JP7830196B2 - silver fine particles - Google Patents

Description

This invention relates to silver nanoparticles used for bonding or wiring between semiconductor elements, high-frequency devices, light-emitting diodes, or semiconductor lasers, etc., and substrates.

Currently, power semiconductor devices using wide-bandgap semiconductors such as silicon carbide (SiC), gallium nitride (GaN), gallium oxide, or diamond are being developed. Compared to semiconductor devices using Si or GaAs, power semiconductor devices have lower on-resistance, enable high-speed switching, and can be miniaturized. Moreover, power semiconductor devices have high heat resistance and can operate at high temperatures of 250 to 300°C.
Solder has traditionally been used to join semiconductor elements to substrates and other materials. However, power semiconductor elements have a higher operating temperature than conventional semiconductor elements, and soldering requires use at a temperature where the solder does not melt. When solder is used for joining, the use of power semiconductor elements is restricted. Thus, joining materials that can be used at high temperatures are also required.

For example, Patent Document 1 describes joining SiC semiconductor elements using eutectic AuGe solder.
In addition to solder, Patent Document 2 describes a thermally conductive paste as a bonding material comprising low-temperature sinterable silver nanoparticles and a thermosetting binder, wherein the thermosetting binder consists of (B1) at least one epoxy resin selected from the group consisting of diglycidyl phthalate, diglycidyl tetrahydrophthalate, diglycidyl hexahydrophthalate and C1-C4 alkyl substituted derivatives thereof, and (B2) at least one curing agent selected from the group consisting of cationic polymerization initiators, amine curing agents and acid anhydride curing agents, and the amount of thermosetting binder is 2 to 7 parts by mass per 100 parts by mass of silver nanoparticles.

Patent No. 5856314 Patent No. 6343041

As described above, when AuGe solder is used as in Patent Document 1, its coefficient of thermal expansion is large, and the volume fluctuates due to temperature changes associated with the operation of the power semiconductor element, causing cracks and other damage, making it impossible to maintain a sufficient bond. Furthermore, AuGe solder has a lower melting point than copper or silver, and its heat resistance is not sufficient.
When a thermosetting binder is included, as in Patent Document 2, the volume shrinkage rate can be reduced, but the volume resistivity is high and the conductivity is insufficient.
Thus, currently, there is no bonding material that satisfies the requirements for heat resistance while also possessing excellent conductivity.

The object of this invention is to provide silver nanoparticles that suppress volume shrinkage and have high conductivity.

To achieve the above objective, one aspect of the present invention provides silver nanoparticles having a particle size of 0.1 μm or more and 1 μm or less as measured by the BET method, a volume resistivity of 10 μΩ·cm or less after firing in the atmosphere at a temperature of 150°C for 1 hour in pellet form, and a volume shrinkage rate of less than 5%.
It is preferable that the content of particles with a particle size of less than 0.1 μm is 40% or less by volume. More preferably, the content of particles with a particle size of less than 0.1 μm is 35% or less by volume, and even more preferably, the content of particles with a particle size of less than 0.1 μm is 5% or less by volume.

According to the present invention, it is possible to provide silver nanoparticles that suppress volume shrinkage and have high conductivity.

This is a schematic diagram illustrating an example of how the silver nanoparticles of the present invention can be used. This is a schematic diagram showing an example of a manufacturing apparatus for silver nanoparticles according to the present invention. This graph shows the particle size distribution of silver nanoparticles in Examples 1 and 2 of the present invention. This is a schematic diagram showing an SEM image of silver nanoparticles according to Example 1 of the present invention. This is a schematic diagram showing an SEM image of silver nanoparticles according to Example 2 of the present invention. This is a schematic diagram showing a conventional SEM image of silver nanoparticles.

The silver nanoparticles of the present invention will be described in detail below based on preferred embodiments shown in the attached drawings.
The figures described below are illustrative examples for illustrating the present invention, and the present invention is not limited to the figures shown below.
The following describes silver nanoparticles.

[Silver fine particles]
The silver nanoparticles have a particle size of 0.1 μm or more and 1 μm or less as measured by the BET method, and after firing in pellet form in air at a temperature of 150°C for 1 hour, their volume resistivity is 10 μΩ·cm or less and their volume shrinkage rate is less than 5%.
In this invention, "atmosphere" refers to the atmosphere generally known as air. The atmosphere is also called the atmospheric environment. The composition of air is 78.08% nitrogen by volume, 20.95% oxygen by volume, 0.93% argon by volume, and 0.03% carbon dioxide by volume. A general measurement error is acceptable for the composition of air.
It is preferable that the content of particles with a particle size of less than 0.1 μm is 40% or less by volume. More preferably, the content of particles with a particle size of less than 0.1 μm is 35% or less by volume, and even more preferably, the content of particles with a particle size of less than 0.1 μm is 5% or less by volume. Furthermore, the lower limit of the content of particles with a particle size of less than 0.1 μm is 0% by volume.
Furthermore, the particle size of the silver nanoparticles is preferably 100 to 400 nm, and more preferably 200 to 400 nm, because this reduces the volume shrinkage rate after firing in the atmosphere (i.e., air) at a temperature of 150°C for 1 hour in the pellet state.
The particle size of silver nanoparticles measured by the BET method is the average particle diameter measured using the BET method. In the BET method, the particle size is calculated from the specific surface area, assuming that the particles are spherical.
The silver nanoparticles exist independently, rather than being dispersed in a solvent or other medium. Therefore, when using silver nanoparticles, a calcined body can be obtained using only the silver nanoparticles.
Furthermore, when using silver nanoparticles in combination with a solvent, the combination of silver nanoparticles and the solvent is not particularly limited, offering a high degree of freedom in solvent selection.

