JP2003176176A - Bismuth layer structure ferroelectric, method of producing the same, memory element and dielectric/ electrostrictive element using the ferroelectric - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bismuth layer structure ferroelectric (BLSF) which is capable of controlling a coercive electric field to a desired value and operat ing under an operation voltage corresponding to each application, to provide a method of producing the same and a dielectric/electrostrictive element using the ferroelectric. <P>SOLUTION: The bismuth layer structure ferroelectric has a chemical composition expressed by a general formula: Sr<SB>1-x</SB>A<SB>2x/3</SB>Bi<SB>2</SB>Ta<SB>2</SB>O<SB>9</SB>(wherein, 0.01≤x≤0.7; A is one of rare earth elements; and V represents vacancy), in which part of Sr of strontium bismuth tantalate (SrBi<SB>2</SB>Ta<SB>2</SB>O<SB>9</SB>) is replaced by a rare earth element and the vacancy V is introduced to the position of Sr. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferroelectric material used in a ferroelectric element such as a non-volatile ferroelectric memory, particularly a bismuth layer structure ferroelectric material and a method for manufacturing the same.

[0002]

2. Description of the Related Art Nonvolatile ferroelectric memory (hereinafter referred to as FeRA)
It is called M. ), Lead zirconate titanate (hereinafter referred to as PZT) and bismuth layered structure ferroelectric (hereinafter referred to as BLSF) are used as the ferroelectric.

Since PZT has a large remanent polarization but a large coercive electric field, deterioration due to repeated use and poor fatigue resistance, platinum cannot be used for the electrode and an expensive material such as iridium is required. Furthermore, since it contains lead, it is not suitable for environmental problems.

On the other hand, BLSF has a smaller remanent polarization than PZT, but is excellent in fatigue resistance. Among them, SrBi
₂ Ta ₂ O ₉ (SBT) is being practically used because it increases the remanent polarization by controlling the composition (excessive Bi).

[0005]

However, the conventional S
In BT, the coercive electric field is smaller than other ferroelectrics, for example, Bi ₄ Ti ₃ O ₁₂ which is one of PZT and BLSF, but it is still insufficient and a reduction in operating voltage is desired.

The decrease in operating voltage is caused by the FeRAM mixed CMOS logic circuit (FeRAM mixed L
It is especially strongly desired in the application field using SI). That is, in the FeRAM mixed LSI, the FeRAM section and the CM
It is desirable that the OS transistor sections operate at the same power supply voltage. However, the CMOS transistor part can operate with a power supply of 3V or 1.8V, but at present, FeR
Since the operating voltage of AM is high, the 1.8V power supply cannot be used to operate the FeRAM embedded LSI. Therefore, P
FeRAM mixed LSI using ZT as a ferroelectric requires 5V for FeRAM operation, so it is compatible with 3V and 5V.
It incorporates two types of transistors.

Further, in a ferroelectric gate FET memory using a ferroelectric thin film for an insulated gate portion, an appropriately large coercive electric field (50-100 kV / cm) and a polarization characteristic with good rectangularity are required. It has not yet been realized to form a ferroelectric thin film that satisfies both characteristics on the SiO ₂ film. Therefore, there is a demand for a technique capable of controlling the coercive electric field in a wide range with an SBT system having good rectangularity and polarization characteristics.

Further, the ferroelectric substance is used as an actuator or the like which utilizes mechanical displacement due to an electric field by utilizing its piezoelectricity. BLSF has small relative permittivity, dielectric loss, and temperature dependence of characteristics, but has characteristics such as large anisotropy of characteristics, and is a high temperature, high frequency, and highly stable piezoelectric ceramic for ultrasonic transmission, and Research is progressing as a high-speed response actuator. However, due to the high operating voltage, a high output power source is required for these applications, making practical implementation difficult.

Therefore, according to the present invention, the coercive electric field can be controlled to a desired value, and the bismuth layered structure ferroelectric (BLSF) capable of operating at an operating voltage according to the application, its manufacturing method, and a memory device using the same. And to provide a piezoelectric / electrostrictive element such as an actuator element.

