Strontium alkylcyclopentadienyls
The substituted metallocenes (alkylcyclopentadienyls) of alkaline earth meatls are known to be volatile and having vapor pressures high enough to be applied as CVD/ ALD precrsors for the growth of corrsponding metal-containing films. However, the first reports of film growth using strintium alkylcyclopentadienyls appeared only recently. Thermal decomposition and carbon incorporation are issues observed when using alkaline earth metallocenes as precursors in MOCVD practice [see 4]
Strontium bis (isopropylcyclopentadienyl) Sr(iPrCp)2
Strontium bis(isopropylcyclopentadienyl) had been used for the deposition of strontium films on glass substrates at 800°C/250Torr. Pure metallic films have been obtained which oxidized quickly in the air [109]
Strontium bis(1,2,4-triisopropylcyclopentadienyl) Sr(1,2,4-iPr3Cp)2
Sr(1,2,4-C5iPr3H2)2 for SrTiO3 by ALD
Sr(1,2,4-C5iPr3H2)2 was applied as the strontium precursor in the growth of deposition of SrTiO3 thin films by ALD, in combination with Ti(OiPr)4 and H2O, at temperatures 250-325°C.Some thermal decomposition of Sr(1,2,4-C5iPr3H2)2 occurred during the deposition. [see 4]
Vehkamäki et al. then evaluated unadducted bis[tris(isopropyl)cyclopentadienyl] strontium [Sr(iPr3Cp)2], for SrTiO3 ALD using [Ti(OiPr)4], as a Ti precursor and H2O as an oxygen source. SrO and SrTiO3 ALD films were obtained At 250°C, with deposition rates of respectively 1.1Å/cycle and about 0.5Å/cycle. However there was some evidence of precursor decomposition, preventing self-limited deposition, even at low temperature such as 250°C. This parasitic CVD effect was even more significant at 300°C and 350°C (M. Vehkamäki et al. Chem. Vap. Deposition 2001, 7 N° 2 pages 75-80).
Sr(iPr3Cp)2 is not an optimized precursor for SrTiO3 and Ba1-xSrxTiO3 ALD because of its too low thermal stability. In particular, a conformal deposition in high aspect ratio trenches is not to be expected.
Strontium bis(tri-isopropyl cyclopentadienyl) Sr(iPr3Cp)2, together with titanium tetrakis(isopropoxide) and water, were used for atomic layer deposition (ALD) of strontium titanate thin films at 250-325°C.
Though completely self-limiting, deposition of SrO could not be achieved because of some minor decomposition of the strontium compound. This decomposition was slow enough to ensure that good control of film stoichiometry was obtained by controlling either
the (Sr-O)/ (Ti-O) pulsing ratio, or the strontium precursor exposure time. The films were strongly (100) textured. After annealing at 500°C in air, SrTiO films with the optimal composition were found to have measured permittivity values of around 180.
[[i]]
[i] M. VEHKAMÄKI; T. HÄNNINEN; M. RITALA Mikko; M. LESKELÄ; T. SAJAVAARA; E. RAUHALA; J. KEINONEN; Chem. Vap. Deposition, 2001, vol. 7, no2, pp. 75-80 (46 ref.)
Strontium bis (1,2,4-triisopropylcyclopentadienyl) thf adduct Sr(1,2,4-C5iPr3H2)2(thf)
Ihanus et al. reported tetrahydrofuran adducts of bis[tris(isopropyl)cyclopentadienyl] strontium and bis[pentamethylcyclopentadienyl] strontium to be useful as precursors for SrS ALD. [239].
