JP6617049B2 - Conductive paste and semiconductor device - Google Patents

Description

  The present invention relates to a conductive paste and a semiconductor device. More specifically, the present invention relates to a conductive paste used for forming a bonding layer, a fine wiring, a conductive bump, and the like of a semiconductor device, and using such a conductive paste. The present invention relates to a semiconductor device including the formed bonding layer.

  Conventionally, Sn—Pb solder has been used for bonding (die bonding) a semiconductor element and a substrate such as a lead frame. However, in recent years, a lead-free bonding material has been demanded from the viewpoint of environmental protection. In addition, with the miniaturization and high density of semiconductor devices, fine wiring formation is required, and a wiring forming material corresponding to this is required. Furthermore, in order to reduce the load on the semiconductor element at the time of die bonding or wiring formation, there is a demand for a material that can be bonded or formed at a low temperature.

In order to solve this problem, various proposals have heretofore been made.
For example, in Patent Document 1, copper fine particles having an average primary particle size of 120 nm and copper fine particles having an average primary particle size of 7 μm are mixed at a mass ratio of 90:10, and glycerol is added to this mixture with a copper fine particle concentration. A conductive paste added so as to be 80% by mass is disclosed.
This document describes that when conductive bumps are formed using such a conductive paste, the generation of coarse voids and cracks is suppressed.

In Patent Document 2, Cu nanoparticles having an average particle diameter of 200 nm and Ni nanoparticles having an average particle diameter of 200 nm are mixed so that the Ni nanoparticles are 2 to 10% by mass. A bonding material paste in which decanol and terpione are added to a mixed powder is disclosed.
This document describes that when such a paste is used, a bonding layer having high bonding strength can be formed at a low temperature.

  The smaller the particle size, the higher the activity of the metal particles, and when the particle size becomes nano-sized, it becomes possible to sinter at a temperature much lower than its melting point. However, the smaller the particle size, the more easily the metal particles are oxidized.

For example, in the case of Cu particles, the surface is usually covered with a natural oxide film (Cu ₂ O). Cu ₂ O is difficult to remove by chemical reaction. Therefore, as described in Patent Document 1, in the combination of Cu nanoparticles having an average particle diameter of 120 nm and coarse Cu particles having an average particle diameter of 7 μm, the inter-particle sintering is inhibited by the natural oxide film, and the bonding layer There is a problem that the strength decreases.

  On the other hand, as described in Patent Document 2, when Cu nanoparticles and Ni nanoparticles are combined, the Cu nanoparticles are sintered because the Ni nanoparticles reduce the natural oxide film on the surface of the Cu nanoparticles. It becomes easy. However, when forming a bonding layer using a paste containing only such nanoparticles, if the sintering proceeds excessively, the shrinkage of the bonding layer increases and voids and cracks are likely to occur in the bonding layer. . In order to avoid this, if the sintering is appropriately performed, the shrinkage can be suppressed, but the density of the bonding layer is lowered. Furthermore, since nanoparticles are generally more expensive than micrometer-sized particles, pastes containing only nanoparticles are expensive.

International Publication No. WO2011 / 114747 JP 2014-175372 A

  The problem to be solved by the present invention includes a conductive paste capable of forming a bonding layer having high bonding strength and a low porosity at low cost, and a bonding layer formed using the conductive paste. It is to provide a semiconductor device.

In order to solve the above problems, the gist of the conductive paste according to the present invention is as follows.
(1) The conductive paste is
Cu-based nanoparticles whose surface is coated with organic molecules and whose average particle diameter is less than 500 nm,
Ni-based coarse particles having an average particle diameter of 1 μm or more;
An organic solvent for dispersing the Cu-based nanoparticles and the Ni-based coarse particles .
(2) The Cu-based nanoparticles include 95 at% or more of Cu, and the Ni-based coarse particles include 95 at% or more of Ni.
(3) When the mass of the Cu-based nanoparticles is W _n and the mass of the Ni-based coarse particles is W _c , the mass ratio (R) of the Ni-based coarse particles (= W _c × 100 / (W _n + W _c )) is 5 mass% or more and less than 20 mass%.

