Copper or copper alloy, bonding wire, method of producing the copper, method of producing the copper alloy, and method of producing the bonding wire

Abstract

Copper or a copper alloy characterized in having an -ray emission of 0.001 cph/cm.sup.2 or less. Since recent semiconductor devices are produced to have higher density and higher capacity, there is greater risk of soft errors caused by the influence of rays emitted from materials positioned near semiconductor chips. In particular, there are strong demands for achieving higher purification of copper and copper alloys which are used near the semiconductor device, such as copper or copper alloy wiring lines, copper or copper alloy bonding wires, and soldering materials, and materials reduced in -ray emission are also demanded. Thus, the present invention elucidates the phenomenon in which rays are emitted from copper or copper alloys, and provides copper or copper alloy reduced in -ray emission which is adaptable to the demanded material, and a bonding wire in which such copper or copper alloy is used as its raw material.

Claims

1. A method of producing a copper of low -ray radiation, comprising the steps of: preparing an electrolytic cell comprising a copper sulfate solution as a starting electrolytic solution, a positive electrode and a negative electrode, and a diaphragm positioned in the electrolytic cell to partition the electrolytic cell into a positive electrode side containing the positive electrode and a negative electrode side containing the negative electrode; performing electrolysis with the electrolytic cell equipped with a crude copper raw material as the positive electrode to obtain an anolyte solution formed by anodic dissolution of the crude copper raw material; extracting and filtering an amount of the anolyte solution to remove lead sulfate precipitates contained in the extracted anolyte solution and to obtain a filtered anolyte solution; bringing back the filtered anolyte solution to the negative electrode side of the electrolytic cell and performing electrolysis to form an electrodeposited copper on the negative electrode; and collecting the electrodeposited copper from the negative electrode and melting and casting the electrodeposited and collected copper to form the copper of low -ray radiation; wherein the copper of low -ray radiation has an -ray count of 0.001 cph/cm.sup.2 or less at a time after a lapse of 30 months from a time of the melting and casting.

2. A method according to claim 1, wherein the copper sulfate solution has a Cu concentration of 30 to 200 g/L.

3. A method according to claim 2, wherein the diaphragm is an anion-exchange membrane through which Pb.sup.2+ ions do not pass.

4. A method according to claim 3, wherein the copper of low -ray radiation has a Pb content of less than 0.01 wtppm, an U content of less than 5 wtppb, and a Th content of less than 5 wtppb.

5. A method according to claim 4, wherein the copper of low -ray radiation has an -ray count of 0.001 cph/cm.sup.2 or less at each time after a lapse of 1 week, 3 weeks, 1 month, 2 months, 6 months, and 30 months from a time of the melting and casting.

6. A method according to claim 5, wherein the copper of low -ray radiation has a purity of 4N (99.99%) or higher.

7. A method of producing a copper alloy of low -ray radiation, comprising the steps of providing a copper produced by the method according to claim 6, melting the copper together with one or more of alloying elements to obtain a copper alloy in a molten state, and casting the copper alloy.

8. A method of producing a bonding wire, comprising the step of subjecting the copper alloy produced according to claim 7 to wire drawing processing to form bonding wire.

9. A method according to claim 1, wherein the diaphragm is an anion-exchange membrane through which Pb.sup.2+ ions do not pass.

10. A method according to claim 1, wherein the copper of low -ray radiation has a Pb content of less than 0.01 wtppm, an U content of less than 5 wtppb, and a Th content of less than 5 wtppb.

11. A method according to claim 1, wherein the copper of low -ray radiation has an -ray count of 0.001 cph/cm.sup.2 or less at each time after a lapse of 1 week, 3 weeks, 1 month, 2 months, 6 months, and 30 months from a time of the melting and casting.

12. A method according to claim 1, wherein the copper of low -ray radiation has a purity of 4N (99.99%) or higher.

13. A method of producing a copper alloy of low -ray radiation, comprising the steps of providing a copper produced by the method according to claim 1, melting the copper together with one or more of alloying elements to obtain a copper alloy in a molten state, and casting the copper alloy.

