JP7220310B2 - Copper paste for non-pressure bonding, method for non-pressure bonding, and method for manufacturing bonded body - Google Patents

Description

TECHNICAL FIELD The present invention relates to a copper paste for pressureless bonding , a pressureless bonding method using the same, and a method for manufacturing a bonded body.

Conventionally, high-melting-point lead solder has been widely used for joining metals, semiconductors, and the like, but from the viewpoint of environmental regulations, etc., there is a demand for a joining material that does not contain lead. As a material capable of low-temperature bonding, a method using metal nanoparticles such as silver is known. Nanoparticles are fused at a temperature lower than their melting point due to the nanosize effect, so low-temperature non-pressure bonding is possible. However, silver nanoparticles are expensive in terms of material cost, and at present, sufficient bonding strength has not been obtained.

Some studies using copper particles as a less expensive bonding material have been reported. In Patent Document 1, copper particles having a particle size on the order of μm are used as a bonding material, and the surfaces of the copper particles are oxidized by in situ synthesis to form nanoparticles, followed by heating in a reducing atmosphere. A joining method is disclosed. Patent Document 2 proposes a method of performing pressureless bonding using a copper paste containing coated nanoparticles and microparticles whose surface is coated with organic molecules to improve dispersibility.

JP 2017-074598 A JP 2014-167145 A

The methods of Patent Documents 1 and 2 cannot be said to have sufficient bonding strength. In view of such problems, an object of the present invention is to provide a copper paste capable of achieving high bonding strength even at low temperature bonding.

The copper paste of the present invention contains metal particles and a dispersion medium. The metal particles include first-class particles and second-class particles. The first particles are copper particles having an average particle size of 1 to 100 μm and having a nanostructure on the surface. The second type particles are copper particles having an average particle size of 0.05 to 5 μm. The average particle size D1 of the first type particles is preferably 2 to 550 times the average particle size D2 of the second type particles.

The nanostructures of the particles of the first kind are formed, for example, by thermal oxidation of copper. Examples of nanostructures include uneven shapes, particle shapes, fiber shapes, and the like.

By preparing a laminate in which the copper paste is provided between members to be joined and heating this laminate in a reducing atmosphere, the copper paste is sintered and the members can be joined.

The copper paste of the present invention is applicable to low temperature pressureless bonding. By using the copper paste of the present invention, high-strength bonding can be achieved.

1 is a scanning micrograph of copper particles; FIG. 3 is a cross-sectional view showing a configuration example of a laminate used for non-pressure bonding. 4 is scanning micrographs of cross sections of bonding layers of Example 1 and Comparative Example 3. FIG.

[Copper paste]
The copper paste of the present invention contains metal particles and a dispersion medium. The metal particles include first-class particles and second-class particles. The average particle size D1 of the first type particles is 1 to 100 μm, and the average particle size D2 of the second type particles is 0.05 to 5 μm. The average particle size D1 of the first type particles is larger than the average particle size D2 of the second type particles. D1 is preferably 2 to 550 times D2. In the specification of the present application, the average particle size is a volume-based cumulative median size (D ₅₀ ) obtained from a particle size distribution measured by a laser diffraction scattering method.

The first particles have a nanostructure on their surface. During heat bonding, the first particles having a nanostructure are fused to the surface, and the gaps between the first particles are filled with the second particles having a small particle diameter, so that there are few gaps in the bonding material and high bonding strength can be achieved.

<Metal particles>
(first-class particles)
The particles of the first kind are copper particles having an average particle size of 1 to 100 μm and having a nanostructure on the surface. Nanostructures on the surface of copper particles include nano-sized irregularities, nanoparticles, nanofibers, and the like. For example, copper particles having a particle diameter of 1 to 100 μm are thermally oxidized to obtain copper particles having a nanostructure of cuprous (cuprous) oxide on the surface.

FIG. 1(A) is a scanning microscope (SEM) photograph of a wet copper powder (“1400YM” manufactured by Mitsui Mining & Smelting Co., Ltd., average particle size 4.2 μm) that has not been heat-treated. The surface of the copper powder before heat treatment is smooth and no nanostructure is formed.

