U.S. patent application number 11/052759 was filed with the patent office on 2005-09-08 for ultrafine metal oxide particle dispersion liquid and ultrafine metal oxide particle thin film.
This patent application is currently assigned to MURATA MANUFACTURING CO., LTD.. Invention is credited to Yamashita, Yasuhisa.
Application Number | 20050194573 11/052759 |
Document ID | / |
Family ID | 34909191 |
Filed Date | 2005-09-08 |
United States Patent
Application |
20050194573 |
Kind Code |
A1 |
Yamashita, Yasuhisa |
September 8, 2005 |
Ultrafine metal oxide particle dispersion liquid and ultrafine
metal oxide particle thin film
Abstract
An ultrafine metal oxide particle dispersion liquid is prepared
by mixing an ultrafine titanium-based composite metal oxide
particle dispersion liquid prepared by hydrolysis in a
microemulsion containing a hydrophobic dispersion medium, water,
and a surfactant and an organic metal compound solution for the
same titanium-based composite metal oxide at a ratio of 1:1 to 1:30
in terms of the composite metal oxide contained.
Inventors: |
Yamashita, Yasuhisa;
(Koka-shi, JP) |
Correspondence
Address: |
DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
1177 AVENUE OF THE AMERICAS (6TH AVENUE)
41 ST FL.
NEW YORK
NY
10036-2714
US
|
Assignee: |
MURATA MANUFACTURING CO.,
LTD.
|
Family ID: |
34909191 |
Appl. No.: |
11/052759 |
Filed: |
February 9, 2005 |
Current U.S.
Class: |
252/500 |
Current CPC
Class: |
C01P 2004/64 20130101;
B01J 13/0008 20130101; B82Y 30/00 20130101; H01G 4/1227 20130101;
B01J 13/0039 20130101; C01G 23/006 20130101; C01P 2006/22
20130101 |
Class at
Publication: |
252/500 |
International
Class: |
H01C 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 4, 2004 |
JP |
2003-060144 |
Claims
1-4. (canceled)
5. An ultrafine metal oxide particle dispersion liquid repared by
mixing an ultrafine titanium-based composite metal oxide particle
dispersion liquid prepared by hydrolysis in a microemulsion
comprising a hydrophobic dispersion medium, water, and a surfactant
and an organic metal compound solution for the same titanium-based
composite metal oxide at a ratio of 1:1 to 1:30 in terms of the
composite metal oxide contained.
6. The ultrafine metal oxide particle dispersion liquid according
to claim 5, wherein the concentration of the surfactant in the
ultrafine titanium-based composite metal oxide particle dispersion
liquid is 10% or less by volume of the hydrophobic dispersion
medium; and the concentration of the ultrafine titanium-based
composite metal oxide particles is 0.05 mol/l or less in terms of
the composite metal oxide.
7. The ultrafine metal oxide particle dispersion liquid according
to claim 6, wherein the titanium-based composite metal oxide is a
perovskite oxide represented by MTiO.sub.3, wherein M indicates at
least one metal element selected from the group consisting of
barium, calcium, and strontium.
8. The ultrafine metal oxide particle dispersion liquid according
to claim 5, wherein the titanium-based composite metal oxide is a
perovskite oxide represented by MTiO.sub.3, wherein M indicates at
least one metal element selected from the group consisting of
barium, calcium, and strontium.
9. An ultrafine metal oxide particle thin film produced with the
ultrafine metal oxide particle dispersion liquid according to claim
8.
10. An ultrafine metal oxide particle thin film produced with the
ultrafine metal oxide particle dispersion liquid according to claim
7.
11. An ultrafine metal oxide particle thin film produced with the
ultrafine metal oxide particle dispersion liquid according to claim
6.
12. An ultrafine metal oxide particle thin film produced with the
ultrafine metal oxide particle dispersion liquid according to claim
5.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to ultrafine metal oxide
particle dispersion liquids in which ultrafine metal oxide
particles are dispersed and ultrafine metal oxide particle thin
films composed of metal oxide nanoparticles and having excellent
dielectric properties.
[0003] 2. Description of the Related Art
[0004] In recent years, higher-performance devices have been
intensively researched and developed with the increasing need for
the miniaturization of devices. For example, composite metal oxides
such as barium titanate and lead titanate zirconate are widely used
for devices such as monolithic capacitors and actuators since such
materials have excellent dielectric and piezoelectric properties.
Development of smaller, higher-performance devices demands the
establishment of techniques for manufacturing thinner,
higher-quality films composed of finer particles.
[0005] One of such thin-film manufacturing techniques is the
preparation of a coating liquid for thin films by a microemulsion
method or metal organic decomposition process (MOD). The
microemulsion method is schematically illustrated in FIG. 1.
[0006] FIG. 1 shows a schematic illustration of a microemulsion and
enlarged sectional views (half) of a droplet 1 and its vicinity
contained in the microemulsion before and after hydrolysis
performed by adding a composite metal alkoxide. FIG. 1 shows the
droplets 1 (in the microemulsion), a surfactant 2, a cosurfactant
3, water 4, a reaction product 5, and a dispersion medium 6, such
as cyclohexane.
