Patent Application
20110162322
The present invention relates to a method for storing a target comprising a rare earth metal or oxide thereof that is easily pulverized due to oxidation or hydroxylation.
Rare earth metals are contained in the earth's crust as a mixed composite oxide. Rare-earth elements are so called because they are separated from relatively rare minerals, but they are not that rare in light of the overall earth's crust. In recent years, rare earth metals are attracting attention as an electronic material, and research and development for using rare earth metals are being promoted.
Among the rare earth metals, lanthanum (La) is attracting particular attention. To briefly introduce lanthanum, lanthanum is a white metal having an atomic number of 57 and an atomic weight of 138.9, and comprises a double hexagonal close-packed structure at normal temperature. Lanthanum has a melting point of 921° C., boiling point of 3500° C., and density of 6.15 g/cm3, its surface is oxidized in the atmosphere, and it gradually melts in water.
Lanthanum is soluble in hot water and acid. Although it is not ductile, it is slightly malleable. Lanthanum's resistivity is 5.70×10−6 Ωcm, and it becomes oxide (La2O3) when burned at 445° C. or higher (refer to Dictionary of Physics and Chemistry). With rare earth elements, it is generally said that compounds with the oxidation number 3 are stable, and lanthanum is also trivalent.
Lanthanum is a metal that is attracting attention as an electronic material such as a metal gate material or a high-dielectric constant (High-k) material. Rare earth elements other than lanthanum also have attributes similar to lanthanum.
A rare earth metal such as lanthanum is a material in which high purification is difficult to achieve since it is easily oxidized during the refining process. In addition, if a rare earth metal such as lanthanum is left in the atmosphere, there is a problem in that the handling thereof is difficult since it will become oxidized and tarnished in a short time.
In recent years, thinning of a gate insulator film is being demanded in the next-generation MOSFET, but with the SiO2 that has been conventionally used as the gate insulator film, the leak current increases due to the tunnel effect, and normal operation is becoming difficult.
Thus, as a substitute for the SiO2 described above, HfO2, ZrO2, Al2O3 and La2O3 with high dielectric constant, high thermal stability, and high energy barrier against the holes and electrons in the silicon have been proposed. in particular, among the foregoing materials, La2O3 is valued highly, and a research on its electrical properties and for its use as a gate insulator film in the next-generation MOSFET has been reported (refer to Non Patent Document 1). Nevertheless, with Non Patent Document 1, the subject of research is a La2O3 film, and it does not make any reference to the properties and behavior of La metal.
It could be said that rare earth metals such as lanthanum and their oxides are still in the research phase. When studying the properties of such rare earth metals and their oxides, if a rare earth metal or oxide thereof itself exists as a sputtering target material, it is possible to form a thin film of such rare earth metal or oxide thereof on a substrate. It will be easy to study the behavior at the interface with the silicon substrate, and the properties of a high-dielectric gate insulator film or the like after forming a rare earth metal compound. There is also a significant advantage in that the freedom of the target as a product will increase.
Nevertheless, even if a lanthanum sputtering target is prepared, as described above, it becomes oxidized in a short time in the atmosphere. Generally speaking, a stable oxide layer is formed on a metal target surface, but since it is extremely thin, it peels off during the initial stage of sputtering and does not affect the sputtering characteristics significantly. However, with a lanthanum sputtering target, the oxide layer becomes thick, and deterioration of the electrical conductivity will occur and thereby cause defective sputtering.
In addition, if the lanthanum sputtering target is left in the atmosphere for a long time, it reacts with the moisture in the air to become covered with white hydroxide powder and ultimately become pulverized, and it may even cause a problem of not allowing normal sputtering to be performed. Thus, after the target is prepared, it is necessary to take measures for preventing oxidation and hydroxylation such as by immediately vacuum-packing or coating the target with fats and oils.
As a method for storing rare earth metals, the standard method is to store the rare earth metals in mineral oil in order to prevent such rare earth metals from being exposed to the air. However, when using a rare earth metal for a sputtering target, it is necessary to clean the target before its use in order to remove the mineral oil. However, there is a problem in that the cleaning of the target itself is difficult due to its reactivity with oxygen, moisture and carbon dioxide.
Accordingly, under normal circumstances, it is necessary to store and package the target based on vacuum packing. However, even in a state where the target is vacuum packed, since pulverization caused by oxidation or hydroxylation will progress even with a small amount of moisture that penetrates the used film, it was difficult to store the sputtering target in enabled condition for a long time.
