U.S. patent application number 10/914160 was filed with the patent office on 2005-01-13 for method for forming dielectric thin film and dielectric thin film formed thereby.
This patent application is currently assigned to Murata Manufacturing Co., Ltd.. Invention is credited to Nishida, Koichi, Shibuya, Koki, Takeshima, Yutaka.
Application Number | 20050009362 10/914160 |
Document ID | / |
Family ID | 26625532 |
Filed Date | 2005-01-13 |
United States Patent
Application |
20050009362 |
Kind Code |
A1 |
Nishida, Koichi ; et
al. |
January 13, 2005 |
Method for forming dielectric thin film and dielectric thin film
formed thereby
Abstract
A method for forming a dielectric thin film includes a film
deposition step of spraying a material solution onto a heated
substrate under a reduced pressure by a two-fluid technique using
an inert gas to deposit a thin film. The material solution is
supplied at a rate that is greater than the vaporization rate of
the solvent in the film deposited on the substrate. The supply of
the material solution is stopped and the solvent remaining in the
film is vaporized remaining solvent. Then, the film is heat-treated
in an oxidizing atmosphere. The substrate is heated to a
temperature in the range of about 100.degree. C. to about
300.degree. C. Thus, a dielectric thin film having reliability can
be formed even if the film thickness is small.
Inventors: |
Nishida, Koichi; (Shiga-ken,
JP) ; Takeshima, Yutaka; (Nagaokakyo-shi, JP)
; Shibuya, Koki; (Omihachiman-shi, JP) |
Correspondence
Address: |
KEATING & BENNETT LLP
Suite 312
10400 Eaton Place
Fairfax
VA
22030
US
|
Assignee: |
Murata Manufacturing Co.,
Ltd.
Nagaokakyo-shi
JP
|
Family ID: |
26625532 |
Appl. No.: |
10/914160 |
Filed: |
August 10, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10914160 |
Aug 10, 2004 |
|
|
|
10339564 |
Jan 10, 2003 |
|
|
|
Current U.S.
Class: |
438/778 ;
257/E21.272 |
Current CPC
Class: |
H01L 21/31691 20130101;
C23C 4/123 20160101; H01L 21/02282 20130101; H01L 21/02323
20130101; C23C 4/18 20130101; H01L 21/02337 20130101 |
Class at
Publication: |
438/778 |
International
Class: |
H01L 021/00; H01L
021/84; H01L 021/31 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 16, 2002 |
JP |
2002-007170 |
Dec 2, 2002 |
JP |
2002-349874 |
Claims
1-8. (canceled).
9. A dielectric thin film formed by a method including a film
deposition step of spraying a material solution including a
starting material and a solvent onto a heated substrate under a
pressure that is less than atmospheric pressure by a two-fluid
technique using an inert gas to deposit a thin film, and a heat
treatment step of subjecting the thin film to heat treatment in an
oxidizing atmosphere, wherein the film deposition step and the heat
treatment step are performed at least two times, the dielectric
thin film having: (a) crystallinity; (b) a thickness of about 200
nm or less; and (c) a relative dielectric constant of about 250 or
more.
10. A dielectric thin film according to claim 9, the dielectric
thin film being formed by repeating at least once the film
deposition and heat treatment steps.
11-19. (canceled).
20. A dielectric thin film formed by a method including a film
deposition step of spraying a material solution including a
starting material and a solvent onto a heated substrate under a
pressure that is less than atmospheric pressure by a two-fluid
technique using an inert gas to deposit a thin film, wherein the
material solution is supplied at a greater rate than the
vaporization rate of the solvent in the film deposited on the
substrate, a solvent vaporization step of stopping the supply of
the material solution and vaporizing the solvent remaining in the
thin film, and a heat treatment step of subjecting the thin film to
heat treatment in an oxidizing atmosphere, the heat treatment step
being performed after the film deposition step and the solvent
vaporization step are repeated at least once, the dielectric thin
film having: (a) crystallinity; and (b) a relative dielectric
constant of about 250 or more.
21. A dielectric thin film according to claim 20, the dielectric
thin film being formed by repeating at least once the film
deposition and heat treatment steps.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to methods for forming a
dielectric thin film, and more particularly to a method for forming
a dielectric thin film using a solution containing starting
materials.
