Sputtering target

Abstract

Provided are a sputtering target that is produced efficiently, has excellent toughness, and can contribute to the magnetic properties of a thin film, and an alloy suitable for the sputtering target. The alloy contains 50 to 70 at% B and 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 CoB phase, a FeB phase, or a CoFeB phase and a plurality of B phases dispersed in the matrix. The circle equivalent diameter of each B phase is 30 μm or less.

Description

Sputtering Target

[0001] The present disclosure relates to sputtering targets and alloys suitable therefor, as well as methods for producing magnetic thin films.

[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 has two magnetic thin films and a shielding layer sandwiched between them. A typical material constituting the shielding layer is MgO. Meanwhile, a typical material constituting the thin films is an alloy containing a large amount of B. This alloy further contains Fe or Co. Because the alloy contains sufficient B, a high tunnel magnetoresistive signal can be achieved in the tunnel magnetoresistive film. Patent Document 1 (WO 2011 / 070860) and Patent Document 2 (JP 2020-132995 A) disclose alloys suitable for magnetic thin films.

[0003] WO2011 / 070860 JP 2020-132995 A

[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 a thin film. On the other hand, B can impair the toughness of the target. A target containing a large amount of B is prone to cracking during use. Furthermore, during the production of this target, defects such as cracking of the compact can occur. This defect impairs the yield of the target.

[0006] An object of the present invention is to provide a sputtering target and an alloy suitable for the same, which can be produced efficiently, has excellent toughness, and can contribute to the magnetic properties of thin films.

[0007] The present disclosure provides the following aspects. [Aspect 1] An alloy suitable for a sputtering target, the alloy comprising: 50 at% to 70 at% B, and one or two elements selected from the group consisting of Co and Fe, the alloy having a metallographic structure with a crystal grain size of 30 μm or less, the metallographic structure including a (CoFe)B phase as a matrix and a plurality of B phases dispersed in the matrix, each of which has an equivalent circle diameter of 30 μm or less. [Aspect 2] The alloy according to Aspect 1, wherein the total content of Co and Fe in the alloy is 30 at% to 50 at%. [Aspect 3] The alloy further contains O, and the O content (ppm by mass) satisfies the following mathematical formula: V １ = -10 × B% + 900 (where B% represents the content of B (at %)) １ The alloy according to aspect 1 or 2, wherein the magnetic thin film thickness is equal to or smaller than the above. [Aspect 4] A sputtering target comprising the alloy according to any one of aspects 1 to 3. [Aspect 5] A method for producing a magnetic thin film, comprising the steps of: preparing a target comprising the alloy according to any one of aspects 1 to 3; and subjecting the target to sputtering.

[0008] 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.

[0009] 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.

[0010] Preferred embodiments are described in detail below.

[0011] [Target] The sputtering target according to this embodiment is composed of an alloy containing B as a major component. This alloy further contains Co or Fe. Preferably, the balance in this alloy is unavoidable impurities. The alloy may have a composition that contains Co but not Fe, a composition that contains Fe but not Co, or a composition that contains Co and Fe. In other words, this alloy is a Co—B alloy, an Fe—B alloy, or a Co—Fe—B alloy.

[0012] [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.

[0013] [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.

[0014] [Metal Structure] The metal structure of this target is a polycrystalline structure, in other words, the metal structure has a plurality (or a large number) of crystal grains.

[0015] 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 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.

[0016] [(CoFe)B Phase] The (CoFe)B phase contains both or either one of 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.

[0017] [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, cracking defects 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 more preferable. 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.

[0018] [Oxygen (O)] The alloy may contain O as an impurity. O bonds with B, and B ２ O ３ It can form a B phase. ２ O ３ The melting point of this B phase is low. ２ O ３The target according to the present embodiment contains a large amount of B, and therefore, the B phase in this target is ２ O ３ Phase is easily formed. ２ O ３ From the viewpoint of suppressing the formation of the phase, it is important to suppress the content of O. From the viewpoint of toughness, it is important that the content of O (ppm by mass) satisfies the following formula: V １ = -10 × B% + 900 = V １ In this formula, B % represents the content of B (at %).

[0019] The O content (ppm by mass) is calculated by the following formula: V ２ = -10 × B% + 800 = V ２ It is more preferable that the value is equal to or smaller than this.

[0020] The O content (ppm by mass) is calculated by the following formula: V ３ = -10 × B% + 750 = V ３ It is particularly preferred that the value is equal to or smaller than this.

[0021] 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.

[0022] [Target Manufacturing Method] In target manufacturing, first, 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.

[0023] This powder is pressurized and heated. Pressurization and heating are preferably performed by 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 hot pressing, spark plasma sintering, hot extrusion, or the like.

