Copper paste for joining, method for joining body to be joined, method for producing joined body, and method for producing copper paste for joining

Abstract

The purpose of the present invention is to provide copper paste for joining that suppresses voids which can occur in a coating film firing step and that can achieve a sufficiently high joining ratio between bodies to be joined. Provided is a copper paste for joining that comprises a solvent and two or more types of copper powders with differing particle sizes, wherein: in one of the types of copper powder, the increase ratio (D2-D1) / D1×100 of a crystallite size D2 (nm) at 250°C with respect to a crystallite size D1 (nm) at 150°C is not less than 5%; the boiling point of the solvent is not lower than 150°C but lower than 300°C; and the proportion of the entire amount of the copper powders with respect to 100 mass% of the copper paste is not less than 88 mass%.

Description

Copper paste for bonding, method for bonding objects to be bonded, method for manufacturing a bonded object, and method for manufacturing a copper paste for bonding

[0001] The present invention relates to a copper paste for bonding. The present invention also relates to a method for bonding objects using the copper paste for bonding, a method for manufacturing a bonded body, and a method for manufacturing the copper paste for bonding.

[0002] In recent years, pastes containing sinterable metal powders have been used as joining materials to replace solder. For example, Patent Document 1 describes a joining copper paste containing submicro copper particles with a volume average particle size of 0.12 μm to 0.8 μm and micro copper particles with a volume average particle size of 2 μm to 50 μm.

[0003] US2018 / 0250751A1

[0004] The paste contains two types of copper particles and a solvent. When this paste is used to bond two objects to be bonded, a coating film of the paste is placed between the objects to be bonded and then fired. In the firing process, the solvent contained in the coating film is volatilized and removed as the temperature rises. If the solvent is not properly removed, spaces called voids are generated in the coating film, which makes it difficult to achieve a sufficient bonding rate with the objects to be bonded. As a result, the bonding strength with the objects to be bonded may be insufficient. This disadvantage is particularly pronounced when the bonding area between the objects to be bonded is large or when bonding is performed without pressure.

[0005] Therefore, an object of the present invention is to provide a copper paste for bonding that can suppress voids that may occur during the firing process of the coating film and can sufficiently increase the bonding rate between objects to be bonded.

[0006] The present invention provides a copper paste for bonding containing two or more types of copper powders having different particle sizes and a solvent, wherein one of the copper powders has an increase rate (D2-D1) / D1x100 of crystallite size D2 (nm) at 250°C relative to crystallite size D1 (nm) at 150°C of 5% or more, wherein the boiling point of the solvent is 150°C or higher and lower than 300°C, and wherein the proportion of the total amount of the copper powder relative to 100% by mass of the copper paste is 88% by mass or higher.

[0007] In addition, the present invention uses the above-mentioned bonding copper paste to bond two objects to be bonded together by 4 mm. ２ The present invention provides a method for joining two objects to be joined, in which the two objects are overlapped with each other with a joining area exceeding 100 mm, and the copper paste is fired without pressure to join the two objects.

[0008] Furthermore, the present invention uses the above-mentioned bonding copper paste to bond two objects to each other by 4 mm. ２ The present invention provides a method for manufacturing a bonded body in which two bonded bodies are overlapped with each other with a bonding area exceeding 100 mm, and the copper paste is fired without pressure to bond the two bonded bodies.

[0009] The present invention also provides a method for producing a copper paste for bonding, which includes a step of mixing copper powder and a solvent, wherein the copper powder comprises copper(I) powder having an increase rate (D2-D1) / D1x100 of crystallite size D2 (nm) at 250°C relative to crystallite size D1 (nm) at 150°C of 5% or more, and copper(II) powder having a particle size different from that of the copper(I) powder, the solvent has a boiling point of 150°C or more and less than 300°C, and the proportion of the total amount of the copper powder relative to 100% by mass of the copper paste for bonding is 88% by mass or more.

[0010] The present invention will be described below based on preferred embodiments. The bonding copper paste (hereinafter sometimes referred to as "paste") of the present invention contains two or more types of copper powder having different particle sizes, a solvent, and, if necessary, an adjuster.

[0011] The shape of the copper powder contained in the paste is not particularly limited, and both spherical and non-spherical powders can be used. Here, the copper powder being spherical means that the circularity coefficient is 0.85 or more. The circularity coefficient is calculated by taking a scanning electron microscope image of primary particles of copper powder, and where S is the area of ​​the two-dimensional projection image of the copper powder and L is the perimeter, and the ratio is 4πS / L. ２ It is calculated using the formula:

[0012] On the other hand, the copper powder being non-spherical means that the circularity coefficient is less than 0.85. Specific examples of non-spherical shapes include flat shapes, polyhedral shapes such as hexahedrons and octahedrons, spindle shapes, irregular shapes, etc. In the present invention, the term "flat shape" refers to a shape having a pair of plate surfaces forming the main surfaces of the particle and side surfaces perpendicular to these plate surfaces, and the plate surfaces and side surfaces may each independently be flat, curved, or irregular.

