TITANIUM (IV) ALKOXIDES

Fig. Vapor pressure of various Ti alkoxides: Ti(OEt)4 (TET), Ti(OiPr)3 (TIPT), Ti(OtBu) (TTTB), Ti(mmp)4

    Titanium (IV) alkoxides are the conventional precursors for the growth of TiO2 and other Ti-O containing materials by MOCVD and ALD.

     Even primary and secondary titanium alkoxides show high vapor pressure and relatively higher thermal stability (as compared to corresponding Hf and Zr alkoxides).

    α-proton of primary ad secondary Ti alkoxides is easily oxidised by ozone, whereas tertiary alkoxides (f.e. Ti(OtBu)4) are relatively unreactive (vaporisation before oxidation).

            A self-regulated ALD growth of TiOx films has been reported for various alkoxide Ti precursors such as Ti(OEt)4, Ti(OiPr)4. The alkoxide Ti precursors are noncorrosive, but their thermal decomposition temperature is relatively low - around 300°C.

       The comparison of vapor pressures of various titanium alkoxides is given in Fig.

Growth rates comparison for various Ti alkoxides

Comparison of growth rates of TiO2 by low pressure (20 mbar) liquid injection MOCVD using several Ti alkoxide precursors

    Comparison of growth rates of TiO2 growth by low pressure (20 mbar) liquid injection MOCVD using several Ti alkoxide precursors (Ti(OiPr)4, Ti(mmp)4 and alkoxide-diketonate Ti(OiP)2(thd)2  was performed . [i]

 [i] A. Jones, J. Mater. Chem.,, 2002,12,  2576–25

Titanium tetrakis(methoxide) Ti(OMe)4

Fig. Formula of Ti(OMe)4

   Titanium (IV) methoxide Ti(OMe)4 (M = 172.00 for monomer) is white solid with melting point mp.200-210°C, was characterized by FTIR and Raman.

     Titanium (IV) methoxide Ti(OMe)4 has oligomeric nature in solid (consists of tetrameric molecules [Ti(OMe)4]4 in the crystal, according to single crystal XRD. According to IR spectra, both in solid and solution (CS2 as solvent) titanium methoxide consists of tetrameric molecules [Ti(OMe)4]4 ). However, in the vapor phase it seems to consist of dimeric molecules [Ti(OMe)4]2 , as only Ti2 species fragments were found in the mass spectra. [[i]]) 

     Due to oligomeric nature [Ti(OMe)4]4 has low volatility and was reported to be unsuitable as MOCVD precursor.  

    However, a surprising observation was made that [Ti(OMe)4]4 appeared to be a useful precursor for ALD of Ti contaning layers. (in the ALD the lower volatility of the precursor is less critical). Good self-limiting ALD growth of TiO2 at 250°C was obtained using titanium methoxide which has no β-hydrogen atoms (only α-H atoms). This shows the important role of β-hydride elimination in titanium alkoxide thermal decomposition. 

Titanium methoxide [Ti(OMe)4]4 showed the highest stability to thermal decomposition among studied titanium alkoxides, and can therefore be used over the widest range of temperatures as Ti ALD precursor.

 [i] D. A. Wright, D. A. Williams, Acta Cryst., 1968, Vol.24, Iss.8, p. 1107-1114, « The crystal and molecular structure of titanium tetramethoxide », https://onlinelibrary.wiley.com/doi/abs/10.1107/S0567740868003766 

Ti(OMe)4 for TiO2 films by ALD

     Titanium tetramethoxide Ti(OMe)4, with water H2O as co-reactant, was applied as Ti precursor for the ALD growth of TiO2 thin films at 200–400 °C temperatures. It was determined that titanium methoxide shows the highest stability with respect to thermal decomposition (as compared with other titanium alkoxides), and thus can be used in large range of temperatures. TiO2 layers deposited from Ti(OMe)4 deposited at 200 °C were amorphous, whereas those deposited at ≥250 °C were polycrystalline with the anatase structure. The prepared TiO2 films contained only minor C and H impurities, except the TiO2 layers deposited at 200 °C The crystalline TiO2 films obtained from Ti(OMe)4 demonstrated photocatalytic activity , for example in decomposing solid stearic acid coated on the film surface, or methylene blue in aqueous solution.[i]

 [i] V. Pore, A. Rahtu, M. Leskelä, M. Ritala, T. Sajavaara, J. Keinonen, Chem. Vapor Dep., 2004, Vol.10, Iss.3,  p.143-148, « Atomic Layer Deposition of Photocatalytic TiO2 Thin Films from Titanium Tetramethoxide and Water », doi.org/10.1002/cvde.200306289, https://onlinelibrary.wiley.com/doi/abs/10.1002/cvde.200306289

