KR100393486B1 - Process for the formation of la-substituted bismuth titanate film with a sol coating method - Google Patents

Abstract

The present invention relates to a method for forming a lanthanum-containing bismuth titanate thin film using a sol coating method. According to the present invention, organic or inorganic salts containing lanthanum, bismuth, and titanium are dissolved in a solvent to form Bi: La: Ti. And a sol mixed in a molar ratio ranging from 3.33 to 3.9: 0.55 to 1.05: 3, and then coated on the substrate, followed by drying and heat treatment to oxidize uniform ferroelectric properties according to a uniform crystal growth direction. The Bi _4-x La _x Ti ₃ O ₁₂ (x = 0.55-1.05) thin film with excellent durability against fatigue can be economically mass-produced.

Description

Economical manufacturing method of lanthanum-containing bismuth titanate thin film by sol coating method {PROCESS FOR THE FORMATION OF LA-SUBSTITUTED BISMUTH TITANATE FILM WITH A SOL COATING METHOD}

The present invention relates to a method for forming a lanthanum-containing bismuth titanate thin film using a sol coating method, and more specifically, organic or inorganic salts containing lanthanum, bismuth and titanium, respectively, in a solvent to dissolve Bi: La: Ti from 3.33 to 3.33. Bi _4-x La _x Ti ₃ O ₁₂ (x = 0.55) with excellent ferroelectric properties and excellent durability against fatigue, using a stable sol in air mixed in a molar ratio of 3.9: 0.55 to 1.05: 3 -1.05) The present invention relates to a method for mass production of thin films economically.

Since research into the application of ferroelectrics to memory chips began in the late 1980s, commercialization of his fertilizer has become more visible in many countries. Basic requirements for such ferroelectric random access memory devices (FRAM devices) include high transition temperature (Tc), high residual polarization (Pr), and low phase formation temperature. ) And low coercive voltage (Vc), and various ferroelectric materials that satisfy these conditions, particularly lead zirconate titanate (PZT) (Zr _x Ti _1-x ) O ₃ ) (Korean Patent Publication No. 99 -233999, Korean Patent Publication No. 99-23743) and SBT (strontium bismuth titanate, SrBi ₂ Ti ₂ O ₉ ), etc. have been studied (James F. Scott et. Al., "Ferroelectric Memories" Science) vol. 246, 1400 (1989)).

When information is input or stored using FRAM devices, the difference in the amount of charge during positive polarization and negative polarization should be large so that there is little possibility of error in signal detection and the design and manufacture of memory devices It becomes easy. The material must have a high residual polarization amount in order for the difference in the amount of charge in the positive and negative polarizations to be large. In addition, in the case of a FRAM device that inputs and stores a telegram by using a difference in the amount of charge in the positive and negative polarizations, it is inevitable to repeatedly switch the polarization several times, and at this time, no fatigue phenomenon occurs in the ferroelectric material. When such fatigue phenomenon occurs, the Pr value is significantly reduced, thereby increasing the possibility of generating an error by making it difficult to detect an electrical signal when reading and storing information.

For the PZT, to be useful as a material of the ferroelectric memory element brought to a high Pr, but if the switching performed by 10 ⁷ times fatigue phenomenon occurs. In order to improve this, electrode materials containing IrO ₂ , RuO ₂ , and composite oxides have been developed, but these materials are not only expensive but also difficult to manufacture, and thus economical. In addition, in the case of the SBT, fatigue resistance is excellent, but the difference in charge amount between the positive and negative polarizations is about 10 μc / cm ^2, which makes it difficult to detect a signal, thereby increasing the possibility of error. It makes it difficult, and the ferroelectric phase generating temperature is higher than 750 ℃, which increases the heat treatment temperature when manufacturing a memory device, thereby providing a cause of increasing the defective rate.

In order to solve this problem, bismuth titanate containing La (BLT, Bi _4-x La _x) , which has high Pr value and does not exhibit fatigue even after 3 x 10 ¹⁰ switching cycles, and has a low phase temperature below 650 ° C Ti ₃ O ₁₂ ) (see B. H Park et al., "Lanthanum-substituted bismuth titanate for use in non-volatile memories", Nature, vol. 401, 14 (1999)).

