JPH08325019A - Production of bismuth layer compound - Google Patents

Abstract

PURPOSE: To produce a layer compound having good crystallinity and improved electrical characteristics by using, as its precursor, a compound which is deposited on a substrate and has a fluorite structure and subjecting the precursor to heat treatment in an oxidizing atmosphere. CONSTITUTION: In this production, source materials such as BiPh3 (triphenylbismuth) used as a Bi source, Sr(DPM)2 (dipivaloyl-strontium) used as an Sr source and Ta(OCH3 )5 used as a Ta source are separately dissolved in an organic solvent such as tetrahydrofuran to obtain their respective solutions each having about 0.1mol/l concn. These solutions are mixed together in liquid volume ratios of Bi:Sr:Ta of 2:1:2 as the initial values and the resultant mixed solution is transferred and introduced into a preheated vaporized. Then, the vaporized solution is introduced together with Ar carrier gas into a reactor evacuated to about 10Torr, and there, the Bi, Sr and Ta sources are deposited on a substrate heated to a about 600 deg.C. Subsequently, an oxidizing gas such as oxygen is allowed to pass through the reactor to obtain a thin film having a fluorite structure. Further, the thin film thus obtained is subjected to heat treatment in a gaseous oxygen stream under ordinary pressure, at 800 deg.C for 1hr to produce a thin film of the objective bismuth layer compound contg. Bi2 SrTa2 O9 as the main phase.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a bismuth layered compound which is useful for an electronic device such as a ferroelectric memory.

[0002]

2. Description of the Related Art Bismuth layered compounds form a group of compounds that are extremely important industrially, such as bismuth-based superconducting oxides having a critical temperature of 110 K or materials for ferroelectric memories. When applying these compounds to electronic devices, development of a thin film process is indispensable.

The structure of this bismuth layered compound is found in, for example, Bi ₂ PbNb ₂ O ₉ , but it is a pseudo-tetragonal system extending columnarly in the c-axis direction, and has a layer of bismuth oxide in the c-axis direction and others. The layer has a repeating structure in which the oxide layer of the element is laminated in a certain order (GASMOLEN
SKII et al., SOVIET PHIYSICS-SOLID STATE, p651-655 (1
961) and ECSUBBARAO, J. Phys. Chem. Solids Pergamon
Press, Vol. 23, p665-676 (1962)). In this repeating structure, the number of bismuth oxide layers in one unit structure and the length of the unit structure vary depending on each bismuth layered compound.

Attempts are currently being made to apply this bismuth layered compound to electronic devices. Among them, a thin film of a bismuth layered compound exhibiting good ferroelectricity is MOD.
It is obtained by a spin coating method such as the (Metal Organic Deposition) method.

[0005]

However, the demand for cleanliness is strict in an actual semiconductor process, and the above-mentioned film forming method by spin coating cannot satisfy this demand.

Therefore, it is necessary to develop a new film forming process, but it is difficult to perform an oxidation reaction on an oxide thin film by an ultrahigh vacuum process (for example, molecular beam epitaxy method or laser ablation). In addition, in order to apply the MOCVD method (metalorganic chemical vapor deposition method) that is often used in semiconductor processes, there are obstacles such as the inability to use hydrogen gas as a carrier and the absence of a good source material. is there.

In order to solve the above-mentioned problem, in the present invention, post-annealing is performed in an oxidizing atmosphere in which the deposition process and the oxidation / crystallization process are separate steps, that is, the oxidation process. This provides a method for producing a bismuth layered compound having physical properties such as good crystallinity and electrical characteristics.

[0008]

The present invention is a method for producing a bismuth layered compound in which a compound having a fluorite structure is used as a precursor and heat treatment is performed to obtain the bismuth layered compound. As a typical example of the bismuth layer _{compound, Bi 2 (Sr a Ba b} Ca c) (Ta d Nb e) 2
O ₉ (a, b, c, d, and e are atomic ratios, have values of 0 to 1, and are represented by a composition formula of a + b + c = 1, d + e = 1). However, since the bismuth layered compound has non-stoichiometry, it is difficult to strictly satisfy the above composition formula,
It is sufficient that the bismuth layered compound has a predetermined layered structure.

According to the above-mentioned structure of the present invention, a bismuth layered compound having good crystallinity and electrical characteristics can be obtained by heat-treating a compound having a fluorite structure as a precursor. .

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In the method for producing a bismuth layered compound of the present invention, a desired bismuth layered compound is obtained by first producing a precursor having a fluorite structure and then heat treating the precursor.

