BONDING WIRE

Abstract

There is provided a metal-coated Al bonding wire which can provide a sufficient bonding reliability of bonded parts of the bonding wire under a high temperature state where a semiconductor device using the metal-coated Al bonding wire is operated. The bonding wire includes a core wire of Al or Al alloy, and a coating layer of Ag, Au or an alloy containing them formed on the outer periphery of the core wire, and the bonding wire is characterized in that when measuring crystal orientations on a cross-section of the core wire in a direction perpendicular to a wire axis of the bonding wire, a crystal orientation <111> angled at 15 degrees or less to a wire longitudinal direction has a proportion of 30 to 90% among crystal orientations in the wire longitudinal direction. Preferably, the surface roughness of the wire is 2 μm or less in terms of Rz.

Claims

1. A bonding wire comprising: a core wire of Al or Al alloy; and a coating layer of Ag, Au or an alloy containing them formed on an outer periphery of the core wire, wherein when measuring crystal orientations on a cross-section of the core wire in a direction perpendicular to a wire axis of the bonding wire, a crystal orientation <111> angled at 15 degrees or less to a wire longitudinal direction has a proportion of 30 to 90% among crystal orientations in the wire longitudinal direction.

2. The bonding wire according to claim 1, wherein a surface roughness of the wire is 2 μm or less in terms of Rz.

3. The bonding wire according to claim 1, wherein a thickness of the coating layer is 1 to 100 nm.

4. The bonding wire according to claim 1, wherein a wire diameter is 50 to 600 μm.

Description

EXAMPLES

[0055] As raw materials of the bonding wire, specifically, as Al used for a core wire, Au and Ag used for a coating layer, materials with a purity of 99.99% by mass or more were prepared separately. An ingot was forged by heating and melting Al, and a coating layer was formed on a surface of the wire using an electrolytic plating method. After that, while wire drawing was performed, a heat treatment during the wire drawing and a thermal refining heat treatment after the wire drawing were performed, thus manufacturing a bonding wire with a final wire diameter shown in Table 1. The coating layer was controlled to have the thickness described in Examples.

[0056] As for the heat treatment during the wire drawing, the heat treatment was performed at a stage where the wire diameter became 800 μm. In Working Examples, the heat treatment was performed at 620° C. for 3 hours with quenching (water cooling). In Comparative Examples 1 to 4, slow cooling (air cooling) was adopted as the cooling condition. In Comparative Examples 5 and 6, no heat treatment was performed. Further, the conditions of the thermal refining heat treatment after the wire drawing were as follows. The standard condition was within a range of 270±10° C. for 10 seconds. As to Working Example Nos. 19 and 20 (the thickness of the coating layer is greater than those of the preferable range of the present invention), the condition of the thermal refining heat treatment was lower in temperature than the standard condition, in order to adjust the crystal orientation <111> proportion.

[0057] The thickness of the coating layer in the resulting bonding wire was measured by ICP analysis to obtain an average film thickness. The metal type and thickness of the coating layer is shown in Table 1.

[0058] The measurement of the crystal orientation <111> proportion (the orientation proportion of crystal in which a crystal orientation <111> is angled at 15 degrees or less to a wire longitudinal direction in a cross-section perpendicular to the wire longitudinal direction) was performed by measuring with EBSD on a cross-section perpendicular to the longitudinal direction of the bonding wire, and calculating the crystal orientation <111> proportion in the above-described procedure using the analysis software dedicated to the measuring device.

[0059] The surface roughness Rz of the wire was evaluated as the longitudinal roughness of the wire with a non-contact ultra-deep shape measuring microscope in accordance with the provisions of JIS B 0601-2001.

[0060] The die abrasion was evaluated by measuring the amount of increase in wire diameter. After drawing a wire of predetermined length, the wire diameter was measured to confirm how the actual wire diameter was increased compared to the target wire diameter. The die abrasion was evaluated according to the degree of increase in the wire diameter and was marked with symbols of “double circle”, “circle” and “triangle” in ascending order in terms of the degree of increase in the wire diameter.

[0061] The bonding wire was bonded using a commercially available wedge bonder. As the sample for evaluation, a copper substrate having a SiC chip mounted thereon was used. Onto the SiC chip, Titanium, nickel and aluminum were deposited from the SiC chip side in advance, and the thicknesses thereof were 0.1, 2 and 4 μm, respectively.

[0062] The bonding property of the bonding wire was evaluated by the wedge bondability. Specifically, its shear strength was evaluated. As for the shear strength, a shear strain was applied to the bonding wire in a state of being wedge bonded in a direction perpendicular to the wire, and the maximum strength that would cause it to fracture was recorded. In the evaluation, the shear strength being 95% or more of the initial bonding strength was marked with a symbol of “double circle”, being 90% to 95% of the initial bonding strength was marked with a symbol of “circle”, and being 70% to 90% of the initial bonding strength was marked with a symbol of “triangle”, and any case of which were determined to be acceptable. On the other hand, the shear strength being less than 70% of the initial bonding strength was determined to be unacceptable.