In general, high-quality ferroelectric thin films are manufactured by high vacuum equipment, pulse laser deposition (PLD) method using a target oxide and a zezer source, and laser ablation method. Difficulties in the production of expensive equipment and target oxides, the difficulty of producing large-area thin films, and the difficulty of achieving a stoichiometric ratio of thin film compositions have been pointed out as disadvantages. Meanwhile, the above document discloses a technique for producing a La-containing bismuth titanate (BLT, Bi _4-x La _x Ti ₃ O ₁₂ ) ferroelectric thin film by a pulse laser deposition method, but manufacturing a high density integrated circuit due to nonuniformity in the crystal growth direction. Problems such as high defect rate due to the change of dielectric properties depending on the position of the device have been pointed out (see August Kingon, "Memories are made of ..." Nature, vol. 401, 658 (1999)). Therefore, in order to commercially use the La-containing bismuth titanate ferroelectric material disclosed in the above document, there is an urgent need for a technique for economically manufacturing a large-area ferroelectric thin film having a uniform crystal growth direction using inexpensive equipment.

The present invention provides a method for economically mass production of La-containing bismuth titanate (BLT, Bi _4-x La _x Ti ₃ O ₁₂ , x = 0.55 to 1.05) thin films with uniform ferroelectricity and durability against fatigue. Its purpose is to.

1 to 3 are graphs showing XRD diffraction patterns, ferroelectric properties, and fatigue properties according to switching, respectively, of the lanthanum-containing bismuth titanate thin film according to the present invention.

In the present invention, a mixture obtained by adding a lanthanum-containing organic or inorganic salt, a bismuth-containing organic or inorganic salt, and a titanium-containing organic or inorganic salt to an acidic water or an alcoholic organic solvent or a mixed solvent thereof is used to prevent volatilization of metal salts. It is heated, completely dissolved and stabilized at a temperature of 150 ° C. or lower in a sealed container in which a condenser is incorporated to form a stable sol in which Bi: La: Ti is mixed in a molar ratio ranging from 3.33 to 3.9: 0.55 to 1.05: 3. And then, the brush is coated on a substrate and then dried and heat treated to provide a method for producing a Bi _4-x La _x Ti ₃ O ₁₂ (x = 0.55 to 1.05) thin film.

Hereinafter, the present invention will be described in more detail.

Since bismuth is a material that is easily volatilized, part of the bismuth is volatilized and disappeared during heat treatment after coating. In order to satisfy the chemical amount of Bi after heat treatment in forming a thin film according to the present invention, an excess of bismuth salt in excess of a stoichiometric amount is required. Should be used. If the stoichiometric amount is not satisfied, crystal defects are generated in the ferroelectric thin film, which causes conductivity in the thin film, thus exhibiting the effect of removing the ferroelectric properties, thus acting as a serious defect. Therefore, according to the present invention, Bi: La: Ti is used in a molar ratio in the range of 3.33 to 3.9: 0.55 to 1.05: 3.

Examples of the lanthanum, bismuth and titanium salts which can be used in the present invention include inorganic salts such as acetates, nitrates, hydrochlorides or hydrates thereof and organic salts such as oxalates, alkoxides and the like.

Since the metal salts are not easily dissolved due to low solubility in solvents, the metal salts and solvents are completely dissolved by heating to a temperature of 150 ° C. or lower, but are performed in a closed container having a volatile condensation device to prevent volatilization of bismuth. It is essential to make a uniform sole.

Solvents may be acidic or alcoholic organic solvents or mixed solvents thereof, and the pH may range from 0.1 to 3 when the sole is used. In order to obtain a uniform coating, the concentration of solute in the formed brush is preferably in the range of 0.3 to 2.0 M. When the concentration is less than 0.3 M, the solute concentration is insufficient when the thin film is manufactured, and when the concentration is 2.0 M or more, complete dissolution of the metal salt is not only difficult, but the viscosity increases, making it difficult to homogeneous coating.

The brush formed in accordance with the present invention is stable for several months with little precipitation of solutes and excellent storage properties can contribute to productivity and quality stabilization during on-site mass production.

In order to manufacture a thin film using the brush, the brush is coated on a suitable substrate, followed by predrying and heat treatment. As the substrate, a conventional electrode metal may be used, and for example, Pt, Ir, Ru, or the like may be used, and the coating may be performed by a conventional dip coating method or spin coating method.