[0010]

The bismuth layered structure ferroelectric material of the present invention comprises strontium bismuth tantalate (S).
A part of Sr of rBi ₂ Ta ₂ O ₉ ) is replaced with a rare earth element A, and a void □ is introduced at the position of Sr, and the chemical composition is represented by the general formula Sr _1-x A _{2x / 3} □ _{x / 3} Bi ₂ Ta ₂
It is characterized by having a chemical composition represented by O ₉ (0.01 ≦ x ≦ 0.7).

In the bismuth layered structure ferroelectric substance of the present invention, the rare earth element A is one or more elements selected from the group consisting of lanthanum La, neodymium Nd, praseodymium Pr and samarium Sm. It is characterized by.

Further, in the bismuth layered structure ferroelectric substance of the present invention, the rare earth element A is lanthanum La and neodymium Nd.

Further, in the bismuth layered structure ferroelectric substance of the present invention, strontium bismuth tantalate (S
A part of Sr of rBi ₂ Ta ₂ O ₉ ) is replaced by the rare earth element A and bismuth Bi, and a hole □ is introduced at the position of Sr, and the chemical composition is represented by the general formula Sr _1-x (A + B
i) _{2x / 3} □ _{x / 3} Bi ₂ Ta ₂ O ₉ (0.01 ≦ x
It has a chemical composition represented by ≦ 0.7).

Further, in the bismuth layered structure ferroelectric substance of the present invention, the rare earth element A is lanthanum La, neodymium Nd, praseodymium Pr and samarium Sm.
It is characterized by being one or more elements selected from the group consisting of

Further, the bismuth layered structure ferroelectric substance of the present invention is characterized in that the rare earth element A is lanthanum La and neodymium Nd.

Furthermore, in the bismuth layered structure ferroelectric substance of the present invention, Ta in the above general formula is characterized in that all or part of Ta is substituted with niobium Nb.

A memory element using the bismuth layered structure ferroelectric material of the present invention is characterized by using any one of the above bismuth layered structure ferroelectric materials.

Further, the piezoelectric / electrostrictive element using the bismuth layered structure ferroelectric substance of the present invention uses a ferroelectric substance in which all or part of Ta in the bismuth layered structure ferroelectric substance is replaced with niobium Nb. It is characterized by that.

Next, the bismuth layered structure ferroelectric Sr _1-x A _{2x / 3} □ _{x / 3} Bi ₂ Ta ₂ O of the present invention described above.
₉ (0.01 ≦ x ≦ 0.7) includes a step of mixing raw material powders containing strontium carbonate SrCO ₃ , bismuth oxide Bi ₂ O ₃ , tantalum oxide Ta ₂ O ₅ and an oxide of a rare earth element. , A step of calcining the mixed raw material powder, a step of crushing the calcined raw material powder into a powder, and molding the powdered raw material powder into pellets by cold isostatic pressing And a step of subjecting the formed pellets to a main firing treatment, and an annealing step of annealing the fired pellets.

Further, the bismuth layered structure ferroelectric Sr _1-x A _{2x / 3} □ _{x / 3} Bi ₂ Ta ₂ O of the present invention.
₉ (0.01 ≦ x ≦ 0.7), the oxide of the rare earth element may be lanthanum oxide La ₂ O ₃ , neodymium oxide Nd ₂ O ₃ , praseodymium oxide Pr ₆ O.
₁₁ and one or more raw material powders selected from the group consisting of samarium oxide Sm ₂ O ₃ .

Further, the bismuth layered structure ferroelectric Sr _1-x (A + Bi) _{2x / 3} □ _{x / 3} Bi _{2 of the} present invention.
The manufacturing method of Ta ₂ O ₉ (0.01 ≦ x ≦ 0.7) is performed by using strontium carbonate SrCO ₃ and bismuth oxide Bi.
₂ O ₃ , tantalum oxide Ta ₂ O ₅ , niobium oxide Nb ₂ O
₅ and the step of mixing the raw material powder containing the oxide of the rare earth element, the step of calcining the mixed raw material powder, the step of crushing the calcined raw material powder into powder, and Characterized by comprising a step of forming the raw material powder into pellets by cold isostatic pressing, a step of subjecting the formed pellets to a main firing treatment, and an annealing step of annealing the fired pellets. It is a thing.