[0024] By preparing a powder having a crystal grain size of 30 μm or less and forming a compact under conditions that make it difficult for the crystal grains to coarsen, 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 forming within a range that does not cause excessive pores in the compact, the growth of crystal grains can be suppressed.

[0025] By preparing a powder having a B content of 70 at% or less and a pure B phase with an equivalent circle diameter of 30 μm or less, and forming a compact using this powder under conditions that make it difficult for the pure B phase to coarsen, it is possible to obtain a target having a metal structure with a maximum equivalent circle diameter Dmax of 30 μm or less. The growth of the pure B phase can be suppressed by lowering the heating temperature during compaction and shortening the heating time within a range that does not cause excessive pores in the compact.

[0026] [Sputtering] The present disclosure is also directed to a method for producing a thin film. This method includes the steps of preparing a target composed of the alloy of the present disclosure described above and subjecting the target to sputtering. This method can produce a magnetic thin film. This method can suppress cracking of the target. In particular, sputtering using a magnetron sputtering device is highly effective in suppressing cracking. This sputtering can produce a thin film suitable for use as a tunnel magnetoresistive film in magnetic heads, magnetic random access memories (MRAMs), and the like.

[0027] 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.

[0028] Example 1: A 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 (outer diameter: 220 mm, inner diameter: 210 mm, length: 200 mm) made of SC (carbon steel) and having a chrome-plated inner surface. 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.

[0029] Examples 2 to 13 and Comparative Examples 2 and 4 Sputtering targets of Examples 2 to 13 and Comparative Examples 2 and 4 were obtained in the same manner as in Example 1, except that raw metals with different compositions were used.

[0030] Comparative Example 1 An ingot was obtained by melting, and then wire-cutting and lathing were performed on the ingot, followed by surface polishing to obtain a sputtering target.

[0031] 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.

[0032] [Grain size] A test piece was cut out from the target. The surface of this test piece was mirror-polished. This surface was etched using a nitric acid alcohol-based etchant. This surface was observed with an optical microscope, and an image of 170 μm in length and 220 μm in width was taken. Three straight lines 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: Dg = (3 L) / N. The results are shown in Table 1 below.

[0033] [Circle-equivalent diameter of B phase] A test piece was cut out from the target. The surface of this test piece was polished. This surface was observed using a scanning electron microscope (SEM), and an image measuring 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. In this specification, the circle-equivalent diameter means the projected area diameter and is defined as the diameter of a circle having the same area as the projected area of ​​a particle (here, B phase). Therefore, the circle-equivalent diameter Dc was measured for each of the multiple B phases observed in one field of view. In this way, the circle-equivalent diameters Dc were measured in five fields of view, and the maximum value Dmax of these was determined. The results are shown in Table 1 below. In the target of Comparative Example 2, no B phase was observed.

[0034] [Transverse Strength] Test pieces were cut out from the target by wire cutting. These test pieces were subjected to a transverse test in accordance with the provisions of "JIS Z 2511, Metal Powder - Method for Measuring the Strength of Green Compacts by Transverse Test." The test conditions were as follows: Test piece thickness t: 3 mm Test piece width W: 3 mm Test piece length: 20 mm Distance between supports: 10 mm. The load P (kN) at which the test piece broke was measured, and the load P was calculated using the following formula: BS = (3 / 2) × P × L / (t ２ x W) BS: flexural strength (MPa) t: thickness of test piece (mm) W: width of test piece (mm) L: distance between supports (mm) P: load at break (kN) The flexural strength BS (MPa) was calculated from the above. The average of the values ​​obtained from three measurements is shown in Table 1 below.

[0035]

[0036] As is clear from Table 1, the sputtering targets of each example are excellent in toughness despite containing a sufficient amount of B. The superiority of these targets is clear from these evaluation results.

[0037] The sputtering targets described above are suitable for producing various magnetic thin films.

Claims

1. An alloy suitable for a sputtering target, said alloy comprising: 50 at% or more and 70 at% or less of B; and one or two elements selected from the group consisting of Co and Fe; said alloy having a metal structure with a crystal grain size of 30 μm or less; said metal structure comprising a (CoFe)B phase as a matrix and a plurality of B phases dispersed in this matrix, each of which has an equivalent circle diameter of 30 μm or less.

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 further contains O, and the O content (ppm by mass) is expressed by the following formula: V １ = -10 × B% + 900 (where B% represents the content of B (at %)) １ 3. The alloy of claim 1 or 2, wherein the .DELTA..times ...

4. A sputtering target comprising the alloy according to any one of claims 1 to 3.

5. A method for producing a magnetic thin film, comprising the steps of: preparing a target made of the alloy according to any one of claims 1 to 3; and subjecting said target to sputtering.

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