STRONTIUM β-DIKETONATES

    Strontium compounds the most investigated for the application as MOCVD precursors  are strontium β-diketonates, such as bis(acetylacetonate) strontium [Sr (acac)2], bis(2,2,6,6-tetramethyl-heptane-3,5-dionate) strontium [Sr(thd)2], and bis(hexafluoroacetylacetonate) strontium [Sr(hfac)2]. However, strontium beta-diketonates are generally solid at room temperature, they often have a very low

volatility and often are oligomers, they generally have a high thermal stability (400°C), which prevents their use in low temperature processes, they generally have a poor reactivity toward moisture, which makes them unsuitable for low temperature H2O ALD processes, and their use in ALD processes using O3 as an oxygen source, results in a high carbon containing strontium carbonate, SrCO3.

Bis(acetylacetonate) strontium [Sr(acac)2]

    Bis(acetylacetonate) strontium [Sr(acac)2], presents some drawbacks such as relatively low volatility and higher ALD/CVD layer growth temperature.

Bis(hexafluoroacetylacetonato) strontium [Sr (hfac)2]

    Bis(hexafluoroacetylacetonate) strontium, [Sr (hfac)2] has similar drawbacks to Sr(acac)2, additionally it contains fluorine atoms that may affect the film quality by unexpected etching during the deposition process or by incorporation of fluorine atoms in the films and might thus lead to surface and interface roughness.

Bis(hexafluoroacetylacetonato) strontium dimethoxyethane adduct [Sr (hfac)2(dme)]

     The addition of a neutral oxygen or of a nitrogen donor complexing ligand in the structure of compounds of the diketonate family, such as cyclic or acyclic ethers, to the metal compound (strontium, barium, calcium), enhance the thermal stability of the final complex. An example of such a compound is {Sr [CF3C(O)CHC(O)CF3]2[CH3OCH2CH2OCH3]}. It is obtained in good yield (94%) and has an acceptable melting point (80-82°C). However, its vapour pressure is in the range of 0.06 Torr at 115°C, considered to be too low for a vapour phase distribution and its use in MOCVD or ALD processes. [242]

Bis(2,2,6,6,-tetramethyl-3,5-heptanedionato) strontium [Sr (thd)2]

Fig. Trimeric structure of [Sr(thd)2]3

Bis(2,2,6,6,-tetramethyl-3,5-heptanedionato) strontium Sr(thd)2 is a white solid trimer at room temperature. Its melting point is reported at 210°C and its vapour pressure of 0.05 Torr at 230°C. Unadducted Sr bis-2,2,2,6-teramethyl-3,5-heptandionate, has a trimeric structure in solid state (Fig. ). [[i]] The TGA curve shows that the most part of the precursor evaporates between 340°C and 410°C (atmospheric pressure) (Fig. ).[244]

[i] J. Brooks, H.O. Davies, T.J. Leedham, A.C. Jones and A. Steiner, Chem. Vap. Deposition, 2000, 6, 66-69.

Sr(thd)2 for SrTiO3 by ALD

    Sr(tmhd)2 was evaluated as strontium ALD precursor at high temperature: 600°C to 725°C. Such a high deposition temperature would require a specific design of the other precursors, which would need to be used at that high temperature, such as titanium precursors for SrTiO3 depositions. Therefore Sr (tmhd)2 was suggested to be not an ideal strontium precursor. [[i]]

     Sr(thd)2 was reported to be successfully applied as precursor for the preparation of SrTiO3 thin layers by ALD at lower temperatures at 250–470°C (in combination with Ti(OiPr)4 and O3). The as-deposited films contained some carbon as SrCO3; good quality SrTiO3 were obtained after thermal annealing reducing the carbon content. [4]

[i] Gao et al. J. Appl. Phys., 2000, vol 87, N°1

Bis(2,2,6,6-tetramethyl-3,5-heptanedionato) strontium bis(2-dimethylaminoethyl)amine adduct Sr(thd)2[HN(CH2CH2NMe2)2]

