| JP56148856 | FILM CARRIER |
| JP02114649 | STRUCTURE OF SEMICONDUCTOR MOUNTING COMPONENT |
| WO/1998/040695 | APPARATUS FOR MEASURING HEIGHT OF BONDING WIRE |
Park, Sung Hyun (Hyundai Apt. 707, Sindaebang-dong Dongjak-gu Seoul 156-787, Sindaebang-dong Dongjak-gu Seoul 156-787, 101-1504, KR)
Park, Pan Ho (601-26, Sindaebang-dong Dongjak-gu Seoul 156-010, KR)
Park, Sung Hyun (Hyundai Apt. 707, Sindaebang-dong Dongjak-gu Seoul 156-787, Sindaebang-dong Dongjak-gu Seoul 156-787, 101-1504, KR)
| 1. | A method of manufacturing a wire used for solid state devices, comprising the steps of: (a) positioning a silver rod longitudinally in a mold ; (b) heating the mold to make the surface of the silver rod be in halfmelted state; (c) injecting melted gold into the mold to wrap the silver rod; (d) cooling slowly the mold until the melted gold infiltrates into the silver rod to form an alloyed layer of gold and silver; (e) annealing the mold; and (f) stretching the silver rod wrapped by the gold after extracting from the mold. |
| 2. | The method set forth in claim 1, wherein volume ratio of silver to gold to wrap the silver rod ranges from 0.1 to 10. |
| 3. | The method set forth in claim 2, wherein the volume ratio of silver to gold to wrap the silver rod is 2. |
| 4. | The method set forth in claim 2, wherein the volume ratio of silver to gold to wrap the silver rod is 3. |
| 5. | The method set forth in claim 2, wherein the volume ratio of silver to gold to wrap the silver rod is 4. |
| 6. | The method set forth in claim 2, wherein the volume ratio of silver to gold to wrap the silver rod is 5. |
| 7. | The method set forth in claim 1, wherein said step (b) heats the mold at'720 C820 C for 20 minutes6 hours, wherein the mold is placed in a vacuous or inert gasfilled room. |
| 8. | The method set forth in claim 1, wherein the mold is annealed at 300C450C for 23 hours, wherein the mold is placed in a vacuous or inert gasfilled room. |
| 9. | The method set forth in claim 1, wherein, in said step (f), the silver rod wrapped by gold is stretched until the diameter of the stretched wire is 0.010 mm 0.070 mm. |
| 10. | A wire for a solidstate device, comprising: a silver rod; a gold layer wrapping the silver rod; and an alloy layer of silver and gold formed at junction plane between the silver rod and the wrapping gold, the alloy layer having been formed through melted gold's getting into fusion with halfmelted surface of the silver rod. |
| 11. | The wire set forth in claim 10, wherein the silver rod is rectangular, square, circular, or halfcircular rod. |
| 12. | The wire set forth in one of claims 10 and 11, wherein the silver rod has several projections around the rod, each projection being formed longitudinally. |
| 13. | The wire set forth in claim 10, wherein the diameter of the wire ranges from 0.010 mm to 0.070 mm. |
| 14. | A method of manufacturing a wire used for solid state devices, comprising the steps of: (a) positioning a silver rod longitudinally in a mold; (b) injecting melted gold into the mold after the mold is heated up; and (d) cooling slowly the mold until the melted gold infiltrates into the silver rod to form an alloy layer of gold and silver. |
| 15. | A method set forth in claim 14, wherein the surface of the silver rod is wrapped by the gold which is tightly connected with the silver rod through the intermediary alloy layer. AMENDED CLAIMS [received by the International Bureau on 14 February 2001 (14.02.2001); original claims 1013 cancelled; remaining claims unchanged (I page)] annealed at 300 C450 C for 23 hours, wherein the mold is placed in a vacuous or inert gasfilled room. |
| 16. | 9 The method set forth in claim 1, wherein, in said step (f), the silver rod wrapped by gold is stretched until the diameter of the stretched wire is 0.010 mm 0.070 mm. |
| 17. | (deleted). |
| 18. | (deleted). |
| 19. | (deleted). |
| 20. | (deleted). |
| 21. | A method of manufacturing a wire used for solid state devices, comprising the steps of: (a) positioning a silver rod longitudinally in a mold; (b) injecting melted gold into the mold after the mold is heated up; and (d) cooling slowly the mold until the melted gold infiltrates into the silver rod to form an alloy layer of gold and silver. |
| 22. | A method set forth in claim 14, wherein the surface of the silver rod is wrapped by the gold which is tightly. |
2. Background Art It is ideal that a bonding wire generally used for solid-state devices is made from only silver since silver has a good thermal and electrical conductivity. However, since silver has low durability and weak resistance against chemicals and oxidation, a silver bonding wire reduces a life span of a semiconductor. Therefore, silver is incongruent to be used for a bonding wire of a solid- state device.
