Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2018, 9, 4602−4606
Gas-Phase Electron-Impact Activation of Atomic Layer Deposition (ALD) Precursors: MeCpPtMe3 Clinton Lien,† Mahsa Konh,‡ Bo Chen,† Andrew V. Teplyakov,‡ and Francisco Zaera*,† †
Department of Chemistry, University of California, Riverside, California 02521, United States Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
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‡
ABSTRACT: The use of gas-phase electron-impact activation of metalorganic complexes to facilitate atomic layer depositions (ALD) was tested for the case of (methylcyclopentadienyl)Pt(IV) trimethyl (MeCpPtMe3) on silicon oxide films. Uptake enhancements of more than 1 order of magnitude were calculated from Xray photoelectron spectroscopy (XPS) data. On the basis of the measured C:Pt ratios, the surface species were estimated to mainly consist of MeCpPt moieties, likely because of the prevalent formation of [MeCpPtMex−nH]+ ions after gas-phase ionization (as determined by mass spectrometry). Counterintuitively, more extensive adsorption was observed on thick SiO2 films than on the native thin SiO2 film that forms on Si(100) wafers, despite the former having virtually no surface OH groups. The adsorption of MeCpPt fragments on silicon oxide surfaces was determined by density functional theory (DFT) calculations to be highly exothermic and to favor attachment to Si−O−Si bridge sites.
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been seen, for instance, in some electron-31,32 and photon-33,34 induced activation processes of adsorbed species. We have recently reported an alternative where electron excitation is carried out in the gas phase, prior to adsorption on the surface.35 We showed that with (methylcyclopentadienyl)Mn(I) tricarbonyl (MeCpMn(CO)3) the carbonyl groups are easily removed this way, and that the remaining MeCpMn fragment is easily adsorbed and activated cleanly once adsorbed on the substrate of interest. The question remains of how general this approach may be. Here we expand the gasphase electron-impact procedure to the deposition of Pt films using (methylcyclopentadienyl)Pt(IV) trimethyl (MeCpPtMe3). This is a popular but somewhat ineffective precursor for Pt film growth, used mainly because of the absence of better alternatives.26,29,36−40 The involvement of methyl, rather than carbonyl, ligands can potentially open new decomposition channels for the precursor during electronimpact activation detrimental to the cleanliness of the ALD process. Below we show that, fortunately, this is not the case, and that the main adsorbed species resulting from exposure of silicon oxide surfaces to gas-phase electron-activated MeCpPtMe3 appears to be the MeCpPt moiety. The main piece of evidence for such a conclusion is the XPS data reported in Figure 1. There, Pt 4f (left panel) and C 1s (right) X-ray photoelectron spectra (XPS) are reported for silicon oxide films exposed to MeCpPtMe3 as is (ion gauge, IG, the device used for the electron bombardment, off) and after gas-phase electron-impact is activated (IG on). The first
tomic layer deposition (ALD) has become a method of choice for the deposition of thin solid films in many practical applications.1−3 In ALD, the surface chemistry involved is divided into two or more complementary and self-limiting steps in order to attain submonolayer control of the film growth.4,5 Like in other chemical vapor deposition (CVD) methods, the challenge in ALD is to identify the appropriate compound to be used for deposition of the element of interest in a clean way, without promoting undesirable side reactions that may lead to incorporation of impurities in the resulting films.6−9 The chosen ALD precursor is required to fulfill two sometimes contradictory properties, namely, it needs to be stable enough to retain its molecular structure while being vaporized for dosing and at the same time it needs to be reactive enough to dissociatively adsorb and react on solid surfaces in order to trigger the chemistry that produces the desired films.10 The identification of viable ALD precursors is particularly difficult with late transition metals, for which the choices of volatile metalorganic compounds are limited and often involve large ligands.11−13 Ligand options in those cases include, on the one hand, bidentate anions such as diketonates, amidinates, and aminoalkoxides, which have proven quite versatile but also fairly reactive, easily decomposing on the surface once adsorption has taken place,14−23 and on the other, aryl moieties such as benzene and methylcyclopentadienyl, which are fairly sturdy but also hard to activate and/or remove from the surface.24−30 One option with the latter group is to activate the complex externally, photolytically, or via electron excitation, for instance. If this approach is taken, however, consideration must be given to the potential nonselective conversion of the ALD precursor, which could lead to ligand decomposition and to the deposition of impurities. This has © XXXX American Chemical Society
