CoFeB alloy sputtering target

Abstract

To provide a sputtering target that is produced efficiently, has excellent toughness, and can contribute to the magnetic properties of a thin film.SOLUTION: The material of a sputtering target is an alloy containing 50 at% or more and 70 at% or less of B. The alloy further contains one or two elements selected from the group consisting of Co and Fe. The alloy has a metallographic structure having a crystal grain size of 30 μm or less. The metallographic structure includes a matrix which is a (CoFe)B phase, and a plurality of B phases dispersed in the matrix. The circle equivalent diameter of each B phase is 30 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This specification discloses a CoFeB alloy sputtering target. This specification also discloses alloys suitable for this target. [Background technology]

[0002] Tunnel magnetoresistive films are used in magnetic heads, magnetic random access memories (MRAMs), and the like. These tunnel magnetoresistive films have a high tunnel magnetoresistive (TMR) signal and a low switching current density (Jc). A typical tunnel magnetoresistive film includes two magnetic thin films and a shielding layer sandwiched between them. A typical material for the shielding layer is MgO. Meanwhile, a typical material for the thin films is an alloy containing a large amount of B. This alloy further contains Fe or Co. Because the alloy contains a sufficient amount of B, a high tunnel magnetoresistive signal can be achieved in the tunnel magnetoresistive film. International Publication WO2011 / 070860 and Japanese Patent Application Laid-Open No. 2020-132995 disclose alloys suitable for magnetic thin films. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication WO2011 / 070860 [Patent Document 2] Patent Publication No. 2020-132995 Summary of the Invention [Problem to be solved by the invention]

[0004] Magnetic thin films can be obtained by sputtering, in which positive ions in a plasma bombard a target, causing atoms to be ejected from the target and then deposited on a substrate to form a thin film.

[0005] As mentioned above, B can contribute to a high tunnel magnetoresistance signal in thin films. However, B can also impair the toughness of the target. Targets containing a large amount of B tend to crack during use. Furthermore, during the manufacturing of such targets, defects such as cracks in the compact can occur. These defects impair the target yield.

[0006] The applicant's intention is to provide a sputtering target that can be efficiently manufactured, has excellent toughness, and can contribute to the magnetic properties of thin films. [Means for solving the problem]

[0007] The alloy disclosed in this specification, which is suitable for sputtering targets, contains 50 at% or more and 70 at% or less of B. This alloy further contains Co or Fe. This alloy has a metal structure with a crystal grain size of 30 μm or less. This metal structure includes a (CoFe)B phase as a matrix and numerous B phases dispersed in this matrix. The equivalent circle diameter of each B phase is 30 μm or less.

[0008] Preferably, the total content of Co and Fe in this alloy is 30 at % or more and 50 at % or less.

[0009] This alloy may further contain O. The O content (ppm by mass) is equal to or smaller than the value V1 calculated by the following formula: V1 = -10 × B% + 900 In this formula, B% represents the content of B (at%).

[0010] This specification also discloses a sputtering target. The material of this target is an alloy containing 50 at% to 70 at% B. This alloy further contains Co or Fe. This alloy has a metal structure with a crystal grain size of 30 μm or less. This metal structure includes a (CoFe)B phase as a matrix and numerous B phases dispersed in this matrix. The circle equivalent diameter of each B phase is 30 μm or less.

[0011] The present specification also discloses a method for producing a magnetic thin film, which comprises the steps of: (1) A step of preparing a target whose material is an alloy containing 50 at% or more and 70 at% or less of B, which further contains Co or Fe, and which has a metallographic structure with a crystal grain size of 30 μm or less, and which includes a (CoFe)B phase as a matrix and a large number of B phases dispersed in the matrix, and each B phase has an equivalent circle diameter of 30 μm or less; and (2) subjecting the target to sputtering Includes. [Effects of the Invention]

[0012] This sputtering target contains sufficient B, so that a thin film with excellent magnetic properties can be obtained from this target. Despite its high B content, this target has excellent toughness. Therefore, this target is less likely to crack during sputtering. Furthermore, defects are less likely to occur during the production of this target.

