Bonding sheet and bonding structure

Abstract

A bonding sheet (1) according to the present invention comprises a copper foil (2) and bonding films (3) that are sinterable and are formed on both surfaces of the copper foil (2). The bonding films (3) contain copper particles and a solid reducing agent. This bonding sheet (1) is used for the purpose of bonding an object (5) to be bonded, said object having at least one metal that is selected from among gold, silver, copper and nickel in the surface. The present invention also provides a bonded structure (10) which is obtained by electrically connecting the object (5) to be bonded, said object having at least one metal that is selected from among gold, silver, copper and nickel in the surface, and the copper foil (2) to each other, with a bonding layer (30) being interposed therebetween, said bonding layer being composed of a sintered structure of the copper particles.

Description

Technical Field

[0001] The present invention relates to a bonding sheet and a bonding structure.

Background Art

[0002] With the recent global trend towards energy conservation, semiconductor devices called power devices are increasingly being used as power conversion and control devices such as inverters. In order to achieve high efficiency and space savings of semiconductor elements, an electronic component called a power module is used, in which a plurality of semiconductor elements are arranged on a metal-ceramics substrate, and a metal layer and metal wires are arranged on the upper surface of the semiconductor elements.

[0003] For the purpose of improving the conduction reliability of a power module, Patent Document 1 discloses a semiconductor device including a first conductor plate, a plurality of semiconductor elements arranged on the first conductor plate, and a first external connection terminal connected to the first conductor plate. This semiconductor device is also disclosed in the same document as having a plurality of semiconductor elements soldered respectively.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] However, since the semiconductor device described in Patent Document 1 uses solder with low thermal conductivity as a bonding material, heat generated during power module driving cannot be sufficiently dissipated, which may lead to failure of the module. In addition to this, there is a risk that the solder melts due to heat during driving and the conduction reliability is impaired.

[0006] Therefore, an object of the present invention is to provide a bonding sheet and a bonding structure excellent in thermal conductivity and conduction reliability.

[0007] The present invention provides a bonding sheet having a copper foil and sinterable bonding films formed on both surfaces of the copper foil, each of the bonding films containing copper particles and a solid reducing agent, and at least one of the bonding films is used for bonding to a bonding object having at least one metal selected from gold, silver, copper, and nickel on its surface.

[0008] Furthermore, the present invention provides a bonding structure in which a first bonding object having at least one metal selected from gold, silver, copper, and nickel on its surface and a second bonding object having at least one metal selected from gold, silver, copper, and nickel on its surface are electrically connected via a bonding layer composed of a sintered structure of copper particles, wherein the following structure (3) is formed in the bonding layer.

Chemical formula

Brief description of the drawings

[0009]

Figure 1

Figure 2

Figure 3

Embodiments for carrying out the invention

[0010] Hereinafter, the bonding sheet and the bonding structure of the present invention will be described based on their preferred embodiments. As shown in FIG. 1, the bonding sheet 1 has a copper foil 2 and bonding films 3, 3 formed on both sides thereof. Each bonding film 3 contains copper particles and a solid reducing agent.

[0011] The bonding sheet 1 is suitably used for bonding to a bonding object 5 (hereinafter, this is also simply referred to as a "bonding object") having at least one kind of metal on its surface selected from gold, silver, copper, and nickel. Further, since the bonding films 3 of the bonding sheet of the present invention are formed on both sides of the copper foil 2, the bonding sheet 1 can be disposed between two bonding objects 5, 5 and used such that the bonding objects 5 are bonded to both sides of the bonding film 3. Examples of the form of the metal present on the surface of the bonding object 5 include a metal layer formed in a layer on the surface of the bonding object 5.

[0012] Each of the bonding films 3, 3 disposed on the bonding sheet 1 has a sinterable structure. "Sinterable" means that the copper particles are present in the bonding film 3 in a state where they are not fused to each other, and by sintering the copper particles in the bonding film 3, a sintered body having a sintered structure in which the copper particles are fused to each other can be formed. By preferably firing the bonding film 3 and the bonding object 5 in the bonding sheet 1 under pressure, the copper foil 2 and the bonding object 5 in the bonding sheet 1 can be bonded to each other.

[0013] After firing, the bonding film 3 becomes a conductive bonding layer 30 having a sintered structure of copper particles, as shown in FIGS. 2(a) and (b). The bonding layer 30 bonds and electrically connects the copper foil 2 and the two bonding objects 5, 5. The bonding sheet 1 can be obtained, for example, by the manufacturing method described later.

[0014] The copper foil 2 constituting the bonding sheet 1 is a copper foil composed of copper and inevitable impurities as the remainder, or a copper foil composed of a copper alloy containing copper and a metal other than copper.

