Method of producing low alpha-ray emitting bismuth, and low alpha-ray emitting bismuth

Abstract

Provided is low alpha-ray emitting bismuth having an alpha dose of 0.003 cph/cm.sup.2 or less. Additionally provided is a method of producing low alpha-ray emitting bismuth, wherein bismuth having an alpha dose of 0.5 cph/cm.sup.2 or less is used as a raw material, the raw material bismuth is melted in a nitric acid solution via 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, the bismuth nitrate solution is passed through a column filled with ion-exchange resin to eliminate polonium contained in the solution by an ion-exchange resin, and bismuth is recovered by means of electrowinning from the solution that was passed through the ion-exchange resin. Recent semiconductor devices are of high density and high capacity, and therefore are subject to increased risk of soft errors caused by the effects of alpha rays emitted from materials in the vicinity of semiconductor chips. In particular, there is a strong demand for higher purification of solder materials used near semiconductor devices, and there is a demand for low alpha-ray emitting materials. Therefore, the present invention aims to elucidate the phenomenon of alpha ray generation from bismuth, and to provide a low alpha-ray emitting, high-purity bismuth that can be applied to the required materials and a production method thereof, as well as to provide an alloy of low alpha-ray emitting bismuth and tin and a production method thereof.

Claims

1. A low alpha-ray dose bismuth, wherein the bismuth has an alpha-ray dose of 0.003 cph/cm.sup.2 or less.

2. The low alpha-ray dose bismuth according to claim 1, wherein the bismuth has a content of Pb of 0.1 ppm or less.

3. The low alpha-ray dose bismuth according to claim 2, wherein the bismuth has a content of each of U and Th of 5 ppb or less.

4. A low alpha-ray dose bismuth-tin alloy, wherein the alloy comprises bismuth according to claim 3 and tin having an alpha-ray dose of less than 0.001 cph/cm.sup.2.

5. A low alpha-ray dose bismuth-tin alloy according to claim 4, wherein the alloy has an alpha-ray dose of 0.0020 cph/cm.sup.2 or less.

6. The low alpha-ray dose bismuth-tin alloy according to claim 5, wherein the alloy has a content of tin of 40 mass % or more and 55 mass % or less.

7. The low alpha-ray dose bismuth according to claim 3, wherein the bismuth has an alpha-ray dose of 0.003 cph/cm.sup.2.

8. A low alpha-ray dose bismuth-tin alloy, wherein the alloy comprises bismuth according to claim 2 and tin having an alpha-ray dose of less than 0.001 cph/cm.sup.2.

9. A low alpha-ray dose bismuth-tin alloy according to claim 8, wherein the alloy has an alpha-ray dose of 0.0020 cph/cm.sup.2 or less.

10. The low alpha-ray dose bismuth-tin alloy according to claim 8, wherein the alloy has a content of tin of 40 mass % or more and 55 mass % or less.

11. The low alpha-ray dose bismuth according to claim 1, wherein the bismuth has a content of each of U and Th of 5 ppb or less.

12. A low alpha-ray dose bismuth-tin alloy, comprising bismuth according to claim 1 and tin having an alpha-ray dose of less than 0.001 cph/cm.sup.2 such that the alloy has an alpha-ray dose of 0.0020 cph/cm.sup.2 or less.

13. The low alpha-ray dose bismuth-tin alloy according to claim 12, wherein the alloy has a content of tin of 40 mass % or more and 55 mass % or less.

14. The low alpha-ray dose bismuth-tin alloy according to claim 13, wherein the alloy has an alpha-ray dose of 0.0018 cph/cm.sup.2.

15. A method of producing low alpha-ray dose bismuth having an alpha-ray dose of 0.003 cph/cm.sup.2 or less, wherein bismuth having an alpha-ray dose of 0.5 cph/cm.sup.2 or less is used as a raw material, the raw material bismuth is melted in a nitric acid solution via 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, the bismuth nitrate solution is passed through a column filled with ion-exchange resin to eliminate polonium contained in the solution by an ion-exchange resin, and bismuth is recovered by means of electrowinning from the solution that was passed through the ion-exchange resin.

16. The method of producing low alpha-ray dose bismuth according to claim 15, wherein volume of the ion-exchange resin is set to be 500 ml or more and 2 L or less when eliminating the polonium contained in the bismuth nitrate solution by the ion-exchange resin.

