Patent Application
20150262752
The present invention relates to a sputtering target suitable for producing a rare-earth magnetic film by sputtering or pulse laser deposition and relates a method of producing the sputtering target.
In recent years, rare-earth magnets having excellent magnetic characteristics have been being reduced in size and have been improved in performance associated with reductions in size and weight of electronic devices. In particular, neodymium magnets have highest magnetic energy products among present magnets and are therefore expected to be applied to the field of energy, such as Micro Electro Mechanical Systems (MEMS) and energy harvest (environmental power generation), and the field of medical instrument.
The rare-earth magnetic thin films are known that they are formed by sputtering (Patent Literature 1, Non-Patent Literature 1) or physical vapor deposition (PVD) such as pulse laser deposition (Patent Literature 2, Non-Patent Literature 2), and various researches and developments for such films have been conducted. Unfortunately, the rare-earth magnetic thin films formed by these methods have not yet obtained magnetic characteristics compatible to those of bulk magnets and have not been yet commercialized at present.
There are many studies on the mechanism of coercive force of neodymium magnets. For example, it is believed that in order to assure coercive force, it is important to isolate main phases from each other in terms of magnetism by uniformly surrounding the main phase of refined Nd2Fe14B with a nonmagnetic Nd-rich phase or to reduce the lattice instability at the interface between a main phase and a Nd-rich phase or the reverse magnetic domain that is formed by impurities (Patent Literature 3, Non-Patent Literature 3).
From these viewpoints, the target materials used in production of rare-earth magnetic thin films preferably include fine and uniform crystalline grains having a high purity. Regarding the purity, it is reported that in particular, a gas component, oxygen, highly affects the magnetic characteristics (Patent Literature 4, Non-Patent Literature 4). In addition, since a large variation in the composition of a thin film affects the magnetic characteristics, it is known that it is important to reduce the void and segregation of a target material and to achieve a constant composition ratio of constituent elements in the thickness direction of the target material.
The target material can be produced by a melting process or a sintering process. The melting process can provide a target material having a high purity and a high density, but has a difficulty in control of the grain diameter and composition. Accordingly, the sintering process, which is excellent in control of grain diameter and composition, is usually employed. However, the sintering process requires a large number of production steps compared to the melting process and has a problem in the production steps of easily causing oxygen contamination, which highly affects the magnetic characteristics of rare-earth magnetic thin films. Thus, effective interruption of oxygen contamination has been required.
SUMMARY OF INVENTION
It is an object of the present invention to provide a sintered compact target that can form rare-earth magnetic thin films, in particular, neodymium magnetic (Nd—Fe—B-based magnetic) thin films, having good magnetic characteristics with excellent mass productivity and reproducibility, and provide a method of producing the sintered compact target.
The present inventors have diligently studied for solving the above-described problems and, as a result, have found that the magnetic characteristics of a rare-earth magnetic thin film can be improved by strictly controlling the crystal grain diameter, relative density, compositional variation, and impurity concentration of a target.
On the basis of these findings, the present invention provides:
The present invention allows stable formation of films by sputtering or pulse laser deposition by strictly controlling the crystal grain diameter, relative density, compositional variation, impurity concentration, etc. of a rare-earth magnetic target, and has excellent effects of improving the magnetic characteristics of rare-earth magnetic thin films and increasing the productivity.
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The rare-earth magnetic target of the present invention comprises neodymium (Nd), iron (Fe), and boron (B) as essential components and can optionally comprise elements that are publicly known as component compositions of rare-earth magnets, for example, rare-earth elements such as Dy, Pr, Tb, Ho and Sm, transition metal elements such as Co, Cu, Cr and Ni, and typical metal elements such as Al.
The target of the present invention is characterized in that the target is constituted of fine and uniform crystalline grains having an average crystal grain diameter of 10 to 200 μm. In general, coercive force is in inverse proportion to the logarithm of square of the crystal grain diameter. Accordingly, a reduction in grain size is effective, and the average crystal grain diameter is reduced to 200 μm or less. The crystalline grains are manufactured by gas atomization having a risk of increasing oxygen content with an increase of the surface area of the gas atomized powder. Accordingly, the average crystal grain diameter is controlled to be 10 μor more.
