JP2016191083A - BISMUTH WITH A REDUCED AMOUNT OF α RAYS AND METHOD FOR PRODUCING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide bismuth with a reduced amount of α rays.SOLUTION: The present invention provides bismuth with a reduced amount of α rays, having an amount of α rays on its surface of 0.0025 cph/cmor less.SELECTED DRAWING: None

Description

  The present invention relates to bismuth with a reduced α dose, which is used for manufacturing semiconductors, and a method for manufacturing the same.

  In recent semiconductor devices and the like, the density is increased and the operating voltage and the cell capacity are reduced. Therefore, there is an increased risk of soft errors due to the influence of α rays from the material in the vicinity of the semiconductor chip. Therefore, for bismuth used as a solder material, there is a demand for a material with high purity and less α-ray emission.

Patent Document 1 discloses that lead, by controlling pH, bismuth concentration in electrolyte, electrolyte temperature, and current density in an electrolyte containing only acid (hydrochloric acid or sulfuric acid) without using hexafluorosilicic acid or additives. Is 1 ppm or less, uranium and thorium are each 5 ppb or less, and α dose is 0.01 cph / cm ² or less.

JP 2013-185214 A

  An object of the present invention is to provide bismuth that can be adapted to the requirements as a solder material and that has a further reduced α dose compared to the prior art.

  The inventor has conducted intensive research on the reduction of α dose of bismuth, and has come up with the idea of reducing the content of polonium in order to reduce the α dose of bismuth. Then, by contacting the bismuth nitrate solution with the cation exchange resin and adsorbing the polonium, the liquid and the resin are separated to obtain the bismuth nitrate solution, and the process of obtaining metal bismuth by electrowinning is repeated at least twice. The inventors have found that bismuth having a very low α dose can be obtained, and have reached the present invention.

Therefore, the present invention includes the following (1) and below.
(1)
Low α-ray bismuth having an α dose of 0.0025 cph / cm ² or less.
(2)
The low α-ray bismuth according to (1), wherein the α dose is 0.0020 cph / cm ² or less.
(3)
Low alpha ray bismuth as described in (1) or (2) whose Pb content is 0.1 ppm or less.
(4)
Low alpha ray bismuth in any one of (1)-(3) whose content of U and Th is 5 ppb or less, respectively.
(5)
Using bismuth having an α dose of 2.0 cph / cm ² or less as a raw material, dissolving the raw material bismuth in a nitric acid solution by electrolysis, having a bismuth concentration of 5 to 50 g / L, pH 0.0 to 0.4 Preparing a bismuth nitrate solution;
A step of bringing a bismuth nitrate solution into contact with an ion exchange resin to obtain an ion exchange bismuth solution in which polonium in the solution is exchanged and removed by the ion exchange resin (first-stage ion exchange resin adsorption step);
Electrolytically collecting from the ion-exchanged bismuth solution and recovering the electrolytically collected bismuth,
Electrolytically collected bismuth is dissolved in a nitric acid solution by electrolysis to adjust a bismuth nitrate solution having a bismuth concentration of 5 to 50 g / L and a pH of 0.0 to 0.4 (electrolytic sampling-bismuth nitrate solution);
Step of obtaining an ion exchange bismuth solution (electrolytic collection-nitric acid-ion exchange bismuth solution) obtained by bringing the electrolytic collection-bismuth nitrate solution into contact with an ion exchange resin and exchanging and removing polonium in the solution with the ion exchange resin (second stage) Ion exchange resin adsorption process),
Electrowinning-from the nitric acid-ion exchange bismuth solution, electrowinning, and collecting the electrowinning bismuth (electrowinning-nitric acid-ion exchange-electrowinning bismuth),
The manufacturing method of the low alpha ray bismuth containing.
(6)
The method according to (5), wherein the flow rate in the second-stage ion exchange resin adsorption step is made lower than the flow rate in the first-stage ion exchange resin adsorption step.
(7)
The method according to (5) or (6), wherein the α dose of the raw material bismuth is 0.6 cph / cm ² or more and 2.0 cph / cm ² or less.

