JP4324796B2 - Ferroelectric film, ferroelectric film manufacturing method, ferroelectric capacitor, and ferroelectric memory - Google Patents

Description

  The present invention relates to a ferroelectric film, a method for manufacturing a ferroelectric film, a ferroelectric capacitor, and a ferroelectric memory.

In recent years, research and development of ferroelectric films such as Pb (Zr, Ti) O ₃ (PZT) and SrBi ₂ Ta ₂ O ₉ (SBT), ferroelectric capacitors and ferroelectric memory devices using the same have been conducted. It is actively done. The structure of the ferroelectric memory device can be roughly classified into 1T, 1T1C, 2T2C, and a simple matrix type. Among them, the 1T type has an internal electric field in the capacitor because of its structure, so that the retention (data retention) is as short as one month, and it is said that the 10-year guarantee required in general semiconductors is impossible. Since the 1T1C type and 2T2C type have almost the same configuration as the DRAM and have a selection transistor, the manufacturing technology of the DRAM can be utilized and the writing speed equivalent to that of the SRAM can be realized. The following small-capacity products have been commercialized.

  Up to now, PZT has been mainly used as a ferroelectric material used in 1T1C type or 2T2C type ferroelectric memory devices. In the case of the same material, the Zr / Ti ratio is 52/48 or 40 /. A mixed region of a rhombohedral crystal and a tetragonal crystal, such as 60, or a composition in the vicinity thereof is used, and an element such as La, Sr, or Ca is doped. This region is used in order to ensure the most necessary reliability of the memory element.

  In other words, the tetragonal region containing Ti is rich in the hysteresis shape, but a Schottky defect due to the ionic crystal structure occurs in the tetragonal region. As a result, a leakage current characteristic or imprint characteristic (so-called hysteresis deformation degree) failure occurs, and it is difficult to ensure reliability. Therefore, the composition of the mixed region of the rhombohedral crystal and the tetragonal crystal as described above or the vicinity thereof is used.

  On the other hand, the simple matrix type has a smaller cell size than the 1T1C type and the 2T2C type, and can be multi-layered with capacitors, so that high integration and low cost are expected. A conventional simple matrix ferroelectric memory device is disclosed in JP-A-9-116107. The publication discloses a driving method in which a voltage of 1/3 of a write voltage is applied to an unselected memory cell when data is written to a memory cell.

In order to obtain a simple matrix ferroelectric memory device, a hysteresis loop with good squareness is indispensable. As a ferroelectric material that can cope with this, Ti-rich tetragonal PZT is considered as a candidate, but it is difficult to ensure reliability as in the above-described 1T1C and 2T2C ferroelectric memories.
JP-A-9-116107

  An object of the present invention is to provide a ferroelectric capacitor and a dielectric memory having good hysteresis characteristics. Another object of the present invention is to provide a ferroelectric film suitable for the above ferroelectric memory and ferroelectric capacitor, and a method for manufacturing the same.

The ferroelectric film according to the present invention is:
(Pb _1-d Bi _d ) (B _1-a X _a ) O ₃ A ferroelectric material represented by the general formula,
B consists of at least one of Zr and Ti,
X consists of at least one of Nb and Ta,
a is in the range of 0.05 ≦ a ≦ 0.4,
d is in the range of 0 <d <1.

  According to the present invention, the above-described Bi having a higher valence than Pb can be replaced with Pb at the A site of PZT, so that the neutrality of the entire crystal structure can be maintained. Can be prevented. Thereby, current leakage of the ferroelectric film can be prevented. In addition, various characteristics such as imprint, retention, and fatigue of the ferroelectric film can be improved.

  In addition, since Bi has a stronger covalent bond with oxygen than Pb, it is considered difficult to escape from the crystal. The strength of the covalent bond is determined by the relative value of the orbital level between the atoms to be bonded. In the perovskite structure, as the 6p orbital level of the atom entering the A site approaches the 2p orbital level of oxygen, the orbital between the A site atom and the oxygen atom closest to the A site is likely to be mixed. Is more difficult to escape from the crystal than Pb. This can also prevent current leakage of the ferroelectric film.

  In the ferroelectric film according to the present invention, the X may exist at a B site having a perovskite structure.

  In the ferroelectric film according to the present invention, Pb and Bi can exist at the A site of the perovskite structure.

In the ferroelectric film according to the present invention,
The d may be in a range of 0 <d ≦ 0.2.

  The ferroelectric film according to the present invention has a tetragonal structure and can be pseudocubic (111) oriented.

  In the ferroelectric film according to the present invention, the X may be Nb.

In the ferroelectric film according to the present invention,
A ferroelectric represented by the general formula _{(Pb 1-d Bi d)} (B 1-a X a) O 3, can include a eutectic of the ferroelectric consisting BiNbO _4.

In the ferroelectric film according to the present invention,
The molar ratio of Bi element to Pb element contained in the eutectic may be 3/7 or more.

In the ferroelectric film according to the present invention,
Crystals have a perovskite structure represented by the general formula _{(Pb 1-d Bi d)} (B 1-a X a) O 3,
The crystal made of BiNbO ₄ may have a bismuth layered perovskite structure.

A method for manufacturing a ferroelectric film according to the present invention includes:
A method of manufacturing a ferroelectric film including a ferroelectric represented by a general formula of (Pb _1-d B _{i d} ) (B _1-a X _a ) O ₃ ,
B consists of at least one of Zr and Ti,
X consists of at least one of Nb and Ta,
a is in the range of 0.05 ≦ a ≦ 0.4,
d is in the range of 0 <d <1;
A sol-gel raw material containing a hydrolyzed / condensed product of metal alkoxide containing at least Zr element and Ti element, polycarboxylic acid or polycarboxylic acid ester, and organic solvent are mixed,
Forming a ferroelectric precursor solution having an ester bond by esterification of the polycarboxylic acid or the polycarboxylic acid derived from the polycarboxylic acid ester with a metal alkoxide.

