JP2005213105A - Polycrystallline metal oxide thin film and its manufacturing method, and non-volatile memory - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polycrystalline metal oxide thin film of a uniform film thickness having a desired film thickness to be used for electronic devices or the like and its manufacturing method, and a non-volatile memory which uses the polycrystalline metal oxide thin film. <P>SOLUTION: The polycrystalline metal oxide thin film, of which the surface is flat and the film thickness is uniform, is obtained by forming the coating film of a dispersion liquid in which the very fine perticles of a metal oxide are dispersed in a sol-gel liquid of a mixture of an organic metal compound, a metal alkoxide, or an organic acid salt and an organic solvent is formed on a substrate by a spin coating method or a dip coating method; and applying a vibration during the processes until the completion of removing the solvent of the coating film, chemical reaction, and crystalization, that is, a heating and drying process and/or a firing process. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a polycrystalline metal oxide thin film, and more particularly to a polycrystalline metal oxide thin film using a sol-gel method or an organic metal fractionation method (MOD), a manufacturing method thereof, and a nonvolatile memory using the same.

  As a method for forming a metal oxide thin film, a physical deposition method (PVD) performed in a vacuum such as a vacuum deposition method, a sputtering method, a molecular beam epitaxial growth method (MBE), a cluster ion beam method (ICB), or a laser ablation method has been conventionally used. ), Vapor phase chemical transport method, metal organic deposition transport method (MOCVD), plasma CVD method, metal phase chemical vapor deposition method (CVD), and liquid phase epitaxial method using melt and solution ( Liquid coating sintering methods such as LPE), printing sintering method, sol-gel method, organometallic decomposition method (MOD), and spray method have been put into practical use.

There are several problems with each of these methods. For example, a physical deposition method (PVD) such as a vacuum evaporation method needs to keep the entire production system at a high vacuum of 10 ⁻² Pa or less, and the evaporation particles flying from the evaporation source have directivity, There are limitations on the size and shape of the material to be formed into a thin film. Furthermore, chemical deposition (CVD) is usually produced in a vacuum environment, as is the case with PVD, so it is necessary to reduce the pressure, and depending on the type of metal oxide to be produced, toxic gases that are dangerous to the human body are used. There is also.

  On the other hand, the liquid coating and sintering method, which is a liquid phase thin film forming method, does not require special vacuum equipment with many restrictions like the above-described method for producing a vapor phase thin film such as the PVD method and the CVD method, and can be used in the atmosphere. Therefore, the thin film can be easily formed, so that it is economical, and the film can be formed even in an uneven and complicated shape. Moreover, the sol-gel method and organometallic decomposition method (MOD), which are a kind of liquid coating sintering method, can form a uniform and high-quality metal oxide thin film in a short time over a large area at a relatively low temperature. .

  In the sol-gel method or organometallic decomposition method (MOD), a liquid in which an organic metal compound, metal alkoxide, or organic acid salt in which the element composition ratio is adjusted is dissolved in a solvent is formed by a spin coating method or a dip coating method. It is applied on top, and this is heated and dried at a few hundred degrees C using a hot plate or the like to remove the solvent. The thin film obtained by drying after coating in this way is simply a mixture of the constituent elements and is not a metal oxide. Thereafter, the thin film is fired at a high temperature of about 500 to 1000 ° C. Thus, a chemical reaction and crystallization are promoted to form a metal oxide thin film.

  Conventionally, there is a method of forming a metal oxide thin film by a spin coating method as described in Patent Document 1, for example. In the spin coating method described in Patent Document 1, even when a metal oxide thin film having a film thickness of about 200 nm is formed, a process of forming a coating film on a substrate and a heating process at 400 ° C. The heat treatment step is repeated four times, and then a step of baking at a temperature of 600 ° C. is necessary, which has a problem that the production cost becomes expensive.

  As mentioned above, the reason why the coating process and the repeated heating and drying process are required is that even if the film thickness is about several hundreds of nanometers, there is a risk that the coating film will crack when heated at once. Because there is.