FIG. 1(B1) is an SEM photograph of the wet copper powder that was sequentially heated in the atmosphere at 100° C. for 10 minutes, 150° C. for 10 minutes, 200° C. for 10 minutes, 250° C. for 10 minutes, and 300° C. for 10 minutes. is. FIG. 1(B2) and FIG. 1(B3) are SEM photographs of wet copper powder that was heat-treated by changing the heating time at 300° C. to 30 minutes and 120 minutes, respectively. FIG. 1(C1) shows 10 minutes at 100° C., 10 minutes at 150° C., 10 minutes at 200° C., 10 minutes at 250° C., 10 minutes at 300° C., 10 minutes at 350° C., and 400° C. in the atmosphere. It is a SEM photograph of the wet copper powder heated sequentially for 10 minutes at. FIG. 1 (C2) and FIG. 1 (C3) are SEM photographs of wet copper powder that was heat-treated by changing the heating time at 400° C. to 30 minutes and 120 minutes, respectively.

In (B1), fine unevenness is formed on the particle surface, and in (B2) and (B3), as the heating time at 300 ° C. increases, the surface unevenness grows into particles. I understand. In (C1) heated at 400° C., finer irregularities are formed than in (B1), and a fine fibrous nanostructure is formed. In (C2) and (C3), it can be seen that the nanofibers grow as the heating time at 400° C. increases.

The melting point of copper is 1085° C., but nanostructures such as nanoscale unevenness, particles, fibers, etc. formed on the surface of copper particles exhibit melting point depression due to size effects, similar to nanoparticles. Therefore, the first particles having a nanostructure on the surface can be fused at a temperature lower than the melting point of copper (for example, about 300° C.) to form a metal bond. In other words, the first particles can be bonded at a low temperature while having a particle diameter on the order of μm. In addition, since the nanostructures are fixed on the surface of the first particles, the problems of aggregation and uneven distribution of metal nanoparticles are less likely to occur.

As described above, a nanostructure can be formed on the surface by heating copper particles having a particle size on the order of μm (hereinafter sometimes referred to as “microcopper particles”).

The shape of the micro copper particles as the raw material of the first particles is not particularly limited, and may be spherical, massive, needle-like, flake-like, or the like. Above all, the shape of the micro copper particles is preferably spherical or flaky because a nanostructure can be easily formed on the surface and the volume of voids between particles can be reduced when the particles are fused together. Note that the term "spherical" includes not only perfect spheres but also substantially spherical shapes with an aspect ratio of 3 or less. "Flake-like" includes plate-like shapes such as plate-like and scale-like shapes.

The particle diameter of the microcopper particles is preferably 1 to 100 μm. Since the particle size of the copper particles hardly changes before and after the nanostructure is formed by heating, the particle size of the micro copper particles is substantially equal to the particle size of the first particles. From the viewpoint of enhancing dispersibility and facilitating the formation of a nanostructure, the particle size of the micro copper particles is preferably 2 µm or more, more preferably 3 µm or more, still more preferably 3.5 µm or more, and particularly preferably 4 µm or more. From the viewpoint of enhancing the fusion between particles and reducing voids during bonding, the particle diameter of the micro copper particles is preferably 60 μm or less, more preferably 50 μm or less, even more preferably 40 μm or less, and particularly preferably 30 μm or less. . A commercially available copper powder may be used as it is as the micro copper particles.

By setting the particle diameter of the micro copper particles within the above range, the particle diameter of the first particles can be within the range of 1 to 100 μm. The particle size of the first particles is preferably 2 to 60 μm, more preferably 3 to 50 μm, still more preferably 3.5 to 40 μm, and particularly preferably 4 to 30 μm.

Nanostructures are formed on the surface by heating the micro-copper particles in an oxidizing atmosphere. The oxidizing atmosphere is an oxygen concentration atmosphere in which copper can be oxidized, and may be the air (oxygen concentration of about 21%). The heating temperature is preferably 200-500°C. The heating time may be appropriately determined according to the heating temperature and the like so that the nanostructure is formed on the surfaces of the microcopper particles, and is, for example, about 5 to 300 minutes.

The reason why nanostructures are formed on the surface of the microcopper particles by heating is not clear, but it is presumed to be related to the difference in thermal expansion coefficient between copper and cuprous oxide (or cuprous oxide). When the micro copper particles are heated in an oxidizing atmosphere, the surfaces of the copper particles are oxidized to form oxide films. When the particles are further heated in this state, oxidation progresses from the surface to the inside of the particles, and both cuprous (sub)oxide on the surface of the particles and copper in the core of the particles thermally expand as the temperature rises. Since copper has a larger coefficient of thermal expansion than copper oxide, as the temperature rises, the copper inside expands the grain boundaries of the oxide film on the surface, and copper precipitates on the surface layer along the expanded grain boundaries. It is thought that copper is oxidized by exposure to an oxidizing atmosphere at , forming nanostructures such as nanoparticles and nanofibers.