[0015] From the viewpoint of achieving both the strength of the copper foil itself and the conductivity of the bonding structure, the copper foil 2 preferably contains copper in an amount of 97% by mass or more, more preferably 99% by mass or more, and still more preferably consists of copper and inevitable impurities as the remainder. From the same viewpoint, the thickness of the copper foil is preferably 0.5 μm or more and 1000 μm or less, and more preferably 1 μm or more and 500 μm or less. The thickness of the copper foil can be measured, for example, by observing a cross-section in the thickness direction of the bonding sheet using a scanning electron microscope.

[0016] As shown in FIG. 1, the bonding film 3 in the bonding sheet 1 may be formed over the entire area of both surfaces of the copper foil 2, may be discontinuously formed on one surface of the copper foil 2 and formed over the entire area of the other surface of the copper foil 2, or may be discontinuously formed on both surfaces of the copper foil 2. When the bonding film 3 is discontinuously formed on the surface of the copper foil 2, the portion where the bonding film 3 is not formed is the portion where the copper foil 2 is exposed.

[0017] As shown in FIG. 1, in the bonding sheet 1, it is preferable that no solid layer intervenes between the copper foil 2 and the bonding film 3. Also, as shown in the same figure, in the bonding sheet 1, it is preferable that no other solid layers such as additional copper foils and additional bonding films exist on the first surface 1A and the second surface 1B of the bonding sheet 1. Note that as long as the effects of the present invention are achieved, the presence of other solid layers on at least one of the surfaces of the bonding sheet 1 is not precluded.

[0018] The copper particles contained in the bonding film 3 can be, for example, (i) copper particles consisting of copper and inevitable impurities as the remainder only, (ii) copper particles consisting of a copper alloy containing copper and other metals other than copper, or (iii) a mixture of (i) and (ii). In the present invention, these modes are collectively referred to simply as "copper particles". Also, depending on the context, "copper particles" refers to the copper particles themselves or copper powder which is an aggregate of copper particles.

[0019] From the perspective of achieving both improved thermal conductivity and improved electrical conductivity, the copper particles preferably contain 50% by mass or more of copper, more preferably 70% by mass or more of copper, and still more preferably 90% by mass or more of copper, with the balance consisting of inevitable impurities. Also, from the same perspective, as the form of inclusion of the copper particles, (i) it is more preferable that the copper particles are an aggregate of copper particles in which the copper and the balance consist only of inevitable impurities. When the aggregate of copper particles contains a plurality of types of particles, the copper content is calculated based on all the copper particles.

[0020] The solid reducing agent contained in the bonding film 3 is solid at 1 atmosphere and room temperature (25°C), and is used to promote the sintering of the copper particles by firing the bonding film 3. For this purpose, it is advantageous that the solid reducing agent has a chemical structure having at least one amino group and a plurality of hydroxyl groups. By using a reducing agent having such a structure, it is possible to suppress the oxidation of the copper particles during sintering as compared with a reducing agent having a plurality of hydroxyl groups and not containing an amino group, so that a dense sintered structure resulting from the promotion of sintering between the copper particles can be obtained. As a result, a bonding structure having high thermal conductivity and electrical reliability can be obtained. "Solid at room temperature (25°C)" means that the melting point of the solid reducing agent is above 25°C.

[0021] The melting point of the solid reducing agent is preferably 300°C or lower. Also, it is preferable that the boiling point of the solid reducing agent is higher than the boiling point of the liquid medium described later. By using a solid reducing agent having such physical properties, when firing the bonding film 3, the reducing agent melts and spreads uniformly in the bonding film 3, so that the sintering of the copper particles is promoted uniformly, and a denser sintered structure is obtained. As a result, a bonding structure that achieves both improved thermal conductivity and electrical reliability and has high heat resistance can be obtained. In addition to this, since the solid reducing agent remains as a solid in the bonding film 3 before firing, the shape retention of the bonding film 3 can be enhanced. As a result, even when the bonding film 3 is pressurized during firing of the bonding film 3, it is difficult for the bonding film 3 to protrude from between the object to be bonded 5 and the copper foil 2, and thickness control becomes even easier, so that a bonding structure having high bonding strength can be obtained.

[0022] The bonding sheet 1 having the above-described configuration contains a solid reducing agent in the bonding film 3, so that when sintering with the object to be bonded 5, the copper particles are sufficiently sintered to form a dense sintered structure. As a result, a sintered structure in which both the thermal conductivity and the electrical conduction reliability are improved can be obtained between the copper foil 2 and the object to be bonded 5. In addition to this, a sintered structure in which a high bonding strength is exhibited between the copper foil 2 and the object to be bonded 5 can be obtained. Further, since both the copper foil 2 and the bonding film 3 contain copper, when the bonding sheet 1 is used in the manufacturing process of the power module, it can be bonded without separately performing a surface treatment process such as metallization for increasing the bonding strength. Therefore, a power module with high thermal conductivity, high electrical conduction reliability, and high bonding strength can be obtained.

