FINE COPPER PARTICLES AND METHOD FOR PRODUCING FINE COPPER PARTICLES

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT International Application No. PCT/JP2024/012027 filed on Mar. 26, 2024, which claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2023-058152 filed on Mar. 31, 2023. The above applications are hereby expressly incorporated by reference, in their entirety, into the present application.

TECHNICAL FIELD

The present invention relates to copper fine particles for use in, for example, bonding between a semiconductor device, a high-frequency device, a light-emitting diode, a semiconductor laser, or the like and a substrate or the like, as well as a method of producing the copper fine particles.

BACKGROUND ART

At present, there have been developed power semiconductor devices using a wide-bandgap semiconductor such as silicon carbide (Sic), gallium nitride (GaN), gallium oxide, or diamond. A power semiconductor device has a lower on-resistance than that of a semiconductor device using Si or GaAs, is capable of high-speed switching, and, in addition, can be reduced in size. Moreover, a power semiconductor device has high heat resistance and can operate at high temperatures of 250° C. to 300° C.

For bonding between a semiconductor device and a substrate or the like, solder has been conventionally used. However, the operation temperature of a power semiconductor device is higher than that of a conventional semiconductor device using Si or GaAs, and accordingly, a power semiconductor device bonded by use of solder is required to be used at a temperature at which solder does not melt. When solder is used as a bonding material, a power semiconductor device needs to be used at a temperature at which solder does not melt, and thus, the use of the power semiconductor device is limited. Thus, a bonding material is also required to be usable at a high temperature.

As a bonding material other than solder, Patent Literature 1 describes a plate-like or sheet-like bonding material containing copper fine particles having copper as a main component and having an average particle size of 300 nm or less. The bonding material further contains a reducing agent for reducing the copper fine particles, and the reducing agent contains one or both of a polyol solvent or an organic acid.

CITATION LIST

Patent Literatures

• • Patent Literature 1: JP 2019-203172 A

SUMMARY OF INVENTION

Technical Problems

The plate-like or sheet-like bonding material of Patent Literature 1 bonds members together under predetermined bonding conditions of pressure, temperature, and time in an inert atmosphere.

However, the bonding material of Patent Literature 1 has the shape of plate or sheet, the size thereof needs to be adjusted according to the size of a portion to be bonded. In addition, the bonding material needs to be aligned with the portion to be bonded, resulting in poor workability. Therefore, there is a demand for bonding in an easy and simple manner, such as painting, and there is a demand for particles usable as a bonding material. Aside from that, when a bonding material is used for die attach (die bonding) or the like, excellent heat dissipation properties are required.

An object of the present invention is to provide copper fine particles applicable to die attach (die bonding) or the like and having excellent heat dissipation properties, as well as a method for producing the copper fine particles.

Solution to Problems

To attain the foregoing object, one embodiment of the invention is copper fine particles having a coating layer constituted of glycol or a glycol polymer, the copper fine particles being present alone.

Preferably, the glycol or the glycol polymer has a molecular weight of 300 or less.

One embodiment of the invention is copper fine particles having a coating layer constituted of glycol or a glycol polymer, the copper fine particles being present alone, wherein a removal percentage of the coating layer at a temperature of 200° C. measured by thermogravimetric and differential thermal analysis in a nitrogen atmosphere is 30 mass % or more.

One embodiment of the invention is copper fine particles having a coating layer constituted of glycol or a glycol polymer, the copper fine particles being present alone, wherein a weight loss percentage at a temperature of 200° C. measured by thermogravimetric and differential thermal analysis in a nitrogen atmosphere is 0.25 mass % or more.

One embodiment of the invention is copper fine particles having a coating layer constituted of glycol or a glycol polymer, the copper fine particles being present alone, wherein a weight increase percentage at a temperature of 200° C. measured by thermogravimetric and differential thermal analysis in an air atmosphere is 5 mass % or less.

One embodiment of the invention is copper fine particles having a coating layer constituted of glycol or a glycol polymer, the copper fine particles being present alone, wherein a particle growth rate in particle size based on BET method after the copper fine particles in a particulate form are heated at a temperature of 200° C. for 1 hour in a nitrogen atmosphere is 105% or more.

One embodiment of the invention is copper fine particles having a coating layer constituted of glycol or a glycol polymer, the copper fine particles being present alone, wherein a volume resistivity after the copper fine particles in a pellet form are baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere is 1.0×10⁻⁴ Ω·cm or less.

One embodiment of the invention is copper fine particles having a coating layer constituted of glycol or a glycol polymer, the copper fine particles being present alone, wherein a volume contraction percentage after the copper fine particles in a pellet form are baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere is 20% or less.

Preferably, a particle size measured by BET method is 20 to 400 nm.

One embodiment of the invention is a method of producing copper fine particles having a coating layer constituted of glycol or a glycol polymer, the method comprising: a step of supplying a carrier gas having copper powder dispersed therein to a thermal plasma flame to thereby produce fine particle bodies; and a step of supplying a surface treating agent containing glycol or a glycol polymer to the fine particle bodies that are in a temperature region in which the surface treating agent does not denature and that are in a state of being dispersed in gas before being collected.

Preferably, a plasma gas of the thermal plasma flame is argon gas, helium gas, hydrogen gas, or nitrogen gas.

Advantageous Effects of Invention

The present invention provides copper fine particles applicable to die attach (die bonding) or the like and having excellent heat dissipation properties, as well as a method for producing the copper fine particles.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing an example of the use form of copper fine particles of the invention.

FIG. 2 is a schematic view showing another example of the use form of the copper fine particles of the invention.

FIG. 3 is a schematic view showing an example of a copper fine particle production apparatus of the invention.

FIG. 4 is a schematic view showing an SEM image of copper fine particles in Example 1.

FIG. 5 is a schematic view showing an SEM image of copper fine particles in Example 2.

FIG. 6A is a schematic view showing an SEM image of the copper fine particles in Example 1 before baking, and

FIG. 6B is a schematic view showing an SEM image of the copper fine particles in Example 1 after baking.

FIG. 7A is a schematic view showing an SEM image of the copper fine particles in Example 2 before baking, and

FIG. 7B is a schematic view showing an SEM image of the copper fine particles in Example 2 after baking.

FIG. 8A is a schematic view showing an SEM image of a pellet of the copper fine particles in Example 1 before baking, and

FIG. 8B is a schematic view showing an SEM image of the pellet of the copper fine particles in Example 1 after baking.

FIG. 9A is a schematic view showing an SEM image of a pellet of the copper fine particles in Example 2 before baking, and

FIG. 9B is a schematic view showing an SEM image of the pellet of the copper fine particles in Example 2 after baking.

FIG. 10 is a graph showing analysis results of crystal structure of the copper fine particles in Example 1 and Example 2 of the invention as obtained by X-ray diffractometry.

FIG. 11 is a graph showing temporal changes in the volume resistivity of the copper fine particles in Example 1 and Example 2 of the invention.

DESCRIPTION OF EMBODIMENTS

Copper fine particles and a method of producing copper fine particles according to the invention are described below in detail based on preferred embodiments shown in the accompanying drawings.

It should be noted that the drawings described below are illustrative to describe the invention and are simplified to describe the invention. Therefore, the invention is not construed to be limited to the drawings described below.

When the term “to” is used for a numerical range below, the range includes numerical values before and after “to.” For instance, the phrase “ε is numerical value ε_α to numerical value ε_β” means that ε is in a range including the value ε_α and the value ε_β, which can be mathematically expressed as ε_α≤ε≤ε_β.

