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WO2022210477A1 - 接合構造体 - Google Patents

接合構造体 Download PDF

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Publication number
WO2022210477A1
WO2022210477A1 PCT/JP2022/014835 JP2022014835W WO2022210477A1 WO 2022210477 A1 WO2022210477 A1 WO 2022210477A1 JP 2022014835 W JP2022014835 W JP 2022014835W WO 2022210477 A1 WO2022210477 A1 WO 2022210477A1
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WO
WIPO (PCT)
Prior art keywords
joined
joint
bonding
bonded
ratio
Prior art date
Application number
PCT/JP2022/014835
Other languages
English (en)
French (fr)
Inventor
真一 山内
圭 穴井
Original Assignee
三井金属鉱業株式会社
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by 三井金属鉱業株式会社 filed Critical 三井金属鉱業株式会社
Priority to KR1020237028510A priority Critical patent/KR20230162592A/ko
Priority to US18/279,012 priority patent/US20240149340A1/en
Priority to JP2023511226A priority patent/JPWO2022210477A1/ja
Priority to EP22780688.2A priority patent/EP4316694A4/en
Priority to CN202280019470.5A priority patent/CN116963856A/zh
Publication of WO2022210477A1 publication Critical patent/WO2022210477A1/ja

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    • B22F7/06—Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression of composite workpieces or articles from parts, e.g. to form tipped tools
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Definitions

  • the present invention relates to a joint structure.
  • the joint structure described in Patent Document 1 can exhibit high joint strength. Maintaining high bonding strength, that is, high bonding reliability is now required.
  • An object of the present invention is to provide a joint structure with high joint reliability.
  • the present invention relates to a joint structure formed by joining two members to be joined and a joint portion formed adjacently between the members to be joined,
  • the joint portion is made of a material mainly composed of copper,
  • the material to be joined and the joint in the end region of the joined structure with respect to the joining ratio Rc between the material to be joined and the joint portion in the central region of the joined structure The ratio (Rs/Rc) of the joint ratio Rs with the part is 0.6 or more and 0.9 or less
  • the present invention provides a bonded structure in which the bonding ratio Rc is 0.3 or more.
  • FIG. 1(a) is a schematic plan view of one embodiment of the bonded structure of the present invention
  • FIG. 1(b) is a schematic cross-sectional view taken along line II in FIG. 1(a).
  • FIG. 2(a) is an enlarged view schematically showing a panorama image of the central region A1 of the interface between the material to be joined 11 and the joint portion 15 in the cross-sectional view in the thickness direction of the joined structure 1 shown in FIG. 1(a).
  • 2(b) is a schematic panorama image of an end region A2 of the interface between the material to be joined 11 and the joint 15 in a cross-sectional view in the thickness direction of the joined structure 1 shown in FIG. 1(a).
  • 3 is an enlarged view shown in FIG.
  • FIG. 3(a) is an ultrasonic image of the joined structure 1 of Example 1 before the thermal cycle test
  • FIG. 3(b) is an ultrasonic image of the joined structure 1 of Example 1 after the thermal cycle test
  • FIG. 4(a) is an ultrasonic image of the joined structure 1 of Comparative Example 1 before the thermal cycle test
  • FIG. 4(b) is an ultrasonic image of the joined structure 1 of Comparative Example 1 after the thermal cycle test.
  • the joint structure 1 includes a first member to be joined 11 (hereinafter also simply referred to as “member to be joined 11”) and a second member to be joined 12 (hereinafter referred to as “member to be joined”). are also simply referred to as “members 12 to be joined”), and a joining portion 15 is formed adjacently between these members 11 and 12 to be joined.
  • the members to be joined 11 and 12 in the joined structure 1 are joined through the joining portion 15 , and no other members are interposed between the members to be joined 11 and 12 and the joining portion 15 .
  • the joint structure 1 of the embodiment shown in the figure is in a state in which a joint portion 15 and a first joint member 11 are laminated in this order on one surface of a second joint member 12. .
  • the material to be bonded 11 shown in FIGS. 1(a) and 1(b) is configured to be smaller in size than the material to be bonded 12, but is not limited to this form, and the materials to be bonded 11 and 12 are identical to each other. It may be a size, and the member 11 to be joined may be configured to be larger than the member 12 to be joined. Furthermore, although the members to be joined 11 and 12 and the joining portion 15 shown in the same figure are all flat, they may be curved if necessary.
  • 1(a) and 1(b) is formed such that its peripheral edge extends outward from the peripheral edge of the member to be joined 11, but is not limited to this form. and the peripheral edge of the workpiece 11 may be aligned with each other.
  • each of the members to be joined 11 and 12 is independently preferably a conductor.
  • the metal forming the surfaces of the members 11 and 12 to be joined at least one of gold, silver, copper, nickel and titanium is preferably independently selected from the viewpoint of improving conductivity.
  • Examples of the materials to be joined 11 and 12 containing such metals include, for example, spacers and heat sinks made of the above metals, semiconductor elements, substrates having at least one of the above metals on their surfaces, and the like. be done.
  • As the substrate for example, an insulating substrate or the like having a metal layer such as copper on the surface of a ceramic or aluminum nitride plate can be used.
  • the semiconductor element contains at least one element among elements such as Si, Ga, Ge, C, N, and As.
  • the metal present on the surfaces of the members 11 and 12 to be joined may be a single metal or an alloy of two or more metals.
  • the material to be bonded 11 is preferably a spacer, a heat sink, or a semiconductor element.
  • the material to be joined 12 is preferably a substrate.
  • the joint portion 15 is a structure containing copper as a main component, and is adjacent between the members to be joined 11 and 12, preferably across the entire plane view.
  • the joint portion 15 preferably contains 50% by mass or more, more preferably 60% by mass or more of copper.
  • the copper content in the junction can be measured by analyzing a cross section in the thickness direction of the junction structure 1 by energy dispersive X-ray spectroscopy (EDX).
  • EDX energy dispersive X-ray spectroscopy
  • Such a joint portion 15 can be formed as a sintered body of metal particles by firing a composition containing metal particles containing copper together with both materials to be joined, as shown in a manufacturing method to be described later. can.
  • the joint 15, which is a sintered body containing such particles may have a large number of pores in its structure.
  • One of the characteristics of the bonded structure of the present invention is that when viewed in cross section in the thickness direction, the bonding ratio between the material to be bonded and the bonding portion is different between the central region and the end regions of the bonded structure. I'm trying.
  • the virtual region V1 represented by the plan view shape of the similar shape is defined as the central region A1 of the joint structure 1 do.
  • the joint ratio at the interface between at least one of the members to be joined 11 and 12 located in the central region A1 and the joint portion 15 is calculated as follows: 1 joining ratio Rc (hereinafter also simply referred to as “joining ratio Rc”).
  • the maximum area A0 is the maximum area of the part where both the materials to be joined 11 and 12 and the joint portion 15 all exist when the joined structure 1 is viewed along the thickness direction Z. As shown in FIG. Therefore, taking the joint structure 1 shown in FIGS. 1(a) and 1(b) as an example, the maximum area A0 coincides with the arrangement area of the material 11 to be joined in plan view.
  • the joint ratio at the interface between at least one of the members to be joined 11, 12 located in the end region A2 and the joint portion 15 is referred to as a second joining ratio Rs (hereinafter also simply referred to as “joining ratio Rs”).
  • the interface targeted for the bonding ratio Rs is the same interface as the interface targeted for the bonding ratio Rc.
  • the ratio (Rs/Rc) of the joining ratio Rs to the joining ratio Rc is preferably 0.6 or more, more preferably 0.7 or more. Also, Rs/Rc is preferably 0.9 or less, more preferably 0.8 or less. That is, in the joint structure 1, the joint ratio of the central region A1 is higher than the joint ratio of the end regions A2 at the interface between at least one of the materials to be joined and the joint portion.
  • the present inventors speculate as follows about the reason why the bonded structure can exhibit high bonding reliability when Rs/Rc is within the range described above.
  • reference numerals shown in FIG. 1 will be used as an example.
  • the joint structure 1 in which the joint 15 is formed between the two members to be joined 11 and 12 typically uses a conductive composition having fluidity as a material for forming the joint 15. It is produced by sintering an object placed between two members 11 and 12 to be joined while applying pressure and increasing the rate of temperature rise.
  • the composition since the composition remains fluid at the initial stage of firing, the composition is extruded so as to be positioned outside the peripheral edges of the members 11 and 12 to be joined by pressurization, and the members to be joined 11 and 12 moves so that the density of the composition increases in the central region A1 in plan view.