[0003] 2. Description of the Related Art
[0004] In order to form a dielectric thin film, using a material
solution, the following methods have been applied:
[0005] (1) Liquid Source CVD (LSCVD): a solution in which organic
metal materials are uniformly dissolved is vaporized to react with
the surface of a substrate, thus forming an oxide thin film.
[0006] (2) Metal Organic Deposition (MOD): a solution in which
organic metal materials are uniformly dissolved is applied, dried,
calcined, and fired to form an oxide thin film.
[0007] (3) A spray method: a material solution is sprayed onto the
surface of a substrate, followed by drying and firing to form an
oxide thin film (for example, a solution is sprayed onto a
substrate by ultrasonic waves, and is subsequently heated to dry
and heat-treated to form a thin film, as disclosed in, for example,
Japanese Unexamined Patent Application Publication No.
9-213643)
[0008] However, the LSCVD method described as method (1) above,
requires the use of expensive compounds having a high vapor
pressure as raw materials because the raw materials must be in a
gas form to be applied onto a substrate. Thus, LSCVD has
disadvantage in that it is very expensive.
[0009] On the other hand, the MOD method described as method (2),
and the spray method, described as method (3), have no vapor
pressure requirement because a solution containing raw materials is
applied in a liquid form onto a substrate. However, the vaporized
solvent passes through the deposited film during drying to form
cavities which are likely to cause a short circuit between
electrodes disposed on both surfaces of the resulting film if the
film thickness is small. It is thus difficult to form a reliable
dielectric thin film.
SUMMARY OF THE INVENTION
[0010] In order to overcome the problems described above, preferred
embodiments of the present invention provide a method for forming a
dielectric thin film having high reliability, and provide a
dielectric thin film formed by the novel method.
[0011] According to a preferred embodiment of the present
invention, a method for forming a dielectric thin film includes the
steps of (a) spraying a material solution including a starting
material and a solvent onto a heated substrate under a reduced
pressure by a two-fluid technique using an inert gas to deposit a
thin film, and (b) subjecting the thin film to heat treatment in an
oxidizing atmosphere.
[0012] By performing this film deposition step, the steps of drying
and calcining the deposited film, which are necessary in known
methods, such as the MOD method and the spray method, can be
eliminated. Thus, the vaporization of the solvent from the
deposited film is prevented and the occurrence of cavities
(loopholes of solvent vapor) is reliably prevented in the thin
film. Consequently, a short circuit between electrodes disposed on
both surfaces of the film is reliably prevented. Thus, a reliable
dielectric thin film can be provided.
[0013] In the method of preferred embodiments of the present
invention, characteristics of the resulting dielectric thin film
depend on conditions for film deposition before heat treatment
rather than on heat treatment conditions. Therefore, by selecting
the conditions for film deposition, a reliable dielectric thin film
can be efficiently formed.
[0014] Also, by heat-treating the deposited thin film in an
oxidizing atmosphere, organic constituents of organic metal
compounds in the material solution are surely burned and removed.
Thus, a reliable, precise dielectric thin film can be provided.
[0015] More specifically, in the method of preferred embodiments of
the present invention, the solvent in the sprayed material solution
is rapidly vaporized to be removed immediately after reaching the
surface of the heated substrate. Thus, a film hardly including any
of the solvent can be deposited on the substrate.
[0016] Thus, the vaporization of the solvent from the deposited
film can be prevented and, thus, the occurrence of cavities in the
resulting thin film is prevented. Consequently, a reliable
dielectric thin film can be achieved in which short circuits do not
occur between electrodes, even if the electrodes are disposed on
both surfaces of the film.
[0017] Preferably, the material solution is sprayed under
conditions that a major portion of the solvent vaporize immediately
soon after the solvent reaches a surface of the substrate.
[0018] By spraying the material solution under conditions allowing
a major portion of the solvent to vaporize immediately, the
vaporization of the solvent is prevented in the deposited thin film
and the occurrence of cavities in the resulting thin film is
reliably prevented and minimized. Thus, a reliable dielectric thin
film can be provided.
[0019] Preferably, the film deposition step and the heat treatment
step are performed two or more times.
[0020] By subjecting one substrate to the film deposition and heat
treatment steps two or more times, a reliable, precise dielectric
thin film in which short circuits do not occur can be provided.