Prior to the description of the embodiments of the present invention, the outline of the present invention will be described.

Fluora which is the basis of fluorite structure
Ito (fluorite) structure is fluorite (CaF ₂) Crystal structure
It has been known for a long time. The crystal structure of fluorite is cubic
The calcium atom is tetrahedral and the fluorine atom is
The structure is arranged in a rectangular shape.

This fluorite structure is formed by many fluorides (MF ₂ ) in addition to fluorite, Ce, Pr,
It is a structure found in an oxide (MO ₂ ) of an element such as Tb or an actinide series element (Th to Am).

Further, since these compounds belong to so-called non-stoichiometric compounds, they are metallic elements and anions (F and O).
It is tolerant of large compositional shifts at both elemental sites. For example, in ZrO ₂ , a considerably large amount of an alkaline earth metal or the like can be dissolved, and in uranium oxide, an oxygen-excessive nonstoichiometric compound can be formed.

The fluorite structure of the above oxide is
Metal atoms are arranged at the sites of calcium atoms of fluorite, and oxygen atoms are arranged at the sites of fluorine atoms, which belong to the same cubic system as fluorite and have high spatial symmetry. The crystal structure of the oxide (BO ₂ ) having this fluorite structure is shown in FIG. From FIG. 1, the metal atom (B) has a face-centered cubic lattice and the oxygen atom (O) has a simple cubic lattice. At the same time, the metal atom (B) is located at the body center of the cube constituted by the oxygen atom (O). It can be seen that the oxygen atom (O) is located at the center of the regular tetrahedron formed by the metal atom (B).

The fluorite-based structure of the bismuth compound, which is the precursor according to the present invention, is basically shown in FIG.
In the crystal structure shown in (1), bismuth and other metal atoms are arranged at the B site, and oxygen atoms are arranged as they are at the oxygen atom sites.

The fluorite-based structure of this precursor has a slight decrease in symmetry from the pure fluorite-structured cubic system due to the above-described compositional shift, etc., and may be tetragonal, orthorhombic, or mono-crystalline. In some cases, the structure may be an orthorhombic system, or may be a structure slightly different from the fluorite structure.

In the preparation of a compound having a fluorite structure as a precursor, various existing film forming methods can be used, particularly when a thin film is formed. The film formation method includes spin coating such as sol-gel method and MOD method, LSMCD (Liquid Source Misted Che)
mical Deposition) method, CVD (Chemical Vapor Deposition) method using metal halide (metal halide), DPM
MO using (DiPivaloylMethanato) etc. as a source
CVD (metalorganic chemical vapor deposition) method, flash CVD method in which a source is transported and mixed in a liquid phase and vaporized by rapidly reducing the pressure in a reaction chamber, further vacuum vapor deposition method, molecular beam vapor deposition method,
A film forming method such as a physical vapor deposition method such as a laser ablation method or a sputtering method can be adopted.

For example, the spin coating method produces a compound having a fluorite structure at about 700 ° C., but the CVD method can produce a compound having a fluorite structure at a low temperature of 600 ° C. or lower.

In the method for producing a bismuth layered compound according to the present invention, the composition of the thin film as a precursor is the same as the composition of the target bismuth layered compound or a composition in the vicinity thereof.

The target bismuth layered compound is
For example, Bi ₂ (Sr, Ba, C used as a ferroelectric substance)
a) In addition to the (Ta, Nb) ₂ O ₉ system, the aforementioned Bi-based oxide superconductors and the like can be mentioned.

The film formation conditions for the compound having a fluorite structure as a precursor are preferably reaction temperature: 300 to 700 ° C. reaction gas pressure: 0.1 to 50 torr carrier gas: in an oxidizing gas containing 5% or more of oxygen. Is.

The annealing conditions of the precursor compound having a fluorite structure are preferably annealing temperature: film formation temperature or higher and 850 ° C. or lower atmosphere: oxidizing atmosphere.

The desired bismuth layered compound can be produced under such conditions.

Here, for example, the composition formula is Bi ₂ SrTa ₂
The case of forming a thin film of a bismuth layer compound that is O ₉ will be taken as an example.

Among the above film forming methods, for example, flash C
When the VD method is used, a so-called as deposi state in which the film is not annealed as it is
precursor) having a fluorite structure can be synthesized. Here, this flash CV
The case of forming a film by the D method will be described.