  According to the present invention, low α-ray bismuth in which α rays are minimized can be obtained. According to the low α-ray bismuth of the present invention, the occurrence of soft errors due to the influence of α rays in the semiconductor device can be significantly reduced.

FIG. 1 is a diagram showing a decay chain (uranium radium decay series) from uranium (U) decay to ²⁰⁶ Pb. FIG. 2 is a graph showing a change in α dose with time after melting and casting of bismuth.

  The present invention will be described in detail below with reference to specific embodiments. The present invention is not limited to the specific embodiments disclosed below.

[Production method of low α-ray bismuth]
According to the present invention, bismuth having an α dose of 2.0 cph / cm ² or less is used as a raw material, the raw material bismuth is dissolved in a nitric acid solution by electrolysis, and has a bismuth concentration of 5 to 50 g / L, pH 0. A step of preparing a 0 to 0.4 bismuth nitrate solution, a step of bringing the bismuth nitrate solution into contact with an ion exchange resin, and obtaining an ion exchange bismuth solution in which polonium in the solution is exchanged and removed by the ion exchange resin; Step of recovering electrolytically collected bismuth by electrolytically collecting from solution, bismuth nitrate having a pH of 0.0 to 0.4 and having a bismuth concentration of 5 to 50 g / L by dissolving electrolytically collected bismuth in a nitric acid solution by electrolysis The step of adjusting the solution (electrowinning-bismuth nitrate solution), the electrowinning-bismuth nitrate solution is brought into contact with the ion exchange resin, and the polonium in the solution is removed. Step of obtaining ion exchange bismuth solution exchanged and removed with on-exchange resin (electrolytic collection-nitric acid-ion exchange bismuth solution), electrolytic collection-electrolytic collection from nitric acid-ion exchange bismuth solution, electrolytic collection bismuth (electrolytic collection-nitric acid) Low ion-ray bismuth can be produced by a method including a step of recovering (ion exchange-electrolytically collected bismuth).

  Thus, using raw material bismuth as a starting material, performing a step of dissolving nitric acid by electrolysis, a step of ion exchange, a step of electrowinning, and performing the second step again following these first step, That is, low alpha-ray bismuth can be manufactured by performing the nitric acid re-dissolution process, the re-ion exchange process, and the re-electrolytic extraction process following these first processes. In the present invention, the α-ray reduction is realized by performing the second process subsequent to the first process, but it is further reduced by repeating the same process set as the second process. It is also within the scope of the present invention to perform alpha radiation.

[Concept of low alpha radiation of bismuth]
Bismuth is all a radioisotope, and there are multiple nuclides involved in alpha radiation. Because of these radioisotopes, the α dose is considered to be high. In order to reduce α-rays, the isotopes involved in these α-ray emissions must be separated and removed. It was considered impossible to produce low bismuth. In response to such common technical knowledge, the present inventor has earnestly studied to reduce the α-ray content of bismuth based on the following idea.

There are many radioactive elements that generate alpha rays, but many are probably not a problem because their half-life is very long or very short, and what actually matters is in the U decay chain (see Figure 1), It will be α rays generated when the polonium isotope ²¹⁰ Po is disintegrated to the lead isotope ²⁰⁶ Pb. Of these, ²⁰⁹ Bi has a very long half-life of 1.9 × 10 ¹⁹ years and is probably not a problem. Among the isotopes involved in α-radiation other than ²⁰⁹ Bi, the longest half-life is ²¹⁰ Bi and the half-life is 5 days (see FIG. 1). The other isotopes ²¹¹ Bi, ²¹² Bi and ²¹⁴ Bi involved in α-ray emission have very short half-lives of 2 minutes, 61 minutes and 20 minutes, respectively. Is probably not a problem. As shown in FIG. 1, ²¹⁰ Bi disintegrates as ²¹⁰ Bi → ²¹⁰ Po → ²⁰⁶ Pb, and α rays are emitted when ²¹⁰ Po disintegrates into ²⁰⁶ Pb. ²⁰⁶ Pb is a stable isotope.