The gist of the semiconductor device according to the present invention is as follows.
(1) The semiconductor device includes:
A semiconductor element;
A substrate,
A bonding layer disposed between the semiconductor element and the substrate;
(2) The bonding layer is obtained by interposing the conductive paste according to the present invention between the semiconductor element and the substrate and firing in a reducing atmosphere,
A matrix containing constituent elements of the Cu-based nanoparticles ;
And Ni-based coarse particles dispersed in the matrix.
The porosity of the bonding layer is preferably 20% or less.

The surface of the Cu-based particles is usually covered with a natural oxide film made of Cu ₂ O. Since Cu ₂ O is not easily reduced, the natural oxide film cannot be removed even if only Cu-based particles having such a natural oxide film are baked in a reducing atmosphere.
On the other hand, the surface of the Ni-based particles is usually covered with a natural oxide film made of NiO. NiO has a function of dissolving Cu ₂ O and oxidizing Cu ⁺ to Cu ²⁺ . Since Cu ²⁺ is more easily reduced than Cu ⁺ , it is easily reduced to Cu by firing in a reducing atmosphere.

  Since the conductive paste according to the present invention includes both Cu-based particles and Ni-based particles, the removal of the natural oxide film and the sintering between the particles proceed appropriately by firing in a reducing atmosphere, and high-strength bonding. A layer is obtained. Moreover, since either one of the Cu-based particles or Ni-based particles is a nanoparticle and the other is a coarse particle, a bonding layer having a low porosity can be obtained. Furthermore, since low-cost coarse particles are used as part of the metal particles, the cost of the conductive paste and the bonding layer formed using the conductive paste can be reduced.

1 is a schematic diagram of a semiconductor device according to an embodiment of the present invention. It is a schematic diagram of the interface formation process of Cu nanoparticle and Ni coarse particle. It is a figure which shows the influence of the mass ratio of the metal coarse particle which acts on the porosity and joining strength.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Conductive paste]
The conductive paste according to the present invention has the following configuration.
(1) The conductive paste is
Metal nanoparticles whose surface is coated with organic molecules and having an average particle diameter of less than 500 nm,
Metal coarse particles having an average particle diameter of 1 μm or more;
An organic solvent for dispersing the metal nanoparticles and the metal coarse particles.
(2) The conductive paste is
(A) using Cu-based nanoparticles containing Cu of more than 50 at% as the metal nanoparticles, and using Ni-based coarse particles containing Ni of more than 50 at% as the metal coarse particles, or
(B) Ni-based nanoparticles containing Ni of over 50 at% are used as the metal nanoparticles, and Cu-based coarse particles containing Cu of over 50 at% are used as the metal coarse particles.

[1.1. Metal nanoparticles]
[1.1.1. Organic molecule]
The metal nanoparticles are covered with organic molecules on the surface. The organic molecule is for suppressing aggregation of the metal nanoparticles in the conductive paste. The organic molecule is preferably one that can be thermally decomposed at a low temperature (specifically, 400 ° C. or lower). The organic molecule is not particularly limited as long as it exhibits such a function.

Examples of organic molecules include fatty acids and aliphatic amines.
Examples of fatty acids include
(A) saturated fatty acids such as octanoic acid, decanoic acid, dodecanoic acid, myristic acid, palmitic acid, stearic acid,
(B) Unsaturated fatty acids such as oleic acid.

As the aliphatic amine, for example,
(A) aliphatic amines such as octylamine, decylamine, dodecylamine, myristylamine, palmitylamine, stearylamine,
(B) Unsaturated aliphatic amines such as oleylamine.

  Metal nanoparticles can be produced by reducing a metal salt in the presence of organic molecules. When fatty acids or aliphatic amines are used as organic molecules during the synthesis of metal nanoparticles, the particle diameter of the metal nanoparticles can be adjusted by changing the carbon number of the hydrocarbon chain contained therein.

