Rational Design of Metalorganic Complexes for the Deposition of

Feb 28, 2019 - Goswami, Noh, Redfern, Otake, Kung, Cui, Chapman, Farha, and Hupp. 0 (0),. Abstract | Full Text HTML | PDF w/ Links | Hi-Res PDF...
0 downloads 0 Views 2MB Size
Download PDF
Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

pubs.acs.org/cm

Rational Design of Metalorganic Complexes for the Deposition of Solid Films: Growth of Metallic Copper with Amidinate Precursors Bo Chen,† Jason P. Coyle,‡ Seań T. Barry,‡ and Francisco Zaera*,† †

Department of Chemistry, University of California, Riverside, California 92521, United States Department of Chemistry, Carleton University, Ottawa, Ontario K1S 5B6, Canada

‡

Downloaded via UNIV OF TEXAS AT DALLAS on March 2, 2019 at 13:41:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: A fourth-generation copper metalorganic compound, Cu(I)-2-(tert-butylimino)-5,5-dimethyl-pyrrolidinate, was designed, and its chemistry on nickel surfaces was characterized, for use as a precursor for atomic layer deposition (ALD) of thin solid metal films. On the basis of surface science studies with similar acetamidinate, guanidinate, and iminopyrrolidinate complexes, it was concluded that the high (and undesirable) reactivity of these when adsorbed on metal surfaces is due to the lability of their C−N bonds, which can be triggered by βhydrogen elimination steps. Accordingly, a ligand was designed without any available hydrogen atoms at these positions. The result is a much more stable reactant. Temperature-programmed desorption (TPD) experiments indicated that dehydrogenation from the new compound on Ni(110) starts only at 450 K, an increase of about 200 K in comparison with any of the earlier-generations ALD precursors. TPD and X-ray photoelectron spectroscopy (XPS) data were used to establish the details of the decomposition mechanism of the ligands, which appears to be initiated by the scission of the iminopyrrolidine C−N bond. Many byproducts are produced, including HCN, N2, iso-butene, and possibly pyrroline and other olefins such as pentenes. However, all of that occurs at relatively high temperatures, leaving an acceptable temperature window for the use of this complex for the deposition of copper films. An increased stability of the new ligands in our new copper ALD precursor was also observed on SiO2 thin films, attesting to the generality of our conclusions. We suggest that our methodology for the rational design of this ALD precursor, based on studies of its surface chemistry, can be easily extended to other cases.

1. INTRODUCTION To address issues of conformality during solid film depositions in applications involving rough topographies at the nanometer scale, researchers have increasingly shifted their interest from physical (e.g., evaporation) to chemical methods. Atomic layer deposition (ALD), where the chemistry used for the deposition is split into two or more self-limiting and complementary steps to gain sub-monolayer control of the film growth,1,2 has shown particular promise for this endeavor,3−5 and has in fact been already incorporated in some manufacturing processes in the microelectronics industry.6−8 One of the main challenges with such chemical deposition methods is the need to avoid the incorporation of impurities, byproducts of the decomposition of the chemicals used, into the growing films.9−11 The choice of the chemical precursors used in ALD is therefore critical.12 The deposition of late transition metals by ALD is particularly difficult, because there are limited choices for precursors, and most of those are metalorganic compounds with ligands containing heteroatoms that are prone to undesirable side reactions.13−15 The design of robust yet active ALD precursors has been generally guided by what is known about their liquid-phase chemistry.15−18 As useful as this approach has been, however, it has shown to have some © XXXX American Chemical Society

limitations, as the chemistry involved in ALD may be quite different to that followed by the same compound in solution: ALD processes take place at gas−surface interfaces, in the absence of any solvent, and the behavior of solid surfaces as ligands (if the analogy can be stretched to that point) can be unique.11 As a consequence, typical inorganic chemistry studies offer only limited predictive power on how precursor candidates will perform in ALD processes. One illustrating example relevant to the systems reported here is metal guanidinates: in solution, these often undergo reversible carbodiimide deinsertion,19−21 but that reaction does not seem to be viable on solid surfaces.22−24 One type of ligand that has shown great promise in chemical vapor deposition (CVD) and ALD processes is amidinates (and the related guanidinates and iminopyrrolidinates). Metal amidinates were proposed for CVD as early as 2001,25 and soon thereafter introduced for uses in ALD.26,27 They are relatively easy to make, can be quite volatile, and their properties can be tuned via modification of the side alkyl moieties.23,28 Unfortunately, they have also shown to be too Received: December 6, 2018 Revised: February 15, 2019

A

DOI: 10.1021/acs.chemmater.8b05065 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials reactive on surfaces for many applications. On oxides such as SiO2 their reactivity may be tempered,29,30 but on metals they can start to decompose even at room temperature.24,31−33 This is a critical problem because, regardless of the starting material, metal ALD eventually takes place on the freshly deposited metal surface. To minimize the propensity of amidinate ligands to decompose during ALD, we have embarked in a long-term project to modify such ligands on the basis of the mechanistic knowledge developed from a study of their surface chemistry using a modern surface science approach.24,30,34−38 Below, we report the successful design of a fourth-generation copper iminopyrrolidinate with high thermal stability on surfaces as a result of this approach. The synthesis of this new precursor was reported in a previous publication,39 but here we provide a detailed account of its surface chemistry aimed to test our hypothesis that additional stability of the ligands can be attained by blocking all of their β-H positions. The breakthrough that we report is that, by redesigning the ligands in these copper amidinate ALD precursors using the mechanistic information extracted from our surface science studies, it was possible to create a new precursor that avoids the undesirable decomposition pathways affecting the performance of all previous versions of these compounds. We contend that our example illustrates a general approach by which ALD precursors can be designed from first principles using basic mechanistic information about their surface chemistry.