A method for manufacturing a ferroelectric film according to the present invention includes:
A method of manufacturing a ferroelectric film including a precursor for forming a ferroelectric film,
The ferroelectric film further includes a crystal made of BiNbO ₄ ,
A sol-gel raw material containing a hydrolyzed / condensed product of metal alkoxide containing at least Zr element and Ti element, polycarboxylic acid or polycarboxylic acid ester, and organic solvent are mixed,
Forming a precursor solution of a ferroelectric having ester bonds by esterification of the polycarboxylic acid or the polycarboxylic acid derived from the polycarboxylic acid and a metal alkoxide.

A method for manufacturing a ferroelectric film according to the present invention includes:
When mixing the sol-gel raw material, the polycarboxylic acid or polycarboxylic acid ester, and the organic solvent, a sol-gel raw material using a bismuth carboxylate can be further included.

In the method for manufacturing a ferroelectric film according to the present invention,
When mixing the sol-gel raw material, the polycarboxylic acid or polycarboxylic acid ester, and the organic solvent, a sol-gel raw material using a lead carboxylate can be further included.

In the method for manufacturing a ferroelectric film according to the present invention,
The polycarboxylic acid or the polycarboxylic acid ester may be a divalent carboxylic acid or a polycarboxylic acid ester.

  The following can be illustrated as polycarboxylic acid used for this invention. Examples of the trivalent carboxylic acid include Trans-aconitic acid, trimesic acid, and examples of the tetravalent carboxylic acid include pyromellitic acid and 1,2,3,4-cyclopentanetetracarboxylic acid. Polycarboxylic acid esters that dissociate in alcohol and act as polycarboxylic acids include divalent dimethyl succinate, diethyl succinate, dibutyl oxalate, dimethyl malonate, dimethyl adipate, dimethyl maleate, and diethyl fumarate. Examples include tributyl tributyl citrate, triethyl 1,1,2-ethanetricarboxylate, trimethyl 1,2,4-benzenetricarboxylate, tetraethyl tetravalent 1,1,2,2-ethanetetracarboxylate, and the like. . These polycarboxylic acid esters are dissociated in the presence of an alcohol and function as a polycarboxylic acid. In addition, the present invention is characterized in that the network is connected by esterification using a polycarboxylic acid. For example, in a single carboxylic acid such as acetic acid or methyl acetate and its ester, the ester network does not grow. It is not included in the invention.

  In the method for producing a precursor composition of the present invention, the divalent carboxylic acid ester is preferably at least one selected from succinic acid esters, maleic acid esters, and malonic acid esters. Specific examples of these esters include dimethyl succinate, dimethyl maleate, and dimethyl malonate.

In the method for manufacturing a ferroelectric film according to the present invention,
In mixing the sol-gel raw material, the polycarboxylic acid or polycarboxylic acid ester, and the organic solvent, a sol-gel raw material further containing Si or Si and Ge can be used.

  The ferroelectric capacitor according to the present invention has the above-described ferroelectric film.

  The ferroelectric capacitor according to the present invention may further include an electrode having a perovskite structure, and the ferroelectric film may be formed on the electrode.

  A ferroelectric memory according to the present invention has the above-described ferroelectric capacitor.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.

1. Ferroelectric Film and Ferroelectric Capacitor FIG. 1 is a cross-sectional view schematically showing a ferroelectric capacitor 100 using a ferroelectric film 101 according to the present embodiment.

  As shown in FIG. 1, the ferroelectric capacitor 100 is formed on the substrate 10, the first electrode 102, the ferroelectric film 101 formed on the first electrode 102, and the ferroelectric film 101. The second electrode 103 is formed.

  The thicknesses of the first electrode 102 and the second electrode 103 are, for example, about 50 to 150 nm.

The ferroelectric film 101 is made of the following general formula (1) having a perovskite crystal structure.
_{(Pb 1-d Bi d)} (B 1-a X a) O 3 ··· (1)
In general formula (1),
B consists of at least one of Zr and Ti,
X consists of at least one of Nb and Ta,
a is in the range of 0.05 ≦ a ≦ 0.4,
d is in the range of 0 <d <1.

  The perovskite type has a crystal structure as shown in FIGS. 2A and 2B. In FIGS. 2A and 2B, the position indicated by A is the A site, and the position indicated by B is the B site. That's it.

  In the general formula (1), Pb and Bi are located at the A site, and B and X are located at the B site. Further, O (oxygen) is located at a position indicated by O in FIGS. 2 (a) and 2 (b). X may be a metal element having a higher valence than Zr and Ti. Examples of metal elements having higher valences than Zr and Ti (+4 valences) are Nb (+5 valences) and Ta (+5 valences).

  A Pb-based perovskite structure such as PZT has a high vapor pressure of Pb, so that Pb located at the A site of the perovskite structure is likely to evaporate. When Pb escapes from the A site, oxygen is simultaneously lost due to the principle of charge neutrality. This phenomenon is called a Schottky defect. For example, when oxygen is deficient in PZT, the band gap of PZT decreases. Due to the decrease in the band gap, the band offset at the metal electrode interface decreases, and the leakage current characteristics of a ferroelectric film made of, for example, PZT deteriorate. The presence of oxygen vacancies generates an oxygen ion current, and charge accumulation at the electrode interface accompanying this ion current causes deterioration of various characteristics such as imprint, retention, and fatigue.

  However, according to the present invention, the above-described Bi having a higher valence than Pb can be replaced with Pb at the A site of PZT, whereby the neutrality of the entire crystal structure can be maintained. Defects can be prevented. Thereby, current leakage of the ferroelectric film 101 can be prevented. In addition, various characteristics such as imprint, retention, and fatigue of the ferroelectric film 101 can be improved.

  In addition, since Bi has a stronger covalent bond with oxygen than Pb, it is considered difficult to escape from the crystal. The strength of the covalent bond is determined by the relative value of the orbital level between the atoms to be bonded. In the perovskite structure, as the 6p orbital level of the atom entering the A site approaches the 2p orbital level of oxygen, the orbital between the A site atom and the oxygen atom closest to the A site is likely to be mixed. Is more difficult to escape from the crystal than Pb. This can also prevent current leakage of the ferroelectric film 101. Since the 6p orbit of Bi is an unoccupied orbit at the A site of the perovskite structure, the 6p orbital can contribute to an increase in the degree of covalent bonding.

