Thermal Chemistry of Copper Acetamidinate ALD Precursors on

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1 Thermal Chemistry of Copper Acetamidinate ALD Precursors on Silicon Oxide Surfaces Studied by XPS Yunxi Yao and Francisco Zaera* Department of Chemistry, University of California, Riverside, CA 92521, USA *Corresponding Author, The thermal surface chemistry of copper(i)-n,n -di-sec-butylacetamidinate, [Cu( s Bu-amd)] 2, a metalorganic complex recently proposed for the chemical-based deposition of copper films, has been characterized on SiO 2 films under ultrahigh vacuum (UHV) conditions by X-ray photoelectron spectroscopy (XPS). Initial adsorption at cryogenic temperatures results in the oxidation of the copper centers, with Cu 2p 3/2 XPS binding energies close to those seen for a +2 oxidation state, an observation that we interpret as the result of the additional coordination of oxygen atoms from the surface to the Cu atoms of the molecular acetamidinate dimer. Either heating to 300 K or dosing the precursor directly at that temperature leads to the loss of one of its two ligands, presumably via hydrogenation/protonation with a hydrogen/proton from a silanol group, or following a similar reaction on a defect site. By approximately 500 K the Cu 2p 3/2, C 1s, and N 1s XPS data suggest that the remaining acetamidinate ligand is displaced from the copper center and bonds to the silicon oxide directly, after which temperatures above 900 K need to be reached to promote further (and only partial) decomposition of those organic moieties. It was also shown that the uptake of the Cu precursor is self-limiting at either 300 or 500 K, although the initial chemistry is somewhat different at the two temperatures, and that the nature 1

2 of the substrate also defines reactivity, with the thin native silicon oxide layer always present on Si(100) surfaces being less reactive than thicker films grown by evaporation because of the lower density of surface nucleation sites. Keywords: Copper acetamidinate, ALD Precursor, Silicon oxide films, X-ray photoelectron spectroscopy, Surface chemistry I. INTRODUCTION There is great interest in the microelectronic industry in developing a viable process for the atomic layer deposition (ALD) of copper-based films, 1-3 but advancement in this direction has been limited by the lack of viable metalorganic precursors. 4,5 Like with other late transition metals, most of the metalorganic complexes of copper require the use of large organic ligands, and those tend to complicate the surface chemistry associated with ALD and to lead to the deposition of undesirable impurities. 6 One promising family of ALD copper precursors has been advanced recently based on amidinates or related ligands to grow metallic copper as well as copper nitride films In connection with this, we 11,12 and others 13,14 have embarked in research projects to elucidate the details of the reaction mechanism of such compounds on solid surfaces. Past work from our group has indicated that both copper acetamidinates and related copper iminopyrrolidinate and guanidinate 18 ALD precursors follow complex multistep decomposition 2

3 pathways on metal surfaces. For one, the dimeric or tetrameric structures in which those compounds exist in the gas phase dissociate and form monomers upon adsorption, even at cryogenic temperatures. Initial decomposition is typically seen at approximately 300 K, at which point either β-hydride elimination or C N scission steps lead to the release of some of the ligands into the gas phase. The remaining adsorbates undergo further decomposition at higher temperatures as the Cu atoms are reduced, and several hydrocarbons as well as HCN and H 2 are released in several stages culminating at temperatures around 600 K. The end result is the deposition of submonolayer coverages of copper in metallic state together with small but measurable amounts of carbon and/or nitrogen containing species. On oxide surfaces, on silicon oxide in particular, the activation of copper acetamidinates seems to follow a much simpler mechanism. According to an infrared absorption (IR) spectroscopy study by Chabal and coworkers, 19,20 the initial surface reaction of copper(i)-n,n -di-secbutylacetamidinate, [Cu( s Bu-amd)] 2, with SiO 2 takes place via displacement of one of the ligands at a silanol site and the formation of a Si O Cu ( s Bu-amd) surface species. A further thermally activated ligand rearrangement, from a bridging to a monodentate structure, was reported to take place at approximately 500 K, but no extensive ligand decomposition was observed on this surface, in contrast with what occurs on metals. In the present report, we summarize our data on a study designed to expand on Chabal s work by characterizing the surface chemistry of [Cu( s Bu-amd)] 2 on thin silicon oxide films using X-ray photoelectron spectroscopy (XPS). Our data are in general consistent with the previous IR study but has led us to a somewhat different interpretation of the rearrangement of the ligands of the precursor on the surface, and also indicate further surface chemistry leading to the deposition of carbon and nitrogen impurities. 3

4 Adsorption at cryogenic temperatures is molecular, with the Cu precursor keeping its initial dimeric structure but adding coordination around the metal atom and increasing its degree of oxidation. An initial set of changes is seen in the adsorbates around 300 K, likely associated with the removal of one of the two ligands and with a partial reduction of the copper atom. Slow decreases in carbon and nitrogen coverages on the surface follow until reaching temperatures about 500 K, at which point a ligand surface rearrangement is proposed to take place, likely a displacement from coordination to the Cu center to direct bonding to the silicon dioxide substrate. A third, previously unidentified, transition is finally seen between 800 and 900 K leading to the removal of some but not all of the surface carbon and nitrogen. Above such temperatures, decomposition is likely to be irreversible and to lead to the deposition of impurities. Below we expand on our findings. II. EXPERIMENTAL All the experiments reported here were performed in a surface-science apparatus described in detail elsewhere. 21,22 The main ultrahigh vacuum (UHV) stainless-steel chamber is turbopumped to a base pressure in the Torr range, and is equipped with a 50-mm radius hemispherical electron energy analyzer (VSW HAC 5000), set at a 50-eV constant pass energy, and a magnesium-anode (hν = ev) X-ray source for X-ray photoelectron spectroscopy (XPS). Most XPS data were fitted to Gaussian peaks to better follow the thermal evolution of the products. Peak areas were scaled by using reported relative sensitivities, 23 and converted to monolayer coverages (ML, normalized to the stoichiometry of the initial Cu precursor) by 4

5 referencing them to the Si 2p XPS signal attenuation seen in control experiments from physical evaporation of Cu metal on SiO 2 /Si(100) films. Because a layer-by-layer model was used in those calculations whereas Cu films are known to form 3D structures instead, it should be stated that this calibration is not fully reliable and is provided here only as a rough guide. Nevertheless, errors in absolute coverages obtained this way are within a factor of two of the real values at worst, and, perhaps more importantly, the relative atomic ratios across the different elements, on which we rely more heavily in our discussion, do not depend on this scaling. For our studies on the thermal chemistry of the Cu precursor on silicon oxide surfaces, thin films were prepared fresh on a Si(100) substrate before each experiment. The Si(100) wafer was glued onto a Ta support plate with a high temperature inorganic adhesive (Aron Ceramic) mixed with a conductive graphite power ( %, Alfa Aesar), and mounted on a manipulator capable of cooling down to below 100 K and of resistively heating to up to 1100 K. The temperature was measured by using a K-type thermocouple glued in between the Ta foil and the Si(100) wafer, and controlled by a homemade proportional integral derivative (PID) circuit. Experiments were carried out on both the native 1 nm-thick silicon oxide film that forms naturally on silicon surfaces and on thicker SiO x films deposited from the gas phase. The latter was done by evaporating silicon using a homemade doser consisting of pure silicon rapped with a tungsten filament (used as a heater) and placed a few centimeters away from the substrate while maintaining an oxygen atmosphere (P O2 = 5 x 10 6 Torr). 24 The surface was maintained at room temperature during deposition, but later annealed in O 2 at 1050 K for 10 min to fully oxidize the deposited silicon. The thickness of the SiO 2 film prepared this way was controlled by the deposition time, and estimated by using the measured SiO 2 /Si XPS signal ratio. 25 Typical Si 2p 5