Figure 1. H2 TPD traces obtained from the thermal decomposition of the four copper amidinate ALD precursors shown on the right on Ni(110) single-crystal surfaces. The numbers correspond to those used in the text, where the complete names are provided.

adjacent to the nitrogen.24,36 This initial step is followed by a sequence of additional bond-breaking reactions over a wide range of temperatures leading to the formation of side products such as alkenes, hydrogen cyanide, acetonitrile, and smaller amidines, and to the deposition of carbon and nitrogen impurities. A summary of the mechanisms proposed for these conversions is provided in Figure S1 (Supporting Information). Ultimately, most attempts to use these compounds for metal ALD have resulted in the growth of poor-quality films.29,44 On the basis of the lessons learned from those studies, a fourth-generation precursor with no β hydrogens was synthesized, Cu(I)-2-(tert-butylimino)-5,5-dimethyl-pyrrolidinate (4). The H2 TPD of this compound adsorbed on Ni(110), also displayed in Figure 1 (top trace), indicates that decomposition in this case is delayed by approximately 200 K, as the onset of hydrogen evolution occurs at ∼450 K. Hydrogen is then detected in a range of temperatures between 450 and approximately 700 K, pointing to a complex and stepwise decomposition mechanism that we will discuss in more detail below (H2 evolution here is reaction-limited, as desorption of hydrogen from H2 dosed on Ni(110) takes place at around 350 K).45 The H2 TPD trace obtained from this new compound (4) is clearly distinct from those acquired with the other amidinate precursors, all of which show a first peak at about 300 K associated with β-hydride elimination steps (and possibly decomposition of the central methyl moiety). Another prominent feature is seen in the H2 TPD traces from adsorbed (1), (2), or (3) at temperatures slightly below 500 K, typically identified with the full dehydrogenation of adsorbed olefins resulting from the fragmentation of the end groups of the amidinate ligands (they are accompanied by molecular desorption of butene, propene, and ethene with (1), (2), and (3), respectively), and additional H2 evolution is also detected at intermediate temperatures, possibly associated with the dehydrogenation of amido or imido surface intermediates. None of this stepwise chemistry is observed with (4), at least not until reaching temperatures in excess of ∼500 K. An additional aspect of the surface chemistry of amidinates and related metalorganic compounds worth mentioning here is that they mostly exist as either dimers or tetramers, not as monomers as often thought of in connection with ALD processes, in the solid phase.19,22,26,39,40 Moreover, a recent work from our laboratory has shown that these oligomeric structures are preserved during the evaporation of the precursors into the gas phase.38 Given the added steric

2. EXPERIMENTAL SECTION All four copper ALD precursors used in this study were synthesized by reaction of the corresponding ligands with an alkyl lithium compound and copper chloride, as described in detail in previous publications;20,24,39,40 line drawings of their structures are provided in Figure S0 (Supporting Information). The temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) experiments were performed in a ultrahigh vacuum apparatus, also described in previous publications.41−43 The Ni(110) surface was cleaned in situ before each experiment by a combination of chemical (O2 and H2 treatments) and physical (sputtering−annealing cycles) procedures until the surface was deemed cleaned by XPS and CO or H2 TPD.36 The SiO2 films were deposited fresh on a Ta substrate each time by evaporating silicon under an O2 environment, as described in more detail elsewhere.30

3. RESULTS AND DISCUSSION The history of this project and the lessons learned from it are briefly summarized by the data shown in Figure 1, which displays the evolution of hydrogen from nickel surfaces (from Ni(110) single-crystal facets) dosed with the four different copper ALD precursors that have been generated in the four rounds of our work, measured in temperature-programmed desorption (TPD) experiments. Here, we use the detection of H2 as a proxy for reactivity, as such desorption is a clear indication of the occurrence of undesirable ligand dehydrogenation reactions. It can be seen that with the first three compounds (the bottom three traces in Figure 1), namely, with Cu(I)-N,N′-di-sec-butyl-acetamidinate (1),36 Cu(I)-N,Ndimethyl-N′,N″-di-iso-propyl-guanidinate (2),24 and Cu(I)-Nsec-butyl-2-iminopyrrolidinate (3),24 hydrogen starts to evolve from the surface at temperatures as low as 250 K. A more detailed study combining data from TPD and XPS experiments afforded the identification of the C−N bonds as the weak links where the ligand decomposition on the Ni surface starts, typically via a β-hydride elimination from the carbon atom B

