JP7293674B2 - bonding wire - Google Patents

Description

The present invention relates to bonding wires used for bonding power semiconductors and the like.

2. Description of the Related Art In a module including a power semiconductor chip such as an IGBT through which a large current flows, ultrasonic bonding using an aluminum bonding wire is applied to bonding electrodes, terminals, and the like.
Since aluminum bonding wires have lower hardness and lower yield strength than copper, they are less likely to generate excessive stress on semiconductors and other components, and have the advantage of being easy to handle.

In recent years, in contrast to such aluminum bonding wires, proposals have been made for bonding wires in which a copper wire having a low volume resistivity is used as a core material and the surface is coated with an aluminum layer (see, for example, Patent Documents 1 and 2). . This is an attempt to obtain ultrasonic bondability and low volume resistivity by coating a copper wire with an aluminum layer.
As a specific method for forming an aluminum film, a method has been proposed in which an aluminum layer is formed on a copper ingot by electroplating and then wire drawing is performed (see Patent Document 2).

JP-A-2000-31194 JP 2014-112581 A WO2010/044305

A bonding wire in which a copper core is coated with an aluminum layer as described in Patent Document 1 or the like is expected to be applied to ultrasonic bonding while obtaining the advantages of copper.
By the way, with aluminum, there is a concern that crystal grains in the structure grow due to heat generation when a large current flows, the strength decreases, and the risk of fracture due to crack growth increases.
According to studies by the present inventors, the growth of crystal grains by heating is particularly promoted for aluminum having a plastically worked structure. Therefore, a bonding wire obtained by coating copper with an aluminum layer and drawing the wire also has the same problem, and a new configuration is required to improve reliability.

SUMMARY OF THE INVENTION An object of the present invention is to provide a bonding wire capable of suppressing the growth of crystal grains in the covering aluminum layer while obtaining the advantage of using copper as a core material.

The inventors of the present invention have found that the growth of crystal grains can be suppressed by using electrolytic aluminum that maintains the form of precipitation formed by electroplating, and have arrived at the present invention. That is, the present invention has a core part made of a copper wire, and an electrolytic aluminum layer part maintaining a deposition form on the surface of the core part, and in the precipitation form, the microstructure of the electrolytic aluminum layer part is a bonding wire having a precipitation structure in which crystal grains are small in a portion near the core portion .

According to the present invention, the low volume resistivity of copper can be used as a bonding wire, and recrystallization of the crystal grains of the aluminum layer due to heat can be suppressed, thereby improving bonding reliability. Therefore, the present invention becomes a useful technique for power semiconductor applications that are placed under increasingly harsh environments.

Fig. 2 is SEM photographs of the cross-sectional microstructure of a bonding wire showing an example of the present invention before a heating test, showing (a) low magnification and (b) high magnification. Fig. 2 is SEM photographs of the cross-sectional microstructure of a bonding wire showing an example of the present invention after a heating test, (a) low magnification and (b) high magnification. SEM photographs of the microstructure of the cross section of the bonding wire of the comparative example before the heating test, (a) low magnification and (b) high magnification. SEM photographs of the cross-sectional microstructure of the bonding wire of the comparative example after the heating test, (a) low magnification and (b) high magnification.

One of the important features of the present invention is the application of electrolytic aluminum that maintains the deposited form as the bonding wire. A detailed description will be given below.
The present inventors focused on the problem of coarsening of crystal grains of aluminum and conducted various studies. As a result, the inventors have found that electrolytic aluminum that maintains the deposited form can suppress the coarsening of crystal grains in comparison with general aluminum bonding wires.

This phenomenon is due to the fact that in general aluminum bonding wires to which plastic working such as wire drawing is applied, crystal grains grow in an attempt to release the residual stress generated by the working. On the other hand, it is presumed that electrolytic aluminum in which the precipitated form is maintained has less residual stress and less driving force for crystal grain growth because such plastic working is not applied. In addition, it was found that the electrolytic aluminum that maintained the investigated deposition form contained a predetermined amount of impurities derived from the plating solution, such as S (sulfur). It was presumed that these impurities are concentrated at grain boundaries and suppress the growth of crystal grains.
Based on these findings, the present inventors used a copper wire having a low volume resistivity as a core material, and formed an electrolytic aluminum layer portion maintaining the deposition form on the surface of the core material portion, thereby achieving the present invention. It has reached the configuration.