With respect to various numerical values, they include a margin of error that is generally acceptable in the relevant technical field, unless otherwise noted.

Copper fine particles and a method of producing copper fine particles are described below.

First Example of Copper Fine Particles

Copper fine particles have a coating layer constituted of glycol or a glycol polymer.

The glycol herein refers to, for instance, diethylene glycol, triethylene glycol, or tetraethylene glycol that is a dehydration condensation product of ethylene glycol, i.e., a dehydration condensation product of two, three, or four molecules of ethylene glycol (HOCH₂CH₂OH). The glycol polymer refers to, for instance, polyethylene glycol that is a polymer of ethylene glycol.

The glycol or glycol polymer exhibits reducing properties. The coating layer constituted of the glycol or glycol polymer has to be present so as to entirely or partially cover the surfaces of the copper fine particles when the copper fine particles are being heated. If the coating layer does not entirely or partially cover the surfaces of the copper fine particles, reducing properties would not be exhibited, so that sintering of the copper fine particles would not proceed, and this may hamper bonding such as die attach. Accordingly, the glycol or glycol polymer is required to be not entirely evaporated (volatilized) at a temperature lower than a bonding temperature.

As the molecular weight of an ethylene glycol polymer increases, the temperature at which the mass starts to sharply decrease due to evaporation or thermal decomposition tends to shift to a high temperature side, and the temperature at which the weight loss percentage becomes 100%, i.e., the temperature at which the ethylene glycol polymer is entirely evaporated or thermally decomposed tends to shift to a high temperature side. Thus, the glycol or glycol polymer preferably has a molecular weight of 300 or less.

Examples of the glycol or glycol polymer with a molecular weight of 300 or less include ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol. Examples thereof also include polyethylene glycol with a molecular weight of 300 or less.

The molecular weight of the glycol or glycol polymer can be selected based on the bonding temperature, for example, the temperature at which the weight loss percentage becomes a specified percentage such as 100% or 50%. In other words, the glycol or glycol polymer can be selected based on the bonding temperature in die attach.

As described above, the copper fine particles have surfaces coated with the glycol or glycol polymer. The presence and the composition of the coating layer of the copper fine particles can be examined using, for instance, a Fourier transform infrared spectrometer (FT-IR).

The copper fine particles preferably have a particle size measured by BET method of 20 to 400 nm. A particle size of 20 to 400 nm is preferred because this leads to good handleability.

The above particle size of the copper fine particles is the average particle size measured using the BET method. In the BET method, particles are assumed to be spherical, and the calculation is made based on the specific surface area. Macsorb HM-1208 (trade name) manufactured by Mountech Co., Ltd. is used to measure the particle size of the copper fine particles.

Owing to the coating layer constituted of the glycol or glycol polymer, the copper fine particles are excellent in the ability to suppress the oxidation of the particle surfaces during baking and the ability to reduce an already-generated oxide on the particle surfaces, so that sintering can be prompted. In addition, the copper fine particles having been sintered have a higher melting point than that of solder or the like and thus have excellent heat resistance. Hence, the copper fine particles are applicable to die attach (die bonding) or the like, and when the copper fine particles are used as a bonding material, this results in excellent electric conductivity and excellent heat dissipation properties, while the heat resistance is satisfied.

In this regard, it is known that in metal, the ratio between the thermal conductivity and the electric conductivity is proportional to the temperature (Wiedemann-Franz law). Thus, in copper which is metal, when the electric conductivity becomes higher, that is, the electric conductivity increases, this means that the thermal conductivity becomes higher. From this, in the copper fine particles, having excellent electric conductivity is equivalent to having excellent heat dissipation properties.

It should be noted that the copper fine particles are not present in a dispersed form in a solvent or the like but are present alone without a solvent or the like. Accordingly, when the copper fine particles are utilized, a sintered body can be obtained from the copper fine particles alone. Also from this aspect, the copper fine particles are favorably applicable to die attach (die bonding) or the like.

In addition, since the copper fine particles are present alone, when the copper fine particles are used in combination with a solvent, there is no particular limitation on the combination with a solvent, and the degree of freedom is high in selection of a solvent.

The copper fine particles are present alone without a solvent or the like as described above. That is, the copper fine particles are present alone, and the copper fine particles being present alone includes the copper fine particles being dispersed in gas.

Second Example of Copper Fine Particles

Copper fine particles have a coating layer constituted of the glycol or glycol polymer, and the removal percentage of the coating layer at a temperature of 200° C. measured by thermogravimetric and differential thermal analysis (TG-DTA) in a nitrogen atmosphere is 30 mass % or more.

The removal percentage of the coating layer is defined by a value obtained by performing the thermogravimetric and differential thermal analysis (TG-DTA) in a nitrogen atmosphere and dividing the weight loss percentage at a temperature of 200° C. by the weight loss percentage at a temperature of 700° C.

When the removal percentage of the coating layer at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis in a nitrogen atmosphere is 30 mass % or more, sintering between the copper fine particles proceeds due to oxidation suppression or reduction of the copper fine particles during baking, and therefore, the copper fine particles can be bonded together well. In addition, since the copper fine particles are bonded together well, the electric conductivity is high, and heat dissipation properties are also high.

The upper limit of the removal percentage of the coating layer at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis is 100 mass %.

For the thermogravimetric and differential thermal analysis (TG-DTA), STA7200 (trade name) of Hitachi High-Technologies Science Corporation is used.

In the invention, a nitrogen atmosphere is an atmosphere where nitrogen gas of 99.99 purity (nitrogen: 99.99 vol %) is constantly circulated and where the oxygen concentration is 3 volume ppm or lower. Hence, in the nitrogen atmosphere, nitrogen may be mixed with a gas which can make the oxygen concentration 3 volume ppm or lower and which or whose decomposed product does not react with copper, e.g., a rare gas or the like. The oxygen concentration can be measured using, for example, PS-820-L (trade name) low concentration oxygen analyzer manufactured by Iijima Electronics Corporation.

When the removal percentage of the coating layer at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis in a nitrogen atmosphere is 30 mass % or more as described above, in addition to the oxidation suppression during baking, the copper fine particles can be reduced even if a copper oxide is present on the surfaces of the copper fine particles during baking, and therefore, the copper fine particles can be bonded together well. Furthermore, since the copper fine particles are bonded together well, it is possible to have high electric conductivity and high heat dissipation properties. In addition, the copper fine particles having been sintered have a higher melting point than that of solder or the like and thus have excellent heat resistance. Hence, the copper fine particles are applicable to die attach (die bonding) or the like, and when the copper fine particles are used as a bonding material, this results in excellent electric conductivity and excellent heat dissipation properties, while the heat resistance is satisfied.

Third Example of Copper Fine Particles

Copper fine particles have a coating layer constituted of the glycol or glycol polymer, and the weight loss percentage at a temperature of 200° C. measured by thermogravimetric and differential thermal analysis in a nitrogen atmosphere is 0.25 mass % or more.

The weight loss percentage at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis in a nitrogen atmosphere is an index indicating the degree of thermal decomposition of the glycol or glycol polymer constituting the coating layer.

When the weight loss percentage at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis in a nitrogen atmosphere is 0.25 mass % or more, the coating layer is thermally decomposed, and in addition to the oxidation suppression during baking, the reduction is possible even if a copper oxide is present on the surfaces of the copper fine particles during baking, and therefore, the copper fine particles can be bonded together well.

The upper limit of the weight loss percentage at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis in a nitrogen atmosphere is 10 mass %.