[0013] The present invention includes two or more types of copper powder with different particle sizes. By using copper powders with such different particle sizes, the copper powder with smaller particle sizes fills the gaps between the copper powder with larger particle sizes, thereby improving the density of the copper powder in the paste. As a result, when the paste is placed between two objects to be joined and fired, the sintering of the copper powder proceeds sufficiently, improving the bonding rate between the copper powders and, as a result, the bonding rate with the objects to be joined. Therefore, the two objects to be joined can be sufficiently joined even without pressure.

[0014] It is preferable that the two or more types of copper powders described above are all spherical. By having all of them spherical, the packing density of the copper powder in the bonding layer when the paste is applied to the bonded bodies and fired can be improved. Furthermore, when the paste is placed between two bonded bodies and fired, the sintering of the copper powder progresses more sufficiently, and the two bonded bodies can be sufficiently bonded even without pressure.

[0015] When the copper powder is spherical, the particle size is determined by the following method. More specifically, 50 or more clearly defined primary particles of copper powder are selected using a scanning electron microscope at a magnification of 10,000 to 150,000 times, and the Heywood diameter of each particle is measured. From the obtained Heywood diameter, the volume of the particles is calculated assuming that the particles are truly spherical, and the volume-cumulative particle size at 50% by volume of the cumulative volume is defined as D. ＳＥＭ５０ It shall be determined as follows.

[0016] When the copper powder contains spherical copper(I) powder and copper(II) powder, the particle size d1 of the copper(I) powder is preferably 0.11 μm or more but less than 1 μm, more preferably 0.11 μm or more but 0.8 μm or less, and even more preferably 0.11 μm or more but 0.6 μm or less. The particle size d2 of the copper(II) powder is preferably 1 μm or more but 10 μm or less, more preferably 1 μm or more but 8 μm or less, and even more preferably 1 μm or more but 6 μm or less.

[0017] Particle size D of cuprous powder and cupric powder ＳＥＭ５０ Specifically, the ratio d2 / d1 of the particle diameter d2 of the coppersecond powder to the particle diameter d1 of the copperfirst powder is preferably 2 or more and 90 or less, more preferably 4 or more and 70 or less, and even more preferably 6 or more and 50 or less. By setting the particle diameters of the two types of copper powder having different particle diameters to the ratio described above, shrinkage cracks are less likely to occur when the paste is applied to one object to be joined and the resulting coating is fired to form a joining layer, and the fillet portions of the joining layer and the object to be joined can be sufficiently joined.

[0018] Of the two or more types of copper powder, one type of copper powder is used in which the increase ratio (D2 - D1) / D1 x 100 of the crystallite size D2 (nm) at 250 °C relative to the crystallite size D1 (nm) at 150 °C is 5.0% or more, preferably 7.5% or more, and more preferably 8.0% or more. By satisfying these conditions, the sinterability of the copper powder constituting the paste is improved. Therefore, when the paste is placed between two bonded bodies and fired, the sintering of the copper powder proceeds more sufficiently, suppressing the generation of voids, and enabling the two bonded bodies to be bonded with a high bonding rate even without pressure. Therefore, this paste is suitable for use when bonding bonded bodies with large bonding areas, as described below, where the impact of void generation is significant. In the following description, for simplicity, the term "increase ratio" refers to the value calculated from "(D2 - D1) / D1 x 100." There is no particular upper limit to the increase ratio, but it can be approximately 100%. Copper powder with an increase rate of 5% or more is not obtained by a special manufacturing method, but can be adjusted by the types of copper source, organic surface treatment agent, reducing agent, organic solvent, etc. used in the manufacture of the copper powder, as well as the reaction time and reaction temperature during the manufacture. Alternatively, copper powder with an increase rate of 5% or more can be appropriately selected from commonly used copper powders.

[0019] The crystallite size of copper powder is determined by analyzing the X-ray diffraction pattern obtained by XRD measurement based on the high-temperature XRD (powder X-ray diffraction) method, and then calculating it using the Scherrer formula. The high-temperature XRD method involves placing the sample in a high-temperature unit capable of heating it, and performing XRD measurement while gradually heating it. The XRD measurement is performed using a fully automated horizontal multipurpose X-ray diffractometer manufactured by Rigaku Corporation, and a high-speed two-dimensional X-ray detector, PILATUS100K / R, manufactured by the same company. The measurement conditions for the X-ray diffraction pattern are as follows: Optical system: Parallel beam X-ray source: CuKα ray Measurement mode: Still mode, fixed at 2θ = 42° (X-ray diffraction intensity corresponding to 2θ = 38 to 48° can be obtained with the two-dimensional detector described above) Exposure time: 60 seconds Collimator: Φ0.2 mm Measurement atmosphere: Nitrogen Measurement temperature: 150, 250°C Heating rate: 10°C / min The measurement temperature is maintained without heating during XRD measurement (exposure time 60 seconds) at each measurement temperature.

[0020] Next, the crystallite size is calculated from the half-width of the X-ray diffraction pattern of the crystal plane (111) of the copper powder obtained by the above-mentioned XRD measurement using the following Scherrer formula: Scherrer formula: D = Kλ / β cos θ D: crystallite diameter K: Scherrer constant (0.94) λ: X-ray wavelength β: half-width [rad] θ: Bragg angle [rad]

[0021] The proportion of copper powder satisfying an increase rate of 5% or more relative to the total amount of copper powder is preferably 10% by mass or more and 80% by mass or less, and more preferably 20% by mass or more and 70% by mass or less, of the total copper powder. This suppresses the generation of voids, improves the sinterability of the copper powder, and enables two bonded bodies to be bonded with a high bonding rate. Furthermore, the two bonded bodies can be bonded sufficiently even without pressure. Furthermore, copper powder with an increase rate of 5% or more does not require a special manufacturing method, but can be obtained by appropriately selecting from commonly used copper powders. Examples of such copper powders include CH-0200 and CH-0200L1 manufactured by Mitsui Mining & Smelting Co., Ltd.