Sr(iPr3Cp)2·2THF (“HyperSr.THF”) for SrTiO3 by ALD
Sr(iPr3Cp)2·2THF (HyperSr.THF) (and for comparison other novel Sr precursors (Sr(iPr3Cp)2 (AbsoluteSr), Sr(iPr3Cp)2.DME (HyperSr.DME), Sr(Me5Cp)2.THF (StarSr.THF),
Sr(Me5Cp)2.DME (StarSr.DME), in which the effect of stabilizing adducts on the properties of Sr cyclopentadienyl precursors and SrO ALD process was evaluated), was evaluated as precursor for deposition of SrTiO3 layers by ALD (in combination with several Ti
precursors - Ti(MeCp)(OMe)3 (PrimeTi), (Ti(Me5Cp)(OMe)3 (StarTi), TiCp(NMe2)3 (TyALD), Ti(Me5Cp)(NMe2)3 (StarTyALD)). The compatibility of Sr(iPr3Cp)2·2THF precursor along with Ti(MeCp)(OMe)3 and (Ti(Me5Cp)(OMe)3), in terms of composition tunability
and material properties of deposited SrTiO3 layers were studied.[i]
[i] R. Katamreddy, Z. Wang, V. Omarjee, P.V. Rao, Ch. Dussarrat, N. Blasco, ECS Trans. 2009, Vol.19, Iss.2, 525-536, « Advanced Precursor Development for Sr and Ti Based Oxide Thin Film Applications », doi: 10.1149/1.3122114, http://ecst.ecsdl.org/content/19/2/525.short
Strontium bis (pentamethylcyclopentadienyl) thf adduct Sr(1,2,4-C5Me5)2(thf)
Bis[pentamethylcyclopentadienyl] strontium tetrahydrofuran adduct was applied as precursors for SrS ALD [239].
Strontium bis (pentamethylcyclopentadienyl) dimethoxyethane adduct Sr(1,2,4-C5Me5)2(dme)
The comparison of thermal behaviour of strontium precursors by TGA is presented in Fig. Dymethoxyethane adduct of strontium pentamethylcyclopentadienyl Sr(Me5Cp)2(DME) is much more volatile than diketonate Sr(methd)2 (>100°C lower sublimation temperature at 1 atm Ar: 290°C vs 410°C). There is no decrease of weight before vaporisation of the complex, meaning there is no dissociation of DME addend from the adduct.
From TGA/DTA fof Sr(Me5Cp)2(DME) under O2 atmosphere (Fig) one can see that the adduct easily reacts with O2 below 100°C, while the second highly exothermic step of oxidation occurs at ~440°C.
Thermal stability of Sr(Me5C)2(DME) was determined by TGA residue after heating for 1 h at given temperature (Fig.). The residue of 2% (temps. <200°C) increased to ~9% at 250°C (means some decomposition occurs at these temperatures
Strontium bis (n-propyltetramethylcyclopentadienyl) Sr(nPrMe4Cp)2
Thermal stability of strontium n-propyltetramethylcyclopentadienyl Sr(nPrMe4Cp)2 was studied by TGA. The complex was stable at 250°C (0% residue after 1h heating), over this temperature however the complex started to rapidly decompose (TGA residue reached 15% at 290°C)
Sr(nPrMe4Cp)2, and for comparison diketonate Sr(thd)2 were tested for atomic layer deposition of SrO, using O2, O3, and H2O as oxygen sources. No deposition was observed with Sr(thd)2 precursors, while SrO films were deposited with Sr(PrMe4Cp)2 with source (bubbler) temperature of 200 °C, at substrate temperatures 250-350 °C with growth rates increasing for in this order: O2 < H2O < O2 + H2O. [240]
Strontium bis(tri-tert-butylcyclopentadienyl) Sr(tBu3Cp)2
Sr(tBu3Cp)2 (+Ti(OMe)4) for SrTiO3, SrO films by ALD
Sr(tBu3Cp)2 was used as Sr precursor (combined with Ti(OCH3)4 as Ti precursor and H2O as O source) for the ALD deposition of SrTiO3 (STO) layers on TiN. After crystallization anneal large single crystal grains and nanocracks were observed in the films. SrTiO3 layer microstructure was significantly improved by introduction of thin SrTiO3 crystalline seed spike annealed at 700 °C, which induces 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 which is thermally stable at this temperature. The use of the seed layer template approach allowed to reduce or totally eliminate the nanocracks; however, 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 Sr(tBu3Cp)2 / Ti(OCH3)4 precursor pulse
ratio. Stoichiometric SrTiO3 had largest refractive index, density, dielectric constant and lowest crystallization temperature. An excess of Sr or Ti resulted in the increase of crystallization onset temperature and contraction or expansion of the perovskite
SrTiO3 cubic cell constant. Incorporation of more Sr in SrTiO3 decreased the leakage current density, but the capacitance-equivalent thickness was increased [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, 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
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