JP7787643B2 - Conductive inkjet ink - Google Patents

Description

The present invention relates to a conductive inkjet ink.

Inkjet printing has traditionally been used as a printing method for drawing images such as patterns and text on a print target. Because inkjet printing can print highly accurate images on demand at low cost and with minimal damage to the print target, its application in a variety of fields is being considered. For example, in recent years, the use of inkjet printing has been considered for forming conductive films that make up the conductive circuit patterns (electrodes, etc.) of electronic components. By using inkjet printing, it is possible to reduce variation in the thickness of the conductive film compared to screen printing. Therefore, it is expected that variation in the conductivity of the conductive film will be reduced and that finer conductive circuit patterns will be printed.

In the manufacture of such electronic components, conductive inkjet ink (hereinafter also referred to as "conductive ink") is used, to which inorganic powder containing metal particles and the like is added as a conductive material. As an example of such a conductive ink, Patent Document 1 discloses an ink containing nanometal powder such as silver or a silver-copper alloy. Patent Document 2 also discloses an ink containing metal oxide microparticles that are reduced by heat treatment, such as silver oxide, copper oxide, palladium oxide, nickel oxide, lead oxide, and cobalt oxide. Generally, to form an appropriate conductive circuit pattern by inkjet printing, the conductive ink must have low viscosity and a high concentration of inorganic powder. Patent Documents 1 and 2 above propose technologies for achieving these inkjet suitabilities.

In addition, conductive inkjet inks are required to have stable dispersion of inorganic powders in the ink in order to ensure ejection properties during printing and conductivity after printing. For example, Patent Document 3 discloses a technique for improving the dispersibility of solid fine particles that have a mixture of acidic and basic adsorption groups on their surfaces by adding a first dispersant that has only acidic or basic adsorption groups, and a second dispersant that has both acidic and basic adsorption groups.

In addition, with conductive inkjet inks, it is desirable to reduce the variation in the thickness of the conductive film in order to reduce variations in the conductivity of the conductive film and to achieve fine conductive circuit patterns. For example, Patent Document 4 discloses the addition of an organic solvent that increases surface tension to reduce the wetting and spreading of a spacer particle dispersion printed by an inkjet device.

Special Publication No. 2008-513565 Japanese Patent Application Laid-Open No. 2012-216425 JP 2015-62871 A JP 2008-111985 A

However, the inventors' research has revealed that increasing the surface tension of conductive inkjet ink makes it difficult to maintain the dispersibility of the inorganic powder contained in the ink. In particular, inorganic powders with high specific gravities (e.g., tungsten powder, palladium powder, platinum powder, molybdenum powder, etc.) are prone to settling and aggregation, which can clog inkjet nozzles. Therefore, it is difficult to simultaneously maintain the dispersibility of the inorganic powder in the ink (i.e., good stability over time) and reduce thickness variation in the conductive film.

The present invention was made in consideration of the above circumstances, and its main objective is to provide a conductive inkjet ink that has excellent stability over time and reduces thickness variation in the printed conductive film.

To achieve the above objective, the present inventors focused on the structure of organic solvents. Generally, organic solvents containing ester or ether bonds are polar, and inks containing such organic solvents have increased surface tension. However, such organic solvents can cause aggregation of inorganic powders. The present inventors hypothesized that such aggregation occurs when electrons are removed from carbon atoms bonded to ester- or ether-bonded oxygen atoms, rendering them in a state in which they can release hydrogen ions. Carbon atoms in a state in which they can release hydrogen ions can react with cationic functional groups (e.g., amine groups) contained in cationic dispersants, thereby inhibiting the action of the cationic dispersants.
Therefore, the present inventors conducted extensive research and found that a conductive inkjet ink containing an organic solvent in which a phenyl group or a phenyl group having a substituent is bonded to a carbon atom in an ester bond or an oxygen atom in an ether bond reduces aggregation of inorganic powder and reduces thickness variations in the conductive film.

That is, the conductive inkjet ink disclosed herein is a conductive inkjet ink used in the production of electronic components, and contains an inorganic powder, a cationic dispersant, and at least one organic solvent, and the organic solvent is a compound represented by the following general formula:
(1) R ₁ -COO-R ₂
(2) R ₁ -O-R ₂
(wherein, in the above general formula, R ₁ is a phenyl group or a phenyl group having a substituent, and R ₂ is an alkyl group having 1 to 5 carbon atoms or an alkyl group having 1 to 5 carbon atoms having a substituent).
The present invention is characterized in that it contains an organic compound represented by any one of the following:
This provides a conductive inkjet ink that has excellent stability over time and reduces variations in the thickness of the printed conductive film.

In one embodiment of the conductive inkjet ink disclosed herein, the surface tension is 31 mN/m or more and 37 mN/m or less. Generally, a surface tension within this range tends to result in reduced stability over time, but the conductive inkjet ink having the configuration disclosed herein maintains good stability over time and can reduce thickness variation in the printed conductive film.

In addition, in a preferred embodiment of the conductive inkjet ink disclosed herein, when the entire organic solvent is taken as 100 wt %, the total proportion of the organic compounds represented by (1) or (2) above is 20 wt % or more. This ensures that a sufficient amount of the organic compound represented by (1) or (2) above is contained, thereby achieving better stability over time and further reducing variation in the thickness of the printed conductive film.

