JP2000212678A - High purity tantalum for thin film formation and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an abnormal discharging phenomenon and the generation of particles at the time of sputtering by sputtering tantalum in which each content of Nb, W, Mo, transition metal elements, high m.p. metal elements, heavy metal elements other than those, radioactive elements such as U and alkali metal elements is controlled to a ratio equal to or below the specified one. SOLUTION: This high purity tantalum for thin film formation contains, by weight, <=10 ppm Nb, W and Mo, <=1 ppm transition metal elements, high m.p. metal elements and heavy metal elements other than those, <=1 ppb radioactive elements such as U and Th and <=1 ppm alkali metal elements such as Na and K. Each content of oxygen and carbon is desirably controlled to <=100 ppm. Ta2O5 is added to an electrolytic bath of fluoride-chloride or the like, and tantalum scrap having >=10% tantalum content is subjected to electrolytic refining at 600 to 1000 deg.C. High purity tantalum powder electrodeposited on a cathode is press-formed and is thereafter uniformly melted by an electron beam or the like to form into an ingot, which is cut into a target shape to obtain a sputtering target.

Description

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, high purity electrolytic deposition (electrodeposition) tantalum is obtained by subjecting a tantalum compound or tantalum scrap to molten salt electrolysis, and the high purity electrolytic deposition (electrodeposition) tantalum is dissolved by an electron beam or the like. The present invention relates to a high-purity tantalum effective for forming a thin film of a semiconductor device and a method for producing the same at low cost by removing volatile components, removing high-melting point metals, radioactive elements, alkali metals, heavy metals and gas components as much as possible. Note that all ppm or ppb described below means ppm by weight or ppb by weight (hereinafter, description of “weight” is omitted).

[0002]

2. Description of the Related Art In recent years, a variety of electronic devices have been born starting from the dramatic progress of semiconductors, and their performance has been improved and new devices have been developed every day. In such a situation, electronic device devices have been miniaturized and the degree of integration has been increasing. Many thin films are formed in many of these manufacturing processes, and tantalum is also used as a thin film for electronic devices as tantalum and its alloy film, tantalum silicide film, or tantalum oxide film due to its unique metallic properties. It is used to form In the case of forming such a thin film of tantalum (including an alloy and a compound), there is no substance contaminating the semiconductor device, that is, the semiconductor device itself requires extremely high purity.

[0003] Thin films used in semiconductor devices and the like tend to be thinner and shorter, and the distance between them is extremely small and the integration density is improved. The problem arises that impurities to be diffused into the adjacent thin film. As a result, the balance between the constituent materials of the self-film and the adjacent film is lost, and a major problem occurs in that the function of the film that must be originally possessed is reduced. In the process of manufacturing such a thin film, the thin film may be heated to several hundred degrees, and the temperature may increase during use of an electronic device incorporating the semiconductor device. Such a rise in temperature further increases the diffusion coefficient of the substance, and causes a serious problem in that the function of electronic devices is reduced due to diffusion.

In particular, in semiconductor devices, impurities such as U and T
Radioactive elements such as h have an adverse effect on MOS due to radiation, and alkali metals and alkaline earth metals such as Na and K are M
The transition interface or heavy metal such as Fe, Ni, Cr, etc. or W, M
Impurity refractory metals such as o and Nb have a problem that interface levels are generated and junction leakage occurs. For this reason, it is necessary to effectively control these so that they do not become a source of contamination to the semiconductor device through the tantalum material.

In general, the above-mentioned tantalum and its alloy film, tantalum silicide film, tantalum oxide film and the like are often formed by physical vapor deposition such as sputtering and vacuum vapor deposition. A film can also be formed by a chemical vapor deposition method such as the external gas phase reaction method. The sputtering method will be described as an example. This sputtering method is a method in which positive ions such as Ar + physically collide with a target provided on a cathode, and metal atoms constituting the target are emitted with the collision energy.

