EP4613402A1

EP4613402A1 - Metal powder, method for producing said metal powder, and metal paste

Info

Publication number: EP4613402A1
Application number: EP23885581.1A
Authority: EP
Inventors: Noriaki Nakamura; Kohei Ogawa; Kenichi Inoue; Hiroshi Murai; Yuichi Makita; Teruaki Koizumi; Yohei Okada; Haruki SHIRATORI; Hidehiro Kamiya
Original assignee: Tanaka Kikinzoku Kogyo KK
Current assignee: Tanaka Precious Metal Technologies Co Ltd
Priority date: 2022-10-31
Filing date: 2023-10-24
Publication date: 2025-09-10
Also published as: WO2024095821A1; CN120129577A; JP2024065141A; EP4613402A4; KR20250107192A; JP7412714B1

Abstract

The present invention relates to a metal powder containing a metal of Au, Ag, Cu, or an alloy thereof, having an average particle size of 0.1 µm or more and 0.4 µm or less, and purity of 99.9% by mass or more. In this metal powder, a presence ratio, in terms of the number of particles, of non-spherical particles in the metal powder having a ratio (b/a) of a minor axis "a" and a major axis "b" of 3 or more is 1% or less. In a preferable aspect, a presence ratio, in terms of the number of particles, of coarse particles having a particle size of 0.5 µm or more is 10% or less. As a method for producing such a metal powder, in a wet reduction method including a metal colloid synthesis step and a metal powder granulation step of granulating metal colloidal particles into a metal powder, surfactants having alkyl groups having carbon numbers falling within specified ranges are preferably used as first and second dispersants in the metal colloid synthesis step and the metal powder granulation step.

Description

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention relates to a metal powder used for electrode/wiring formation, bonding, sealing and the like performed in the field of electronics such as a semiconductor device and a semiconductor element, and a method for producing the same, and in particular to a metal powder in which a presence ratio of non-spherical particles is reduced, and a method for producing the same.

DESCRIPTION OF THE RELATED ART

In recent years, use of metal pastes in various processes of element bonding, formation of electrodes and wirings, and hermetical sealing in various applications in electric/electronic components, semiconductor devices, semiconductor elements, power devices, MEMs and the like has been expanded. Regarding a metal paste for such application, the present applicant has already proposed that a metal paste containing a metal (such as gold, silver, palladium, or copper) of high purity (of 99.9% by mass or more), and obtained by mixing a metal powder of a submicron order (1 µm or less) with an organic solvent is useful for the aforementioned applications (for example, Patent Documents 1 to 3).
For example, in the production process of a semiconductor device, in bonding process such as die bonding or flip-chip bonding for bonding a semiconductor chip onto a base material (such as a substrate or an IC driver), a metal paste is applied for forming a desired shape or pattern on the base material by photoetching or the like. Then, the metal paste is dried, and appropriately temporarily sintered to form a bump, and a semiconductor chip is placed on this bump. Thereafter, the resultant is heated under pressure to sinter the metal powder constituting the bump, thus forming a metal powder sintered body, that is, a bonding medium. In such a bonding process, the metal paste proposed by the present applicant ensures a low-temperature sinterability by specifying the purity and the average particle size of the metal particle as described above, and thus, contributes to temperature reduction in the bonding process. The sintering temperature of a metal powder used in a metal paste is correlated with the particle size of the metal powder, and is liable to increase as the particle size is larger. Besides, the purity of the metal powder affects the plastic deformability of the metal powder in sintering, and affects the denseness of the metal powder sintered body obtained after the sintering. Therefore, the low-temperature sinterability is ensured and resistance increase of the bonding medium that is a conductor is suppressed by specifying the average particle size and using the metal powder with high purity.
As a method for producing a metal powder with the average particle size thus controlled, methods based on a wet reduction method are also known. For example, in a method for producing a metal powder (gold powder) by a wet reduction method described in Patent Document 4, a reducing agent and a metal salt are supplied to a solution in which gold ultrafine particles (colloidal particles) are dispersed as nuclear particles, and thus, gold is precipitated on the surfaces of the nuclear particles to obtain a gold powder. In this method, a gold powder of a submicron order can be produced by adjusting the particle size and the number of the nuclear particles, and the concentration and the amount of the gold compound solution supplied.

Prior Art Document

Patent Document

Patent Document 1
Japanese Patent No. 5613253
Patent Document 2
Japanese Patent Application Laid-Open No. 2013-206765
Patent Document 3
Japanese Patent Application Laid-Open No. 2021-025091
Patent Document 4
Japanese Patent Application Laid-Open No. Hei 9-20903

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

As described above, metal pastes and metal powders revealed so far have basic required characteristics such as the low-temperature sinterability and electric conductivity (low resistance). Under circumstances where the use of metal pastes has been expanding in the field of electronics, however, higher quality and versatility are demanded. For example, the behavior of a metal powder in application of a metal paste does not directly affect properties such as electric resistance of a bump or bond portion to be formed, but is deemed as a significant item, and there are many points to be considered. Therefore, the present invention reveals a metal powder and a metal paste having higher quality, to be used in various processes of bonding, electrode formation, and sealing performed in the field of electronics, and a method for producing the same.

Solution to Problem

Regarding the aforementioned problem, the present inventors have decided to examine, as the structure of a metal powder produced by a wet reduction method, improvement of the particle size distribution of the metal powder for the following reason. Even in the bonding process of the field of electronics in which low-temperature bonding is required, it is not only the average particle size of a metal powder that should be evaluated. According to the examination made by the present inventors, in a metal powder produced by a wet reduction method, most of particles are well-shaped spherical metal particles, but some particles may be in a non-spherical shape (rod shape, plate shape, rectangular shape and the like). It is presumed that these irregular-shaped particles do not affect the average particle size, and probably little affect the sintering temperature and the like. When a presence ratio of non-spherical particles in shapes different from that of surrounding spherical particles is increased, however, it is determined in an electron microscope image of the entire metal powder that the appearance is abnormal. In addition to such a problem of the appearance, the behavior of the metal powder in the process of applying and sintering the metal paste may be affected. For example, when a metal paste is applied to a base material having a hole or a trench, an irregular-shaped particle may hinder the metal powder from being filled in the hole or the trench. Besides, the shape of an electrode, a bump or the like formed from the metal powder may be affected, or incompatibility may be caused because a rod-shaped particle bridges bumps or the like.
Through these examinations, the present inventors have presumed that the structural element of a metal paste or metal powder with higher quality is that it contains spherical particles with formation of non-spherical particles suppressed. Under this policy, the present inventors have decided to optimize raw materials and production conditions in a method for producing a metal powder based on a conventional wet reduction method. As a result, a metal powder in which a presence ratio of non-spherical particles is appropriately suppressed has been found, resulting in accomplishing the present invention.
The present invention that solves the above-described problem is drawn to a metal powder containing a metal of Au, Ag, or Cu, or an alloy thereof having an average particle size of 0.1 µm or more and 0.4 µm or less, and purity of 99.9% by mass or more, wherein a presence ratio, in terms of the number of particles, of non-spherical particles in the metal powder having a ratio (b/a) of a minor axis "a" and a major axis "b" of 3 or more is 1% or less. Now, the metal powder of the present invention, a method for producing the same, and a metal paste using the metal powder will be described in more detail.

