated upon thermal activation and how metal nitride films are formed. Previous studies using infrared absorption spectroscopy have suggested two parallel reactions to account for the elimination of the amido ligands. The first involves β-hydride elimination from a dialkylamido group to produce an imine, N-methyl methyleneimine (H2CdNCH3, MMI) in the case of dimethylamido groups, with the concomitant transfer of a hydrogen atom to the metal center.18,29 In the second pathway, a dialkylamido ligand incorporates a hydrogen atom from a neighboring ligand to detach from the metal center as a dialkylamine (dimethylamine, DMA, NH(CH3)2, when starting with dimethylamido ligands).17 In the presence of ammonia, a reactant often used for the deposition of metal nitrides in CVD and ALD, the amido ligands can also be displaced via a transamination reaction.16,22 All these reactions have by now been well established. Nevertheless, they do not account for all the details of how the metal nitrides are formed in the chemical film growth processes. One outstanding question, Received: February 16, 2011 Revised: March 21, 2011 Published: April 04, 2011 8240

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The Journal of Physical Chemistry C for instance, is how the metal center is reduced from its original state, Ta(V) in the case of PDMAT, to the oxidation state seen in the final metal nitride, Ta(III) in TaN. In previous studies with titanium tetrachloride30 and with TEMAT22 we have suggested that this occurs via either disproportionation or reductive elimination reactions, but that has yet to be fully confirmed. Other side reactions must also occur to account for the carbon impurities often detected in the grown metal nitride films, but those are not known at present. In this report, key results from our study of the surface chemistry of PDMAT on a Ta surface are presented. A number of experiments were carried out under ultrahigh vacuum (UHV) conditions, specifically temperature-programmed desorption (TPD) and isothermal kinetic measurements using an effusive molecular beam setup.31,32 A number of reactions were identified, including several C
H, C
N, and Ta
N bond-cleavage reactions and also hydrogenation and β-hydride elimination steps. Coadsorption experiments with NH3, ND3, and 15NH3, used to help examine the proposed transamination of the dimethylamide ligands, are also discussed.

2. EXPERIMENTAL SECTION The experiments were performed in a molecular-beam ultrahigh-vacuum (UHV) apparatus described previously.31,33,34 The main stainless-steel chamber, approximately 6 L in volume, is pumped with a 340 L/s turbo-molecular pump (Seiko Seiki STP300). It is equipped with an ion gun (Leybold) for sample cleaning and with a quadrupole mass spectrometer (UTI-100C) for gas analysis. A personal computer is set up to receive data for up to 15 different masses as well as a reading of the temperature of the surface during the TPD and kinetic experiments. A 1.2 mm thick Ta foil (Alfa Aesar, 99.95% pure) was mounted on the sample manipulator, which is capable of X
Y
Z
θ motion. This sample could be heated resistively to temperatures of up to 1200 K and cooled to as low as 100 K through liquid nitrogencooled feedthroughs. A K-type thermocouple was spotwelded to the backside of the foil for temperature measurements, and a homemade temperature controller used to set or ramp its temperature. An effusive collimated molecular beam doser was used for all surface exposures as well as for the isothermal kinetic experiments. This doser consists of a multichannel microcapillary array mounted on a stainless-steel tube connected to a leak valve. The doser is fitted with a flag so that the beam can be blocked and unblocked at will. The PDMAT (Sigma-Aldrich, 99.9%) was heated to 313 K in the gas manifold feeding the doser to reach a sufficiently high vapor pressure for gas introduction into the UHV chamber. During the PDMAT exposures, the chamber temperature was maintained at 333 K to avoid condensation on its walls. NH3 (Matheson, 99.99%,), ND3 (Sigma-Aldrich, 99%), and 15NH3 (Sigma-Aldrich, 98%) were introduced into the beam through a separate leak valve. The Ta foil was cleaned by sequential Arþ ion sputtering for 10 min and annealing to 1200 K under vacuum for 10 min prior to each TPD and isothermal kinetic experiment. Temperature-programmed desorption (TPD) and isothermal kinetic measurements were performed. For the TPD experiments, the PDMAT was dosed at a surface temperature of either 110 or 300 K and heated at a rate of 5 K/s. The TPD signals are reported in arbitrary units, but intensity factors are provided in each trace for relative comparisons. The isothermal experiments

