Ultrahigh-Purity Copper and Process for Producing the Same, and Bonding Wire Comprising Ultrahigh-Purity Copper
Provided is ultrahigh purity copper having a hardness of 40 Hv or less, and a purity of 8N or higher (provided that this excludes the gas components of O, C, N, H, S and P). With this ultrahigh purity copper, the respective elements of O, S and P as gas components are 1 wtppm or less. Also provided is a manufacturing method of ultrahigh purity copper based on two-step electrolysis using an electrolytic solution comprised of copper nitrate solution, including the procedures of adding hydrochloric acid in an electrolytic solution comprised of copper nitrate solution; circulating the electrolytic solution; and performing two-step electrolysis while eliminating impurities with a filter upon temporarily setting the circulating electrolytic solution to a temperature of 10° C. or less. The present invention provides a copper material that is compatible with the thinning (wire drawing) of the above, and is capable of efficiently manufacturing ultrahigh purity copper having a purity of 8N (99.999999 wt %) or higher, providing the obtained ultrahigh purity copper, and providing a bonding wire for use in a semiconductor element that can be thinned.
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The present invention relates to ultrahigh purity copper having a purity of 8N (99.999999 wt %) or higher that can be subject to thinning without breaking, and the manufacturing method of such ultrahigh purity copper. The present invention also relates to a bonding wire comprised of such ultrahigh purity copper.
BACKGROUND ARTConventionally, a gold wire has been used as the bonding wire for electrically connecting a silicon chip as the semiconductor element and the lead frame. Nevertheless, from the perspective that a gold wire is expensive and the strength thereof is inferior to copper, proposals have been made for switching to a copper wire (Non-Patent Documents 1 and 2).
High purity copper is characterized in that it is soft since the recrystallization temperature is low, it has favorable workability since there is hardly any brittleness in the intermediate temperature range, and it has high thermal conductivity since the electrical resistance is small at ultracold temperatures. It is also characterized by characteristic improvement due to addition of infinitesimal quantity of elements and great impact of impurity contamination on the characteristics.
The foregoing conventional technologies utilize such characteristics of high purity copper to manufacture a bonding wire for semiconductor elements.
Nevertheless, a copper wire also entails several drawbacks; for instance, a copper wire is easily oxidized when forming the tip thereof into a ball shape, and there is a problem in that the silicon chip will be damaged when the ball shape is inferior or because the ball is too hard.
Proposals have been made to overcome the foregoing problems that arise during the shaping of the ball and with regard to the hardness of copper; namely, employing a protective atmosphere (reduction or inert atmosphere), and realizing copper having a purity level of 4N to 6N by eliminating impurities so as to soften such copper.
As conventional technologies of copper bonding wires, numerous documents have been disclosed; for instance, a copper bonding wire wherein the impurities are 10 ppm or less (refer to Patent Document 1), a copper bonding wire wherein the impurities containing B are 50 ppm or less (refer to Patent Document 2), a bonding wire for use in a semiconductor element wherein the copper purity is 99.999% or higher and the S content is 0.0005% or less (refer to Patent Document 3), a copper bonding wire comprised of high purity copper subject to annealing during the wire drawing process (refer to Patent Document 4), a bonding wire for use in a semiconductor device wherein the total content of impurities is 5 ppm or less, and the rupture strength is 18 to 28 kg/mm2 (refer to Patent Document 5), a copper bonding wire with adjusted crystal grains wherein the impurities are 10 ppm or less (refer to Patent Document 6), a bonding wire for use in a semiconductor element wherein the copper purity is 99.999% or higher, the S content is 0.0005% or less, and 0.02% of In, Hf and Mg is added thereto (refer to Patent Document 7), a bonding wire for use in a semiconductor element wherein the copper purity is 99.999% or higher, the S content is 0.0005% or less, and the alkali metals elements and alkali earth metal elements are 0.0001% or less (refer to Patent Document 8), a bonding wire for use in a semiconductor element wherein the copper purity is 99.999% or higher, the S content is 0.0005% or less, In and Mg are less than 0.02%, and Be, B, Zr, Y, Ag, Si, Ca and rare earths are added thereto (refer to Patent Document 9), a bonding wire for use in a semiconductor element wherein the coefficient of extension during rupture is 3% to 15% (refer to Patent Document 10), a metal wire for wire bonding wherein a gold plating layer is provided on the copper wire (refer to Patent Document 11), a copper thin wire superior in ultrasonic weldability wherein the copper purity is 99.999% or higher, and the tensile strength is less than 23 kg/mm2 (refer to Patent Document 12), a bonding wire containing additive elements in an amount of 10 ppm or less (refer to Patent Document 13), a bonding wire for use in a semiconductor integrated circuit element wherein the copper purity of the core is 99.99% to 99.999%, and the coating layer is 99.999% or higher (refer to Patent Document 14), a copper alloy ultrathin wire for use in a semiconductor device wherein the copper purity is 99.9999% or higher, the content of Fe and Ag is 0.1% to 3.0%, and containing Mg in an amount of 0.5 to 400 ppm (refer to Patent Document 15), a copper bonding wire wherein the copper purity is 99.9999% or higher, and containing rare earths or Zr in an amount of 0.0002% to 0.002% (refer to Patent Document 16), and a copper bonding wire that does not damage the electrode tip using a porous metal (refer to Patent Document 17), among others.