A method for obtaining the bonding wire of the present invention will be described below.
First, prepare a copper wire as a core material. Copper wires of various compositions and sizes, such as oxygen-free copper and copper alloys, can be used depending on the application and purpose. Copper wires generally have a diameter of about 0.1 to 0.5 mm, and can be used.
The surface of the copper wire may be subjected to a treatment such as cleaning in order to improve characteristics such as ease of electrolytic deposition and adhesion.
An electrolytic aluminum layer is formed by electroplating on the surface of the core member made of copper wire. However, when the electrolytic aluminum layer portion is formed by electroplating, a plating solution made of an aqueous solvent cannot be applied depending on the ionization tendency.
Plating solutions used for this purpose include solution-based plating solutions in which aluminum salts are dissolved in solvents such as dimethylsulfone and toluene, ionic liquid-based plating solutions containing imidazolium salts and aluminum salts, potassium chloride, sodium chloride, and aluminum. A molten salt-based plating solution obtained by melting salt at a high temperature can be used.

According to the study of the present inventor, in addition to the viewpoint of forming an aluminum layer that is difficult to grow grains, from the viewpoint of easiness of washing after formation and uniformity of the formed aluminum layer, a solution system using dimethyl sulfone as a solvent is most preferred. Specifically, a plating solution containing (1) dialkyl sulfone, (2) aluminum halide, and (3) nitrogen-containing compound can be used. The nitrogen-containing compound used here is an ammonium halide, a primary amine hydrohalide, a secondary amine hydrohalide, or a tertiary amine hydrohalide, having the general formula: R ¹ R ² R ³ R ⁴ At least selected from the group consisting of quaternary ammonium salts represented by N·X (R ¹ to R ⁴ are the same or different alkyl groups, X is a counter anion for the quaternary ammonium cation) and nitrogen-containing aromatic compounds One can be used. (See Patent Document 3)

In addition, since this plating solution is hygroscopic and may deteriorate or decompose due to moisture absorption, it is particularly important to control the moisture content in the plating bath or the container for storing the solution. It is desirable that the dew point in the tank be −40° C. or lower.
An electrolytic aluminum layer can be formed by using a copper wire as a cathode and aluminum as an anode and applying a direct current between the electrodes. At this time, the current density may be adjusted according to the film thickness and speed. For example, the current density can generally be on the order of 80-600 mA/cm ² . Further, in order to suppress deterioration of the plating solution, it is desirable to use aluminum with a purity of 97 mass % or higher for the anode.

The thickness of the electrolytic aluminum layer to be formed is preferably 2 μm to 30 μm, more preferably 5 to 20 μm. If the thickness is less than 2 μm, the aluminum layer is too thin, and there are cases where sufficient bonding strength cannot be obtained, or ultrasonic bonding is difficult. On the other hand, if the thickness is 30 μm or more, it takes time to form the wire by electroplating, and in addition, when the wire is bent, the electrolytic aluminum layer is too thick, cracks may occur, and the wire may be difficult to handle.

By the above method, the electrolytic aluminum layer formed on the surface of the core material of the copper wire can contain S=0.01 to 0.15 mass %. When the content is within this range, embrittlement does not occur, and it is also effective in suppressing grain formation.