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Figure 1. Temperature-programmed desorption (TPD) spectra for PDMAT adsorbed on a clean Ta foil surface at 300 K. Left panel: raw traces for 2, 14, 15, 16, 26, 27, 28, 43, and 45 amu recorded after a 5.0 L dose. Right panel: TPD traces for hydrogen, methane, hydrogen cyanide, ethylene, N-methyl methyleneamine (MMI), and dimethylamine (DMA), obtained after deconvolution. An additional trace for molecular PDMAT desorption after dosing at 110 K is shown at the bottom of both panels. The heating rate was 5 K/s.

were performed by facing the Ta sample directly toward the collimated beam doser and stabilizing it at a fixed surface temperature. The PDMAT beam was then started at t = 0 s, by setting the background pressure at a value of P = 5  10
9 Torr (which corresponds to a flux of F = 0.005 ML/s). By comparing the TPD area obtained after direct-beam versus background exposures, an enhancement factor of 5 was estimated with the former;33 in this report, exposures and pressures are reported after multiplying the raw pressure reading by this enhancement factor. The isothermal kinetic measurements were initiated by unblocking the beam (by removing the flag from its path) at t = 30 s. The beam was blocked again when the surface reactions reached steady state at t = 450 s (Figure 3), and this unblocking and blocking of the beam was repeated a few times afterward to check on the steady-state behavior of the system. In experiments with codosed ammonia, both gases were introduced simultaneously through the same molecular beam doser by using two parallel leak valves behind the capillary array. The background partial pressures used for the dosing of the ammonia were either 5  10
8 or 5  10
7 Torr, as reported in the appropriate text and figures. Given that there is significant overlap within the cracking patterns in the mass spectra of the reactant and products and that this directly affects the interpretation of the TPD and isothermal desorption spectra, a deconvolution procedure was required to separate the contributions of the individual compounds to the overall kinetics.32 The following procedure was followed: (1) appropriate masses were selected to represent each of the potential reactants and products to minimize overlap; (2) the raw data were deconvoluted using the mass fragmentation pattern of the selected molecules, as measured independently in control experiments; and (3) the extracted data were divided by the appropriate sensitivity factors to obtain the relative yields for each molecule.

3. RESULTS Figure 1 displays the results from a typical TPD experiment with PDMAT on the clean Ta foil surface. In this case, 5.0 L 8241

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Figure 2. Peak temperatures (left panel) and desorption yields (right panel) for each of the desorbing species seen in the TPD data in Figure 1 as a function of initial PDMAT dose.