As shown in FIG. 1, as the heating temperature increases and the heating time increases, the nanostructure on the surface of the micro copper particles tends to grow. In addition, there is a tendency to form fibrous nanostructures (nanofibers) as the heating temperature rises. When nanofibers are formed on the surface of the first particles, there is a tendency for the copper particles to be particularly improved in fusion bondability. In order to form a fibrous nanostructure, the heating rate is reduced (for example, 5° C./min or less), or the temperature is raised stepwise to 350° C. or higher, and the temperature is 350° C. or higher. It is preferable to heat for 10 minutes or longer. It is thought that by gently raising the temperature, the deposition rate of the metal from the inside of the particle to the surface layer is controlled, and the deposit tends to grow in the form of fibers.

When an uneven or particulate nanostructure is formed on the surface of the microcopper particles, the particle diameter of the nanostructure is preferably 500 nm or less, more preferably 200 nm or less. When nanofibers are formed on the surfaces of microcopper particles, the fiber diameter is preferably 100 nm or less, more preferably 50 nm or less. Although the length of the fiber is not particularly limited, it is, for example, 10 μm or less, preferably 5 μm or less. If the size of the nanostructure is within the above range, good bondability at low temperatures (for example, about 200 to 500° C.) can be ensured. The size of the nanostructures is measured based on SEM images of the particles.

(Second-class particles)
The second kind particles are copper particles having an average particle size of 0.05 to 5 μm. The second type particles may or may not have a nanostructure on the surface. The second type particles have the effect of filling the gaps between the particles when the first type particles are fused to reduce the volume of the gaps. Therefore, particles having an average particle diameter smaller than that of the first type particles are used as the second type particles.

In order to effectively fill the gaps between the fused first particles, the ratio D1/D2 between the average particle diameter D2 of the second particles and the average particle diameter D1 of the first particles is preferably 2 or more. 5 or more is more preferable, and 3 or more is even more preferable. On the other hand, D1/D2 is preferably 550 or less, more preferably 300 or less, still more preferably 100 or less, and particularly preferably 50 or less, from the viewpoint of reducing the ratio of grain boundaries during bonding to ensure bonding strength.

From the viewpoint of ensuring dispersibility and suppressing aggregation and reducing grain boundaries during bonding, the average particle size of the second type particles is preferably 0.07 μm or more, more preferably 0.1 μm or more, and 0.2 μm. The above is more preferable.

It is preferable that the second type particles have fusibility in a temperature range of 400° C. or less. When the second type particles do not have a nanostructure on the surface, the average particle diameter D2 is preferably 4 μm or less, more preferably 3 μm or less, and even more preferably 2 μm or less, in order to lower the melting point due to the size effect. When the second type particles have a nanostructure on the surface like the first type particles, the nanostructure can realize low-temperature fusion bonding, so the average particle size D2 of the second type particles is 5 μm or less. and the range of D1/D2 is the above range.

The shape of the second type particles is not particularly limited, and may be spherical, massive, needle-like, flake-like, or the like. Above all, the shape of the second type particles is preferably spherical or flaky because the volume of voids between particles can be reduced when the particles are fused together. As described above, the second type particles may have a nanostructure formed on their surface.

The shape of the second type particles may be the same as or different from the shape of the first type particles. For example, both the first particles and the second particles may be spherical, both the first particles and the second particles may be flaky, the first particles may be flaky, and the second particles may be flaky. The particles may be spherical, or the particles of the first kind may be spherical and the particles of the second kind may be flaky.

As the second type particles, commercially available copper powder having an average particle size of 0.05 to 5 μm may be used as it is. In addition, a commercially available copper powder having a nanostructure formed on its surface by heat oxidation can also be used.

