JP2018145518A - Cu-Ni alloy sputtering target - Google Patents

Abstract

A Cu alloy sputtering target capable of forming a Cu alloy film having excellent weather resistance and suppressing generation of micro arc discharge during film formation is provided.
SOLUTION: Ni is contained in the range of 16% by mass to 55% by mass, and the balance is composed of Cu and inevitable impurities, and the hydrogen content of the inevitable impurities is less than 5 ppm by mass, A Cu-Ni alloy sputtering target, wherein the oxygen content is 500 ppm by mass or less.
[Selection figure] None

Description

  The present invention relates to a Cu—Ni alloy sputtering target.

Conventionally, Al is widely used as a flat panel display such as a liquid crystal or an organic EL panel, or a wiring film such as a touch panel. Recently, miniaturization (narrowing) and thinning of the wiring film have been attempted, and a wiring film having a lower specific resistance than before has been demanded.
With the miniaturization and thinning of the wiring film described above, a wiring film using Cu or Cu alloy, which is a material having a specific resistance lower than that of Al, is provided.

However, a Cu wiring film made of Cu or Cu alloy having a low specific resistance has a problem that it has low weather resistance and is likely to be discolored particularly in an atmosphere having humidity. For this reason, a protective film is provided on the Cu wiring film.
For example, Patent Document 1 discloses a Cu wiring protective film made of a Cu alloy containing Ni and Al and / or Ti and Cu, and a Cu alloy sputtering target for forming the Cu wiring protective film.

JP 2014-105362 A

  Recently, the size of the glass substrate on which the wiring film is formed has been increased, and along with this, the sputtering target itself for forming the protective film tends to increase in size. However, when the conventional Cu alloy sputtering target as described in Patent Document 1 is enlarged, micro arc discharge (abnormal discharge) and splash may occur depending on sputtering conditions, and film formation cannot be performed satisfactorily. was there. That is, when a large sputtering target is used, a large amount of power is applied. Therefore, if voids are generated on the sputtering surface of the sputtering target or inclusions such as oxides are present, Arc discharge tends to occur, the sputtering target melts locally and particles are generated, and the Cu alloy film may not be formed satisfactorily. The same applies to a sputtering target that is not large in size when a large amount of power is applied to improve the film formation efficiency.

  The present invention has been made in view of the above-described circumstances, and is a Cu alloy sputtering target capable of forming a Cu alloy film having excellent weather resistance and suppressing generation of micro arc discharge during film formation. The purpose is to provide.

  In order to solve the above problems, the Cu alloy sputtering target of the present invention contains Ni in a range of 16 mass% or more and 55 mass% or less, and the remainder is Cu-Ni alloy sputtering having a composition consisting of Cu and inevitable impurities. The target is characterized in that the hydrogen content of the inevitable impurities is less than 5 ppm by mass and the oxygen content is 500 ppm by mass or less.

  According to the Cu—Ni alloy sputtering target of the present invention having such a configuration, since the main components are two elements of Cu and Ni, variation in composition is reduced. Further, since the Ni content is in the range of 16 mass% or more and 55 mass% or less, the weather resistance of the formed Cu—Ni alloy film is increased.

  Furthermore, among the inevitable impurities, the hydrogen content is less than 5 ppm by mass, and the oxygen content is limited to 500 ppm by mass or less, so it is possible to suppress the occurrence of micro arc discharge during sputter deposition, Sputter deposition can be performed stably.

Here, in the Cu—Ni alloy sputtering target of the present invention, the number of voids having a maximum diameter of 2 μm or more is preferably 1 or less per 1 mm ² region in the sputtering surface.
In this case, since the amount of large voids having a maximum diameter of 2 μm or more is small, it is possible to more reliably suppress the occurrence of micro arc discharge during sputtering film formation.

Moreover, in the Cu-Ni alloy sputtering target of this invention, it is preferable that carbon content is 500 mass ppm or less among the said inevitable impurities.
Large Cu alloy sputtering targets are usually manufactured through casting and hot rolling processes, but when cracks occur during hot rolling, micro arc discharge occurs at the cracks, so they are used as sputtering targets. You may not be able to do it. By restricting the carbon content, the hot rollability can be improved, and the occurrence of cracks during hot rolling can be suppressed.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the Cu-Ni alloy sputtering target which can form the Cu-Ni alloy film excellent in the weather resistance, and can suppress generation | occurrence | production of the micro arc discharge at the time of film-forming. .