Monomeric seven-coordinate Sr(thd)2[HN(CH2CH2NMe2)2] complex was obtained  by heating ethanol solvate Sr(thd)2[HN(CH2CH2NMe2)2](EtOH) at 130°C, the latter one was synthesized by the reaction of Sr(OEt)2(EtOH)4 with H-tmhd in the oresence of  HN(CH2CH2NMe2)2. The complex Sr(thd)2[HN(CH2CH2NMe2)2] is stable at temperatures below 220 °C, according to the combined TGA and thermal stability tests. [[i]]

[i] B. Luo, D. Yu, B. E. Kucera, S. A. Campbell, W. L. Gladfelter, Chem. Vapor Deposition, 2007, Vol. 13, Iss. 8, p.381–388, “Synthesis and Structures of Bis(2-dimethylaminoethyl)amine Adducts of Strontium Bis(2,2,6,6-tetramethylheptane-3,5-dionate) and Their Use in the CVD of Cubic Strontium-Doped Hafnium Dioxides”

Sr(thd)2[HN(CH2CH2NMe2)2] for HfO2(Sr) by MOCVD

Sr(thd)2[HN(CH2CH2NMe2)2] was applied as  a liquid (at 115 to 175 °C) Sr precursor for the deposition of  high-κ dielectric strontium hafnium oxide films (in combination with Hf(OtBu)4 as hafnium precursor) by cold-wall, low-pressure MOCVD. The atomic ratios of the films ranged from 0 to 0.83 by variation of Sr/(Sr + Hf) ratios. Layers with low Sr doping, (x ≤ 0.15) were crystalline (Sr-stabilized cubic hafnia structure), whereas layers with higher Sr content were amorphous, as was determined by XRD. The proportion of the cubic phase increased the dielectric constant of the films; with maximum value of k=25 obtained for the layer grown with a ratio Sr/(Sr + Hf) = 0.07.

Bis (methoxyethoxy-2,2,6,6-tetramethylheptane-3,5-dionato) strontium [Sr(methd)2]

Fig. Comparison of vapor pressures: diketonate Sr(methd)2 vs alkylcyclopentadienyls Sr(Cp*)2(DME) and Sr(nPrMe4)2

     To overcome some of the drawbacks of conventional strontium beta-diketonates, Lee et al. proposed to use strontium bis(methoxyethoxy-2,2,6,6-tetramethylheptane-3,5-dionate) [Sr(methd)2] as MOCVD precursor .

     Sr(methd)2 is a liquid at room temperature and it thermogravimetric analysis showed a good thermal stability with full evaporation without residue at 398°C. The comparison of estimated vapor pressure of strontium alkylcyclopentadienyls with Sr diketonate Sr(methd)2 vapor pressure is given in Fig. The alkylcyclopentadienyls Sr(Cp*)2(DME) and Sr(nPrMe4Cp)2 are clearly more volatile than the β-diketonate Sr(methd)2.

Sr(methd)2 for Ba1-xSrxTiO3 by thermal MOCVD

     Lee et al. evaluated the behaviour of Sr(methd)2 in Ba1-xSrxTiO3 (BST) MOCVD from 350°C to 600°C, with Ba(methd)2 and Ti(mpd)2(thd)2 as Ba and Ti precursors respectively. However, Sr(methd)2 is very viscous at room temperature (105 cp), which makes its handling as a liquid unlikely. Moreover, its poor volatility makes its use non convenient in ALD processes with good step coverage in deep trenches with high aspect ratio. [[i]]

[i] Lee et al., J. Mater. Res. , 1999, Vol. 14, n°10

Sr(methd)2 for Ba1-xSrxTiO3 by liquid injection MOCVD

     Sr(methd)2 is also reported to be applicable as precursor for deposition of Ba1-xSrxTiO3 films (combined along with new Ti precursors) by liquid injection MOCVD. (Sr(methd)2 was dissolved in n-butyl acetate as solvent). The vaporizer temperature and the precursor solution delivery tube were kept at quite high temperatures (280°C). [[i]]