Received: July 6, 2018 Accepted: July 30, 2018
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DOI: 10.1021/acs.jpclett.8b02125 J. Phys. Chem. Lett. 2018, 9, 4602−4606
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The Journal of Physical Chemistry Letters
signal from pure platinum as the standard, a layer-by-layer model, and reported values for electron inelastic mean free paths43 and relative electron energy analyzer sensitivities.44 Enhancement factors of approximately 5 and 12 were seen with the IG on (to promote gas-phase excitation of the Pt ALD precursor) with the thin and thick SiO2 films, respectively. There was also a clear increase in Pt deposition under similar conditions on the thick versus thin SiO2 films, a factor of ∼3.6 with the IG on (∼1.4 with the IG off). This is surprising, because the thick oxide films used here were determined to have a low density of surface silanol (Si−OH) groups, the sites where most ALD precursors adsorb and initiate their surface conversion.45−47 More on this below. Finally, in most cases the atomic C:Pt ratio is close to 6, the value expected if only the MeCpPt moiety remains coordinated to the Pt atoms (the original ALD precursor minus the three methyl groups). The one exception to this stoichiometry is the case of deposition on the thick SiO2 film with the IG on, in which case the C:Pt ratio is only about 3.4. Possibly, some adsorbates in that case lose their MeCp ligand either in the gas phase, prior to bonding to the surface, or upon interaction with it. Regardless, the lower C:Pt ratio obtained in that case speaks to the promise of cleaner Pt films when grown on the thick SiO2 films under IGon conditions. Of course, the ultimate goal is to protonate or oxidize the MeCp− ligand in the second half of the ALD cycle for its clean removal;48 therefore, the main goal here is to not deposit strongly bonded residues that would be difficult to make react with the second ALD agent. The stoichiometry of the adsorbates that result from exposure of the SiO2 surfaces to the electron-impact-activated MeCpPtMe3 precursor may be explained by taking a closer look at that activation process. This can be done by examining the electron-impact mass spectrum of MeCpPtMe3, given that the ionizer in most mass spectrometers has the same design and follow the same physical principle as the nude Bayard− Alpert IG used here to activate the ALD precursors in the gas phase. A mass spectrum of MeCpPtMe3 obtained by us using a quadrupole mass filter is reported in Figure 3. The high-mass end of the data is dominated by ions resulting from the loss of
Figure 1. Pt 4f (left) and C 1s (right) XPS traces from SiO2 films exposed to MeCpPtMe3. Data are provided for four cases: on thin (bottom) and thick (top) films and from experiments carried out with the IG on (solid red traces) and off (dashed blue traces). Turning on the IG, a way to activate the MeCpPtMe3 ALD precursor via electron impact in the gas phase, leads to a significant increase in Pt deposition probability. Also, more extensive deposition is observed on the thick SiO2 film.
observation that becomes clear from these data is that the deposition of Pt (and carbon) is greatly enhanced by turning the IG on. It is also evident that more extensive deposition takes place on the thick (300 nm) SiO2 film than on the thin (∼1 nm) SiO2 film that is native to silicon wafers. The Pt 4f7/2 peaks are centered at binding energy (BE) values of 71.3−71.7 eV, slightly higher than that of Pt(0) (70.9 eV) but significantly lower than what would correspond to oxidized Pt (73.8 eV for PtO).41 One possible interpretation is that the deposited Pt is in a metallic state but that the BE is affected by the size of the Pt nanoparticles that form on the surface, although this is unlikely given that the overall coverage is below one monolayer.42 Alternatively, the adsorbed Pt atoms may have a partial positive charge because of their binding to surface oxygen atoms. The BE for the C 1s XPS peak is 284.6 eV, consistent with it corresponding to the MeCp ligand.41 Figure 2 summarizes the results from a quantitative analysis of the areas of the XPS peaks in Figure 1, using the Pt 4f XPS
Figure 3. Electron-impact mass spectrum of MeCpPtMe3 recorded with a quadrupole mass spectrometer. Because of the low sensitivity of the instrument to high masses, it was necessary to degrade the resolution above 200 amu, but the signal intensities were calibrated to match those for the settings used to acquire the low-amu end. The main observation from these data relevant to our ALD studies is that the majority of the ions detected are produced via the loss of one or more methyl ligands while retaining the MeCpPt moiety intact.