A. Structure of Metal Powder of Present Invention

As described above, the metal powder of the present invention contains a metal of Au, Ag, or Cu, or an alloy of these having high purity (of 99.9% by mass or more). The constituent metal of the metal powder is specified as Au, Ag, or Cu because these are metals that can be sintered at a comparatively low temperature when in the form of a fine powder, and these are all metals having favorable conductivity. The metal powder may contain an alloy of the metals, and a metal powder containing an alloy obtained by mutually alloying Au, Ag, and Cu, or an alloy containing Au, Ag or Cu as a principal component (80% by mass or more) can be used. Au is particularly preferred. Au is comparatively soft, and can be formed into a dense sintered body through plastic deformation occurring in sintering, and is excellent in conductivity, and therefore, a bond portion, a bump, or the like having low resistance can be formed therefrom.
The purity of the metal powder is specified as 99.9% by mass or more because a low-purity metal containing an impurity has high hardness, and plastic deformation is difficult to proceed in forming a sintered body to be used as a bonding material or the like. The purity herein refers to, in a metal powder containing any one of Au, Ag, and Cu, a concentration of the metal element, and in a metal powder containing an alloy of Au, Ag, and Cu, concentrations of the metals and the alloy element.
The average particle size of the metal powder is specified as 0.1 µm or more and 0.4 µm or less for optimizing a sintering temperature of the metal powder. The sintering temperature of a metal powder is liable to increase as the average particle size increases. In the present invention, as a range in which the low-temperature sinterability is particularly good, the upper limit of the average particle size is set to 0.4 µm. On the other hand, the lower limit is set to 0.1 µm because a metal powder having an average particle size smaller than this value easily aggregates when formed into a paste. In the present invention, a particle in the metal powder having a particle size of 0.5 µm or more beyond the upper limit (0.4 µm) of the average particle size is determined as a coarse particle.
The average particle size of the metal powder of the present invention is a number average particle size (M_N). For measuring the average particle size of a metal powder, the metal powder is observed and imaged with a microscope (such as an optical microscope, or an electron microscope (SEM, or TEM)), and a plurality of metal particles included in a photograph or image are arbitrarily selected to measure the particle size. In this observation and particle size measurement, it is preferable that a plurality of (preferably 5 or more) observation regions are set, and that a plurality of (N: preferably 100 or more) metal particles are observed and measured in each region. The observation magnification is preferably 10000 times or more, and more preferably 20000 or more and 30000 or less. For the measurement of the particle size of the metal powder, the particle size of each metal particle in a photograph or image may be measured, or computer software such as image analysis software may be used. As the particle size, a particle size obtained by a biaxial method employing calculation based on a major axis and a minor axis of a particle in the image, or a Feret diameter (caliper diameter) based on the length of a side of a rectangle circumscribing a particle in the image, or the like can be employed. As the Feret diameter, at least any one of a minimum Feret diameter, a maximum Feret diameter, and a mean Feret diameter is preferably obtained.
In the present invention, the presence ratio of non-spherical particles is equal to or less than a prescribed value. In the present invention, a particle in the metal powder having the ratio (b/a) of the minor axis "a" and the major axis "b" of the particle of 3 or more is defined as a non-spherical particle. In the metal powder of the present invention, the presence ratio, in terms of the number of particles, of non-spherical particles is 1% or less. Even when the presence ratio is as small as 1%, the non-spherical particles may cause a problem in applying or sintering process of the metal paste in addition to a defect in appearance of the entire metal powder. The determination of the non-spherical particle through the measurement of the ratio between the minor axis "a" and the major axis "b" can be performed together with the measurement of the particle size of the metal powder and the measurement of the average particle size described above. At this point, when the biaxial method is employed as the method for calculating the particle size of the metal powder, a minor axis "a"nd a major axis measured are set as the minor axis "a" and the major axis "b", respectively. Alternatively, when the Feret diameter is used as the particle size, the minimum Feret diameter and the maximum Feret diameter are set as the minor axis "a" and the major axis "b", respectively.
In the metal powder of the present invention, a presence ratio of non-spherical particles as well as coarse particles in the metal powder is preferably limited. The metal powder of the present invention specified in the average particle size naturally contains metal particles having particle sizes equal to or larger than the average particle size, and it is difficult to completely eliminate the presence of coarse particles. When the presence ratio of metal particles much more larger than the average particle size is large, the sinterability can be affected. Therefore, as a factor that may affect the quality of the metal powder, it is preferable to consider the presence of coarse particles as well as irregular-shaped particles. In the present invention, a coarse particle refers to a particle of 0.5 µm or more, and the presence ratio, in terms of the number of particles, of such coarse particles is preferably 10% or less. For the calculation of the particle size of the coarse particle, the same method as that described above can be employed.
Besides, when the metal powder of the present invention is one obtained through a production method of the present invention employing a wet reduction method described below, a compound or a derivative derived from a surfactant used as a dispersant may be bonded to the surface thereof. The surfactant is present on the surface of the powder from a stage prior to forming the metal powder into a metal paste. Specifically, an alkylamine salt or a quaternary ammonium salt having an alkyl group having a carbon number of 14 or more and 18 or less may be bonded to at least a part of the surface of the metal powder. Besides, an alkylamine salt or a quaternary ammonium salt having an alkyl group having a carbon number of 12 or more and less than 14 may be also bonded to at least a part of the surface of the metal powder. Such compounds do not affect the sintering property of the metal powder, but indicates that a prescribed surfactant is used in the production process (metal colloid synthesis step described below). When such a surfactant is bonded to the metal powder, the amount of the surfactant is preferably 0.01% or more and 5% or less, and more preferably 0.03% or more and 3% or less in terms of a mass ratio. When the compound is attached in an excessive amount, the sintering of the metal powder subsequently performed may be affected.

B. Method for Producing Metal Powder of Present Invention

As described above, for the problem that a metal powder having higher quality than a conventional one needs to be found, the present inventors have improved a wet reduction method employed as the production method. In the method for producing a metal powder by a wet reduction method, a reducing agent and a metal salt are supplied to a solution in which colloidal particles of a metal are dispersed as nuclear particles for grain growth, thereby forming (granulating) a metal powder. Besides, the synthesis of metal colloidal particles is also basically based on the wet reduction method, and a metal salt used as a raw material is mixed with a reducing agent in a solvent for reduction-precipitating the metal to obtain metal colloidal particles. In the synthesis step of metal colloidal particles, a dispersant is mixed together with the metal salt and the reducing agent. The dispersant is an additive for suppressing coarsening of particles through excessive aggregation by bonding to the surfaces of the metal colloidal particles having been reduction-precipitated. In the present invention, in consideration of binding properties to various metals and the like, a surfactant having an alkyl group (cationic surfactant) is used as the dispersant.
In the method for producing a metal powder by the wet reduction method, the dispersant is an essential additive in the metal colloid synthesis step, and binds to metal colloidal particles formed through reduction-precipitation, and the resultant in that state is used in the subsequent metal powder granulation step. Also in the metal powder granulation step, a dispersant the same as or different from that used in the metal colloid synthesis step is added in many cases for suppressing aggregation of a metal powder during the growth process. Besides, as described below, a reaction solution obtained through the synthesis step of metal colloidal particles is assumed to be used in the whole amount or a part thereof. In this case, a portion of the dispersant not having bonded to metal colloidal particles in the metal colloid synthesis step remains in the reaction solution in the metal powder granulation step.
Accordingly, it is deemed that the dispersant is contained in the reaction solutions of both the synthesis step of metal colloidal particles and the granulation step of the metal powder. The present inventors have examined the influence, on the metal colloidal particles and the gold powder, of the carbon number of an alkyl group of the surfactant used as the dispersant in the metal colloid synthesis step and the metal powder granulation step, resulting in obtaining the following findings (i) and (ii):

(i) In the metal colloid synthesis step, when a surfactant having a small carbon number of an alkyl group is used, coarse metal colloidal particles are easily produced, and the particle size distribution is liable to be wide.

