RU2790266C1 - Method for producing ferrite films - Google Patents

Abstract

FIELD: synthesis technology.

SUBSTANCE: invention relates to a technology for the synthesis of anisotropic (with an easy magnetization axis directed perpendicular to the film plane) BaFe₁₂O₁₉ films by vapor phase deposition. Such a material can be used in the development of planar nonreciprocal microwave devices with a self-bias effect, in spintronic devices as a magnetic dielectric. The method for obtaining ferrite films includes the production of a target, treatment of a single-crystal substrate with argon ions, sputtering of the target onto a heated substrate, supply of a controlled flow of oxygen ions to the substrate area, further crystallization annealing of the film, while a target of the same composition, a single-crystal substrate, is used to obtain a film of barium hexaferrite BaFe₁₂O₁₉ Al₂O₃ crystallographic orientation (001), the substrate is heated to a temperature of 300-350°C during the deposition process, after deposition of a 70-90 nm film on the substrate, the deposition process is interrupted, after which the film is held for 5 minutes at the substrate temperature, then the deposition is repeated many times layers of 70-90 nm and exposure at a temperature of 300-350°C for 5 min until the required film thickness, and crystallization annealing is carried out in air at a temperature of 800-900°C for 1-3 hours.

EFFECT: obtaining high-quality thick (up to ~1 μm and more) anisotropic films of hexagonal ferrites BaFe₁₂O₁₉ with the direction of the magnetic moment perpendicular to the plane of the film.

1 cl, 3 dwg, 1 ex

Description

The invention relates to microwave microelectronics, and more specifically to technologies for the synthesis of anisotropic (with easy magnetization axis directed perpendicular to the surface) BaFe ₁₂ O ₁₉ films by vapor deposition. Such films can be used in the development of planar nonreciprocal microwave devices with a self-bias effect and in spintronic devices as a magnetic dielectric.

The problem of fabricating films of hexagonal barium ferrite with a high degree of crystallographic texture and, accordingly, anisotropy of magnetic properties is very nontrivial.

The following methods for producing ferrite films are known: “Method for producing nanosized films of ferrite garnet containing Bi” (see Ukrainian patent No. 66219. Prokopov A.R., Shaposhnikov A.N., Karavaynikov A.V., Bull. No. 24, 2011) and “Method for obtaining nanoscale films of Bi-containing ferite-garnets” (RF patent No. 2532185. Kostishin V.G., Chitanov D.N., Komlev A.S., Yudanov N.A., Trukhan V. M., Shelkovaya T.V.). However, these methods have the following disadvantages:

1) thick films of ferrites cannot be obtained by these methods;

2) films of hexagonal ferrites cannot be obtained by these methods.

Closest to the proposed one is the "Method for obtaining nanosized ferrite films" (see RF patent No. 2532187. Kostishin V.G., Panina L.V., Morchenko A.T., Chitanov D.N., Yudanov N.A. and Adamtsov A.Yu.; bulletin No. 30, 27.10.2014). The method includes manufacturing a target of a given composition, treating a single-crystal strontium titanate substrate with argon ions, sputtering the target onto a substrate heated to a temperature of 700-750°C, and annealing the resulting film in an oxygen atmosphere for 1.0 hour at a temperature of 500-550°C at normal atmospheric pressure.

The present method has the following disadvantages:

1) films of hexagonal ferrites cannot be obtained by this method;

2) BaFe ₁₂ O ₁₉ hexaferrite films with an easy magnetization axis perpendicular to the film surface cannot be obtained by this method;

3) this method cannot be used to obtain thick (thickness ~1 μm or more) BaFe ₁₂ O ₁₉ hexaferrite films with an easy magnetization axis perpendicular to the film surface.

The purpose of the present invention is to develop a method for producing thick (thickness ~1 μm or more) anisotropic films of hexagonal ferrites BaFe ₁₂ O ₁₉ with the direction of the magnetic moment perpendicular to the plane of the film.

This goal is achieved by the fact that in the proposed method for producing ferrite films, including the manufacture of a BaFe ₁₂ O ₁₉ hexaferrite target, the use of a single-crystal Al ₂ O ₃ substrate of crystallographic orientation (001), treatment of the substrate with argon ions, sputtering of the target onto a heated substrate with further crystallization annealing of the film the film deposition process carried out at relatively low substrate temperatures (about 300°C) is interrupted periodically.

In principle, obtaining an anisotropic hexaferrite film using the proposed technology is possible by any method of ion deposition. The following can be cited as general requirements for a particular method: 1) the use of conditions (chemical composition of the target, residual pressure in the chamber, operating pressure in the chamber, etc.) that ensure the formation of a film on the substrate that is as close as possible in composition to stoichiometric BaFe ₁₂ O ₁₉ ; 2) use as a substrate of a single-crystal plate Al ₂ O ₃ crystallographic orientation (001). After fulfilling these basic conditions, it is possible to use the directly proposed method, which consists in the following: after heating the substrate to 300–350°C, a 70–90 nm film of barium hexaferrite is deposited. After reaching the specified thickness, the deposition process is interrupted, and the film is kept for 5 minutes at the same temperature of 300-350°C. The film of the required thickness is obtained after repeated repetition of the previous operations: deposition of 70-90 nm and exposure at 300-350°C for 5 minutes. After that, it is necessary to carry out crystallization annealing of the film in air at 800–900°C for 1–3 h.

