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Particulate silver powder and method of manufacturing same

Particulate silver powder and method of manufacturing same description/claims

The present invention relates to a spherical, fine, particulate powder of silver (especially one of a particle diameter on the nanometer order) and a liquid dispersion thereof, particularly to a particulate silver powder low in corrosive components that is a suitable interconnect forming material for fabricating fine circuit patterns, especially for forming interconnects by the ink-jet method, and a method of manufacturing the powder. The particulate silver powder of the present invention is also a suitable material for forming interconnects on LSI substrates, and electrodes and interconnects of flat panel displays (FPDs), and for filling in fine trenches, via holes and contact holes. It is also suitable for use as a coloring material for automobile paints and the like. Moreover, its low impurity content and toxicity level make it useful as a carrier for adsorbing biochemical substances and the like in the fields of medical treatment, diagnostics and biotechnology.

When the size of solid substance particles reaches the ultrafine nanometer order (called “nanoparticles” in the following), the specific surface area of the particles becomes very great, so that, even though they are solids, their interface with a gas or liquid becomes extremely large. Their surface characteristics therefore strongly affect the properties of the solid substance.

It is known that the melting point of metal nanoparticles is dramatically reduced from that in the bulk state. In comparison with conventional micrometer-order particles, therefore, metal nanoparticles offer not only fine interconnect formation capability but also other features such as low-temperature sinter capability. Owing to the low resistance and excellent weatherability of silver nanoparticles, and also their low price compared with other noble metal nanoparticles, silver nanoparticles are seen as metal nanoparticles with particular promise as the next-generation material for fine interconnects.

Known methods of manufacturing nanometer-order particles of silver are broadly divided into vapor phase methods and liquid phase methods. Vapor phase methods are ordinarily methods that conduct deposition in a gas. Patent Document 1 describes a method of vaporizing silver in a helium or other inert gas atmosphere, under a reduced pressure of around 0.5 torr. Patent Document 2 teaches a liquid phase method for obtaining a silver colloid by reducing silver ions in an aqueous phase using an amine and transferring the obtained silver deposition phase to an organic solvent phase (polymeric dispersant). Patent Document 3 describes a method in which a reducing agent (alkali metal borohydride or ammonium borohydride) is used to reduce a silver halide in a solvent in the presence of a thiol type protective agent.

Patent Document 1: JP 2001-35255A

Patent Document 2: JP 11-319538 A

Patent Document 3: JP 2003-253311A

The silver particles obtained by the vapor phase method of Patent Document 1 are 10 nm or less in diameter and have good dispersibility in solvent. However, the technology requires a special apparatus. This makes it difficult to synthesize large quantities of silver nanoparticles for industrial use. In contrast, the liquid phase method is basically suitable for large-volume synthesis but has a problem in that the metal nanoparticles in the liquid have a strong tendency to agglomerate, making it difficult to obtain a monodispersed nanoparticle powder. In most cases, citric acid is used as the dispersant when manufacturing metal nanoparticles, and the metal ion concentration in the liquid is usually very low as 10 mmole/L (0.01 mole/L) or lower. This is a barrier to its industrial application.

Although the foregoing method of Patent Document 2 achieves synthesis of stably dispersed silver nanoparticles by using a high metal ion concentration of 0.1 mole/L or greater and a high reaction mixture concentration, it suppresses agglomeration by using a polymeric dispersant having a high number average molecular weight of several tens of thousands. The use of a dispersant of high molecular weight is not a problem when the silver nanoparticles are to be used as a coloring material, but when the particles are to be used in circuit fabrication applications, a firing temperature that is equal to or higher than the polymer boiling point is required and, in addition, pores readily form in the interconnects after the firing, so that problems of high resistance and breakage arise, making the particles not altogether suitable for fine interconnect applications.

The foregoing method of Patent Document 3 conducts the reaction at a relatively high reactant concentration of 0.1 mole/L or greater and disperses the obtained silver particles of 10 nm or less using a dispersant. As a suitable dispersant, Patent Document 3 proposes a thiol type dispersant, which can be readily vaporized by low-temperature firing at the time of interconnect formation because it has a low molecular weight of around 200. However, the thiol type surfactant contains sulfur (S), and sulfur causes corrosion of interconnects and other electronic components, making it an unsuitable element for interconnect formation applications. The method is therefore not suitable for interconnect formation applications.

Moreover, the liquid phase method uses silver nitrate, silver halide or the like as the silver source material (Patent Documents 2 and 3), so that the reaction solution unavoidably contains many ions originating from the silver source material, such as I−, Cl−, SO42−, NO3− and CN−. In the case of nanoparticles, specific surface area is extraordinarily large, solid-liquid separation and washing are difficult, and the reactivity of the ions with silver is high since most of the ions originating from the starting material originally formed compounds with silver. Because of these and other factors, the ions originating from the starting material are found to be adsorbed on the silver particles after reaction or to have reacted with them and become present as inclusions therein. A liquid dispersion prepared using the particles similarly contains the ions as impurities. On the other hand, the increasingly sophisticated performance capabilities of electronic equipment in recent years has placed severe demands on the components of such equipment, and the content of constituents and elements that degrade reliability is required to be managed on the ppm order. Under such circumstances, it is undesirable for corrosive components like I−, Cl−, SO42−, NO3− and CN− to be entrained in the liquid dispersion of the particulate silver powder.

Therefore, an object of the present invention is to overcome such problems by providing a nanoparticle powder of silver low in corrosive components that is suitable for fine interconnect formation applications, and a liquid dispersion thereof, at low cost and in large quantities. Moreover, since monodispersion of spherical silver nanoparticles of uniform diameter is preferable, another object is to provide a liquid dispersion of such silver particles.

In accordance with the present invention, there is provided a particulate silver powder having an average particle diameter measured by TEM observation (DTEM) of 200 nm or less, preferably 100 nm or less, more preferably 30 nm or less, an aspect ratio of less than 2.50, and a {(DTEM)/(Dx)} of 5.0 or less (where (Dx) denotes X-ray crystallite size), which particulate silver powder has a content of each of I−, Cl−, SO42−, NO3− and CN− of 100 ppm or less. The particulate silver powder preferably has a TEM average particle diameter (DTEM) of 100 nm, preferably 30 nm or less, and has adhered to the surfaces of the particles thereof an organic protective agent (typically a fatty acid or an amine compound) having a molecular weight of 100 to 400.

The present invention further provides a liquid dispersion of silver particles obtained by dispersing such particulate silver powder in an organic solvent (typically a non-polar or low-polarity solvent), which liquid dispersion of silver particles has an average particle diameter (D50) measured by the dynamic light-scattering method of 200 nm or less, a degree of dispersion {(D50)/(DTEM)} of 5.0 or less, and a content of each of I−, Cl−, SO42−, NO3− and CN− of 100 ppm or less.

Further, as a method of manufacturing such a particulate silver powder, the present invention provides a method of manufacturing a particulate silver powder, wherein a silver compound other than silver nitrate (typically silver carbonate or silver oxide) is reduced in an organic solvent (typically an alcohol or polyol) having a boiling point of 85° C. or greater at a temperature of 85° C. or greater and in the presence of an organic protective agent (typically a fatty acid or amino compound having a molecular weight of 100 to 400).

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