[0021] If the material solution includes an organic metal compound,
very few cavities or voids are inevitably formed in the thin film
when the organic constituents of the organic metal compound are
removed, even if the solvent can be efficiently removed in the film
deposition step. However, by repeating the film deposition step and
the heat treatment step, the cavities or voids, which are likely to
cause short circuits, are filled, so that the occurrence of short
circuits can be reliably prevented.
[0022] Preferably, the material solution includes at least one
metallic element in a total concentration of approximately 0.01
mol/L or less.
[0023] By using a material solution including approximately 0.01
mol/L or less of the metallic element, the size of lumps formed by
the solidification of drops of the sprayed material solution can be
reduced and, thus, a dielectric thin film having a uniform, small
thickness can be achieved.
[0024] Since the substrate is heated in the method of preferred
embodiments of the present invention, a material solution having an
excessively high concentration is likely to result in lumps having
a grain size as large as several micrometers. This makes it
difficult to form a dielectric thin film having a thickness as
small as a submicron. Also, the surface morphology is significantly
degraded due to the resulting rough surface.
[0025] The inventors conducted an experiment to determine a
suitable concentration of the material solution for forming a film
having an adequately small thickness. As a result, it has been
discovered and shown that, by using a material solution having a
metallic element concentration of approximately 0.01 mol/L or less,
the size of lumps formed on the surface of the substrate can be
reduced so that the thickness of the dielectric thin film becomes
uniform and small. Therefore, it is preferable to use a material
solution having a metallic element concentration of approximately
0.01 mol/L or less.
[0026] According to another preferred embodiment of the present
invention, a method for forming a dielectric thin film includes the
steps of (a) spraying a material solution including a starting
material and a solvent onto a heated substrate under a reduced
pressure by a two-fluid technique using an inert gas to deposit a
thin film, (b) stopping the supply of the material solution and
vaporizing the solvent remaining in the thin film, and (c)
subjecting the thin film to heat treatment in an oxidizing
atmosphere.
[0027] The heat treatment step is performed after the film
deposition step and the solvent vaporization step is preferably
repeated once or more times. Also, the material solution is
supplied at a rate that is greater than the vaporization rate of
the solvent in the film deposited on the substrate.
[0028] By supplying the material solution at a rate that is greater
than the vaporization rate of the solvent in the film to deposit a
thin film, then vaporizing the solvent remaining in the film with
the material solvent not supplied, and heat-treating the film in an
oxidizing atmosphere, the vaporization of the solvent is prevented
in the deposited thin film and, thus, the occurrence of cavities is
reliably prevented in the resulting thin film. Thus, a reliable
dielectric thin film is provided.
[0029] Although the method includes the solvent vaporization step
after the film deposition step, the solvent is proactively
vaporized by heating the substrate during film deposition. The
deposited film is gradually dried from the substrate side while the
solvent is vaporized. Thus, the occurrence of cavities can be
prevented in the solvent vaporization step effectively, and
consequently, a reliable dielectric thin film is provided.
[0030] In this method, the supply rate of the material solution can
be increased to increase the speed of film formation.
[0031] The speed of film formation in this method is a few times to
several tens of times higher than that of other processes of
preferred embodiments of the present invention.
[0032] Also, the material solution can be efficiently adhered to
the substrate to increase raw material efficiency.
[0033] Preferably, the film deposition step is performed at a
pressure of approximately 13.3 kPa (100 Torr) or less.
[0034] By setting the pressure in the film deposition step at
approximately 13.3 kPa or less, a reliable dielectric thin film
which has excellent characteristics and does not exhibit short
circuits even if the thickness is small is provided.
[0035] Preferably, the thickness of a film deposited at one time is
approximately 50 nm or less.
[0036] By setting the thickness of a film deposited at one time to
be approximately 50 nm or less, cavities are surely prevented and
thus, a reliable dielectric thin film can be achieved. If the
thickness deposited at one time becomes larger than approximately
50 nm, the solvent vaporized from the film easily passes through
the thin film to undesirably form cavities.
[0037] Preferably, the solvent vaporization step is performed at a
pressure lower than the pressure in the film deposition step.
[0038] By setting the pressure in the solvent vaporization step
lower than the film deposition step, the solvent can be efficiently
vaporized and removed.
