JP2002319597A - Bonding wire and manufacturing method thereof - Google Patents

Abstract

(57) [Problem] To provide a bonding wire that can suppress occurrence of leaning failure during wire bonding even with a narrow bonding pitch, is resistant to wire flow, and has excellent tensile strength. SOLUTION: When reducing the diameter of a wire cast material and performing final annealing, at least one intermediate annealing is performed in the middle of the diameter reducing wire drawing, and at the time of each annealing reaching the final annealing through the intermediate annealing. Up to 75 to 99.997%. The resulting bonding wire was heated for 15 to 25 seconds in an atmosphere of
The tensile strength (high-temperature strength) measured in a temperature atmosphere of 3K (250 ° C) is higher than the 0.2% proof stress measured in a temperature atmosphere of 298K (25 ° C). The bonding wire can be evaluated and managed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bonding wire for a semiconductor element used for electrically connecting an electrode on the semiconductor element to an external electrode such as a package.

[0002]

2. Description of the Related Art Conventionally, a bonding wire used to connect an electrode on a semiconductor element to an external lead is:
After melt-casting, it is subjected to roll processing to produce a wire cast material having a predetermined composition, which is reduced in diameter by a diamond die or the like to a predetermined wire diameter of, for example, 15 μm or 30 μm, and further processed by final annealing to reduce processing strain. Manufactured with removal.

[0003] The bonding wire manufactured in this way has been designed to be thinner and to achieve higher tensile strength in order to cope with the miniaturization of semiconductor elements and packages. That is, as the size of the semiconductor element becomes smaller, the size of the electrode on the element becomes smaller. Therefore, the size of a ball melt-formed at the tip of the wire during wire bonding must be reduced in accordance with the size of the electrode. Therefore, the wire diameter of the bonding wire must be reduced, and in order to prevent disconnection of the thinned bonding wire,
It is necessary to increase the tensile strength per unit area.

[0004]

In recent years, with the progress of high integration of semiconductor devices and miniaturization of packages,
The wire interval (bonding pitch) has become narrower, and the loop length of the bonding wire has become longer, such as 4 mm or 5 mm. For this reason, even if the bending or leaning is extremely small in angle, the bonding wires adjacent to each other easily come into contact with each other, and there is a problem that short-circuit failure occurs frequently.

[0005] Leaning failure is, as shown in FIG.
When the bonding wire 1 after the wire bonding is observed from a direction parallel to the loop, the bonding wire 1 is inclined in a lateral direction at a portion immediately above the ball 2 adhered to the pad of the semiconductor element 4 (directly above the ball 3). This means that the upper loop portion 1a of the inclined bonding wire 1 is close to the upper loop portion 1b of the adjacent wire. Since such a state causes an electrical short, a package in which a leaning failure has occurred is treated as a defective product, which is a factor that greatly reduces the product yield.

[0006] Whether or not the bonding wire is liable to cause a leaning defect has been evaluated by measuring the tensile strength immediately above the ball after conducting a wire bonding test. For this reason, the evaluation management of the bonding wire which is unlikely to cause a leaning defect is extremely troublesome.

[0007] In the case of a bonding wire whose elongation rate is increased by annealing to soften, the tendency of the leaning failure to decrease is recognized. However, the annealing reduces the tensile strength of the bonding wire at the same time, so that a wire flow is likely to occur in a later resin molding step, and there is a problem that a neck collapse failure occurs.

As shown in FIG. 3, when the bonding wire 1 after wire bonding is observed from a direction perpendicular to the loop, the loop is formed by the second bonding with the lead frame 5 or the like just above the ball 3 as shown in FIG. It is a phenomenon of being pulled by the side and falling down. When this neck collapse failure occurs in the outer wire 1c of the staggered bonding, it is close to the inner wire 1d, so that it is likely to cause an electrical short as in the case of the leaning failure.

The present invention has been made in view of such conventional circumstances,
An object of the present invention is to provide a bonding wire having excellent tensile strength, excellent in straightness, capable of suppressing a leaning defect, and at the same time, having a small wire flow and preventing occurrence of a neck falling defect, and a method of manufacturing the same. And

[0010]

In order to achieve the above object, a bonding wire provided by the present invention is 523K.
After heating for 15 to 25 seconds in a temperature atmosphere of
It is characterized by a higher than 0.2% proof stress measured in a temperature atmosphere of 8K.

