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`Samsung Electronics Co., Ltd. v. Demaray LLC
`Samsung Electronic's Exhibit 1056
`Exhibit 1056, Page 1
`
`

`

`
`
`US. Patent
`
`
`
`
`Jun. 25,2002
`
`
`
`Sheet 1 0f5
`
`
`
`US 6,409,965 B1
`
`
`
`
`
`THEORETIC DENSITY
`
`
`
`RATIO 97%
`
`
`
`._THEORETIC DENSITY
`. -RATIO 95%
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`
`DEPOSITIONRATE(A/s)
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`
`LIFETIME OF TARGET (kWh)
`
`
`
`Ex. 1056, Page 2
`
`Ex. 1056, Page 2
`
`

`

`
`US. Patent
`
`
`
`
`
`Jun. 25, 2002
`
`
`
`
`Sheet 2 0f5
`
`
`
`US 6,409,965 B1
`
`
`
`PERMEABILITY
`OFRATEEARTHALLOYPOWDER
`
`
`
`MAGNETIC
`
`
`
`
`
`
`CONTENT OF RATE EARTH METALS
`
`
`
`
`
`
`
`IN RATE EARTH ALLOY POWDER (IN WEIGHT 0/0)
`
`Ex. 1056, Page 3
`
`Ex. 1056, Page 3
`
`

`

`
`US. Patent
`
`
`
`
`
`Jun. 25, 2002
`
`
`
`
`Sheet 3 0f5
`
`
`
`US 6,409,965 B1
`
`
`
`E
`t
`
`2
`g
`(J
`
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`:-
`a:
`E
`
`I—
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`
`CONTENT OF RATE EARTH ALLOY POWDER
`
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`
`IN ALLOY POWDER (IN WEIGHT %)
`
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`
`
` £2
`E
`
`
`9.
`
`*2
`a:
`E
`
`E
`C:
`
`
`2
`
`E
`
`
`
`8B
`
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`
`
`CONTENT OF RATE EARTH ALLOY POWDER
`
`
`
`
`HAVING THE MAGNETIC PERMEABILITY
`
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`
`
`
`
`OF 1.5 IN ALLOY POWDER (IN WEIGHT %)
`
`
`
`Ex. 1056, Page 4
`
`E
`._J
`a
`5
`
`g
`D-
`
`g
`5;
`
`E
`
`2
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`
`
`
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`E:
`3
`E
`E
`E
`o
`
`
`:
`
`E
`
`g
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`Ex. 1056, Page 4
`
`

`

`
`US. Patent
`
`
`
`
`
`Jun. 25, 2002
`
`
`
`
`Sheet 4 0f5
`
`
`
`US 6,409,965 B1
`
`
`
`
`
`Ex. 1056, Page 5
`
`Ex. 1056, Page 5
`
`

`

`
`US. Patent
`
`
`
`
`
`Jun. 25, 2002
`
`
`
`
`Sheet 5 0f5
`
`
`
`US 6,409,965 B1
`
`
`Fig. 6A
`
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`
`Fig. 6 B
`
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`
`0.?
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`Fig. 6C
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`12
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`
`10
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`
`Fig. GD
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`mm
`
`Ex. 1056, Page 6
`
`Ex. 1056, Page 6
`
`