[1.1.2. Average particle size]
The average particle diameter of the metal nanoparticles affects the strength and porosity of the bonding layer formed from the conductive paste.
Here, the “average particle diameter of the metal nanoparticles” is the average value of the diameters of 200 metal nanoparticles randomly extracted by microscopic observation (for example, TEM observation) (diameter of the minimum circumscribed circle of the particles). Say.

  In general, as the average particle size of the metal nanoparticles becomes smaller, the sinterability is improved, and a bonding layer having high strength and low porosity can be obtained. For this purpose, the average particle size of the metal nanoparticles needs to be less than 500 nm. The average particle size is preferably less than 400 nm, more preferably less than 300 nm, and even more preferably less than 200 nm.

  The average particle diameter of the metal nanoparticles is preferably as small as possible. However, in general, as the average particle size of the metal nanoparticles becomes smaller, the manufacturability and handleability decrease. For example, if the metal nanoparticles become too small, the metal nanoparticles may oxidize in the paste and sintering may not proceed, and the natural oxide film cannot be sufficiently removed by heat treatment during bonding, resulting in bonding strength and conductivity. , Thermal conductivity, etc. will decrease. Therefore, the average particle diameter of the metal nanoparticles is preferably 10 nm or more. The average particle diameter is preferably 30 nm or more, and more preferably 50 nm or more.

[1.1.3. composition]
In the present invention, the metal nanoparticles are composed of Cu-based nanoparticles or Ni-based nanoparticles.
Here, the “Cu-based nanoparticle” refers to a particle made of a metal or alloy containing 50 at% or more of Cu and having an average particle diameter of less than 500 nm.
The “Ni-based nanoparticle” refers to a particle made of a metal or alloy containing Ni of more than 50 at% and having an average particle diameter of less than 500 nm.

Generally, as the amount of Cu in the Cu-based nanoparticles increases, the thermal conductivity and electrical conductivity improve. The amount of Cu is preferably 70 at% or more, more preferably 90 at% or more, and further preferably 95 at% or more. The Cu-based nanoparticles may be substantially composed only of Cu (pure Cu).
When the Cu-based nanoparticles are made of a Cu alloy, examples of the alloy element include Ag, Ni, Sn, Co, and Zn.

Similarly, as the amount of Ni in the Ni-based nanoparticle increases, the reduction of Cu is promoted, and the bonding layer is more easily sintered. The amount of Ni is preferably 70 at% or more, more preferably 90 at% or more, and further preferably 95 at% or more. The Ni-based nanoparticles may be substantially composed of Ni (pure Ni).
When the Ni-based nanoparticles are made of a Ni alloy, examples of the alloy element include Cu, Ag, and Sn.

[1.2. Coarse metal particles]
[1.2.1. Average particle size]
The average particle diameter of the coarse metal particles affects the strength and porosity of the bonding layer formed from the conductive paste.
Here, “average particle diameter of coarse metal particles” means the diameter of 200 coarse metal particles randomly extracted by microscopic observation (for example, SEM observation, TEM observation, etc.) ) Mean value.

  The bonding layer is obtained by printing (applying) the conductive paste on the substrate surface, and firing at a temperature at which the coarse metal particles are not substantially sintered (shrinkage, grain growth) and the metal nanoparticles are sintered. It is formed. In general, the smaller the metal coarse particles, the more voids between the metal coarse particles in the printed layer of the conductive paste. Therefore, it becomes difficult to fill the voids between the coarse metal particles with the metal nanoparticles during firing, and the porosity of the bonding layer increases. In order to obtain a bonding layer having a low porosity, the average particle diameter of the coarse metal particles needs to be 1 μm or more. The average particle diameter is preferably 2 μm or more, more preferably 3 μm or more.

  On the other hand, if the average particle diameter of the coarse metal particles becomes too large, it becomes difficult to print the conductive paste smoothly. Therefore, the average particle diameter of the coarse metal particles is preferably less than 30 μm. The average particle size is preferably less than 20 μm, more preferably less than 10 μm.