In addition, in one preferred embodiment of the conductive inkjet ink disclosed herein, the proportion of the organic solvent is 30 wt% or more and 70 wt% or less when the entire conductive inkjet ink is taken as 100 wt%. This achieves excellent stability over time and a higher level of reduced variation in the thickness of the printed conductive film.

In addition, in a preferred embodiment of the conductive inkjet ink disclosed herein, the inorganic powder contains at least one of tungsten powder and palladium powder. Tungsten powder and palladium powder are inorganic powders with high specific gravities that are prone to settling and aggregation. However, the technology disclosed herein can suppress the settling and aggregation of these powders, thereby providing a conductive inkjet ink containing tungsten powder and/or palladium powder that has excellent stability over time and reduced thickness variation in the printed conductive film.

FIG. 1 is a cross-sectional view schematically showing an agitator/pulverizer used in the production of conductive ink. 1 is an overall view schematically illustrating an example of an inkjet device. FIG. 3 is a cross-sectional view schematically showing an inkjet head of the inkjet device in FIG. 2.

Preferred embodiments of the technology disclosed herein are described below. Matters necessary for implementing the present invention other than those specifically mentioned in this specification can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. When a numerical range is stated in this specification as "A to B (A and B are arbitrary numerical values)," this is the same as the general interpretation, and means "greater than A and less than B (including a range greater than A but less than B)."

1. Conductive Inkjet Ink The conductive inkjet ink (conductive ink) disclosed herein contains at least (A) an inorganic powder, (B) a dispersant, and (C) an organic solvent. In addition to these, other components such as a binder resin may also be included. Typically, the conductive ink is in a state in which the inorganic powder is dispersed in an organic vehicle component (a mixture of a dispersant, an organic solvent, and a binder resin). Each component contained in the conductive ink disclosed herein will be described below.

(A) Inorganic Powder The inorganic powder is a material that constitutes the main component (base material) of the printed layer (conductive circuit pattern) after firing. In the conductive ink disclosed herein, conductive metal powders can be preferably used as the inorganic powder. Examples of metals in such metal powders include tungsten (W), palladium (Pd), platinum (Pt), molybdenum (Mo), rhodium (Rh), cobalt (Co), nickel (Ni), iron (Fe), chromium (Cr), gold (Au), silver (Ag), copper (Cu), ruthenium (Ru), osmium (Os), iridium (Ir), and aluminum (Al). Among these, W, Pd, Pt, Mo, Co, Ni, Fe, and Cr have melting points of 1400°C or higher, and are therefore suitable for use in plasma-resistant electronic components that require high-temperature firing at 1200°C or higher during the manufacturing process. Furthermore, among these, W, Pd, Pt, and Mo have particularly high melting points and excellent heat resistance, making them more suitable for use in plasma-resistant electronic components. However, because these metal powders have very high specific gravities (specific gravities of 10 or more), they are prone to precipitation and aggregation, and their stability over time is likely to decrease. However, the technology disclosed herein can suppress precipitation and aggregation even in metal powders with high specific gravities. Therefore, even high-melting-point metals with high specific gravities, such as W, Pd, Pt, and Mo, can be suitably used in the conductive ink disclosed herein. Note that the metal powders may be used alone or in combination of two or more types.

Furthermore, the inorganic powder may contain inorganic particles other than the metal particles constituting the metal powder, as long as the effects of the technology disclosed herein are not impaired. Examples of such inorganic particles include ceramic particles such as ZrO ₂ , Al ₂ O ₃ , Ag ₂ O, Cu ₂ O, PdO, NiO, and CoO. These ceramic particles have a higher melting point than general metal particles, so mixing them with metal particles can improve the heat resistance of the conductive film. When inorganic particles such as ceramic particles are added, the proportion of metal particles (metal powder) contained in the inorganic powder, when the total inorganic powder is taken as 100 wt %, is typically 50 wt % or more, preferably 70 wt % or more, more preferably 80 wt % or more, and even more preferably 90 wt % or more. This allows the conductive film to achieve high levels of conductivity and heat resistance.

Furthermore, from the viewpoint of properly exerting the dispersibility-improving effect of the dispersant, it is preferable that the inorganic powder has a specific surface area of a certain level or more. For example, the specific surface area of the inorganic powder is preferably 1 m ² /g or more, more preferably 1.2 m ² /g or more, and particularly preferably 1.5 m ² /g or more. On the other hand, if the specific surface area is too large, aggregation of inorganic particles is more likely to occur. From this viewpoint, the upper limit of the specific surface area of the inorganic powder is preferably 5 m ² /g or less, more preferably 4 m ² /g or less, and particularly preferably 3 m ² /g or less. In this specification, "specific surface area" refers to the BET specific surface area measured based on the BET method specified in JIS Z8830:2013.

Furthermore, the particle size of inorganic particles is one factor that can affect ejection performance and long-term stability. Specifically, if the particle size of inorganic particles is too large, the ejection orifices of the inkjet device may become clogged with inorganic particles, significantly reducing ejection performance. Therefore, the average particle size of inorganic particles contained in conductive ink is preferably 500 nm or less, more preferably 450 nm or less, even more preferably 400 nm or less, and particularly preferably 350 nm or less, and may be, for example, 320 nm or less. On the other hand, if the particle size of inorganic particles is too small, the inorganic particles tend to aggregate, which can lead to a decrease in long-term stability. From this perspective, the average particle size of inorganic particles is preferably 150 nm or more, more preferably 170 nm or more, even more preferably 180 nm or more, and particularly preferably 200 nm or more, and may be, for example, 220 nm or more. Note that, in this specification, "average particle size" refers to the average particle size determined by dynamic light scattering (DLS) analysis. The average particle size based on the DLS method can be measured in accordance with JIS Z 8828:2013.