`

`
`1
`SPUTTERING TARGET AND ITS
`
`
`
`MANUFACTURING METHOD
`
`
`BACKGROUND OF THE INVENTION
`
`
`1. Field of the Invention
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`This invention relates to a sputtering target and its manu-
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`facturing method especially suitable for application to an
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`alloy target used for sputtering in a manufacturing process of
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`a magneto-optical recording medium.
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`2. Description of the Related Art
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`For years, price strategies have been important in the field
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`of AV recording mediums such as mini discs (MDs) intended
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`for general customers, and reduction of prices has been
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`demanded continuously. To meet
`the demand for price
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`reduction of mediums, reduction of manufacturing costs of
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`mediums has been required. Regarding cost reduction of
`mediums, thickness of targets used for fabrication of record-
`
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`ing materials of mediums is an important factor.
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`is,
`That
`for manufacturing a medium, magnetron
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`sputtering, among others, is used for stacking its recording
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`material. For deposition of the recording medium by mag-
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`netron sputtering, a target containing the recording material
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`is used. Such a target includes as its major component a rare
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`earth element which is a rare element and expensive.
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`Conventionally, however, only 30% to 50% of the target was
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`sputtered and contributed to deposition of the film, and the
`remainder of the target was abandoned. Therefore,
`it has
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`been demanded to increase the ratio of a target actually
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`sputtered and contributing deposition of a film by increasing
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`the thickness of the target.
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`However, targets conventionally used for manufacturing
`
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`mediums had magnetic permeability around 5. Therefore,
`
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`the maximum thickness of a target enabling stable discharge
`
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`of plasma and sputtering was about 8 mm.
`Under the circumstances, various researches and devel-
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`opments have been made toward fabrication of targets
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`having a thickness of 8 mm or more, and various proposals
`
`
`were presented.
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`For example, there was proposes a technique for manu-
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`facturing a target made by hot-pressing alloy powder
`
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`obtained by mechanical crushing and having a magnetic
`
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`
`permeability not exceeding 3 (Japanese Patent Laid-Open
`
`
`
`
`
`
`Publication No. hei-10-251847 (Literature 1)). Literature 1
`
`
`
`
`
`teaches a method for manufacturing a magneto-optical
`
`
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`
`
`recording alloy target made by hot-pressing alloy powder
`
`
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`made by mechanical crushing and having a magnetic per-
`
`
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`
`meability not exceeding 3, and a method for manufacturing
`
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`
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`
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`a magneto-optical recording alloy target made by mechani-
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`
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`cally crushing used targets into alloy powder and mixing it
`
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`with new alloy powder and having a magnetic permeability
`
`
`not exceeding 3.
`
`
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`Literature 1 also teaches that a target having a low
`
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`magnetic permeability, low containment of oxygen, single-
`sintered structure of a rare earth metal and a transition metal,
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`and a thickness not
`less than 8 mm can be made by
`
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`dissolving a source material and used targets, then making a
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`quickly cooled alloy, and mechanically crushing the quickly
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`cooled alloy into alloy powder.
`However,
`the Inventor conducted various experiments
`
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`concerning alloy powder as taught by Literature 1, and
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`found that the alloy powder described in Literature con-
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`tained an unacceptably large amount of metal impurities for
`
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`practical use.
`
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`For example, there was proposed a technique for manu-
`
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`
`
`
`facturing a target made by hot-pressing alloy powder
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
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`
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`45
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`50
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`55
