Metal bonding method and metal bonded structure

The gap between first and second bonding portions is filled with a disperse solution obtained by dispersing copper micro-particles into a solution for copper oxide elution, so as to elute copper oxide configured as the outermost layer of the first bonding portion and copper oxide configured as the outermost layer of the second bonding portion, and copper oxide formed on the surface of each copper micro-particle. Pressure is applied to the first and second bonding portions using a press machine so as to raise the pressure of the disperse solution. At the same time, heat is applied under a relatively low temperature condition of 200° C. to 300° C., so as to remove the components contained in the disperse solution except for copper, thereby depositing copper. Thus, a first base portion and a second base portion are bonded via a copper bonded portion containing copper derived from the copper micro-particles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a metal bonding method and a metal bonded structure.

2. Description of the Related Art

As an electrically conductive material used to form a wiring layer that is a component of a wiring substrate, or used to form an electrode surface of each electrode of a semiconductor chip or the like, copper is widely employed. As a conventional metal bonding method for electrically connecting a first bonding member to be bonded such as a wiring layer of a wiring substrate or the like to a second bonding member to be bonded such as an element electrode of a semiconductor chip, examples of such a conventional metal bonding methods include: a method in which the bonding faces are solder-bonded via solder; a method in which the bonding faces are bonded to each other by applying pressure while heating the bonding faces at a high temperature; and a method in which the bonding faces are activated by means of ion irradiation or the like in a vacuum so as to bond the bonding faces to each other; and so forth.

RELATED ART DOCUMENTS

Patent Documents

[Patent Document 1]Japanese Patent Application Laid Open No. 2003-100811[Patent Document 2]International Patent Application WO 2004/110925[Patent Document 3]Japanese Patent Application Laid Open No. 2006-334652[Patent Document 4]Japanese Patent Application Laid Open No. 2001-225180

With such a method in which a copper member is bonded to another copper member via solder, a Cu—Sn alloy layer occurs at a bonded interface between each copper layer and the adjacent solder layer. Such a Cu—Sn alloy layer has relatively large electric resistance, and poor ductility, leading to a problem of poor electrical characteristics and/or a problem of poor connection reliability at such a bonded portion. With such a method in which the bonding faces are bonded to each other by applying pressure while heating the bonding faces at a high temperature, in some cases, such an arrangement leads to a problem of damage of the wiring substrate or the semiconductor chip due to the application of heat or the application of pressure. With such a method in which the bonding faces are activated in a vacuum so as to bond the bonding faces to each other, such an arrangement requires large-scale equipment such as a vacuum apparatus, leading to an unavoidable increase in costs.

SUMMARY OF THE INVENTION

An embodiment of the present invention relates to a metal bonding method. The metal bonding method comprises: preparing a disperse solution obtained by dispersing copper micro-particles into a solution for oxide elution into which an oxide with copper oxide as a principal component can be eluted; filling a gap between a first bonding portion formed of a first metal material and a second bonding portion formed of a second metal material with the disperse solution; further reducing a distance between the first bonding portion and the second bonding portion in a state in which the gap between them is filled with the disperse solution; and applying energy to the gap between the first bonding portion and the second bonding portion in the state in which the gap between the first bonding portion and the second bonding portion is reduced, so as to bond the first bonding portion and the second bonding portion.

DETAILED DESCRIPTION OF THE INVENTION

Description will be made below regarding an embodiment of the present invention with reference to the drawings. It should be noted that, in all the drawings, the same components are denoted by the same reference symbols, and redundant description will be omitted as appropriate.

FIGS. 1 and 2are process diagrams each showing a metal bonding method according to an embodiment. Description will be made regarding the metal bonding method according to the embodiment with reference to drawings.

First, as shown inFIG. 1A, a first bonding portion10and a second bonding portion20are prepared. The first bonding portion10includes a first base portion12formed of a metal with copper as a principal component, and a first coating portion14configured to coat the surface of the bonding face side of the first base portion12. Furthermore, the second bonding portion20includes a second base member22formed of a metal with copper as a principal component, and a second coating portion24configured to coat the surface of the bonding face side of the second base portion22. The first coating portion14and the second coating portion24are each formed of an oxide material with copper oxide as a principal component. Here, “with copper as a principal component” and “with copper oxide as a principal component” mean that the material contains copper or copper oxide with a concentration that is greater than 50%.