Nevertheless, all of these conventional technologies relate to a purity level of 5N to 6N. With that said, it should be known that it is not technically easy to industrially manufacture the foregoing high purity copper having a purity level of 5N to 6N. These technologies conform to the objective of conventional bonding wires.
In recent years, there are demands of copper bonding subject to thinning. Nevertheless, even with the foregoing high purity copper having a purity level of 5N to 6N, there is a problem in that the copper itself has a higher hardness in comparison to gold, and will rupture during the wire drawing process. In order to overcome this problem, the copper must possess a certain level of softness (low hardness). In addition, a high purity copper bonding wire having a purity level of 5N to 6N also had the problem of generating cracks in the silicon under the Al pad during adhesion.
Accordingly, practically speaking, the conventional technologies had problems in not being able to obtain a copper bonding wire that supersedes the performance of Au.
[Non-Patent Document 1] “Practical Application Technology of Copper Wire Bonding” (No. 42, 1989) pages 77 to 80
[Non-Patent Document 2] “Development of Copper Bonding Wire” Electronics Special Feature (No.10, 1991-1) pages 53 to 56
[Patent Document 1] Japanese Patent Laid-Open Publication No. S61-251062
[Patent Document 2] Japanese Patent Laid-Open Publication No. S61-255045
[Patent Document 3] Japanese Patent Laid-Open Publication No. S62-104061
[Patent Document 4] Japanese Patent Laid-Open Publication No. S62-20858
[Patent Document 5] Japanese Patent Laid-Open Publication No. S62-22469
[Patent Document 6] Japanese Patent Laid-Open Publication No. S62-89348
[Patent Document 7] Japanese Patent Laid-Open Publication No. S62-127436
[Patent Document 8] Japanese Patent Laid-Open Publication No. S62-216238
[Patent Document 9] Japanese Patent Laid-Open Publication No. S62-127438
[Patent Document 10] Japanese Patent Laid-Open Publication No. S63-3424
[Patent Document 11] Japanese Patent Laid-Open Publication No. S63-56924
[Patent Document 12] Japanese Patent Laid-Open Publication No. S63-72858
[Patent Document 13] Japanese Patent Laid-Open Publication No. S63-3424
[Patent Document 14] Japanese Patent Laid-Open Publication No. S63-236338
[Patent Document 15] Japanese Patent Laid-Open Publication No. H3-291340
[Patent Document 16] Japanese Patent Laid-Open Publication No. H4-247630
An object of the present invention is to obtain a copper material that is compatible with the thinning (wire drawing) of the above. The conventional high purity copper having a purity level of 5N to 6N, which in itself is extremely high purity copper, was able to achieve a certain degree of softness by eliminating impurities as much as possible.