[0039] Preferably, the substrate is heated to a temperature in the
range of about 100.degree. C. to about 300.degree. C.
[0040] By heating the substrate to about 100.degree. C. to about
300.degree. C. in the film deposition step, a reliable dielectric
thin film can be achieved which has excellent characteristics and
does not exhibit short circuits even if the thickness is small.
[0041] A temperature of higher than about 300.degree. C. leads to
the decomposition of the solvent, thus allowing the thin film to
include a large amount of decomposition products. The decomposition
products remain in the thin film even after heat treatment,
consequently increasing leak current in comparison with in the case
of a temperature of about 300.degree. C. or less. Accordingly, it
is preferable to set the temperature for heating the substrate at
about 300.degree. C. or less.
[0042] The temperature for heating the substrate can be reduced to
about 100.degree. C. without problems. However, a temperature of
lower than about 100.degree. C. reduces the vaporization rate of
the solvent of the material solution to allow a large amount of the
solvent to remain in the thin film. This significantly degrades the
surface morphology and the resulting dielectric thin film is liable
to exhibit a short circuit. Accordingly, it is preferable to set
the temperature for heating the substrate in the range of about
100.degree. C. to about 300.degree. C.
[0043] Preferably, the heat treatment is performed at a temperature
of about 500.degree. C. or more.
[0044] By setting the temperature for heat treatment at about
500.degree. C. or more, organic constituents of an organic metal
compound included in the material solution can be surely burned and
removed. Thus, a reliable, precise dielectric thin film can be
achieved.
[0045] Preferably, the material solution includes (a) titanium and
barium; or (b) titanium and strontium.
[0046] By using a material solution including titanium and barium;
or titanium and strontium, a reliable barium titanate or strontium
titanate dielectric thin film having various uses can be
efficiently formed.
[0047] Other preferred embodiments of the present invention are
directed to a dielectric thin film formed by the method according
to the preferred embodiments described above. The dielectric thin
film of such a preferred embodiment preferably has (a)
crystallinity; (b) a thickness of about 200 nm or less; and (c) a
relative dielectric constant of about 250 or more.
[0048] The dielectric thin film has high reliability and can be
suitably used for multilayer ceramic electronic components such as
monolithic ceramic capacitors.
[0049] The dielectric thin film may be formed by repeating the film
deposition and heat treatment steps one or more times.
[0050] By subjecting one substrate to the film deposition and heat
treatment steps two or more times, the resulting dielectric thin
film has high reliability and does not exhibit a short circuit even
if electrodes are disposed on both surfaces thereof. Thus, the
dielectric thin film can be suitably used for multilayer ceramic
electronic components such as monolithic ceramic capacitors.
[0051] Also, other preferred embodiments of the present invention
are directed to another dielectric thin film formed by other
methods according to preferred embodiments of the present
invention. This dielectric thin film according to another preferred
embodiment preferably has (a) crystallinity; and (b) a relative
dielectric constant of about 250 or more.
[0052] The dielectric thin film has high reliability and does not
exhibit a short circuit even if electrodes are disposed on both
surfaces thereof. Thus, the dielectric thin film can be suitably
used for multilayer ceramic electronic components such as
monolithic ceramic capacitors.
[0053] Preferably, the dielectric thin film includes (a) titanium
and barium; or (b) titanium and strontium.
[0054] The resulting barium titanate or strontium titanate
dielectric thin film has high reliability and various uses.
[0055] Other features, elements, characteristics and advantages of
the present invention will become more apparent from the following
detailed description of preferred embodiments thereof with
reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] FIG. 1 is a schematic illustration of an apparatus for
forming a dielectric thin film by a method according to a preferred
embodiment of the present invention;
[0057] FIG. 2 is a schematic illustration of a two-fluid nozzle
used in the apparatus shown in FIG. 1; and
[0058] FIG. 3 is a graph showing the relationship with time between
the supply rate of a material solution and the internal pressure of
a film-forming chamber in the film-deposition and solvent
vaporization steps in Example 4.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0059] The present invention will now be further illustrated with
reference to preferred embodiments thereof.
[0060] FIG. 1 shows an apparatus for forming dielectric thin films,
and the dielectric thin films of Examples were formed with this
apparatus by a method according to preferred embodiments of the
present invention.