Furthermore, the bismuth layered structure ferroelectric Sr _1-x (A + Bi) _{2x / 3} □ _{x / 3} Bi _{2 of the} present invention.
In the method for producing Ta ₂ O ₉ (0.01 ≦ x ≦ 0.7), the oxide of the rare earth element is lanthanum oxide La ₂
One or a plurality of raw material powders selected from the group consisting of O ₃ , neodymium oxide Nd ₂ O ₃ , praseodymium oxide Pr ₆ O ₁₁ and samarium oxide Sm ₂ O ₃ .

Next, the bismuth layered structure ferroelectric Sr _1-x A _{2x / 3} □ _{x / 3} Bi ₂ Ta ₂ O of the present invention described above.
₉ (0.01 ≦ x ≦ 0.7), and a method for manufacturing a thin film ferroelectric is as follows: Strontium alkoxide Sr (Oi
Pr) ₂ , bismuth alkoxide Bi (OnBu) ₃ , tantalum alkoxide Ta (OEt) ₅ , and lanthanum alkoxide La (OnBu) ₃ are dissolved in a solvent at a predetermined ratio to obtain a metal alkoxide solution, and the metal alkoxide solution is added to the solution. Cause hydrolysis and polycondensation reaction,
It is characterized by comprising a step of obtaining a stock solution, a step of spin-coating the stock solution on a semiconductor substrate, a step of drying the spin-coated thin film, and a step of baking the dried thin film. .

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will now be described in detail with reference to examples.

The bismuth layered structure ferroelectric substance (BL of the present invention
SF) is strontium bismuth tantalate (SrBi
₂ Ta ₂ O ₉ ) has a composition in which a part of Sr is replaced with a rare earth element and a hole (defect) is introduced at the position of Sr, that is, (Sr _1-x A _{2x / 3} □ _{x / 3} ) Bi ₂ Ta ₂
It has a composition of O ₉ . Here, A in the formula represents a rare earth element, □ represents a hole, and x is 0.5 or less. In the following, in the case of lanthanum La as a rare earth element, that is, (Sr _1-x La _{2x / 3} □ _{x / 3} ) Bi ₂ Ta
The manufacturing method of ₂ O ₉ will be described.

Raw material powders each having a purity of 99.99%
Strontium carbonate SrCO_Three, Bismuth oxide Bi_Two
O_Three, Tantalum oxide Ta_TwoO₅And lanthanum oxide La
_TwoO _ThreePrepare these oxides and carbonates by the following procedure
To prepare a sample.

The above raw material powder is added in a predetermined ratio, that is,
Weigh to the desired metal molar ratio and mix. This mixing is done by weighing the raw material powder and zirconia (ZrO ₂ ) balls (balls with a diameter of 5 mm and 10 mm) in a container (poly bottle).
Mix for 6-12 hours by placing, adding ethanol, capping and rotating the plastic bottle. During this rotary mixing,
The raw material powder is uniformly mixed by randomly moving heavy zirconia balls. Next, it is dried. In this step, ethanol is evaporated while rotating the raw material powders mixed by a rotary evaporator. The evaporation of ethanol is to prevent sedimentation due to the difference in specific gravity of the raw material powder.

The dried mixed raw material powder is calcined.
In this step, the mixed raw material powder is mixed in air or in oxygen 10
Heat treatment at 00 ℃ for 4-7 hours. The powder thus calcined is placed in a plastic bottle together with zirconia balls in the same manner as in the above mixing step, ethanol is added, and the mixture is mixed for 6-12 hours. By this step, the calcined powder is finely pulverized by the zirconia balls to be pulverized. After that, the calcined powder is dried in the same manner as the above-mentioned drying step.