  • the composition was efficiently sintered while the central region A1 in plan view had a relatively dense structure, and sufficient adhesion to the members 11 and 12 to be joined was exhibited.
  • a joint portion 15 is formed between the members to be joined 11 and 12 over the entire area of the members to be joined 11 and 12 in plan view.
  • the bonding ratio Rc is preferably 0.3 or more, more preferably 0.4 or more, and still more preferably 0.5 or more, and the higher the bonding ratio, the better, from the viewpoint of developing sufficient bonding strength with the material to be bonded.
  • the bonding ratio Rs is preferably 0.35 or more and 0.6 or less, more preferably 0.35 or more and 0.55 or less, and still more preferably 0.35 or more and 0.5 or less. Since the elastic modulus of the end region A2 of the joined body becomes moderately low due to the joining ratio Rs being such a ratio, it is possible to prevent stress concentration in the end region A2 and the occurrence of cracks. be able to.
  • the ratio Rs/Rc, the first bonding ratio Rc, and the second bonding ratio Rs described above define the relationship between at least one of the materials 11 and 12 to be bonded and the bonding portion 15 at least one of the interfaces. It is preferable that at least the interface G1 between the material to be bonded 11 and the bonding portion 15 satisfies the ratio and each bonding ratio. More preferably, it satisfies the above preferred range.
  • Such a joint structure 1 is produced by subjecting a conductive composition containing metal particles containing copper together with both the members to be joined 11 and 12 to a predetermined pressure, as shown in the manufacturing method described later, for example. can be obtained by firing at a heating rate of
  • the first joining ratio Rc and the second joining ratio Rs can be measured by the following method.
  • the joint structure 1 to be measured is embedded in a resin, and then a maximum area A0, a central area A1, and an end area A2 are defined in the plan view of the joint structure 1 by optical microscope observation.
  • the resin-embedded joined structure is cut in the thickness direction, the cut surface is polished, and the polished surface is processed by ion milling, for example, as shown in FIG. Prepare an observation surface.
  • the position of the interface G1 or the interface G2 between the workpiece to be bonded and the bonding portion to be measured is visually observed using an electron microscope at an arbitrary position within the central region A1 on the observation surface described above. Identify.
  • a panoramic image is obtained by photographing and arranging a plurality of images with a magnification that makes the object clear so that the total length Lt of the measurement field is about 30 ⁇ m, for example.
  • one of the interfaces G1 and G2 between the workpieces 11 and 12 and the joint 15 is measured, and the total length La along the plane direction X of the part where the interface is unclear is calculated. This is taken as the joining length L1 of the central region A1.
  • the joint length L1 is obtained by subtracting the total length of the parts where the interfaces G1 and G2 between the workpieces 11 and 12 to be measured and the joint 15 are clear from the total length Lt of the measurement field.
  • FIG. 2A shows a schematic diagram of a panoramic image in the central area A1, taking the interface G1 between the first workpiece 11 and the joint 15 as an example.
  • the schematic diagram also shows the total length Lt, the length La, and the joint length L1.
  • the ratio (L1/Lt) of the joint length L1 in the central area to the total length Lt of the measurement visual field is defined as the first joint ratio Rc in the present invention.
  • the same interface as the interface subjected to the measurement of the joint ratio Rc is determined by visual observation using an electron microscope.
  • the total length Lt of the measurement field is about 30 ⁇ m, for example, in the same manner as described above.
  • the total length Lb along the plane direction X of the part where the interface between the workpieces 11 and 12 to be measured and the joint 15 is unclear is measured, and the total length Lb of the part is measured.
  • the bonding length L2 may be calculated by subtracting the total length of the parts where the interfaces between the workpieces 11 and 12 and the bonding portion 15 are clear from the total length Lt of the measurement field. good.
  • the bonding length L2 can be calculated by visually observing the electron microscope image described above and the binarized image described later, or by using software as necessary.
  • FIG. 2B shows a schematic diagram of a panoramic image of the end area A2, taking the interface G1 between the first workpiece 11 and the joint 15 as an example.
  • the schematic diagram also shows the total length Lt, the length Lb, and the joint length L2. Then, the ratio (L2/Lt) of the joining length L2 of the end area A2 to the total length Lt of the measurement field is defined as the joining ratio Rs in the present invention.
  • the interface G1 between the material to be bonded 11 and the bonding portion 15 is clear is determined by binarizing the interface G1 between the material to be bonded 11 to be measured and the bonding portion 15 in the electron microscopic image.
  • the interface G1 between the material to be joined 11 and the joint portion 15 is clear if a boundary between a region represented by the same color and a region represented by a different color is observed when the image is viewed along the thickness direction. and On the other hand, if the boundary is not observed, it is assumed that the interface G1 between the workpiece 11 and the joint 15 is unclear.
  • the interface G2 between the material to be joined 12 and the joint 15 can also be observed in the same manner as described above to determine whether the interface G2 is clear or unclear.
  • the method of measuring the bonding ratios Rc and Rs has been described with a cross-sectional view of the bonded structure 1 in the thickness direction.
  • the portion 15 is continuously joined over the entire area of the maximum area A0 in plan view. Therefore, for the joint structure 1, a virtual circle centered on the centroid of the maximum area A0 in a plan view and virtual radial lines that pass through the center of the virtual circle and cut the virtual circle every 30° are considered.
  • both the bonding ratios Rc and Rs measured by observing cross sections of the bonded structure in plan view cut in the thickness direction at intervals of 30° along the imaginary radial line are within the above ranges.
  • This manufacturing method includes a coating step of applying a bonding composition containing copper on the surface of one of the materials to be bonded (for example, the material to be bonded 12) to form a coating film, and drying the coating film to form a dry coating. It is roughly divided into three processes: a drying process for forming a film, and a bonding process in which the other material to be bonded (for example, the material to be bonded 11) is further laminated on the dried coating film and heated while being pressed while being bonded in this state. be done.
  • a bonding composition containing copper is applied to the surface of one of the materials to be bonded to form a coating film (application step).
  • the bonding composition preferably comprises metal particles containing copper and a liquid medium. Details of the bonding composition will be described later.
  • the method of applying the bonding composition it is preferable to adopt a method that allows the formed coating film to have a uniform thickness and can be applied to the entire surface to be coated.
  • coating methods include screen printing, dispense printing, gravure printing, and offset printing.
  • the bonding composition is preferably in the form of paste or ink containing a liquid medium.
  • the thickness of the coating film to be formed is preferably set to 1 ⁇ m or more and 250 ⁇ m or less, more preferably 5 ⁇ m or more and 150 ⁇ m or less, immediately after coating, from the viewpoint of forming a bonded structure stably having high bonding strength. preferable.
  • the formed coating film is dried to obtain a dry coating film (drying process).
  • a dry coating film in which the amount of the liquid medium in the coating film is reduced.
  • a dry coating film is one in which the ratio of the liquid medium to the total weight of the film is 9% by weight or less. Since the content of each constituent material other than the liquid medium is substantially the same in the coating film and the dry coating film obtained by drying the coating film, the ratio of the liquid medium in the dry coating film is, for example, before and after drying. It can be calculated by measuring the mass change of the coating film.
  • a method of volatilizing the liquid medium by utilizing the volatility of the liquid medium can be adopted.
  • the proportion of the liquid medium in the dry coating film after the liquid medium has been removed is preferably 9 parts by mass or less, and preferably 7 parts by mass or less, as described above, with respect to the total mass of the coating film of 100 parts by mass. is more preferably 5 parts by mass or less.
  • This step can be changed as appropriate depending on the composition of the bonding composition to be used, but it is preferable to perform this step at 40° C. or more and 150° C. or less under atmospheric pressure for 1 minute or more and 60 minutes or less. This is preferable in that deterioration of properties of the article and the materials to be joined together with the articles to be dried hardly occurs, and the bonding strength between the materials to be joined can be increased while maintaining the desired physical properties of the materials to be joined.
  • the other member to be joined is laminated on the dry coating film and joined (joining step).
  • the dry coating film is obtained through the above-described steps, the other member to be joined is laminated on the dry coating film, and the first member to be joined and the second member to be joined are joined together.
  • a laminate is obtained in which the dried coating film is interposed therebetween.
  • This laminate is, for example, formed by laminating a coating film and the first member to be joined in this order on one surface of the second member to be joined.
  • the laminated body is heated at a predetermined heating rate while being pressurized to sinter the metal powder contained in the dried coating film, thereby forming a joining portion for joining two members to be joined together.
  • the atmosphere during sintering is preferably a reducing gas atmosphere such as hydrogen or formic acid, or an inert gas atmosphere such as nitrogen or argon.