[0023] From the viewpoint of achieving both high thermal conductivity and high electrical conduction reliability in the sintered structure, the bonding film 3 preferably contains an amino alcohol compound as a solid reducing agent, and more preferably contains an amino alcohol compound represented by the following chemical formula (1) or (2). The "amino alcohol compound" refers to an organic compound having at least one amine of primary to tertiary amines and at least one alcohol of primary to tertiary alcohols in one chemical structure.

[0024]

Chemical formula

[0025] In Chemical formula (1) or (2), R 1 or R 6 each independently represents a hydrogen atom, a hydroxyl group, a hydrocarbon group having 1 to 10 carbon atoms, or a hydrocarbon group having 1 to 10 carbon atoms and having a hydroxyl group. Examples of the hydrocarbon group include saturated or unsaturated aliphatic groups. This aliphatic group may be linear or branched. Examples of the hydrocarbon group in R 1 or R 6 include a methyl group, an ethyl group, a propyl group, and the like.

[0026] In formula (2), R 7 represents a hydrocarbon group having 1 to 10 carbon atoms or a hydrocarbon group having 1 to 10 carbon atoms and a hydroxyl group. Examples of the hydrocarbon group include saturated or unsaturated aliphatic groups. This aliphatic group may be linear or branched. R 7 Examples of the hydrocarbon group in R

[0027] In chemical formula (1), from the viewpoint of enhancing the sinterability of copper particles, R 1 to R 5 at least two of them contain a hydroxyl group. That is, at least two of R 1 to R 5 are a hydroxyl group or a hydrocarbon group having 1 to 10 carbon atoms and a hydroxyl group. Also in formula (2), at least two of R 1 to R 6 contain a hydroxyl group. That is, at least two of R 1 to R 6 are a hydroxyl group or a hydrocarbon group having 1 to 10 carbon atoms and a hydroxyl group. Particularly in chemical formula (1), it is preferable that at least two of R 1 to R 5 are hydrocarbon groups having 1 to 4 carbon atoms and a hydroxyl group. Also in chemical formula (2), it is preferable that at least two of R 1 to R 6 are hydrocarbon groups having 1 to 4 carbon atoms and a hydroxyl group. In this case, the hydroxyl group in the hydroxyalkyl group is preferably bonded to the end of the alkyl group.

[0028] From the viewpoint of enhancing the sinterability of copper particles, the reducing agent represented by chemical formula (1) preferably has three or more of R 1 to R 5 containing a hydroxyl group, more preferably four or more containing a hydroxyl group, and R 1 to R 5It is more preferable that all of them contain a hydroxyl group. From the same viewpoint, in the reducing agent represented by Chemical Formula (2), it is preferable that three or more of R 1 to R 6 contain a hydroxyl group, and it is more preferable that four or more contain a hydroxyl group.

[0029] Specific examples of the amino alcohol compound represented by Chemical Formula (1) or (2) include bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane (BIS-TRIS, melting point: 104°C, boiling point: over 300°C, corresponding to Chemical Formula (1)), 2-amino-2-(hydroxymethyl)-1,3-propanediol (TRIS, melting point: 169 to 173°C, boiling point: over 300°C, corresponding to Chemical Formula (1)), 1,3-bis(tris(hydroxymethyl)methylamino)propane (BIS-TRIS propane, melting point: 164 to 165°C, boiling point: over 300°C, corresponding to Chemical Formula (2)), and the like. Among these, from the viewpoint of enhancing the sinterability between copper particles and obtaining a bonding structure having a dense bonding layer, it is preferable to use bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane (BIS-TRIS) as the solid reducing agent.

[0030] The above-described solid reducing agent can be used alone or in combination of two or more. In any case, from the viewpoint of enhancing the sinterability of copper particles, the proportion of the solid reducing agent in the bonding film is preferably 0.1 part by mass or more, more preferably 1 part by mass or more, based on 100 parts by mass of copper particles. Also, from the viewpoint of exhibiting suitable coating performance on the copper foil while maintaining the proportion of copper particles in the bonding composition, it is practical to set it to 10 parts by mass or less, preferably 8 parts by mass or less, and more preferably 5 parts by mass or less.

[0031] The bonding film 3 may further contain a liquid medium having a boiling point of less than 300°C at 1 atm. The liquid medium is used to improve the formability of the bonding film 3 when forming the bonding film. From such a viewpoint, it is preferable that the above-described liquid medium is liquid at 1 atm and room temperature (25°C).

[0032] When a liquid medium is contained in the bonding film, from the viewpoint of suppressing the oxidation of copper particles, it is also preferable that the liquid medium is a non-aqueous medium.