  In general, the thinner the coating film, the lower the internal stress and the fewer the number of defects. However, when the coating film becomes thicker, defects that serve as nucleation centers for crack propagation are generated. Therefore, in the conventional spin coating method and dip coating method, the film thickness formed by one coating is 100 nm or less, and it is difficult to form a thick film beyond this.

  Further, for example, in Patent Document 2, a dispersion in which a fine ceramic is mixed with an organometallic sol-gel solution is applied to a substrate, and then heated to hydrolyze and thermally decompose to remove organic substances. A method for producing a crystalline metal oxide coating is disclosed.

However, in this method, fine powder ceramics aggregate in the coating film forming step and the coating film heating step, and a thick film coating can be formed, but there is a problem that a uniform film thickness cannot be obtained. Non-uniformity in functional polycrystalline metal oxide thin films used in microscale or nanoscale electronic devices such as memories, shutters, switches, etc., where electrodes must be stacked on the polycrystalline metal oxide thin film The film thickness is a big problem from the viewpoint of the performance and reliability of the electronic device.
JP-A-5-257103 Japanese National Patent Publication No. 11-502262

  The present invention relates to a production method for producing a polycrystalline metal oxide thin film having a desired film thickness and a particularly uniform film thickness as a polycrystalline metal oxide thin film used for electronic devices and the like, and the polycrystalline metal oxide A thin film and a nonvolatile memory using the same are provided.

  The present invention includes a step of forming a coating film on a substrate with a dispersion in which fine powder particles of a metal oxide are dispersed in a sol-gel liquid in which an organic metal compound, a metal alkoxide, or an organic acid salt and an organic solvent are mixed. And a step of applying vibration to the coating film during the whole period or a part of the period until the solvent removal, chemical reaction, and crystallization of the coating film are completed. It is a manufacturing method of a crystalline metal oxide thin film.

  And the step of applying vibration to the coating film is the whole period or a part of the heating and drying process of the coating film, the whole period or a part of the baking process of the coating film, or the coating film It is the manufacturing method characterized by being the whole period of each heat drying process and baking process, or each partial period.

  In addition, the formation of the coating film on the substrate is a manufacturing method by a spin coating method or a dip coating method, and the vibration applied to the coating film is ultrasonic.

  Further, the organic solvent is a medium boiling point solvent or a high boiling point solvent, and as the organic solvent, isobutyl alcohol, isoamyl alcohol, tetrachloroethylene, methyl isobutyl ketone, diethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol It is at least one of monobutyl ether, ethylene glycol monobutyl ether acetate, and triethylene glycol monobutyl ether.

  The polycrystalline metal oxide thin film manufactured by the above manufacturing method has a flat thin film surface and a uniform film thickness.

  The thin film has metal oxide crystal grains composed of at least two or more groups having different average grain sizes of crystal grains, and the crystal grains of each group are made of the same element, and the average grains The average crystal grain size of the group with the largest diameter is more than twice the average crystal grain size of the group with the smallest average grain size, and the crystal grains of the group with the smallest average grain size are It is characterized in that the crystal grains of a group having a larger average particle diameter are cross-linked and bonded, or the crystal grains of the group having the smallest average particle diameter are a matrix.

  Further, the metal oxide is characterized by having a perovskite structure, an ilmenite structure, or a spinel structure, and the metal oxide having a perovskite structure is a ferroelectric material, a supergiant magnetoresistive (CMR) material, And high temperature superconducting (HTSC) materials, and any one of strontium barium titanate, strontium zirconate, praseodymium calcium manganate, barium calcium gadolinium cobaltate The metal oxide having the perovskite structure contains at least one or more additive elements of chromium, vanadium, scandium or other transition metals.

  On the other hand, a metal oxide having an ilmenite structure is a ferroelectric material, and the metal oxide contains at least one or more additive elements of magnesium, indium, scandium, zinc, copper, and iron. It contains lithium niobate or lithium tantalate.

  The concentration of the additive element is also more than 0 mol% and not more than 10 mol%.

  The metal oxide having a spinel structure is any one of magnesium titanate, chromium magnesium acid, aluminum magnesium oxide, and iron cobaltate.

  In the nonvolatile memory, the polycrystalline metal oxide thin film is formed between a pair of electrodes.