[0061] The apparatus has a film-forming chamber A for forming
dielectric thin films, a material feeder B for supplying a material
solution including a starting material of a dielectric thin film
and a solvent, and a two-fluid nozzle C for atomizing the material
solution and supplying the atomized solution to the film-forming
chamber A, an inert gas feeder D for supplying an inert gas (Ar gas
in the Examples) used to atomize the material solution to the
two-fluid nozzle C, an oxygen gas feeder E for supplying oxygen gas
to the film-forming chamber A, and an exhaust system F for
exhausting the gas from the film-forming chamber A and for sucking
in the gas until a degree of vacuum in the chamber A reaches a
predetermined degree of vacuum.
[0062] In the film-forming chamber A, a stage 2 is arranged to
place a substrate 1 thereon. The stage 2 includes a heater for
heating the substrate 1 to a predetermined temperature. The
film-forming chamber A also has a pressure gauge 3 for indicating
the internal pressure thereof.
[0063] The material feeder B includes a material vessel 5 to place
the material solution 4 in and a pump 6 for delivering the material
solution 4 to the two-fluid nozzle C, and a flow controlling valve
7.
[0064] The two-fluid nozzle C is intended to atomize the material
solution 4 by a two-fluid technique, and its end is positioned
inside the film-forming chamber A so that the spray of the atomized
material solution 4 is emitted to the substrate 1.
[0065] As shown in FIG. 2, the two-fluid nozzle C includes a
central tube 22a through which the material solution 4 passes and
an external tube 22b with a gas inlet 22c disposed outside the
central tube 22a. While the gas introduced from the gas inlet 22c
passes through the space between the central tube 22a and the
external tube 22b, the material solution delivered through the
central tube 22a is mixed with the gas and splayed. Thus, the
material solution is introduced in an atomized form into the
film-forming chamber A. In this preferred embodiment of the present
invention, the central tube 22a preferably has an inside diameter
of about 0.25 mm and an outside diameter of about 1.59 mm, and the
external tube 22b has an inside diameter of about 2.30 mm. Hence,
the material solution passes through a substantially cylindrical
region having a diameter of about 0.25 mm and the gas passes
through a toroidal region having an inside diameter of about 1.59
mm and an outside diameter of about 2.30 mm.
[0066] The inert gas feeder D is intended to supply the inert gas
used for spraying the material solution in the two-fluid nozzle C.
The inert gas feeder D includes a pressure control valve 8 and a
mass flow controller 9 so that the inert gas is delivered to the
two-fluid nozzle C at a predetermined speed.
[0067] The oxygen gas feeder E is intended to supply oxygen
(O.sub.2) gas to the film-forming chamber A to make the inside of
the chamber an atmosphere of oxygen gas during heat treatment. The
oxygen gas feeder E includes a pressure control valve 10 and a mass
flow controller 11 so that the oxygen gas is supplied to the
film-forming chamber A at a predetermined speed.
[0068] The exhaust system F is intended to reduce the internal
pressure of the film-forming chamber A to a predetermined vacuum
and to exhaust gas from the film-forming chamber A. The exhaust
system F has a pressure control valve 12, a liquid nitrogen trap
13, and a rotary pump (vacuum pump) 14.
[0069] A method for forming a dielectric thin film with this
apparatus will now be described.
EXAMPLE 1
[0070] (1) TiO(acac).sub.2, Ba(acac).sub.2.multidot.2H.sub.2O, and
Sr(acac).sub.2.multidot.2H.sub.2O were weighed out in amounts
equivalent to about 0.000362 mol of barium and strontium in total
and about 0.00155 of titanium, and were dissolved in
2-methoxyethanol. The total volume of the solution was adjusted to
approximately 1000 mL, and thus, a material solution was
prepared.
[0071] (2) The internal pressure of the film-forming chamber was
reduced to about 0.013 kPa (0.1 Torr) or less with the rotary pump
14. A Si substrate 1 coated with a SiO.sub.2 layer on the surface
thereof and having a Ti layer and a Pt electrode on the rear
surface thereof in that order, that is, a Pt/Ti/SiO.sub.2/Si
substrate, was placed on the stage 2 and heated to about
240.degree. C. Ar gas was supplied to the film-forming chamber A at
a flow rate of about 7 L/min with the mass flow controller 9, and
the internal pressure of the film-forming chamber A was maintained
at about 2.66 kPa (20 Torr) with the pressure control valve 12.