[1.2.2. composition]
In the present invention, when the metal nanoparticles are made of Cu-based nanoparticles, the metal coarse particles are made of Ni-based coarse particles. On the other hand, when the metal nanoparticles are made of Ni-based nanoparticles, the metal coarse particles are made of Cu-based coarse particles.
Here, the “Cu-based coarse particles” are particles made of a metal or an alloy containing Cu of more than 50 at% and having an average particle diameter of 1 μm or more.
The “Ni-based coarse particles” are particles made of a metal or alloy containing Ni of more than 50 at% and having an average particle diameter of 1 μm or more.

Generally, as the amount of Cu in the Cu-based coarse particles increases, thermal conductivity and electrical conductivity are improved. The amount of Cu is preferably 70 at% or more, more preferably 90 at% or more, and further preferably 95 at% or more. The Cu-based coarse particles may be substantially composed only of Cu (pure Cu).
When the Cu-based coarse particles are made of a Cu alloy, examples of the alloy element include Ag, Ni, Sn, Co, and Zn.

Similarly, as the amount of Ni in the Ni-based coarse particles increases, the reduction of Cu is promoted, and the bonding layer is more easily sintered. The amount of Ni is preferably 70 at% or more, more preferably 90 at% or more, and further preferably 95 at% or more. The Ni-based coarse particles may be substantially composed only of Ni (pure Ni).
When the Ni-based coarse particles are made of a Ni alloy, examples of the alloy element include Cu, Ag, and Sn.

[1.2.3. Mass ratio of coarse metal particles]
The mass ratio of the coarse metal particles affects the strength and porosity of the bonding layer formed from the conductive paste.
Here, the “ mass ratio (R) of coarse metal particles” refers to a value represented by the following formula (1).
R (%) = W _c × 100 / (W _n + W _c ) (1)
However,
W _n is the mass of the metal nanoparticles ,
W _c is the mass of the metal coarse particles .

In general, as the mass ratio of the coarse metal particles increases, a bonding layer having a lower porosity can be obtained. Moreover, the shrinkage | contraction at the time of baking is suppressed and the production | generation of the void and a crack in a joining layer can be suppressed. In order to obtain such an effect, the mass ratio of the coarse metal particles is preferably 5 mass % or more. The mass ratio is preferably 7 mass % or more , and more preferably 10 mass% or more .
On the other hand, when the mass ratio of the metal coarse particles becomes excessive, the voids between the metal coarse particles cannot be filled with the metal nanoparticles, and the porosity of the bonding layer increases. This also impairs the electrical and thermal conductivity characteristics of the bonding layer. Therefore, the mass ratio of the coarse metal particles is preferably 30 mass % or less . The mass ratio is preferably 20 mass % or less .

[1.3. Organic solvent]
[1.3.1. composition]
The organic solvent is for dispersing the metal nanoparticles and the metal coarse particles. The organic solvent is preferably one that can be volatilized or thermally decomposed at a low temperature (specifically, 400 ° C. or lower). The organic solvent is not particularly limited as long as it exhibits such a function.

As an organic solvent, for example,
(A) alkanes such as tetradecane, hexadecane, dodecane,
(B) aliphatic alcohols such as 1-butanol, decanol, octanol, hexanol, isopropyl alcohol,
(C) Cyclic alcohols such as terpionol and cyclohexanol.
Any one of these may be used, or two or more thereof may be used.

[1.3.2. Amount of organic solvent]
The amount of the organic solvent affects the handleability of the paste.
Here, the “amount (%) of organic solvent” refers to the ratio (= W _S × 100 / W _T ) of the mass (W _S ) of the organic solvent to the total mass (W _T ) of the conductive paste.

  Generally, when the amount of the organic solvent is too small, the viscosity of the paste becomes excessively large. On the other hand, when the amount of the organic solvent is too large, the viscosity of the paste becomes excessively small. The optimum amount of the organic solvent varies depending on the average particle diameter and particle shape of the metal particles, the type of the organic solvent, the presence or absence of other additives, and the like. The amount of the organic solvent is usually 10 to 50%.