Ti(OMe)4 (+Bi[N(SiMe3)2]3) for Bi4Ti3O12 films by ALD

     Titanium methoxide Ti(OMe)4 (combined with Bi(N(SiMe3)2)3 as Bi source and H2O as O source), was applied as Ti precursor for the growth of bismuth titanate thin films by ALD. Self-limited growth of Bi−Ti−O layers was achieved at a deposition temperature of 190 °C. Surface morphologies atypical for amorphous ALD films were obtained for the as-deposited Bi−Ti−O films, related to either partial Bi reduction or to weak ordering of Bi−Ti−O layers during growth. Either pyrochlore Bi2Ti2O7-type structures, mixtures of pyrochlore- and layered perovskite Bi4Ti3O12, or phase-pure Bi4Ti3O12 were obtained depending on the Ti/(Bi + Ti) ratio in the Bi−Ti−O films postannealed at 750°C. The formation of phase-pure Bi4Ti3O12 was favoured by growing slightly Bi-rich films. 51 nm thick Bi−Ti−O layers deposited on Pt/SiO2/Si substrates demonstrated remanent polarization Pr = 0.5 μC/cm2, coercive field Ec = 24 kV/cm and  leakage current density <1 μA/cm2 (up to ±1.6 V applied bias) after postannealing in O2 at 600°C. [i]

 [i] M. Vehkamäki, T. Hatanpää, M. Kemell, M. Ritala, M. Leskelä, Chem. Mater., 2006, 18 (16), pp 3883–3888, DOI: 10.1021/cm060966v, « Atomic Layer Deposition of Ferroelectric Bismuth Titanate Bi4Ti3O12 Thin Films », https://pubs.acs.org/doi/abs/10.1021/cm060966v

Ti(OMe)4 (+Ba(tBu3Cp)2) for BaTiO films by ALD

      Titanium tetrakis(methoxide) Ti(OMe)4, combined with Ba(tBu3C5H2)2 as Ba precursor and H2O as O source) was applied as titanium precursor for the growth of barium titanate BaTiO thin films by ALD at 340 °C. Binary reactions of Ba(tBu3C5H2)2 and H2O forming BaO layers by ALD were first studied separately; the process included hydration/dehydration cycles strongly influenced by the growth temperature. When Ti(OMe)4 - H2O growth cycles were mixed in optimal way with Ba(tBu3C5H2)2 – H2O cycles, self-limiting growth of amorphous barium titanate layers was achieved. Post-deposition annealing at 600 °C resulted on the crystallization of the as-deposited amorphous BaTiO films and increase of dielectric permittivities from 15 to 70. 32 nm thick Ba–Ti–O film in a Pt electrode stack annealed at 600 °C demonstrated charge density 1.9 μC cm–2 (equivalent oxide thickness of 1.8 nm) and leakage current density ≤ 1 × 10–7 A cm–2 at 1 V bias. [[i]]

 [i] M. Vehkamäki, T. Hatanpää, M. Ritala, M. Leskelä, S. Väyrynen, E. Rauhala, Chem. Vapor Dep., 2007, Vol.13, Iss.5, p.239-246, « Atomic Layer Deposition of BaTiO3 Thin Films—Effect of Barium Hydroxide Formation « , doi.org/10.1002/cvde.200606538, onlinelibrary.wiley.com/doi/abs/10.1002/cvde.200606538

Ti(OMe)4 (+Sr(tBu3Cp)2) for SrTiO, TiO2 films by ALD

   Ti(OCH3)4 was used as Ti precursor (with Sr(tBu3Cp)2 as Sr precursor and H2O as O source) for the growth of SrTiO3 (STO) films by ALD on TiN (SrTiO3 is a promising candidate as a high-k dielectric for DRAM applications). Large single crystals grains and nanocracks were observed in the layers after crystallization anneal. However, introduction of thin STO crystalline seed spike annealed at 700 °C allowed to improve the microstructure by inducing formation of much smaller grains in the top layer after post-deposition anneal. The seed approach was as well applied for a layer directly deposited in crystalline state at 370 °C, with a Ti(Me5Cp)(OMe)3 precursor thermally stable at this temperature. The use of the seed layer template approach allowed to were reduce or totally eliminate the nanocracks; however, the leakage current was reduced only using Ti(OCH3)4 as precursor.[i]

      Growth and saturation behavior of SrTiO3 and binary oxides (SrO, TiO2) using Ti(OCH3)4 , Sr(tBu3Cp)2 and H2O as precursors, was evaluated by ellipsometry thickness measurements. The amount of Sr and Ti incorporated in STO films was controlled by the precursor pulse ratio. Stoichiometric SrTiO3 had largest refractive index, density, dielectric constant and lowest crystallization temperature. An excess of Ti or Sr resulted in an increase in the crystallization onset temperature and contraction or expansion of the perovskite SrTiO3  cubic cell constant. Leakage current density was reduced (but the capacitance-equivalent thickness increased) by incorporation of more Sr in SrTiO3.[ii]

 [i] M. Popovici, K. Tomida, J. Swerts, P. Favia, A. Delabie, H. Bender, Ch. Adelmann, H. Tielens, B. Brijs, B. Kaczer, M.A. Pawlak, M.‐S. Kim, L. Altimime, S. Van Elshocht, J.A. Kittl, Phys. Status Solidi A, 2011, Vol.208, Iss.8, p.1920-1924, « A comparative study of the microstructure–dielectric properties of crystalline SrTiO3 ALD films obtained via seed layer approach « , doi.org/10.1002/pssa.201026710, https://onlinelibrary.wiley.com/doi/abs/10.1002/pssa.201026710

[ii] M. Popovici, S. Van Elshocht, N. Menou, J. Swerts, D. Pierreux, A. Delabie, B. Brijs, T. Conard, K. Opsomer, J.W. Maes, D. J. Wouters, J. A. Kittl, J. Electrochem. Soc. 2010, Vol.157, Iss.1, G1-G6,  « Atomic Layer Deposition of Strontium Titanate Films Using Sr(tBu3Cp) 2 and Ti ( OMe ) 4 », doi: 10.1149/1.3244213, http://jes.ecsdl.org/content/157/1/G1.short  

Titanium tetrakis(ethoxide) Ti(OEt)4

Fig. Formula of Ti(OEt)4

     Titanium ethoxide Ti(OEt)4 (M = 228.12 for monomer) is colorless moisture sensitive liquid (density d = 1.09 at 20°C). 