DOI: 10.1021/acs.chemmater.8b05065 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

the ligands are removed from the surface (the organic adsorbates mask some of the XPS and AES signals at low temperatures). The evolution of the H2 yield, obtained by integration of the TPD trace and included in the right panel of Figure 2 for comparison, traces closely the reduction of Cu(I) to Cu(0), especially considering that, as a kinetic experiment, the H2 TPD signal is expected to lag the equilibrium population of the resulting surface species. This indicates that the decomposition of the ligands is rate-limiting in terms of the reactions leading to the direct deposition of copper on the nickel substrate. Presumably, the ligands, or the fragments resulting from their thermal activation, migrate to the Ni substrate as the Cu(I) cations bond to the surface and are reduced to their metallic state. The surface chemistry that follows can be inferred, at least in part, from the information accrued from a more extensive TPD study of this system. The traces for some key fragments are reported in Figure 3, and a more complete

hindrance and the potential blocking of access to the metal cations resulting from the new arrangement of the ligands, it is not straightforward to envision the way in which these precursors are activated on solid surfaces; this is the subject of the ongoing research in our laboratory.11,38,46 Our most recent work suggests that, on metals, they may adsorb as dimers but follow surface chemistry similar to what would be expected for monomers.46 For the sake of simplicity, our discussion in this article starts from the monomeric structure adsorbed on the Ni(100) substrate. Nevertheless, it is conceivable that, alternatively, coordination of amidinate ALD precursors to solid surfaces may initially take place via atoms within the ligand (most likely one of the nitrogens in the amidinate, as calculated for adsorption on silicon oxide),38 and that ligand decomposition may be required to release the metal cations and afford direct coordination to the surface. One straightforward way to evaluate this step is by following the ensuing reduction of the metal cations to their final metallic state using X-ray photoelectron spectroscopy (XPS). The appropriate data for the thermal conversion of (4) on Ni(110) are reported in Figure 2. There, both Cu 2p3/2 XPS

Figure 2. Cu 2p3/2 XPS (left) and Cu L3VV AES (center) data from a saturated monolayer of Cu(I)-2-(tert-butylimino)-5,5-dimethyl-pyrrolidinate (4) adsorbed at room temperature on the Ni(110) surface versus the temperatures used in subsequent annealing treatments. Right: Summary of the quantitation of the XPS data in terms of Cu(0) and Cu(I) apparent surface coverages versus annealing temperature. Also shown is the progress of the yield of desorbing H2 from TPD experiments as a function of temperature. This clearly traces the reduction of copper cations in the ALD precursor, suggesting that ligand removal is the rate-limiting step for the coordination and reduction of the copper atoms on the surface.

Figure 3. TPD traces for selected masses from a layer (20 L) of (4) adsorbed on Ni(110) at 100 K. The several peaks seen in these spectra (and those in Figure S2) indicate desorption of a number of gas-phase products, including H2, HCN, N2, iso-butene, pentene, 5,5dimethyl-pyrroline, and an iminopyrrolidine dimer.

set of data is provided in Figure S2 (Supporting Information). In spite of the high level of noise in the data (mainly due to the low yields of the desorbing products), clear desorption signals are observed for many fragments, an indication of multistep decomposition chemistry. In particular, the broad peak for the 27 (and 26) amu fragment at around 615 K corresponds to HCN, the feature at 830 K in the 14 amu trace (also seen in the 28 amu trace) to N2, the signal at 550 K in the 230 amu data (also seen in many other traces, especially those for 181, 191, 207, 222, 230, 263, 181, and 334 amu) to an iminopyrrolidine dimer (from coupling of the monomers via their nitrogen atom originally bonded to Cu), and the peak in the 56 amu trace, also at 550 K, to iso-butene. A number of additional hydrocarbons are inferred from the weak signals seen in the traces for other masses as well. We speculate that the early (∼500 K) signal seen in the 55 and 56 amu traces may originate from pentenes, and that desorption at 550 K in the 67 amu spectra is possibly from 5,5-dimethyl-pyrroline. Finally, a small amount of butane may be produced at 600 K (43 amu). These all can be seen as fragments resulting from the scission of key bonds in the original iminopyrrolidinate

(left panel) and Cu L3VV Auger electron spectroscopy (AES, center) results are shown as a function of annealing temperature after monolayer saturation at 300 K, together with a summary of the estimated relative coverages of Cu(0) and Cu(I) on the surface versus annealing temperature (from the Cu 2p3/2 XPS data; right panel). Clear shifts in both XPS and AES peaks are seen, from a binding energy (BE) of 933.5 to a value of 932.2 eV and a kinetic energy (KE) of 933.5 to a new KE of 932.2 eV, respectively, at around 500 K, signifying the reduction of copper ions at this temperature (the peaks resulting from deconvolution of the XPS data reported in the left panel of Figure 2 may suggest minor decomposition at lower temperatures, but that is an artifact from the deconvolution of the overlapping XPS peaks; the AES data show a clear transition at 500 K). There is also an apparent increase in total copper coverage, but this is only a reflection of the exposure of these atoms to the solid−vacuum interface as C