Below, the Cu-Ni alloy sputtering target which is one Embodiment of this invention is demonstrated in detail.
The Cu—Ni alloy sputtering target according to the present embodiment is used when, for example, forming a protective film laminated on a wiring film such as a flat panel display or a touch panel, or a Cu wiring film made of Cu or Cu alloy. It is what is used.
In addition, the Cu-Ni alloy sputtering target which is this embodiment has a flat plate shape, and is a large sputtering target having an area of the sputtering surface of 100,000 mm ² or more.

  The Cu—Ni alloy sputtering target according to the present embodiment contains Ni in a range of 16 mass% to 55 mass%, with the balance being composed of Cu and inevitable impurities. And in the Cu-Ni alloy sputtering target which is this embodiment, content of hydrogen is less than 5 mass ppm among unavoidable impurities, and content of oxygen is 500 mass ppm or less.

In the Cu—Ni alloy sputtering target according to this embodiment, the number of voids having a maximum diameter of 2 μm or more is set to 1 or less per 1 mm ² region in the sputtering surface.
Furthermore, in the Cu—Ni alloy sputtering target according to the present embodiment, the carbon content of inevitable impurities is set to 500 ppm by mass or less.

  Next, the reason why the composition of the Cu—Ni alloy sputtering target according to this embodiment and the number of voids are defined as described above will be described.

(Ni content: 16 mass% or more and 55 mass% or less)
Ni is an element having an effect of improving the weather resistance of Cu. By containing Ni, discoloration of the formed Cu—Ni alloy film can be suppressed.
Here, when the Ni content is less than 16% by mass, the weather resistance is not sufficiently improved, and the discoloration of the formed Cu—Ni alloy film may not be sufficiently suppressed. On the other hand, when the Ni content exceeds 55% by mass, the magnetism becomes too strong, and it may be difficult to form a sputter film.
For this reason, in the present embodiment, the Ni content is set in the range of 16 mass% to 55 mass%. In order to reliably improve the weather resistance of Cu, the Ni content is preferably in the range of 20% by mass to 50% by mass, and more preferably in the range of 25% by mass to 45% by mass. Further preferred.

(Hydrogen content: less than 5 ppm by mass)
It is presumed that hydrogen contained in the raw material for producing the Cu—Ni alloy sputtering target remained. The hydrogen content of the Cu—Ni alloy sputtering target correlates with the number of voids, and when the hydrogen content is small, the number of voids tends to decrease. If voids exist, electric charges are likely to accumulate in the surrounding Cu-Ni alloy part. Therefore, when the number of voids increases, electric charges accumulate around the voids during sputter deposition, and the number of occurrences of micro arc discharge increases. There is a fear.
For this reason, in this embodiment, the hydrogen content is set to less than 5 ppm by mass. In order to reliably suppress micro arc discharge and stably form a sputter film, the hydrogen content is preferably less than 4 ppm by mass, and more preferably less than 3 ppm by mass. The lower limit of the hydrogen content is usually 0.1 mass ppm or more. Even if the hydrogen content is less than 0.1 mass ppm, the effect of reducing the number of voids is not improved. On the other hand, the work for reducing the hydrogen content becomes complicated and the production cost may increase.

(Oxygen content: 500 mass ppm or less)
It is assumed that oxygen is present as a remaining oxide after a part of the refractory of the melting furnace is caught in the ingot during melting and casting of the Cu and Ni oxides and the target constituting the Cu-Ni alloy sputtering target. Is done. Since these oxides easily emit secondary electrons, if the content of oxide in the Cu-Ni alloy sputtering target is excessive, the amount of secondary electrons emitted increases during sputtering film formation. There is a risk of increasing the number of occurrences of micro arc discharge.
For this reason, in this embodiment, the oxygen content is set to 500 mass ppm or less. In order to reliably suppress micro arc discharge and stably form a sputter film, the oxygen content is preferably 300 mass ppm or less, and more preferably 50 mass ppm or less. The lower limit of the oxygen content is usually 0.1 mass ppm or more. Even if the oxygen content is less than 0.1 mass ppm, the effect of reducing the emission amount of secondary electrons is not improved. On the other hand, the work for reducing the oxygen content becomes complicated and the production cost may increase. is there.

