Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2000-07-21
2001-06-19
Richter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S396000
Reexamination Certificate
active
06248928
ABSTRACT:
DESCRIPTION
This invention concerns a method for manufacturing precursors for use in metalorganic chemical vapour deposition (MOCVD) techniques.
Strontium bis-tetramethylheptanedionate, Sr(thd)
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
3
, and strontium bismuth tantalate, SrBi
2
Ta
2
O
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 &bgr;-diketonate precursors, such as Sr(thd)
2
, the purity of the compound and evaporation characteristics are critically dependent on the method of manufacture of the precursor.
Group IIA &bgr;-diketonates may be prepared by an aqueous route, such as by addition of &bgr;-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 &bgr;-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)
2
.
The theoretical process is as follows:
Sr+
x
E
t
OH→Sr(OE
t
)
2−
x
E
t
OH
Sr(OE
t
)
2×
x
E
t
OH+2
thd
H→Sr(
thd
)
2
+x
E
t
OH
However, the resultant precursor is not Sr(thd)
2
but a trimeric species {Sr(thd)
2
}
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 &bgr;-diketonate precursor suitable for use in MOCVD techniques.
According to this invention there is provided a method of manufacturing a strontium &bgr;-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 &bgr;-diketone to form a strontium &bgr;-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 &bgr;-diketone used in the method of the invention preferably has the formula
wherein R
1
and R
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 &bgr;-diketones for use in the method of the invention include the following:
R
1
R
2
CH
3
CH
3
acetylacetone (acacH)
CF
3
CH
3
trifluoroacetylacetone (tfacH)
CF
3
CF
3
hexafluoroacetylacetone (hfacH)
CH
3
C(CH
3
)
3
dimethylheptanedione (dhdH)
C(CH
3
)
3
C(CH
3
)
3
tetramethylheptanedione (thdH)
CH
3
CF
2
CF
2
CF
3
heptafluoroheptanedione (fhdH)
CF
2
CF
2
CF
3
CF
2
CF
2
CF
3
tetradecafluorononanedione (tdfndH)
C(CH
3
)
3
CF
3
trifluorodimethylhexanedione (tpmH)
CF
3
CF
2
CF
3
octafluorohexanedione (ofhdH)
C(CH
3
)
3
CF
2
CF
3
pentafluorodimethylheptanedione
(ppm)
CF
3
CF
2
CF
2
CF
3
decafluoroheptanedione (dfhd)
C(CH
3
)
3
CH
2
CH
2
CH
2
OCH
3
dimethylmethoxyoctanedione
(dmmodH)
CCl
3
CH
3
trichloropentanedione (tclacH)
Ph
Ph
diphenylpropanedione (dppH)
The preferred &bgr;-diketone for use in produding a strontium precursor for MOCVD according to the invention is tetramethylheptanedione (thdH).
It is believed that the use of a sterically hindered alcohol as reactant and solvent leads to the formation initially of Sr(thd)
2
—alcohol adduct as an intermediate, which prevents formation of unwanted [Sr(thd)
2
] thdH adduct as a product.
REFERENCES:
patent: 5453494 (1995-09-01), Kirlin et al.
patent: 5518536 (1996-05-01), Doellein
Drinker Biddle & Reath LLP
Inorgtech Limited
Richter Johann
Witherspoon Sikarl A.
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