6 and O 1s XPS traces from such films are shown in Figure 1 for a range of thicknesses going from 0 to 10 nm. Most of the experiments reported here were performed on 10-nm thick SiO 2 films. The copper(i)-n,n -di-sec-butylacetamidinate, [Cu( s Bu-amd)] 2, precursor was provided to us by Prof. Roy Gordon from Harvard University, who synthesized it by sequential reactions between N,N -di-sec-butylacetamidine and CH 3 Li and CuCl. 26 The surfaces were cleaned before each experiment by combinations of high-temperature annealing and Ar + ion sputtering, after which the SiO 2 films were deposited and the Cu precursor adsorbed. Dosing was carried out by backfilling of the chamber using a leak valve, and controlled by fixing the precursor temperature at 365 K; the gas lines behind the valve were kept at approximately 330 K in order to minimize condensation. The final exposures were determined by the exposure time, which was in the range of tens of minutes; they amounted to approximately 10 L (1 L = 1 x 10 6 Torr s) in most cases, sufficient for the deposition of 3-5 monolayers of the precursor on the surface. III. RESULTS AND DISCUSSION The first set of experiments carried out to characterize the thermal chemistry of the [Cu( s Buamd)] 2 precursor on silicon oxide films was for a saturated layer and as a function of annealing temperature. The raw Cu 2p 3/2 XPS (left panel) and Cu L 3 VV Auger electron (AES, right panel) spectra measured on a ~2 nm thick SiO 2 /Si(100) film are reported in Figure 2, and the evolution versus temperature of the coverages calculated by deconvolution and integration of the peaks is summarized in Figure 3. At cryogenic temperatures, specifically at 100 K, the Cu 2p 3/2 XPS 6

7 trace is dominated by a broad peak at a binding energy (BE) of ev (Figure 2, left panel, second from bottom trace). Because of the low temperature of adsorption used here, the expectation is for the copper precursor to remain intact upon adsorption. It should be noted, however, that the observed BE value is more consistent with a +2 oxidation state than with the +1 value of the central metal ion in the original precursor. 27 In our previous work on the adsorption of [Cu( s Bu-amd)] 2 on metals different values of BE, between and ev, were observed, whereas a study with Cu(acac) 2, a precursor containing Cu 2+ ions, yielded a BE of the molecular precursor at ev, essentially the same as that seen here. To help with the assignment of the oxidation state of the copper atoms on the surface, additional spectra were obtained for the L 3 VV AES line (Figure 2, right panel). Surprisingly, after lowtemperature adsorption, this feature was observed at exceptionally low kinetic energies (KE), around 911 ev at 100 K and at ev at 150 K. Those numbers correspond to Auger parameter values (Cu 2p 3/2 BE + L 3 VV AES) of and ev, respectively, more typical of Cu + compounds with highly electronegative anions such as Cl or CN. 27 This may cast some doubts on the interpretation of the Cu 2p 3/2 XPS spectra provided above, but, given that the for copper Auger parameter is less consistent than the XPS binding energies in predicting oxidation states for Cu, and also based on the subsequent trends seen with this Cu ALD precursor as a function of temperature (see below), we still favor an explanation where molecular adsorption is accompanied with a nominal oxidation of the copper centers. We propose that the adsorption takes place via retention of the dimeric form but with a somewhat distorted molecular geometry, with the ligands moving away from its original planar configuration 26 to allow for the copper centers each to bond to an oxygen atom from the silicon 7

8 oxide surface. An oxygen or silanol group from that surface may coordinate to the copper center via one of its electron lone pairs, forming a dative bond and leading to a charge redistribution akin to the partial oxidation of the original Cu + ion. In order to learn about the surface chemistry of the ligands themselves, our XPS characterization of the adsorbed species was complemented with spectra recorded for the C 1s and N 1s signals. The raw traces together with their fits to Gaussian peaks are provided in Figure 4, and the results from quantitative analysis of the signal intensities are summarized in Figure 5 (normalized to the stoichiometry of the copper precursor, that is, divided by factors of 10 and 2 for the C and N data, respectively). It should be indicated that although in principle the C 1s signals from the original acetamidinate ligand can be deconvoluted into a minimum of three peaks corresponding to carbon atoms coordinated to two, one, and no nitrogen atoms, respectively, 16 here we decided to group all those together into one single broad Gaussian feature in order to simplify the analysis: only two peaks, at approximately and ev, assigned to the coordinated acetamidinate ligands in the original dimer and in other coordination modes (to a Cu monomer or to the SiO x surface), respectively, were fitted to the C 1s traces. Analogously, two peaks at approximately and ev were used to fit the N 1s XPS traces. After adsorption of the [Cu( s Bu-amd)] 2 precursor at 100 K, the C 1s and N 1s signals actually peak at and ev, respectively, values that we associate with the two acetamidinate ligands coordinated to two copper atoms as in the original dimer. Upon heating the surface to 150 K, the Cu 2p 3/2 XPS peak shifts slightly to lower BE, to about ev, and develops a small shoulder at ev (Figure 2, left panel, third from bottom 8

