JPH1129328A - Coating solution for forming thin film of ferroelectric material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a coating solution free from the problem of crystal deposition and gelatinization, causing little change in the solution viscosity with time and having improved storage stability by including bismuth nitrate, an organometallic compound and a diol compound. SOLUTION: This solution is produced by dissolving bismuth nitrate, an organometallic compound selected from strontium compound, tantalum compound, barium compound, lead compound, niobium compound and titanium compound, at least one kind of dial compound selected from ethylene glycol, diethylene glycol and 1,3-propanediol in an amount of 0.1-5.0 equivalent based on the amount of the metal in the coating solution and optionally 0.1-1.0 equivalent, preferbly 0.1-0.3 equivalent (based on the metal in the solution) of acetylacetone and a doping agent comprising a compound containing at least one kind of element selected from Ba, Pb, Ti and Nb.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a bismuth strontium tantalate (SB) used for a semiconductor memory or the like.
T) A coating solution capable of forming a thin film.

[0002]

2. Description of the Related Art Ferroelectrics have many functions such as spontaneous polarization, high dielectric constant, electro-optic effect, piezoelectric effect, and pyroelectric effect, and are widely applied to device development. Above all, ferroelectric nonvolatile memories using such thin films have been actively developed. Bismuth layered compounds such as bismuth strontium tantalate (SBT) have attracted attention as ferroelectric materials having good ferroelectric properties and particularly excellent polarization inversion resistance.

The problem of forming an SBT thin film is that the thin film forming temperature, that is, the crystallization temperature is generally very high, such as 750 to 800 ° C., so that it has a large thermal effect on the substrate, and a ferroelectric nonvolatile memory or the like is manufactured. Pt / SiO used at the time
_{For a 2} / Si or Pt / Ti / SiO ₂ / Si substrate, P
Damage to the t film occurs. In addition, there is a problem that the surface of the thin film is roughened due to the growth of crystal grains, and as a result, electric characteristics such as an increase in leak current are deteriorated. JP-A-8-2909
No. 02, JP-A-8-290903 and JP-A-8-290964 describe a thin-film dielectric having a composition represented by a very broadly defined general formula. Although SBT thin films are included, there is no description in these documents that gives specific suggestions on low-temperature deposition of SBT thin films.

On the other hand, formation of such a ferroelectric thin film generally requires a solution obtained by dissolving or reacting an organic metal compound such as a metal alkoxide or a metal salt of 2-ethylhexanoic acid in an organic solvent or a solution obtained by hydrolysis. Widely used (Proceedings of the 13th Ferroelectric Application Conference, p.1
25-126 (1996) and Proceedings of the 12th Ferroelectric Application Conference, pp. 139-140 (1995)). However, these organometallic compounds, especially bismuth alkoxide and bismuth 2-ethylhexanoate, are very expensive. Further, since bismuth alkoxide is hardly soluble in an organic solvent, preparation of a coating solution for forming a thin film is not easy. A coating solution using bismuth nitrate, which is inexpensive, has also been announced. However, since bismuth nitrate is hardly soluble in organic solvents, a coating solution for forming a thin film is prepared using a mixed solvent of an organic solvent and acetic acid ( JP-A-8-23073). In this case, there are problems such as poor coating characteristics to the substrate and poor storage stability of the coating solution.

[0005]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a bismuth raw material using inexpensive bismuth nitrate.
An object of the present invention is to provide a bismuth strontium tantalate (SBT) thin film forming coating solution capable of forming a thin film at a low temperature of 50 ° C. or lower. It is a further object of the present invention to provide a coating solution for forming an SBT thin film which is free from crystal precipitation and gelation, has little change over time in solution viscosity, and has excellent storage stability.

