JP3845721B2 - Method for producing composite oxide nanocrystallite thin film by laser ablation - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a composite oxide thin film comprising nanocrystals on the surface of various substrate materials such as glass, metal, ceramics, and polymers.
[0002]
[Prior art]
An oxide having a perovskite structure represented by the chemical composition of ABO ₃ (A: rare earth element or alkaline earth metal element, B: transition metal element) has unique functionality. For example, LaFeO ₃ is a functional material that is actually applied to sensors for environmental measurement and automobile waste gas detection as a highly sensitive gas sensor material. On the other hand, BaTiO ₃ is a promising electronic / optical material applied to a ferroelectric memory because it has ferroelectric characteristics, or to an optical waveguide because it has a low refractive index. This oxide microcrystal has not yet been obtained by a satisfactory production method.
In order to improve the functional properties of perovskite oxides and to make them electronic devices, it is very important to make thin films. Since the functionality of an oxide having a perovskite structure is mainly due to its unique crystal structure, it is very necessary to prepare a thin film having crystallinity. In addition, it is considered that the characteristics of various functional materials obtained by making the film thin can be further improved by setting the size of the constituted crystal to the nanometer order.
Incidentally, in the manufacturing method of a thin film of oxide, placed the material to be formed on the substrate and on the substrate in a vacuum chamber, have it been conducted to carry out the vacuum deposition on a substrate is evaporated by irradiating a laser over light material (JP-A-62-222058).
In irradiation of the laser over the laser deposition method, in particular, a laser ablation method is used. This is because a target material is evaporated by irradiating a target with a laser pulse having a large laser density at a constant period, and a thin film having a composition close to the target composition can be formed on the substrate.
The characteristics of various functional materials obtained by thinning by this manufacturing method can be further improved.
In this manufacturing method, a target having the same composition or the same composition as the metal ratio of the ceramic compound thin film is irradiated with a high energy laser beam. In this case as well, a laser deposition method, particularly a laser ablation method is used. This is because the target material is evaporated by irradiating the target with a laser pulse having a large laser density, and a thin film having a composition close to the target composition can be formed on the substrate. And in such a method, when forming a ceramic compound thin film on a board | substrate, vapor deposition atmosphere gas is sprayed on the surface of a target (Unexamined-Japanese-Patent No. 5-270830).
Regarding the production of a single crystal thin film or a superconducting thin film, it is preferable that the target main surface has an acute angle at a position separated from the base so that the main surface of the target forms an acute angle with the main surface of the base. (Japanese Patent Laid-Open Nos. 2001-31497 and 2001-31492).
Also known is a method of forming a thin film on a substrate by irradiating the target material with beam light while adjusting the pressure of the atmospheric gas and the distance between the substrate and the target material (Japanese Patent Laid-Open No. 2000-144387). issue).
Conventionally, thin films have been formed by sputtering or laser ablation as a sol-gel method or vapor phase method, but in order to further crystallize this, it is necessary to perform heat treatment at a high temperature or heat the substrate during preparation of the thin film. It has been considered essential.
In particular, the oxide thin film production method having a perovskite structure by laser ablation is usually produced by heating a substrate from about 300 ° C. to about 1000 ° C. in an oxygen atmosphere of about several Pa to several tens of Pa (for example, “Laser Ablation and its Application”, p.248-263), published by Corona, edited by the Electrochemical Society Laser Ablation and its Industrial Application Research Special Committee, is essential for thin film crystallization. has been considered the case, crystals and perform such heating its size increases in growth, was not necessarily desirable.
[0003]
[Problems to be solved by the invention]
An object of the present invention is to provide a method of forming nanometer-order microcrystals of an oxide thin film having a perovskite structure on a substrate surface without particularly heating the substrate. And the manufacturing method consists of a very simple combination of steps, and the target product can be obtained in high yield.
[0004]
[Means for Solving the Problems]
The present inventors diligently investigated the above problems and found the following matters.
Laser ablation in the presence of argon gas in a target having the same composition ratio as the oxide having the perovskite structure (A: rare earth element or alkaline earth metal element, B: transition metal element) represented by the chemical composition of ABO ₃ It has been found that an oxide microcrystal having a perovskite structure can be manufactured on a substrate which is irradiated with a laser beam by a method and is perpendicular to the target surface and has an off-axis arrangement. In particular, it is important to perform under a pressure condition of 50 Pa to 200 Pa, and to use a substrate that is perpendicular to the target surface and has an off-axis arrangement, and these conditions. I found it important to combine.
[0005]
According to the present invention, the following method is provided.
(1) Argon gas is present in a target made of an oxide having a perovskite structure represented by the chemical composition of ABO ₃ (A: rare earth element or alkaline earth metal element, B: transition metal element), and 50 Pa to 200 Pa. A laser ablation method is used to irradiate a laser beam under a pressure condition of 5 to produce an oxide microcrystalline thin film having a perovskite structure on a substrate that is perpendicular to the target surface and has an off-axis arrangement. A method for producing a microcrystalline thin film made of an oxide having a perovskite structure.