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`60
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`65
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`US 6,409,965 B1
`
`
`
`
`2
`
`
`
`
`
`
`obtained by mechanical crushing and having a magnetic
`
`
`
`
`
`
`permeability not exceeding 3 (Japanese Patent Laid-Open
`
`
`
`
`
`
`Publication No. hei-10-251847 (Literature 1)). Literature 1
`
`
`
`
`
`teaches a method for manufacturing a magneto-optical
`
`
`
`
`
`
`
`recording alloy target made by hot-pressing alloy powder
`
`
`
`
`
`
`
`made by mechanical crushing and having a magnetic per-
`
`
`
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`
`
`
`meability not exceeding 3, and a method for manufacturing
`
`
`
`
`
`
`
`a magneto-optical recording alloy target made by mechani-
`
`
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`
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`
`
`cally crushing used targets into alloy powder and mixing it
`
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`with new alloy powder and having a magnetic permeability
`
`
`not exceeding 3.
`
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`The magneto-optical recording alloy target having a mag-
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`netic permeability not larger than 3, which is disclosed in
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`Literature 1, also involves the problem that a sufficient leak
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`magnetic flux cannot be obtained in any magnetron sputter-
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`ing apparatus, and sputtering of this target is impossible.
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`That is, although the Inventor made a target having the
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`magnetic permeability of 2.1 and the thickness of 10 mm
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`from alloy powder as taught by Literature 1, and tried
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`sputtering by setting this target in place of a sputtering
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`apparatus, it could not sputter this target.
`
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`teach a
`Furthermore, although Literature 1 does not
`
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`
`
`magneto-optical recording alloy target having a magnetic
`
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`permeability not larger than 2.3, according to the Inventor’s
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`knowledge, a 10 mm thick target having a magnetic perme-
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`ability around 2.3 as taught by Literature 1 cannot be
`
`sputtered.
`OBJECTS AND SUMMARY OF THE
`
`
`
`INVENTION
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`It is therefore an object of the invention to provide a
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`sputtering target and its manufacturing method enabling
`
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`effective use of expensive rare earth metals, not adversely
`
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`affecting the environment, contributing to reduction of the
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`manufacturing cost, and ensuring a target with a magnetic
`
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`
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`permeability low enough for sputtering.
`The Inventor made researches toward solution of the
`
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`
`
`above-indicated problems involved in the conventional tech-
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`niques. A summary thereof is shown below.
`
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`
`
`According to the Inventor’s knowledge, in order to effec-
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`tive use an expensive rare earth metal for fabricating a target
`whose thickness is not less than 10 mm, it is desirable to
`
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`produce recycled alloy powder by using used targets, and
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`fabricate new targets by using the recycled alloy powder. So,
`
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`the Inventor conducted various experiments regarding rare
`
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`earth alloy power containing recycled alloy powder.
`The Inventor first made reviews about theoretic density
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`ratios of targets fabricated. According to the Inventor’s
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`knowledge, rare earth alloy powder is very readily oxidized,
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`and if a target substantially made of rare earth alloy powder
`has a low theoretic density ratio, oxidation of the target itself
`
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`progresses. Therefore, any medium made by using this target
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`cannot have a satisfactory property. Relation between depo-
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`sition rate in a sputtering process and target
`lifetime is
`shown in FIG. 1,
`taking two different cases where the
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`theoretic density ratio of the target is 95% and 97%, respec-
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`tively.
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`It is apparent from FIG. 1 that, when the theoretic density
`
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`ratio is high,
`i.e. 97%, fluctuation in deposition rate of
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`sputtering is small. In other words, if thickness of the target
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`is constant, the target lifetime is longer, and the manufac-
`turing cost of mediums can be reduced when the theoretic
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`density ratio is high.
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`Therefore, it is preferable for a target to have a theoretic
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`
`
`density ratio not lower than 95%, and more preferably not
`
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`
`
`Ex. 1056, Page 7
`
`Ex. 1056, Page 7
`
`