In the formation of this sputtered film, if impurities are present in the tantalum (alloy / compound) target, coarse particles floating in the sputter chamber adhere to the substrate and short-circuit the thin-film circuit, or form a thin-film projection. The amount of generated particles causing the increase increases, and if oxygen, carbon, hydrogen, nitrogen, and the like, which are gas components, are present in the target, during sputtering, an abnormal discharge which is considered to be caused by the sudden occurrence of the gas is caused, and the uniform discharge occurs. The problem that a film is not formed occurs.

In view of the above, it has been desired to produce high-purity tantalum having a reduced content of impurities such as transition metals, alkali metals, and other impurity elements as well as oxygen, which are impurities. Conventionally, as a method of producing this high-purity tantalum, potassium tantalum fluoride (K ₂ TaF ₇ ) crystal is reduced with sodium, and this is ingot-formed by electron beam melting. However, this method must use expensive potassium tantalum fluoride (K ₂ TaF ₇ ) crystals,
Furthermore, the manufactured tantalum is a radioactive element such as U or Th,
There is a problem that a large amount of impurities such as W, Mo, Nb and other high melting point metals and heavy metals exist. In addition, there is a method of reducing tantalum oxide (Ta ₂ O ₅ ) with carbon, but the product produced by this method has a further lower purity.
For this reason, high purity of true tantalum on an industrial scale has not been realized.

[0008]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, in particular, suppresses contaminants caused by interdiffusion of materials constituting a laminated thin film, and exhibits an abnormal discharge phenomenon when forming a film by sputtering. It is an object of the present invention to provide high-purity tantalum that can be used as a target or the like capable of limiting particles and particles as much as possible, and a method for producing the same.

[0009]

In order to solve the above-mentioned problems, the present invention provides a method in which each element of 1 Nb, W and Mo contains 10 ppm.
Hereinafter, the contents of transition metal elements, high melting point metal elements and heavy metal elements other than those described above are 1 ppm or less, the contents of radioactive elements such as U and Th are 1 ppb or less, and the contents of alkali metal elements such as Na and K. 2. The high-purity tantalum 2 for forming a thin film, wherein the content of oxygen and carbon as gas components is 100 ppm or less. A method for producing high-purity tantalum for forming a thin film, characterized in that tantalum scrap is subjected to molten salt electrolysis to obtain high-purity tantalum by electrolytic deposition, and then dissolved by an electron beam or the like to remove volatile components. ,I will provide a.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION As a raw material of tantalum used in the present invention, tantalum scrap can be used.
The tantalum scrap may have a tantalum content of 10% or more. In addition, the form may be an oxide such as Ta ₂ O ₅ , a chloride, or a fluoride in addition to tantalum metal containing impurities. As a bath for molten salt electrolysis, a fluoride-chloride or fluoride bath can be used. For example, Ta ₂ O ₅ is added to the electrolytic bath to perform electrolytic purification. The electrolysis temperature varies depending on the composition of the bath.
Implement within the range. Graphite is used for the anode, and Ni rod or the like is used for the cathode. During the electrolysis, CO and CO ₂ gases are generated from the anode. Then, high-purity tantalum powder electrodeposited on the cathode can be obtained.

After the high-purity tantalum powder obtained by the molten salt electrolysis is pressed once, it is uniformly dissolved by electron beam melting, vacuum arc melting, plasma melting, induction melting using a water-cooled copper crucible, and the volatile components are further reduced. Remove. This is cast into an ingot. In any case, the dissolution is performed in a vacuum or in an inert atmosphere to prevent entry of oxygen. Impurities such as gas components and easily volatile alkali metal elements are further reduced in this step. This allows
Each element of Nb, W, Mo is 10 ppm or less, each content of transition metal element, high melting point metal element or other heavy metal element is 1 ppm or less, content of radioactive elements such as U and Th is 1 ppb or less, Na, High-purity tantalum in which the content of each of the alkali metal elements such as K is 1 ppm or less is obtained.