The content of inorganic powder in the conductive ink is not particularly limited and can be adjusted appropriately depending on the printing purpose (the conductive circuit pattern to be formed). Specifically, as the content of inorganic powder in the conductive ink increases, it becomes easier to form a conductive circuit pattern (conductive film) of appropriate thickness with fewer print runs. From this perspective, the content of inorganic powder relative to the total amount of conductive ink (100 wt%) is preferably 30 wt% or more, more preferably 35 wt% or more, and particularly preferably 40 wt% or more. On the other hand, as the content of inorganic powder decreases, stability over time and ejection performance tend to improve. From this perspective, the content of inorganic powder relative to the total amount of conductive ink is preferably 70 wt% or less, more preferably 60 wt% or less, and particularly preferably 55 wt% or less.

(B) Dispersant: A dispersant is an organic compound that uniformly disperses inorganic particles in the ink and inhibits aggregation and sedimentation of the inorganic particles. The conductive ink disclosed herein preferably contains a cationic dispersant, and may further contain a nonionic dispersant.

A cationic dispersant is a dispersant having a cationic functional group. An example of a cationic dispersant is an amine dispersant. An amine dispersant is a chain-like organic compound having an amine group at at least one end. The amine group of such a cationic dispersant adheres to the surface of metal particles with weakly acidic surfaces, causing steric hindrance and suppressing aggregation of the metal particles. This improves stability over time. Examples of metal particles that may have weakly acidic surfaces include tungsten particles, palladium particles, and molybdenum particles. The surfaces of these particles can become weakly acidic due to oxidation by oxygen in the air. Therefore, cationic dispersants are suitable for use with these particles. Conventional cationic dispersants can be used without particular restrictions as the cationic dispersant. Examples include fatty acid amine dispersants and polyester amine dispersants with a weight-average molecular weight of approximately 1×10 ³ to 5×10 ⁴ . More specific examples that can be suitably used include Hypermer KD-1 and Hypermer KD-3 manufactured by Croda Japan Co., Ltd., and S-Leam AD Series (registered trademark) AD-508E manufactured by NOF Corporation. The cationic dispersants may be used alone or in combination of two or more.

The nonionic dispersant can enhance the steric hindrance of the metal particle surface by adhering to the cationic dispersant adhered to the surface of the metal particle. Specifically, by adhering an amine group at one end of the cationic dispersant to the surface of the metal particle and then adhering a nonionic dispersant to the other end of the cationic dispersant, a greater steric hindrance can be achieved. This can more effectively suppress the aggregation of metal particles, thereby further improving stability over time. As the nonionic dispersant, any conventionally known nonionic dispersant can be used without particular limitation. Examples include ether-based dispersants, ester-based dispersants, ether-ester dispersants, and nitrogen-containing dispersants having a weight-average molecular weight of approximately 1×10 ³ to 1×10 ^4. Specific examples include Hypermer KD-13 and Hypermer KD-14 manufactured by Croda Japan Co., Ltd.

While there are no particular restrictions on the proportion of dispersant in the organic vehicle component (a mixture of dispersant, organic solvent, and binder resin), a high dispersant proportion can impair the conductivity of the conductive ink. Therefore, when the total organic vehicle component is 100 wt%, the dispersant proportion is typically 5 wt% or less, preferably 4 wt% or less, and may be, for example, 3 wt% or less. Furthermore, if the dispersant proportion is low, the dispersant's effect of suppressing metal particle aggregation may not be fully exerted. Therefore, the dispersant proportion should be, for example, 0.5 wt% or more, and preferably 1 wt% or more. The dispersant proportion mentioned here refers to cationic dispersant alone, or to a mixed dispersant consisting of a cationic dispersant and a nonionic dispersant.

Furthermore, when using a mixed dispersant that combines a cationic dispersant and a nonionic dispersant, it is preferable that the amount of nonionic dispersant be such that it can sufficiently adhere to the cationic dispersant. Therefore, when the entire mixed dispersant is taken as 100 wt%, the proportion of nonionic dispersant should be, for example, 20 wt% or more, preferably 50 wt% or more, and more preferably 60 wt% or more. On the other hand, if the amount of nonionic dispersant is too high, the amount of cationic dispersant will be relatively small, and the cationic dispersant will not be able to sufficiently adhere to the surface of the metal particles. Therefore, the proportion of the nonionic dispersant should be, for example, 90 wt% or less, preferably 80 wt% or less, and more preferably 70 wt% or less.

(C) Organic Solvent The organic solvent contained in the conductive inkjet ink disclosed herein is composed of at least one organic compound.
(1) R ₁ -COO-R ₂
(2) R ₁ -O-R ₂
(wherein, in the general formula, R ₁ is a phenyl group or a phenyl group having a substituent, and R ₂ is an alkyl group having 1 to 5 carbon atoms or an alkyl group having 1 to 5 carbon atoms having a substituent).
The present invention contains at least one organic compound represented by any one of the following formulae:

_R1 is typically a phenyl group ( _{C6H5 group} ), and the phenyl group may have one or two substituents. Examples of such substituents include a hydroxyl group, an aldehyde group, a carboxyl group, an acetyl group, a methyl group, an ethyl group, and a propyl group. When _R1 contains a substituent, the number of substituents is typically one, but may be two. The orientation of the substituent may be in the ortho, meta, or para position.

