ANHYDROUS MONONUCLEAR TRIS(BETA-DIKETONATE) BISMUTH COMPOSITIONS FOR DEPOSITION OF BISMUTH-CONTAINING FILMS, AND METHOD OF MAKING THE SAME

Abstract: Anhydrous mononuclear tris( beta -diketonato) bismuth complexes, useful as precursors for chemical vapor deposition of bismuth, for producing Bi-containing films of significantly improved stoichiometry, morphology and functional character, as compared to films obtained from dinuclear tris( beta -diketonato) bismuth complexes of the prior art.

Description: ANHYDROUS MONONUCLEAR TRIS(BETA-DIKETONATE) BISMUTH COMPOSITIONS FOR DEPOSITION OF BISMUTH-CONTAINING FILMS, AND METHOD OF MAKING THE SAME DESCRIPTION Field Of The Invention This invention relates to the synthesis and production of anhydrous mononuclear tπs(β- diketonate) bismuth compositions, e.g., anhydrous mononuclear tπs(2,2,6,6-tetramethyl- 3,5-heptanedιonato) bismuth. Such bismuth-containing compositions have utility as precursors for chemical vapor deposition of bismuth, bismuth oxide, bismuth-containing oxides and bismuth-containing chalcogonides. Description of the Related Art Ferroelectric random access memoπes (FRAMs) rely on high-integrity ferroelectric thin- iilms as critical components of memoiy cell architecture Electrical performance ot ferroelectric oxides such as SrBι 2 Ta 2 O 9 (SBT) show a strong dependence on the identity of the precursor used in depositing the Bi component in the ferroelectric material. For example, the use of a Bi precursor such as tπphenyl bismuth results in poor stoichiometπc control, high substrate temperatures to decompose the precursor, strong surface dependence, and extreme dependence of the precursoi incorporation efficiency on reactor pressure and partial pressure of oxygen duπng deposition. To ameliorate the foregoing deficiencies, the art has continued to seek other bismuth precursors. One such bismuth-containing precursor candidate is Bi tπs(2,2,6,6-tetramethyl-3,5- heptanedionato). As used hereinafter, the hgand "2,2,6,6-tetramethyl-3,5- heptanedionato" is sometimes referred to by the designation "thd." It is an object of the present invention to provide an improved class of bismuth precursors for deposition of Bi for applications such as ferroelectric thin films and devices. Other objects and advantages of the present invention will be more fully apparent horn the ensuing disclosure and appended claims. SUMMARY OF THE INVENTION The present invention relates to anhydrous mononuclear tπs(β-dιketonate) bismuth compositions, and to a method of synthesis thereof. Such anhydrous mononuclear bismuth precursors have been discovered as novel compositions having unexpectedly supeπor properties in relation to dinuclear (β-diketonate) bismuth compositions of the pπor art, in respect of their volatilization and deposition characteristics which render the anhydrous mononuclear bismuth precursors of the present invention particularly suitable as CVD precursors. The anhydrous mononuclear bismuth compositions of the present invention therefore constitute a substantial advance in the art over the dinuclear tπs(β- diketonate) bismuth precursors heretofore available. The β-diketonato ligand of the anhydrous mononuclear tπs(β-diketonate) bismuth compositions of the present invention may be any suitable type, including the illustrative β-diketonato ligand species set out in Table I below: TABLE I β-diketonato ligand Abbreviation 2,2,6,6-tetramethyl-3,5-heptanedιonato thd 1 J J -tnfluoro-2,4-pentanedιonato tfac 1 J J ,5 ,5 ,5-hexafluoro-2,4-pentanedιonato hfac 6,6,7,7,8,8,8-heptafluoro-2,2-dιmethyl-3,5-octanedιonato fod 2,2,7-tπmethyl-3.5-octanedιonato tod lJJ,5,5,6,6J,7J-decafluoro-2J-heptanedιonato dfhd 1 J J -tπfluoro-6-methyl-2J-heptanedιonato tfmhd The anhydrous mononuclear tπs(β-dιketonato) bismuth compositions of the invention have utility as precursors for the vapor-phase deposition of bismuth, as for example in the formation of feiroelectπc thin films of SιBi2Ta2θo (SBT), or in the formation ot superconductor films containing Bi In such applications, the use ot the anhydious mononuclear Bi source material piovides foi bettei thermal transpoit and f lash vapoπzation leading to Bi-containing films of significantly improved stoichiometiy, morphology and ferroelectric/superconducting chaiactei The synthesis ot anhydrous mononuclear tπs (β-diketonato) bismuth complexes ol the present invention may be earned out in an aprotic solvent medium under anaerobic conditions, by reacting the corresponding Na(β-dιketonato) compound with a Bi tπhalide compound Punfication of the isolated reaction product bismuth complex, e