Bonding wire for high-speed signal line

Abstract

A bonding wire for a high-speed signal line for connecting a pad electrode of a semiconductor device and a lead electrode on a circuit board contains palladium (Pd), platinum (Pt), silver (Ag), and a trace additive element.

Claims

1. A bonding wire for a high-speed signal line for connecting a pad electrode of a semiconductor device and a lead electrode on a circuit board, consisting essentially of: a trace additive element; 0.8 to 2.5 mass % of palladium (Pd), 0.1 to 0.7 mass % of platinum (Pt); and a remaining being silver (Ag) with purity of 99.99 mass % or more, wherein the bonding wire comprises a core and a skin region in a doughnut shape segregated on the core so that a high purity silver segregation layer covers a lower purity silver on a surface of the bonding wire, and the skin region contains an alloy having a higher content of silver (Ag) than a content of silver (Ag) in the core to thereby transmit super-high frequency.

2. The bonding wire for the high-speed signal line according to claim 1, wherein the skin region is a surface segregation layer.

3. A bonding wire for a high-speed signal line for connecting a pad electrode of a semiconductor device and a lead electrode on a circuit board, consisting essentially of: a trace additive element; 0.8 to 2.5 mass % of palladium (Pd); 0.1 to 0.7 mass % of platinum (Pt); and at least one element selected from the group consisting of rhodium (Rh), iridium (Ir), ruthenium (Ru), copper (Cu), nickel (Ni), iron (Fe), magnesium (Mg), zinc (Zn), aluminum (Al), indium (In), silicon (Si), germanium (Ge), beryllium (Be), bismuth (Bi), selenium (Se), cerium (Ce), yttrium (Y), lanthanum (La), calcium (Ca) and europium (Eu), wherein a total amount of the trace additive element is 5 to 300 mass ppm, and a remaining is silver (Ag) with purity of 99.99 mass % or more, the bonding wire comprises a core and a skin region in a doughnut shape segregated on the core so that a high purity silver segregation layer covers a lower purity silver on a surface of the bonding wire, and the skin region contains an alloy having a higher content of silver (Ag) than a content of silver (Ag) in the core to thereby transmit super-high frequency.

4. The bonding wire for the high-speed signal line according to claim 1, wherein a ratio of palladium (Pd) to platinum (Pt) is within a range of 5:1 to 10:1.

5. The bonding wire for the high-speed signal line according to claim 1, wherein the high-speed signal has a frequency of 1 to 15 GHz.

6. The bonding wire for the high-speed signal line according to claim 1, wherein the pad electrode comprises an aluminum (Al) metal with purity of 99.9 mass % or more or an aluminum (Al) alloy comprising 0.5 to 2.0 mass % of silicon (Si) or copper (Cu) and the remaining is aluminum (Al) with purity of 99.9 mass % or more.

7. The bonding wire for the high-speed signal line according to claim 1, wherein the pad electrode is an electrode pad whose outer layer is gold (Au), palladium (Pd), or platinum (Pt).

8. The bonding wire for the high-speed signal line according to claim 1, wherein a thickness of the skin region is 10 nm.

9. The bonding wire for the high-speed signal line according to claim 2, wherein a thickness of the surface segregation layer is 10 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional schematic view illustrating distribution of a highly-concentrated silver (Ag) layer of the present invention;

(2) FIG. 2 is a view illustrating voltage changes of an implementation product 1 and a comparison product 39 over time in a graphical form; and

(3) FIG. 3 illustrates results of qualitative analysis of the implementation product 1 near the outermost surface.

DESCRIPTION OF THE EMBODIMENT

(4) AgPdPt ternary alloys having element composition as illustrated in Table 1 (each containing silver (Ag) with purity of 99.99 mass % or more added with 5 to 300 ppm of a trace additive element of rhodium (Rh), ruthenium (Ru), iridium (Ir), copper (Cu), nickel (Ni), iron (Fe), magnesium (Mg), zinc (Zn), aluminum (Al), manganese (Mn), indium (In), silicon (Si), germanium (Ge), tin (Sn), beryllium (Be), bismuth (Bi), selenium (Se), cerium (Ce), titanium (Ti), yttrium (Y), calcium (Ca), lanthanum (La), europium (Eu) or antimony (Sb)) were melted and continuously cast under an inert atmosphere, to have the diameter of 8 mm, similarly to a normal pure gold bonding wire. The continuously-cast thick wires were continuously subjected to a cold and wet continuous wire drawing process by a diamond die until the diameter is reduced to 20 m as a final wire diameter, that is, reduction of area is 99.99% or more, and were subjected to predetermined refining heat treatment, so as to manufacture bonding wires 1 to 38 having the wire diameter of 20 M (hereinafter referred to as implementation products) according to the present invention.

