Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2000-02-15
2001-10-30
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S363000, C205S367000
Reexamination Certificate
active
06309529
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for producing a sputtering target material, and more particularly to a method for producing a sputtering target material which is used for producing precious metal thin film.
BACKGROUND ART
Recently, a trend toward size reduction of electronic and electric apparatus has become more pronounced, along with demand for enhancing integration of LSIs, which are employed to control functions of these apparatus. In other words, electric circuits as well as each and every electronic element must have a higher degree of integration.
In the course of enhancing integration, precious metals such as ruthenium or iridium have been employed as materials which are more useful for providing an electrode to a wafer of semiconductor devices, since precious metals provide a thin-film electrode having excellent electrode properties.
A variety of methods, such as vacuum vapor deposition and CVD, have been employed for forming thin film contained in semiconductor devices. Of these methods, sputtering, which is one type of physical vapor deposition method, is most widely employed at present. Sputtering is a method for forming a metal thin film, and comprises the steps of causing particles such as argon ions to collide with a target constituting the material of a thin film to be produced, and depositing metal particles on a substrate released by exchange of momentum. Therefore, since the properties of the formed thin film are apt to vary depending on characteristics of the target material, such as purity, high purity is a critical requirement for a target material.
A precious metal target material has conventionally been produced through either one of the following typical methods; i.e., powder metallurgy, in which precious metal powder is shaped and sintered through hot-pressing (HIP); and a casting method in which a compact of ruthenium powder formed by hot-pressing is melted in a crucible by means of irradiation with an electron beam and solidified.
In the case of the casting method, the purity of a target material is easily controlled, to thereby obtain a target material of comparatively high purity. However, much energy is required for melting a precious metal having a high melting point, and during casting a raw material must be provided in an amount greater than that found in actual products. In addition, the method has a drawback in that the number of production steps is high, to thereby elevate production cost and product price.
Powder metallurgy can produce a sputtering target material at lower energy cost than can casting. Furthermore, powder metallurgy has an advantage of providing high yield. However, a binder cannot be used during production of a sputtering target material through powder metallurgy, since the material must have high purity. Thus, a metal powder constituting a sputtering target material must be sintered and solidified without use of a binder. Without use of a binder, appropriately shaping and sintering a metal powder is very difficult, as is determining parameters for several process conditions.
Even though a powder can be sintered without use of a binder, a raw material powder easily becomes contaminated or easily adsorbs impurities, and very careful storage of the raw material for producing a sputtering target material is required. Thus, when a sputtering target material is produced through powder metallurgy, it is difficult to produce a target material of uniform structure and purity which permits use of the target for producing an electronic element in the electronic industry, unless production conditions, including storage of raw material powder, are very strictly controlled. In addition, production through powder metallurgy comprises cumbersome steps, such as producing a raw material powder and hot-pressing. These steps increase production cost, to thereby disadvantageously elevate product price.
As described above, although several practical methods for producing a precious metal target material have been developed, these methods are not necessarily satisfactory in view of product properties and production cost. Thus, demand exists for a more efficient method for producing a precious metal target material.
DISCLOSURE OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a method for producing a sputtering target material, which method simplifies production steps and produces a high-purity target material.
The present inventors have conducted earnest studies in order to attain the above object, and have found that a sputtering target material can be directly produced through electrodeposition of a precious metal or a precious metal alloy from a molten salt mixture comprising a precious metal salt and a solvent salt.
The invention as recited in claim
1
is drawn to a method for producing a sputtering target material which comprises electrolyzing a molten salt mixture comprising a precious metal salt and a solvent salt, to thereby obtain a precious metal or a precious metal alloy serving as the target material. Briefly, the method enables direct production of a sputtering target material through electrolysis of a molten salt.
The reason why a molten salt mixture comprising a precious metal salt and a solvent salt is used will be described. If a high-purity raw material compound is used and appropriate electrolysis conditions are employed, a high-purity precious metal can be deposited, to thereby directly provide a high-quality target material through a single step. Furthermore, in the present invention, since a precious metal of interest is deposited electrochemically, a metal component which is more reactive than a precious metal is not mingled in the formed deposit. Therefore, the present invention provides a method for producing a sputtering target material containing a considerably low amount of a radioactive isotope, such as an isotope of thorium or an isotope of uranium, which may affect characteristics of a semiconductor. In contrast, during production through casting, metallic impurities such as radioactive isotopes may be mingled into the formed deposit regardless of the electrochemical properties of the impurities. Thus, the method of the present invention provides an excellent effect for preventing incorporation of such impurities.
Another advantage of electrolysis of a molten salt employed in the present invention is that a metal of interest can be obtained through electrodeposition performed at a temperature much lower than the melting point of a precious metal without requiring a high temperature such that the precious metal melts. Briefly, a target material can be produced with less energy than that required in casting. In addition, in the present invention, molds having a shape corresponding to the target of interest are not required, and a target material having a shape nearly equal to that of a final target is directly obtained by employing a cathode having a shape corresponding to that of the final target. In a final step, the thus-obtained target material is subjected to simple physical polishing, to thereby obtain a final product. The number of these steps carried out in the method of the present invention is much lower than that carried out in conventional methods.
The method of the present invention will next be compared with steps carried out in powder-sintering. The surface of powder which is sintered in the method is easily oxidized and contaminated. Therefore, the powder must be stored with maximum care under rigorous control. From the general view of fabrication process, a powder-sintering method requires a number of steps such as adjusting the particle size of powder, shaping the powder, and sintering the shaped powder. In contrast, the method of the present invention employing electrolysis of a molten salt can omit a considerable number of steps for producing a sputtering target.
In the method of the present invention, the term “solvent salt” refers to a molten salt such as a chloride or a cyanide compound which serves as an ionic conductor du
Hagiwara Ken
Hara Noriaki
Matsuzaka Ritsuya
Yarita Somei
Arent Fox Kintner & Plotkin & Kahn, PLLC
Tanaka Kikinozoku Kogyo K.K.
Valentine Donald R.
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