JP5408147B2 - Bonding wire - Google Patents

Description

  The present invention relates to a bonding wire, and more particularly to a bonding wire having a core material mainly composed of copper and an outer layer in which palladium is coated on the core material.

  Conventionally, a gold wire having a wire diameter of about 15 to 75 μm has been used as a bonding wire used for connecting an electrode on a semiconductor element and an external electrode. Gold wire has been used favorably for its high chemical stability and is easy to handle in the atmosphere. However, the composition of the gold wire is very expensive because 99 mass% to 99.99 mass% is occupied by gold, and the plastic ball grid array package (the gold wire usage amount of IO pins is very large). In the plastic ball grid (Alley Package) and low-price memory packages, there was a request to reduce the amount of gold used and the cost by using gold wires with a thinner wire diameter.

  Moreover, although the electrical specific resistance of gold is about 2.3 μΩcm, the electrical resistance increases as the wire diameter is reduced. Therefore, there is a limit to the thinning of the semiconductor package that preferably does not change the current value, and from the viewpoint of reducing heat generation due to the higher density of the package, the electrical specific resistance is about 1.7 μΩcm, which is lower than that of gold, and cost-effective. There was also a request to replace it with cheap copper. Under these circumstances, copper bonding wires have been developed and commercialized in recent years.

  However, although copper bonding wires are superior in electrical conductivity and inexpensive compared to gold bonding wires, they are harder than gold and the balls formed on the wire tips are hard, which damages the aluminum electrode pads on the silicon chip. There was a problem of giving. In order to obtain a soft ball, it is necessary to use high-purity copper, but there has been a problem that wire loop controllability and tensile properties are deteriorated due to coarsening of crystal grains due to a decrease in impurities.

  In order to solve this problem, a method has been proposed in which copper having a purity of 5N to 6N is used to improve wire loop controllability and tensile properties by refinement of copper crystal grains by addition of sulfur (see, for example, Patent Document 1). . In addition, soft balls using high-purity copper are easy to oxidize, and there is a problem that the reliability of bonding with the aluminum electrode pad is inferior, so there is a method of preventing ball oxidation by adding phosphorus to solve such problems. It has been proposed (see, for example, Patent Document 2).

  However, since the surface of the copper bonding wire is easily oxidized, and there is a problem that the bonding property of the stitch bonding is lowered by the oxidation, the configurations described in Patent Documents 1 and 2 are put to practical use due to the deterioration of the bonding property due to the surface oxidation. It was difficult.

  Therefore, in order to prevent oxidation of the copper surface, a method of forming a palladium film on the surface of a high purity copper fine wire has been proposed (see, for example, Patent Document 3). In addition, in order to solve the problem of the formation of a ball having low sphericity and the problem of oxidation of exposed copper due to partial loss of palladium coating during ball formation, 1 to 500 ppm by mass of P is added to the copper core material, And the method of coat | covering a copper core material with the intermediate | middle layer of palladium or platinum, and the skin layer of gold | metal | money is proposed (for example, refer patent document 4).

JP-A-7-86325 JP 2010-153539 A JP-A-62-97360 Japanese Patent No. 4349641

  However, in the configuration described in Patent Document 3 described above, a ball having a low sphericity is generated during assembly of the semiconductor element, or the ball after bonding does not exhibit a uniform color and is partially copper-colored. In other words, there is a problem that a strong portion, that is, a portion having a low palladium concentration may occur.

  In addition, in the configuration described in Patent Document 4 described above, the presence of the gold skin layer and the addition of several hundred mass ppm of phosphorus improve the sphericity of the ball formed and the bondability of stitch bonding. When a large amount is added, voids are easily formed in the ball, and there is a problem that there is a concern about long-term reliability of bonding.

  In other words, the conventional bonding wire in which the copper core material is coated with palladium has a low bonding stability to the aluminum electrode pad due to the oxidation of the wire itself or the ball surface, and the correspondence of bonding stability is not compatible with loop stability. was there.

  Accordingly, an object of the present invention is to provide a bonding wire that forms a ball having high bondability without causing pad damage by preventing a hardness increase due to oxidation of the ball surface and ensuring sphericity. .