Nevertheless, although it cannot necessarily be said that the further purification of copper will lead to the reduction of hardness that will be compatible with thinning or the like, the present invention attempted to achieve higher purification of the high purity copper having a purity level of 5N to 6N. In other words, the present invention aims to develop technology for efficiently manufacturing ultrahigh purity copper having a purity of 8N (99.999999 wt %) or higher, and provide extremely soft ultrahigh purity copper obtained thereby which in particular enables thinning. The present invention also aims to provide a bonding wire for use in a semiconductor element capable of preventing the generation of cracks in the silicon during bonding, inhibiting the heat generation caused by the low resistivity of wires, and preventing oxidization during heat treatment.
In order to achieve the foregoing objects, the present invention, based on the premise of realizing high purification through electrolytic refining, provides ultrahigh purity copper having a hardness of 40 Hv or less, and a purity of 8N or higher (provided that this excludes the gas components of O, C, N, H, S and P). If the hardness is 40 Hv or less, it is possible to prevent the generation of cracks in the silicon during bonding and enable thinning. The ultrahigh purity copper of this invention is able to achieve this object.
It is desirable that the respective elements of O, S and P as gas components are 1 wtppm or less. These gas components (impurities) create a compound with Cu, and become deposited as a heterophase in the Cu. Although such heterophase deposits are extremely trivial in the ultrahigh purity copper, rupture will occur during the thinning process with such heterophase deposits as the origin. In addition, since the Cu will harden, cracks will easily occur in the silicon during bonding. Accordingly, it is necessary to reduce the foregoing gas components as much as possible.
Moreover, with the ultrahigh purity copper of the present invention, it is possible to achieve a recrystallization temperature of 200° C. or less. Thus, annealing can be performed at low temperatures during the manufacture process such as the wire drawing process, and it is thereby possible to lower the hardness even further.
As a result of the foregoing configuration, thinning is enabled, and it is possible to obtain ultrahigh purity copper that is soft and suitable as a bonding wire.
Upon manufacturing the ultrahigh purity copper of the present invention, electrolytic solution comprised of copper nitrate solution added with hydrochloric acid is used, and electrolysis is performed with two-step electrolysis. The electrolytic solution is circulated. The liquid temperature of the circulated solution is temporarily set to be 10° C. or less, and impurities such as AgCl are deposited and eliminated with a filter so as to obtain a further purified electrolytic solution. The electrolysis temperature is set to be 10 to 70° C. Like this, by performing electrolysis more than twice, it is possible to obtain high purity copper having a purity level of 8N or higher. If the electrolysis temperature is less than 10° C., impurities such as oxygen will increase and the electrodeposited Cu will become brittle. If the electrolysis temperature exceeds 70° C., the evaporation of the electrolytic solution will become a significant loss. Accordingly, it is desirable to set the electrolysis temperature to 10 to 70° C.
[Effect of the Invention]As described above, the present invention yields a significant effect of being able to efficiently manufacture ultrahigh purity copper having a purity level of 8N (99.999999 wt %) or higher. It is thereby possible to reduce the hardness and obtain a copper material that is compatible with thinning (wire drawing).
In particular, thinning of a bonding wire for a semiconductor element is enabled. In addition, the present invention yields a superior effect of being able to prevent the generation of cracks in the silicon during bonding, inhibit the heat generation caused by the low resistance of the wires, and prevent the oxidization during heat treatment.
Since the improvement of corrosion resistance (oxidation resistance) will not require the heating atmosphere to be a strict Ar+H2 atmosphere, an additional effect is yielded in that the labor efficiency is also favorable.
BEST MODE FOR CARRYING OUT THE INVENTIONA bulk copper raw material having a purity level of 4N was used as the anode, a copper plate was used as the cathode, and an electrolytic solution comprised of copper nitrate solution was used to perform electrolysis based on two-step electrolysis. Since only a purity level of 5 to 7N can be obtained in the first step, electrolysis must be performed with two or more steps. The copper raw material (4N) primarily contains large amounts of O, P and S.
In the primary electrolysis, the bath temperature was set to 15° C. or less, a nitric acid series electrolytic solution was used, and hydrochloric acid was added to the electrolytic solution in an amount of 0.1 to 100 ml/L. This is in order to eliminate Ag as AgCl.