  FIG. 3 shows the relative values of the 6p orbital levels of Bi and Pb when the 2p orbital level of the oxygen atom is used as a reference. The relative value of the 6p orbital level of Bi and Pb is a value based on the orbital energy level of each atom obtained by the first principle calculation based on the density functional method. The 6p orbital level of Bi is closer to the 2p orbital level of oxygen than the 6p orbital level of Pb. Thus, the 6p orbital level of the atoms entering the A site approaches the 2p orbital level of oxygen, so that the covalent bond with oxygen is enhanced. When the covalent bond between the A site atom and the oxygen atom is increased, the ferroelectricity such as the Curie temperature and the polarization moment of the ferroelectric film 101 can be enhanced. Note that the 6s orbitals of Bi and Pb are occupied orbitals at the A site, and therefore do not contribute effectively to the degree of covalent bonding and do not increase the ferroelectricity.

The added amount a of X is
0.10 ≦ a ≦ 0.30
Is preferably in the range of
0.20 ≦ a ≦ 0.25
It is more preferable that

  The case where X is a +5 valent element is, for example, Nb or Ta, but a preferable element is Nb.

  When the addition amount N of Nb is in the range of 0 <a ≦ 0.2, the ferroelectric film 101 is made of a perovskite type single phase film, and Nb exists at the B site of the perovskite structure.

  The additive amount d of Bi is preferably 0 <d ≦ 0.5, particularly 0.05 ≦ d ≦ 0.30.

  When the additive amount d of Bi is in the range of 0 <d ≦ 0.2, the ferroelectric film 101 is made of a perovskite type single phase film, and Bi is present at the A site of the perovskite structure.

On the other hand, when the additive amount d of Bi is in the range of 0.3 ≦ d <1, the ferroelectric film 101 is not only a crystal represented by the general formula (1) but also a crystal made of BiNbO _4. Including. This is because when the addition amount of Nb and Bi increases, the added Nb and Bi exceed the range of the solid solubility limit, and some of the added Nb and Bi do not enter the B site or A site of the perovskite structure. is there. As a result, the ferroelectric film 101 includes a eutectic having a crystal composed of the general formula (1) and a crystal composed of BiNbO ₄ . A crystal made of BiNbO ₄ has a bismuth layered perovskite structure as shown in FIG.

  The thickness of the ferroelectric film 101 is, for example, about 50 to 150 nm.

Specific examples of those represented by the general formula (1), for example, _{(Pb 1-d Bi d)} (Zr, Ti) is made of a 1-a _Nb a _{O 3.} This, _Pb having a perovskite crystal structure _{(Zr 1-p Ti p)} O 3 ( hereinafter, also referred to as "PZT".) In, is obtained by adding Nb and Bi.

The amount of Nb added is indicated by a in the above formula. The amount of Bi added is indicated by d in the above formula. P indicating the composition ratio of Zr and Ti is
0.3 ≦ p ≦ 1.0
Is preferably in the range of
0.5 ≦ p ≦ 0.8
More preferably, it is the range.

The general formula (1) described above is represented by (Pb _1-d Bi _d ) (B _1-a X _a ) O ₃ , and the atoms at the A site are not lost, but the atoms at the A site are missing. It can also be made. In other words, the composition formula in this case is
_{(Pb 1-d Bi d)} 1-b (B 1-a X a) O 3
It is shown by the general formula of In this case, the deficiencies b, a, and d of the A site are
b = (a + d) / (2 + d)
Is satisfied.
A is
0.05 ≦ a ≦ 0.4
Where d is
0 <d <1
Range.

Furthermore, O can be lost. That is, the composition formula in that case is
_{(Pb 1-d Bi d)} 1-b (B 1-a X a) O 3-c
Where a, b, c, and d are
b = (a + d + 2c) / (2 + d)
The oxygen deficiency c is
0 ≦ c ≦ 0.05
Preferably,
0 <c ≦ 0.03
Range. A is
0.05 ≦ a ≦ 0.4
Where d is
0 <d <1
Range.

2. Method for Manufacturing Ferroelectric Film and Ferroelectric Capacitor Next, a method for manufacturing a ferroelectric film and a ferroelectric capacitor in the present embodiment will be described.

  (1) First, the substrate 10 is prepared. As the substrate 10, for example, silicon can be used.

  (2) Next, the first electrode 102 is formed on the substrate 10. The first electrode 102 is preferably made of a metal oxide having a perovskite structure. Since the ferroelectric film 101 is formed on the first electrode 102 having a perovskite structure, a high-quality crystal is likely to grow. Thereby, current leakage of the ferroelectric film 101 can be prevented.

  The first electrode 102 can be formed by, for example, a laser ablation method. That is, a target including a desired electrode material is prepared. Then, the target is irradiated with laser light, and atoms including oxygen atoms and metal atoms are knocked out of the target to generate plumes. Then, the plume is emitted toward the substrate 10 and brought into contact therewith. As a result, the first electrode 102 is formed by epitaxial growth on the substrate 10.

As the material of the first electrode 102 having a perovskite _{structure, SrRuO 3, Nb-SrTiO 3} , La-SrTiO 3, Nb- (La, Sr) CoO 3, LaNiO 3, or PbBaO ₃ or the like can be used. Here, Nb-SrTiO ₃ is obtained by doping SrTiO ₃ with Nb, La-SrTiO ₃ is obtained by doping SrTiO ₃ with La, and Nb- (La, Sr) CoO ₃ is represented by (La, Sr). CoO ₃ is doped with Nb. The first electrode 102 is not limited to one having a perovskite structure, and may be formed using, for example, Pt, Ir, or IrO 2 _X.

  Further, as a method for forming the first electrode 102, an ion beam assist method, a sputtering method, a vacuum deposition method, or the like may be used instead of the laser ablation method.

  (3) Next, the ferroelectric film 101 is formed on the first electrode 102.