The weight loss percentage is obtained by the thermogravimetric and differential thermal analysis (TG-DTA).

The nitrogen atmosphere is as described above.

Owing to the weight loss percentage of the copper fine particles at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis in a nitrogen atmosphere being 0.25 mass % or more, the coating layer constituted of the glycol or glycol polymer is thermally decomposed at a temperature of 200° C. and the copper fine particles can be bonded together well, and it is possible to have high electric conductivity and high heat dissipation properties. In addition, the copper fine particles having been sintered have a higher melting point than that of solder or the like and thus have excellent heat resistance. Hence, the copper fine particles are applicable to die attach (die bonding) or the like, and when the copper fine particles are used as a bonding material, this results in excellent electric conductivity and excellent heat dissipation properties, while the heat resistance is satisfied.

Fourth Example of Copper Fine Particles

Copper fine particles have a coating layer constituted of the glycol or glycol polymer, and the weight increase percentage at a temperature of 200° C. measured by thermogravimetric and differential thermal analysis in an air atmosphere is 5 mass % or less.

The weight increase percentage at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis in an air atmosphere is an index indicating the degree of oxidation of the copper fine particles.

When the weight increase percentage at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis in an air atmosphere is 5 mass % or less, the oxidation of the copper fine particles is suppressed during sintering at a temperature of 200° C. in a nitrogen atmosphere, so that the copper fine particles can be bonded together well.

The lower limit of the weight increase percentage at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis in an air atmosphere is 0 mass %.

The upper limit of the weight increase percentage at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis in an air atmosphere is 25 mass %.

The weight increase percentage is obtained by the thermogravimetric and differential thermal analysis (TG-DTA).

The air atmosphere refers to the atmosphere generally called the air. The composition of the air atmosphere, i.e., air, includes 78.08 vol % of nitrogen, 20.95 vol % of oxygen, 0.93 vol % of argon, and 0.03 vol % of carbon dioxide. For the composition of air, an ordinary measurement error is acceptable.

Owing to the weight increase percentage of the copper fine particles at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis in an air atmosphere being 5 mass % or less, the oxidation is suppressed, the copper fine particles can be bonded together well, and it is possible to have high electric conductivity. In addition, the copper fine particles having been sintered have a higher melting point than that of solder or the like and thus have excellent heat resistance. Hence, the copper fine particles are applicable to die attach (die bonding) or the like, and when the copper fine particles are used as a bonding material, this results in excellent electric conductivity and excellent heat dissipation properties, while the heat resistance is satisfied.

Fifth Example of Copper Fine Particles

Copper fine particles have a coating layer constituted of the glycol or glycol polymer, and the particle growth rate in particle size based on the BET method after the copper fine particles in a particulate form are heated at a temperature of 200° C. for 1 hour in a nitrogen atmosphere is 105% or more.

The particle growth rate can be determined from the particle size before baking and that after baking. The particle growth rate is obtained using the following formula.

Particle growth rate (%)=((particle size after baking)/(particle size before baking))×100

When the particle growth rate is 105% or more, the copper fine particles can be sintered by 1-hour baking at a temperature of 200° C. in a nitrogen atmosphere.

The upper limit of the particle growth rate is for example 500%.

The nitrogen atmosphere is as described above.

Owing to the particle growth rate in particle size based on the BET method after the copper fine particles in a particulate form are heated at a temperature of 200° C. for 1 hour in a nitrogen atmosphere being 105% or more, the copper fine particles can be sintered by 1-hour heating at a temperature of 200° C. in a nitrogen atmosphere, and it is possible to have high electric conductivity and high heat dissipation properties. In addition, the copper fine particles having been sintered have a higher melting point than that of solder or the like and thus have excellent heat resistance. Hence, the copper fine particles are applicable to die attach (die bonding) or the like, and when the copper fine particles are used as a bonding material, this results in excellent electric conductivity and excellent heat dissipation properties, while the heat resistance is satisfied.

Sixth Example of Copper Fine Particles

Copper fine particles have a coating layer constituted of the glycol or glycol polymer, and the volume resistivity after the copper fine particles in a pellet form are baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere is 1.0×10⁻⁴ Ω·cm or less.

The lower limit of the volume resistivity of the copper fine particles is 1.6×10⁻⁶ Ω·cm.

When the volume resistivity is 1.0×10⁻⁴ Ω·cm or less, the electric conductivity and the heat dissipation properties are excellent.

When the volume resistivity is obtained, the copper fine particles are formed into a cylindrical pellet, and the pellet is placed in an electric furnace and baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere.

The pellet is prepared by pressing and retaining the copper fine particles at pressure of 127 MPa for 10 seconds with a pressing machine.

The volume resistivity is a value obtained by measurement by the four-terminal method using the pellet. For example, Loresta EP ((device name), MCP-T360 (model)) manufactured by Mitsubishi Chemical Corporation is used as a measurement device. By measuring the volume resistivity of the pellet before baking and that after baking, the change in volume resistivity following the baking can be measured.

The copper fine particles have a volume resistivity of 1.0×10⁻⁴ Ω·cm or less after the copper fine particles in a pellet form are baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere, and the electric conductivity is high. In addition, the copper fine particles having been sintered have a higher melting point than that of solder or the like and thus have excellent heat resistance. Hence, the copper fine particles are applicable to die attach (die bonding) or the like, and when the copper fine particles are used as a bonding material, this results in excellent electric conductivity and excellent heat dissipation properties, while the heat resistance is satisfied.

Seventh Example of Copper Fine Particles

Copper fine particles have a coating layer constituted of the glycol or glycol polymer, and the volume contraction percentage after the copper fine particles in a pellet form are baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere is 20% or less. The lower limit of the volume contraction percentage is 0%.

When the volume contraction percentage is 20% or less, cracking or the like caused by volume contraction can be suppressed.

The volume contraction percentage is a value obtained as follows: the copper fine particles are pressed and retained at pressure of 127 MPa for 10 seconds with a pressing machine as described above to prepare a cylindrical pellet, the thickness and the diameter of the cylindrical pellet are measured with a caliper, and the calculation is made based on the volume of the pellet before baking and that after baking. The volume contraction percentage is obtained by the following formula. An electric furnace is used to bake the pellet. The nitrogen atmosphere is as described above.

Volume ⁢ contraction ⁢ percentage ⁢ ( % ) = 100 - ( ( volume ⁢ after ⁢ baking / volume ⁢ before ⁢ baking ) × 100 )

Owing to the volume contraction percentage of the copper fine particles after the copper fine particles in a pellet form are baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere being 20% or less, it is possible to suppress volume contraction and have high electric conductivity. In addition, the copper fine particles having been sintered have a higher melting point than that of solder or the like and thus have excellent heat resistance. Hence, the copper fine particles are applicable to die attach (die bonding) or the like, and when the copper fine particles are used as a bonding material, this results in excellent electric conductivity and excellent heat dissipation properties, while the heat resistance is satisfied.

The lower limit of the volume contraction percentage after the copper fine particles in a pellet form are baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere is for example 0%.

[Use Form of Copper Fine Particles]

FIG. 1 is a schematic view showing an example of the use form of the copper fine particles of the invention, and FIG. 2 is a schematic view showing another example of the use form of the copper fine particles of the invention.

The copper fine particles are used as a bonding material, for example. More specifically, the copper fine particles are used as a bonding material to bond between a substrate 50 and a power semiconductor device 52 shown in FIG. 1. The copper fine particles are used for die attachment.

