Methods for manufacturing dielectric powders

Details Methods for manufacturing dielectric powders Methods for manufacturing dielectric powders

2000-10-30

2003-07-01

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Oxygen or compound thereof

Metal containing

C501S010000, C501S136000, C501S137000

Reexamination Certificate

active

06585951

ABSTRACT:

FIELD OF THE INVENTION
The invention pertains to methods for manufacturing dielectric materials.
BACKGROUND
Barium strontium titanate (BST) is used in electronic devices, particularly in electronic devices such as capacitors, because of its high dielectric constant (between approximately 200 and 6,000). Other applications of BST include capacitor-varistors, positive temperature coefficient (PTC) resistors, thick film multilayer ceramic capacitors (MLC), embedded capacitance laminates, and bulk ceramic transducers. Thick films of BST have also been used in gigabit dynamic random access memory devices (DRAMs), in bypass capacitors, in filters, and in GaAs microwave monolithic integrated circuit (MMICs).
Practical applications of BST require inexpensive manufacturing processes that are applicable to small geometry devices. One method of maintaining a selected value of capacitance while shrinking the size of a capacitor is the use of small BST particles. Unfortunately, such small BST particles cannot be manufactured by conventional ceramic processes that produce BST particles that tend to stick together or agglomerate, increasing the effective particle size.
Conventional sputtering processes used to form BST films produce reproducible films, but the quality of these films depends on the quality of the sputtering target. In addition, the deposition rate of such BST films can be low, restricting the use of BST films to devices that are produced in low volumes. Improved BST sputtering targets are needed to achieve the higher deposition rates necessary for high volume production.
A significant factor in producing a satisfactory sputtering target is the quality of the BST powder used to make the target. The performance of a sputtering target depends on BST particle size, size distribution, chemical homogeneity, and sintering temperature. The sintering temperature of a BST target can be lowered by using small particles. Nanometer-sized BST particles are infrequently used because such particles are much more expensive than larger, agglomerated particles that have a wide range of particle sizes. Existing processes for nanometer-sized BST particles involve high temperature processing, expensive organo-metallic precursors, hazardous and lengthy process steps, and hazardous reagents, and produce irregularly shaped or agglomerated particles.
One prior art method of manufacturing BST particles is referred to as an oxalic precipitation method. In this method, Ba
0.6
Sr
0.4
TiO (OH)
2
C
2
O
4
·4H
2
O precipitate is prepared by co-precipitation of cations as a mixed oxalate in an oxalic acid solution. The precipitate is calcined at 900° C.-1000° C. to convert the precipitate into BST. See, for example, P. K. Gallagher et al., “Preparation of Semiconducting Titanates by Chemical Method,” J. Am. Ceram. Soc., vol. 46, pp 359-365 (1963) and F. Schrey, “Effect of pH on the Chemical Preparation of Barium-Strontium Titanate,” J. Am. Cer. Soc., vol. 48, pp 401-405 (1965). The oxalic precipitation methods described in these references are complex, and the organic compounds used are expensive. In addition, the BST particles produced by these methods are irregularly shaped, strongly agglomerated, and are large, having dimensions of between 500 nm and 800 nm.
U.S. Pat. No. 4,677,083 discloses another method of preparing BST powders. In this method, a TiCl
4
solution is mixed with barium nitrate and strontium nitrate at a pH of 14. This mixture is held at a temperature of 100° C. for 4 hours, followed by drying at 100° C. for 24 hours, forming a product that includes KCl and KNO
3
as by-products. These by-products are removed and the remaining product is heat-treated at 800° C. for 2 hours. This process requires removal of the KCl and KNO
3
by-products prior to heat treatment. In addition, this process involves dilution of a TiCl
4
