JP6128421B2 - Sputtering target material for heat sink layer formation of thermally assisted magnetic recording media - Google Patents

Description

  The present invention relates to a sputtering target material for forming a heat sink layer in a heat-assisted magnetic recording medium.

In recent years, a perpendicular magnetic recording system has been put into practical use in response to a demand for a high recording density of a magnetic recording apparatus by realizing an advanced information society. The amount of generated digital information is increasing at an annual rate of about 50% or more, and further higher recording density is required. In order to increase the recording density, it is necessary to increase the recording capacity per unit area. For this purpose, it is preferable to make the crystal grain size of the recording layer fine.
However, when the crystal grain size of the recording layer is made fine, there is a problem of thermal fluctuation in which magnetically recorded data disappears due to the influence of ambient heat. In order to prevent this, a material having high magnetic anisotropy energy may be used for the recording layer. However, since the limit of the write magnetic field of the head is exceeded, a new problem that recording becomes difficult occurs. As a method for solving these problems, a heat-assisted magnetic recording method has been proposed.

FIG. 1 shows an example of the layer structure of the heat-assisted magnetic recording medium. On the nonmagnetic substrate 1, a heat sink layer 2, a soft magnetic backing layer 3, an orientation control layer 4, a recording layer 5, and a protective layer 6 are formed in this order from the bottom. Further, the layer structure is not limited to that shown in FIG. 1, and the arrangement of the heat sink layer 2 and the soft magnetic backing layer 3 may be appropriately switched.
In the heat-assisted magnetic recording method, a material having high magnetic anisotropy energy such as Fe—Pt or Co—Pt is used for the recording layer 5. As described above, when the magnetic anisotropy energy is high, there is a restriction on the write magnetic field of the head. Therefore, when writing information, the recording layer 5 is heated to reduce the coercive force, and recording is performed from the head during this time. Information is written and recorded by applying a magnetic field. In order to realize this method, in order not to lose the written information, the heated recording layer 5 must be quickly cooled after writing, and the coercivity once lowered must be increased. Therefore, a heat sink layer 2 is formed on the heat-assisted magnetic recording medium for the purpose of quickly absorbing the heat of the recording layer 5.

  For this heat sink layer, an element having high thermal conductivity is effective, and such element includes Cu. However, when a Cu film is used as the heat sink layer, the surface roughness of the film increases due to crystallization of the Cu film, and the orientation control layer (for example, Cr or MgO) or the recording layer disposed on the heat sink layer is the heat sink layer. As a result, there may be a problem that signal noise increases. In order to solve this problem, a heat sink layer in which 0.1 to 1 atomic% of Zr is added to Cu has been proposed (see, for example, Patent Document 1).

US Publication No. 2007/0026263

The heat sink layer disclosed in Patent Document 1 described above is excellent in that high thermal conductivity and a smooth surface can be obtained by adding Zr to Cu. However, according to the study of the present inventor, when a heat sink layer is formed by sputtering using an alloy sputtering target material in which Zr alone is added to Cu, the surface roughness increases depending on the amount of Zr added. The problem that the smooth surface required as a layer may not be obtained may be confirmed.
An object of the present invention is to provide a sputtering target material for forming a heat sink layer of a heat-assisted magnetic recording medium, which requires high thermal conductivity and a smooth surface.

The present inventor selects Zr, Cr as an additive element that has Cu having a high thermal conductivity as a main component and further reduces the surface roughness of the sputtering target material for forming the heat sink layer of the heat-assisted magnetic recording medium. As a result of studying these addition ranges, the present invention has been achieved.
That is, the present invention provides a composition formula in the atomic ratio _{Cu 100-x-y -Zr x} -Cr y, is represented by 0.10 ≦ x ≦ 5.00,0.10 ≦ y ≦ 1.00, the balance It is an invention of a sputtering target material for forming a heat sink layer of a heat-assisted magnetic recording medium comprising inevitable impurities.

  INDUSTRIAL APPLICABILITY The present invention can provide a sputtering target material that can form a heat sink layer of a heat-assisted magnetic recording medium having high thermal conductivity and low surface roughness, and is useful for increasing the recording density of the heat-assisted magnetic recording medium. Technology.

It is the figure which represented typically an example of the layer structure of a heat-assisted magnetic recording medium. Sample No. of Comparative Example It is a field emission type scanning electron microscope image of the heat sink layer formed into a film using 2 sputtering target materials. Sample No. of the present invention example. 5 is a field emission scanning electron microscope image of a heat sink layer formed using a sputtering target material of No. 5. Sample No. of the present invention example. It is a field emission type scanning electron microscope image of the heat sink layer formed into a film using one sputtering target material. Sample No. of Comparative Example It is an atomic force microscope image of the heat sink layer formed into a film using 2 sputtering target materials. Sample No. of the present invention example. It is an atomic force microscope image of the heat sink layer formed into a film using 1 sputtering target material.