[0011] In the bonding wire of the present invention, it is preferable that the tensile strength measured in the 523K temperature atmosphere after heating in the 523K temperature atmosphere for 15 to 25 seconds is 200 MPa or more in nominal stress.

In a 523K temperature atmosphere, 15 to 25
The tensile strength measured after heating for two seconds in the same temperature atmosphere has long been referred to as high-temperature strength or hot strength, and has been widely used worldwide as an index for estimating the characteristics of a bonding wire during bonding. Also,
0.2% proof stress is the relationship between applied stress and strain,
This is the stress at which a permanent set of 0.2% remains when unloaded on the way. Nominal stress means a value obtained by dividing an external force by an initial cross-sectional area before deformation.

[0013] The method of manufacturing a bonding wire according to the present invention is a method of manufacturing a bonding wire in which a wire cast material is reduced in diameter to a predetermined wire diameter and then finally annealed. And one-time intermediate annealing, and the area reduction rate of the wire in each diameter reduction drawing from the wire cast material to the first intermediate annealing, from the intermediate annealing to the next intermediate annealing, and from the last intermediate annealing to the final annealing Is 75 to 99.997%.

In the above method of manufacturing a bonding wire according to the present invention, it is preferable that the temperature of the intermediate annealing is a temperature not lower than the primary recrystallization temperature of the wire casting material and not exceeding the secondary recrystallization temperature.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS At the time of wire bonding, a plastic deformation is applied to the upper portion of the ball by a capillary action, and a loop of the bonding wire is formed. Therefore, the present inventors have determined that 523K (250
C.) for 15 to 25 seconds and then the relationship between the tensile strength (hereinafter referred to as high-temperature strength) measured in the same 523 K temperature atmosphere and the tensile strength immediately above the ball after wire bonding is investigated. As a result, as shown in FIG. 1, it was found that both had a strong positive correlation.

Therefore, the relationship between the high-temperature strength and leaning failure was examined in more detail. As a result, the high-temperature strength higher than the 0.2% proof stress measured in a temperature atmosphere of 298 K (25 ° C.) was determined as the characteristic of the bonding wire. It was found that when having, leaning defects were drastically reduced. Also,
It has been found that a bonding wire having such high-temperature strength and having a high-temperature strength of 200 MPa or more at a nominal stress also significantly reduces neck collapse failure due to wire flow during resin molding, and is more preferable.

That is, the bonding wire of the present invention is characterized in that the high-temperature strength is higher than the 0.2% proof stress measured in a temperature atmosphere of 298 K, and the high-temperature strength is preferably 200 MPa or more in nominal stress. . Such a bonding wire is excellent in straightness and allows the wire to be deformed normally without breaking immediately above the ball, and the loop follows the capillary more correctly even in severe capillary operation for forming a loop. Therefore, leaning failure can be prevented.

Therefore, according to the present invention, instead of measuring the tensile strength immediately above the ball after the actual wire bonding, the high-temperature strength that can be easily measured from the bonding wire itself is used as a reference, thereby reducing the leaning failure. It is possible to easily evaluate and manage a bonding wire that is unlikely to occur.

The bonding wire of the present invention having such high-temperature strength is preferably formed by reducing the diameter by reducing the straightness of the wire and adjusting the turbulence of the wire structure, which is a major cause of the occurrence of leaning failure. It can be manufactured by performing annealing in the middle of the wire process. According to the conventional general manufacturing method, the bonding wire manufactured by reducing the diameter of the wire cast material to a predetermined final wire diameter without any annealing in the middle and then performing the final annealing is 0.2%. It has no higher temperature strength than proof stress.

Next, a method for manufacturing a bonding wire according to the present invention will be specifically described. In the method of the present invention, the wire cast material is reduced in diameter to a predetermined wire diameter and then finally annealed to produce a bonding wire. One or more intermediate annealings are performed during the diameter reducing and drawing step. Moreover, the area processing rate of the wire from the wire cast material to the final annealing through the intermediate annealing, that is, the area processing rate from the wire cast material to the first intermediate annealing, the area from the intermediate annealing to the next intermediate annealing. The processing rate and the area processing rate from the last halfway annealing to the final annealing are each in the range of 75 to 99.997%.