    Vapor pressure of Ti(OEt)4 is 1.0 Torr/119°C.  

   Vapor pressure equation: logP (Torr) = 17.0-6683/T(K)  

   Ti(OEt)4 is thermally stable at 220°C at least for 1h. 

    TGA of Ti(OEt)4 under atmosphere of O3(4%) (2slm) was performed): exothermic peaks at 131°C (-19%) and 204°C (-35% total weight) were observed. 

    Titanium ethoxide Ti(OEt)4 , despite its relatively low vapor pressure, was applied as ALD and MOCVD precursor for the growth of TiO2 and other Ti- containing layers . 

Ti(OEt)4 for TiO2 films by ALD

   Titanium ethoxide Ti(OEt)4 was tested as precursor for the atomic layer epitaxy (ALE) growth of TiO2 thin films. No completely self-limited growth was observed, what was explained by partial Ti(OEt)4 precursor decomposition. However, due to slowness of decomposition the advantages of ALE growth were still present: accurate thickness control, film uniformity, high film density, reproducibility of TiO2 growth. [i]

 [i] M. Ritala, M. Leskela, E. Rauhala, Chem. Mater., 1994, 6 (4), pp 556–561, « Atomic layer epitaxy growth of titanium dioxide thin films from titanium ethoxide », DOI: 10.1021/cm00040a035, https://pubs.acs.org/doi/abs/10.1021/cm00040a035

Ti(OEt)4 for Pb(ZrxTi1−x)O3 films by MOCVD

      Titanium ethoxide Ti(OEt)4, combined with [Pb(thd)2] and [Zr(thd)4] as Pb and Zr sources, was applied as safe and stable Ti precursor for the hot‐wall MOCVD growth of perovskite structure Pb(ZrxTi1−x)O3 films at temperatures as low as 550 °C on sapphire, Pt/Ti/SiO2/Si, and RuOx/SiO2/Si substrates.  The growth rates ranged from 10.0 to 20.0 nm/min. Good compositional uniformity across the bulk of the films and absence of C contamination was demonstrated by Auger electron spectroscopy (AES) depth profile.  The AES spectra also showed The Zr/Ti ratio in the Pb(ZrxTi1−x)O3 films was easily controlled by the precursor temperatures and flow rate of diluent gas. Optical constants were determined by a UV‐VIS‐NIR spectrophotometry. As‐deposited Pb(ZrxTi1−x)O3 films were dense and had uniform and fine grain size. Spontaneous polarization of 23.3 μC/cm3 and a coercive field of 64.5 kV/cm was obtained for the 600 °C annealed film (Pb/Zr/Ti=50/41/9).[i]

 [i] Ch.H. Peng,  S.B. Desu, Appl. Phys. Lett. 61, 16 (1992); « Low‐temperature metalorganic chemical vapor deposition of perovskite Pb(ZrxTi1−x)O3 thin films », https://doi.org/10.1063/1.107646 ,  https://aip.scitation.org/doi/abs/10.1063/1.107646

Ti(OEt)4 for Pb(ZrxTi1−x)O3 films by CW and HW MOCVD

      In another report titanium ethoxide [Ti(OEt)4], with [Pb(thd)2] and [Zr(thd)4] as Pb and Zr co-precursors, was used as Ti source for the deposition of ferroelectric Pb(ZrxT1-x)O3 films by both cold-wall and hot-wall MOCVD at temperatures as low as 550°C. The chosen MO sources proved to be safe and stable precursors. The as‐deposited Pb(ZrxT1-x)O3 films had single-phase perovskite structure and were smooth, specular, crack-free, and uniform, and adhered well to the substrates. The stoichiometry of the films was easily controlled by precursor molar rate (f.e. varying the precursor temperature and/or the carrier gas flow rate) AES depth profile showed good Pb(ZrxT1-x)O3 layer compositional uniformity and absence of C contamination. As-deposited layers were dense and had uniform fine grains (≅0.1 μm). High refractive index (n= 2.413) and low extinction coeflicient (k= 0.0008) at a wavelength of 632.8 nm were obtained for PZT films grown on sapphire. Spontaneous polarization 23.3 μC/cm2 and coercive field 64.5 kV/cm was obtained for the PZT (82/18) film annealed at 600°C. [i]

 [i] Ch.H. Peng, S.B. Desu,  J. Amer. Ceram. Soc., Vol.77, Iss.7, 1994, p.1799-1812, « Metalorganic Chemical Vapor Deposition of Ferroelectric Pb(Zr,Ti)O3 Thin Films », https://doi.org/10.1111/j.1151-2916.1994.tb07054.x,  https://ceramics.onlinelibrary.wiley.com/doi/abs/10.1111/j.1151-2916.1994.tb07054.x 