[i] U.S. patent 6,689,427

Sr(methd)2 for SrO by ALD

    Using Sr(methd)2 as a strontium precursor, Kil et al. overcame the usual carbon incorporation in the O3 ALD process with strontium β-diketonates, by using remote plasma activated oxygen radicals as oxidant source, and obtained an atomic layer deposition of SrO in the 150-275°C temperature range at 0.78Å/cycle  The results were however not satisfactory enough, it was rather preferable to get a precursor with a higher reactivity to H2O.[[i]]

[i] Kil et al., Chem. Vap. Dep., 2002, 8, n°5

Strontium 1,1,1,2,2,3,3,7,7,8,8,9,9,9-tetradecafluorononane-4,6-dionate [Sr(TDFND)2H2O]

Strontium 1,1,1,2,2,3,3,7,7,7-decafluoroheptane-2,4-dionate [Sr(DFHD)2H2O]

     Strontium complexes of the β-diketones 1,1,1,2,2,3,3,7,7,8,8,9,9,9-tetradecafluorononane-4,6-dione (HTDFND)   and 1,1,1,2,2,3,3,7,7,7-decafluoroheptane-2,4-dione (HDFHD) were prepared and characterised analytically and spectroscopically. Simultaneous thermal analysis Sr(TDFND)2H2O] and [Sr(DHFD)2H2O] at 1 atm revealed that the complexes lose water but then sublime, generally completely without decomposition. 

         [Sr(TDFND)2H2O] and ([Sr(DFHD)2H2O] complexes were demonstrated to be  suitable precursors for the growth of SrF2 on Si substrates by MOCVD. The grown SrF2 layers had complete (111) orientation.  Changes in the film growth rate with time were attributed to precursor decomposition (in case of [Sr(DFHD)2H2O]). [[i]]

[i]  S.C. Thompson, D.J. Cole-Hamilton, D. D. Gilliland, M.L. Hitchman, J.C. Barnes, « Advanced Materials for Optics and Electronics, 1992vol. 1, Iss. 2, p.81–97, « Stable and volatile β-diketonate complexes of copper, calcium, strontium, barium and yttrium for use as chemical vapour deposition precursors » 

Strontium bis(2,2,6,6-tetramethylheptane-3,5-dionate) (N,N’,N’’,N’’’,N’’’’- pentamethyldiethylenetriamine adduct Sr(thd)2(PMDETA)

    Sr(thd)2 adduct with pentamethyldiethylenetriamine Sr(thd)2(PMDETA) is volatile , and  in combination with Ba(thd)2(PMDETA) and Ti(OiPr)4, Ti(OiPr)2(thd)2 or Ti(dmae)4 was applied as Sr precursor for the growth of BST ((Ba,Sr)TiO3) thin films by MOCVD.

Sr(thd)2(PMDETA) for (Ba,Sr)TiOx by liquid injection MOCVD

     Sr(thd)2(PMDETA)  (PMDETA=pentamethyldiethylenetriamine)) was applied as Sr precursor (combined with Ti(dmae)4 (dmae=dimethylaminoethoxide) and Ba(thd)2-PMDETA)  for the deposition of barium strontium titanate (Ba,Sr)TiOx (BST) thin films by direct liquid injection MOCVD.The incorporation of Sr and Ba into the BST film was almost constant (Sr/(Ba+Sr)=0.5) at growth temperatures 420–500 °C. The step coverage and properties of BST films obtained using Sr(thd)2(PMDETA)  as Sr source were studied.[i]

[i] J.-H. Lee, J.-Y. Kim, J. Vacuum Sci. Tech. A 17, 3033 (1999); « Metalorganic chemical vapor deposition of barium strontium titanate thin films with a more coordinatively saturated Ti precursor, Ti(dmae)4 (dmae=dimethylaminoethoxide) »,  doi.org/10.1116/1.582001, avs.scitation.org/doi/abs/10.1116/1.582001 

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