Communication pubs.acs.org/cm
Thermal Atomic Layer Deposition of Titanium Films Using Titanium Tetrachloride and 2‑Methyl-1,4-bis(trimethylsilyl)-2,5-cyclohexadiene or 1,4-Bis(trimethylsilyl)-1,4-dihydropyrazine Joseph P. Klesko,† Christopher M. Thrush,‡ and Charles H. Winter*,† †
Department of Chemistry and ‡Department of Electrical and Computer Engineering, Wayne State University, Detroit, Michigan 48202, United States S Supporting Information *
he growth of electropositive metal films from chemical precursors is extremely challenging due to the difficult reductions of the precursor metal ions and the lack of powerful reducing coreagents that can transform the metal ions to the metals rapidly.1−5 Titanium is a typical electropositive metal and has a very negative electrochemical potential (Ti(II) ↔ Ti(O), E0 = −1.631 V6). Metallic titanium films have numerous applications in microelectronic devices, including recent reports of titanium/aluminum alloy liners for advanced transistor structures.7−9 Due to the very small feature sizes and threedimensional architectures of future transistors, titanium metal must be deposited by atomic layer deposition (ALD) to meet thickness uniformity and conformality requirements.7−9 ALD is a film growth technique that relies upon complementary, selflimiting reactions between two chemical precursors to afford the desired material through a layer-by-layer growth approach.10−12 ALD affords inherently conformal coating of shaped substrates and enables subnanometer control of film thicknesses because of the self-limiting growth mechanism. There have been few reports of metallic titanium film growth from chemical precursors. The chemical vapor deposition (CVD) of titanium/aluminum alloy films was demonstrated from TiCl4 and AlH3(NMe3) at temperatures of 65−127 °C, but the films were alloyed with aluminum due to competitive thermal decomposition of AlH3(NMe3) to aluminum metal.13 An alternate CVD approach to titanium films used TiCl4 and sodium vapor, which afforded films that were contaminated with NaCl.14 The only reported ALD growth of titanium films employed TiCl4 and H2 plasma between 20 and 200 °C.15,16 Thermal ALD is generally preferred over plasma ALD, since the latter can afford substrate damage from highly reactive species and can also result in poor conformal coverage of high aspect ratio features due to surface recombination of H atoms.1,17 In a series of recent papers, Mashima and co-workers have described the use of 1,4-bis(trimethylsilyl)-2-methyl-2,5-cyclohexadiene (1, Chart 1) and 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (2, Chart 1) and as salt-free reagents for the reduction
T
of early transition metal halide complexes from the highest oxidation states to lower oxidation states.18−21 Compound 1 was also used to reduce TiCl4 to surface-bound TiCl2.22 Herein, we describe the low temperature ALD growth of metallic titanium films using TiCl4 and 1 and 2 as precursors. Compounds 123 and 224,25 were prepared by literature procedures. The boiling point of 1 is 70−72 °C at 0.12 Torr,23 whereas 2 melts at 63 °C and evaporates without residue at about 80 °C at 0.05 Torr. The decomposition points of 1 and 2 are >300 and >265 °C,26 respectively, and thus have sufficient volatility and thermal stability to serve as ALD precursors.10−12 Mashima demonstrated that 1 and 2 react with metal halides with elimination of ClSiMe3,18−21 so it is likely that intermediate 2-methyl 1,4-cyclohexadienyl or 1,4-dihydropyrazinyl complexes are formed, which then eliminate aromatic toluene or pyrazine to reduce the metal center by two electrons per ligand. Accordingly, the electrochemical reduction potentials of toluene or pyrazine should give an estimate of the reducing power of 1 and 2. The cyclic voltammagram of toluene and