In order to achieve the above object, a bonding wire according to an embodiment of the present invention is a bonding wire having a core material mainly composed of copper and an outer layer coated with palladium on the core material,
The core material contains sulfur, and the concentration of the sulfur is in the range of 0.0001 to 0.0007% by mass.

A bonding wire according to another embodiment of the present invention is a bonding wire having a core material mainly composed of copper and an outer layer coated with palladium on the core material,
The core material contains phosphorus and sulfur, the phosphorus concentration is in the range of 0.001 to 0.015 mass%, and the sulfur concentration is in the range of 0.0001 to 0.0007 mass%. Features.

A bonding wire according to another embodiment of the present invention is a bonding wire having a core material mainly composed of copper and an outer layer coated with palladium on the core material,
The core material contains phosphorus and sulfur, the phosphorus concentration is in the range of 0.002 to 0.008 mass%, and the sulfur concentration is in the range of 0.0001 to 0.0003 mass%. Features.

  In the outer layer, the thickness of the region where the concentration of palladium is 50 at% or more with respect to the total amount of metal elements contained in the core material and the entire outer layer may be 50 to 250 nm.

  The core material may contain gold or palladium, and the gold or palladium concentration may be in the range of 0.0001 to 10% by mass.

  ADVANTAGE OF THE INVENTION According to this invention, while being able to prevent the oxidation of a wire surface and hardening of ball bonding, ball sphericity and bondability can be improved.

It is a section lineblock diagram showing an example of a bonding wire concerning an embodiment of the present invention. FIG. 1A is a cross-sectional configuration diagram in the longitudinal direction of the bonding wire according to the present embodiment. FIG. 1B is a cross-sectional configuration diagram in the radial direction of the bonding wire according to the present embodiment.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

  FIG. 1 is a cross-sectional configuration diagram illustrating an example of a bonding wire according to an embodiment of the present invention. 1A is a cross-sectional configuration diagram in the longitudinal direction of the bonding wire according to the present embodiment, and FIG. 1B is a cross-sectional configuration diagram in the radial direction of the bonding wire according to the present embodiment.

  1A and 1B, the bonding wire according to the present embodiment includes a core material 10 and an outer layer 20. The bonding wire according to the present embodiment has a configuration in which the outer layer 20 is coated so as to surround the outer periphery of the core material 10.

  The core material 10 is a part that bears the center of electrical connection between the semiconductor element and the pad in the bonding wire, and is composed mainly of copper (Cu). The outer layer 20 covers the core material 10 to prevent copper from coming into direct contact with oxygen and oxidizing, and is composed of palladium (Pd).

  In FIG. 1, the boundary line 30 between the core material 10 and the outer layer 20 is clearly shown, but in reality, the boundary between the core material 10 and the outer layer 20 is formed by the copper of the core material 10 and the palladium of the outer layer 20. The diffusion region 35 is formed in a state in which a mixture of. In the diffusion region 35, the ratio of copper increases toward the inside, and the ratio of palladium increases toward the outside. For example, the boundary line 30 may be assumed to have a 1: 1 ratio of copper of the core material 10 to palladium of the outer layer 20.

  The core material 10 of the bonding wire according to the present embodiment contains copper as a main component, but contains sulfur (S). Moreover, you may contain phosphorus (P) in addition to sulfur as needed.

  The reason for adding sulfur to the core 10 mainly composed of copper is to obtain a ball hardness necessary for sufficient bonding to a semiconductor element in ball bonding. Further, the addition of sulfur has an effect of increasing wire strength, and also has an effect of promoting good ball formation by stabilizing the tail strength at the time of stitch bonding which is the second bonding. The amount of sulfur added to the core material 10 is preferably a sulfur concentration of 0.0001 to 0.0007% by weight. Addition of sulfur serves to make the crystal grain structure of the copper core material 10 fine and stable, but when the amount of sulfur addition is large, the ball hardness increases excessively, which is not preferable.

In addition, it is also effective to add phosphorus to the core material 10 to which sulfur is added in order to improve ball sphericity and ball bondability. Phosphorus has the effect of preventing ball surface oxidation and enhancing ball sphericity. The effect of preventing the oxidation of the core material 10 by phosphorus is higher when the gas used for forming the ball is a forming gas (5 vol% (H ₂ + N ₂ ) mixed gas).