g., by recrystallization, should also be earned out in an aprotic medium undei anaeiobic conditions. In a specific method aspect, the present invention relates to a method of synthesis of anhydrous mononuclear tns(2,2,6,6-tetramethyl-3,5-heptanedιonato) bismuth, by reaction of Na(2,2,6,6-tetramethyl-3,5-heptanedιonato) and BiCl3 in an aprotic solvent under anaerobic conditions. The anhydrous mononuclear tns(2J,6,6-tetramethyl-3,5-heptanedιonato) bismuth precursor of the invention may be usefully employed tor depositing bismuth or a bismuth-containing him on a substrate, by vaponzmg the anhydrous mononucleai tπs(β- diketonato) bismuth to form a vaporized precursor, and contacting the vaporized precursor with the substrate to deposit bismuth or a bismuth-containing film thereon. Such deposition may employ liquid de veiy and flash vaponzation of the anhydrous mononuclear tπs(β-dιketonato) bismuth precursor to form the piecursoi vapoi. and the deposition may be effected by various techniques such as chemical vapor deposition (CVD), including any of vanous assisted (e g., plasma-assisted, photoactivated, ion beam-assisted, etc.) CVD methods BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 and 2 are x-ray crystallographic structural depictions reported in the literature (PRIOR ART) for the dinuclear bismuth complex [Bι(thd)3]?, which is a crystalline white solid with a melting point of 1 17°C Figure 3 is a plot of precursor transport (TGA) and melting point (DSC) cuives for a representative dinuclear bismuth complex [Bι(thd)3]2 of the pnor art Figure 4 is an x-ray crystallographic structural depiction for the anhydrous mononuclear tns(2,2,6,6-tetramethyl-3,5-heptanedιonato) bismuth complex of the piesent invention This matenal represents a new form of Bι(thd) 3 Figure 5 is a plot of a melting point (DSC) curve for anhydrous mononuclear tns(2, 2,6,6- tetramethyl-3,5-heptanedιonato) bismuth complex of the piesent invention, exhibiting a melting point of about 139°C Figure 6 is a plot of melting point (DSC) curves for the anhydrous mononuclear tns(2,2,6,6-tetramethyl-3,5-heptanedιonato) bismuth complex ot the present invention in "tiesh" synthesized condition (curve A), after exposuie to air/moisture foi 48 hours (curve B) and after exposure to air/moisture for 96 hours (curve C), showing the complete conveision of the anhydrous mononucleai tπs(2,2,6,6-tetramethyl-3,5- heptanedionato) bismuth complex after 96 hours of air/moisture exposure into the lower melting point dinuclear tns(2,2,6,6-tetιamethyl-3,5-heptanedιonato) bismuth complex, [Bι(thd) 3 ] 2 DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF The present invention relates to the discovery of anhydrous mononuclear forms ot tπs(β- diketonato) bismuth compositions, e g , tns(2,2,6,6-tetramethyl-3,5-heptanedιonato) bismuth, which overcome deficiencies of the prior art dinuclear bismuth complex [Bι(thd)3]2. As a result of its mononuclear form, the bismuth complex of the present invention provides improved thermal transport and more controlled and reproducible gas-phase concentrations than are possible with the pnor art dinuclear complex. These charactenstics provide the mononuclear bismuth complexes of the present invention with the ability to form Bi-containing films ot supeπor stoichiometry, morphology and functional performance charactenstics, in relation to the films of the pnor art formed from the aforementioned dinuclear bismuth complex [Bι(thd)3]2 As a precursor tor the deposition ol Bi or Bi-contain g films, the previously commercially available dinuclear Bi-thd complex is markedly interior in respect ot its volatility, transport and vapoπzation properties, being less volatile and moie easily decomposed during transpoit and flash vapouzauon than desiied, and pioducing highei levels of residue in process equipment upstieam of the deposition chambei The ready susceptibility of the pnoi art dinuclear bismuth complex [Bι(thd)3]2 to decomposition of the precursor is severely detrimental in a liquid delivery Hash vaporization piocess, where such decomposition leads to premature clogging of the vapoπzei as well as undesirable changes in gas-phase concentrations dunng the deposition process Such gas-phase concentration variations in turn yield undesirable giadients in deposited film thickness, and in film stoichiometry where the bismuth component is deposited in a multicomponent film. Unexpectedly, the discovery of the mononuclear Bι(β-dιketonate) 3 complexes of the present invention has provided a solution to such deficiencies of the pnor art Bι(thd) 3 precursor matenal. The