JP5616165B2 - Silver bonding wire - Google Patents

Description

  The present invention relates to a silver (Ag) bonding wire used for connecting an electrode on a semiconductor element and an external electrode or a silver bump wire used for connecting a stud bump and a lead provided on a semiconductor element.

In general, since the diameter of a bonding wire used for connecting an electrode on a semiconductor element and an external electrode is as very small as 15 to 75 μm, it is necessary to be a metal having good conductivity and excellent workability. Conventionally, gold (Au) has been used as a material because of its chemical stability and ease of handling in the atmosphere.
However, since the Au bonding wire is very expensive because 99% to 99.99% of the weight is Au, there has been a strong demand from the industry to replace the material with an inexpensive material.

In response to these demands, copper (Cu) bonding wires have recently been developed and used in discrete packages such as power ICs and transistors that require thick wires. However, since Cu is a metal that easily oxidizes, Cu bonding wires are packaged in a sealed bag sealed with nitrogen gas so that the surface does not oxidize due to long-term storage, or the wire surface is oxidized during use to provide good bonding properties. In order not to deteriorate, there is a problem that the winding length per winding is set as short as about 500 m to be used up before oxidation starts, and workability and bonding properties are inferior to those of Au wires. For the purpose of improving such problems, a metal-coated Cu wire in which the surface of a copper core wire is coated with a different metal has been proposed.
For example, a bonding wire in which the surface of a copper core wire is coated with palladium (Pd) or a palladium alloy directly or through an intermediate layer, the surface of the copper core wire is coated with gold, or the surface of the copper core wire is coated with platinum. Are known.

  The metal-coated Cu wire whose surface is coated with Pd has solved the problem of uncovered bare Cu for long-term storage and lengthening of the winding length, while Pd coated to prevent oxidation is used when molten balls are formed. Since it melts at a high concentration on the surface of the Cu ball and forms an alloy layer with a high concentration of Pd in the vicinity of the surface after solidification, the surface hardness becomes higher than that of bare Cu, resulting in a problem that the bondability deteriorates. .

Further, in plastic deformation of the ball during ball bonding, the hardness increase due to work hardening due to so-called solid solution hardening is significant compared to the bare Cu ball, and the structure of the aluminum pad that is the adherend of the ball is a low-k material (carbon In the case of a multilayer film structure of silicon dioxide (SiO ₂ ) whose main component is silicon and silicon dioxide (SiO ₂ ), the so-called pad damage in which the low-k material is destroyed during ball bonding. The problem also occurred.

  For this reason, as a result of intensive investigations to remedy such adverse effects, the present inventors have found that the reason why such pad damage occurs is that the ball surface hardness of purity 99.99% Cu is 59 Hv and purity is 99.99. In addition to being harder than 41Hv of% Au, the atoms that dissolve in the ball surface are 140 pm Pd whose radius is larger than 135 pm of Cu, so that the Cu lattice is distorted by compressive stress. However, I thought that it might be a dislocation movement barrier during plastic deformation.

  There are several elements that have an electrical conductivity equal to or higher than that of Au and have a hardness lower than that of Cu, but silver (Ag) can be cited in view of oxidation resistance. The electrical resistivity is 2.2 μΩcm for Au, while the purity is 99.99% Ag is 1.6 μΩcm, the electrical conductivity is as high as 1.7 μΩcm for Cu, and the ball surface hardness is 41 Hv for Au. On the other hand, Ag is 44 Hv, which is lower than 59 Hv of Cu.

  On the other hand, the idea of using Ag as a bonding wire has been around since 1975 (see, for example, Patent Document 1), but when left in the air for a long time, it sulfides and discolors the surface, and the bonding property during bonding is improved. Because of the instability, a method of coating Au with the purpose of suppressing surface discoloration has been proposed. For example, the surface of an Ag core wire is coated with 2.5 to 35% by weight of Au in a diameter of 20 to 80 μm. 99.99% or more of pure gold on the surface of a bonding wire (for example, see Patent Document 2) or an Ag-Au alloy core wire in which 0.03 to 10 w / o of Au is added to 99.9% or more of pure silver. A coated bonding wire has been proposed (see, for example, Patent Document 3). Furthermore, a method of coating the surface of the Ag core wire with a white metal has been proposed (for example, see Patent Document 4).