(other metal particles)
The copper paste may contain metal particles other than the first type particles and the second type particles. Metal particles other than copper particles include copper nanoparticles, particles of nickel, silver, gold, palladium, platinum, and the like. The average particle size of metal particles other than copper particles is preferably about 0.01 to 50 μm. The amount of the metal particles other than the copper particles is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less with respect to 100 parts by mass of the total amount of the metal particles. In other words, the content of the copper particles (including copper particles having an oxide nanostructure on their surfaces) with respect to the total amount of 100 parts by mass of the metal particles is preferably 80 parts by mass or more, more preferably 90 parts by mass or more, and 95 parts by mass. Part or more is more preferable. When the amount of the copper particles is within the above range, it becomes easy to ensure the bonding strength.

(Contents of first-class particles and second-class particles)
As described above, the second type particles have the effect of filling the gaps between the fused first type particles. The contents of the first type particles and the second type particles in the metal particles may be set according to the particle size ratio D1/D2 of the two so that the second type particles have the above action.

The content of the first particles is preferably 20 to 95 parts by mass, more preferably 30 to 90 parts by mass, even more preferably 35 to 85 parts by mass, with respect to 100 parts by mass of the total amount of metal particles, and 40 to 80 parts by mass. is particularly preferred. If the content of the first-class particles is within the above range, when the copper paste for pressureless bonding is sintered, high bonding strength and connection reliability can be achieved due to fusion between the first-class particles.

The content of the second type particles is preferably 5 to 80 parts by mass, more preferably 10 to 70 parts by mass, even more preferably 15 to 65 parts by mass, and 20 to 60 parts by mass with respect to 100 parts by mass of the total amount of metal particles. is particularly preferred. If the content of the second type particles is within the above range, the voids between the fused first type particles are easily filled with the second type particles when the copper paste for pressureless bonding is sintered. . Therefore, the porosity is reduced, and the bonding strength can be improved. In addition, the nanostructure on the surface of the first type particles is also fused with the second type particles, increasing the bonding area. Therefore, the bonding strength tends to increase compared to the case where only the first particles are present.

In order to promote fusion between the first particles and to efficiently fill the gaps between the first particles with the second particles, the amount of the second particles must be less than the amount of the first particles. 0.05 to 5 times is preferable, 0.1 to 2 times is more preferable, 0.2 to 1.5 times is more preferable, and 0.25 to 1.3 times is particularly preferable.

<Dispersion medium>
The copper paste contains a dispersion medium (solvent) for dispersing the metal particles. The dispersion medium is not particularly limited as long as it can disperse the metal particles and volatilize during sintering of the paste, and various aqueous solvents and organic solvents can be used. The boiling point of the dispersion medium is preferably about 150 to 400°C. A plurality of solvents having different boiling points may be mixed and used as a dispersion medium.

Specific examples of the dispersion medium include chain hydrocarbons, aromatic hydrocarbons, alicyclic hydrocarbons, chain alcohols, aromatic alcohols, alicyclic alcohols, polyhydric alcohols such as glycols and triols, ethers, and glycol ethers. , amines, amides, aldehydes, ketones and the like.

Among these, glycol or glycol ether is preferably used as the dispersion medium because of its excellent dispersibility of the copper particles. Glycols include alkylene glycols such as ethylene glycol and propylene glycol, and polyalkylene glycols (mainly having a molecular weight of 1000 or less) such as polyethylene glycol and polypropylene glycol. Glycol ethers include polyalkylene glycol alkyl ethers such as diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monoethyl ether, tripropylene glycol monobutyl ether, and ester derivatives thereof ( For example, diethylene glycol monobutyl ether acetate) can be mentioned.

The amount of the dispersion medium is about 5 to 100 parts by mass, preferably about 7 to 70 parts by mass, per 100 parts by mass of the metal particles. If the content of the dispersion medium is within the above range, the metal particles can be appropriately dispersed, and the viscosity of the copper paste can be adjusted within an appropriate range.

<Additive>
The copper paste may contain various additives as necessary. Examples of additives include antioxidants, surfactants, antifoaming agents, ion trapping agents, and the like.

As will be described later, the copper paste of the present invention promotes fusion of copper particles by heating in a reducing atmosphere. The copper paste may contain a reducing agent for the purpose of promoting fusion of copper. Reducing agents include sulfide, thiosulfate, oxalic acid, formic acid, ascorbic acid, aldehyde, hydrazine and its derivatives, hydroxylamine and its derivatives, dithiothreitol, phosphite, hydrophosphite, phosphorous acid and its derivatives. , lithium aluminum hydride, diisobutylaluminum hydride, sodium borohydride and the like.