[0063] The high-temperature long-term hysteresis was performed by a power cycle test. The power cycle test involved repeatedly heating and cooling the semiconductor device in which an Al bonding wire had been bonded. Specifically, the semiconductor device was heated over 2 seconds until the temperature of the bonded parts of the bonding wires in the semiconductor device became 140° C., and then was cooled over 5 seconds until the temperature of the bonded parts became 30° C. This heating and cooling cycle was repeated 200,000 times.

[0064] After performing the high-temperature long-term hysteresis, the bonding shear strength of the first bonded part was measured to evaluate the reliability of the bonded part. The shear strength was measured as a comparison with the initial shear strength of the bonded part. In the evaluation, the shear strength being 95% or more of the initial bonding strength was marked with a symbol of “double circle”, being 90% to 95% of the initial bonding strength was marked with a symbol of “circle”, being 70% to 90% of the initial bonding strength was marked with a symbol of “triangle”, and being less than 70% of the initial bonding strength was marked with a symbol of “cross” in the column “reliability test” in Table 1. The evaluations of “double circle” and “circle” were determined to be acceptable and the others were determined to be unacceptable.

TABLE-US-00001 TABLE 1 Wire Specification Wire Crystal Texture Thickness Orientation Surface Wire of coating <111> roughness Wire Properties Remarks diameter layer (nm) proportion Rz Die Bonding Reliability (Heat treatment No. (μm) Ag Au (%) (μm) abrasion property test conditions) Working  1  50 10 0 33 1.5 O O O Example  2  50 5 0 50 1.8 ⊚ ⊚ ⊚  3  50 1 0 86 1.1 O O O  4  50 0 10 31 1.7 O O O  5  50 0 5 53 1.1 ⊚ ⊚ ⊚  6  50 0 1 90 1.3 O O O  7 200 40 0 32 1.8 O O O  8 200 20 0 51 1.0 ⊚ ⊚ ⊚  9 200 1 0 89 1.7 O O O 10 200 0 40 31 1.7 O O O 11 200 0 20 52 1.3 ⊚ ⊚ ⊚ 12 200 0 1 87 1.8 O O O 13 600 100 0 32 1.4 O O O 14 600 50 0 52 1.7 ⊚ ⊚ ⊚ 15 600 1 0 88 1.6 O O O 16 600 0 100 33 1.8 O O O 17 600 0 50 51 1.1 ⊚ ⊚ ⊚ 18 600 0 1 86 1.9 O O O 19 200 120 0 32 1.5 O Δ O Termal refining heat treatment_low temperature 20 200 0 120 34 1.7 O Δ O ditto 21 200 0.5 0 88 2.3 Δ Δ O 22 200 0 0.5 79 2.5 Δ Δ O Comparative  1 200 120 0 29 1.5 O Δ Δ Slow cooling Example after heat treatment during wire drawing  2 200 0 120 28 1.7 O Δ Δ ditto  3 200 100 0 26 1.2 Δ Δ X ditto  4 200 0 100 27 1.4 Δ Δ X ditto  5 200 1 0 93 1.8 Δ Δ X No heat treatment during wire drawing  6 200 0 1 91 1.9 Δ Δ X ditto

[0065] The results are shown in Table 1. Values outside the range of the present invention are underlined.

[0066] All of Working Example Nos. 1 to 22 had the crystal orientation <111> proportion being within the range of the present invention, and they achieved favorable results in all of die abrasion, bonding property, and reliability test. In particular, Working Example Nos. 1 to 18 had the thickness of the coating layer being within the preferable range of the present invention, and all of them achieved favorable results in the reliability test. Further, Working Example Nos. 2, 5, 8, 11, 14 and 17 had the thickness of the coating layer and the crystal orientation <111> proportion being within the more preferable range of the present invention, and they achieved evaluation of “double circle” in all of the die abrasion, bonding property, and reliability test. Working Example Nos. 19 and 20 had the thickness of the coating layer being thicker than those under the preferable condition of the present invention. Although the crystal orientation <111> proportion thereof was within the range of the present invention by performing the thermal refining heat treatment at a lower temperature than those under the standard condition, they merely achieved evaluation of “triangle” in the bonding property.

[0067] As for Working Example Nos. 21 and 22 in which the thickness of the coating layer was below the preferable lower limit, the evaluation results of die abrasion, surface properties, and bonding property were “triangle”.

[0068] Comparative Example Nos. 1 and 2 had the thickness of the coating layer being thicker than those under the preferable condition of the present invention, and had the crystal orientation <111> proportion being out of the lower limit of the range of the present invention since the thermal refining heat treatment was performed under the standard condition. As for them, the result of the reliability test was “triangle” (unacceptable), and the result of the bonding property was “triangle”.

[0069] Comparative Example Nos. 3 and 4 had the crystal orientation <111> proportion being out of the lower limit of the range of the present invention since the cooling condition after the heat treatment during the wire drawing was slow cooling (air cooling). As for them, the result of the reliability test was “cross”, and both results of the die abrasion and the bonding property were “triangle”.

[0070] Comparative Example Nos. 5 and 6 had the crystal orientation <111> proportion being out of the upper limit of the range of the present invention since the heat treatment during the wire drawing was not performed. As for them, the result of the reliability test was “cross”, and both results of the die abrasion and the bonding property were “triangle”.