The volume resistivity is preferably 9 μΩ·cm or less, more preferably 8 μΩ·cm or less, and most preferably 7 μΩ·cm or less. The lower limit of the volume resistivity is 1.47 μΩ·cm.
Furthermore, the volume shrinkage rate is preferably 3% or less, more preferably 1% or less, and most preferably 0%. The lower limit of the volume shrinkage rate is 0%.
Furthermore, it is preferable that the silver nanoparticles contain 40% or less of particles with a particle size of less than 0.1 μm by volume, as this suppresses the occurrence of cracks after firing.
In silver nanoparticles, the proportion of particles with a particle size of less than 0.1 μm is determined from the volume-based particle size distribution obtained by acquiring SEM (scanning electron microscope) images of the silver nanoparticles and performing image analysis on the SEM images. In other words, the proportion of particles with a particle size of less than 0.1 μm is determined as a percentage of the total volume of the silver nanoparticles.

For firing, the silver nanoparticles are formed into cylindrical pellets, which are then placed in an electric furnace and fired at 150°C for one hour in the open air.
The pellets are produced by pressing silver nanoparticles in a press machine and holding them at a pressure of 127 MPa for 10 seconds.
The volume resistivity is a value obtained by measuring pellets using the four-terminal method. For example, the Loresta EP (MCP-T360) manufactured by Mitsubishi Chemical Corporation is used as the measuring device. By measuring the volume resistivity of the pellets before and after firing, the change in volume resistivity after firing can be measured.

The volume shrinkage rate is calculated by forming cylindrical pellets from silver nanoparticles using a press machine as described above, holding them at a pressure of 127 MPa for 10 seconds. The thickness and diameter of the cylindrical pellets are measured with calipers, and the volume shrinkage rate is calculated from the volume of the pellets before and after firing. The volume shrinkage rate is obtained by the following formula. An electric furnace is used for firing the pellets.
Volume shrinkage rate (%) = 100 - ((Volume after firing / Volume before firing) × 100)

Silver nanoparticles have a particle size of 0.1 μm to 1 μm as measured by the BET method, and after firing in pellet form in air at 150°C for 1 hour, their volume resistivity is 10 μΩ·cm or less, and their volume shrinkage rate is less than 5%. This suppresses volume shrinkage and provides high conductivity. Furthermore, silver nanoparticles have a higher melting point than solder and other materials, and exhibit superior heat resistance. Therefore, when silver nanoparticles are used as a bonding material, it is possible to achieve excellent conductivity while satisfying heat resistance requirements.
Furthermore, it is preferable that the proportion of particles with a particle size of less than 0.1 μm is 40% or less by volume, because this ensures that larger particles are more uniform, thereby further suppressing volume shrinkage after firing of the silver nanoparticles.

Figure 1 is a schematic diagram showing an example of how the silver nanoparticles of the present invention can be used.
Silver nanoparticles are used, for example, in the bonding between the substrate 50 and the power semiconductor element 52 shown in Figure 1. Silver nanoparticles are also used in die attachments.
The silver nanoparticles constitute a bonding portion 54 that joins the substrate 50 and the power semiconductor element 52. The bonding portion 54 is formed by firing the silver nanoparticles in air at a temperature of 150°C for 1 hour. The bonding portion 54 joins the substrate 50 and the power semiconductor element 52, physically fixing them together.
Silver nanoparticles have a higher melting point and higher heat resistance compared to solder and resin. Furthermore, as mentioned above, the volume resistivity of silver nanoparticles after firing in pellet form at 150°C for 1 hour in air is 10 μΩ·cm or less, and the volume shrinkage rate is less than 5%. As a result, even if temperature changes occur due to the operation of the power semiconductor element 52, the volume fluctuation of the junction 54 is small, and the occurrence of cracks is suppressed. This maintains the junction and provides high durability. In addition, because the volume resistivity of silver nanoparticles is low after firing, they also have excellent thermal conductivity, and the heat generated by the power semiconductor element 52 can be efficiently conducted to the substrate 50 by the junction 54.
The substrate 50 is, for example, a ceramic substrate such as _Si3N4 on _which copper wiring is provided.
The power semiconductor element 52 is a semiconductor element that uses a semiconductor such as silicon carbide (SiC), gallium nitride (GaN), gallium oxide, or diamond.
Furthermore, the silver nanoparticles are not limited to bonding with power semiconductor elements 52, but can also be used for bonding with high-frequency devices, light-emitting diodes, or semiconductor lasers. Silver nanoparticles are suitable for bonding with devices that generate a lot of heat or have high operating temperatures.
Furthermore, silver nanoparticles can be used not only for bonding but also for various types of wiring, such as signal wiring and conductive wiring.

[Method for manufacturing silver nanoparticles]
Next, an example of a method for producing silver nanoparticles will be described based on Figure 2, but the method for producing silver nanoparticles of the present invention is not limited to a method using the silver nanoparticle production apparatus shown in Figure 2.
Figure 2 is a schematic diagram showing an example of a silver nanoparticle manufacturing apparatus according to the present invention.
The above-mentioned silver nanoparticles can be obtained using the silver nanoparticle manufacturing apparatus 10 shown in Figure 2 (hereinafter simply referred to as the manufacturing apparatus 10).