At this time, as the substrate, for example, one obtained by sequentially depositing Ti and Pt on silicon by a sputtering method is used. As a CVD source, BiP as a Bi source
h ₃ (triphenylbismuth), Bi (O-Tol) ₃
, Sr (DPM) ₂ (dipivaloyl-strontium), Sr (Me ₅ C ₅ ) ₂ .2THF, etc. as Sr source,
Ta (OCH ₃ ) ₅ , Ta (O-iPr) as Ta source
Select the appropriate one from the _5th grade. Here, as the Bi source, Bi (O-iPr) ₃ , Bi (O-tC)
_{4 H 9) 3, Bi (} O-tC 5 H 11) 3, Bi (O-T
When a raw material containing oxygen such as ol) ₃ is used, a compound having a fluorite structure may be produced even in an oxidizing atmosphere.

These source materials are dissolved in an organic solvent or the like, conveyed in a liquid state, and introduced into the reactor together with an Ar (argon) carrier gas. At this time, preferably 0.
The source solution is vaporized by rapidly reducing the pressure to about 1 to 10 torr, and deposited on the substrate in a vapor phase state. When pure oxygen is used as the oxidizing gas, the flow rate is adjusted and supplied so that the oxygen partial pressure is about 50% of the total pressure.

Regarding the mixing ratio of each source, it is preferable to analyze the composition of a trial-produced thin film by using an EPMA (electron probe microanalyzer), a fluorescent X-ray device, etc., and compare it with the target composition, and based on that, Set the correct mix ratio by adjusting the mix ratio.

If the supply composition ratio of the source material is appropriate,
At a substrate temperature of about 300 to 700 ° C., a fluorite structure single phase or a thin film close to single phase can be obtained.

The precursor thus obtained is heat-treated at 800 ° C. for 1 hour in an oxidizing atmosphere, preferably in an oxygen stream at atmospheric pressure, to give the bismuth layered compound Bi ₂ SrTa ₂ O. A thin film having ₉ as the main phase can be obtained.

By forming a platinum electrode on the thus-obtained thin film by, for example, a sputtering method, the electrical characteristics can be measured, and a clear hysteresis can be observed at this time.

In general, the products produced by the CVD method are various due to complicated multistage elementary processes between sources,
There are many factors that cause compositional deviations such as nucleation in the gas phase, generation of by-products and re-evaporation from the substrate, and it is difficult to obtain the target phase stably in the as-deposited state. Even if annealed, the target phase cannot always be obtained.

Therefore, the production method according to the present invention in which a precursor having a fluorite type structure is used as a crystal nucleus to heat-treat the crystal nucleus to obtain a bismuth layer compound of a target phase is useful.

Further, it is not uncommon for the bismuth oxide system to exhibit optimum electrical characteristics at a position slightly deviating from the theoretical composition. In fact, even in the thin film of the bismuth layered compound of the present invention shown here, bismuth is slightly present.

The compound having a fluorite structure as a precursor may have a cubic fluorite structure or a tetragonal or orthorhombic structure with some asymmetry.

Next, a specific example of the method for producing the bismuth layered compound according to the present invention will be described. This specific example is an example in the case of forming a thin film of a bismuth layered compound having a composition of Bi ₂ SrTa ₂ O ₉ .

On the (100) plane of the naturally oxidized silicon, 100 nm of Ti and 100 nm of Pt are sequentially deposited by a sputtering method at room temperature to form a substrate.

As a source material for the CVD method, for example, Bi
Ph ₃ , Sr (DPM) ₂ , Ta (O-iPr) _{5 and the} like are selected, and these source materials are dissolved in an organic solvent such as THF (tetrahydrofuran) to prepare a solution having a concentration of about 0.1 M / l. To do.

With this solution as the initial value, Bi: Sr: Ta
= 2: 1: 2, mixed with a liquid volume ratio, conveyed, and introduced into a vaporizer. At this time, the vaporizer is appropriately heated so that the source material does not deposit and adhere to the inner wall of the vaporizer.

The vaporized solution is introduced into the reactor from the vaporizer together with the Ar carrier gas.

The vaporizer and reactor are depressurized to about 10 torr to vaporize the source solution. Then, the vaporized source solution is transported and deposited on the substrate in a vapor phase state.

At this time, the temperature of the substrate is set to about 600.degree.

On the other hand, an oxidizing gas such as pure oxygen, a balance gas of oxygen (a mixed gas of Ar + 20% oxygen, etc.), ozone, N ₂ O, NO _2, etc. is directly introduced into the reactor without passing through the vaporizer.