[0072] (3) The material solution was continuously supplied to the
film-forming chamber A with the pump 6 at a rate of about 2 mL/min
for approximately 360 minutes to deposit a thin film on the
Pt/Ti/SiO.sub.2/Si substrate 1.
[0073] After stopping the supply of the material solution, the
temperature of the stage 2 was reduced to room temperature. The
film-forming chamber A was purged with Ar gas and the internal
pressure of the film-forming chamber A was increased to atmospheric
pressure.
[0074] (5) With the film-forming chamber A opened and exposed to
the atmosphere, the stage 2 was heated at about 650.degree. C. for
approximately 3 hours to heat-treat (fire) the dielectric thin film
while the gas flow ratio of Ar gas to O.sub.2 gas was adjusted to
about 4 to 1. Thus, a barium-strontium titanate (BST) dielectric
thin film (Sample A) was formed.
[0075] The resulting BST dielectric thin film (Sample A) was
provided with twenty Pt electrodes of about 500 .mu.m in diameter
on the surface thereof by sputtering. The capacitance of each Pt
electrode was measured. As a result, eighteen out of the twenty
electrodes were short-circuited. Hence, the percentage of short
circuits was 90%.
[0076] The points where no short circuit had occurred were
subjected to measurements of tan .delta. at 1 kHz, thicknesses of
the dielectric film, relative dielectric constants, and leak
currents at an applied voltage of about 1.0 V. The results are as
follows:
[0077] (1) tan .delta. at 1 kHz: about 10.5%
[0078] (2) dielectric thin film thickness: about 200 nm
[0079] (3) relative dielectric constant: about 150
[0080] (4) leak current at an impressed voltage of 1.0 V: about
1.times.10.sup.-4 A.multidot.cm.sup.-2
[0081] Also, the composition of the dielectric thin film was
analyzed, and consequently the proportion of the metal elements
constituting the film was (barium+strontium):titanium=52:48.
[0082] The dielectric thin film of Example 1 exhibited a high
percentage of short circuits of 90%, and the relative dielectric
constant of 150 and the leak current of about 1.times.10.sup.-4
A.multidot.cm.sup.-2 are not necessarily satisfactory results.
However, these results are better than those of a dielectric film
formed by applying a material solution in a liquid form onto a
substrate as in the MOD method or the spray method. Therefore, by
selecting conditions for film deposition and heat treatment, a
dielectric thin film that is more reliable than the dielectric thin
films formed by a known method, such as the MOD method or the spray
method, can be efficiently formed.
EXAMPLE 2
[0083] A film was deposited for approximately 90 minutes using the
same material solution as in Example 1 under the same conditions as
in Example 1, including the substrate temperature, vacuum, Ar flow
rate, and supply rate of the material solution. The resulting film
was subjected to heat treatment under the same conditions as in
Example 1, and subsequently the same film deposition step and heat
treatment step were repeated. Hence, the film deposition step and
the heat treatment step were performed twice. Thus, a BST
dielectric thin film (Sample B) was formed.
[0084] The resulting BST dielectric thin film (Sample B) was
provided with twenty Pt electrodes of about 500 .mu.m in diameter
on the surface thereof by sputtering. The capacitance of each Pt,
electrode was measured. As a result; four out of the twenty
electrodes were short-circuited. Hence, the percentage of short
circuits was 20%.
[0085] The points where no short circuit had occurred were
subjected to measurements of tan .delta. at 1 kHz, thicknesses of
the dielectric thin film, relative dielectric constants, and leak
currents at an applied voltage of about 1.0 V. The results are as
follows:
[0086] (1) tan .delta. at 1 kHz: about 4.2%
[0087] (2) dielectric film thickness: about 80 nm
[0088] (3) relative dielectric constant: about 250
[0089] (4) leak current at an impressed voltage of 1.0 V: about
1.times.10.sup.-6 A.multidot.cm.sup.-2
[0090] Thus, Sample B, which is formed by repeating the film
deposition step and the heat treatment, exhibited better
characteristics than those of Sample A, that is, a tan .delta. of
4.2%, a relative dielectric constant of about 250, and a leak
current of about 1.times.10.sup.-6 A.multidot.cm.sup.-2.