The manufacturing apparatus 10 includes a plasma torch 12 for generating a thermal plasma flame, a material supply device 14 for supplying raw material powder of silver nanoparticles into the plasma torch 12, a chamber 16 that functions as a cooling tank for generating primary silver nanoparticles 15, a cyclone 19 for removing coarse particles having a particle size greater than or equal to an arbitrarily defined particle size from the primary silver nanoparticles 15, and a recovery unit 20 for recovering secondary silver nanoparticles 18 having a desired particle size that have been classified by the cyclone 19. The chamber 16 and the cyclone 19 are connected by a connecting pipe 21a. The cyclone 19 and the recovery unit 20 are connected by a connecting pipe 21b connected to an inner pipe 19e.
The manufacturing apparatus 10 further includes a supply unit 40 that supplies a surface treatment agent to primary silver particles 15 or secondary silver particles 18.
The primary silver particles 15 and the secondary silver particles 18 are both fine particles obtained during the manufacturing process of the fine particles of the present invention. The fine particles of the present invention are obtained by surface-treating the primary silver particles 15 or the secondary silver particles 18, i.e., the surface-treated silver particles 30.
For the material supply device 14, chamber 16, cyclone 19, and recovery unit 20, for example, various devices described in Japanese Patent Application Publication No. 2007-138287 can be used.

In this embodiment, for example, silver powder is used as a raw material for producing fine particles.
The average particle size of the silver powder is appropriately set so that it evaporates easily in a thermal plasma flame. The average particle size of the silver powder is measured using laser diffraction and is, for example, 100 μm or less, preferably 50 μm or less, and more preferably 15 μm or less.

The plasma torch 12 consists of a quartz tube 12a and a high-frequency oscillation coil 12b surrounding it. A supply pipe 14a (described later) for supplying fine particle raw material powder into the plasma torch 12 is located in the center of the upper part of the plasma torch 12. A plasma gas supply port 12c is formed around the supply pipe 14a (on the same circumference), and the plasma gas supply port 12c is ring-shaped. A power supply (not shown) that generates a high-frequency voltage is connected to the high-frequency oscillation coil 12b. When a high-frequency voltage is applied to the high-frequency oscillation coil 12b, a thermal plasma flame 24 is generated. The thermal plasma flame 24 evaporates the raw material (not shown), turning it into a mixture in a gaseous state. The plasma torch 12 is a processing unit that uses a gaseous method to turn the raw material into a mixture in a gaseous state.

The plasma gas supply unit 22 supplies plasma gas into the plasma torch 12. The plasma gas supply unit 22 is connected to the plasma gas supply port 12c via piping 22a. Although not shown in the diagram, the plasma gas supply unit 22 is equipped with a supply volume adjustment unit, such as a valve, for adjusting the supply amount. The plasma gas is supplied from the plasma gas supply unit 22 through the ring-shaped plasma gas supply port 12c into the plasma torch 12 from the directions indicated by arrows P and S.

For example, a mixture 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 unit 22. From the plasma gas supply unit 22, hydrogen gas and argon gas are supplied into the plasma torch 12 via piping 22a and the plasma gas supply port 12c from the directions indicated by arrows P and S. Note that only argon gas may be supplied in the direction indicated by arrow P.
Furthermore, since the plasma gas used depends on the silver nanoparticles, it is not essential to use a mixed gas as described above, and a single type of gas may suffice as the plasma gas.
When a high-frequency voltage is applied to the high-frequency oscillation coil 12b, a thermal plasma flame 24 is generated inside the plasma torch 12.

The temperature of the thermal plasma flame 24 must be higher than the boiling point of the raw material powder. On the other hand, a higher temperature of the thermal plasma flame 24 is preferable because it makes it easier for the raw material powder to enter a gaseous state, but the temperature is not particularly limited. For example, the temperature of the thermal plasma flame 24 can be set to 6000°C, and theoretically it is thought that it can reach about 10000°C.
Furthermore, the pressure atmosphere inside the plasma torch 12 is preferably below atmospheric pressure. Here, the atmosphere below atmospheric pressure is not particularly limited, but for example, it is 0.5 to 100 kPa.

Furthermore, the outside of the quartz tube 12a is surrounded by concentrically formed tubes (not shown). Cooling water is circulated between these tubes and the quartz tube 12a to water-cool it, preventing the quartz tube 12a from becoming too hot due to 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 via a supply pipe 14a. The material supply device 14 supplies raw materials into the thermal plasma flame 24 inside the plasma torch 12.
The material supply device 14 is not particularly limited as long as it can supply the raw material into the thermal plasma flame 24. For example, the raw material may be supplied into the thermal plasma flame 24 in a state where it is dispersed in particulate form.

When the raw material is in powder form, for example, a material supply device 14 that supplies silver powder in powder form can be, as described above, for example, one disclosed in Japanese Patent Application Publication No. 2007-138287. In this case, the material supply device 14 includes, for example, a storage tank (not shown) for storing the raw material, a screw feeder (not shown) for quantitatively transporting the raw material, a dispersion unit (not shown) for dispersing the raw material transported by the screw feeder into primary particles before it is finally scattered, and a carrier gas supply source (not shown).