  First, a sol-gel raw material containing a hydrolyzed / condensed product of metal alkoxide containing at least Zr element and Ti element, a polycarboxylic acid or polycarboxylic acid ester, and an organic solvent are mixed to form a polycarboxylic acid or polycarboxylic acid ester. A ferroelectric precursor solution having an ester bond by esterification of the derived polycarboxylic acid and a metal alkoxide is formed.

  The formation reaction of the precursor solution can be roughly divided into a first-stage alkoxy group substitution reaction as shown in FIG. 5 and a second-stage esterification reaction as shown in FIG. 6 to form a polymer network. Including. 5 and 6 show an example in which dimethyl succinate is used as a polycarboxylic acid ester and n-butanol is used as an organic solvent for convenience. Dimethyl succinate is nonpolar but dissociates in alcohol to form dicarboxylic acid.

  In the first-stage reaction, as shown in FIG. 5, both are ester-bonded by esterification of dimethyl succinate and a metal alkoxide of the sol-gel raw material. That is, dimethyl succinate dissociates in n-butanol, and a proton is added to one carbonyl group (first carbonyl group). A substitution reaction of the first carbonyl group and the alkoxy group of the metal alkoxide occurs, and a reaction product in which the first carboxyl group is esterified and an alcohol are generated. Here, the “ester bond” means a bond (—COO—) between a carbonyl group and an oxygen atom.

  In the second stage reaction, as shown in FIG. 6, a substitution reaction between the other carboxyl group (second carboxyl group) remaining in the first stage reaction and the alkoxy group of the metal alkoxide occurs, A reaction product in which the carboxyl group is esterified and an alcohol are produced.

  Thus, a polymer network in which the hydrolysis / condensation products of metal alkoxide contained in the sol-gel raw material are ester-bonded is obtained by the two-stage reaction. Therefore, this polymer network has ester bonds in a moderately orderly manner in the network. In addition, since dimethyl succinate dissociates in two steps and the first carboxyl group has a larger acid dissociation constant than the second carboxyl group, the first stage reaction has a higher reaction rate than the second stage reaction. Therefore, the second stage reaction proceeds more slowly than the first stage reaction.

  In the present embodiment, the following method can be employed to promote the esterification reaction described above.

  (A) Increase the concentration or reactivity of the reactants. Specifically, the reactivity is increased by increasing the degree of dissociation of the polycarboxylic acid or polycarboxylic acid ester by increasing the temperature of the reaction system. The temperature of the reaction system depends on the boiling point of the organic solvent, but is preferably higher than room temperature and lower than the boiling point of the organic solvent. As temperature of a reaction system, it can be 100 degrees C or less, for example, Preferably it is 50-100 degreeC.

  (B) The reaction by-product is removed. Specifically, esterification further proceeds by removing water and alcohol generated together with esterification.

  (C) Physically accelerate the molecular motion of the reactants. Specifically, the reactivity of the reactant is enhanced by irradiating energy rays such as ultraviolet rays.

The organic solvent used in the method for producing the precursor composition of the present embodiment can be an alcohol. When alcohol is used as the solvent, both the sol-gel raw material and the polycarboxylic acid or polycarboxylic acid ester can be dissolved well.
Although it does not specifically limit as alcohol, Monovalent alcohols, such as butanol, methanol, ethanol, and propanol, or a polyhydric alcohol can be illustrated. Examples of such alcohols include the following.

Monohydric alcohols;
As propanol (propyl alcohol), 1-propanol (boiling point 97.4 ° C), 2-propanol (boiling point 82.7 ° C),
As butanol (butyl alcohol), 1-butanol (boiling point 117 ° C.), 2-butanol (boiling point 100 ° C.), 2-methyl-1-propanol (boiling point 108 ° C.), 2-methyl-2-propanol (melting point 25.4) ℃, boiling point 83 ℃),
As pentanol (amyl alcohol), 1-pentanol (boiling point 137 ° C.), 3-methyl-1-butanol (boiling point 131 ° C.), 2-methyl-1-butanol (boiling point 128 ° C.), 2,2 dimethyl-1 -Propanol (boiling point 113 ° C), 2-pentanol (boiling point 119 ° C), 3-methyl-2-butanol (boiling point 112.5 ° C), 3-pentanol (boiling point 117 ° C), 2-methyl-2-butanol (Boiling point 102 ° C.),
Polyhydric alcohols;
Ethylene glycol (melting point −11.5 ° C., boiling point 197.5 ° C.), glycerin (melting point 17 ° C., boiling point 290 ° C.).

  The polycarboxylic acid or the polycarboxylic acid ester is preferably a divalent carboxylic acid or a polycarboxylic acid ester.

  The amount of polycarboxylic acid or polycarboxylic acid ester used depends on the composition ratio of the sol-gel raw material and the ferroelectric, but the total molar ion concentration of the polycarboxylic acid, for example, PZT sol-gel raw material, PbNb sol-gel raw material, and PbSi sol-gel raw material The polyionic acid molar ion concentration is preferably 1 ≧ (polycarboxylic acid molar ion concentration) / (total molar ion concentration of raw material solution), more preferably 1: 1. The addition amount of polycarboxylic acid can be 0.35 mol, for example.

  The amount of polycarboxylic acid or polycarboxylic acid ester added is desirably equal to or greater than the total number of moles of raw material solutions to be combined. The ratio of the molar ion concentration of the two is 1: 1, and all the raw materials are bonded, but since the ester exists stably in the acidic solution, in order to make the ester exist stably, the total number of moles of the raw material solution It is preferable to add a large amount of polycarboxylic acid. Here, the number of moles of polycarboxylic acid or polycarboxylic acid ester is a valence. That is, in the case of a divalent polycarboxylic acid or polycarboxylic acid ester, one molecule of polycarboxylic acid or polycarboxylic acid ester can bind two molecules of raw material molecules. If it is polycarboxylic acid ester, it will be 1: 1 with 0.5 mol of polycarboxylic acid or polycarboxylic acid ester with respect to 1 mol of raw material solutions. In addition, the polycarboxylic acid or the polycarboxylic acid ester is not an acid from the beginning, but dissociates the ester of the polycarboxylic acid in the alcohol to form a polycarboxylic acid. In this case, the number of moles of alcohol to be added is preferably 1 ≧ (number of moles of alcohol / number of moles of polycarboxylic acid ester). This is because, in order to sufficiently dissociate all the polycarboxylic acid esters, the larger the number of moles of alcohol, the more stable dissociation. Here, the number of moles of alcohol also means the so-called molar ion concentration divided by the valence of alcohol.