[0007] The microemulsion method is performed by adding water to a
hydrophobic medium together with a surfactant to prepare a
microemulsion in which fine water droplets (the droplets 1 in FIG.
1) are dispersed and then introducing and reacting a starting
material in the water droplets through, for example, hydrolysis to
produce crystallized ultrafine metal oxide particles having a small
particle size distribution. A transparent dispersion liquid in
which the resultant ultrafine metal oxide particles (also called
metal oxide nanoparticles) are well dispersed and do not aggregate
can be prepared by adjusting the content of water in the
microemulsion to the minimum amount of water required for the
hydrolysis.
[0008] This transparent dispersion liquid may be deposited by, for
example, spin coating to form a thin film composed of fine
nanoparticles having a size of about 20 nm.
[0009] On the other hand, MOD is a method for forming a thin film
by applying (for example, by spin coating), drying, and firing a
MOD coating liquid prepared by dissolving a metal alkoxide or
carboxylate in, for example, an aromatic solvent.
[0010] Japanese Unexamined Patent Application Publication No.
10-87329, for example, discloses a coating liquid containing
organic metal compounds produced by, for example, reacting an
alkoxide or complex of a specific metal with an carboxylic
anhydride, glycol, a .beta.-diketone, or a dicarboxylic monoester.
The organic metal compounds have high solubility to organic
solvents, and the coating liquid has high storage stability. In
addition, the coating liquid may be partially hydrolyzed for use as
a sol-gel coating liquid to reduce the content of organic
components in the coating liquid.
[0011] For example, a barium titanate (BaTiO.sub.3) nanoparticle
thin film formed with a transparent barium titanate nanoparticle
dispersion liquid prepared by the microemulsion method with the
minimum amount of water required for the hydrolysis has high
crystallinity and excellent dielectric properties though the film
is composed of fine particles having a size of 20 nm or less. This
is probably because a gradual reaction does not allow the particles
to take in excess water. In addition, this dispersion liquid,
having high dispersibility and transparency, enables the production
of an excellent thin film having uniformity and small surface
roughness.
[0012] A coating liquid prepared by a conventional microemulsion
method, however, undesirably requires the repetition of coating
many times to form a thin film having a desired thickness because
the concentration of the coating liquid is low and therefore the
rate of deposition is low, namely 10 to 20 nm for each coating.
[0013] A higher rate of deposition for each coating is possible by
the following approaches:
[0014] (1) The hydrophobic medium in the microemulsion is removed
to condense the microemulsion, thereby increasing the concentration
of the composite metal nanoparticles produced.
[0015] (2) The content of water in the microemulsion and the amount
of metal alkoxide added, which corresponds to the content of water,
are increased to increase the concentration of the composite metal
nanoparticles.
[0016] (3) The amount of metal alkoxide added is increased with the
composition of the microemulsion unchanged to increase the amount
of residual metal alkoxide relative to that of the composite metal
nanoparticles and increase the concentration of the composite metal
nanoparticles.
[0017] According to the first approach, the removal of the
hydrophobic medium increases the proportion of the residual
surfactant, thus undesirably increasing the viscosity of the
dispersion liquid and producing porous films due to loss of the
surfactant during firing.
[0018] According to the second approach, the content of water for
the hydrolysis in the microemulsion is increased to increase the
possible amount of metal alkoxide added. In this approach, however,
an increased content of water relative to that of metal alkoxide
causes aggregation at the beginning of the addition of the metal
alkoxide to the microemulsion to produce a cloudy dispersion liquid
unless the metal alkoxide is exceptionally rapidly and uniformly
added. Such a cloudy dispersion liquid is extremely difficult to
disperse again.
[0019] According to the third approach, a dispersion liquid
prepared by adding an excessive amount of metal alkoxide stock
solution with the composition of the microemulsion unchanged
contains a larger amount of residual unreacted metal alkoxide. In
the microemulsion method, a metal alkoxide, which is readily
hydrolyzed, is used because the hydrolysis must be performed with a
small amount of water. The residual unreacted metal alkoxide is
therefore readily hydrolyzed by moisture in air. As a result, this
dispersion liquid undesirably exhibits low storage stability and
repeatability.
[0020] On the other hand, a stock solution prepared by MOD has
about several to tens of times as high a concentration as that
prepared by the microemulsion method, thus providing a higher rate
of deposition, namely 30 to 80 nm for each coating. In addition,
the stock solution prepared by MOD is directly used in principle,
thus allowing uniform deposition and less leakage current.
Furthermore, MOD can be modified for improving the storage
stability and preventing striation, and has also been studied for
crystallization with high solubility at relatively low
temperatures. MOD, however, has a fundamental problem: this method
poses difficulty in providing good electrical properties, though
producing an apparently good-quality film that is relatively dense
and smooth. This is probably because the resultant film has
insufficient crystallinity.