As the carbon number of an alkyl group of a surfactant used is larger, the particle size of metal colloidal particles is liable to be smaller. Besides, in accordance with the increase of the carbon number, the particle size distribution becomes narrower, and hence, metal colloidal particles having small variation in the particle size are produced. When the particle size of the metal colloidal particles produced is smaller, the number of particles increases. Since a large number of metal colloidal particles having a small particle size are produced, the metal colloidal particles effectively exhibit catalytic action in the subsequent metal powder granulation step, which promotes uniform grain growth, and thus, a metal powder in a favorable shape is formed.
(ii) On the other hand, when a surfactant having a large carbon number of an alkyl group is used in the metal powder granulation step, the shape of the resultant metal powder is affected, and the growth of non-spherical particles is promoted.
On the contrary, when a surfactant having a small carbon number of an alkyl group is used in the metal powder granulation step, the shape of the particles of the resultant metal powder is less affected.
In the metal powder of the present invention, the presence ratio of non-spherical particles is reduced, and it is preferable that the presence ratio of coarse particles is also reduced. In order to produce such a metal powder, it is necessary to suppress the growth of non-spherical particles with the variation in the particle size distribution reduced. Referring to the above-described two findings, it is probably preferable to use, as a dispersant, a surfactant having an alkyl group having a large carbon number in the metal colloid synthesis step, and to use, as a dispersant, a surfactant having an alkyl group having a small carbon number in the metal powder granulation step.
Based on the above-described examination, the present inventors have found that a carbon number of 14 is suitable as a threshold value in the carbon number of an alkyl group of a surfactant for distinguishing the effect of improving the particle size distribution and suppressing the production of coarse particles from the effect of suppressing production of non-spherical particles. In the metal colloid synthesis step, a surfactant having an alkyl group having a carbon number of 14 or more is determined as an essential dispersant, and on the other hand, in the metal powder granulation step, a surfactant having an alkyl group having a carbon number less than 14 is used, and thus, the metal powder of the present invention can be produced.
Specifically, the method for producing a metal powder of the present invention includes a metal colloid synthesis step of synthesizing metal colloidal particles by reacting a metal salt with a reducing agent in a first solvent containing a first dispersant; and a metal powder granulation step of forming a metal powder from the metal colloidal particles by adding a metal salt, a reducing agent, and an optional second dispersant to a second solvent containing the metal colloidal particles having been synthesized in the metal colloid synthesis step. The first solvent used in the metal colloid synthesis step contains, as the first dispersant, at least a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less, and the second solvent used in the metal powder granulation step contains, as the second dispersant, a surfactant having an alkyl group having a carbon number of 12 or more and less than 14. The respective steps of the method for producing a metal powder of the present invention will now be described.

(a-1) Metal Colloid Synthesis Step

As described above, the metal colloid synthesis step is a step of synthesizing metal colloidal particles serving as nuclei of a metal powder based on the wet reduction method. In the metal colloid synthesis step, a metal salt are reacted with a reducing agent in the presence of a dispersant in a first solvent to reduction-precipitate a metal. Examples of the metal salt of Au used as the raw material include chloroauric acid salt, gold sulfite, and gold cyanite. Besides, examples thereof of Ag include silver chloride, silver nitrate, and silver acetate, and examples thereof of Cu include copper chloride, copper nitrate, and copper sulfate. As the reducing agent, hydroxylammonium chloride, sodium borohydride, dimethylamine boron, trisodium citrate dihydrate, or the like can be used. These can be mixed in the form of a solution. The first solvent is not limited as long as it is a solvent capable of dissolving the metal salt, the reducing agent, and the dispersant. A polar solvent is preferably used, and specifically, water or an organic solvent such as alcohol, and a mixed solvent of water and an organic solvent are preferable solvents.
In the metal colloid synthesis step, a dispersant including a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less is essentially contained as the first dispersant in the solvent (reaction solution). A surfactant having an alkyl group having a carbon number less than 14 increases variation in the particle size of metal colloidal particles in the metal colloid synthesis step, which leads to increase of the particle size. When such metal colloidal particles are grown in the metal powder granulation step, the presence ratio of non-spherical particles in the metal powder may increase. On the other hand, when a surfactant having an alkyl group having a carbon number of more than 18 is used as the dispersant in the metal colloid synthesis step, if the dispersant is present in the reaction solution in the metal powder granulation step, the growth of non-spherical particles such as rod-shaped particles is promoted to increase the presence ratio of non-spherical particles. For these reasons, in the present invention, a dispersant including a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less is used as the first dispersant in the metal colloid synthesis step. The specific composition of the first dispersant will be described below together with the composition of the second dispersant.
The reaction solution in the metal colloid synthesis step may contain the first dispersant (surfactant having an alkyl group having a carbon number of 14 or more and 18 or less), and in addition, a surfactant having an alkyl group having a carbon number of 12 or more and less than 14 may be present in the reaction solution.
The metal colloidal particles are synthesized by mixing the metal salt, the reducing agent, and the first dispersant with the first solvent. The order of mixing these is not especially limited, and for example, the metal salt (solution) may be added to a mixture solution of the dispersant and the reducing agent.
As a preferable composition of the reaction solution in the metal colloid synthesis step, a metal concentration in the reaction solution is preferably 0.01 g/L or more and 1 g/L or less, and more preferably 0.01 g/L or more and 0.1 g/L or less. When the gold concentration in the reaction solution is low, homogeneous metal colloidal particles can be formed, but when the metal concentration is too low, the reaction of producing colloid is difficult to proceed, and hence, the above-described range is preferred. Besides, the amount of the reducing agent is preferably 2.5 times or more and 10 times or less as compared with the molar concentration of the metal in the reaction solution. When the concentration of the reducing agent is high, homogeneous metal colloidal particles are liable to be synthesized, but when the concentration is excessively high, a part of the reducing agent remains unreacted, which may cause aggregation of the particles. The concentration of the dispersant is preferably 0.1 g/L or more and 10 g/L or less. When the concentration is less than 0.1 g/L, the function as the dispersant cannot be exhibited, and even when the concentration is more than 10 g/L, the function as the dispersant is not affected. The concentration of the dispersant is more preferably 1 g/L or more and 10 g/L or less.
As the reaction conditions for the metal colloidal particle synthesis, a reaction temperature is preferably 80°C or more and 90°C or less. When the temperature is less than 80°C, the reaction of producing metal colloidal particles is difficult to proceed, and when the temperature is more than 90°C, the reactivity does not change, and a problem in the production may occur, for example, the reaction solution may largely vaporize.

