Sputtering target, granular film and perpendicular magnetic recording medium

Abstract

The sputtering target contains 0.05 at % or more of Bi, a total content of metal oxides is 10 vol % to 70 vol %, and the balance contains at least Ru.

Description

[Technical Field]

[0001] This specification discloses techniques relating to sputtering targets, granular films, and perpendicular magnetic recording media. [Background technology]

[0002] Magnetic recording media, such as perpendicular magnetic recording media, which record magnetic fields perpendicular to the recording surface, may be composed of multiple layers, including upper and lower recording layers and other layers. These layers are formed by sequentially depositing films on a substrate by sputtering using sputtering targets appropriate for each layer. Among these, sputtering targets may be used, in which the metal phase is made of a metal primarily composed of Co and the oxide phase contains a specific metal oxide. Examples of such sputtering targets include those described in Patent Documents 1 to 4.

[0003] Recently, so-called ECC (Exchange-Coupled Composite) media have been used for recording layers, in which ferromagnetic oxide layers containing Co and Ru or Ru-based oxides are alternately formed with exchange coupling control layers using granular films containing Co and Ru or Ru-based oxides. Furthermore, between the Ru-based intermediate layer and the bottommost recording layer using the Co-based ferromagnetic layer, a nonmagnetic onset layer made of a granular film containing Co and Ru or Ru-based oxides is used to improve the separation between the magnetic grains of the recording layer. Such layers are described, for example, in Patent Documents 1 to 4. The granular films used in these exchange coupling control layers and onset layers are required to have the same high crystal orientation of the magnetic grains as the ferromagnetic oxide layer formed thereon and to promote good separation between the magnetic grains. The ferromagnetic oxide layer here refers to a layer having a saturation magnetization of approximately 400 emu / cc or more at room temperature, and the exchange coupling control layer and onset layer refer to layers having a saturation magnetization of approximately 300 emu / cc or less at room temperature (see Patent Document 3).

[0004] Such layers are generally formed from sputtering targets consisting of Co and Ru, or Ru as the primary material, with the addition of non-magnetic metals such as Pt and Cr, and metal oxides such as SiO2, TiO2, and BO3. This allows the CoPt magnetic grains in the ferromagnetic oxide layer formed on top to have good crystal orientation while remaining non-magnetic. Furthermore, metal oxides such as SiO2, TiO2, and BO3 are simultaneously sputtered and fill the spaces between the magnetic grains, forming a so-called granular structure. This weakens the exchange coupling between the magnetic grains, resulting in a recording layer capable of retaining high-density recording bits. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-009086 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-053969 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-176858 [Patent Document 4] Japanese Patent Application Laid-Open No. 2011-123959 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in sputtering targets in which metal oxides such as SiO2, TiO2, and B2O3 are added to Co or Ru as described above, the separation of magnetic particles is insufficient to further improve recording density. Therefore, there is room for further improvement in this type of sputtering target.

[0007] In order to solve these problems, this specification proposes a sputtering target, a granular film, and a perpendicular magnetic recording medium that can improve the crystal orientation and separation of magnetic grains. [Means for solving the problem]

[0008] The sputtering target disclosed in this specification contains Bi at 0.05 at % or more. and 10at% or less The total content of metal oxides is 10 vol % to 70 vol %, with the remainder containing at least Ru.

[0009] The granular film disclosed in this specification contains Bi at 0.05 at % or more. and 10at% or less The total content of metal oxides is 10 vol % to 70 vol %, with the remainder containing at least Ru. The perpendicular magnetic recording medium disclosed in this specification comprises the above-mentioned granular film. [Effects of the Invention]

[0010] Granular films and perpendicular magnetic recording media produced using the above sputtering targets can suppress grain growth while maintaining the crystalline orientation of the magnetic grains, thereby improving the grain size distribution of the magnetic grains and the separation between the magnetic grains. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the sputtering target, granular film, and perpendicular magnetic recording medium described above will be described in detail. In one embodiment, the sputtering target contains 0.05 at% or more of Bi, a total content of metal oxides is 10 vol% to 70 vol%, and the remainder contains at least Ru. By performing sputtering using a sputtering target containing Bi in this manner, it is possible to improve the separation of magnetic particles in the film formed on the top of the sputtered film while maintaining the crystal orientation.