  The method of forming a molten ball at the tip of a bonding wire can be broadly divided into a method using a hydrogen torch and a method using an electric torch. Currently, in order to accurately control the ball size, the current and time are adjusted. Electric torches that can freely set the melting energy have become mainstream. In addition, with the recent high integration of semiconductors, the size of the ball requires a very small ball having a diameter of 50 μm or less.

  Under these conditions, when a ball is formed on the tip of a metal-coated Ag wire whose surface is coated with Au or Pt, the melting point of the surface-coated metal is higher than that of the Ag wire. Similarly, Au and Pt are concentrated and dissolved in the vicinity of the Ag ball surface, and the ball is hardened and hardened during the subsequent ball bonding, and pad damage with the low-k material cannot be avoided. There was also a trade-off relationship that the occurrence of pad damage increased with increasing coating metal thickness to prevent sulfidation. This is because the ball surface hardness exceeds 60 Hv at which pad damage is likely to occur.

  As a measure for reducing the price of bonding wires, an Ag alloy wire to which a precious metal is added at a high concentration has been proposed (see, for example, Patent Document 5). For example, when 40% by mass of Au was added to Ag, it was possible to form a spherical ball in the atmosphere, but the electrical resistivity value was 7.7 μΩcm, and the purity required by the market was 99% Au. The problem that the wire greatly exceeds 3.1 μΩcm and the work hardening at the time of ball bonding is large, and the pad damage with the low-k material cannot be avoided.

Japanese Utility Model Publication No. 52-33264 JP 51-85669 A Japanese Patent Laid-Open No. 1-110741 JP 56-26459 A JP-A-11-288896

When a ball is formed in the atmosphere, a true spherical ball cannot be obtained unless a large amount of noble metal is contained. However, even if pure Ag is used in a mixed atmosphere of 5% hydrogen and 95% nitrogen, it becomes a true sphere. However, in order to use hydrogen gas, it is necessary not only to modify the wire bonder, but also to make capital investments such as measures for safety in rooms where hydrogen is used and measures for storing hydrogen gas.
On the other hand, if the only gas used is nitrogen, since many semiconductor assembly plants already use nitrogen gas, the necessary investment can be limited to the installation of the gas nozzle on the wire bonder. All the above problems can be solved if a soft ball with no spherical oxidation can be formed only with nitrogen gas, and a wire having an electrical resistivity of 3.1 μΩcm or less and an insulating film can be formed on the surface thereof. The Rukoto.

In addition, when an attempt is made to form a ball at the tip of an Ag wire having a purity of 99.99% or more by plasma discharge in the atmosphere, the surface of the Ag wire that has become hot changes to black (hereinafter referred to as blackening), and the ball becomes wrinkled ) And pad damage is likely to occur.
In order to prevent this, ball formation in nitrogen gas was tried, but blacking occurred on the wire surface as in the air, and the ball roundness increased compared to the air, but the sharpness at the bottom of the ball was eliminated. Was not.
On the other hand, in a mixed gas of 5% hydrogen and 95% nitrogen, no blackening occurred on the wire surface, and a ball having a true spherical gloss was formed.

  Therefore, the problem to be solved by the present invention is that the electrical resistivity of Ag having high electrical conductivity is not lowered so much, the surface of Ag is chemically stabilized, and the wire surface is blackened in nitrogen gas. It is another object of the present invention to provide an Ag bonding wire that can form a true spherical ball with no sharp point at the bottom of the ball and that does not soften with time and does not age soften easily.

In order to solve the above problems, one of the Ag bonding wires according to the present invention contains (gold) Au in a range of 10,000 ppm to 55000 ppm and bismuth (Bi) in a range of 1 ppm to 100 ppm. The remainder was a bonding wire made of silver (Ag) containing inevitable impurities.
Another Ag bonding wire according to the present invention contains (gold) Au in a range from 10,000 to 55000 ppm by mass, bismuth (Bi) in a range from 1 to 100 ppm by mass, and further contains 20000 ppm of palladium (Pd). It was set as the bonding wire which consists of silver (Ag) which contains below ppm and the remainder contains an unavoidable impurity.
In the present invention, the bonding wire may be a bump wire.