The copper paste contains resin components such as polyester resins, polyurethane resins such as blocked isocyanate, epoxy resins, acrylic resins, polyacrylamide resins, polyether resins, melamine resins, and terpene resins. may be These resin components can act as binders for the metal particles. In addition, since the copper paste of the present invention can fill the gaps between the first type particles with the second type particles having a smaller particle size than the first type particles, even if it does not contain a resin component, High bondability can be realized. In particular, when high conductivity is required for the joint, the copper paste preferably does not substantially contain a resin component. The content of the resin in the copper paste is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, still more preferably 3 parts by mass or less, and particularly preferably 1 part by mass or less, relative to 100 parts by mass of the metal particles.

<Preparation of copper paste>
A copper paste can be prepared by mixing the above metal particles and optional additives with a dispersion medium. The metal particles may be dispersed all at once in the dispersion medium, or the remainder may be added after part of the metal particles are dispersed. Alternatively, the first type particles may be added after the second type particles are dispersed, or the dispersion liquid of the first type particles and the dispersion liquid of the second type particles may be mixed.

After mixing each component, a stirring treatment may be performed. Moreover, before and after mixing each component, aggregates may be removed by a classification operation.

For agitation, Ishikawa stirrer, Silverson stirrer, cavitation stirrer, planetary rotation stirrer, ultra-thin film high-speed rotary disperser, ultrasonic disperser, Raikai machine, twin-screw kneader, bead mill, ball mill, A stirring/kneading device such as a three-roll mill, homogenizer, planetary mixer, super-high-pressure disperser, thin-layer shear disperser, wet ultra-atomizer, supersonic jet mill, etc. may be used.

The classification operation can be performed using filtration, natural sedimentation, and centrifugation. Filtration filters include water combs, metal meshes, metal filters, and nylon meshes.

[Joining using copper paste]
The above copper paste can be used as a conductive paste for forming various wirings and conductive films, and as a bonding material for bonding between a plurality of members. In particular, the above copper paste can realize high bondability by sintering in a reducing atmosphere, and is suitably used as a paste for pressureless bonding.

In pressureless bonding, a laminate is prepared in which copper paste is arranged between the first member and the second member, and the copper paste and the second member are arranged in the direction in which the weight of the first member acts, or The laminate is heated in a reducing atmosphere while applying a pressure of 0.01 MPa or less. When the copper paste is sintered by heating in a reducing atmosphere, fusion bonding between metal particles progresses and the first member and the second member are joined.

(Preparation of laminate)
FIG. 2 is a cross-sectional view showing a configuration example of a laminate 10 in which a copper paste 5 is arranged between the first member 1 and the second member 2. As shown in FIG. Such a laminate can be prepared, for example, by providing the above copper paste 5 in a predetermined region of the second member 2 and placing the first member 1 thereon.

The first member 1 and the second member 2 are not particularly limited, and various metal materials, semiconductor materials, ceramic materials, or resin materials can be used. Specific examples of the second member include semiconductor substrates such as silicon substrates; metal substrates such as copper substrates; lead frames; metal plate-attached ceramic substrates (e.g., DBC); Power supply members such as blocks and terminals, radiator plates, water cooling plates, and the like can be used. Specific examples of the first member include diodes, rectifiers, thyristors, MOS gate drivers, power switches, power MOSFETs, IGBTs, Schottky diodes, power modules composed of fast recovery diodes, transmitters, amplifiers, sensors, and analog integrated circuits. , semiconductor lasers, and LED modules. The first member and the second member are not limited to the above. Moreover, what was mentioned above as an example of the first member may be the second member, and what was mentioned above as an example of the second member may be the first member.

The first member 1 and the second member 2 may contain metal on the surfaces in contact with the copper paste 5 (bonding material). Metals include copper, nickel, silver, gold, palladium, platinum, lead, tin, cobalt, manganese, aluminum, beryllium, titanium, chromium, iron, molybdenum and alloys thereof.

Methods for providing the copper paste 5 as a bonding material on the second member 2 include screen printing, transfer printing, offset printing, letterpress printing, intaglio printing, gravure printing, stencil printing, soft lithography, jet printing, dispenser, and comma coating. , slit coating, die coating, gravure coating, bar coating, play coating, spin coating, and electrodeposition coating.

