Bonding wire for semiconductor device

Abstract

A bonding wire includes a Cu alloy core material, and a Pd coating layer formed on the Cu alloy core material. The bonding wire contains at least one element selected from Ni, Zn, Rh, In, Ir, and Pt. A concentration of the elements in total relative to the entire wire is 0.03% by mass or more and 2% by mass or less. When measuring crystal orientations on a cross-section of the core material in a direction perpendicular to a wire axis of the bonding wire, a crystal orientation <100> angled at 15 degrees or less to a wire axis direction has a proportion of 50% or more among crystal orientations in the wire axis direction. An average crystal grain size in the cross-section of the core material in the direction perpendicular to the wire axis of the bonding wire is 0.9 m or more and 1.3 m or less.

Claims

1. A bonding wire for a semiconductor device, the bonding wire comprising: a Cu alloy core material; and a Pd coating layer formed on a surface of the Cu alloy core material, wherein the bonding wire contains at least one element selected from Ni, Zn, Rh, Ir, and Pt, a concentration of the elements in total relative to the entire wire is 0.03% by mass or more and 2% by mass or less, or the bonding wire contains In, a concentration of In is 0.07% by mass or more and 2% by mass or less relative to the entire wire, when measuring crystal orientations on a cross-section of the core material in a direction perpendicular to a wire axis of the bonding wire, a crystal orientation <100> angled at 15 degrees or less to the wire axis direction has a proportion of 50% or more among crystal orientations in the wire axis direction, and an average crystal grain size in the cross-section of the core material in the direction perpendicular to the wire axis of the bonding wire is 0.9 m or more and 1.3 m or less.

2. The bonding wire for a semiconductor device according to claim 1, wherein a strength ratio defined by the following Equation (1) is 1.1 or more and 1.6 or less:
strength ratio=ultimate strength/0.2% offset yield strength(1).

3. The bonding wire for a semiconductor device according to claim 1, wherein a thickness of the Pd coating layer is 0.015 m or more and 0.150 m or less.

4. The bonding wire for a semiconductor device according to claim 1, further comprising an alloy skin layer containing Au and Pd on the Pd coating layer.

5. The bonding wire for a semiconductor device according to claim 4, wherein a thickness of the alloy skin layer containing Au and Pd is 0.0005 m or more and 0.050 m or less.

6. The bonding wire for a semiconductor device according to claim 1, wherein the bonding wire contains at least one element selected from Ga, Ge, As, Te, Sn, Sb, Bi, and Se, a concentration of the at least one element in total relative to the entire wire is 0.1 ppm by mass or more and 100 ppm by mass or less, and Sn10 ppm by mass; Sb10 ppm by mass; and Bi1 ppm by mass.

7. The bonding wire for a semiconductor device according to claim 1, wherein the bonding wire further contains at least one element selected from B, P, Mg, Ca, and La, and a concentration of each of the at least one element relative to the entire wire is 1 ppm by mass or more and 100 ppm by mass or less.

8. The bonding wire for a semiconductor device according to-claim 1, wherein Cu is present on an outermost surface of the bonding wire.

Description

EXAMPLES

(1) The following specifically describes the bonding wire according to an embodiment of the present invention with reference to examples.

(2) (Sample)

(3) First, the following describes a method for manufacturing a sample. Cu as a raw material of the core material with a purity of 99.99% by mass or more and containing inevitable impurities as the remainder was used. Au, Pd, Ni, Zn, Rh, In, Ir, and Pt with a purity of 99% by mass or more and containing inevitable impurities as the remainder were used. Ni, Zn, Rh, In, Ir, and Pt as additive elements to the core material are mixed so that the wire or the core material will have a target composition. Regarding the addition of Ni, Zn, Rh, In, Ir and Pt, they can be mixed singly.

(4) Alternatively, they may be mixed so as to be a desired amount using a Cu master alloy containing the additive elements manufactured in advance if the element has a high melting point as a single body or if the element is added in an infinitesimal amount. Working examples listed in Table 3 further contain one or more of Ga, Ge, As, Te, Sn, Sb, Bi, Se, B, P, Mg, Ca, and La.