  According to the polycrystalline metal oxide thin film and the method for producing the same according to the present invention, in the process until the solvent removal of the coating film, chemical reaction, and crystallization are completed, that is, in the heating drying process and / or the baking process of the coating film. The simple method of applying vibration eliminates the aggregation of metal oxide fine powder particles, and produces a polycrystalline metal oxide thin film with a desired film thickness and a uniform film thickness. It has the effect of becoming. In addition, a microscale or nanoscale electronic device such as a non-volatile memory in which an electrode or the like is formed on the polycrystalline metal oxide thin film has an effect of high performance, low cost, and high reliability. .

  Hereinafter, a polycrystalline metal oxide thin film according to an embodiment of the present invention, a manufacturing method thereof, and a nonvolatile memory using the same will be described.

(Embodiment 1)
1 is a schematic cross-sectional view of a polycrystalline metal oxide thin film manufactured by the manufacturing method according to Embodiment 1. FIG. As a comparative example, a polycrystalline metal oxide thin film manufactured using a conventional technique will also be described.

  First, FIG. 4 is a schematic cross-sectional view of a polycrystalline metal oxide thin film manufactured by a conventional manufacturing method, in which a metal is contained in a sol-gel solution in which an organic metal compound, a metal alkoxide, or an organic acid salt and an organic solvent are mixed. A dispersion liquid in which fine oxide particles are dispersed is dropped to form a coating film on the substrate 41 by a spin coating method or a dip coating method, and then a polycrystalline metal oxide through a heating and drying step and a firing step. The thin film 42 is manufactured.

  The polycrystalline metal oxide thin film 42 is a metal comprising fine powder particles 43 of metal oxide and crystal grains 44 having a small crystal grain size formed by hydrolysis or the like from an organometallic compound, metal alkoxide, or organic acid salt. The metal oxide fine powder particles 43 agglomerate to form a protruding portion 45, and a polycrystalline metal oxide thin film 42 having a flat surface and a uniform film thickness is generated by the generation of the protruding portion 45 due to the aggregation. It was something I could not get.

  In contrast, in the present invention, a dispersion liquid in which fine powder particles of a metal oxide are dispersed in a sol-gel liquid in which an organic metal compound, a metal alkoxide, or an organic acid salt and an organic solvent are mixed is dropped onto the substrate 11. Then, a coating film is formed on the substrate 11 by a spin coating method or a dip coating method, and a process until solvent removal, chemical reaction, and crystallization are completed on this coating film, that is, a temperature of 100 to 300 ° C. of the coating film. When vibration was applied in the heating and drying step and / or the baking step at a temperature of 400 to 700 ° C., as shown in FIG. 1, a polycrystalline metal oxide thin film 12 having a flat thin film surface and a uniform film thickness was obtained. Obtained. Thus, the thin film surface becomes flat and the film thickness becomes uniform because the organic solvent is volatilized and the organic metal compound, metal alkoxide, or organic acid salt is converted into metal from the sol by a chemical reaction such as hydrolysis. In the process of forming oxide crystals, it is considered that the increase in the cohesive energy of fine powder particles of metal oxide is suppressed by application of vibration energy.

  The polycrystalline metal oxide thin film 12 produced by the production method of the present invention comprises a crystal grain 14 obtained from fine powder particles of a metal oxide dispersed in a coating liquid, an organometallic compound, and a metal alkoxide. Or crystal grains 13 formed from an organic acid salt by hydrolysis or the like.

  The polycrystalline metal oxide thin film 12 of the present invention is hydrolyzed from crystal grains 14 of a group having a large crystal grain size obtained from fine powder particles of metal oxide and an organic metal compound, metal alkoxide, or organic acid salt. It consists of a group of crystal grains 13 having a small crystal grain size formed by decomposition or the like. The crystal grains of both groups are composed of the same element, and the average grain size of the crystal grains 14 obtained from the fine powder particles of the metal oxide was formed from an organometallic compound, a metal alkoxide, or an organic acid salt. The crystal grains 13 in the group having a smaller average particle diameter than twice the average grain diameter of the crystal grains 13 are cross-bonded to each other.