The thickness of the copper paste 5 (the thickness after drying the dispersion medium, that is, the thickness of the bonding layer) is, for example, about 1 to 1000 μm. The thickness of the bonding layer can be 10 μm or more, 30 μm or more, 50 μm or more, 70 μm or more, or 100 μm or more. The coating thickness can be 700 μm or less, 500 μm or less, 400 μm or less, 300 μm or less, or 200 μm or less.

The copper paste for non-pressure bonding provided on the second member may be dried as appropriate for the purpose of suppressing flow during sintering and generation of voids. The gas atmosphere during drying may be the air, an oxygen-free atmosphere, or a reducing atmosphere. Drying may be performed at normal temperature and normal pressure, or may be accelerated by heating or reducing pressure.

The placement of the first member 1 on the paste 5 provided on the second member 2 may be performed using a chip mounter, a flip chip bonder, or the like, or manually using various jigs.

(sintering)
The copper paste is sintered by heating the laminate. By heating in a reducing atmosphere, the nanostructure of copper (sub)oxide formed on the surface of the first particles is reduced to form a nanostructure of copper. of fusion progresses. When the second type particles have an oxide film or a nanostructure of cuprous oxide on the surface, the surfaces of the second type particles are also reduced in a reducing atmosphere, promoting fusion.

The reducing atmosphere includes an atmosphere in which a reducing gas such as hydrogen or formic acid is present. The reducing atmosphere gas may be a mixed gas of a reducing gas such as hydrogen or formic acid and an inert gas such as nitrogen or rare gas. When the paste contains a reducing agent, heating may be performed in an oxidation-inhibiting atmosphere instead of using a reducing gas. In this case, the reducing agent is volatilized by heating to create a reducing atmosphere. The oxidation-suppressing atmosphere includes an atmosphere of an inert gas such as nitrogen or a rare gas, or under vacuum.

The maximum temperature reached during heating is preferably 200 to 500° C., more preferably 230 to 450, from the viewpoint of promoting volatilization of the dispersion medium and fusion of the metal particles while suppressing thermal damage to the first member and the second member. °C is more preferred, and 250 to 400°C is even more preferred.

The holding time in the above temperature range is preferably 1 minute or longer, more preferably 5 minutes or longer, from the viewpoint of sufficiently advancing volatilization of the dispersion medium and fusion of the metal particles. Although the upper limit of the heating holding time is not particularly limited, it is preferably 60 minutes or less from the viewpoint of yield, process efficiency, and the like.

The die shear strength of the joined body in which the first member and the second member are joined via a copper paste sintered body (joining material) is preferably 20 MPa or more, more preferably 23 MPa or more, and even more preferably 25 MPa or more. By using the above copper paste, high shear strength can be realized by non-pressure bonding at a low temperature below the melting point of copper.

As presumed factors for realizing such high bonding strength, low-temperature fusion bonding is possible because the first particles, which have a relatively large particle size, have a nanostructure on the surface, and the second particles, which have a relatively small particle size, are possible. The particles of the second kind form a microstructure in which the particles of the first kind enter the gaps between the particles of the first kind, and sintering between the particles proceeds, so that a dense bonding layer with few gaps is formed. . The cross section of the bonding layer after sintering the copper paste preferably has a porosity of 25% or less, more preferably 20% or less, and even more preferably 15% or less. The porosity can be calculated from the SEM observation image of the joint cross section (see FIG. 3).

In addition to the small number of voids, the use of microparticles reduces volume shrinkage during sintering and suppresses strain in the bonding layer, which is thought to contribute to the improvement in bonding strength. In addition, since the second type particles can be fused at a low temperature and have a particle size that is difficult to cause aggregation, the nanostructure on the surface of the first type particles and the second type particles are fused and It is considered that the fusion between the particles of the second kind progresses and the bonding strength further increases. Furthermore, the ratio D1/D2 of the average particle size D1 of the first type particles and the average particle size D2 of the second type particles is within a predetermined range, so that the proportion of grain boundaries in the bonding layer is small. is thought to have contributed to the increase in

The joining method of the present invention can be applied to manufacture of various electronic components and semiconductor devices. That is, a bonded body obtained by bonding a plurality of parts by sintering the copper paste of the present invention can be an electronic part, a semiconductor device, or the like. The bonded body of the present invention has a high die shear strength at the bonded portion and is excellent in connection reliability. In addition, since the bonding material is mainly made of copper and the voids between the first particles are filled with the second particles, the porosity is low, so high thermal conductivity and high electrical conductivity can be achieved.

