JP2641000B2 - Gold alloy fine wire for bonding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gold alloy fine wire having excellent heat resistance and used for connecting an electrode on a semiconductor element to an external lead, and more particularly to vibration fatigue during a semiconductor device assembling operation after bonding. The present invention relates to a gold alloy thin wire for bonding, in which the strength of a ball neck portion is improved in order to greatly reduce the disconnection caused by the wire.

[0002]

2. Description of the Related Art Conventionally, a gold alloy thin wire has been mainly used as a bonding wire for connecting between an electrode on a semiconductor element and an external lead. As a technique for bonding a gold alloy thin wire, a thermocompression bonding method is a typical method. In the thermocompression bonding method, the tip of a gold alloy thin wire is heated and melted with an electric torch, and a ball is formed by surface tension.
In this method, after the ball portion is pressure-bonded to the electrode of the semiconductor element heated within the range of 0 ° C., the connection with the lead side is further performed by ultrasonic pressure bonding.

[0003] In recent years, with the improvement of bonding technology, there has been a strong demand for improving the characteristics of gold alloy thin wires. For example, as a result of the portion immediately above the ball being exposed to a high temperature during bonding, crystal grains are coarsened, and the strength of the gold alloy wire is deteriorated. As a countermeasure for this, a gold alloy thin wire having a larger pull strength than a conventional gold alloy thin wire using the pull strength after bonding as an index is desired.

Further, with the recent generalization of thin semiconductor devices, gold alloy fine wires made by adding a large amount of elements to high-purity gold have been developed and used for the purpose of reducing the loop height after bonding. I have. For example, calcium 5-1
Gold bonding wire containing 00 ppm by weight (JP-A-53-105968). One of lanthanum, cerium, praseodymium, neodymium and samarium.
Gold bonding wire (JP-A-58-154242) containing 3 to 100 ppm by weight of one or more species and 1 to 60 ppm by weight of one or more kinds of germanium, beryllium and calcium. .
However, the pull strength decreases due to the decrease in the loop height, and the hardness of the ball increases due to the addition of a large amount of alloying elements, which significantly deteriorates the bonding strength after bonding and lowers the reliability of the semiconductor device. Therefore, a gold alloy thin wire for bonding having a low loop height and high reliability, which has the same bonding strength as a conventional gold alloy thin wire having a relatively high loop height, is desired.

[0005]

The inventors of the present invention have studied the various gold wires for bonding that have been conventionally proposed, and as a result, these gold wires for bonding have a high content without a specific additive element. Although it has a higher tensile strength than a pure gold wire, it may cause breakage during assembly of the semiconductor device as a result of coarsening of the crystal grains directly above the ball, resulting in large variations in pull strength after bonding. It was confirmed that there was a problem that the reliability was low and the value was low.

In addition, a gold alloy fine wire for bonding to which a large amount of an element for increasing the recrystallization temperature of gold is added in order to lower the loop height after bonding has a problem that the pull strength is reduced due to precipitates at the crystal grain boundaries after bonding. To do,
An oxide layer is formed on the ball surface by oxidation of the additional element during the formation of the ball as the amount of the additional element increases, so that sufficient bonding cannot be performed during thermocompression bonding with the electrode. In addition, the deformation rate decreases during crimping and shrinkage holes are easily formed at the tip of the ball, which reduces the joint area with the electrode of the semiconductor element, and reduces the shear strength. Breakage occurs due to vibrations caused by such factors as the flow resistance of the sealing resin applied to the gold alloy thin wire during resin sealing, and the difference in the coefficient of thermal expansion of the constituent materials of the semiconductor device due to various subsequent thermal cycles. The thin gold alloy wire may be broken by a shear force or the like, and as a result, there is a problem that the reliability of the semiconductor device is reduced.

[0007]

Means for Solving the Problems The present inventors have proposed that by adding aluminum and calcium in combination, a bonding metal having a high strength at the ball neck portion, a small variation in the ball neck portion, and a fine crystal grain at the ball neck portion is provided. It has been confirmed that by obtaining an alloy thin wire and controlling the amount of addition, a gold alloy thin wire for bonding having a low loop height can be industrially easily manufactured, and the above-mentioned problems can be solved.

That is, the gist of the present invention is as follows. (1) Contains 3 to 50 ppm by weight of aluminum and 3 to 30 ppm by weight of calcium, with the balance being gold and unavoidable impurities
Gold alloy thin wires for bonding made of materials . (2) Aluminum as a first group element is 3 to 50 weights
Ppm and 3 to 30 ppm by weight of calcium, and yttrium, lanthanum, cerium,
Neodymium, it contains Jisupurodjiumu and 3-30 wt ppm 1 or more kinds of beryllium, and the total amount of the first group of elements and elements of the second group Ri <br/> Ah at 9-100 wt ppm, the balance Is a gold alloy wire for bonding consisting of gold and unavoidable impurities .