[0012] (Sputtering target composition) The metal components of the sputtering target mainly contain Co and / or Ru, but it is essential that Bi is also included. The inclusion of Bi suppresses the growth of metal grains, making it easier to reduce the size of the magnetic grains in the recording layer formed on top. In particular, when film formation is performed at a high substrate temperature to improve crystallinity, the size of the metal grains does not increase, allowing for both small grain size and good crystallinity. Furthermore, not only does this facilitate segregation of metal oxides at grain boundaries, but it also allows for the creation of films with minimal grain boundary width dispersion. This allows for the dispersion of fine metal grains with uniform grain size distribution through oxide grain boundaries with uniform widths. As a result, the grain size dispersion of the magnetic grains in the recording layer formed on top can be reduced, and a recording layer with oxide grain boundaries with uniform widths can be formed.

[0013] This is thought to be due not only to the low melting points of Bi and Bi oxide itself, but also to the fact that Bi oxide can combine with other major oxides to lower its melting point, and while Bi does not readily form alloys with Co or Ru, Co and Ru have good wettability with Bi oxide. Generally, one of the reasons for the large particle size dispersion is that metal oxides with high melting points solidify first, hindering the growth of magnetic particles primarily composed of Co or Ru. However, lowering the melting point of the metal oxide facilitates the mobility of the metal oxide, preventing particle growth and reducing particle size dispersion. Furthermore, the good wettability of Bi oxide with Co and Ru prevents magnetic particles surrounded by the oxide from becoming rounded, resulting in a film in which oxides of uniform width are formed around polygonal magnetic particles. The above is a possible explanation, but the present invention is not limited to such a theory.

[0014] The Bi content is 0.05 at% or more in terms of Bi equivalent. Bi may be contained as a metal component or an oxide component, but when it is contained as both a metal component and an oxide component, the above content means the total of the Bi element in those components. If the Bi content is less than 0.05 at%, the improvement in spatial separation between metal particles is insufficient. On the other hand, if the Bi content is too high, there is a concern that the hcp structure of the metal particles will not be stable. Therefore, the Bi content is preferably 0.5 at% or more, and can be, for example, 0.5 at% to 10 at%. Due to the above-mentioned effects of adding Bi, it is preferable that part or all of Bi is contained as a metal oxide.

[0015] The sputtering target contains at least Ru as a metal component. It may also contain Co. This is to make the metal particles have the same hcp crystal structure as the CoPt formed on top. However, Co may not be included.

[0016] The metal components of the sputtering target may contain, in addition to the above-mentioned Bi and Ru, and in some cases Co, in order to optimize the crystal lattice constant and wettability with the magnetic particles and oxide grain boundaries that form the upper layer, one or more elements selected from the group consisting of Pt, Au, Ag, B, Cu, Cr, Ge, Ir, Mn, Mo, Nb, Ni, Pd, Re, Rh, Ta, W, and V, in a total amount of 0.5 at% to 30 at%.

[0017] Sputtering targets generally contain the above-mentioned metals and metal oxides. The total content of the metal oxides is 10 vol% to 70 vol% by volume. The total content of the metal oxides can be 1 mol% to 30 mol%. When a film formed using this sputtering target is formed on a film having a hcp structure such as Ru, it has a granular structure in which the metal oxides are dispersed in the above-mentioned metals. If the metal oxide content is too low, the separation of the metal particles may be insufficient, and the magnetic cluster size of the recording layer formed using this may be large. On the other hand, if the metal oxide content is too high, the proportion of metal particles is low, which reduces the crystallinity of the magnetic particles formed on top, and the magnetic particles formed on top may not obtain sufficient saturation magnetization and magnetic anisotropy, resulting in insufficient playback signal strength and thermal stability.

[0018] The oxide volume fraction can be calculated from the density and molecular weight of each component contained in the sputtering target, but can also be calculated from the area ratio of the oxide phase in any cut surface of the sputtering target. In this case, the volume ratio of the oxide phase in the sputtering target can be the area ratio in the cut surface.

[0019] Specific examples of the metal oxide include oxides of Co, Cr, Si, Ti, B, and Ta. Therefore, the sputtering target may contain an oxide of at least one element selected from the group consisting of Co, Cr, Si, Ti, B, and Ta. Examples of such metal oxides include SiO2, TiO2, and BO3.

[0020] In particular, when an oxide of Ti is contained, the separation of metal particles is improved, so it is preferable that the sputtering target contains an oxide of Ti such as TiO2. In addition, since oxides of Si and B can make the oxide layer amorphous and contribute to the formation of grain boundaries with a uniform width and in line with the shape of the metal particles, it is preferable that the sputtering target contains an oxide of either SiO2 or B2O3.