When reviewing the conventional background art, there are the following methods; namely, a method of covering the hollow-cathode-type sputtering target with a resin bag (refer to Patent Document 1), a method of coating the target with a plastic protective film (refer to Patent Document 2), a method of packaging the target using a film having a surface that does not release particles (refer to Patent Document 3), a method of preparing a target storage container with a transparent lid of acrylic resin and screwing shut the storage container (refer to Patent Document 4), and a method of encapsulating the sputtering target in a bag (refer to Patent Document 5). Nevertheless, since the foregoing Documents disclose that the target is encapsulated with a resin lid or a resin film, they are insufficient as a method for storing a target comprising a rare earth metal or oxide thereof.
[Non Patent Document 1] Written by Eisuke Tokumitsu and two others, “Research on Oxide Material for High-k Gate Insulator Film” The Institute of Electrical Engineers of Japan, Research Paper of Electronic Materials, Vol. 6-13, Pages 37 to 41, Published on Sep. 21, 2001
[Patent Document 1] International Publication No. W02005/037649
[Patent Document 4] Japanese Laid-Open Patent Publication No. H8-246135
[Patent Document 5] Japanese Laid-Open Patent Publication No. H4-231461
An object of this invention is to provide technology for allowing the long-term storage of a sputtering target in enabled condition by devising the method for storing a target comprising a rare earth metal or oxide thereof, and thereby inhibiting the pulverization of the target caused by the oxidation and hydroxylation of such target due to residual air or ingress of air.
The present invention provides:
The present invention additionally provides:
The present invention further provides:
When encapsulating and storing a conventional target comprising a rare earth metal or oxide thereof in a hermetic container or with a plastic film, if it goes unattended for a long time, it will react with oxygen and moisture and become covered with white hydroxide powder, and there is a problem in that normal sputtering cannot be performed. Nevertheless, the target that is stored in a storage container or a film-type seal according to the present invention yields a significant effect of being able to avoid the foregoing problems.
Rare earths, in particular lanthanum and lanthanum oxide, are known to have extremely strong hygroscopic properties (reactivity with moisture). Thus, the challenge to date was how to prevent lanthanum and lanthanum oxide from absorbing moisture as much as possible, and how to store lanthanum and lanthanum oxide in an environment with minimal moisture.
Nevertheless, as a result of taking advantage of the fact that lanthanum oxide has a higher hygroscopic property than lanthanum, the oxidation or hydroxylation of the lanthanum target body can be prevented by coating, mounting or placing lanthanum oxide (powder or sintered compact of a plate or block) on the target surface upon storing the lanthanum target.
Moreover, when storing a lanthanum oxide target, moisture can be absorbed and eliminated more effectively by enclosing the powdered or granular lanthanum oxide of larger surface area and, therefore, degradation of the target caused by hydroxylation can be prevented.
Specifically, the method for storing a target comprising a rare earth metal or oxide thereof according to the present invention is to introduce, as a desiccant, oxide of the same rare earth metal as the material of the rare earth metal or its oxide target to be stored into a storage container or a film-type seal.
Here, even if lanthanum oxide reacts with moisture to be hydroxylated and pulverized and it adheres to the target surface, since it is a compound of the identical metal in a powdered form, the foregoing powder will not cause any contamination since it can be eliminated easily. This is a significant advantage in comparison to the case of using a desiccant made of other metals.
Moreover, in cases where the contamination by other rare earth metals will not cause a problem: it is possible to introduce oxide of rare earth metal having a greater hygroscopic property than the material of the rare earth metal or its oxide target to be stored, as a desiccant, into a storage container or a film-type seal for the target, and store the target by sealing the storage container or the film-type seal.
If the target is made of two or more types of rare earth metals or their oxides, an oxide of the rare earth metal with the greatest hygroscopic property can be used as a desiccant.
As an encapsulating and storing method for target, it is preferable to prevent the ingress of outside air as much as possible. Vacuum sealing can be performed as one such method. Moreover, when vacuum sealing and storing the target, it is preferable to once replace the inside of the container or the film-type seal with inert gas having a dew point of −80° C. or less and thereafter perform vacuum sealing. As means for encapsulating and storing the target, flexible film may be used in the form of a hermetic bag and subject to vacuum sealing.
Although the case of performing vacuum sealing was explained above, inert gas having a dew point of −80° C. or less may be filled and sealed as the method for storing a target. All of the foregoing methods prevent the ingress of outside air.
As described above, contact with outside air is blocked and the ingress of moisture in the outside air is inhibited as much as possible. Nevertheless, even if there is slight ingress of outside air, hydroxylation of the target body can be inhibited by mounting or filling the foregoing rare earth oxide to be used as the desiccant in the space created when the target is encapsulated.