(5) Examples 1 to 14 are the implementation products according to claim 1, examples 15 to 38 are the implementation products according to claim 2, and examples 8 to 14 and 27 to 38 are the implementation products according to claim 3.

(6) TABLE-US-00001 TABLE 1 Main element mass % Trace additive element mass % No. Ag Pd Pt Rh Ru Ir Cu Ni Fe Mg Zn Al Mn In Imple- 1 Balance 0.8 0.3 0 0 0 0 0 0 0 0 0 0 0 mentation 2 Balance 1.0 0.3 0 0 0 0 0 0 0 0 0 0 0 products 3 Balance 1.2 0.6 0 0 0 0 0 0 0 0 0 0 0 4 Balance 1.5 0.1 0 0 0 0 0 0 0 0 0 0 0 5 Balance 1.8 0.6 0 0 0 0 0 0 0 0 0 0 0 6 Balance 2.0 0.5 0 0 0 0 0 0 0 0 0 0 0 7 Balance 2.5 0.7 0 0 0 0 0 0 0 0 0 0 0 8 Balance 0.8 0.1 0 0 0 0 0 0 0 0 0 0 0 9 Balance 1.0 0.1 0 0 0 0 0 0 0 0 0 0 0 10 Balance 1.2 0.2 0 0 0 0 0 0 0 0 0 0 0 11 Balance 1.5 0.3 0 0 0 0 0 0 0 0 0 0 0 12 Balance 1.8 0.2 0 0 0 0 0 0 0 0 0 0 0 13 Balance 2.0 0.2 0 0 0 0 0 0 0 0 0 0 0 14 Balance 2.2 0.3 0 0 0 0 0 0 0 0 0 0 0 15 Balance 0.8 0.3 0 0 0 60 0 0 0 0 0 0 0 16 Balance 0.8 0.3 0 0 0 0 60 0 0 0 0 0 0 17 Balance 1.0 0.3 100 0 0 0 0 0 0 0 0 0 0 18 Balance 1.0 0.3 0 0 0 0 0 0 0 0 0 0 0 19 Balance 1.2 0.6 0 0 0 0 0 0 0 0 0 0 0 20 Balance 1.2 0.6 0 0 0 0 0 0 0 0 0 0 0 21 Balance 1.5 0.1 0 0 0 0 0 0 0 0 0 0 0 22 Balance 1.5 0.1 0 0 0 0 0 0 0 0 0 0 0 23 Balance 1.8 0.6 0 0 0 0 0 60 0 0 0 0 0 24 Balance 1.8 0.6 0 0 0 0 0 0 0 0 0 0 0 25 Balance 2.0 0.5 0 0 60 0 0 0 0 0 0 0 0 26 Balance 2.0 0.5 0 50 0 0 0 0 0 0 0 0 0 27 Balance 0.8 0.1 0 0 0 0 0 0 0 0 0 0 0 28 Balance 0.8 0.1 0 0 0 0 0 0 0 0 0 0 0 29 Balance 1.0 0.1 0 0 0 0 0 60 0 0 0 0 0 30 Balance 1.0 0.1 0 0 0 0 0 0 0 0 0 0 0 31 Balance 1.2 0.2 0 0 0 0 0 0 60 0 0 0 0 32 Balance 1.2 0.2 0 0 0 0 0 0 0 0 0 50 0 33 Balance 1.5 0.3 0 0 0 60 0 0 0 0 0 0 0 34 Balance 1.5 0.3 0 0 0 0 60 0 0 0 0 0 0 35 Balance 1.8 0.2 0 0 0 0 0 0 0 0 0 0 0 36 Balance 1.8 0.2 0 0 0 0 0 0 0 60 0 0 0 37 Balance 2.0 0.2 0 0 0 0 0 0 0 0 60 0 0 38 Balance 2.0 0.2 0 0 0 0 0 0 0 0 0 0 60 Compar- 39 Balance 0.5 0.5 0 0 0 0 0 0 0 0 0 0 0 ison 40 Balance 0.7 0.2 0 0 0 0 0 0 0 0 0 0 0 products 41 Balance 