9 trace), and so do the C 1s and N 1s peaks, which now appear at and ev, respectively, but also have small (10% of total intensity) shoulders at and ev (Figure 4, second from bottom traces). The peak positions and signal intensities of all these features then remain the same up to 250 K, suggesting the presence of the same di-coordinated intermediate on the surface over the K temperature range. However, a clear change is seen in the L 3 VV AES traces between 150 and 200 K, with a new peak developing at a KE of ev (Figure 2, right panel, second and third traces from bottom), a value more in line with that seen for Cu 2+ salts. 27 It is possible that the adsorbed monolayer may be partially covered with condensed precursors below 150 K, masking the true AES peak for the new surface intermediate, and that molecular desorption occurs fully between 150 and 200 K and exposes the first adsorbate monolayer. TPD experiments with this and other precursors have identified condensed multilayer desorption under similar conditions. 16,17 Alternatively, the increase in the KE of the L 3 VV peak may be interpreted as the result of a shift in the valence band of the copper atoms as they rearrange slightly on the surface, perhaps developing a more covalent bond with the surface oxygen atoms. It should also be noted that the Cu 2p 3/2 XPS peak at 934.0, although representing a minority (~20% of the total surface Cu) species at this stage, is evident at temperatures as low as 150 K. That species does become dominant at higher temperatures, as we discuss next. The spectra recorded after annealing at 300 K clearly reflect a transition in the nature of the surface species, and by 350 K that change has fully taken place. The main contribution to the Cu 2p 3/2 XPS spectra becomes the ev peak, which is accompanied by a L 3 VV AES peak at ev (for an Auger parameter of ev; Figure 2, 7 th traces from bottom). The 9

10 compounds with the closest reported values for all those numbers are still Cu 2+ salts, but perhaps with less electronegative counterions. 27 It is possible that at this point a new surface species forms, still with highly oxidized copper atoms but with different coordination. More likely, though, the same basic molecular structural unit of the Cu acetamidinate remains intact, but in monomeric form. The C 1s and N 1s XPS signals also change in this temperature range, and become dominated by signals centered at and ev (Figure 4, 6 th traces from bottom). All these values are much closer to those reported for the species resulting from low-temperature adsorption of the precursor on metal surfaces, 16 suggesting that, as it happens in those cases (and as mentioned above), the dimer on the silicon dioxide substrate splits and forms monomers. In terms of the XPS signal intensities, the Cu 2p 3/2 peak grows by over 50% from the values seen at lower temperatures (Figure 3), indicating greater exposure to the vacuum environment (presumably thanks to the removal of some ligands), whereas both C 1s and N 1s XPS signals decrease gradually by 20-30% (Figure 5). It is likely that at this point some acetamidinate ligands are displaced from their coordination sites on the Cu centers by either OH surface groups or other defect surface sites, producing free acetamidine (which is released to the gas phase) and a Si O Cu( s Bu-amd) monomer directly coordinated to an oxygen surface atom. Based on their IR experiments, the Chabal group proposed the formation of a similar surface intermediate upon adsorption with direct bonding of both copper atoms to respective surface oxygen atoms in a complex that has lost one of the acetamidinate ligand. 20 Although their experiments were carried out at higher temperatures, starting at 375 K, their proposed Si O N( s But)=C(CH 3 ) N( s But) Cu O Si di-copper bonded structure, with its increased Cu oxidation state, is consistent with our XPS measurements, suggesting that this dissociative adsorption modality may start at approximately 300 K. 10

11 The next transition in surface chemistry in this copper acetamidinate-silicon oxide system is seen at approximately 500 K. The change here is somewhat subtler, manifested by a red shift of the Cu 2p 3/2 XPS main peak by about 0.3 ev, to ev (Figure 2, left panel, 6 th trace from top). The Cu L 3 VV shifts in the opposite direction, to a new value of about ev (Figure 2, right panel, 6 th trace from top), so the Auger parameter remains virtually invariable. Nevertheless, a clear change in surface chemistry occurs at this temperature, as also indicated by red shifts in the C 1s and N 1s XPS comparable to that seen in the Cu 2p 3/2 spectra (Figure 4). The intensities of the XPS signals stabilize after losing virtually all contributions from the high BE peaks, now at and ev, something that happens gradually in the K temperature range (Figure 5). All these changes coincide with the thermally activated rearrangement reported by Chabal and coworkers from a bridging to a monodentate structure. 20 However, we do not see evidence in our XPS data of any loss of copper atoms from the surface, as they propose in their mechanism. Instead, we suggest that the observed transition may be a displacement of the acetamidinate ligands from its coordination to copper ions to a species bonded directly to the silica surface. One difference between our study and theirs is that in their case they added hydrogen from the gas phase, thus facilitating the hydrogenation of the organic ligands. Nevertheless, we still see a chemical transformation in the same temperature range in the absence of H 2, one that does not involve the loss of any copper (or acetamidinate ligands) from the surface or any significant oxidation state changes. It should be said that the opposite shifts observed at this stage for the Cu 2p 3/2 XPS and Cu L 3 VV AES signals (Figure 2) is consistent with a partial reduction of the copper atoms, which is what would result from the removal of the acetamidinate ligands from their sphere of coordination. Nevertheless, this reduction does not go 11