[0091] Also, the composition of the dielectric thin film was
analyzed, and consequently the proportion of the metal elements
constituting the film was (barium+strontium):titanium=52:48.
EXAMPLE 3
[0092] A film was deposited for approximately 60 minutes using the
same material solution as in Examples 1 and 2 under the same
conditions as in Example 1, including the substrate temperature,
vacuum, Ar flow rate, and supply rate of the material solution. The
resulting film was subjected to heat treatment under the same
conditions as in Example 1, and subsequently the same film
deposition step and heat treatment were repeated twice. Hence, the
film deposition step and the heat treatment were performed three
times. Thus, a BST dielectric thin film (Sample C) was formed.
[0093] The resulting BST dielectric thin film (Sample C) was
provided with twenty Pt electrodes of about 500 .mu.m in diameter
on the surface thereof by sputtering. The capacitance of each Pt
electrode was measured. As a result, three out of the twenty
electrodes were short-circuited. Hence, the percentage of short
circuits was 15%.
[0094] The points where no short circuit had occurred were
subjected to measurements of tan .delta. at 1 kHz, thicknesses of
the dielectric film, relative dielectric constants, and leak
currents at an applied voltage of about 1.0 V. The results are as
follows:
[0095] (1) tan .delta. at 1 kHz: about 2.7%
[0096] (2) dielectric film thickness: about 90 nm
[0097] (3) relative dielectric constant: about 250
[0098] (4) leak current at an impressed voltage of 1.0 V: about
1.times.10.sup.-7 A.multidot.cm.sup.-2
[0099] Thus, Sample C, which is formed by performing the film
deposition step and the heat treatment three times, exhibited still
better characteristics than those of Samples A and B, that is, a
tan .delta. of about 2.7%, a relative dielectric constant of 250,
and a leak current of about 1.times.10.sup.-7
A.multidot.cm.sup.-2.
[0100] Also, the composition of the dielectric thin film was
analyzed, and consequently the proportion of the metal elements
constituting the film was (barium+strontium):titanium=52:48.
[0101] In Examples 1, 2, and 3, a solution in which
TiO(acac).sub.2, Ba(acac).sub.2.multidot.2H.sub.2O, and
Sr(acac).sub.2.multidot.2H.sub.2O dissolved in 2-methoxyethanol was
used as the material solution. However, the material solution is
not limited to this, and various constituents may be used to
achieve the same effects.
[0102] In Examples 1, 2, and 3, the Pt/Ti/SiO.sub.2/Si substrate
was used as a substrate of the dielectric film. However, the type
of the substrate is not particularly limited and various types of
substrates may be used.
EXAMPLE 4
[0103] (1) The following compounds were used as Ba, Sr, Ti raw
materials:
[0104] Ba(C.sub.5H.sub.7O.sub.2).sub.2.multidot.2H.sub.2O;
[0105] Sr(C.sub.5H.sub.7O.sub.2).sub.2.multidot.2H.sub.2O; and
[0106] TiO(C.sub.5H.sub.7O.sub.2).sub.2
[0107] (2) The Ba, Sr, and Ti raw materials were compounded at a
ratio of about 1:1:3, and dissolved in 2-mthoxyethanol (boiling
point at atmospheric pressure: 125.degree. C.) while stirring for
about 60 minutes using a magnetic stir. Thus a material solution
having a total molarity of about 0.005 mol/L was prepared.
[0108] (3) The material solution was sprayed onto a substrate at a
rate that is greater than the vaporization rate of the solvent in
the deposited film on the substrate, as shown in Table 1, with the
film-forming apparatus shown in FIG. 1. Thus, a dielectric thin
film was deposited. The temperature of the substrate was set at
about 300.degree. C. The spray gas was Ar, and the substrate was an
R-plane sapphire substrate provided with a Pt film having a
thickness of 200 nm on the surface thereof by sputtering.
1 TABLE 1 Ba raw material Ba
(C.sub.5H.sub.7O.sub.2).sub.2.2H.sub.2O Sr raw material Sr
(C.sub.5H.sub.7O.sub.2).sub.2.2H.sub.2O Ti raw material TiO
(C.sub.5H.sub.7O.sub.2).sub.2 Solvent 2-methoxyethanol Ba:Sr:Ti
mole ratio 1:1:3 Material concentration 0.005 mol/L Material
solution supply rate 1 g/min Ar gas flow rate 10 L/min Film
deposition temperature 170.degree. C. Film-forming chamber pressure
10.64 kPa (80 Torr) Film-deposition time 30 min
[0109] The above-described conditions leads to a wet film because a
portion of the solvent remains in the film.