(5) The Cu alloy as the core material was manufactured to give a wire diameter of a few millimeters by continuous casting. The obtained alloy of a few millimeters was drawn to manufacture a wire with a diameter of 0.3 to 1.4 mm. A commercially available lubricant was used for the wire drawing, and a wire drawing speed was 20 to 150 m/min. In order to remove an oxide film on the wire surface, pickling treatment with hydrochloric acid or the like was performed, and the Pd coating layer was formed by 1 to 15 m so as to cover the entire surface of the Cu alloy as the core material. Furthermore, for partial wires, the alloy skin layer containing Au and Pd was formed by 0.05 to 1.5 m on the Pd coating layer. For the formation of the Pd coating layer and the alloy skin layer containing Au and Pd, electroplating was used. A commercially available semiconductor plating solution was used for a plating solution. Thereafter, wire drawing was performed mainly using dies with an area reduction rate of 10 to 21%, and furthermore, one to three pieces of heat treatment were performed at 200 to 500 C. during the wire drawing to perform working to a diameter of 20 m. After working, heat treatment was performed so that breaking elongation would finally be about 5 to 15%. A method of heat treatment was performed while successively sweeping the wire and was performed while causing a N.sub.2 or Ar gas to flow. A wire feeding speed was 10 to 90 m/min, a heat treatment temperature was 350 to 500 C., and a heat treatment time was 1 to 10 seconds.

(6) (Method of Evaluation)

(7) The contents of Ni, Zn, Rh, In, Ir, Pt, Ga, Ge, As, Te, Sn, Sb, Bi, Se, B, P, Mg, Ca, and La in the wire were analyzed as the concentrations of the elements contained in the entire bonding wire using an ICP emission spectrometer.

(8) For the concentration analysis of the Pd coating layer and the skin alloy layer containing Au and Pd, Auger electron spectrometry was performed while trimming the bonding wire from its surface in the depth direction by sputtering or the like. From an obtained concentration profile in the depth direction, the thickness of the Pd coating layer, the thickness of the skin alloy layer containing Au and Pd were determined.

(9) The orientation proportion of the crystal orientation <100> angled at 15 degrees or less to the wire longitudinal direction among the crystal orientations in the wire longitudinal direction in the cross-section of the core material in the direction perpendicular to the wire axis of the bonding wire was calculated by observing crystal orientations of an observation surface (that is, the cross-section of the core material in the direction perpendicular to the wire axis) by EBSD. For the analysis of EBSD measurement data, exclusive software (OIM analysis manufactured by TSL Solutions, for example) was used. The average crystal grain size in the cross-section of the core material in the direction perpendicular to the wire axis was calculated by observing the crystal orientations on the observation surface by EBSD. For the analysis of EBSD measurement data, exclusive software (OIM analysis manufactured by TSL Solutions, for example) was used. The crystal grain size was obtained by performing an arithmetic mean on an equivalent diameter of crystal grains contained in a measurement area (the diameter of a circle equivalent to an area of a crystal grain; a circle-equivalent diameter).

(10) The 0.2% offset yield strength and the ultimate strength were evaluated by performing a tensile test with an inter-mark distance of 100 mm. A universal material test machine Type 5542 manufactured by Instron was used for a tensile test apparatus. The 0.2% offset yield strength was calculated using exclusive software installed in the apparatus. A load at the time of breaking was determined to be the ultimate strength. The strength ratio was calculated from the following Equation (1)
Strength ratio=ultimate strength/0.2% offset yield strength.(1)

(11) The evaluation of the wedge bondability in the wire bonded part was determined by performing 1,000 pieces of bonding on wedge bonding parts of a BGA substrate and by the occurrence frequency of peeling of the bonded parts. The used BGA substrate was plated with Ni and Au. In this evaluation, assuming bonding conditions more rigorous than normal, a stage temperature was set to 150 C., which was lower than a general set temperature range. In the evaluation, a case in which 11 or more failures occurred was determined to be problematic to be marked with a symbol of cross, a case of 6 to 10 failures was determined to be practicable but somewhat problematic to be marked with a symbol of triangle, a case of 1 to 5 failures was determined to be no problem to be marked with a symbol of circle, and a case in which no failure occurred was determined to be excellent to be marked with a symbol of double circle in the column wedge bondability in Table 1.

(12) The bonding reliability of the ball bonded part in a high-temperature and high humidity environment or a high-temperature environment was determined by manufacturing a sample for bonding reliability evaluation, performing HTS evaluation, and by the bonding longevity of the ball bonded part. The sample for bonding reliability evaluation was manufactured by performing ball bonding onto an electrode formed by forming an alloy of Al-1.0% Si-0.5% Cu as a film with a thickness of 0.8 m on a Si substrate on a general metallic frame using a commercially available wire bonder and sealing it with a commercially available epoxy resin. A ball was formed while causing a N.sub.2+5% H.sub.2 gas to flow at a flow rate of 0.4 to 0.6 L/min, and its size was within the range of a diameter of 33 to 34 m.