mononuclear Bι(β-dιketonate) 3 complexes of the present invention are markedly supeπor to the corresponding dinuclear species, providing improved vaporizer performance, extended vaponzer lifetimes and better control of the Bi-containing film stoichiometry than [Bt(thd) 3 ] 2 . As a result, the Bι(β-dιketonate) 3 mononuclear complexes of the present invention lead to a more highly reproducible CVD process and improved quality ot the deposited film, based on film stoichiometry and uniformity While the anhydrous mononuclear tns(β-dιketonato) bismuth complexes ot the present invention are usefully employed in a wide vanety of chemical vapor deposition piocesses for the formation of bismuth-containing films, the complexes have particular utility as a precursor for the vapor-phase deposition of bismuth or bismuth oxide in the formation ot terroelectnc thin lilms and Bi-based superconducting thin film matenals A most preferred use of such mononuclear tns(β-dιketonato) bismuth complexes is in the formation of ferroelectnc Bi-contaming thin films for the manufacture of devices such as ferroelectric random access memones The same matenals may be used to deposit bismuth-containing chalcogemdes and skutteurdites. The mononuclear tπs(β-dιketonato) bismuth complexes of the invention may be deposited in any suitable manner For example, the bismuth deposition process may employ liquid delivery and flash vaponzation of the anhydrous mononuclear tns(β- diketonato) bismuth precursor to form the precursor vapor, and the deposition itself may be effected by chemical vapor deposition (CVD), including any of vaπous assisted (e.g., plasma-assisted) CVD methods, or m any other suitable manner. For purposes of liquid delivery, the anhydrous mononuclear tπs(β-dιketonato) bismuth may be dissolved in any suitable solvent medium, e.g., a single solvent or a multicomponent solvent mixture, compatible with such bismuth reagent A suitable multicomponent solvent mixture for such purpose is a solvent composition comprising solvent species A, B and C, wherein A is a C6-C8 alkane, B is a C8-C f2 alkane, and C is a glyme-based solvent (glyme, diglyme, tetraglyme, etc.) or a polyamine, in the proportion A:B.C wherein A is from about 3 to about 7 parts by volume, B is from about 2 to about 6 parts by volume, and C is from 0 to about 3 parts by volume. A highly preferred solvent composition ot such type comprises octane, decane and a polyamine in approximately 5:4: 1 proportion by volume. Polyamine species potentially useful as component C in the above-described solvent composition include N, N, N', N'-tetramethylethylenediamine, N, N, N', N", N"- pentamethyldiethylenetnamine, N, N, N\ N", N'", N"'-hexamethyltπethylenetetramιne, or other suitable polyamine component. The synthesis of anhydrous mononticleai tπs (β-diketonato) bismuth complexes of the present invention may be readily earned out in an aprotic solvent medium under anaerobic conditions, and at room temperature and pressure, by reaction of the corresponding Na(β-dιketonato) compound with BX 3 , wherein X is halo (bromo, chloro, lodo, or fluoro), and most preferably chloro. Punfication of the isolated reaction product bismuth complex, e.g., by recrystal zation, is also advantageously performed in an aprotic medium under anaerobic conditions. The aprotic solvent may suitably compnse one or more alkanes such as pentane, octane, or decane, aliphatic or cyclic ethers such as tetrahydrofuran, aryl solvent species such as benzene or toluene, and/or any other suitable aprotic solvent(s) for solution ot the Na(β- diketonato) and B1X3 starting matenals, wherein such solvents do not preclude the reaction of such Na(β-dιketonato) and B1X3 starting matenals to form the mononucleai (β-diketonato) bismuth product. The foregoing synthesis reaction is earned out under anaerobic conditions, i.e., in the substantial absence, and preferably substantially complete absence, ot oxygen Such anaerobic conditions may for example compnse carrying out the reaction under inert or oxygen-free atmospheie, such as under a nitrogen or aigon blanket over the reaction vessel containing the reactants and the aprotic solvent medium. Similarly, the solvents and reactants should be dry and free from water and other protic source constituents As an illustrative example of the foregoing synthesis, anhydrous mononucleai tns(2,2,6,6-tetramethyl-3,5-heptanedιonato) bismuth may be synthesized in accordance with the present invention by reaction of Na(2,2,6,6-tetramethyl-3,5-heptanedιonato) with B1CI3 in octane, and under a nitrogen gas environment. Subsequent to its