pyrazine in DMF containing 0.1 M NBu4ClO4 showed reductions at E1/2 = −2.46 and −2.32 V, respectively, versus Cp2Fe.26 Treatment of TiCl4 with 1 or 2 (2 equiv) in toluene at 23 °C led to the instant formation of black precipitates. While these precipitates were too reactive to identify, the high reactivity of TiCl4 toward 1 and 2 is appropriate for ALD.1,5,10−12 Encouraged by the reducing capability of 1 and 2 and the solution reactions with TiCl4, film depositions using TiCl4 and 1 were explored. Growth conditions, full ALD plots, and film characterization data are contained in the Supporting Information.26 Precursor pulse lengths, substrate temperatures, and the number of cycles were varied to assess the growth behavior. The growth rate was probed as a function of the pulse lengths of TiCl4 and 1 at 180 °C, using 3000 cycles and 5.0 and 10.0 s N2 purges after the pulses of TiCl4 and 1, respectively. Similar growth rates were observed on Si(100) with native oxide, thermal SiO2 (100 nm)/Si, Pt (15 nm)/SiO2 (100 nm)/ Si, and Ru (10 nm)/Si. The studies described herein were carried out on Si(100) with native oxide and thermal SiO2 (100 nm)/Si substrates. A plot of growth rate versus TiCl4 pulse length showed a growth plateau at ≥0.2 s pulse lengths, with a saturative growth rate of 0.06 Å/cycle (Figure 1a). A similar plot
Chart 1. Structures of 1 and 2
Received: May 7, 2015 Revised: June 25, 2015 Published: July 15, 2015 © 2015 American Chemical Society
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DOI: 10.1021/acs.chemmater.5b01707 Chem. Mater. 2015, 27, 4918−4921
Communication
Chemistry of Materials
Figure 2. XPS titanium 2p ionization region of a 82 nm thick titanium film grown at 140 °C on Si(100) with native oxide showing binding energies at intervals of argon ion sputtering.
formed upon air exposure of a pristine titanium surface demonstrated that 3 keV argon ion sputtering progressively reduces TiO2 to Ti2O3 and then TiO but does not lead to titanium metal.27 The titanium 2p binding energies between 10 and 60 min of Ar ion sputtering are consistent with those reported for TiO2, Ti2O3, TiO, and titanium metal.28 After sputtering for ≥70 min, the titanium 2p3/2 ionizations exactly matched metal (453.8 eV32) and were not consistent with TiC (454.8 eV32), TiN (454.9 eV32), or TiSi2 (453.2 eV33). A plot of Ti(IV), Ti(III), Ti(II), and Ti(0) ionization intensities versus argon ion sputtering time revealed the rapid emergence of Ti(0) at about 50 min, confirming the presence of Ti(0) in the bulk of the film.26 An XPS depth profiling study showed low carbon (2.9−3.0%), nitrogen (700 °C.35 The low temperatures of our titanium ALD processes imply at most very thin TiSi2 interfacial layers. This is the first report of a thermal titanium ALD process. Upon the basis of the reduction potential of toluene (E1/2 = −2.46 V), most metal ions in the periodic table should be reduced by the 2-methyl-1,4-cyclohexadienyl dianion,6 which results upon double desilylation of 1. A similar argument can be made for 2. Accordingly, 1 and 2 should be powerful reducing coreagents for the ALD growth of metal films. This work should lead to significant advances in the ALD growth of electropositive metal and element films that have been previously difficult or impossible. ALD film growth studies of other metal and element films using 1, 2, and related derivatives are ongoing in our laboratory.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the U.S. National Science Foundation (Grant No. CHE-1212574) and SAFC Hitech. We thank Professor Daniel J. Mindiola for a helpful discussion.
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ASSOCIATED CONTENT
S Supporting Information *
Precursor and film characterization data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01707.