As described above, an important feature of the present invention is that Cu is a main component of a sputtering target material for obtaining a high thermal conductivity required for a heat-sink layer of a heat-assisted magnetic recording medium and a smooth surface roughness. In addition, Zr and Cr are selected as additive elements, and appropriate addition ranges thereof are found.
The sputtering target material of this invention has Cu as a main component. The reason is that Cu has a high thermal conductivity and is inexpensive and relatively easily available. Au, Ag, and Al are examples of metal elements with high thermal conductivity, but Au and Ag are expensive and difficult to obtain. In addition, Al has a low melting point of 660 ° C., and in the process of manufacturing a heat-assisted magnetic recording medium, the substrate temperature may be heated to close to 700 ° C. during the formation of the recording layer. The surface roughness may increase due to growth. Therefore, Cu is the main component in the present invention.

  In the sputtering target material of the present invention, a specific range of Zr is added to Cu. There is a general correlation between the crystal grain size and the surface roughness of a film formed by sputtering using a sputtering target material, and the smaller the crystal grain size of the film, the smaller the surface roughness. In particular, Zr can reduce the surface roughness because the crystal grain size becomes fine even with a small addition amount, and the decrease in thermal conductivity due to the addition of Zr can also be kept low. Moreover, although a heat sink layer is so preferable that a heat conductivity is high, since heat conductivity falls by addition of the element to Cu, the one where addition amount is small is preferable.

In the present invention, the lower limit of the amount of Zr added is 0.10 atomic%. This is because if the amount of Zr added is less than 0.10 atomic%, the crystal grain size of the formed heat sink layer increases and the surface roughness tends to increase.
In the present invention, the upper limit of the amount of Zr added is set to 5.00 atomic%. This is because when Zr is added to Cu in an amount exceeding 5.00 atomic%, the heat sink layer to be formed becomes amorphous and the surface roughness is reduced, but the thermal conductivity is remarkably lowered. Therefore, in the present invention, the upper limit of the amount of Zr added is set to 5.00 atomic%.

In the present invention, Cr is added to Cu in combination with Zr. In the present invention, in order to reduce the surface roughness of the heat sink layer to be obtained and to ensure high thermal conductivity, the addition range of Cr is set to 0.10 to 1.00 atomic%.
According to the study by the present inventor, it was confirmed that when Zr was added to Cu, the formed heat sink layer was not an amorphous phase but a microcrystal. As an example, FIG. 2 shows the result of observing the surface of a heat sink layer formed using a sputtering target material in which 0.50 atomic% of Zr is added to Cu with a field emission scanning electron microscope (hereinafter referred to as FE-SEM). Show. As shown in the arrow part of FIG. 2, the heat sink layer formed with the sputtering target material to which Zr is added alone can confirm a dent along the crystal grain boundary on the film surface. This is presumed to be the cause of roughening the film surface.

The sputtering target material of the present invention reduces the surface roughness of the heat sink layer by precipitating grain boundary precipitates at the crystal grain boundaries in order to reduce dents formed on the surface of the heat sink layer to be formed. Can do. Since Cr is separated from the parent phase Cu, it is likely to precipitate at the crystal grain boundary, and since the diffusion rate in Cu is relatively fast, it can be quickly precipitated at the crystal grain boundary to reduce the surface roughness. Therefore, in the present invention, Cr was selected as an element to be added in combination with Zr to Cu.
In the present invention, the lower limit of the amount of Cr added is 0.10 atomic%. This is because when the amount of Cr added is less than 0.10 atomic%, the effect of Cr grain boundary precipitation is small, and the dents along the crystal grain boundary cannot be sufficiently filled, so that the surface roughness is large. Because it is easy to become.
On the other hand, as an example of adding 1.00 atomic% of Cr added in combination to Cu, film formation is performed using a sputtering target material whose composition formula in atomic ratio is represented by Cu-0.25% Zr-1.00% Cr. An FE-SEM photograph of the heat sink layer is shown in FIG. It can be seen that when 1.00 atomic% of Cr combined with Zr is added to Cu, shallow and light voids that are not harmful are generated on the surface of the heat sink layer, as indicated by the arrows in FIG.
According to the inventor's study, when the amount of Cr added exceeds 1.00 atomic%, the recesses along the grain boundaries are filled by the grain boundary precipitation of Cr, while deep voids are formed to increase the surface roughness. Confirmed to do. And when Cr content was 1.00 atomic% or less, it discovered that the depth of the void produced on the surface of a heat sink layer can be made shallow, and the surface smoothness required as a heat sink layer can be maintained. Thereby, in this invention, the upper limit of the addition amount of Cr was set to 1.00 atomic%.