According to the Ag bonding wire according to the present invention, 10,000 to 55000 ppm by mass of Au is added to Ag. Therefore, the wire surface is not blackened by heating at the time of ball formation, and the electrical resistivity value is currently widely used. Further, it is 3.1 μΩcm or less, comparable to that of a 99% pure Au wire, and the ball hardness can be made softer than that of a 99.99% pure Cu wire, thereby eliminating pad damage during ball bonding.
Moreover, since Bi is added in an amount of 1 to 100 ppm by mass, aging softening is not caused, and changes in mechanical characteristics during storage at room temperature can be prevented. Furthermore, by adding Pd to 20000 mass ppm or less, even if the joint between the Ag wire and the Al pad is left in a high humidity condition, it does not rapidly deteriorate.

It is the front view of a semiconductor package used for evaluation of the silver bonding wire which concerns on this invention, and explanatory drawing of an electrical resistance measurement.

Ag of the Ag bonding wire according to the present invention has a purity of 99.99% by mass or more. This is to keep the electrical resistivity low.
The addition of 10000 to 55000 mass ppm of Au to Ag does not cause the surface of the wire to become black due to heating during ball formation, and the electrical resistivity value of 99% purity Au wire currently widely used. This is because the hardness is set to 1 μΩcm or less and the ball hardness is softer than that of a 99.99% pure Cu wire.
If the added amount of Au is less than 10000 mass ppm, blackening of the wire surface cannot be prevented, and if it exceeds 55000 mass ppm, the electrical specific resistance increases.
Therefore, the addition amount of Au shall be 10,000 mass ppm or more and 55000 mass ppm or less.

  Bi is added in an amount of 1 to 100 ppm by mass because Ag containing 10000 to 55000 ppm by mass of Au exhibits aging softening and thus prevents changes in mechanical properties during storage at room temperature. If the amount added is less than 1 ppm by mass, the change in characteristics cannot be suppressed. If the amount added exceeds 100 ppm by mass, Bi precipitates on the ball surface, increasing the hardness of the ball and causing pad damage during ball bonding. Therefore, the addition amount of Bi is set to 1 mass ppm or more and 100 mass ppm or less.

  In the joint portion between the Ag wire and the Al pad, Al is thermally diffused in the Ag, and voids are generated in the joint surface to reduce the joint strength and increase the electrical resistivity. However, the diffusion of Al into Ag can be prevented by adding Pd to 20000 mass ppm or less. If it is added in excess of 20000 mass ppm, the hardness of the ball increases and pad damage occurs during ball bonding. Therefore, the amount of Pd added is 20000 mass ppm or less.

  Hereinafter, the present invention will be described in detail with reference to examples and comparative examples for an Ag bonding wire according to the present invention.

(Examples and Comparative Examples)
Table 1 shows various additive elements and addition amounts in the examples of the Ag bonding wires of the present invention and the Ag bonding wires of the comparative examples.

Sample No. 1 to Sample No. 12 showing Examples of the present invention were, in a vacuum melting continuous casting furnace, put Ag having a purity of 99.99% by mass or more as a raw material in a carbon crucible having a purity of 99.999% or more, After high-frequency melting at a vacuum degree of 1 × 10 ⁻⁴ Pa or less and sufficient degassing at a molten metal temperature of 1100 ° C. or more and a holding time of 10 minutes or more, Au, Bi, and Pd are preliminarily given the predetermined composition shown in Table 1. Then, the mixture was poured into a crucible and dissolved, and then an inert gas was injected to return to atmospheric pressure, and cast to 8 mmφ by continuous casting.
Next, the wire was subjected to wire drawing, the diameter was reduced to 16 μm, and an Ag bonding wire annealed in an air atmosphere so that the elongation rate was 6% was obtained.
In this example, a bonding wire having a diameter of 16 μm and an elongation rate of 6% was evaluated. However, even if the bonding wire diameter is up to about 30 μm or the elongation rate is up to about 10%, The effect is not affected.

Sample No. 13 to Sample No. 19 as comparative examples had the compositions shown in Table 1, and the production conditions other than the composition were produced under the same conditions as in the examples.
The bonding wire was evaluated using the semiconductor package shown in FIG. In the semiconductor package 10 shown in FIG. 1, a silicon chip 3 having an aluminum electrode 4 is disposed in the center of the sealing epoxy resin 6, and six silver platings are applied to the sealing epoxy resin 6. A lead 5 is provided. Each aluminum electrode 4 and lead 5 are connected by a silver bonding wire 1. In connecting the silver bonding wire 1 on the aluminum electrode 4, a silver bonding wire ball 2 is formed on the aluminum electrode 4.