`

`
`
`US 6,409,965 B1
`
`
`3
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`lower than 97%. Discussion is continued below, selecting
`
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`the case of the theoretic density ratio not lower than 97%.
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`For the purpose of determining composition of rare earth
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`alloy powder, the Inventor conducted an experiment about
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`dependency of magnetic permeability of rare earth alloy
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`powder upon quantity of rare earth metals contained in rare
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`earth alloy powder. A result of the experiment is shown in
`FIG. 2.
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`It apparent from FIG. 2 that magnetic permeability of rare
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`earth alloy powder is 5 or higher when it contains about 20
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`weight % of rare earth metals, but decreases to 2 or lower
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`when the content of earth metals therein is 35 weight % or
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`higher. Therefore, in order to maintain a magnetic perme-
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`the
`ability not higher than 2 in rare earth alloy powder,
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`content of rare earth metal in the rare earth alloy powder is
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`preferably not lower than 35 weight %, and more preferably
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`not lower than 40 weight %.
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`The Inventor also conducted an experiment about mag-
`20
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`netic permeability and theoretic density ratio of targets upon
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`rare earth alloy powder contained in alloy powder, using
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`targets prepared by using rare earth alloy powder. A result of
`
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`the experiment is shown in FIG. 3. In FIG. 3, values of
`
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`magnetic permeability of targets are plotted with I whereas
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`values of theoretic density ratio are plotted with o.
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`It is apparent from FIG. 3 that permeability of targets is
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`larger than 2 when the content of rare earth alloy powder in
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`alloy powder is less than 65 weight %, and becomes 2 or less
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`when the content of the rare earth alloy powder in alloy
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`powder is not less than 65 weight %, or preferably not less
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`than 70 weight %, taking errors into account.
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`Thus, content of rare earth alloy powder in alloy powder
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`as the source material of a target should be not less than 65
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`weight %, and more preferably not less than 70 weight %.
`35
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`The Inventor also conducted an experiment about mag-
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`netic permeability and theoretic density ratio of targets. FIG.
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`4 shows dependency of magnetic permeability and theoretic
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`density ratio of targets upon content of rare earth alloy
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`powder in alloy powder, when magnetic permeability of the
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`rare earth alloy powder is controlled not to exceed 2 (more
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`specifically around 1.5).
`In FIG. 4, values of magnetic
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`permeability of targets are plotted with I whereas values of
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`theoretic density ratio are plotted with o.
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`It is apparent from FIG. 4 that magnetic permeability of
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`targets becomes larger than 2 when amount of rare earth
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`alloy powder, having a magnetic permeability not larger than
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`2, contained in alloy powder used as the source material of
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`targets is less than 50 weight %, and becomes 2 or less when
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`50 weight % or more.
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`Therefore, content of rare earth alloy powder having a
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`magnetic permeability not higher than 2 in alloy powder
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`used as the source material of targets is preferably controlled
`not to be lower than 50 weight %.
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`Through those various experiments and researches, the
`Inventor has come to know that, in order to control magnetic
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`permeability of a target not to exceed 2 while using recycled
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`alloy powder, it is necessary to control the content of rare
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`earth alloy powder including recycled alloy powder con-
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`tained in alloy powder used as the source material of the
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`target not to exceed 65 weight %.
`The invention has been made through those researches
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`and accompanying experiments.
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`According to the first aspect of the invention, there is
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`provided a sputtering target made of alloy powder which
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`contains at least 65 weight percent of at least one kind of rare
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`earth alloy powder made of at least one kind of rare earth
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`10