  • the sintering temperature is preferably 150° C. or higher and 350° C. or lower, more preferably 200° C. or higher and 350° C. or lower, and still more preferably 230° C. or higher and 300° C. or lower.
  • the temperature increase rate during sintering is preferably 120°C/min or more and 2000°C/min or less, more preferably 240°C/min or more and 1500°C/min in the process from 40°C to reaching the sintering temperature described above. Below, more preferably 360° C./min or more and 1000° C./min or less.
  • This heating rate is faster than the heating rate employed in the manufacture of conventional bonded structures. By carrying out at such a speed, it is possible to manufacture a bonded structure that satisfies the aforementioned Rs/Rc relationship in a short period of time.
  • the pressure applied during sintering is preferably more than 6 MPa and 40 MPa or less, more preferably 8 MPa or more and 30 MPa or less, still more preferably 10 MPa or more and 20 MPa or less, from the start of heating to the end of sintering.
  • the sintering time is preferably 0.5 minutes or more and 120 minutes or less, more preferably 1 minute or more and 60 minutes or less, and preferably 1 minute or more and 30 minutes or less, provided that the sintering temperature is within the above range. .
  • the joint structure 1 in which the joint 15 as a sintered body of metal particles containing copper constituting the joint composition is formed between the two members 11 and 12 to be joined is obtained.
  • the above-described Rs/Rc relationship is set between the first workpiece 11 and the bonding portion 15 and between the second workpiece 12 and the bonding portion 15 respectively. It is possible to suitably obtain a joined structure 1 which is filled.
  • the bonding portion 15 contains copper, and when the bonding composition contains a solid reducing agent to be described later, the following structure (1) is formed in the bonding layer.
  • R 3 to R 5 are each independently 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. represents For the details of R 3 to R 5 , the explanations of chemical formulas (2) and (3) below are appropriately applied. Moreover, * represents a binding site with copper.
  • Whether or not the structure (1) is formed in the bonding layer can be confirmed by subjecting the cross section of the bonding portion to mass spectrometry or the like using TOF-SIMS.
  • TOF-SIMS mass spectrometry
  • BIS-TRIS is used as a reducing agent
  • a fragment of m/z 152 due to “CN(Cu) 2 ” is observed in the mass spectrum on the positive electrode side in TOF-SIMS.
  • the bonding composition contains metal powder composed of metal particles containing copper, and preferably further contains a reducing agent and a liquid medium.
  • the metal powder used in the present invention preferably consists of aggregates of metal particles containing copper.
  • the metal powder may further contain at least one metal selected from gold, silver, palladium, aluminum, nickel and tin, if necessary.
  • the shape of the metal particles that make up the metal powder used in the present invention is, for example, spherical, flake-like, dendrite-like (dendritic), rod-like, and the like. These can be used singly or in combination.
  • the particle size is preferably 0.03 ⁇ m or more in terms of volume cumulative particle size D SEM50 at a cumulative volume of 50% measured by image analysis of scanning electron microscope observation, It is more preferably 0.05 ⁇ m or more. Moreover, it is preferably 20 ⁇ m or less, more preferably 10 ⁇ m or less. With such a particle size, a coating film having a thin and uniform thickness can be formed with high productivity, and the bonding strength between the materials to be bonded can be further increased.
  • the volume cumulative particle size D50 at a cumulative volume of 50 % by volume measured by a laser diffraction scattering particle size distribution measurement method is preferably 0.3 ⁇ m or more. It is more preferably 5 ⁇ m or more, and even more preferably 0.7 ⁇ m or more.
  • the particle size of the flake-shaped copper particles is preferably 100 ⁇ m or less, more preferably 70 ⁇ m or less, and even more preferably 50 ⁇ m or less, in terms of D50 .
  • the term “flake” refers to a shape having a pair of plate surfaces forming the main surfaces of the particles and a side surface orthogonal to these plate surfaces. can be a face.
  • the reducing agent contained in the bonding composition is preferably solid at 1 atm and room temperature (25°C). Such a reducing agent is used to promote sintering of the metal particles by firing the composition, thereby developing high bonding strength.
  • the reducing agent is advantageously of a chemical structure having at least one amino group and multiple hydroxyl groups.
  • Solid at room temperature (25°C) means that the melting point of the reducing agent is greater than 25°C.
  • the solid reducing agent is also referred to as "solid reducing agent”.
  • the melting point of the solid reducing agent is preferably below the sintering temperature of the metal powder.
  • the boiling point of the solid reducing agent is preferably higher than the boiling point of the liquid medium to be described later.
  • an aminoalcohol compound represented by the following chemical formula (2) or (3) From the viewpoint of efficiently developing high joint strength when pressure bonding is performed, and from the viewpoint of developing high conductivity reliability after pressure bonding when a conductor is used as the material to be bonded, solid reduction As the agent, it is preferable to use an aminoalcohol compound represented by the following chemical formula (2) or (3).
  • R 1 to R 6 each independently represent a hydrogen atom, a hydroxyl group, a hydrocarbon group having 1 to 10 carbon atoms, or a hydroxyl group having 1 to 10 carbon atoms. represents a hydrocarbon group.
  • R 7 represents a hydrocarbon group having 1 to 10 carbon atoms or a hydrocarbon group having 1 to 10 carbon atoms and having a hydroxyl group.
  • Hydrocarbon groups include saturated or unsaturated aliphatic groups. The aliphatic group may be linear or branched.
  • aminoalcohol compound represented by the chemical formula (2) or (3) include bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane (BIS-TRIS, melting point: 104°C, boiling point: over 300°C, corresponding to chemical formula (2)), 2-amino-2-(hydroxymethyl)-1,3-propanediol (TRIS, melting point: 169-173°C, boiling point: over 300°C, corresponding to chemical formula (2)), 1 , 3-bis(tris(hydroxymethyl)methylamino)propane (BIS-TRIS propane, melting point: 164-165°C, boiling point: over 300°C, corresponding to chemical formula (3)).
  • bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane as the solid reducing agent from the viewpoint of obtaining a bonded body that exhibits high bonding strength while enhancing the sinterability of metal particles.
  • the proportion of the solid reducing agent in the bonding composition is preferably 0.1 parts by mass or more with respect to 100 parts by mass of the metal powder, from the viewpoint of enhancing the sinterability of the metal particles. Preferably, it is more preferably 1 part by mass or more.
  • it is realistic to make it 10 parts by mass or less, and 8 parts by mass or less. It is preferable to set it as, and it is more preferable to set it as 5 mass parts or less.
  • the bonding composition preferably further contains a liquid medium.
  • the liquid medium is preferably a non-aqueous solvent from the viewpoint of improving the coating properties of the bonding composition, the solubility of the solid reducing agent, and the efficiency of forming a dry coating film due to the appropriate volatility of the liquid medium.
  • monohydric or polyhydric alcohols more preferably polyhydric alcohols.
  • polyhydric alcohols examples include propylene glycol, ethylene glycol, hexylene glycol, diethylene glycol, 1,3-butanediol, 1,4-butanediol, dipropylene glycol, tripropylene glycol, glycerin, polyethylene glycol 200 and polyethylene glycol 300. etc.
  • a liquid medium can be used individually by 1 type or in combination of 2 or more types.
  • the liquid medium is preferably 10 parts by mass or more and 40 parts by mass or less, more preferably 10 parts by mass or more and 35 parts by mass or less with respect to 100 parts by mass of the metal powder.
  • the bonding composition may contain other components such as a binder component, a surface tension modifier, an antifoaming agent, and a viscosity modifier as long as the effects of the present invention are exhibited.
  • the ratio of the other components is preferably 0.1 parts by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the metal powder.
  • a bonded body having such a bonding portion is suitably used in an environment exposed to high temperatures, such as an electronic circuit mounted on a vehicle or an electronic circuit mounted with a power device, by taking advantage of its high bonding strength and thermal conductivity. be done.
  • Example 1 (1) Preparation of bonding composition
  • copper powder spherical copper powder with a D SEM50 of 0.14 ⁇ m and D50 of 4.9 ⁇ m and an aspect ratio (ratio of the length of the main surface to the thickness of the particle) of 13 A mixture with flaky copper powder was used.
  • the content ratio of the spherical copper powder and the flaky copper powder in the mixture of copper powder was 70% by mass of spherical copper powder:30% by mass of flaky copper powder.
  • Bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane was used as a reducing agent at a ratio of 2.5 parts to 100 parts of copper powder.
  • a liquid medium a mixture of polyethylene glycol 300 (1 part per 100 parts of copper powder) and hexylene glycol (31.6 parts per 100 parts of copper powder) was used.