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples.

[Preparation of copper particles]
Commercially available copper powders A to I below were prepared.
A: “Tn-Cu100” manufactured by Taiyo Nippon Sanso (spherical copper powder with an average particle size of 0.1 μm)
B: "1050YP" manufactured by Mitsui Kinzoku Co., Ltd. (flake-shaped copper powder with an average particle size of 0.9 μm)
C: "MAC03K" manufactured by Mitsui Kinzoku Mining (spherical copper powder with an average particle size of 3.0 μm)
D: "MAC03KP" manufactured by Mitsui Kinzoku Co., Ltd. (flake-shaped copper powder with an average particle size of 4.0 μm)
E: "1400YM" manufactured by Mitsui Mining & Smelting (spherical copper powder with an average particle size of 4.2 μm)
F: "1400YP" manufactured by Mitsui Mining & Smelting (flake-shaped copper powder with an average particle size of 5.2 μm)
G: "MA-CF" manufactured by Mitsui Kinzoku Mining (flake-shaped copper powder with an average particle size of 21.1 μm)
H: "Cu6500" made by CuLox (spherical copper powder with an average particle size of 50 μm)
I: "MACNS" manufactured by Mitsui Kinzoku Co., Ltd. (spherical copper powder with an average particle size of 64 μm)

[Preparation of copper particles having a nanofiber structure on the surface]
While stirring the above particles E in the atmosphere, 10 minutes at 100 ° C., 10 minutes at 150 ° C., 10 minutes at 200 ° C., 10 minutes at 250 ° C., 10 minutes at 300 ° C., and 350 ° C. for 10 minutes. minutes, and heated at 400° C. for 30 minutes. When the particles R after heating were observed with an SEM, almost no agglomerates were observed, and as shown in FIG. 1(C2), a fibrous nanostructure was formed on the surface.

Particles P, Q, S, T, and U having fibrous nanostructures on their surfaces were obtained by heating the particles C, D, F, G, H, and I under the same conditions as in the preparation of the particles R. , V were produced.

[Example 1]
50 parts by mass of particles R as the first particles, 50 parts by mass of particles B as the second particles, and 30 parts by mass of tripropylene glycol monomethyl ether (MFTG; boiling point 242.4° C.) as a dispersion medium were mixed. Under reduced pressure, using a stirrer ("Mazerustar KK-V300" manufactured by Kurabo Industries), the mixture was planetarily stirred for 2 minutes at a revolution speed of 1340 rpm and a rotation speed of 737 rpm to obtain a copper paste for pressureless bonding.

[Examples 2 to 9 and Comparative Examples 1 to 6]
The blending amounts of the metal particles and the solvent were changed as shown in Table 1 (the numerical values of the blending in Table 1 are parts by mass). A copper paste was obtained in the same manner as in Example 1 except for the above.

[evaluation]
(Preparation of sample for die shear strength test)
0.009 g of copper paste is applied to the center of a 20 mm × 20 mm copper plate (thickness 1 mm), and a Cu chip having a thickness of 1 mm and a size of 5 × 5 mm is brought into contact therewith, and then the Cu chip is applied with a load of 10 g. A laminate was formed by pressing lightly.

This laminate was placed in a furnace of a reduction bonding apparatus (“RB-100” manufactured by Ayumi Industry), heated from room temperature to 130° C. in 4 minutes, and then held at 130° C. for 5 minutes for pre-drying. . After that, the temperature was raised from 130° C. to 300° C. in 10 minutes. The samples of Examples 1 to 9, Comparative Examples 1 to 3, and Comparative Example 6 were heated from room temperature to 300° C. in an air atmosphere. The samples of Comparative Examples 4 and 5 were heated from room temperature to 300° C. in a nitrogen atmosphere. After the temperature was raised to 300° C., formic acid vapor was introduced into the furnace to create a formic acid atmosphere, and heating was performed at 300° C. for 30 minutes. After the inside of the furnace was replaced with nitrogen gas and cooled to 35° C. or lower, the sample was taken out.

(Measurement of die shear strength)
Using a universal bond tester (Nordson Advanced Technologies 4000 series) equipped with a DS-100 load cell, the die shear strength of the above sample was measured under the conditions of a measurement speed of 1 mm/min and a measurement height of 200 μm. .