  The heat conductivity of the heat sink layer in the heat-assisted magnetic recording medium is preferably as high as possible from the viewpoint of cooling efficiency of the recording layer. It is known that the thermal conductivity at room temperature required for the heat sink layer may be 100 W / m · K or more, and preferably 200 W / m · K or more. As described above, the sputtering target material of the present invention can increase the thermal conductivity of the obtained heat sink layer to 200 W / m · K or more by adding Zr and Cr to Cu in a specific range.

Further, it is known that in heat-assisted magnetic recording media, the surface roughness of the heat sink layer is preferably as small as possible in order to reduce noise in the recording layer. The preferable surface roughness of the heat sink layer needs to be adjusted within a range in which the thermal conductivity does not significantly decrease, and it is known that 0.7 nm or less is preferable, and 0.5 nm or less is more preferable. .
The surface roughness referred to here refers to the arithmetic average roughness (Ra) defined by JIS B 0601-2001, and can be measured using, for example, an atomic force microscope (hereinafter referred to as AFM). .

  As a method for producing a sputtering target material for forming a heat sink layer of the heat-assisted magnetic recording medium of the present invention, a melt casting method or a powder sintering method can be applied. In the melt casting method, it is possible to produce a cast ingot or a bulk body obtained by applying plastic processing or pressure processing to the cast ingot. In addition, for powder sintering, an alloy powder having a final composition is manufactured by a gas atomization method and used as a raw material powder, or a mixed powder in which a plurality of alloy powders and pure metal powders are mixed to have a final composition of a sputtering target material. It can be a raw material powder.

  As a method for sintering the raw material powder, it is possible to use pressure sintering such as hot isostatic pressing, hot pressing, discharge plasma sintering, and extrusion press sintering. In order to obtain a relative density of 98% or more that can be stably used as a sputtering target material, the sintering temperature is 850 to 1050 ° C., the pressing pressure is 20 MPa or more, and the sintering time is 1 to 10 hours. It is desirable.

First, raw materials each having a purity of 99.9% or more were mixed and melted in a vacuum melting furnace, and then an ingot having a composition formula in terms of atomic ratio of Cu-0.25% Zr-0.25% Cr was produced by casting. . Next, the ingot was machined to produce a sputtering target material (sample No. 1) for forming a heat sink layer of a heat-assisted magnetic recording medium having a diameter of 164 mm and a thickness of 4 mm.
As comparative examples, raw materials each having a purity of 99.9% or more were mixed and melted in a vacuum melting furnace, and then an ingot having a composition formula in terms of atomic ratio of Cu-0.50 atomic% Zr was produced by casting. Next, the ingot was machined to produce a sputtering target material (sample No. 2) for forming a heat sink layer of a heat-assisted magnetic recording medium having a diameter of 164 mm and a thickness of 4 mm.

The following evaluation was performed using each sputtering target material produced above.
(1) Surface Roughness Evaluation Each sputtering target material is placed in a chamber of a DC magnetron sputtering apparatus (model number: C-3010) manufactured by Canon Anelva Co., Ltd., and the vacuum reach is 2 × 10 ⁻⁵ Pa or less. After evacuating until it became, sputtering was performed under the conditions of Ar gas pressure 0.6 Pa and input power 1000 W. Using a 2.5-inch glass substrate, first, an underlayer (Ni-37.5Ta atom%) having a film thickness of 20 nm was formed at room temperature. A heat sink layer having a thickness of 100 nm was formed on the underlayer using the sputtering target material 1. Next, each sample was vacuum-heated at 660 ° C. with an infrared lamp heater in a sputtering apparatus, and a sample for surface roughness evaluation was produced. Thereafter, the extent of the measurement area 500 nm ^2, the arithmetic mean roughness defined by JIS B 0601-2001 (Ra) of Seiko Instruments Co., Ltd. of AFM (model number: SPA300) was measured by. In addition, the measurement of arithmetic average roughness (Ra) measured 3 points | pieces per sample, and employ | adopted the average value.

(2) Thermal conductivity evaluation The 2.5-inch glass was formed without forming the above-mentioned Ni-37.5 atomic% Ta undercoat film in the same manner as the sputtering apparatus and sputtering conditions used in the surface roughness evaluation. A 100 nm heat sink layer was formed on the substrate at room temperature. Next, each sample was subjected to a vacuum heat treatment at 660 ° C. in a vacuum furnace to prepare a sample for thermal conductivity evaluation.
The specific resistance of each obtained sample was measured at room temperature by the 4-probe method, and the thermal conductivity was calculated according to the Wiedemann-Franz rule. The measurement results of the surface roughness and the calculation results of the thermal conductivity are shown in Table 1, FE-SEM images are shown in FIGS. 2 and 4, and AFM images are shown in FIGS.