14. A method of producing a bonding wire, comprising the step of subjecting the copper alloy produced according to claim 13 to wire drawing processing to form bonding wire.

Description

BRIEF DESCRIPTION OF DRAWINGS

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

(2) FIG. 2 This is a diagram showing the -rays amount that is emitted when the decay chain of .sup.210Pb.fwdarw..sup.210Bi.fwdarw..sup.210Po.fwdarw..sup.206Pb is reconstructed from a state without hardly any isotope .sup.210Po of polonium.

(3) FIG. 3 This is a diagram showing the relation of Pb content in Cu and -ray emission.

(4) FIG. 4 This is a diagram schematically showing the lead removal process based on diaphragm electrolysis.

DETAILED DESCRIPTION

(5) While there are numerous radioactive elements that generate rays, many of them have an extremely long half-life or an extremely short half-life, and in reality do not cause problems. What are problematic are the rays that are generated in the U decay chain (refer to FIG. 1) during the decay from the isotope .sup.210Po of polonium to the isotope .sup.206Pb of lead.

(6) With copper or copper alloy wiring lines, copper or copper alloy bonding wires, and soldering materials that are used near the semiconductor device, materials have been developed in which copper or copper alloy is used as the raw material, and a low -ray copper or copper alloy material is being demanded.

(7) For example, oxygen-free copper of 4N to 5N is normally used as the raw material of a copper or copper alloy bonding wire, and contains lead in an amount of 0.1 wtppm or more, and the generation of rays also exceeds 0.001 cph/cm.sup.2. Moreover, conventionally, since it was considered that low -rays were not even required, it could be said that there was no motive for reducing the rays.

(8) As described above, Po has extremely high sublimability, and when heated in the production process; for instance, during the melting/casting process, Po becomes sublimated. It is considered that, if the isotope .sup.210Po of polonium is eliminated in the production process, the decay from the isotope .sup.210Po of polonium to the isotope .sup.206Pb of lead will not occur, which in turn will not cause the generation of rays (refer to U decay chain of FIG. 1).

(9) Nevertheless, in a state where there is hardly any isotope .sup.210Po of polonium, decay of .sup.210Pb.fwdarw..sup.210Bi.fwdarw..sup.210Po.fwdarw..sup.206Pb will occur. In addition, it has been confirmed that it takes approximately 27 months (little over 2 years) for this decay chain to become an equilibrium state (refer to FIG. 2).

(10) In other words, when the isotope .sup.206Pb of lead (half-life of 22 years) is contained in the copper material, decay of .sup.210Pb.fwdarw..sup.210Bi (half-life of 5 days).fwdarw..sup.210Po (half-life of 138 days) will advance pursuant to the lapse of time, and .sup.210Po is generated as a result of the decay chain being reconstructed. Thus, rays caused by the decay from the isotope .sup.210Po of polonium to the isotope .sup.206Pb of lead are generated.

(11) Accordingly, the problem cannot be resolved even if the -ray emission is low immediately after the product is produced, and there is a problem in that the -ray emission gradually increases with the lapse of time, and the risk of soft errors will increase. The foregoing period of 27 months (little over 2 years) is not necessary a short period of time.

(12) The problem of the -ray emission gradually increasing with the lapse of time even if the -ray emission is low immediately after the product is produced is a result of the isotope .sup.210Pb of lead of the U decay chain shown in FIG. 1 being contained in the material, and it could be said that the foregoing problem cannot be resolved unless the content of the isotope .sup.210Pb of lead is reduced as much as possible.

(13) FIG. 3 shows the relation of the Pb content in Cu and the -ray emission. The straight line shown in FIG. 3 shifts up and down depending on the ratio of the isotope .sup.214Pb, .sup.210Pb, .sup.209Pb, .sup.208Pb, .sup.207Pb, .sup.206Pb, .sup.204Pb of lead, and it has been confirmed that the straight line shifts more upward as the ratio of the isotope .sup.210Pb of lead is greater.