[1.4. Other additives]
The conductive paste may contain components other than the metal nanoparticles, the metal coarse particles, and the organic solvent as necessary.
As other ingredients,
(A) an organic binder for suppressing aggregation of metal nanoparticles in the conductive paste, or for adjusting the viscosity of the conductive paste;
(B) a surfactant for improving the dispersibility of the metal nanoparticles and the metal coarse particles in the conductive paste;
and so on.

[2. Bonding layer]
When the conductive paste according to the present invention is interposed between a semiconductor element and a substrate and fired in a reducing atmosphere, a bonding layer is obtained. At this time, if necessary, the paste layer between the semiconductor element and the substrate may be fired while being pressed.

[2.1. Firing conditions]
In order to obtain a bonding layer with high strength and low porosity, it is necessary to advance the sintering of the metal nanoparticles while reducing the natural oxide film on the surface of the metal particles. For this purpose, the firing needs to be performed in a reducing atmosphere. Examples of the reducing atmosphere include an H ₂ atmosphere, a formic acid atmosphere, and an atmosphere obtained by diluting them with an inert gas such as Ar or N ₂ .

The firing needs to be performed at a temperature at which the sintering of the metal nanoparticles proceeds without causing the sintering (shrinkage and grain growth) of the coarse metal particles. Moreover, the firing temperature is preferably a temperature at which the heat load on the semiconductor element is small. The optimum firing temperature varies depending on the composition of the metal nanoparticles and the coarse metal particles, the average particle diameter of the metal nanoparticles, and the like.
For example, in the case of a combination of Cu-based nanoparticles composed of pure Cu and Ni-based coarse particles composed of pure Ni, the firing temperature is preferably 300 ° C to 400 ° C.

[2.2. Bonding layer structure]
If the firing conditions are inappropriate, the metal nanoparticles are not sufficiently sintered, resulting in a structure in which bridges made of metal nanoparticles are formed around the coarse metal particles. Therefore, a bonding layer having a low porosity cannot be obtained. The increase in the porosity causes a decrease in the durability of the bonding layer.
In contrast, when the firing conditions are optimized, the sintering of the metal nanoparticles proceeds sufficiently, and a continuous layer (matrix) containing the constituent elements of the metal nanoparticles is formed around the coarse metal particles. At this time, plastic deformation of the coarse metal particles may occur due to the pressurization, but sintering between the coarse metal particles does not occur, so the bonding layer shrinks substantially (the spacing between the coarse metal particles due to element diffusion). It does not happen. As a result, a bonding layer including a matrix containing the constituent elements of the metal nanoparticles and coarse metal particles dispersed in the matrix is obtained.

[2.3. Porosity]
When the firing conditions are optimized, a bonding layer having a low porosity can be obtained. Specifically, the porosity becomes 20% or less or 15% or less by optimizing the firing conditions.

[3. Semiconductor device]
A semiconductor device according to the present invention includes:
A semiconductor element;
A substrate,
A bonding layer disposed between the semiconductor element and the substrate;

[3.1. Semiconductor element]
In the present invention, the semiconductor element is not particularly limited, and the present invention can be applied to any semiconductor element. Examples of semiconductor elements include power elements, LSIs, resistors, capacitors, and the like.

[3.2. substrate]
In the present invention, the substrate to which the semiconductor element is bonded is not particularly limited, and the present invention can be applied to any substrate.
As a substrate, for example,
(A) "Lead frame" such as copper alloy lead frame,
(B) “Ceramic substrate on which an electrode is formed” such as a DBC (Direct Bond Copper: registered trademark) substrate, an active metal bonding (AMC) substrate,
(C) “Mounting substrates” such as alumina substrates with electrodes, low temperature co-fired ceramics (LTCC) substrates, glass epoxy substrates,
(D) a copper plate for mounting a plurality of semiconductor elements having different thicknesses on the same lead frame and providing a space necessary for wire bonding such as a signal line;
and so on.