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`4
`element and at least two kinds of elements selected from the
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`group consisting of Fe, Co, Ni, Cr and Si, and contains at
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`least one kind of recycled alloy powder prepared by using a
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`target used at least once for sputtering.
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`In the first aspect of the invention, magnetic permeability
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`of the target is typically a value not larger than 2.
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`In the first aspect of the invention, thickness of the target
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`ensuring a sufficient
`leak magnetic flux intensity and
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`enabling sputtering is typically not less than 8 mm and not
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`more than 20 mm, or preferably not less than 10 mm and not
`more than 15 mm.
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`According to the second aspect of the invention, there is
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`provided a manufacturing method of a sputtering target
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`which manufactures a target from alloy powder which
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`contains at least 65 weight percent of at least one kind of rare
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`earth alloy powder made of at least one kind of rare earth
`element and at least two kinds of elements selected from the
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`group consisting of Fe, Co, Ni, Cr and Si, and contains at
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`least one kind of recycled alloy powder prepared by using a
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`target used at least once for sputtering.
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`In the second aspect of the invention, for the purpose of
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`ensuring a practical level of the content of metal impurities
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`in the alloy powder used for making the target, the recycled
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`alloy powder is prepared by powdering by an atomizing
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`method an ingot prepared by a target used at least once for
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`sputtering and a material not used before for making a target.
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`From the standpoint of making fine powder minimized in
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`composition segregation, typically used as the atomizing
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`method is a gas atomizing method. However, a single-roll
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`method or a centrifugal disc method is also usable.
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`In the present
`invention, recycled alloy powder is a
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`substance obtained by powdering an ingot prepared from a
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`target used at least once for sputtering, and a new material
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`not used before for making a target. In the present invention,
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`recycled alloy powder typically contains rare earth alloy
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`powder prepared from a target used at least once for sput-
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`tering by 30 weight % or more, or preferably by 50 weight
`% or more.
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`In the present invention, a rare earth element is the generic
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`name of lanthanoids, Sc (scandium) and Y (yttrium) to
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`lanthanoids. More specifically, it is the general name of La
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`(lanthanum, Ce (cerium), Pr
`(praseodymium), Nd
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`(neodymium), Pm (promethium), Sm (samarium), Eu
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`(europium), Gd (gadolinium), Tb (terbium), Dy
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`(dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb
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`(ytterbium), Lu (lutetium), Y and Sc.
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`In the present invention, alloy powder typically contains
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`50 weight % or more of rare earth alloy powder having a
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`magnetic permeability not larger than 2. In order to control
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`the magnetic permeability of the rare earth alloy powder not
`to exceed 2, content of rare earth metal in the rare earth alloy
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`powder is typically not less than 30 weight %, and prefer-
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`ably not less than 35 weight %.
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`In the present invention, in order to make a target having
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`a magnetic permeability not larger than 2, magnetic perme-
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`ability of alloy powder is controlled not to exceed 2.
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`In the present invention, content of metal impurities in the
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`target is typically not more than 0.1 weight %, and content
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`of metal impurities in the alloy powder is not more than 0.1
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`weight %.
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`In the present invention, theoretic density ratio of the
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`target is typically not less than 97%, and preferably not less
`than 98%.
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`In the present invention, the sputtering apparatus using
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`the target is typically a magnetron sputtering apparatus.
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`Ex. 1056, Page 8
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`Ex. 1056, Page 8
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`