DOI: 10.1021/acs.chemmater.8b05065 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

so far. This figure displays C 1s (left panel) and N 1s (center) XPS traces obtained after annealing a saturated monolayer of (4) adsorbed on Ni(110) to selected temperatures (together with the peaks obtained from data deconvolution) as well as a summary of the XPS peak areas in terms of surface coverages, normalized to the approximately half-a-monolayer coverage established for Cu in Figure 2 (all expressed in precursor monomer monolayer equivalents, based on a value of ∼0.5 ML at saturation). The molecular contribution to these XPS signals was extracted from fitting three (BE = 284.9, 285.9, and 266.9 eV) and two (BE = 398.4 and 399.9 eV) peaks to the C 1s and N 1s traces of condensed (4), respectively, with fixed 7:2:1 and 3:1 area ratios, and the remaining signal was then analyzed for the identification of other surface species. Additional peaks at BE = 283.6 and 284.6 eV were seen in the C 1s XPS traces starting at 450 K, which we roughly associate with imino/ unsaturated and alkyl carbons, respectively, although the former is likely to also reflect direct bonding to the Ni surface: the red shift in BE may be due to an increase in electron density because metallic Ni is more electron-rich than the Cu(I) cation of the metalorganic complex. In the case of the N 1s XPS data, a new peak also develops starting at 450 K, at BE = 397.2 eV, a value consistent with imino- or nitrile-type bonds on the surface. All of these new XPS features persist until 800 K. Moreover, the two C 1s peaks trace each other closely in terms of their intensities, suggesting that their signals come from species resulting from a single event, possibly the breaking of the iminopyrrolidine C−N bond. On the other hand, the signal of the imino N 1s XPS peak drops in relative terms (compared to that of the C 1s features) at around 550 K, possibly as HCN desorbs from the surface. It is quite likely that the C 1s XPS peaks in the high-temperature region correspond to more than one surface species, perhaps to both nitrile and olefin surface intermediates. As with the TPD data, the corresponding C 1s and N 1s XPS data for the protonated ligand (5), reported in Figure S4 (Supporting Information), display many similarities to those for (4). A comparative summary of the XPS area results is provided in Figure S5 (Supporting Information). The main differences between the two cases, for (4) versus (5), are seen at low temperatures, as molecular desorption of the heavier Cu complex takes place at higher temperatures and may involve a monomer−dimer interconversion. This low-temperature chemistry, also evidenced in the TPD data, is not the focus of our present report, and will be discussed in more detail in a future publication. What is relevant to our discussion is the detection of alkyl and imino carbon atoms and amino and imino nitrogen atoms. The carbon XPS peaks show similar behavior with (4) versus (5) after annealing at 450 K or above, but both alkyl and imino carbons are seen with (5) at temperatures as low as 300 K. This means that the iminopyrrolidinate ligand can be easily activated on the surface at relatively low temperatures but is prevented from doing so with (4) because of its coordination to the Cu cation. Similar trends to those reported for the C 1s XPS are observed in the N 1s XPS data, but an additional new peak was also detected with (5) at BE = 398.3 eV starting at 300 K that we ascribe to amino nitrogen atoms. The route that produces such species does not seem to be available to (4), again because in that case the ligand is coordinated to Cu at these low temperatures. It is also worth pointing out that the C 1s and N 1s XPS peak intensities approach values close to zero with both (4) and (5) after annealing at above 800 K. It is tempting to make the

ligand, specifically at the iminopyrrolidine C−N bond, at the amino-tert-butyl C−N bond, and at the C−N and C2−C3 bonds of the pyrroline ring. As mentioned above, the coincidence of the onsets for H2 evolution and Cu(I) reduction reported in Figure 2 suggests that activation of (4) starts with the displacement of the organic ligands from the adsorbed metalorganic unit to the Ni surface, and that their thermal decomposition takes place afterward on that metal. To further check on this interpretation of the data, comparative TPD experiments were carried out with the protonated ligand, 2-(tert-butylimino)-5,5,-dimethylpyrrolidine (5). TPD traces from (5) adsorbed on Ni(110) for some key masses are reported in Figure S3 (Supporting Information), together with the corresponding data for (4) for comparison. Many similarities can be identified between the TPD data of the two compounds, especially above 300 K (after molecular desorption), but some differences are also apparent. Desorption of H2, HCN, N2, and iso-butene all is seen in the 2, 27, 28, and 56 amu traces, respectively, in both cases, although the yields are lower and the desorption takes place at slightly lower temperatures in the case of the free protonated ligand (the desorption maxima are seen at 570 vs 580 K for H2, at 580 vs 615 K for HCN, at 800 vs 830 K for N2, and at 500 vs 550 K for iso-butene). These differences may be accounted for by differences in the coverage of the 2-(tert-butylimino)-5,5dimethyl-pyrrolidinate species on the surface, likely because the protonated ligand is stable and mostly desorbs molecularly, and/or by the presence of coadsorbed Cu atoms in the experiments with (4). Desorption of 5,5-dimethyl-pyrrolidine is also clearly captured in both cases by the peak at 550 K in the 67 amu traces. On the other hand, no evidence for ligand dimer formation was observed with (5), as it was with (4), again possibly because of the lower surface coverage associated with the former case. Overall, it can be said that (4) and (5) appear to follow the same thermal chemistry on Ni(110), a result supportive of the idea that the decomposition of the ligands in (4) takes place on the Ni surface rather than on the Cu complex. The complementary XPS data reported in Figure 4 lends further support to the general features of the thermal chemistry of (4) on Ni(110) that have been identified in our discussion