Provided that the first base portion12and the second base portion22are formed of copper-based metal, the forms of the first base portion12and the second base portion22are not restricted in particular. For example, the first base portion12and the second base portion22may each be configured as a deposited layer formed of copper on a substrate such as a silicon substrate using a sputtering method or the like. Also, the first base portion12and the second base portion22may each be configured as an external terminal portion of a wiring layer formed by patterning a copper sheet such as a copper foil. Specifically, the first coating portion14and the second coating portion24are each configured as a thin film formed of Cu2O, and each having a thickness of 10 nm, for example. The first coating portion14and the second coating portion24may each be configured as an artificial coating film or a natural coating film. With the present embodiment, the first coating portion14and the second coating portion24are each configured as a natural oxide film, which is formed by oxidation of copper in the atmosphere.

Next, as shown inFIG. 1B, the gap between the first coating portion14and the second coating portion24is filled with a disperse solution30obtained by dispersing copper micro-particles32into a solution for oxide elution into which an oxide with copper oxide as a principal component can be eluted.

Specifically, after the disperse solution30is dropped onto or applied to the surface of the second coating portion24of the second bonding portion20, the first bonding portion10is mounted on top of the second bonding portion20onto which the disperse solution30was dropped or to which it was applied, such that the first coating portion14side of the first bonding portion10faces the second coating portion24of the second bonding portion20. Thus, the gap between the first bonding portion10and the second bonding portion20is filled with the disperse solution30.

With the present embodiment, as a solution into which the oxide with copper oxide as a principal component can be eluted, ammonia water is prepared. The concentration of the ammonia water is 0.2% to 10%, for example. Next, 1 g of copper micro-particles is added to the ammonia water such that the copper micro-particles are dispersed into the ammonia water. The copper micro-particles have an average grain size of 5 μm, for example. Examples of a method for dispersing such copper micro-particles into ammonia water include an ultrasonic dispersion method, a stirring method, and so forth. It should be noted that the surface of each copper micro-particle is coated by a natural oxide film, i.e., a copper oxide film.

After the first coating portion14and the second coating portion24are left for a period of time on the order of 1 minute at room temperature, as shown inFIG. 1C, the copper oxide, which forms the first coating portion14, is eluted into the solution30, thereby removing the first coating portion14. In the same way, the copper oxide, which forms the second coating portion24, is eluted into the solution30, thereby removing the second coating portion24. By eluting the copper oxide that forms the first coating portion14and the copper oxide that forms the second coating portion24into the disperse solution30, copper that forms a base portion12and copper that forms a base portion22are respectively exposed as the outermost face (bonding face side exposed face) of the first bonding portion10and the outermost face (bonding face side exposed face) of the second bonding portion20. Furthermore, the copper oxide formed on the surface of each copper micro-particle is eluted into the disperse solution30, whereby copper is exposed on the outermost face of each copper micro-particle32. In the disperse solution30, an ammonia ion that functions as a ligand and a copper ion form a copper complex. With the present embodiment, such a copper complex is considered to be configured as a thermo-degradable tetraamine copper complex ion represented by [Cu(NH3)4]2+. It should be noted that ammonia water is inactive with respect to copper. Thus, copper which is a component of the first base portion12, copper which is a component of the second base portion22, and copper which is a component of the core of each copper microparticle32, do not react with the ammonia water and remains as a component of the respective portions.

Next, as shown inFIG. 2A, pressure is applied to the first bonding portion10and the second bonding portion20by means of a press machine so as to further reduce the distance between the first bonding portion10and the second bonding portion20from the state shown inFIG. 1C. In the pressing, a pressure of 1 MPa is applied, for example.

Next, as shown inFIG. 2B, heating is performed at a relatively low temperature of 200° C. to 300° C. in a state in which pressure is applied to the first bond portion10and the second bond portion20, so as to remove the components contained in the disperse solution30except for copper, thereby depositing or otherwise recrystallizing copper. With the present embodiment, the heating provides evaporation of water. Furthermore, the heating provides thermal decomposition of the tetraamine copper complex ion, thereby providing evaporation of the ammonia component. This encourages solid-phase diffusion, and gradually increases the concentration of copper contained in the solution30. Furthermore, pressing by means of the press machine gradually reduces the distance between the outermost face of the first bonding portion10and the outermost face of the second bonding portion20. With the present embodiment, the energy required to bond the first bonding portion10and the second bonding portion20is applied in the form of heating. The method for applying the energy to the gap between the first bonding portion10and the second bonding portion20is not restricted to heating. Also, a method in which ultrasonic vibration energy is supplied by means of an ultrasonic vibration apparatus may be applied, for example.

