Metalorganic chemical vapor deposition precursors

A method of manufacturing a strontium .beta.-diketonate precursor suitable for MOCVD techniques comprises the step of reacting strontium with a sterically hindered alcohol to produce strontium alkoxide, subsequently reacting the strontium alkoxide with a .beta.-diketone to form a strontium .beta.-diketonate alcohol adduct and removing the alcohol from the adduct.

DESCRIPTION
 This invention concerns a method for manufacturing precursors for use in
 metalorganic chemical vapour deposition (MOCVD) techniques.
 Strontium bis-tetramethylheptanedionate, Sr(thd).sub.2, has important
 applications as a precursor for growth of ferroelectric, dielectric and
 superconducting oxide films by MOCVD. Examples of these oxides are barium
 strontium titanate, (Ba,Sr) TiO.sub.3, and strontium bismuth tantalate,
 SrBi.sub.2 Ta.sub.2 O.sub.9.
 MOCVD precursors should be volatile and evaporate cleanly without leaving
 any significant involatile residues. The precursors should also be pure
 i.e. free from extraneous organic impurities, which might interfere with
 the MOCVD process.
 In the case of Group IIA .beta.-diketonate precursors, such as
 Sr(thd).sub.2, the purity of the compound and evaporation characteristics
 are critically dependent on the method of manufacture of the precursor.
 Group IIA .beta.-diketonates may be prepared by an aqueous route, such as
 by addition of .beta.-diketone to an aqueous solution of metal chloride or
 metal hydroxide or by addition of Na(thd) to hydrated metal chloride.
 However, these routes are unsuitable for producing precursors for MOCVD as
 they lead to hydrated species, which form involatile residues on
 evaporation during MOCVD. This is a serious problem as it leads to changes
 in the precursor gas-phase concentration over time and having poor
 uniformity oxide layers.
 Preferred routes to producing high purity Group IIA .beta.-diketonates are,
 therefore, non-aqueous and are carried out in hydrocarbon or alcohol
 solvents with rigorous exclusion of air and moisture during synthesis.
 An alternative non-aqueous route called `labile ligand displacement` has
 been proposed for producing Sr(thd).sub.2.
 The theoretical process is as follows:
EQU Sr+xEtOH.fwdarw.Sr(OEt).sub.2- xEtOH
EQU Sr(OEt).sub.2.times. xEtOH+2thdH.fwdarw.Sr(thd).sub.2 +xEtOH
 However, the resultant precursor is not Sr(thd).sub.2 but a trimeric
 species {Sr(thd).sub.2 }.sub.3 (thdH), which contains adducted neutral
 thdH ligand. This ligand is lost during evaporation of the precursor
 during MOCVD, which is an undesirable complication in the process. FIG. 1
 of the accompanying drawings illustrates the effect.
 An object of this invention is to provide a method of manufacturing a
 strontium .beta.-diketonate precursor suitable for use in MOCVD
 techniques.
 According to this invention there is provided a method of manufacturing a
 strontium .beta.-diketonate precursor suitable for use in MOCVD techniques
 comprising the steps of reacting strontium with a sterically hindered
 alcohol to produce strontium alkoxide, subsequently reacting the strontium
 alkoxide with a .beta.-diketone to form a strontium .beta.-diketonate
 alcohol adduct and removing the alcohol from the adduct.
 The alcohol used in the reaction may also serve as solvent for the
 reaction.
 The alcohol chosen for use in the method of the invention is preferably a
 secondary or tertiary alcohol in order to provide the desired steric
 hindrance. Iso-propanol is a preferred alcohol for use in the invention,
 although other bulky alcohols such as iso-butanol or tertiary butanol may
 also be suitable. Indeed any alcohol more sterically hindered than
 methanol or ethanol could be suitable for use in the method of the
 invention.
 The .beta.-diketone used in the method of the invention preferably has the
 formula
 ##STR1##
 wherein R.sub.1 and R.sub.2 are the same or different and are straight or
 branched, optionally substituted, alkyl groups or, optionally substituted,
 phenyl groups. Example of suitable substituents include chlorine, fluorine
 and methoxy.
 Examples of suitable .beta.-diketones for use in the method of the
 invention include the following:

The invention will be yet further described by means of the following
 Example.
 EXAMPLE
 87.6 (1 mole) of strontium metal was dissolved in 3 liters of dry
 iso-propanol. The mixture was brought to the boil under reflux in order to
 dissolve all the strontium metal. 368 g (2 moles) of
 tetramethylheptanedione (thdH) was then added and the mixture stirred at
 room temperature for 1 hour.
 The volume of the mixture was reduced to 800 cm.sup.3 by vacuum
 distillation (50.degree. C.) and the mixture set aside to crystallize.
 The resulting crystals were identified by x-ray diffraction as the
 monomeric iso-propanol adduct of Sr (thd).sub.2, [Sr(thd).sub.2 ]
 (i-PrOH). The structure for this adduct is shown in FIG. 3 of the
 drawings.
 The yield was 295 g (65% based on Sr metal).
 The crystals were then pumped under vacuum at between 30-80.degree. C. to
 remove partially the adducted iso-propanol, resulting in a
 microcrystalline solid.
 A 100 g portion of the monocrystalline solid was then dissolved in 1 liter
 of dry nonane. The solution was heated at 60.degree. C. under reduced
 pressure (20 mm Hg) and the residual iso-propanol was removed by
 distillation at a head temperatue of approximately 23.degree. C. The
 removal of iso-propanol was continued until the distillation lead
 temparature rose to approximately 54.degree. C.
 The volume of the nonane solution was reduced in vacuo (maximum temperature
 65.degree. C.) until crystals were seen to form in the reaction vessel.
 The mixture was then set aside to cool. The resulting crystals were
 filtered and dried in vacuo to give Sr(thd)2 as a white microcrystalline
 solid, which gave the following microanalysis results for four different
 samples: