CN1175481C - Method of making wires for solid state devices - Google Patents

Abstract

The present invention relates to a method of manufacturing wires used for solid-state devices. This manufacturing process positions a silver rod vertically at the center of a mold, inserts the mold containing the silver rod into an electrical furnace, heats the mold until the surface of the rod becomes in half-melted or pre-heated state, injects melted gold into the mold to wrap the silver rod, cooling slowly the silver rod in the electrical furnace until the melted gold infiltrates into the silver rod, anneals the silver rod in an annealing furnace, and stretches the annealed silver rod to necessary diameter after extracting from the mold.

Description

Method of making wires for solid state devices

technical field

The present invention relates to a method of manufacturing connecting wires for solid state devices. In particular, this connecting wire consists of a silver rod and a gold-coated layer on the surface of the silver rod, wherein an alloy layer resulting from molten gold and silver (solid solution) is formed between the two metals.

technical background

Due to silver's good thermal and electrical conductivity, it is ideal that the connecting wires generally used in solid-state devices be made entirely of copper. However, since silver has poor durability and poor resistance to chemical corrosion and oxidation, silver connecting wires can reduce the service life of semiconductors. Therefore, silver is not suitable for making connecting wires of solid-state devices.

Now, to solve these problems, the connecting wires on solid-state devices are made entirely of gold. However, gold is expensive so as to increase the manufacturing cost of semiconductors, and gold has inherent limitations in improving tensile strength, electrical conductivity, and thermal conductivity that are essential for connecting wires. While gold is superior to silver in terms of durability and resistance to chemical corrosion and oxidation, gold is less thermally/electrically conductive than silver. Therefore, gold cannot be reliably used as an optimal connection wire on solid-state devices.

Korean Patent Application No. 93-21794 discloses a conventional method of attaching a thin gold layer to the surface of a silver wire. In this method, gold rods are rolled into 0.5mm thick gold foils with a diameter of 3.5mm. Gold foil is then coated on the surface of the silver rod, which eliminates air bubbles, and heated to 700°C-800°C. By reheating the silver rod covered with gold foil to 500°C-600°C, each contact surface of the silver rod and gold foil is bonded to each other by diffusion bonding.

However, the resulting silver rods covered with gold foil can only be used to make parts. Since this common method utilizes diffusion at the contact of silver and gold, it is preferable to increase the heating temperature to 1063°C, the melting point of gold. However, since the melting point of silver is 960°C, if the heating temperature is increased to 1063°C, the shape of the silver rod will be deformed.

Korean Patent Application No. 99-17837 discloses another conventional method, which is used in the production process of parts, in which gold foil is partially contacted with the surface of silver wire. According to this method, gold foil is wrapped around a silver rod that is grooved inward, gold wires are inserted into the grooves, and diffusion occurs. However, this method cannot be used for connecting wires of semiconductors, and can only be used to coat the surface of parts with a gold layer.

According to this disclosed conventional method, the adhesive strength of the mutual contact surfaces is so weak that it cannot provide sufficient adhesive strength for use as fine wires in solid-state devices.

Contents of the invention

An object of the present invention is to provide a simple and convenient method of manufacturing connecting wires that can be applied to the manufacturing process of solid-state devices. According to this method, the connecting wire can be less expensive and more durable and resistant to chemical corrosion and oxidation. Moreover, the connection wire manufactured according to the present invention can satisfy the requirements of thermal conductivity, electrical conductivity, resistivity, tensile strength, etc., which are essential to the field of solid-state devices. In addition, it can provide sufficient bonding strength for fine wires.

The method includes the following steps: placing a silver rod of a specific length vertically in the center of the mold; inserting the mold with the silver rod into an electric heating furnace; heating the mold so that the surface of the silver rod is in a semi-molten or preheated state; injecting molten gold into the mold In order to coat the silver rod; cool the silver rod in an electric heating furnace, and the molten gold infiltrates the silver rod at the same time; anneal the cooled silver rod in the annealing furnace; stretch the annealed silver rod after taking it out of the mold.

The volume ratio of silver to gold coating the silver rod ranges from 0.1 to 10. However, it is preferred that the volume ratio of silver to gold is 2, 3, 4 or 5.