0.5 1.5 0 0 0 0 0 0 0 0 0 0 0 42 Balance 3.5 0.3 0 0 0 0 0 0 0 0 0 0 0 43 Balance 2.7 1.0 0 0 0 0 0 0 0 0 0 0 0 44 Balance 3.0 1.5 0 0 0 0 0 0 0 0 0 0 0 45 Balance 1.5 0.3 550 0 0 0 0 0 0 0 0 0 0 46 Balance 1.5 0.3 0 0 0 350 0 0 0 0 0 0 0 47 Balance 1.5 0.3 0 0 0 0 0 0 0 0 0 0 0 Trace additive element mass % No. Si Ge Sn Be Bi Se Ce Te Y Ca La Eu Sb Imple- 1 0 0 0 0 0 0 0 0 0 0 0 0 0 mentation 2 0 0 0 0 0 0 0 0 0 0 0 0 0 products 3 0 0 0 0 0 0 0 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 0 0 0 0 0 7 0 0 0 0 0 0 0 0 0 0 0 0 0 8 0 0 0 0 0 0 0 0 0 0 0 0 0 9 0 0 0 0 0 0 0 0 0 0 0 0 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 11 0 0 0 0 0 0 0 0 0 0 0 0 0 12 0 0 0 0 0 0 0 0 0 0 0 0 0 13 0 0 0 0 0 0 0 0 0 0 0 0 0 14 0 0 0 0 0 0 0 0 0 0 0 0 0 15 0 0 0 0 0 0 0 0 0 0 0 0 0 16 0 0 0 0 0 0 0 0 0 0 0 0 0 17 0 0 0 0 0 0 0 0 0 0 0 0 0 18 0 0 0 0 100 0 0 0 0 0 0 0 0 19 0 0 0 0 0 0 0 0 0 100 0 0 0 20 0 0 0 0 0 0 0 0 0 0 100 0 0 21 50 0 0 0 0 0 50 0 0 0 0 0 0 22 0 0 0 10 0 0 0 0 25 0 0 0 0 23 0 0 0 0 0 30 0 0 0 0 0 0 0 24 0 0 0 0 0 60 0 15 0 0 0 0 0 25 0 0 0 0 0 0 0 0 0 0 0 40 0 26 0 0 0 0 0 0 0 0 0 0 50 0 0 27 0 0 100 0 0 0 0 0 0 0 0 0 0 28 0 0 0 0 0 0 0 0 0 0 0 100 0 29 0 0 0 0 0 0 0 0 0 0 0 0 0 30 0 60 0 0 0 0 0 0 0 0 0 0 0 31 0 0 0 0 0 0 0 0 0 0 0 0 0 32 0 0 0 0 0 0 0 0 0 0 0 0 0 33 0 0 0 0 0 0 0 0 30 0 0 0 0 34 0 0 0 0 0 0 30 0 0 0 0 0 0 35 0 0 0 0 75 0 25 0 0 0 0 0 0 36 0 0 0 10 0 0 0 0 0 0 0 0 0 37 0 0 0 0 0 0 0 0 0 0 0 0 20 38 0 0 0 0 0 0 0 0 0 40 0 0 0 Compar- 39 0 0 0 0 0 0 0 0 0 0 0 0 0 ison 40 0 0 0 0 0 0 0 0 0 0 0 0 0 products 41 0 0 0 0 0 0 0 0 0 0 0 0 0 42 0 0 0 0 0 0 0 0 0 0 0 0 0 43 0 0 0 0 0 0 0 0 0 0 0 0 0 44 0 0 0 0 0 0 0 0 0 0 0 0 0 45 0 0 0 0 0 0 0 0 0 0 0 0 0 46 0 0 0 0 0 0 0 0 0 0 100 100 0 47 0 0 0 0 0 0 0 0 0 0 0 0 0

Comparative Example

(7) Similarly to the examples, bonding wires 39 to 47 as illustrated in Table 1, having element composition that is outside a composition range of the present invention were manufactured as comparison products (hereinafter referred to as comparison products).