Then, electrolysis was performed at pH of 0 to 4, and current density of 0.1 to 10 A/dm2. The electrolytic solution was circulated, the electrolytic solution of the circuit was cooled with a cooler to a temperature of 10° C. or less to deposit AgCl, and a filter was used to eliminate such AgCl and the like. It is thereby possible to obtain high purity copper having a purity level of 5N to 6N.
Subsequently, this copper having a purity level of 5N to 6N was used to perform electrolysis once again under the same electrolytic conditions as those described above. The electrolytic solution was similarly circulated, the electrolytic solution of the circuit was cooled with a cooler to a temperature of 10° C. or less to deposit AgCl, and a filter was used to eliminate such AgCl and the like.
According to the foregoing process, it was possible to obtain electrodeposited copper (deposited on the cathode) having a purity level of 8N or higher. In other words, it was possible to realize a purity level of 8N (99.999999 wt %) or higher excluding gas components, and to make all metallic component as impurities to be 0.01 wtppm or less.
In addition, the electrodeposited copper obtained by the foregoing electrolysis may also be subject to vacuum melting such as by electron beam melting. As a result of performing such vacuum melting, it is possible to effectively eliminate alkali metals such as Na and K and other volatile impurities, as well as gas components such as Cl. As a result of performing degasification with reducible gas as necessary, it is possible to eliminate and reduce the gas components.
The ultrahigh purity copper having a purity level of 8N or higher manufactured as described above satisfies the condition of having a hardness of 40 Hv or less, and it is also possible to achieve a recrystallization temperature of 200° C. or less.
EXAMPLESExamples of the present invention are now explained. These Examples merely illustrate a preferred example, and the present invention shall in no way be limited thereby. In other words, all modifications, other embodiments and modes covered by the technical spirit of the present invention shall be included in this invention.
Example 1A bulk copper raw material having a purity level of 4N was used as the anode, and a copper plate was used as the cathode to perform electrolysis. The content of raw material impurities is shown in Table 1. As shown in Table 1, the copper raw material (4N) primarily contained large amounts of O, P and S. Table 1 also shows the hardness and recrystallization temperature at this impurity level. The hardness at a purity level of 4N was 45 Hv, and the recrystallization temperature was 450° C. The diameter of the wire rod capable of being subject to thinning was 200 μm. The wires at this level were hard and generated cracks in the silicon during bonding (adhesion).
A nitric acid series electrolytic solution having a bath temperature of 30° C. was used, and 1 ml/L of hydrochloric acid was additionally added thereto. Electrolysis was performed at a pH of 1.3, and a current density of 1 A/dm2. The electrolytic solution was circulated, the electrolytic solution of the circuit was temporarily cooled with a cooler to a temperature of 2° C., and a filter was used to eliminate the deposited AgCl and the like. Thereby, as shown in Table 1, it was possible to obtain high purity copper having a purity level of 6N. Table 1 shows the hardness and recrystallization temperature of copper having a purity level of 6N. Hardness of the 6N copper was 42 Hv, and the recrystallization temperature was reduced to 230° C. The diameter of the wire rod capable of being subject to thinning was 40 μm. Some cracks were generated in the silicon during bonding (adhesion), but this was not as significant as the 4N Cu.
Subsequently, this copper having a purity level of 6N was used to perform electrolysis once again under the same electrolytic conditions as those described above. The electrolytic solution was similarly circulated, the electrolytic solution of the circuit was temporarily cooled with a cooler to a temperature of 0° C., and a filter was used to eliminate the deposited AgCl and the like. Thereby, as shown in Table 1, it was possible to obtain ultrahigh purity copper having a purity level of 9N.
Table 1 shows the hardness and recrystallization temperature of copper having a purity level of 9N. The hardness was 38 Hv, and the recrystallization temperature decreased to 150° C. The diameter of the wire rod capable of being subject to thinning was 2 μm, and improved significantly. No cracks were generated in the silicon during bonding (adhesion).
Example 1 achieved all conditions of the present invention; namely, the hardness being 40 Hv or less and the recrystallization temperature being 200° C. or less.