The conditions for making the surface of the silver rod in a semi-molten state are: a heating temperature of 720° C. to 820° C., a heating time of 20 minutes to 6 hours, and a vacuum electric heating furnace. The annealing conditions are 300°C-450°C and 2-3 hours. The diameter of the wire stretched from molten gold-infiltrated silver rod is 0.016-0.070mm, which is the most suitable diameter range for connecting wires in solid-state devices.

In addition, wires with a diameter of 0.016 mm smaller than the critical diameter of 0.018 mm for pure gold connecting wires can be manufactured according to the present invention, because this manufacturing method can stretch the wires to a diameter of 0.01 mm, which is known commercially. The minimum diameter in the optimized manufacturing process. Therefore, the present invention can be used in the field of semiconductor manufacturing where thinner connection wires are required as chips become smaller.

The wire manufactured according to the present invention includes: a silver rod in the center of the wire; a silver-gold alloy layer formed by infiltrating molten gold into the silver rod to a certain depth; and covering the surface of the alloy layer with durability and resistance to chemical corrosion and oxidation gold layer.

The central silver rod covered by the outer gold layer can be a rectangular, square, circular or semicircular rod. And, preferably, the silver rod has more than two protrusions, so as to increase the contact area between the gold layer and the silver rod.

According to the wire manufacturing method of the present invention, wires can be manufactured more conveniently, the limitations of gold in terms of reliability and price can be solved, and the limitations of silver in durability, chemical corrosion resistance and oxidation resistance can be solved, and the durability and chemical resistance can be satisfied. Standard for corrosion and oxidation resistance. In addition, the wire produced according to the method of the present invention has very good resistivity and tensile strength required in the semiconductor field, and has good thermal conductivity and electrical conductivity. Also, the bonding alloy layer of the wire has sufficient adhesive strength so that it is possible to manufacture extremely fine wires that are much thinner than those currently used.

Description of drawings

The accompanying drawings are included to help a better understanding of the invention and constitute a part of this specification, illustrate preferred embodiments of the invention, and together with the description explain the principle of the invention.

In the attached picture:

Fig. 1 has shown the flow chart of the manufacturing process that is used for the wire of solid-state device according to the present invention;

Figure 2 is a perspective view of a silver rod used in the present invention;

Fig. 3 is the perspective view of mold;

Figure 4 is a perspective view of a silver rod inserted into a mold;

Fig. 5 is the plan view of the casting mold that silver rod and molten gold are housed inside;

Fig. 6 is the vertical sectional view of the casting mold that silver rod and molten gold are housed inside;

Figure 7 shows the heat balance of gold and silver;

Fig. 8 is a thousand times enlarged view of the wire cross section manufactured according to the present invention;

Fig. 9 is a re-enlarged view of a part of the cross section in Fig. 8;

Fig. 10 is the experimental curve of the electrical resistivity of the fabricated wire versus temperature; and

FIG. 11 shows the difference between the thickness of the gold-silver alloy layer formed in the present invention and the thickness of the gold-silver diffusion layer formed in the conventional invention.

Description of preferred embodiments

The accompanying drawings show preferred embodiments of the present invention and explain the principles of the present invention in conjunction with the description.

Before describing in detail the manufacturing process of the semiconductor connection wire, the characteristics of gold (Au) and silver (Ag) will be introduced below.

Gold is so durable and resistant to chemical attack and oxidation that it is even as stable in oxidizing agents as it is in water. Its electrical conductivity is 67% of that of silver, its electrical resistivity is 2.35 μΩ·cm, and its thermal conductivity is 3.15 watts/cm°C.

The crystal structure of gold is FCC (face-centered cubic), the lattice constant is 4.07864 Å (25°C), and the atomic radius is 1.44 Å.

Silver is less durable and less resistant to chemical corrosion and oxidation than gold. However, silver has the strongest thermal and electrical conductivity among all metals, and its resistivity is 1.59 μΩ·cm. The crystal structure of silver is also FCC, the lattice constant is 14.0862 Å (25°C), and the atomic radius is 1.44 Å.