(2) The method for producing a microcrystalline thin film comprising an oxide having a perovskite structure according to (1), wherein the oxide having a perovskite structure is LaFeO ₃ .
(3) The method for producing a microcrystalline thin film comprising an oxide having a perovskite structure according to (1), wherein the oxide having a perovskite structure is BaTiO ₃ .
[0006]
Delete [0007]
DETAILED DESCRIPTION OF THE INVENTION
The target product of the present invention is an oxide having a perovskite structure represented by the following chemical composition of ABO ₃ (A: rare earth element or alkaline earth metal element, B: transition metal element). These rare earth elements, alkaline earth metal elements, and transition metal elements indicate that elements belonging to the periodic table can be used. Among these, LaFeO _{3 ,} BaTiO _{3 ,} which are currently attracting attention in various fields, are shown. And so on.
[0008]
The raw material of the present invention targets LaFeO _{3 and} BaTiO ₃ having the same composition as the target substance.
Generally, a LaFeO ₃ target is produced by producing a powder by an ordinary method, molding and firing the powder. Specifically, a LaFeO ₃ powder is used as a raw material, which is uniaxially molded and sintered at 1000 ° C. is used.
Similarly, for the BaTiO ₃ target, generally, a sintered body obtained by producing a powder by an ordinary method, molding and sintering the powder is used. For example, a target manufactured by Furuuchi Chemical Co., Ltd. produced by a normal solid phase reaction method can be used as a raw material.
As the target oxide, a metal-doped oxide and a substituted perovskite oxide can also be used.
[0009]
In the present invention, the target is irradiated with laser light by a laser ablation method, and an oxide having a perovskite structure is accumulated on the substrate.
The crystal structure of the obtained thin film is identified by thin film X-ray diffraction (X-ray diffractometer RAD-C manufactured by Rigaku Corporation). In addition, the size of the crystallite constituting the thin film shows the result estimated using the Scherrer equation. The stoichiometric ratio of metal atoms in the thin film was determined by X-ray photoelectron spectroscopy (PHI X-ray photoelectron spectrometer 5600ci) analysis.
[0010]
In the present invention, the reactor shown in FIG. 1 is used. The material 4 constituting the target is placed on a table placed on the rotating shaft 6 (FIG. 2).
Laser over light emanating from the laser over generator 11, through the window 10, poured into the reaction chamber 2 through the condenser lens, and is irradiated onto the target. To perform laser over ablation, an excimer laser over is preferably used. The excimer laser over, KrF excimer laser over (wavelength 248 nm) and ArF excimer laser over (wavelength 193 nm) is used.
Laser over ablation, high energy density by irradiating (pulse energy 1.01J / cm ² of about or more) laser light to the target material, in which occupies to perform the melting and desorption of the target materials.
In the present invention, an ArF excimer laser is used, and the result is that the repetition frequency is normally about 10 Hz, the pulse width is 17 ns, and the number of pulses is about 2,000. About these conditions, it can change suitably.
[0011]
In the reaction chamber, the inside is evacuated with an ultra-high vacuum exhaust system, Ar gas is introduced, and the pressure of the rare gas can be changed in a wide range between 1 Pa and 200 Pa. The pressure is kept constant within this range.
[0012]
Substrate placement is very important for obtaining nanocrystalline thin films. In the present invention, an off-axis arrangement is adopted in which the substrate is vertically set with respect to the target surface. By taking this position, a thin film microcrystal can be obtained.
Entire substrate needs to enter within a distance of 50mm from the irradiation spot position of the laser over that on the target. By adopting such a substrate arrangement, the uniformity of the size of the precipitated particles can be improved and the deposition efficiency of the thin film can be increased.
Further, the uniformity of the film thickness can be greatly improved by rotating the substrate. The rotation speed of the substrate is in the range of 10 to 100 rpm. When the rotation speed is outside this range, resulting that film may become not uniform.
[0013]
Experiments confirmed that the morphology of nanoparticles obtained by laser ablation strongly depends on the atmospheric pressure.
LaFeO ₃ target, a raw material powder comprising LaFeO ₃ powder, 20mm diameter at a pressure of 140 MPa, and uniaxially molding the 5mm thick pellets are those obtained by sintered for 3 hours at 1000 ° C. in air. This was of the composition described above. And the composition obtained in the condition to the target, were laser ablation under the following pressure conditions.
In the pressure range of 1-10 Pa, almost no crystallized fine particles were observed and the thin film was amorphous. Next, diffraction peaks based on LaFeO ₃ nanocrystallites were observed in the range of 50 to 200 Pa.
When the crystal sizes of the thin films prepared at 50 Pa and 100 Pa were estimated , it was clarified that films composed of nanocrystallites having a size of 6.8 nm and 7.2 nm , respectively, of 10 nm or less were formed.