[0022] The copper powder may have a surface treatment agent attached to its surface, which can prevent excessive aggregation of the copper powder particles.

[0023] Suitable surface treatment agents for suppressing aggregation between copper powder particles in the present invention include, for example, various fatty acids, aliphatic amines, and complexing agents with an affinity for copper. In particular, saturated or unsaturated fatty acids or aliphatic amines having 6 to 18 carbon atoms, especially 10 to 18 carbon atoms, and copper salts thereof are preferred from the viewpoint of improving oxidation resistance. Specific examples of such fatty acids or aliphatic amines include benzoic acid, pentanoic acid, hexanoic acid, octanoic acid, nonanoic acid, decanoic acid, lauric acid, palmitic acid, oleic acid, stearic acid, pentylamine, hexylamine, octylamine, decylamine, laurylamine, oleylamine, and stearylamine. Complexing agents with an affinity for copper include, for example, amino acids such as glycine, and dimethylglyoxime. These fatty acids, aliphatic amines, and complexing agents can be used alone or in combination of two or more.

[0024] In the present invention, it is preferable to use a solvent with a boiling point of 150°C or higher but lower than 300°C. This allows the solvent to remain up to a predetermined temperature during sintering of the paste, thereby suppressing the generation of voids due to bumping and the like. In addition, gas escape occurs gradually, thereby suppressing the generation of voids. Furthermore, a liquid bridging force acts between copper particles present in the solvent, causing an adhesive force to act between adjacent copper particles. As a result, when the paste is applied to the bonded bodies and sintered, the voids in the generated bonding layer are reduced, and the bonding rate of the bonding layer, i.e., between the sintered copper particles and the bonded bodies, is increased. As a result, two bonded bodies can be bonded with sufficient bonding strength even without pressure. For this reason, a 4mm ２ The paste of the present invention is preferably used when joining objects to be joined having a joining area exceeding 100° C. From the same viewpoint, it is preferable that the above-mentioned solvent does not contain a solvent having a boiling point of 300° C. or higher.

[0025] Examples of solvents having a boiling point of 150°C or higher but lower than 300°C include monoalcohols, polyhydric alcohols, ketones, ethers, polyhydric alcohol alkyl ethers, polyhydric alcohol aryl ethers, aliphatic organic acids, esters, nitrogen-containing heterocyclic compounds, amides, amines, and saturated hydrocarbons. These solvents can be used alone or in combination. Among them, at least one of alcohols such as propylene glycol, ethylene glycol, hexylene glycol, diethylene glycol, 1,3-butanediol, 1,4-butanediol, dipropylene glycol, tripropylene glycol, terpineol, and dihydroterpineol, ethers such as ethyl carbitol and butyl carbitol, and aliphatic organic acids is preferred, and aliphatic organic acids are particularly preferred. The use of these solvents removes oxide films that may be present on the surfaces of the copper powder, allowing highly active copper powder to be precipitated and sintered, making it easier to obtain a bonded body with reduced electrical resistance.

[0026] Examples of the aliphatic organic acid include carboxylic acids. Examples of carboxylic acids include branched primary carboxylic acids, secondary carboxylic acids, and tertiary carboxylic acids. Secondary or tertiary carboxylic acids are preferred, and tertiary carboxylic acids are even more preferred. By using such carboxylic acids, the bulkiness of the carboxylic acid's molecular structure can maintain adequate particle dispersion, which makes it easier to suppress the generation of voids during sintering of copper powder particles, thereby improving the bonding rate with the bonded bodies. In addition, the sinterability of the copper paste can be sufficiently improved, making it easy to obtain a bonding layer that simultaneously achieves high levels of bonding strength with the bonded bodies. Examples of carboxylic acids suitable for the present invention include one or more of the following: branched and saturated aliphatic monocarboxylic acids such as isobutyric acid, pivalic acid, 2,2-methylbutyric acid, isopentanoic acid, isohexanoic acid, isoheptanoic acid, isooctanoic acid, isononanoic acid, isodecanoic acid, and neodecanoic acid; branched and unsaturated aliphatic monocarboxylic acids such as methacrylic acid; and unsaturated tricarboxylic acids such as aconitic acid. These can be used alone or in combination.

[0027] The ratio of the total amount of copper powder to the paste is preferably 88% by mass or more, more preferably 92% by mass or more, even more preferably 95% by mass or more, and may be 96% by mass or more. Furthermore, the ratio of the total amount of copper powder to the paste is more preferably 98% by mass or less. By setting the ratio of copper powder in the paste within the above range, the ratio of copper powder in the paste increases and the ratio of solvent decreases, thereby suppressing the occurrence of voids and improving the bonding rate between the bonded bodies and the copper powder, i.e., the bonding layer formed by sintering the copper powder, and the bonded bodies, and increasing the contact area. This allows the two bonded bodies to be sufficiently bonded even without pressure, and a bond of 4 mm ２The bonding strength of the two objects to be bonded can be sufficiently improved even when the bonding area is large and exceeds 100 m / s. Note that the two types of copper powder constituting the paste are both spherical, and the ratio d2 / d1 of the particle diameter d2 of the coppersecond powder to the particle diameter d1 of the copperfirst powder is set to 2 or more and 90 or less, preferably 6 or more and 50 or less, so that good coatability is achieved even when the copper powder concentration in the paste is as high as described above.