[0021] Furthermore, Bi may be present in the target in the form of an oxide. In other words, the above metal oxides may contain Bi. Bi oxide forms a complex oxide with other metal oxides, which is expected to lower the melting point and improve the sinterability of the target. It is also expected to promote segregation of the oxide grain size even in the sputtered film state. Furthermore, it is preferable to include Co oxide in order to keep some or all of Bi stable as an oxide.

[0022] (Method for manufacturing sputtering targets) The sputtering target described above can be manufactured by, for example, a powder sintering method, and a specific example of the manufacturing method will be described below.

[0023] First, metal powders are prepared, including Bi powder, Ru powder, and optionally Co powder, and, if necessary, one or more powders selected from the group consisting of Pt, Au, Ag, B, Cu, Cr, Ge, Ir, Mn, Mo, Nb, Ni, Pd, Re, Rh, Ta, W, and V.

[0024] The metal powder may be a powder of not only a single element but also an alloy, and a particle size within the range of 1 μm to 150 μm is preferable because it enables uniform mixing and prevents segregation and coarse crystallization. If the particle size of the metal powder is larger than 150 μm, the oxide particles described below may not be uniformly dispersed, and if it is smaller than 1 μm, the oxidation of the metal powder may cause the sputtering target to deviate from the desired composition.

[0025] Furthermore, as the oxide powder, for example, TiO2 powder, SiO2 powder, Bi2O3 and / or B2O3 powder, etc. are prepared. The particle size of the oxide powder is preferably in the range of 1 μm to 30 μm. This allows the oxide particles to be more uniformly dispersed in the metal phase when mixed with the metal powder and pressure sintered. If the particle size of the oxide powder is larger than 30 μm, coarse oxide particles may be generated after pressure sintering, while if it is smaller than 1 μm, aggregation of the oxide powder particles may occur.

[0026] Next, the metal powder and oxide powder are weighed to obtain the desired composition, and mixed and pulverized using a known method such as a ball mill. At this time, it is desirable to fill the inside of the container used for mixing and pulverization with an inert gas to suppress oxidation of the raw material powder as much as possible. This allows for the production of a mixed powder in which the desired metal powder and oxide powder are uniformly mixed.

[0027] The mixed powder thus obtained is then pressurized and sintered in a vacuum or inert gas atmosphere to form a desired shape such as a disk. Various pressure sintering methods can be used here, such as hot press sintering, hot isostatic sintering, and plasma discharge sintering. Among these, hot isostatic sintering is effective in terms of increasing the density of the sintered body.

[0028] The temperature held during sintering is preferably in the range of 600 to 1500° C., and more preferably 700 to 1400° C. The time for holding the temperature within this range is preferably 1 hour or longer. The pressure applied during sintering is preferably 10 MPa or more, more preferably 20 MPa or more. This allows the oxide particles to be more uniformly dispersed in the metal phase.

[0029] The sintered body obtained by the above pressure sintering is subjected to cutting or other machining using a lathe or the like to form it into a desired shape, thereby producing a sputtering target in the shape of a disk or the like.

[0030] (Granular film) By using the sputtering target as described above and performing sputtering in a sputtering device, typically a magnetron sputtering device, it is possible to form a granular film having a non-magnetic oxide layer structure.

[0031] The granular film that becomes the non-magnetic oxide layer has substantially the same composition as the above-mentioned sputtering target. More specifically, the granular film contains 0.05 at% or more, preferably 0.5 at% or more, of Bi, and a total of 10 vol% to 70 vol% of metal oxides surrounding numerous metal particles composed primarily of Co and Ru, or Ru. The total metal oxide content in this granular film can be 1 mol% to 30 mol%. The amount of Bi added can be adjusted to obtain the desired value. Increasing the amount of Bi reduces the crystallinity of the metal particles, but this also depends on the amount of other nonmagnetic metals and oxides. Therefore, it is difficult to specify a maximum amount of Bi to be added. However, adding approximately 10 at% Bi may deteriorate the hcp structure crystals composed primarily of Co and Ru. Therefore, the Bi content in the magnetic film can be, for example, 0.5 at% to 10 at%. Bi may be present partially or entirely as an oxide. A granular film is a film having a structure in which metal particles are dispersed and metal oxides are embedded between the metal particles.