  • a pasty bonding composition was obtained by mixing the above copper powder, reducing agent and liquid medium.
  • the viscosity of the bonding composition at a shear rate of 10 s -1 was 34 Pa ⁇ s at 25°C.
  • a 4.9 mm square semiconductor element SiC chip, thickness 0.38 mm, CPW5-1200 manufactured by Wolfspeed Co., Ltd., square in plan view
  • a pressure of 20 MPa is applied in the thickness direction, and the temperature is raised from 40° C. to 280° C. at a temperature elevation rate of 960° C./min in a nitrogen atmosphere, and then baked at 280° C. for 5 minutes.
  • each member to be joined and the joint portion were formed adjacent to each other over the entire plane view.
  • Mass spectrometry by TOF-SIMS confirmed that the joint portion, which is a sintered body of the dried coating film, had the chemical structure represented by the structure (1).
  • Example 2 A bonded structure having the structure shown in FIGS. 1(a) and 1(b) was obtained in the same manner as in Example 1, except that the pressure applied during sintering was changed to 10 MPa in manufacturing the bonded structure. . It was confirmed by TOF-SIMS mass spectrometry that the chemical structure represented by structure (1) was formed in the junction.
  • Example 1 A joined structure was obtained in the same manner as in Example 1, except that the pressure applied during sintering was changed to 6 MPa in the production of the joined structure. It was confirmed by TOF-SIMS mass spectrometry that the chemical structure represented by structure (1) was formed in the junction.
  • TCT thermal cycle test
  • the obtained image data was binarized using image processing software Image-J, and the black area ratio (conjugation ratio after 10 cycles of TCT; %) in 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. After that, File-Open was selected to open the image data for calculating the bonding rate, and then the range (A) of the SiC chip mounting portion in the image was specified. Next, after selecting Edit-Copy to system and copying the specified range (A), File-New-System clipboard was selected and the image of the specified range (A) was pasted.
  • Image-Type-8bit was selected to clarify the junction, and after the image was converted, Image-Adjust-Threshold was selected to adjust the threshold of the image to 110.
  • a red range (B) existing within the range (A) of the SiC chip mounting portion in the adjusted image was specified.
  • the red area (B) is the SiC chip bonding portion, and the bonding rate is calculated by (B)/(A) ⁇ 100. A higher bonding rate indicates a higher bonding reliability even when excessive temperature changes occur. Table 1 shows the results.

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Abstract

接合構造体（１）は、２つの被接合材（１１，１２）と、これらの被接合材間に隣接して形成された接合部（１５）とが接合されてなる。接合部（１５）は、銅を主体とする材料からなる。接合構造体（１）の厚み方向（Ｚ）断面視において、該接合構造体の中央域（Ａ１）における被接合材と接合部との接合割合（Ｒｃ）に対する、該接合構造体の端部域（Ａ２）における被接合材と接合部との接合割合（Ｒｓ）の比（Ｒｓ／Ｒｃ）が０．６以上０．９以下である。接合割合（Ｒｃ）が０．３以上である。

Description

接合構造体
　本発明は、接合構造体に関する。
　近年の世界的な省エネルギー化の流れに伴い、インバータなど電力変換・制御装置としてパワーデバイスと呼ばれる半導体デバイスが盛んに用いられている。このようなデバイスに適用可能な技術として、本出願人は、銅を含み且つ所定の化学構造を有する接合部によって２つの被接合材が接続された接合構造を提案した（特許文献１参照）。
国際公開第２０２０／０３２１６１号パンフレット
　特許文献１に記載の接合構造は高い接合強度が発現可能であるが、例えば当該接合構造を備えた半導体デバイスを自動車用途等に用いる際には、温度変化の激しい環境に長期間置かれても接合強度が高く維持されること、即ち、高い接合信頼性が求められるようになってきている。
　本発明の課題は、高い接合信頼性を有する接合構造体を提供することにある。
　本発明は、２つの被接合材と、これらの被接合材間に隣接して形成された接合部とが接合されてなる接合構造体であって、
　前記接合部は、銅を主体とする材料からなり、
　前記接合構造体の厚み方向断面視において、該接合構造体の中央域における前記被接合材と前記接合部との接合割合Ｒｃに対する、該接合構造体の端部域における前記被接合材と前記接合部との接合割合Ｒｓの比（Ｒｓ／Ｒｃ）が、０．６以上０．９以下であり、
　前記接合割合Ｒｃが０．３以上である、接合構造体を提供するものである。