[0033] From the viewpoint of having both the formability of the bonding film and the appropriate volatility of the liquid medium, the liquid medium is preferably a monohydric or polyhydric alcohol, and more preferably a polyhydric alcohol. Examples of the polyhydric alcohol include propylene glycol (boiling point: 188 °C), ethylene glycol (boiling point: 197 °C), hexylene glycol (boiling point: 197 °C), diethylene glycol (boiling point: 245 °C), 1,3-butanediol (boiling point: 207 °C), 1,4-butanediol (boiling point: 228 °C), dipropylene glycol (boiling point: 231 °C), tripropylene glycol (boiling point: 273 °C), glycerin (boiling point: 290 °C), polyethylene glycol 200 (boiling point: 250 °C), polyethylene glycol 300 (boiling point: 250 °C), etc. The liquid medium can be used alone or in combination of two or more. Among these, from the viewpoint of enhancing the shape retention of the bonding film 3 and enhancing the dispersibility of the components in the bonding film 3 to form a uniform and dense sintered structure, it is preferable that the liquid medium contains one or more of hexylene glycol and polyethylene glycols such as polyethylene glycol 200 and polyethylene glycol 300.

[0034] When the bonding film contains a liquid medium, the content of the liquid medium in the bonding film is preferably 9 parts by mass or less, and more preferably 7 parts by mass or less with respect to 100 parts by mass of the copper particles from the viewpoint of enhancing the shape retention of the bonding film. The content ratio of the liquid medium in the bonding film can be calculated, for example, by measuring the mass change of the coating film before and after drying because when manufactured according to the manufacturing method described later, the content of each constituent material other than the liquid medium in the coating film and the bonding film 3 which is the dried coating film is substantially the same.

[0035] The shape of the copper particles contained in the bonding film 3 is, for example, spherical, flat (flake-shaped), dendritic (branched), rod-shaped, etc., and these can be used alone or in combination of multiple. The shape of the copper particles depends on the manufacturing method. For example, when using the wet reduction method or the atomization method as the manufacturing method of copper particles, spherical particles are easily obtained. When using the electrolytic reduction method, dendritic or rod-shaped particles are easily obtained. Flat particles can be obtained, for example, by applying mechanical external force to spherical particles to cause plastic deformation.

[0036] The copper particles preferably have a spherical shape. In this case, the particle size of the spherical copper particles can be measured by image analysis of a scanning electron microscope image. Specifically, after reading the image data of the copper particles obtained by a scanning electron microscope using Mac-View manufactured by Mountech Co., Ltd., more than 50 copper particles on the image data are randomly selected, and the particle size (Heywood diameter) of the particles, the area S of the two-dimensional projection image of the particles, and the perimeter L of the two-dimensional projection image of the particles are measured respectively. Then, from the obtained Heywood diameter, the volume when the particles are assumed to be a perfect sphere is calculated, and the volume cumulative particle size D at 50% by volume of the cumulative volume of the volume is SEM50 set as.

[0037] Also, whether the copper particles are spherical or not is determined by the circularity coefficient 4πS / L from the area S and the perimeter L of each particle randomly selected by the above method 2 is calculated, and further the arithmetic mean value thereof is calculated. When the arithmetic mean value of the circularity coefficient is 0.85 or more, particularly 0.90 or more, the copper particles are defined as spherical.

[0038] The particle size of the spherical copper particles is the volume cumulative particle size D at 50% by volume of the cumulative volume described above SEM50It is preferably represented by [a certain value] and is 30 nm or more and 200 nm or less, more preferably 40 nm or more and 180 nm or less. With such a configuration, a dense sintered structure can be formed when joining the copper foil and the object to be joined. As a result, the joined structure using the joining sheet can exhibit high thermal conductivity, heat resistance, and high joining strength. In addition, since a dense sintered structure can be formed even under relatively low sintering conditions, an excessive heat load on the object to be joined during manufacturing can be suppressed, and a joined structure capable of exhibiting desired performance can be obtained.

[0039] It is also preferable that the copper particles include those having a flat shape. At this time, the flat copper particles have a volume cumulative particle size D at 50% by volume in the cumulative volume measured by the laser diffraction scattering method for particle size distribution 50 of 0.3 μm or more and 100 μm or less, more preferably 0.5 μm or more and 70 μm or less, and even more preferably 0.7 μm or more and 50 μm or less. By including particles having such a particle size, the joined structure using the joining sheet can exhibit both high thermal conductivity and electrical conduction reliability, and can also exhibit heat resistance and high joining strength. In addition, a dense sintered structure can be formed even under relatively low sintering conditions. The flat shape refers to a shape having a pair of plate surfaces forming the main surface of the particle and side surfaces intersecting these plate surfaces, and the plate surfaces and the side surfaces can each independently be a flat surface, a curved surface, or a concave-convex surface.

[0040] D 50 The measurement of [D] is performed as follows: For example, 0.1 g of the measurement sample and 50 mL of pure water are mixed and dispersed for 1 minute using an ultrasonic homogenizer (manufactured by Nippon Seiki Co., Ltd., US-300T). Then, as a laser diffraction scattering type particle size distribution measuring device, for example, MT3300 EXII manufactured by Microtrac Bell is used to measure the particle size distribution.