If necessary, the ingot thus manufactured is cut into a target shape, and the surface is polished to obtain a tantalum sputtering target for forming a thin film with a limited amount of impurities. This can be used as a target of high purity tantalum silicide by powder metallurgy using high purity silicon. When performing sputtering, this tantalum target (including a compound such as an alloy and a silicide) is brazed to a copper backing plate and inserted into a sputtering chamber. For example, tantalum silicide is used as a wiring material for VLSI and MOS devices. , Can be used as a gate electrode or a drain electrode.

Since such a tantalum thin film is adjacent to various thin films used in a semiconductor device, the above-mentioned radioactive elements such as U and Th, alkali metals such as Na and K,
It is particularly necessary to reduce transition metals such as Fe, Ni and Cr or high melting point metals or heavy metals such as W, Mo and Nb which become impurities. This can reduce contaminants or influence generated from the tantalum (including compounds such as alloys and silicide) film and form a dense film that effectively functions as a wiring or electrode material.

Next, the present invention will be described in detail based on examples and comparative examples. This embodiment is merely an example, and the present invention is not limited to this example. That is, the present invention is limited only by the scope of the claims, and covers all aspects or modifications other than the examples included in the present invention. (Example 1) Using 150 kg of Ta ₂ O ₅ as a raw material, electrolytically deposited tantalum was obtained using an electrolytic bath of KCl-KF-K ₂ TaF ₇ -Ta ₂ O ₅ . The Ta concentration in the electrolytic bath is 8% by weight. Electrolysis conditions are a temperature of 720 ° C. and a current density of 0.1 A / cm ² . Ta concentration of 5 to 10 wt%
Ta ₂ O ₅ was added semi-continuously to keep Generation of CO and CO ₂ gases was confirmed from the anode. By this molten salt electrolysis, a high-purity tantalum powder as an electrolytic deposit of about 50 kg was obtained on the cathode side. This high-purity tantalum powder was press-molded into a predetermined shape so as to be compatible with an electron beam melting apparatus, and then uniformly melted with an electron beam to obtain an ingot.

Table 1 shows the results of analysis of Ta ₂ O ₅ as a raw material, tantalum electrolytic deposits by molten salt electrolysis, and impurities in the ingot after the electron beam. As is clear from Table 1, at the stage of the Ta ₂ O ₅ raw material, it becomes a radioactive element such as U and Th, an alkali metal such as Na and K, a transition metal such as Fe, Ni, and Cr, or a high melting point metal or an impurity. W, M
The heavy metals such as o and Nb, and the gas components of oxygen and carbon are all high, but all the impurity elements are significantly reduced after the electrolysis of the molten salt. At this stage, oxygen, alkali metals such as Na and K, and transition metals such as Fe, Ni, and Cr have not yet reached the intended reduction amount (K increases due to contamination from the bath). However, after electron beam melting, volatile components have been removed and oxygen has been reduced to 20 ppm. From the above, each element of Nb, W, and Mo is 10 ppm
Hereinafter, each content of a transition metal element, a high melting point metal element or a heavy metal element other than the above is 1 ppm or less, each content of a radioactive element such as U or Th is 1 ppb or less, and each content of an alkali metal element such as Na or K is contained. The initial purpose of obtaining high-purity tantalum for forming a thin film having an amount of 1 ppm or less has been achieved.

[0016]

[Table 1]

Example 2 Using 100 kg of Ta scrap as a raw material, NaF-KF-LiF-K ₂ TaF was used.
_Using the electrolytic bath No. ₇ , electrolytically deposited tantalum was obtained. The Ta concentration in the electrolytic bath is 8% by weight. Electrolysis condition is temperature 800 °
C, the current density is 0.1 A / cm ² . A tantalum-containing metal was used for the anode. By this molten salt electrolysis, high-purity tantalum powder as an electrolytic deposit of about 20 kg was obtained on the cathode side. This high-purity tantalum powder was press-molded into a predetermined shape so as to be compatible with an electron beam melting apparatus, and then uniformly melted with an electron beam to obtain an ingot.