[0013] This sputtering target contains sufficient B, so that a thin film with excellent magnetic properties can be obtained from this target. Despite its high B content, this target has excellent toughness. Therefore, this target is less likely to crack during sputtering. Furthermore, defects are less likely to occur during the production of this target. DETAILED DESCRIPTION OF THE INVENTION

[0014] Preferred embodiments are described in detail below.

[0015] [target] The material of the sputtering target according to this embodiment is an alloy containing B as a major component. This alloy further contains Co or Fe. Preferably, the balance is unavoidable impurities. The alloy may have a composition containing Co but not Fe, a composition containing Fe but not Co, or a composition containing Co and Fe. In other words, this alloy is a Co-B alloy, an Fe-B alloy, or a Co-Fe-B alloy.

[0016] [Boron (B)] This target contains a large amount of B. A thin film containing a large amount of B can be formed from this target. B can contribute to the magnetism of this thin film. In a tunnel magnetoresistive film having this thin film, B can contribute to a high tunnel magnetoresistive signal. From this viewpoint, the B content in the alloy is preferably 50 at% or more, more preferably 53 at% or more, and particularly preferably 55 at% or more. Excess B promotes the precipitation of excessive B phase or the precipitation of matrix B phase. These B phases inhibit the toughness of the target. From the viewpoint of toughness, the B content is preferably 70 at% or less, more preferably 67 at% or less, and particularly preferably 65 at% or less.

[0017] [Cobalt (Co) and Iron (Fe)] A thin film containing Co or Fe can be formed from a target containing Co or Fe. Co and Fe can contribute to the magnetism of this thin film. From this viewpoint, the total content of Co and Fe in the alloy is preferably 30 at% or more, more preferably 33 at% or more, and particularly preferably 35 at% or more. From the viewpoint that the alloy can contain sufficient B, the total content of Co and Fe is preferably 50 at% or less, more preferably 47 at% or less, and particularly preferably 45 at% or less.

[0018] [Metal structure] The metal structure of this target is a polycrystalline structure, in other words, the metal structure has a large number of crystal grains.

[0019] The crystal grain size Dg in this metal structure is 30 μm or less. In other words, this metal structure is fine. This target has excellent toughness despite containing a B phase. From the viewpoint of toughness, the crystal grain size Dg is more preferably 22 μm or less, and particularly preferably 17 μm or less. The smaller the crystal grain size Dg, the better. The crystal grain size Dg can be calculated statistically based on observation of the metal structure with an optical microscope. The measurement conditions will be described later.

[0020] [(CoFe)B phase] The (CoFe)B phase contains both or either Co and Fe. The molar ratio of Co atoms to Fe atoms in the (CoFe)B phase is 0:1 or more and 1:0 or less. In other words, the matrix composition is a CoB phase, an FeB phase, or a CoFeB phase. In the CoB phase, Co atoms and B atoms are bonded together, and the molar ratio of Co atoms to B atoms is 1:1. In the FeB phase, Fe atoms and B atoms are bonded together, and the molar ratio of Fe atoms to B atoms is 1:1. In the CoFeB phase, Co atoms or Fe atoms are bonded together with B atoms, and the molar ratio of the sum of Co atoms and Fe atoms to B atoms is 1:1.

[0021] [B phase] As described above, the B phase is dispersed in the matrix. The circle-equivalent diameter Dc of each B phase is 30 μm or less. In other words, the maximum value Dmax of these circle-equivalent diameters Dc is 30 μm or less. These B phases do not significantly impair the toughness of the target. This target is less likely to crack during sputtering. During the production of this target, defects such as cracks in the compact are less likely to occur. This target has excellent production yield. From these viewpoints, the maximum value Dmax is more preferably 20 μm or less, and particularly preferably 15 μm or less. The smaller the maximum value Dmax, the better. The maximum value Dmax can be calculated statistically based on observation of the metallographic structure with a scanning electron microscope. The measurement conditions will be described later.

[0022] Oxygen The alloy may contain O as an impurity. O may combine with B to form a B2O3 phase. The melting point of the B2O3 phase is low. A target containing this B2O3 phase has poor toughness. Since the target according to this embodiment contains a large amount of B, the B2O3 phase is likely to form in this target. From the viewpoint of suppressing the B2O3 phase, it is important to suppress the O content. From the viewpoint of toughness, it is preferable that the O content (ppm by mass) is equal to or smaller than the value V1 calculated by the following formula: V1 = -10 × B% + 900 In this formula, B% represents the content of B (at%).