Therefore, a complete solid solution of gold and silver in any mixing ratio can be made. In addition, since the elongation ratios of gold and silver are equal to each other (50:50), there will be no stretching problem after annealing, and fine wires can also be produced with gold and silver.

Silver acts as the conduction pathway due to having the best electrical conductivity of all metals, while gold acts to protect the inner silver core due to its good durability and resistance to chemical corrosion and oxidation.

The detailed fabrication process for connecting wires on solid-state devices made of gold and silver is described below.

First, a silver rod 10 is manufactured from silver with a purity of 99%. As shown in FIG. 2, there are a plurality of vertical protrusions 11 around the silver rod 10 to enlarge the contact area with the molten gold.

In addition to the shape shown in FIG. 2, the silver rod 10 can also have various shapes, such as quadrangular, ie rectangular or square, hexagonal, octagonal, circular and semicircular like a half moon.

As shown in FIG. 4 , the silver rod 10 of this shape is placed vertically and tightly fixed in the mold 40 , and the upper part 50 of the silver rod 10 (shown in FIG. 6 ) protrudes a little from the mold 40 . The casting mold 40 containing the silver rod 10 is placed in a vacuum electric heating furnace (not shown). The mold is heated at 720° C.-820° C. for 20 minutes to 3 hours until the surface of the silver rod in the mold 40 is in a semi-molten state.

The reason for heating the silver rod 10 at 720°C-820°C is that if the heating temperature is higher than the melting point of silver at 960°C, the shape of the silver rod 10 will be distorted, and below this heating temperature, the silver rod 10 cannot become semi-molten state. Although a heating time of 20 minutes to 3 hours is preferred, a heating time outside this range may also be selected depending on the diameter of the casting mold 40 and the silver rod 10 .

After the surface of the silver rod 10 is in a semi-molten state, molten gold with a purity of 99.9% is quickly injected into the mold 40 . Heat balance is established in a vacuum or an electric heating furnace full of inert gas, whereby the particles of molten gold 20 penetrate into the semi-molten surface of the silver rod 10 and fuse with the semi-molten silver particles, so that the contact surface between gold and silver An alloy solid solution layer with a constant thickness is formed on it. The alloy layer tightly connects the outer gold layer and the inner silver rod 10 .

The reason for leaving the mold 40 that has been injected with molten gold 20 in the electric furnace that is no longer heated is to allow the mold 40 to cool slowly, and the reason for the electric furnace to be evacuated or filled with inert gas is to prevent the surface of the silver rod 10 from being oxidized.

When the electric heating furnace is sufficiently cooled, the silver rod 10 and the gold-clad layer 20 are integrated through the closely connected alloy layer 30 . Then, the mold 40 is put into an annealing furnace (not shown), annealed at 300° C.-450° C. for 2-3 hours, and then slowly cooled. It is required that the annealing process can eliminate the stress in the lattice structure, coordinate the elongation of silver and gold, and improve the ductility.

If the annealing temperature and/or time used exceed the above respective ranges, the coarse grain structure is unsatisfactory, the internal stress cannot be relieved, the elongation rates of gold and silver become different from each other, and the ductility cannot be improved.

After the above process is completed, the upper part 50 protruding a little from the mold 40 is connected to a stretching machine (not shown), and stretched to a diameter of 0.016mm-0.070mm. The stretched wire is wound on the wire spool and used as the connection wire on the chip.

Fig. 8 is an enlarged view of the cross-section of a wire with a diameter of 30.2 μm produced by the above steps thousands of times. As shown in the figure, a gold-silver alloy layer is formed on the surface of the silver rod fused with finance; the fusion depth, that is, the thickness of the alloy layer, is much thicker than that of the diffusion layer formed in conventional methods such as hot pressing.

Fig. 9 is a re-enlarged view of a part of the cross-section of the wire. This figure clearly shows that an alloy layer formed from gold-silver solid solution is formed on the contact surface of the silver rod 10 and the gold-clad 20 .

According to the type of the wire used for the semiconductor, the amount of gold coating on the surface of the silver rod 10 can be adjusted. As mentioned above, the available ratios for silver and gold range from 0.1 to 10. If high storage capacity or low resistivity is required, the proportion of gold should be reduced.