(Carbon content: 500 mass ppm or less)
Carbon is an element that may cause cracking in the manufacturing process of the Cu—Ni alloy sputtering target of the present embodiment. Cracking proceeds from the outer periphery of the rolled Cu-Ni alloy sheet during rolling of the Cu-Ni alloy. If it is a fine crack, it can be removed by machining or the like, but if the carbon content exceeds 500 ppm, the crack tends to increase, and depending on the machining, it may be difficult to completely remove the crack. . If cracks exist in the Cu—Ni alloy sputtering target, the number of occurrences of micro arc discharge starting from the cracks may increase during sputtering film formation. That is, the low carbon content means that the amount of cracks that cause micro-arc discharge during sputtering film formation is small.
For this reason, in this embodiment, the carbon content is set to 500 mass ppm or less. In order to surely suppress the occurrence of cracks and suppress the occurrence of micro arc discharge during sputtering film formation, the carbon content is preferably 300 mass ppm or less, and is preferably 50 mass ppm or less. Further preferred. The lower limit of the carbon content is usually 3 ppm by mass or more. Even if the carbon content is less than 3 ppm by mass, the effect of reducing fine cracks is not improved. On the other hand, the work for reducing the carbon content becomes complicated and the production cost may increase.

(Number of voids)
In the Cu—Ni alloy sputtering target according to the present embodiment, if a void having a maximum diameter of 2 μm or more exists on the sputtering surface, the electric charge is concentrated around the void and the micro arc discharge is likely to occur. .
For this reason, in this embodiment, the number of voids having a maximum diameter of 2 μm or more is limited to one or less per 1 mm ² region in the sputtering surface.

Next, an example of a method for producing the Cu—Ni alloy sputtering target according to the present embodiment will be described.
The Cu—Ni alloy sputtering target according to this embodiment is manufactured through processes such as a melt casting process, a hot rolling process, a leveler processing process / cold rolling process, a heat treatment process, and a machining process. Below, each process is demonstrated.

(Melting casting process)
In the melt casting process, Cu and Ni are melt cast to obtain a Cu—Ni alloy ingot.
First, the melting raw material is weighed so that the above-described target composition is obtained. As a melting raw material, it is preferable to use Cu having a purity of 99.99% by mass or more and Ni having a purity of 99.9% by mass or more.
It is preferable to use oxygen-free copper as a melting material of Cu.

As a Ni melting material, it is preferable to use a material obtained by subjecting electrolytic Ni purified by an electrolytic method to a hydrogen reduction treatment.
Since the electrolytic Ni normally contains hydrogen in excess of 10 ppm by mass, the hydrogen in the electrolytic Ni remains in the Cu—Ni alloy ingot when used as a melting raw material. It is because there is a possibility of generating. If voids are generated in the Cu-Ni alloy ingot, the voids may remain on the target and cause micro arc discharge during sputtering film formation.
The mechanism by which voids are generated in the Cu—Ni alloy ingot by the hydrogen in the electrolytic Ni is considered as follows. Since electrolytic Ni contains excess hydrogen that is higher than its solubility, when Cu and electrolytic Ni are dissolved to form a molten Cu-Ni alloy, the hydrogen in electrolytic Ni is the molten Cu-Ni alloy (liquid phase). Blend into. On the other hand, since hydrogen has a low solubility in the solid phase of the Cu—Ni alloy, bubbles of hydrogen gas are generated at the boundary between the liquid phase and the solid phase when the melt of the Cu—Ni alloy solidifies. Solidification of the molten metal and generation of bubbles proceed simultaneously, and bubbles that have not been discharged to the outside of the molten metal are left as voids in the Cu—Ni alloy ingot.

  As the hydrogen reduction treatment of the electrolytic Ni, the electrolytic Ni is heated and melted, held at a temperature higher than the melting temperature of Ni by 10 to 50 ° C. for 10 to 30 minutes, and then cooled and solidified to form a Ni ingot. Can be used. In the electrolytic Ni hydrogen reduction treatment, it is preferable to use an induction melting furnace as the heating device. The hydrogen reduction treatment is preferably performed in a vacuum atmosphere or an inert gas atmosphere. The Ni ingot obtained by this hydrogen reduction treatment preferably has a hydrogen content of 10 mass ppm or less, particularly 5 mass ppm or less.