Figure 2. Quantitation of the areas of the XPS spectra in Figure 1 in terms of the surface coverages attained for Pt and C. Again, enhancements in the extent of deposition are seen on the thick (versus thin) SiO2 films and with the IG on. In addition, the C:Pt atomic ratios are 6 or less, suggesting that all of the methyl groups are removed from the original precursor once adsorption takes place on the surface. 4603
DOI: 10.1021/acs.jpclett.8b02125 J. Phys. Chem. Lett. 2018, 9, 4602−4606
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The Journal of Physical Chemistry Letters one or more methyl groups upon ionization. Indeed, the dominant peaks are those for [MeCpPt−nH] + and [MeCpPtMe−nH]+, which together account for almost 90% of all of the ions resulting from methyl moiety losses. Adding the contributions from [MeCpPtMe 2 −nH] + and [MeCpPtMe3−nH]+, it is estimated that ions from methyl losses account for approximately 70% of all of the ions with mass above 200 amu. Another 25% comes from PtMex+ fragments, the product of the removal of the MeCp ligand together with some of the methyl groups; therefore, only 5% originate from further MeCp decomposition. It should be noted that because of the poor sensitivity to high-mass ions of our quadrupole our experiments required changing the sensitivity and resolution of the instrument to detect those, a fact that reduces the accuracy of the yield calculations. Fortunately, a more detailed study reported in the literature corroborates our conclusions: an estimated 91% of all ions (including the low-amu region) were reported there to correspond to [MeCpPtMex−nH]+ and less than 1% to products from MeCp decomposition.49 It should also be mentioned that PtCxHy+ ions, platinum-containing clusters that have lost the MeCp ring, could be alternative sources of Pt during these film depositions. However, given that their fraction in the mass spectra is small, approximately 5% of the total, they are not expected to be major contributors to that process. In the end, the lesson derived from this mass spectroscopy characterization of the gas-phase electron-impact activation of the MeCpPtMe3 ALD precursor is that, like with MeCpMn(CO)3, the resulting ion products are mainly metal centers coordinated to the cyclopentadienyl ligand (with perhaps some additional CH3 or CO ligands, which are likely lost upon adsorption on the surface). Consequently, electron bombardment in the gas phase seems to be a reasonably clean approach for the formation of activated precursors from the original organometallic complexes to facilitate the ALD process. We also believe that our proposed methodology may be fairly general, applicable to many other precursors, at least to those with strongly bonded and stable ligands such as cyclopentadienyls. In any case, the cleanliness of the ionization process with any potential ALD precursor can be easily checked by recording its electron-impact mass spectrum, as illustrated here. Finally, the thermodynamic viability of the adsorption of the MeCpPt fragment on silicon oxide surfaces was probed by quantum mechanical density functional theory (DFT) calculations. The energetics of the relevant adsorption steps is reported in Figure 4. First to notice is the fact that the molecular adsorption of the MeCpPtMe3 precursor on a model cluster system representing a fully oxidized silicon surface with only silanol and Si−O−Si bridge sites exposed is exothermic (ΔEads,molec = −70.5 kJ/mol, left-side reaction) and therefore favorable. Despite the simplicity of the computational model used and the single possible adsorption configuration presented, this computational approach, which includes Grimme’s D3 dispersion corrections,50 yielded results qualitatively consistent with a previous detailed theoretical investigation of the adsorption of the same precursor molecule on silica surfaces.51 The most likely reason for the inefficiency of the regular ALD process with this precursor is the high activation barriers expected toward dissociation of the adsorbed molecule; a recent DFT study found that the removal of the MeCp ring requires approximately 75 kJ/mol.52 Our calculations also show that adsorption of the MeCpPt
Figure 4. DFT calculations of the adsorption energies of the MeCpPtMe3 ALD precursor and the MeCpPt fragment expected from electron-impact activation on a silicon oxide cluster. Stronger adsorption is clearly seen with the latter. Also evident from these calculations is a preference for adsorption on Si−O−Si bridge sites over the terminal silanol groups typically believed to act as nucleation sites in ALD.