17. The method of producing low alpha-ray dose bismuth according to claim 16, wherein the rate of passing the bismuth nitrate solution through the column filled with the ion-exchange resin is 5 L/h or more and 8 L/h or less when eliminating the polonium contained in the bismuth nitrate solution by the ion-exchange resin.

18. The method of producing low alpha-ray dose bismuth according to claim 17, wherein the raw material bismuth is melted in the nitric acid solution via electrolysis to eliminate elements having an electric potential nobler than bismuth.

19. The method of producing low alpha-ray dose bismuth according to claim 18, wherein elements having an electric potential baser than bismuth are eliminated via electrowinning.

20. The method of producing low alpha-ray dose bismuth according to claim 15, wherein the rate of passing the bismuth nitrate solution through the column filled with the ion-exchange resin is 5 L/h or more and 8 L/h or less when eliminating the polonium contained in the bismuth nitrate solution by the ion-exchange resin.

21. The method of producing low alpha-ray dose bismuth according to claim 15, wherein the raw material bismuth is melted in the nitric acid solution via electrolysis to eliminate elements having an electric potential nobler than bismuth.

22. The method of producing low alpha-ray dose bismuth according to claim 15, wherein elements having an electric potential baser than bismuth are eliminated via electrowinning.

Description

DETAILED DESCRIPTION

(1) FIG. 1 This is a diagram showing the decay chain (uranium/radium decay chain) from the decay of uranium (U) up to .sup.206Pb.

(2) FIG. 2 This is a diagram showing the transition of the alpha dose based on the lapse of time after the melting and casting of bismuth.

DESCRIPTION OF EMBODIMENTS

(3) While there are numerous radioactive elements that generate alpha rays, most of them practically pose no problem because their half-life is extremely long or extremely short, and the practical problem is the alpha rays that are generated during the disintegration from the polonium isotope .sup.210Po to the lead isotope .sup.206Pb in the U decay chain (see FIG. 1).

(4) Bismuth is entirely a radioactive isotope, and there are multiple nuclides that are responsible for the alpha ray emission. Since it is considered that the alpha dose becomes high due to these radioactive isotopes, the isotopes that are responsible for the alpha ray emission must be isolated and eliminated in order to reduce the alpha dose. Thus, it was considered that it is impossible to industrially produce bismuth having a low alpha dose.

(5) Among the isotopes that are responsible for the alpha ray emission, .sup.209Bi is the only isotope with a long half-life, and since its half-life is extremely long at 1.910.sup.19 years, it is practically harmless.

(6) Other than .sup.209Bi, an isotope having a long half-life that is responsible for the alpha ray emission is .sup.210Bi, and its half-life is 5 days (see FIG. 1). The half-life of other isotopes .sup.211Bi, .sup.212Bi, and .sup.214Bi that are responsible for the alpha ray emission is each extremely short at 2 minutes, 61 minutes, and 20 minutes, and their daughter nuclides and granddaughter nuclides similarly have an extremely short half-life, and are practically harmless.

(7) As shown in FIG. 1, .sup.210Bi disintegrates in the manner of .sup.210Bi.fwdarw..sup.210Po.fwdarw..sup.210Pb, and alpha rays are emitted when .sup.210Po disintegrates to .sup.206Pb. .sup.206Pb is a stable isotope. Nevertheless, as a result of examining the alpha dose emitted from bismuth, it was discovered that bismuth yields a unique alpha dose change that cannot be found in other metals (see FIG. 2).

(8) Normally, for instance, in the case of tin, the alpha dose is low immediately after melting and casting, and the alpha dose increases pursuant to the lapse of time. Nevertheless, in the case of bismuth, the alpha dose is high immediately after melting and casting, and decreases pursuant to the lapse of time. As a result of examining this phenomena, it was discovered that the radioactive elements in bismuth that are responsible for the alpha ray emission was mainly polonium.

(9) While it was discovered that most of the alpha rays emitted from bismuth is polonium, the alpha dose of bismuth does not fall below a certain level even after a prolonged time which is sufficiently longer than the half-life of .sup.210Po; that is, a long period of time in which .sup.210Po hardly becomes decayed any more. This is considered to be because .sup.210Pb exists in bismuth, and the decay of .sup.210Pb.fwdarw..sup.210Bi.fwdarw..sup.210Po.fwdarw..sup.206Pb occurs.

(10) In other words, when the lead isotope .sup.210Pb (a half-life of 22.3 years) is contained in the material, disintegration (FIG. 1) of .sup.210Pb.sup.2.fwdarw..sup.210Bi (a half-life of 5 days).fwdarw..sup.210Po (a half-life of 138 days) will advance pursuant to the lapse of time, and the decay chain becomes reconstructed and .sup.210Po is generated. Thus, alpha rays are generated based on the disintegration from the polonium isotope .sup.210Po to the lead isotope .sup.206Pb.