(1 L = 1  10
6 Torr 3 s) of PDMAT was dosed at a surface temperature of 300 K to reach monolayer saturation yet avoid multilayer condensation. The left panel shows the raw data for selected masses, including those that were ultimately chosen for the deconvolution analysis. Signals for 2, 16, 27, 43, and 45 amu were used for hydrogen (H2), methane (CH4), hydrogen cyanide (HCN), N-methyl methyleneimine (H2CdNCH3, MMI), and dimethylamine (HN(CH3)2, DMA), respectively. Ethylene was followed by the signal for 28 amu; that mass can in principle be ascribed to several products, but the transfer of the signal from the 28 amu to the 29 amu trace in isothermal desorption experiments with PDMAT plus deuterium-labeled ammonia (Figure 7) indicated that in our case it does indeed correspond to ethylene. The traces for 14 (CH2þ) and 15 (CH3þ) amu, which display intensities 17% and 85% as large as those seen for 16 amu, were identified as fragments of CH4. The trace for 26 amu was assigned mostly to HCN, but it also included some contribution from C2H4.35 The TPD traces obtained after deconvolution of the raw data in the left panel of Figure 1 are shown in the right panel of that same figure. It can be seen from simple inspection of Figure 1 that the deconvolution procedure results, at least in some cases, in noticeable changes in peak shapes and/or peak temperatures. For example, the peak temperatures ultimately obtained for the desorption of HCN and C2H4 are slightly shifted when compared to the values seen in the original 27 and 28 amu traces, to higher and lower temperatures, respectively, because of the signal overlaps of both signals from both species. In any case, the main reaction products identified from the thermal decomposition of PDMAT on the Ta surface were hydrogen, methane, hydrogen cyanide, ethylene, and MMI. Contrary to expectation, the desorption of DMA is negligible compared to that of the other products. Finally, multilayer and monolayer PDMAT desorption was also observed, at 250 and 350 K, respectively, in a separate experiment where the tantalum compound was exposed at 110 K (Figure 1, bottom traces). The molecular desorption of similar organometallic compounds has been observed at similar temperatures.29,36 Figure 2 summarizes the data from our TPD experiments in the form of peak maxima (left panel) and desorption yields (right panel) as a function of initial PDMAT exposure. The peak temperatures are all approximately constant as a function of

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Figure 3. Typical isothermal kinetic curves obtained in molecular-beam experiments with PDMAT on a Ta foil. Left panel: time evolution of the raw signals for various amu's during PDMAT exposure at 550 K. Right panel: corresponding kinetic traces for the partial pressures of the key desorbing products obtained after deconvolution of the data on the left. In these experiments, the beam was initially unblocked after 30 s and blocked and unblocked again a few times after reaching steady state, as indicated by the arrows.

exposure, although some shifts to slightly higher temperatures with increasing surface coverage were seen, in particular for H2, CH4, C2H4, and HCN. Desorption of both DMA and MMI starts at 350 K and peaks around 490 and 505 K, respectively. H2, CH4, and C2H4 all desorb at higher temperatures and show peaks between 560 and 580 K. HCN peaks at 630 K, a temperature at least 50 K higher than those seen for all the other species. In terms of desorption yields, those follow an approximately Langmuir uptake and reach saturation after PDMAT exposure of approximately 1.5
2.0 L (Figure 2, right). No significant changes in selectivity with exposure were observed. The results from a typical set of isothermal desorption measurements are shown in Figure 3. As indicated in the Experimental Section, the PDMAT beam was started at t = 0 s, unblocked so it was allowed to directly impinge on the surface at t = 30 s, blocked again after reaching steady state at t = 450 s, and further unblocked and blocked at periods of approximately Δt ∼ 50 s to extract the rates for the steady-state reactions from any contributions due to background gases. Like with the TPD results, the raw molecular-beam data in the left panel of Figure 3 were deconvoluted and plotted in terms of the partial pressures of the relevant species in the right panel of Figure 3. It should be indicated that in this molecular beam setup the partial pressures are directly proportional to the reactions rates, and the steadystate rate values are indicated by the jumps seen between the times when the flag is in the blocked versus unblocked positions. There is additional kinetic information in the transients observed right after each blocking and unblocking of the beam as well,37 but analysis of those data is beyond the scope of this report. The results from the isothermal kinetic measurements are complementary to those derived from the TPD studies. The same main products were seen, namely, hydrogen, methane, hydrogen cyanide, ethylene, MMI, and DMA. The signals for hydrogen, methane, and ethylene increase sharply at first, right after the first beam unblocking at t = 30 s, but then decrease exponentially, with different time constants, as time elapses. This 8242

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Figure 4. Steady-state desorption rates for each of the products detected in molecular-beam kinetic measurements such as those in Figure 3 as a function of surface temperature. Shown are data obtained by using beams of PDMAT alone (9) and mixed with NH3 at two different partial pressures, 5  10
8 (0) and 5  10
7 (O) torr.