SUMMARY OF THE INVENTION
[0021] The present inventor has found that the above problem can be
solved with an ultrafine metal oxide particle dispersion liquid
prepared by mixing an ultrafine titanium-based composite metal
oxide particle dispersion liquid (A) prepared by the microemulsion
method and an organic metal compound solution (B) for the same
titanium-based composite metal oxide with high storage stability at
a ratio of A:B in the range of 1:1 to 1:30 in terms of the
composite metal oxide contained, thus completing the present
invention. Using this dispersion liquid, an excellent
titanium-based composite metal oxide nanoparticle thin film that
has high dielectric properties and causes less leakage current can
be produced at a higher rate of deposition for each coating.
[0022] According to this approach, the concentration of the
dispersion liquid can be increased with no increase in the
proportion of the surfactant in the microemulsion, thus increasing
the rate of deposition for each coating. In addition, a dense,
smooth thin film that has excellent electrical properties and
causes less leakage current can be achieved because voids between
the particles are filled with the MOD solution.
[0023] Accordingly, the present invention provides an ultrafine
metal oxide particle dispersion liquid prepared by mixing an
ultrafine titanium-based composite metal oxide particle dispersion
liquid prepared by hydrolysis in a microemulsion containing a
hydrophobic dispersion medium, water, and a surfactant and an
organic metal compound solution for the same titanium-based
composite metal oxide at a ratio of 1:1 to 1:30 in terms of the
composite metal oxide contained.
[0024] Ultrafine particles herein refer to those having an average
diameter of, for example, 100 nm or less.
[0025] Using the ultrafine metal oxide particle dispersion liquid
according to the present invention, an excellent thin film that has
high dielectric properties and causes less leakage current can be
produced more efficiently at a higher rate of deposition for each
coating.
[0026] The concentration of the surfactant in the ultrafine
titanium-based composite metal oxide particle dispersion liquid is
preferably 10% or less by volume of the hydrophobic dispersion
medium. For such a concentration, a smaller amount of surfactant
remains, thus reducing the viscosity of the dispersion liquid and
preventing the production of porous films due to loss of the
surfactant during firing. In addition, the concentration of the
ultrafine titanium-based composite metal oxide particles is
preferably 0.05 mol/l or less in terms of the composite metal
oxide. For such a concentration, the resultant composite metal
oxide nanoparticles can be highly dispersed to provide a
transparent dispersion liquid.
[0027] The titanium-based composite metal oxide is preferably a
perovskite oxide represented by MTiO.sub.3, wherein M indicates at
least one metal element selected from the group consisting of
barium, calcium, and strontium.
[0028] The present invention further provides an ultrafine metal
oxide particle thin film produced with the above ultrafine metal
oxide particle dispersion liquid. This film has excellent
dielectric properties, including a high relative dielectric
constant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 shows schematic illustrations of a microemulsion and
a droplet before and after hydrolysis;
[0030] FIGS. 2A to 2D are sectional views for illustrating a
process of producing a monolithic capacitor including ultrafine
metal oxide particle thin films formed using an ultrafine metal
oxide particle dispersion liquid of the present invention as a
coating liquid; and
[0031] FIG. 3 is a sectional view of a thin-film device including
an ultrafine metal oxide particle thin film formed using an
ultrafine metal oxide particle dispersion liquid of the present
invention as a coating liquid.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] Preferred embodiments of the present invention will now be
described in detail.
[0033] A microemulsion used in the present invention is a
water-in-oil (W/O) microemulsion containing a hydrophobic
dispersion medium, water, and a surfactant. Ultrafine
titanium-based composite metal oxide particles are formed in the
microemulsion through hydrolysis. This microemulsion is mixed with
an organic metal compound solution for the same titanium-based
composite metal oxide at a ratio of 1:1 to 1:30 in terms of the
composite metal oxide contained.
[0034] Examples of the hydrophobic dispersion medium used include
petroleum hydrocarbons such as kerosene; nonpolar hydrocarbons such
as cyclohexane, hexane, cyclopentane, benzene, and octane; ethers
such as diethyl ether and isopropyl ether; and their mixtures.
[0035] Examples of the surfactant used include ionic surfactants
such as AOT (sodium bis(2-ethylhexyl) sulfosuccinate) and SDS
(CH.sub.3(CH.sub.2).sub.11OSO.sub.3Na) and nonionic surfactants
such as NP-n (n=1-10)
((p--C.sub.9H.sub.19)--C.sub.6H4--O--(CH.sub.2CH.sub.2O).su-
b.nCH.sub.2CH.sub.2OH) and polyoxyethylene(n) lauryl ether
(C.sub.12H.sub.25(OCH.sub.2CH.sub.2).sub.nOH). Among them, nonionic
surfactants are preferred because ionic surfactants leave
unnecessary components in the film produced.
[0036] The starting material for hydrolysis in the present
invention is a titanium-based composite metal alkoxide prepared by
mixing different metal alkoxides in an alcohol. Examples of the
alcohol used include ethanol, propanol, butanol, and isopropyl
alcohol; preferably, an alcohol corresponding to the individual
metal alkoxides is used.