[0006]

As a result of intensive studies to solve the above-mentioned problems, it has been found that a diol compound is contained in a solution containing bismuth nitrate and an organometallic compound, so that the solution is 750 ° C. or lower, further 700 ° C. We have succeeded in developing a coating solution for forming a thin film of bismuth strontium tantalate (SBT) that can form a thin film even at the following low temperatures. Further, by further adding acetylacetone to this solution, it was possible to obtain a coating solution for forming an SBT thin film, which was free from crystal precipitation and gelation, had little change in solution viscosity with time, and had excellent storage stability. That is, the present invention provides a coating solution for forming a bismuth strontium tantalate (SBT) thin film, characterized in that a diol compound is contained in a solution containing bismuth nitrate and an organometallic compound, and acetylacetone is further contained as desired. is there.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The bismuth nitrate used in the present invention includes bismuth nitrate pentahydrate, bismuth nitrate dihydrate, bismuth nitrate hemihydrate, bismuth nitrate anhydrate and the like. Preferably, it is bismuth nitrate pentahydrate. Further, examples of the organometallic compound used in the present invention are shown. Examples of the strontium compound include strontium dimethoxide, strontium diethoxide, strontium dipropoxide, strontium alkoxide such as strontium dibutoxide, and 2-ethylhexane strontium, strontium n-octanoate, strontium naphthenate, and the like. Preferably, strontium dipropoxide and 2-ethylhexane strontium are used. Examples of the tantalum compound include tantalum pentamethoxide, tantalum pentaethoxide, tantalum pentapropoxide, tantalum alkoxide such as tantalum pentapropoxide, tantalum 2-ethylhexanoate, and tantalum n-hexanoate. Preferably, tantalum pentaethoxide, 2
-Tantalum ethylhexanoate. Examples of the barium compound include barium alkoxides such as barium dimethoxide, barium diethoxide, and barium dipropoxide, barium octylate, and barium n-hexanoate. Preferably, it is barium dipropoxide. Examples of the lead compound include lead alkoxides such as lead diethoxide, lead dipropoxide, and lead dibutoxide, and lead 2-ethylhexanoate, anhydrous lead acetate, and lead acetate trihydrate. Preferred are anhydrous lead acetate and lead acetate trihydrate. Examples of the niobium compound include niobium alkoxides such as niobium pentamethoxide, niobium pentaethoxide, niobium pentapropoxide, and niobium pentabutoxide, niobium octylate, and niobium n-hexanoate. Preferably, it is niobium pentaethoxide. Examples of the titanium compound include titanium alkoxides such as titanium tetramethoxide, titanium tetraethoxide, titanium tetrapropoxide, and titanium tetrabutoxide. Preferably, titanium tetrapropoxide is used. In addition, Ca, C
d, La, Y, Sm, Dy, Ce, Sb, Hf, Sn,
Examples include W, Mo, Yb, Eu, Pr, and Tb, and a compound containing these elements may be added as long as the present invention is not impaired.

Examples of the organic solvent used in the coating solution according to the present invention include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl. ether,
Propylene glycol monobutyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether,
Examples include ethylene glycol dibutyl ether and propylene glycol monomethyl ether acetate, and preferred is ethylene glycol monomethyl ether. These organic solvents may be used alone or in combination of two or more.

Examples of the diol compound used in the present invention include ethylene glycol, propylene glycol,
1,3-propanediol, 1,2-butanediol,
Examples thereof include 1,3-butanediol, 2,3-butanediol, tetramethylene glycol, pentamethylene glycol, hexylene glycol, octylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol. Preferred are ethylene glycol, diethylene glycol and 1,3-propanediol.

Next, a method for producing the coating solution of the present invention will be described. The coating solution of the present invention is produced by dissolving bismuth nitrate in a mixed solution of a diol compound and acetylacetone, and mixing a solution obtained by adding the organic solvent and a solution obtained by dissolving the organic metal compound other than bismuth in an organic solvent. However, the order of preparation and the reaction method are not limited. In addition, this solution can be used as it is, but if necessary, it can be hydrolyzed to adjust the concentration to obtain a coating solution. Further, it is known that a thin film having good electric properties can be obtained by reducing Strontium and increasing Bismuth in SBT containing SBT and a doping agent. By changing the amount, a thin film having an arbitrary composition ratio can be easily formed. The amount of the diol compound to be added is usually 0.1 to 5.0 equivalents, preferably 0.1 to 0.1, relative to the amount of metal in the coating solution.
Two equivalents. The addition amount of acetylacetone is usually used in an amount of 0.1 to 1.0 equivalent to the amount of metal in the coating solution,
Preferably it is 0.1 to 0.3 equivalent.