Figure 5. Left panel: Selectivities, in terms of fractional rates, for the desorption of the products in the molecular beam experiments with pure PDMAT summarized in Figure 4 as a function of reaction temperature. Right panel: Temperature dependence of the reaction rates in Arrhenius form. The solid lines correspond to linear fits to the data, from which activation energies (reported in the inset) were estimated.

indicates that, on a clean Ta surface, PDMAT decomposes to produce mainly H2, CH4, and C2H4 (and perhaps a small amount of HCN), and deposits a number of surface species before reaching steady-state behavior. In contrast, the MMI trace shows the opposite behavior, in that its desorption yield increased slowly with time after the initial unblocking of the beam. Clearly, the activity of the surface switches to a more selective decomposition pathway involving β-hydride elimination to the imine as the surface species from the initial decomposition build up. DMA desorption is negligible at this temperature. The temperature-dependent reaction rates of the desorbing molecules under steady state are summarized in Figure 4 (filled squares). A threshold for steady-state reactivity is seen at about 450 K (although a small initial transient is observed at the beginning of the kinetic runs at temperatures as low as 400 K), after which most of the rates increase exponentially. Then, above 600 K, the rates for H2, HCN, and MMI production still continue

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Figure 6. Isothermal kinetic traces at steady state obtained for the conversion of PDMAT on a Ta foil at 600 K alone (left) and mixed with ammonia (middle). The traces obtained with a beam of pure ammonia are also provided for reference (right).

to increase, even if the slopes of the increments decrease somewhat; however, the rates for CH4 and DMA level off, and that for C2H4 goes down. These trends result in significant changes in selectivity with increasing reaction temperature; the products that follow different kinetic behavior versus temperature must originate from different, perhaps competitive, reaction pathways. The left panel of Figure 5 displays the isothermal rate data versus temperature in relative terms, as fractions calculated by dividing the steady-state rates for each product by the sum of all rates. This way of representing the data highlights the changes in selectivity seen as a function of reaction temperature. Hydrogen is by far the major reaction product detected from decomposition of PDMAT on the Ta surface at all temperatures, always amounting to more than 60% of all the molecules detected in the gas phase. As for the other species, their ratios change significantly with temperature. Methane production accounts for approximately 10
20% of the total yield but goes up first, between 400 and 500 K, and decreases monotonically afterward. Significant amounts (>10%) of MMI are also made starting at 450 K, but the yield of that product goes down with increasing temperature at the expense of the formation of both ethylene and HCN. The trend followed by the DMA rate mirrors that of MMI but with absolute yields only a third or less of those of the imine. The right panel in Figure 5 shows the Arrhenius plots obtained for the rate data in Figure 4. Apparent activation energies for the production of all species, estimated from the slopes in this graphic, are also reported. An interesting observation from these data is that, in terms of their energetics, the products can be paired up into three groups: (1) MMI and DMA, for which Ea ∼ 30
55 kJ/mol, (2) H2 and CH4, which are produced with activation energies of about 70 kJ/mol, and (3) HCN and C2H4, with activation energies of about 90 kJ/mol. These three groups could also be identified by the way selectivity evolves as a function of temperature, as mentioned in the previous paragraph, in particular in terms of how the production of MMI and DMA is replaced by HCN and C2H4 formation as the reaction temperature is increased. This suggests at least two, perhaps three, different sets of elementary steps in the overall reaction mechanism, involving a number of different intermediates. The trends in activation energies are also well matched by the trends seen in 8243

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Figure 7. Isothermal kinetic traces at steady state from experiments with isotope-labeled ammonia. Traces are shown for the conversion at 575 K of PDMAT alone (left) and in mixtures with NH3 (second from left), ND3 (second from right), and 15NH3 (right). Raw data are reported for 27, 28, 29, 43, 44, 45, 46, and 47 amu.