The dry powder is molded into pellets (diameter 10 mm, thickness 3-5 mm) by a molding process. In this step, a compactor is filled with 1-2 g of powder, and a pressure of 50-200 MPa is applied to produce a compact (pellet). Specifically, a powder sample is pressure-treated from above with a pressure of about 100 MPa by uniaxial pressure molding, and then a pressure of 200 MPa is applied by cold isotropic pressure molding.
In this cold isostatic pressing, a sample pellet is put in oil and isotropically pressurized.

The formed pellets are subjected to a main baking treatment. In this step, the pellets are 1200-125 in air or oxygen.
Heat treatment is performed at 0 ° C. to produce a dense sintered body. The density of the obtained sintered body reaches 95-99% of that of a single crystal without air holes, and the air hole ratio, that is, the porosity is about 1-5%. The heat treatment specifically fires the pellets in an alumina crucible. To prevent the pellets from coming into contact with the crucible,
Spread a powder of the same composition as the pellet to a thickness of about several mm,
Put pellets on it. Further, a powder having the same composition as that of the pellet is applied onto the pellet, and the pellet is wrapped with the powder and heat-treated to prevent volatilization of the pellet raw material.

The fired pellets are annealed in the air or oxygen in the annealing step.
It is slowly cooled to -700 ℃ over 5-10 hours. This step reduces oxygen vacancies, which are oxygen vacancies contained in the pellet.

The annealed pellets are polished in a polishing process using carbon powder to a thickness of 0.1-0.3 mm. That is, the surface of the glass is scraped by sprinkling carbon powder on glass, adding water, and rubbing the pellets on this.

Platinum or gold is sputtered to a thickness of 100-200 nm on the front and back surfaces of the polished pellets to form electrodes to complete a ferroelectric element.

An electric field was applied between the electrodes of the ferroelectric element thus constructed and the polarization characteristics were measured.
A hysteresis curve as shown by the solid line was obtained. In the same figure, for comparison, the hysteresis characteristic of the conventional SBT type ferroelectric is shown by a dotted line. The composition of the measured ferroelectric was (Sr _1-x La _{2x / 3} □ _{x / 3} ) Bi ₂ Ta ₂ O.
₉ , the substitution amount x is 0.5, that is, Sr
_0.5 La _0.33 Bi ₂ Ta ₂ O ₉ and coercive electric field E
c = 20 kV / cm and remanent polarization Pr = 8 μC / cm ² were obtained. This value is the value of the conventional SBT dielectric, that is,
Ec (33 kV / cm) and Pr when x is 0 and not substituted
This corresponds to 62% and 115% of (7 μC / cm ² ), respectively. That is, the coercive electric field could be reduced to ⅔ or less while the residual polarization was slightly increased. Regarding the conventional SBT type ferroelectric, a composition in which bismuth Bi is excessive is researched and developed in order to increase the remanent polarization, but this composition Sr _0.73 Bi _{. 2.18} Ta ₂
Value at O ₉ Ec = 45 kV / cm, Pr = 12 μC / c
Compared with m ² , Pr is 2/3, but Ec is 45.
% And less than half.

In a ferroelectric substance, generally, when the coercive electric field is made small, the remanent polarization also becomes small. As a result of analyzing the crystal structure of the ferroelectric substance of the present invention manufactured as described above, there are two types of strain of the crystal lattice that exhibits ferroelectricity, one that is strongly related to the remanent polarization and one that is strongly related to the coercive electric field. I found out that there is. It has been clarified that the coercive electric field is reduced because the latter strain is reduced by the La substitution and the introduction of the holes.

According to the above ferroelectric substance of the present invention, the coercive electric field Ec = 20 kV / cm could be achieved. Therefore, when this material is formed into a thin film having a thickness of 100 nm, polarization reversal at 0.2 to 0.3 V is achieved. Is possible. When this thin film is used in a capacitor type ferroelectric memory, it can be operated with a power supply voltage of 1 V or less.

FeRA for IC cards and portable equipment
In the M-embedded LSI, downsizing of the element and reduction of power consumption can be realized by using a low voltage of 1 V or less, so that FeRAM is put into practical use in these application fields.

The manufacturing method of the ferroelectric substance in the above-mentioned embodiment is suitable for small-scale production which can be applied in the laboratory, but the present invention is not limited to this, and is suitable for mass production. It goes without saying that manufacturing methods can also be used.