Next, as shown inFIG. 2C, after the completion of removal of components contained in the disperse solution30except for the copper component, the outermost face of the first bonding portion10and the outermost face of the second bonding portion20are bonded to each other via a copper bonded portion40formed of the copper derived from the copper oxide and the copper derived from the copper micro-particles32shown inFIG. 2B. The copper bonded portion40exhibits high orientation and high stability. Furthermore, the copper bonded portion40contains copper derived from the copper micro-particles. Thus, the copper bonded portion40has a copper grain size that is smaller than the copper grain size of the first bonding portion10and the copper grain size of the second bonding portion20. The thickness of the copper bonded portion40in the final stage is adjusted according to the quantity of copper micro-particles contained in the disperse solution. For example, the copper bonded portion40is configured to have a thickness of 1 to 10 μm. After the completion of bonding via the copper bonded portion40, the heating is stopped, and the copper bonded portion40is cooled until its temperature reaches on the order of room temperature. It should be noted that the period of time from the start of heating up to the stop of heating is 10 minutes, for example. After the cooling, the pressure application is stopped, whereby the bonding step for the first bonding portion10and the second bonding portion20is completed.

With the metal bonding method described above, such an arrangement is capable of bonding a pair of copper members at a relatively low temperature without involving large-scale equipment such as a vacuum apparatus or the like. Specifically, by elution of the first coating portion14and the second coating portion24into the disperse solution30, copper is exposed on the bonding face of the first bonding portion10and the bonding face of the second bonding portion20. In other words, the bonding face of the first bonding portion10and the bonding face of the second bonding portion20are activated. After the bonding face of the first bonding portion10and the bonding face of the second bonding20are activated, the first bonding portion10and the second bonding portion20are bonded to each other via the copper bonded portion40containing copper derived from the copper micro-particles32. The copper bonded portion40has a thickness that corresponds to the copper derived from the copper micro-particles32. Thus, even if voids occur between the bonding face of the first bonding portion10and the copper bonded portion40or between the bonding face of the second bonding portion20and the copper bonded portion40, such an arrangement prevents such voids from occurring in the form of a line. Thus, such an arrangement provides improved reliability of the connection between the first bonding portion10and the second bonding portion20.

The thickness of the copper bonded portion40can be adjusted by adjusting the copper derived from the copper micro-particles. Thus, such an arrangement allows the bonding distance between the first bonding portion10and the second bonding portion20to be adjusted as necessary in a simple manner.

Even if the bonding face of the first bonding portion10or the bonding face of the second bond portion20has irregularity on the order of the diameter of each copper micro-particle32or otherwise several times the diameter of each copper micro-particle32, these recesses are filled by the copper micro-particles32. Thus, such an arrangement allows the first bonding portion10and the second bonding portion20to be bonded to each other without polishing the bonding faces.

[Solution Used for Metal Bonding]

With the metal bonding method according to the aforementioned embodiment, ammonia water is used as a solution to be used for metal bonding. However, the present invention is not restricted to such an arrangement. Rather, a desired solution may be employed provided that the solution contains a ligand that can form a complex with copper. Examples of such a solution include a carboxylic acid aqueous solution.

Examples of carboxylic acids used to prepare such a carboxylic acid aqueous solution include: monocarboxylic acid such as acetic acid, and the like; dicarboxylic acid such as oxalic acid, malonic acid, succinic acid, glutaric acid, phthalic acid, maleic acid, and the like; and oxycarboxylic acid such as tartaric acid, citric acid, lactic acid, salicylic acid, and the like.

With such an arrangement, such a carboxylic acid aqueous solution preferably contains carboxylic acid which is able to function as a multidentate ligand. With such a carboxylic acid aqueous solution containing carboxylic acid which is able to function as a multidentate ligand, the carboxylic acid and copper form a chelate, thereby generating a copper complex having markedly improved stability. As a result, such an arrangement is capable of reducing the temperature required for the bonding. It should be noted that the fact that tartaric acid forms a chelate is described in “The Iwanami Dictionary of Physics and Chemistry”, 4th ed., p. 593 (Iwanami Shoten). Also, the fact that tartaric acid, oxalic acid, or the like, forms a chelate is described in “Inorganic chemistry”, Vol. 2, p. 666, written by R. B. Heslop, K. Jones, translated by Yoshihiko Saito. Here, chelation represents a reaction in which a multidentate ligand forms a ring, thereby generating a complex having markedly improved stability.

More specifically, in a case in which citric acid is employed as the solution to be used for metal bonding, in the step shown inFIG. 1B, a disperse solution is employed that has been obtained by adding 1 g of copper micro-particles having an average grain size of 5 μm to 20 mL of a citric acid solution (concentration of 10%).