Figure 4. C 1s (left panel) and N 1s (center) XPS traces for (4) adsorbed on Ni(110) after annealing to the indicated temperatures. The peaks obtained from deconvolution of the data are also reported (thin lines). Right: Summary of the XPS peak intensities versus annealing temperature. Besides a decrease of the signal associated with (4) as it desorbs from the surface, additional features were identified for alkyl and unsaturated carbons and for imino- or nitriletype nitrogens. D

DOI: 10.1021/acs.chemmater.8b05065 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

nature of the substrate where the film growth is performed. We have chosen nickel as a representative of the metals used in many applications: cobalt47 and ruthenium48,49 have been suggested as potential diffusion barriers for copper, for instance, and we have previously reported that the chemistry of these precursors on Ni and Co is similar.36 We have also shown that the mechanism for the thermal decomposition of other copper amidinates on copper substrates shares many common features with that on nickel substrates, even if activations starts at higher temperatures on the former.37,50 This is relevant because copper films end up growing on copper surfaces after several ALD cycles regardless of what the initial substrate is. We contend that the difference in reactivity seen between our new (4) precursor and the other copper amidinates is also likely to exist on copper substrates. More challenging is to extrapolate the thermal chemistry seen on metals to other types of surfaces, oxides or semiconductors, for instance. We have, for instance, reported a significant lower degree of decomposition of (3) on SiO2 films (compared to that on either nickel or copper substrates), 30,50,51 and a much simpler decomposition mechanism has been proposed on the same oxide for (1) on the basis of infrared absorption spectroscopy studies.29,52 In Figure 6, we report results from initial experiments designed to

argument that this is indicative of the cleaning of the surfaces at these high temperatures, but in fact may be more likely a reflection of the diffusion of atomic carbon (and nitrogen) into the bulk of the nickel crystal. In fact, similar behavior was seen with (3).24 The fact is that it is not desirable to ever reach such high temperatures during ALD processes anyway; ligand removal should be achieved via reactions with the second ALD agent instead. On the basis of the TPD and XPS data reported above, we have constructed a scheme highlighting the main features of the thermal chemistry of (4) on Ni(110) surfaces (Figure 5).

Figure 5. Proposed reaction mechanism for the thermal decomposition of (4) on Ni surfaces.

The focus in this scheme is on the decomposition of the organic ligands after molecular desorption, that is, above room temperature; the starting point is the molecular monomer adsorbed on the Ni(110) surface. Evidence for the start of decomposition chemistry at 450 K is indicated by the onset of the desorption of molecular hydrogen in the TPD spectra and by the reduction of the Cu(I) cations to metallic Cu(0) seen in the Cu 2p3/2 XPS and Cu L3VV AES data. Several pieces of evidence were presented to support the idea that whereas some of the iminopyrrolidinate ligands dimerize and desorb at 550 K, the majority of these are displaced from the original metalorganic compound to the Ni surface. The first step of the iminopyrrolidinate decomposition is likely the breaking of the iminopyrrolidine C−N bond on that metal to create tert-butylimido and 5,5-dimethyl-pyrroline adsorbed species (or similar moieties). These intermediates then appear to undergo further decomposition soon thereafter; the tert-butyl imido fragments seems to undergo clean C−N bond scission and dehydrogenation steps to produce iso-butene, whereas the pyrroline may break into two major fragments, yielding HCN and pentene, respectively. Complete dehydrogenation of the adsorbed surface species is over by about 700 K, and any atomic nitrogen remaining on the surface recombines at around 830 K and desorbs as N2. All of this chemistry takes place in a relatively narrow range of temperatures, at least in comparison to what is seen with other amidinate precursors ((1), (2), or (3)). So far, we have discussed the thermal chemistry of copper amidinate ALD precursors on nickel surfaces. The question arises as to how general our conclusions may be in terms of the

Figure 6. H2 TPD traces from the thermal decomposition of (3) and (4) on SiO2 thin films.

probe the differences in thermal chemistry of (4) on SiO2 compared to that of the other copper amidinate precursors. Like in Figure 1, we have here chosen to follow the evolution of H2 as a reflection of the fragmentation of the ligands on the substrate. It can be clearly seen that (4) is much less prone to decomposition than (3) on SiO2; the threshold for H2 evolution shifts by about 100 K, from ∼500 K with (3) to ∼600 K with (4). It appears that (4) is more stable toward thermal decomposition when adsorbed on solid surfaces regardless of the nature of the substrate. We also believe that our conclusion is general and applicable to many other systems. In addition, we argue that it should be straightforward to extend our approach of designing stable ligands for ALD precursors based on surface science mechanistic studies to the film growth of other elements. E