The copper fine particles constitute a bonding portion 54 bonding the substrate 50 and the power semiconductor device 52 together. The bonding portion 54 is formed by baking the copper fine particles at a temperature of 200° C. for 1 hour in a nitrogen atmosphere, for example. The bonding portion 54 bonds the substrate 50 and the power semiconductor device 52 together, so that the substrate 50 and the power semiconductor device 52 are physically fixed together.

The copper fine particles are also usable to bond a single substrate 50 with a plurality of semiconductor devices as shown in FIG. 2. In FIG. 2, three semiconductor devices 53a, 53b, and 53c are shown as an example. The three semiconductor devices 53a, 53b, and 53c are different in size. The three semiconductor devices 53a, 53b, and 53c are bonded to the substrate 50 through bonding portions 54.

Since the three semiconductor devices 53a, 53b, and 53c are different in size, if bonding materials forming the bonding portions 54 are in the shape of a plate or sheet, the bonding materials need to be sized to fit the three semiconductor devices 53a, 53b, and 53c and also need to be adjusted in position, resulting in poor workability.

In contrast, when the copper fine particles are used, the bonding material does not have to be sized to fit the three semiconductor devices 53a, 53b, and 53c and also does not have to be adjusted in position, thus leading to easy and simple bonding.

The copper fine particles sintered have a higher melting point than that of solder and have high heat resistance. In addition, the copper fine particles have a volume contraction percentage of 20% or less after the copper fine particles in a pellet form are baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere as described above. Because of this, even when the power semiconductor device 52 is operated and causes a temperature change, the bonding portion 54 would have a small volume change, so that cracking or the like is suppressed. Hence, even when the temperature of the atmosphere exceeds the melting point of solder, the bond between the power semiconductor device 52 and the substrate 50 and the bond between the semiconductor devices 53a, 53b, and 53c and the substrate 50 are maintained, and high durability is also obtained. Furthermore, since the copper fine particles having been baked have a low volume resistivity, the thermal conductivity is excellent, and the bonding portion 54 allows heat generated in the power semiconductor device 52 to be efficiently conducted to the substrate 50.

The substrate 50 is, for example, a ceramic substrate of Si₃N₄ or the like on which copper wiring is provided.

The power semiconductor device 52 is, for example, a semiconductor device using a semiconductor such as silicon carbide (SiC), gallium nitride (GaN), gallium oxide, or diamond.

The semiconductor devices 53a, 53b, and 53c are semiconductor devices using a typical silicon substrate. The semiconductor devices 53a, 53b, and 53c may be power semiconductor devices.

The use of the copper fine particles is not limited to bonding of the power semiconductor device 52 or the semiconductor devices 53a, 53b, and 53c, and the copper fine particles can be also used for bonding of a high-frequency device, a light-emitting diode, a semiconductor laser, or the like. The copper fine particles have excellent thermal conductivity as described above and are therefore suitable for bonding of an object that generates a large amount of heat or an object that operates at high temperature.

The copper fine particles are also usable for various wiring including signal wiring, conductive wiring, and the like in addition to the bonding between the power semiconductor device and the substrate and the bonding between the semiconductor devices and the substrate as described above.

[Method of Producing Copper Fine Particles]

Next, an example of the method of producing copper fine particles is described with reference to FIG. 3, but the method of producing copper fine particles according to the invention is not limited to a production method using a copper fine particle production apparatus 10 shown in FIG. 3. FIG. 3 is a schematic view showing an example of a copper fine particle production apparatus of the invention.

With the copper fine particle production apparatus 10 (hereinafter, simply referred to as “production apparatus 10”) shown in FIG. 3, the foregoing copper fine particles can be obtained.

The production apparatus 10 includes a plasma torch 12 generating a thermal plasma flame, a material supply device 14 supplying feedstock powder of copper fine particles into the plasma torch 12, a chamber 16 serving as a cooling tank for use in producing primary fine particles 15 of copper, a cyclone 19 removing, from the primary fine particles 15 of copper, coarse particles having a particle size equal to or larger than an arbitrarily specified particle size, and a collecting section 20 collecting secondary fine particles 18 of copper having a desired particle size as obtained by classification by the cyclone 19. The chamber 16 is connected to the cyclone 19 through a connection tube 21a. The cyclone 19 is connected to the collecting section 20 through a connection tube 21b that is connected to an inner tube 19e.

The production apparatus 10 further includes a supply section 40 supplying a surface treating agent to the primary fine particles 15 of copper or the secondary fine particles 18 of copper.

The primary fine particles 15 of copper and the secondary fine particles 18 of copper are both fine particle bodies in the middle of the production process of the fine particles of the invention. Those obtained by surface treating the primary fine particles 15 of copper or the secondary fine particles 18 of copper, i.e., surface treated copper fine particles 30 are the copper fine particles of the invention. The copper fine particles 30 have surfaces coated with the glycol or glycol polymer and are configured to have a coating layer constituted of the glycol or glycol polymer.

For the material supply device 14, the chamber 16, the cyclone 19, and the collecting section 20, those devices described in, for example, JP 2007-138287 A may be used.

In the present embodiment, for example, copper powder is used as the feedstock in the production of the fine particles.

The copper powder is appropriately designed to have an average particle size which allows easy evaporation of the powder in a thermal plasma flame. The average particle size of the copper powder is measured by a laser diffraction method and is, for example, 100 μm or less, preferably 50 μm or less, and more preferably 15 μm or less.

The plasma torch 12 is constituted of a quartz tube 12a and a high frequency oscillation coil 12b surrounding the outside of the quartz tube. A supply tube 14a to be described later which is for supplying feedstock powder of the fine particles into the plasma torch 12 is provided on the top of the plasma torch 12 at the central part thereof. A plasma gas supply port 12c is formed in the peripheral portion of the supply tube 14a (on the same circumference). The plasma gas supply port 12c is in a ring shape. To the high frequency oscillation coil 12b, a power source (not shown) that generates a high frequency voltage is connected. When a high frequency voltage is applied to the high frequency oscillation coil 12b, a thermal plasma flame 24 is generated. The feedstock (not shown) is evaporated by the thermal plasma flame 24 and transformed into a mixture in a gas phase state. The plasma torch 12 constitutes a treatment section transforming the feedstock into a mixture in a gas phase state by means of a gas-phase process.

A plasma gas supply section 22 is configured to supply plasma gas into the plasma torch 12. The plasma gas supply section 22 is connected to the plasma gas supply port 12c through a pipe 22a. Although not illustrated, the plasma gas supply section 22 is provided with a supply amount adjuster such as a valve for adjusting the supply amount. The plasma gas is supplied from the plasma gas supply section 22 into the plasma torch 12 through the plasma gas supply port 12c of ring shape in the direction indicated by arrow P and the direction indicated by arrow S.

For example, a mixed gas of hydrogen gas and argon gas is used as the plasma gas. In this case, hydrogen gas and argon gas are stored in the plasma gas supply section 22. Hydrogen gas and argon gas are supplied from the plasma gas supply section 22 into the plasma torch 12 in the direction indicated by arrow P and the direction indicated by arrow S after passing through the pipe 22a and then the plasma gas supply port 12c. Argon gas alone may be supplied in the direction indicated by arrow P.

As the plasma gas, a gas suitable for the copper fine particles, such as argon gas, helium gas, hydrogen gas, or nitrogen gas, is selected and used; thus, it is not essential to use a mixed gas as the plasma gas as described above, and a single gas may be used as the plasma gas.

When a high frequency voltage is applied to the high frequency oscillation coil 12b, the thermal plasma flame 24 is generated in the plasma torch 12.