[0041] When flat copper particles are included, it is preferable that the ratio of the length of the major axis to the length of the minor axis on the plate surface of the flat copper particles (hereinafter, this is also referred to as the "aspect ratio") is 2 or more and 80 or less, and it is preferably 5 or more and 40 or less. By further including particles of such a shape, a densely sintered bonding layer can be formed, and improvement in thermal conductivity and improvement in electrical conduction reliability can be realized.

[0042] The lengths of the major axis and the minor axis in the flat copper particles are obtained as follows. That is, the particles to be measured are observed with a scanning electron microscope, and when considering a virtual circumscribed rectangle in each two-dimensional projection image while rotating 360 degrees in the direction horizontal to the plate surface of the particles, for the one in which one side of the circumscribed rectangle becomes the maximum, the long side is taken as the major axis and the short side is taken as the minor axis. Similarly, 50 or more of the particles are randomly selected and the major axis and the minor axis are measured respectively, and they are obtained from the arithmetic mean values of these.

[0043] Next, the manufacturing method of the bonding sheet 1 described above will be explained. The manufacturing method of the bonding sheet 1 includes, for example, a coating step of applying a bonding composition to the surface of the copper foil 2 to form a coating film, and a drying coating film forming step of drying the coating film to form a bonding film 3 which is a dried coating film.

[0044] As a manufacturing method of the bonding sheet 1, for example, (a) as a first coating step, after applying a bonding composition to one surface of the copper foil 2 to form a coating film, a first drying coating film forming step is performed, and then, as a second coating step, after applying a bonding composition to the other surface of the copper foil 2 to form a coating film, a second drying coating film forming step may be performed. Instead of this, (b) after applying a bonding composition to both surfaces of the copper foil 2 to form a coating film, a drying coating film forming step may be performed. The following explanation is applicable to both the aspects of (a) and (b) above.

[0045] First, a bonding composition containing copper particles and a solid reducing agent is applied to the surface of the copper foil 2 to form a coating film. There are no particular restrictions on the means of applying the bonding composition, and known coating means can be used. For example, screen printing, dispenser printing, gravure printing, offset printing, etc. can be used. From the viewpoint of efficiently performing the coating step of the bonding composition, the bonding composition is preferably in the form of a paste or ink containing a liquid medium. The bonding composition can be obtained by mixing the above-described copper particles and solid reducing agent, and, if necessary, the above-described liquid medium.

[0046] From the viewpoints of obtaining a bonding film having high shape retention and forming a bonding structure that stably exhibits thermal conductivity and electrical conduction reliability, the thickness of the coating film to be formed is preferably set to be 1 μm or more and 1000 μm or less immediately after coating, and more preferably 5 μm or more and 700 μm or less.

[0047] When the bonding composition contains a liquid medium, the content of the liquid medium in the bonding composition is preferably 40 parts by mass or less, more preferably 35 parts by mass or less, based on 100 parts by mass of the copper particles, from the viewpoint of imparting appropriate viscosity to the bonding composition and enhancing the shape retention of the coating film when the bonding composition is applied on the copper foil. Also, the content of the liquid medium in the bonding composition is preferably 10 parts by mass or less.

[0048] From the viewpoints of enhancing the coatability and shape retention of the coating film on the copper foil, the bonding composition preferably has a viscosity of 20 Pa·s or more and 200 Pa·s or less, and more preferably 25 Pa·s or more and 180 Pa·s or less, at a shear rate of 10 s -1 and at 25°C when unheated. The viscosity of the bonding composition can be measured with a rheometer (viscoelasticity measuring device) using a parallel plate type sensor.

[0049] The bonding composition may contain other components as long as the effects of the present invention can be achieved. Examples of other components include a binder component, a surface tension regulator, an antifoaming agent, a viscosity regulator, and the like. The proportion of other components is preferably such that the total amount thereof is 0.1 part by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the copper particles.

[0050] Next, the coating film formed on the copper foil surface is dried to obtain a bonding sheet 1 having a bonding film 3 and a copper foil 2. In this step, by drying the coating film, at least a part of the liquid medium is removed from the coating film, and a bonding film 3 with a reduced amount of the liquid medium in the coating film is formed on the surface of the copper foil 2. By removing the liquid medium from the coating film, the shape retention of the bonding film 3 can be further enhanced. Furthermore, when the bonding sheet 1 having the copper foil 2 and the bonding film 3 is bonded to the object to be bonded 5 by firing, the adhesion of the bonding layer 30 can be enhanced, and both the thermal conductivity and the electrical conductivity reliability can be made high. As described above, it is preferable that the proportion of the liquid medium with respect to 100 parts by mass of the copper particles in the bonding film 3 is 9 parts by mass or less. In this step, the coating film and the bonding film 3 which is the dried coating film have substantially the same content of each constituent material other than the liquid medium. Also, the copper particles in the bonding film formed in this step are not fused to each other and are sinterable.