DOI: 10.1021/acs.chemmater.8b05065 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials ORCID

4. CONCLUSIONS A synergistic study involving the synthesis and surface science mechanistic work led, after four iterations, to the design of a thermally stable copper amidinate precursor, Cu(I)-2-(tertbutylimino)-5,5-dimethyl-pyrrolidinate (4). Previous research on earlier complexes pointed to the most reactive position in these compounds being the C−N bonds, especially when accompanied by hydrogen atoms at the β-C positions. Accordingly, all β-hydrogen atoms were substituted to block that decomposition pathway. The result was an increase of about 200 K in the onset of the dehydrogenation pathways on nickel surfaces, as indicated by H2 TPD data (Figure 1). Decomposition of (4) on Ni(110) starts at 450 K and is immediately accompanied by the reduction of the Cu cation to a metallic state (Figure 2), presumably because of a transfer of the ligands to the Ni surface. After an early desorption of some of the ligands as dimers, fragmentation of the remaining adsorbed species takes place and yields a number of desorbing products, including HCN, N2, iso-butene, pentene, and dimethyl-pyrroline (Figure 3). C 1s and N 1s XPS data afforded the identification of additional surface intermediates, products of selective bond fragmentation (Figure 4). TPD and XPS data from studies of the thermal chemistry of the protonated iminopyrrolidinate ligand (5) on Ni(110) displayed many similar features to those obtained with (4), supporting the idea that the decomposition chemistry takes place on the Ni surface. All of this information was used to advance the basic features of a reaction mechanism for the surface chemistry of (4) on Ni (Figure 5). Additional TPD studies indicated that the added stability of the new ALD precursor also delays its thermal decomposition on SiO2 (Figure 6), indicating that the conclusions derived from our studies on the added stability of the new copper iminopyrrolidinate ALD precursor may be general regardless of the nature of the substrate where the films are grown. Amidinate ligands are common and can be used to make viable ALD precursors for many metals. Therefore, we believe that the conclusions derived here in terms of the need to block their β-H positions to increase their thermal stability on metal surfaces may be general and can be extended to many other cases. More broadly, we illustrate here the usefulness of studying and understanding the surface chemistry of ALD precursors to their design. Ours is, we believe, a rare case of a successful implementation of synthetic strategies in ALD based on surface chemistry research.

■

Seán T. Barry: 0000-0001-5515-4734 Francisco Zaera: 0000-0002-0128-7221 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

■

REFERENCES

Financial support for this project was provided by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering (MSE) Division, under award no. DE-SC0001839.

(1) Leskelä, M.; Ritala, M. Atomic Layer Deposition Chemistry: Recent Developments and Future Challenges. Angew. Chem., Int. Ed. 2003, 42, 5548−5554. (2) Zaera, F. Mechanisms of Surface Reactions in Thin Solid Film Chemical Deposition Processes. Coord. Chem. Rev. 2013, 257, 3177− 3191. (3) George, S. M. Atomic Layer Deposition: An Overview. Chem. Rev. 2010, 110, 111−131. (4) 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. (5) Meng, X.; Wang, X.; Geng, D.; Ozgit-Akgun, C.; Schneider, N.; Elam, J. W. Atomic Layer Deposition for Nanomaterial Synthesis and Functionalization in Energy Technology. Mater. Horiz. 2017, 4, 133− 154. (6) Putkonen, M. ALD Applications Beyond Outside IC Technology- Existing and Emerging Possibilities. ECS Trans. 2009, 25, 143−155. (7) Leskelä, M.; Niinistö, J.; Ritala, M. 4.05 - Atomic Layer Deposition. In Comprehensive Materials Processing; Hashmi, S., Batalha, G. F., Van Tyne, C. J., Yilbas, B., Eds.; Elsevier: Oxford, 2014; Vol. 4; pp 101−123. (8) Tullo, A. H.; McCoy, M.; Tremblay, J.-F. Electronic Chemical Makers Help Keep Gordon Moore’s Prediction Alive: Materials for Photolithography, Atomic Layer Deposition, and Chemical Mechanical Planarization Aid Semiconductor Miniaturization. Chem. Eng. News 2015, 93, 31−37. (9) Zaera, F. The Surface Chemistry of Atomic Layer Depositions of Solid Thin Films. J. Phys. Chem. Lett. 2012, 3, 1301−1309. (10) Knapas, K.; Ritala, M. In Situ Studies on Reaction Mechanisms in Atomic Layer Deposition. Crit. Rev. Solid State Mater. Sci. 2013, 38, 167−202. (11) Barry, S. T.; Teplyakov, A. V.; Zaera, F. The Chemistry of Inorganic Precursors During the Chemical Deposition of Films on Solid Surfaces. Acc. Chem. Res. 2018, 51, 800−809. (12) Koponen, S. E.; Gordon, P. G.; Barry, S. T. Principles of Precursor Design for Vapour Deposition Methods. Polyhedron 2016, 108, 59−66. (13) 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. (14) Hämäläinen, J.; Ritala, M.; Leskelä, M. Atomic Layer Deposition of Noble Metals and Their Oxides. Chem. Mater. 2014, 26, 786−801. (15) Gordon, R. G. ALD Precursors and Reaction Mechanisms. In Atomic Layer Deposition for Semiconductors; Hwang, S. C., Ed.; Springer: Boston, MA, 2014; pp 15−46. (16) Putkonen, M.; Niinistö, L. Organometallic Precursors for Atomic Layer Deposition. Top. Organomet. Chem. 2005, 9, 125−145. (17) 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.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b05065. Line drawing of the four precursors; proposed thermal decomposition mechanisms for (1), (2), and (3) on Ni(110); TPD traces; C 1s and N 1s XPS data for (4) and (5); and summary of the N 1s and C 1s XPS peak intensities for (4) and (5) adsorbed on Ni(110) (PDF)

