JP2018066052A - LOW α RAY BISMUTH - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide bismuth having reduced α ray amount.SOLUTION: There are provided a manufacturing method of low α ray bismuth including a process for manufacturing a bismuth nitrate solution by electrolyzing raw material metal bismuth, a process for deposition removing polonium ions from the bismuth nitrate solution by contacting the bismuth nitrate solution with bismuth or a metal simple element of a more noble metal element than bismuth, a process for recovering metal bismuth by electrolytic extracting the same from the bismuth nitrate solution from which the polonium ions are deposition removed, the low α ray bismuth having surface α ray amount of 0.001 cph/cmor less, Pb content of 0.1 ppm or less, and contents of U and Th of 5 ppb or less respectively.SELECTED DRAWING: None

Description

  The present invention relates to bismuth with a reduced α dose, which is used in the manufacture of semiconductors and the like.

  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. Patent Document 2 discloses that a bismuth nitrate solution was electrolytically purified with a titanium cathode and a bismuth anode and then dissolved in a vacuum to achieve an α dose of 0.01 cph / cm ² or less.

JP 2013-185214 A International Publication No. 2014/069357 A1

  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.

  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, in the bismuth nitrate solution, Bi or a metal element having a potential more noble than Bi and a potential lower than Po is introduced, and Po is substituted on the metal element to be removed from the liquid. The present inventors have found that bismuth with a reduced α dose can be obtained by recovering Bi from the solution after removal of Po.

Therefore, the present invention includes the following (1) and below.
(1)
Low α-ray bismuth having a surface α dose of 0.001 cph / cm ² or less, a Pb content of 0.1 ppm or less, and U and Th contents of 5 ppb or less, respectively.
(2)
The low alpha ray bismuth as described in (1) whose surface alpha dose is less than 0.0008 cph / cm < ² >.
(3)
The low α-ray bismuth according to (1), wherein the surface α dose is less than 0.0005 cph / cm ² .

Furthermore, this invention includes the following (11) or less.
(11)
A step of electrolyzing the raw metal bismuth to prepare a bismuth nitrate solution;
A step of bringing a bismuth nitrate solution into contact with bismuth or a simple metal element of a metal element more precious than bismuth to precipitate and remove polonium ions from the bismuth nitrate solution;
From the bismuth nitrate solution from which polonium ions have been deposited and removed, electrolytically collecting and recovering metal bismuth,
The manufacturing method of the low alpha ray bismuth containing.
(12)
The production method according to (11), wherein the bismuth nitrate solution has a pH of 0.0 to 1.3.
(13)
The production method according to (11) or (12), wherein the bismuth nitrate solution has a bismuth concentration of 5 to 120 g / L.
(14)
The production method according to any one of (11) to (13), wherein the metal element nobler than bismuth is a metal element lower than polonium.
(15)
The manufacturing method according to any one of (11) to (14), wherein the metal element nobler than bismuth is Cu.

  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.

[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 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)”.

[Surface α ray measurement]
The surface alpha ray measurement of the metal bismuth in this invention can be measured, for example using the Gas Flow Proportional Counter model 8600A-LB made from Ordella as described in an Example. In this apparatus, the gas used was 90% argon-10% methane, and the measurement time was 504 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 hours to 504 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.

As described in the examples, the detector area of the measuring apparatus was 1008 cm ² and the α-ray counting efficiency was 90%. Since the background α-ray count was 1250 counts / 500 hours under the measurement conditions, the detection limit was 0.00036 cph / cm ² . This detection limit (Detection Limit, LLD, LOD) can be calculated according to formulas and references described in JEDEC JESD221 “Alpha Radiation Measurement in Electronic Materials”, p. At the detection limit where the possibility of error is 5%, the formula n is 3.29. Assuming that the α dose of the sample is small, the sample count number is equal to the background count number, and the sample and background measurement times are equal, the detection limit is expressed by the following equation: 3.29 × √2 × {√ ( BG value)} / 0.90 (counting efficiency) / detector area / measurement time. Since the α-ray measurement value is considered to follow a Poisson distribution, √ (BG value) corresponds to the standard deviation of BG. For example, if the measured value is 1250 [counts], 500 [h], and the detector area is 1008 [cm ² ], the detection limit is LOD = 4.66 × (√1250) / [500 × 1008 × 0.90] = 0.00036 cph / cm ² ].
References: Michihisa Uemoto, “Concept of detection limit and lower limit of quantification”, Bunseki, 2010, 5, P.216-221

[Production method of low α-ray bismuth]
According to the present invention, the process of preparing a bismuth nitrate solution by electrolyzing the raw metal bismuth, bismuth nitrate solution, bismuth or a simple metal element having a lower potential than polium and having a lower potential than polonium And a step of precipitating and removing polonium ions from the bismuth nitrate solution, and recovering metal bismuth by electrolytic extraction from the bismuth nitrate solution from which the polonium ions have been precipitated and removed. Wire bismuth can be produced.