A copper wire (manufactured by Nilaco Corporation) with a diameter of 0.25 mm was prepared as a core material. The surface of the copper wire was degreased and washed and then dried to a clean state. Next, a plating solution containing dimethylsulfone, aluminum chloride and ammonium chloride was prepared. The plating solution used was prepared in a molar ratio of dimethylsulfone:aluminum chloride:ammonium chloride=10:3.8:0.2. The prepared copper wire was used as a cathode, aluminum with a purity of 99 mass % was used as an anode, and electricity was passed between the cathode and the anode for 10 minutes. The energization was carried out under the condition that the liquid temperature was kept at 95 to 100° C. with a current density of 100 mA/cm ² between both electrodes. As a result, an electrolytic aluminum layer portion was formed on the surface of the copper wire to obtain the bonding wire of the present invention. The obtained bonding wire was washed with running water and dried with an air blow.

The cross-sectional microstructure of the obtained bonding wire was observed by SEM (scanning electron microscope, JSM-7800F, manufactured by JEOL Ltd.). Observation conditions were an acceleration voltage of 5 kV and a working distance of 4 mm. The results are shown in FIG. As shown in FIG. 1, in the bonding wire of the present invention, an electrolytic aluminum layer portion 2 is confirmed on the surface of a core portion 1 made of copper wire. The microstructure of the electrolytic aluminum layer portion 2 is a typical precipitation structure in which crystal grains are small in the portion near the core portion, and the precipitation form is maintained. It turned out that it was uniformly formed on the surface of the core material of the copper wire.
Also, the components of the obtained bonding wires were analyzed. EDX (energy dispersive X-ray spectrometer, JED-2300 SD30, manufactured by JEOL Ltd.) attached to the SEM was used for the measurement. The measurement conditions were an acceleration voltage of 5 kV, a working distance of 10 mm, the number of scans of 50, and a measurement area of 10 μm×100 μm. As a result, S=0.1 mass% and Cl=0.1 mass% (remainder=Al) were detected.

Next, in order to evaluate the thermal grain growth of the bonding wire of the present invention, a heating and holding test was conducted. A constant temperature dryer (manufactured by Tokyo Rikagaku Co., Ltd., VOS-301SD) was used for the test, and the heating and holding conditions were 240° C. for 30 minutes in an air atmosphere. When the microstructure of the bonding wire after the test was observed in the same manner as above, as shown in FIG. 2, there was no noticeable change in the microstructure compared to FIG. 1 before the test, and the electrolytic aluminum layer maintained the deposition form. Part 2 was confirmed. From this result, it is believed that the electrolytic aluminum layer portion of the present invention, which maintains the deposition form, has little residual stress due to the fact that plastic working is not applied, and the growth of crystal grains in the electrolytic aluminum layer portion is suppressed. Estimated. When the composition of the electrolytic aluminum layer was analyzed by EDX in the same manner as above, S = 0.1 mass% and Cl = 0.1 mass% (remainder = Al) were detected, and it was confirmed that there was no change before and after the heating and holding test. bottom. From this result, it was presumed that the presence of a predetermined amount of impurities in the electrolytic aluminum layer also affected the suppression of grain growth.

Next, for comparison, a similar heating and holding test was performed on a commercially available aluminum wire with a purity of 99% and a diameter of 0.45 mm (manufactured by The Nilaco Corporation), and the cross-sectional microstructures before and after the test were compared. The results are shown in FIGS. 3 and 4. FIG. The crystal grains of the aluminum wire cross section 3 shown in FIG. 4 after the heating test are larger than the crystal grains of the aluminum wire cross section 3 shown in FIG. 3 before the heating test. As is clear from this result, the aluminum wire was coarsened by recrystallization of crystal grains after the heating test. As a result, it is expected that the physical properties of the wire will change significantly, making it easier for the wire to soften or break. As is clear from comparison with FIGS. 1 and 2, the bonding wire of the present invention is presumed to have high bonding reliability because the growth of crystal grains in the electrolytic aluminum layer is suppressed.

1 core part 2 electrolytic aluminum layer part 3 aluminum wire cross section

Claims (1)

Having a core part made of a copper wire and an electrolytic aluminum layer part maintaining a deposition form on the surface of the core part ,
A bonding wire characterized in that, in the deposition mode, the microstructure of the electrolytic aluminum layer portion is a deposition structure in which crystal grains are smaller in a portion near the core portion.

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