■

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. F

DOI: 10.1021/acs.chemmater.8b05065 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials (18) Hatanpäa,̈ T.; Ritala, M.; Leskelä, M. Precursors as Enablers of ALD Technology: Contributions from University of Helsinki. Coord. Chem. Rev. 2013, 257, 3297−3322. (19) Edelmann, F. T. Chapter 3: Advances in the Coordination Chemistry of Amidinate and Guanidinate Ligands. In Advances in Organometallic Chemistry; Anthony, F. H., Mark, J. F., Eds.; Academic Press: Amsterdam, 2008; Vol. 57; pp 183−352. (20) Coyle, J. P.; Monillas, W. H.; Yap, G. P. A.; Barry, S. T. Synthesis and Thermal Chemistry of Copper (I) Guanidinates. Inorg. Chem. 2008, 47, 683−689. (21) Willcocks, A. M.; Robinson, T. P.; Roche, C.; Pugh, T.; Richards, S. P.; Kingsley, A. J.; Lowe, J. P.; Johnson, A. L. Multinuclear Copper(I) Guanidinate Complexes. Inorg. Chem. 2012, 51, 246−257. (22) Whitehorne, T. J. J.; Coyle, J. P.; Mahmood, A.; Monillas, W. H.; Yap, G. P. A.; Barry, S. T. Group 11 Amidinates and Guanidinates: Potential Precursors for Vapour Deposition. Eur. J. Inorg. Chem. 2011, 2011, 3240−3247. (23) Barry, S. T. Amidinates, Guanidinates and Iminopyrrolidinates: Understanding Precursor Thermolysis to Design a Better Ligand. Coord. Chem. Rev. 2013, 257, 3192−3201. (24) Kim, T.; Yao, Y.; Coyle, J. P.; Barry, S. T.; Zaera, F. Thermal Chemistry of Cu(I)-Iminopyrrolidinate and Cu(I)-Guanidinate Atomic Layer Deposition (ALD) Precursors on Ni(110) SingleCrystal Surfaces. Chem. Mater. 2013, 25, 3630−3639. (25) Sadique, A. R.; Heeg, M. J.; Winter, C. H. Monomeric and Dimeric Amidinate Complexes of Magnesium. Inorg. Chem. 2001, 40, 6349−6355. (26) Lim, B. S.; Rahtu, A.; Gordon, R. G. Atomic Layer Deposition of Transition Metals. Nat. Mater. 2003, 2, 749−754. (27) Lim, B. S.; Rahtu, A.; Park, J.-S.; Gordon, R. G. Synthesis and Characterization of Volatile, Thermally Stable, Reactive Transition Metal Amidinates. Inorg. Chem. 2003, 42, 7951−7958. (28) Edelmann, F. T. Chapter Two - Recent Progress in the Chemistry of Metal Amidinates and Guanidinates: Syntheses, Catalysis and Materials. In Advances in Organometallic Chemistry; Anthony, F. H., Mark, J. F., Eds.; Academic Press: Amsterdam, 2013; Vol. 61; pp 55−374. (29) 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. (30) Yao, Y.; Zaera, F. Thermal Chemistry of Copper Acetamidinate Atomic Layer Deposition Precursors on Silicon Oxide Surfaces Studied by XPS. J. Vac. Sci. Technol., A 2016, 34, No. 01A101. (31) Elko-Hansen, T. D. M.; Ekerdt, J. G. XPS Investigation of the Atomic Layer Deposition Half Reactions of Bis(N-tert-Butyl-N′Ethylpropionamidinato) Cobalt(II). Chem. Mater. 2014, 26, 2642− 2646. (32) Guo, Z.; Li, H.; Chen, Q.; Sang, L.; Yang, L.; Liu, Z.; Wang, X. Low-Temperature Atomic Layer Deposition of High Purity, Smooth, Low Resistivity Copper Films by Using Amidinate Precursor and Hydrogen Plasma. Chem. Mater. 2015, 27, 5988−5996. (33) Gordon, P. G.; Kurek, A.; Barry, S. T. Trends in Copper Precursor Development for CVD and ALD Applications. ECS J. Solid State Sci. Technol. 2015, 4, N3188−N3197. (34) 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. (35) Ma, Q.; Guo, H.; Gordon, R. G.; Zaera, F. Uptake of Copper Acetamidinate ALD Precursors on Nickel Surfaces. Chem. Mater. 2010, 22, 352−359. (36) Ma, Q.; Guo, H.; Gordon, R. G.; Zaera, F. Surface Chemistry of Copper(I) Acetamidinates in Connection with Atomic Layer Deposition (ALD) Processes. Chem. Mater. 2011, 23, 3325−3334. (37) Ma, Q.; Zaera, F.; Gordon, R. G. Thermal Chemistry of Copper(I)-N,N′-Di-sec-Butylacetamidinate on Cu(110) Single-Crystal Surfaces. J. Vac. Sci. Technol., A 2012, 30, No. 01A114. (38) Chen, B.; Duan, Y.; Yao, Y.; Ma, Q.; Coyle, J. P.; Barry, S. T.; Teplyakov, A. V.; Zaera, F. Activation of the Dimers and Tetramers of