When melting and casting the weighed Cu and Ni, it is preferable to use an induction melting furnace in order to sufficiently mix the melting raw materials and make the composition of the molten metal uniform.
Here, at the time of melting, by preventing oxidation of Cu and Ni which are alloy constituent components, the occurrence of oxide mixing is suppressed.
In order to prevent oxidation during melting, it is preferable to dissolve in a vacuum atmosphere or an inert gas atmosphere. In addition, when dissolving in an air atmosphere in consideration of productivity, a method of keeping the molten metal in a reducing atmosphere by using a carbon crucible or coating the molten metal surface with carbon particles and carbon powder may be adopted. Good.

  In addition, it is industrially difficult to reliably prevent oxidation of the above-described alloy elements, and there is a possibility that the refractory material of the melting furnace, the above-described carbon powder, and the like are also involved in the ingot and become non-metallic inclusions. is there. In order to prevent the inclusion of these nonmetallic inclusions, it is preferable to float and separate the nonmetallic inclusions by a tundish or a distributor using a vertical continuous casting machine. It is also preferable to float and separate nonmetallic inclusions by slowing the cooling rate as in the unidirectional solidification method.

(Hot rolling process)
In the hot rolling step, the Cu—Ni alloy ingot obtained in the melt casting step is cut into a predetermined length and then hot rolled.
The conditions for hot rolling are preferably a rolling reduction per pass of 10% to 20% and a hot rolling temperature of 550 ° C to 1000 ° C. The rolling reduction is a value calculated from the following equation.
Reduction ratio (%) = {(thickness of Cu—Ni alloy before hot rolling pass−thickness of Cu—Ni alloy after hot rolling pass) / thickness of Cu—Ni alloy before hot rolling pass } × 100

  By the hot rolling process, a Cu—Ni alloy rolled plate having an average crystal grain size of 100 μm or less and a Vickers hardness of 60 Hv or more and 120 Hv or less can be obtained.

(Leveler processing process / cold rolling process, heat treatment process)
The Cu—Ni alloy rolled plate obtained in the hot rolling step described above may be subjected to leveler processing or cold rolling processing in order to improve the flatness of the rolled plate.
In addition, when leveler processing or cold rolling processing is performed, in order to adjust the average crystal grain size and Vickers hardness, heat treatment is performed under a condition of holding at a temperature of 550 ° C. or higher and 850 ° C. or lower for 1 to 2 hours. Then, it is preferable to cool in the air.

(Machining process)
In the machining step, grinding and polishing are performed on the surface to be the sputtered surface of the Cu—Ni alloy rolled plate obtained as described above. It is preferable to adjust the surface roughness of the sputter surface so that the maximum height Rz is 5 μm or less.

Through the steps as described above, the Cu—Ni alloy sputtering target according to this embodiment is manufactured.
This Cu—Ni alloy sputtering target is soldered to a Cu backing plate, attached to a sputtering apparatus, and a Cu—Ni alloy film is formed by sputtering on an opposing substrate.
Here, the Cu—Ni alloy film formed by sputtering has the same composition as the above-described Cu—Ni alloy sputtering target.

  According to the Cu—Ni alloy sputtering target of the present embodiment configured as described above, since the components are two elements of Cu and Ni, the variation in composition is reduced. Further, since the Ni content is in the range of 16 mass% or more and 55 mass% or less, the weather resistance of the formed Cu—Ni alloy film is increased.

  Further, among the inevitable impurities, the hydrogen content is less than 5 ppm by mass, the oxygen content is limited to 500 ppm by mass or less, and the amount of voids and oxides is small. It is possible to suppress the occurrence of micro arc discharge due to the oxide and the oxide, and it is possible to stably perform the sputtering film formation.

In the Cu—Ni alloy sputtering target according to the present embodiment, the number of voids having a maximum diameter of 2 μm or more is 1 or less per 1 mm ² region in the sputtering surface. The occurrence of micro arc discharge at the time can be more reliably suppressed.

  Furthermore, in the Cu—Ni alloy sputtering target of this embodiment, since the carbon content of the inevitable impurities is 500 mass ppm or less, the hot rolling property can be improved, and hot rolling is performed. The occurrence of cracks at the time can be suppressed.