(Embodiment 2)
FIG. 2 is a schematic cross-sectional view of a polycrystalline metal oxide thin film manufactured by the manufacturing method according to the second embodiment.

  The manufacturing method according to the second embodiment is the same method as the manufacturing method of the first embodiment described above, and the dispersion liquid in which the amount of fine metal oxide particles in the dispersion liquid is reduced is dispersed in the substrate 21. Dropping onto the film to form a coating film by spin coating or dip coating, and the process until solvent removal, chemical reaction, and crystallization are completed on this coating film, that is, the coating drying process as described above In the firing step, vibration is applied in the same manner as in the first embodiment to obtain a polycrystalline metal oxide thin film 22 having a flat thin film surface and a uniform film thickness.

  Similarly to the polycrystalline metal oxide thin film 12, this polycrystalline metal oxide thin film 22 is also composed of a crystal grain 24 of a large crystal grain size obtained from fine powder particles of a metal oxide, an organic metal compound, and a metal alkoxide. Or a crystal grain 23 of a small crystal grain size formed by hydrolysis or the like from an organic acid salt, and a crystal grain having an average grain size of two types. The crystal grains of both groups are composed of the same element, and the average grain size of the crystal grains 24 obtained from the fine powder particles of the metal oxide is formed from an organometallic compound, a metal alkoxide, or an organic acid salt. The average grain size of the crystal grains 23 is twice or more, and the crystal grains 23 serve as a matrix to form a metal structure in which the crystal grains 24 are dispersed.

  In any of the above embodiments, the metal oxide fine powder particles to be dispersed in the dispersion liquid are preferably primary fine particles, and the size of the metal oxide fine powder particles is the polycrystalline to be produced. It is appropriately selected according to the thickness of the metal oxide thin film. Further, the vibration applied to the coating film is given by electric means or mechanical means, which may be ultrasonic vibration, and the frequency and duration of the applied vibration are the polycrystalline metal oxide thin film to be manufactured. The material is appropriately selected depending on the materials of 12 and 22.

  The period during which the vibration is applied may be the whole period in each of the heat drying process and / or the baking process of the coating film, or may be the most part or part of the period in each process. What is necessary is just to select the appropriate period when a protrusion part does not generate | occur | produce by aggregation.

  In addition, in order to prevent the organic solvent in the sol-gel liquid from being vaporized before the polycrystalline metal oxide thin film is flattened during heating by applying vibration, as the organic solvent, Medium boiling solvent such as isobutyl alcohol, isoamyl alcohol, tetrachloroethylene, methyl isobutyl ketone having a boiling point of 100 to 150 ° C, or diethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, ethylene glycol mono High boiling solvents such as butyl ether acetate and triethylene glycol monobutyl ether are preferred.

  In addition, in the sol-gel liquid in which the organic metal compound, the metal alkoxide, or the organic acid salt and the organic solvent are mixed, the hydrolysis is coordinated with the catalyst or the metal atom in order to control the reaction rate and the reaction form. A chelating agent may be added.

  In the first and second embodiments, the case where the polycrystalline metal oxide thin film has a crystal grain size composed of crystal grains composed of two types of groups has been described. The grains may be dispersed with crystal grains composed of a group having two or more kinds of average grain sizes. That is, the polycrystalline metal oxide thin film may be composed of crystal grains composed of two or more groups having an average grain size of the same element. When the average particle size of the metal oxide fine powder particles is composed of two or more groups, the average crystal particle size of the group with the largest average particle size is an organometallic compound, metal alkoxide, or organic More than twice the average crystal grain size of the group with the smallest average grain size formed from the acid salt, and the smallest group with the average grain size formed from the organometallic compound, metal alkoxide, or organic acid salt The crystal grains cross-link and bond the crystal grains of a group having a larger average particle diameter than that, or the crystal grains of the group having the smallest average particle diameter serve as a matrix.

  In order for the polycrystalline metal oxide thin films 12 and 22 in the first and second embodiments to change the electric resistance value by electric means, the metal oxide has a perovskite structure, an ilmenite structure, or Those having a crystal structure such as a spinel structure are preferable, and in the case of having a perovskite structure, at least one of a ferroelectric material, a supergiant magnetoresistive (CMR) material, and a high temperature superconducting (HTSC) material. In particular, any one of strontium barium titanate, strontium zirconate, praseodymium calcium manganate, and barium calcium gadolinium cobaltate is preferable. Furthermore, the metal oxide may contain at least one or more additive elements of chromium, vanadium, scandium, or other transition metals.