In these days, bonding wires for solid-state devices are made from only gold to solve such problems. However, gold is so expensive that the manufacturing price of a semiconductor is increased, and it also has an innate restraints in improving the tensile strength, the resistivity and thermal conductivity, which are essential to a bonding wire. While gold is superior to silver in the view of durability and the resistance against chemicals and oxidation, it has lower thermal/electrical
conductivity than silver. Therefore, it cannot be safely used as an optimal bonding wire in solid-state devices.
A conventional method of attaching a thin gold layer onto the surface of a silver wire, was disclosed in Korea Patent Application No. 93-21794. In this method, a gold bar is rolled out to foil of 0.5 mm thickness and 3.5 mm diameter. This foil is then wound around a silver rod, from which bubbles are eliminated, heated up to 700C- 800 C. By re-heating the silver rod wound by the gold foil up to 500 C-600 C, each contacting surface of the silver rod and gold foil is stuck to each other through diffusion connection.
However, the resulting silver rod wound with gold foil can be used for only making accessories. Because the conventional method uses the diffusion at junction of silver and gold, it is preferable to increase the heating temperature up to the gold melting temperature, 1063 C.
However, if the heating temperature is increased to 1063 C, the shape of silver rod is distorted since the melting temperature of silver is 960C.
Another conventional method, which is invented for applying to accessory manufacturing process, of partly connecting the gold foil to the surface of a silver wire is disclosed in Korea Patent Application No. 99-17837.
According to this method, gold foil winds a silver rod in which grooves are formed inward, gold wires are inserted into the grooves, and then, they undergo the diffusion However, this method cannot be used for a bonding wire of a semiconductor, but can be used for only coating an accessory with gold layer.
Adhesive strength of contacting sides connected each other according to the disclosed conventional methods is
so weak that it cannot give enough adhesive strength to be used for a microscopic wire in a solid-state device.
3. Disclosure of Invention It is an object of the present invention to provide a simple and easy wire manufacturing method applicable to solid-state device manufacturing process. According to the present method, a bonding wire can be made cheaper, and is more durable and resistant against chemical and oxidation. In addition, a boding wire manufactured by the present invention can satisfy the thermal and electrical conductivity, resistivity, and tensile strength which are essential to the field of solid-state device, furthermore. it can give enough adhesive strength for a microscopic wire.
The present method comprises following steps of: positioning a silver rod of specific length vertically at the center of a mold; inserting the mold containing the silver rod into an electrical furnace ; heating the mold to make the surface of the rod be in half-melted or pre- heated state; injecting melted gold into the mold to wrap the silver rod; cooling down the silver rod in the electrical furnace while the melted gold infiltrates into the silver rod; annealing the cooled silver rod in an annealing furnace; and stretching the annealed silver rod after extracting from the mold.
The volume ratio of silver to gold wrapping the silver rod ranges from 0.1 to 10. However, it is preferable that the volume ratio of silver to gold is 2,3,4, or 5.
The condition making the surface of a silver rod be into half-melted state is 720C-820 C of heating temperature, 20 minutes hours of heating time, and vacuous electrical furnace. And, the annealing condition
is 300 C ~450 C and 2-3 hours. The diameter of a wire stretched from a silver rod the melted gold infiltrated into is 0.016-0.070 mm which is most suitable to bonding wires for solid-state devices.
In addition, according to the present invention, it is possible to manufacture a wire of 0.016 mm diameter smaller than a gold-only bonding wire whose critical diameter is 0.018 mm, since this manufacturing method makes it possible to stretch a wire until its diameter is 0.01 mm which is the smallest in the known commercial manufacturing process. Therefore, this invention can be applicable to a semiconductor manufacturing field requiring thinner wire since chip size is much more smaller.
A wire manufactured according to the present invention consists of a silver rod located at the center of a wire, a silver and gold alloyed layer formed through melted gold's infiltrating into the silver rod up to some depth, and a gold layer, which covers the alloyed layer, having durability and resistance against chemicals and oxidation.