The raw material, along with the carrier gas under extrusion pressure from the carrier gas supply source, is supplied to the thermal plasma flame 24 in the plasma torch 12 via the supply pipe 14a.
The material supply device 14 is not particularly limited in its configuration, as long as it can prevent the aggregation of raw materials and disperse them into the plasma torch 12 while maintaining a dispersed state. An inert gas such as argon gas is used as the carrier gas. The carrier gas flow rate can be controlled using a flow meter such as a float-type flow meter. The carrier gas flow rate value is the scale value of the flow meter.

Chamber 16 is located adjacent to the lower part of the plasma torch 12. Within Chamber 16, primary silver particles 15, which are particulate matter, are generated from the aforementioned gaseous mixture without the use of a cooling gas. Chamber 16 functions as a cooling tank. The cooling gas, also known as a rapid cooling gas, is typically argon gas or similar.

The gas supply unit 28 supplies a temperature-regulating gas containing an inert gas to, for example, the connecting pipe 21a or the connecting pipe 21b. The gas supply unit 28 supplies the temperature-regulating gas containing an inert gas to the primary silver particles 15 or the secondary silver particles 18.
The gas supply unit 28 includes, for example, a valve 28a and a first gas supply pipe 28b and a second gas supply pipe 28c connected to the valve 28a. The first gas supply pipe 28b is connected to a connecting pipe 21a, and the second gas supply pipe 28c is connected to the connecting pipe 21b.
By switching valve 28a, temperature-regulating gas is supplied to either the first gas supply pipe 28b or the second gas supply pipe 28c, and the temperature-regulating gas is supplied to either the connecting pipe 21a or the connecting pipe 21b.

The gas supply unit 28 further includes a compressor or a pressure-applying device (not shown) such as a blower that applies extrusion pressure to the temperature-regulating gas supplied to the first gas supply pipe 28b or the second gas supply pipe 28c.
Furthermore, the gas supply unit 28 includes a storage unit (not shown) for storing temperature-regulating gas and a pressure control valve for controlling the amount of gas supplied. The temperature-regulating gas is, for example, argon gas.
The desired gas temperature can be adjusted by the temperature-regulating gas supplied from the gas supply unit 28 into the connecting pipe 21a or the connecting pipe 21b.

As shown in Figure 2, the chamber 16 is equipped with a cyclone 19 for classifying primary silver particles 15 to a desired particle size. This cyclone 19 comprises an inlet pipe 19a for supplying primary particles 15 from the chamber 16, a cylindrical outer cylinder 19b connected to the inlet pipe 19a and located at the top of the cyclone 19, a frustoconical section 19c extending downward from the bottom of the outer cylinder 19b with a gradually decreasing diameter, a coarse particle recovery chamber 19d connected to the bottom of the frustoconical section 19c for recovering coarse particles having a particle size greater than or equal to the desired particle size, and an inner pipe 19e protruding from the outer cylinder 19b and connected to a recovery section 20 (described in detail later). The chamber 16 and the inlet pipe 19a are connected by a connecting pipe 21a, and the primary particles 15 move to the cyclone 19 through the connecting pipe 21a. The connecting pipe 21a is a transport path for the primary particles 15.

An airflow containing primary particles 15 is blown in from the inlet pipe 19a of the cyclone 19 along the inner circumferential wall of the outer cylinder 19b, thereby forming a downward-flowing swirling flow as this airflow flows from the inner circumferential wall of the outer cylinder 19b toward the frustoconical portion 19c, as indicated by the arrow T in Figure 2.
Then, when the aforementioned downward swirling flow reverses and becomes an upward flow, due to the balance between centrifugal force and drag, the coarse particles cannot ride the upward flow and descend along the side of the frustum 19c, where they are collected in the coarse particle collection chamber 19d. Furthermore, fine particles that are more affected by drag than by centrifugal force are discharged outside the cyclone 19 along with the upward flow along the inner wall of the frustum 19c through the inner tube 19e and connecting tube 21b.

Furthermore, negative pressure (suction force) is generated from the recovery unit 20, which will be described in detail later, through the inner tube 19e and the connecting tube 21b. This negative pressure (suction force) then draws in the fine particles separated from the swirling airflow, as indicated by the symbol U, and sends them to the recovery unit 20 through the inner tube 19e and the connecting tube 21b.

A recovery unit 20 is provided on the extension of the inner tube 19e, which is the outlet for the airflow inside the cyclone 19, to recover silver nanoparticles 30 having a desired particle size on the order of nanometers. The recovery unit 20 comprises a recovery chamber 20a, a filter 20b provided inside the recovery chamber 20a, and a vacuum pump 29 connected via a tube provided below inside the recovery chamber 20a. The silver nanoparticles 30 sent from the cyclone 19 are drawn into the recovery chamber 20a by the vacuum pump 29 and are recovered while remaining on the surface of the filter 20b.
Furthermore, in the manufacturing apparatus 10 described above, the number of cyclones used is not limited to one, but may be two or more.