A target is generally bonded to a backing plate. For instance, when using a flexible film in the form of a hermetic bag and performing vacuum sealing thereto, unevenness will inevitably arise between the target and the backing plate, and a space is likely to be formed. Outside air is easily accumulated in such space. Pulverization of the target tends to advance from such space. Thus, it is desirable to fill the rare earth oxide as the desiccant at such uneven part or in such space.
In this respect, it should be easy to understand that the rare earth oxide to be used as the desiccant is preferably in the form of powder or granule of large surface area. Nevertheless, it is effective to simply place a small piece of rare earth oxide at a location where outside air is easily accumulated.
Moreover, although it is most effective to place the rare earth oxide and the target to be in direct contact, adhesion of powder to the target surface may cause the generation of particles during sputtering. In the foregoing case, a sufficient effect can be yielded even by enclosing the rare earth oxide packed with a permeable film as with a general desiccant.
The method for storing a target according to the present invention is particularly effective for a lanthanum target or a target containing lanthanum as the rare earth metal configuring the target. Moreover, the foregoing rare earth oxide to be used as the desiccant is lanthanum oxide. This is ironic, but lanthanum oxide that is most easily hydroxylated has the greatest inhibitory effect on hydroxylation of a target comprising a rare earth metal or oxide thereof in the method for storing a target comprising a rare earth metal or oxide thereof.
It is also important, as the method for storing a target comprising a rare earth metal or oxide thereof, to keep the amount of moisture penetration through the flexible film to be used in encapsulating and storing the target or the amount of moisture ingress into the storage container from the outside to be 0.1 g/m2·24 h or less and to prevent the ingress of moisture as much as possible.
Table 1 shows the favorable examples of the flexible film to be used in encapsulating and storing the target, as well as the non-favorable examples.
In Table 1, the examples having properties of GX Barrier (product name) or higher are effective. As shown in Table 1, GX Barrier (product name) and a bag containing Al foil are favorable. Table 1 shows representative examples, and it goes without saying that other flexible films may be used so as long as they satisfy the foregoing conditions.
Examples of implementing the present invention are now explained.
Incidentally, these Examples are merely illustrative, and the present invention shall in no way be limited thereby. In other words, various modifications and other embodiments based on the technical spirit claimed in the claims shall be covered by the present invention as a matter of course.
This is an example where the surface and side face of a La target are thinly coated with La oxide powder and vacuum packing is performed thereto. A specific example is shown in
As shown in
This is an example where La oxide powder (as with a silica gel bag) is placed in a space due to the unevenness with BP and vacuum packing is performed thereto. A specific example is shown in
As shown in
This is an example where a La oxide target is placed in a metallic container, La oxide powder is filled around the periphery of the La target, ambient air is once replaced with argon having a dew point or −80° C. or less, and vacuum sealing is subsequently performed thereto. A specific example is shown in
As shown in
This is an example where a metal alloy target comprising La and Er is placed in a metallic container, a sintered La oxide block is placed at the uneven part between the target and BP, air in the container is replaced with argon gas having a dew point of −80° C. or less, and vacuuming is subsequently performed thereto.
As shown in
This is an example where the La target was subject to vacuum sealing with a film. A specific example is shown in
This is an example where the La target was subject to vacuum sealing with a film, and silica gel was placed as a desiccant. A specific example is shown in
When silica gel is placed as shown in
This is an example where the La2O3 target was subject to vacuum sealing with a film. A specific example is shown in
As evident from the foregoing Examples and Comparative
Examples, upon storing a target comprising a rare earth metal or oxide thereof, it is evident that the introduction of oxide of the same rare earth metal as the material of the target comprising the rare earth metal or oxide thereof to be stored as a desiccant into a hermetic container or seal is extremely effective. It is thereby possible to effectively inhibit the pulverization of the target caused by the oxidation and hydroxylation of such target due to residual air or ingress of air.
Conventionally, if a rare earth metal or a rare earth metal oxide sputtering target is left in the atmosphere for a long time, it reacts with the moisture in the air and becomes covered with white hydroxide powder, and there is a problem in that normal sputtering cannot be performed. Nevertheless, the method for storing a target comprising a rare earth metal or oxide thereof according to the present invention enables to avoid the foregoing problems.
The method for storing a target comprising a rare earth metal or oxide thereof according to the present invention is to introduce, as a desiccant, oxide of the same rare earth metal as the material of the target comprising the rare earth metal or oxide thereof to be stored into a storage container or a film-type seal. It is thereby possible to effectively inhibit the condition where the target reacts with the moisture in the atmosphere and becomes covered with white hydroxide powder.
Accordingly, targets can be stably supplied as an electronic material such as a metal gate material or a high-dielectric constant (High-k) material, and the present invention is extremely useful industrially.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-277725 | Oct 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/068248 | 10/23/2009 | WO | 00 | 3/16/2011 |