[0110] (4) The supply of the material solution was stopped
simultaneously with the completion of film deposition, and the
pressure control valve 12 of the film-forming chamber A, shown in
FIG. 1, was fully opened to remove the solvent remaining in the
film by vaporization, with the substrate temperature maintained at
about 300.degree. C. The solvent in the film is rapidly vaporized
and thus, the surface of the film is dried. The internal pressure
of the film-forming chamber A becomes about 2.66 kPa (20 Torr) in
the solvent vaporization step.
[0111] (5) The film deposition step described in (3) and the
solvent vaporization step described in (4) were repeated a
plurality of times to form a dielectric thin film having a
thickness of about 500 nm.
[0112] FIG. 3 shows the relationship with time between the supply
rate of the material solution and the internal pressure of the
film-forming chamber in the film-deposition step and the solvent
vaporization step in Example 4. In Example 4, the total time for
forming the thin film was approximately 60 minutes, including the
total film-deposition time of approximately 30 minutes.
[0113] The resulting film having a thickness of about 500 nm,
formed in Example 4 by repeating the steps of film deposition (3)
and solvent vaporization (4) a plurality of times, did not exhibit
cracks even when examined by microscopy.
[0114] Thus, by spraying the material solution onto the substrate
at a rate that is greater than the vaporization rate of the solvent
in the film on the substrate and by repeating these film deposition
and drying steps, a dielectric thin film having no cracks can be
efficiently formed.
[0115] (6) The thin film was subjected to heat treatment in an
oxidizing atmosphere (in the air in Example 4) at about 650.degree.
C. for approximately 3 hours. Thus, a multiple oxide dielectric
thin film was completed which has reliability and a high dielectric
constant.
COMPARATIVE EXAMPLE
[0116] A dielectric thin film having a thickness of about 500 nm
was formed by continuously depositing the material solution for
approximately 30 minutes under the same conditions as in Example
4.
[0117] In this instance, while a dielectric thin film of about 500
nm in thickness was formed at a time, a large number of cracks
having a width of about 200 nm to about 2 .mu.m occurred. This is
probably because when a deposited film having a relatively large
thickness is dried, the solvent is vaporized to reduce the volume
of the film, with a low adhesion between the resulting thin film
and the Pt film on the substrate.
[0118] The thin film having the large number of cracks was
subjected to heat treatment in an oxidizing atmosphere (in the air)
at about 650.degree. C. for approximately 3 hours to complete a
dielectric thin film. The resulting dielectric thin film exhibited
a dielectric constant lower than that of the film of Example 4 and,
thus, the reliability was unsatisfactory.
[0119] It is undesirable to excessively reduce the supply rate of
the material solution because the solvent in the spray is
excessively vaporized to seriously reduce the efficiency of
material deposition on the substrate, even if the film-deposition
step and the solvent vaporization step are repeated and then heat
treatment is performed, as in Example 4.
[0120] In Example 4, a solution having a predetermined
concentration in which
Ba(C.sub.5H.sub.7O.sub.2).sub.2.multidot.2H.sub.2O,
Sr(C.sub.5H.sub.7O.sub.2).sub.2.multidot.2H.sub.2O, and
TiO(C.sub.5H.sub.7O.sub.2).sub.2 are dissolved in 2-methoxyethanol
was used as the material solution. However, the material solution
is not limited to this, and various constituents may be used to
lead to the same effects.
[0121] In Examples 4, an R-plane sapphire substrate provided with a
Pt film at a thickness of about 200 nm by sputtering was used as a
substrate of the dielectric thin film. However, the type of the
substrate is not particularly limited and various types of
substrates may be used.
[0122] It should be understood that the foregoing description is
only illustrative of the present invention. Various alternatives
and modifications can be devised by those skilled in the art
without departing from the present invention. Accordingly, the
present invention is intended to embrace all such alternatives,
modifications and variances which fall within the scope of the
appended claims.
* * * * *