The rare-earth magnetic target of the present invention is characterized in that the target has a relative density of 97% or more, preferably 99% or more. Stable deposition by sputtering or pulse laser deposition can be achieved by reducing, for example, voids of the target and increasing the density, and the compositional variations in the resulting thin films can be reduced.
The target of the present invention is also characterized in that the compositional variation of neodymium in the thickness direction is low and, preferably, that the coefficient of variation is 10% or less. Herein, the coefficient of variation is determined by measuring component compositions at a plurality of arbitrary positions along the thickness direction of the target and being calculated from the obtained average value and standard deviation of the neodymium compositions by an expression: coefficient of variation=(standard deviation)/(arithmetic mean value)×100 (%). The control of the thus-calculated coefficient of variation to 10% or less can also reduce the compositional variation in a rare-earth magnetic thin film and can prevent the deterioration of magnetic characteristics.
The target of the present invention is further characterized in that the target is a high-purity target having a low impurity content, in particular, that the content of oxygen, which is a gas component, is 1000 wtppm or less.
It is known that among gas components, oxygen considerably affects magnetic characteristics. Accordingly, stable and good magnetic characteristics can be provided by reducing the oxygen content as much as possible.
The rare-earth magnetic target of the present invention can be produced by, for example, as follows.
Neodymium (Nd) and iron (Fe) each having a purity of 3N5 (99.95%) or more, preferably 4N (99.99%) or more, and more preferably 4N5 (99.995%) or more and boron (B) or ferroboron having a purity of 3N (99.9%) or more are prepared as essential raw materials.
An alloy ingot is then produced by melting and casting the raw materials in a high vacuum of about 2×10−4 Torr or less. The alloy ingot is then re-melted, followed by gas atomization in an inert gas to produce a fine powder. Subsequently, the thus-prepared fine powder is sintered by hot pressing or hot isostatic pressing into a sintered compact. The sintered compact is machined by, for example, surface polishing into a rare-earth magnetic target for forming thin films.
In the melting and casting in above, a cold crucible melting process using a water-cooled copper crucible is desirably employed. This melting process can prevent impurity contamination from a crucible, compared to a vacuum induction heating using an ordinary magnesium crucible or aluminum crucible, and can produce an ingot having a high purity.
In the production of a fine powder, as described above, it is preferred to employ a gas atomizing method involving rapid solidification by high speed jetting of an inert gas from a nozzle in an inert gas atmosphere. The residual oxygen in the atmosphere tends to form a native oxide film on the surface of the fine grain during the rapid solidification. It is therefore important to perform vacuuming immediately after the input of the raw materials in an atomization apparatus, and then introduce an inert gas.
The inert gas herein can be nitrogen gas, argon gas, helium gas, or a gas mixture thereof. The average crystal grain diameter can be controlled to 10 to 200 μm by varying the pressure of the gas jetted at a high speed in the gas atomization apparatus from 0.5 to 2 MPa. The average crystal grain diameter herein refers to the 50% cumulative diameter of measured grain size distribution.
Subsequently, the resulting fine powder is sintered by hot pressing or hot isostatic pressing. In order to prevent contamination with oxygen, the sintering is performed in an inert atmosphere or in a high vacuum of about 5×10−4 Torr or less. Regarding sintering conditions, when the pressure was 15 MPa, a temperature of 600° C. left an unsintered portion, whereas a temperature of 650° C. or more realized complete sintering. However, partial seizure occurs in the mold by increasing the sintering temperature and the sintering pressure. The pressure not causing seizure at a temperature of 950° C. was 25 MPa. The sintering conditions that allow sintering not causing seizure and providing a relative density of 97% or more are a temperature of 700° C. or more and 950° C. or less and a pressure of 10 MPa or more and 25 MPa or less as shown in
The resulting sintered compact is machined by, for example, grinding or polishing into a target form suitable for a use. The resulting target can be used for forming rare-earth magnetic thin films by sputtering or pulse laser deposition.
The present invention will now be described on the basis of an Example and a Comparative Example. The Example is merely illustrative, and the invention is not intended to be limited thereby. That is, the present invention is limited only by the claims and includes various modifications within the scope of the present invention in addition to the following Example.