Sample No. of the present invention example. 1 and Comparative Sample No. As shown in Table 1, the heat conductivity of the heat sink layer formed with the sputtering target material No. 2 has a value of 200 W / m · K or more, and has sufficient characteristics as the heat sink layer of the heat-assisted magnetic recording medium. It was confirmed that it was obtained.
Sample No. of Comparative Example In the heat sink layer formed of the sputtering target material in which only Zr was added to Cu of 2, a dent along the crystal grain boundary was observed as indicated by an arrow in FIG. 2. Further, the heat sink layer formed of the sputtering target material in which only Zr is added to Cu of the comparative example has a large surface roughness (Ra) as shown in Table 1 and FIG. 5, and the surface smoothness cannot be ensured. Was confirmed.
On the other hand, sample No. As shown in FIG. 4, it was confirmed that the dent along the crystal grain boundary was reduced in the heat sink layer formed of the sputtering target material in which a specific amount of Zr and Cr was added to 1 Cu. In addition, Sample No. As shown in Table 1 and FIG. 6, the heat sink layer formed of the sputtering target material 1 has a small surface roughness (Ra) and a smooth surface, and is used as a heat sink layer of a heat-assisted magnetic recording medium. It was confirmed that it was effective.

Zr powder and Cr powder were mixed with Cu powder so as to be substantially the same as the target composition of the heat sink layer shown in Table 2, and filled in a pressurized vessel made of mild steel. Then, after performing pressure sintering by hot isostatic pressing under conditions of a temperature of 950 ° C. and a pressure of 120 MPa for 1 hour to produce a sintered body, the obtained sintered body was machined to have a diameter of 164 mm, Sputtering target materials (samples No. 3 to No. 5) for forming a heat sink layer of a heat-assisted magnetic recording medium of the present invention having a thickness of 4 mm were produced.
As a comparative example, only Cr powder was mixed with Cu powder and filled into a pressure vessel made of mild steel. Then, after performing pressure sintering by hot isostatic pressing under conditions of a temperature of 950 ° C. and a pressure of 120 MPa for 1 hour to produce a sintered body, the obtained sintered body was machined to have a diameter of 164 mm, A sputtering target material (sample No. 6) for forming a heat sink layer of a heat-assisted magnetic recording medium having a thickness of 4 mm was produced.

The following evaluation was performed using each sputtering target material produced above.
(1) Surface Roughness Evaluation After a surface roughness measurement sample was prepared in the same manner as in Example 1, the arithmetic average roughness defined in JIS B 0601-2001 using the AFM described in Example 1 ( Ra) was measured. The arithmetic average roughness (Ra) was measured with an area of 10 μm ² . Further, the arithmetic average roughness (Ra) was similarly measured with an area of 10 μm ² for the heat sink layers formed using the sputtering targets of Sample 1 and Sample 2 described in Example 1.
(2) Evaluation of thermal conductivity A sample for measuring thermal conductivity was prepared and evaluated by the same method as in Example 1. Table 2 shows the calculation results of the thermal conductivity.

Sample No. 1 in which a specific amount of Zr and Cr were added in combination to Cu in the inventive example. 1, no. 4 and no. In the heat sink layer formed using the sputtering target material of No. 5, sample No. 5 in which only Zr was added to Cu of the comparative example. It can be seen that the thermal conductivity higher than that in the case of film formation using 2 is obtained, and that the higher the thermal conductivity, the better the sputtering target for forming the heat sink layer, which is preferable.
Further, in the present invention example in which a specific amount of Zr and Cr was added to Cu, in Sample No. It was confirmed that the heat sink layer formed using the sputtering target material No. 5 can reduce the depth of the void and reduce the surface roughness (Ra) as indicated by the arrow in FIG. In addition, as can be seen from Table 2, the sample No. 1 and sample no. 3-No. When the sputtering target No. 5 is used, the surface roughness (Ra) can be made smaller than when the film is formed using the sputtering target of Sample 2 in which only Zr is added to Cu of the comparative example, and smoothness is improved. It was confirmed that an excellent heat sink layer could be formed.
In contrast, sample No. 1 in which only Cr was added to Cu. It was confirmed that the heat sink layer formed using the sputtering target No. 6 had a large surface roughness (Ra) and could not ensure smoothness.

DESCRIPTION OF SYMBOLS 1 Nonmagnetic board | substrate 2 Heat sink layer 3 Soft magnetic backing layer 4 Orientation control layer 5 Recording layer 6 Protective layer

Claims (1)

Composition formula in atomic ratio _{Cu 100-x-y -Zr x} -Cr y, 0.10 ≦ x ≦ 0.50, is represented by 0.10 ≦ y ≦ 1.00, the balance being unavoidable impurities A sputtering target material for forming a heat sink layer of a thermally assisted magnetic recording medium.