  In the manufacturing method of the precursor composition of this embodiment, the raw material which consists of metal carboxylates can further be included. Typical examples of such metal carboxylates include lead acetate and lead octylate, which are the lead carboxylates described above.

  In the method for producing a precursor composition of the present embodiment, an organometallic compound (MOD raw material) can be used together with the sol-gel raw material. As such an organometallic compound, for example, niobium octylate can be used. As shown in FIG. 7, niobium octylate has a structure in which Nb is covalently bonded by two atoms and an octyl group is present in the other part. In this case, Nb—Nb has two atoms bonded to each other, but since no more networks exist, it is treated as a MOD raw material.

  Formation of a network of carboxylic acid and MOD raw material proceeds mainly by an alcohol exchange reaction. For example, in the case of niobium octylate, it reacts between a carboxylic acid and an octyl group (alcohol exchange reaction), and esterification of R—COO—Nb proceeds. Thus, in this embodiment, by esterifying the MOD raw material, the molecules of the MOD raw material can be bonded to the precursor network by condensation of the MOD raw material and the alkoxide.

Furthermore, in the method for producing a precursor composition of the present embodiment, a sol-gel raw material containing Si or Si and Ge can be used as a sol-gel raw material containing a hydrolyzed / condensed product of metal alkoxide. As such a sol-gel solution may be used alone sol-gel solution for PbSiO _3, or both the sol-gel solution and PbGeO ₃ sol-gel solution for PbSiO _3. By using such a sol-gel raw material containing Si or Ge, the temperature during film formation can be lowered, and the ferroelectric can be crystallized from about 450 ° C.

In the method for producing a precursor composition of the present embodiment, in order to obtain PZTN, a sol-gel solution in which at least a sol-gel solution for PbZrO ₃ , a sol-gel solution for PbTiO ₃ , and a sol-gel solution for PbNbO ₃ is used is used. be able to. Also in this case, the above-described sol-gel raw material containing Si or Si and Ge can be further mixed.

When Ta is introduced instead of Nb, a sol-gel solution for PbTaO ₃ can be used as the sol-gel raw material.

  Since the precursor of the precursor composition obtained in this embodiment has an appropriate ester bond between a plurality of molecular networks, a reversible reaction is possible. Therefore, in the precursor, the polymerized precursor (polymer network) can be decomposed into a metal alkoxide condensate by proceeding the reaction in the left direction shown in FIG.

  According to the manufacturing method of the ferroelectric film 101 of the present embodiment, the hydrolysis / condensation products (plural molecular networks) of the metal alkoxide of the sol-gel raw material are condensed by an ester bond in an organic solvent with polycarboxylic acid. A polymer network is obtained. Therefore, this polymer network has moderate ester bonds between a plurality of molecular networks derived from the hydrolysis / condensation product. The esterification reaction can be easily performed by temperature control or the like.

  In addition, as described above, the precursor composition of the present embodiment has an appropriate ester bond between a plurality of molecular networks, so that a reversible reaction is possible. Therefore, in the composition remaining after the formation of the ferroelectric film, the polymerized precursor (polymer network) can be decomposed into a metal alkoxide (or a molecular network composed of a condensate thereof). Such metal alkoxides (or molecular networks composed of condensates thereof) can be reused as precursor raw materials, so that harmful substances such as lead can be reused, and there are great advantages from the environmental viewpoint. .

  In the ferroelectric capacitor, a tetragonal structure and pseudo-cubic (111) orientation are preferable for maintaining the squareness of the hysteresis loop. This is because the 90-degree domain in the ferroelectric film 101 can be eliminated if this arrangement is adopted.

Further, in order to form a pseudo-cubic (111) oriented ferroelectric capacitor, Pt having a (111) orientation may be used as the first electrode 102. Alternatively, a transition metal oxide having a perovskite structure having a (111) orientation may be used as the first electrode 102. Examples of these transition metal oxides are SrRuO ₃ and Nb-doped SrTiO ₃ . On the (111) -oriented first electrode 102, the ferroelectric film 101 can be easily grown by pulling the underlying (111) -oriented direction.

  The ferroelectric film 101 can be formed by crystallizing the precursor solution by applying heat treatment or the like.

  Specifically, a series of steps including a mixed solution coating step, an alcohol removal step, a drying heat treatment step, and a degreasing heat treatment step are performed a desired number of times, and then baked by crystallization annealing to form the ferroelectric film 101. The conditions in each process are as follows, for example.

  In the mixed solution coating step, the mixed solution is coated by a coating method such as spin coating. First, the mixed solution is dropped on the first electrode 102. Spin is performed for the purpose of spreading the dropped solution over the entire surface of the substrate. The rotation speed of the spin is, for example, about 500 rpm. Next, the mixed solution is applied onto the first electrode 102 by performing spinning for a desired time at a reduced rotational speed. The number of rotations at this time is, for example, 50 rpm or less. The drying heat treatment step is performed at 150 to 180 ° C. The drying heat treatment is performed using a hot plate or the like in an air atmosphere. Similarly, the degreasing heat treatment step is performed in an air atmosphere on a hot plate maintained at 300 ° C to 350 ° C. Firing for crystallization is performed using thermal rapid annealing (RTA) or the like in an oxygen atmosphere.

  The thickness of the sintered ferroelectric film 101 can be about 50 to 150 nm. The ferroelectric film 101 can also be formed by using, for example, a sputtering method, a molecular beam epitaxy method, or a laser ablation method.

(4) Next, the second electrode 103 is formed on the ferroelectric film 101. The second electrode 103 can be formed by, for example, a sputtering method or a vacuum deposition method. As the upper electrode, it is preferable to use a material mainly made of Pt. The second electrode 103 is not limited to Pt, and Ir, IrO _x , SrRuO ₃ , Nb—SrTiO ₃ , La—SrTiO ₃ , Nb— (LaSr) CoO ₃ , LaNiO ₃ , PbBaO _3, etc. These known electrode materials can also be used.