As mentioned above, although embodiment of this invention was described, this invention is not limited to this, It can change suitably in the range which does not deviate from the technical idea of the invention.
For example, in the present embodiment, a large sputtering target having a flat plate shape and an area of the sputtering surface of 100000 mm ² or more has been described, but the shape of the Cu—Ni alloy sputtering target is not particularly limited, and is a disk shape. Alternatively, it may have a rectangular flat plate shape or a cylindrical shape. Further, the area of the sputter surface is not limited to the above range.

  Below, the result of the evaluation test evaluated about the effect of the Cu-Ni alloy sputtering target which concerns on this invention is demonstrated.

[Invention Examples 1 to 8, Comparative Examples 1 to 4]
(1) Hydrogen reduction treatment of electrolytic Ni (production of hydrogen reduced Ni ingot)
Electrolytic Ni (purity: 99.99% by mass or more) was prepared. The hydrogen content in the electrolytic Ni was in the range of 11 to 15 ppm by mass.
Electrolytic Ni was put into an alumina crucible. Next, electrolytic Ni is heated and melted in a vacuum atmosphere using a high-frequency induction heating furnace, held at 1570 to 1590 ° C. for 2 to 15 minutes, and then cooled and solidified to obtain a hydrogen-reduced Ni ingot. It was. Table 1 shows the melt retention time of the hydrogen reduction treatment and the hydrogen content of the obtained hydrogen reduced Ni ingot. However, in Comparative Example 1, the hydrogen reduction treatment of electrolytic Ni was not performed. The hydrogen content of electrolytic Ni and hydrogen-reduced Ni ingots was analyzed by an inert gas melting-thermal conductivity method.

(2) Production of Cu-Ni alloy ingot The hydrogen-reduced Ni ingot obtained in (1) above and oxygen-free copper (purity: 99.99 mass%) have the composition shown in Table 1 below. Thus, it put into the melting crucible shown in Table 1. However, in Comparative Example 1, electrolytic Ni having a hydrogen content of 11 ppm was used as the Ni raw material. Next, using a high frequency induction heating furnace, heating is performed in the atmosphere shown in Table 1 below, and the obtained molten metal is poured into a vertical continuous casting machine by a distributor and has a rectangular cross section of 180 mm × 60 mm. An ingot was produced. After cutting and removing the top part and bottom part of this ingot, it cut | disconnected to the length suitable for hot rolling, and obtained the Cu-Ni alloy ingot of 180 mm x 60 mm x length 600mm.

(3) Manufacture of Cu—Ni alloy rolled plate After heating the Cu—Ni alloy ingot obtained in (2) above at 850 ° C. for 1 hour, hot rolling is performed with a rolling reduction rate of 1 pass to 11 to 15%. It implemented and obtained the Cu-Ni alloy rolled sheet of 190 mm x 1900 mm x thickness 18mm. The temperature of the rolled sheet after each final hot rolling pass was 600 to 700 ° C.
In Example 7 of the present invention, cracks having a size that can be visually confirmed occurred in a part of the rolled Cu—Ni alloy sheet.

(4) Production of Evaluation Target A target material having a diameter of 125 mm and a thickness of 5 mm was cut out from the obtained Cu—Ni alloy rolled plate using a machining center and a lathe. After the obtained Cu—Ni alloy sputtering target was soldered to a Cu backing plate, the sputtering surface was further polished to obtain an evaluation target. In the polishing process, the surface roughness was reduced by changing the grains from coarse (# 150) to fine (# 800) in order, and then the dust adhered by the polishing was washed and removed.
In Example 7 of the present invention, the cracked portion of the rolled Cu—Ni alloy plate was removed when the target material was cut out.

(5) Evaluation For the target for evaluation thus obtained, the content of inevitable impurities (hydrogen, oxygen, carbon), the number of voids, the number of micro arc discharges (number of abnormal discharges), and the weather resistance of the film are as follows: It evaluated as follows.

(Inevitable impurity content)
The hydrogen content was analyzed by an inert gas melting-thermal conductivity method using a sample piece taken by cutting a Cu-Ni rolled sheet. Oxygen and carbon were analyzed by an infrared absorption method using sample pieces collected in the same manner.