The center silver rod wrapped by the outside gold layer, may be a rectangular, a square, a circular, or a half- circular rod. In addition, it is preferable that the silver rod has more than two projections formed around to increase the contacting surface between the gold layer and the silver rod.
With the present wire manufacturing method, it is possible to manufacture a wire more conveniently and to resolve the restraints of gold in the view of reliability and price and those of silver in the view of durability and resistance against chemical and oxidation to meet the standard of durability and resistance against chemical
and oxidation. In addition, a wire manufactured through the present method has far better resistivity and tensile strength which are required in the field of semiconductor, and also has good thermal and electrical conductivity.
Furthermore, the connecting alloy layer of a wire is adhesive enough to make an extremely micro wire which is much thinner than currently used wire.
4. Brief Description of Drawings The accompanying drawings, which are included to provide a further understanding of the invention, illustrate the preferred embodiment of this invention, and together with the description, serve to explain the principles of the present invention.
In the drawings: FIG. 1 depicts a flowchart of manufacturing process of a wire used in solid-state devices according to the present invention ; FIG. 2 is a perspective view of a silver rod used in the present invention; FIG. 3 is a perspective view of a mold; FIG. 4 is a perspective view of a silver rod inserted in a mold; FIG. 5 is a plan view of a mold including a silver rod and melted gold in it; FIG. 6 is a vertical section of a mold including a silver rod and melted gold in it; FIG. 7 depicts the thermal equilibrium of gold and silver; FIG. 8 is a several thousand times magnified picture taken from a cross section of a wire manufactured according to the present method; FIG. 9 is a re-magnified picture taken from a part of the cross section of FIG. 8;
FIG. 10 shows an experimental graph of resistivity of a manufactured wire versus temperature ; and FIG. 11 is a graph showing the difference between thickness of an alloy layer of gold and silver formed by the present invention and that of the diffused layer of gold and silver formed by conventional invention.
5. Modes for Carrying out the Invention The accompanying drawings illustrate the preferred embodiments of the present invention, and together with the description, serve to explain the principles of the present invention.
Before detailed explanation of manufacturing a wire for a semiconductor, the characteristics of gold (Au) and silver (Ag) are explained as follows.
Gold is so good durable and resistant against chemical and oxidation that it is invariant in even oxidizer as well as in water. Its electrical conductivity is 67% of silver, and the resistivity is 2.35 liq-cm, and the thermal conductivity is 3.15 Watts/cmC.
The crystal structure of gold is FCC (Face Centered Cubic), its lattice constant is 4.07864A (25 C), and its atomic radius is 1.44Å.
Silver has lower durability and resistance against chemical and oxidation than gold. However, the thermal and electrical conductivity is highest among all metals and its resistivity is 1.59 pQ-cnl. The crystal structure of silver is also FCC, its lattice constant is 4.0862A (25 C)/and its atomic radius is 1.44A.
Therefore, it is possible to make a complete solid solution from both gold and silver regardless of their any mixing ratio. Furthermore, because the stretching rate is equal to each other (50: 50), which causes no
stretching problem, after annealing is done, it is also possible to manufacture microscopic wire with gold and silver.
While silver plays a role of a conducting path for its highest electrical conductivity among all metals, gold protects inner silver for its good durability and resistance against chemicals and oxidation.
The detailed manufacturing process of a wire, used for solid-state device, made from gold and silver is described hereinbelow.
A silver rod 10 is made with silver of 99% purity, at first. The shape of the silver rod 10 has several vertical projections 11 around the rod as depicted in FIG.
2 to enlarge area to be contacted with melted gold.
Instead of the shape of FIG. 2, the silver rod 10 may have various shapes such as a four-cornered rod, that is, rectangle or square, a hexagon, an octagon, a circular, and half-circular one like a half moon.
Such-shaped silver rod 10 is positioned vertically and fixed tightly in the inner of a mold 40 as shown in FIG.
4 with the upper part 50 (shown in FIG. 6) of the silver rod 10 protruded a little from the mold 40. The mold 40 including the silver rod 10 is then inserted into a vacuum electric furnace (not figured). After the mold 40 is heated at 720 C-820C for 20 minutes hours until the surface of the silver rod 10 in the mold 40 comes to half-melted state.