The essence of the proposed technical solution is as follows.

When heated to 300°C, hematite Fe ₂ O ₃ begins to crystallize from the amorphous Ba-Fe-O system. Hematite itself is characterized by surface crystallization, and, moreover, it spontaneously forms with the (001) texture. The presence of the (001) crystallographic orientation of the Al ₂ O ₃ single crystal substrate will only enhance the formation of the (001) texture. Providing a 5-minute interval is necessary so that the crystallites have time to form, because otherwise the incident particles (atoms, clusters of atoms, reflected ions, etc.) will destroy the bonds formed and limit crystallization. When an additional film layer is deposited on its surface, the formation of hematite (001) nuclei also occurs. At the end of the deposition process, the film will consist of an amorphous matrix and inclusions of hematite (001) crystallites throughout its volume, which will serve as a “template” for the growth of hexaferrite (001) during crystallization annealing. Deviations of such parameters as the thickness of one layer, exposure time and substrate temperature from the recommended ones are undesirable, since they can provoke an increase in the probability of formation of non-oriented crystallites, which will lead to a deterioration in the degree of texture of the final film.

The method is implemented as follows. A BaFe ₁₂ O ₁₉ hexaferrite target is fabricated using the ceramic technology method. Single-crystal Al ₂ O ₃ substrate is treated in a vacuum chamber with argon ions 10-20 eV. A single-crystal substrate of crystallographic orientation (001) is placed in a substrate holder with a heater. In the vacuum chamber create a working pressure (Ar gas) (0.5-0.7) mTorr. Using a heater, the substrate is heated to a temperature of (300-350)°C. Next, the target material is deposited on the substrate by sputtering the target with an argon beam using an ion current source. In order to facilitate the crystallization of stoichiometric BaFe ₁₂ O ₁₉ hexaferrite, a controlled flow of oxygen ions is supplied to the substrate region using an ion source. After deposition of a 70–90 nm film of barium hexaferrite onto a substrate, the deposition process is terminated and the film is held for 5 min at a substrate temperature of 300–350°C. The film of the required thickness is obtained after repeated repetition of the previous operations: deposition of 70-90 nm and exposure at 300-350°C for 5 minutes. After that, it is necessary to carry out crystallization annealing of the film in air at 800-900°C for 1-3 hours.

In FIG. 1 shows the X-ray diffraction pattern of a film obtained by the proposed technology on a single-crystal Al ₂ O ₃ (001) substrate. As can be seen, the hexaferrite reflections are represented only by the (001) family.

The proposed technical solution has the following distinctive features:

1. Single-crystal Al ₂ O ₃ substrate of crystallographic orientation (001) is used;

2. During the deposition process, the substrate is heated to a temperature of 300-350°C;

3. The film deposition process is interrupted every 70-90 nm, after which the film is held for 5 minutes at the substrate temperature;

4. The number of deposited layers of 70-90 nm is determined by the required film thickness;

5. Crystallization annealing is carried out in air at a temperature of 800-900°C for 1-3 hours.

The use of these features to achieve this goal is unknown to the authors.

Example.

Using the proposed technology, films of barium hexaferrite were obtained on an Al ₂ O ₃ (001) substrate. Ion-beam sputtering was used as a synthesis method. The target was a disk of stoichiometric BaFe ₁₂ O ₁₉ obtained by standard ceramic technology. The distance from the substrate to the target was 37 mm, the residual pressure was 0.08 mTorr, the operating pressure (Ar gas) was 0.6 mTorr, the ion source voltage was 1.4 kV, the discharge current was 30 mA, the compensator cathode current was 2 .8 A, deposition rate - 20 nm/min. The thickness of one layer was 80 nm, the number of layers was 13 (total thickness ~1 μm), the substrate temperature was 300–320°C, and the exposure time between deposition of each layer was 5 min. Crystallization annealing was carried out in a muffle furnace in air at a temperature of 900°C for 2 h (heating rate 300°C/h, natural cooling).

In FIG. 1 is an X-ray diffraction pattern of the resulting film showing the presence of a high degree of (001) type hexaferrite texture. In FIG. 2 shows a snapshot of the surface of the film obtained by the proposed technology, in Fig. 3 is a snapshot of a sample obtained without interrupting the deposition process and with a thickness of only 600 nm. As can be seen from FIG. 3, the absence of interruptions in the deposition process led to the formation of differently oriented grains in the film. At the same time, the surface of the film obtained by the proposed technology consists of intergrown flat hexagonal grains, confirming their (001) orientation.

Claims (1)

A method for producing ferrite films, including the manufacture of a target, treatment of a single-crystal substrate with argon ions, sputtering of a target onto a heated substrate, supply of a controlled flow of oxygen ions to the substrate region, further crystallization annealing of the film, characterized in that a target is used to obtain a film of barium hexaferrite BaFe ₁₂ O ₁₉ of the same composition, a single-crystal Al ₂ O ₃ substrate of crystallographic orientation (001), the substrate during deposition is heated to a temperature of 300-350°C, after deposition of a 70-90 nm film on the substrate, the deposition process is interrupted, after which the film is held for 5 min at the substrate temperature, then repeatedly repeat the deposition of layers of 70-90 nm and holding at a temperature of 300-350°C for 5 minutes until the required film thickness, and crystallization annealing is carried out in air at a temperature of 800-900°C for 1-3 h.

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