(a-2) Metal Powder Granulation Step

The metal powder granulation step is a step of producing a metal powder by growing the metal colloidal particles having been synthesized in the metal colloid synthesis step. In the metal powder granulation step, a metal salt and a reducing agent are added to a second solvent containing the metal colloidal particles to grow the metal colloidal particles, and thus, a metal powder is granulated. As the metal salt and the reducing agent added in this step, the same ones as the metal salt and the reducing agent used in the metal colloid synthesis step described above can be used. The metal salt and the reducing agent may not be, however, those of the same types (compositions).
In dispersing the metal colloidal particles in the second solvent in the metal powder granulation step, the entire reaction solution synthesized in the metal colloid synthesis step may be directly used in the metal powder granulation step. Alternatively, a part of the reaction solution synthesized in the metal colloid synthesis step may be taken out so that the reaction solution thus taken out may be used in the metal powder granulation step. In this case, the second solvent is the same as the first solvent.
Alternatively, with a part or the whole of the reaction solution synthesized in the metal colloid synthesis step used, a new solvent may be additionally added thereto. The additional addition of a solvent can be performed for purpose of adjusting the concentrations of the metal salt, the reducing agent, and the like, and adjusting the liquid amount for ensuring the handleability of the reaction solution. As the solvent added in this case, the same type of solvent as the first solvent can be used, and the same solvent as the first solvent may be used. A mixed solvent of the solvent added here and the first solvent corresponds to the second solvent. It goes without saying that the same solvent as the first solvent can be added here. Moreover, only the metal colloidal particles are separated and collected from the reaction solution synthesized in the metal colloid synthesis step, and these metal colloidal particles may be dispersed in a new solvent. Also in this case, the second solvent is preferably a polar solvent such as water or an alcohol similarly to the first solvent.
In the metal powder granulation step, a surfactant having an alkyl group having a carbon number of 12 or more and less than 14 is required to be contained as the second dispersant in the second solvent. As described above, a surfactant having an alkyl group having a large carbon number has the function of promoting the growth of non-spherical particles, and hence a surfactant having a carbon number of more than 14 is not positively used. It is, however, preferable that a dispersant is present in the process for growing the metal colloidal particles to the metal powder. A surfactant having an alkyl group having a carbon number of 12 or more and less than 14 functions as the dispersant for the metal powder without promoting the growth of non-spherical particles, and hence is used as the essential dispersant (second dispersant) in this step.
It is, however, not essential to add the second dispersant (surfactant having an alkyl group having a carbon number of 12 or more and less than 14) in the metal powder granulation step. When a part or the whole of the reaction solution in the metal colloid synthesis step is used in the metal powder granulation step, and the reaction solution contains the surfactant having an alkyl group having a carbon number of 12 or more and less than 14, the surfactant functions as the second dispersant. In this case, there is no need to perform an operation for adding the second dispersant. In particular, when a mixed dispersant described below is used, there is no need to perform an operation for adding the second dispersant in some cases. In the present invention, there is a requirement that a surfactant having an alkyl group having a carbon number of 12 or more and less than 14 is contained in the reaction solution in the metal powder granulation step. The details of the second dispersant will be described below together with those of the first dispersant.
The reaction solution in the metal powder granulation step may contain the second dispersant (surfactant having an alkyl group having a carbon number of 12 or more and less than 14), and in addition, a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less may be present in the reaction solution.
A metal powder is produced by mixing a metal salt, a reducing agent, and an optional second dispersant with the second solvent described above. The order of mixing these is not especially limited. As a preferable composition of the reaction solution in the metal powder granulation step, the metal concentration is preferably 10 g/L or more and 150 g/L or less. The metal salt added in the metal powder granulation step is a precursor for growing the fine metal colloidal particles into a metal powder having a prescribed average particle size. Therefore, the metal concentration in the metal powder granulation step can be set in accordance with the average particle size of the metal powder to be produced. An excessively high metal concentration may, for example, cause ununiform nucleus production, and hence the above-described range is preferred. The metal concentration in the reaction solution in the metal powder granulation step refers to a sum of the mass of the metal contained in the metal salt added in the metal powder granulation step, and the mass of the metal colloidal particles serving as the nuclei. Besides, the amount of the reducing agent to be mixed is preferably 2.5 times or more and 5 times or less as compared with the molar concentration of the metal in the reaction solution. When the amount of the reducing agent is too small, a portion of the metal salt may remain unreacted. Alternatively, when the amount of the reducing agent is too large, the reaction easily occurs rapidly, and hence not only the particle size is difficult to control but also safe and stable production may become difficult.
The concentration of the dispersant in the reaction solution in the metal powder granulation step is preferably 1/80 times or more and 1/6 times or less as compared with the metal concentration in the reaction solution. When the concentration value of the dispersant in the reaction solution in the metal colloid synthesis step is used as a reference, the concentration of the dispersant is preferably 1/50 times or more and 2 times or less as compared with that. The dispersant is used in the metal powder granulation step not for the effect of suppressing aggregation of the particles but for maintaining the particle size distribution, and there is no need for the concentration of the dispersant to exceed the metal concentration. In this regard, the concentration of the dispersant in the metal powder granulation step is different from the dispersant concentration in the metal colloid synthesis step. A dispersant concentration at some level is, however, necessary for maintaining the particle size distribution. In particular, in the metal powder granulation step, a considerable amount of the metal salt is added to increase the metal concentration for growing the fine metal colloidal particles into a metal powder of a submicron order, and therefore, the dispersant is preferably added. The surfactant having an alkyl group having a carbon number of 14 or more and 16 or less specified in the present invention can suppress the production of non-spherical particles and coarse particles, and therefore, is allowed to be positively added in the metal powder granulation step. For these reasons, the concentration of the dispersant in the reaction solution is preferably within the above-described range.
The concentration of the dispersant in the reaction solution in the metal powder granulation step is calculated based on the total amount of the dispersant contained in the reaction solution, and it does not matter whether or not the dispersant is bonded to the metal colloidal particles.
A reaction temperature in the metal powder granulation step is preferably 80°C or more and 90°C or less. This is for the following reasons: when the temperature is lower than 80°C, a portion of the metal salt may remain unreacted even if the conditions of the amounts of the metal salt and the reduction agent and the like are favorable. When the temperature is more than 90°C, the reaction easily occurs rapidly, and hence stable production of the metal powder may become difficult.