The above _R2 may be an alkyl group having 1 to 5 carbon atoms, but is preferably an alkyl group having 1 to 3 carbon atoms. In addition, such an alkyl group may have a substituent. There is no particular limitation on the number of such substituents, but for example, the alkyl group may have 1 to 3 substituents, and preferably has one substituent. Examples of such substituents include a hydroxyl group, an aldehyde group, a carboxyl group, an acetyl group, a phenyl group, a methyl group, an ethyl group, and a propyl group.

The molecular weight of the organic compound represented by either (1) or (2) above is, for example, 400 or less, preferably 300 or less, and more preferably 250 or less. The lower limit of the molecular weight may be approximately 10 or more (more specifically, 10 or more), since the molecular weight of the organic compound represented by (2) above having a phenyl group as _R1 and a methyl group as _R2 is the smallest molecular weight.

The surface tension of the organic compound described in either (1) or (2) above is not particularly limited, but is preferably 30 mN/m or more, preferably 33 mN/m or more, more preferably 35 mN/m, and particularly preferably 37 mN/m or more. This increases the surface tension of the conductive ink and reduces thickness variations in the printed conductive film. On the other hand, if the surface tension of such an organic compound is too high, the surface tension of the conductive ink will increase excessively, which may cause clogging of the nozzles of the inkjet device. Therefore, for example, the surface tension of such an organic compound is preferably 50 mN/m or less, and preferably 45 mN/m or less. Note that this surface tension can be measured, for example, using a static surface tensiometer (DYNEMASTER DY-300, manufactured by Kyowa Interface Science Co., Ltd.).

Specific examples of organic compounds represented by (1) above include methyl benzoate, ethyl benzoate, propyl benzoate, butyl benzoate, benzyl benzoate, methyl salicylate, ethyl salicylate, and dimethyl phthalate. Specific examples of organic compounds represented by (2) above include butyl phenyl ether, propylene glycol monophenyl ether, and 2-phenoxyethanol.

The organic solvent used in the conductive ink disclosed herein may be one of the organic compounds described in (1) or (2) above, either singly or in combination. In addition to the organic compounds described in (1) or (2), organic solvents (organic compounds) that can be used in conventional inkjet inks may also be used singly or in combination. From the perspective of preventing rapid drying during the drying process and forming a stable dried film (conductive film), it is preferable to use an organic solvent with a relatively high boiling point (e.g., boiling point: 140°C to 260°C). From the perspective of improving printing accuracy, it is preferable for the organic solvent to have a relatively low viscosity (approximately 0.5 mPa·s to 20 mPa·s) and a moderate surface tension (approximately 20 mN/m to 40 mN/m). Suitable examples of such organic solvents include glycol acetate and aliphatic monoalcohols. Examples of glycol acetates include ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, ethylene glycol monobutyl ether acetate, diethylene glycol monomethyl ether acetate, diethylene glycol monoethyl ether acetate, diethylene glycol monobutyl ether acetate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, propylene glycol monobutyl ether acetate, dipropylene glycol monomethyl ether acetate, butyl monoglycol acetate (BMGAC), butyl diglycol acetate (BDGAC), and 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate. Examples of aliphatic monoalcohols include straight-chain or branched aliphatic alcohols such as methanol, ethanol, propanol, isopropanol, butanol, n-amyl alcohol, hexanol, heptanol, n-octanol, 2-ethylhexanol, isooctanol, nonanol, decanol, isoundecanol, lauryl alcohol, cetyl alcohol, and stearyl alcohol. Glycol ether organic solvents such as diethylene glycol dibutyl ether can also be used.

The organic solvent is the main component of the organic vehicle component. Therefore, when the total organic vehicle component is taken as 100 wt%, the proportion of the organic solvent is typically 80 wt% or more, preferably 90 wt% or more, more preferably 95 wt% or more, and may be, for example, 96 wt% or more.

When an organic solvent is used in combination with an organic compound described in either (1) or (2) above and another organic compound, if the proportion of the organic compound described in either (1) or (2) above is low, the effect of improving stability over time and reducing thickness variation in the conductive film may be insufficient. Therefore, when the total organic solvent is taken as 100 wt%, the proportion of the organic compound described in either (1) or (2) above is preferably 20 wt% or more, and may be, for example, 40 wt% or more. There is no particular upper limit to this proportion, but it may be, for example, 95 wt% or less, 85 wt% or less, or 75 wt% or less.

Furthermore, if the content of the organic solvent containing the organic compound described in either (1) or (2) above in the conductive ink is low, the effect of improving stability over time and reducing thickness variation in the conductive film may be insufficient. Therefore, when the entire conductive ink is taken as 100 wt%, the proportion of the organic solvent containing the organic compound described in either (1) or (2) above is preferably 30 wt% or more, more preferably 40 wt% or more, and even more preferably 45 wt% or more. Furthermore, the upper limit of the content of such organic compound is not particularly limited, but may be, for example, 70 wt% or less, or may be 65 wt% or less, or may be 60 wt% or less.