(14) Accordingly, it is important to reduce the ratio of the isotope .sup.210Pb of lead in the copper, and, since the isotope .sup.210Pb of lead can also be reduced by reducing Pb to be 0.1 ppm or less, the -ray emission will not increase with the lapse of time.

(15) Moreover, the abundance ratio of the isotope .sup.206Pb of lead being small means that the ratio of the U decay chain shown in FIG. 1 is relatively small, and it is considered that the isotope .sup.210Pb of lead belonging to this system will also decrease.

(16) The -ray emission of copper that was subject to melting/casting can consequently achieve 0.001 cph/cm.sup.2 or less. To achieve an -ray emission of the foregoing level is the basis of the present invention, and it could be said that the conventional technologies do not disclose or even suggest the foregoing achievement based on the foregoing recognition.

(17) Specifically, provided is copper or a copper alloy having an -ray emission of 0.001 cph/cm.sup.2 or less in all cases; namely, 1 week after, 3 weeks after, 1 month after, 2 months after, and 6 months after the melting/casting, and 30 months, which is over 27 months after the melting/casting until when the decay chain of .sup.210Pb.fwdarw..sup.210Bi.fwdarw..sup.210Po.fwdarw..sup.206Pb becomes equilibrium in a state where there is no isotope .sup.210Po of polonium which generates rays caused by the decay to the isotope .sup.206Pb of lead.

(18) Note that, with a hydrochloric acid bath that is used in the electrolytic refining for low pregelatinization (reduction of -ray emission) of tin and the like, since Pb is not deposited, Pb cannot be eliminated with the diaphragm electrolysis of the present case. Moreover, while a nitric acid bath is also used in the electrolytic refining of Cu, since Pb is also not deposited in the foregoing case, Pb cannot be eliminated with the diaphragm electrolysis of the present case. In the present case, Pb is eliminated based on diaphragm electrolysis as a result of causing Pb to be deposited, and low pregelatinization (reduction of -ray emission) is thereby achieved.

(19) In addition, caution is required upon measuring the -ray emission. This is because rays (hereinafter referred to as background (BG) rays as needed) are sometimes output from the -ray measuring device (equipment). The foregoing -ray emission in the present invention is the substantial -ray emission that does not include the rays output from the -ray measuring device. Thus, the term -ray emission as used in this specification shall be used in the foregoing meaning.

(20) While the -ray emission that is emitted from copper or copper alloy was explained above, alloys containing copper or copper alloy are also strongly affected by the -ray emission. While the influence of the -ray emission is sometimes mitigated by components other than copper in which the -ray emission is small or hardly occurs, with regard to a copper alloy in which 40% or more of copper is at least contained in the alloy component, it would be desirable to use the copper with low -ray emission according to the present invention.

(21) The refining of copper is performed based on the diaphragm electrolysis described below.

(22) A copper sulfate solution is used as an electrolytic solution, a diaphragm is provided between a positive electrode and a negative electrode, deposits, particularly lead sulfate, in the electrolytic solution extracted from the positive electrode side are removed, and the electrolytic solution is thereafter supplied to the negative electrode side.

(23) The diaphragm electrolysis of the present invention is unique in that a copper sulfate solution (for instance, Cu concentration of 30 to 200 g/L) is used, and an anion-exchange membrane, through which Pb.sup.2+ ions do not pass, is used as the diaphragm. Even if diaphragm electrolysis is performed, when a cation-exchange membrane is used, Pb.sup.2+ ions will pass therethrough, and lead will get mixed into the electrodeposited copper on the cathode side. Thus, as described above, it is necessary to use an anion-exchange membrane. Moreover, elimination of the lead sulfate as the deposits from the electrolytic solution is performed via filtration using a filter.