Further, in the above embodiment, (Sr
_1-xA_{2x / 3}□_{x / 3}) Bi_TwoTa _TwoO₉Composition
The ferroelectric substance having is explained, but a part of Ta
Replaced all with Nb (Sr_1-xA_{2x / 3}□
_{x / 3}) Bi_Two(Ta_2-yNb_y) O₉The composition of
The electric body also has a similar hysteresis characteristic.

This ferroelectric substance is used as a raw material as described above.
Niobium oxide Nb is used as an example raw material._TwoO ₅And add the above
It can be manufactured similarly to the example.

It should be noted that (Sr _1-x A _{2x / 3} □ _{x / 3} ) B
A ferroelectric having a composition of i ₂ Ta ₂ O ₉ is suitable for use as a memory element, while a ferroelectric in which Ta is partially or wholly replaced with Nb is suitable for use as a piezoelectric element. That is, Sr in which Ta of strontium bismuth tantalate (SrBi ₂ Ta ₂ O ₉ ) was replaced with Nb
Bi ₂ Nb ₂ O ₉ has been conventionally expected as a piezoelectric material. However, as a problem in applying this material to a piezoelectric body, poling (polarization reversal in the sample) is difficult because the coercive electric field Ec is large, and the piezoelectric ability of this system cannot be derived. Therefore, as in the above-described embodiment of the present invention, by replacing Sr with La accompanied by holes, Ec can be lowered, which can facilitate poling and improve piezoelectric characteristics. This material is also useful as a lead-free piezoelectric material that does not contain harmful lead,
Because it can reduce the use of lead in electronic devices,
The ripple effect of making a large contribution to the improvement of the global environment can also be obtained.

In the above example, strontium tantalate.
Umbismuth (SrBi_TwoTa_TwoO ₉) Sr part of
The case of replacing with the lantern La has been described.
By substituting a rare earth element other than tan La, various
Hysteresis characteristics, especially, the coercive electric field value is controlled to various values.
Can be controlled. These ferroelectrics include Sr
Of neodymium Nd, praseodymium Pr and
A ferroelectric substance obtained by substituting it with malium Sm is obtained. these
Although the starting materials for ferroelectrics are different, the manufacturing method for them is La
It is almost the same as the case of the substitutional ferroelectric. Therefore starting material
Only will be described below.

First, when substituting with neodymium Nd
Is 99.99% pure strontium carbonate as raw material powder.
Um SrCO_Three, Bismuth oxide Bi_TwoO_Three, Tantalum oxide
Le Ta _TwoO₅And neodymium oxide Nd_TwoO_ThreeUsing
It In addition, niobium oxide Nb _TwoO₅You can add
Yes.

Next, when substituting with praseodymium Pr, as raw material powder, strontium carbonate SrCO _{3 having} a purity of 99.99%, bismuth oxide Bi ₂ O ₃ , tantalum oxide Ta ₂ O ₅ and praseodymium Pr ₆ O _{11 are used.}
To use. Further, niobium oxide Nb ₂ O ₅ may be added thereto.

Further, when replacing with samarium Sm, strontium carbonate SrCO ₃ , bismuth oxide Bi ₂ O ₃ , tantalum oxide Ta ₂ O ₅ and samarium oxide Sm ₂ O ₃ having a purity of 99.99% are used as raw material powders. . Further, niobium oxide Nb ₂ O ₅ may be added thereto.

FIG. 2 shows the hysteresis characteristic of the ferroelectric substance produced by using these raw material powders in the substantially same procedure as in the above-mentioned embodiment. From the figure and FIG. 1, the values of the coercive electric field are lanthanum La and neodymium Nd (2Ec = 58 kV / c).
m), samarium Sm (2Ec = 63 kV / cm), praseodymium Pr (2Ec = 74 kV / cm), and cerium Ce (2Ec = 92 kV / cm). Thus, in these dielectrics, the value of the coercive electric field can be controlled to 28.5 to 46 kV / cm.