[0051] In order to dry and remove the liquid medium, a drying method such as natural drying, hot air drying, infrared irradiation, hot plate drying, etc. that utilizes the volatility of the liquid medium can be used to volatilize the liquid medium. This step can be appropriately changed according to the composition of the bonding composition used, but it is preferably carried out at a temperature below the melting point of the copper particles. For example, it can be carried out at 60°C or higher and 150°C or lower, under atmospheric pressure, for 1 minute or more and 30 minutes or less in an air atmosphere.

[0052] The thickness of the bonding film 3 is preferably 0.5 μm or more, more preferably 3 μm or more, from the viewpoint of enhancing the adhesion to the object to be bonded 5 and obtaining a bonding structure 10 having excellent thermal conductivity and electrical reliability. Further, the thickness of the bonding film 3 is preferably 980 μm or less, more preferably 600 μm or less, from the viewpoint of realizing space saving of electronic components. The thickness of the bonding film 3 can be adjusted, for example, by appropriately adjusting the thickness of the coating film formed in the above-described process. The thickness of the bonding film 3 can be measured, for example, by observing a cross section in the thickness direction of the bonding sheet 1 using a scanning electron microscope.

[0053] Next, a method for manufacturing the bonding structure 10 will be described. As one embodiment, the bonding sheet 1 obtained through the above-described process and the object to be bonded 5 are bonded together to obtain a bonding structure 10 having the structures shown in FIGS. 2(a) and (b). The bonding structure 10 in the present embodiment includes a first object to be bonded 51 having at least one kind of metal selected from gold, silver, copper, and nickel on its surface, and a second object to be bonded 52 having at least one kind of metal selected from gold, silver, copper, and nickel on its surface, which are electrically connected through a bonding layer 30 formed of a sintered structure of copper particles.

[0054] Examples of the object to be bonded 5 in the bonding structure 10 include spacers, heat sinks, semiconductor elements made of metals such as gold, silver, or copper, and substrates having at least one kind of metal selected from gold, silver, copper, and nickel on their surfaces. As the substrate, for example, an insulating substrate having copper on the surface of a ceramic or aluminum nitride plate, such as a DBC (Direct Bonded Copper) substrate, can be used.

[0055] In the manufacturing process of the bonding structure 10, first, when the bonding sheet 1 is obtained through the above-described process, as shown in FIGS. 2(a) and (b), the bonding film 3 in the bonding sheet 1 and the object to be bonded 5 are opposed to each other to obtain a laminate 10A in which the bonding film 3 is disposed between the object to be bonded 5 and the copper foil 2 in the bonding sheet 1.

[0056] In the embodiment shown in Fig. 2(a), the laminate 10A is arranged such that the first surface 1A, which is the surface where the bonding film 3 exists in the bonding sheet 1, and the first object to be bonded 51 are in surface contact with each other. In addition to this, the laminate 10A is arranged such that the second surface 1B, which is the surface where the bonding film 3 exists in the bonding sheet 1, and the second object to be bonded 52 are in surface contact with each other. That is, in the laminate 10A shown in Fig. 2(a), the bonding sheet 1 is arranged between the first object to be bonded 51 and the second object to be bonded 52. In the state where the laminate 10A is formed, none of the bonding films 3 are sintered.

[0057] In the embodiment shown in Fig. 2(b), the laminate 10A is arranged such that the first surface 1A, which is the surface where the bonding film 3 exists in the first bonding sheet 1S, and the first object to be bonded 51 are in surface contact with each other. In addition to this, the laminate 10A is arranged such that the second surface 1B, which is the surface where the bonding film 3 exists in the first bonding sheet 1S, and the third object to be bonded 53 are in surface contact with each other. Furthermore, the laminate 10A is arranged such that the first surface 1C, which is the surface where the bonding film 3 exists in the second bonding sheet 1T, and the third object to be bonded 53 are in surface contact with each other. In addition to this, the laminate 10A is arranged such that the second surface 1D, which is the surface where the bonding film 3 exists in the second bonding sheet 1T, and the second object to be bonded 52 are in surface contact with each other. That is, in the laminate 10A shown in Fig. 2(b), the third object to be bonded 53 is arranged between the first object to be bonded 51 and the second object to be bonded 52, and the bonding sheets 1S and 1T are respectively arranged between the first object to be bonded 51 and the third object to be bonded 53, and between the second object to be bonded 52 and the third object to be bonded 53. Also in this embodiment, in the state where the laminate 10A is formed, none of the bonding films 3 are sintered.

[0058] As the object 5 to be joined in the laminate 10A, the same object as the object 5 to be joined in the joining structure 10 can be used. In the embodiment shown in FIG. 2(b), the third object 53 to be joined is preferably a semiconductor element or a substrate having at least one metal of gold, silver, copper, and nickel on its surface.

[0059] Next, the laminate 10A is heated to sinter the copper particles contained in the joining film 3, thereby forming a joining layer 30 that joins the object 5 to be joined and the copper foil 2.