[0032] The metal oxide in the granular film may contain an oxide of at least one element selected from the group consisting of Co, Cr, Si, Ti, B, and Ta. Among these, the metal oxide preferably contains an oxide of Ti, Si, or B. The total content of the metal oxides is 10 vol% to 70 vol%.

[0033] The granular film may further contain one or more elements selected from the group consisting of Pt, Au, Ag, B, Cu, Cr, Ge, Ir, Mn, Mo, Nb, Ni, Pd, Re, Rh, Ta, W, and V in a total amount of 0.5 at % to 30 at %.

[0034] Such granular films can be used for a variety of purposes, including as part of the intermediate layer and recording layer among the adhesion layer, soft magnetic layer, seed layer, underlayer (e.g., Ru layer), intermediate layer, recording layer, and protective layer on the substrate that constitute a magnetic recording medium (i.e., perpendicular magnetic recording medium). In particular, by using them below the ferromagnetic layer in the recording layer, they can help improve both the magnetic anisotropy and the magnetic grain separation of the ferromagnetic layer. Specifically, they are suitable for use as an onset layer between the intermediate layer and recording layer, which aims to improve the magnetic grain separation of the bottommost ferromagnetic layer, or as an exchange coupling control layer, which is used to adjust the interlayer magnetic coupling between the multiple ferromagnetic layers that constitute the recording layer. The granular films that constitute such layers preferably have a saturation magnetization of 300 emu / cc or less. Ferromagnetic layers generally have a saturation magnetization of approximately 400 emu / cc or more at room temperature.

[0035] (Perpendicular magnetic recording media) Perpendicular magnetic recording media, unlike conventional horizontal magnetic recording methods that record magnetic fields horizontally relative to the recording surface, record magnetic fields perpendicular to the recording surface, enabling higher-density recording and therefore being widely adopted in hard disk drives and other devices. Specifically, perpendicular magnetic recording media are typically constructed by sequentially laminating an adhesion layer, a soft magnetic layer, a seed layer, an underlayer such as a Ru layer, an intermediate layer, a recording layer, and a protective layer on a substrate such as aluminum or glass. The sputtering target described above is suitable for depositing an onset layer beneath the recording layer. The recording layer can also have a so-called ECC media structure, in which multiple ferromagnetic oxide layers and nonmagnetic oxide layers are alternately stacked. In this case, the sputtering target described above is also suitable for depositing the nonmagnetic oxide layers between the ferromagnetic oxide layers. [Example]

[0036] Next, the sputtering target described above was prototyped and its performance was confirmed, which will be described below. However, the description here is for illustrative purposes only and is not intended to be limiting.

[0037] As examples, we fabricated a Ru-(Co-Pt-)Bi-oxide sputtering target and a Ru-(Co-Pt-)Bi2O3-oxide sputtering target, and as a comparative example, we fabricated a Ru-(Co-Pt-) oxide sputtering target. The compositions of each sputtering target are shown in Table 1.

[0038] [Table 1]

[0039] To explain the specific manufacturing method of these sputtering targets in detail, the specified metal powders and metal oxide powders were first weighed and placed in a 10-liter ball mill pot along with zirconia balls as a milling medium, and mixed by rotating for 24 hours. The mixed powder was then removed from the ball mill and filled into a cylindrical carbon mold with a diameter of 190 mm, and sintered by hot pressing. The hot pressing conditions were a vacuum atmosphere, a heating rate of 300°C / hour, a holding temperature of 1000°C, and a holding time of 2 hours. A pressure of 30 MPa was applied from the start of the heating period until the end of the holding period. After the holding period, the powder was allowed to cool naturally in the chamber. The resulting sintered body was then cut to prepare a sputtering target. As a raw material, Bi metal powder was used in Examples 1 to 7, and Bi oxide powder was used in Examples 8 to 13.

[0040] A magnetron sputtering system (Canon Anelva C-3010) was used to deposit Cr-Ti (6 nm), Ni-W (5 nm), and Ru (20 nm) films in that order on a glass substrate. Each of the sputtering targets was then sputtered at 300 W in an Ar 3.0 Pa atmosphere to form a 1 nm-thick granular film. An 11 nm-thick magnetic film of Co-20Pt-3TiO2-3SiO2-3B2O3 was then deposited as a ferromagnetic layer, followed by a 3 nm Ru protective film to prevent oxidation. In Comparative Example 7, the ferromagnetic layer was deposited directly on the Ru film.