the TPD results, except perhaps for the case of C2H4, which desorbs at lower temperatures than HCN. The fact that the rate of C2H4 production decreases above 650 K seems to be related with this discrepancy. We suggest that adsorbed HCN may in fact be an intermediate involved in ethylene formation. Additional isothermal kinetics experiments were performed to test the role of ammonia in the chemical vapor deposition of tantalum nitride using the PDMAT precursor. Figure 6 shows sections of the steady-state traces obtained for a surface temperature of 550 K by using beams of pure PDMAT (left), a mixture of PDMAT þ NH3 (center), and pure NH3 (right); a summary of the rates measured at other temperatures in the first two cases is provided in Figure 4. The first thing to notice here is that the effects of ammonia addition on the conversion of PDMAT start to become apparent only at surface temperatures of 550 K and above. At that point, the production of CH4, and to a lesser extent H2 and HCN, all decrease, and that of DMA increases. There are additional changes in the transient kinetics seen upon blocking and unblocking of the beam, in particular a slower response in the rates of production of H2, CH4, C2H4, and HCN, and perhaps a slight acceleration in the rates of MMI and DMA formation, with the added ammonia. This could possibly be a reflection of a transamination step, the displacement of the dimethylamido ligands by amido moieties, preceding further surface chemistry. No significant detection of products is seen in molecular beams with pure ammonia, as expected; the traces are provided in the right-hand side of Figure 6 for reference. To better identify the source of the hydrogen and nitrogen atoms observed in the final products from reactions between PDMAT and ammonia, comparative isothermal kinetic experiments were performed by using ammonia isotope labeled at the hydrogen and nitrogen positions, namely, with NH3, ND3, and 15 NH3 (Figure 7). Substitution of NH3 in the beam for ND3 results in an increase in intensity of the signal for 29 amu at the expense of that for 28 amu, a good indication that these traces correspond to ethylene and that the production of that species involves a hydrogenation (deuteration) step with a hydrogen (deuterium) atom from the ammonia. Moreover, the signal growth in the trace for 46 amu (DN(CH3)2), the increase in the signal for 45 amu (DN(CH3)(CH2þ)), and the decrease in

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Figure 8. Left panel: Isothermal kinetic traces for 2, 3, 4, 16, 17, and 18 amu from isotope-labeling experiments similar to those in Figure 7, except with a lower ammonia partial pressure (5  10
8 instead of 5  10
7 Torr). Results from blank experiments with both NH3 and ND3 alone are also reported. Right panel: calculated yields for the different isotopomers of hydrogen and methane estimated from the data on the left.

Figure 9. Proposed reaction scheme for the thermal chemistry of PDMAT on tantalum surfaces.

intensity in the trace for 44 amu (HN(CH3)(CH2þ)) all show that hydrogenation of the dimethylamido ligands to DN(CH3)2 also requires a hydrogen (deuterium) atom from the ammonia. On the other hand, substitution of normal ammonia, NH3, by nitrogen-labeled ammonia, 15NH3, does not lead to any significant changes in the isotopic composition of any of the products, which means that no nitrogen atoms from the ammonia are incorporated into any of the desorbing species. Finally, a follow up on the isotope-labeling experiments looking at the evolution of the light products was done by performing experiments using a lower ammonia pressure (Figure 8). The emphasis here was on following the production of hydrogen and methane. A new signal was detected for 3 amu in the experiments with PDMAT and ND3, indicating the formation of some HD, most likely involving deuterium atoms generated via H
D exchange on certain surface intermediates. A measurable intensity in the trace for 17 amu, which corresponds to CH3D, was also observed; this amounted to about 20% of all the methane produced. On the other hand, no CH2D2 formation was detected, implying that methane is produced via 8244

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The Journal of Physical Chemistry C the hydrogenation (deuteration) of methyl, not methylene, surface groups.