  (5) Next, post-annealing can be performed in an oxygen atmosphere using RTA or the like, if necessary. As a result, a good interface between the second electrode 103 and the ferroelectric film 101 can be formed, and the crystallinity of the ferroelectric film 101 can be improved.

  Through the above steps, the ferroelectric film 101 and the ferroelectric capacitor 100 according to the present embodiment can be manufactured.

  According to the ferroelectric capacitor 100 of the present embodiment, the crystallization temperature can be lowered and the squareness of hysteresis can be improved. Further, the improvement in the squareness of hysteresis by the ferroelectric capacitor 100 is effective in the stability of disturbance, which is important for driving a simple matrix ferroelectric memory device.

3. Ferroelectric Memory FIGS. 8A and 8B are diagrams showing the configuration of a simple matrix ferroelectric memory device 300 in the embodiment of the present invention. 8A is a plan view thereof, and FIG. 8B is a cross-sectional view taken along the line AA of FIG. 8A. As shown in FIGS. 8A and 8B, the ferroelectric memory device 300 has a predetermined number of word lines 301 to 303 formed on the substrate 308 and a predetermined number of the word lines. Bit lines 304 to 306. The ferroelectric film 307 described in the above embodiment is inserted between the word lines 301 to 303 and the bit lines 304 to 306, and strong in the intersection region between the word lines 301 to 303 and the bit lines 304 to 306. A dielectric capacitor is formed.

  In the ferroelectric memory device 300 in which memory cells configured by this simple matrix are arranged, writing to and reading from the ferroelectric capacitors formed in the intersecting regions of the word lines 301 to 303 and the bit lines 304 to 306 are as follows: This is performed by a peripheral drive circuit, a read amplifier circuit (not shown) or the like (referred to as “peripheral circuit”). This peripheral circuit may be formed by a MOS transistor on a substrate different from the memory cell array and connected to the word lines 301 to 303 and the bit lines 304 to 306, or a single crystal silicon substrate may be used as the substrate 308. By using the peripheral circuit, it is possible to integrate the peripheral circuit on the same substrate as the memory cell array.

  FIG. 9 is a cross-sectional view showing an example of a ferroelectric memory device 400 in which the memory cell array is integrated with peripheral circuits on the same substrate in the present embodiment.

  In FIG. 9, a MOS transistor 402 is formed on a single crystal silicon substrate 401, and this transistor formation region becomes a peripheral circuit portion. The MOS transistor 402 includes a single crystal silicon substrate 401, source / drain regions 405, a gate insulating film 403, and a gate electrode 404. The ferroelectric memory device 400 includes an element isolation oxide film 406, a first interlayer insulating film 407, a first wiring layer 408, and a second interlayer insulating film 409.

  The ferroelectric memory device 400 includes a memory cell array including ferroelectric capacitors 420. The ferroelectric capacitor 420 includes a lower electrode (first electrode or second electrode) 410 serving as a word line or a bit line, From a ferroelectric film 411 including a ferroelectric phase and a paraelectric phase, and an upper electrode (second electrode or first electrode) 412 formed on the ferroelectric film 411 and serving as a bit line or a word line Composed.

  Further, the ferroelectric memory device 400 includes a third interlayer insulating film 413 on the ferroelectric capacitor 420, and the memory cell array and the peripheral circuit portion are connected by the second wiring layer 414. In the ferroelectric memory device 400, a protective film 415 is formed on the third interlayer insulating film 413 and the second wiring layer 414.

  According to the ferroelectric memory device 400 having the above configuration, the memory cell array and the peripheral circuit portion can be integrated on the same substrate. Although the ferroelectric memory device 400 shown in FIG. 9 has a configuration in which a memory cell array is formed on the peripheral circuit portion, of course, no memory cell array is arranged on the peripheral circuit portion, It is good also as a structure which is in contact with the circuit part planarly.

  Since the ferroelectric capacitor 420 used in the present embodiment is composed of the ferroelectric film according to the above-described embodiment, the squareness of hysteresis is very good and it has a stable disturb characteristic. Furthermore, the ferroelectric capacitor 420 has less damage to peripheral circuits and other elements due to lower process temperature, and less process damage (especially hydrogen reduction), thereby suppressing deterioration of hysteresis due to damage. Can do. Therefore, by using the ferroelectric capacitor 420, the simple matrix ferroelectric memory device 300 can be put into practical use.

  FIG. 10A shows a structural diagram of a 1T1C type ferroelectric memory device 500 as a modification. FIG. 10B is an equivalent circuit diagram of the ferroelectric memory device 500.

  As shown in FIG. 10A, the ferroelectric memory device 500 includes a capacitor 504 (1C) including a lower electrode 501, an upper electrode 502 connected to a plate line, and the ferroelectric film 503 of the above-described embodiment. ), And a memory element having a structure similar to that of a DRAM including a switching transistor element 507 (1T) having a gate electrode 506 connected to the data line 505 and one of the source / drain electrodes connected to the word line. . The 1T1C type memory can perform writing and reading at a high speed of 100 ns or less, and the written data is nonvolatile. Therefore, it is promising for replacement of the SRAM.

  As mentioned above, although an example of embodiment of this invention was described, this invention is not limited to these, Various aspects can be taken within the range of the summary.

4). Experimental Example Hereinafter, an experimental example of the present invention will be described.

4.1. Example 1
The ferroelectric film according to the present embodiment was performed using the following raw material solution.

A solution in which a condensation polymer such as lead acetate (Pb) and zirconium butoxide (Zr) for forming a PbZrO ₃ perovskite crystal with Pb and Zr is dissolved in a solvent such as n-butanol in an anhydrous state (hereinafter referred to as PZ solution). ), A PbTiO ₃ perovskite crystal composed of Pb and Ti, a solution obtained by dissolving a condensation polymer such as lead acetate (Pb) and titanium isopropoxide (Ti) in a solvent such as n-butanol in an anhydrous state (hereinafter referred to as “PbTiO ₃ perovskite crystal”) And PT solution) were mixed at a ratio of (PZ solution) :( PT solution) = 45: 55 to prepare solution 1.