(b) Specific Compositions of First and Second Dispersants

As described above, in the method for producing a metal powder of the present invention, the carbon numbers of alkyl groups of the first and second dispersants (surfactants) essentially contained in the reaction solutions in each of the metal colloid synthesis step and the metal powder granulation step are specified.
In the present invention, preferable examples of the surfactant having an alkyl group specifically include an alkylamine salt and a quaternary ammonium salt of cationic surfactants.
Examples of the surfactant preferable as the first dispersant based on the carbon number of an alkyl group thereof (14 or more and 18 or less) include the following. Examples of the alkylamine salt include tetradecylamine acetate (having a carbon number of an alkyl group of 14), pentadecylamine acetate (having a carbon number of an alkyl group of 15), hexadecylamine acetate (having a carbon number of an alkyl group of 16), heptadecylamine acetate (having a carbon number of an alkyl group of 17), octadecylamine acetate (having a carbon number of an alkyl group of 18), tetradecylamine hydrochloride (having a carbon number of an alkyl group of 14), pentadecylamine hydrochloride (having a carbon number of an alkyl group of 15), hexadecylamine hydrochloride (having a carbon number of an alkyl group of 16), heptadecylamine hydrochloride (having a carbon number of an alkyl group of 17), and octadecylamine hydrochloride (having a carbon number of an alkyl group of 18). Examples of the quaternary ammonium salt include tetradecyl trimethyl ammonium salt (having a carbon number of an alkyl group of 14), pentadecyl trimethyl ammonium salt (having a carbon number of an alkyl group of 15), hexadecyl trimethyl ammonium salt (having a carbon number of an alkyl group of 16), heptadecyl trimethyl ammonium salt (having a carbon number of an alkyl group of 17), and octadecyl trimethyl ammonium salt (having a carbon number of an alkyl group of 18).
Examples of a surfactant preferable as the second dispersant based on the carbon number of an alkyl group thereof (12 or more and less than 14) include the following. Examples of the alkylamine salt include dodecylamine acetate (having a carbon number of 12), tridecylamine acetate (having a carbon number of 13), dodecylamine hydrochloride (having a carbon number of 12), and tridecylamine hydrochloride (having a carbon number of 13). Examples of the quaternary ammonium salt include dodecyl trimethyl ammonium salt (having a carbon number of an alkyl group of 12), and tridecyl trimethyl ammonium salt (having a carbon number of an alkyl group of 13).
In the present invention, the phrase that "the first solvent of the metal colloid synthesis step contains a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less" refers to that the first solvent may contain at least any one of surfactants having an alkyl group having a carbon number of 14 or more and 18 or less, and need not contain all of these. The first solvent may contain a plurality of surfactants having carbon numbers falling within the aforementioned range. For example, the first solvent may contain two surfactants, that is, a surfactant having an alkyl group having a carbon number of 14 (such as tetradecylamine acetate) and a surfactant having an alkyl group having a carbon number of 16 (such as hexadecylamine acetate). Similarly, the second solvent in the metal powder granulation step may contain at least any one of surfactants having an alkyl group having a carbon number of 12 or more and less than 14. The ranges of the carbon numbers of the alkyl groups of the first and second dispersants are preferably a carbon number of 14 or more and 16 or less in the first dispersant, and a carbon number of 12 in the second dispersant.
As the usage forms of the first and second dispersants (surfactants), the surfactant having an alkyl group having a carbon number of 14 or more and 18 or less is mixed as the first dispersant with the first solvent in the metal colloid synthesis step. Then, in the metal powder granulation step, the surfactant having an alkyl group having a carbon number of 12 or more and less than 14 is added as the second dispersant to the second solvent.
Besides, a form of the dispersant useful in the present invention can be a mixed dispersant of a mixture of the surfactant having an alkyl group having a carbon number of 14 or more and 18 or less and the surfactant having an alkyl group having a carbon number of 12 or more and less than 14. This mixed dispersant can be used only in the metal colloid synthesis step, or in both the metal colloid synthesis step and the metal powder granulation step. As described above, when a part or the whole of the reaction solution of the metal colloid synthesis step is used in the metal powder granulation step, the dispersant of the metal colloid synthesis step is also included in the metal powder granulation step. In the metal colloid synthesis step, it is essential to add the surfactant having an alkyl group having a carbon number of 14 or more and 18 or less, but when the dispersant used at this stage contains the surfactant having an alkyl group having a carbon number of 12 or more and less than 14, this surfactant is contained in the reaction solution in the metal powder granulation step, and effectively functions. Thus, there is no need to add a dispersant in the metal powder granulation step. Besides, when the dispersant is added in the metal powder granulation step, there is no need to separately use the dispersants in the respective steps, which is convenient for control of chemical agents.
In the metal powder granulation step, however, a surfactant having an alkyl group having a large carbon number may promote the growth of non-spherical particles, and hence, the mixed dispersant preferably has an appropriate composition. Specifically, it is preferable that the mixed dispersant contains a surfactant having an alkyl group having a carbon number of 12 or more and less than 14 in an amount of 50% or more and 80% or less in terms of % by mass, and a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less in an amount of 20% or more and 50% or less in terms of % by mass. In consideration of the influence of the surfactants having the carbon numbers in these ranges, the ratio of the surfactant having an alkyl group having a carbon number of 12 or more and less than 14 is preferably high in the mixed dispersant.
The mixed dispersant can contain, in addition to the surfactant having an alkyl group having a carbon number of 12 or more and less than 14 and the surfactant having an alkyl group having a carbon number of 14 or more and 18 or less, a surfactant having an alkyl group having a carbon number of 10 or more and less than 12. The surfactant having an alkyl group having a carbon number of 10 or more and less than 12 is an unpreferable dispersant in the metal colloid synthesis step, but can function as a dispersant in the metal particle granulation step. Besides, the surfactant having an alkyl group having a carbon number of 10 or more and less than 12 has high solubility in various solvents, and easily disappears at a low temperature in sintering the metal paste. Therefore, the surfactant having an alkyl group having a carbon number of 10 or more and less than 12 is useful for adjusting the solubility in a solvent and the volatility of the mixed dispersant, and can be used as the dispersant only when the mixed dispersant is used. When the mixed dispersant contains the surfactant having an alkyl group having a carbon number of 10 or more and less than 12, the content thereof is preferably 10% or less in terms of % by mass.
Moreover, it is more preferable that the mixed dispersant contains a surfactant having an alkyl group having a carbon number of 12 or more and less than 14 in an amount of 50% or more and 80% or less in terms of % by mass, and a surfactant having an alkyl group having a carbon number of 14 or more and 16 or less in an amount of 20% or more and 50% or less. Among surfactants having an alkyl group having a carbon number of 14 or more and 18 or less, a surfactant having an alkyl group having a carbon number of 18 or more is liable to grow non-spherical particles in the metal powder granulation step, and therefore, the content thereof is thus limited.
When a metal powder is produced by using the mixed dispersant, the surfactant having an alkyl group having a carbon number of 14 or more and 18 or less is bonded to the metal powder thus produced, and the presence ratios of the surfactants having the respective carbon numbers in the surfactants bonded to the metal powder are different from mixing ratios of the surfactants having the respective carbon numbers in the mixed dispersant in many cases. This is because a surfactant having a smaller carbon number is liable to bind to a metal powder preferentially than a surfactant having a larger carbon number in the metal powder granulation step.
Through the metal powder granulation step described so far, a metal powder having desired average particle size and particle size distribution is produced. Thereafter, the metal powder is collected, and appropriately washed with alcohol or the like, and thus, the metal powder can be obtained. This metal powder may be subjected to a post-treatment with a cyanide solution for removing chlorine described in Patent Document 2.