[0023] It is more preferable that the O content (ppm by mass) is equal to or smaller than the value V2 calculated by the following formula. V2 = -10 × B% + 800

[0024] It is particularly preferable that the O content (ppm by mass) is equal to or smaller than the value V3 calculated by the following formula. V3 = -10 × B% + 750

[0025] The O content is measured by gas analysis using non-dispersive infrared absorption spectroscopy after fusion in an inert gas. The size of the test piece used for this analysis is 3 mm x 3 mm x 30 mm. This size is cut out from the target by wire cutting.

[0026] [Target manufacturing method] In manufacturing a target, first, a powder is prepared. This powder can be manufactured by atomization, pulverization, or the like. Examples of atomization methods include gas atomization, water atomization, and disk atomization. Gas atomization and disk atomization are preferred from the viewpoint of preventing impurities from being mixed into the powder. Atomization in an inert gas atmosphere is preferred from the viewpoint of preventing impurities from being mixed into the powder. Gas atomization is preferred from the viewpoint of mass productivity.

[0027] This powder is pressurized and heated. A preferred method of pressurization and heating is hot isostatic pressing (HIP). This pressurization and heating produces a sputtering target that is a sintered body. This target is processed to a predetermined size and is then subjected to sputtering. The target may also be produced by a hot press method, a spark plasma sintering method, a hot extrusion method, or the like.

[0028] By preparing a powder with a crystal grain size of 30 μm or less and forming a compact under conditions that prevent the crystal grains from coarsening, a target having a metal structure with a crystal grain size Dg of 30 μm or less can be obtained. By lowering the heating temperature and shortening the heating time during compaction within a range that does not cause excessive pores in the compact, the growth of crystal grains can be suppressed.

[0029] By preparing a powder in which the B content is 70 at% or less and the equivalent circle diameter of the pure B phase is 30 μm or less, and by forming a compact under conditions that make it difficult for this pure B phase to coarsen, it is possible to obtain a target having a metal structure in which the maximum equivalent circle diameter Dc is 30 μm or less. The growth of the pure B phase can be suppressed by lowering the heating temperature and shortening the heating time during compaction within a range that does not cause excessive pores in the compact.

[0030] [Sputtering] This specification is also directed to a method for manufacturing a thin film, the method comprising: (1) A step of preparing a target whose material is an alloy containing 50 at% or more and 70 at% or less of B, which further contains Co or Fe, and which has a metallographic structure with a crystal grain size of 30 μm or less, and which includes a (CoFe)B phase as a matrix and a large number of B phases dispersed in the matrix, and each B phase has an equivalent circle diameter of 30 μm or less; and (2) subjecting the target to sputtering This manufacturing method can produce a magnetic thin film. This manufacturing method can suppress cracking of the target. In particular, the suppression of cracking is highly effective in sputtering using a magnetron sputtering device. This sputtering method can produce a thin film suitable for tunnel magnetoresistive films in magnetic heads, magnetic random access memories (MRAMs), and the like. [Example]

[0031] The effects of the sputtering targets according to the examples will be explained below, but the scope of the present disclosure should not be construed as being limited based on the descriptions of these examples.

[0032] [Example 1] The raw metal was placed in an alumina crucible. The raw metal was induction-heated and melted in an argon gas atmosphere. The molten metal was discharged from the nozzle of the crucible and sprayed with high-pressure argon to obtain a powder. The powder was classified to adjust the particle size to 300 μm or less. The powder was filled into a capsule with an outer diameter of 220 mm, an inner diameter of 210 mm, and a length of 200 mm, made of SC, and the inner surface was chrome-plated. The capsule was degassed and then sealed. The powder was subjected to hot isostatic pressing (HIP) at a temperature of 1200°C and a pressure of 140 MPa for 3 hours to obtain a sintered body. The sintered body was wire-cut and lathe-machined, and then flat-polished to obtain a sputtering target. The target had a disk shape. The target had a diameter of 180 mm and a thickness of 7 mm. The composition of this target is shown in Table 1 below.

[0033] [Examples 2-13 and Comparative Examples 2 and 4] Sputtering targets of Examples 2-13 and Comparative Examples 2 and 4 were obtained in the same manner as in Example 1, except that raw material metals with different compositions were used.

