JPH06349312A - Palladium wire for semiconductor element - Google Patents

Abstract

PURPOSE:To obtain a characteristic same as or more than a gold alloy fine line in the overall bonding property by making the tensile extension percentage in a palladium fine line within a specific scope. CONSTITUTION:This palladium, fine line for semiconductor element has the tensile extension percentage in the line direction within the scope 5 to 14%. The reason to set the above tensile extension percentage at 4 to 15% is like this. When the extension percentage is less than 5%, the strength is too high, and a poor connection or an unstable ball formation are generated in the second bonding condition and when it is more than 15%, the wire strength is lowered and a wire bending following the reduction of the linear advancing property of a loop immediately after the bonding is made remarkable. By using such a palladium fine line as a bonding wire, a good connection with a lead frame, and a continued bonding property can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a palladium thin wire used for bonding in manufacturing a semiconductor device.

[0002]

2. Description of the Related Art Currently, gold alloy fine wires are used as bonding wires for joining electrodes on semiconductor elements and external leads.
High-purity copper fine wires and aluminum alloy fine wires are used. Since it is difficult to form a normal ball in the air with an aluminum alloy thin wire, it is common to use a wedge bonding method in which the thin wire is ultrasonically thermocompression bonded to the electrode part on the semiconductor element as a bonding method. Productivity is lower than that of. In the case of a gold alloy fine wire or a high-purity copper fine wire, the tip of the fine wire is heated and melted by the heat input of the arc to form a ball by the surface tension, and then it is placed on the electrode of the semiconductor element heated in the range of 150 to 300 ° C. A general method is to ultrasonically press bond this ball portion and then ultrasonically press the connection with the external lead side. However, in the case of a copper thin wire, when a ball is formed in the air, the copper is easily oxidized and the ball shape is unstable, and the copper ball portion is hardened by the oxidation and the silicon chip under the electrode is damaged during bonding. Therefore, it is necessary to blow an inert gas at the time of ball formation to prevent oxidation. As a result, gold can stably form a spherical ball in the atmosphere, has low electric resistance, and has excellent characteristics such as corrosion resistance and spreadability. Therefore, as a bonding wire for a semiconductor element, a gold alloy fine wire is used. Is the mainstream.

In order to use it as a semiconductor element such as a transistor or an IC, it is sealed with resin for the purpose of protecting the Si chip, the bonding wire, and the lead frame at the portion where the Si chip is attached. At the time of resin sealing, the resin heated at a high temperature flows into the mold at a high speed while maintaining a high viscosity, so that the connected wire is deformed to generate a flow. In order to cope with high-density mounting of semiconductor elements and narrow pitches, it is required to elongate gold alloy thin wires, thin wires, or increase loops. Generally, when the wire becomes longer, the wire sags or bends immediately after bonding,
There is a tendency that wire flow during resin sealing becomes large.
It has been pointed out that the conventional gold alloy fine wire has a limit in satisfying the conditions of lengthening and suppressing sagging and bending of the wire at the same time.

In addition, the environmental conditions under which semiconductor elements are used are becoming more and more severe. For example, semiconductor elements used in the vicinity of the engine room of automobiles may be used in high temperature or high humidity environments. When a thin wire is used, the long-term reliability of the joint with the aluminum electrode under high temperature environment is considered to be a problem. More recently, LS
In semiconductors such as I, the density has been increasing, and in some semiconductor devices, the demand for low-priced and highly-reliable wire bonding materials has been increasing in consideration of the increase in the fine dose of gold alloy used. .

[0005]

As a material that can be substituted for the gold alloy fine wire, the material cost is lower than that of gold, the ball portion of a true sphere can be stably formed, and an appropriate loop shape can be obtained. It is necessary that the strength and the reliability of the joint be equal to or superior to the gold alloy thin wire. Palladium has potential as a material that satisfies these various conditions. The use of palladium as a bonding wire material is disclosed in JP-A-59-59.
It is disclosed in Japanese Patent Laid-Open No. 177339, Japanese Patent Laid-Open No. 59-2014454, etc., and is proposed including improvement of characteristics by adding several kinds of alloying elements to palladium. As a result of the inventors of the present invention studying the use of a palladium thin wire for a bonding wire, the palladium thin wire has high strength, the sagging and bending of the wire immediately after bonding, and the wire flow at the time of resin sealing are compared with the gold alloy thin wire. It was confirmed that the amount of the gold alloy was reduced, and the long-term reliability of the joint with the aluminum electrode under high temperature environment was better than that of the gold alloy thin wire.