The coating solution of the present invention is spin-coated on a substrate of glass, sapphire, titanium, platinum or the like, dried, subjected to intermediate firing for the purpose of removing organic components, and finally to crystallization by final firing. As a result, an SBT thin film containing SBT and a doping agent can be formed.

[0012]

Examples and Comparative Examples The present invention will be described in more detail with reference to examples together with comparative examples, but the present invention is not limited to these examples.

Example 1 10 mmol of strontium diisopropoxide and 20 mmol of tantalum pentaethoxide were respectively dissolved in 100 ml of ethylene glycol monomethyl ether, heated and refluxed, concentrated, cooled to room temperature, and mixed with each other. Separately, 20 mmol of bismuth nitrate pentahydrate was added and dissolved in 0.31 g of ethylene glycol and 0.98 g of acetylacetone. Thereafter, 100 ml of ethylene glycol monomethyl ether was added, and the mixture was stirred at 100 ° C. for 2 hours, concentrated by dehydration and cooled to room temperature. This solution was added to a mixed solution of strontium and tantalum and stirred for 4 hours. After the reaction, the solution was concentrated to a concentration of 10 wt% in terms of SBT, the concentration was adjusted, and the solution was filtered through a 0.2 μm filter to remove sulfur.
A coating solution for forming a BT thin film was prepared. This solution did not show any precipitation or gelation of crystals due to the change with time, and further showed almost no change in the solution viscosity with time, confirming that the solution had excellent storage stability.

Next, the coating solution is applied to Pt / SiO ₂ / Si
The substrate was spin-coated at 3000 rpm for 30 seconds, and dried in a clean oven at 150 ° C. for 30 minutes.
Thereafter, intermediate firing was performed at 400 ° C. for 1 hour in a diffusion furnace.
The above steps were repeated until the desired film thickness was obtained. In this embodiment, the process was repeated three times. Finally, main firing is performed at 650 ° C. for 1 hour in an oxygen atmosphere in a diffusion furnace, and 216
A nm thin film was formed. When the X-ray diffraction analysis of this thin film is performed, the X-ray diffraction pattern shown in FIG. 1 is obtained, and the bismuth layered perovskite structure peculiar to the ferroelectric material and particularly (10
It was confirmed that the SBT film was oriented in 5).

Example 2 10 mmol of strontium diisopropoxide and 20 mmol of tantalum pentaethoxide were respectively dissolved in 100 ml of ethylene glycol monomethyl ether, heated and refluxed, concentrated, cooled to room temperature, and mixed with each other. On the other hand, 20 mmol of bismuth nitrate pentahydrate was added and dissolved in 0.52 g of diethylene glycol and 0.98 g of acetylacetone. Thereafter, 100 ml of ethylene glycol monomethyl ether was added, and the mixture was stirred at 100 ° C. for 2 hours, dehydrated and concentrated, and cooled to room temperature. This solution was added to a mixed solution of strontium and tantalum and stirred for 4 hours.
After the reaction, this solution was concentrated so as to have a concentration of 10 wt% in terms of SBT, the concentration was adjusted, and the solution was filtered through a 0.2 μm filter to prepare a coating solution for forming an SBT thin film. This solution did not show any precipitation or gelation of crystals due to the change with time, and further showed almost no change in the solution viscosity with time, confirming that the solution had excellent storage stability.

Next, the coating solution is made of Pt / SiO ₂ / Si
The substrate was spin-coated at 3000 rpm for 30 seconds, and dried in a clean oven at 150 ° C. for 30 minutes.
Thereafter, intermediate firing was performed at 400 ° C. for 1 hour in a diffusion furnace.
The above steps were repeated until the desired film thickness was obtained. In this embodiment, the process was repeated three times. Finally, main firing is performed at 650 ° C. for 1 hour in an oxygen atmosphere in a diffusion furnace, and 228
A nm thin film was formed. When the X-ray diffraction analysis of this thin film is performed, the X-ray diffraction pattern shown in FIG. 2 is obtained, and the bismuth layered perovskite structure peculiar to the ferroelectric substance, particularly,
It was confirmed that the SBT film was oriented in the following manner.