The reason why the silver rod 10 is heated at 720C- 820 C is that the shape of the silver rod 10 would be distorted if it is heated at higher temperature than the silver melting temperature of 960 C, and that the surface of the silver rod 10 can not become half-melted at below
the heating temperature. Even though it is preferable that the heating time is 20 minutes-3 hours, the heating time out of this range can be selected depending on the dimensions of the mold 40 and the silver rod 10.
After the surface of the silver rod 10 becomes half- melted state, melted gold of purity 99.9% is injected into the mold 40 instantaneously. In the vacuous or inert gas-filled electrical furnace, the thermal equilibrium is established so that the particles of the melted gold 20 infiltrate into the half-melted surface of the silver rod 10 and get into fusion with the particles of half-melted silver, which results in an alloyed solid solution layer 30 of constant thickness at junction plane of silver and gold. The alloyed layer would connect the outer gold layer with the inner silver rod 10 tightly.
The reason why the mold 40 the melted gold 20 is injected in is left in the electrical furnace not being heated is to cool the mold 40 slowly, and the reason why the electrical furnace is vacuous or filled with an inert gas is to prevent the surface of the silver rod 10 from being oxidized.
When the furnace is cooled enough, the silver rod 10 and the wrapping gold 20 become in a body through the tight-coupling alloy layer 30. Then, the mold 40 is inserted into an annealing furnace (not figured), annealed at 300C-450C for 2-3 hours, and cooled slowly.
This annealing process is required to get rid of inner stress of intergranular structure and to match the stretching rates of silver and gold, and to increase ductility.
If the annealing temperature and/or time out of aforementioned each range is applied, the open grain
structure is unsatisfactory, the inner stress is not removed, the stretching rates of gold and silver becomes different each other, and the ductility is not improved.
When the above processes are finished, the upper part 50 protruded a little from the mold 40 is linked to a stretching machine (not figured), and is stretched until its diameter becomes 0.016 mm-0. 070 mm. The stretched wire is wound around a spool and is then used for bonding wires on a chip.
FIG. 8 is a several-thousand-times magnified picture taken from a cross section of a wire of 30.2 gm diameter manufactured through the above-explained processes. This picture shows that the alloy layer of gold and silver has been formed through gold's getting into fusion with the surface of the silver rod, and that the fused depth, that is, the thickness of the alloy layer is much thicker than that of a diffused layer formed by a conventional method, such as thermal compression.
FIG. 9 is a picture taken from a part of the cross section of the wire after magnified much more. This picture shows clearly that the alloy layer 30 made from solid solution of gold and silver is formed at the contacting surface of the silver rod 10 and the wrapping gold 20.
The amount of gold wrapping the surface of the silver rod 10 can be adjusted depending upon what type of semiconductor the manufactured wire is used in. The possible range of the ratio of silver to gold is from 0.1 to 10 as aforementioned. In case that large memory capacity or much low resistivity is required, the fraction of gold should be lowered.
Considering the memory capacity and/or resistivity, the
volume ratio of silver to gold may be selected among 2,3, 4 and 5. In case that much smaller fraction of gold should be used, an appropriate volume ratio higher than the above-mentioned ones is chosen.
Temperature according to the volume ratio of silver to gold can be calculated by the following equation.
Calorie Q = C (specific heat) X m (mass) X At (temperature), where the specific heat : Gold = 0.0312 cal/gr, Silver = 0.0556 cal/gr, and the specific gravity: Gold = 19.3 gr/cd, Silver = 10.5 gr/cd.
Example (It is supposed that the volume of gold is 100 ci) Mass of gold : 100 cuf X 19.3 gr/cS = 1930gr, Mass of silver : 200cd X 10.5 gr/cm'= 2100 gr if the ratio of silver to gold is 2, 300 cd x 10.5 gr/cd = 3150 gr if the ratio is 3, 400 cm3 # 10.5 gr/cd = 4200 gr if the ratio is 4, and 500cS X 10.5 gr/ci = 5250 if the ratio is 5.
As explained above, the silver rod 10 and the mold 40, both having volume variation in accordance with temperature, have been already heated up to the temperature of 720 C-820 C at which the surface of the silver rod 10 becomes half-melted, in the electrical furnace before the melted gold 20 is injected into the mold 40. Therefore, even though the mass and the temperature are in reverse proportion to each other, the special temperature control for the volume variance of the silver rod is not necessary since the infiltrating
depth of melted gold into silver is almost equal everywhere in the thermal equilibrium state of FIG. 7.
To manufacture the aimed high-quality wires to be used in solid-state device, the silver rod 10 should be located at the center of solid solution of gold to wrap the entire surface of the silver rod 10.