fragment is much more exothermic than adsorption of the intact precursor, and that adsorption on bridged Si−O−Si sites is more favorable than bonding on terminal silanol (Si−OH) groups: ΔHads,Si−O−Si = −136.5 kJ/mol (right-side reaction) versus ΔHads,Si−OH = −115.7 kJ/mol (center). The latter observation is somewhat surprising because surface OH groups are usually the preferred nucleation sites in ALD (as mentioned already above), but it does explain why more effective uptake was seen here on the thick SiO2 film than on the native oxide; the former is almost completely devoid of surface silanol groups, as determined by infrared absorption spectroscopy and by hydrophobicity (contact-angle) measurements (data not shown). In conclusion, we have shown that gas-phase electronimpact activation of MeCpPtMe3 affords a more efficient uptake on silicon oxide surfaces, thus facilitating Pt ALD processes. It was also shown that the deposited layer is as clean or cleaner than that obtained without such activation, specifically in terms of its carbon content. We explain this observation in terms of the relatively simple distribution of secondary ions formed in the gas phase, which is dominated by [MeCpPtMex−nH]+ fragments. The fact that the methyl ligands do not seem to interfere in the deposition or contribute to an increase in carbon deposition points to the potential generality of the ion-impact activation process. The next step in this research will be to investigate how the addition of the gas-phase electron-impact activation of MeCpPtMe3 affects the overall ALD process. Such ALD is typically done using oxidizing agents, but it would be interesting to explore if reducing agents such as H2 may become effective as well, once the metal precursor is partially activated and possibly fully demethylated.53
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EXPERIMENTAL METHODS The XPS experiments were performed in a two-tier ultrahigh vacuum (UHV) apparatus described in previous publications.40,54,55 The gas dosing of the MeCpPtMe3 precursor was 4604
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(4) Leskelä, M.; Ritala, M. Atomic Layer Deposition Chemistry: Recent Developments and Future Challenges. Angew. Chem., Int. Ed. 2003, 42, 5548−5554. (5) Zaera, F. Mechanisms of Surface Reactions in Thin Solid Film Chemical Deposition Processes. Coord. Chem. Rev. 2013, 257, 3177− 3191. (6) Zaera, F. The Surface Chemistry of Thin Film Atomic Layer Deposition (ALD) Processes for Electronic Device Manufacturing. J. Mater. Chem. 2008, 18, 3521−3526. (7) Zaera, F. The Surface Chemistry of Atomic Layer Depositions of Solid Thin Films. J. Phys. Chem. Lett. 2012, 3, 1301−1309. (8) Knapas, K.; Ritala, M. In Situ Studies on Reaction Mechanisms in Atomic Layer Deposition. Crit. Rev. Solid State Mater. Sci. 2013, 38, 167−202. (9) 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. (10) Koponen, S. E.; Gordon, P. G.; Barry, S. T. Principles of Precursor Design for Vapour Deposition Methods. Polyhedron 2016, 108, 59−66. (11) 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. (12) Hämäläinen, J.; Ritala, M.; Leskelä, M. Atomic Layer Deposition of Noble Metals and Their Oxides. Chem. Mater. 2014, 26, 786−801. (13) Gordon, R. G. ALD Precursors and Reaction Mechanisms. In Atomic Layer Deposition for Semiconductors; Hwang, S. C., Ed.; Springer US: Boston, MA, 2014; pp 15−46. (14) Kwon, J.; Dai, M.; Halls, M. D.; Langereis, E.; Chabal, Y. J.; Gordon, R. G. In Situ Infrared Characterization During Atomic Layer Deposition of Lanthanum Oxide. J. Phys. Chem. C 2009, 113, 654− 660. (15) 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. (16) Ma, Q.; Zaera, F. Chemistry of Cu(acac)2 on Ni(110) and Cu(110) Surfaces: Implications for Atomic Layer Deposition Processes. J. Vac. Sci. Technol., A 2013, 31, 01A112. (17) 