`

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`US 6,409,965 B1
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`Explained below are embodiments of the invention with
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`reference to the drawings. In all figures illustrating the
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`embodiments, the same or equivalent parts or elements are
`labeled with common reference numerals.
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`First explained is a manufacturing method of a sputtering
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`target according to the first embodiment of the invention.
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`In the sputtering target manufacturing method according
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`to the first embodiment, a target of a quaternary FeTbCoCr
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`5
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`However, the target can be made in any other sputtering
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`apparatus, such as opposed-electrodes sputtering apparatus,
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`electron cyclotron resonance (ECR) sputtering apparatus,
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`high-frequency sputtering apparatus,
`reactive sputtering
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`apparatus, bias sputtering apparatus, collimate sputtering
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`apparatus or long-distance (LD) sputtering apparatus.
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`In the present invention, the sputtering target is preferably
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`made of alloy powder which contains at least one kind of
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`rare earth alloy powder selected from the group consisting of
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`FeTbCo, FeTbCr, FeTbCoCr, FeGdCo, FeDyCo and FeGd-
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`CoSi by 65 weight % or more, and containing at least one
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`kind of recycled alloy powder prepared by using a target
`which is made of at least one kind of rare earth alloy selected
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`from the group consisting of FeTbCo, FeTbCr, FeTbCoCr,
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`FeGdCo, FeDyCo and FeGdCoSi used at least once for
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`sputtering.
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`According to the sputtering target and its manufacturing
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`method having the above summarized structures according
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`to the invention, since the sputtering target is made of alloy
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`powder which contains at least one kind of rare earth alloy
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`powder made of at least one kind of rare earth element and
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`least
`two kinds of elements selected from the group
`at
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`consisting of Fe, Co, Ni, Cr and Si, and contains at least one
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`kind of recycled alloy powder prepared from a target used at
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`least once for sputtering, the sputtering target can be lowered
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`in magnetic permeability, and increased in thickness.
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`The above, and other, objects, features and advantage of
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`the present invention will become readily apparent from the
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`following detailed description thereof which is to be read in
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`connection with the accompanying drawings.
`BRIEF DESCRIPTION OF THE DRAWINGS
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`FIG. 1 is a graph showing relation between deposition rate
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`and target lifetime in a sputtering process according to the
`invention;
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`FIG. 2 is a graph showing dependency of magnetic
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`permeability of rare earth alloy powder used for manufac-
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`turing a target upon content of rare earth metals;
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`FIG. 3 is a graph showing dependency of magnetic
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`permeability and theoretic density ratio of a target according
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`to the invention upon content of rare earth alloy powder in
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`alloy powder;
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`FIG. 4 is a graph showing dependency of magnetic
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`permeability and theoretic density ratio of a target according
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`to the invention upon content of rare earth alloy powder
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`having a magnetic permeability not larger than 2 in alloy
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`powder;
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`FIG. 5 is a diagram roughly showing configuration of a
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`gas atomizing apparatus for preparing recycled alloy powder
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`according to the invention; and
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`FIGS. 6A through 6D are diagrams for explaining a
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`compression sintering process for manufacturing a target
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`according to the invention.
`DETAILED DESCRIPTION OF THE
`
`
`PREFERRED EMBODIMENTS
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`6
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`alloy used at least once for magnetron sputtering and new
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`metals which are the same metals (Fe, Tb, Co, Cr) compos-
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`ing the quaternary alloy are blended to contain the used
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`target by 30 weight % or more, such as 50 weight %, for
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`example,
`in the first embodiment, and an ingot is made
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`therefrom in a vacuum melting furnace (not shown). As
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`explained later, this ingot is processed into recycled alloy
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`powder containing rare earth alloy powder such that the
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`content of rare earth metals in the rare earth alloy powder
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`forming the recycled alloy powder is at least 35 weight %,
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`such as 42 weight %, for example, in the first embodiment.
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`After that,
`the ingot
`is powdered in an inactive gas
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`atmosphere like argon (Ar) gas by a gas atomizing process
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`to prepare recycled alloy powder of FeTbCoCr alloy. Mag-
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`netic permeability of this recycled alloy powder has been
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`confirmed to be 1.5 by measurement. A gas atomizing
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`apparatus used for the gas atomizing process is explained
`below.
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`As shown in FIG. 5, the gas atomizing apparatus accord-
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`ing to the invention includes a vacuum melting furnace 1,
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`granulating chamber 2, cyclone 3 and container 4 which are
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`connected in sequence.
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`The vacuum melting furnace 1 is a furnace for melting
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`materials by high-frequency induction heating, for example.
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`A nozzle 5 is provided directly under the vacuum melting
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`furnace 1, and a gas inlet 6 for jetting a gas is provided neat