As a result of investigating the α dose emitted from bismuth, it was found that the α dose change peculiar to bismuth, which is not seen in other metals, is observed (see FIG. 2). Usually, for example, in the case of tin, the α dose is low immediately after melting and casting, and the α dose increases with time. However, in the case of bismuth, the α dose is high immediately after melting and casting, and the α dose decreases with time. As a result of intensive studies based on the α dose change peculiar to bismuth, it was found that most of the radioactive elements in bismuth involved in α-radiation are polonium. However, even when the time is sufficiently longer than the half-life of ²¹⁰ Po, that is, for a long time such that ²¹⁰ Po hardly disintegrates, the α dose of bismuth does not fall below a certain level.

The present inventor considered that this cause is due to the presence of ²¹⁰ Pb in bismuth and the decay of ²¹⁰ Pb → ²¹⁰ Bi → ²¹⁰ Po → ²⁰⁶ Pb. That is, when the lead isotope ²¹⁰ Pb (half-life 22.3 years) is contained in the material, ²¹⁰ Pb → ²¹⁰ Bi (half-life 5 days) → ²¹⁰ Po (half-life 138 days) over time As the decay of the arsenic (Fig. 1) proceeds and the decay chain is reconstructed to yield ²¹⁰ Po, α-rays are generated due to the decay of the polonium isotope ²¹⁰ Po to the lead isotope ²⁰⁶ Pb, which is the bismuth α The hypothesis was reached that it was the main cause of the line.

  According to this hypothesis, it is derived that removal of polonium is effective for lowering α-rays of bismuth, and preferably removal of lead is also effective for lowering α-rays of bismuth. Based on this idea, the present inventor has reached the present invention, which makes it possible to produce bismuth with a low α dose industrially. The unit notation of “ppm” used in the present invention means “weight ppm (wtppm)”. The α-ray count number of metal bismuth indicates an α dose when measured using a Gas Flow Proportional Counter model 8600A-LB manufactured by Ordela. In this apparatus, the gas used was 90% argon-10% methane, and the measurement time was 104 hours for both the background and the sample. The first 4 hours of the measurement time was set as the time required for the measurement chamber purge, and thereafter, the time required from 5 to 104 hours was set as the time required for data measurement. Since a very small amount of α-rays (background (BG) α-rays) are emitted from the measuring device, the value obtained by subtracting the background α-ray count from the measurement data of the α-ray count was evaluated as the α-ray count of metal bismuth. . The α-ray count number of metal bismuth means the result measured within 3 months from the electrowinning.

[Raw material bismuth]
As the raw material bismuth, metal bismuth having a low α dose is desirable. For example, the surface α dose can be 2.0 cph / cm ² or less, preferably 0.6 cph / cm ² or more and 2.0 cph / cm ² or less. These can be used as appropriate depending on the target α-ray reduction value.

[Nitric acid dissolution]
The raw material bismuth is dissolved in nitric acid by electrolysis to prepare a bismuth nitrate solution having a bismuth concentration of 5 to 50 g / L and a pH of 0.0 to 0.4. By this electrolysis, raw material bismuth can be dissolved in a nitric acid solution, and elements that are more potential than bismuth can be removed. The bismuth concentration of the nitric acid solution is preferably 5 to 50 g / L, and if it is lower than 5 g / L, the production efficiency is poor, and if it is higher than 50 g / L, precipitation of the bismuth compound occurs and the yield deteriorates. The pH of the nitric acid solution is preferably 0.0 to 0.4. When the pH is lower than 0.0, a large amount of chemical is required. When the pH is higher than 0.4, the solubility of bismuth is lowered and a sufficient bismuth concentration is obtained. It becomes difficult to obtain.

[Ion exchange]
A bismuth nitrate solution is brought into contact with an ion exchange resin, and polonium (Po) in the solution is exchanged and removed with the ion exchange resin to obtain a bismuth solution (ion exchange bismuth solution). The contact with the ion exchange resin can be performed by a known means. For example, the bismuth nitrate solution may be passed through a column packed with the ion exchange resin to make contact, or the ion exchange resin is introduced into the bismuth nitrate solution. After charging, solid-liquid separation may be performed.