Here, the area processing rate (%) of the wire is
{1- (wire diameter after wire drawing reduction / wire diameter before wire drawing reduction) ² } ×
Defined at 100.

As described above, the reason why the area reduction rate of the wire until each annealing is in the range of 75 to 99.997% is as follows.
If it is less than 75%, a sufficient fiber structure cannot be obtained in the wire structure when the wire is reduced in diameter to a predetermined wire diameter, so that the straightness of the wire is reduced and a stable loop cannot be obtained. Also, if annealing is performed after the wire area processing rate exceeds 99.997%, the strain is saturated and the wire structure is disturbed,
This is because the effects of intermediate annealing and final annealing are reduced.

If the annealing temperature during the intermediate annealing is lower than the primary recrystallization temperature of the wire casting material, it is not possible to sufficiently remove the processing strain that causes the fiber structure in the wire to be disordered. Further, when the annealing temperature exceeds the secondary recrystallization temperature, the crystal in the wire becomes coarse, so that the strength of the wire after annealing is significantly reduced as compared with that before annealing. Therefore, it is desirable that the annealing temperature of the intermediate annealing is not lower than the primary recrystallization temperature of the wire casting material and not higher than the secondary recrystallization temperature.

Here, the primary recrystallization temperature is a temperature at which processing strain is removed to form a new crystal.
m is about 0.4 to 0.5 Tm. The secondary recrystallization temperature is a temperature equal to or higher than the primary recrystallization temperature and at which abnormally large crystals occur.

If the area processing rate is within the above range, the intermediate annealing may be repeated a plurality of times, and the effect of improving the straightness of the wire is maintained. The intermediate annealing may be either continuous annealing or annealing for a certain period of time. However, in any of the methods, it is necessary to sufficiently remove the distortion up to the center of the wire.

With respect to the composition of the bonding wire, a wire containing gold as a main component is preferable. Since a bonding wire containing gold as a main component has a good fibrous structure in the wire, it has high straightness and can stably obtain a loop with less leaning. Be, Ca added to gold,
Minor components such as Ce, La, and Pd increase the wire strength as the added amount increases. However, if too much is added, problems such as bonding failure during bonding may occur. preferable. Specifically, Be is 2 to 10 ppm by weight, and Ca is 10 to 35 ppm.
It is preferable to adjust to the range of ppm by weight.

[0027]

【Example】Example 1  20 Ca in high purity Au of 99.999% by weight or more
Composition containing 10 ppm by weight of Be and 10 ppm by weight of Be (hereinafter referred to as “Be”).
(Hereinafter referred to as “4N composition”) and the same high purity Au
30 ppm by weight of Ca, 5 ppm by weight of Be and Pd
500 ppm by weight (hereinafter referred to as “3N composition”)
F) for the gold alloy
By rolling, a wire cast material having a wire diameter of 25 mm was manufactured.

As shown in Table 1 below, each of the obtained wire castings was subjected to diameter reduction drawing and intermediate annealing to a final wire diameter of 25 μm, and the elongation at room temperature was 4 to 6%. The final annealing (continuous annealing) was performed so that Table 1 below shows the wire diameter at the time of each annealing, the area processing rate up to each annealing, the temperature and time or the method of each annealing for each sample of the present invention example and the comparative example. Finally, a polyoxylene alkyl ether equivalent to a monomolecular film thickness was applied to the surface of the wire to obtain a gold bonding wire.

[0029]

[Table 1]

The gold bonding wire of each sample was measured for 0.2% proof stress at a temperature of 298K, tensile strength and elongation at the same temperature, and heated for 15 to 25 seconds at a temperature of 523K. After that, the tensile strength (high-temperature strength) in the same temperature atmosphere was measured, and the nominal stress was obtained. In addition, for each bonding wire, after performing 3820 bonding at a wire interval of 60 μm and a loop length of 5 mm using a capillary having an inner diameter of 30 μm, the interval between adjacent wires at the top of the loop was measured with a measuring microscope, and the interval was measured. Is 40 μm
The following wires were determined to have poor leaning, and the numbers thereof are shown in Table 2 below. The wire bonder used was UTC300 manufactured by Shinkawa Co., Ltd., and the loop mode was set to “SQR” and the loop height was set to 280 μm.