[0028] From the viewpoint of improving the coating property or printability of the paste, the shear rate is set to 10 s －１ The viscosity at a shear rate of 10 s is preferably 10 Pa·s or more, and more preferably 15 Pa·s or more. －１ The viscosity value at this time is preferably 800 Pa·s or less, more preferably 700 Pa·s or less, and may be 200 Pa·s or less. The viscosity is measured using a rheometer (viscoelasticity measuring device). The viscosity of the paste can be measured using a rheometer MARS III manufactured by Thermo Scientific. The measurement conditions for the viscosity of the paste are as follows: Measurement mode: Shear rate dependency measurement Sensor: Parallel type (Φ20 mm) Measurement temperature: 25°C Gap: 0.300 mm Shear rate: 0.05 to 120.01 s －１ Measurement time: 2 minutes

[0029] The paste may contain an appropriate modifier for adjusting various properties as described above, such as a reducing agent, a viscosity modifier, or a surface tension modifier.

[0030] The reducing agent is preferably one that promotes sintering of copper powder, and examples thereof include monoalcohols, polyhydric alcohols, amino alcohols, citric acid, oxalic acid, formic acid, ascorbic acid, aldehydes, hydrazine and its derivatives, hydroxylamine and its derivatives, dithiothreitol, phosphite, hydrophosphite, phosphorous acid and its derivatives, etc.

[0031] The viscosity adjuster is preferably one that can adjust the viscosity of the paste, preferably within the above-mentioned viscosity range, and examples thereof include ketones, esters, alcohols, glycols, hydrocarbons, and polymers.

[0032] Examples of the surface tension adjuster include polymers such as acrylic surfactants, silicone surfactants, alkyl polyoxyethylene ethers, and fatty acid glycerol esters, and monomers such as alcohols, hydrocarbons, esters, and glycols.

[0033] There are no particular limitations on the type of the objects to be bonded. In general, it is preferable that the surfaces of the objects to be bonded contain a metal. For example, a member having a surface made of metal can be used as at least one of the two objects to be bonded. Note that the term "metal" as used herein refers to a metal itself that does not form a compound with other elements, or an alloy of two or more metals. Examples of such metals include copper, silver, gold, aluminum, palladium, nickel, and alloys made of a combination of two or more of these.

[0034] When at least one of the two objects to be joined is a member having a surface made of metal, the surface made of metal may be made of one type of metal, or may be made of two or more types of metal. When made of two or more types of metal, the surface may be an alloy. In general, the surface made of metal is preferably flat, but in some cases it may be curved.

[0035] Specific examples of the bonded bodies include spacers and heat sinks made of the above-mentioned metals, semiconductor elements, and substrates having at least one of the above-mentioned metals on their surfaces. Examples of the substrate include an insulating substrate having a metal layer such as copper on the surface of a ceramic or aluminum nitride plate. When a semiconductor element is used as the bonded body, the semiconductor element contains one or more elements such as Si, Ga, Ge, C, N, and As. One of the bonded bodies is preferably a substrate. The other bonded body is preferably a spacer, heat sink, or semiconductor element.

[0036] Furthermore, a dried paste containing metal fine particles and a solvent can be used as at least one of the objects to be joined. Specifically, a member having a metal surface can be used as one of the objects to be joined, and a dried paste containing metal fine particles and a solvent can be used as the other object to be joined. When a dried paste is used, it is preferable to apply the paste to a support substrate made of a metal such as copper and dry it to obtain a dried paste.

[0037] Next, a preferred method for joining the objects to be joined will be described. First, a paste is applied to a first object to be joined to form a coating film (coating film forming step). The first object to be joined can be composed of any of the above-mentioned objects to be joined, and is preferably a spacer, a heat sink, a semiconductor element, or a substrate having at least one of the above-mentioned metals on its surface, etc.

[0038] Once the coating film is formed on the surface of the first bonded body, the second bonded body is placed on the coating film to form a laminate in which the first bonded body, the coating film, and the second bonded body are stacked in this order (laminate formation step). The second bonded body can be the same as the first bonded body described above, without any particular restrictions. When the first bonded body is, for example, a substrate, the second bonded body is preferably, for example, a spacer, a heat sink, or a semiconductor element. Furthermore, in the present invention, the first bonded body and the second bonded body are bonded using the paste described above, and since the generation of voids during bonding can be suppressed, the bonding area where the problem of void generation becomes significant is 4 mm ２ Furthermore, it is suitable for use when using a bonded object having a bonding area of ​​25 mm or more, where the problem of void generation becomes particularly pronounced. ２ Even if the bonding area is larger than this, it is significant in that the generation of voids is suppressed. ２ is.