Neodymium having a purity of 3N5, iron having a purity of 4N, and ferroboron having a purity of 2N were prepared as raw materials. All the raw materials were in block forms. These materials were weighed to give a composition of Nd15—Fe75—B10. The materials were then fed into a cold crucible furnace as a water-cooled copper crucible, and subjected to melting in a vacuum of 1×10×4 Torr at 1320° C. for 60 minutes or more to produce about 6 kg of an alloy ingot.
Subsequently, the upper, bottom, and side portions of the ingot were ground and was then cut into blocks. The blocks were fed in a gas atomization apparatus. The apparatus was evacuated to 1×10−2 Torr, and an inert gas was then introduced into the apparatus. The temperature of the apparatus was increased to 1420° C. and was maintained for about 10 minutes. Subsequently, an inert gas was jetted at about 1.5 MPa to the dropped molten metal to give a fine powder having an average grain diameter of about 60 μm as shown in
Subsequently, the fine powder was charged in a mold for pressing, was applied with a pressure of 15 MPa in a vacuum atmosphere, and was sintered at a temperature of 900° C. for 2 hours, followed by cooling to ordinary temperature. The side portions and the upper and bottom surfaces of the resulting sintered compact were ground and polished into a disk-shaped target having a diameter of 76 mm and a thickness of 4 mm as shown in
Subsequently, the compositional variations of Nd, Fe, and B in the thickness direction of the resulting target were measured with an EPMA within a range of 1.54 mm from the surface at 4-μm intervals. The results are shown in
Subsequently, the target was attached to a backing plate, and a Ta buffer layer, a NdFeB layer (40 nm), and a Ta cap layer were continuously formed by sputtering at an Ar pressure of 1×10−2 Torr on a Si substrate provided with a thermal oxide film. A Ta layer was separately formed using a tantalum target. The B-H curve of the rare-earth magnetic thin film showed a coercive force of 1.1 T. Thus, good magnetic characteristics were achieved.
A target was produced by a melting process, unlike Example 1. Neodymium having a purity of 3N5, iron having a purity of 4N, and ferroboron having a purity of 2N were prepared as raw materials, and were weighed to give a composition of Nd15—Fe75—B10. The materials were then fed into a cold crucible furnace as a water-cooled copper crucible, and subjected to melting in a vacuum of 1×10−4 Torr at 1320° C. for 60 minutes or more to produce about 6 kg of an alloy ingot. In order to prevent microcracking in the ingot, the alloy ingot was cooled by furnace cooling. Subsequently, the upper, bottom, and side portions of the ingot were ground, and the side portions and the upper and bottom surfaces were then ground and polished into a disk-shaped target having a diameter of 76 mm and a thickness of 4 mm. The average crystal grain diameter of the target was about 210 μm in observation with an SEM as in Example 1. The relative density of the target material measured by an Archimedes method was 100%.
Furthermore, the compositional variations of Nd, Fe, and B in the thickness direction of the resulting target were measured as in Example 1 with an EPMA within a range of 1.54 mm from the surface at 4-μm intervals. The results in the target material of Comparative Example 1 were that the coefficient of variation of the Nd composition was 30%, the coefficient of variation of the Fe composition was 32%, and the coefficient of variation of the B composition was 35%. Thus, the compositions significantly varied compared to those in the target material produced by the sintering process. The results of measurement of the gas component concentrations of the target by an LECO method were that the oxygen concentration was 340 ppm, the carbon concentration was 120 ppm, the nitrogen concentration was 10 ppm, and the hydrogen concentration was 40 ppm. Thus, gas components were contained therein.
Subsequently, the target was attached to a backing plate, and a Ta buffer layer, a NdFeB layer (40 nm), and a Ta cap layer were continuously formed as in Example 1 on a Si substrate provided with a thermal oxide film to form a rare-earth magnetic thin film. The B-H curve of the rare-earth magnetic thin film showed a coercive force of 0.7 T. Thus, good magnetic characteristics were not achieved.
The sintered compact target of the present invention can form a high-quality rare-earth magnetic thin film having good magnetic characteristics by sputtering or pulse laser deposition and is therefore useful in the field of energy, such as Micro Electro Mechanical Systems (MEMS) and energy harvest (environmental power generation), and the field of medical instrument.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-012788 | Jan 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/076186 | 9/27/2013 | WO | 00 |