Ti(OEt)4 for W-doped TiO2 films by AACVD

     Titanium ethoxide Ti(OEt)4, in combination with dopant concentrations of W(OEt)6 (both supplied in toluene solution), was used as Ti precursor for the aerosol-assisted CVD deposition of W-doped TiO2 films (having both transparent conducting oxide (TCO) and photocatalytic properties) at 500 °C. The deposted W-doped TiO2 films possessed anatase structure and had good n-type electrical conductivities, according to Hall effect measurements. Lowest resistivity (0.034 Ω.cm), reasonable charge carrier mobility (14.9 cm3/V.s) and concentration (×1019 cm−3) were obtained for the TiO2(W) film doped with 2.25 at.% W. The presence of both W6+ and W4+ in the TiO2 matrix was indicated by XPS, with the substitutional doping of W4+ inducing an expansion of the anatase unit cell (as was found by XRD). Good photocatalytic activity under UV-light illumination was demonstrated for the grown W-doped TiO2 films by higher rate degradation of resazurin redox dye compared to  undoped TiO2. [i]

 [i] S. Sathasivam, D.S. Bhachu, Y. Lu, N. Chadwick, Sh.A. Althabaiti, A.O. Alyoubi, S.N. Basahel, C.J. Carmalt, I.P. Parkin, Scientific Reports, 2015, Vol.5, Article number: 10952, « Tungsten Doped TiO2 with Enhanced Photocatalytic and Optoelectrical Properties via Aerosol Assisted Chemical Vapor Deposition », https://www.nature.com/articles/srep10952

Titanium tetrakis(isopropoxide) Ti(OiPr)4

Fig. Formula of Ti(OiPr)4

Titanium tetrakis(isopropoxide) Ti(OiPr)4 (M = 284.22) is colorless moisture sensitive liquid with density d = 0.96 (20°C), melting point 14-17°C, boiling point 58°C/ 1 Torr.

    Ti(OiPr)4 has vapor pressure  1.0 Torr/62°C, vapor pressure equation logP (Torr) = 8.38-2811/T(K). Ti(OiPr)4 is thermally stable at 260°C at least for 1h.

Ti(OiPr)4 was studied by TGA under O3 (4%) atmosphere, 2 slm): exothermic peaks were observed at 61°C (-27%) and 232°C (-65% total weight).

Ti(OiPr)4 is the most widely employed metal-organic titanium precursor MOCVD and ALD. For instance, it has been used for the growth of a TiO2 by MOCVD, LACVD (laser-assisted chemical vapour deposition), ALD , BaTiO3 by PACVD, SrTiO3 by ALD

Surface decomposition mechanism of Ti(OiPr)4

Ti(OiPr)4 for Pb-Zr-Ti-O films by MOCVD

    Titanium tetrakis(isopropoxide) Ti(OiPr)4, combined with Pb(thd)2 and Zr(OtBu)4 as Pb, Zr sources and O2 as oxidant, was applied as Ti precursor for the deposition of Pb-Zr-Ti-O (PZT) layers by MOCVD at 600-700°C and pressure 1-2 Torr. Perovskite structure PZT layers with well-developed crystalline grains were obtained on Pt substrates.[i]

       Ti(OiPr)4 with Pb(thd)2, Zr(OtBu)4, as Pb,Zr co-precursors and O2 as oxidizing agent, was used as Ti precursor for the deposition of uniform (thickness variation <5%) large-area (6–8 inch) Pb(Zr,Ti)O3 (PZT) and TiO2 thin films using a single wafer MOCVD system. The step coverage of MOCVD-grown PZT films was much better compared to RF sputtering.[ii]

[i] H. Itoh, K. Kashihara, T. Okudaira, K. Tsukamoto, Y. Akasaka, Intern. Electr. Dev. Meeting 1991 ,[Tech. Digest], MOCVD for PZT thin films by using novel metalorganic sources, DOI:  10.1109/IEDM.1991.235296, https://ieeexplore.ieee.org/abstract/document/235296

[ii] T. Shiosaki, M. Fujimoto, M. Shimizu, M. Fukagawa, K. Nakaya, E. Tanikawa, Integr. Ferroelectrics, An International Journal, 1994, Vol.5, Iss. 1, p.39-45, « Large-area growth of Pb(Zr,Ti)O3 thin films by MOCVD »,https://www.tandfonline.com/doi/abs/10.1080/10584589408018678 

Titanium tetrakis(n-butoxide) Ti(OnBu)4

Fig. Formula of Ti(OnBu)4

    Titanium tetrakis(n-butoxide) (or tetrabutyl orthotitanate ) Ti(OnBu)4 (M = 340.32) is viscous colorless to pale-yellow liquid (density d= 0.998 g/mL at 20 °C) , refractive index n20/D 1.491(lit.) , flash point 77 °C, melting point -55°C,

Boiling point of  Ti(OnBu)4: 206 °C/10 Torr, 312°C/ 760 Torr.   

Vapor pressure of Ti(OnBu)4: 0.5Torr/140°C, 19Torr/200°C.