[0039] The laminate formed as described above is subjected to a first heating step in which the laminate is heated. In this step, the laminate is generally heated from around room temperature to remove the solvent contained in the coating. In this step, sintering of the copper particles contained in the coating does not occur.

[0040] In this process, the laminate is heated incrementally (first heating process). In this specification, "incrementally heated" refers to heating that continuously increases from the start of heating in the first heating process until the maximum temperature described below is reached. After the maximum temperature is reached, the temperature may be kept constant (for example, for 10 minutes to 120 minutes). It is also acceptable for the heating temperature to temporarily remain constant or to decrease during the first heating process. By performing the first heating process incrementally, the solvent contained in the coating film can be gradually removed, which has the advantage of making it less likely for voids to occur when sintering the copper particles in the second heating process described below.

[0041] The heating in this step may be performed so that the temperature increases linearly, exponentially, or logarithmically over time, or a combination of these may form a temperature rise curve. Regardless of the heating mode, it is preferable to perform the heating without pressure and in an inert atmosphere. "Without pressure" refers to a state in which no external force other than the weight of the second bonded body is applied between the first bonded body and the second bonded body in the laminate. Furthermore, performing the first heating step in an inert atmosphere, in combination with the second heating step in the atmosphere described below, can effectively suppress the generation of voids in the sintered body formed by firing. In contrast, performing the first heating step in a reducing atmosphere simultaneously removes the solvent from the coating film and sinters the copper particles in the coating film, making the sintering of voids more likely to occur in the sintered body.

[0042] Examples of the inert atmosphere used in the first heating step include a nitrogen gas atmosphere and a rare gas atmosphere such as argon or neon. From the viewpoint of economy, it is preferable to use a nitrogen gas atmosphere. The inert atmosphere used in this heating step does not contain any gas other than the inert gas, such as a reducing gas such as hydrogen gas, or an oxidizing gas such as oxygen gas or air, except for gases that are inevitably mixed in.

[0043] The heating of the laminate in the first heating step is preferably carried out at a temperature rise rate of 0.01°C / s or more and 1°C / s or less, since this allows the solvent contained in the coating film to be gradually removed. From the viewpoint of making this advantage even more pronounced, the temperature rise rate in this step is more preferably 0.01°C / s or more and 0.8°C / s or less, and even more preferably 0.01°C / s or more and 0.6°C / s or less. When the temperature rise rate in the first heating step is not constant, it is sufficient that the average value of the temperature rise rate falls within the above range.

[0044] The end temperature of the first heating step, in other words, the maximum temperature in the first heating step, is preferably set to 110°C or higher and Bp°C or lower, where Bp is the boiling point of the solvent contained in the copper paste, because this allows for good evaporation of the solvent and moisture absorbed by the copper paste, making it easier to suppress the occurrence of voids. To further enhance this advantage, the lower limit of the maximum temperature in the first heating step is preferably 140°C, more preferably 160°C.

[0045] In particular, when the solvent contained in the copper paste is an aliphatic organic acid, it is preferable to set the maximum heating temperature in the first heating step to 240°C or less, from the viewpoint of suppressing the generation of voids due to the rapid evaporation of the solvent component.

[0046] The laminate that has been subjected to the first heating step is then subjected to a second heating step (second heating step), in which the laminate is heated to a temperature equal to or higher than the heating temperature in the first heating step, causing the copper particles in the coating to sinter together, forming a sintered body, i.e., a bonding layer.

[0047] The second heating step is distinguished from the first heating step by the difference in the atmosphere. Specifically, the first heating step is performed in an inert atmosphere, whereas the second heating step is performed in a reducing atmosphere. In this way, the present manufacturing method effectively suppresses the generation of voids in the sintered body by employing a combination of an inert atmosphere and a reducing atmosphere.

[0048] The reducing atmosphere used in the second heating step is an atmosphere containing a reducing gas. Examples of reducing gases include hydrogen, formic acid, carbon monoxide, and ammonia. One of these reducing gases can be used alone, or two or more can be used in combination. Among these reducing gases, using a formic acid-containing gas is preferable because the oxide film present on the surface of the copper particles is easily removed by the formic acid, thereby increasing the surface activity of the copper particles and facilitating sintering between the copper particles at low temperatures.

[0049] The reducing atmosphere used in the second heating step may contain only a reducing gas, or may contain other gases in addition to the reducing gas. Examples of other gases include the inert gas used in the first heating step. Specifically, nitrogen gas, rare gas, etc. can be used. When the reducing atmosphere used in the second heating step is a mixed atmosphere of a reducing gas and an inert gas, the concentration of the reducing gas in the mixed atmosphere is preferably 1 vol% or more, more preferably 2 vol% or more. This is because the oxide film present on the surface of the copper particles is easily removed by the reducing gas such as formic acid, thereby increasing the surface activity of the copper particles and facilitating sintering between the copper particles at low temperatures.

[0050] The temperature change rate in the second heating step itself is preferably -0.1°C / s or more and 1.0°C / s or less, preferably -0.1°C / s or more and 0.5°C / s or less, assuming that the heating temperature in the second heating step is equal to or higher than the heating temperature in the first heating step, in order to ensure that sintering proceeds appropriately without unevenness. From the viewpoint of further enhancing this advantage, it is even more preferable that the temperature increase rate in the second heating step is -0.1°C / s or more and 0.3°C / s or less. If the temperature change rate in the second heating step is not constant, it is sufficient that the average value of the temperature change rate falls within the above range. Note that the temperature change rate also includes 0°C / s.