Drying of the coated thin film may be carried out at a temperature in the range of room temperature to 650 ℃, heat treatment may be carried out at a temperature in the range of 400 to 700 ℃ under an air or oxygen atmosphere. The coating thin film is present in an amorphous phase at a temperature of 400 ° C. or lower, but when subjected to a high temperature heat treatment, the volatile component evaporates and crystallizes to become a ferroelectric phase. Therefore, when the heat treatment temperature is lower than 400 ℃, the ferroelectric phase is not formed, the coating thin film remains in an amorphous phase, and when the heat treatment is performed at a temperature higher than 700 ℃, the lower metal electrode may be damaged in the form of oxidation or peeling. Not desirable

In order for the lanthanum-containing bismuth titanate thin film obtained after the heat treatment to exhibit ferroelectricity and durability against fatigue, the thin film composition should be Bi _4-x La _x Ti ₃ O ₁₂ (x = 0.55 to 1.05). If the lanthanum content (x value) is less than 0.55 mole, there is a problem that fatigue occurs after long-term (10 ⁹ cycles) switching. If the lanthanum content (x value) exceeds 1.05 mole, the fatigue phenomenon does not occur but the residual polarization value may decrease. have.

The lanthanum-containing bismuth titanate thin film prepared according to the present invention exhibits homogeneous ferroelectric properties as it has a homogeneous crystal growth direction, and does not exhibit fatigue phenomenon even after several switching. As described above, the method according to the present invention targets It is low in cost because there is no need for forming and firing raw materials for fabrication, it is easy to control stoichiometrically of thin film composition, and only needs relatively low temperature basic melting equipment below 150 ℃, and high vacuum is required. It is free of large area coating and is an economical and effective method for mass production of high quality lanthanum-containing bismuth titanate thin films having homogeneous ferroelectric properties.

Hereinafter, although an Example further demonstrates this invention, these Examples do not limit this invention.

Preparation of Coating Sol

Preparation Example 1

Bismuth acetate, lanthanum acetate hydrate, and chelated titanum diisopropoxide in dilute acetic acid solution at pH 1 were 3.33 to 3.9: 0.55 to 1.05 based on Bi: La: Ti. The mixture was mixed to a molar ratio of 3 :, and then heated to 100 ° C. in a closed vessel equipped with a condenser to prevent volatilization of the metal salts for complete dissolution and stabilization of the metal salts. At this time, the titanium isopropoxylated when exposed to the air, it is rapidly oxidized and is not uniformly distributed in the sol, so that the composition tends to be non-uniform, so that the composition of the sol is used after complexing with acetylacetonate before use. In addition, in order to prevent cracking due to rapid solvent volatilization during thin film molding, 0.2 vol% of ethylene glycol was added as a drying rate adjusting agent for controlling the drying rate.

The concentration of the sole was variously adjusted to 0.3, 0.6, 1.0, and 2.0 M concentrations based on the total amount of the solute, thereby preparing various lanthanum-containing bismuth titanate thin film preparation brushes according to the present invention.

Preparation Example 2

Except for using bismuth nitrate hydrate, lanthanum nitrate hydrate, diisopropoxylated titanium complex salt and a nitric acid aqueous solution of pH 0.9 as the starting material in Preparation Example 1 to form a sol according to the present invention.

Preparation Example 3

In Preparation Example 1, except that bismuth chloride, lanthanum chloride chloride, diisopropoxylated titanium complex salt and aqueous hydrochloric acid solution of pH 0.9 were used as starting materials, the sol according to the present invention was formed.

Preparation Example 4

In ethanol, bismuth ethoxide, lanthanum 2-methoxyethoxide, and titanum ethoxide were used in the range of 3.33 to 3.9: 0.55 to 1.05: 3 based on Bi: La: Ti. After mixing to a molar ratio of, the mixture was heated to 100 ° C. in a closed vessel equipped with a condenser to prevent volatilization of the metal salts for complete dissolution, hydration, polymerization and stabilization of the metal salts. In addition, in order to prevent cracking due to rapid solvent volatilization during thin film molding, 0.2 vol% of ethylene glycol was added as a drying rate adjusting agent for controlling the drying rate.

The concentration of the sole was variously adjusted to 0.3, 0.6, 1.0, and 2.0 M concentrations based on the total amount of the solute, thereby preparing various lanthanum-containing bismuth titanate thin film preparation brushes according to the present invention.