The supply unit 40 supplies the surface treatment agent St to the silver fine particles within the chamber 16, either downstream of the first gas supply pipe 28b in the connecting pipe 21a, or downstream of the second gas supply pipe 28c in the connecting pipe 21b. Here, the side of the chamber 16 with respect to the connecting pipe 21a is referred to as the upstream side, and the side of the cyclone 19 is referred to as the downstream side.
The supply unit 40 includes, for example, a valve 41 and a first supply pipe 41a, a second supply pipe 41b, and a third supply pipe 41c connected to the valve 41. The first supply pipe 41a is connected to the side surface 16b of the chamber 16. The second supply pipe 41b is connected to the connecting pipe 21a downstream of the first gas supply pipe 28b, and the third supply pipe 41c is connected to the connecting pipe 21b downstream of the second gas supply pipe 28c. The first supply pipe 41a is connected in the chamber 16, for example, at a height of approximately the same as or lower than the position where the connecting pipe 21a is connected. The surface treatment agent St is supplied into the chamber 16 from the inner wall 16a of the chamber 16 via the first supply pipe 41a.
Let _P1 be the connection point in the connecting pipe 21a of the second supply pipe 41b, and let _P2 be the connection point in the connecting pipe 21b of the third supply pipe 41c. The connection point _P2 of the third supply pipe 41c is downstream of the connection point _P1 of the second supply pipe 41b.
The supply unit 40 supplies the surface treatment agent St to the primary silver particles 15 in the chamber 16, the primary silver particles 15 passing through the connecting pipe 21a, or the secondary silver particles 18 passing through the connecting pipe 21b.
The supply unit 40 supplies the surface treatment agent St in a temperature range suitable for the surface treatment agent St. The surface treatment agent St adheres to the primary silver particles 15 or secondary silver particles 18, the primary silver particles 15 or secondary silver particles 18 are surface-treated, the fusion of the silver particles is prevented, and silver particles 30 are obtained.
The method of supplying the surface treatment agent St by the supply unit 40 is not particularly limited, and an example is a method of forming the surface treatment agent St into droplets and spraying them onto the secondary silver particles 18.

As described above, the surface treatment agent St is supplied within a suitable temperature range. The suitable temperature range is the temperature range in which the surface treatment agent St can prevent the fusion of silver nanoparticles. Therefore, as long as the fusion of silver nanoparticles can be prevented, the surface treatment agent St may be introduced from a temperature range in which it denatures, or from a temperature range in which it does not denature.
Furthermore, the surface condition of surface-treated microparticles can be examined, for example, using an FT-IR (Fourier Transform Infrared Spectrophotometer).

The temperature range that can prevent the fusion of the silver nanoparticles described above is the temperature range in which the primary nanoparticles 15 can be coated with organic matter produced by the modification of the surface treatment agent St or with the surface treatment agent St itself. The temperature range in which the surface treatment agent St does not modify is the temperature range determined based on the temperature measured by differential thermal-thermogravimetric analysis (TG-DTA).
The temperature range in which the surface treatment agent St does not denature is defined as the temperature range in which the weight loss rate of the surface treatment agent St, measured by differential thermal-thermogravimetric simultaneous measurement, is 50% by mass or less. More preferably, the weight loss rate is 30% by mass or less, and even more preferably 10% by mass or less.
For simultaneous differential thermal and thermogravimetric measurements, the STA7200 (product name) from Hitachi High-Tech Science Corporation is used.

The surface treatment agent St is not particularly limited, but examples include an organic acid alone and a solution of an organic acid, as well as an organic substance having an amine group and a solution of an organic substance having an amine group.
Furthermore, if the organic acid is in liquid form when used, it is not necessarily required to dissolve it in a solvent, as in an aqueous solution; the organic acid can be used on its own. The same applies to surface treatment agents St other than organic acids, such as acidic substances, basic substances, natural resins, and synthetic resins; if they are in liquid form when used, they can be used on their own.
An example of an organic substance having an amine group is dodecylamine.

(Dispersant alone and dispersant solution)
For example, a dispersant having only amine groups can be used. The following can be used as dispersants. When the dispersant has amine groups, the amine value of the dispersant is preferably 10 to 100, and more preferably 10 to 60.

Examples of dispersants containing only amine groups include DISPERBYK-102, DISPERBYK-160, DISPERBYK-161, DISPERBYK-162, DISPERBYK-2163, DISPERBYK-2164, DISPERBYK-166, DISPERBYK-167, DISPERBYK-168, and DISPER BYK-2000, DISPERBYK-2050, DISPERBYK-2150, DISPERBYK-2155, DISPERBYK-LPN6919, DISPERBYK-LPN21116, DISPERBYK-LPN21234, DISPERBYK-9075, DISPERBYK-9077 (all manufactured by Big Chemie); EFKA Examples include EFKA 4015, EFKA 4020, EFKA 4046, EFKA 4047, EFKA 4050, EFKA 4055, EFKA 4060, EFKA 4080, EFKA 4300, EFKA 4330, EFKA 4340, EFKA 4400, EFKA 4401, EFKA 4402, EFKA 4403, and EFKA 4800 (all manufactured by BASF); and Adisper® PB711 (manufactured by Ajinomoto Fine Techno Co., Ltd.).

Examples of polymeric dispersants containing amine groups include DISPERBY K-142, DISPERBY K-145, DISPERBY K-2001, DISPERBY K-2010, DISPERBY K-2020, DISPERBY K-2025, DISPERBY K-9076, and Anti-Terra-205 (all manufactured by Bic Chemie); SOLSPERSE 24000 (manufactured by Lubrizol Corporation); and Azisper (registered trademark) PB821, Azisper PB880, and Azisper PB881 (all manufactured by Ajinomoto Fine Techno Co., Ltd.).