The elongation rate of gold alloy fine wires generally used in the past is in the range of 2 to 6%, and the gold alloy fine wire adjusted within this range has an appropriate loop shape and good workability. Is. Therefore, in the case of palladium used as a bonding wire, a fine wire having a similar elongation rate is considered, and for example, Japanese Patent Application No. 3-158694.
Even in the palladium thin wire proposed in Japanese Patent Laid-Open No. 10-31100, only the result regarding the case where the elongation rate is 4% is used, and in the conventional literature, the elongation rate is limited to less than 5% for the palladium thin wire. However, it is difficult to obtain a sufficient bonding area when bonding to the inner lead of the lead frame with a palladium thin wire whose elongation rate is adjusted to less than 5%, which is in the above range, and continuous bondability may deteriorate. It is found that there is a high risk of contact with the Si chip due to the low loop height.

[0007]

From the above-mentioned viewpoints, the present inventors have conducted research to develop a palladium thin wire capable of improving continuous bondability and achieving a good loop shape. As a good mechanical property, by making the elongation of the wire 5% or more,
It has been found that the bonding characteristics as a whole can be as good as or better than those of the gold alloy fine wire. That is, the present invention is based on the above findings, and its gist is that the tensile elongation in the linear direction at room temperature is 5
It is a palladium thin wire for semiconductor elements, which is characterized by being -14%. Preferably, the tensile elongation range is 7 to 1
It is a palladium thin wire for a semiconductor element, which is 3%.

[0008]

The structure of the present invention relating to the palladium thin wire will be further described below. Since the fine palladium wire has higher tensile strength and hardness than gold, when the fine palladium wire is bonded (second bonding) to the lead frame, the deformability of the wire is low and it is difficult to obtain a sufficient bonding area. If the load and the ultrasonic output are set high in order to obtain sufficient bonding strength, the ball shape becomes unstable.

A good second bonding is one in which a wire is deformed by ultrasonic pressure bonding using a capillary and bonded on a lead frame, and then the wire is pulled upward to break at the most deformed portion immediately below the capillary. At this time, the length of the wire portion (tail portion) protruding from the capillary tip governs the state of arc discharge during ball formation. When the load or ultrasonic output increases, the wire cross-sectional area in the part deformed by the capillary near the wire joint becomes noticeable, and the length of the tail part varies, resulting in ball shape and size. Becomes unstable. Therefore, as an appropriate range of the joining conditions, it is necessary to ensure that sufficient joining strength is ensured and that spherical balls can be continuously produced.
Good spreadability is good for gold alloy fine wires, so good continuous bonding is easy.However, if palladium thin wires with low deformability are used, the appropriate range becomes narrower.
It may be difficult to obtain good bonding in the second bonding, and in addition, the ball may not be formed and a defect may occur during continuous bonding.

Comparing the mechanical properties of palladium with that of gold, for example, using high-purity palladium, which has a purity of at least 99.95% by weight and is readily available as a commercial product, the final wire diameter is extended to 30 μm. In a palladium wire adjusted to have an elongation of about 4% by continuous annealing after being drawn, the tensile strength is 80% or more higher than that of a general-purpose gold wire having the same elongation, and the Vickers hardness is higher than that of a gold alloy wire. The value is about 60% higher.

The main mechanical properties of the bonding wire include strength and elongation, and it is considered effective to reduce the strength in order to accelerate the deformation of the wire. However, the strength of the wire is susceptible to various conditions in the manufacturing process. For wire production, the wire is thinned in the wire drawing process using a die, intermediate annealing of the thin wire is added during the process to improve the yield of the manufacturing process, and the wire drawing process is continued to the final wire diameter, followed by continuous annealing. Adjust mechanical properties. When the intermediate annealing conditions are different, the strength of the as-drawn thin wire at the final wire diameter changes greatly, and even the strength after continuous annealing is affected, but the elongation rate is affected by the wire drawing and intermediate annealing conditions. It is hard to receive, and this tendency is remarkable in the fine palladium wire. The continuous bondability or bondability of the palladium wire is evaluated by the elongation rate of the wire based on the above reason. Further, by controlling the conditions of continuous annealing, the target elongation rate can be easily achieved.

[0012] In the wire in which the heat treatment temperature of the palladium fine wire is increased to increase the elongation, the mechanical strength is lowered and the deformability of the wire is improved, so that the bonding area of the second bonding is easily increased and It has been found that variations in tail length are also reduced and continuous bondability is improved. Further, in the case of a fine palladium wire, as the elongation increases, the loop height rises and it is possible to easily prevent contact with the silicon chip, and the neck portion immediately above the ball portion stands upright, which reduces damage to the neck portion. It was confirmed that there was a tendency.

The elongation rate of the fine palladium wire according to the present invention in the direction of the wire is set to 5 to 14%. If the elongation rate is less than 5%, the strength is high, and for the above reason, the bonding failure or instability during the second bonding is unstable. This is because ball formation occurs, and if it exceeds 14%, the wire strength becomes low, so that the wire bending becomes remarkable due to the deterioration of the straightness of the loop immediately after bonding, and the like. Further, it has been confirmed that in the palladium fine wire having an elongation rate within the above range, the wire flow during resin sealing is suppressed to be lower than that of the gold alloy fine wire.