If storage capacity and/or resistivity are considered, the volume ratio of silver and gold can be selected as 2, 3, 4 and 5. If the amount of gold is required to be very low, an appropriate volume ratio greater than that described above can be selected.

According to the volume ratio of silver and gold, the temperature can be calculated by the following formula.

Calorie Q=C (specific heat)×m (mass)×Δt (temperature), wherein, specific heat: gold=0.0312 cal/g, silver=0.0556 cal/g, specific gravity: gold=19.3 g/cm ³ , silver= 10.5 g/cm ³ .

Example: (assuming the volume of gold is 100cm ³ )

The mass of gold: 100cm ³ ×19.3 grams/cm ³ =1930 grams

Silver mass: If the silver-gold ratio is 2,200cm ³ ×10.5g/cm ³ =2100g,

If the silver-gold ratio is 3,300cm ³ ×10.5g/cm ³ =3150g,

If the silver-gold ratio is 4,400cm ³ x 10.5g/cm ³ = 4200g,

If the silver-gold ratio is 5,500 cm ³ ×10.5 g/cm ³ =5250 g.

As mentioned above, in the electric heating furnace, before the molten gold 20 is injected into the mold 40, the silver rod 10 and the mold 40 whose volume changes with temperature have been heated to 720°C-820°C. At this temperature, the surface of the silver rod 10 is already at semi-molten state. Therefore, although mass and temperature are inversely proportional, there is no need for special temperature control of the volume change of the silver rod, because in the temperature equilibrium state shown in Fig. .

In order to manufacture high-quality wires for solid-state devices, the silver rod 10 should be in the center of a solid solution of gold, and the gold should completely cover the surface of the silver rod 10 .

In order to examine the excellent characteristics of the wires manufactured according to the present invention, the test results of tensile load and resistivity are listed below.

Table 1 lists the tensile loads of the wires fabricated according to the above procedure. This experiment was completed by the Bureau of Technology and Standards under the Korean Ministry of Industry, Commerce and Energy. The tensile loads of wires with diameters of 30.2 μm, 33.1 μm, 37.5 μm and 49.5 μm were 28, 38, 42 and 80, respectively.

[Table 1] Tensile loads of wires manufactured according to the present invention Diameter (μm) pull 30.2 28 33.1 38 37.5 42 49.5 80

Table 2 and Table 3 list the parameters of HeraeusOriental Gold Wire and Tanaka Bonding Wire, which are widely used in solid-state devices in the world. Compared with Table 1, under the same diameter, the tensile load of the wire made by the present invention is almost twice that of the HeraeusOriental Gold Wire wire or the Tanaka Bonding Wire wire.

[Table 2] Characteristics of Heraeus Oriental Gold Wire Diameter (μm) room temperature High temperature (250°C) Elongation (%) Tensile load (g) Elongation (%) Tensile load (g) 20 4.1 8.1 1.3 6.2 25 4.0 12.9 1.6 11.1 30 4.3 19.9 1.8 16.8 33 4.5 21.5 1.9 18.2 38 4.8 26.5 2.2 20.3

[Table 3] Characteristics of Tanaka Bonding Wire (H: Hard, M: Moderate, S: Soft) diameter weight hardness Elongation tensile load (μm) Mil mg/20cm % gr 18±1 0.7±0.05 0.87-1.09 h 0.5-2.5 >4.0 m 2.0-5.0 >2.5 S 4.0-8.0 >1.5 20±1 0.8±0.05 1.10-1.34 h 0.5-2.5 >7.0 m 2.0-6.0 >4.0 S 6.0-11.0 >2.0 23±1 0.9±0.05 1.47-1.75 h 0.5-2.5 >8.0 m 2.0-7.0 >5.0 S 7.0-11.0 >4.0 25±1 1.0±0.05 1.75-2.05 h 0.5-2.5 >11.0 m 2.0-8.0 >7.0 S 8.0-12.0 >5.0 28±1 1.1±0.05 2.21-2.55 h 0.5-2.5 >14.0 m 2.0-8.0 >8.0 S 8.0-12.0 >6.0 30±1 1.2±0.05 2.55-2.92 h 0.5-2.5 >15.0 m 2.0-8.0 >9.0 S 8.0-12.0 >7.0 33±1 1.3±0.05 3.10-3.50 h 0.5-2.5 >17.0 m 2.0-8.0 >11.0 S 8.0-12.0 >8.0 38±1 1.5±0.05 4.15-4.62 h 0.5-3.0 >26.0 m 3.0-8.0 >15.0 S 8.0-15.0 >12.0 50±1 2.0±0.05 6.98-8.20 h 0.5-3.0 >45.0 m 3.0-10.0 >28.0 S 10.0-20.0 >20.0