It should be noted that, if such a solution that is used to elute an oxide with copper oxide as a principal component can also be used to elute an oxide of a metal that differs from copper, e.g., aluminum oxide, either one of the first bonding portion10or the second bonding portion20may be formed of aluminum-based metal. Specifically, citric acid can be used to elute aluminum oxide. Thus, by employing citric acid as a solution for elution of an oxide with copper oxide as a principal component, such an arrangement enables aluminum-aluminum bonding and aluminum-copper bonding. In this case, the bonding portions are respectively formed of copper and aluminum.

EXAMPLE

A disperse solution is prepared by dispersing 1 g of copper micro-particles having an average grain size of 5 μm into 20 mL of citric acid solution (concentration of 10%), so as to perform copper-copper bonding using the aforementioned metal bonding method.FIGS. 3A and 3Bshow cross-sectional SEM images obtained by observing the bonded face of a metal bonded structure thus obtained.FIG. 3Bis an enlarged view of a relevant part (region enclosed by the rectangle inFIG. 3A) of the bonded region shown inFIG. 3A. The region A and the region B respectively correspond to the bonding portions formed of copper. The region C corresponds to a bonded region containing copper derived from the copper micro-particles, which also corresponds to the copper bonded portion40shown inFIG. 2C. The region C has a thickness of approximately 3 μm. It has been confirmed that the region C has copper grains having a size that is smaller than the copper grain size in the region A and the copper grain size in the region B. Furthermore, it has been confirmed that the region C having thickness along the layered direction has randomly scattered voids between the adjacent copper grains, and that such voids do not occur in the form of a line. It should be noted that the region C that functions as the copper bonded portion40is defined by the uneven shape of the copper bonding face that functions as the first bonding portion10or the uneven shape of the copper bonding face that functions as the second bonding portion20. Thus, the region C is not necessarily configured as a linear band region. In some cases, the region C is configured in a wavy shape.

The present invention is not restricted to the aforementioned embodiment. Also, various kinds of modifications such as design modifications may be made based on the knowledge of those skilled in this art, which are also encompassed within the technical scope of the present invention.

It should be noted that the invention according to the present embodiment may be specified according to the items described below.

[Item 1] A metal bonding method comprising:

preparing a disperse solution obtained by dispersing copper micro-particles into a solution for oxide elution into which an oxide with copper oxide as a principal component can be eluted;

filling a gap between a first bonding portion formed of a first metal material and a second bonding portion formed of a second metal material with the disperse solution;

further reducing a distance between the first bonding portion and the second bonding portion in a state in which the gap between them is filled with the disperse solution; and

applying energy to the gap between the first bonding portion and the second bonding portion in the state in which the gap between the first bonding portion and the second bonding portion is reduced, so as to bond the first bonding portion and the second bonding portion using the copper micro-particles.

[Item 2] A metal bonding method according to Item 1, wherein, in the preparation of the disperse solution, micro-particles each having a structure in which a core formed of copper is coated by copper oxide are dispersed into the oxide-eluting solution, thereby eluting the copper oxide that coats each core.
[Item 3] A metal bonding method according to Item 1 or 2, further comprising cooling the bonded portion after the first bonding portion and the second bonding portion are bonded to each other.
[Item 4] A metal bonding method according to any one of Items 1 through 3, wherein the oxide-eluting solution is inactive with respect to copper.
[Item 5] A metal bonding method according to any one of Items 1 through 4, wherein the oxide-eluting solution contains a ligand that can form a complex with copper.
[Item 6] A metal bonding method according to Item 5, wherein the complex is thermally degradable.
[Item 7] A metal bonding method according to any one of Items 1 through 6, wherein the solution is configured as ammonia water, or otherwise as a carboxylic acid aqueous solution.
[Item 8] A metal bonding method according to Item 7, wherein carboxylic acid contained in the aforementioned carboxylic acid aqueous solution functions as a multidentate ligand.
[Item 9] A metal bonding method according to Item 8, wherein the multidentate ligand forms at least two coordinate bonds with a single copper ion.
[Item 10] A metal bonded structure comprising:

a first bonding portion formed of a first metal material;

a second bonding portion formed of a second metal material; and

a bonded portion formed as a region between the first bonding portion and the second bonding portion, and having a plurality of voids in a scattered manner.

[Item 11] A metal bonded structure according to Item 10, wherein the bonded portion is formed of copper or otherwise an alloy formed of copper and the first metal material or the second metal material.