Synthesis of titanium (IV) n-butoxide: Ti(OnBu)4 is produced by reacting titanium tetrachloride with butanol: TiCl4 + 4 nBuOH -> Ti(OnBu)4 + 4HCl

 Titanium tetrakis(n-butoxide) Ti(OnBu)4 is soluble in most organic solvents,  decomposes in water and reacts violently with oxidizing materials. Ti(OnBu)4  reacts violently with nitric and sulfuric acid, inorganic peroxides and hydroxides, bases, amides, amines, boranes and isocyanates. It is irritating to skin and eyes, and causes nausea and vomiting if swallowed. LD50 is 3122 mg/kg (rat, oral) and 180 mg/kg (mouse, intravenal).

Ti(OnBu)4 was applied as Ti precursor for the growth of TiO2, SrTiO3 by MOCVD.

Ti(OnBu)4 for Fe-doped TiO2 nanoparticles by MOCVD

     Ti(OnBu)4  was applied as precursor for the growth of Fe-doped TiO2 nanoparticles on quartz tube walls by MOCVD at temperature 400-700°C (using FeCp2 as Fe source, directly mixed in various concentrations (0.25, 0.5, 1.5 and 2.5 g/L with  Ti(OnBu)4 ), with N2 carrier gas (400sccm) and O2 (100sccm) as oxidant. Undoped TiO2 nanoparticles were amorphous at 400° and had anatase structure at 700°C C deposition temperature. Fe-doping unduced phase transition of TiO2 nanoparticles from amorphous to anatase structure, and at 700°C from anatase to rutile structure, as determrined by XRD. Increasing of growth temperature as well as increase of Fe doping concentration resulted in decrease of TiO2 nanoparticle size, according to TEM and XRD.

[i]

[i] S. H. Othman, S. Abdul Rashid, T. I. M. Ghazi, and N. Abdullah, J. Appl. Sci., 2010, vol. 10, no. 12, pp. 1044–1051, « Effect of fe doping on phase transition of TiO2 nanoparticles synthesized by MOCVD,”, http://docsdrive.com/pdfs/ansinet/jas/2010/1044-1051.pdf 

Titanium tetrakis(tert-butoxide) Ti(OtBu)4

Fig. Formula of Ti(OtBu)4

    Titanium tetrakis(tert-butoxide) Ti(OtBu)4 (M = 340.34) is colorless moisture sensitive liquid (density d = 0.89) with melting point 4°C, boiling point 70°C/ 0.2 Torr, 160-165°C/ 15 Torr. 

Vapor pressure:  0.1 Torr/ 40°C, 1.0Torr/ 68°C (other data 1 Torr/ 75°C), 5Torr/ 95°C. Vapor pressure equation logP (Torr) = 9.35-3189/T(K)

Ti(OtBu)4 is thermally stable at 260°C at least 1h. TGA of Ti(OtBu)4  under atmosphere of O3 (4%, 2 slm) revealed only one endothermal peak at 149.4°C (-96.4%) – indicating vaporisation only.

Fig. 1H NMR spectrum of Ti(OtBu)4

All protons in Ti(OtBu)4 are magnetically equivalent, according to 1H NMR (Fig.).

Ti(OtBu)4) was  applied as precursor for the deposition of Ti-containing films like TiO2, PbTiO3, Pb(Zr,Ti)O3  by MOCVD.

Ti(OtBu)4 for PbTiO3, Pb(Zr,Ti)O3 films by MOCVD

     Titanium tetrakis(tert-butoxide) Ti(OtBu)4, combined with PbEt4, Zr(OtBu)4 as Pb, Zr sources and O2 as oxidizing agent), was used for the growth of epitaxial PbZrxTi1−xO3   (PZT) thin films by cold-wal horizontal flow MOCVD at 700°C, using 0.5mm thick SrTiO3 substrates (001 oriented within 60.5°)... The properties for the layers were examined as a function of the Zr/(Zr+Ti) ratio in the gas phase. Two sets of PZT films having thicknesses 50–100 and 700–1400 nm, containing 0%, 40%, 60%, and 100% Zr, were grown and investigated. The refractive index n was found from ellipsometry of thin PZT films and from reflectivity measurements for the thicker PZT films (results were obtained for the energies from 1.55 to 3.72 eV, and with a Cauchy-fit extrapolation down to 0.62 eV). The refractive-index curves demonstrated a systematic variation with composition. For all compositions, n is close to 3.2 at 3.72 eV (333 nm), while at 1.55 eV (800 nm) n is 2.35 for PZ (x=1) and 2.61 for PT (x=0). The optical band gap of PZT layers (3.6±0.1 eV) was found to be practically independent of composition. [i]

[i] M.P. Moret, M.A.C. Devillers, J.Appl.Phys. 92, 468 (2002);« Optical properties of PbTiO3, PbZrxTi1−xO3, and PbZrO3 films deposited by metalorganic chemical vapor on SrTiO3 »,  doi.org/10.1063/1.1486048  

Titanium tetrakis(neopentoxide) Ti(ONep)4

Fig. Dimeric sructure of [Ti(ONep)4]2 in solid

     Titanium tetrakis(neopentoxide) Ti(OCH2tBu)4, (or Ti(ONep)4),  an extremely soluble and sublimable metal alkoxide, was synthesized in high yield by an alcoholysis exchange between Ti(OPri)4 and HONep. [For comparison, metathesis reaction between TiCl4 and NaONep yielded halogen-substituted Ti3(O)(Cl)(ONp)9·C7H8, which adopts a standard M3(μ3-X)2(μ-X)3X6 structure].