(13) For the HTS evaluation, the manufactured sample for bonding reliability evaluation was exposed to a high-temperature environment with a temperature of 200 C. using a high-temperature thermostatic device. A shear test on the ball bonded part was performed every 500 hours, and a time when a value of shear strength was half the shear strength initially obtained was determined to be the bonding longevity of the ball bonded part. The shear test after the high-temperature and high-humidity test was performed after removing the resin by acid treatment and exposing the ball bonded part.

(14) A tester manufactured by DAGE was used for a shear tester for the HTS evaluation. An average value of measurement values of 10 ball bonded parts randomly selected was used for the value of the shear strength. In the evaluation, the bonding longevity being 500 to 1,000 hours was determined to be practicable but desirably to be improved to be marked with a symbol of triangle, being 1,000 to 3,000 hours was determined to be practically no problem to be marked with a symbol of circle, and being 3,000 hours or more was determined to be especially excellent to be marked with a symbol of double circle.

(15) For the evaluation of ball formability (FAB shape), a ball before performing bonding was collected and observed, and the presence or absence of voids on the ball surface and the presence or absence of deformation of the ball, which is primarily a perfect sphere, were determined. The occurrence of any of the above was determined to be faulty. The formation of the ball was performed while a N.sub.2 gas was blownsprayed at a flow rate of 0.5 L/min in order to reduce oxidation in a melting process. The size of the ball was 34 m. For one condition, 50 balls were observed. A SEM was used for the observation. In the evaluation of the ball formability, a case in which five or more failures occurred was determined to be problematic to be marked with a symbol of cross, a case of three or four failures was determined to be practicable but somewhat problematic to be marked with a symbol of triangle, a case of one or two failures was determined to be no problem to be marked with a symbol of circle, and a case in which no failure occurred was determined to be excellent to be marked with a symbol of double circle in the column FAB shape in Table 1.

(16) The bonding longevity of the ball bonded part in the high-temperature and high-humidity environment with a temperature of 130 C. and a relative humidity of 85% can be evaluated by the following HAST evaluation. For the HAST evaluation, the manufactured sample for bonding reliability evaluation was exposed to a high-temperature and high-humidity environment with a temperature of 130 C. and a relative humidity of 85% using an unsaturated type pressure cooker tester and was biased with 5 V. A shear test on the ball bonded part was performed every 48 hours, and a time when a value of shear strength was half the shear strength initially obtained was determined to be the bonding longevity of the ball bonded part. The shear test after the high-temperature and high-humidity test was performed after removing the resin by acid treatment and exposing the ball bonded part.

(17) A tester manufactured by DAGE was used for a shear tester for the HAST evaluation. An average value of measurement values of 10 ball bonded parts randomly selected was used for the value of the shear strength. In the evaluation, the bonding longevity being 144 hours or more and less than 288 hours was determined to be practically no problem to be marked with a symbol of circle, being 288 hours or more and less than 384 hours was determined to be excellent to be marked with a symbol of double circle, and being 384 hours or more was determined to be especially excellent to be marked with a symbol of a pair of double circles in the column HAST in Table 1.

(18) The evaluation of a crushed shape of the ball bonded part was determined by observing the ball bonded part after bonding from immediately above and by its circularity. For a bonding counterpart, an electrode in which an Al-0.5% Cu alloy was formed as a film with a thickness of 1.0 m on a Si substrate was used. The observation was performed using an optical microscope, and 200 sites were observed for one condition. Being elliptic with large deviation from a perfect circle and being anisotropic in deformation were determined to be faulty in the crushed shape of the ball bonded part. In the evaluation, a case in which one to three failures was determined to be no problem to be marked with a symbol of circle, and a case in which a favorable perfect circle was obtained for all was determined to be especially excellent to be marked with a symbol of double circle in the column crushed shape in Table 1.