(Organic solvents)
There are no particular restrictions on organic solvents, and they can be appropriately selected depending on the purpose. Examples of organic solvents include alcohols such as methanol, ketones such as acetone, alkyl halides, amides such as formamide, sulfoxides such as dimethyl sulfoxide, heterocyclic compounds, hydrocarbons, esters such as ethyl acetate, and ethers. These may be used individually or in combination of two or more.

(Organic acids by themselves and organic acid solutions)
When using an organic acid, which is an acidic substance, as a surface treatment agent, for example, an aqueous solution is sprayed from the supply unit 40 using pure water as the solvent. In this case, the organic acid is preferably water-soluble and has a low boiling point, and is preferably composed only of C, O, and H. Examples of organic acids that can be _used include L _{- ascorbic acid (C6H8O6 )} , formic acid ( _CH2O2 ), glutaric acid ( _C5H8O4 ), _succinic acid ( _{C4H6O4 )} , _oxalic acid ( _{C2H2O4 )} , DL-tartaric acid ( _C4H6O6 ), lactose monohydrate, _{maltose monohydrate, maleic acid (C4H4O4 ) ,} D- _{mannitol ( C6H14O6} ), _citric acid ( _{C6H8O7 )} , malic acid ( _C4H6O5 ), _malonic acid ( _{C3H4O4 ) ,} and _aliphatic carboxylic acids _. It is preferable to use at least one of _the above _- mentioned organic acids.
For example, argon gas is used as the atomizing gas to atomize an aqueous solution of an organic acid, but it is not limited to argon gas; inert gases such as nitrogen gas can also be used.

The system may include a sensor (not shown) for measuring the temperature of the transport path for the primary silver particles 15 or secondary silver particles 18. The temperature measurement result from this sensor is used to determine whether or not the temperature is within a suitable range for the surface treatment agent St. In this case, the temperature measurement result is output to, for example, the supply unit 40. The supply unit 40 can determine whether or not the temperature is within a suitable range for the surface treatment agent St based on the temperature measurement result from the sensor for the transport path for the primary silver particles 15 or secondary silver particles 18. If the temperature of the transport path for the primary silver particles 15 or secondary silver particles 18 is within a temperature range unsuitable for the surface treatment agent St, the flow rate of the temperature-regulating gas supplied from the gas supply unit 28 is changed, for example.
As described above, the temperature measurement result from the sensor is used to determine whether or not the temperature range is suitable for the surface treatment agent St. Therefore, it is preferable to install the sensor upstream of the connection position _P1 in the connecting pipe 21a of the second supply pipe 41b. For this reason, the sensor is installed, for example, in the connecting pipe 21a.
While the configuration of a sensor is not particularly limited as long as it can measure temperature, a short measurement time is preferable. For this reason, sensors such as resistance thermometers, radiation thermometers, infrared radiation temperature sensors, and thermistors can be used.

Next, an example of a method for producing silver nanoparticles using the manufacturing apparatus 10 described above will be explained.
First, as a raw material powder for silver nanoparticles, for example, silver powder with an average particle size of 15 μm or less is put into the material supply device 14.
For example, argon gas and hydrogen gas are used as the plasma gas, and a high-frequency voltage is applied to the high-frequency oscillation coil 12b to generate a thermal plasma flame 24 inside the plasma torch 12.
Next, silver powder is transported as a gas using, for example, argon gas, and supplied to the thermal plasma flame 24 in the plasma torch 12 via the supply pipe 14a. The supplied silver powder evaporates in the thermal plasma flame 24 to become a mixture in a gaseous state, and primary silver particles 15 are generated in the chamber 16 from the mixture in the gaseous state without the use of a cooling gas.

The primary silver particles 15 obtained in the chamber 16 are then blown into the cyclone 19 through the connecting pipe 21a and the inlet pipe 19a, along the inner wall of the outer cylinder 19b, along with the airflow. As a result, this airflow flows along the inner wall of the outer cylinder 19b, as shown by arrow T in Figure 2, forming a swirling flow and descending. When the descending swirling flow reverses and becomes an upward flow, due to the balance between centrifugal force and drag, the coarse particles cannot ride the upward flow and descend along the side of the frustoconical section 19c, where they are collected in the coarse particle collection chamber 19d. Fine particles that are more affected by drag than centrifugal force are discharged from the inner wall of the frustoconical section 19c along with the upward flow on the inner wall, leaving the cyclone 19.
The discharged secondary silver particles 18 are sucked in the direction indicated by the symbol U in Figure 1 by the negative pressure (suction force) from the recovery unit 20 by the vacuum pump 29, and pass through the inner tube 19e and connecting tube 21b.

As the primary silver particles 15 or secondary silver particles 18 pass through the connecting pipe 21a or connecting pipe 21b, a temperature-regulating gas is supplied from the gas supply unit 28 through the first gas supply pipe 28b or the second gas supply pipe 28c into the connecting pipe 21a or connecting pipe 21b, thereby cooling the primary silver particles 15 or secondary silver particles 18. After the temperature-regulating gas brings the primary silver particles 15 or secondary silver particles 18 to a temperature range suitable for the surface treatment agent, the surface treatment agent St is further supplied from the supply unit 40 into the chamber 16, the connecting pipe 21a, or the connecting pipe 21b to the primary silver particles 15 or secondary silver particles 18, for example, in the form of a spray, thereby surface-treating the primary silver particles 15 or secondary silver particles 18.
Surface-treated primary silver particles 15 or secondary silver particles 18, i.e., silver particles 30, are sent to the recovery unit 20, where they are collected by the filter 20b of the recovery unit 20. In this way, silver particles are obtained.