As the ion exchange resin, a cation exchange resin can be suitably used. Examples of the cation exchange group of the cation exchange resin include —SO ₃ H group and —COOH group. In a preferred embodiment, a resin having —SO ₃ H groups as cation exchange groups is preferred. The resin of the ion exchange resin is not particularly limited as long as it can withstand the above treatment, and examples thereof include styrene, divinylbenzene, acrylic, and methacrylic resins. In a preferred embodiment, styrenic resins are preferred.

  The capacity of the ion exchange resin can be appropriately selected and used in accordance with the volume and concentration of the bismuth nitrate solution, but can preferably be 500 mL or more and 2 L or less per 100 L of the bismuth nitrate solution, and the capacity is lower than 500 mL. Decreases the polonium resin adsorbability, lowers the polonium removal efficiency, and if the capacity is higher than 2L, the amount of resin used increases and the processing cost increases, and the amount of bismuth adsorbed on the resin increases and the yield decreases. To do.

  In the case of using an ion exchange resin column, the flow rate of the bismuth nitrate solution through the column is preferably 8 L / h or more and 15 L / h or less, and processing speed is required at a rate lower than 8 L / h. If it exceeds h, the liquid passing speed is too high, and polonium that passes without being adsorbed increases, and may not be completely removed during reion exchange described later.

[Electrolytic sampling]
Electrolytic collection is performed from a solution (ion exchange bismuth solution) separated into solid and liquid after ion exchange to recover bismuth (electrolytically collected bismuth). By this electrowinning, an element that is lower in potential than bismuth can be removed. Thereby, bismuth can be highly purified effectively. Known conditions can be used as the conditions for electrolytic collection.

[Nitric acid redissolution]
Electrolytically collected bismuth is dissolved (re-dissolved) in a nitric acid solution by electrolysis to prepare a bismuth nitrate solution having a bismuth concentration of 5 to 50 g / L and a pH of 0.0 to 0.4 (electrolytic sampling-bismuth nitrate solution). To do. This nitric acid re-dissolution can be performed in the same procedure as the above-mentioned nitric acid dissolution.

[Reion exchange]
The electrolytic collection-bismuth nitrate solution is brought into contact with an ion exchange resin to obtain an ion exchange bismuth solution in which polonium in the solution is exchanged and removed by the ion exchange resin (electrolytic collection-nitric acid-ion exchange bismuth solution). The capacity of the ion exchange resin can be appropriately selected and used according to the volume and concentration of the bismuth nitrate solution, as in the above-described ion exchange, but is preferably 500 mL to 2 L per 100 L of the bismuth nitrate solution. . When an ion exchange resin column is used, the flow rate is preferably 4 L / h or more and 8 L / h or less, and processing time is required when the flow rate is lower than 4 L / h. When the flow rate exceeds 8 L / h, the flow rate is too high. Thus, polonium passing without being adsorbed increases. Since the concentration of polonium in the liquid is thinner in the second time than in the first ion exchange, it is necessary to slow down the liquid flow rate.

[Re-electrolysis collection]
Electrolytically collected from the electrolytically collected nitric acid-ion exchange bismuth solution to collect electrolytically collected bismuth (electrolytically collected-nitric acid-ion exchange-electrolytically collected bismuth). This re-electrolytic collection can be performed in the same procedure as the above-described electrolytic collection.

[Low α-ray bismuth]
The low α-ray bismuth according to the present invention has a surface α dose of 0.0025 cph / cm ² or less, preferably 0.0020 cph / cm ² or less, more preferably 0.0018 cph / cm ² or less. Such a low α dose metal bismuth has not been produced so far. The low α-ray bismuth according to the present invention preferably has a Pb content of 0.1 ppm or less. The low α-ray bismuth according to the present invention preferably has U and Th contents of 5 ppb or less, respectively. By setting it as such content, it can be set as the low alpha ray bismuth reduced extremely including the surface alpha dose which will reach | attain with time.

  Even when the low α-ray bismuth of the present invention is used as an alloy, the α-ray derived from bismuth can be similarly excellently reduced in α-ray. Alloys containing the low alpha ray bismuth of the present invention are also within the scope of the present invention.