(24) As described above, the present invention can eliminate lead to a level of 0.1 ppm by performing diaphragm electrolysis of using an anion-exchange membrane and using a copper sulfate solution as the electrolytic solution. The copper or copper alloy of the present invention obtained as described above yields a superior effect of being able to considerably reduce the occurrence of soft errors in the semiconductor device caused by the influence of -ray emission.

(25) Note that, as described above, while diaphragm electrolysis of using an anion-exchange membrane and using a copper sulfate solution as the electrolytic solution is an effective method, it should be easy to understand that the present invention is not limited to this method so as long as the production method can achieve an -ray emission of 0.001 cph/cm.sup.2 or less.

EXAMPLES

(26) Examples of the present invention are now explained. Note that these Examples are merely illustrative, and the present invention is not limited to these Examples. In other words, the present invention covers all modes or modifications other than the following Examples within the scope of the technical concept of the present invention.

Example 1

(27) Crude copper (purity of approximately 99%) after being refined in a revolving furnace in the copper refining process was used as a raw material anode, and electrolytic refining was performed using a copper sulfate solution. Since the lead contained in the crude copper is deposited as lead sulfate, diaphragm electrolysis using an anion-exchange membrane was performed in order to prevent the deposits from getting caught in the electrodeposit.

(28) The crude copper was subject to electrodissolution using the positive electrode, the resulting solution of a predetermined copper concentration was extracted with a pump and filtered, and the solution, which is free of deposits, was delivered to the negative electrode to obtain an electrodeposit. Consequently, a copper electrodeposit having a purity of 4N with a low lead concentration was obtained. The contents of Pb, U, and Th were respectively <0.01 wtppm, <5 wtppb, and <5 wtppb.

(29) The collected electrodeposited copper was washed and dried, subject to melting/casting at a temperature of 1200 C., and the time-dependent change of the -ray emission from immediately after the melting/casting was checked. The sample to be subject to -ray measurement was obtained by rolling a plate that was subject to melting/casting to a thickness of approximately 1.5 mm, and cut out into a plate of 310 mm310 mm. The surface area thereof is 961 cm.sup.2. The obtained plate was used as the sample to be subject to -ray measurement.

(30) As the -ray measuring device, the Gas Flow Proportional Counter Model 8600A-LB manufactured by Ordela was used. The used gas was 90% argon and 10% methane, the measurement time was 104 hours for both the background and sample, and the first 4 hours were the hours required for purging the measurement chamber, and the measurement was performed from 5 hours after to 104 hours after. In other words, used for the calculation of the -ray emission was data from 5 hours after to 104 hours after of samples 1 week after, 3 weeks after, 1 month after, 2 months after, and 6 months after the melting/casting.

(31) With regard to the foregoing samples, as a result of measuring the -ray emission 1 week after, 3 weeks after, 1 month after, 2 months after, and 6 months after the melting/casting, and 30 months, which is over 27 months after the melting/casting until when the decay chain of .sup.210Pb.fwdarw..sup.210Bi.fwdarw..sup.210Po.fwdarw..sup.206Pb becomes equilibrium in a state where there is no isotope .sup.210Po of polonium which generates rays caused by the decay to the isotope .sup.206Pb of lead, the -ray emission was, at maximum, 0.001 cph/cm.sup.2, and satisfied the conditions of the present invention.

(32) Moreover, when producing a copper alloy ingot, as the alloy elements to be added, normally several 10 to several 100 wtppm of one or more types selected from Al, Ag, B, Ba, Be, Bi, Ca, Ce, Co, Cr, Dy, Eu, Gd, Ge, In, Ir, La, Mg, Mo, Nd, Ni, P, Pd, Pt, Rh, Ru, Sb, Si, Sn, Sr, Y, Ti, Yb, Zn, and Zr are added.

(33) Upon producing this copper alloy, it is important to cause the contents of Pb, U, and Th contained in the copper alloy, which contains Cu as the base, to be respectively <0.01 wtppm, <5 wtppb, and <5 wtppb when performing the melting/casting process. In this Example, as a result of similarly measuring the -ray emission upon producing the foregoing copper alloy, the -ray emission was also, at maximum, 0.001 cph/cm.sup.2.