The surface tension of the organic solvent as a whole is not particularly limited, but is preferably 30 mN/m or more, preferably 31 mN/m or more, and more preferably 32 mN/m or more. The surface tension of the organic solvent as a whole is preferably 40 mN/m or less, preferably 37 mN/m or less, more preferably 36 mN/m or less, and may be 35 mN/m or less. The surface tension of the organic solvent as a whole affects the surface tension of the conductive ink, so by keeping the surface tension within the above range, the surface tension of the conductive ink can be suitably adjusted.

(D) Other Components The conductive ink disclosed herein may further contain known additives that can be used in inkjet inks (typically, conductive inkjet inks), as long as the effects of the technology disclosed herein are not impaired. Examples of such additives include binder resins.

The binder resin is a component that can improve the adhesion and strength of the conductive film (dried film) printed with conductive ink when the conductive film dries. As long as the effects of the technology disclosed herein are not impaired, binder resins that can be used in conventional conductive inks can be used without particular restrictions. From the perspective of forming a conductive film with excellent adhesion, the glass transition temperature of the binder resin is preferably 40°C or higher, more preferably 50°C or higher, and particularly preferably 60°C or higher. Furthermore, if binder resin remains after baking, it can cause a decrease in the quality of manufactured electronic components (e.g., increased resistance). For this reason, it is preferable that the binder resin be easily burned off during baking (e.g., heating at 500°C to 2000°C). Examples of such binder resins include polyvinyl acetal resin and acrylic resin. Specific examples of polyvinyl acetal resin include polyvinyl butyral resin and polyvinyl formal resin (vinylon).

As the weight-average molecular weight Mw of the binder resin increases, the fixation and strength of the dried film tend to improve. From this perspective, the weight-average molecular weight Mw of the binder resin is preferably 0.5×10 ⁴ or more, more preferably 1×10 ⁴ or more, even more preferably 1.5×10 ⁴ or more, and particularly preferably 2×10 ⁴ or more. On the other hand, as the weight-average molecular weight Mw of the binder resin decreases, the ink viscosity tends to decrease. From this perspective, the weight-average molecular weight Mw of the binder resin is preferably 20×10 ⁴ or less, more preferably 15×10 ⁴ or less, even more preferably 10×10 ⁴ or less, and particularly preferably 5×10 ⁴ or less.

The amount of binder resin contained can be adjusted as appropriate as long as it does not impair the effects of the technology disclosed herein. For example, when the total organic vehicle component is 100 wt%, the lower limit of the binder resin is, for example, 0.1 wt% or more, and may be 0.5 wt% or more, or even 1 wt% or more. On the other hand, the upper limit of the binder resin is, for example, 3 wt% or less, and may be 2 wt% or less, or even 1.5 wt% or less.

The conductive ink disclosed herein may contain additives other than the binder resin. The types and amounts of additives other than the binder resin can be changed as appropriate based on conventional technical common sense, and as they do not characterize the present invention, detailed explanations will be omitted.

The content of organic vehicle components (dispersant, organic solvent, and resin binder) in the conductive ink is not particularly limited and can be adjusted appropriately depending on the printing purpose, similar to the content of the inorganic powder described above. Therefore, when the entire conductive ink is taken as 100 wt%, the organic vehicle components are preferably 30 wt% or more, more preferably 40 wt% or more, and particularly preferably 45 wt% or more. Furthermore, the upper limit of the organic vehicle components is, for example, 70 wt% or less, preferably 65 wt% or less, and more preferably 60 wt% or less.

<Conductive ink properties>
The surface tension of the conductive ink disclosed herein is preferably 30 mN/m or more, and preferably 31 mN/m or more, from the viewpoint of reducing thickness variations of the conductive film. Furthermore, from the viewpoint of suppressing clogging of the inkjet ejection orifices, the surface tension of the conductive ink is, for example, 40 mN/m or less, preferably 37 mN/m or less, more preferably 36 mN/m or less (e.g., 35.2 mN/m or less), even more preferably 35 mN/m or less (e.g., 34.8 mN/m or less), and particularly preferably 34 mN/m or less.

The viscosity of the conductive ink is not particularly limited, but if the viscosity of the conductive ink is too high, it will tend to adhere to the nozzles of the inkjet device, which may result in reduced printing accuracy. Therefore, the viscosity of the conductive ink should be, for example, 30 mPa·s or less, preferably 25 mPa·s or less, and more preferably 20 mPa·s or less, and may be, for example, 16 mPa·s or less or 15 mPa·s or less. Furthermore, if the viscosity is too low, the ejected ink will tend to spread, which may result in reduced printing accuracy. Therefore, the viscosity of the conductive ink should be, for example, 5 mPa·s or more, preferably 7 mPa·s or more, more preferably 9 mPa·s or more, and may be 10 mPa·s or more. The viscosity of the ink can be measured using a B-type viscometer.

2. Preparation of Conductive Ink Next, the procedure for preparing (manufacturing) the conductive ink disclosed herein will be described. The conductive ink disclosed herein is prepared by mixing the above-mentioned components and then crushing and dispersing the inorganic powder. Figure 1 is a cross-sectional view that schematically shows an agitator/pulverizer used to manufacture the conductive ink. Note that the following description shows an example of a means for preparing the conductive ink disclosed herein and is not intended to limit the technology disclosed herein.