  Hereinafter, the present invention will be described in detail with reference to examples. The present invention is not limited to the examples illustrated below.

[Example 1]
[Purification of bismuth]
Raw material bismuth having an α dose of 1.81 cph / cm ² was dissolved in a nitric acid solution by electrolysis to remove elements that are more noble than bismuth. As this bismuth nitrate solution, a Bi concentration of 40.0 g / L, pH = 0.3, and 100 L were used.
This bismuth nitrate solution was passed at 15 L / h through a column packed with 2 L of a cation exchange resin (Diaion SK-1B, cation exchange group: sulfonic acid group (—SO ³ H), manufactured by Mitsubishi Chemical Corporation).
Next, using the liquid 1 after passing through the liquid, electrolytic collection was performed at 25 A and 0.48 A / cm ² to remove an element lower in potential than bismuth, thereby obtaining metal bismuth.
The obtained bismuth was dissolved in a nitric acid solution by electrolysis to remove elements that are more noble than bismuth. As this bismuth nitrate solution, a Bi concentration of 39.2 g / L, pH = 0.3, and 100 L were used.
This bismuth nitrate solution was passed through a column packed with 2 L of cation exchange resin at 8 L / h.
Next, by using the liquid 2 after passing through the above, electrolytic extraction was performed at 25 A and 0.48 A / cm ² to remove elements lower in potential than bismuth to obtain metal bismuth.

[Α-ray measurement]
The α dose of the obtained metal bismuth was measured with an α ray measuring device. The α-ray count number of metal bismuth indicates an α dose when measured using a Gas Flow Proportional Counter model 8600A-LB manufactured by Ordela. In this apparatus, the gas used was 90% argon-10% methane, and the measurement time was 104 hours for both the background and the sample. The first 4 hours of the measurement time was set as the time required for the measurement chamber purge, and thereafter, the time required from 5 to 104 hours was set as the time required for data measurement. Since a very small amount of α-rays (background (BG) α-rays) are emitted from the measuring device, the value obtained by subtracting the background α-ray count from the measurement data of the α-ray count was evaluated as the α-ray count of metal bismuth. . As a result, Table 1 shows the α dose of raw material bismuth and the α dose of bismuth obtained by purification. As shown in Table 1, the surface α dose of the raw material was 1.81 cph / cm ² , but it was 0.52 cph / cm ² after the first purification and 0.0018 cph / cm ² after the second purification. The decrease in α dose was significant. When bismuth obtained by GDMS (glow discharge mass spectrometry) was analyzed, the Pb content was 0.1 ppm or less, and the U and Th contents were 5 ppb or less, respectively.

[Example 2]
[Purification of bismuth]
Raw material bismuth having an α dose of 1.95 cph / cm ² was dissolved in a nitric acid solution by electrolysis to remove elements that are more noble than bismuth. The bismuth nitrate solution used had a Bi concentration of 42.6 g / L, pH = 0.3, and 100 L.
This bismuth nitrate solution was passed through a column packed with 500 mL of cation exchange resin at 8 L / h.
Next, using the liquid 3 after passing through the liquid, electrolytic extraction was performed at 50 A and 0.95 A / cm ² to remove elements lower in potential than bismuth to obtain metal bismuth.
The obtained bismuth was dissolved in a nitric acid solution by electrolysis to remove elements that are more noble than bismuth. As this bismuth nitrate solution, a Bi concentration of 41.3 g / L, pH = 0.3, and 100 L were used.
This bismuth nitrate solution was passed through a column packed with 500 mL of cation exchange resin at 4 L / h.
Next, using the liquid 4 after passing through the above, electrolytic extraction was performed at 50 A and 0.95 A / cm ² to remove an element which is lower in potential than bismuth, thereby obtaining metal bismuth.

[Α-ray measurement]
The α-dose of the obtained metal bismuth was measured by the same method as in Example 1 with an α-ray measuring device. Table 2 shows the α dose of raw material bismuth and the α dose of bismuth obtained by purification. As shown in Table 2, the surface α dose of the raw material was 1.95 cph / cm ² , but it was 0.33 cph / cm ² after the first purification and 0.0005 cph / cm ² after the second purification. The decrease in α dose was significant. Moreover, when the bismuth obtained by GDMS (glow discharge mass spectrometry) was analyzed, content of Pb was 0.1 ppm or less. The contents of U and Th were each 5 ppb or less.