  Each sample was evaluated by bonding using a Shinkawa UTC-1000 that can form a ball while blowing nitrogen gas to the tip of the silver bonding wire. The discharge conditions were such that the flow rate of nitrogen gas was 0.6 liter / min and the discharge current was adjusted so that a ball having a diameter of 50 μm was obtained.

Regarding the blackening of the heat effect at the time of ball formation, the state of discharge was photographed with a high-speed camera fx-K5 made by Nack, and the presence or absence of blackening was visually judged on the reproduced image.
The shape of the ball 2 was judged by observing with a microscope whether it was spherical or bowl-shaped, and whether or not the bottom of the ball 2 was sharpened.
Regarding the ball hardness, Vickers hardness was measured using MVK-H3 manufactured by Akashi for the ball 2 having a spherical shape immediately after bonding. 50Hv or less is a good product.

  The electrical specific resistance value used a simple method of calculating from the electrical resistance and solid wire diameter of a wire having a length of 300.0 mm, and the measuring instrument used was DEGITAL MALTIMETER made by ADVANTEST. An electrical specific resistance value of 3.0 μΩcm or less is a good product.

  The presence or absence of aging softening was measured by measuring the elongation of a drawn wire having a diameter of 230 μm using a UTM-4-100 made by TOYO BALDWIN, and after 2 weeks, the change in elongation was 5 % Or more, it was judged that there was aging softening.

For the bonding reliability, the shear strength at the bonding interface was measured using Dage-SERIES-5000PXY manufactured by Dage. The evaluation samples were compared as “initial” before being held at a high temperature, and “left” after being held at 175 ° C. for 240 hours using an oven DKM600 manufactured by Yamato.
Per unit area shear strength 7.00kg / mm ² or more good (○ mark), was determined 10.0 kg / mm ² or more and excellent (◎ mark).

The reliability was measured using the DIGITAL HITESTER manufactured by HIOKI using the measurement circuit shown in FIG. A value of 8.5Ω or less was judged good (◯), and a value of 7.0Ω or less was judged good (◎).
In addition, in the case where the characteristics of pad damage, initial hardness, specific resistance, and weather softening do not satisfy the above criteria, the evaluation of bonding (reliability) and reliability is not performed. "showed that.
These results are shown in Table 2.

Sample Nos. 1 to 12 as examples show good results in all the characteristics described in Table 2.
In Sample No. 13, which is a comparative example, aging softening occurred because Bi was not added.
Similarly, Bi is not added in Sample Nos. 18 and 19, but in Sample No. 18, Ca is added by 5 ppm by mass, and in Sample No. 19, Pt is added by 9000 ppm by mass. As a result, aging softening occurred as in sample No. 13.
In Sample No. 14, since Bi was added beyond a specific range, the hardness of the ball was increased and pad damage was generated, and subsequent evaluations were not possible.
Sample No. 15 had an addition amount of Au of only 5000 ppm by mass, the wire surface was blackened, and the ball tip became bowl-shaped.
Sample No. 16 had an excess of 75,000 mass ppm of Au, and the specific resistance value was as high as 3.2 μΩcm.
In Sample No. 17, the amount of Pd added was excessive at 30000 mass ppm, the initial hardness was increased to 55 Hv, and the specific resistance value was increased to 3.7 μΩcm.

  The above-described examples and comparative examples have been described with respect to the bonding wire used for connecting the electrode on the semiconductor element and the external electrode. However, in the bump wire used for connecting the stud bump and the lead provided on the semiconductor element. Is expected to exert excellent effects as well and make a great contribution to the semiconductor industry.

1: Silver bonding wire 2: Ball 3: Silicon chip 4: Aluminum electrode 5: Lead 6: Sealing epoxy resin 7: Resistance measuring instrument 10: Semiconductor package

Claims (3)

  A silver bonding wire characterized by containing Au in a range from 10,000 ppm to 55000 ppm, Bi in a range from 1 ppm to 100 ppm, and the balance consisting of Ag and inevitable impurities.   A silver bonding wire characterized by containing Au in a range from 10000 mass ppm to 55000 mass ppm, Bi in a content from 1 mass ppm to 100 mass ppm, further containing Pd in an amount of 20000 mass ppm, and the balance comprising Ag and inevitable impurities. . The silver bonding wire according to claim 1, wherein the bonding wire is a bump wire.

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