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Communication
AUTHOR INFORMATION
Corresponding Author
*(C.H.W.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 4920
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Chemistry of Materials (17) Profijt, H. B.; Potts, S. E.; van de Sanden, M. C. M.; Kessels, W. M. M. Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges. J. Vac. Sci. Technol., A 2011, 29, 050801. (18) Saito, T.; Nishiyama, H.; Tanahashi, H.; Kawakita, K.; Tsurugi, H.; Mashima, K. 1,4-Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadienes as Strong Salt-Free Reductants for Generating Low-Valent Early Transition Metals with Electron-Donating Ligands. J. Am. Chem. Soc. 2014, 136, 5161−5170. (19) Tsurugi, H.; Tanahashi, H.; Nishiyama, H.; Fegler, W.; Saito, T.; Sauer, A.; Okuda, J.; Mashima, K. Salt-Free Reducing Reagent of Bis(trimethylsilyl)cyclohexadiene Mediates Multielectron Reduction of Chloride Complexes of W(VI) and W(IV). J. Am. Chem. Soc. 2013, 135, 5986−5989. (20) Tsurugi, H.; Saito, T.; Tanahashi, H.; Arnold, J.; Mashima, K. Carbon Radical Generation by d0 Tantalum Complexes with α-Diimine Ligands through Ligand-Centered Redox Processes. J. Am. Chem. Soc. 2011, 133, 18673−18683. (21) Arteaga-Müller, R.; Tsurugi, H.; Saito, T.; Yanagawa, M.; Oda, S.; Mashima, K. New Tantalum Ligand-Free Catalyst System for Highly Selective Trimerization of Ethylene Affording 1-Hexene: New Evidence of a Metallacycle Mechanism. J. Am. Chem. Soc. 2009, 131, 5370−5371. (22) Popoff, N.; Espinas, J.; Gouré, E.; Boyron, O.; Le Roux, E.; Basset, J.-M.; Gauvin, R. M.; De Mallmann, A.; Taoufik, M. Shifting From Ziegler−Natta to Philips-Type Catalyst? A Simple and Safe Access to Reduced Titanium Systems for Ethylene Polymerization. Macromol. Rapid Commun. 2011, 32, 1921−1924. (23) Laguerre, M.; Dunogues, J.; Calas, R.; Duffaut, N. Silylation d’Hydrocarbures Mono-Aromatiques Mono- ou Disubstitues. J. Organomet. Chem. 1976, 112, 49−59. (24) Sulzbach, R. A.; Iqbal, A. F. M. 1,4-Bis(trimethylsilyl)-1,4dihydropyrazine by Reductive Silylation of Pyrazine. Angew. Chem., Int. Ed. Engl. 1971, 10, 127. (25) Kaim, W. Effects of Cyclic 8π-Electron Conjugation in Reductively Silylated Nitrogen Heterocycles. J. Am. Chem. Soc. 1983, 105, 707−713. (26) Precursor characterization and film growth data are contained in the Supporting Information. (27) McCafferty, E.; Wightman, J. P. An X-ray Photoelectron Spectroscopy Sputter Profile Study of the Native Air-Formed Oxide Film on Titanium. Appl. Surf. Sci. 1999, 143, 92−100. (28) Benito, N.; Palacio, C. Mixed Ti-O-Si Oxide Films Formation by Oxidation of Titanium-Silicon Interfaces. Appl. Surf. Sci. 2014, 301, 436−441. (29) Xia, J.; Masaki, N.; Jiang, K.; Yanagida, S. Deposition of a Thin Film of TiOx from a Titanium Metal Target as Novel Blocking Layers at Conducting Glass/TiO2 Interfaces in Ionic Liquid Mesoscopic TiO2 Dye-Sensitized Solar Cells. J. Phys. Chem. B 2006, 110, 25222−25228. (30) Godfroid, T.; Gouttebaron, R.; Dauchot, J. P.; Leclère, P.; Lazzaroni, R.; Hecq, M. Growth of Ultrathin Ti Films Deposited on SnO2 by Magnetron Sputtering. Thin Solid Films 2003, 437, 57−62. (31) Pouilleau, J.; Devilliers, D.; Garrido, F.; Durand-Vidal, S.; Mahé, E. Structure and Composition of Passive Titanium Oxide Films. Mater. Sci. Eng., B 1997, B47, 235−243. (32) Handbook of X-ray Photoelectron Spectroscopy; Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., Murlenberg, G. E., Eds; Perkin-Elmer Corporation: Eden Prairie, MN, 1979. (33) Yang, W. Y.; Iwakuro, H.; Yagi, H.; Kuroda, T.; Nakamura, S. Study of Oxidation of TiSi2 Thin Film by XPS. Jpn. J. Appl. Phys. 1984, 23, 1560−1567. (34) Knisley, T. J.; Saly, M. J.; Heeg, M. J.; Roberts, J. L.; Winter, C. H. Volatility and High Thermal Stability in Mid- to Late-First-Row Transition-Metal Diazadienyl Complexes. Organometallics 2011, 30, 5010−5017. (35) Tam, P. L.; Cao, Y.; Nyborg, L. XRD and XPS Characterization of Transition Metal Silicide Thin Films. Surf. Sci. 2012, 606, 329−336.
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