C. Metal Paste Using Metal Powder of Present Invention

A metal paste of the present invention is produced by mixing the above-described metal powder with an organic solvent used as a dispersion medium. In the production of the metal paste, the metal powder and the organic solvent can be mixed at room temperature. When the above-described additive is to be added, it may be added simultaneously with the metal powder and the organic solvent, or added after mixing the metal powder and the organic solvent.
The content of the metal powder in the metal paste is preferably 80% by mass or more and 99% by mass or less in terms of the mass (based on the mass of the entire paste). When the content is less than 80% by mass, bleeding that the solvent exudes from the paste during a process of paste application or the like may occur, and in addition, a void is formed during temperature increase, and hence a bond portion in a favorable bonded state is difficult to obtain. Besides, when the content is more than 99% by mass, aggregation of the metal powder may occur in some cases. The content of the metal powder is more preferably 87 to 96% by mass.
The organic solvent used as the dispersion medium is preferably an organic solvent having a boiling point of 200 to 400°C (under atmospheric pressure). When the boiling point of the organic solvent is less than 200°C, the evaporation rate is so high that the metal particles may aggregate, and the solvent may vaporize at the stage of the paste application, and hence is difficult to handle. On the other hand, an organic solvent having a boiling point more than 400°C may remain in the bond portion even after being heated.
Preferable specific examples of the organic solvent usable in the present invention include branched saturated aliphatic dihydric alcohols and monoterpene alcohols. More specifically, propylene glycol, 1,2-butanediol, 1,3-butanediol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 2,3-pentanediol, 2,4-pentanediol, 1,2-hexanediol, 1,3-hexanediol, 1,4-hexanediol, 1,5-hexanediol, 1,6-hexanediol, and derivatives or the like thereof such as 2,4-diethyl-1,5-pentanediol are used as the branched saturated aliphatic dihydric alcohols. Besides, citronellol, geraniol, nerol, menthol, terpineol (α, β), carveol, twill alcohol, pinocampheol, β-phenethyl alcohol, dimethyl octanol, hydroxycitronellol, 2,4-diethyl-1,5-pentanediol, trimethylpentanediol monoisobutyrate, and derivatives thereof are used as the monoterpene alcohols. Alternatively, a compound obtained through a condensation reaction between a monovalent carboxylic acid and polyhydric alcohol is effectively used, and examples thereof include triethylene glycol di-2-ethylhexanoate, and triethylene glycol di-2-ethylbutanoate. Since the boiling point of an organic solvent is liable to depend on the carbon number thereof, each solvent to be used preferably has a carbon number of 5 to 20. From this viewpoint, an aromatic hydrocarbon may be used, and for example, alkyl benzene can be used without any functional problem.
As the organic solvent, one organic solvent may be used, or a mixture of two or more organic solvents having different boiling points may be used. When the organic solvent consists of a solvent having a low boiling point and a solvent having a high boiling point, the organic solvent having a low boiling point is removed by volatilization in a treatment for adjusting the content of the metal particles, and thus, the adjustment can be eased.
The metal paste of the present invention contains, as the basic composition, the two constituent elements of the metal powder and the organic solvent, and may appropriately contain an additive. As the additive, one or more selected from acrylic resins, cellulose resins, and alkyd resins may be contained. When such a resin is further added, the aggregation of the metal powder in the paste can be prevented to form a more homogeneous bond portion. An example of the acrylic resins includes a methyl methacrylate polymer, an example of the cellulose resins includes ethyl cellulose, and an example of the alkyd resins includes a phthalic anhydride resin. Among these, ethyl cellulose is particularly preferred.
The metal paste of the present invention is useful for various applications such as bonding, sealing, and formation of an electrode, a bump, and a wiring in the field of electronics and the like. When used in such application, the metal paste of the present invention is applied to and dried on an object such as a substrate, or a member to be bonded. A dried material of the metal powder thus obtained works as a bonding material, a sealing material, and a precursor of a bump.
The dried material of the metal powder is provided in a state according to the application, and then the metal powder is sintered by heating under pressure. For example, the metal paste is applied and dried to form a bonding material in the form of a bump, a semiconductor device or chip is placed on the bonding material, and the resultant is heated under pressure to form a bond portion made of a sintered body of gold. The heating temperature for the sintering is preferably 150°C or more and 300°C or less.

Advantageous Effects of Invention

As described so far, a metal powder of the present invention is a metal powder in which a presence ratio of non-spherical particles in the shape of a rod, a plate or the like is reduced. The present invention can attain favorable sinterability as required for properties exhibited after forming the metal powder into a metal paste, and in application to bonding, sealing, and the like. The metal powder of the present invention can be produced by optimizing a dispersant used in a wet reduction method.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates SEM images of appearances of gold colloidal particles synthesized using surfactants different in the carbon number of an alkyl group in First Embodiment;
Fig. 2 illustrates SEM images of appearances of gold powders of Example 1 and Comparative Example produced in First Embodiment; and
Fig. 3 illustrates SEM images of appearances of gold powders produced using mixed dispersants in Examples 2 and 3 in Second Embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment: A preferred embodiment of the present invention will now be described. In the present embodiment, a gold (Au) powder was produced as a metal powder by a wet reduction method (including a metal colloid synthesis step and a metal powder granulation step). In this embodiment, a suitable range of the carbon number of an alkyl group contained in a surfactant used as a dispersant in the metal colloid synthesis step was detected, and then, contents of non-spherical particles obtained by using surfactants different in the carbon number in the metal powder granulation step were evaluated.

[Examination of Suitable Conditions in Gold Colloid Synthesis Step]

An alkylamine acetate was mixed as a dispersant with 20 mL of pure water. To the thus obtained dispersant solution, 2 mg of hydroxylammonium chloride was added as a reducing agent, and the resultant was heated and stirred at 80°C for dissolution. Then, to the solution containing the dispersant and the reducing agent, 0.45 mL of a chloroauric acid aqueous solution (Au content: 0.32 mg (3.6 mM)) was added, and the resultant was stirred for 2 hours with the temperature kept at 80°C to synthesize gold colloidal particles.
In this colloid synthesis step, five alkylamine acetates of decylamine acetate (having a carbon number of an alkyl group of 10), dodecylamine acetate (having a carbon number of an alkyl group of 12), tetradecylamine acetate (having a carbon number of an alkyl group of 14), hexadecylamine acetate (having a carbon number of an alkyl group of 16), and octadecylamine acetate (having a carbon number of an alkyl group of 18) were used in the present embodiment as the alkylamine acetate used as the dispersant. In preparing the dispersant solution, each alkylamine acetate was mixed in an amount of 0.13 mmol.
The gold colloidal particles synthesized by using the respective alkylamine acetates were observed with an SEM, and measured for the particle size distribution and the average particle size (M_N). 5 µL of each solution of the gold colloidal particles thus produced was dispensed, and placed on a grid to be dried, and the resultant was observed with a TEM (transmission electron microscope (JEM-1400 manufactured by JOEL Ltd.)) (acceleration voltage: 120 V, magnification: 150000 times). Then, based on a plurality of photographs taken in the observation with the TEM, a vertical Feret diameter was measured in 200 particles of each sample to create a particle size distribution. As the average particle size of the gold colloidal particles, the median diameter was employed. Since decylamine acetate (having a carbon number of an alkyl group of 10) suffered occurrence of precipitation and adherence of precipitated particles to the wall of a reactor, it was difficult to collect the particles, so that neither the SEM observation nor the measurement of the particle size distribution was viable. Fig. 1 shows SEM images of gold colloidal particles synthesized using the respective alkylamine acetates (having carbon numbers of an alkyl group of 12, 14, 16, and 18). Besides, in the particle size distributions of the gold colloidal particles synthesized with the respective alkylamine acetates, the ratio of gold colloidal particles having a particle size of 10 nm or less, and the measurement results of the average particle size are shown in Table 1. [Table 1]