[0060] The atmosphere during sintering is preferably an inert gas atmosphere such as nitrogen. The sintering temperature is preferably less than 300°C, more preferably 150°C or more and less than 300°C, still more preferably 200°C or more and less than 300°C, and even more preferably 230°C or more and less than 300°C. The sintering time is preferably 30 minutes or less, more preferably 0.5 minute or more and 25 minutes or less, still more preferably 1 minute or more and 20 minutes or less, provided that the sintering temperature is within the above range.

[0061] Sintering is preferably performed under pressure. At this time, the pressure applied to the joining film is preferably 0.001 MPa or more, more preferably 0.001 MPa or more and 20 MPa or less, still more preferably 0.01 MPa or more and 15 MPa or less.

[0062] The joining layer 30 formed through the above steps has a sintered structure of the copper particles contained in the joining film 3. That is, the joining layer 30 is composed of a sintered body of the copper particles contained in the joining film 3 and is formed by sintering the joining film 3.

[0063] When the laminate 10A of the embodiment shown in Fig. 2(a) is sintered, the copper particles contained in each joining film 3 are sintered, and the portion where the joining sheet 1 is disposed becomes a composite joining layer 1L having a copper foil 2 and joining layers 30 formed on both surfaces of the copper foil 2. Therefore, in the joining structure 10 obtained by sintering the laminate 10A of the embodiment shown in Fig. 2(a), the first joining object 51 and the second joining object 52 are electrically connected via a composite joining layer 1L having a copper foil 2 and joining layers 30, 30 formed on both surfaces of the copper foil 2.

[0064] Also, when the laminate 10A of the embodiment shown in Fig. 2(b) is sintered, the copper particles contained in each joining film 3 are sintered, and each joining sheet 1S, 1T becomes a composite joining layer 1L having a copper foil 2 and joining layers 30 formed on both surfaces of the copper foil 2. Therefore, in the joining structure 10 obtained by sintering the laminate 10A of the embodiment shown in Fig. 2(b), the first joining object 51 and the third joining object 53, and the second joining object 52 and the third joining object 53 are electrically connected via the composite joining layer 1L, respectively.

[0065] When the laminate 10A of the embodiments shown in Figs. 2(a) and (b) is sintered, when the solid reducing agent represented by the above-described chemical formula (1) or (2) is contained in the joining film 3, the following structure (3) derived from copper in the copper particles and the solid reducing agent is formed in the joining layer 30.

[0066]

Chemical formula

[0067] In the formula, R 3 or R 5 each independently represents a hydrogen atom, a hydroxyl group, a hydrocarbon group having 1 to 10 carbon atoms, or a hydrocarbon group having 1 to 10 carbon atoms and having a hydroxyl group. The details of R 3 or R 5 are appropriately applied to the descriptions of the above-described chemical formulas (1) and (2). Further, * represents a bonding site with copper.

[0068] Whether the structure (3) is formed in the bonding layer 30 can be confirmed by performing mass spectrometry or the like by TOF-SIMS on the cross-section of the bonding layer. For example, when BIS-TRIS is used as a solid reducing agent, a fragment with a molecular weight of 152 due to C-N(Cu)2 is observed in the mass spectrum on the positive electrode side in TOF-SIMS.

[0069] The thickness of the bonding layer 30 is preferably adjusted so that the copper foil and the object to be bonded are securely bonded and have sufficiently high conductivity and thermal conductivity. Specifically, the thickness of the bonding layer 30 is preferably 0.1 μm or more and 950 μm or less, and more preferably 1 μm or more and 500 μm or less. Since the liquid medium in the bonding film 3 does not exist during the manufacturing process of the bonding layer 30, the thickness of the bonding layer 30 is the same as the thickness of the bonding film 3 or thinner than the thickness of the bonding film 3.

[0070] The thickness of the bonding layer 30 can be appropriately controlled, for example, by adjusting the thickness of the film formed using the above-described bonding composition, the thickness of the bonding film 3, or the pressing conditions during firing. Further, the thickness of the bonding layer 30 is measured by embedding the bonding layer in resin and then polishing it, and observing the polished surface with an electron microscope.

[0071] The bonding structure 10 having the above-described bonding sheet 1 and the above-described bonding layer 30 is preferably used in an environment exposed to high temperatures, for example, an in-vehicle electronic circuit or an electronic circuit on which a power device is mounted, taking advantage of its high thermal conductivity and electrical reliability characteristics. The bonding structure 10 is preferably used as a power module structure. Examples of the power module structure include a mode in which a plurality of bonding structures 10 and electrodes such as lead frames are electrically connected via metal wires made of metals such as gold and copper. The metal wires are arranged singly or in plurality on an object to be bonded 5 such as a semiconductor element or a substrate having a metal such as copper on its surface, and electrically connect the bonding structure 10 and the electrodes, respectively.