  When the metal oxide has an ilmenite structure, it is preferably a ferroelectric material, and lithium niobate containing at least one additional element of magnesium, indium, scandium, zinc, copper, and iron. Or lithium tantalate is good.

  Further, when the metal oxide has a perovskite structure or an ilmenite structure, the concentration of the additive element is preferably more than 0 mol% and 10 mol% or less.

  Moreover, when it has a spinel type structure, it is preferably any of magnesium titanate, chromium magnesium oxide, aluminum magnesium acid, and iron cobaltate.

(Embodiment 3)
Next, a nonvolatile memory according to Embodiment 3 of the present invention will be described. FIG. 3 is a cross-sectional view showing a configuration of the nonvolatile memory 30 according to the third embodiment.

  The nonvolatile memory 30 includes a lower electrode 32 and an upper electrode 33 made of platinum, silver, iridium or the like formed on a substrate 31 made of silicon or silicon whose surface is covered with silicon oxide, and faces each other. A polycrystalline metal oxide thin film 34 whose electric resistance value changes is formed between the pair of electrodes by the electric means described in the first or second embodiment.

  The thicknesses of the lower electrode 32, the upper electrode 33 and the polycrystalline metal oxide thin film 34 are each preferably 10 nm to 1 μm, and the thickness of the silicon oxide is preferably 0.1 to 1 μm.

  In manufacturing the nonvolatile memory 30, after forming the lower electrode 32 on the substrate 31, the polycrystalline metal oxide thin film 34 is formed by the method according to the first or second embodiment, and the upper electrode is formed thereon. The electrode 33 may be formed.

  The polycrystalline metal oxide thin film 34 has an electric resistance value between the lower electrode 32 and the upper electrode 33 by applying a voltage pulse or current pulse, applying a DC voltage or DC current, applying an AC voltage or AC current, or the like. Change. Accordingly, by changing the electrical resistance value of the polycrystalline metal oxide thin film 34 by the electrical means, the high resistance state is related to the theoretical “0”, and the low resistance state is related to the theoretical “1”, thereby storing digital information. Can be done.

  In this way, by constructing a non-volatile memory with a polycrystalline metal oxide thin film with a flat and uniform film thickness without protrusions due to aggregation, high-performance and highly reliable microscale and nanoscale electrons You can get a device.

  When a nonvolatile memory is configured, it is preferable that an adhesion layer such as titanium or tantalum is formed between the substrate 31 and the lower electrode 32.

  The adhesion layer such as titanium or tantalum also has an effect of suppressing diffusion or chemical reaction between the substrate 31 and the polycrystalline metal oxide thin film 34.

  Since the present invention can be manufactured by a simple method in which a polycrystalline metal oxide thin film having a desired thickness and a uniform thickness is subjected to vibration, a non-volatile structure in which an electrode or the like is formed on the polycrystalline metal oxide thin film It is useful for microscale and nanoscale electronic devices such as memories.

Schematic cross-sectional view of a polycrystalline metal oxide thin film according to Embodiment 1 of the present invention Schematic cross-sectional view of a polycrystalline metal oxide thin film according to Embodiment 2 of the present invention Sectional drawing which shows the structure of the non-volatile memory which concerns on Embodiment 3 of this invention. Schematic cross-sectional view of a polycrystalline metal oxide thin film manufactured by a conventional manufacturing method

Explanation of symbols

11, 21, 31 Substrate 12, 22, 34 Polycrystalline metal oxide thin film 13, 14, 23, 24 Crystal grain 30 Non-volatile memory 32 Lower electrode 33 Upper electrode

Claims (23)