12 all the way to metallic copper (for which the reported values are and ev, respectively), 27 presumably because of the ionic character of the remaining bonding to the surface oxygen atoms. Finally, one last clear transition is seen in these annealing experiments at 900 K. At that point, the Cu 2p 3/2 XPS signal increases by approximately 10% (Figure 3) while the C 1s and N 1s XPS peaks decrease in intensity by about 20-30% (Figure 5). The Cu 2p 3/2 peak shifts slightly, to ev, as does the Cu L 3 VV AES signal, in that case to ~916.7 ev (Figure 2, 2 nd traces from bottom); these values are still associated with copper atoms in an oxidized state, most likely a charge value of approximately +1. The C 1s and N 1s XPS peaks also change in shape and position (Figure 4, 2 nd from top traces), but not in a very significant way, suggesting that decomposition of the organic surface moieties is limited even at such high temperatures. Nevertheless, we speculate that the new surface species that form above 900 K may not be easy to hydrogenate and remove from the surface, as required for a clean ALD process. Interestingly, that still provides a wide window of temperatures where limited surface chemistry takes place and where ALD processes could be feasible. The catch is that such low reactivity would only be seen in the initial stages of any Cu film growth on silica, since much higher activity is likely once a thin copper film is deposited on the surface. 17 Similar annealing experiments were carried out after adsorption at 300 K. At such temperatures, the molecularly adsorbed precursor is already activated, and only the monomeric species coordinated to the surface oxygen sites is detected. Under those circumstances, all the XPS spectra remain virtually invariant in the K temperature range (data not shown). The 12

13 Cu 2p 3/2 XPS peaks in this case are quite similar to those reported above when starting from the precursor adsorbed at 100 K, both in terms of peak position and peak intensity (Figure 3). The C 1s and N 1s XPS signals, on the other hand, are both much weaker (Figure 5). This signifies the fact that upon exposure of the surface to the Cu precursor at 300 K, the latter reacts immediately at the nucleation sites and loses most of its ligands to the gas phase (after hydrogenation/protonation). The reaction seems to still be self-limiting, because the final Cu coverage obtained at 300 K is the same as that seen when starting at 100 K (Figure 3). It is also not totally clean, because of the residual carbon and nitrogen left on the surface, which correspond to approximately 30 and 80% of the Cu coverage, respectively (Figure 5). Notice the lack of stoichiometry in these numbers, a fact that could be explained in part by photoelectron shielding effects if the adsorbates adopt a specific geometry on the surface (the copper and nitrogen atoms would appear to be more exposed than the carbon atoms), but that also suggest that some degree of ligand decomposition does take place on the surface. High-temperature annealing after the uptake of the [Cu( s Bu-amd)] 2 precursor at 150 K on a thin (native) SiO 2 film leads to less deposition of Cu atoms ( ~ 0.15 instead of 0.35 ML, data not shown), but approximately the same amounts of carbon and nitrogen as in the case of the deposition at 300 K on the thicker oxide film. Because the native oxide is expected to have a different (lower) density of defect sites and/or silanol surface groups than the thicker films deposited on the Si(100) substrate, these differences point to the crucial role of the those surface sites for the nucleation required to initiate the thermal activation of the copper precursor. 13