4. DISCUSSION In this work, the thermal chemistry of PDMAT on the surface of a Ta foil has been characterized under UHV conditions by a combination of TPD and isothermal kinetics measurements using effusive collimated molecular beams. The data corroborate some general conclusions published previously on the surface chemistry of early transition-metal amido compounds but also provide some new insights into the mechanism of the thermal conversion of those compounds on metal surfaces. A proposed reaction scheme is provided in Figure 9 to guide the following discussion. The first thing to note from the data reported here is that H2, CH4, C2H4, HCN, N-methyl methyleneimine (MMI), and dimethylamine (DMA) were identified as the main products in both types of experiments. On the basis of their kinetics of formation, those products were grouped in three categories: (1) MMI and DMA, which are produced mainly at low temperatures, (2) HCN and C2H4, which dominate mostly at higher temperature, and (3) H2 and CH4, which are seen to form all throughout. This suggests that the first group may constitute the primary products of PDMAT thermal decomposition and that the others may originate from further decomposition of those and other intermediates on the surface. The production of both the amine and the imine has been reported previously with several amido complexes containing either dimethylamido16,17,22,29,36,38,39 or analogous dialkylamido7,9,20,23,25,26,28 ligands. It is widely believed that MMI comes from β-hydride elimination from an amido ligand, although it is still not clear if that happens on the surface or while the ligand is still bonded to the original metal center in the organometallic precursor. As an example of the data available in the literature about this reaction, infrared absorption spectroscopy studies have shown that heating TDMAT in the gas phase above 550 K leads to the growth of peaks at 1590 and 1276 cm
1 for CdN and Ti
N
C stretching vibrations, respectively.17 Although the observation and identification of the peak for Ti
N
C has not been reliable,19,29 the CdN stretching vibrational peak has been seen repeatedly.7,20 The formation of DMA most likely takes place via hydrogenation of the amido ligands. Such hydrogenation may follow intramolecular hydrogen-transfer pathways but more probably involves the incorporation of a hydrogen atom from the surface. In either case, it has been suggested in previous reports that the hydrogen required for this reaction may originate from the β-hydride elimination step responsible for the production of MMI. However, if this is the case, and the overall reaction is a disproportionation of two amido ligands into DMA and MMI, the number of DMA molecules produced this way should match that of MMI. In contrast, although DMA was detected in our experiments, its yield was at most 1/3 that of MMI, and that ratio became even smaller in reactions carried out at temperatures above 550 K. This implies that the probability for the amido ligands to be hydrogenated is not very high (hydrogen desorption dominates instead) and also that a significant fraction of intact amido ligands may survive up to high temperatures. Another observation from our work that corroborates previous observations is the fact that DMA generation is enhanced by NH3 addition to the gas mixture. This may perhaps be