(8) Incidentally, the comparison product 47 is formed by continuously casting (reducing the diameter of) an element wire obtained by subjecting the continuously-cast thick wire having the diameter of 8 mm, similar to those of the examples, to pickling by dilute nitric acid at 80 C., and does not have the surface segregation layer on its outer layer. Therefore, the comparison product 47 is different from the implementation products in that it is subjected to the pickling, although its composition is within the composition range of the present invention.

(9) Incidentally, the refining heat treatment in the present invention and the comparative example is heat treatment for adjusting the bonding wire to have a predetermined elongation value, in the measurement by a tensile breaking testing machine, while adjusting temperature and speed in a tube furnace, similarly to the case of a pure gold wire. After the refining heat treatment, the highly-concentrated silver (Ag) layer segregated in the doughnut shape on the surface of the implementation product was not lost.

(10) [Checking of Surface Segregation Layer]

(11) The AgPdPt three-element alloy having the composition of the implementation product 1 was continuously cast under the inert atmosphere to obtain the thick wire having the diameter of 8 mm. This thick wire was subjected to the continuous wire drawing process while being water-cooled, and subjected to the refining heat treatment so that a percentage of elongation becomes 4%, and the bonding wire having the diameter of 20 m was obtained. With regard to the silver (Ag) element of the bonding wire, the Auger analysis was performed in the depth direction from its outer layer toward its center. The results are illustrated in FIG. 1.

(12) As is clear from FIG. 1, it is found out that the surface segregation layer of highly-concentrated silver (Ag) exists from the surface of the implementation product to the depth of about 10 nm.

(13) [Checking of Silver Sulfide]

(14) The bonding wire of the implementation product 1 was left to stand in a room-temperature atmosphere for 30 days. Then, the thickness of the silver sulfide (Ag.sub.2S) film at the outermost surface was measured by the Sequential Electrochemical Reduction Analysis, by using a sulfide film thickness measuring machine (QC-200 manufactured by Cermatronics Boeki Co., Ltd.). As a result of this, the silver sulfide (Ag.sub.2S) film was not detected. This is illustrated by a thick black line (a curved line in an L shape) in FIG. 2.

(15) The bonding wire of the comparison product 39 was left to stand similarly to the implementation product 1, and the thickness of the silver sulfide (Ag.sub.2S) film was measured. As a result of this, the silver sulfide (Ag.sub.2S) film was detected. This is illustrated by a line with diamond shape marks (a step-wise curved line) in FIG. 2.

(16) FIG. 2 is a view illustrating voltage changes over time in a graphical form. With the comparison product 39 in which silver sulfide (Ag.sub.2S) is formed, such a phenomenon occurs that the voltage does not change over time in a section where the voltage is 0.25 to 0.80 V. Meanwhile, with the bonding wire of the implementation product 1, the above-described step-wise phenomenon does not occur in the above-described section of the voltage, and the voltage changes over time. Thus, it is found out that silver sulfide (Ag.sub.2S) is not formed in the outermost surface of the bonding wire of the implementation product 1.

(17) In addition, the outermost surface of the implementation product 1 is subjected to qualitative analysis to check a detection amount of sulfur (S). This is illustrated in FIG. 3.

(18) As illustrated in FIG. 3, it is found out that sulfur (S) exists in the outermost surface of the bonding wire of the implementation product 1. However, silver sulfide (Ag.sub.2S) is not detected in the results in FIG. 2, and therefore, it is found out that sulfur (S) of the bonding wire of the implementation product 1 is in a state of the silver sulfide layer that is unstable and physically adsorbed, without reacting with silver (Ag) existing in the outermost surface and forming the strong silver sulfide (Ag.sub.2S) film. Further, as is clear from FIG. 3, the metal element in the outermost surface of the bonding wire of the implementation product 1 is silver (Ag), without including palladium (Pd) or platinum (Pt), and substantially forms the highly-concentrated silver (Ag) layer, which is most suited for the high-speed signal layer.