(11) Accordingly, it is also important to reduce the ratio of the lead isotope .sup.210Pb in bismuth in addition to reducing polonium. By reducing the content of Pb to be 0.1 ppm or less, it is consequently possible to reduce the lead isotope .sup.210Pb, and the alpha dose of bismuth can thereby be reduced even more.

(12) Upon producing low alpha-ray emitting bismuth, it is desirable to use bismuth having an alpha dose of 0.5 cph/cm.sup.2 or less as the raw material. Since the isolation of Po becomes difficult when the alpha dose of the raw material is high, it is preferable to use a raw material having a relatively low alpha dose; that is, an alpha dose of 0.5 cph/cm.sup.2 or less. However, it is obvious that the upper limit of the alpha dose of the raw material is not limited since it can be adjusted according to the intended alpha dose reduction.

(13) A bismuth nitrate solution having a bismuth concentration of 5 to 50 g/L and a pH of 0.0 to 0.4 is prepared via electrolysis. The raw material bismuth is melted in a nitric acid solution via the foregoing electrolysis to eliminate elements having an electric potential nobler than bismuth.

(14) The reason why the bismuth concentration is set to be 5 to 50 g/L is because when the bismuth concentration is lower than 5 g/L, the production efficiency will deteriorate, and when the bismuth concentration is greater than 50 g/L, precipitation of the bismuth compound will occur and deteriorate the yield. Moreover, the reason why the pH is set to be 0.0 to 0.4 is because when the pH is lower than 0.0, more chemicals are required, and when the pH is greater than 0.4, the solubility of bismuth will decrease, and it becomes difficult to achieve a sufficient bismuth concentration.

(15) Next, the bismuth nitrate solution is passed through a column that is filled with ion-exchange resin, and polonium (Po) in the solution is eliminated by the ion-exchange resin. Upon performing this elimination by the ion-exchange resin, the volume of the ion-exchange resin is preferably 500 mL or more and 2 L or less. At a low volume that is lower than 500 mL, polonium absorption into the resin will deteriorate, and it is not possible to efficiently eliminate polonium. Moreover, at a high volume that is higher than 2 L, an increase in the amount of resin used causes an increase in the treatment cost, and the amount of bismuth that becomes adsorbed into the resin will increase so that the yield will deteriorate, and therefore it would be inappropriate.

(16) Moreover, the rate of passing the bismuth nitrate solution through the resin column (solution velocity) for the elimination by ion exchange is preferably 5 L/h or more and 8 L/h or less. At a low rate that is less than 5 L/h, treatment time will be longer and the treatment efficiency will deteriorate, and therefore it is undesirable. Meanwhile, when the rate exceeds 8 L/h, the solution velocity is too fast and polonium will pass through the ion-exchange resin without being adsorbed so that polonium-adsorbing efficiency of the resin will deteriorate, and therefore it is undesirable. Bismuth is recovered by means of electrowinning from the solution that was passed through the ion-exchange resin. As a result of performing electrowinning, it is possible to eliminate elements having an electric potential baser than bismuth. It is thereby possible to effectively eliminate Po, and produce bismuth having a low alpha dose.

(17) Accordingly, it is possible to obtain low alpha-ray emitting bismuth having an alpha dose of 0.003 cph/cm.sup.2 or less. The obtained bismuth can also cause the content of Pb to be 0.1 ppm or less, and the contents of U and Th to each be 5 ppb or less, and the present invention covers all of the above.

(18) Moreover, by mixing and melting the foregoing low alpha-ray emitting bismuth having an alpha dose of 0.003 cph/cm.sup.2 or less, and tin having an alpha dose of less than 0.001 cph/cm.sup.2, it is possible to produce a low alpha-ray emitting bismuth-tin alloy having an alpha dose of 0.0020 cph/cm.sup.2 or less.

(19) Here, while there is no particular limitation in the method of producing the low alpha-ray emitting tin that is used for producing the low alpha-ray emitting bismuth-tin alloy of the present invention, for instance, it is desirable to use low alpha-ray emitting tin having a purity of 5N or higher and an alpha dose of less than 0.001 cph/cm.sup.2 as described in Japanese Unexamined Patent Application Publication No. 2010-156052 A, which is obtained by leaching commercially available tin having a purity level of 3N in acid such as sulfuric acid to obtain an electrolyte, and performing electrolytic refining while adsorbing impurities contained in the electrolyte with adsorption materials such as oxides, activated carbon and carbon.