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. (18) Duan, Y.; Gao, F.; Teplyakov, A. V. Role of the Deposition Precursor Molecules in Defining Oxidation State of Deposited Copper in Surface Reduction Reactions on H-Terminated Si(111) Surface. J. Phys. Chem. C 2015, 119, 27018−27027. (19) 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. (20) 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, 01A101. (21) 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. (22) Duan, Y.; Teplyakov, A. V. Deposition of Copper from Cu(I) and Cu(II) Precursors onto HOPG Surface: Role of Surface Defects and Choice of a Precursor. J. Chem. Phys. 2017, 146, 052814. (23) Qin, X.; Zaera, F. Chemistry of Ruthenium Diketonate Atomic Layer Deposition (ALD) Precursors on Metal Surfaces. J. Phys. Chem. C 2018, 122, 13481. (24) Aaltonen, T.; Rahtu, A.; Ritala, M.; Leskelä, M. Reaction Mechanism Studies on Atomic Layer Deposition of Ruthenium and Platinum. Electrochem. Solid-State Lett. 2003, 6, C130−C133. (25) Elam, J. W.; Pellin, M. J.; Elliott, S. D.; Zydor, A.; Faia, M. C.; Hupp, J. T. Mechanism for Zirconium Oxide Atomic Layer
carried out in the auxiliary chamber, used as the ALD reactor, whereas the spectra were acquired in the main volume, using the aluminum anode of a nonmonochromatized dual Mg−Kα/ Al−Kα X-ray excitation source and a Leybold EA11 semispherical electron energy analyzer equipped with multichannel detection. Gas-phase electron-impact excitation of the Pt precursor was done during dosing in the ALD reactor by using a non-line-of-sight nude Bayard−Alpert-type IG operating at an ionization potential of 150 eV. The mass spectrum of the MeCpPtMe3 was acquired using a UTI 100C quadrupole mass spectrometer installed in a second UHV instrument, using an ionization energy of 70 eV. 5 6 , 5 7 The trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPtMe3) precursor was purchased from Sigma-Aldrich (98% purity), purified in situ in our gas manifold via repeated freeze−pump−thaw cycles, and dosed via a leak valve. The thin SiO2 films used here were the native (∼1 nm thick) layers that form on Si(100) samples, which were cut from commercial Si(100) wafers (Si-Tech). The samples with p-type (boron-doped) 300 nm thick SiO2/Si(100) films were obtained from University Wafer. The DFT calculations were carried out using the Gaussian 09 suite of programs.58 They utilized the B3LYP functional59,60 with Grimme’s D3 dispersion corrections50 and the LANL2DZ basis set. A Si9O7H14 cluster model was used to represent a fully oxidized silicon surface with two silanol groups and a bridge Si−O−Si group available for interaction with adsorbed species. Following the optimization of this cluster, the positions of silicon atoms representing the four bottom layers of the silicon substrate were fixed to avoid unrealistic distortions, and then the fragments of interest were added to the models. The values of the adsorption energies were calculated by referencing the absolute energies of the fully optimized surface-bound fragments (or molecules) to the energies of the separate fully optimized Si9O7H14 cluster model and the molecular fragments being considered.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Andrew V. Teplyakov: 0000-0002-6646-3310 Francisco Zaera: 0000-0002-0128-7221 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support for this project was provided by a grant from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering (MSE) Division, under Award No. DE-FG02-03ER46599. The work in A.V.T.’s group at the University of Delaware was partially supported by the National Science Foundation (DMR1609973 (GOALI)).
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The Journal of Physical Chemistry Letters
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DOI: 10.1021/acs.jpclett.8b02125 J. Phys. Chem. Lett. 2018, 9, 4602−4606