To manufacture the conductive ink disclosed herein, the ink precursor slurry (including paste and suspension) is first prepared by weighing and mixing the components described above. The conductive ink is then prepared by stirring the slurry and crushing the inorganic powder using an agitator/miller 100 such as the one shown in Figure 1. Specifically, fine crushing beads (e.g., zirconia beads with an average particle size of 10 μm to 50 μm) are added to the slurry, and the slurry is then supplied from the supply port 110 into the agitator vessel 120. The agitator vessel 120 contains a shaft 134 with multiple agitator blades 132. One end of the shaft 134 is attached to a motor (not shown). Operating the motor rotates the shaft 134, stirring the slurry with the multiple agitator blades 132 while sending it downstream in the liquid feed direction D. During this agitation, the inorganic particles are crushed by the crusher beads, and the finely divided inorganic powder is dispersed in the slurry.

The slurry sent downstream in the liquid feed direction D then passes through filter 140. As a result, inorganic particles and disintegrating beads that have not been atomized are collected by filter 140, and conductive ink with sufficiently dispersed inorganic powder is discharged from outlet 150. In this process, by adjusting the pore size of filter 140 and the average particle size of the disintegrating beads, the "average particle size of the inorganic powder" and "specific surface area of the inorganic powder" in the conductive ink can be adjusted to the desired range.

3. Uses of the Conductive Ink Next, uses of the conductive ink disclosed herein will be described. The conductive ink disclosed herein is used in the manufacture of electronic components. In this specification, "used in electronic components" not only refers to an embodiment in which the conductive ink disclosed herein is directly printed on the surface of an inorganic substrate, but also includes an embodiment in which the conductive ink is indirectly attached to the surface of an inorganic substrate via an intermediate material such as transfer paper.

(a) Printing Fig. 2 is a general view showing an example of an inkjet device, and Fig. 3 is a cross-sectional view showing an inkjet head of the inkjet device shown in Fig. 2.

The conductive ink disclosed herein is printed onto the surface of a printing target using an inkjet device 1 such as the one shown in Figure 2. There are no particular restrictions on the material or shape of the inorganic substrate W to be printed, and any substrate that can be used as a substrate for general electronic components can be used without particular restrictions. Note that when the conductive ink disclosed herein contains a high-melting point metal (W, Pd, Pt, Mo, etc.), it is particularly suitable for use on inorganic substrates W (such as alumina substrates and aluminum nitride substrates) that are subjected to high-temperature firing at 1200°C or higher.

Next, the structure of the inkjet device 1 shown in Figure 2 will be described. The inkjet device 1 is equipped with an inkjet head 10 that stores conductive ink. The inkjet head 10 is housed inside a print cartridge 40. The print cartridge 40 is inserted into a guide shaft 20 and is configured to move back and forth along the axial direction X of the guide shaft 20. Although not shown, the inkjet device 1 also includes a moving means for moving the guide shaft 20 in the vertical direction Y. This allows the inkjet device 1 to eject conductive ink onto a desired position on the inorganic substrate W.

The inkjet head 10 shown in FIG. 2 may be, for example, a piezoelectric inkjet head such as that shown in FIG. 3. This piezoelectric inkjet head 10 includes a storage section 13 for storing ink within a case 12, which is connected to a discharge section 16 via a liquid supply path 15. The discharge section 16 includes a discharge port 17 that opens to the outside of the case 12, and a piezoelectric element 18 is disposed opposite the discharge port 17. In this inkjet head 10, the piezoelectric element 18 is vibrated to discharge the ink from the discharge section 16 through the discharge port 17 toward the inorganic substrate W. The conductive ink disclosed herein has a high level of stability over time, allowing for highly accurate discharge over a long period of time. Therefore, the conductive ink disclosed herein can print highly precise patterns (images) with minimal film thickness variation on the surface of the inorganic substrate W.

(b) Drying Treatment Next, a drying treatment is performed in which the inorganic substrate W to which the ink is attached is heated at a predetermined temperature. This removes the organic solvent from the ink and forms a dry film on the inorganic substrate W. As described above, it is preferable that a binder resin be added to the conductive ink in order to improve the fixation of the dry film to the surface of the inorganic substrate W. The heating temperature in the drying treatment is preferably set to a temperature (for example, 50°C to 150°C, preferably 60°C to 80°C) at which the organic solvent is removed and at which sintering of the inorganic powder does not occur.

(c) Firing In the manufacturing method disclosed herein, the firing temperature can be appropriately adjusted depending on the type of inorganic powder contained in the conductive ink. For example, the inorganic substrate W after forming a dried film can be fired at 500°C to 2000°C. Furthermore, if the inorganic powder contained in the conductive ink is composed of a high-melting-point metal (W, Pd, Pt, Mo, etc.), these metal powders do not melt even at a firing temperature of 1200°C, so high-temperature firing at 1200°C or higher (e.g., 1300°C to 1500°C) can be performed. Therefore, conductive inks containing high-melting-point metal powders can be suitably used in the manufacture of plasma-resistant electronic components (e.g., electrostatic chucks) that are fired at high temperatures of 1200°C or higher.
By such baking, the binder resin that may be contained in the conductive ink is burned off, and the inorganic powder is sintered and fixed to the surface of the inorganic base material W. As a result, it is possible to manufacture an electronic component having a conductive circuit pattern with reduced thickness variation.