[0037] Examples of the composite metal alkoxide used, which is not
particularly limited, include barium titanium methoxide, barium
titanium ethoxide, barium titanium propoxide, barium titanium
butoxide, strontium titanium methoxide, strontium titanium
ethoxide, calcium titanium methoxide, calcium titanium ethoxide,
magnesium titanium methoxide, and magnesium titanium ethoxide.
[0038] The organic metal compound solution for the same
titanium-based composite metal oxide is exemplified by a solution
containing a mixture of a titanium alkoxide and an organic compound
of a metal other than titanium, such as barium, strontium, calcium,
and magnesium. Examples of the solvent used for the solution
include lower carboxylic acid solvents, alcohol solvents,
polyalcohol solvents, ether solvents, ketone solvents, ester
solvents, N-methyl-2-pyrrolidone, and N,N-dimethylacetamide.
[0039] Examples of lower carboxylic acid solvents include butyric
acid, acetic acid, propionic acid, and valeric acid.
[0040] Examples of alcohol solvents include methanol, ethanol,
propanol, butanol, amyl alcohol, cyclohexanol, and
methylcyclohexanol.
[0041] Examples of polyalcohol solvents include ethylene glycol
monomethyl ether, ethylene glycol monoacetate, diethylene glycol
monomethyl ether, diethylene glycol monoacetate, propylene glycol
monoethyl ether, propylene glycol monoacetate, dipropylene glycol
monoethyl ether, and methoxybutanol.
[0042] Examples of ether solvents include methylal, diethyl ether,
dipropyl ether, dibutyl ether, dianol ether, diethyl acetal,
dihexyl ether, trioxane, and dioxane.
[0043] Examples of ketone solvents include acetone, methyl ethyl
ketone, methyl propyl ketone, methyl isobutyl ketone, methyl amyl
ketone, methyl cyclohexyl ketone, diethyl ketone, ethyl butyl
ketone, trimethyl nonanone, acetonitrile acetone, dimethyl oxide,
phorone, cyclohexanone, and diacetone alcohol.
[0044] Examples of ester solvents include ethyl formate, methyl
acetate, ethyl acetate, butyl acetate, cyclohexyl acetate, methyl
propionate, ethyl hydroxyisobutyrate, ethyl acetoacetate, ethyl
lactate, methoxybutyl acetate, diethyl oxalate, and diethyl
malonate.
[0045] The content of water in the microemulsion of the present
invention is preferably 0.95 to 3 times the amount of water
required for the hydrolysis of the starting material, namely a
composite metal alkoxide.
[0046] The amount of water required for the hydrolysis of, for
example, barium isopropoxide (Ba(isop).sub.2) and titanium
isopropoxide (Ti(isop).sub.4) is determined by the following
chemical formula:
Ba(isop).sub.2+Ti(isop).sub.4+3H.sub.2O.fwdarw.BaTiO.sub.3+6
isopropanol
[0047] In this example, therefore, the hydrolysis of 1 mol of
barium isopropoxide and 1 mol of titanium isopropoxide requires 3
mol of water. Accordingly, the content of water in the
microemulsion must be 2.85 to 9 mol, namely 0.95 to 3 times the
amount of water required for the hydrolysis.
[0048] It may be preferred to adjust the content of water to 1
times or less. If the content of water is 1 times or less, the
hydrolysis completely consumes water to provide a significantly
clear dispersion liquid containing an unreacted residue or
amorphous particles having insufficient crystallinity. The
unreacted or amorphous phase may infiltrate between the ultrafine
particles constituting the resultant film to increase the density
of the film and function as a sintering aid.
[0049] Nevertheless, the content of water is preferably at least
0.95 times because a higher proportion of unreacted or amorphous
phase involves a lower proportion of crystal phase. More
preferably, the content of water is 1.05 to 1.2 times. Within this
range, the resultant dispersion liquid has high clarity and
dispersibility and contains ultrafine composite metal oxide
particles with high crystallinity.
[0050] Minimizing the amount of water remaining after the
hydrolysis also has the effect of inhibiting variations in the
composition of the composite metal oxide synthesized. In the
synthesis of barium titanate by hydrolysis, for example, a modified
stock solution containing an excessive amount of barium is
generally used because barium partially dissolves into water; the
Ba/Ti ratio is an important characteristic of the resultant
material. If substantially no water remains after the hydrolysis as
in the present invention, no barium ions dissolve into water, so
that uniform target ultrafine particles can be produced with an
unmodified stock solution. If, therefore, the content of water in
the microemulsion is 0.95 to 3 times, preferably 1.05 to 1.2 times,
the amount of water required for the hydrolysis, the resultant
particles have substantially no variations in composition due to
the dissolution of water-soluble components into water.
[0051] In addition, at least one alcohol is preferably added to the
microemulsion as another surfactant, namely cosurfactant.
[0052] FIG. 1 shows a schematic illustration of the microemulsion
and enlarged sectional views (half) of a droplet and its vicinity
contained in the microemulsion before and after the hydrolysis
performed by adding a composite metal alkoxide. FIG. 1 shows
droplets 1 (contained in the microemulsion), a surfactant 2, a
cosurfactant 3, water 4, a reaction product 5, and a dispersion
medium 6, such as cyclohexane.