[0031]

[Table 2]

As can be seen from the above results, one or more intermediate annealings were performed at a wire area addition efficiency of 75 to 99.997% during the processing from the wire cast material to the final annealing. In the bonding wires of Samples 1 to 3 of the invention, the high-temperature strength (523K) is adjusted to be higher than the 0.2% proof stress measured in a temperature atmosphere of 298K,
The nominal stresses were all 200 MPa or more, the number of leaning defects was small, and the straightness was good.

Incidentally, in Samples 2 and 3, the intermediate annealing was performed at a temperature not exceeding the secondary recrystallization temperature, and the strength was slightly higher than that of Sample 1 subjected to the intermediate annealing at a temperature exceeding the secondary recrystallization temperature. Was obtained. Further, as shown by the results of Samples 1 to 3, the area processing rate was 75 to 99.9.
Even if the intermediate annealing is performed a plurality of times within the range of 97%, the effect of improving straightness does not decrease.

On the other hand, while working from the wire cast material to the final wire diameter, the samples 4 and 6 of the comparative example, in which no annealing was performed on the way, when the area processing rate of the wire exceeded 99.997%. In Sample 5 of the comparative example where the final annealing was performed, and in Sample 7 of the comparative example where the area annealing rate of the wire exceeded 99.997%, the intermediate annealing was performed and the final annealing was performed at less than 75%. High temperature strength (523% proof strength)
K) is low, and the number of leaning defects exceeds 100,
Good straightness could not be obtained.

[0035]Example 2  High purity Au having a purity of 99.999% by weight or more and a predetermined amount of
High frequency induction heating with trace components such as Be, Ca, Ce, La
Melted in a heating furnace to obtain an alloy ingot having the composition shown in Table 3 below
Was. These alloy ingots are subjected to groove roll rolling to have a wire diameter of 2
A 5 mm wire casting was produced.

The obtained cast wire material was subjected to diameter reduction and drawing by a die while performing intermediate annealing under the annealing conditions described below until the final wire diameter, and finally to a diameter of 28 μm.
m extra fine line. Thereafter, each ultrafine wire is subjected to final annealing (continuous annealing) so that the elongation at room temperature is 5 to 10%, and the surface of the wire is coated with polyoxylen alkyl ether in the same manner as in Example 1, and the gold bonding wire is applied. And

[0037]

[Table 3]

The conditions of the halfway annealing performed during the above-described diameter reduction wire drawing are as follows. Condition 1: When the wire diameter is 4.0 mm (area processing rate 9
(7.4%), the intermediate annealing (60 minutes) at a temperature of 390 ° C. is performed so that the Vickers hardness of the wire surface after annealing becomes 70% of that before annealing. Condition 2: After the same halfway annealing as in Condition 1 when the wire diameter was 4.0 mm, the elongation was 10% when the wire diameter was further 0.3 mm (area reduction ratio 99.4%). Annealing (continuous annealing) is performed at a temperature of 560 ° C. Condition 3: When the wire diameter is 0.3 mm (area processing rate 9
9.99%) at a temperature of 560 such that the elongation is 5%.
Intermediate annealing (continuous annealing) is performed at ℃. Condition 4: No intermediate annealing is performed until the final wire diameter.

The gold bonding wire of each sample obtained in this manner was used at a temperature of 298 K in an atmosphere of 0.2 mm.
After measuring the tensile strength and elongation at 2% proof stress and the same temperature for 15 to 25 seconds at a temperature of 523 K, the tensile strength (high temperature strength) at the same temperature was measured, and the nominal stress was measured. I asked. The results obtained are shown in Table 4 below.

[0040]

[Table 4]

As can be seen from the above results, Samples 8 to 10 of the present invention were measured in an atmosphere at a temperature of 298K.
The high temperature strength (523K) is adjusted to be higher than the 2% proof stress, and the nominal stresses are all 200 MPa or more. On the other hand, the high-temperature strength (523K) of the comparative examples 11 to 14 was lower than the 0.2% proof stress, and the high-temperature strength (523K) of the sample 14 not subjected to the intermediate annealing was significantly higher than the tensile strength (298K). And the nominal stress is also 200 MPa or less.