The flow rates of the Ar carrier gas and the oxidizing gas are each 500 sccm by a mass flow controller.
Adjust the supply to a level and supply.

In this way, it is possible to obtain a thin film having a fluorite structure which is excellent in crystallinity and is close to a single phase.

The X-ray diffraction pattern and the electron beam diffraction pattern of this thin film are shown in FIG. 2 and FIG. 3, respectively. In FIG. 2, F
The peak described as is a peak of a bismuth compound having a fluorite structure, and the peak described as Pt is a peak due to Pt of the substrate below the thin film. After the symbols F and Pt, the crystal planes corresponding to the peaks are shown.

From FIGS. 2 and 3, (111), (20
0), (220), (311), (222), (40
Peaks due to reflection on each surface of (0) are observed, which clearly shows the characteristics of the fluorite structure.

Then, the thin film made of the compound having the fluorite structure is heat-treated at 800 ° C. for 1 hour in an oxygen stream at atmospheric pressure.

As a result, the desired composition Bi ₂ SrTa ₂ O was obtained.
A thin film of a bismuth layered compound having ₉ as a main phase could be obtained.

An X-ray diffraction pattern of a thin film made of this bismuth layered compound is shown in FIG. In FIG. 4, Pt is shown in FIG.
Similarly to the above, the peak is due to platinum, and Bi is the peak due to the bismuth layered compound. The X-ray diffraction pattern of FIG. 4 is shown in FIG.
It can be seen from the comparison with the X-ray diffraction pattern of 1) that a phase change occurred due to annealing in an oxidizing atmosphere and the position of the peak, that is, the crystal structure of the bismuth compound changed. Further, the observation result of the selected area diffraction image of this bismuth layered compound was in good agreement with the calculation, and it was confirmed that a bismuth layered compound having a good crystal structure was obtained.

Further, in order to evaluate the characteristics of the thin film produced in this example, an upper platinum electrode having a thickness of about 200 nm was formed on the thin film by the sputtering method, and the hysteresis characteristic of the thin film was measured.

FIG. 5 shows a graph of hysteresis characteristics.
In FIG. 5, the applied voltage is changed from 1 to 5 V and the hysteresis is measured. 2Pr value (plus remanent polarization Pr ⁺
A negative residual polarization Pr ^- difference between) is 10 [mu] C / cm
It is about ² . Further, in FIG. 6, the magnitude of the applied voltage and γ = 2P in the measurement of the hysteresis characteristic shown in FIG.
A graph shows the relationship with the value of r / (Ps ⁺ -Pr ⁺ ).
Ps ⁺ is the polarization at the maximum applied voltage. From the viewpoint of the hysteresis shape, the larger the γ value, the larger the output. Therefore, FIG. 6 shows that this thin film is preferably used in the vicinity of an applied voltage of 1V. However,
The position of the peak of the γ value, that is, the operating voltage can be controlled by the film thickness of the thin film. In addition, the effective output of the actual device largely depends on the operating voltage, the bit line capacitance and the like.

(Comparative Example) Bi _{2 was} prepared in the same manner as in the above-described embodiment except that the precursor was made to have an amorphous structure by setting the substrate temperature at the time of depositing the precursor on the substrate by the CVD method to 280 ° C. A SrTa ₂ O ₉ thin film was prepared.
When the γ value in the hysteresis characteristic of this thin film was measured in the same manner as in FIG. 6, it was confirmed that the γ value was small in this comparative example, even though the atomic composition ratio was the same as in the above-mentioned example. It was When a thin film having such a small γ value is used for a ferroelectric memory, the output is remarkably reduced, and it is difficult to secure sufficient reliability.

Although the bismuth layer compound of interest has the composition of Bi ₂ SrTa ₂ O _{9 in} the above-mentioned embodiments, the bismuth layer compound of other compositions is similarly obtained as a thin film of the composition of interest. You can

Although the thin film of the bismuth layered compound is formed in the above-mentioned embodiment, the manufacturing method of the present invention can be applied not only to the thin film but also to the production of the bulk bismuth layered compound. An example is shown below.

The composition ratio of the target bismuth layered compound is obtained.
As described above, the raw material bismuth oxide (Bi ₂O₃), Hydroxide
Trontium (Sr (OH)₂) Or strontium nitrate
Tium (Sr (NO₃)₂), Tantalum oxide (Ta₂O
_Five) Is weighed, mixed well and crushed in a mortar
From this, a powder raw material is produced. As a raw material for strontium
Is not a commonly used carbonate and decomposes at relatively low temperatures
Use hydroxide or nitrate.