solution at a high pH, increasing the cost of this method. The BST particles produced by this process are irregularly shaped and the particle size varies over a wide range.
Another process for the precipitation of BST particles includes synthesis of BaTi(C
6
H
4
O
2
)
3
and SrTi (C
6
H
4
O
2
)
3
by reacting titanium tetrachloride and C
6
H
4
(OH)
2
in a toluene solvent with boiling water suspensions of BaCO
3
and SrCO
3
, respectively. This produces aqueous solutions of BaTi (C
6
H
4
O
2
)
3
and SrTi (C
6
H
4
O
2
)
3
that are freeze dried, forming a mixture of solid complexes. The mixture is subjected to pyrolysis and calcination at 700° C. for 2 hours. This process produces strongly agglomerated, irregularly shaped BST particles having dimensions of about 800 nm. This method and the BST particles produced by this method are discussed in N. J. Ali and S. J. Milne, “Synthesis and Processing Characteristics of Ba
0.65
Sr
0.35
TiO
3
Powders from Catecholate Precursors,” J. Am. Ceram. Soc., vol. 76, pp. 2321-2326 (1993).
Another method of producing BST particles involves the synthesis of an oxalate precipitate, Ba
1−x
Sr
x
TiO
3
(C
2
O
4
)
2
·4H
2
O (wherein 0.0≦x≦0.3), by mixing TiO(NO
3
)
2
, Ba(NO)
2
, Sr(NO)
2
, and a titanyl oxalate solution to produce a precipitate that is calcinated for 2 hours at 900° C. This process is described in T. Noh et al., “Chemical Preparation of Barium-Strontium Titanate,” Bull. Korean Chem. Soc., vol. 16, pp. 1180-1184 (1995). This method produces strongly agglomerated BST particles having dimensions of between about 13 nm and 40 nm.
In another method, BST particles are prepared by a vapor-phase hydrolysis of precursors obtained from alkoxide-hydroxide. This method involves preparation of Ba(OH)
2
·8H
2
O and Sr(OH)
2
·8H
2
O at 300° C. to obtain precursors Ba(OH)
2
·H
2
O and Sr(OH)
2
, respectively. These precursors are dissolved in methanol mixed with Ti-isopropoxide in a dry N
2
environment, and maintained for 15 hours at room temperature. The methanol is then evaporated, producing a powder precursor that is then slowly hydrolyzed at 100° C. in an N
2
environment. The hydrolyzed powder is then calcined at 900° C. for 2 hours, and ball milled in alcohol for 24 hours to obtain BST particles. The BST particles produced by this method are strongly agglomerated and are not separable by ultrasonication. This method is described more fully in T. Hayashi et al., “Preparation of Ba
1−x
Sr
x
TiO
3
Particles by Vapor-Phase Hydrolysis of Precursors Formed from Alkoxide-Hydroxide,” Jpn. J. Appl. Phys., vol. 37, pp. 5232-5236 (1998).
In summary, solution-based BST powder synthesis methods involve difficult chemical precipitation steps, and produce BST particles that are large, irregularly shaped, or strongly agglomerated. In addition, these methods produce BST particles having varying chemical compositions and use expensive organic precursors and hazardous process steps. None of these methods produce acceptably sized and shaped BST particles and new methods and apparatus for the production of such particles are needed.
SUMMARY OF THE INVENTION
Methods form manufacturing nanometer-sized barium strontium titanate Ba
1−x
Sr
x
TiO
3
(BST) particles and particles of other materials, such as barium ferrites, metal oxides (e.g., iron oxide, tin oxide), and other semiconductors, insulators, ferrolectrics are provided. The methods include mixing BST particle components and matrix components that are melted to form a glassy matrix containing BST particles or precursors thereof. The BST particle components include oxides, hydroxides, and carbonates of barium, strontium, and titanium. Matrix components include materials comprising sodium, particularly sodium salts such as sodium borate (Na
2
O·2B
2
O
3
), sodium carbonate (Na
2
CO
3
), and sodium silicate (Na
2
O·SiO
2
·9H
2
O), selected according to a predetermined molar ratio. The mixture of particle components and matrix components is melted and the molten mixture is quenched in ice water or by another method to produce an amorphous material, typically in the form of flakes or other irregular solids. The amorphous material is annealed to form BST particles in the amorphous materi

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