図１（ａ）は、本発明の接合構造体の一実施形態の模式的な平面図であり、図１（ｂ）は図１（ａ）におけるＩ－Ｉ線での模式的な断面図である。 図２（ａ）は、図１（ａ）に示す接合構造体１の厚み方向断面視において、被接合材１１と接合部１５との界面の中央域Ａ１のパノラマ像を模式的に示す拡大図であり、図２（ｂ）は、図１（ａ）に示す接合構造体１の厚み方向断面視において、被接合材１１と接合部１５との界面の端部域Ａ２のパノラマ像を模式的に示す拡大図である。 図３（ａ）は、実施例１の接合構造体１の冷熱サイクル試験前の超音波画像であり、図３（ｂ）は実施例１の接合構造体１の冷熱サイクル試験後の超音波画像である。 図４（ａ）は比較例１の接合構造体１の冷熱サイクル試験前の超音波画像であり、図４（ｂ）は比較例１の接合構造体１の冷熱サイクル試験後の超音波画像である。
　以下に、本発明の接合構造体を、その好ましい実施形態に基づき説明する。
　図１（ａ）及び（ｂ）には、本発明の接合構造体の一実施形態が示されている。同図に示す実施形態では、接合構造体１は、第１の被接合材１１（以下、これを単に「被接合材１１」ともいう。）と、第２の被接合材１２（以下、これを単に「被接合材１２」ともいう。）とを有し、これらの被接合材１１，１２の間に隣接して形成された接合部１５を有する。接合構造体１における被接合材１１，１２は接合部１５を介して接合されており、各被接合材１１，１２と接合部１５との間に他の部材は介在していない。同図に示す実施形態の接合構造体１は、第２の被接合材１２の一方の面上に、接合部１５及び第１の被接合材１１がこの順で積層された状態となっている。
　図１（ａ）及び（ｂ）に示す被接合材１１は、その寸法が被接合材１２よりも小さく構成されているが、この形態に限られず、各被接合材１１，１２が互いに同一の寸法であってもよく、被接合材１１が被接合材１２よりも大きく構成されていてもよい。更に、同図に示す各被接合材１１，１２、並びに接合部１５はいずれも平面であるが、必要に応じて曲面となっていてもよい。
　また、図１（ａ）及び（ｂ）に示す接合部１５は、その周縁が被接合材１１の周縁よりも外方に延出して形成されているが、この形態に限られず、接合部１５の周縁と被接合材１１の周縁とが一致した状態で形成されていてもよい。
　各被接合材１１，１２の種類に特に制限はないが、好ましくは各被接合材１１，１２のうち少なくとも一方において、より好ましくは被接合材１１，１２の双方において、接合部１５と接触する面に金属を含む。この場合、両被接合材１１，１２は、接合部１５を介して電気的に接続されていることも好ましい。つまり、各被接合材１１，１２はそれぞれ独立して、好ましくは導電体である。各被接合材１１，１２の表面を形成する金属としては、導電性の向上の観点から、それぞれ独立して、金、銀、銅、ニッケル及びチタンの少なくとも一種が好ましく挙げられる。
　このような金属を含む各被接合材１１，１２としては、それぞれ独立して、例えば上述の金属からなるスペーサーや放熱板、半導体素子、並びに上述した金属の少なくとも一種を表面に有する基板等が挙げられる。基板としては、例えば、セラミックス又は窒化アルミニウム板の表面に銅等の金属層を有する絶縁基板等を用いることができる。被接合材として半導体素子を用いる場合、半導体素子は、Ｓｉ、Ｇａ、Ｇｅ、Ｃ、Ｎ、Ａｓ等の元素のうち一種以上を含む。被接合材１１，１２の表面に存在する金属は、金属単体であってもよく、あるいは２種類以上の金属の合金であってもよい。
　被接合材１１は、スペーサー、放熱板、又は半導体素子のいずれかであることが好ましい。被接合材１２は、好ましくは基板である。
　接合部１５は、銅を主体として含む構造体であり、各被接合材１１，１２の間に隣接して、好ましくは平面視全域に介在している。接合部１５は、銅を好ましくは５０質量％以上、より好ましくは６０質量％以上含むものである。接合部における銅の含有量は、接合構造体１の厚み方向断面を対象として、エネルギー分散型Ｘ線分光法（ＥＤＸ）によって分析することで測定することができる。
　このような接合部１５は、例えば後述する製造方法にて示すように、銅を含む金属粒子を含む組成物を両被接合材とともに焼成して、金属粒子どうしの焼結体として形成することができる。このような粒子を含んで形成された焼結体である接合部１５は、その構造中に多数の空孔を有していてもよい。
　本発明の接合構造体は、これを厚み方向に断面視したときに、被接合材と接合部との接合割合が、接合構造体の中央域と端部域とで互いに異なることを特徴の一つとしている。
　詳細には、図１（ａ）に示すように、接合構造体１の各被接合材１１，１２と接合部１５とが接合されている最大領域Ａ０の平面視における図心と、最大領域Ａ０の平面積を５０％に縮小した相似形の平面視における図心とを一致させたときに、当該相似形の平面視形状で表される仮想領域Ｖ１を、接合構造体１の中央域Ａ１とする。そして、接合構造体１の中央域Ａ１を厚み方向Ｚに断面視したときに、上述の中央域Ａ１に位置する少なくとも一方の被接合材１１，１２と接合部１５との界面における接合割合を第１接合割合Ｒｃ（以下、これを単に「接合割合Ｒｃ」ともいう。）とする。
　なお、最大領域Ａ０は、接合構造体１を厚み方向Ｚに沿ってみたときに、両被接合材１１，１２と接合部１５とのすべてが存在する部位の最大の領域である。したがって、図１（ａ）及び（ｂ）に示される接合構造体１を例にとると、最大領域Ａ０は、被接合材１１の平面視における配置領域と一致する。
　また、図１（ａ）に示すように、接合構造体１の平面視において、最大領域Ａ０の平面視における図心と、最大領域Ａ０の平面積を９０％に縮小した相似形の平面視における図心とを一致させたときに、当該相似形の平面視形状で表される仮想領域Ｖ２を考える。このとき、接合構造体１の平面視において、当該仮想領域Ｖ２の外方であり且つ最大領域Ａ０の周縁（同図中、被接合材１１の周縁１１ｅに相当）よりも内方に位置する領域を、接合構造体１の端部域Ａ２とする。そして、接合構造体１の端部域Ａ２を厚み方向Ｚに断面視したときに、上述の端部域Ａ２に位置する少なくとも一方の被接合材１１，１２と接合部１５との界面における接合割合を第２接合割合Ｒｓ（以下、これを単に「接合割合Ｒｓ」ともいう。）とする。接合割合Ｒｓの対象となる界面は、接合割合Ｒｃの対象とした界面と同一の界面とする。
　このとき、接合割合Ｒｃに対する接合割合Ｒｓの比（Ｒｓ／Ｒｃ）が、好ましくは０．６以上、より好ましくは０．７以上である。また、Ｒｓ／Ｒｃは好ましくは０．９以下、より好ましくは０．８以下である。つまり、接合構造体１は、少なくとも一方の被接合材と接合部との界面において、中央域Ａ１の接合割合が端部域Ａ２の接合割合よりも高いものである。
　Ｒｓ／Ｒｃが上述した範囲であることによって、接合構造体が高い接合信頼性を発現できる理由について、本発明者は以下のように推測している。以下の説明では図１にて図示した符号を例にとり説明する。
　２つの被接合材１１，１２の間に接合部１５が形成された接合構造体１は、典型的には、接合部１５の形成材料として流動性を有する導電性の組成物を用い、この組成物を２つの被接合材１１，１２の間に配した状態で、加圧しながら昇温速度を早くした状態で焼成して作製される。このとき、組成物は焼成初期では流動性が残存しているので、加圧によって、被接合材１１，１２の周縁外方に位置するように組成物が押し出されたり、被接合材１１，１２の平面視中央域Ａ１において組成物の密度が高まるように移動したりする。この状態で焼結されることによって、平面視中央域Ａ１では比較的密な構造となりつつ、組成物が効率的に焼結して、被接合材１１，１２との密着性が十分に発現した接合部１５が各被接合材１１，１２の間に該被接合材１１，１２の平面視全域に形成される。特に、Ｒｓ／Ｒｃが上述した特定の範囲にあると、接合構造体１全体としての接合強度が維持されると共に、接合構造体１の端部域Ａ２における応力集中が緩和されることとなるため、温度変化環境においても接合部の高い接合機能が維持され、高い接合信頼性を維持することができる。
　これに対し、加圧が不十分である条件や、昇温速度が適切でない条件で接合部１５が形成される場合には、接合構造体１の平面視中央域Ａ１に位置する接合部１５が密な構造となり難く、Ｒｓ／Ｒｃが１に近い値となる。そうすると、接合部１５の端部域Ａ２における応力集中の緩和ができなくなり、接合信頼性を維持することが困難となる。
　接合割合Ｒｃは、好ましくは０．３以上、より好ましくは０．４以上、更に好ましくは０．５以上であり、被接合材との十分な接合強度を発現する点から高ければ高いほど好ましい。
　接合割合Ｒｓは、好ましくは０．３５以上０．６以下、より好ましくは０．３５以上０．５５以下、更に好ましくは０．３５以上０．５以下である。接合割合Ｒｓがこのような割合となっていることによって、接合体の端部域Ａ２の弾性率が適度に低くなるので、端部域Ａ２に応力が集中して割れが発生するのを防止することができる。