  However, sufficient ball hardness cannot be obtained under the condition of phosphorus addition without sulfur. For example, in ball bonding in which an electrode is heated to about 150 ° C. with a plastic ball grid array package (PBGA) or the like, ultrasonic waves are not sufficiently transmitted to the ball, resulting in insufficient ball bonding. As a result, in a pull strength test after being left at high temperature, it was confirmed by experiments that a ball lift that would cause the ball to peel off from the aluminum pad was likely to occur. Therefore, it is preferable to add both sulfur and phosphorus to the copper core material 10 instead of phosphorus alone.

  In addition, as the amount of phosphorus and sulfur added to the core material 10, the phosphorus concentration is in the range of 0.001 to 0.015 wt%, and the sulfur concentration is in the range of 0.0001 to 0.0007 wt%. More preferably, the phosphorus concentration is in the range of 0.002 to 0.008 wt%, and the sulfur concentration is in the range of 0.0001 to 0.0003 wt%. When the amount of phosphorus increases, in the pull strength test after standing at high temperature, the metal peeling that the base metal layer of the aluminum pad such as the Ti layer breaks and peels easily occurs.

  Further, in the outer layer 20, if the thickness of the region where the concentration of palladium with respect to the total amount of metal elements contained in the bonding wire is 50 at% or more is in the range of 50 to 250 nm, the eccentricity at the time of ball formation is suppressed, and further stitching is performed. Bondability with the lead can be improved during bonding. This is because the formation of copper oxide is suppressed by coating the oxidizable copper surface with palladium having oxidation resistance.

  The inventors have empirically grasped that the stitch bondability is deteriorated by the copper oxide decentering the ball and the copper oxide becoming an inclusion between the pad and the bonding wire. When the thickness of the outer layer 20 is less than 50 nm, since the film thickness of the outer layer 20 is insufficient, the copper of the core material is oxidized, and there is a high possibility that the eccentricity of the ball and the deterioration of the stitch bondability will be caused. . On the other hand, when the thickness of the outer layer 20 exceeds 250 nm, the heat transfer between the palladium and the copper becomes non-uniform, and the ball is eccentric. Furthermore, by forming an alloy in the vicinity of the ball surface, the surface hardness increases, and the pad electrode on the chip side is damaged and destroyed, so the bonding device can stop due to an error and deteriorate workability. High nature. Therefore, in the outer layer 20, it is preferable that the thickness of the region where the concentration of palladium with respect to the total amount of metal elements in the bonding wire is 50 at% or more is in the range of 50 to 250 nm.

  As a method for coating the core material 10 with palladium, a physical vapor deposition method typified by sputtering, ion plating, vacuum vapor deposition, a chemical vapor deposition method typified by plasma CVD (Chemical Vapor Deposition), plating, and the like can be considered. An appropriate method may be used depending on the case. However, as a technique for mass production by simply covering palladium in the longitudinal direction of the bonding wire with a low-cost equipment and mass-producing it, a vacuum equipment is unnecessary, a film can be formed in a short time, and palladium can be formed in the process. Plating methods that allow easy weight balance management and low weight loss are preferred.

In addition, as a method for measuring the thickness of the coated palladium, a method of analyzing while digging in the depth direction from the surface of the bonding wire by sputtering or the like, or a line analysis at the cross section of the bonding wire can be used. . In this embodiment, the concentration change of palladium and copper from the bonding wire surface toward the core material is measured, and the region where the palladium concentration is 50 at% or more is defined as the palladium coating thickness. To improve the accuracy of these quantitative analyses, electron microanalysis (EPMA), energy dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (EDG) AES, Auger Electron Spectroscopy), a transmission electron microscope (TEM), etc. can be used. In particular, since the AES method has a high spatial resolution, it is effective for concentration analysis of the finest region on the outermost surface. For investigation of the average composition, inductively coupled plasma atomic emission spectroscopy (ICP-AES)
Alternatively, quantitative analysis can be performed using glow discharge mass spectrometry (GDMS).

