In the Chemical Vapor Deposition (CVD) fabrication of alkaline earth metal (i.e. barium, strontium, etc.) containing materials there is a growing need for cleanly volatile sources of these metals that can deliver steady gas phase mass transport of metal-containing vapor during the CVD process. It is especially important that the metal containing vapor is stable and, therefore, of constant chemical formulation. This permits precise control over the elemental composition of the final product. Examples of alkaline earth metal containing materials produced in the CVD process are high temperature superconducting ceramic thin films and ultralow loss heavy metal fluoride glasses for use in high transmittance optical fibers. In the former case a vapor containing the "volatile" organometallic compound barium bis(hexafluoroacetylacetonate), i.e. ba(hfac).sub.2, along with volatile compounds of yttrium and copper, is contacted with oxygen gas at 10 torr pressure and 500.degree. C. to deposit a thin film of high temperature superconducting ceramic of the formulation yBa.sub.2 Cu.sub.3 O.sub.x onto a sapphire substrate. K. Shinohara et al Jpn. J. Appl. Phy., 27 (9), L1683 (1988). In the latter case a vapor containing the volatile compound barium bis(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione) along with a volatile zirconium compound is contacted with hydrogen fluoride gas at 10-15 kPa pressure at 210.degree. C. to yield BaF.sub.2 and ZrF.sub.4 respectively. These fluorides are then further processed into high transmittance optical fibers. K. Fujiura et al Jpn. J. Appl. Phy., 28 (1), L147-149 (1989).
The CVD process of forming metal fluorides from volatile sources is a well established technology for metals that readily form volatile complexes such as Be, Al, Zr; see for example U.S. Pat. No. 4,718,929. However, in general, a major requirement in any CVD processing of alkaline earth metal compounds of barium, calcium, and strontium is the need for cleanly volatile sources of these elements. Even though alkaline earth .beta.-diketonate complexes have been utilized in CVD processes, it is well known that they tend to decompose upon sublimation so their effectiveness as precursors for CVD processes is limited by this problem. A. P. Purdy et al Inorg. Chem. (28) 2799-2803 (1989).
One approach towards solving the problem of preparing volatile alkaline earth metal compounds has been to synthesize .beta.-diketonate complexes of these elements wherein the .beta.-diketonate ligands contain bulky groups (i.e. t-butyl or fluorinated groups such as CF.sub.3 or -CF.sub.2 CF.sub.2 CF.sub.3). Alkaline earth complexes such as barium bis(2,2,6,6-tetramethyl-3,5-heptanedionate) or barium bis(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6,-dione) that bear these structural features have been utilized as volatile sources in CVD processes as taught by Y. Hisanori et al Supercond. Sci. Technology 2, 115-117 (1989) and A. J. Panson et al Appl. Phys. Lett. 53 (18), 1756-8 (1988). These two strategies of using sterically demanding ligands and fluorocarbon character are well known to increase the volatility of metal .beta.-diketonate complexes. Sievers, R. E., Science, 201, 217 (1978). This is thought to occur by effecting a reduction in intermolecular association via the shielding of metal centers from each other. However, in the case of alkaline earth metal complexes, such ligands are limited in their ability to promote volatility since they cannot completely supply the degree of coordination or steric shielding that is demanded by these metals. This lack of complete shielding leads to ligands becoming at least partially coordinated (i.e. shared) between two or more metal centers. This in turn leads to polymeric or highly associated involatile compounds. This effect is well known for other alkaline earth complexes that contain simple ligands (i.e. methyl or ethyl organic groups) which are unable to supply the degree of shielding required to stabilize the complexes against polymerization. Huffman, J., Organometallics, vol 8 no. 8 2045 (1989).
The volatilities of metal .beta.-diketonate complexes can be increased by coordinating small neutral molecules to the metal centers of these compounds to form volatile "adducts". This helps to prevent ligands being shared between metal centers and thereby promotes more monomeric and hence more volatile compounds. Examples of such small molecules which have been used to increase the volatility of both magnesium and zinc .beta.-diketonates are tetrahydrofuran (THF) and 1,2-dimethoxyethane (diglyme) as taught in U.S. Pat. Nos. 4,501,602 and 4,558,144. Recently TNO Co. (Division of Technology for Society, Dept. of Chemistry, Zeist, The Netherlands) claim to have prepared volatile derivatives of various alkaline earth .beta.-diketonates by treating parent complexes with various linear polyether glymes such as tetraglyme. However, a problem with the approach of adding simple linear coordinating molecules to alkaline earth .beta.-diketone complexes is that under heating conditions they can dissociate from the metal centers thereby precluding the sublimation of the entire adduct. This also causes the chemical composition of the vapor emitting from the sublimation process to be constantly changing. This translates to a loss of mass transfer control in the CVD process.
USPTO Backgrounds
USPTO Backgrounds