[Test Example]
Test examples relating to the technology disclosed herein will be described below. Note that the following test examples are not intended to limit the technology disclosed herein.

<Preparation of Conductive Ink>
In this test, 14 types of conductive inks (Examples 1 to 14) containing inorganic powder, organic solvent, binder resin, and dispersant were prepared. Specifically, inorganic powder and organic vehicle components (organic solvent, binder resin, and dispersant) were mixed in a weight ratio of 53:47 to prepare a slurry. The slurry was then subjected to a crushing and dispersion process (rotation speed: 1500 rpm, mixing time: 4 hours) using crushing beads (zirconia beads with an average particle size of 30 μm), followed by filtration under pressure to obtain the conductive inks of Examples 1 to 14. The proportions of each component in the organic vehicle (i.e., the proportions of the organic solvent, binder resin, and dispersant relative to the total organic vehicle components as 100 wt%) are shown in Table 2 below. The materials used in preparing these conductive inks are described in detail below.

(Inorganic powder)
In this test, tungsten (W) powder (specific surface area: 2.9 m ² /g) or palladium (Pd) powder (specific surface area: 1.5 m ² /g) was used as the inorganic powder. The types (constituent elements) of the inorganic powders (metal powders) used in each example are shown in Table 2.

(organic solvent)
In this test, the organic compounds listed below were used either alone or in combination as organic solvents. The composition ratio (wt%) of the organic compounds constituting each organic solvent is shown in Table 2, with the total organic solvent being 100 wt%. Note that _R1 below represents a phenyl group, and _R2 represents an alkyl group or an alkyl group containing a substituent.
<Organic Compound Represented by R ₁ —COO—R ₂ >
Methyl benzoate, benzyl benzoate (organic compounds represented by R ₁ -O-R ₂ )
Propylene glycol monophenyl ether (manufactured by Nippon Nyukazai Co., Ltd., product name: PhFG), 2-phenoxyethanol (organic compound having a phenyl group (containing neither an ester bond nor an ether bond))
Benzyl alcohol (an organic compound having an ester bond and/or an ether bond (not containing a phenyl group))
Butyl diglycol acetate (BDGAC), diethyl maleate, butyl monoglycol acetate (BMGAC), diethylene glycol dibutyl ether (manufactured by Toho Chemical Industry Co., Ltd., product name: Hisorb BDB), dimethyl adipate, diethyl malonate

The surface tension (mN/m) of the above organic compounds was measured using a static surface tensiometer (DYNEMASTER DY-300, manufactured by Kyowa Interface Science Co., Ltd.). The results are shown in Table 1. Table 2 also shows the average surface tension of the organic solvents calculated from the surface tension values of each organic compound shown in Table 1 and the composition ratios of the organic solvents shown in Table 2.

(binder resin)
The binder resin used was a polyvinyl acetal resin (manufactured by Sekisui Chemical Co., Ltd., model: BL-S). The molecular weight of this polyvinyl acetal resin was 2.3 x 10 ⁴ and the glass transition temperature was 66°C.

(Dispersant)
In this test, at least one of the dispersants listed below was used as the dispersant. The type of dispersant used in each example and the proportion (wt %) of the dispersant relative to the total organic vehicle are shown in Table 2.
<Cationic Dispersant>
Dispersant C1: Hypermer KD-1 manufactured by Croda Japan Co., Ltd.
Dispersant C2: NOF Corporation: Esreem AD Series (registered trademark) AD-508E
<Nonionic dispersant>
Dispersant N1: Hypermer KD-13 manufactured by Croda Japan Co., Ltd.

<Evaluation test>
A. Ink Properties (A-1) Viscosity The viscosity (mPa s) of the prepared conductive ink was measured using a Brookfield viscometer while maintaining the temperature at 25°C. The rotor rotation speed of the Brookfield viscometer was set to 5 rpm. The results are shown in Table 2.

(A-2) Surface Tension The surface tension (mN/m) of the prepared conductive ink was measured using a static surface tensiometer (DYNEMASTER DY-300, manufactured by Kyowa Interface Science Co., Ltd.). The results are shown in Table 2.

(A-3) Average particle size The average particle size of the prepared conductive ink was measured by dynamic light scattering (DLS). The results are shown in Table 2. Note that for Examples 9 to 11, where particles in the conductive ink aggregated, it was not possible to measure the average particle size.

(A-4) Measurement over time (measurement of stability over time)
A portion of the prepared conductive ink was collected in two storage bottles, one of which was stored at 25°C and the other at 60°C. The average particle size was measured for each of the inks stored at 25°C and 60°C after one week, two weeks, three weeks, and four weeks. The results are shown in Table 2, where an average particle size of 320 nm or less after four weeks in both the 25°C and 60°C environments is marked with "Good" and an average particle size of greater than 320 nm is marked with "Poor." The average particle size was measured using DLS. The results are shown in Table 2.

B. Printing Test (B-1) Discharge Test An inkjet printer (Fujifilm Corporation: Material Printer DMP-2831) was used to evaluate the continuous discharge performance and intermittent discharge performance (open time discharge performance) of the conductive ink of each example. In both performance evaluations, the conductive ink of each example was printed in the form of a film on the surface of an alumina substrate under discharge conditions of 10 pl/dot and 1200 dpi. At this time, the discharge state was visually observed using a camera attached to the inkjet printer. Note that for Examples 9 to 11, the particles in the conductive ink were aggregated, making it impossible to perform the discharge test.