`the nozzle 5.
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`The granulating chamber 2 is a processing chamber for
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`powdering a material molten in the vacuum melting furnace
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`1 by spraying it in form of fine mist and thereby preparing
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`a powder material. The cyclone 3 and the container 4 are
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`used for storing a powdered material.
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`By using the gas atomizing apparatus having the above-
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`explained configuration, recycled alloy powder of FeTb-
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`CoCr alloy is prepared.
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`That is, an ingot prepared from the used target and new
`metals of the same components is melted in the vacuum
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`melting furnace 1. After that, the molten material of the used
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`target is continuously sprayed into the granulating chamber
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`2 from the vacuum melting furnace 1 through the nozzle 5,
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`and an inactive gas such asAr gas, for example, is jetted into
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`the granulating chamber 2 through the gas inlet 6. As a
`result, the molten material form fine mist.
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`Thereafter, the molten material in form of fine mist is
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`quickly cooled and powdered into recycled alloy powder 7
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`while it falls down. Then, the recycled alloy powder 7 is
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`introduced into the cyclone 3. Thereafter, the recycled alloy
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`powder 7 falls from the cyclone 3 into the container 4 and
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`is stored there. Grain size and grain size profile of the
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`globular grains 7 is controlled taking account of the tem-
`perature of the molten material, flow rate of the molten
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`material from the nozzle 5, nozzle diameter, flow rate of the
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`jetted inactive gas, and so forth. After that, the recycled alloy
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`powder 7 is classified to form an optimum grain size profile.
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`After that, the recycled alloy powder 7 is blended with
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`powder containing Fe, Tb, Co and Cr, for example, and by
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`dry mixture thereof in an Ar gas atmosphere, alloy powder
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`for manufacturing the target is prepared. Respective mate-
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`rials of this alloy powder are blended such that the alloy
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`powder contains rare earth alloy powder by at least 65
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`weight %, or preferably by at least 70 weight %, and rare
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`earth alloy powder having a magnetic permeability not
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`larger than 2 by at
`least 50 weight %.
`In this first
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`embodiment, the alloy powder is prepared to contain 80
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`weight %, for example, of recycled alloy powder having a
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`magnetic permeability of 1.5 and contain another material as
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`65
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`Ex. 1056, Page 9
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`Ex. 1056, Page 9
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`the sputtering
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`7
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`its remainder part, which is adjusted to make a predeter-
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`mined target composition. Namely,
`they are blended to
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`enable fabrication of a target containing 66 atomic % of Fe,
`24 atomic % of Tb, 7 atomic % of Co and 3 atomic % of Cr.
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`As a result, mixed powder satisfying the above-indicated
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`requirements is prepared.
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`Subsequently, using this mixed powder,
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`target is manufactured.
`That is, first as shown in FIG. 6A, using a container 11
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`storing mixed powder 10, the mixed powder 10 is supplied
`into a carbon mold 12 from above it. Next, as shown in FIG.
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`6B, a compression punch rod 13 is inserted into the carbon
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`mold 12 from above the carbon mold and the mixed powder
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`10 supplied therein. After that, as shown in FIG. 6C, the
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`carbon mold 12 is set
`in a predetermined compression
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`sintering apparatus (not shown), and heated and sintered
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`there under a pressure applied onto the mixed powder 10
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`through the compression punch rod 13. After it is sintered,
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`as shown in FIG. 6D, the pressure is removed, and it is
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`cooled. Then, a sintered material 15 made by sintering the
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`mixed powder 10 is removed from the carbon mold 12.
`After that, the outer circumferential surface and top and
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`bottom surfaces of the sintered material 15 prepared as
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`explained above is cut approximately by 1 mm. As a result,
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`the intended column-shaped sputtering target, containing 66
`atomic % of Fe, 24 atomic % of Tb, 7 atomic % of Co and
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`3 atomic % of Cr, and having the diameter of 127 mm and
`thickness of 10 mm, is obtained.
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`Using sputtering targets actually made in this manner,
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`magnetic permeability thereof were measured.
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`For this measurement of magnetic permeability, after a
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`sample of the size of 8x6><0.5 mm is cut out from an eroded
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`portion of a target, its magnetism is removed by a demag-
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`netizer (not shown). After that, setting the sample in position
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`of a vibration sample magnetometer (VSM), a magnetic
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`field is applied to have a magnetic curve drawn. After that,
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`a tangential line is drawn along the initial portion of the
`magnetic curve, value of the Y axis relative to a coercive
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`force is read, and value of the magnetic permeability is
`calculated.
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`As a result of the measurement of magnetic permeability
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`by the VSM apparatus, the target prepared by the manufac-
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`turing method according to the first embodiment was con-
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`firmed to have the magnetic permeability of 1.2.
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`Additionally, theoretic density ratio, content of oxygen
`and content of metal impurities were also measured. As a
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`result, theoretic density ratio was 97%, content of oxygen
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