Ultrahigh-purity copper and process for producing the same, and bonding wire comprising ultrahigh-purity copper

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.

Conventionally, 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

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