[0053] Adding at least one alcohol as a cosurfactant allows water
droplets to be dispersed more stably in the preparation of the
microemulsion. In addition, when the hydrolysis consumes water in
the microemulsion to produce ultrafine composite metal oxide
particles, the cosurfactant infiltrates between the surfactant and
the composite metal oxide particles to keep the surfactant
surrounding the particles. The composite metal oxide particles can
therefore be dispersed as stably as water.
[0054] The cosurfactant infiltrates the hydrophilic part of the
surfactant to reduce the interface energy to water and relieve the
steric hindrance of the long carbon chain of the hydrophilic part.
The alcohol used as the cosurfactant preferably has about four to
ten carbon atoms, though the suitable number of carbon atoms
depends on the length of the carbon chain of the hydrophilic part.
An alcohol having less than four carbon atoms is undesirable
because it is excessively hydrophilic and dissolves from the
interface into water. On the other hand, an alcohol having more
than ten carbon atoms is undesirable because it is excessively
hydrophobic and has larger steric hindrance.
[0055] The starting material used is preferably a composite metal
alkoxide formed by mixing metal alkoxides before the
hydrolysis.
[0056] It is known that a barium alkoxide is readily polymerized in
an alcohol. According to J. Am. Ceram. Soc., 77 [2] pp. 603-605 and
Jpn. J. Appl. Phys. Vol. 36, pp. 5939-5942, a crystal of
BaTi(OCH(CH.sub.3).sub.2- )--C.sub.6H.sub.6 can be obtained by
maturing an isopropanol solution of barium and titanium in benzene.
These documents have also indicated that benzene, which does not
substantially dissolve metal alkoxides, has the effects of
promoting the stabilization and precipitation of the
BaTi(OCH(CH.sub.3).sub.2)--C.sub.6H.sub.6 crystal and inhibiting
the polymerization of the barium alkoxide.
[0057] Adding benzene in such an amount as to precipitate no
crystal can inhibit the polymerization of a barium alkoxide to
provide a uniform barium titanium double alkoxide more readily. To
produce uniform ultrafine barium-containing composite metal oxide
particles, therefore, a barium-containing composite alkoxide stock
solution is preferably adjusted by adding benzene, which inhibits
the polymerization of the barium alkoxide, to the alcohol solvent.
A different material having the same effect may be used instead of
benzene.
[0058] The microemulsion shown in FIG. 1 is mixed with the organic
metal compound solution (MOD solution) for the same titanium-based
composite metal oxide at a ratio of 1:1 to 1:30 in terms of the
composite metal oxide contained (the titanium-based composite metal
oxide versus the organic metal compound) to prepare an ultrafine
metal oxide particle dispersion liquid according to the present
invention.
[0059] The concentration of the surfactant in the ultrafine metal
oxide particle dispersion liquid of the present invention is
preferably 10% or less by volume of the hydrophobic dispersion
medium. Its lower limit is a minimum value at which the surfactant
can function effectively. In addition, the concentration of the
ultrafine titanium-based composite metal oxide particles is
preferably 0.05 mol/l or less in terms of the composite metal
oxide.
[0060] An ultrafine metal oxide particle thin film according to the
present invention is produced with the ultrafine metal oxide
particle dispersion liquid of the present invention.
[0061] The ultrafine particles contained in the ultrafine metal
oxide particle dispersion liquid may be heat-treated at a
relatively low temperature, for example 600.degree. C. or less,
since they have high crystallinity.
[0062] The ultrafine metal oxide particle thin film of the present
invention is composed of ultrafine metal oxide particles with an
average diameter of 15 to 50 nm, and has ferroelectric
properties.
[0063] As the ferroelectric properties, the ultrafine metal oxide
particle thin film of the present invention preferably has a
remanent polarization (2Pr) of 2 .mu.C/cm.sup.2 or more and a
relative dielectric constant of 300 or more at a measurement
frequency of 1 kHz at room temperature.
[0064] The ultrafine metal oxide particle thin film of the present
invention, including ultrafine particles with an average diameter
of 15 to 50 nm and having a remanent polarization (2Pr) of 2
.mu.C/cm.sup.2 or more and a relative dielectric constant of 300 or
more, can function sufficiently in ferroelectric devices such as
thin-film capacitors and monolithic capacitors. This ultrafine
metal oxide particle thin film therefore enables the production of
smaller, thinner-film devices. In particular, a thin-film capacitor
including the film, which is composed of a larger number of
particles, can achieve higher reliability, a smaller layer
thickness, and a smaller device size.
[0065] The ultrafine metal oxide particle thin film of the present
invention is preferably produced by synthesizing finest possible
crystallized metal oxide particles, for example ceramic
nanoparticles of a titanate-containing perovskite oxide; depositing
the particles on a substrate while keeping them highly dispersed;
and adding energy, such as heat, for growing the particles to a
diameter of 15 to 50 nm to facilitate the densification and
crystallization of the deposit.