[Raw metal 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 0.2 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. As the raw material metal bismuth, for example, bismuth having a purity of 3N5 or 5N can be used.

[Electrolysis and nitric acid dissolution]
The raw metal bismuth is dissolved in nitric acid by electrolysis to prepare a bismuth nitrate solution. This nitric acid dissolution by electrolysis can also remove a certain amount of elements that are more potential than bismuth. This electrolysis can use a known means, and can be carried out, for example, according to the method disclosed in Patent Document 2. Alternatively, for example, bismuth nitrate can be prepared by the following conditions:
Electrolyzer: PVC (Catholite and Anorite are separated by anion exchange membrane)
Cathode: Titanium Anode: Bismuth Current density: 0.1 to 1.0 A / dm ²
Catholite pH (starting): 0.0-1.5
Anorite pH (at start): 0.0-1.5
Anion exchange membrane: Ceremion AMT

[Bismuth nitrate solution]
In a preferred embodiment, the bismuth nitrate solution obtained by electrolysis has a bismuth concentration of 5 to 120 g / L, preferably 20 to 120 g / L. When it is lower than 5 g / L, the production efficiency is poor, and when it is higher than 120 g / L, precipitation of the bismuth compound occurs, resulting in poor yield. In a preferred embodiment, the bismuth nitrate solution obtained by electrolysis is pH 0.0 to 1.3, preferably pH 0.5 to 1.2, pH 0.5 to 1.1. If the pH is lower than 0.0, Bi or Cu for substitution deposition may dissolve in the nitric acid solution at a high rate, which is not preferable. On the other hand, if the pH exceeds 1.3, precipitation of the Bi compound tends to occur when the Bi concentration in the liquid is high, which is not preferable.

[Metallic elements precious than bismuth]
The metal element nobler than bismuth is preferably a metal element baser than polonium, for example, Cu. Being more noble than bismuth means that the standard electrode potential has a higher value than bismuth, and being baser than polonium means that the standard electrode potential has a lower value than polonium.

[Contact of metals with bismuth or metal elements nobler than bismuth]
When the bismuth nitrate solution is brought into contact with a metal of a metal element nobler than bismuth, polonium ions are deposited and removed as a metal. This contact can be performed by a known means for bringing a liquid into contact with a solid. For example, the above-mentioned noble metal element is introduced into a bismuth nitrate solution, for example, in a granular, plate-like, rod-like, or net-like form so as to be excellent in handleability and secure a large contact surface area. May be suspended or held in the flow path of the bismuth nitrate solution. The contact time can be appropriately selected by those skilled in the art in consideration of the surface area of contact. Further, even when the bismuth nitrate solution is brought into contact with the bismuth metal alone, the polonium ions are deposited and removed as a metal. The contact with metal bismuth can be performed by the same means as the contact with a metal element nobler than bismuth.

[Electrolytic sampling]
From the bismuth nitrate solution from which the polonium ions have been deposited and removed, electrolytic collection is performed to recover metal bismuth. Electrolytic extraction can use a well-known means, for example, can be implemented according to the method of patent document 2. However, the preferred bismuth concentration and pH of the bismuth nitrate solution in electrowinning are as described above.

