All-alkoxide synthesis of strontium-containing metal oxides

Details All-alkoxide synthesis of strontium-containing metal oxides All-alkoxide synthesis of strontium-containing metal oxides

1999-03-24

2001-04-03

Beck, Shrive (Department: 1762)

Coating processes

Heat decomposition of applied coating or base material

Reexamination Certificate

active

06210752

ABSTRACT:

BACKGROUND OF THE INVENTION
The present application is generally directed to a method of making metal-oxide, ceramic materials using only alkoxide precursors and more particularly, to a method of making strontium-containing metal oxide ceramic materials using a strontium neo-pentoxide precursor.
Metal-oxide, ceramic thin film materials have been increasingly investigated for use in various electronic and electro-optical applications. Enhanced development of these metal-oxide, thin film materials, often ferroelectric in nature, is necessary to yield important advances in miniaturization of memory devices, higher capacity memory devices, true memory capacity, increased radiation hardness and very fast switching. However, formation of the desired metal-oxide thin films typically requires rather extreme temperature processing, which is not always conducive to integration with conventional silicon-wafer processing technologies. Typically, at higher temperatures, the stability of the underlying silicon wafer and structures thereon are disrupted and render the final device useless.
While both physical and chemical methods exist for the production of metal-oxide ceramic materials, chemical solution routes (i.e., sol-gel methods routes) to electroceramic thin films are advantageous due to the low processing temperatures required for conversion to the ceramic phase, the flexibility in altering the stoichiometry or inclusion of dopants, and the relatively inexpensive experimental setup required. Polymeric sol-gel processing differs from other chemical methods by the poly-condensation of molecular species within the liquid phase, which often results in the crystallization of complex oxides at significantly reduced temperatures. This is in contrast to many of the other chemical methods of preparation, such as metalorganic decomposition and oxalate or citrate salt decomposition methods.
Several sol-gel methods exist for production of some metal-oxide ceramic thin films but uniform solution routes that require lower processing temperatures are of continued interest. A method using only alkoxide precursors is preferred to produce a precursor solution for the production of ceramic oxide thin films (i.e., ferroelectric and perovskite thin films), in part because of the subsequent low processing temperatures. Another added benefit is that, by using an all alkoxide precursor solution, side reactions can be limited and reproducible, highly uniform, ceramic thin films can be produced. Metal alkoxides are known to be useful precursor compounds in producing high quality metal-oxide ceramic thin films. However, the chemical structural characteristics and properties of these precursor compounds are important in determining the ease of preparation, the processing parameters necessary to prepare the metal oxide thin films and the quality and properties of the resulting thin films.
Alkoxide routes for some metal-oxide ceramic thin films are not utilized due to the low solubility of commercially available precursors. One method to circumvent this problem is to modify these precursors. The methoxy ethanol (MeOEtOH) route reported by Frey et al. (Frey, M., and Payne, D., Chem. Mater., 1995, 7, 123-129) is one general method that produces high quality precursor solutions. However, the process of Frey et al. using MeOEtOH entails the complex synthesis and hazardous (teratogenic) nature of the MeOEtOH.
Nonaka et al. (U.S. Pat. No. 4,920,093, issued on Apr. 24, 1990) also describes an all-alkoxide route for forming metal oxide thin films by a sol-gel method, specifically for forming a thin film system of copper and a rare earth metal with barium, strontium, and calcium, together with a compound that has hydrolysis inhibitory action. Nonaka et al., noting problems in obtaining thin films having a homogeneous composition, uses alkoxides and alkyoxyalkoxides dissolved in alcohols to form adequate thin films.
Boyle (U.S. Pat. No. 5,683,614, issued on Nov. 4, 1997) describes a sol-gel synthesis of bismuth-strontium-tantalum oxide; however, only the tantalum precursor compound is an alkoxide (an ethoxide) with bismuth acetate and strontium acetate precursor compounds required to achieve sufficient solubility for the described process.
Paz de Araujo et al. (U.S. Pat. No. 5,468,679, issued on Nov. 21, 1995) describes a multi-step, sol-gel process for fabricating ferroelectric materials, including strontium-containing metal-oxides, as layered superlattices by using a precursor comprising a titanium 2-methoxyethoxide in a solvent. de Araujo et al. note that short-chain metal alkoxides, such as ethoxides and propoxides, usually hydrolyze too readily to permit convenient handling and long term storage and longer chain metal ligands may not be soluble in water or may contain too much organic material to produce high quality films. They therefore conclude that only metal alkoxides with intermediate length ligands, such as 2-methoxyethoxides, are suitable for precursor solutions that can produce thin films of high quality. de Araujo et al. also teach that the precursor solution to the metal-oxides can contain metal moieties from a group of strontium, calcium, barium, bismuth, cadmium, lead, titanium, tantalum, hafnium, tungsten, niobium, zirconium, bismuth, scandium, yttrium, lanthanum, antimony, chromium, thallium, oxygen, chlorine, fluorine and mixtures thereof.
The present invention provides a method for producing strontium-containing metal-oxide, ceramic materials by using alkoxide precursors, including a strontium neo-pentoxide precursor, that can be used to produce metal oxide ceramic thin films quickly and at low temperatures.
SUMMARY OF THE INVENTION
According to the present invention, a method is provided of making a strontium-containing, metal-oxide ceramic thin film on a substrate comprising the steps of forming a precursor liquid by mixing a strontium alkoxide dissolved in a solvent and at least one metal alkoxide dissolved in a solvent, said at least one metal alkoxide selected from the group consisting of alkoxides of calcium, barium, bismuth, cadmium, lead, titanium, tantalum, hafnium, tungsten, niobium, zirconium, yttrium, lanthanum, antimony, chromium and thallium; depositing a thin film of said precursor liquid on a substrate; and heating said thin film in the presence of oxygen to make a strontium-containing, metal-oxide ceramic thin film. The strontium alkoxide is a novel strontium neo-pentoxide, Sr
5
(O)[OCH
2
C(CH
3
)
3
]
8
(solvent)
5
, said solvent being selected from a group consisting of Lewis basic solvents, preferably an amine solvent such as pyridine and a furan solvent such as tetrahydrofuran.
One metal-oxide ceramic thin film prepared was a barium-strontium-titanium precursor solution spin-cast deposited on a Si/SiO
2
/Pt substrate and heated to approximately 550-700° C. Multiple layers can be formed by the method of the present invention. Another metal-oxide ceramic thin film prepared was a bismuth-strontium-tantalum precursor spin-cast deposited on a substrate and heated to form the ceramic thin film. The strontium alkoxide was selected from the group consisting of Sr
5
(O)(OCH
2
C(CH
3
)
3
)
8
(C
4
H
8
O)
4
(HOCH
2
C(CH
3
)
3
) or Sr
5
(O)(OCH
2
C(CH
3
)
3
)
8
(C
5
H
5
N)
4
(HOCH
2
C(CH
3
)
3
), the bismuth alkoxide was Bi(OR)
3
, where R is selected from the group OC
6
H
3
((CH(CH
3
)
2
)
2
, OC
6
H
3
(CH
3
)
2
, or OCH
2
C(CH
3
)
3
, and the tantalum alkoxide was selected from the group consisting of Ta(OC
2
H
5
)
5
, Ta(OC
2
H
5
)(ONp)
4
, and Ta(OCH
2
C(CH
3
)
3
)
5
.


REFERENCES:
patent: 5039654 (1991-08-01), Mizuta et al.
patent: 5645634 (1997-07-01), Ogi et al.
patent: 5807495 (1998-09-01), Ogi et al.
patent: 5811153 (1998-09-01), Hashimoto et al.

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