synthesis, the anhydrous mononuclear tns (β-diketonato) bismuth complexes of the invention may be converted to the corresponding dinuclear species, if desired, by exposure of the mononuclear tns (β-diketonato) bismuth complex to air and/or protic media such as water, isopropanol, etc. Referring now to the drawings, Figures 1 and 2 are x-ray ciystallographic stiuctural depictions reported in the literature (PRIOR ART) tor the dinuclear bismuth complex [Bι(thd) 3 ] 2 , which is a ciystalhne white solid with a melting point ol 117°C Figure 3 is a plot of precursor tianspoit (TGA) and melting point (DSC) curves tor a representative dinuclear bismuth complex [Bι(thd)3]2 of the prior art, having a melting point ot 117°C. The literature has vanously leported the melting point of such material as being in the range of 112-115°C. This matenal is interior in precursoi tiansport properties, as reflected by the TGA plot, showing a gradual weight loss ovei a wide temperature range during the STA examination Figure 4 is an x-ray crystal lographic structuial depiction tor the anhydrous mononucleai tns(2,2,6,6-tetramethyl-3,5-heptanedιonato) bismuth complex of the present invention. This matenal represents a new form of Bι(thd) 3 Figure 5 is a plot of a melting point (DSC) curve for anhydrous mononuclear tπs(2,2,6,6- tetramethyl-3,5-heptanedιonato) bismuth complex of the present invention, exhibiting a melting point of about 139°C The IR and NMR characteristics of this material aie consistent with the mononuclear character of Bι(thd) 3 . Figure 6 is a plot of melting point (DSC) curves for the anhydrous mononuclear tns(2,2,6,6-tetramethyl-3,5-heptanedιonato) bismuth complex of the present invention in "fresh" synthesized condition (curve A), after exposure to air/moisture for 48 hours (curve B) and after exposure to air/moisture for 96 hours (curve C), showing the complete conversion of the anhydrous mononuclear tπs(2,2,6,6-tetramethyl-3,5- heptanedionato) bismuth complex after 96 hours of air/moisture exposure into the lower melting point dinuclear tπs(2,2,6,6-telιamethyl-3,5-heptanedιonato) bismuth complex. [Bι(thd) 3 ] 2 Thus, ambient air/moisture exposure gradually converts the mononuclear precursor to the lower melting dinuclear matenal having a melting point in the range of 117-120°C Prolonged direct air exposure over an extended penod of time is also observed to result in the formation of a white, wet solid and eventually a cleai liquid The dissolution ol solid mononuclear Bι(thd) matenal in isopropanol oi othei protic solvent has also been found to effect a conversion of the mononuclear material to the dinuclear complex, [Bι(thd) 3 ] 2 Single crystal x-ray diffraction staictural determination conclusively show the Bι(thd) 3 matenal of the present invention to be of mononuclear form. Such matenal has a melting point which is more than 20°C above that reported in the literature. The pπoi art literature reports an x-ray crystal structure which is a dinuclear complex, [Bι(thd) 3 ] 2 , with excess Hthd ligand in the crystal lattice The pnor art literature reports the previously known Bi-thd complex as being air and moisture stable. This is untrue for the dinuclear complex, [Bι(thd) 3 ] 2 , and is also untrue for the anhydrous mononucleai Bι(thd) 3 material of the present invention, as shown in the plot of Figure 6. The mononuclear Bι(thd) 3 can be converted to the known dinuclear complex, [Bι(thd) 3 ] 2 by air exposuie or exposure to protic media such as isopropanol. The Bι(thd) 3 matenal ot the present invention thus is a mononuclear complex heretofore unknown, and represents a significant advance in the art m the provision of a precursoi matenal having highly attractive charactenstics for vaponzation, vapor-phase transport, and bismuth deposition, in a wide vanety of applications in which bismuth or bismuth- containing films aie employed in the fabncation ot microelectronic, terroelectnc and superconducting devices or structures INDUSTRIAL APPLICABILITY The anhydrous mononuclear tπs(β-dιketonato) bismuth complexes of the piesent invention have utility tor chemical vapor deposition of bismuth-containing films, for fabncation of microelectronic devices such as high-integπty ferroelectric thin-film random access memory devices and bismuth-containing high-temperatuie superconductors The bismuth precuisois ot the present invention permit the formation of bismuth-containing films for such applications, having significantly improved stoichiometry, morphology and ferroelectnc/superconducting character, relative to films produced using precursoi s of the pnor art.