  Further, a bonding wire may be configured by adding gold or palladium to the core material 10. A bonding wire in which gold or palladium is added to the core material 10 suppresses the formation of copper oxide due to the effect of oxidation resistance of gold or palladium. Can be increased. The amount of gold or palladium added to the core material 10 is preferably in the range of 0.0001 to 10% by mass. This is because if too much gold or palladium is added, pad damage is likely to occur due to an increase in ball hardness due to alloying. In addition, since copper oxide production | generation can be suppressed by adding gold | metal | money or palladium to the core material 10, it becomes possible to make thin the thickness which coat | covers palladium on a copper surface, and simplification of a manufacturing method becomes easy. However, the thickness of the region where the palladium concentration is 50 at% or more is not necessarily limited.

  Table 1 shows the components and configuration of the bonding wire according to the embodiment of the present invention. Table 2 shows the components and configuration of the bonding wire according to the comparative example. In Tables 1 and 2, the leftmost number indicates the number of the example and the number of the comparative example. Further, in this embodiment, the same reference numerals are given to the same components as those described so far, and the description thereof will be omitted.

実施例１〜１４及び比較例１５〜２０は、以下のような製造工程により製造した。まず、パラジウム被膜を有する銅ボンディングワイヤで、純度９９．９９９９％の電気銅を使用して表１又は表２に示すリン、硫黄、金及びパラジウム含有量になるように添加して連続熔解鋳造し、ダイスによる引き抜き伸線加工により線径０．２ｍｍまで縮径し、線速１０ｍ／ｍｉｎで連続加熱処理を行った。これにより、銅を主成分とする芯材１０が完成する。

Examples 1 to 14 and Comparative Examples 15 to 20 were manufactured by the following manufacturing process. First, using a copper bonding wire with a palladium coating, add 99.9999% pure copper to the phosphorus, sulfur, gold and palladium contents shown in Table 1 or Table 2 and continuously melt and cast. The wire diameter was reduced to 0.2 mm by drawing and drawing with a die, and continuous heat treatment was performed at a wire speed of 10 m / min. Thereby, the core material 10 which has copper as a main component is completed.

  Next, degreasing, chemical polishing, and acid activity, which are pretreatments for plating, were performed to expose the copper base, and 0.1 μm-thick palladium strike plating was performed, followed by palladium plating with a predetermined thickness. Strike plating is plating that is performed at a high current density in a short time in order to improve the adhesion of palladium plating, and forms a thin and highly adhesive coating. Next, heat treatment was performed, the diameter was reduced to 20 μm by drawing and drawing with a die, and continuous annealing was performed at a linear speed of 50 m / min in a nitrogen gas atmosphere so that the elongation rate was 10%. The plating thickness was adjusted by changing the plating time.

  As described in the bonding wires according to this embodiment, the bonding wires according to Examples 1 to 14 have a sulfur concentration of 0.0001 to 0.0007% by mass and a thickness of a region where the palladium concentration is 50 at% or more. It comprised so that it might become 50-250 nm. Among these, the bonding wires according to Examples 2 to 5 and Examples 7 to 14 have a phosphorus concentration of 0.0001 to 0.015% by mass. In particular, the bonding wires according to Examples 4 to 5, Examples 8 to 10, and Example 14 have a phosphorus concentration of 0.002 to 0.008 mass%. In addition, the bonding wires according to Examples 11 to 14 have a gold or palladium concentration of 0.0001 to 10% by mass.

  On the other hand, the bonding wires according to Comparative Examples 15, 18, and 19 have a sulfur concentration of less than 0.0001% by mass, and the bonding wires according to Comparative Examples 16, 17, and 20 have a sulfur concentration exceeding 0.0007% by mass. It deviates from the scope of wire bonding according to the embodiment. Further, the bonding wires according to Comparative Examples 17 and 18 have a phosphorus concentration exceeding 0.015 mass%. In the bonding wire according to Comparative Example 19, the thickness of the region where the palladium concentration is 50 at% or more is less than 50 nm, and in Comparative Example 20, the thickness of the region where the palladium concentration is 50 at% or more exceeds 250 nm. Thus, the bonding wires according to Comparative Examples 15 to 20 deviate from the range described in the bonding wires according to the present embodiment in any item.

