Liquid Compounds for CVD of Alkaline Earth Metals
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impurities can segregate on the surface of a solid, further degrading their reproducibility as vapor sources. In fact, most practical applications of CVD use liquid sources, rather than solids. Solid sources can also be dissolved in a liquid solvent, and the liquid solution can subsequently be flash-vaporized. This approach, however, introduces large amounts of solvent vapors into the CVD reactor. The solvent vapors can introduce impurities, such as carbon, into the deposited material. Hazards, such as flammability or toxicity, may be introduced by adding solvents. Solvents also increase the effort needed to dispose of waste from the process. Alkaline earth metals are essential components of many important materials, such as those listed in the abstract. Unfortunately, volatile liquid compounds have not been available for the alkaline earth metals, so CVD of these materials has had to rely on inconvenient solid sources.' A room-temperature liquid barium compound was reported,2 but it has been found to be nonvolatile. Numerous low-melting solid barium compounds have been prepared, but all reported compounds are solid at room temperature.' Liquid mixtures of certain alkaline earth compounds have been made, and can be flash vaporized as CVD precursors. While these mixtures are useful, it would be easier to purify and analyze pure liquid alkaline earth compounds. The first pure liquid barium compounds were reported recently.' They are barium betadiketonates bound to N,N',N"-trialkyldiethylenetriamines. When the alkyl chains are more than
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Mat. Res. Soc. Symp. Proc. Vol. 574 -1999 Materials Research Society
four carbons long, the resulting barium compounds are liquid at room temperature. These liquid compounds were successfully flash-vaporized as vapor sources for CVD of barium-containing materials, such as barium titanate. In the present paper, similar liquid compounds are reported for strontium, calcium and magnesium, and methods for their synthesis are given. These beta-diketonates bound to partially alkylated polyamines are liquid at room temperature and monomeric in hydrocarbon solution. One of the strontium compounds crystallized just below room temperature. It was found to be monomeric by X-ray crystallography, with four oxygen atoms and three nitrogen atoms bound to each strontium atom. All of the new compounds can be flash-vaporized as CVD precursors. Some of the magnesium compounds can even be distilled intact at low pressures, although the barium, strontium and calcium compounds dissociate into metal P3-diketonate and polyamine under slow distillation. As an example, barium strontium titanate films were prepared by CVD using some of the new liquid compounds as precursors. SYNTHESIS OF NEW LIQUID PRECURSORS Dialkylethylenediamines were prepared by the following reaction: 0 H 2N
0
0
0
/
NH 2 +
R
OR
RN H
N H
R
iAH
0O
+ '
R
I
H
H20
H
H
H
H
N)
RNH
N H
Synthesis of N,N'-dihexylethylenediamine (dheda): In a 500-mL flask, ethylenediamine (7.79 mL, 120 mmol) was dissolved in 150 mL of te
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