Next, using the gold bonding wire of each of the above samples, a wire gap of 80 μm and a loop height of 200 μm were respectively formed by a UTC-300 type wire bonder manufactured by Shinkawa Corporation.
m, a loop length of 5 mm, and 6240 wires were wire-bonded in parallel. Observation was made from a direction parallel to the loop using a measuring microscope, and a wire inclined left and right until the gap between the upper portions of the loop became 35 μm or less was determined to be poor in leaning, and the number thereof was measured. Observation was made from a direction perpendicular to the loop, and the number of neck falling defects was measured. Table 5 below shows the numbers of the thus obtained leaning defects and neck falling defects, and the total defect rates thereof.

Further, as described above, the loop height is 200 μm.
In addition, the gold bonding wire of each sample bonded with a loop length of 5 mm was subjected to resin molding under general conditions perpendicular to the loop, and then the wire flow rate was measured. The wire flow rate was obtained by calculating the value obtained by reducing the maximum distance displaced from the position before the mold by the loop length with respect to the curved loop caused to flow through the resin, and the average value of the five points was shown as a percentage. The determined wire flow rates are also shown in Table 5.

[0044]

[Table 5]

From the above results, Samples 8 to 10 of the present invention were obtained.
Gold bonding wire is 0.2% proof stress (298K)
High-temperature strength (523K) and a high nominal stress (523K) exceeding 200 MPa, and one-tenth of the leaning failure is lower than that of the comparative sample 11 which is a conventional product.
It can be seen that it has decreased sharply to below. In addition, the occurrence of neck collapse failure observed with a conventional wire having a small number of leaning defects as in sample 14 is eliminated in samples 8 to 10 of the present invention, and the wire flow rate is generally considered to be good. 3%
It fits below.

[0046]

According to the present invention, excellent straightness can be suppressed, and leaning defects can be suppressed.
It is possible to provide a bonding wire excellent in tensile strength and a method for manufacturing the same, which can prevent the occurrence of neck collapse failure.

Further, when evaluating and managing a bonding wire in which a leaning failure is unlikely to occur, it is not necessary to measure the tensile strength immediately above the ball after wire bonding as in the related art, so that the bonding wire before wire bonding can be easily measured. The evaluation and management can be easily performed based on the measurable high-temperature strength.

Therefore, in response to the high integration of the semiconductor element and the miniaturization of the package, the defect in the wire bonding step and the resin molding step can be reduced even if the bonding pitch is as narrow as 100 μm or less. In this case, the yield and reliability of the product can be improved.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between tensile strength immediately above a ball and high-temperature strength in a bonding wire having a wire diameter of 25 μm.

FIG. 2 is a side view schematically showing a state of a bonding wire having a leaning defect.

FIG. 3 is a side view schematically showing a state in which a neck of a bonding wire is poorly collapsed.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Bonding wire 1a, 1b Upper part of loop 1c Outer wire 1d Inner wire 2 Ball 3 Upper part of ball 4 Semiconductor element 5 Lead frame

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Eiji Murase Inventor 1-6-1 Suehirocho, Ome-shi, Tokyo Sumitomo Metal Mining Co., Ltd. F-term (reference) 5F044 FF04 FF10

Claims (5)

[Claims] 1. The method according to claim 1, wherein after heating in a temperature atmosphere of 523K for 15 to 25 seconds, a tensile strength measured in the temperature atmosphere is higher than a 0.2% proof stress measured in a temperature atmosphere of 298K. Bonding wire. 2. The bonding wire according to claim 1, wherein a tensile strength measured at a temperature of 523 K for 15 to 25 seconds and subsequently measured at the temperature of the atmosphere is 200 MPa or more at a nominal stress. . 3. The bonding wire according to claim 1, wherein the bonding wire is mainly composed of gold. 4. A method for producing a bonding wire in which a wire cast material is reduced in diameter to a predetermined wire diameter and then finally annealed, wherein at least one intermediate annealing is performed during the diameter reducing and drawing step. The area reduction rate of the wire from 75% to 99.997% in the wire diameter reduction from the wire casting material to the first intermediate annealing, from the intermediate annealing to the next intermediate annealing, and from the last intermediate annealing to the final annealing The manufacturing method of the bonding wire characterized by the above-mentioned. 5. The bonding wire according to claim 4, wherein the temperature of the intermediate annealing is a temperature not lower than the primary recrystallization temperature of the wire casting material and not exceeding the secondary recrystallization temperature. Manufacturing method.