To verify the excellent features of a wire manufactured according to the present invention, the experimental results about tension load and resistivity is presented.
Table 1 shows the tension load of a wire manufactured by the above-explained processes. This test has been conducted by Agency for Technology and Standards, parts of Ministry of Commerce, Industry and Energy in South Korea. The tension loads of wires with diameter 30.2 Xm, 33.1 Xl, 37.5 Xm and 49.5 um are 28,38,42 and 80, respectively.
[Table 1] Tension load of a wire manufactured by the present invention Diameter (m) Tension 30.2 28 33. 1 38 37.5 42 49.5 80 Tables 2 and 3 show the features of Heraeus Oriental Gold Wire and Tanaka Bonding Wire, which are world widely used for solid-state devices. Compared with Table 1, the tension load of a wire manufactured through the present invention is almost twice than that of Heraeus Oriental Gold Wire or Tanaka Bonding Wire for a wire having same diameter.
[Table 2] Features of Heraeus Oriental Gold Wire Diameter Room temp. High temp. (250C) (un) Stretching Tension Stretching Tension rate (%) load (gr) rate (%) load (gr) 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] Features of Tanaka Bonding Wire (H: hard, M: medium, S: soft) Diameter Weight Stretching Tension State of rate load µm mil mg/20cm hardness % gr 0.5-2. 5 > 4.0 18#1 0.7#0.05 0.87- M 2.0-5.0 # 2.5 1.09 s 4.0-8.0 > 1.5 0.5-2. 5 # 7. 0 20~ 1 0.8~0.05 1.10- M 2.0-6.0 # 1.34 S 6.0-11.0 # 2.0 H 0.5-2.5 # 8.0 23#1 0.9#0.05 1.47- M 2.0-7.0 # 5.0 1.75 7.0-11.0 > 4.0 0. 5-2.5 #11.0 1.75- 25 1 1.0 0.05 M 2. 0-8. 0 >7. 0 2.05 S 8. 0-12.0 > 5. 0 H 0.5-2.5 >14.0 28#1 1.1#0.05 2.21- M 2.0-8.0 # 8.0 2.55 8.0-12. 0 > 6.0 H 0. 5-2.5 >15.0 301 1.2#0.05 2.55- M 2.0-8.0 > 9.0 33#1 1.3# 0.05 2.92 S 8.0-12.0 # 7.0 0.5-2. 5 >17.0 33# 1 1.3# 0.05 3.10- M 2.0-8.0 #11.0 3.50 S 8.0-12.0 > 8.0 38 1 1. 5~ 0.05 4. 15- H 0.5-3.0 >26.0 4.62 M 3. 0-8.0 >15. 0 S 8. 0-15.0 >12.0 H 0. 5-3.0 >45.0 6.98- 50 1 2.0+ 0.05 M 3.0-10.0 >28.0 8.20 S 10.0-20.0 #20.0
The comparison of features of Table 1 with those of Tables 2 and 3 shows that the tension load, one of the most important characteristics for a wire used for solid- state device, is twice greater than that of conventional products. The breakage rate of conventional gold wires is about 20% during the packaging process due to the wire bonding process and/or the pressure caused from a molding object. Therefore the improvement in tension load reduces the breakage rate remarkably, thereby increasing yield of semiconductor.
FIG. 10 is a graph of resistivity of a wire manufactured through the present invention versus temperature. This test has been also done by Agency for Technology and Standards, parts of Ministry of Commerce, Industry and Energy in South Korea. The graph obtained through authorities'test shows that the resistivity of a wire is 1.9201i., on the average and never exceeds 1.93011 Q-cm. The sample wire used for this test is 0.3 mm wide, 235.5 art thick, and 68 mm long. The test conditions of 6 K/min of ramp rate and 1 mA of current are used.
The resistivity of a wire manufactured through the present process is evidently far better than that of conventional wires being used at present, because the resistivity of Tanaka Bonding Wire is known as 2.31-3.02p Q-cm and the resistivity of other gold wires are not much different from that of Tanaka Bonding Wire.
FIG. 11 is a graph showing the difference between the thickness of an alloy layer of gold and silver formed by
the present invention and that of a diffused gold layer into silver according to conventional inventions. As shown in FIG. 11, the thickness T1'of an alloy layer formed through melted gold's infiltrating into half- melted silver surface is larger than T2'of the diffused layer, so that the coherence between gold and silver is naturally better than the previous.
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