  A solution 2 was prepared by mixing bismuth octylate and niobium octylate in a molar ratio of (bismuth octylate) :( niobium octylate) = 1: 1 together with Si element in a solvent such as n-butanol. .

Next, Solution 1 and Solution 2 were mixed with dimethyl succinate to prepare a raw material solution. Further, dimethyl succinate was mixed at a ratio of 0.5 mol / l with respect to each raw material solution metal element concentration of 1 mol / l. Thereafter, each raw material solution was sealed and held at 90 ° C. for 30 minutes, and then cooled to room temperature to sufficiently promote esterification.
The ratio of the solution 1 and the solution 2 is as follows.
Raw material solution 1; solution 1: solution 2 = 100: 0
Raw material solution 2; Solution 1: Solution 2 = 95: 5
Raw material solution 3; Solution 1: Solution 2 = 90: 10
Raw material solution 4; Solution 1: Solution 2 = 80: 20
A sample was prepared by the method shown in FIG. 11 using the prepared raw material solutions 1 to 4.

  That is, a lower electrode made of platinum was formed by sputtering, a raw material solution was applied to a substrate by spin coating, and a drying process was performed at 150 to 180 ° C. (150 ° C.) using a hot plate to remove alcohol. Then, degreasing heat treatment was performed at 300 to 350 ° C. (300 ° C.) using a hot plate. Then, the said application | coating process, the drying process, and the degreasing heat processing were performed in multiple times (all 3 times) as needed, and the coating film of the desired film thickness was obtained. Further, a ferroelectric film sample having a film thickness of 150 nm was obtained by crystallization annealing (firing). Firing for crystallization was performed at 650 to 700 ° C. (650 ° C.) using rapid thermal annealing (RTA) in an oxygen atmosphere. Further, an upper electrode made of platinum is formed by a sputtering method, and recovery annealing is performed at 650 to 700 ° C. (650 ° C.) using RTA, whereby a ferroelectric capacitor sample (hereinafter referred to as “capacitor”). Also called “sample”).

  The following characteristics were examined using these samples.

  (A) The crystallinity of the three types of ferroelectric films using the raw material solution 2 to the raw material solution 4 was examined by X-ray diffraction. The results are shown in FIGS. FIG. 12 is a diagram showing the crystallinity of a ferroelectric film of a sample using the raw material solution 2. FIG. 13 is a diagram showing the crystallinity of a ferroelectric film of a sample using the raw material solution 3. FIG. 14 is a diagram showing the crystallinity of a ferroelectric film sample using the raw material solution 4.

  From FIG. 12 to FIG. 14, the peak of (111) similar to PZT was recognized in any sample using the raw material solution 2 to the raw material solution 4, and it was confirmed that a perovskite single phase film was formed. That is, it was confirmed that Bi and Nb were completely dissolved in PZT, Bi was substituted for the A site, and Nb was substituted for the B site.

  (B) The hysteresis calculated | required about the capacitor sample using the raw material solution 1-the raw material solution 4 in FIGS. 15-18 is shown. FIG. 15 shows a hysteresis characteristic of a sample using the raw material solution 1. FIG. 16 shows the hysteresis characteristics of the sample using the raw material solution 2. FIG. 17 shows the hysteresis characteristics of the sample using the raw material solution 3. FIG. 18 shows a hysteresis characteristic of a sample using the raw material solution 4. From FIG. 15 to FIG. 18, it was confirmed that all the capacitor samples have good hysteresis characteristics. In particular, as the addition amount of Bi and Nb increased, the squareness of hysteresis improved.

4.2. Example 2
Similar to Example 1, a raw material solution was prepared by mixing Solution 1 and Solution 2 at the following ratio.
Raw material solution 5; Solution 1: Solution 2 = 70: 30
Raw material solution 6; Solution 1: Solution 2 = 40: 60
Raw material solution 7; Solution 1: Solution 2 = 10: 90
Next, a capacitor sample was formed in the same manner as in Example 1, and the following characteristics were examined using the formed sample.

(A) The crystallinity of three types of ferroelectric films using the raw material solution 5 to the raw material solution 7 was examined by X-ray diffraction. The results shown in FIG. 19 were obtained for all the ferroelectric films. According to FIG. 19, peaks of (112), (212), (211) similar to BNO (BiNbO ₄ ) and a peak of (111) similar to PZT are recognized, and the obtained ferroelectric film Has been confirmed to contain a eutectic in which a bismuth layered perovskite crystal made of BNO and a perovskite crystal made of PZT are mixed. Here again, Bi and Nb are dissolved in PZT, Bi replaces part of the A site, and Nb replaces part of the B site.

  (B) The hysteresis calculated | required about the capacitor sample using the raw material solution 5-the raw material solution 7 in FIGS. 20-22 is shown. FIG. 20 shows the hysteresis characteristics of the sample using the raw material solution 5. FIG. 21 shows hysteresis characteristics of a sample using the raw material solution 6. FIG. 22 shows the hysteresis characteristics of the sample using the raw material solution 7. From FIG. 20 to FIG. 22, it was confirmed that any capacitor sample has a good hysteresis characteristic.