　上述の比Ｒｓ／Ｒｃ、第１接合割合Ｒｃ及び第２接合割合Ｒｓは、少なくとも一方の被接合材１１，１２と接合部１５との少なくとも一方の界面における関係を規定するものであるが、上述した比及び各接合割合は、被接合材１１と接合部１５との界面Ｇ１において少なくとも満たしていることが好ましく、これに加えて、被接合材１２と接合部１５との界面Ｇ２においても同様に上述の好適な範囲を満たしていることがより好ましい。
　このような接合構造体１は、例えば後述する製造方法にて示すように、銅を含む金属粒子を含む導電性組成物を両被接合材１１，１２とともに所定の圧力を付与した状態で、所定の昇温速度で焼成することによって得ることができる。
　第１接合割合Ｒｃ及び第２接合割合Ｒｓは、以下の方法で測定することができる。
　まず測定対象の接合構造体１を樹脂包埋したあと、光学顕微鏡観察によって、接合構造体１の平面視において、最大領域Ａ０、中央域Ａ１及び端部域Ａ２をそれぞれ画定する。その後、最大領域Ａ０の平面視での図心（図１（ａ）中、第１の被接合材１１の平面視における図心に相当する）を通過する位置で、樹脂包埋した接合構造体を厚み方向に切断し、当該切断面を研磨して、更にイオンミリングにより研磨面を加工した後、例えば図１（ｂ）で示されるように、図心を通過する厚み方向断面視に相当する観察面を作製する。
　次いで、上述した観察面における中央域Ａ１内の任意の位置において、測定対象となる被接合材と接合部との界面Ｇ１又は界面Ｇ２の位置を、電子顕微鏡を用いて目視にて観察することで特定する。
　次いで、電子顕微鏡観察において、例えば測定視野の全長Ｌｔが３０μｍ程度となるように、対象が明瞭となる倍率の画像を複数撮影して並べることで、パノラマ像を取得する。この状態で、被接合材１１，１２と接合部１５との界面Ｇ１，Ｇ２のうち一方の界面を測定対象として、当該界面が不明瞭である部位の面方向Ｘに沿う長さＬａの合計を測定し、これを中央域Ａ１の接合長さＬ１とする。これに代えて、接合長さＬ１は、測定視野の全長Ｌｔから、測定対象となる被接合材１１，１２と接合部１５との界面Ｇ１，Ｇ２が明瞭である部位の長さの合計を差し引くことによって算出してもよい。接合長さＬ１は、上述した電子顕微鏡画像や後述する二値化画像を対象として、目視したり、必要に応じてソフトウェアを用いたりすることで算出することができる。
　図２（ａ）には、第１の被接合材１１と接合部１５との界面Ｇ１を例にとり、中央域Ａ１におけるパノラマ像の模式図が示されている。同模式図では、全長Ｌｔ、長さＬａ及び接合長さＬ１を併せて示している。
　そして、測定視野の全長Ｌｔに対する中央域の接合長さＬ１の比（Ｌ１／Ｌｔ）を、本発明における第１接合割合Ｒｃとする。
　同様に、上述した観察面における端部域Ａ２内の任意の位置において、被接合材１１，１２と接合部１５との界面Ｇ１，Ｇ２のうち、接合割合Ｒｃの測定に供した界面と同一界面を、電子顕微鏡を用いて目視で観察することによって特定する。
　端部域Ａ２内の測定においても同様に、電子顕微鏡観察において、例えば上述の方法と同様にして測定視野の全長Ｌｔが３０μｍ程度となるようにパノラマ像を取得する。この状態で、測定対象となる被接合材１１，１２と接合部１５との界面が不明瞭である部位の面方向Ｘに沿う長さＬｂの合計を測定し、これを端部域Ａ２の接合長さＬ２とする。これに代えて、接合長さＬ２は、測定視野の全長Ｌｔから、各被接合材１１，１２と接合部１５との界面が明瞭である部位の長さの合計を差し引くことによって算出してもよい。接合長さＬ２は、上述した電子顕微鏡画像や後述する二値化画像を対象として、目視したり、必要に応じてソフトウェアを用いたりすることで算出することができる。
　図２（ｂ）には、第１の被接合材１１と接合部１５との界面Ｇ１を例にとり、端部域Ａ２のパノラマ像の模式図が示されている。同模式図では、全長Ｌｔ、長さＬｂ及び接合長さＬ２を併せて示している。
　そして、測定視野の全長Ｌｔに対する端部域Ａ２の接合長さＬ２の比（Ｌ２／Ｌｔ）を、本発明における接合割合Ｒｓとする。
　上述した「被接合材１１，１２と接合部１５との界面Ｇ１，Ｇ２が不明瞭である部位」とは、接合構造体１の製造時において、金属粒子と被接合材とが十分に焼結した部位を意味する。一方、「被接合材１１，１２と接合部１５との界面Ｇ１，Ｇ２が明瞭である部位」とは、接合構造体１の製造時において、金属粒子と被接合材とが焼結していない部位を意味する。
　被接合材１１と接合部１５との界面Ｇ１が明瞭であるか否かは、電子顕微画像において、測定対象の被接合材１１と接合部１５との界面Ｇ１において、二値化した二値化画像を厚み方向に沿ってみたときに、同色で表される領域と異なる色で表される領域との境界が観察されれば、被接合材１１と接合部１５との界面Ｇ１が明瞭であるとする。一方、前記の境界が観察されなければ、被接合材１１と接合部１５との界面Ｇ１が不明瞭であるとする。
　被接合材１２と接合部１５との界面Ｇ２についても、上述の方法と同様に観察して、界面Ｇ２が明瞭又は不明瞭と判定することができる。
　なお説明の便宜上、接合割合Ｒｃ，Ｒｓの測定方法に関して、接合構造体１の厚み方向の一断面視で説明したが、典型的には、接合構造体１における各被接合材１１，１２と接合部１５とは最大領域Ａ０の平面視全域で連続して接合されている。したがって、接合構造体１は、最大領域Ａ０の平面視での図心を中心とする仮想円と、該仮想円の中心を通り且つ該仮想円を３０°おきに切断する仮想放射状線を考えたときに、平面視での接合構造体を該仮想放射状線で３０°おきに厚み方向に切断した断面を観察して測定した各接合割合Ｒｃ，Ｒｓがいずれも上述の範囲であることが好ましい。
　次に、接合構造体の製造方法について説明する。本製造方法は、一方の被接合材（例えば被接合材１２）の表面上に、銅を含む接合用組成物を塗布して塗膜を形成する塗布工程、該塗膜を乾燥させて乾燥塗膜を形成する乾燥工程、及び該乾燥塗膜上に他方の被接合材（例えば被接合材１１）を更に積層し、この状態で加圧しながら加熱して接合させる接合工程の３つに大別される。
　まず、一方の被接合材の表面上に、銅を含む接合用組成物を塗布して塗膜を形成する（塗布工程）。接合用組成物は、好ましくは銅を含む金属粒子及び液媒体を含むものである。接合用組成物の詳細は後述する。
　接合用組成物の塗布方法は、形成される塗膜の厚みが均一となり、且つ塗布予定面の全域に塗布できる方法を採用することが好ましい。このような塗布方法としては、例えばスクリーン印刷、ディスペンス印刷、グラビア印刷、オフセット印刷などが挙げられる。均一な塗布性の向上の観点から、接合用組成物は、液媒体を含むペースト状又はインク状のものであることが好ましい。
　形成する塗膜の厚みは、高い接合強度を安定的に有する接合構造体を形成する観点から、塗布直後において、１μｍ以上２５０μｍ以下に設定することが好ましく、５μｍ以上１５０μｍ以下に設定することが更に好ましい。また、被接合材と接合体とが隣接して密着していない部分が発生せずに、被接合材と接合体との接合面全域で接合部を形成しやすくして、接合対象との接合強度を接合面全域で高める観点から、塗膜の平面視における塗布面積は、接合対象となる被接合材のうち最も平面積が小さい被接合材の底面積以上の面積で塗布することが好ましい。
　次に、形成した塗膜を乾燥させて乾燥塗膜を得る（乾燥工程）。本工程では、乾燥により該塗膜から液媒体の少なくとも一部を除去して、塗膜中の液媒体の量が低減した乾燥塗膜を得る。塗膜から液媒体を除去することで、乾燥塗膜の保形性を高めることができ、またその状態で被接合材どうしを接合するので、接合強度を高いものとすることができる。乾燥塗膜とは、膜の全質量に対する液媒体の割合が９質量％以下のものである。塗膜と、該塗膜を乾燥させた乾燥塗膜とは、液媒体以外の各構成材料の含有量は実質的に同一であるので、乾燥塗膜における液媒体の割合は、例えば、乾燥前後の塗膜の質量変化を測定して算出することができる。
　液媒体を乾燥して除去するためには、該液媒体の揮発性を利用して液媒体を揮発させる方法を採用でき、例えば、自然乾燥、熱風乾燥、赤外線の照射、ホットプレート乾燥等の乾燥方法を用いることができる。
　液媒体が除去された後の乾燥塗膜における該液媒体の割合は、塗膜の全質量１００質量部に対して、上述のとおり９質量部以下であることが好ましく、７質量部以下であることが更に好ましく、５質量部以下であることが更に好ましい。
　本工程は、用いる接合用組成物の組成に応じて適宜変更可能であるが、大気雰囲気下で、４０℃以上１５０℃以下、大気圧、１分以上６０分以下で行うことが、接合用組成物及びこれとともに乾燥に供される被接合材の変質が生じ難く、被接合材の所望の物性を維持しつつ、被接合材どうしの接合強度を高められる点で好ましい。