A bulk copper raw material having a purity level of 4N level was used as the anode, and a copper plate was used as the cathode to perform electrolysis. The content of raw material impurities is shown in Table 2. As shown in Table 2, the copper raw material (4N) primarily contained large amounts of O, P and S. Table 2 also shows the hardness and recrystallization temperature at this impurity level. The hardness at a purity level of 4N was 47 Hv, and the recrystallization temperature was 450° C. The diameter of the wire rod capable of being subject to thinning was 200 μm.
A nitric acid series electrolytic solution having a bath temperature of 25° C. was used, and 1 ml/L of hydrochloric acid was additionally added thereto. Electrolysis was performed at a pH of 2.0, and a current density of 1.5 A/dm2. The electrolytic solution was circulated, and, as with Example 1, the electrolytic solution of the circuit was temporarily cooled with a cooler to a temperature of 5° C., and a filter was used to eliminate the deposited AgCl and the like. Thereby, as shown in Table 2, it was possible to obtain high purity copper having a purity level of 5N5. Table 2 shows the hardness and recrystallization temperature of copper having a purity level of 5N5. Hardness of the 5N5 copper was 43 Hv, and the recrystallization temperature was reduced to 280° C. The diameter of the wire rod capable of being subject to thinning was 50 μm.
Subsequently, this copper having a purity level of 5N5 was used to perform electrolysis once again under the same electrolytic conditions as those described above. The electrolysis was performed by setting the electrolytic solution to a temperature of 20° C. The electrolytic solution was similarly circulated, the electrolytic solution of the circuit was temporarily cooled with a cooler to a temperature of 3° C., and a filter was used to eliminate the deposited AgCl and the like. Thereby, as shown in Table 2, it was possible to obtain ultrahigh purity copper having a purity level of 8N.
Table 2 shows the hardness and recrystallization temperature of copper having a purity level of 8N. The hardness was 40 Hv, and the recrystallization temperature decreased to 180° C. The diameter of the wire rod capable of being subject to thinning was 5 μm, and improved significantly.
Example 2 achieved all conditions of the present invention; namely, the hardness being 40 Hv or less and the recrystallization temperature being 200° C. or less.
A bulk copper raw material having a purity level of 4N was used as the anode, and a copper plate was used as the cathode to perform electrolysis. The content of raw material impurities is shown in Table 3. As shown in Table 3, the copper raw material (4N) primarily contained large amounts of O, P and S. Table 3 also shows the hardness and recrystallization temperature at this impurity level. The hardness at a purity level of 4N was 45 Hv, and the recrystallization temperature was 450° C. The diameter of the wire rod capable of being subject to thinning was 200 μm.
A nitric acid series electrolytic solution having a bath temperature of 20° C. was used, and 1 ml/L of hydrochloric acid was additionally added thereto. Electrolysis was performed at a pH of 1.3, and a current density of 1 A/dm2. The electrolytic solution was circulated but not cooled, and used as the electrolytic solution as is. Thereby, as shown in Table 3, it was possible to obtain high purity copper having a purity level of 4N5. Table 3 shows the hardness and recrystallization temperature of copper having a purity level of 4N5. Hardness of the 4N5 copper was 44 Hv. The recrystallization temperature was 430° C., and did not decrease significantly. The diameter of the wire rod capable of being subject to thinning was 150 μm, but not very effective.
Subsequently, this copper having a purity level of 4N5 was used to perform electrolysis once again under the same electrolytic conditions as those described above. Thereby, as shown in Table 3, it was possible to obtain high purity copper having a purity level of 5N.
Table 3 shows the hardness and recrystallization temperature of copper having a purity level of 5N. The hardness was high at 43 Hv, and the recrystallization temperature decreased to 420° C. The diameter of the wire rod capable of being subject to thinning was 120 μm, and insufficient.
As described above, Comparative Example 1 did not satisfy all conditions of the present invention; namely, the hardness being 40 Hv or less and the recrystallization temperature being 200° C. or less, and did not achieve sufficient softness for realizing the thinning of a bonding wire. In addition, numerous cracks were generated in the silicon under the Al pad during heat bonding.
A bulk copper raw material having a purity level of 4N level was used as the anode, and a copper plate was used as the cathode to perform electrolysis. The content of raw material impurities is shown in Table 4. As shown in Table 4, the copper raw material (4N) primarily contained large amounts of O, P and S. Table 4 also shows the hardness and recrystallization temperature at this impurity level. The hardness at a purity level of 4N was 45 Hv, and the recrystallization temperature was 450° C. The diameter of the wire rod capable of being subject to thinning was 200 μm.