Preparation Example 5

In Preparation Example 4, except that 2-methoxyethoxylated bismuth, 2-methoxyethoxylated lanthanum, and titanium methoxylated were used as starting materials, and methanol was used as a solvent. A brush was formed.

Preparation Example 6

In Preparation Example 4, except that bismuth ethoxylated, lanthanum 2-methoxyethoxylated and titanium methoxylated were used as starting materials, and 2-methoxyethanol was used as a solvent. Formed.

Preparation Example 7

In Preparation Example 4, a sol according to the present invention was formed in the same manner except that bismuth isopropoxylated, lanthanum isopropoxylated and titanium isobutoxylated were used as starting materials and propanol was used as a solvent.

The brush according to the present invention obtained in Preparation Examples 1 to 7 was maintained in a homogeneous state without precipitation phenomenon even when left in the air at room temperature for at least 3 months.

Comparative Production Examples 1 to 3

In the preparation examples 1 to 7, respectively, except that the concentration of the sole was changed to 0.1M, 0.2M, 2.5M, 3.0M (when heated to 100 ° C.) and 1.0M (when dissolved at room temperature) Brushes for forming a comparative lanthanum-containing bismuth titan thin film were formed.

Formation of Lanthanum-containing Bismuth Titanium Thin Films

Examples 1-7 and Comparative Examples 1-7

The sol solutions according to the present invention obtained in Preparation Examples 1 to 7 and the comparative sol solutions obtained in Comparative Preparation Examples 1 to 7 were formed on a Pt metal thin film (Pt / Ti / SiO ₂ / Si substrate) by a rotation coating method. Uniformly coated. At this time, when the solute concentration of the brush was 0.3 M, it was coated with a thickness of 500-600Å, 0.6M if 800-900Å, 1.0M if 1,000-1,100Å, and 2.0M 1,300-1,500Å. .

In the case of coating using the soles of Preparation Examples 1 to 7 according to the present invention, a homogeneous coating layer can be obtained, whereas in the case of using the brushes of Comparative Preparation Examples 1 to 7, the concentration of 0.3 M or less is heat treated. When the porous thin film was obtained and had a concentration of 2.0 M or more, a heterogeneous coating layer was obtained due to the increase in viscosity, solute imperfection, and the like. In addition, in the case of a brush having a concentration of 1.0M and not subjected to heat treatment for stabilization, coating was impossible due to the occurrence of precipitation of solute and disproportionation of the sole.

Crystallization of Lanthanum-containing Bismuth Titanium Thin Films

Example 8

The thin film coated according to Example 1 using the brush of Preparation Example 1 was dried at 250 ° C. for 5 minutes and then heat-treated at 400, 450, 500, 550, 600 and 650 ° C. for 60 minutes in an air atmosphere. Thin films with the composition of Bi _4-x La _x Ti ₃ O ₁₂ (x = 0.55-1.05) were obtained.

After drying at 250 ° C., the thin film showed an amorphous phase, and the thin films heat-treated at 400 to 650 ° C. showed ferroelectric crystal phases. This is not only the lowest heat treatment temperature of all ferroelectric materials commercially available to date, but it can also suppress the diffusion by heating between the thin film layers in the actual process of growing many different types of multilayer thin films composed of different materials on metal electrodes. There is an important advantage.

FIG. 1 is a diagram showing an XRD diffraction pattern of a thin film having a composition of Bi _3.25 La _0.75 Ti ₃ O ₁₂ having a lanthanum content of 0.75 mol in the thin films prepared as described above. It can be clearly seen from FIG. 1 that the thin film obtained by heat treatment at 400 to 700 ° C. after coating by the sol coating method according to the present invention exhibits a ferroelectric crystal phase.

In addition, it can be seen from the XRD diffraction pattern of FIG. 1 that all ferroelectric phases are grown almost in the C-axis direction, which can be presented as an important advantage to lower the defective rate by securing quality uniformity in memory device production, as well as non-uniform crystal growth. This is an important step to solve the problem of increasing the defective rate of the highly integrated memory device.

Comparative Example 8

Except that the heat treatment temperature was 350 ℃ and 800 ℃ in Example 8 was carried out in the same manner to obtain a coating thin film.

The thin film heat-treated at 350 ° C. showed an amorphous phase and the thin film heat-treated at 800 ° C. showed damage and peeled.

