JP5152897B2 - Copper bonding wire - Google Patents

Description

  The present invention relates to a copper bonding wire used for connecting an electrode on a semiconductor element and an external electrode.

In general, the diameter of a bonding wire used for connection between an electrode on a semiconductor element and an external electrode is very thin, 15 to 75 μm. In addition, because of chemical stability and ease of handling in the air, a gold wire has been conventionally used. It was used.
However, since gold wire is very expensive because 99% to 99.99% of the weight is gold, there has been a demand from the industry to replace it with inexpensive copper as a material.

In addition, aluminum or aluminum alloy is often used as the electrode material on the semiconductor element, and when the reliability of the joint surface of the intermetallic compound produced by joining a bonding wire to the surface of the aluminum or the like was evaluated, As a result, gold has deteriorated faster than copper as the metal constituting the bonding wire. For this reason, it is desirable to replace the bonding wire with the copper wire in order to improve the reliability of the bonding between the bonding wire and the electrode material. There was also a request from the industry.

The greatest adverse effect of changing the material used as the bonding wire from gold to copper is that the occurrence rate of chip cracks is higher in copper than in gold.

For this reason, in order to correct such an adverse effect, in order to prevent the generation of oxide on the ball surface formed at the tip of the copper wire at the time of ball bonding, it is industrially easily available and the purity is relatively high as 99.99% or higher. For this reason, oxygen-free copper having an oxygen concentration of less than 10 ppm, which lowers the ball hardness, is generally used.
For example, Patent Document 1 discloses that high-purity copper having a purity of 99.999% or higher is obtained by refining high-purity electrolytic copper obtained by electrolytic refining by zone melting after repeating electrolytic refining several times. A method for reducing the occurrence rate of chip cracks by obtaining a material has been proposed.

In this way, the adoption of oxygen-free copper wire and high purity of copper realizes softening of the ball and contributes to a significant reduction in pad damage, and has been used as a copper bonding wire for power ICs and transistors. ing.

  On the other hand, evaluation for application of copper bonding wires to semiconductor packages such as PBGA (Plastic Ball Grid Array package) and QFN (Quad Flat Non lead package), whose production volume has been rapidly increasing in recent years, has started.

However, when PCT (Pressure Cooker Test) was performed on these semiconductor packages, the ball joints were corroded, and the problems of electrical insulation were revealed.
More specifically, a silicon chip on which high-purity aluminum is vapor-deposited is die-bonded to a silver-plated lead frame, and between the aluminum and the silver-plated lead, a conventional 4N purity copper bonding wire is used. When wire bonding was performed and this was left in a constant temperature and humidity environment of 85 ° C. and 85% without resin sealing, the ball joints were corroded by standing for up to 168 hours, and the ball was corroded in the shear strength measurement test. The phenomenon of peeling off on the surface was observed.
This is presumably because any semiconductor package is sealed with resin on one side, and moisture has entered the package from the gap between the lead and the resin.
Japanese Patent Laid-Open No. 60-244054

For this reason, as a result of diligent investigations to correct such adverse effects, the present inventors conducted elemental qualitative analysis by EPMA (Electron Probe X-ray Microanalyzer) on the aluminum pad surface and the back surface of the ball where the ball peeled off due to corrosion. As a result, chlorine was detected in all the samples, and the chlorine present in the copper bonding wire eluted from the copper wire due to the reaction with moisture. As a result, the present invention was adopted.
That is, the problem to be solved by the present invention is to provide a copper bonding wire capable of suppressing the corrosion of the ball joint even under high humidity.

In order to solve the above-mentioned problems, a copper bonding wire according to the present invention uses raw material copper in a carbon crucible using a vacuum melting continuous casting furnace, melts at a high frequency at a vacuum degree of 1 × 10 ⁻⁴ Pa or less, and a molten metal temperature 1150. After degassing at a temperature of 10 ° C. or higher and a holding time of 10 minutes or longer, it is continuously cast to a predetermined diameter by returning to atmospheric pressure with an inert gas, and the amount of chlorine detected by glow discharge mass spectrometry is 1 mass ppm or less After preparing an oxygen-free copper cast wire, the diameter was reduced without acid washing to the final wire diameter, the chlorine content was adjusted to a predetermined elongation by annealing in forming gas, and the chlorine content was 1 mass ppm or less It is a copper bonding wire.

  According to the copper bonding wire of the present invention, since the chlorine concentration contained in the copper wire or attached to the surface is low, corrosion of the ball bonding interface is suppressed even under high humidity. Therefore, even in a single-side resin-sealed package such as PBGA or QFN that easily enters moisture, the reliability of the ball bonding interface is increased, and a copper bonding wire can be applied.