[0051] The maximum heating temperature in the second heating step is preferably 250° C. or less in order to prevent voids that may occur due to the sudden evaporation of the remaining solvent. The heating time in the second heating step is preferably 10 minutes or more and 180 minutes or less.

[0052] By subjecting the laminate to the second heating step, a sintered body is produced from the paste coating, and the sintered body bonds the first and second bonded bodies. This results in the desired bonded body. Completion of the second heating step, i.e., initiation of the cooling step, is achieved by switching the atmosphere in the system to an inert atmosphere and lowering the temperature in the system. To lower the temperature in the system, for example, heating may be stopped and the system may be allowed to cool naturally, or a cooling gas (this gas is an inert gas) may be circulated. During the cooling step, maintaining the system in an inert atmosphere is preferable in terms of preventing oxidation of the bonded body. The type of inert atmosphere may be the same as in the first heating step. Heating in the second heating step is performed without pressure. "Under pressure" is the same as described above.

[0053] As described above, by using the paste of the present invention, voids that may occur during the film firing process are suppressed, resulting in a high bonding rate of the bonding layer. This allows for a sufficiently high bonding strength between the bonded objects. Furthermore, as a secondary effect of the high bonding rate, the paste also exhibits excellent thermal and electrical properties. Such a paste of the present invention is suitable for use in, for example, automotive electronic circuits and electronic circuits equipped with power devices.

[0054] In addition to the above-described embodiments, the present invention further discloses the following copper paste for bonding, a method for bonding bodies, a method for manufacturing a bonded body, and a method for manufacturing a copper paste for bonding. [1] A copper paste for bonding containing two or more types of copper powders having different particle sizes and a solvent, wherein one of the copper powders has a crystallite size D2 (nm) at 250°C relative to a crystallite size D1 (nm) at 150°C, the increase rate (D2 - D1) / D1 x 100 being 5% or more, and the boiling point of the solvent is 150°C or higher but lower than 300°C, and the proportion of the total amount of the copper powder relative to 100% by mass of the copper paste is 88% by mass or more. [2] The copper paste for bonding according to [1], wherein all of the copper powders are spherical. [3] The copper paste for bonding according to [2], wherein the copper powders include a cuprous powder and a cupric powder, and wherein the ratio d2 / d1 of the particle size d2 of the cupric powder to the particle size d1 of the cuprous powder is 2 or higher but 90 or lower. [4] The copper paste for bonding according to any one of [1] to [3], wherein the proportion of copper powder having the increase rate (D2-D1) / D1 x 100 of 5% or more is 10% by mass or more and 80% by mass or less, relative to 100% by mass of the total amount of the copper powder. [5] The copper paste for bonding according to any one of [1] to [4], wherein the proportion of the total amount of the copper powder relative to 100% by mass of the copper paste is 92% by mass or more. [6] The copper paste for bonding according to any one of [1] to [5], which does not contain a solvent having a boiling point of 300°C or higher. [7] The copper paste for bonding according to any one of [1] to [5], wherein the proportion of the total amount of the copper powder having the increase rate (D2-D1) / D1 x 100 of 5% or more is 10% by mass or more and 80% by mass or less, relative to 100% by mass of the total amount of the copper powder. －１ [8] The copper paste for bonding according to any one of [1] to [6], wherein the viscosity value at 0.5 mm is 10 Pa·s or more and 800 Pa·s or less. ２ [9] The copper paste for bonding according to any one of [1] to [8], which is for bonding objects to be bonded that are greater than 4 mm.

[10] The copper paste for bonding according to any one of [1] to [9], which is for pressureless bonding.

[11] The copper paste for bonding according to any one of [1] to [9], which is for bonding two objects to be bonded that are greater than 4 mm. ２

[11] A method for joining two objects to be joined, in which two objects to be joined are overlapped with each other with a joining area exceeding 4 mm and the copper paste is fired without pressure.２

[12] A method for producing a copper paste for bonding, comprising the step of mixing copper powder and a solvent, wherein the copper powder comprises cuprous powder having a crystallite size D2 (nm) at 250°C relative to a crystallite size D1 (nm) at 150°C, where D2 is an increase rate (D2 - D1) / D1 x 100) of 5% or more, and cupric powder having a particle size different from that of the cuprous powder, the solvent has a boiling point of 150°C or higher but lower than 300°C, and the proportion of the total amount of the copper powder relative to 100% by mass of the copper paste for bonding is 88% by mass or more.

[0055] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to such examples. Furthermore, unless otherwise specified, "%" means "% by mass."