It is necessary for the thermal plasma flame 24 to have a higher temperature than the boiling point of the feedstock powder. A higher temperature of the thermal plasma flame 24 is preferred because the feedstock powder is more easily transformed into a gas phase state; however, there is no particular limitation on the temperature. For instance, the thermal plasma flame 24 may have a temperature of 6,000° C., and in theory, the temperature is deemed to reach 10,000° C. or thereabout.

The ambient pressure inside the plasma torch 12 is preferably up to atmospheric pressure. For the ambiance at a pressure up to atmospheric pressure, the pressure is not particularly limited and is, for example, 0.5 to 100 kPa.

The outside of the quartz tube 12a is surrounded by a concentrically formed tube (not shown), and cooling water is circulated between this tube and the quartz tube 12a to cool the quartz tube 12a with the water, thereby preventing the quartz tube 12a from having an excessively high temperature caused by the thermal plasma flame 24 generated in the plasma torch 12.

The material supply device 14 is connected to the top of the plasma torch 12 through the supply tube 14a. The material supply device 14 is configured to supply the feedstock into the thermal plasma flame 24 in the plasma torch 12.

The material supply device 14 is not particularly limited as long as it can supply the feedstock into the thermal plasma flame 24, and, for example, the material supply device 14 supplies the feedstock into the thermal plasma flame 24 with the feedstock being dispersed in a particulate form.

When the feedstock is powder, for instance, as described above, the device disclosed in JP 2007-138287 A may be used as the material supply device 14 that supplies the feedstock, e.g., copper powder in a powdery form. In this case, the material supply device 14 includes, for example, a storage tank (not shown) storing the feedstock, a screw feeder (not shown) transporting the feedstock in a fixed amount, a dispersion section (not shown) dispersing the feedstock transported by the screw feeder to transform it into the form of primary particles before the feedstock is finally sprayed, and a carrier gas supply source (not shown).

Together with a carrier gas to which push-out pressure is applied from the carrier gas supply source, the feedstock is supplied into the thermal plasma flame 24 in the plasma torch 12 through the supply tube 14a.

The configuration of the material supply device 14 is not particularly limited as long as the device can prevent the feedstock from agglomerating, thus making it possible to spray the feedstock into the plasma torch 12 with the dispersed state maintained. An inert gas such as argon gas is used as the carrier gas, for example. The flow rate of the carrier gas can be controlled using, for example, a flowmeter such as a float type flowmeter. The flow rate value of the carrier gas is indicated by a reading on the flowmeter.

The chamber 16 is provided below and adjacent to the plasma torch 12, and in the chamber 16, the primary fine particles 15 of copper being fine particle bodies are generated from the above-described mixture in a gas phase state with no use of a cooling gas. The chamber 16 serves as a cooling tank. A cooling gas is also called a quenching gas, and argon gas or the like is adopted.

There is provided a gas supply section 28 supplying a temperature adjusting gas including an inert gas into the connection tube 21a or the connection tube 21b, for instance. The gas supply section 28 supplies the temperature adjusting gas including an inert gas to the primary fine particles 15 of copper or the secondary fine particles 18 of copper.

The gas supply section 28 includes, for instance, a valve 28a, a first gas supply tube 28b, and a second gas supply tube 28c, the first and second gas supply tubes 28b and 28c being connected to the valve 28a. The first gas supply tube 28b is connected to the connection tube 21a, and the second gas supply tube 28c is connected to the connection tube 21b.

By switching the valve 28a, the temperature adjusting gas is supplied to either one of the first gas supply tube 28b and the second gas supply tube 28c, and the temperature adjusting gas is supplied into the connection tube 21a or the connection tube 21b.

The gas supply section 28 further includes a pressure application device (not shown) applying push-out pressure to the temperature adjusting gas supplied into the first gas supply tube 28b or the second gas supply tube 28c. The pressure application device is, for instance, a compressor, a blower, or the like.

Moreover, the gas supply section 28 includes a storage section (not shown) storing the temperature adjusting gas, and a pressure control valve (not shown) controlling the amount of gas supply. The temperature adjusting gas is an argon gas for instance.

Owing to the temperature adjusting gas supplied from the gas supply section 28 to the connection tube 21a or the connection tube 21b, the gas temperature can be adjusted to a desired temperature.

As shown in FIG. 3, the cyclone 19 is provided to the chamber 16 to classify the primary fine particles 15 of copper based on a desired particle size. The cyclone 19 includes an inlet tube 19a supplying the primary fine particles 15 from the chamber 16, a cylindrical outer tube 19b connected to the inlet tube 19a and positioned at an upper portion of the cyclone 19, a truncated conical part 19c continuing downward from the bottom of the outer tube 19b and having a gradually decreasing diameter, a coarse particle collecting chamber 19d connected to the bottom of the truncated conical part 19c and collecting coarse particles having a particle size equal to or larger than the above-mentioned desired particle size, and an inner tube 19e connected to the collecting section 20 to be detailed later and projecting from the outer tube 19b. The chamber 16 is connected to the inlet tube 19a via the connection tube 21a, and the primary fine particles 15 move to the cyclone 19 through the connection tube 21a. The connection tube 21a is a transport path of the primary fine particles 15.

A gas stream containing the primary fine particles 15 is blown from the inlet tube 19a of the cyclone 19 to flow along the inner peripheral wall of the outer tube 19b, and accordingly, this gas stream flows in the direction from the inner peripheral wall of the outer tube 19b toward the truncated conical part 19c as indicated by arrow T in FIG. 3, thus forming a downward swirling stream.

When the downward swirling stream is inverted to an upward stream, coarse particles cannot follow the upward stream due to the balance between the centrifugal force and drag, fall down along the lateral surface of the truncated conical part 19c, and are collected in the coarse particle collecting chamber 19d. Fine particles having been affected by the drag more than the centrifugal force are discharged to the outside of the cyclone 19 through the inner tube 19e and the connection tube 21b along with the upward stream on the inner wall of the truncated conical part 19c.

The apparatus is configured such that negative pressure (suction force) is exerted from the collecting section 20, which will be detailed later, through the inner tube 19e and the connection tube 21b. Due to the negative pressure (suction force), the fine particles separated from the swirling gas stream are sucked as indicated by arrow U and sent to the collecting section 20 through the inner tube 19e and the connection tube 21b.

On the extension of the inner tube 19e which is an outlet for the gas stream in the cyclone 19, the collecting section 20 is provided to collect the copper fine particles 30 having a desired particle size on the order of nanometers. The collecting section 20 includes a collecting chamber 20a, a filter 20b provided in the collecting chamber 20a, and a vacuum pump 29 connected through a tube 20c provided at a lower portion of the inside of the collecting chamber 20a. The copper fine particles 30 delivered from the cyclone 19 are sucked by the vacuum pump 29 to be introduced into the collecting chamber 20a, and remain on the surface of the filter 20b to be collected. It should be noted that the number of cyclones used in the production apparatus 10 is not limited to one and may be two or more. Alternatively, no cyclone may be used.

The supply section 40 supplies a surface treating agent St to copper fine particles at a position downstream of the first gas supply tube 28b in the connection tube 21a or downstream of the second gas supply tube 28c in the connection tube 21b. Owing to the surface treating agent St, copper fine particles having surfaces coated with the glycol or glycol polymer are formed. Here, with respect to the connection tube 21a, the chamber 16 side is referred to as an upstream side, and the cyclone 19 side is referred to as a downstream side.