Thus, the copper bonding wire which concerns on the present Example and comparative example produced was evaluated by performing the bonding test. In the bonding test, Bonder used IConn made by Kürick & Sofa. The gas used for ball formation was 100% nitrogen (N ₂ ) gas. The bonder used has the function of spraying the gas used for ball formation to the bonding site, and in this evaluation, we used gas spraying to the bonding site to evaluate under the same conditions as the customer's use conditions. As a semiconductor element for ball bonding evaluation, a commercially available test wafer in which the pad material is Al-0.5% Cu containing 0.5% copper and aluminum film thickness is 0.6 μm is used. Using. For the stitch bonding evaluation, a lead frame silver-plated to a thickness of 3 μm was used.

  The bond strength of the bond is obtained by using a bond tester 5000 from Daisy, and the shear strength of the ball bonded in the Y direction, which is the vibration direction of the bonder transducer, and the stitch pull performed by pulling the hook in the vicinity of the stitch bonding. The strength was measured. In the Y direction, aluminum is likely to be pushed out by the ball during ball bonding, and it is easy to trigger joint destruction during the bondability evaluation. Further, in stitch bonding in the Y direction, the wire is easily slipped on the silver plating. Thus, in the Y direction, both ball bonding and stitch pull bonding tend to be insufficient compared to the X direction. Therefore, as a sample for comparing and evaluating the bondability, the evaluation was performed mainly using a wire bonded in the Y direction.

  Ball sphericity was evaluated based on the ball shape observed from the horizontal direction and the crushed ball shape observed from vertically above. The ball bondability was evaluated based on the shear angle in the Y direction and the presence or absence of metal peeling. Here, the ball shape was evaluated by observing a ball formed to have a diameter 1.7 times the wire diameter from the horizontal direction. Further, the shape of the crushed ball was evaluated by observing a ball-bonded ball from above in a vertical direction so that the diameter of the crushed ball is twice the wire diameter. Furthermore, the presence or absence of metal peeling was evaluated by the presence or absence of peeling of the metal film from the lower layer of the aluminum pad, which is determined to be caused by pad damage in the failure mode in the pull strength measurement in the Y direction.

  Table 3 shows the bonding test results of the bonding wires according to Examples 1 to 14 in comparison with the bonding wires according to Comparative Examples 15 to 19.

表３において、ボール形状については、１００個のボールを水平方向から観察し、ボンディングワイヤの軸中心とボールの軸中心が一致しない、いわゆる芯ずれ又は鏃状ボールが発生した場合を×、ボールの底部がわずかに尖ったボールが発生した場合を△、いずれの異常も発生しない場合を○とした。

In Table 3, regarding the ball shape, 100 balls were observed from the horizontal direction, and the case where a so-called misalignment or bowl-shaped ball in which the axis center of the bonding wire and the axis center of the ball did not coincide with each other occurred ×, The case where a ball with a slightly sharp bottom was generated was marked as △, and the case where no abnormality occurred was marked as ◯.

  As for the shape of the crushed ball, 100 balls were observed from above, and a case where a ball in which the difference between the longest diameter Y and the shortest diameter X of the same crushed ball exceeded 3 μm occurred was marked as x. Further, the difference between the longest diameter Y and the shortest diameter X of the same crushed ball was divided by the average diameter obtained by averaging the differences between the average diameter Xbar in the X direction and the average diameter Ybar in the Y direction of each ball (( Y−X) / ((Xbar + Ybar) / 2)) × 100, the case where the so-called roundness exceeds 3% is represented by Δ. Furthermore, the case where any abnormality did not occur was evaluated as ◯.

The shear strength was measured for 24 balls. The average crushing diameter calculated from the average crushing diameter in the X direction and the average crushing diameter in the Y direction was taken as the diameter, and the area of the circle calculated using this diameter was considered as the bonding area. The shear strength was evaluated by the strength per unit area (unit: kgf / mm 2) obtained by dividing the shear strength (unit: gf) of one whole ball by the bonding area (unit: square mm). Then, among the 24 balls was measured, × a case where 9 kgf / mm ² below the ball has occurred, 9 or 10.5kgf / mm less than ² where the ball has occurred △, all balls 10. The case where it became 5 kgf / mm < ² > or more was set as (circle).