(Continuous discharge performance)
In the evaluation test for continuous discharge performance, whether the conductive ink could be discharged continuously for 10 minutes was evaluated. The results are shown in Table 2, with a rating of "◎" for continuous discharge for 10 minutes, a rating of "◯" for continuous discharge for 5 minutes or less, and a rating of "×" for discharge that could not be achieved from the start.

(Intermittent discharge performance)
Conductive inks that were rated as "Excellent" or "Good" for the continuous discharge performance were evaluated for intermittent discharge performance (open time discharge performance). In the test to evaluate intermittent discharge performance, after the 10-minute continuous discharge test, a 10-minute discharge stop period was set, and whether discharge was possible again after that was evaluated. The results are shown in Table 2, with "Good" indicating that discharge was possible and "Poor" indicating that discharge was not possible.

(B-2) Film Thickness Variation Test Using conductive inks that were evaluated as "◎" or "◯" for the continuous discharge performance and the intermittent discharge performance, a line having a width of 3 mm, a length of 3 cm, and a thickness (film thickness) of 5 μm was printed using the inkjet printer. The thickness of the printed line was measured using a laser microscope, and the coefficient of variation (CV value) of the thickness was calculated. The results are shown in Table 2. Note that a lower CV value indicates smaller variation.

<Overall rating>
Conductive inks that received a rating of "good" in the time-lapse measurement, a rating of "good" in the continuous discharge performance, a rating of "good" in the intermittent discharge performance, and a CV value of less than 0.1 in the film thickness variation test were given an overall rating of "good". Conductive inks that received a rating of "good" in the time-lapse measurement, a rating of "good" in the continuous discharge performance, a rating of "good" in the intermittent discharge performance, and a CV value of less than 0.1 in the film thickness variation test were given an overall rating of "good". Conductive inks that received a rating of "bad" in any of the time-lapse measurement, continuous discharge performance, and intermittent discharge performance, or that received a CV value of 0.1 or greater in the film thickness variation test were given an overall rating of "bad". The overall ratings are shown in Table 2.

As shown in Table 2, Examples 2 to 8 and 13 to 14 exhibited excellent temporal stability, and also achieved favorable results in the ejection test and film thickness variation test. This demonstrates that the inclusion of an organic compound represented by the above-described general formula R ₁ -COO-R ₂ or R ₁ -O-R ₂ results in a conductive ink with excellent temporal stability and reduced thickness variation of the conductive film. Furthermore, particularly in Examples 2 to 4, 7 to 8, and 13 to 14, the continuous ejection performance was superior, and film thickness variation was further reduced. This demonstrates that it is more preferable for the conductive ink to have a surface tension of 31 mN/m or more but less than 34.9 mN/m (e.g., 34 mN/m or less).

Furthermore, in Examples 9 to 11, the conductive ink aggregated. This shows that even if the surface tension of the organic solvent is 32.1 mN/m to 36.5 mN/m, if the organic solvent is composed only of an ester bond- and/or ether bond-containing compound (not containing a phenyl group), the stability over time is not good. Furthermore, even if an organic solvent containing a phenyl group-containing compound is used, as in Example 12, if the compound does not fall under the organic compound represented by the above-mentioned general formula: R ₁ -COO-R ₂ or R ₁ -O-R ₂ , the stability over time is not good.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above.

REFERENCE SIGNS LIST 1 Inkjet device 10 Inkjet head 12 Case 13 Storage section 15 Liquid delivery path 16 Discharge section 17 Discharge port 18 Piezo element 20 Guide shaft 40 Print cartridge 100 Stirring and grinding machine 110 Supply port 120 Stirring container 132 Stirring blade 134 Shaft 140 Filter 150 Discharge port

Claims (5)

1. A conductive inkjet ink for use in the manufacture of electronic components, comprising:
The ink contains an inorganic powder, a cationic dispersant, and at least one organic solvent,
the organic solvent contains an ester bond and/or an ether bond-containing compound,
The ester bond and/or ether bond-containing compound has the following general formula:
(1) R ₁ -COO-R ₂
(2) R ₁ -O-R ₂
(wherein, in the general formula, R ₁ is a phenyl group or a phenyl group having a substituent, and R ₂ is an alkyl group having 1 to 5 carbon atoms or an alkyl group having 1 to 5 carbon atoms having a substituent).
It contains an organic compound represented by any of the following:
In the ester bond- and/or ether bond-containing compound, the total proportion of the organic compound represented by (1) or (2) is 20 wt % or more and 75 wt % or less,
The inorganic powder includes at least one of a tungsten powder and a palladium powder .
Conductive inkjet ink. The conductive inkjet ink according to claim 1, having a surface tension of 31 mN/m or more and 37 mN/m or less. The conductive inkjet ink according to claim 1 or 2, wherein the total proportion of the organic compounds represented by (1) or (2) is 20 wt % or more when the entire organic solvent is taken as 100 wt %. The conductive inkjet ink described in any one of claims 1 to 3, wherein the proportion of the organic solvent is 30 wt% or more and 70 wt% or less when the entire conductive inkjet ink is taken as 100 wt%. The conductive ink-jet ink according to any one of claims 1 to 4 , further comprising a nonionic dispersant.