EXAMPLES

(20) The Examples and Comparative Examples of the present invention are now explained. Note that the Examples and Comparative Examples are merely exemplifications, and the present invention is not limited to the Examples. In other words, the present invention covers all modes and modifications other than the Examples within the scope of the technical spirit of the present invention. Moreover, the Comparative Examples are outside the conditions of the present invention, but have been prepared for facilitating the understanding of the effects of the present invention.

Example 1

(21) Raw material bismuth having an alpha dose of 0.483 cph/cm.sup.2 was melted in a nitric acid solution via electrolysis to eliminate elements having an electric potential nobler than bismuth. As the bismuth nitrate solution, 100 L of a solution having a Bi concentration of 40.2 g/L and a pH of 0.3 was used.

(22) The bismuth nitrate solution was passed through, at a rate of 5 L/h, a column filled with 2 L of cation-exchange resin of DIAION SK-1 B manufactured by Mitsubishi Chemical Corporation, which is one type of strongly acidic cation-exchange resin and has a sulfonate group (SO3H) as the exchange group.

(23) Subsequently, electrowinning was performed at 25 A and 0.48 A/cm.sup.2 using the aforementioned solution (filtrate) 1, which was passed through the cation-exchange resin, to eliminate elements having an electric potential baser than bismuth and obtain metal bismuth.

(24) The alpha dose of the obtained metal bismuth was thereafter measured using an alpha ray measurement device. The alpha dose of the raw material bismuth and the alpha dose of the refined bismuth are shown in Table 1.

(25) As shown in Table 1, while the surface alpha dose of the raw material was 0.483 cph/cm.sup.2, the alpha dose after refinement was 0.003 cph/cm.sup.2, and the reduction of the alpha dose was notable. Moreover, as a result of analyzing the obtained bismuth using a GDMS (glow discharge mass spectrometric analysis method), the content of Pb was 0.1 ppm or less.

(26) TABLE-US-00001 TABLE 1 Surface alpha dose (cph/cm.sup.2) Raw material 0.483 After refining 0.003

Example 2

(27) Raw material bismuth having an alpha dose of 0.462 cph/cm.sup.2 was melted in a nitric acid solution via electrolysis to eliminate elements having an electric potential nobler than bismuth. As the bismuth nitrate solution, 100 L of a solution having a Bi concentration of 41.6 g/L and a pH of 0.3 was used.

(28) The bismuth nitrate solution was passed through, at a rate of 8 L/h, a column filled with 500 mL of cation-exchange resin of DIAION SK-1 B manufactured by Mitsubishi Chemical Corporation.

(29) Subsequently, electrowinning was performed at 50 A and 0.97 A/cm.sup.2 using the aforementioned filtrate 2 to eliminate elements having an electric potential baser than bismuth and obtain metal bismuth.

(30) The alpha dose of the obtained metal bismuth was thereafter measured using an alpha ray measurement device. The alpha dose of the raw material bismuth and the alpha dose of the refined bismuth are shown in Table 2.

(31) As shown in Table 2, while the surface alpha dose of the raw material was 0.462 cph/cm.sup.2, the alpha dose after refinement was 0.003 cph/cm.sup.2, and the reduction of the alpha dose was notable.

(32) Moreover, as a result of analyzing the obtained bismuth using a GDMS (glow discharge mass spectrometric analysis method), the content of Pb was 0.1 ppm or less.

(33) TABLE-US-00002 TABLE 2 Surface alpha dose (cph/cm.sup.2) Raw material 0.462 After refining 0.003

Comparative Example 1

(34) Raw material bismuth having an alpha dose of 0.710 cph/cm.sup.2 was melted in a nitric acid solution via electrolysis to eliminate elements having an electric potential nobler than bismuth. As the bismuth nitrate solution, 100 L of a solution having a Bi concentration of 40.3 g/L and a pH of 0.3 was used.

(35) The bismuth nitrate solution was passed through, at a rate of 10 L/h, a column filled with 400 mL of cation-exchange resin of DIAION SK-1 B manufactured by Mitsubishi Chemical Corporation.

(36) Subsequently, electrowinning was performed at 50 A and 0.97 A/cm.sup.2 using the aforementioned filtrate 3 to eliminate elements having an electric potential baser than bismuth and obtain metal bismuth.