     The crystal and molecular structure of titanium tetrakis(neopentoxide) was determined by single crystal XRD: it appeared to be dimer [Ti(μ-ONp)(ONp)3)]2 , which adopts a typical M2(μ-X)2(X)6 fused axial−equatorial edge-shared geometry, with each Ti metal center being 5 coordinated.

Ti(ONep)4 was determined to be a monomer in solution (based on molecular weight solution measurements as well as 1H, 13C, 17O (natural abundance), and 47,49Ti solution and solid-state NMR investigations).[i]

Ti(ONep)4) has high volatility (vaporizes at 120°C allowing to reach 21µ/h growth rate of TiO2) and was  applied for the Ti-containing film growthlike TiO2  by MOCVD.

 [i] T.J. Boyle, T.M. Alam, Eric R. Mechenbier, Inorg. Chem., 1997, 36 (15), pp 3293–3300, « Titanium(IV) Neopentoxides. X-ray Structures of Ti3(μ3-O)(μ3-Cl)(μ-OCH2CMe3)3(OCH2CMe3)6 and [Ti(μ-OCH2CMe3)(OCH2CMe3)3]2 », DOI: 10.1021/ic960924g, https://pubs.acs.org/doi/abs/10.1021/ic960924g

Ti(ONep)4 for TiO2 films by MOCVD

     Titanium(IV) neopentoxide, [Ti(μ-ONep)(ONep)3]2 has relatively high volatility at low temperatures and was tested as  precursor (with direct vaporization without a carrier gas) for the growth of TiO2 thin films by lamp-heated cold-wall MOCVD. Neopentoxide ONep derivatives were found to be competitive precursors for the deposition of TiO2 layers (in comparison to other metal-organic precursors). [Ti(ONep)4]2 precursor was vaporized at 120 °C, allowing to reach TiO2 growth rate ∼0.35 μm/min (21µm/h) at substrate temperature 330°C; the grown film consisted of the anatase phase of TiO2 with < 1 % C impurity. [i]

 [i] J.J. Gallegos III, T.L. Ward, T.J. Boyle, M.A. Rodriguez, L.P. Francisco, Chem. Vapor Deposition, 2000, Vol.6, Iss.1, p.21-26 , « Neo‐pentoxide Precursors for MOCVD Thin Films of TiO2 and ZrO2 », doi.org/10.1002/(SICI)1521-3862(200002)6:1<21::AID-CVDE21>3.0.CO;2-C, onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1521-3862(200002)6:1%3C21::AID-CVDE21%3E3.0.CO;2-C

Titanium bis(isopropoxide) bis(1-methoxy-2-methyl-2-propoxide) [Ti(OiPr)2(mmp)2]

Fig. Molecular structure of Ti(OiPr)2(mmp)2

    Ti(OPri)2(mmp)2 is a white solid which evaporates at 250ºC at atmospheric pressure and decomposes at temperatures < 400ºC. The structure of Ti(OPri)2(mmp)2 is shown in Fig.

    Ti(OPri)2(mmp)2 was reported to be vaporizable at 200ºC (as 0.1 M octane solution) in MOCVD conditions (special vaporizer) when using it as Ti precursor for the growth of TiTaO films by AVD (see below).

Ti(OiPr)2(mmp)2 for TiO2, TiTaO layers by AVD

Fig. MIM Capacitance density vs. thickness of TiTaO (grown by AVD using Ti(OiPr)2(mmp)2 /TBTDET)

     Ti(OiPr)2(mmp)2 was applied as Ti precursor for the growth of TiO2 and TiTaO layers (in combination with Ta(=NtBu)(Net2)3 (TBTDET) as Ta source and O2 as oxidant) on Si and 20nmTiN/SiO2/Sisubstrates by atomic vapor deposition (AVD = liquid injection MOCVD).[[i]] Commercial AIXTRON Tricent MOCVD reactor (allowing growth on the substrates up to 200 and 300mm) was used for the TiO2 and TiTaO layer growth. The growth conditions were as below: precursor octane solution concentration 0.1M/injection frequency 1Hz for Ti(OiPr)2(mmp)2 (0.2M / 2 Hz for TBTDET), precursor evaporation temperature 200°C, deposition temperature 400°C, growth pressure 10 mbar, Ar carrier gas (total flow 2000sccm), O2 flow 500sccm.

   TiO2 films grown at 400°C using Ti(OiPr)2(mmp)2 precursor were polycrystalline with anatase structure of the layers (as determined by XRD) – typical crystallization temperature for TiO2 thin films. 

The growth of Ti-Ta-O layers on Si substrates using Ti(OiPr)2(mmp)2 and TBTDET was performed at 400°C (temperature fitting well the thermal budget of the BEOL process), and annealed at  different temperatures (700 to 1000°C). The as deposited TiTaO films were X-Ray amorphous, whereas starting from 700°C annealing temperature they started to crystallise in the phase which is closest to the orthorhombic Ta2O5 phase.

   The composition of Ti(OiPr)2(mmp)2 /TBTDET-deposited TiTaO films was studied by XPS: the determined Ta/Ti ratio was 1.5.