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explained by a transamination reaction, but the mechanistic details of how that may happen have not been determined, or even discussed much, in the literature.16,18,29,40
42 Regardless, a couple of interesting observations from the data in Figure 4 add to our knowledge on the kinetics of this step. First, the effect of ammonia starts to be noticeable only at surface temperatures of 550 K or above. This is approximately 100 K higher than the onset of reactivity seen for pure PDMAT on the tantalum surface. Ammonia decomposition is therefore the likely limiting step in the CVD conversion of PDMAT þ NH3 mixtures to tantalum nitride films, which sets the minimum temperature required in the design of ALD processes. The second observation is that the main role of ammonia is to enhance hydrogenation of the amido ligands to form DMA at the expense of decomposition to methane. The two reactions appear to compete, at least above 550 K. The next issue to be discussed here is the formation of methane, which comes from decomposition of amido ligands and requires the scission of C
N bonds. To the best of our knowledge, methane production has not been reported in ALD or CVD studies with amido complexes, but it has been seen in surface-science studies on the decomposition of PDMAT on a Ni foil36 and also during the thermal chemistry of TDMAT on a W(100) surface.29 Theoretical calculations have suggested a facile generation of methane based on the idea that the cleavage of nitrogen
carbon bonds is energetically favorable when compared to most metal
nitrogen bond scission steps.39 A couple of observations from our work are relevant to the elucidation of the mechanism for methane formation: (1) CH4 appears to be made at the expense of DMA since the yields for those two show opposite trends upon ammonia addition to the reaction mixture (Figure 4); and (2) the hydrogen required to hydrogenate the methyl groups in the amido ligands appears to originate mainly from the original PDMAT since the addition of deuterated ammonia leads to only minimal production of CH3D (Figure 8). We suggest that the dimethylamido ligands may decompose either on the original Ta metal center or on the surface to produce methane and a methyleneimido, H2CdN(ads), surface species. The latter intermediate could dehydrogenate further to HCN and/or participate in coupling reactions, as discussed next. Next, we turn our attention to the formation of ethylene. This is a novel product not detected in most previous studies. We are only aware of one report, a surface-science characterization of the chemistry of PDMAT on a nickel foil, where such product was seen,36 and no discussion was provided there as to its possible origin. Here we suggest that ethylene may be produced by a reaction between two of the H2CdN(ads) groups that form after methane desorption, in a similar fashion as seen with alcohols and aldehydes on other early transition metal surfaces. One possibility is that this could occur via the direct coupling of the two methylene groups in adjacent methyleneimido surface intermediates, a reaction that would generate H2CdCH2 and leave two N(ads) atoms on the surface.43,44 However, that does not explain the incorporation of deuterium atoms seen during reactions of PDMAT with ND3 (Figure 7). More likely, some of the methyleneimido intermediates may dehydrogenate further to yield HCN(ads), and the methylidyne (HC) moiety in that product may then insert into the carbon
nitrogen bond in H2CdN(ads) to produce H2CdC(H)-N(ads); that vinyl moiety can subsequently be hydrogenated (deuterated) to produce ethylene, leaving behind adsorbed atomic nitrogen. This may at 8245

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The Journal of Physical Chemistry C first appear to be an esoteric mechanism; however, HCN was in fact detected in the gas phase in our experiments, and the type of insertion step proposed here is quite similar to that seen with oxygenated reactants on vanadium surfaces.43,45,46 On the other hand, no coupling reactions to produce hydrazines, or insertions to yield diaminomethanes, were seen in this work. Such reactions have been recently reported by us in gas-phase studies with amido complexes of titania and tantalum.22 In terms of the relative yields of the reactions observed here, the production of hydrogen from total decomposition of the adsorbed PDMAT dominates at all temperatures. That is not a desirable outcome in CVD or ALD. However, after accounting for the stoichiometry of the PDMAT precursor, the situation is not quite as dire because each amido ligand can produce up to three H2 molecules and each PDMAT twelve. Still, accounting for those factors, H2 evolution always represents at least 40% of all the converted amido ligands (that minimum measured at 500 K). Under the best of circumstances, with PDMAT alone, the maximum fraction of amido ligands transforming into MMI and DMA amounts to only about one-third of the total; that occurs at 450 K. In terms of the removal of carbon and nitrogen atoms from the surface, the former desorbs mostly as MMI and DMA at low temperatures (∼70% of all desorbing carbon at 450 K) but as methane and ethylene at higher temperatures (∼70% at 575 K). The nitrogen is removed mainly as MMI (60
80%, less at higher temperatures). It should be noted that these stoichiometries do not match that calculated by hydrogen mass balance, which means that some carbon and some nitrogen atoms are left behind on the surface after complete thermal conversion of the adsorbed PDMAT. Finally, we discuss the kinetics of the PDMAT thermal conversion on the tantalum surface in relation to CVD and ALD processes. The fact that the initial desorption (after the first flag removal) of H2 and CH4 in molecular beam experiments on the clean Ta is seen at temperature as low as 400 K (data not shown) sets a high limit for the dissociative adsorption of the TDMAT precursor. In fact, that may happen even at 300 K, given that the TPD in Figure 1 shows products from surface species that form after dosing PDMAT at that temperature. This means that temperatures as low as 300
400 K could be used in the first half-cycle of ALD processes with TDMAT, at least on surfaces of early transition metals. On the other hand, significant steadystate conversion starts at about 500 K. At that point, a CVD regime is reached, where the PDMAT precursor can sustain a self-decomposition process that leads to the growth of tantalum films but also possibly deposits carbon and nitrogen on the surface. These observations suggest a window for ALD between approximately 300 and 500 K. It should also be indicated that the relative yields for MMI and DMA, the most desirable gas-phase products (because they remove the PDMAT ligands cleanly from the surface), decrease monotonically with temperature, so the lower the temperature the better for ALD and CVD. Moreover, 500 K is the threshold temperature where other more dehydrogenated products, HCN and C2H4 in particular, start to form, and those should be avoided if possible because they may decompose rather than desorb molecularly and leave carbon and nitrogen behind on the surface. The next relevant temperature identified from our data in terms of CVD and ALD processes is >550 K, the point at which addition of ammonia to the reaction mixture starts to have an effect on the surface reactions of PDMAT, minimizing methane production at the expense of DMA formation (the ammonia