Metal Amidinate Atomic Layer Deposition Precursors Upon Adsorption on Silicon Oxide Surfaces. J. Vac. Sci. Technol., A 2017, 35, No. 01B124. (39) Coyle, J. P.; Pallister, P. J.; Kurek, A.; Sirianni, E. R.; Yap, G. P. A.; Barry, S. T. Copper Iminopyrrolidinates: A Study of Thermal and Surface Chemistry. Inorg. Chem. 2013, 52, 910−917. (40) Li, Z.; Barry, S. T.; Gordon, R. G. Synthesis and Characterization of Copper(I) Amidinates as Precursors for Atomic Layer Deposition (ALD) of Copper Metal. Inorg. Chem. 2005, 44, 1728− 1735. (41) Zaera, F. A Thermal Desorption and X-Ray Photoelectron Spectroscopy Study of the Surface Chemistry of Iron Pentacarbonyl. J. Vac. Sci. Technol., A 1989, 7, 640−645. (42) Kim, T.; Zaera, F. X-Ray-Initiated Metal-Promoted Thin Film Growth. J. Phys. Chem. C 2012, 116, 8594−8600. (43) Yao, Y.; Guerrero-Sánchez, J.; Takeuchi, N.; Zaera, F. Coadsorption of Formic Acid and Hydrazine on Cu(110) SingleCrystal Surfaces. J. Phys. Chem. C 2018. (44) Wang, H.; Gordon, R. G.; Alvis, R.; Ulfig, R. M. Atomic Layer Deposition of Ruthenium Thin Films from an Amidinate Precursor. Chem. Vap. Deposition 2009, 15, 312−319. (45) Christmann, K.; Schober, O.; Ertl, G.; Neumann, M. Adsorption of Hydrogen on Nickel Single Crystal Surfaces. J. Chem. Phys. 1974, 60, 4528−4540. (46) Guerrero-Sánchez, J.; Takeuchi, N.; Zaera, F. DFT Study of the Surface Adsorption and Dissociation of Copper(I) Acetamidinates on Cu(110) Surfaces. J. Phys. Chem. C 2019, 123, 4341−4348. (47) O’Sullivan, E. J.; Schrott, A. G.; Paunovic, M.; Sambucetti, C. J.; Marino, J. R.; Bailey, P. J.; Kaja, S.; Semkow, K. W. Electrolessly Deposited Diffusion Barriers for Microelectronics. IBM J. Res. Dev. 1998, 42, 607−620. (48) Li, H.; Farmer, D. B.; Gordon, R. G.; Lin, Y.; Vlassak, J. Vapor Deposition of Ruthenium from an Amidinate Precursor. J. Electrochem. Soc. 2007, 154, D642−D647. (49) Waechtler, T.; Ding, S.-F.; Hofmann, L.; Mothes, R.; Xie, Q.; Oswald, S.; Detavernier, C.; Schulz, S. E.; Qu, X.-P.; Lang, H.; et al. ALD-Grown Seed Layers for Electrochemical Copper Deposition Integrated with Different Diffusion Barrier Systems. Microelectron. Eng. 2011, 88, 684−689. (50) Yao, Y.; Coyle, J. P.; Barry, S. T.; Zaera, F. Effect of the Nature of the Substrate on the Surface Chemistry of Atomic Layer Deposition Precursors. J. Chem. Phys. 2017, 146, No. 052806. (51) Yao, Y.; Coyle, J. P.; Barry, S. T.; Zaera, F. Thermal Decomposition of Copper Iminopyrrolidinate Atomic Layer Deposition (ALD) Precursors on Silicon Oxide Surfaces. J. Phys. Chem. C 2016, 120, 14149−14156. (52) Dai, M.; Kwon, J.; Langereis, E.; Wielunski, L.; Chabal, Y. J.; Li, Z.; Gordon, R. G. In-Situ FTIR Study of Atomic Layer Deposition (ALD) of Copper Metal Films. ECS Trans. 2007, 11, 91−101.

G

DOI: 10.1021/acs.chemmater.8b05065 Chem. Mater. XXXX, XXX, XXX−XXX