Dispersant (carbon number of alkyl group) Average Particle Size (nm) Ratio of Fine Particles

C12 14.6 16.5%

C14 12.0 45.0%

C16 10.3 63.0%

C18 9.4 93.0%
Referring to Fig. 2 and Table 1, the larger the carbon number of the alkylamine used as the dispersant is, the smaller the average particle size of the gold colloidal particles is liable to be. It is confirmed that as the carbon number increases, the ratio of gold colloidal particles having a smaller particle size (of 10 nm or less) increases, and the particle size distribution becomes narrower. These trends well match the above-described findings (i) and (ii). As for the carbon number of the alkylamine, it was found, by comparison between the carbon number of 12 and the carbon number 14, that the ratio of the gold colloidal particles having a smaller particle size rapidly increased when the carbon number was 14, and therefore, it is presumed that the threshold value of the carbon number is suitably set to 14. As described above, for growing the metal powder in the metal powder granulation step after the metal colloid synthesis step, it is probably preferable that variation in the particle size distribution is reduced. It was confirmed, based on these results of the examination, that a surfactant having an alkyl group having a large carbon number is suitably used in the metal colloid synthesis step.

[Examination of Suitable Conditions in Gold Powder Granulation Step]

Therefore, it was decided to granulate a gold powder based on gold colloidal particles synthesized by using a dispersant having an alkyl group having a carbon number of 18 (octadecylamine acetate) in the gold colloid synthesis step (Example 1).
3 mL (15%) of a reaction solution resulting from the gold colloid synthesis step was dispensed, a dispersant solution obtained by dissolving the dispersant in 70 mL of pure water was added thereto, and a solution obtained by dissolving 5 g of hydroxylammonium chloride used as the reducing agent in 8 mL of pure water was further added to the resultant. Thereafter, in a state where the liquid temperature was stabilized at 80°C, 20 mL of a chloroauric acid aqueous solution (Au content: 2.25 g (571 mM)) used as a gold salt for granulation was added thereto, and the resultant was stirred for 30 minutes to produce a gold powder. Thereafter, the gold powder was collected.
In the gold powder granulation step, two alkylamine acetates, that is, dodecylamine acetate (having a carbon number of an alkyl group of 12) and octadecylamine acetate (having a carbon number of an alkyl group of 18) were used as the dispersant in the present embodiment. In preparing the dispersant solution, each alkylamine acetate was mixed in an amount of 1 mmol.
After producing the gold powder in the gold powder granulation step, the gold powder was collected by centrifugation, and was subjected to SEM observation and measurement of the particle size distribution and the average particle size. At this point, image analysis software (used software: MIPAR manufactured by Lightstone Corp.) was used for measuring, in an SEM image, particle sizes (maximum Feret diameters) of (400 or more) gold particles included in the image. Then, the number average particle size (M_N) of the gold powder was calculated. Besides, non-spherical particles were determined with the minimum Feret diameter and the maximum Feret diameter regarded as the minor axis "a" and the major axis "b", respectively to calculate the presence ratio of non-spherical particles.
Fig. 2 shows SEM images of a gold powder produced by using, as the dispersant in the gold powder granulation step, dodecylamine acetate (having a carbon number of an alkyl group of 12) (Example 1), and a gold powder produced by using octadecylamine acetate (having a carbon number of an alkyl group of 18) (Comparative Example). Referring to the SEM images of Fig. 2, it is understood that most of particles have favorable spherical shapes in the gold powder produced by using dodecylamine acetate (having a carbon number of an alkyl group of 12). On the contrary, some non-spherical (rod-shaped) particles are found in the gold powder produced by using octadecylamine acetate (having a carbon number of an alkyl group of 18). In these gold powders, the presence ratio of non-spherical particles was 0% in the gold powder obtained using dodecylamine acetate (having a carbon number of an alkyl group of 12) in the gold powder granulation step, and was 8.15% in the gold powder obtained using octadecylamine acetate (having a carbon number of an alkyl group of 18) in the gold powder granulation step.
It was confirmed, based on the results of the present embodiment described so far, that in the method for producing a metal powder including the metal colloid synthesis step and the metal powder granulation step, a surfactant having an alkyl group having a large carbon number is preferably used in the former step, and a surfactant having an alkyl group having a small carbon number is preferably used in the latter step.
Second Embodiment: In the present embodiment, two mixed dispersants were used as the dispersant in both the metal colloid synthesis step and the metal powder granulation step to produce gold powders (Examples 2 and 3). The dispersants used in the present embodiments were mixed dispersants of decylamine chloride (having a carbon number of an alkyl group of 10), dodecylamine acetate (having a carbon number of an alkyl group of 12), tetradecylamine acetate (having a carbon number of an alkyl group of 14), and hexadecylamine chloride (having a carbon number of an alkyl group of 16), and had the following components:

Mixed Dispersant of Example 2
- tetradecylamine acetate (carbon number of 14): 27% by mass
- dodecylamine acetate (carbon number of 12): balance
Mixed Dispersant of Example 3
- decylamine chloride (carbon number of alkyl group of 10): 5% by mass
- dodecylamine acetate (carbon number of alkyl group of 12): 60% by mass
- tetradecylamine acetate (carbon number of alkyl group of 14): 25% by mass
- hexadecylamine chloride (carbon number of alkyl group of 16): 10% by mass

[Gold Colloid Synthesis Step]

To 147.2 mL of pure water, 0.32 g of the mixed dispersant dissolved in 12.8 mL of pure water was added. To the resultant dispersant solution, 0.016 g of hydroxylammonium chloride, used as a reducing agent, dissolved in 5.0 mL of pure water was added, and the resultant was heated and stirred at 80°C for dissolution. Then, this solution was mixed with 4.5 mL of a chloroauric acid aqueous solution (Au content: 0.0064 mg (7.2 mM)), and the resultant was stirred for 1 hour with the temperature kept at 80°C. Thus, a red transparent solution of gold colloidal particles was obtained.

[Gold Powder Granulation Step]

To the whole amount of the solution of the gold colloidal particles (reaction solution), a solution obtained by dissolving 2.56 g of a mixture of the same surfactants, used as the dispersant, in 102.4 mL pure water was added, and 20 g of hydroxylammonium chloride, used as the reducing agent, dissolved in about 200 mL of pure water was added thereto. Thereafter, in a state where the liquid temperature was stabilized at 80°C, 100 mL of a chloroauric acid aqueous solution (Au content: 22 g (1120 mM)) used as a gold salt for granulation was added thereto, and the resultant was stirred for 30 minutes to produce a gold powder. Thereafter, the gold powder was collected.

The collected gold powder was subjected to SEM observation and measurement of the average particle size in the same manner as in First Embodiment, and the presence ratio of non-spherical particles was calculated. In the present embodiment, a presence ratio of coarse particles was also measured and calculated. Fig. 3 illustrates SEM images of the gold powders of Examples 2 and 3 produced in the present embodiment. The measurement results of the average particle size and the like are shown in Table 1. Table 1 also shows the results of First Embodiment (Example 1).