  For pad damage, 24 balls were observed, and a crack or destruction of the silicon part or a metal peeling mode occurred x, a ball lift mode in which the ball peeled off the aluminum pad occurred Δ, A case where all the modes were out of stitches or wires was marked as ◯.

  Table 3 shows the evaluation results of the ball sphericity and ball bondability, but the bonding wires according to Examples 1 to 14 have a ball shape, a crushed ball shape, shear strength in the Y direction, and metal peeling. In all evaluation items of presence / absence, a good result of ◯ or Δ is obtained, and the wire is excellent in ball sphericity and bondability. Here, in the bonding wires according to Examples 2 to 5 and Examples 7 to 14 in which the concentration of phosphorus is in the range of 0.001 to 0.015% by mass, the bonding wire has an excellent ball shape, and sulfur and As a result, the effect of adding phosphorus was demonstrated. In particular, in the bonding wires according to Examples 4, 5 and 10 in which the concentration of phosphorus is in the range of 0.002 to 0.008% by mass, all evaluation items are ◯, and the effect of adding phosphorus is The result was fully demonstrated. In addition, in the bonding wires according to Examples 1 to 14 where the thickness of the region where the concentration of palladium is 50 at% or more is in the range of 50 to 250 nm, the result is excellent in the crushed ball shape and the shear strength in the Y direction. Also in the bonding wires according to Examples 11 to 14 in which gold or palladium is added to copper, the eccentricity and irregular shape at the time of ball formation are suppressed and the bondability is improved.

  On the other hand, in Table 3, the bonding wires according to Comparative Examples 15 to 19 are x or Δ in all evaluation items, and the bonding wire according to this example is compared with the bonding wire according to the comparative example. It can be seen how good the ball bondability is.

  As described above, according to the wire bonding according to the present embodiment, the sphericity is high and the strength is sufficient, and the possibility of damaging the pads of the semiconductor element to be bonded can be significantly reduced.

  The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the present invention. Can be added.

  The present invention can be used for a bonding wire for a semiconductor device that connects an electrode on a semiconductor element and an external electrode.

10 core material 20 outer layer 30 boundary line 35 diffusion region

Claims (5)

A bonding wire having a core material mainly composed of copper and an outer layer coated with palladium on the core material,
The core material contains sulfur, the concentration of sulfur is Ri range der of 0.0001 to 0.0007 mass%,
In the outer layer, the bonding wire concentration of palladium thickness of the region is at least 50at% relative total metal elements contained in the whole of the core material and the outer layer, characterized in 50~250nm der Rukoto . A bonding wire having a core material mainly composed of copper and an outer layer coated with palladium on the core material,
The core material containing phosphorus and sulfur, in the range the concentration of the phosphorus in the 0.001 to 0.015 mass%, the concentration of sulfur is Ri der range of 0.0001 to 0.0007 wt%,
In the outer layer, the bonding wire concentration of palladium thickness of the region is at least 50at% relative total metal elements contained in the whole of the core material and the outer layer, characterized in 50~250nm der Rukoto . A bonding wire having a core material mainly composed of copper and an outer layer coated with palladium on the core material,
The core material containing phosphorus and sulfur, in the range the concentration of the phosphorus is from 0.002 to 0.008 wt%, the concentration of sulfur is Ri der range of 0.0001 to 0.0003 wt%,
In the outer layer, the bonding wire concentration of palladium thickness of the region is at least 50at% relative total metal elements contained in the whole of the core material and the outer layer, characterized in 50~250nm der Rukoto . The bonding wire according to any one of claims 1 to 3 , wherein the core material contains gold or palladium, and the concentration of the gold or palladium is in a range of 0.0001 to 10 mass%. . A bonding wire having a core material mainly composed of copper and an outer layer coated with palladium on the core material,
The core material contains sulfur, and the concentration of the sulfur is in the range of 0.0001 to 0.0007% by mass;
The said core material contains gold | metal | money or palladium and the density | concentration of this gold | metal | money or palladium is the range of 0.0001-10 mass%, The bonding wire characterized by the above-mentioned.

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