[0056] [Example 1] (1) Preparation of Paste Copper powder having an average primary particle diameter D ＳＥＭ５０ Spherical cuprous powder (manufactured by Mitsui Mining & Smelting Co., Ltd., CH-0200L1) with an average primary particle diameter D ＳＥＭ５０ The copper powder was mixed with spherical cupric powder (CS20 manufactured by Mitsui Mining & Smelting Co., Ltd.) having a diameter of 2.2 μm at a mass ratio of 30:70. The ratio of the particle size of the cupric powder to the particle size of the cuprous powder was 15.7. The increase in the crystallite size D2 (nm) at 250°C relative to the crystallite size D1 (nm) of the cuprous powder at 150°C (D2-D1) / D1×100 was 10.1%. Neodecanoic acid (Versatic 10 manufactured by Hexion, boiling point 270°C to 280°C) was used as the solvent. Neodecanoic acid was added to 100% copper paste at 7.5% (copper powder ratio to 100% copper paste: 92.5%), and the mixture was pre-mixed with a spatula. Then, using a THINKY CORPORATION ARE-500 rotary / revolution vacuum mixer, two cycles of mixing (1000 rpm x 1 minute) and degassing mode (2000 rpm x 30 seconds) were performed to form a paste. This paste was further processed using a three-roll mill to further disperse and mix the mixture, thereby preparing the copper paste of the present example.

[0057] (2) Application to First Bonded Body The copper paste was applied by screen printing to a chip mounting portion of a copper lead frame (thickness: 2.0 mm) as a first bonded body to form a coating film. The coating film was formed into a rectangle of 6 mm x 10 mm. The thickness of the coating film was 100 μm.

[0058] (3) Production of Laminate A 5 mm square SiC chip (thickness: 0.2 mm) was placed on the coating film as a second object to be bonded, and the thickness of the coating film was adjusted to 50 μm using a Digimatic indicator (manufactured by Mitutoyo Corporation).

[0059] (4) First Heating Step The laminate was placed in a heating furnace. 100% nitrogen gas was circulated through the heating furnace. The flow rate of the nitrogen gas was 3 L / min. Under this condition, the laminate was heated from room temperature (25°C) to 200°C, and the temperature was maintained for 10 minutes after reaching 200°C. The temperature was increased linearly over time at a rate of 0.1°C / sec. The laminate was heated without pressure.

[0060] (5) Second Heating Step After the temperature inside the heating furnace was maintained at 200°C for 10 minutes, the gas flowing through the heating furnace was switched to a reducing gas. The temperature increase was stopped, and the temperature inside the heating furnace was maintained at 200°C. Nitrogen gas containing 3 vol% formic acid was used as the reducing gas. The flow rate of the reducing gas was 0.5 L / min. In this step, the temperature inside the heating furnace was maintained constant at 200°C for 60 minutes. The laminate was heated without pressure.

[0061] (6) Cooling Step After the second heating step was performed for 60 minutes, the gas flowing through the heating furnace was switched to 100% nitrogen gas. Heating in the heating furnace was stopped. The heating furnace was then cooled, and after the temperature in the furnace had dropped to room temperature, the bonded body was removed from the furnace.

[0062] Example 2 A bonded body was obtained in the same manner as in Example 1, except that neodecanoic acid was added so that the ratio of the total amount of cuprous powder and cupric powder to 100% of the paste was 92.0%.

[0063] Example 3 A bonded body was obtained in the same manner as in Example 1, except that neodecanoic acid was added so that the total amount of cuprous powder and cupric powder was 90.0% relative to 100% of the paste.

[0064] Example 4 A bonded body was obtained in the same manner as in Example 1, except that neodecanoic acid was added so that the ratio of the total amount of cuprous powder and cupric powder to 100% of the paste was 88.0%.

[0065] [Example 5] (2) Application to the first bonded object and (3) Production of the laminate in Example 1 were changed to the following method. Specifically, the copper paste was applied to the chip mounting portion of a copper lead frame (thickness: 2.0 mm) serving as the first bonded object using a dispenser (Musashi Engineering: S-SIGMA-CM3-V5). Next, a 5 mm square SiC chip (thickness: 0.2 mm) serving as the second bonded object was placed on the applied copper paste so that the ground surface of the SiC chip was in close contact with the copper paste. The thickness of the coating after placing the SiC chip was 50 μm. The copper paste was distributed over the entire ground surface of the SiC chip. Furthermore, in (5) Second Heating Step in Example 1, the maintenance time at 200°C was changed from 60 minutes to 120 minutes. Otherwise, a bonded object was obtained in the same manner as in Example 1.

[0066] Example 6 A bonded body was obtained in the same manner as in Example 5, except that in (1) Preparation of the paste in Example 5, neodecanoic acid was added in an amount of 5.0% relative to 100% of the copper paste (ratio of copper powder to 100% of the copper paste: 95.0%).

[0067] Example 7 A bonded body was obtained in the same manner as in Example 5, except that in (1) Preparation of the paste in Example 5, neodecanoic acid was added in an amount of 4.0% relative to 100% of the copper paste (ratio of copper powder to 100% of the copper paste: 96.0%).

[0068] Comparative Example 1 A bonded body was obtained in the same manner as in Example 1, except that neodecanoic acid was added so that the ratio of the total amount of cuprous powder and cupric powder to 100% of the paste was 86.0%.

[0069] Comparative Example 2 A bonded body was obtained in the same manner as in Example 1, except that neodecanoic acid was added so that the ratio of the total amount of cuprous powder and cupric powder to 100% of the paste was 84.0%.