[3.3. Bonding layer]
The bonding layer is obtained by interposing the conductive paste according to the present invention between the semiconductor element and the substrate and firing in a reducing atmosphere. Since the details of the bonding layer are as described above, the description thereof is omitted.

[3.4. Adhesion layer]
[3.4.1. composition]
An adhesion layer may be further formed on the surface of the semiconductor element on the bonding layer side and / or on the surface of the substrate on the bonding layer side.
Here, the “adhesion layer” refers to a layer having a function of improving the bonding strength between the semiconductor element or the substrate and the bonding layer.

  The adhesion layer is preferably a layer containing any one or more elements selected from the group consisting of Ni, Ag, and Co. All of these elements have an action of removing the natural oxide film on the surface of the Cu-based particles. Therefore, when such an adhesion layer is interposed at the interface between the bonding layer and the semiconductor element or the substrate, the bonding layer and the semiconductor element or the substrate can be bonded more firmly.

[3.4.2. thickness]
The thickness of the adhesion layer affects the bonding strength. If the thickness of the adhesive layer is too thin, high bonding strength cannot be obtained. Therefore, the thickness of the adhesion layer is preferably 1 nm or more.
On the other hand, increasing the thickness of the adhesive layer more than necessary not only has no profit, but also increases the production cost or increases the electrical resistance of the adhesive layer. Therefore, the thickness of the adhesion layer is preferably 10 μm or less. The thickness of the adhesion layer is preferably 200 nm or less.

[3.4.3. Method for forming adhesion layer]
The method for forming the adhesion layer is not particularly limited. As a method for forming the adhesion layer, for example,
(A) a method of forming an adhesion layer on the surface of a semiconductor element or a substrate using a sputtering method, a vacuum deposition method, or the like;
(B) A method of forming an adhesion layer on the surface of a semiconductor element or substrate using an electroplating method or an electroless plating method (plating method),
(C) Using an inkjet method, a spin coating method, a dip method, a screen printing method, or the like, a paste or ink containing a material for the adhesion layer is applied to the surface of a semiconductor element or substrate, and this is applied to an inert gas atmosphere or reducing property. A method of heating in a gas atmosphere (coating method),
and so on.

[3.5. Concrete example]
FIG. 1 is a schematic diagram of a semiconductor device according to an embodiment of the present invention. In FIG. 1, the semiconductor device 10 includes a semiconductor element 12, lead frames 14a and 14b, a copper plate 16, and bonding layers 18a to 18c. The copper plate 16 is for providing a space necessary for mounting a plurality of semiconductor elements 12 having different thicknesses on the same lead frames 14a and 14b and performing wire bonding such as signal lines. The semiconductor device 10 is normally used in a state where these components are sealed with a resin (not shown).

The bonding layer 18 a is inserted between the lower lead frame 14 a and the semiconductor element 12. An adhesion layer may be formed on each of the upper surface of the lead frame 14 a and the lower surface of the semiconductor element 12.
In addition, the bonding layer 18 b is inserted between the semiconductor element 14 a and the copper plate 16. An adhesion layer may be formed on each of the upper surface of the semiconductor element 12 and the lower surface of the copper plate 16.
Further, the bonding layer 18c is inserted between the copper plate 16 and the upper lead frame 14b. An adhesion layer may be formed on each of the upper surface of the copper plate 16 and the lower surface of the lead frame 14b.

The semiconductor device 10 shown in FIG.
(A) The conductive paste according to the present invention is applied to the lower surface of the semiconductor element 12 and both surfaces of the copper plate 16, respectively.
(B) The lead frame 14a, the semiconductor element 12, the copper plate 16, and the lead frame 14b are stacked in this order to form a laminate.
(C) If necessary, it can be produced by heating the laminate in a reducing atmosphere while applying pressure in the lamination direction.