Also in this embodiment, the composition (Sr
_1-x A _{2x / 3} □ _{x / 3} ) Bi ₂ Ta ₂ O ₉ A composition in which part or all of Ta is replaced with Nb (Sr
_{1-x A 2 x / 3} □ x / 3) Bi 2 (Ta 2-y Nb y)
The ferroelectric substance of O ₉ also has similar hysteresis characteristics.

In the above embodiment, part of Sr of strontium bismuth tantalate (SrBi ₂ Ta ₂ O ₉ ) is 1 part.
I explained about the case of replacing with rare earth elements of
It is also possible to substitute with two kinds of elements. An example in which lanthanum La and Nd-based solid solution are substituted will be described below.

The raw material powders used each have a purity of 99.9.
9% strontium carbonate SrCO _Three, Bismuth oxide B
i_TwoO_Three, Tantalum oxide Ta_TwoO₅, Lanthanum oxide La
_TwoO _ThreeAnd neodymium oxide Nd_TwoO_ThreePrepare this
In the same manner as in the above example, weigh and mix them at a predetermined ratio and dry.
Drying, calcination, powdering by mixing and crushing, drying, molding, book
Each process of baking, annealing, polishing, and electrode formation is performed. This
So composition Sr_1-x(La_y, Nd_1-y)
_{2x / 3}□_{x / 3}Bi_TwoTa_TwoO₉(X is the composition of 0.7
Hereinafter, a ferroelectric having y = 0 to 1 and □ is a hole is manufactured.
To be done.

FIG. 3 shows hysteresis characteristics when the composition ratio y of lanthanum La and neodymium Nd is changed between 0 and 1. As is clear from the figure, the value of the coercive electric field increases as the value of y increases.

In this embodiment also, the composition Sr
_1-x(La_y, Nd_1-y)_{2x / Three}□_{x / 3}Bi_Two
Ta_TwoO₉Substitute all or part of Ta in Nb with Nb
Composition Sr_1-x(La_y, Nd_1-y)_{2x / 3}□
_{x / 3}Bi_Two(Ta_2-yNb _y) O₉The same for ferroelectrics
It has excellent hysteresis characteristics.

In the above examples, a part of Sr of strontium bismuth tantalate (SrBi ₂ Ta ₂ O ₉ ) is replaced with the rare earth element A and bismuth Bi, and the chemical composition is represented by the general formula Sr _1-x ( A + Bi) _{2x / 3}
□ A bismuth layer structure ferroelectric having a chemical composition represented by _{x / 3} Bi ₂ Ta ₂ O ₉ (0.01 ≦ x ≦ 0.7) may be used. Such a ferroelectric material can control the coercive electric field to various values while maintaining a large remanent polarization Pr, as shown in FIG. The composition of the ferroelectric substance in the figure is Sr _0.5 La _0.15 Bi _0.18 Bi ₂ Ta ₂ O.
Represented by ₉ .

Further, in the above embodiments, part of Sr of strontium bismuth tantalate (SrBi ₂ Ta ₂ O ₉ ) may be replaced with the rare earth element A and Ta may be replaced with Nb.

Further, in the above embodiments, the bismuth layered structure ferroelectric substance of the present invention is described as an embodiment in which it is molded in a bulk shape, but the present invention is not limited to such a form, and a memory When applied to devices,
It can also be formed as a thin film on a semiconductor substrate. The thin film manufacturing method by the sol-gel method will be briefly described below.

The following metal alkoxides are used as raw materials,
Strontium alkoxide Sr (OiPr) ₂ , bismuth alkoxide Bi (OnBu) ₃ , tantalum alkoxide Ta (OEt) ₅ , lanthanum alkoxide La
(OnBu) ₃ is dissolved in a methoxypropanol solvent at a predetermined ratio to prepare a metal alkoxide solution of 0.1 to 1 mol / Kg. Further, in order to stabilize the solution, about 0 to 2 mol of acetylacetone is added to 1 mol of the contained metal.

Then, about 0 to 2 mol of water is added to the above solution with respect to 1 mol of the contained metal to cause partial hydrolysis and polycondensation reaction to obtain a stock solution (sol). When water is not added here, hydrolysis and polycondensation reactions are caused by water in the air after the next spin coating step.

