Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation
Reexamination Certificate
2000-09-21
2002-06-25
VerSteeg, Steven H. (Department: 1753)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Post sintering operation
C420S540000, C148S514000, C419S006000, C419S010000, C419S066000, C204S298130
Reexamination Certificate
active
06409965
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a sputtering target and its manufacturing method especially suitable for application to an alloy target used for sputtering in a manufacturing process of a magneto-optical recording medium.
2. Description of the Related Art
For years, price strategies have been important in the field of AV recording mediums such as mini discs (MDs) intended for general customers, and reduction of prices has been demanded continuously. To meet the demand for price reduction of mediums, reduction of manufacturing costs of mediums has been required. Regarding cost reduction of mediums, thickness of targets used for fabrication of recording materials of mediums is an important factor.
That is, for manufacturing a medium, magnetron sputtering, among others, is used for stacking its recording material. For deposition of the recording medium by magnetron sputtering, a target containing the recording material is used. Such a target includes as its major component a rare earth element which is a rare element and expensive. Conventionally, however, only 30% to 50% of the target was sputtered and contributed to deposition of the film, and the remainder of the target was abandoned. Therefore, it has been demanded to increase the ratio of a target actually sputtered and contributing deposition of a film by increasing the thickness of the target.
However, targets conventionally used for manufacturing mediums had magnetic permeability around 5. Therefore, the maximum thickness of a target enabling stable discharge of plasma and sputtering was about 8 mm.
Under the circumstances, various researches and developments have been made toward fabrication of targets having a thickness of 8 mm or more, and various proposals were presented.
For example, there was proposes a technique for manufacturing a target made by hot-pressing alloy powder 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 permeability not exceeding 3, and a method for manufacturing a magneto-optical recording alloy target made by mechanically crushing used targets into alloy powder and mixing it with new alloy powder and having a magnetic permeability not exceeding 3.
Literature 1 also teaches that a target having a low magnetic permeability, low containment of oxygen, single-sintered structure of a rare earth metal and a transition metal, and a thickness not less than 8 mm can be made by dissolving a source material and used targets, then making a quickly cooled alloy, and mechanically crushing the quickly cooled alloy into alloy powder.
However, the Inventor conducted various experiments concerning alloy powder as taught by Literature 1, and found that the alloy powder described in Literature contained an unacceptably large amount of metal impurities for practical use.
For example, there was proposed a technique for manufacturing a target made by hot-pressing alloy powder 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 permeability not exceeding 3, and a method for manufacturing a magneto-optical recording alloy target made by mechanically crushing used targets into alloy powder and mixing it with new alloy powder and having a magnetic permeability not exceeding 3.
The magneto-optical recording alloy target having a magnetic permeability not larger than 3, which is disclosed in Literature 1, also involves the problem that a sufficient leak magnetic flux cannot be obtained in any magnetron sputtering apparatus, and sputtering of this target is impossible.
That is, although the Inventor made a target having the magnetic permeability of 2.1 and the thickness of 10 mm from alloy powder as taught by Literature 1, and tried sputtering by setting this target in place of a sputtering apparatus, it could not sputter this target.
Furthermore, although Literature 1 does not teach a magneto-optical recording alloy target having a magnetic permeability not larger than 2.3, according to the Inventor's knowledge, a 10 mm thick target having a magnetic permeability around 2.3 as taught by Literature 1 cannot be sputtered.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a sputtering target and its manufacturing method enabling effective use of expensive rare earth metals, not adversely affecting the environment, contributing to reduction of the manufacturing cost, and ensuring a target with a magnetic permeability low enough for sputtering.
The Inventor made researches toward solution of the above-indicated problems involved in the conventional techniques. A summary thereof is shown below.
According to the Inventor's knowledge, in order to effective use an expensive rare earth metal for fabricating a target whose thickness is not less than 10 mm, it is desirable to produce recycled alloy powder by using used targets, and fabricate new targets by using the recycled alloy powder. So, the Inventor conducted various experiments regarding rare earth alloy power containing recycled alloy powder.
The Inventor first made reviews about theoretic density ratios of targets fabricated. According to the Inventor's knowledge, rare earth alloy powder is very readily oxidized, and if a target substantially made of rare earth alloy powder has a low theoretic density ratio, oxidation of the target itself progresses. Therefore, any medium made by using this target cannot have a satisfactory property. Relation between deposition rate in a sputtering process and target lifetime is shown in
FIG. 1
, taking two different cases where the theoretic density ratio of the target is 95% and 97%, respectively.
It is apparent from
FIG. 1
that, when the theoretic density ratio is high, i.e. 97%, fluctuation in deposition rate of sputtering is small. In other words, if thickness of the target is constant, the target lifetime is longer, and the manufacturing cost of mediums can be reduced when the theoretic density ratio is high.
Therefore, it is preferable for a target to have a theoretic density ratio not lower than 95%, and more preferably not lower than 97%. Discussion is continued below, selecting the case of the theoretic density ratio not lower than 97%.
For the purpose of determining composition of rare earth alloy powder, the Inventor conducted an experiment about dependency of magnetic permeability of rare earth alloy powder upon quantity of rare earth metals contained in rare earth alloy powder. A result of the experiment is shown in FIG.
2
.
It apparent from
FIG. 2
that magnetic permeability of rare earth alloy powder is 5 or higher when it contains about 20 weight % of rare earth metals, but decreases to 2 or lower when the content of earth metals therein is 35 weight % or higher. Therefore, in order to maintain a magnetic permeability not higher than 2 in rare earth alloy powder, the content of rare earth metal in the rare earth alloy powder is preferably not lower than 35 weight %, and more preferably not lower than 40 weight %.
The Inventor also conducted an experiment about magnetic permeability and theoretic density ratio of targets upon rare earth alloy powder contained in alloy powder, using targets prepared by using rare earth alloy powder. A result of the experiment is shown in FIG.
3
. In
FIG. 3
, values of magnetic permeability of targets are plotted with ▪ whereas values of theoretic density ratio are plotted with ∘.
It is apparent from
FIG. 3
that permeability of targets is larger than 2 when the content of rare earth alloy powder in alloy powder is less than 6
Kimura Hitoshi
Nagata Takahiro
Sasaki Manabu
Yokoyama Norio
Sonnenschein Nath & Rosenthal
Sony Corporation
VerSteeg Steven H.
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