14 Next, the uptake of the Cu precursor was probed as a function of dose at several substrate temperatures. Figure 6 shows the Cu 2p 3/2 XPS data recorded as a function of exposure time for deposition on a 10-nm thick SiO 2 film at 300 (left panel) and 500 (right) K, and Figure 7 summarizes the data in terms of the resulting Cu coverages for the two main species identified by spectra deconvolution, that is, for the peaks at and ev BEs. The total uptake in both cases is similar, perhaps being slightly more effective at 300 than 500 K, and the final species are also the same, dominated by the Cu + ions corresponding to the ev BE (although the Cu L 3 VV peaks are seen at surprisingly low KEs, Figure 8). However, a significant difference is seen at the early exposures times, where most of the Cu adsorbed at 300 K is still in the form of the initial precursor (the adsorbate that yields a Cu 2p 3/2 peak at ev). In contrast, at 500 K, the first copper species are already dissociated, and only after those reach a threshold coverage does the ev BE copper start to build up. In the end, though, that latter species reaches the same limiting value at both temperatures, about ML, with the rest corresponding to the activated adsorbate. The deposition also seems to be self-limited, leveling off at a total Cu coverage of approximately 0.35 ML, the same as in the annealing experiments. Finally, the uptake in the early stages (10 min) of dosing was tested as a function of temperature on the native oxide film (Figure 9). Because this surface has a lower density of nucleation sites, it is expected to display lower reactivity, as discussed above, but the new data indicate that similar Cu coverages can be reached if high enough temperatures (500 K) are used. As opposed to the data in Figure 7, here there is a strong temperature effect, with significant Cu uptake only being possible at 450 K or above. This deposition is mainly in the form of the activated monomeric species directly exchanged with nucleation surface sites, likely silanol groups. 14

15 Together with the annealing results discussed above, these data point to the importance of the nucleation sites in facilitating the activation of the Cu complex and the deposition of Cu on the surface. We suggest that it would be desirable to pretreat the silicon oxide surfaces before Cu ALD when possible, perhaps by UV/ozonolysis, 28 in order to maximize the surface density of nucleation sites. IV. CONCLUSIONS X-ray photoelectron spectroscopy (XPS) was used to characterize the thermal chemistry of copper(i)-n,n -di-sec-butylacetamidinate, [Cu( s Bu-amd)] 2, a promising precursor for the chemical deposition of copper-containing films, on the surfaces of SiO 2 thin films. Experiments based on annealing of a saturated layer of the adsorbate at different temperatures, starting at 100 K, identified clear chemical transformation at three distinct temperatures, around 300, 500, and 900 K. According to the Cu 2p 3/2 XPS data, the copper atoms of the acetamidinate precursor adsorbed at low temperatures display a high (+2) oxidation state, an observation that we interpret as the result of additional coordination of the two Cu atoms in the intact molecular dimer to oxygen atoms on the silicon oxide substrate. A subtle electronic change is observed around 200 K in the Cu L 3 VV Auger electron spectra (AES), indicating a modification of the valence band structure, possibly toward a more covalent bonding to the surface, but significant changes in the nature of the adsorbed species are seen only around 300 K, manifested by changes in all Cu 2p 3/2, C 1s, and N 1s XPS peak positions and peak intensities. First, the Cu 2p 3/2 XPS shifts to lower binding energies (and the Cu L 3 VV AES peak to higher kinetic energies), a clear indication of 15

16 the partial reduction of the Cu centers. In addition, about a third of the signal intensities of the C 1s and N 1s XPS features are lost between about 300 and 500 K, likely because of the hydrogenation (protonation) of one of the two acetamidinate ligands and of its desorption as the corresponding acetamidine to the gas phase. A third transition is then observed between 500 and 600 K. This conversion was previously suggested to correspond to a rearrangement from a bidentate to a monodentate binding of the ligand and to the reduction and loss of one of the two Cu atoms of the original dimer, 20 but that is incompatible with our XPS data. Because there are no appreciable losses of Cu, C, or N in this temperature range, and because the oxidation state of the copper centers does not change in any significant way either, we believe that the observed transition is associated with a migration of the acetamidinate ligand from the Cu atoms to the silicon oxide surface instead. Finally, the resulting adsorbates seem to be quite stable: the final transition is seen at 900 K, and it involves only relatively minor losses of carbon and nitrogen from the surface. Exposure of the surface to the Cu precursor at 300 K (instead of 100 K) seems to bypass the first transition and lead to the formation of the coordinated monomer species directly. Also, because decomposition occurs as the surface is exposed to the precursor gas, the elimination of the ligand seems to be more efficient: the same amount of Cu is deposited this way, but with much less carbon or nitrogen. Another important observation is that the nature of the silicon dioxide surface plays a critical role in the initial activation of the precursor. Indeed, experiments on the thin native oxide formed naturally on Si(100) wafers, which are believed to have lower density of the surface nucleation sites required for metal film growth, lead to lower copper deposition. Finally, uptakes of the Cu acetamidinate ALD precursor on the silicon oxide films at 300 and 16