The above stock solution is spin-coated on a Si single crystal substrate to form a film, which is gelled. That is, this spin coating was carried out on a Si substrate coated with a Pt electrode (a SiO ₂ layer having a thickness of about 500 nm, a Ti thin film having a thickness of about 500 nm, and a Pt thin film having a thickness of about 200 nm deposited thereon). Rotation speed per minute 200-4
While rotating at 000 rpm (the number of rotations per minute),
The stock solution is dropped and gelled.

In the drying step, the spin-coated thin film is dried while gradually raising the temperature to 50 to 400 ° C. on a hot plate. Specifically, 2 minutes at 60 ° C, 2 minutes at 80 ° C, 2 minutes at 120 ° C, 2 minutes at 250 ° C.
Min., Dry at 400 degrees for 2 minutes.

The dried thin film has a temperature of 600-
Insert in an electric furnace kept at 800 ℃, and in oxygen 1-6
Hold for about 0 minutes and perform firing (rapid thermal annealing). This firing raises the temperature from room temperature to the heat treatment temperature in about 10 seconds. Further, similar firing may be performed by irradiating infrared rays with a rapid thermal annealing device.

In order to obtain a thin film having a target thickness (about 10 to 300 nm), the above steps from spin coating to firing are repeated.

On the surface of the thin film after firing, platinum or gold is sputtered on the sample to form a thin film having a thickness of 100 to 200 nm as an electrode.

The thin film to which the electrode is attached is annealed in oxygen at 600 to 800 ° C. for about 5 to 30 minutes in the same manner as firing to obtain a thin film.

As described above, when substituting a part of Sr of strontium bismuth tantalate (SrBi ₂ Ta ₂ O ₉ ) with a rare earth element, the rare earth element is selected and manufactured, and thereby polarization with good rectangularity is obtained. A moderately large coercive electric field can be achieved together with the characteristics. Therefore, by selecting such a ferroelectric substance, a ferroelectric gate FET memory can be realized.

Since the coercive electric field can be controlled in a wide range, the characteristics can be selected in a wide range from low voltage drive type to high voltage drive high output type even in the field of piezoelectric / electrostrictive elements such as actuator elements. Becomes As a result, various applications can be put to practical use by making use of the features of BLSF that have not been realized so far.

[0065]

EFFECTS OF THE INVENTION The bismuth layered structure ferroelectric material of the present invention described above can be manufactured by selecting rare earth elements, and can achieve a moderately large coercive electric field with good rectangular polarization characteristics. Realization of body gate FET memory becomes possible.

Since the coercive electric field can be controlled in a wide range, the characteristics can be selected in a wide range from low voltage drive type to high voltage drive high output type even in the field of piezoelectric / electrostrictive elements such as actuator elements. Becomes As a result, applications that make the most of the features of BLSF that have not been realized so far are put to practical use.

[Brief description of drawings]

FIG. 1 shows the polarization characteristics of the ferroelectric element of the present invention as compared with the conventional SB.
It is a hysteresis curve shown in comparison with the characteristics of a T-based ferroelectric.

FIG. 2 is a hysteresis curve showing polarization characteristics of a ferroelectric element according to another embodiment of the present invention in comparison with characteristics of a conventional SBT type ferroelectric.

FIG. 3 is a hysteresis curve showing polarization characteristics of a ferroelectric element that is still another embodiment of the present invention.

FIG. 4 is a hysteresis curve showing polarization characteristics of a ferroelectric element according to still another embodiment of the present invention in comparison with characteristics of a conventional SBT type ferroelectric.