[0072] An example of the power module structure will be described with reference to FIG. 2(b). A semiconductor element is used as the first object to be joined 51, a heat sink made of a metal such as copper is used as the second object to be joined 52, and a DBC substrate is used as the third object to be joined 53. Then, a joining structure 10 can be formed in which a composite joining layer 1L, which is a sintered body of the joining sheet 1, is formed at least between both objects to be joined 51 and 52 and between both objects to be joined 52 and 53.

[0073] As described above, the present invention has been described based on its preferred embodiments, but the present invention is not limited to the above embodiments. For example, in the joining structure 10 obtained by sintering the laminate 10A of the embodiment shown in FIG. 2(b), the composite joining layers 1L are arranged between the objects to be joined 51, 52, and 53, respectively, but the present invention is not limited to this form. Specifically, the composite joining layer 1L may be arranged between the objects to be joined 51 and 53, and only the joining layer 30 may be arranged between the objects to be joined 52 and 53. Also, only the joining layer 30 may be arranged between the objects to be joined 51 and 53, and the composite joining layer 1L may be arranged between the objects to be joined 52 and 53.

[0074] Further, as shown in FIG. 3, a joining structure 10 in which only the joining layer 30 is arranged between two objects to be joined 5 and 5 may be employed. As a manufacturing method of the joining structure 10, for example, a joining composition is applied to the first object to be joined 51 to form a coating film, and the coating film is dried to obtain a joining film 3. Then, the second object to be joined 52 is arranged on the joining film 3, and the joining film 3 is sintered and joined together with the objects to be joined 51 and 52 to form the joining structure 10. In the present embodiment, each object to be joined 51 and 52 is electrically connected only through the joining layer 30. This embodiment has an advantage that post-processing such as removing the excess joining layer 30 (or the joining film 3) formed during the manufacturing process is not required because only the joining film 3 serving as the joining layer 30 needs to be formed to have the minimum necessary dimensions as compared with the design using the joining sheet 1. The descriptions of the joining composition, the joining film 3, each object to be joined 5, and the joining layer 30 are appropriately applied to the above-described embodiments.

[0075] Also, at least one surface of the copper foil 2 may be surface-treated with an organic compound such as benzotriazole for the purpose of corrosion prevention or the like, as long as the effects of the present invention are not impaired.

Industrial Applicability

[0076] According to the present invention, a joining sheet and a joining structure excellent in thermal conductivity and electrical connection reliability are provided.

Claims

1. A bonding sheet having a copper foil and a sinterable bonding film formed on both sides of the copper foil, each of the bonding films contains copper particles, a solid reducing agent, and a liquid medium, the copper particles include spherical copper particles and flat copper particles, the volume cumulative particle size DSEM50 at 50% by volume measured by image analysis of a scanning electron microscope image of the spherical copper particles is 30 nm or more and 200 nm or less, the volume cumulative particle size D50 at 50% by volume by the laser diffraction scattering particle size distribution measurement method of the flat copper particles is 0.7 μm or more and 50 μm or less, the aspect ratio of the flat copper particles is 5 or more and 40 or less, the solid reducing agent is an amino alcohol compound, the liquid medium is a polyhydric alcohol, the ratio of the solid reducing agent in the bonding film is 0.1 part by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the copper particles, the content of the liquid medium in the bonding film is 9 parts by mass or less with respect to 100 parts by mass of the copper particles, a bonding sheet, wherein at least one of the bonding films is used for bonding to a bonding object having at least one metal of gold, silver, copper, and nickel on its surface.

2. The bonding sheet according to claim 1, wherein both sides of the bonding film are used for bonding to the bonding object.

3. The bonding sheet according to claim 1 or 2, wherein the liquid medium contains one or more of hexylene glycol and polyethylene glycol.

4. A bonding structure in which a first bonding object having at least one metal of gold, silver, copper, and nickel on its surface and a second bonding object having at least one metal of gold, silver, copper, and nickel on its surface are electrically connected via a bonding layer composed of a sintered structure of copper particles, the first bonding object and the second bonding object are electrically connected via a composite bonding layer having a copper foil and the bonding layer formed on both sides of the copper foil, the first bonding object and the second bonding object are any combination selected from a spacer, a heat sink, a semiconductor element, and a substrate, a bonding structure, in which the following structure (3) is formed in the bonding layer. 【Chemical 1】 (In the formula, R ３ or R ５ each independently represents a hydrogen atom, a hydroxyl group, a hydrocarbon group having 1 to 10 carbon atoms, or a hydrocarbon group having 1 to 10 carbon atoms and having a hydroxyl group, and * represents a bonding site with copper.)

5. A third bonding object is further disposed between the first bonding object and the second bonding object, Whether the first object to be joined and the third object to be joined, and the second object to be joined and the third object to be joined are electrically connected via the joining layer or are electrically connected via the composite joining layer, The first object to be joined is the semiconductor element, The second object to be joined is the heat sink, The joining structure according to claim 4, wherein the third object to be joined is an insulating substrate.

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