[0034] [Comparative Example 1] An ingot was obtained by melting, and the ingot was subjected to wire cutting and lathe processing, and further subjected to surface polishing to obtain a sputtering target.

[0035] Comparative Example 3 A target of Comparative Example 3 was obtained in the same manner as in Example 1, except that the temperature of the hot isostatic pressing treatment was set to 1350°C.

[0036] [Crystal grain size] A test piece was cut out from the target. The surface of this test piece was mirror-polished. This surface was then etched using a nitric acid alcohol-based etchant. This surface was observed under an optical microscope, and an image measuring 170 μm in length and 220 μm in width was taken. Three straight lines, each with a length L of 220 μm, were drawn on this image, and the number N of intersections between these lines and the grain boundaries was counted. The grain size Dg was calculated based on the following formula. The results are shown in Table 1 below. Dg = (3 L) / N

[0037] [Circle equivalent diameter of phase B] A test piece was cut out from the target. The surface of this test piece was polished. This surface was observed with a scanning electron microscope (SEM), and an image of 100 μm in length and 140 μm in width was taken. The circle-equivalent diameter Dc of the B phase was measured by image analysis. The circle-equivalent diameter Dc was measured in five fields of view, and the maximum value Dmax was determined. The results are shown in Table 1 below. In the target of Comparative Example 2, no B phase was observed.

[0038] [Transverse bending strength] Test pieces were cut out from the target by wire cutting. These test pieces were subjected to a flexural test in accordance with the provisions of "JIS Z 2511, Metal powder - Method for measuring green strength by flexural test." The test conditions were as follows: Test piece thickness: 3 mm Test piece width W: 3 mm Test piece length: 20 mm Distance between fulcrums: 10mm The load P (kN) when the test piece broke was measured, and the flexural strength BS (MPa) was calculated using the following formula. The average values ​​obtained from three measurements are shown in Table 1 below. BS = (3 / 2) × P × L / (t2 × W) BS: bending strength (MPa) t: thickness of test piece (mm) W: Width of test piece (mm) L: Distance between fulcrums (mm) P: Load at break (kN)

[0039] [Table 1]

[0040] As is clear from Table 1, the sputtering targets of each example are excellent in toughness despite containing a sufficient amount of B. These evaluation results clearly demonstrate the superiority of these targets. [Industrial Applicability]

[0041] The sputtering targets described above are suitable for producing various magnetic thin films.

Claims

1. An alloy containing 50 at% or more and 70 at% or less of B, The alloy further contains one or two elements selected from the group consisting of Co and Fe, The balance of the alloy is unavoidable impurities, and The alloy has a metal structure with a crystal grain size of 30 μm or less, the metal structure includes a (CoFe)B phase as a matrix and a large number of B phases dispersed in the matrix, An alloy suitable for a sputtering target, in which the equivalent circle diameter of each B phase is 30 μm or less.

2. 2. The alloy according to claim 1, wherein the total content of Co and Fe in said alloy is 30 at % or more and 50 at % or less.

3. The alloy according to claim 1 or 2, further containing O, and the O content (ppm by mass) is equal to or smaller than a value V1 calculated by the following formula: V1 = -10 × B% + 900 (In this formula, B% represents the content of B (at%).)

4. A sputtering target whose material is an alloy containing 50 at% or more and 70 at% or less of B, The alloy further contains one or two elements selected from the group consisting of Co and Fe, The balance of the alloy is unavoidable impurities, and The alloy has a metal structure with a crystal grain size of 30 μm or less, the metal structure includes a (CoFe)B phase as a matrix and a large number of B phases dispersed in the matrix, A sputtering target in which the equivalent circle diameter of each B phase is 30 μm or less.

5. (1) A step of preparing a target whose material is an alloy containing 50 at% or more and 70 at% or less of B, the alloy further containing one or two elements selected from the group consisting of Co and Fe, the remainder of the alloy being unavoidable impurities, the alloy having a metal structure with a crystal grain size of 30 μm or less, the metal structure including a (CoFe)B phase as a matrix and a large number of B phases dispersed in this matrix, and each B phase having an equivalent circle diameter of 30 μm or less; and (2) A step of subjecting the target to sputtering A method for producing a magnetic thin film, comprising:

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