The supply section 40 includes, for example, a valve 41, a first supply tube 41b, and a second supply tube 41c, the first and second supply tubes 41b and 41c being connected to the valve 41. The first supply tube 41b is connected to the connection tube 21a at a position downstream of the first gas supply tube 28b, and the second supply tube 41c is connected to the connection tube 21b at a position downstream of the second gas supply tube 28c.

The connection position of the first supply tube 41b to the connection tube 21a is defined as P₁, and the connection position of the second supply tube 41c to the connection tube 21b is defined as P₂. The connection position P₂ of the second supply tube 41c is situated downstream of the connection position P₁ of the first supply tube 41b.

The supply section 40 supplies the surface treating agent St to fine particle bodies (the primary fine particles 15 of copper or the secondary fine particles 18 of copper) that are in a temperature region in which the surface treating agent St does not denature and that are in the state of being dispersed in gas before being collected. More specifically, the supply section 40 supplies the surface treating agent St to the primary fine particles 15 of copper passing through the connection tube 21a or the secondary fine particles 18 of copper passing through the connection tube 21b. The primary fine particles 15 of copper passing through the connection tube 21a or the secondary fine particles 18 of copper passing through the connection tube 21b are fine particle bodies in the state of being dispersed in gas before being collected.

The surface treating agent St is supplied by the supply section 40 in a temperature region in which the surface treating agent St does not denature, and the surface treating agent St adheres to the primary fine particles 15 of copper or the secondary fine particles 18 of copper. Thus, the primary fine particles 15 of copper or the secondary fine particles 18 of copper are surface-treated, and the copper fine particles 30 having surfaces coated with the glycol or glycol polymer are formed. Coating of the glycol or glycol polymer also prevents the copper fine particles 30 from fusing together.

The surface condition of the surface treated copper fine particles 30 can be examined using, for instance, a Fourier transform infrared spectrometer (FT-IR).

The method of supplying the surface treating agent St by the supply section 40 is not particularly limited, and in an exemplary method, the surface treating agent St is transformed into droplets and sprayed to the secondary fine particles 18 of copper.

As described above, the surface treating agent St is supplied in a temperature region in which the surface treating agent St does not denature. The temperature region in which the surface treating agent St does not denature is specifically a temperature region in which the glycol or glycol polymer does not denature.

The temperature region in which the surface treating agent St does not denature refers to a temperature region determined based on the temperature measured by thermogravimetric and differential thermal analysis (TG-DTA).

The temperature region in which the surface treating agent St does not denature is defined to be a temperature region where the weight loss percentage is 50 mass % or less in the thermogravimetric and differential thermal analysis of the surface treating agent St. The weight loss percentage is preferably 30 mass % or less and more preferably 10 mass % or less.

For the thermogravimetric and differential thermal analysis, STA7200 (trade name) of Hitachi High-Tech Science Corporation is used.

For the surface treating agent St, the glycol or glycol polymer to constitute a coating layer (not shown) is used. More specifically, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol and the like are used for the surface treating agent St. The surface treating agent St may contain an organic solvent.

(Organic Solvent)

The organic solvent is not particularly limited and can be appropriately selected according to the purpose. Examples of the organic solvent include alcohols such as ethanol and methanol, ketones such as acetone, alkyl halides, amides such as formamide, sulfoxides such as dimethyl sulfoxide, a heterocyclic compound, hydrocarbons, esters such as ethyl acetate, and ethers. These may be used singly or in combination of two or more.

The production apparatus 10 may include a sensor (not shown) that measures the temperature of the transport path for the primary fine particles 15 of copper or that for the secondary fine particles 18 of copper. The sensor's measurement result of temperature is utilized in determination as to whether the temperature falls in the temperature region where the surface treating agent St does not denature. In this process, the measurement result of temperature is output to the supply section 40, for example. The supply section 40 can determine whether the temperature falls in the temperature region where the surface treating agent St does not denature based on the sensor's measurement result of temperature of the transport path for the primary fine particles 15 of copper or the sensor's measurement result of temperature of the transport path for the secondary fine particles 18 of copper. When the temperature of the transport path for the primary fine particles 15 of copper or that for the secondary fine particles 18 of copper is in a temperature region where the surface treating agent St denatures, the flow rate of the temperature adjusting gas supplied from the gas supply section 28 is changed, for example.

Since the sensor's measurement result of temperature is used in determination as to whether the temperature falls in the temperature region where the surface treating agent St does not denature as described above, the sensor is preferably provided at a position upstream of the connection position P₁ of the first supply tube 41b to the connection tube 21a. Accordingly, the sensor is provided to the connection tube 21a for instance.

While the configuration of the sensor is not particularly limited as long as the temperature can be measured, a short measuring time is preferred. Thus, examples of usable sensors include a resistance thermometer, a radiation thermometer, an infrared radiation thermometer, and a thermistor.

Next, one example of the method of producing copper fine particles using the production apparatus 10 above is described.

First, for example, copper powder having an average particle size of 15 μm or less is charged into the material supply device 14 as the feedstock powder of the copper fine particles.

As the plasma gas, for example, argon gas and hydrogen gas are used, and a high frequency voltage is applied to the high frequency oscillation coil 12b to generate the thermal plasma flame 24 in the plasma torch 12.

Next, the copper powder is transported with gas, e.g., argon gas used as the carrier gas and supplied to the thermal plasma flame 24 in the plasma torch 12 through the supply tube 14a. The copper powder supplied is evaporated in the thermal plasma flame 24 to be transformed into a mixture in a gas phase state, from which the primary fine particles 15 of copper are generated in the chamber 16 with no use of cooling gas.

Then, the primary fine particles 15 of copper thus obtained in the chamber 16 pass through the connection tube 21a and are blown in through the inlet tube 19a of the cyclone 19 together with a gas stream along the inner peripheral wall of the outer tube 19b, and accordingly, this gas stream flows along the inner peripheral wall of the outer tube 19b as indicated by arrow T in FIG. 3, thus forming a swirling stream which goes downward. When the downward swirling stream is inverted to an upward stream, coarse particles cannot follow the upward stream due to the balance between the centrifugal force and drag, fall down along the lateral surface of the truncated conical part 19c, and are collected in the coarse particle collecting chamber 19d. Fine particles having been affected by the drag more than the centrifugal force are discharged along the inner wall of the truncated conical part 19c to the outside of the cyclone 19 together with the upward stream on the inner wall.

Due to the negative pressure (suction force) applied by the vacuum pump 29 through the collecting section 20, the discharged secondary fine particles 18 of copper are sucked in the direction indicated by arrow U in FIG. 3 to pass through the inner tube 19e and the connection tube 21b.

When the primary fine particles 15 of copper pass through the connection tube 21a or the secondary fine particles 18 of copper pass through the connection tube 21b, the temperature adjusting gas is supplied from the gas supply section 28 into the connection tube 21a through the first gas supply tube 28b to cool the primary fine particles 15 of copper or into the connection tube 21b through the second gas supply tube 28c to cool the secondary fine particles 18 of copper.

After, with the temperature adjusting gas, the temperature of the primary fine particles 15 of copper or the secondary fine particles 18 of copper is changed to fall in the temperature region where the surface treating agent St does not denature, the surface treating agent St is supplied in the form of, for example, spray from the supply section 40 to the primary fine particles 15 of copper in the connection tube 21a to thereby surface treat the primary fine particles 15 of copper. Or, the surface treating agent St is supplied in the form of, for example, spray from the supply section 40 to the secondary fine particles 18 of copper in the connection tube 21b to thereby surface treat the secondary fine particles 18 of copper.