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Claims (14)

[Claims] 1. Strontium bismuth tantalate (S
A part of Sr of rBi ₂ Ta ₂ O ₉ ) is replaced with a rare earth element A, and a void □ is introduced at the position of Sr, and the chemical composition is represented by the general formula Sr _1-x A _{2x / 3} □ _{x / 3} Bi ₂ Ta ₂
A bismuth layered structure ferroelectric having a chemical composition represented by O ₉ (0.01 ≦ x ≦ 0.7). 2. The rare earth element A is lanthanum La, neodymium Nd, praseodymium Pr and samarium S.
2. The bismuth layered structure ferroelectric substance according to claim 1, which is one or more elements selected from the group consisting of m. 3. The bismuth layered structure ferroelectric substance according to claim 2, wherein the rare earth element A is lanthanum La and neodymium Nd. 4. Strontium bismuth tantalate (S
A part of Sr of rBi ₂ Ta ₂ O ₉ ) is replaced by the rare earth element A and bismuth Bi, and a hole □ is introduced at the position of Sr, and the chemical composition is represented by the general formula Sr _1-x (A + B
i) _{2x / 3} □ _{x / 3} Bi ₂ Ta ₂ O ₉ (0.01 ≦ x
A bismuth layered structure ferroelectric having a chemical composition represented by ≦ 0.7). 5. The rare earth element A is lanthanum La, neodymium Nd, praseodymium Pr and samarium S.
The bismuth layered structure ferroelectric substance according to claim 4, which is one or more elements selected from the group consisting of m. 6. The bismuth layered structure ferroelectric substance according to claim 5, wherein the rare earth element A is lanthanum La and neodymium Nd. 7. The bismuth layered structure ferroelectric material according to claim 1, wherein Ta in the general formula is wholly or partially substituted with niobium Nb. 8. A memory device using the bismuth layered structure ferroelectric material according to claim 1. Description: 9. A piezoelectric / electrostrictive element using the bismuth layered structure ferroelectric material according to claim 7. Description: 10. A step of mixing a raw material powder containing strontium carbonate SrCO ₃ , bismuth oxide Bi ₂ O ₃ , tantalum oxide Ta ₂ O ₅ and an oxide of a rare earth element, and a step of calcining the mixed raw material powder. And a step of crushing the calcined raw material powder into powder, a step of molding the powdered raw material powder into pellets by cold isostatic pressing, and a main-baking treatment of the formed pellets. And the process of
The method for manufacturing a bismuth layered structure ferroelectric material according to claim 1, further comprising an annealing step of annealing the fired pellets. 11. The rare earth oxide includes lanthanum oxide La ₂ O ₃ , neodymium oxide Nd ₂ O ₃ , praseodymium oxide Pr ₆ O ₁₁ and samarium oxide Sm ₂ O.
11. The method for producing a bismuth layered structure ferroelectric substance according to claim 10, which is one or a plurality of raw material powders selected from the group consisting of ₃ . 12. A step of mixing raw material powders containing strontium carbonate SrCO ₃ , bismuth oxide Bi ₂ O ₃ , tantalum oxide Ta ₂ O ₅ , niobium oxide Nb ₂ O ₅ and an oxide of a rare earth element, and the mixed raw materials. A step of calcining the powder, a step of crushing the calcined raw material powder into a powder, a step of molding the powdered raw material powder into a pellet by cold isostatic pressing, 5. The method according to claim 4, further comprising a step of subjecting the fired pellets to a main firing treatment and an annealing step of annealing the fired pellets.
A method for producing the bismuth layered structure ferroelectric material according to claim 1. 13. The rare earth oxide is lanthanum oxide La ₂ O ₃ , neodymium oxide Nd ₂ O ₃ , praseodymium oxide Pr ₆ O ₁₁ and samarium oxide Sm ₂ O.
13. The method for producing a bismuth layered structure ferroelectric material according to claim 12, wherein the raw material powder is one or more raw material powders selected from the group consisting of ₃ . 14. A strontium alkoxide Sr (O
iPr) ₂ , bismuth alkoxide Bi (OnBu) ₃ ,
A step of dissolving tantalum alkoxide Ta (OEt) ₅ and lanthanum alkoxide La (OnBu) ₃ in a solvent at a predetermined ratio to obtain a metal alkoxide solution, and causing the metal alkoxide solution to undergo a hydrolysis and polycondensation reaction,
A bismuth layered structure comprising a step of obtaining a stock solution, a step of spin-coating the stock solution on a semiconductor substrate, a step of drying the spin-coated thin film, and a step of baking the dried thin film. Ferroelectric manufacturing method.