[0066] In such a film deposition method, for example, a dispersion
liquid prepared by a microemulsion (ME) method and containing
highly dispersed metal oxide nanoparticles with a uniform particle
size distribution is mixed with a MOD solution at the above ratio.
The resultant coating liquid, for example the ultrafine metal oxide
particle dispersion liquid of the present invention, is directly
deposited by, for example, spin coating to form a thin film. This
film is then heat-treated with, for example, a rapid thermal
annealing (RTA) furnace.
[0067] In particular, the ultrafine metal oxide particle thin film
of the present invention is preferably produced by synthesizing
ultrafine metal oxide particles with an average diameter of 30 nm
or less, for example ceramic nanoparticles of a titanate-containing
perovskite oxide; depositing the ceramic nanoparticles on a
substrate while keeping them highly dispersed in a dispersion
medium; and adding energy to the resultant ceramic nanoparticle
thin film by, for example, heat treatment to grow the particles to
a diameter of 15 to 50 nm.
[0068] As described above, fine ceramic particles, or ceramic
nanoparticles, have a strong tendency to aggregate, and aggregated
particles are extremely difficult to disperse again. To synthesize
target ceramic nanoparticles while keeping them highly dispersed in
a dispersion medium, a starting material may be reacted in
nanosized fine reaction spaces contained in the dispersion medium.
The above microemulsion method is desirable as such a method. For
the above W/O microemulsion, which contains a hydrophobic
dispersion medium, water, and a surfactant, water droplets with a
diameter of several to tens of nanometers can be thermally stably
dispersed in the hydrophobic dispersion medium.
[0069] For example, a barium titanium composite alkoxide stock
solution may be added dropwise to this microemulsion to synthesize
crystallized barium titanate nanoparticles for use in monolithic
capacitors. These nanoparticles are very fine and uniform because
they have the diameter corresponding to the water droplets.
[0070] In addition, as described above, the synthesized barium
titanate nanoparticles can be kept stably dispersed without
aggregation by adjusting the content of water in the microemulsion
to 0.95 to 3 times the amount of water required for the
hydrolysis.
[0071] The synthesized nanoparticles are preferably as fine as
possible, namely 30 nm or less, more preferably 10 nm or less, in
average diameter because they must be grown within the range not
exceeding 50 nm by the subsequent heat treatment. A dispersion
liquid containing such nanoparticles is applied onto a substrate
by, for example, spin coating, dip coating, or screen printing and
is heat-treated for removing the organic solvent to achieve higher
crystallinity, thus providing a dense barium titanate nanoparticle
thin film.
[0072] The average diameter of the particles contained in the film
may be adjusted after the deposition with, for example, an electric
furnace, an infrared furnace, or an RTA furnace. In particular, an
infrared furnace and an RTA furnace are desirable because these
furnaces can provide a rapid temperature rise to suppress the
growth of the particles to a smaller degree at the same temperature
by short-time heat treatment.
[0073] The thin film of the ultrafine metal oxide particles
synthesized by the microemulsion method is dense, in spite of
nanoparticles, because these particles are deposited while keeping
them highly dispersed. This film has also been crystallized and
therefore forms no intermediate layer after, for example, heat
treatment, thus preventing poor densification and large surface
roughness. In addition, this film suffers no deterioration in
electric properties due to the formation of a different phase at
the interface to the substrate. Furthermore, in spite of a small
average particle diameter, namely not more than 50 nm, this film is
highly densified and crystallized to provide excellent dielectric
properties and reliability because finest possible crystallized
nanoparticles are grown by, for example, heat treatment.
[0074] Unlike powder samples, this ultrafine metal oxide particle
thin film can exhibit ferroelectric properties even if the average
diameter of the nanoparticles contained in the film is smaller than
a generally accepted critical size. This possibility is related to
effects such as stress from the substrate. In practice, an
ultrafine metal oxide particle thin film produced according to the
present invention has been confirmed to exhibit ferroelectric
properties and excellent dielectric properties even though the
particles contained in the film had an average diameter not more
than 50 mn.
[0075] In addition, the above process of producing an ultrafine
metal oxide particle thin film is advantageous in that this process
includes fewer and simpler steps, namely a series of steps of
synthesis, dispersion liquid, and deposition, and requires simpler
equipment.
[0076] An example of electronic devices including the ultrafine
metal oxide particle thin film of the present invention will now be
described. The ultrafine metal oxide particle thin film of the
present invention may be applied to various electronic devices.
FIGS. 2A to 2D show an example of a monolithic capacitor including
this film.
[0077] A ceramic layer 8, an internal electrode 9a, another ceramic
layer 8, an internal electrode 9b, another ceramic layer 8, and
another internal electrode 9a are deposited in the stated order on
a substrate 7, such as an alumina substrate, shown in FIG. 2A.
Referring to FIG. 2B, this deposition process is repeated to form a
ceramic-metal laminate 10 including the internal electrodes 9a and
9b and ceramic layers 8 that are alternately laminated on the
substrate 7.