Table 2 shows the results of analysis of Ta scrap as a raw material, tantalum electrolytic deposits by molten salt electrolysis, and impurities of the ingot after the electron beam. As is clear from Table 2, at the stage of the Ta scrap raw material, radioactive elements such as U and Th, alkali metals such as Na and K, Fe, N
The transition metal such as i, Cr or the high melting point metal or the heavy metal such as W, Mo, Nb, which is an impurity, and the gas components of oxygen and carbon are all high. I have. At this stage, as in Embodiment 1, oxygen, alkali metals such as Na and K, Fe, Ni,
Transition metals such as these have not yet reached the desired reduction (Na, K rather rise due to contamination from the bath).
However, after the electron beam melting, the volatile components were removed and the oxygen was reduced to 40 ppm. From the above, Nb,
Each element of W and Mo is 10 ppm or less, and each content of transition metal element, high melting point metal element or heavy metal element other than the above is 1
ppm or less, each content of radioactive elements such as U and Th is 1p
The initial object of obtaining high-purity tantalum for forming a thin film having a content of alkali metal elements such as pb or less and an alkali metal element such as Na or K of 1 ppm or less was achieved. As described above, the present invention has an excellent effect that contaminants from a tantalum or tantalum compound film used for a semiconductor device or the like can be extremely reduced.

[0019]

[Table 2]

Next, a disk-shaped tantalum target was molded from the tantalum ingots obtained in Examples 1 and 2 above, mounted on a backing plate, and sputtered to examine the sheet resistance of the thin film and the number of generated particles. . Table 3 shows the results. For comparison, sputtering was performed under the same conditions using a tantalum target having a purity of 99.9%. The results are also shown in Table 3.
As is clear from Table 3, in the comparative example, 350 μΩ ·
cm, whereas the sheet resistances of the highly pure Examples 1 and 2 are 250 μΩ · cm and 260 μΩ · cm, respectively.
cm, which confirms the favorable properties of this example. In addition, the number of generated particles was 50 / wafer in the comparative example, but was 10 / wafer and 15 / wafer in Examples 1 and 2, indicating that the number of generated particles was extremely low.

[0021]

[Table 3]

[0022]

According to the present invention, the combined use of the molten salt electrolysis method and the dissolving method such as an electron beam can reduce the content of each of Nb, W, and Mo to 10 ppm or less, a transition metal element, a high melting point metal element and other elements. Each content of heavy metal element is 1 ppm or less,
Each of radioactive elements such as U and Th is 1 ppb or less, Na, K
It is possible to obtain a high-purity tantalum for forming a thin film in which the content of each of the alkali metal elements such as is 1 ppm or less. By improving the purity of tantalum, contaminants from tantalum when forming a laminated thin film such as a semiconductor device can be effectively prevented, and abnormal discharge during sputtering used when forming the thin film is performed. Phenomena and particle generation can be effectively suppressed, and the present invention has an excellent effect that it can be easily manufactured at low cost even from scrap.

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[Procedure amendment]

[Submission date] March 12, 1999 (1999.3.1.
2)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Correction contents]

[0016]

[Table 1]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0019

[Correction method] Change

[Correction contents]

[0019]

[Table 2]

Claims (3)

[Claims] 1. Each element of Nb, W, and Mo is 10 weight pp.
m or less, each content of transition metal element, high melting point metal element and heavy metal element other than the above is 1 wt ppm or less, each content of radioactive elements such as U and Th is 1 wt ppb or less, Na, K
High-purity tantalum for forming a thin film, wherein each content of an alkali metal element such as the above is 1 wt ppm or less. 2. The content of oxygen and carbon as gas components is 1
2. The high-purity tantalum for forming a thin film according to claim 1, wherein the content is not more than 00 ppm by weight. 3. A high-purity tantalum for thin film formation, wherein a tantalum compound or tantalum scrap is subjected to molten salt electrolysis to obtain electrolytically deposited high-purity tantalum, and then dissolved by an electron beam or the like to remove volatile components. Production method.