In addition, at a pressure of 50 Pa or higher, a tendency for the crystal size to increase with increasing pressure was also observed.
The above results are shown in FIG . FIG. 3 is an X- ray diffractogram of LaFeO ₃ thin films deposited under 200 mJ / pulse conditions under different argon pressures .
It has been shown that the results of X-ray diffraction of LaFeO ₃ thin films obtained by changing the pressure of argon change .
From the above results , it can be said that the pressure range of 50 Pa to 200 Pa is a preferable range.
[0014]
FIG. 4 is a graph showing the relationship between the La / Fe atomic ratio in the thin film and the argon pressure for the LaFeO ₃ thin film deposited under 200 mJ / pulse condition under different argon pressures .
The amorphous sample prepared at 1 Pa had a La / Fe atomic ratio of about 1.6, which was greater than the stoichiometric ratio 1 of LaFeO ₃ perovskite.
Furthermore, as the pressure increased, the La / Fe atomic ratio in the thin film became about 1 as shown in the figure.
Therefore, it was clarified that the thin film prepared at a pressure of 50 Pa to 200 Pa was a LaFeO ₃ perovskite nano-microcrystalline thin film composed of crystals having a size of 10 nm or less.
From the above results, it can be said that the LaFeO ₃ perovskite nano-microcrystalline thin film production method is effective to be performed under a pressure condition of 50 Pa to 200 Pa.
[0015]
Next, the results for the BaTiO ₃ perovskite thin film will be described.
A target made by Furuuchi Chemical Co., Ltd. (purity 99.9%, density 55%) made by pressing BaTiO ₃ powder at a pressure of 150 to 200 MPa and sintering in air at 1100 ° C for 2 hours ) Was used.
BaTiO ₃ perovskite thin films were prepared under various pressure conditions.
The crystal structure of the obtained thin film is the one identified by thin film X-ray diffraction (X-ray diffractometer RAD-C manufactured by Rigaku Corporation).
Although a continuous film was obtained at a pressure of 1 Pa, the diffraction peak based on the BaTiO ₃ perovskite was very small, which probably contained almost no nanocrystallites in the film, or the deposited film was very thin. It was thought to be a membrane.
Next, in the pressure range of 10 Pa or more, many diffraction peaks were observed, and a crystalline thin film was deposited. In particular, the the BaTiO ₃ perovskite crystallite size was estimated in the thin film prepared in 10Pa is 7.2 nm, the size revealed that following BaTiO ₃ perovskite nanocrystals 10nm is formed in the thin film.
In the pressure range of 50-200 Pa, the crystallite size of BaTiO ₃ perovskite tended to increase with increasing pressure. However, in these pressure ranges, a BaCO ₃ diffraction peak was simultaneously observed in addition to the BaTiO _{3 in} the thin film, even though the Ba / Ti atomic ratio in the thin film determined by X-ray photoelectron spectroscopy was 1. Since there is no carbon component in the target, BaCO ₃ was formed by the reaction of carbon dioxide in the air and the Ba component in the perovskite in the thin film when the sample was removed from the reaction chamber after film formation. It can be said.
From the above results, it can be said that it is important that the BaTiO ₃ perovskite thin film is produced in a pressure range of 50 to 200 Pa.
[0016]
【The invention's effect】
According to the present invention, nanometer-order microcrystals of an oxide thin film having a perovskite structure can be formed on a substrate surface without particularly heating the substrate. And the manufacturing method consists of a very simple combination of steps, and the target product can be obtained in high yield.
[Brief description of the drawings]
FIG. 1 is a diagram showing an apparatus used in the present invention. FIG. 2 is a diagram showing a relationship between a target and a substrate of the present invention. FIG. 3 is an X- ray diffraction diagram of a LaFeO ₃ thin film when the argon pressure is changed . 4] A diagram showing the relationship between the La / Fe atomic ratio in the thin film and the argon pressure Explanation of code 1 Production apparatus 2 Reaction chamber 3 Base material 4 Target material 5 Unit 6 Axis 7 Motor 8 Vacuum pump 9 Pipe 10 Window 11 Laser 12 Lens 13 Laser light

Claims (3)

A pressure condition of 50 Pa to 200 Pa in the presence of argon gas in a target made of an oxide having a perovskite structure represented by the chemical composition of ABO ₃ (A: rare earth element or alkaline earth metal element, B: transition metal element) Then, a laser ablation method is used to produce a microcrystalline thin film of an oxide having a perovskite structure on a substrate that is perpendicular to the target surface and has an off-axis arrangement. A method for producing a microcrystalline thin film comprising an oxide having a perovskite structure. 2. The method for producing a microcrystalline thin film comprising an oxide having a perovskite structure according to claim 1, wherein the oxide having a perovskite structure is LaFeO ₃ . 2. The method for producing a microcrystalline thin film comprising an oxide having a perovskite structure according to claim 1, wherein the oxide having a perovskite structure is BaTiO ₃ .

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