This powder raw material is pressure-molded into pellets and then kept at 700 ° C. for 3 days in the atmosphere.

After the heating is completed, it is quickly taken out of the furnace and put into liquid nitrogen for rapid cooling.

As a result, bismuth strontium tantalum oxide having a fluorite structure Can be obtained.

By using this oxide of fluorite structure as a precursor and heat-treating this at 800 to 1000 ° C. in an atmospheric pressure oxygen stream, a bismuth layered compound having a nearly single phase could be obtained.

In addition, the composition of the bismuth layered compound is Bi
₂ Sr (Ta, Nb) ₂ O ₉ may be used. in this case,
Although the value of the coercive electric field was larger than that of Bi ₂ SrTa ₂ O ₉ , the other characteristics were almost the same.

The method for producing the bismuth layered compound of the present invention is not limited to the above-mentioned examples, and various other configurations can be adopted without departing from the scope of the present invention.

[0064]

According to the above-described method of forming an insulating film of the present invention, it is difficult to directly obtain a compound having a fluorite structure as a precursor in an oxidizing atmosphere, which is difficult to obtain directly by a usual method. A bismuth layered compound having excellent electrical characteristics can be obtained.

In addition, the above-mentioned composition deviation property, that is, the fluorite structure has a wide tolerance for composition deviation, and the bismuth layered compound exhibits optimum electrical characteristics when the composition deviates slightly from the theoretical composition. Utilizing this fact, the present invention can form a bismuth layered compound having a composition exhibiting optimum electrical characteristics via a precursor made of a compound having a fluorite structure. Therefore, according to the present invention, a ferroelectric thin film having good electric characteristics can be obtained.

In addition, due to the tolerance of the composition deviation, the process margin, that is, the tolerance of the manufacturing conditions can be made large, and the electronic device made of the bismuth layer compound can be manufactured more easily.

[Brief description of drawings]

FIG. 1 is a crystal structure diagram of a metal oxide having a fluorite structure.

FIG. 2 is an X-ray diffraction pattern of a thin film of a compound having a fluorite structure according to the present invention.

FIG. 3 is an electron diffraction pattern of a thin film of a compound having a fluorite structure according to the present invention.

FIG. 4 is an X-ray diffraction pattern of a bismuth layered compound thin film produced by annealing.

FIG. 5 is a graph showing measurement results of hysteresis of a bismuth layer compound thin film.

FIG. 6 is a graph showing the relationship between the applied voltage and the γ value in the measurement of FIG.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical indication location H01L 27/108 H01L 39/24 ZAAB 21/8242 9276-4M 27/10 651 39/24 ZAA (72 ) Inventor Yuji Ikeda 6-735 Kitashinagawa, Shinagawa-ku, Tokyo Inside Sony Corporation

Claims (8)

[Claims] 1. A method for producing a bismuth layered compound, which comprises using a compound having a fluorite structure as a precursor and heat-treating the precursor to obtain a bismuth layered compound. 2. Bi ₂ (Sr _a Ba _b Ca _c ) (Ta _d
Nb _e ) ₂ O ₉ represented by the composition formula, a, b, c, d,
The method for producing a bismuth layered compound according to claim 1, wherein e is an atomic ratio and takes a value of 0 to 1 to obtain a bismuth layered compound in which a + b + c = 1 and d + e = 1. 3. The method for producing a bismuth layered compound according to claim 1, wherein a bismuth layered compound represented by the composition formula Bi ₂ SrTa ₂ O ₉ is obtained. 4. The method for producing a bismuth layered compound according to claim 1, wherein the heat treatment is performed in an oxidizing atmosphere. 5. The compound having a fluorite structure is formed by a chemical vapor deposition method at a reaction temperature of 300 to 700 ° C., and the heat treatment is performed in an oxidizing atmosphere to obtain a compound having the fluorite structure. The method for producing a bismuth layered compound according to claim 1, wherein the method is carried out at a temperature equal to or higher than the reaction temperature. 6. The method for producing a bismuth layered compound according to claim 5, wherein the reaction gas pressure in the chemical vapor deposition is 0.1 to 50 Torr. 7. The method for producing a bismuth layered compound according to claim 5, wherein an oxidizing gas containing 5% or more of oxygen is used as a carrier gas in the chemical vapor deposition. 8. The method for producing a bismuth layered compound according to claim 1, wherein a bismuth layered compound represented by a composition formula of Bi ₂ Sr (Ta, Nb) ₂ O ₉ is obtained.