Examples 9-14 and Comparative Examples 9-14

A thin film was obtained in the same manner as in Example 8, using the soles of Preparation Examples 2 to 7 and Comparative Preparation Examples 2 to 7.

As in Example 9 and Comparative Example 9, the thin film heat-treated at a temperature of 400 to 650 ℃ according to the present invention shows a ferroelectric phase, while the thin film heat-treated at a temperature of 350 ℃ remains an amorphous phase, heat treatment at 800 ℃ The thin film exhibited a phenomenon that the lower metal electrode was damaged and peeled off.

Durability test on dielectric properties and fatigue phenomena of thin films

The residual polarization value of the ferroelectric lanthanum titanate-containing bismuth titanate thin films (heat treatment temperature of 650 ° C.) obtained in Example 8 and the occurrence of fatigue phenomena after 10 ¹⁰ switching were measured by the electro-residual polarization measurement method. This was within the range of 15-20 μc / cm ² and no fatigue phenomenon occurred.

FIG. 2 shows ferroelectric characteristics (polarization-field curve) of a thin film having a composition of Bi _3.25 La _0.75 Ti ₃ O ₁₂ , wherein a residual polarization value (2P _r ) representing a quality characteristic of a memory device is 28 μc / cm ² is shown. It is generally known that the ferroelectric thin film formation temperature is about 750 ° C. and the ferroelectric thin film can be formed at a temperature lower than SBT having a residual polarization value of 20 μc / cm ² or less. It means that the ferroelectric thin film can be formed. In addition, the residual polarization value of the thin film according to the present invention is higher than 24 μc / cm ² reported in the prior art (Nature).

3a and 3b show the fatigue characteristics according to the switching of the thin film, it can be seen that even after 3.5 x 10 ¹⁰ switching, not only does the fatigue phenomenon occur but also the electric field-polarization characteristic hardly changes.

Meanwhile, as in Example 8 (heat treatment temperature of 650 ° C.), a thin film was prepared separately so that the lanthanum content in the lanthanum-containing bismuth titanate thin film was 0.35 mol and 1.15 mol, which is outside the scope of the present invention, and these were lanthanum according to the present invention. When the content is 0.55 to 1.05 moles and the residual polarization value and the fatigue phenomenon after 10 ¹⁰ switching was compared, the results are shown in the table below.

La content (mol) Residual Polarization Value (μc / cm ² ) 10 ¹⁰ Fatigue after switching The present invention 0.55 10 to 15 none 0.65 10 to 18 none 0.75 12 to 20 none 0.85 13 to 20 none 0.95 13 to 20 none 1.05 10 to 18 none Control 0.35 5 to 8 10 ⁹ Fatigue after switching 1.15 5 to 10 none

From the above table, it can be seen that the thin film according to the present invention has ferroelectricity and durability against fatigue phenomenon.

The lanthanum-containing bismuth titanate thin film prepared according to the present invention is a ferroelectric thin film exhibiting high residual polarization values without fatigue phenomenon even after long-term switching, and the method of the present invention does not require forming and firing of raw materials for target fabrication. Low cost of raw materials, easy stoichiometric control of thin film composition, uniform crystal growth direction of thin film to suppress defects due to quality variation, and relatively low temperature melting equipment below 200 ℃ It requires only high vacuum, no high vacuum, and easy to coat with large area, making it an economical and effective method for mass production of lanthanum-containing bismuth titanate thin films.

Claims (6)

Lanthanum salts, bismuth salts and titanium salts are dissolved in acidic or alcoholic organic solvents or mixed solvents thereof to form a sol in which Bi: La: Ti is mixed in a molar ratio ranging from 0.33 to 3.9: 0.55 to 1.05: 3. Forming, coating the brush on a substrate and then drying and heat-oxidizing to prepare a thin film having a composition of formula (I): Bi _4-x La _x Ti ₃ O ₁₂ Wherein x is a number ranging from 0.55 to 1.05. The method of claim 1, The lanthanum salt, bismuth salt or titanium salt is an inorganic acid salt, an organic acid salt or an alkoxide. The method of claim 1, And the concentration of the sole is in the range of 0.3 to 2 M. The method of claim 1, Dissolution of metal salts by heat treatment in a closed vessel with a condenser. The method of claim 1, Drying is carried out at a temperature from room temperature to 650 ° C. The method of claim 1, Heat treatment at a temperature in the range from 400 to 700 ° C. under an oxygen or air atmosphere.