The surface-treated primary fine particles 15 of copper or the surface-treated secondary fine particles 18 of copper, i.e., the copper fine particles 30 are transported to the collecting section 20 and collected by the filter 20b in the collecting section 20. The copper fine particles 30 are obtained in this manner.

When the copper fine particles 30 are collected by the collecting section 20, the internal pressure of the cyclone 19 is preferably equal to or lower than the atmospheric pressure. For the particle size of the copper fine particles 30, an arbitrary particle size on the order of nanometers is specified according to the purpose.

While the primary fine particles of copper are formed using the thermal plasma flame as a heat source in the invention, the primary fine particles of copper may be formed by a different gas-phase process. Thus, the method of forming the primary fine particles of copper is not limited to the one using the thermal plasma flame as long as it is a gas-phase process, and may be a method using a flame process, for example. The method of forming the primary fine particles using the thermal plasma flame is called a thermal plasma process.

The flame process herein is a method in which a flame is used as a heat source and fine particles are synthesized by putting a copper-containing feedstock through the flame. In the flame process, the copper-containing feedstock is supplied to the flame so that copper fine particles are generated in the flame, thus obtaining the primary fine particles 15 of copper. Then, the surface treating agent St is supplied to the primary fine particles 15 of copper or the secondary fine particles 18 of copper to thereby produce the copper fine particles.

In the flame process, for the surface treating agent, the same agent as that stated for the thermal plasma process described above can also be used.

The present invention is basically configured as above. While the copper fine particles and the method of producing copper fine particles according to the invention are described above in detail, the invention is by no means limited to the foregoing embodiments, and it should be understood that various improvements and modifications are possible without departing from the scope and spirit of the invention.

EXAMPLES

The copper fine particles of the invention are more specifically described below.

In Examples, copper fine particles of Examples 1 and 2 and copper fine particles of Comparative Example 1 were produced. To produce the copper fine particles of Examples 1 and 2 and the copper fine particles of Comparative Example 1, the production apparatus 10 shown in FIG. 3 was used. The production conditions are shown below.

Example 1

In Example 1, copper powder having an average particle size of 5 μm was used as the feedstock powder. The average particle size of the copper powder is a value measured with a particle size distribution meter. As the particle size distribution meter, MT3300 (trade name) manufactured by MicrotracBEL Corp. was used.

The production conditions for the copper fine particles were as follows: an input power to plasma was set to be constant at 18 kW, and the pressure in the plasma torch was fixed at 60 kPa.

Argon gas was used as the carrier gas. The flow rate of the argon gas was set to 5 L/minute (standard state converted value).

Argon gas and hydrogen gas were used as the plasma gas. The flow rate of the argon gas was set to 200 L/minute (standard state converted value), and the flow rate of the hydrogen gas was set to 5 L/minute (standard state converted value).

Argon gas was used as the temperature adjusting gas. The flow rate of the argon gas was set to 200 L/minute (standard state converted value).

In Example 1, triethylene glycol (molecular weight: 150.17, boiling point: 276° C.) was used as a surface treating agent. Using ethanol as a solvent, a solution containing triethylene glycol (concentration of triethylene glycol: 5 W/W %) was sprayed to the secondary fine particles 18 of copper (see FIG. 3) through the second supply tube 41c (see FIG. 3) using a spray gas. Argon gas was used as the spray gas.

For the triethylene glycol, that manufactured by Tokyo Chemical Industry Co., Ltd. was used. For the ethanol, that manufactured by JUNSEI CHEMICAL CO., LTD. (product code 17065-1283) was used.

Example 2

Example 2 is different from Example 1 in that polyethylene glycol (molecular weight: about 200, boiling point: 250° C.) was used as a surface treating agent. In Example 2, using ethanol as a solvent, a solution containing polyethylene glycol (concentration of polyethylene glycol: 5 W/W %) was sprayed to the secondary fine particles 18 of copper (see FIG. 3) through the second supply tube 41c (see FIG. 3) using a spray gas. Argon gas was used as the spray gas.

The flow rate of argon gas used as the temperature adjusting gas was set to 380 L/minute (standard state converted value) in Example 2.

For the polyethylene glycol, that manufactured by Tokyo Chemical Industry Co., Ltd. was used. For the ethanol, that manufactured by JUNSEI CHEMICAL CO., LTD. (product code 17065-1283) was used.

Comparative Example 1

Comparative Example 1 is different from Example 1 in that terpineol (molecular weight: 154.25, boiling point: 219° C.) was used as a surface treating agent. In Comparative Example 1, using ethanol as a solvent, a solution containing terpineol (terpineol concentration: 5 W/W %) was sprayed to the secondary fine particles 18 of copper (see FIG. 3) through the second supply tube 41c (see FIG. 3) using a spray gas. Argon gas was used as the spray gas.

The flow rate of argon gas used as the temperature adjusting gas was set to 200 L/minute (standard state converted value) in Comparative Example 1.

For the terpineol, that manufactured by FUJIFILM Wako Pure Chemical Corporation was used. For the ethanol, that manufactured by JUNSEI CHEMICAL CO., LTD. (product code 17065-1283) was used.

Scanning electron microscope (SEM) images of the copper fine particles in Examples 1 and 2 were obtained. The SEM images were obtained using Regulus (registered trademark) 8220 (trade name) manufactured by Hitachi High-Technologies Corporation. FIG. 4 shows an SEM image of the copper fine particles in Example 1, and FIG. 5 shows an SEM image of the copper fine particles in Example 2.

To measure the particle sizes of the copper fine particles in Examples 1 and 2 and the copper fine particles in Comparative Example 1 by the BET method, Macsorb HM-1208 (trade name) manufactured by Mountech Co., Ltd. was used.

The copper fine particles in Examples 1 and 2 were baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere, and then SEM images thereof were obtained. The SEM images were obtained using Regulus (registered trademark) 8220 (trade name) manufactured by Hitachi High-Technologies Corporation. FIG. 6A is a schematic view showing an SEM image of the copper fine particles in Example 1 before baking, and FIG. 6B is a schematic view showing an SEM image of the copper fine particles in Example 1 after baking. FIG. 7A is a schematic view showing an SEM image of the copper fine particles in Example 2 before baking, and FIG. 7B is a schematic view showing an SEM image of the copper fine particles in Example 2 after baking.

The copper fine particles in Examples 1 and 2 and the copper fine particles in Comparative Example 1, all in a particulate form, were baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere, and then the particle sizes based on the BET method were measured. The results thereof are shown in Table 1 below.

The particle growth rate was determined from the particle size before baking and that after baking. The particle growth rate was obtained using the following formula.

Particle growth rate (%)=((particle size after baking)/(particle size before baking))×100

For the copper fine particles in Examples 1 and 2 and the copper fine particles in Comparative Example 1, measurements were made for the removal percentage of the coating layer at a temperature of 200° C. by the thermogravimetric and differential thermal analysis, the weight loss percentage at a temperature of 200° C. by the thermogravimetric and differential thermal analysis, and the weight increase percentage at a temperature of 200° C. by the thermogravimetric and differential thermal analysis. The results thereof are shown in Table 1 below.

The removal percentage of the coating layer at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis is defined by a value obtained by performing the thermogravimetric and differential thermal analysis (TG-DTA) in a nitrogen atmosphere and dividing the weight loss percentage at a temperature of 200° C. by the weight loss percentage at a temperature of 700° C.