(Void number)
Samples for observing the structure were cut out from the respective portions obtained by equally dividing the sputtering surface of the evaluation target into four equal parts, and the sputtering surfaces were polished. Next, the sputtered surface of each polished sample was observed with a dark field image at a magnification of 100 times with an optical microscope. Since this is a dark field image, if there is a void (dent) of a certain size or more on the sputter surface, that portion is detected as a point that shines white. A maximum void length of about 2 μm or more was detected by this method. An area of 0.5 mm × 0.5 mm (= 0.25 mm ² ) was arbitrarily extracted from the sputtering surface of each sample, and the number of voids detected in the area was counted. The case where the number of voids detected in a total area of 1 mm ² extracted from the sputter surfaces of four samples was 1 or less was evaluated as “OK”, and the case where two or more voids were evaluated as “NG”. The evaluation results are shown in Table 2.

(Number of micro arc discharges)
A target for evaluation was attached to the sputtering apparatus, pre-sputtering was performed for 30 minutes from the start of use under the conditions of ultimate vacuum: 5 × 10 ⁻⁴ Pa, gas pressure: argon 0.3 Pa, sputtering power: DC 1000 W, and then micro The number of arc discharges was examined. The micro arc discharge was detected by adding a micro arc monitor manufactured by Landmark Technology Co., Ltd. to the sputtering power source and detecting a decrease in the discharge voltage. A micro arc in which the arc discharge energy is 50 MJ or less obtained by repeating 50 minutes of discharge for 1 minute and stopping for 1 minute to simulate intermittent sputtering film formation, which is an actual operation mode. The number of discharges was counted. Table 2 shows the results of counting the number of micro arc discharges.

(Membrane weather resistance)
A 50 mm × 50 mm × 0.7 mm non-alkali glass substrate is placed opposite to the evaluation target so that the distance between the substrates is 60 mm, the ultimate vacuum: 5 × 10 −4 Pa, the gas pressure: argon 0.3 Pa, and sputtering. Power: Sputtering was performed under the condition of DC 600 W, and a Cu—Ni alloy film having a thickness of 150 nm was formed on the substrate.
After the Cu-Ni alloy film was subjected to a constant temperature and humidity test for 250 hours under a constant temperature and humidity condition of a temperature of 70 ° C. and a relative humidity of 90%, the surface of the Cu—Ni alloy film was visually observed. Then, the case where the color change was recognized was evaluated as “NG”, and the case where the color change was not confirmed was evaluated as “OK”. The evaluation results are shown in Table 2.

In Comparative Example 1 in which the hydrogen content was 5 mass ppm or more, the number of voids increased and the number of micro arc discharges increased.
In Comparative Example 2 in which the oxygen content exceeds 500 mass ppm, the number of micro arc discharges is increased, and the formed Cu-Ni alloy film is discolored after the constant temperature and humidity test, and the weather resistance is insufficient. It was.
In Comparative Example 3 in which the Ni content was less than 16% by mass, the formed Cu—Ni alloy film was discolored after the constant temperature and humidity test, and the weather resistance was insufficient.
In Comparative Example 4 where the Ni content exceeds 55 mass%, sputtering could not be performed. It is presumed that the magnetism has become stronger.

  On the other hand, in the example of the present invention in which the contents of Ni, hydrogen, and oxygen are within the scope of the present invention, the number of voids is small, the number of micro arc discharges is suppressed, and stable film formation is performed. I was able to. The formed Cu—Ni alloy film was excellent in weather resistance.

  From the above, according to the present invention example, a Cu—Ni alloy sputtering target capable of forming a Cu—Ni alloy film excellent in weather resistance and suppressing the occurrence of micro arc discharge during film formation is provided. It was confirmed that it was possible.

Claims (3)

Ni is contained in the range of 16 mass% or more and 55 mass% or less, and the balance has a composition consisting of Cu and inevitable impurities,
The Cu-Ni alloy sputtering target characterized in that the hydrogen content of the inevitable impurities is less than 5 ppm by mass, and the oxygen content is 500 ppm by mass or less. ^2. The Cu—Ni alloy sputtering target according to claim 1, wherein the number of voids having a maximum diameter of 2 μm or more is 1 or less per 1 mm ² region in the sputtering surface.   The Cu-Ni alloy sputtering target according to claim 1 or 2, wherein a carbon content of the inevitable impurities is 500 ppm by mass or less.