Hereinafter, the configuration of the present invention will be further described. The high-purity gold used in the present invention contains gold having a purity of at least 99.995% by weight or more, with the balance being unavoidable impurities. When the purity is less than 99.995% by weight, it is affected by impurities contained therein. In particular, the effect of the alloy element addition amount according to the present invention cannot be sufficiently exerted with a high-loop gold alloy wire having a relatively small amount of alloy element addition.

[0010] Aluminum has a large solid solution limit in gold and is conventionally known to have an effect of raising the recrystallization temperature. However, adding aluminum alone has no sufficient effect. By adding calcium together with aluminum in high-purity gold, the strength of the ball neck portion is increased, the pull strength is improved, and disconnection due to various vibrations before the resin sealing step can be reduced. This effect is not sufficient if the amount of aluminum added is less than 3 ppm by weight under addition of calcium, and the crystal grains at the ball neck are not stably refined. Further, when the aluminum addition amount exceeds 50 ppm by weight,
When the ball is formed, the ball does not become a true sphere, and a strong oxide of aluminum and calcium is formed on the ball surface, so that it is difficult for both new metal surfaces to come out when the ball and the electrode are pressed during bonding with the electrode. Since the joining strength is reduced, the range of the added amount of aluminum is set to 3 to 50 ppm by weight.

[0011] Calcium is known to be an element for improving heat resistance, but if it is less than 3 ppm by weight, the effect of refining the crystal of the ball neck cannot be obtained. Also,
When added in an amount of more than 30 ppm by weight, the crystal at the ball neck portion is refined to improve the heat resistance, but shrinkage holes are formed at the ball tip portion at the time of forming the ball. An object is generated, and the joint interface area between the ball and the electrode is reduced, so that the joint strength (shear strength) is reduced. Further, when the hardness of the ball increases as the amount of calcium added increases and a load necessary to secure sufficient bonding with the electrode is applied, the semiconductor element below the electrode may be cracked. Therefore, the amount of calcium added was in the range of 3 to 30 ppm by weight. The above-mentioned problems do not occur even if the contents of aluminum and calcium are the respective upper limits. It is considered that such an effect is obtained by the composite addition because aluminum increases the amount of calcium dissolved in gold and increases the strength of the crystal grain boundaries.

The purpose of the addition of the elements of the second group is to further facilitate the drawing by work hardening of the fine gold alloy wire during the drawing. The elements of the second group, when added alone, are not effective in improving the room temperature strength due to work hardening during wire drawing unless added in large amounts. However, when added together with the elements of the first group, work hardening at the time of drawing becomes large, and disconnection due to insufficient tensile strength of the gold wire at the time of drawing can be prevented. Further, there is an effect of increasing the high-temperature strength together with the effect of further improving the heat resistance by adding calcium, and as a result, the above-described effect of the present invention in which aluminum and calcium are added in combination can be further improved.

When the addition amount of these second group elements is less than 3 ppm by weight, the effect of accelerating work hardening is unstable, and the combined effect of improving heat resistance is not sufficient. On the other hand, when the addition exceeds 30 ppm by weight, shrinkage holes at the tip of the ball, which are not generated by the addition amount of only the first group of elements, easily occur, and as a result, the shear strength is reduced.
Therefore, the addition amount of the element of the second group is 3 to 30 weight p
pm.

If the total amount of the elements of the first and second groups is less than 9 ppm by weight, the crystal grains in the ball neck portion are not stably refined and the effect is not sufficient.
Exceeding the limit, the ball does not become a true sphere when the ball is formed by the electric torch, and a sufficient bonding area cannot be obtained when bonding with the electrode on the semiconductor element, the bonding strength is significantly reduced, and the electrode comes into contact with other electrodes. Therefore, the total amount of the elements of the first group and the second group is set in the range of 9 to 100 ppm by weight.

[0015]

Embodiments will be described below. Gold purity 9
Using 9.995% by weight or more of electrolytic gold, the master alloys containing the above-mentioned respective additional elements were individually melted in a high-frequency vacuum melting furnace and cast. In addition, melting and casting a mother alloy to which calcium and aluminum are added in combination has an effect of improving the yield of calcium addition.

By using a predetermined amount of the thus obtained master alloy containing each additional element and electrolytic gold having a gold purity of not less than 99.995% by weight, a gold alloy having the chemical components shown in Table 1 was subjected to a high-frequency vacuum melting furnace. After the ingot is rolled and the ingot is rolled, the wire is drawn at room temperature to form a gold alloy fine wire having a final wire diameter of 25 μmφ, and is continuously annealed in an air atmosphere to have an elongation value of about 4%. Adjust so that

With respect to the obtained gold alloy thin wire, the room temperature tensile strength, the loop height, the vibration rupture rate, the ball shape and the joint strength were examined, and the results are shown in Table 1. After joining the electrodes on the semiconductor device and the external leads using a high-speed automatic bonder, the height of the formed loop and the electrode surface of the semiconductor device are 80 lines using an optical microscope. The loop height was determined by measuring the difference between the two.