(37) The alpha dose of the obtained metal bismuth was thereafter measured using an alpha ray measurement device. The alpha dose of the raw material bismuth and the alpha dose of the refined bismuth are shown in Table 3.

(38) As shown in Table 3, while the surface alpha dose of the raw material was 0.710 cph/cm.sup.2, the alpha dose after refinement was 0.005 cph/cm.sup.2, and the alpha dose decreased, but was insufficient.

(39) TABLE-US-00003 TABLE 3 Surface alpha dose (cph/cm.sup.2) Raw material 0.710 After refining 0.005

Comparative Example 2

(40) Raw material bismuth having an alpha dose of 0.510 cph/cm.sup.2 was melted in a nitric acid solution via electrolysis to eliminate elements having an electric potential nobler than bismuth. As the bismuth nitrate solution, 100 L of a solution having a Bi concentration of 40.5 g/L and a pH of 0.2 was used.

(41) The bismuth nitrate solution was passed through, at a rate of 9 L/h, a column filled with 450 mL of cation-exchange resin of DIAION SK-1B manufactured by Mitsubishi Chemical Corporation.

(42) Subsequently, electrowinning was performed at 50 A and 0.97 A/cm.sup.2 using the aforementioned filtrate 4 to eliminate elements having an electric potential baser than bismuth and obtain metal bismuth.

(43) The alpha dose of the obtained metal bismuth was thereafter measured using an alpha ray measurement device. The alpha dose of the raw material bismuth and the alpha dose of the refined bismuth are shown in Table 4.

(44) As shown in Table 4, while the surface alpha dose of the raw material was 0.510 cph/cm.sup.2, the alpha dose after refinement was 0.004 cph/cm.sup.2, and the alpha dose decreased, but was insufficient.

(45) TABLE-US-00004 TABLE 4 Surface alpha dose (cph/cm.sup.2) Raw material 0.510 After refining 0.004

Example 3

(46) 2 kg of low alpha-ray emitting bismuth having an alpha dose of 0.003 cph/cm.sup.2 which was refined with the method described in Example 1, and 2 kg of tin having an alpha dose of 0.0008 cph/cm.sup.2 were filled in a graphite crucible, and the bismuth and tin were alloyed by being mixed and melted at 300 C.

(47) The alpha dose of the bismuth-tin alloy that was alloyed in Example 3 was 0.0018 cph/cm.sup.2, and a bismuth-tin alloy having a low alpha dose was obtained.

(48) It has been confirmed that, as a material for use in semiconductor chips which are recently of high density and with reduced operating voltage and cell volume, the bismuth-tin alloy having a low alpha dose produced as described above is effective in reducing the occurrence of soft errors caused by alpha rays, and the alpha dose has been reduced to a favorable level as a semiconductor material.

(49) TABLE-US-00005 TABLE 5 Surface alpha dose (cph/cm.sup.2) Refined Bi raw material 0.003 Refined Sn raw material 0.0008 BiSn alloy 0.0018

Comparative Example 3

(50) 2 kg of bismuth having an alpha dose of 0.005 cph/cm.sup.2 which was refined with the method described in Comparative Example 1, and 2 kg of tin having an alpha dose of 0.002 cph/cm.sup.2 were used, and the bismuth and tin were alloyed with the same method as the method described in Example 3.

(51) The alpha dose of the bismuth-tin alloy that was alloyed in Comparative Example 3 was 0.004 cph/cm.sup.2, and it was not possible to obtain a bismuth-tin alloy having a low alpha dose that is sufficiently usable, as a material for use in semiconductor chips which are recently of high density and with reduced operating voltage and cell volume, for reducing the occurrence of soft errors caused by alpha rays.

(52) TABLE-US-00006 TABLE 6 Surface alpha dose (cph/cm.sup.2) Refined Bi raw material 0.005 Refined Sn raw material 0.0020 BiSn alloy 0.0040

INDUSTRIAL APPLICABILITY

(53) As described above, the present invention can provide bismuth that is suitable for low alpha-ray emitting materials. Recent semiconductor devices are of high density and with reduced operating voltage and cell volume, and therefore are subject to increased risk of soft errors caused by the effects of alpha rays emitted from materials in the vicinity of semiconductor chips. However, by using low alpha-ray emitting bismuth, it is possible to yield an effect of reducing the occurrence of soft errors in semiconductor devices caused by the effects of alpha rays. In particular, bismuth as a low alpha-ray emitting material can be suitably used as the raw material of bismuth-tin for solder materials and the like.