Example 3 10 mmol of strontium diisopropoxide and 20 mmol of tantalum pentaethoxide were each dissolved in 100 ml of ethylene glycol monomethyl ether, heated to reflux, concentrated, cooled to room temperature, and mixed with each other. On the other hand, 20 mmol of bismuth nitrate pentahydrate was added and dissolved in 0.37 g of 1,3-propanediol and 0.98 g of acetylacetone. Thereafter, 100 ml of ethylene glycol monomethyl ether was added, and the mixture was stirred at 100 ° C. for 2 hours, concentrated by dehydration and cooled to room temperature. This solution was added to a mixed solution of strontium and tantalum and stirred for 4 hours. After the reaction, this solution was concentrated so as to have a concentration of 10 wt% in terms of SBT, the concentration was adjusted, and the solution was filtered through a 0.2 μm filter to prepare a coating solution for forming an SBT thin film. This solution did not show any precipitation or gelation of crystals due to the change with time, and further showed almost no change in the solution viscosity with time, confirming that the solution had excellent storage stability.

Next, the coating solution is applied to Pt / SiO ₂ / Si
The substrate was spin-coated at 3000 rpm for 30 seconds, and dried in a clean oven at 150 ° C. for 30 minutes.
Thereafter, intermediate firing was performed at 400 ° C. for 1 hour in a diffusion furnace.
The above steps were repeated until the desired film thickness was obtained. In this embodiment, the process was repeated three times. Finally, main firing is performed at 650 ° C. for 1 hour in an oxygen atmosphere in a diffusion furnace, and 228
A nm thin film was formed. When the X-ray diffraction analysis of this thin film is performed, the X-ray diffraction pattern shown in FIG. 3 is obtained, and the bismuth layered perovskite structure peculiar to the ferroelectric substance, particularly, (105)
It was confirmed that the SBT film was oriented in the following manner.

COMPARATIVE EXAMPLE 10 mmol of strontium diisopropoxide and 20 mmol of tantalum pentaethoxide were dissolved in 100 ml of ethylene glycol monomethyl ether, heated and stirred under reflux, concentrated, cooled to room temperature, and mixed with each other. Next, 20 mmol of bismuth nitrate pentahydrate was added to the mixed solution of strontium and tantalum and mixed, followed by stirring at room temperature. After the reaction, this solution was concentrated so as to have a concentration of 10 wt% in terms of SBT, the concentration was adjusted, and the solution was filtered through a 0.2 μm filter to prepare a coating solution for forming an SBT thin film. Crystals precipitated. That is, this coating solution is not practical because of poor storage stability. After adding bismuth nitrate pentahydrate, the solution is heated to 60
At ℃, the solution became cloudy and no coating solution could be prepared.

[0020]

The coating solution for forming an SBT thin film according to the present invention comprises:
A coating solution for forming an SBT thin film on a substrate, which has the advantage that a thin film can be formed at a low temperature, has no precipitation or gelation of crystals, has a small change in solution viscosity over time, and has excellent storage stability. It is suitable as.

[Brief description of the drawings]

FIG. 1 is an X-ray diffraction pattern of an SBT thin film of Example 1 of the present invention.

FIG. 2 is an X-ray diffraction pattern of the SBT thin film of Example 2 of the present invention.

FIG. 3 is an X-ray diffraction pattern of the SBT thin film of Example 3 of the present invention.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kiyoto Mori 1-7-1 Inari, Soka City, Saitama Prefecture Kanto Chemical Co., Ltd. Central Research Laboratory

Claims (5)

[Claims] 1. A coating solution for forming a bismuth strontium tantalate (SBT) thin film, comprising bismuth nitrate, an organometallic compound and a diol compound. 2. The solution according to claim 1, further comprising acetylacetone. 3. The solution according to claim 1, further comprising a doping agent. 4. The doping agent is Ba, Pb, Ti, Nb.
The solution according to claim 3, wherein the solution is at least one selected from the group consisting of: 5. The diol compound is ethylene glycol,
Characterized in that it is at least one selected from diethylene glycol and 1,3-propanediol,
The solution according to claim 1.