A nitric acid series electrolytic solution having a bath temperature of 35° C. was used, and 1 ml/L of hydrochloric acid was additionally added thereto. Electrolysis was performed at a pH of 1.3, and a current density of 1 A/dm2. The electrolytic solution was only circulated. Thereby, as shown in Table 4, it was possible to obtain high purity copper having a purity level of 4N5. Table 4 shows the hardness and recrystallization temperature of copper having a purity level of 4N5. Hardness of the 4N5 copper was 44 Hv. The recrystallization temperature decreased to 430° C. The diameter of the wire rod capable of being subject to thinning was 150 μm.
Subsequently, this copper having a purity level of 4N5 was used to perform electrolysis once again under the same electrolytic conditions as those described above. In addition, the electrolytic solution of the circuit was temporarily cooled with a cooler to a temperature of 15° C., and a filter was used to eliminate the deposited AgCl and the like. Thereby, as shown in Table 4, it was possible to obtain high purity copper having a purity level of 6N.
Table 4 shows the hardness and recrystallization temperature of copper having a purity level of 6N. The hardness was high at 42 Hv, and the recrystallization temperature decreased to 230° C. The diameter of the wire rod capable of being subject to thinning was 40 μm, and insufficient.
As described above, Comparative Example 2 did not satisfy the conditions of the present invention; namely, the hardness being 40 Hv or less and the recrystallization temperature being 200°0 C. or less, and did not achieve sufficient softness for realizing the thinning of a bonding wire. In addition, numerous cracks were generated in the silicon under the Al pad during heat bonding.
As described above, the present invention yields a significant effect of being able to efficiently manufacture ultrahigh purity copper having a purity of 8N (99.999999 wt %) or higher. It is thereby possible to obtain a copper material with reduced hardness and compatible with thinning (wire drawing). In particular, thinning of a bonding wire for a semiconductor element is enabled. In addition, the present invention yields a superior effect of being able to prevent the generation of cracks in the silicon during bonding, inhibit the heat generation caused by the low resistance of the wires, and prevent the oxidization during heat treatment.
Accordingly, the present invention is suitable for a copper bonding wire for a semiconductor element that requires thinning.
Claims
1. Ultrahigh purity copper having a hardness of 40 Hv or less, and a purity of 8N or higher (provided that this excludes the gas components of O, C, N, H, S and P).
2. The ultrahigh purity copper according to claim 1, wherein the respective elements of O, S and P as gas components are 1 wtppm or less.
3. The ultrahigh purity copper according to claim 1 or claim 2, wherein the recrystallization temperature is 200° C. or less.
4. A bonding wire comprised of the ultrahigh purity copper according to any one of claims 1 to 3.
5. A manufacturing method of ultrahigh purity copper based on two-step electrolysis using an electrolytic solution comprised of copper nitrate solution, including the procedures of adding hydrochloric acid in an electrolytic solution comprised of copper nitrate solution; circulating the electrolytic solution; and performing two-step electrolysis while eliminating impurities with a filter upon temporarily setting the circulating electrolytic solution to a temperature of 10° C. or less.
6. The manufacturing method of ultrahigh purity copper according to any one of claims 1 to 3 based on two-step electrolysis using an electrolytic solution comprised of copper nitrate solution, including the procedures of adding hydrochloric acid in an electrolytic solution comprised of copper nitrate solution; circulating the electrolytic solution; and performing two-step electrolysis while eliminating impurities with a filter upon temporarily setting the circulating electrolytic solution to a temperature of 10° C. or less.
Type: Application
Filed: Apr 24, 2006
Publication Date: Nov 5, 2009
Applicant: NIPPON MINING & METALS CO., LTD. (Tokyo)
Inventors: Yuichiro Shindo (Ibaraki), Kouichi Takemoto (Ibaraki)
Application Number: 11/915,628
International Classification: C22C 9/00 (20060101); C25D 21/06 (20060101); C25D 3/38 (20060101);