　続いて、乾燥塗膜上に他方の被接合材を積層して接合する（接合工程）。詳細には、上述の工程を経て乾燥塗膜が得られたら、他方の被接合材を該乾燥塗膜上に積層して、第１の被接合材と第２の被接合材と、これらの間に乾燥塗膜が介在して配された積層体を得る。この積層体は、例えば、第２の被接合材の一方の面上に、塗膜及び第１の被接合材がこの順で積層されているものである。
　そして、積層体を加圧しながら所定の昇温速度で加熱して、乾燥塗膜に含まれる金属粉を焼結させることで、２つの被接合材どうしを接合する接合部を形成する。
　焼結時の雰囲気は、水素やギ酸等の還元ガス雰囲気であるか、又は窒素やアルゴン等の不活性ガス雰囲気であることが好ましい。
　焼結温度は、好ましくは１５０℃以上３５０℃以下、より好ましくは２００℃以上３５０℃以下、更に好ましくは２３０℃以上３００℃以下である。
　焼結時の昇温速度は、４０℃から上述の焼結温度に到達するまでの過程において、好ましくは１２０℃／分以上２０００℃／分以下、より好ましくは２４０℃／分以上１５００℃／分以下、更に好ましくは３６０℃／分以上１０００℃／分以下である。この昇温速度は、従来の接合構造体の製造に採用される昇温速度よりも速いものである。このような速度で行うことによって、上述したＲｓ／Ｒｃの関係を満たした接合構造体を短い時間で製造することができる。
　焼結時に加える圧力は、加熱開始から焼結終了までにわたって、好ましくは６ＭＰａ超４０ＭＰａ以下、より好ましくは８ＭＰａ以上３０ＭＰａ以下、更に好ましくは１０ＭＰａ以上２０ＭＰａ以下を維持することが好ましい。
　焼結時間は、焼結温度が上述の範囲にある状態を条件として、好ましくは０．５分以上１２０分以下、より好ましくは１分以上６０分以下、好ましくは１分以上３０分以下である。
　以上の工程を経て、接合用組成物を構成する銅を含む金属粒子の焼結体としての接合部１５が２つの被接合材１１，１２の間に形成された接合構造体１となる。上述した好適な製造方法によれば、上述したＲｓ／Ｒｃの関係を第１の被接合材１１と接合部１５との間、並びに第２の被接合材１２と接合部１５との間でそれぞれ満たした接合構造体１を好適に得ることができる。接合部１５は銅を含むものであり、また、接合用組成物が後述する固体還元剤を含む場合には、以下の構造（１）が接合層に形成される。
Figure JPOXMLDOC01-appb-C000002
　式（１）中、Ｒ３ないしＲ５は、それぞれ独立して、水素原子、水酸基、炭素原子数１以上１０以下の炭化水素基、又は水酸基を有する炭素原子数１以上１０以下の炭化水素基を表す。Ｒ３ないしＲ５の詳細は、後述する化学式（２）及び（３）の説明が適宜適用される。また、＊は、銅との結合部位を表す。
　接合層に前記構造（１）が形成されているか否かは、接合部の断面を対象として、ＴＯＦ－ＳＩＭＳによる質量分析等を行うことにより確認することができる。例えば還元剤としてＢＩＳ－ＴＲＩＳを用いる場合、ＴＯＦ－ＳＩＭＳでの正極側のマススペクトルにおいて「Ｃ－Ｎ（Ｃｕ）２」に起因するｍ／ｚ１５２のフラグメントが観察される。
　接合用組成物は、銅を含む金属粒子からなる金属粉を含んでおり、好ましくは還元剤及び液媒体を更に含むものである。
　本発明において用いられる金属粉は、銅を含む金属粒子の集合体からなることが好ましい。金属粉は、必要に応じて金、銀、パラジウム、アルミニウム、ニッケル及びスズの少なくとも一種の金属が更に含まれていてもよい。
　本発明に用いられる金属粉を構成する金属粒子の形状は、例えば、球状、フレーク状、デンドライト状（樹枝状）、棒状等である。これらは単独で又は複数組み合わせて用いることができる。
　金属粒子が球状である場合の粒径は、走査型電子顕微鏡観察の画像解析によって測定された累積体積５０容量％における体積累積粒径ＤＳＥＭ５０で表して、０．０３μｍ以上であることが好ましく、０．０５μｍ以上であることがより好ましい。また、２０μｍ以下であることが好ましく、１０μｍ以下であることがより好ましい。このような粒径であることによって、薄型且つ均一な厚さを有する塗膜を生産性高く形成でき、被接合材どうしの接合強度を一層高めることができる。
　また、銅粒子がフレーク状である場合の粒径は、レーザー回折散乱式粒度分布測定法による累積体積５０容量％における体積累積粒径Ｄ５０が、０．３μｍ以上であることが好ましく、０．５μｍ以上あることがより好ましく、０．７μｍ以上であることが更に好ましい。またフレーク状の銅粒子の粒径は、Ｄ５０で表して、１００μｍ以下であることが好ましく、７０μｍ以下であることがより好ましく、５０μｍ以下であることが更に好ましい。このような粒径の粒子を含むことによって、粒子どうしが隙間なく配された塗膜を形成することができ、緻密な構造を有する接合部を形成でき、被接合材どうしの高い接合強度を発現できる。フレーク状とは、粒子の主面を形成している一対の板面と、これらの板面に直交する側面とを有する形状を指し、板面及び側面はそれぞれ独立して、平面、曲面又は凹凸面でありうる。
　接合用組成物に含まれる還元剤は、１気圧、室温（２５℃）において固体であることが好ましい。このような還元剤は、該組成物の焼成による金属粒子どうしの焼結を促進させて、高い接合強度を発現させるために用いられる。この目的のために、還元剤は少なくとも１個のアミノ基及び複数の水酸基を有する化学構造のものであることが有利である。「室温（２５℃）において固体」とは、還元剤の融点が２５℃超であることを指す。以下、固体状の還元剤を「固体還元剤」ともいう。
　固体還元剤の融点は、金属粉の焼結温度以下であることが好ましい。また、固体還元剤の沸点は、後述する液媒体の沸点よりも高いことも好ましい。このような物性を有する固体還元剤を用いることによって、接合用組成物の塗膜を乾燥させて乾燥塗膜を得る際に、固体還元剤を接合用組成物中に固体として残留させることができ、その結果、該接合用組成物の乾燥塗膜の保形性を高くすることができる。
　加圧接合を行ったときの高い接合強度を効率的に発現させる観点、及び、被接合材として導電体を用いたときに、加圧接合後の高い導電信頼性を発現させる観点から、固体還元剤として、以下の化学式（２）又は（３）で表されるアミノアルコール化合物を用いることが好ましい。
Figure JPOXMLDOC01-appb-C000003
　化学式（２）又は（３）中、Ｒ１ないしＲ６は、それぞれ独立に、水素原子、水酸基、炭素原子数１以上１０以下の炭化水素基、又は水酸基を有する炭素原子数１以上１０以下の炭化水素基を表す。また、式（２）中、Ｒ７は、炭素原子数１以上１０以下の炭化水素基、又は水酸基を有する炭素原子数１以上１０以下の炭化水素基を表す。炭化水素基としては飽和又は不飽和の脂肪族基が挙げられる。この脂肪族基は直鎖状のものであってもよく、あるいは分岐鎖状のものであってもよい。
　化学式（２）又は（３）で表されるアミノアルコール化合物の具体例としては、ビス（２－ヒドロキシエチル）イミノトリス（ヒドロキシメチル）メタン（ＢＩＳ－ＴＲＩＳ、融点：１０４℃、沸点：３００℃超、化学式（２）に該当）、２－アミノ－２－（ヒドロキシメチル）－１，３－プロパンジオール（ＴＲＩＳ、融点：１６９～１７３℃、沸点：３００℃超、化学式（２）に該当）、１，３－ビス（トリス（ヒドロキシメチル）メチルアミノ）プロパン（ＢＩＳ－ＴＲＩＳ　ｐｒｏｐａｎｅ、融点：１６４～１６５℃、沸点：３００℃超、化学式（３）に該当）などが挙げられる。これらのうち、金属粒子どうしの焼結性を高めつつ、高い接合強度を発現する接合体を得る観点から、固体還元剤としてビス（２－ヒドロキシエチル）イミノトリス（ヒドロキシメチル）メタンを用いることが好ましい。
　上述した固体還元剤は、一種を単独で用いることができ、あるいは二種以上を組み合わせて用いることができる。いずれの場合であっても、接合用組成物における固体還元剤の割合は、金属粒子どうしの焼結性を高める観点から、金属粉１００質量部に対して０．１質量部以上であることが好ましく、１質量部以上であることが更に好ましい。また、接合用組成物中に占める金属粉の割合を維持しつつ、被接合材への好適な塗布性能を発揮する観点から、１０質量部以下とすることが現実的であり、８質量部以下とすることが好ましく、５質量部以下とすることが更に好ましい。
　塗膜の塗布性を高める観点から、接合用組成物は、液媒体を更に含むことが好ましい。液媒体は、接合用組成物の塗布性、固体還元剤の溶解性、及び液媒体の適度な揮発性に伴う乾燥塗膜の形成効率の向上を兼ね備える観点から、非水溶媒であることが好ましく、一価又は多価のアルコールであることがより好ましく、多価アルコールであることが更に好ましい。