Sectional drawing which shows the ferroelectric capacitor concerning embodiment. (A), (b) is explanatory drawing of a perovskite crystal structure. The figure which shows the relative value of the 6p orbital level of Bi and Pb. Explanatory drawing of Bi layered perovskite crystal structure. The figure which shows the production | generation reaction of the precursor solution of the ferroelectric film concerning this Embodiment. The figure which shows the production | generation reaction of the precursor solution of the ferroelectric film concerning this Embodiment. The figure which shows the metal carboxylic acid used by this Embodiment. 1A and 1B are a plan view and a cross-sectional view schematically showing a simple matrix ferroelectric memory device in an embodiment. 1 is a cross-sectional view showing an example of a ferroelectric memory device in which a memory cell array is integrated on a same substrate together with peripheral circuits in an embodiment. Sectional drawing which shows typically the 1T1C type ferroelectric memory in the modification of embodiment, and its equivalent circuit schematic. The figure which shows the formation method of the sample of the Example concerning this Embodiment. The figure which shows the crystallinity of the ferroelectric film of the sample using the raw material solution 2. FIG. The figure which shows the crystallinity of the ferroelectric film of the sample using the raw material solution 3. FIG. The figure which shows the crystallinity of the ferroelectric film of the sample using the raw material solution 4. FIG. The figure which shows the hysteresis characteristic of the sample using the raw material solution 1. FIG. The figure which shows the hysteresis characteristic of the sample using the raw material solution 2. FIG. The figure which shows the hysteresis characteristic of the sample using the raw material solution 3. FIG. The figure which shows the hysteresis characteristic of the sample using the raw material solution 4. FIG. The figure which shows the crystallinity of the ferroelectric film of the sample using the raw material solutions 5-7. The figure which shows the hysteresis characteristic of the sample using the raw material solution 5. FIG. The figure which shows the hysteresis characteristic of the sample using the raw material solution 6. FIG. The figure which shows the hysteresis characteristic of the sample using the raw material solution 7. FIG.

Explanation of symbols

10 Substrate, 100 Ferroelectric capacitor, 101 Ferroelectric film, 102 First electrode, 103 Second electrode, 300 Ferroelectric memory device, 301, 302, 303 Word line, 304, 305, 306 Bit line, 307 Strong Dielectric film, 308 substrate, 400 ferroelectric memory device, 401 single crystal silicon substrate, 402 MOS transistor, 403 gate insulating film, 404 gate electrode, 405 source / drain region, 406 oxide film for element isolation, 407 first Interlayer insulating film, 408 first wiring layer, 409 second interlayer insulating film, 410 lower electrode, 411 ferroelectric film, 412 upper electrode, 413 third interlayer insulating film, 414 second wiring layer, 415 protection Film, 420 ferroelectric capacitor, 500 ferroelectric memory device, 501 bottom electrode, 502 on Electrode, 503 ferroelectric film, 504 a capacitor, 505 data lines, 506 a gate electrode, 507 a transistor element

Claims (18)

Represented by the general formula _{(Pb 1-d Bi d)} (B 1-a X a) O 3,
B consists of at least one of Zr and Ti,
X consists of at least one of Nb and Ta,
a is in the range of 0.05 ≦ a ≦ 0.4,
d is a ferroelectric film in a range of 0 <d <1. In claim 1,
X is a ferroelectric film present at the B site of the perovskite structure. In claim 1 or 2,
Pb and Bi are ferroelectric films present at the A site of the perovskite structure. In claim 3,
The d is a ferroelectric film in a range of 0 <d ≦ 0.2. In any of claims 1 to 4,
A ferroelectric film having a tetragonal structure and having a pseudo cubic (111) orientation. In any of claims 1 to 5,
The ferroelectric film in which X is Nb. In any one of Claims 1 thru | or 6.
Comprising a ferroelectric represented by the general formula _{(Pb 1-d Bi d)} (B 1-a X a) O 3, a eutectic of the ferroelectric consisting BiNbO _4, the ferroelectric film. In claim 7,
The ferroelectric film, wherein a ratio of the number of moles of Bi element to Pb element contained in the eutectic is 3/7 or more. In claim 7 or 8,
Crystals have a perovskite structure represented by the general formula _{(Pb 1-d Bi d)} (B 1-a X a) O 3,
The BiNbO ₄ crystal is a ferroelectric film having a bismuth layered perovskite structure. A _{(Pb 1-d Bi d)} (B 1-a X a) method for producing _{O 3} in general formula shown Ru ferroelectric film,
B consists of at least one of Zr and Ti,
X consists of at least one of Nb and Ta,
a is in the range of 0.05 ≦ a ≦ 0.4,
d is in the range of 0 <d <1;
A sol-gel raw material containing a hydrolyzed / condensed product of metal alkoxide containing at least Zr element and Ti element, polycarboxylic acid or polycarboxylic acid ester, and organic solvent are mixed,
A method for producing a ferroelectric film, comprising: forming a precursor solution of a ferroelectric having an ester bond by esterification of the polycarboxylic acid or the polycarboxylic acid derived from the polycarboxylic acid ester and a metal alkoxide. In claim 10,
The ferroelectric film further includes a crystal made of BiNbO ₄ ,
A sol-gel raw material containing a hydrolyzed / condensed product of metal alkoxide containing at least Zr element and Ti element, polycarboxylic acid or polycarboxylic acid ester, and organic solvent are mixed,
A method for producing a ferroelectric film, comprising: forming a precursor solution of a ferroelectric having an ester bond by esterification of the polycarboxylic acid or the polycarboxylic acid derived from the polycarboxylic acid ester and a metal alkoxide. In claim 10 or 11,
A method for producing a ferroelectric film, which further includes a sol-gel raw material using a bismuth carboxylate when the sol-gel raw material, the polycarboxylic acid or polycarboxylic acid ester, and the organic solvent are mixed. In any one of claims 10 to 12,
A method for producing a ferroelectric film, which further comprises a sol-gel raw material using a lead carboxylate when the sol-gel raw material, the polycarboxylic acid or polycarboxylic acid ester, and the organic solvent are mixed. In any one of claims 10 to 13,
The method for producing a ferroelectric film, wherein the polycarboxylic acid or the polycarboxylic acid ester is a divalent carboxylic acid or a polycarboxylic acid ester. In any of claims 10 to 14,
A method for producing a ferroelectric film, wherein a sol-gel raw material further containing Si or Si and Ge is used when mixing the sol-gel raw material, the polycarboxylic acid or polycarboxylic acid ester, and the organic solvent.   A ferroelectric capacitor comprising the ferroelectric film according to claim 1. In claim 16,
It further has an electrode having a perovskite structure,
The ferroelectric capacitor, wherein the ferroelectric film is formed on the electrode.   A ferroelectric memory comprising the ferroelectric capacitor according to claim 16.

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