(19) TABLE-US-00001 TABLE 1 Crystal structure Film thickness of <100> alloy skin layer Proportion Average Film thickness of containing Au of wire C crystal Wire component (% by mass) Pd coating layer and Pd section grain size No. Ni Zn Rh ln lr Pt Total (m) (m) (%) (m) Working 1 0.7 0.7 0.015 92 1.1 Example 2 1.2 1.2 0.050 72 0.9 3 1.0 1.0 0.100 71 1.0 4 0.5 0.5 0.150 72 1.1 5 0.1 0.1 0.015 75 1.2 6 0.03 0.03 0.050 63 1.3 7 1.1 0.3 1.4 0.100 75 1.0 8 1.2 0.8 2.0 0.150 65 0.9 9 0.7 0.1 0.8 0.015 51 1.2 10 0.6 0.1 0.05 0.75 0.100 97 1.2 11 0.8 0.8 0.3 1.9 0.150 80 1.1 12 0.05 0.05 0.05 0.15 0.015 70 1.2 13 1.0 0.1 0.3 1.4 0.015 54 1.0 14 0.5 0.5 0.015 0.0005 91 1.1 15 1.2 1.2 0.050 0.0010 70 0.9 16 0.7 0.7 0.100 0.0100 69 1.1 17 0.3 0.3 0.150 0.0500 70 1.2 18 0.1 0.1 0.015 0.0005 76 1.2 19 0.05 0.05 0.050 0.0010 64 1.3 20 0.5 0.3 0.8 0.100 0.0100 74 1.1 21 1.2 0.1 1.3 0.150 0.0500 64 1.2 22 0.7 0.01 0.71 0.015 0.0005 50 1.1 23 0.6 0.1 0.05 0.75 0.050 0.0010 98 1.0 24 0.8 0.8 0.3 1.9 0.100 0.0100 85 0.9 25 0.05 0.05 0.05 0.15 0.150 0.0500 74 1.3 26 1.0 0.1 0.3 1.4 0.015 0.0100 51 0.9 Mechanical characteristics Yield strength Maximum yield 0.2% Yield ratio Wire quality strength {circle around (1)} strength {circle around (2)} {circle around (1)}/{circle around (2)} Wedge FAB Crushed No. (mN/m.sup.2) bondability HTS shape HAST shape Working 1 0.19 0.16 1.19 Example 2 0.22 0.17 1.29 3 0.24 0.16 1.50 4 0.29 0.24 1.21 5 0.30 0.22 1.36 6 0.31 0.20 1.55 7 0.33 0.28 1.18 8 0.34 0.27 1.26 9 0.35 0.22 1.59 10 0.33 0.30 1.10 11 0.34 0.28 1.21 12 0.35 0.22 1.59 13 0.35 0.23 1.52 14 0.20 0.18 1.11 15 0.21 0.17 1.24 16 0.22 0.15 1.47 17 0.28 0.24 1.17 18 0.29 0.22 1.32 19 0.30 0.19 1.58 20 0.33 0.28 1.18 21 0.34 0.26 1.31 22 0.35 0.23 1.52 23 0.30 0.20 1.50 24 0.33 0.29 1.14 25 0.34 0.25 1.36 26 0.35 0.25 1.40

(20) TABLE-US-00002 TABLE 2 Crystal structure Film thickness <100> Film thickness of alloy skin Proportion Average of Pd layer containing of wire C crystal Wire component (% by mass) coating layer Au and Pd section grain size No. Ni Zn Rh ln lr Pt Total (m) (m) (%) (m) Comparative 1 0.7 0.7 0.015 50 0.8 Example 2 1.2 0.8 2.0 0.150 49 1.5 3 0.6 0.1 0.05 0.75 0.100 51 0.7 4 0.03 0.03 0.050 45 0.9 5 0.7 0.1 0.8 0.015 40 1.1 6 0.8 0.8 0.3 1.9 0.150 30 1.3 7 1.2 1.2 0.050 41 1.0 8 1.1 0.3 1.4 0.100 45 1.4 9 0.05 0.05 0.05 0.15 0.015 48 1.6 Mechanical characteristics Yield Maximum 0.2% strength yield Yield ratio Wire quality strength {circle around (1)} strength {circle around (2)} {circle around (1)}/{circle around (2)} Wedge No. (mN/m.sup.2) bondability HTS FAB shape HAST Crushed shape Comparative 1 0.20 0.12 1.09 X Example 2 0.29 0.16 1.81 X 3 0.34 0.19 1.08 X 4 0.21 0.12 1.75 X 5 0.30 0.17 1.76 X 6 0.35 0.19 1.84 X 7 0.21 0.12 1.75 X 8 0.30 0.18 1.67 9 0.34 0.20 1.70