[4. Action]
Since Cu nanoparticles are excellent in sinterability and heat resistance and are low in cost, use of Cu nanoparticles as a bonding material for semiconductor elements has been studied. However, when bonding is performed using only Cu nanoparticles, voids and cracks may be generated in the bonding layer due to sintering shrinkage and the like, which may reduce the environmental resistance against oxidation and the like. As a countermeasure, a method of mixing Cu nanoparticles and Cu coarse particles has been proposed as described in Patent Document 1. However, this method has a drawback in that the natural oxide film present on the surface of the coarse particles hinders sintering in the bonding layer, and the strength of the bonding layer is greatly reduced.

  In contrast, when a combination of Cu-based nanoparticles and Ni-based coarse particles, or a combination of Ni-based nanoparticles and Cu-based coarse particles is used, high strength and low voids are generated without causing voids or cracks in the bonding layer. Rate of bonding layer is obtained. This is considered to be due to the following reason.

In FIG. 2, the schematic diagram of the interface formation process of Cu nanoparticle and Ni coarse particle is shown. The surface of the Cu-based nanoparticles is usually covered with a natural oxide film made of Cu ₂ O (see FIG. 2 (a)). Since Cu ₂ O is not easily reduced, the natural oxide film cannot be removed even if only Cu nanoparticles provided with such a natural oxide film are baked in a reducing atmosphere.
On the other hand, the surface of the Ni particles is usually covered with a natural oxide film made of NiO (see FIG. 2A). NiO has a function of dissolving Cu ₂ O and oxidizing Cu ⁺ to Cu ²⁺ . Since Cu ²⁺ is more easily reduced than Cu ⁺ , it is easily reduced to Cu by firing in a reducing atmosphere.

Therefore, when heating is performed in a reducing atmosphere while the Cu nanoparticles and the Ni coarse particles are in contact, a neck is formed at the interface between the Cu nanoparticles and the Ni coarse particles, and at the same time, the Cu in Cu ₂ O becomes a NiO layer. The metal Cu begins to precipitate in the NiO layer (see FIG. 2B).
If heating is continued, Cu surface diffusion and reduction further proceed. As a result, the diameter of the neck portion increases, and at the same time, the natural oxide film on the surface of the Cu nanoparticles is completely removed, and the NiO layer gradually changes to a Cu—Ni alloy layer (FIG. 2C).

When the heating is further continued, the surface diffusion of Cu further proceeds, and the surface of the Ni coarse particles is covered with the Cu layer or the Cu—Ni alloy layer (FIG. 2D). In the actual bonding layer, there are a large number of Cu nanoparticles around the Ni coarse particles, and as the surface diffusion proceeds, the Cu nanoparticles change into a continuous matrix surrounding the Ni coarse particles. .
When Cu-based nanoparticles and Ni-based nanoparticles are made of Cu alloy and Ni alloy, respectively, and in the case of a combination of Ni-based nanoparticles and Cu-based coarse particles, sintering proceeds by the same process. To do.

  Since the conductive paste according to the present invention includes both Cu-based particles and Ni-based particles, the removal of the natural oxide film and the sintering between the particles proceed appropriately by firing in a reducing atmosphere, and high-strength bonding. A layer is obtained. Moreover, since either one of the Cu-based particles or Ni-based particles is a nanoparticle and the other is a coarse particle, a bonding layer having a low porosity can be obtained. Furthermore, since low-cost coarse particles are used as part of the metal particles, the cost of the conductive paste and the bonding layer formed using the conductive paste can be reduced.

(Examples 1-2, Reference Examples 3-4, Comparative Examples 1-5)
[1. Preparation of sample]
[1.1. Examples 1-2, Reference Examples 3-4]
As the metal nanoparticles, Cu nanoparticles modified with fatty acid and amine and having an average particle diameter of 140 nm were used. Further, Ni particles (manufactured by High Purity Chemical Research Laboratory) having an average particle diameter of 2 to 3 μm were used as the coarse metal particles. Predetermined amounts of Ni particles were mixed with Cu nanoparticles, and paste was made using α-terpioneer and 1-decanol. The mass ratio of the Ni particles was 5 mass % (Example 1), 10 mass% (Example 2) , 20 mass% (Reference Example 3) , or 30 mass% (Reference Example 4) .
Next, a SiC chip and a Cu plate whose surfaces were each coated with a Ni film were prepared. A paste was applied to the surface of the substrate so that a bonding layer was formed between the Ni film of the SiC chip and the Ni film of the substrate, and the SiC chip and the Cu plate were laminated. The obtained laminated body was fired in a hydrogen atmosphere to obtain a joined body.