[0041] The saturation magnetization Ms, coercive force Hc, and magnetic anisotropy Ku of each sample were measured. Measurements were performed using a vibrating sample magnetometer (VSM) and a torque magnetometer (TRQ) manufactured by Tamagawa Seisakusho Co., Ltd. Note that the saturation magnetization Ms refers to the saturation magnetization of the entire sample fabricated by laminating multiple layers, including the ferromagnetic layer, as described above, and is not the saturation magnetization of the granular film alone. It can be seen that the samples of Examples 1 to 13 exhibit higher magnetic anisotropy than the samples of Comparative Examples 1 to 7. This indicates that the provision of the layers of the Examples improves the crystallinity of the magnetic particles in the ferromagnetic layer. Furthermore, the fact that the saturation magnetization and coercive force are almost unchanged despite the higher magnetic anisotropy compared to Comparative Examples 1 to 6 is thought to indicate that the size of the magnetic particles in the ferromagnetic layer is smaller and the separation is improved.

[0042] Next, the average particle size and particle size distribution of the magnetic particles were determined from TEM images obtained by energy dispersive X-ray spectroscopy (TEM-EDX) using a transmission electron microscope manufactured by JEOL Ltd. As shown in Table 1, when Bi was added, the magnetic particles tended to have smaller particle sizes and generally smaller particle size distributions than when Bi was not added. Normally, when the particle size of the magnetic particles is small, the particle size distribution increases, resulting in an increase in small particles with poor crystallinity, which reduces magnetic anisotropy. However, in Examples 1 to 13, the addition of Bi resulted in a small particle size distribution despite the small particle size of the magnetic particles, and thus higher magnetic anisotropy than in the comparative examples. Therefore, it is believed that Examples 1 to 13 improved the crystallinity of the metal particles in the non-magnetic film while improving separation, thereby maintaining or improving the magnetic properties of the upper ferromagnetic layer.

[0043] From the above, it was found that the sputtering targets of Examples 1 to 13 suppress the growth of metal grains in the nonmagnetic oxide layer and uniformly segregate the metal oxide to the grain boundaries, making it possible to produce a film with small grain size and little grain size dispersion, which in turn suppresses the growth of magnetic grains in the ferromagnetic layer formed thereon, making it possible to form a film with small grain size dispersion.This shows that it is possible to produce a ferromagnetic layer with small average grain size and high magnetic anisotropy, and furthermore, it is possible to reduce grain size dispersion and improve separation between particles.

Claims

1. A sputtering target containing 0.05 at % or more and 10 at % or less of Bi, a total content of metal oxides being 10 vol % to 70 vol %, the metal oxides including an oxide of at least one element selected from the group consisting of Co, Cr, Si, Ti, B and Ta, and the remainder being Ru for forming a film containing metal particles having at least an hcp crystal structure.

2. 2. The sputtering target according to claim 1, comprising Co.

3. 3. The sputtering target according to claim 1, wherein part or all of Bi is contained as a metal oxide.

4. The sputtering target according to any one of claims 1 to 3, containing 0.5 at% or more of Bi.

5. The sputtering target according to any one of claims 1 to 4, further comprising 0.5 at% to 30 at% of one or more elements selected from the group consisting of Pt, Au, Ag, B, Cu, Cr, Ge, Ir, Mn, Mo, Nb, Ni, Pd, Re, Rh, Ta, W, and V.

6. A granular film containing metal particles having an hcp crystal structure, the film containing Bi at 0.05 at% or more and 10 at% or less, a total content of metal oxides being 10 vol% to 70 vol%, the metal oxides comprising an oxide of at least one element selected from the group consisting of Co, Cr, Si, Ti, B and Ta, and the remainder being at least Ru.

7. The granular film according to claim 6, which contains Co.

8. 8. The granular film according to claim 6, wherein a part or all of Bi is contained in the form of a metal oxide.

9. The granular film according to any one of claims 6 to 8, containing 0.5 at % or more of Bi.

10. The granular film according to any one of claims 6 to 9, further comprising 0.5 at% to 30 at% of one or more elements selected from the group consisting of Pt, Au, Ag, B, Cu, Cr, Ge, Ir, Mn, Mo, Nb, Ni, Pd, Re, Rh, Ta, W, and V.

11. A perpendicular magnetic recording medium comprising the granular film according to any one of claims 6 to 10.

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