(21) TABLE-US-00003 TABLE 3 Film Film thickness of Crystal structure thickness alloy skin <100> of Pd layer Proportion Average coating containing of wire C crystal Wire component (% by mass) layer Au and Pd section grain size No. Ni Zn Rh ln lr Pt Total Others (m) (m) (%) (m) Working 27 0.7 0.7 Ga: 0.007 0.100 88 0.9 Example 28 1.1 1.1 Ge: 0.008 0.050 75 1.0 29 0.7 0.7 As: 0.003 0.050 72 1.0 30 1.2 1.2 Te: 0.001 0.150 67 1.2 31 0.5 0.5 Sn: 0.0007 0.015 66 1.0 32 0.05 0.05 Sb: 0.0008 0.050 74 1.1 33 1.0 1.0 Bi: 0.00008 0.100 80 1.1 34 0.8 0.8 Se: 0.0001 0.100 92 0.9 35 0.05 0.05 Ga: 0.003 0.100 72 1.2 Te: 0.0008 36 0.08 0.08 Ge: 0.003 0.150 0.0050 55 1.3 Sb: 0.0007 37 0.1 0.1 As: 0.001 0.150 0.0100 82 1.1 Se: 0.001 38 0.08 0.08 B: 0.0008 0.050 74 1.1 39 1.2 1.2 P: 0.004 0.050 77 1.2 40 0.05 0.05 Mg: 0.005 0.100 91 1.0 41 0.5 0.5 Ca: 0.003 0.015 68 1.0 42 0.1 0.1 La: 0.003 0.100 0.0100 91 0.9 43 0.05 0.05 P: 0.006 0.050 0.0050 68 1.1 B: 0.0008 45 0.6 0.6 P: 0.003 0.015 0.0100 57 1.3 Ca: 0.001 Mechanical characteristics Yield Maximum strength yield 0.2% Yield ratio strength {circle around (1)} strength {circle around (2)} {circle around (1)}/{circle around (2)} Wire quality No. (mN/m.sup.2) Wedge bondability HTS FAB shape HAST Crushed shape Working 27 0.22 0.18 1.22 Example 28 0.25 0.17 1.47 29 0.30 0.21 1.43 30 0.31 0.24 1.29 31 0.29 0.22 1.32 32 0.35 0.29 1.21 33 0.31 0.22 1.41 34 0.27 0.19 1.42 35 0.30 0.19 1.58 36 0.33 0.25 1.32 37 0.32 0.25 1.28 38 0.34 0.23 1.48 39 0.29 0.20 1.45 40 0.33 0.28 1.18 41 0.23 0.19 1.21 42 0.26 0.21 1.24 43 0.29 0.19 1.53 45 0.33 0.24 1.38

(22) (Evaluation Results)

(23) The bonding wires according to Working Examples 1 through 26 in Table 1 each include the Cu alloy core material and the Pd coating layer formed on the surface of the Cu alloy core material, the thickness of the Pd coating layer being in the preferable range of 0.015 to 0.150 m and all the FAB shape having been favorable. It has been revealed that the high-temperature reliability of the ball bonded part by the HTS evaluation is favorable when the bonding wire contains at least one element selected from Ni, Zn, Rh, In, Ir, and Pt and the concentration of the elements in total relative to the entire wire is 0.03 to 2% by mass.

(24) In Working Examples 1 through 26, the area reduction rate at the time of wire drawing was 10% or more, and the heat treatment temperature after wire drawing was a low temperature of 500 C. or less, whereby, the crystal orientation <100> angled at 15 degrees or less to the wire longitudinal direction among the crystal orientations in the wire longitudinal direction could be 50% or more when measuring crystal orientations on the cross-section of the core material in the direction perpendicular to the wire axis of the bonding wire, and the average crystal grain size in the cross-section of the core material in the direction perpendicular to the wire axis of the bonding wire could be 0.9 to 1.3 m. Consequently, the strength ratio (=ultimate strength/0.2% offset yield strength) in all cases was in the range of 1.1 to 1.6 even though the wire contained Ni, Zn, Rh, In, Ir, and Pt. Consequently, the wedge bondability was favorable in all cases.

(25) In contrast, in Comparative Examples 4 through 6 in Table 2, the heat treatment temperature was a high temperature of 600 C. or more, whereby the <100> orientation proportion in the wire longitudinal direction was less than 50%. In Comparative Examples 2 and 7 through 9, the heat treatment temperature was a high temperature of 620 C. or more, whereby the <100> orientation proportion in the wire longitudinal direction was less than 50%, and the average crystal grain size in the cross-section of the core material was more than 1.3 m. Consequently, in all Comparative Examples 2 and 4 through 9, the strength ratio was more than 1.6, and the wedge bondability was faulty.

(26) In Comparative Examples 1 and 3, when the die area reduction rate was less than 10%, the average crystal grain size in the cross-section of the core material was less than 0.9 m, the strength ratio was less than 1.1, and the wedge bondability was faulty in both cases.