[0078] The ceramic layers 8 are formed by the process for producing
the ultrafine metal oxide particle thin film of the present
invention while the internal electrodes 9a and 9b are formed by
chemical vapor deposition (CVD), evaporation, or sputtering. The
ceramic layers 8 and the internal electrodes 9a and 9b each have a
thickness of, for example, 1 .mu.m or less. The internal electrodes
9a and 9b are patterned with a mask and are alternately led to the
opposite ends.
[0079] Referring to FIG. 2C, the substrate 7 is selectively removed
by, for example, etching to leave the ceramic-metal laminate 10.
Referring to FIG. 2D, external electrodes 11a and 11b are formed at
both ends of the ceramic-metal laminate 10 by, for example, dipping
or sputtering to produce a very small monolithic ceramic capacitor
12 in which the internal electrodes 9a are connected to one
external electrode 11a and the internal electrodes 9b are connected
to the other external electrode 11b.
[0080] FIG. 3 shows an example of a dielectric thin-film device
including the ultrafine metal oxide particle thin film of the
present invention.
[0081] A substrate 13 that constitutes the lower layer of the
dielectric thin-film device and a platinum film 14 that functions
as a lower electrode on the substrate 13 are prepared by the
following process. A surface of a monocrystalline silicon board 15
is oxidized to form a silicon oxide film 16 that functions as a
buffer layer to prevent silicon from diffusing into the lower
electrode, namely the platinum film 14. An aluminum oxide film 17
having a thickness of 1,000 .ANG. is formed on the silicon oxide
film 16 by sputtering to enhance the adhesion between the silicon
oxide film 16 and the platinum film 14. The platinum film 14 is
formed at a thickness of 3,000 .ANG. on the substrate 13 including
the silicon board 15, the silicon oxide film 16, and the aluminum
oxide film 17.
[0082] A dielectric thin film 18 composed of ultrafine metal oxide
particles with an average diameter of 15 to 50 nm according to the
present invention is formed on the platinum film 14. Platinum
electrodes 19 are formed as upper electrodes on the dielectric thin
film 18 by sputtering.
EXAMPLES
[0083] The present invention will now be described in detail with
examples below, though the present invention is not limited to
these examples.
Example 1
[0084] First, 4 g of barium isopropoxide was mixed into 160 ml of
isopropyl alcohol in a glove box containing an argon atmosphere to
prepare a barium alkoxide solution. An equimolar amount of titanium
isopropoxide solution was added dropwise into the barium alkoxide
solution, which was mixed overnight to obtain a light yellow,
transparent barium-titanium composite alkoxide stock solution.
[0085] A W/O microemulsion was prepared by mixing water,
cyclohexane, as a dispersion medium, NP-10
((p--C.sub.9H.sub.19)--C.sub.6H.sub.4--O--(CH.su-
b.2CH.sub.2O).sub.10CH.sub.2CH.sub.2OH), as a surfactant, and
1-octanol, as a cosurfactant, at a ratio of 0.2:150:7.5:9,
respectively, while bubbling the mixture with an argon gas. The
barium-titanium composite alkoxide stock solution was then added to
the microemulsion with a micropipette so that the content of water
in the microemulsion was 1.2 times the amount of water required for
the hydrolysis of the barium-titanium composite alkoxide. This
mixture was stirred in a glove box containing an argon atmosphere
for one day to prepare an ultrafine barium titanate particle
dispersion liquid (A).
[0086] This dispersion liquid (A) was transparent light brown and
contained highly dispersed ultrafine barium titanate particles
produced by the hydrolysis. A portion of the dispersion liquid (A)
was mixed with ethanol to cause precipitation and was centrifuged
and washed with an organic solvent to obtain a crystal phase.
According to powder X-ray diffractometry, the resultant crystal
phase was a barium titanate crystal. In addition, according to
observation by high-resolution scanning electron microscopy (SEM),
the resultant ultrafine particles had an extremely small diameter,
namely about 8 nm, and a uniform particle size distribution.
[0087] Equimolar amounts of n-barium butyrate and titanium
isopropoxide were mixed into isoamyl butyrate. Then, 10 g of
2,4-pentadione was added to prepare 200 ml of an MOD solution (B)
having a concentration of 0.3 mol/l in terms of barium titanate
(BT).
[0088] The dispersion liquid (A) and the solution (B) were mixed at
a ratio of 1:1, 1:10, and 1:30 in terms of BT to prepare coating
liquids for barium titanate nanoparticle thin films.
[0089] The resultant coating liquids were used to form ultrafine
barium titanate particle thin films by spin coating. Thin films
having a thickness of 240 nm were formed by repeating a process of
spin coating and heat treatments at 150.degree. C. and 450.degree.
C. and firing the films at 700.degree. C. for 20 minutes. The
number of coatings was adjusted so that the final thickness of each
film reached 240 nm.
[0090] Upper electrodes were formed on the barium titanate
nanoparticle thin films by Pt sputtering to evaluate their
electrical properties. The relative dielectric constant and
dielectric loss of each film were measured with an LCR meter at a
measurement frequency of 1 kHz at room temperature. The
ferroelectric properties of each film were evaluated by hysteresis
measurement. These evaluations are shown in Table 1 below.