When the silver nanoparticles 30 are collected in the collection unit 20, the internal pressure inside the cyclone 19 is preferably below atmospheric pressure. Furthermore, the particle size of the silver nanoparticles 30 can be any size on the order of nanometers, depending on the purpose.
In this invention, primary silver nanoparticles are formed using a thermal plasma flame as the heat source, but primary silver nanoparticles can also be formed using other gas-phase methods. Therefore, as long as a gas-phase method is used, it is not limited to using a thermal plasma flame; for example, a manufacturing method for forming primary silver nanoparticles using a flame method is also acceptable. The method of manufacturing primary silver nanoparticles using a thermal plasma flame is called the thermal plasma method.

Here, the flame method is a method of synthesizing fine particles by using a flame as a heat source and passing a silver-containing raw material through the flame. In the flame method, a silver-containing raw material is supplied to the flame, and silver particles are generated in the flame, and the growth of the silver particles is suppressed to obtain primary silver fine particles 15. Furthermore, a surface treatment agent St is supplied to the primary silver fine particles 15 or secondary silver fine particles 18 to produce silver fine particles.
Furthermore, in the flame method, the same surface treatment agent as in the thermal plasma method described above can be used.

The present invention is basically configured as described above. While the silver nanoparticles of the present invention have been described in detail above, the present invention is not limited to the embodiments described above, and various improvements or modifications may be made without departing from the spirit of the invention.

The silver nanoparticles of the present invention will be described in more detail below.
In this example, silver nanoparticles of Examples 1 to 3 and conventional silver nanoparticles were manufactured. The manufacturing apparatus 10 shown in Figure 2 was used to manufacture the silver nanoparticles of Examples 1 to 3 and the conventional silver nanoparticles. The manufacturing conditions are shown below.

In Example 1, silver powder with an average particle size of 15 μm was used as the raw material powder. The average particle size of the silver powder was measured using a particle size analyzer. A Microtrac-Bell MT3300 was used as the particle size analyzer.
The manufacturing conditions for the silver nanoparticles were as follows: the plasma input was kept constant at 18 kW, and the pressure inside the plasma torch was fixed at 60 kPa.
Argon gas was used as the carrier gas. The argon gas flow rate was set to 5 liters/minute (at standard conditions).
Argon and hydrogen gases were used as plasma gases. The flow rate of argon gas was set to 200 liters/minute (at standard conditions), and the flow rate of hydrogen gas was set to 5 liters/minute (at standard conditions).
Argon gas was used as the temperature control gas. The argon gas flow rate was set to 240 liters/minute (converted to standard conditions).
In Example 1, citric acid was used as the organic acid. Pure water was used as the solvent, and an aqueous solution containing citric acid (citric acid concentration 3.76 W/W%) was sprayed onto primary silver particles from a second supply pipe 41b (see Figure 2) connected to the side surface 16b of the chamber 16 using a spray gas. Argon gas was used as the spray gas.

Example 2 was the same as Example 1, except for the following:
In Example 2, the internal pressure of the plasma torch was fixed at 85 kPa. The flow rate of the argon gas used for temperature control was set to 15 liters/minute (converted to standard conditions).

Example 3 was identical to Example 2, except for the following: Example 3 did not use a temperature-regulating gas.

In the conventional silver nanoparticle setup, the plasma input was kept constant at 14 kW, and the internal pressure of the plasma torch was fixed at 40 kPa.
Argon gas was used as the carrier gas. The argon gas flow rate was set to 5 liters/minute (at standard conditions).
Argon and hydrogen gases were used as plasma gases. The flow rate of argon gas was set to 170 liters/minute (at standard conditions), and the flow rate of hydrogen gas was set to 5 liters/minute (at standard conditions).
Argon and methane gases were used as cooling gases. The flow rate of argon gas was set to 300 liters/minute (at standard conditions), and the flow rate of methane gas was set to 5.7 liters/minute (at standard conditions).
In contrast, conventional silver nanoparticles did not use temperature-regulating gases or organic acids.