Hereinafter, the copper bonding wire according to the present invention will be described.
[Examples and Comparative Examples]
Table 1 shows an example of the present invention and a comparison with conventional oxygen-free copper wires currently used in the market.
The amount of chlorine and the total amount of impurities were measured by inserting a wire sample having a diameter of 30 μm into an aluminum cap and pressing it into a flat plate by performing a 20-t press, and performing preliminary discharge for about 1 hour in the apparatus before measurement. After removing the sample surface about several microns, the removed surface was measured by glow discharge mass spectrometry. The total amount of impurities was the sum of the detected amounts of each element detected by glow discharge mass spectrometry above the detection limit.

Sample Nos. 1 to 3 were obtained by placing raw material copper in a carbon crucible in a vacuum melting continuous casting furnace, melting at high frequency at a vacuum degree of 1 × 10 ⁻⁴ Pa or less, and a molten metal temperature of 1150 ° C. or higher and a holding time of 10 minutes or longer After degassing, return to atmospheric pressure with inert gas, oxygen-free copper cast wires with different total impurities and less than 1ppm of chlorine by glow discharge mass spectrometry, cast to 8mmφ by continuous casting, without acid cleaning The copper bonding wire was reduced in diameter to 30 μm, annealed in a forming gas of 5% hydrogen and 95% nitrogen, and the elongation was adjusted from 13 to 14%.
The chlorine content of the oxygen-free copper cast wire used in the present invention is also required to be 1 ppm or less. If the oxygen content of the oxygen-free copper casting wire exceeds 1 ppm, then no special treatment such as dechlorination will be applied even if acid cleaning is not performed so as not to increase the chlorine content. This is because, as far as) the amount of chlorine present in the obtained copper bonding wire exceeds 1 ppm.
Therefore, in the copper bonding wire according to the present invention, a treatment for increasing the amount of chlorine such as acid cleaning is not suitable.

  Sample No. 4 is an oxygen-free copper cast wire having a chlorine content of about 3 ppm as cast by glow discharge mass spectrometry, which has been cast by the above continuous casting. A copper bonding wire having a diameter of 30 μm, annealed in a forming gas of 5% hydrogen and 95% nitrogen and adjusted to an elongation of 14%.

  Sample Nos. 5 and 6 are oxygen-free copper cast wires with a chlorine content of about 2 ppm cast by glow discharge mass spectrometry, which have been cast by the above-described continuous casting, and then reduced in diameter after hydrochloric acid cleaning and ultrasonic cleaning with pure water. The copper bonding wire is 30 μm, annealed in a forming gas of 5% hydrogen and 95% nitrogen to adjust the elongation to 14%.

  Sample No. 7 is an oxygen-free copper wire having a chlorine content of about 1 ppm as cast by glow discharge mass spectrometry, which has been cast by the above continuous casting. Thus, the copper bonding wire is 30 μm in diameter and annealed in a forming gas of 5% hydrogen and 95% nitrogen to adjust the elongation to 14%.

As for the corrosion rate, as shown in FIG. 1, a silicon chip 2 on which high-purity aluminum is deposited is die-bonded on a silver-plated lead frame 4, and a copper bonding wire 1 is interposed between the aluminum and the silver-plated lead 3. Wire-bonding is carried out with a resin, and this is left in a constant temperature and humidity environment at 85 ° C. and 85%, and a ball shear strength measurement test is performed after 120 hours and 192 hours, and the interface between the ball and the aluminum electrode is observed with an optical microscope. The number of corrosion balls generated was divided by the number of ball shear measurements to determine the corrosion occurrence rate.
Before wire bonding, the lead frame with a silicon chip was subjected to plasma cleaning in an argon / nitrogen atmosphere for 2 minutes to clean the surface.

  As a result, in the comparative example, during ball shear strength measurement after standing up to 192 hours in a constant temperature and humidity environment of 85 ° C. and 85%, ball peeling due to the occurrence of corrosion of the ball joint was observed. No ball flaking due to corrosion was observed.

  According to the copper bonding wire according to the present invention, the amount of chlorine detected by glow discharge mass spectrometry is 1 ppm by mass or less, and the amount of chlorine eluted from the copper wire by reaction with moisture is low. Since the influence on the corrosion of the steel is suppressed and the problem of electrical insulation of the ball joint in PCT is solved, the industrial utility value is great.

It is a front view of the semiconductor package using the copper bonding wire which concerns on this invention.

Explanation of symbols

1: Copper bonding wire 2: Aluminum deposited silicon chip 3: Silver plated lead 4: Lead frame

Claims (1)

After high-frequency melting at a vacuum degree of 1 × 10 ⁻⁴ Pa or less, degassing at a molten metal temperature of 1150 ° C. or more and a holding time of 10 minutes or more, then returning to atmospheric pressure with an inert gas and continuously casting, the chlorine content is 1 mass ppm with the following oxygen-free copper cast wire, reduced diameter without acid washing until the final wire diameter, the amount of chlorine and adjusting to a predetermined elongation and annealed in a forming gas below 1 ppm by mass copper bonding wire.

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