[Low α-ray bismuth]
Low α-rays bismuth produced according to the present invention, the surface α dose 0.001 cph / cm ² or less, preferably, for example, less than ^{0.001cph / cm 2, 0.0008cph / cm} 2 or less, 0.0008cph / cm less than ^{2, 0.0007cph} / cm ² or less, 0.0005cph / cm ² or less, less than 0.0005cph / cm ^2, it is 0.0004cph / cm ² or less. 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 produced according to the present invention is used as an alloy, the α-ray derived from bismuth can achieve the same excellent low α-ray emission. The production of the alloy containing the low alpha bismuth of the present invention is 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]
[Cu]
Raw material bismuth (purity: 4N) having an α dose of 0.34 cph / cm ² was dissolved in a nitric acid solution by electrolysis under the following conditions.
Electrolyzer: PVC (Catholite and Anorite are separated by anion exchange membrane)
Cathode: Titanium Anode: Bismuth Current density: 0.95 A / dm ²
Catholite pH (at start): 1.0
Anorite pH (at start): 1.0
Anion exchange membrane: Ceremion AMT
The resulting bismuth nitrate solution had a Bi concentration of 43.3 g / L and pH = 1.1, and 100 L of this was used in the next experiment.
To this bismuth nitrate solution, 2.2 kg of copper particles (particle size: 5 mm to 20 mm, average particle size: about 10 mm) was added and immersed for 23 hours.
Next, copper particles were removed from the liquid after the above treatment, and electrowinning was performed at 50 A and 0.80 A / cm ² to obtain metal bismuth.
And metal bismuth was measured with the method mentioned later by (alpha) dose with the alpha ray measuring apparatus. The surface α dose of the raw material was 0.34 cph / cm ² , but after purification, it was 0.0007 cph / cm ² , and the α dose was significantly reduced. 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]
[Bi]
Raw material bismuth (purity: 4N) having an α dose of 0.34 cph / cm ² was dissolved in a nitric acid solution by electrolysis under the same conditions as in Example 1.
The obtained bismuth nitrate solution had a Bi concentration of 46.2 g / L and pH = 1.1, and 100 L of this was used in the next experiment.
2 kg of bismuth flakes (flat shape, flat surface size of about 15 mm × about 15 mm, average thickness of about 3 mm, surface α dose of 1 cph / cm ² or less) were added to this bismuth nitrate solution, and immersed for 96 hours.
Next, bismuth flakes were removed from the liquid after the above treatment, and electrowinning was performed at 50 A and 0.80 A / cm ² to obtain metal bismuth.
Then, the α dose of metal bismuth was measured with an α ray measuring device. The surface α dose of the raw material was 0.34 cph / cm ² , but after purification, it was 0.0004 cph / cm ² , and the α dose was significantly reduced. 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.

[Comparative Example 1]
[Ru]
Raw material bismuth (purity: 4N) having an α dose of 0.34 cph / cm ² was dissolved in a nitric acid solution by electrolysis under the same conditions as in Example 1.
The obtained bismuth nitrate solution had a Bi concentration of 44.2 g / L and pH = 1.0, and 100 L of this was used in the next experiment.
A Ti plate with a Ru-plated surface was inserted into this bismuth nitrate solution and immersed for 24 hours.
Next, the Ti plate was removed from the treated liquid, washed with water and dried.
And the alpha dose was measured with the alpha ray measuring apparatus for the Ti plate after the said immersion, water washing, and drying. The surface α dose of the Ru-plated Ti plate was 0.027 cph / cm ² before immersion, but 0.023 cph / cm ² after immersion, and the α dose hardly changed. It was found that Ru has no effect of substitution adsorption of Po and Po cannot be removed.

[Α dose measurement]
The measurement samples used in the above examples and comparative examples were prepared as described below.
First, in Example 1 and Example 2, a metal Bi is added to a nitric acid solution, and a nitric acid Bi solution is prepared by electrolysis. Then, copper particles or Bi flakes are added to the nitric acid Bi solution, respectively. After removing Po, the electrodeposited Bi was cast into a 310 mm × 310 mm mold to a thickness of about 1.5 mm, and a square thin metal Bi sample having a side of 310 mm and a thickness of about 1.5 mm was produced. .

As the α-ray measuring apparatus, GasFlow Proportional Counter model 8600A-LB manufactured by Ordella was used. The gas used is 90% argon-10% methane, the measurement time is 504 hours for both the background and the sample, and the first 4 hours is the time required for the measurement chamber purge. Was used for α dose calculation. 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 measuring device had a detector area of 1008 cm ² and an α ray counting efficiency of 90%. Since the background α-ray count was 1250 counts / 500 hours under the measurement conditions, the detection limit was 0.00036 cph / cm ² .
A measurement sample is a type of device that is placed in a sealed sample chamber and detects alpha rays emitted from the surface of the sample with a detector placed at the top of the sample chamber. Is something.
In the α ray measurement of the sample in the present invention, the sample of the size and shape described above is placed in the measurement chamber of the measurement apparatus so that one main surface of the sample faces the detection surface of the upper α ray detector. I went. At this time, the surface on the back side of the sample is placed in contact with the measurement chamber, the side surface of the sample is perpendicular to the α-ray detection surface, and the area is relative to the main surface facing the detector. Is considerably small, and the range of α rays is sufficiently small, the wraparound from the back and side surfaces is negligible, and the value obtained by normalizing the detected α dose with the area of the main surface is in the present invention. α dose was used.

  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 (3)

Low α-ray bismuth having a surface α dose of 0.001 cph / cm ² or less, a Pb content of 0.1 ppm or less, and U and Th contents of 5 ppb or less, respectively. The low α-ray bismuth of claim 1, wherein the surface α dose is less than 0.0008 cph / cm ² . The low α-ray bismuth of claim 1, wherein the surface α dose is less than 0.0005 cph / cm ² .