[1.2. Comparative Examples 1-5]
A joined body was produced in the same manner as in Examples 1 to 4 except that Cu particles (manufactured by High Purity Chemical Research Laboratories) having an average particle diameter of 1 μm were used instead of Ni particles. The mass ratio of the Cu particles was 0 mass % (Comparative Example 1), 5 mass % (Comparative Example 2), 10 mass% (Comparative Example 3), 20 mass% (Comparative Example 4), or 30 mass% (Comparative Example 5).

[2. Test method and results]
The joining strength of the joined body was evaluated by a shear test. Moreover, the cross section of the joined body was observed with a scanning electron microscope, and the porosity was determined. The porosity is defined as the ratio of the area of the void to the area of the bonding layer. Table 1 shows the results. FIG. 3 shows the influence of the mass ratio of coarse metal particles on the porosity and bonding strength. Table 1 and FIG. 3 show the following.

Regardless of the metal species of the added coarse particles, the mass ratio of the coarse particles increased and the bonding strength tended to decrease. The joined body using the paste to which Ni coarse particles were added had a greater bonding strength than the joined body using the paste to which the same proportion of Cu coarse particles was added.
Moreover, the porosity in the joining layer decreased by adding coarse particles. The mass ratio with the smallest porosity was smaller in the paste to which Ni coarse particles were added. From this result, the joined body using the paste added with Ni coarse particles forms a dense joint layer and has high strength as compared with the joined body using the paste added with Cu coarse particles. Was shown to be possible.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

  The conductive paste according to the present invention can be used for manufacturing a bonding layer, a fine wiring, a conductive bump, and the like of a semiconductor device.

DESCRIPTION OF SYMBOLS 10 Semiconductor device 12 Semiconductor element 14a, 14b Lead frame (board | substrate)
16 Copper plate (substrate)
18a-18c bonding layer

Claims (5)

A conductive paste having the following configuration.
(1) The conductive paste is
Cu-based nanoparticles whose surface is coated with organic molecules and whose average particle diameter is less than 500 nm,
Ni-based coarse particles having an average particle diameter of 1 μm or more;
An organic solvent for dispersing the Cu-based nanoparticles and the Ni-based coarse particles .
(2) The Cu-based nanoparticles include 95 at% or more of Cu, and the Ni-based coarse particles include 95 at% or more of Ni.
(3) When the mass of the Cu-based nanoparticles is W _n and the mass of the Ni-based coarse particles is W _c , the mass ratio (R) of the Ni-based coarse particles (= W _c × 100 / (W _n + W _c )) is 5 mass% or more and less than 20 mass%. The conductive paste according to claim 1, wherein an average particle diameter of the Ni-based coarse particles is less than 30 μm. A semiconductor device having the following configuration.
(1) The semiconductor device includes:
A semiconductor element;
A substrate,
A bonding layer disposed between the semiconductor element and the substrate;
(2) The bonding layer is obtained by interposing the conductive paste according to claim 1 or 2 between the semiconductor element and the substrate and firing in a reducing atmosphere,
A matrix containing constituent elements of the Cu-based nanoparticles ;
And Ni-based coarse particles dispersed in the matrix. The semiconductor device according to claim 3, wherein the bonding layer has a porosity of 20% or less. An adhesion layer containing at least one element selected from the group consisting of Ni, Ag, and Co is formed on the surface of the semiconductor element on the bonding layer side and / or on the surface of the substrate on the bonding layer side. The semiconductor device according to claim 3 or 4 .

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