    The obtained TiTaO layers were applied for the preparation of MIM capacitor structures; the dependence of MIM capacitance density vs. TiTaO layer thickness was investigated, the very high dielectric constant k=50 was extracted. (Fig.). The voltage linearity coefficient α was below the limit of 100 ppm/V2 for the MIM capacitors with TiTaO layer thickness higher than 37 nm (such MIMs demonstrated very high capacitance density of 11 fF/μm2 and relatively low leakage currents in the range of 10-7 A/cm2 at 1V. (such capacitance value much higher than the ones obtained for reported SrTaO and HfO2 MIM capacitors  - making Ti(OiPr)2(mmp)2 /TBTDET-deposited TiTaO material  promising as high-k dielectric for MIM applications.)

[i] Mindaugas Lukošius, PhD Thesis (Univerity Oldenburg) , 2010, « Atomic Vapor Depositions of Metal Insulator Metal capacitors: Investigation, Development and Integration », http://oops.uni-oldenburg.de/949/1/lukato10.pdf

Ti(OiPr)2(mmp)2 for Bi4Ti3O12 by MOCVD

    Ti(OPri)2(mmp)2 (and for comparison Ti(mmp)4) were tested for the liquid injection MOCVD growth of Bi4Ti3O12 layers, as Ti alkoxide precursors having bidentate alkoxide ligands mmp, thus increasing the coordinative saturation of a metal and reducing its moisture sensitivity (as an alternatives to conventional Ti(OR)4 precursors which are highly moisture sensitive due to unsaturated 4-coordinate Ti centres). Both Ti(OPri)2(mmp)2 and Ti(mmp)4 are Ti precursors well matched to Bi(mmp)3 in terms of similar physical properties (volatility) and decomposition characteristics. Also for comparison Ti alkoxide-diketonate [Ti(OPri)2(thd)2] was tested (which contains the chelating [thd] group, making Ti coordination more fully saturated and thus precursor less moisture sensitive than the parent Ti(OPri)4). A further advantage of Ti(OPri)2(mmp)2 or Ti(mmp)4 / Bi(mmp)3 precursor system is the presence of the same ligand in each complex, what minimises unfavourable precursor interactions in solution.

      Thermogravimetric analysis (TGA) showed that all complexes (Ti(OPri)2(mmp)2 as well as Ti(mmp)4,  Ti(OiPr)2(thd)2 and Bi(mmp)3) have well-matched volatilities, evaporating in similar temperature range and being fully evaporated at < 300°C.

       The variation in oxide growth rates with substrate temperature for Ti(OiPr)2(mmp)2 and for comparison for Ti(mmp)4 ,  Ti(OPri)2(thd)2 and Bi(mmp)3 is presented in Fig. 2. It was found that Ti(OiPr)2(mmp)2 as well as Ti(mmp)4 deposit oxides at good growth rates in the same temperatures range as Bi(mmp)3 (350-600°C), and at significantly lower temperature than the more thermally stable Ti(OiPr)2(thd)2.

    The solid composition of Bi-titanate grown using Ti(OPri)2(mmp)2/ Bi(mmp)3 or Ti(mmp)4 / Bi(mmp)3 pair showed little variation with substrate temperature down to as low as 300°C (according to EDX measurments). However, when using the more thermally stable Ti(OPri)2(thd)2 (+ Bi(mmp)3), a marked decrease in Ti content at lower substrate temperatures was observed for the grown Bi-titanate layers (with no Ti incorporation at all at 300°C). It was concluded that the Bi(mmp)3 / Ti(mmp)3 combination was the best matched and most suitable for the depositon of Bi-titanate by liquid injection CVD. Due to increased desorption of Bi at higher substrate temperatures, the composition of Bi4Ti3O12 films grown from Bi(mmp)3 / Ti(mmp)4 became self-limiting at 600°C, with only little increase in Bi content in solid, when the solution Bi(mmp)3 mole fraction was increased from 0.57 to 0.78.[i]

[i] A.C. Jones, P.A. Williams, N.L. Tobin, P.R. Chalker, P. Marshall P.J. Wright, P.A. Lane, P. Donohue, L.M. Smith, H.O. Davies, « Development of Improved Precursors for the MOCVD of Bismuth Titanate », https://www.electrochem.org/dl/ma/203/pdfs/2062.pdf

Ti(OiPr)2(mmp)2 for (Bi,La)TiOx by MOCVD

    Titanium bis(isopropoxide) bis(1-methoxy-2-methyl-2-propoxide) [Ti(OiPr)2(mmp)2], combined with BiPh)3 and La(thd)3(tetraglyme) as Bi, La source and O2 as oxidiser, was applied as Ti precursor for the deposition of Lanthanum-substituted bismuth titanate (BLT) (Bi,La)TiOx thin films on Ir/Ti/SiO2/Si substrates by liquid injection MOCVD. The growth conditions were following: Ti/Bi/La precursor octane solution concentrations 0.01/0.02/0.01 M (obtained growth rate 125nm/h) or 0.02/0.04/0.01 M (growth rate 200-250nm/h), precursor vaporizer temperature 245°C, showerhead temperature 230°C, gas line temperature 200°C, deposition temperature 500°C, process pressure 5mbar, N2 carrier gas 2000sccm, O2 flow 300sccm. The composition of deposited (Bi,La)TiOx films was studied by XPS. Off-c-axis oriented crystallized (Bi,La)TiOx films were obtained on (111) Ir/Ti/SiO2/Si substrates at 500°C, according to XRD. Smooth surface (necessary for producing ferroelectric capacitors for FeRAM applications) was demonstrated for the grown (Bi,La)TiOx films by SEM.[i]