Comparing the characteristics of Table 1 with Table 2 and Table 3 shows that the tensile load of the wire manufactured according to the present invention - one of the most important characteristics of connecting wires for solid-state devices - is twice that of conventional products. During the packaging process, the breakage rate of conventional gold wires due to the stress caused by the wire bonding process and/or molded objects is 20%. Therefore, an increase in the tensile load can significantly reduce the disconnection rate, thereby increasing the yield of semiconductors.

Figure 10 shows resistivity versus temperature curves for wires made in accordance with the present invention. This test was also completed by the Bureau of Technology and Standards under the Korean Ministry of Industry, Commerce and Energy. The curve obtained through the authoritative test shows that the average resistivity of the wire is 1.920μΩ·cm, not exceeding 1.930μΩ·cm. The sample wire used for the test is 0.3 mm wide, 235.5 μm thick, and 68 mm long. The experimental conditions are heating rate 6K/min, current intensity 1mA.

Because the resistivity of Tanaka Bonding Wire is known to be 2.31-3.02 μΩ·cm, and the resistivity of other gold wires is similar to this, the resistivity of the wire manufactured according to the present invention is much better than that of the traditional wire.

The graph in Figure 11 shows the difference between the thickness of the gold-silver alloy formed by the present invention and the thickness of the diffused gold layer that enters the silver in the existing invention. As shown in Figure 11, the thickness "T1" of the alloy layer formed by the infiltration of molten gold into the surface of semi-molten silver is greater than the thickness "T2" of the diffusion layer, therefore, the adhesion of gold and silver is naturally better than that of previous products.

Claims (11)

1. the manufacture method of a solid state device usefulness lead comprises the steps:

(a) vertically be placed in the mold silver-colored rod;

(b) heated mold makes silver-colored excellent surface become semi-molten state;

(c) the fusion gold is injected mold to envelope the silver rod;

(d) slowly cool off mold, infiltrate silver-colored clavate up to the fusion gold and become the electrum layer;

(e) mold is annealed; With

(f) from mold, after the taking-up, stretch by the silver-colored rod of gold coating.

2. method according to claim 1 is characterized in that, the scope of the volume ratio of the gold of silver and coated with silver rod is 0.1 to 10.

3. method according to claim 2 is characterized in that, the volume ratio of the gold of described silver and coated with silver rod is 2.

4. method according to claim 2 is characterized in that, the volume ratio of the gold of described silver and coated with silver rod is 3.

5. method according to claim 2 is characterized in that, the volume ratio of the gold of described silver and coated with silver rod is 4.

6. method according to claim 2 is characterized in that, the volume ratio of the gold of described silver and coated with silver rod is 5.

7. method according to claim 1 is characterized in that, in the described step (b), 720 ℃ of-820 ℃ of following heated mold 20 minutes-6 hours, wherein mold was placed on vacuum or is full of in the space of inert gas.

8. method according to claim 1 is characterized in that, mold was annealed down 2-3 hour at 300 ℃-450 ℃, and wherein mold is placed on vacuum or is full of in the space of inert gas.

9. method according to claim 1 is characterized in that, in described step (f), the silver rod that stretches and to be coated by gold becomes 0.010mm-0.070mm up to the diameter of the lead that is stretched.

10. the manufacture method of a solid state device usefulness lead comprises the steps:

(a) vertically be placed on silver-colored rod in the mold;

(b) after the mold heating, the fusion gold is injected in the mold; With

(d) slowly cool off mold, infiltrate silver-colored clavate up to the fusion gold and become the electrum layer.

11. method according to claim 10 is characterized in that, the surface of silver rod is coated by gold, and gold is combined closely by intermediate alloy layer and silver-colored rod.

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