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provides the hydrogen required for such hydrogenation). This is a desirable change, because it promotes further clean removal of the amido groups in the PDMAT precursor, but it is not particularly helpful for ALD because 550 K is a temperature at which extensive decomposition of PDMAT may already occur in the first ALD half-cycle. Simply, ammonia is not a particularly good choice of reactant for ALD with these amido precursors; a more reactive nitrogen-containing agent is needed. Ammonia addition to the reaction mixture may still help in CVD, though. Increasing the reaction temperature above 600 K does not lead to any significant further changes in product selectivity. Similarly, increments in pressure or exposures of NH3 do not appear to help much as far as film deposition is concerned: the relative yields of the different products in our experiments are the same with 5  10
8 and 5  10
7 Torr NH3. It may be that the amount of ammonia that adsorbs on the surface reaches saturation at low ammonia pressure, and that beyond that there are no further changes in kinetics.

5. CONCLUSIONS The thermal chemistry of pentakis(dimethylamido)tantalum (PDMAT) on a tantalum surface was investigated. Hydrogen, methane, hydrogen cyanide, ethylene, N-methyl methyleneimine (MMI), and dimethylamine (DMA) were all identified as desorbing products in both temperature-programmed desorption (TPD) and isothermal molecular-beam kinetic experiments. On the basis of both the peak maxima in the TPD spectra and the activation energy determined from the isothermal kinetic measurement, these desorbing species can be paired up into three groups, namely, MMI and DMA, which are produced at low temperature, C2H4 and HCN, which are observed at higher temperatures, and H2 and CH4, detected all throughout the whole temperature range studied. The first group is estimated to correspond to the formation of primary products, whereas the rest originate from further secondary reactions occurring on the surface at higher temperatures. Steady reactivity is detected starting at about 450 K, in chemistry dominated by the known hydrogenation and β-hydride elimination steps responsible for DMA and MMI production. Additional chemistry is triggered at higher temperatures, however, to yield methane, ethylene, and hydrogen cyanide (in addition to H2). It is proposed here that some adsorbed dimethylamido intermediates first disproportionate to CH4 þ H2CdN(ads) and that the latter methyleneimido species then dehydrogenates to HCN. Ethylene formation is suggested to occur via insertion of a methylidyne, HC(ads), moiety from HCN into the methylene
nitrogen bond of the methyleneimido surface species to form a vinyl intermediate, which is later hydrogenated with surface hydrogen to ethylene. The addition of ammonia to the reaction mixture leads to changes in reactivity only above 550 K, at which point the main effect is the partial suppression of methane formation at the expense of an increase in DMA production. Isotope labeling experiments were used to establish that the extra hydrogen in the DMA production originates from the added ammonia. Methane formation, on the other hand, takes place via an intramolecular hydrogen transfer instead. By using 15NH3, it was also established that none of the nitrogen-containing products detected in the gas phase incorporate nitrogen atoms from the added ammonia, suggesting that those atoms remain on the surface and incorporate into the growing tantalum nitride films. 8246

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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