Table 1 shows the compositions of the copper pastes of Examples and Comparative Examples, the average particle size ratio D1/D2 of the metal particles in the copper pastes, and the die shear strength of the bonded samples. In addition, in Table 1, the content of the first particles having a relatively large particle size is underlined. For the samples of Example 1 and Comparative Example 3, cross-sectional SEM observation was performed, and the porosity was calculated from the SEM photographs of FIGS. 3(A) and 3(B).

In Comparative Examples 1 and 2, in which only copper particles without nanostructures were used, the die shear strength of the bonded samples was less than 20 MPa. In Comparative Example 3 using only micro copper particles having a nanostructure on the surface, the shear strength was even lower than in Comparative Examples 1 and 2. The porosity of the joint cross section of Comparative Example 3 obtained from FIG. 3(B) was 30.4%.

In Examples 1 to 9 containing copper particles (second type particles) having a smaller particle size than the first type particles in addition to microparticles (first type particles) having a nanostructure on the surface as metal particles, any Also, the die shear strength increased to 20 MPa or more. The porosity of the joint cross section of Example 1 obtained from FIG. It was confirmed that the porosity was reduced in From these results, by using micro copper particles with a nanostructure formed on the surface by thermal oxidation and copper particles with a relatively small particle size, the voids between the micro copper particles are relatively small particles It can be seen that the bonding between the metal particles is strengthened and the bonding strength is increased as the metal particles are filled.

Examples 8 and 9 using micro copper particles having a nanostructure on the surface as the second-type particles having a relatively small particle size also exhibited high bonding strength as in the other examples. On the other hand, in Comparative Examples 4 and 5, as in Comparative Examples 1 and 2, two types of copper particles having different particle sizes were used, but the die shear strength was even lower than in Comparative Examples 1 to 3. rice field. From these results, in Examples, the nanostructure formed on the surface of the copper particles has the effect of promoting the fusion between the microparticles, so that the bonding strength increased, whereas the nanostructure It is considered that in Comparative Examples 4 and 5 using microparticles that were not bonded, the bonding strength was lowered because the microparticles did not fuse with each other.

In Comparative Example 6, in which particles V having a nanostructure on the surface and an average particle size of 64 μm and particles A having an average particle size of 0.1 μm were used, microparticles having a nanostructure on the surface and particles having a small particle size Although copper particles were used in combination, the bonding strength was insufficient. In Comparative Example 6, it is considered that the bonding strength decreased because the particle diameters D1/D2 of the two types of particles were large and the grain boundary ratio was high. From this result, it can be seen that high bondability can be achieved by setting the ratio D1/D2 of the average particle sizes of the first type particles to the second type particles within a predetermined range.

Claims (5)

A copper paste used for non-pressure bonding between a plurality of members,
Metal particles containing first type particles and second type particles having an average particle size smaller than the first type particles, and a dispersion medium ,
The first particles are copper particles having an average particle diameter of 1 to 100 μm and having a fibrous nanostructure formed of copper oxide on the surface,
The second type particles are copper particles having an average particle size of 0.05 to 5 μm,
The average particle size of the first type particles is 2 to 550 times the average particle size of the second type particles,
The content of the copper particles with respect to 100 parts by mass of the total amount of the metal particles is 80 parts by mass or more,
A copper paste for pressureless bonding , wherein the content of the first type particles is 20 to 95 parts by mass and the content of the second type particles is 5 to 80 parts by mass with respect to the total amount of 100 parts by mass of the metal particles. . 2. The copper paste for pressureless bonding according to claim 1, wherein the average particle size of the first type particles is 3 to 50 μm, and the average particle size of the second type particles is 0.1 to 5 μm. 3. The copper paste for pressureless bonding according to claim 1, which contains 5 to 100 parts by mass of the dispersion medium with respect to 100 parts by mass of the metal particles. Prepare a laminate provided with the copper paste according to any one of claims 1 to 3 between the first member and the second member,
A pressureless bonding method, wherein the laminate is heated in a reducing atmosphere to sinter the copper paste. Prepare a laminate provided with the copper paste according to any one of claims 1 to 3 between the first member and the second member,
A method for manufacturing a joined body, wherein the laminated body is heated in a reducing atmosphere to sinter the copper paste, thereby joining the first member and the second member without pressure .

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