17 500 K appear to both be self-limiting, stopping at about a third of a monolayers of Cu, but to follow different initial reactions, with the adsorption at 300 K (but not at 500 K) being initiated by the surface-coordinated dimer. ACKNOWLEDGEMENTS Financial support for this project was provided by a grant from the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering (MSE) Division, under Award Number DE-FG02-03ER REFERENCES 1. S. P. Murarka, Mater. Sci. Eng. R 19, 87 (1997). 2. R. Rosenberg, D. C. Edelstein, C.-K. Hu, and K. P. Rodbell, Annu. Rev. Mater. Sci. 30, 229 (2000). 3. H. Kim, J. Vac. Sci. Technol. B 21, 2231 (2003). 4. B. D. Fahlman, Curr. Org. Chem. 10, 1021 (2006). 5. T. J. Knisley, L. C. Kalutarage, and C. H. Winter, Coord. Chem. Rev. 257, 3222 (2013). 6. Q. Ma and F. Zaera, J. Vac. Sci. Technol., A 31, 01A112 (2013). 7. B. S. Lim, A. Rahtu, and R. G. Gordon, Nat. Mater. 2, 749 (2003). 8. Y. Au, Y. Lin, and R. G. Gordon, J. Electrochem. Soc. 158, D248 (2011). 17

18 9. S. T. Barry, Coord. Chem. Rev. 257, 3192 (2013). 10. K. Shima, H. Shimizu, T. Momose, and Y. Shimogaki, ECS J. Solid State Sci. Technol. 4, P20 (2015). 11. F. Zaera, J. Phys. Chem. Lett. 3, 1301 (2012). 12. F. Zaera, Coord. Chem. Rev. 257, 3177 (2013). 13. S. M. George, Chem. Rev. 110, 111 (2010). 14. K. Bernal Ramos, M. J. Saly, and Y. J. Chabal, Coord. Chem. Rev. 257, 3271 (2013). 15. Q. Ma, H. Guo, R. G. Gordon, and F. Zaera, Chem. Mater. 22, 352 (2010). 16. Q. Ma, H. Guo, R. G. Gordon, and F. Zaera, Chem. Mater. 23, 3325 (2011). 17. Q. Ma, F. Zaera, and R. G. Gordon, J. Vac. Sci. Technol. A 30, 01A114 (2012). 18. T. Kim, Y. Yao, J. P. Coyle, S. T. Barry, and F. Zaera, Chem. Mater. 25, 3630 (2013). 19. M. Dai, J. Kwon, E. Langereis, L. Wielunski, Y. J. Chabal, Z. Li, and R. G. Gordon, ECS Trans. 11, 91 (2007). 20. M. Dai, J. Kwon, M. D. Halls, R. G. Gordon, and Y. J. Chabal, Langmuir 26, 3911 (2010). 21. F. Zaera, J. Vac. Sci. Technol. A 7, 640 (1989). 22. T. Kim and F. Zaera, J. Phys. Chem. C 116, 8594 (2012). 23. in Practical Surface Analysis. Volume 1. Auger and X-ray Photoelectron Spectroscopy, edited by D. Briggs and M. P. Seah (John Wiley and Sons, Chichester, UK, 1990). 24. J. W. He, X. Xu, J. S. Corneille, and D. W. Goodman, Surf. Sci. 279, 119 (1992). 25. Z. H. Lu, J. P. McCaffrey, B. Brar, G. D. Wilk, R. M. Wallace, L. C. Feldman, and S. P. Tay, Appl. Phys. Lett. 71, 2764 (1997). 26. Z. Li, S. T. Barry, and R. G. Gordon, Inorg. Chem. 44, 1728 (2005). 18

19 27. Handbook of X-Ray Photoelectron Spectroscopy, edited by C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder, and G. E. Muilenberg (Perkin-Elmer Corporation, Eden Prairie, MN, 1978). 28. L. Guo and F. Zaera, Nanotechnology 25, (2014). 19