[Table 2]

	Carbon Number of Alkyl Group of Surfactant		Average Particle Size (M_N:nm)	Presence Ratio of Non-Spherical Particles	Presence Ratio of Coarse Particles
	Colloid Synthesis	Metal Powder Granulation	Average Particle Size (M_N:nm)	Presence Ratio of Non-Spherical Particles	Presence Ratio of Coarse Particles
Example 1	C18	C12	230	0%	0%
Example 2	Mixed Dispersant (C14+C12)		216	0%	0.8%
Example 3	Mixed Dispersant (C16+C14+C12+C10)		210	0%	0.2%
Comparative Example	C18	C18	127	8.15%	0%

It is understood, from Table 2, that a favorable gold powder free from non-spherical particles can be produced also by using the mixed dispersant containing a mixture of the surfactants having carbon numbers of an alkyl group falling within different ranges. In Examples 2 and 3, the same mixed dispersant was used both in the metal colloid synthesis step and the metal powder granulation step. It is deemed that the dispersants having the respective carbon numbers in the mixed dispersant effectively function in the respective steps. Besides, it was also confirmed that an alkylamine salt having a carbon number of an alkyl group of 10 may be mixed when a mixed dispersant is used.
Also from the viewpoint of the presence ratio of coarse particles, it is deemed that the production of coarse particles can be suppressed by appropriately setting the carbon number of an alkyl group of a surfactant used as the dispersant as in Examples 1 to 3.

[Property Evaluation of Metal Paste]

Next, the gold powders produced in Examples 2 and 3 of Second Embodiment were used to produce gold pastes. The gold paste was produced by mixing, with each gold powder, menthanol (dihydroterpineol) as an organic solvent. The blending rate of the organic solvent was 10% by weight. Then, each of the thus produced gold pastes was applied to and sintered on a substrate to form a bump, and the shape and resistance value thereof were measured. In applying the gold paste, an Al₂O₃ plate in a disc shape having a diameter of 2 inches was used as the substrate, and with this substrate covered with a metal mask (of stainless steel) having a thickness of 350 µm, and a rectangular hole of 5 mm × 20 mm, the paste was applied on the entire surface of the substrate. In this application step, the gold paste was dropped onto the metal mask, and spread on the metal mask with a squeegee so that the gold paste could be filled in the hole of the metal mask. After applying the gold paste, an excessive portion of the paste was wiped off, the metal mask was removed, and the resultant was dried by heating at 100°C for 1 hour, followed by heating at 230°C for 30 minutes for sintering.
When the cross section of a gold bump resulting from the sintering was observed with a metallographic microscope, it was found that the bump was in a rectangular shape following the shape of the hole of the metal mask, and the bump surface was in a favorable shape free from roughness. The gold bump was measured for a volume resistivity with a resistivity meter (Loresta GP MCP-T610, manufactured by Nittoseiko Analytech Co., Ltd.), and as a result, the resistivity was about 7.0 µΩ·cm in all Examples, and thus, it was confirmed that all the bumps were good conductive materials. It was confirmed that when a gold powder in which non-spherical particles and coarse particles are excluded is used, a good application property is obtained, and a homogeneous sintered body is obtained also in sintering.

Industrial Applicability

A gold powder of the present invention contains a gold powder of spherical particles in which the presence ratio of non-spherical particles is suppressed. The gold powder of the present invention is, owing to the particle shape and exclusion of coarse particles, not only improved in the appearance when observed with a microscope but also excellent in stability and filling property in applying a metal paste. A method for producing a gold powder of the present invention is achieved by optimization of a dispersant used in synthesizing gold colloidal particles corresponding to nuclei of the gold powder. A gold paste of the present invention has the aforementioned properties with low-temperature sinterability retained. The gold paste of the present invention is useful for respective processes of bonding, sealing, and formation of electrodes and wirings in various applications to electric/electronic components, semiconductor devices, semiconductor elements, power devices, MEMs and the like.

Claims

A metal powder comprising a metal of Au, Ag, Cu, or an alloy thereof having an average particle size of 0.1 µm or more and 0.4 µm or less, and purity of 99.9% by mass or more,
wherein a presence ratio, in terms of the number of particles, of non-spherical particles in the metal powder having a ratio (b/a) of a minor axis "a" and a major axis "b" of 3 or more is 1% or less.
The metal powder according to claim 1, wherein a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less is bonded to at least a part of a surface of the metal powder.
The metal powder according to claim 2, wherein the surfactant is an alkylamine salt or a quaternary ammonium salt.
A metal paste comprising the metal powder defined in any one of claims 1 to 3, and an organic solvent.
A method for producing a metal powder comprising:
a metal colloid synthesis step of synthesizing metal colloidal particles by reacting a metal salt with a reducing agent in a first solvent containing a first dispersant; and

a metal powder granulation step of forming the metal colloidal particles into a metal powder by adding a metal salt, a reducing agent, and an optional second dispersant to a second solvent containing the metal colloidal particles synthesized in the metal colloid synthesis step,

wherein the first solvent in the metal colloid synthesis step contains, as the first dispersant, at least a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less, and

the second solvent in the metal powder granulation step contains, as the second dispersant, a surfactant having an alkyl group having a carbon number of 12 or more and less than 14.
The method for producing a metal powder according to claim 5, wherein the second solvent in the metal powder granulation step contains a part or the whole of a reaction solution produced in the metal colloid synthesis step.
The method for producing a metal powder according to claim 5 or 6,
wherein in the metal colloid synthesis step, a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less is mixed with the first solvent as the first dispersant, and

in the metal powder granulation step, a surfactant having an alkyl group having a carbon number of 12 or more and less than 14 is added to the second solvent as the second dispersant.
The method for producing a metal powder according to claim 5 or 6, wherein in the metal colloid synthesis step, a mixed dispersant containing a surfactant having an alkyl group having a carbon number of 12 or more and less than 14 and a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less is mixed with the first solvent as the first dispersant.
The method for producing a metal powder according to claim 8, wherein in the metal powder granulation step, a mixed dispersant containing a surfactant having an alkyl group having a carbon number of 12 or more and less than 14 and a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less is mixed with the second solvent as the second dispersant.
The method for producing a metal powder according to claim 8,
wherein the mixed dispersant contains:
a surfactant having an alkyl group having a carbon number of 12 or more and less than 14 in an amount of 50% or more and 80% or less in terms of % by mass;

a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less in an amount of 20% or more and 50% or less in terms of % by mass; and

a balance of a surfactant having an alkyl group having a carbon number of 10 or more and less than 12.
The method for producing a metal powder according to claim 9,
wherein the mixed dispersant contains:
a surfactant having an alkyl group having a carbon number of 12 or more and less than 14 in an amount of 50% or more and 80% or less in terms of % by mass;

a surfactant having an alkyl group having a carbon number of 14 or more and 18 or less in an amount of 20% or more and 50% or less in terms of % by mass; and

a balance of a surfactant having an alkyl group having a carbon number of 10 or more and less than 12.
The method for producing a metal powder according to claim 5 or 6, wherein the surfactant is an alkylamine salt or a quaternary ammonium salt.
The method for producing a metal powder according to claim 7, wherein the surfactant is an alkylamine salt or a quaternary ammonium salt.
The method for producing a metal powder according to claim 8, wherein the surfactant is an alkylamine salt or a quaternary ammonium salt.
The method for producing a metal powder according to claim 9, wherein the surfactant is an alkylamine salt or a quaternary ammonium salt.