[0091] In addition, the withstand voltage of each film was
evaluated by applying a voltage of 200 kV/cm onto the film. In this
test, broken films were determined as "Bad", and unbroken films
were determined as "Good".
[0092] Furthermore, the storage stability of each coating liquid
was evaluated after storage at 25.degree. C. for ten days in a
glass vial sealed in air. Coating liquids in which foreign
materials occurred or those that distinctly changed in color were
determined as "Bad", and unchanged coating liquids were determined
as "Good".
Comparative Example 1
[0093] A dispersion liquid (A) and solution (B) were prepared in
the same manner as Example 1, and were mixed at a ratio of 1:0,
0:1, 1:0.5, and 1:35 in terms of barium titanate to prepare coating
liquids for barium titanate nanoparticle thin films. As in Example
1, thin films were formed with the coating liquids and were
evaluated for electrical properties.
Comparative Example 2
[0094] A dispersion liquid (A) was prepared in the same manner as
Example 1. Cyclohexane was removed from the dispersion liquid (A)
with an evaporator until its volume was reduced to 1/5 of the
original volume, thus obtaining a coating liquid for barium
titanate nanoparticle thin films. As in Example 1, a thin film was
formed with the coating liquid and was evaluated for electrical
properties.
Comparative Example 3
[0095] A microemulsion and composite alkoxide stock solution were
prepared in the same manner as Example 1. The composite alkoxide
stock solution was then added to the microemulsion in an amount of
5 times that equivalent to the content of water. This mixture was
stirred in a glove box containing an argon atmosphere for one day
to prepare a dispersion liquid (A).
[0096] As in Example 1, a thin film was formed with the dispersion
liquid (A) and was evaluated for electrical properties. The storage
stability of the dispersion liquid (A) was also evaluated as in
Example 1.
[0097] Table 1 shows the evaluations of Example 1 and Comparative
Examples 1 to 3.
1 TABLE 1 Dispersion A: Relative solution B Number of Withstand
dielectric Dielectric Storage Comprehensive (in terms of BT)
coatings voltage constant loss (%) Hysteresis stability evaluation
Example 1 1:1 10 Good 760 2 Good Good Good Example 1 1:10 8 Good
650 3 Good Good Good Example 1 1:30 6 Good 480 3 Good Good Good
Comparative 1:0 20 Good 790 2 Good Good Bad Example 1 Comparative
0:1 4 Good 180 3 Bad Good Bad Example 1 Comparative 1:0.5 16 Good
780 3 Good Good Bad Example 1 Comparative 1:35 5 Good 290 3 Bad
Good Bad Example 1 Comparative 1:0 9 Bad 600 8 -- Bad Bad Example 2
Comparative 1:0 10 Bad 540 5 Good Bad Bad Example 3
[0098] When the microemulsion was used alone, the resultant film
had excellent dielectric properties, including a high relative
dielectric constant. The deposition of the film, however, required
20 coatings to reach a thickness of 240 nm.
[0099] When the coating liquids prepared by mixing the dispersion
liquid and the MOD solution at ratios of 1:1 to 1:30 in terms of BT
were used, thin films having the same thickness could be formed by
ten or fewer coatings. In addition, these films had relative
dielectric constants not less than 300 and dielectric losses of 3%
or less.
[0100] When the amount of the MOD solution mixed was less than 1
times that of the dispersion liquid, the deposition of a thin film
required more than 10 coatings. When, on the other hand, the amount
of the MOD solution mixed was more than 30 times that of the
dispersion liquid, the resultant film had a relative dielectric
constant less than 300 and exhibited substantially no
hysteresis.
[0101] When the MOD solution was used alone, a thin film having the
same thickness could be formed by fewer coatings. This film,
however, had insufficient electrical properties. When the
hydrophobic medium contained in the microemulsion was removed by
evaporation to increase its concentration, a thin film could be
formed by fewer coatings. This film, however, was porous because
the proportion of the surfactant in the microemulsion was
increased. As a result, the film exhibited a lower withstand
voltage and a larger dielectric loss.
[0102] When the amount of composite alkoxide stock solution added
to the microemulsion was 5 times the amount equivalent to that of
water required for the hydrolysis, a thin film could be formed by
fewer coatings. This film, however, exhibited an insufficient
withstand voltage due to residual carbon and a dielectric loss not
less than 3%. After a period of storage, the coating liquid changed
in color from light brown to dark brown. This change revealed that
the coating liquid had poor storage stability.
[0103] According to the present invention, as described above, an
ultrafine metal oxide particle dispersion liquid is prepared by
mixing an ultrafine titanium-based composite metal oxide particle
dispersion liquid (A) prepared by the microemulsion method and an
organic metal compound solution (B) for the same composite metal
oxide with high storage stability at a ratio of A:B in the range of
1:1 to 1:30 in terms of the composite metal oxide contained. This
dispersion liquid enables the production of an excellent
titanium-based composite metal oxide nanoparticle thin film that
has high dielectric properties and causes less leakage current at a
higher rate of deposition for each coating.
* * * * *