For Examples 1 to 3, SEM images of the obtained silver nanoparticles were acquired. The SEM images were obtained using a Regulus 8220 manufactured by Hitachi High-Technologies Corporation.
The particle size distribution was calculated by image analysis of the SEM images of the silver nanoparticles of Examples 1 to 3. The results are shown in Figure 3. Figure 3 is a graph showing the particle size distribution of the silver nanoparticles of Examples 1 and 2 of the present invention. The particle size distribution of the silver nanoparticles shown in Figure 3 is the particle size distribution obtained on a volume basis.
In Figure 3, reference numeral 62 indicates the cumulative particle size distribution for Example 1, and reference numeral 63 indicates the frequency particle size distribution for Example 1. In Example 2, the particle size determined by the BET method was 192 nm.
Reference numeral 64 indicates the cumulative particle size distribution of Example 2, and reference numeral 65 indicates the frequency particle size distribution of Example 2. The particle size of Example 2 measured by the BET method was 356 nm. The particle size of Example 3 measured by the BET method was 386 nm. A Macsorb HM-1208 manufactured by Mountec Co., Ltd. was used to measure the particle size of the silver nanoparticles of Examples 1 to 3 by the BET method.
In the silver nanoparticles of Examples 1 to 3, the proportion of particles with a particle size of less than 0.1 μm was first determined by image analysis of SEM images of the silver nanoparticles to obtain the particle size distribution based on volume. Next, the proportion of particles with a particle size of less than 0.1 μm (100 nm) was determined for the silver nanoparticles from the particle size distribution based on volume.
As shown in Figure 3, Examples 1 and 2 have a low proportion of particles with a particle size of less than 0.1 μm (100 nm). It has also been confirmed that Example 3 has a low proportion of particles with a particle size of less than 0.1 μm (100 nm).

For Examples 1-3 and conventional silver nanoparticles, the volume resistivity was measured before firing after forming them into cylindrical pellets, and the volume resistivity and volume shrinkage rate were measured after firing in air (i.e., in air) at a temperature of 150°C for 1 hour. The results are shown in Table 1 below. The composition of the air was as described above.
Table 1 below shows the particle sizes of Examples 1 to 3, as well as the volume-based content percentage of particles with a particle size of less than 0.1 μm (100 nm). In Table 1 below, the "conventional silver nanoparticles" mentioned above are referred to as the "conventional example."
In measuring the volume resistivity, silver nanoparticles were first pressed at a pressure of 127 MPa for 10 seconds to produce cylindrical pellets. A Mitsubishi Chemical Corporation Loresta EP (MCP-T360) was used as the measuring device, and the volume resistivity of the pellets before and after firing was measured using the four-terminal method.
The pellets were placed in an electric furnace and fired in the atmosphere at a temperature of 150°C for 1 hour.

In measuring the volume shrinkage rate, first, silver nanoparticles were pressed using a press machine and held at a pressure of 127 MPa for 10 seconds to produce cylindrical pellets. The thickness and diameter of the cylindrical pellets were measured with calipers to obtain the volume before firing. After firing, the thickness and diameter of the cylindrical pellets were also measured with calipers to obtain the volume after firing. The volume shrinkage rate was calculated from the volumes of the pellets before and after firing. The following formula was used to calculate the volume shrinkage rate. The pellets were placed in an electric furnace and fired in air at a temperature of 150°C for 1 hour.
Volume shrinkage rate (%) = 100 - ((Volume after firing / Volume before firing) × 100)
The density was measured as follows: The thickness and diameter of the cylindrical pellets before firing were measured with calipers, and the mass of the pellets was measured with an electronic balance. The density of the cylindrical pellets before firing was calculated from the volume and mass of the cylindrical pellets. After firing, the thickness and diameter of the cylindrical pellets were measured with calipers, and the mass of the pellets was measured with an electronic balance. The density of the cylindrical pellets after firing was calculated from the volume and mass of the cylindrical pellets after firing.

Figure 4 is a schematic diagram showing an SEM image of silver nanoparticles according to Example 1 of the present invention, and Figure 5 is a schematic diagram showing an SEM image of silver nanoparticles according to Example 2 of the present invention. Figure 6 is a schematic diagram showing an SEM image of conventional silver nanoparticles.
As shown in Figures 4 to 6, the silver nanoparticles of Examples 1 and 2 have larger particle sizes and fewer small particles than conventional silver nanoparticles. It was also confirmed that Example 3 has larger particle sizes and fewer small particles than conventional silver nanoparticles.
As shown in Table 1, the silver nanoparticles of Examples 1 to 3, after being formed into cylindrical pellets and fired in air at a temperature of 150°C for 1 hour, exhibited lower volume resistivity and lower volume shrinkage compared to conventional silver nanoparticles.

10. Manufacturing equipment for silver nanoparticles (manufacturing equipment)
12 Plasma torch 12a Quartz tube 12b High-frequency oscillation coil 12c Plasma gas supply port 14 Material supply device 14a Supply pipe 15 Primary particles 16 Chamber 16a Inner wall 16b Side 18 Secondary particles 19 Cyclone 19a Inlet pipe 19b Outer cylinder 19c Truncated cone section 19d Coarse particle recovery chamber 19e Inner pipe 20 Recovery section 20a Recovery chamber 20b Filter 21a, 21b Connecting pipes 22 Plasma gas supply section 22a Piping 24 Thermal plasma flame 28 Gas supply section 28a Valve 28b First gas supply pipe 28c Second gas supply pipe 29 Vacuum pump 30 Silver particles 40 Supply section 41 Valve 41a First supply pipe 41b Second supply tube 41c Third supply tube 50 Substrate 52 Power semiconductor element 54 Joint St Surface treatment agent

Claims (1)

The content of particles with a particle size of less than 0.1 μm is 0.2 to 3.0% by volume.
The particle size measured by the BET method was 192–386 nm.
The density before firing is 7.19 to 7.66 g/cm³ ^, and the volume resistivity before firing is 31.0 to 68.1 μΩ·cm.
The proportion of particles with a particle size of less than 0.1 μm is determined as a percentage of the total volume of silver nanoparticles, based on the volume-based particle size distribution obtained by image analysis of SEM images of the silver nanoparticles .