[i] C.-P. Yeh, T. Grinys, M. Lisker, E.P. Burte, Mater. Res. Soc. Symp. Proc., Vol. 1110, 1110-C03-20, « Fabrication of Ferroelectric BLT Thin Films by Direct Liquid Injection MOCVD », https://www.researchgate.net/profile/Chia_Pin_Yeh/publication/269076887_Fabrication_of_ferroelectric_BLT_thin_films_by_direct_liquid_injection_MOCVD/links/54b507d00cf2318f0f97100e/Fabrication-of-ferroelectric-BLT-thin-films-by-direct-liquid-injection-MOCVD.pdf

Titanium tetrakis(methoxymethylpropanolate) Ti(mmp)4

Fig. TGA of Ti(mmp)4 (and for comparison Ti(OiPr)2(mmp)2, Ti(OiPr)2(thd)2

Titanium tetrakis(methoxymethylpropanolate) Ti(mmp)4 (M=) is liquid.

The TGA of Ti(mmp)4 and Ti(OiPr)2(mmp)2 and Ti(OiPr)2(thd)2 is shown in Fig.  The main amount of Ti(mmp)4 evaporates at 210-240°C, the remaining mass is ~8%.

Ti(mmp)4 for Pb(Zr,Ti)Ox by MOCVD

      Titanium tetrakis(methoxymethylpropanolate) Ti(MMP)4, combined in a cocktail solution with Pb(DMAMP)2 and  Zr(MMP)4 in ECH (ethylciclohexane) solvent, was applied as titanium precursor for the growth of ferroelectric PZT thin films by liquid delivery MOCVD. The cocktail solution perfectly vaporized above 290°C and was stable for >3 months. At 400°C substrate temperature, the grown layers were crystallized into perovskite PZT phase with preferred 111-orientation. The cocktail solution approach proved to be suitable for PZT deposition at a low substrate temperature.[i]

     Titanium tetrakis(1-methoxy-2-methyl-2-propoxide) [Ti(mmp)4] combined with [Pb(thd)2] and Zr(mmp)4 were applied as Ti, Pb and Zr precursors (supplied by liquid delivery source-supply system) for the deposition of Pb(Zr,Ti)O3 (PZT) films into sub-micron SiO2/TiAlN/Ti/SiO2/Si trench structures by pulsed-MOCVD, the conformality of thickness and composition was investigated. The slopes of the straight lines (plotted in the Arrhenius coordinates) for the constituents Ti, Pb, Zr in PZT film were almost the same both in lower and higher region of deposition temperature (Td).  This means that by using Ti(mmp)4-Pb(thd)2-Zr(mmp)4-O2 source system, the layer composition change with the Td can be suppressed. The step coverage/composition conformality of PZT films at two Tds (450 and 540°C as lower and higher temperatures) was similar: the sidewall-bottom step coverage (SCs-b) was ~70 % (worse due to crystallization at the bottom of the trench (underlayer at this area is not SiO2 but crystalline TiAlN), whereas the sidewall step coverage (SCsw) was very good (>90 %); composition conformality along the depth direction of the sidewall was also very good. [ii]

[i] Y. Otani, K. Uchiyama, S. Okamura, T. Shiosaki, Integr. Ferroelectrics: An International Journal, 2006, Vol. 81, Iss. 1, p.261-270 ; DOI: 10.1080/10584580600663326, « Low temperature deposition of Pb(Zr,Ti)O3 thin films by liquid delivery MOCVD using a cocktail source with Pb(DMAMP)2, Zr(MMP)4 and Ti(MMP)4 », https://www.tandfonline.com/doi/abs/10.1080/10584580600663326

[ii] A. Nagai, G. Asano, J. Minamidate, Ch. J. Choi, MRS Online Proceeding Library, 2004, Archive 830, « Step Coverage and Composition of Pb(Zr,Ti)O3 Capacitors Prepared on Sub-Micron Three-Dimensional Trench Structure by Metalorganic Chemical Vapor Deposition », DOI: 10.1557/PROC-830-D2.2, https://www.researchgate.net/publication/283402281_Step_Coverage_and_Composition_of_PbZr_TiO3_Capacitors_Prepared_on_Sub-Micron_Three-Dimensional_Trench_Structure_by_Metalorganic_Chemical_Vapor_Deposition 

Ti(mmp)4 for BiTiOx by MOCVD

     Ti(mmp)4 (and for comparison Ti(OPri)2(mmp)2 ), in combination with Bi(mmp)3 as Bi precursor, was applied as Ti source for the growth of bismuth titanate (BiTiOx) thin films by liquid injection MOCVD on Si(100) substrates at 300-600°C temperatures. Bismuth titanate layers grown at substrate temperatures > 500°C were shown to consist predominantly of the Bi4Ti3O12 phase.[i]

[i] P.A. Williams, A.C.C. Jones, N.L. Tobin, P.A. Marshall, MRS Symp. Proc., 2002, Vol.748 (Symposium U – Ferroelectric Thin Films XI),  U12.3, « Development of Improved Precursors for the MOCVD of Bismuth Titanate »,  https://doi.org/10.1557/PROC-748-U12.3

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