(19) [Aluminum Splash Test]

(20) Each of the implementation products 1 to 38 and the comparison products 39 to 47 was set in a commercially-available wire bonder. Under an atmosphere of blowing nitrogen, the free air ball (FAB) was formed aiming at 38 m on a 70 m-square aluminum pad (having a gold (Au) layer with the thickness of 20 nm evaporated onto its surface), formed by an Al-1.0 mass % Si-0.5 mass % Cu alloy, on the surface of a dummy semiconductor IC (having a test pattern embedded in a wafer, which is abbreviated as TEG), and ball bonding was performed under the conditions of substrate heating temperature of 200 C., loop length of 5 mm, loop height of 220 m, crimping ball diameter of 50 m, and crimping ball height of 10 m. With regard to a measurement method of the amount of aluminum splash, the crimping ball of each of the wires was observed from immediately above, by using a commercially available SEM, to measure the position of aluminum that protrudes to the maximum extent from the crimping ball, by setting the peripheral portion of the crimping ball as a base point. When the protruded amount of aluminum is less than 2 m, it is judged as (very good), when it is 2 m or more and less than 4 m, it is judged as (good), and when it is 4 m or more, it is judged as x (poor). Evaluation results of the aluminum splash test are illustrated in Table 2.

(21) [Chip Damage Test]

(22) In addition, chip damage of the samples was examined. According to a chip damage test, the aluminum pad was dissolved by aqueous sodium hydroxide, and then the chip was observed by the stereoscopic microscope. With regard to the chip damage test in Table 2, when there is a flaw or a crack, even if only slightly, it is judged as x (poor), and when there is neither flaw nor crack, it is judged as (good). The results are illustrated in Table 2.

(23) TABLE-US-00002 TABLE 2 Evaluation items Signal Aluminum Chip waveform Shear strength splash damage deterioration test of crimping No. test test test ball Implemen- 1 tation 2 products 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 Compar- 39 X X ison 40 X X products 41 X 42 X X X 43 X X X 44 X X X 45 X X X 46 X X X 47 X
[Signal Waveform Deterioration Test]

(24) Next, a signal waveform deterioration test was performed by a four-terminal method. The implementation product wires and the comparison product wires (each having the wire diameter of 20 m and the length of 100 mm) were used as the samples. At the time of measurement, a commercially-available function generator was used to propagate a pulse waveform of 10 GHz and 2 V to the implementation product wires and the comparison product wires, and the signal waveform was measured by a predetermined commercially-available digital oscilloscope and probe that can measure the pulse waveform in a 10 GHz band. A probe interval for the measurement was allowed to be 50 mm. With regard to a degree of signal waveform deterioration, a delay time of the output signal waveform propagating through the wire was measured, until it reached an input voltage value. From the results of the experiment, it is confirmed that the signal delay time of the conventional wire (15 ppm of Ca, 20 ppm of Eu, and 99.999 mass % of Au as the balance) is 20%. Thus, with regard to the judgment of the signal delay time, when the delay time is 20% or less of that of the conventional wire, it is judged as (good), and when the delay time is more than 20%, it is judged as x (poor). Evaluation results of the signal waveform deterioration test for the implementation product wires and the comparison product wires are illustrated in Table 2.

(25) [Shear Strength Test of Crimping Ball]

(26) The members and the evaluation device that are similar to those used in the aluminum splash test were used to perform the bonding of each of the implementation product wires and the comparison product wires to a dedicated IC chip by using the wire bonder. With regard to the 100 products, Multipurpose bondtester (BT) (Series 4000) manufactured by Dage Japan Co., Ltd. was used to perform shear strength evaluation of the crimping ball at the time of the ball bonding. The results of the shear strength evaluation of the crimping ball are illustrated in Table 2.

(27) In Table 2, ball shear means a shear load value of first bonding. (very good) means that the value is 12 kg/mm.sup.2 or more, (good) means that the value is 10 kg/mm.sup.2 or more and less than 12 kg/mm.sup.2, and x (poor) means that the value is less than 10 kg/mm.sup.2 or ball stripping is caused.

(28) It is found out that, as is clear from the results in Table 2, in the signal waveform deterioration test, no deterioration is found in any of the implementation products 1 to 38 according to the present invention, but the deterioration is found in all of the comparison products 39 to 47.

(29) With regard to the aluminum splash test, the chip damage test and the shear strength test of the crimping ball, all the implementation products 1 to 38 according to the present invention have the excellent results. However, it is found out that the comparison products 39 and 40 have unfavorable results with regard to the shear strength test of the crimping ball, and the comparison products 42 to 46 have unfavorable results with regard to the aluminum splash test and the chip damage test.

INDUSTRIAL APPLICABILITY

(30) The bonding wire of the present invention is most suitable for transmitting super-high frequency signals of several GHz to over 10 GHz, which has a wide variety of applications as the bonding wire for signals, suitable for transmitting the high-frequency signals.