The thermogravimetric and differential thermal analysis was used to measure the removal percentage of the coating layer at a temperature of 200° C. and the weight loss percentage and the weight increase percentage at a temperature of 200° C. The weight increase percentage was obtained by performing the thermogravimetric and differential thermal analysis (TG-DTA) in an air atmosphere.

For the thermogravimetric and differential thermal analysis (TG-DTA), STA7200 (trade name) of Hitachi High-Tech Science Corporation was used.

The copper fine particles in Examples 1 and 2 and the copper fine particles in Comparative Example 1 were separately formed into cylindrical pellets, and the volume resistivity and the volume contraction percentage of each pellet were measured after being baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere. The results thereof are shown in Table 1 below.

A nitrogen atmosphere was an atmosphere where nitrogen gas of 99.99 purity (nitrogen: 99.99 vol %) was constantly circulated, and in the atmosphere, the oxygen concentration was 3 volume ppm or lower. The oxygen concentration was measured using a low concentration oxygen analyzer PS-820-L (trade name) manufactured by Iijima Electronics Corporation.

For measuring the volume resistivity, first, the copper fine particles were pressed and retained at pressure of 127 MPa for 10 seconds with a pressing machine to thereby prepare a cylindrical pellet. Loresta EP ((device name), MCP-T360 (model)) manufactured by Mitsubishi Chemical Corporation was used as a measurement device, and the volume resistivity of the pellet was measured by the four-terminal method before and after baking.

The pellet was placed in an electric furnace and baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere.

In measurement of the volume contraction percentage, first, the copper fine particles were pressed and retained at pressure of 127 MPa for 10 seconds with a pressing machine to thereby prepare a cylindrical pellet. The thickness and the diameter of the cylindrical pellet were measured with a caliper, and the volume contraction percentage was calculated based on the volume of the pellet before baking and that after baking. In the calculation of the volume contraction percentage, the following formula was used. The pellet was placed in an electric furnace and baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere.

Volume contraction percentage (%)=100−((volume after baking/volume before baking)×100)

The copper fine particles in Examples 1 and 2 were separately formed into cylindrical pellets, and an SEM image of each pellet before baking and that after baking were obtained. The results thereof are shown in FIGS. 8A and 8B and FIGS. 9A and 9B.

FIG. 8A is a schematic view showing an SEM image of a pellet of the copper fine particles in Example 1 before baking, and FIG. 8B is a schematic view showing an SEM image of the pellet of the copper fine particles in Example 1 after baking. FIG. 9A is a schematic view showing an SEM image of a pellet of the copper fine particles in Example 2 before baking, and FIG. 9B is a schematic view showing an SEM image of the pellet of the copper fine particles in Example 2 after baking.

FIGS. 8A and 8B and FIGS. 9A and 9B confirm that the copper fine particles are bonded together and thus the copper fine particles have been sintered as shown in FIG. 8B and FIG. 9B. Although not shown, a cylindrical pellet was formed also for Comparative Example 1, and an SEM image of the pellet before baking and that after baking were obtained. In Comparative Example 1, sintering of the copper fine particles was not confirmed. Thus, sintering of the copper fine particles was checked by observation using the SEM images. The results thereof are shown in Table 1 below.

For the copper fine particles of Examples 1 and 2 and the copper fine particles of Comparative Example 1, the crystal structure was analyzed by X-ray diffractometry. FIG. 10 shows analysis results of crystal structure of the copper fine particles in Examples 1 and 2 of the invention as obtained by X-ray diffractometry. FIG. 10 shows a diffraction pattern 60 of Example 1 and a diffraction pattern 62 of Example 2.

FIG. 10 confirms that Examples 1 and 2 are copper fine particles. With the copper fine particles of Examples 1 and 2, the crystal structure was analyzed by X-ray diffractometry three weeks later, and it was confirmed that analysis results of the crystal structure similar to FIG. 10 were obtained, although not shown.

The copper fine particles in Examples 1 and 2 were again separately formed into pellets 1 week after and 2 weeks after the production of the copper fine particles, each pellet was baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere, and the volume resistivity was measured; then, the results shown in FIG. 11 were obtained. No big change was seen in volume resistivity in both Examples 1 and 2. FIG. 11 is a graph showing temporal changes in the volume resistivity of the copper fine particles in Examples 1 and 2 of the invention.

	TABLE 1
			Comparative
	Example 1	Example 2	Example 1

Surface treating agent	Tri-	Poly-	Terpineol
	ethylene	ethylene
	glycol	glycol

Before	BET (m2/g)	2.07	1.91	2.72
baking	Particle size (nm)	344	374	262
After	BET (m2/g)	1.92	1.71	3.00
baking	Particle size (nm)	372	416	238

Removal percentage of coating	38.6	47.8	26.8
layer (mass %)
Weight loss percentage (mass	0.489	0.651	0.213
%)
Weight increase percentage	2.78	1.43	5.93
(mass %)
Particle growth rate (%)	108	111	90.7

After	Volume	3.17 ×	2.96 ×	9.45 ×
baking	resistivity (Ωcm)	10−5	10−5	10−5
	Volume	11	14	2.7
	contraction
	percentage (%)

Sintering of copper fine	Sintered	Sintered	Not
particles			sintered

As shown in Table 1, the copper fine particles in Examples 1 and 2 have particle growth rates larger than that of the copper fine particles in Comparative Example 1 and also have been confirmed to be sintered, the particle growth rate in each example having been determined using the particle size before baking and the particle size after 1-hour heating of the particles in a particulate form at a temperature of 200° C. in a nitrogen atmosphere.

For the copper fine particles in Examples 1 and 2, sintering was confirmed after the particles were separately formed into cylindrical pellets and each pellet was baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere. The copper fine particles in Examples 1 and 2 had smaller volume resistivities and larger volume contraction percentages than those of the copper fine particles in Comparative Example 1 after being separately formed into cylindrical pellets and baked at a temperature of 200° C. for 1 hour in a nitrogen atmosphere.

In Comparative Example 1, the removal percentage of the coating layer at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis was less than 30 mass %. This fact reveals that the copper fine particles cannot be bonded together well in Comparative Example 1.

In addition, in Comparative Example 1, since the weight loss percentage at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis is less than 0.25 mass %, the copper fine particles cannot be bonded together well. In addition, in Comparative Example 1, since the weight increase percentage at a temperature of 200° C. measured by the thermogravimetric and differential thermal analysis is more than 5 mass %, the copper fine particles cannot be bonded together well.

REFERENCE SIGNS LIST

• • 10 copper fine particle production apparatus (production apparatus)
  • 12 plasma torch
  • 12a quartz tube
  • 12b high frequency oscillation coil
  • 12c plasma gas supply port
  • 14 material supply device
  • 14a supply tube
  • 15 primary fine particle
  • 16 chamber
  • 18 secondary fine particle
  • 19 cyclone
  • 19a inlet tube
  • 19b outer tube
  • 19c truncated conical part
  • 19d coarse particle collecting chamber
  • 19e inner tube
  • 20 collecting section
  • 20a collecting chamber
  • 20b filter
  • 21a, 21b connection tube
  • 22 plasma gas supply section
  • 22a pipe
  • 24 thermal plasma flame
  • 28 gas supply section
  • 28a valve
  • 28b first gas supply tube
  • 28c second gas supply tube
  • 29 vacuum pump
  • 30 copper fine particle
  • 40 supply section
  • 41 valve
  • 41b first supply tube
  • 41c second supply tube
  • 50 substrate
  • 52 power semiconductor device
  • 53 semiconductor device
  • 54 bonding portion
  • 60, 62 diffraction pattern
  • St surface treating agent