20 FIGURE CAPTIONS Figure 1. (Color online) Si 2p (left panel) and O 1s (right) X-ray photoelectron spectroscopy (XPS) traces from silicon oxide films of different thicknesses. These films were deposited on a Si(100) wafer surface via evaporation of silicon in the presence of O 2 gas followed by annealing under the same O 2 environment. The film thickness can be controlled by varying the deposition time, and estimated by the XPS signal intensities, as mentioned in the text. Most experiments reported here were performed on a 10-nm thick film, but some data were acquired on a ~2-nm thick film, and a few experiments were also carried out on the native oxide. Figure 2. (Color online) Cu 2p 3/2 XPS (left panel) and Cu L 3 VV Auger electron spectroscopy (AES, right) data for 10 L of copper(i)-n,n -di-sec-butylacetamidinate, [Cu( s Buamd)] 2, adsorbed on a ~2-nm SiO 2 film at 100 K as a function of annealing temperature. The XPS raw data are reported as dots, whereas the solid lines correspond to fits to two Gaussian peaks, from which binding energies (BEs) and coverages were calculated. Figure 3. (Color online) Summary of the information extracted from the data in Figure 2 in terms of Cu coverages versus annealing temperature. Reported are the total Cu coverage (yellow solid circles and brown solid squares) as well as the two main contributions identified by our Gaussian peak fitting procedure, namely, the features centered at approximately (top-filled circles and squares) and (bottom 20

21 filled symbols) ev. Results are reported for annealing experiments carried out after adsorption at 100 (circles, light colors) and 300 (squares, darker colors) K. The two Cu 2p 3/2 BEs are associated with the original precursor dimer and a monomer after loosing one of the two ligand, both bonded to oxygen atoms on the silicon dioxide surface. Figure 4. (Color online) C1s (left panel) and N 1s (right) XPS data from the same experiments as in Figure 2. The raw data are reported as dots, the total fit as solid lines, and the fitted Gaussian peaks as broken lines. As a simplification, only two broad peaks were fitted to each trace regardless of the different environment around the different carbon (and possibly nitrogen) atoms; this affords an easier discussion in terms of the change in chemical environment of the ligand with temperature. Figure 5. (Color online) Summary of coverage data versus annealing temperature for carbon (left panel) and nitrogen (right), extracted from the data in Figure 4. These data are complementary to those provided for Cu in Figure 3. The legend of the symbols is similar to that used for the Cu data as well. In addition to results from adsorption at 100 (circles) and 300 (squares) K, an additional set of points (green solid triangles) is also provided for adsorption at 150 K on a native SiO 2 /Si(100) film. Figure 6. (Color online) Cu 2p 3/2 XPS traces for the uptake of [Cu( s Bu-amd)] 2 adsorbed on a ~10-nm SiO 2 film at 300 (left panel) and 500 (right) K as a function of time of exposure. Again, the raw data are reported as dots and the fits as solid lines. 21

22 Figure 7. (Color online) Uptake curves in the form of Cu coverages versus exposure time, a measure of precursor dose, extracted from the data in Figure 6. Data are plotted both for the total coverages (solid symbols) and for each of the two main copper components (top- and bottom-filled symbols for the low monomer species and high adsorbed dimer BE peaks, respectively). The blue data correspond to uptake at 300 K, the red to deposition at 500 K. Figure 8. (Color online) Cu L 3 VV AES traces for the uptake of [Cu( s Bu-amd)] 2 adsorbed on a ~10-nm SiO 2 film at 300 (left panel) and 500 (right) K as a function of time of exposure, in correspondence to the Cu 2p 3/2 XPS data in Figure 6. Figure 9. (Color online) Left: Cu 2p 3/2 XPS traces recorded after 10 min of exposure of a native SiO 2 /Si(100) film to [Cu( s Bu-amd)] 2 at the indicated temperatures. Like before, the raw data are reported as dots and the fits as solid lines. Right: Summary of the total Cu coverage and the contributions from the monomer and dimer forms of the adsorbed precursor as a function of deposition temperature, extracted from the data in the left panel. 22

23 Figure 1 23

24 Figure 2 24

25 Figure 3 25

26 Figure 4 26

27 Figure 5 27

28 Figure 6 28

29 Figure 7 29

30 Figure 8 30

31 Figure 9 31