The vibration rupture rate is determined by measuring the iron-42% nickel lead frame (bonding span: 2 mm, having six IC packages in which 64 inner lead pins are arranged in four directions) in a cassette in which five semiconductor chips are mounted. The electrode on the chip and the inner lead were joined by the gold alloy thin wire by automatic high-speed bonding of the above-mentioned 25 μmφ gold alloy thin wire, and then stored again in the cassette, and the cassette was fixed to the vibration tester, and the frequency was 100 Hz. ,
After the gravitational acceleration was vibrated for 1 hour at each of the levels of 1G, 2G, and 3G, the disconnection state of the joint was inspected with an optical microscope, and the ratio of the number of disconnections was evaluated in percentage.

The shape of the ball is determined by using a high-speed automatic bonder, observing a gold alloy ball obtained by arc discharge using an electric torch with a scanning electron microscope. X that cannot be joined to the electrode of good shape and x that is good
The evaluation was made with marks. After fixing the semiconductor element to be measured with the lead frame after high-speed automatic bonding with a jig, the central part of the gold alloy fine wire after bonding is pulled, and the tensile strength when the fine wire breaks 100 pieces is measured as the pull strength. The evaluation was based on the variation. A jig was moved in parallel with the semiconductor element at a position 5 μm above the electrode of the fixed semiconductor element to shear and break the bonded gold ball, and the maximum load at the time of peeling was measured by 100 shear strengths. And the result of obtaining the variation.

Tables 1 and 2 (continued from Table 1) show the evaluation results of the gold alloy thin wires produced within the composition of the present invention.
Table 4 (continued from Table 3) shows the evaluation results of the gold alloy thin wires containing the added amount outside the composition of the present invention. From the experience of the prior art, in the case of a gold alloy thin wire, the pull strength tends to decrease as the loop height decreases. The same tendency is shown in the results of Tables 1 to 4. In comparison of the pull strengths of the gold alloy thin wires having almost the same loop height, the pull strengths in Table 2 are all as shown in Table 4.
Is smaller than the value and the variation is small. Also, when an excessive amount of alloying elements is added, the shear strength is reduced and the variation is increased.

It has been empirically known that even in the case of vibration disconnection, the tendency of the disconnection occurrence rate to increase as the loop height decreases becomes smaller.
When the disconnection is performed at the vibration intensity of 1G, the reliability of the manufactured semiconductor device is low, and when the disconnection is performed at the vibration intensity of 2G,
If the disconnection rate is 20% or less, the reliability of the semiconductor device is sufficiently satisfied. In Tables 2 and 4, with respect to the disconnection occurrence rates of the gold alloy thin wires having substantially the same loop height, the results of the present invention are lower than those of the present invention, and the reliability of the semiconductor device is sufficient.

It is considered that there is no problem if the shear strength is usually 50 g or more in the case of a gold alloy thin wire of 25 μm. In the case of Table 2, all satisfy the value of 50 g or more, but in the case of Table 4, the value may be less than 50 g, which is insufficient as a bonding gold alloy thin wire. In the evaluation of the ball shape, all the components within the range of the present invention shown in Table 1 form a normal ball (see Table 2), but the components in Table 3 show abnormal balls such as shrinkage holes at the ball tip. (Table 4
(See Reference).

As described above, when the component composition of the present invention is deviated, the bonding strength between the pull strength and the shear strength is insufficient.
Obviously, the occurrence rate of vibration disconnection is high, which lowers the reliability of the manufactured semiconductor device.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

[0028]

The gold alloy thin wire of the present invention has a small loop height variation, a high bonding strength, a small variation, a low rate of vibration disconnection, a normal ball shape and a stable ball shape. Bonding is possible, and the same effect is obtained even when the wire diameter is 18 to 30 μm, so that it has industrially useful characteristics.

Claims (2)

(57) [Claims] 1. An aluminum alloy containing 3 to 50 ppm by weight of aluminum and 3 to 30 ppm by weight of calcium, with the balance being gold.
Gold alloy wire for bonding made of impurities . 2. The method according to claim 1, wherein the first group of elements is aluminum.
50 ppm by weight and 3 to 30 ppm by weight calcium
And contains at least 3 to 30 ppm by weight of one or more of yttrium, lanthanum, cerium, neodymium, dysprosium and beryllium as the second group of elements, and the total amount of the first group of elements and the second group of elements is 9-100 weight p
pm der is, Bondi <br/> ring for gold alloy thin line the balance of gold and unavoidable impurities.