Claims: THE CLAIMS 1. Anhydrous mononuclear tris(β-diketonato) bismuth. 2. An anhydrous mononuclear tris(β-diketonato) bismuth complex, wherein the β- diketonato ligand is selected from the group consisting of: 2,2,6,6-tetramethyl-3,5-heptanedionato; lJJ-trifluoro-2,4-pentanedionato; 1 J J ,5,5,5-hexafluoro-2,4-pentanedionato; 6,6,7,7, 8,8, 8-heptafluoro-2,2-dimethyl-3,5-octanedionato; 2,2,7-trimethyl-3,5-octanedionato; lJJ,5,5,6,6,7,7,7-decafluoro-2J-heptanedionato; and lJJ-trifluoro-6-methyl-2,4-heptanedionato. 3. Anhydrous mononuclear tris(2,2,6,6-tetramethyl-3,5-heptanedionato) bismuth. 4. Tris(2,2,6,6-tetramethyl-3,5-heptanedionato) bismuth having a melting point of about 139°C. 5. Tris(2,2,6,6-tetramethyl-3,5-heptanedionato) bismuth having the x-ray crystallographic structure shown in Figure 4. 6. A method of synthesis of an anhydrous mononuclear tris (β-diketonato) bismuth complex, comprising reacting a corresponding Na(β-diketonato) compound with a Bi trihalide compound in an aprotic solvent medium under anaerobic conditions for sufficient time and at sufficient temperature to yield said anhydrous mononuclear tris (β- diketonato) bismuth complex as a reaction product thereof. 7. A method according to claim 6, wherein the aprotic solvent medium comprises a solvent selected from the group consisting of alkanes, aliphatic ethers, cyclic ethers, and aryl compounds. 8. A method according to claim 6, wherein the aprotic solvent medium comprises a solvent selected from the group consisting of pentane, octane, decane, tetrahydrofuran, benzene and toluene. 9. A method according to claim 6, wherein the Na(β-diketonato) compound comprises a β-diketonato moiety selected from the group consisting of: 2,2,6,6-tetramethyl-3,5-heptanedionato; 1 J J -trifluoro-2,4-pentanedionato; 1 J J ,5,5,5-hexafluoro-2,4-pentanedionato; 6,6,7,7,8,8,8-heptafluoro-2J-dimethyl-3,5-octanedionato; 2,2,7-trimethyl-3,5-octanedionato; lJJ,5,5,6,6,7,7,7-decafluoro-2,4-heptanedionato; and lJJ-trifluoro-6-methyl-2,4-heptanedionato. 10. A method according to claim 6, wherein the Bi trihalide compound is BiCl 3 . 11. A method of synthesis of anhydrous mononuclear tris(2,2,6,6-tetramethyl-3,5- heptanedionato) bismuth, comprising reaction of Na(2,2,6,6-tetramethyl-3,5- heptanedionato) and BiCl3 in an aprotic solvent medium under anaerobic conditions. 12. A method according to claim 11, wherein the aprotic solvent medium comprises a solvent selected from the group consisting of alkanes, aliphatic ethers, cyclic ethers, and aryl compounds. 13. A method according to claim 11, wherein the aprotic solvent medium comprises a solvent selected from the group consisting of pentane, octane, decane, tetrahydrofuran, benzene and toluene.

Inventors: BAUM THOMAS H, BHANDARI GAUTAM, CHAPPUIS MARGARET, BAUM, THOMAS, H., BHANDARI, GAUTAM, CHAPPUIS, MARGARET
Applicants: ADVANCED TECH MATERIALS, ADVANCED TECHNOLOGY MATERIALS, INC.
Application year: 1998
Main classification: C07F9
IPC codes: C07F9