[0070] [Bonding Rate of the Bonding Layer] The bonding rate of the bonding layer was measured as an indicator of whether void generation during sintering of copper particles was suppressed. The bonded structures obtained in the examples and comparative examples were observed from the backside of the copper lead frame using an ultrasonic flaw detector (Hitachi Power Solutions, Model: FineSAT III). Observation was performed using a 75 MHz probe and a reflection method. When observing the peeling state of the bonding layer, the gain was set to 25-35 dB, and the delay and width of the S gate were adjusted so that the S gate peak position was on the surface of the copper lead frame. The F gate delay was adjusted to specify the observation range of the bonding layer, and the width was set to a peak width of 1.5 wavelengths. The Z-axis coordinate of the probe was adjusted to maximize the amplitude of the observed peak, and observation was performed. The contrast of the observed image was adjusted using the automatic function. The obtained image data was binarized using image processing software Image-J, and the proportion of black areas within the observed area was calculated. That is, after starting Image-J, Analyze-Set measurement was selected, and Area, Area fraction, and Limit to Threshold were checked. Then, File-Open was selected, and the image data for calculating the bonding rate was opened, and the range (A) of the SiC chip mounting area in the image was specified. Next, Edit-Copy to system was selected, the specified range (A) was copied, and then File-New-System clipboard was selected to paste the image of the specified range (A). Then, in order to clarify the bonding area, Image-Type-8bit was selected, the image was converted, and then Image-Adjust-Threshold was selected to adjust the image threshold to 110. Then, a black area (B) existing within the area (A) of the SiC chip mounting area in the adjusted image was designated. The black area (B) became the SiC chip bonding area, and the bonding rate was calculated as (B) / (A) x 100. A high bonding rate indicates that the occurrence of voids was suppressed.

[0071]

[0072] As is clear from the results shown in Table 1, the bonded bodies obtained in each Example had a higher bonding rate than the bonded bodies of the Comparative Examples. It can be seen that the bonded bodies obtained in each Example have progressed in sintering of the copper powder in the bonding layer compared to the bonded bodies obtained in the Comparative Examples, and the occurrence of voids is suppressed. Although not shown in the table, the breaking strength of each of the bonded bodies obtained in the Examples was measured. A bond tester, Condor Sigma, manufactured by XYZTEC Corporation was used for the measurement. Breaking strength (MPa) was calculated by the formula: breaking load (N) / bonding area (mm ２ In all of the examples, the breaking strength exceeded 25 MPa, and the bonding strength was also excellent.

[0073] As described above in detail, the present invention provides a copper paste for bonding that can suppress the generation of voids that may occur during the firing process of the coating film and can sufficiently increase the bonding rate between the objects to be bonded, particularly when the bonding area between the objects to be bonded is large or when bonding is performed without applying pressure, and that can suppress the generation of voids and can sufficiently increase the bonding rate.

Claims

1. A copper paste for bonding containing two or more types of copper powders having different particle sizes and a solvent, wherein one of the copper powders has an increase rate (D2-D1) / D1x100 of crystallite size D2 (nm) at 250°C relative to crystallite size D1 (nm) at 150°C of 5% or more, the boiling point of the solvent is 150°C or higher and lower than 300°C, and the proportion of the total amount of the copper powder to 100% by mass of the copper paste is 88% by mass or higher.

2. The copper paste for bonding according to claim 1, wherein the copper powder is spherical.

3. The copper paste for bonding according to claim 2, wherein the copper powder comprises copper(I) powder and copper(II) powder, and the ratio d2 / d1 of the particle diameter d2 of the copper(II) powder to the particle diameter d1 of the copper(I) powder is 2 or more and 90 or less.

4. The copper paste for bonding according to claim 1, wherein the proportion of copper powder having an increase rate (D2-D1) / D1 x 100 of 5% or more is 10% by mass or more and 80% by mass or less, relative to 100% by mass of the total amount of copper powder.

5. The copper paste for bonding according to claim 1, wherein the ratio of the total amount of the copper powder to 100% by mass of the copper paste is 92% by mass or more.

6. The copper bonding paste according to claim 1, which does not contain a solvent having a boiling point of 300°C or higher.

7. Shear rate 10 s －１ The copper paste for bonding according to claim 1, wherein the viscosity value at time t is 10 Pa·s or more and 800 Pa·s or less.

8. Bonding area is 4mm ２ The copper paste for bonding according to claim 1, which is for bonding objects having a bonding strength exceeding 1000 MPa.

9. The copper paste for bonding according to claim 1, which is for pressureless bonding.

10. Two objects to be joined are joined together by using the joining copper paste according to any one of claims 1 to 9. ２ The method for joining two objects to be joined comprises overlapping the two objects with a joining area exceeding 100 mm, and then firing the copper paste without applying pressure to join the two objects.

11. Two objects to be joined are joined together by using the joining copper paste according to any one of claims 1 to 9. ２ and then firing the copper paste without pressure to bond the two objects to be bonded.

12. A method for producing a copper paste for bonding, comprising a step of mixing copper powder and a solvent, wherein the copper powder comprises copper(I) powder having an increase rate (D2-D1) / D1x100 of crystallite size D2 (nm) at 250°C relative to crystallite size D1 (nm) at 150°C of 5% or more, and copper(II) powder having a particle size different from that of the copper(I) powder, the solvent has a boiling point of 150°C or higher but lower than 300°C, and the proportion of the total amount of the copper powder relative to 100% by mass of the copper paste for bonding is 88% by mass or more.