[Comparative Example 1]
[Purification of bismuth]
Raw material bismuth having an α dose of 2.25 cph / cm ² was dissolved in a nitric acid solution by electrolysis to remove elements that are more noble than bismuth. The bismuth nitrate solution used had a Bi concentration of 42.6 g / L, pH = 0.3, and 100 L.
This bismuth nitrate solution was passed through a column packed with 300 mL of cation exchange resin at 17 L / h.
Next, using the liquid 5 after passing through the above, electrolytic extraction was performed at 50 A and 0.95 A / cm ² to remove an element that was lower in potential than bismuth, and metal bismuth was obtained.
The obtained bismuth was dissolved in a nitric acid solution by electrolysis to remove elements that are more noble than bismuth. As this bismuth nitrate solution, a Bi concentration of 40.3 g / L, pH = 0.3, and 100 L were used.
This bismuth nitrate solution was passed through a column packed with 300 mL of cation exchange resin at 10 L / h.
Next, using the liquid 6 after passing through the above, electrolytic extraction was performed at 50 A and 0.95 A / cm ² to remove an element lower in potential than bismuth to obtain metal bismuth.

[Α-ray measurement]
The α dose of the obtained metal bismuth was measured with an α ray measuring device. Table 3 shows the α dose of raw material bismuth and the α dose of bismuth obtained by purification. As shown in Table 3, the surface α dose of the raw material was 2.25 cph / cm ² , but it was 0.78 cph / cm ² after the first purification and 0.0042 cph / cm ² after the second purification. Although the α dose decreased, it was not sufficient as compared with the examples.

  According to the present invention, low α-ray bismuth in which α rays are minimized can be obtained. The present invention is industrially useful.

Claims (7)

Low α-ray bismuth having an α dose of 0.0025 cph / cm ² or less. The low alpha ray bismuth of Claim 1 whose alpha dose is 0.0020 cph / cm < ² > or less.   The low alpha ray bismuth of Claim 1 or 2 whose Pb content is 0.1 ppm or less.   Low alpha ray bismuth in any one of Claims 1-3 whose content of U and Th is 5 ppb or less, respectively. Using bismuth having an α dose of 2.0 cph / cm ² or less as a raw material, dissolving the raw material bismuth in a nitric acid solution by electrolysis, having a bismuth concentration of 5 to 50 g / L, pH 0.0 to 0.4 Preparing a bismuth nitrate solution;
A step of bringing a bismuth nitrate solution into contact with an ion exchange resin to obtain an ion exchange bismuth solution in which polonium in the solution is exchanged and removed by the ion exchange resin (first-stage ion exchange resin adsorption step);
Electrolytically collecting from the ion-exchanged bismuth solution and recovering the electrolytically collected bismuth,
A step of dissolving electrolytically collected bismuth in a nitric acid solution by electrolysis to adjust a bismuth nitrate solution having a bismuth concentration of 5 to 50 g / L and having a pH of 0.0 to 0.4 (electrolytic sampling-bismuth nitrate solution);
Step of obtaining an ion exchange bismuth solution (electrolytic collection-nitric acid-ion exchange bismuth solution) obtained by bringing the electrolytic collection-bismuth nitrate solution into contact with an ion exchange resin and exchanging and removing polonium in the solution with the ion exchange resin (second stage) Ion exchange resin adsorption process),
Electrowinning-from the nitric acid-ion exchange bismuth solution, electrowinning, and collecting the electrowinning bismuth (electrowinning-nitric acid-ion exchange-electrowinning bismuth),
The manufacturing method of the low alpha ray bismuth containing.   6. The method according to claim 5, wherein the flow rate in the second-stage ion exchange resin adsorption step is lower than the flow rate in the first-stage ion exchange resin adsorption step. The method according to claim 5 or 6, wherein the α dose of the raw material bismuth is 0.6 cph / cm ² or more and 2.0 cph / cm ² or less.