Comparative Example 1

(34) Commercially available oxygen-free copper was subject to melting/casting, and an -ray sample was prepared with the same method as Example 1. The contents of Pb, U, and Th were respectively 1 wtppm, <5 wtppb, and <5 wtppb.

(35) As a result of checking the time-dependent change of the -ray emission from immediately after the melting/casting process, the -ray emission was 0.001 cph/cm.sup.2 or less immediately after the melting/casting process, but gradually increased. This is considered to be because, while the -ray emission had temporarily decreased since Po evaporated in the melting/casting process, since Pb is contained in an amount of 1 wtppm, the decay chain was constructed once again and the -ray emission had consequently increased. It was hence not possible to achieve the object of the present invention.

Comparative Example 2

(36) Crude copper (purity of approximately 99%) after being refined in a revolving furnace in the copper refining process was used as a raw material anode, and electrolytic refining was performed using a copper sulfate solution without using a diaphragm. Consequently, the contents of Pb, U, and Th were respectively 0.2 wtppm, <5 wtppb, and <5 wtppb.

(37) As a result of checking the time-dependent change of the -ray emission from immediately after the melting/casting process, the -ray emission was 0.001 cph/cm.sup.2 or less immediately after the melting/casting process, but gradually increased. This is considered to be because, while the -ray emission had temporarily decreased since Po evaporated in the melting/casting process, since Pb is contained in an amount of 0.2 wtppm, the decay chain was constructed once again and the -ray emission had consequently increased. It was hence not possible to achieve the object of the present invention.

Example 2

(38) The copper ingot prepared with the method of Example 1 was subject to wire drawing processing to obtain a wire having a diameter of 25 m. As a result of covering a sample tray of the -ray measuring device with the prepared wires and performing a measurement with the same method as Example 1, the -ray emission did not increase and was stably 0.001 cph/cm.sup.2 or less. Accordingly, this processed copper wire can be effectively used as a copper bonding wire.

(39) Moreover, the copper alloy ingot produced in Example 1 doped with several 10 to several 100 wtppm of one or more types selected from Al, Ag, B, Ba, Be, Bi, Ca, Ce, Co, Cr, Dy, Eu, Gd, Ge, In, Ir, La, Mg, Mo, Nd, Ni, P, Pd, Pt, Rh, Ru, Sb, Si, Sn, Sr, Y, Ti, Yb, Zn, and Zr was subject to wire drawing processing, and a result of performing a measurement, the -ray emission did not increase and was stably 0.001 cph/cm.sup.2 or less. This means that, careful component adjustment is important in the production stage of the copper alloy, and it is also important to cause the contents of Pb, U, and Th contained in the copper alloy, which contains Cu as the base, to be respectively <0.01 wtppm, <5 wtppb, and <5 wtppb.

Comparative Example 3

(40) The copper ingots prepared with the methods of Comparative Example 1 and Comparative Example 2 was subject to wire drawing processing to obtain wires having a diameter of 25 m. As a result of covering a sample tray of the -ray measuring device with the prepared wires and performing a measurement, the -ray emission was roughly 0.001 cph/cm.sup.2 immediately after the wire drawing processing, but gradually increased. Accordingly, it cannot be said that the foregoing processed copper wire is an effective material as a copper bonding wire.

(41) As described above, since the present invention yields a superior effect of being able to provide copper or a copper alloy which is adaptable to materials reduced in rays and a bonding wire which uses such copper or copper alloy as its raw material; it is possible to eliminate the influence of rays on semiconductor chips as much as possible. Accordingly, the present invention can considerably reduce the occurrence of soft errors in the semiconductor device caused by the influence of -ray emission, and is useful as a material for locations, in which copper or a copper alloy is used, such as copper or copper alloy wiring lines, copper or copper alloy bonding wires, and soldering materials.