In general, a region having a low elongation rate in terms of mechanical properties of a thin wire corresponds to a region in which the strength changes greatly. Therefore, strict control of heat treatment conditions is required to suppress variations in strength. In addition, if the heat treatment temperature is increased to obtain a high elongation and the recrystallization of the wire progresses too much, the wire is easily deformed and the workability in the wire manufacturing process and the bonding process deteriorates. Is concerned. Therefore, the range of the elongation rate of the palladium thin wire at room temperature is more preferably 7 to 13%.

[0015]

EXAMPLES Examples will be described below. Purity is about 9
Using 9.95% by weight of palladium as a raw material, melting and casting in a high-frequency vacuum melting furnace, rolling the ingot, and then performing wire drawing at room temperature, adding an intermediate annealing step for fine wires, if necessary, and The wire drawing process was continued to form a fine palladium wire having a final wire diameter of 30 μm, and the wire was continuously annealed in the atmosphere to adjust the elongation to fall within the range of the present invention. As an example of the manufacturing conditions, in the palladium thin wire of the second embodiment, the intermediate annealing is performed at about 750 ° C. and the continuous annealing temperature is about 650 ° C. when the wire diameter is about 1.5 mm. Table 1 shows the results of examination of the mechanical properties of the wires, ball shape, loop height, wire bending, continuous bondability, etc. of the obtained palladium thin wires.

Regarding the mechanical characteristics of the wire, the wire diameter is 3
Tensile test at normal temperature using 0μm palladium wire
The test was performed 0 times and evaluated by the average value of the breaking strength and the elongation rate. Balls made with a high-speed automatic bonder sprayed with an inert gas and sufficiently suppressed oxidation were observed with a scanning electron microscope, and abnormal ball shapes or marked ball diameter variations were marked with a triangle. The good ones were evaluated by ○. The loop height is determined by measuring the top height of each loop formed and the electrode surface of the semiconductor element with an optical microscope after joining 100 between the electrode on the semiconductor element and the external lead, and measuring the height of both of them. The loop height, which is the difference in height, and its variation were evaluated. For wire bending, observe the wire bonded so that the bonding distance (span) at both ends of the wire is 2.5 mm from the direction almost perpendicular to the semiconductor element. Use the projector to set the distance of the perpendicular to the maximum bend
The average value of 0 measurements was shown. Regarding continuous bondability, by connecting 500 wires using a high-speed automatic bonder, 10 or more bonding defects are marked with X, 1 to 10 are marked with Δ, and no defects are generated. It is written with a circle.

[0017]

[Table 1]

In the observation result of the loop shape, the maximum loop height is 200 μm in the palladium wire of this embodiment.
It is high at m or more, and a sufficient upright portion is obtained in the neck portion directly above the ball. Moreover, the wire bending is suppressed to a low level and good straightness is maintained. On the other hand, in the comparative example whose elongation rate is lower than the range of the present invention, the maximum loop height is 1
A low value of 80 μm or less was observed, and it was recognized that the neck portion was inclined in the direction of the joint portion with the lead frame. Further, when the elongation exceeds the range of the present invention, the wire bending is high and its variation is large. In this example, the sphericity of the ball was high and the size was constant, and good results were obtained in terms of continuous bondability, but in the comparative example in which the elongation rate is lower than the range of the present invention, the ball shape does not become a sphere. In some cases, some of them had a large variation, and some of them had large variations, and defective bonding due to insufficient bonding strength with the lead frame was also recognized.

In this embodiment, only the case of a high-purity palladium thin wire is shown to clarify the effect of the present invention, but the present invention is not limited to this. In the future, it is possible to use palladium fine wires alloyed by adding elements for the purpose of improving the function as a bonding wire, or to use inexpensive low-purity palladium as a material, and use low-purity palladium with a purity of about 99.9% as a material. The same tendency has been confirmed for the effects of the present invention when used or when a palladium alloy thin wire to which several kinds of elements are added is used.

[0020]

As described above, by using the palladium thin wire according to the present invention as a bonding wire, good bonding with the lead frame and improvement of continuous bonding property are achieved, and also when the wire is bent and the resin is sealed. The present invention provides a palladium thin wire which has characteristics equal to or better than those of gold alloy thin wires and which is suitable for high-density mounting of semiconductors, by simultaneously achieving reduction of wire flow and high looping.

Claims (1)

[Claims] 1. A palladium thin wire for a semiconductor device, which has a tensile elongation in the linear direction in the range of 5 to 14%.