Forming a coating film on a substrate with a dispersion in which fine powder particles of a metal oxide are dispersed in a sol-gel liquid in which an organic metal compound, a metal alkoxide, or an organic acid salt and an organic solvent are mixed; A step of applying vibration to the coating film during the whole period or a part of the period until the solvent removal, chemical reaction and crystallization are completed. Thin film manufacturing method. The method for producing a polycrystalline metal oxide thin film according to claim 1, wherein the step of applying vibration to the coating film is the entire period or a part of the heating and drying process of the coating film. 2. The method for producing a polycrystalline metal oxide thin film according to claim 1, wherein the step of applying vibration to the coating film is the whole period or a part of the baking process of the coating film. 2. The method for producing a polycrystalline metal oxide thin film according to claim 1, wherein the step of applying vibration to the coating film is a whole period or a part of each period of the heating and drying process and the baking process of the coating film. Method. Formation of a coating film on a substrate of a dispersion liquid in which fine powder particles of metal oxide are dispersed in a sol-gel liquid in which an organic metal compound, a metal alkoxide, or an organic acid salt and an organic solvent are mixed is performed by a spin coating method or The method for producing a polycrystalline metal oxide thin film according to any one of claims 1 to 4, wherein the method is a dip coating method. The method for producing a polycrystalline metal oxide thin film according to any one of claims 1 to 5, wherein the vibration applied to the coating film is an ultrasonic wave. The method for producing a polycrystalline metal oxide thin film according to any one of claims 1 to 6, wherein the organic solvent is a medium boiling point solvent or a high boiling point solvent. The organic solvent is at least one of isobutyl alcohol, isoamyl alcohol, tetrachloroethylene, methyl isobutyl ketone, diethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, ethylene glycol monobutyl ether acetate, triethylene glycol monobutyl ether The method for producing a polycrystalline metal oxide thin film according to any one of claims 1 to 7, wherein: A polycrystalline metal oxide thin film produced by the production method according to claim 1. There are metal oxide crystal grains composed of at least two kinds of groups having different average grain sizes, and each group of crystal grains is composed of the same element, and the group having the largest average grain diameter The polycrystalline metal oxide thin film according to claim 9, wherein the average crystal grain size is at least twice the average crystal grain size of the group having the smallest average grain size. 11. The polycrystalline metal oxide thin film according to claim 10, wherein the crystal grains of the group having the smallest average particle diameter are crosslinked and bonded to the crystal grains of the group having a larger average particle diameter. The polycrystalline metal oxide thin film according to claim 10, wherein the crystal grains of the group having the smallest average particle diameter are a matrix. The polycrystalline metal oxide thin film according to any one of claims 9 to 12, wherein the metal oxide has a perovskite structure. The polycrystalline metal oxide thin film according to any one of claims 9 to 12, wherein the metal oxide has an ilmenite structure. The polycrystalline metal oxide thin film according to claim 9, wherein the metal oxide has a spinel structure. The metal oxide having a perovskite structure is any one of a ferroelectric material, a super giant magnetoresistance (CMR) material, and a high temperature superconducting (HTSC) material. Polycrystalline metal oxide thin film. 17. The metal oxide having a perovskite structure is any one of strontium barium titanate, strontium zirconate, praseodymium calcium manganate, and barium calcium gadolinium cobaltate. A polycrystalline metal oxide thin film according to claim 1. 18. The polycrystalline metal oxide thin film according to claim 17, wherein the metal oxide having a perovskite structure contains at least one or more additive elements of chromium, vanadium, scandium or other transition metals. The polycrystalline metal oxide thin film according to claim 14, wherein the metal oxide having an ilmenite type structure is a ferroelectric material. 15. The metal oxide having an ilmenite structure is lithium niobate or lithium tantalate containing at least one additional element of magnesium, indium, scandium, zinc, copper, and iron. The polycrystalline metal oxide thin film according to claim 19. The polycrystalline metal oxide thin film according to claim 18 or 20, wherein the concentration of the additive element is more than 0 mol% and not more than 10 mol%. The polycrystalline metal oxide thin film according to claim 15, wherein the metal oxide having a spinel structure is any one of magnesium titanate, chromium magnesium acid, aluminum magnesium acid, and iron cobaltate. A non-volatile memory, wherein the polycrystalline metal oxide thin film according to any one of claims 9 to 22 is formed between a pair of electrodes.

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