　多価アルコールとしては、例えばプロピレングリコール、エチレングリコール、ヘキシレングリコール、ジエチレングリコール、１，３－ブタンジオール、１，４－ブタンジオール、ジプロピレングリコール、トリプロピレングリコール、グリセリン、ポリエチレングリコール２００、ポリエチレングリコール３００などが挙げられる。液媒体は一種を単独で又は二種以上を組み合わせて用いることができる。
　接合用組成物に液媒体を含む場合、接合用組成物を被接合材上に塗布したときの塗膜の保形性と、形成した塗膜の厚さの均一性を高める観点から、液媒体の含有量は、金属粉１００質量部に対して１０質量部以上４０質量部以下であることが好ましく、１０質量部以上３５質量部以下であることが更に好ましい。
　接合用組成物は、本発明の効果が奏される限りにおいて、例えばバインダー成分、表面張力調整剤、消泡剤、粘度調整剤などの他の成分を含んでいてもよい。他の成分の割合は、その総量が、金属粉１００質量部に対して０．１質量部以上１０質量部以下であることが好ましい。
　このような接合部を有する接合体は、その高い接合強度や熱伝導性の特性を活かして、高温に曝される環境、例えば車載用電子回路やパワーデバイスが実装された電子回路に好適に用いられる。
　以下、実施例により本発明を更に詳細に説明する。しかしながら本発明の範囲は、かかる実施例に制限されない。特に断らない限り、「％」及び「部」はそれぞれ「質量％」及び「質量部」を意味する。
　　〔実施例１〕
（１）接合用組成物の調製
　銅粉として、ＤＳＥＭ５０が０．１４μｍの球状銅粉と、Ｄ５０が４．９μｍでアスペクト比（粒子の厚みに対する主面の長さの比）が１３のフレーク状銅粉との混合物を用いた。銅粉の混合物中に占める球状銅粉とフレーク状銅粉の含有割合は、球状銅粉７０質量％：フレーク状銅粉３０質量％とした。
　還元剤としてビス（２－ヒドロキシエチル）イミノトリス（ヒドロキシメチル）メタンを、銅粉１００部に対して２．５部の割合で用いた。
　液媒体として、ポリエチレングリコール３００（銅粉１００部に対して１部）とヘキシレングリコール（銅粉１００部に対して３１．６部）との混合物を用いた。
　上述した銅粉、還元剤及び液媒体を混合してペースト状の接合用組成物を得た。
　せん断速度１０ｓ－１における接合用組成物の粘度は２５℃において３４Ｐａ・ｓであった。
（２）接合構造体の製造
　第２の被接合材として、２０ｍｍ四方の平面視正方形の銅板（厚み１ｍｍ）の中央に、スクリーン印刷によって接合用組成物を塗布して塗膜を形成した。塗膜は、厚み１００μｍのメタルマスクを用いて、銅板の中央部を含む領域に、６ｍｍ四方の正方形に形成した。この塗膜を熱風乾燥機中、１１０℃、１０分で乾燥させて液媒体を一部除去し、室温下に放置して、乾燥塗膜を得た。また、乾燥塗膜中の液媒体の含有量を確認した結果、５質量％以下であった。
　次いで、第１の被接合材として、Ａｇメタライズによって形成された銀層を表面全域に有する４．９ｍｍ四方の平面視正方形の半導体素子（ＳｉＣチップ、厚み０．３８ｍｍ、Ｗｏｌｆｓｐｅｅｄ社製、ＣＰＷ５－１２００－Ｚ０５０Ｂ）を乾燥塗膜上に載置し、積層体とした。この状態下に２０ＭＰａの圧力を厚み方向に加え、窒素雰囲気下、昇温速度９６０℃／分で４０℃から２８０℃まで昇温し、その後、２８０℃で５分間焼成を行い、図１（ａ）及び（ｂ）に示す構造を有する接合構造体を作製した。この接合構造体は、各被接合材と接合部とが平面視全域で隣接して形成されていた。また乾燥塗膜の焼結体である接合部は、前記の構造（１）で表される化学構造が形成されていることを、ＴＯＦ－ＳＩＭＳによる質量分析によって確認した。
　　〔実施例２〕
　接合構造体の製造において、焼結時の圧力付与を１０ＭＰａに変更した以外は、実施例１と同様の方法で、図１（ａ）及び（ｂ）に示す構造を有する接合構造体を得た。接合部は、前記の構造（１）で表される化学構造が形成されていることを、ＴＯＦ－ＳＩＭＳによる質量分析によって確認した。
　　〔比較例１〕
　接合構造体の製造において、焼結時の圧力付与を６ＭＰａに変更した以外は、実施例１と同様の方法で接合構造体を得た。接合部は、前記の構造（１）で表される化学構造が形成されていることを、ＴＯＦ－ＳＩＭＳによる質量分析によって確認した。
　〔接合割合の測定〕
　実施例及び比較例の接合構造体について、株式会社システムインフロンティア製のソフトウェアＭｅａｓｕｒｅｍｅｎｔＡｄｖｉｓｅｒを用いて、被接合材と接合部との界面における各接合割合Ｒｃ，Ｒｓ並びにこれらの比Ｒｓ／Ｒｃを、上述した方法でそれぞれ測定及び算出した。その結果を測定視野の全長Ｌｔとともに以下の表１に示す。
　なお説明の便宜上、以下の表中では、ＳｉＣチップ（第１の被接合材１１）と接合部１５との間の接合割合のみを示している。
〔接合信頼性の評価〕
　接合構造体に対して、冷熱サイクル試験（ＴＣＴ）を１０サイクル行い、そのときの焼結体からの剥離の状態を超音波探傷装置（日立パワーソリューションズ社製、型番：ＦｉｎｅＳＡＴ III）を用い、７５ＭＨｚのプローブにて反射法によって、銅板の裏面側から観察した。ＴＣＴでは、（１）－４０℃・１５分間、（２）＋１２５℃・１５分間、を１サイクルとした。
　接合層の剥離状態を観察する際、ゲインの値を２５～３５ｄＢの値にした後、Ｓゲートのピーク位置が銅板の表面となるようＳゲートのディレイと幅を調節した。接合層の観察範囲を指定するためＦゲートのディレイを調整し、幅を１．５波長分のピーク幅に設定した。観察ピークの振幅が最大となるようプローブのＺ軸座標を調整し、観察を行った。観察像のコントラストはオート機能を用いて調節した。
　上述の装置では、接合状態が良好であるものは色が濃く（黒色に）観察され、クラックや剥離が発生し接合状態が悪い領域は色が薄く（白色に）観察される。このうち、実施例１及び比較例１のＴＣＴ前後の画像データを図３及び図４に示す。
　また、得られた画像データを画像処理ソフトウェアＩｍａｇｅ－Ｊを用いて二値化し、観察した面積中における黒色の面積割合（ＴＣＴ１０サイクル後の接合率；％）を算出した。すなわち、Ｉｍａｇｅ－Ｊを起動した後、Ａｎａｌｙｚｅ－Ｓｅｔ　ｍｅａｓｕｒｅｍｅｎｔを選択し、Ａｒｅａ、Ａｒｅａ ｆｒａｃｔｉｏｎ、Ｌｉｍｉｔ ｔｏ Ｔｈｒｅｓｈｏｌｄにチェックを入れた。その後、Ｆｉｌｅ－Ｏｐｅｎを選択し、接合率を算出する画像データを開いた後に、画像中のＳｉＣチップ搭載部の範囲（Ａ）を指定した。次いで、Ｅｄｉｔ－Ｃｏｐｙ ｔｏ ｓｙｓｔｅｍを選択し、指定した範囲（Ａ）をコピーした後、Ｆｉｌｅ－Ｎｅｗ－Ｓｙｓｔｅｍ ｃｌｉｐｂｏａｒｄを選択して指定した範囲（Ａ）の画像を貼り付けた。その後、接合部を明確にするために、Ｉｍａｇｅ－Ｔｙｐｅ－８ｂｉｔを選択し、画像を変換した後、Ｉｍａｇｅ－Ａｄｊｕｓｔ－Ｔｈｒｅｓｈｏｌｄを選択して画像の閾値を１１０に調整した。そして、調整後の画像におけるＳｉＣチップ搭載部の範囲（Ａ）内に存在する赤色範囲（Ｂ）を指定した。前記赤色範囲（Ｂ）がＳｉＣチップ接合部となり、その接合率は（Ｂ）／（Ａ）×１００で算出される。接合率が高いほど、過度な温度変化が発生した場合でも接合信頼性が高いことを示す。結果を表１に示す。
Figure JPOXMLDOC01-appb-T000004
　本発明によれば、高い接合信頼性を有する接合構造体が提供される。

Claims (3)

  1. 　２つの被接合材と、これらの被接合材間に隣接して形成された接合部とが接合されてなる接合構造体であって、
    　前記接合部は、銅を主体とする材料からなり、
    　前記接合構造体の厚み方向断面視において、該接合構造体の中央域における前記被接合材と前記接合部との接合割合Ｒｃに対する、該接合構造体の端部域における前記被接合材と前記接合部との接合割合Ｒｓの比（Ｒｓ／Ｒｃ）が、０．６以上０．９以下であり、
    　前記接合割合Ｒｃが０．３以上である、接合構造体。
  2. 　前記被接合材の少なくとも一方は、その表面に金、銀、銅、ニッケル及びチタンの少なくとも一種の金属を有する、請求項１に記載の接合構造体。
  3. 　前記接合部に以下の式（１）で示される構造が形成されている、請求項１又は２に記載の接合構造体。
    Figure JPOXMLDOC01-appb-C000001
     　（式（１）中、Ｒ３ないしＲ５は、それぞれ独立に、水素原子、水酸基、炭素原子数１以上１０以下の炭化水素基、又は水酸基を有する炭素原子数１以上１０以下の炭化水素基を表し、
    　＊は、銅との結合部位を表す。）
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US20240149340A1 (en) 2024-05-09
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