Composite Substrate, Method for Manufacturing the Same, and Semiconductor Device Comprising the Composite Substrate

Abstract

To provided a composite substrate and a method for manufacturing the same.SOLUTION: A composite substrate 14 is formed from a composite material containing diamond particles 10 and metal, and has a base layer 180 with a first surface 180A and a second surface 180B opposite the first surface, a bottom surface 82B bonded to the first surface of the base layer, a flat layer 82 with a top surface 82A having a surface roughness Ra of 10 nm or less, and an insulating layer 84 bonded directly to the top surface of the flat layer.SELECTED DRAWING: Figure 4A

Description

Technical Field

[0001] The present disclosure relates to a composite substrate containing diamond and metal, a method for manufacturing the same, and a semiconductor device including the composite substrate.

Background Art

[0002] As the light output of semiconductor light-emitting elements increases, heat dissipation of semiconductor elements has become an important issue. Heat dissipation of semiconductor elements is an important issue not only for semiconductor light-emitting elements but also for semiconductor integrated circuit elements that execute arithmetic processing at high speed and monolithic microwave integrated circuit elements that oscillate microwaves.

[0003] In order to dissipate heat from semiconductor elements that generate heat during operation in this way, the development of heat dissipation members such as composite substrates that dissipate the heat of semiconductor elements has been promoted. The heat dissipation member is required to have high thermal conductivity. As a promising material for the composite substrate, the development of a composite material containing diamond and metal has been promoted. In this composite material, diamond particles having a higher thermal conductivity than metal are dispersed in a metal such as copper (Cu).

[0004] Patent Document 1 discloses an example of a composite material in which diamond particles are dispersed in Cu. Patent Document 2 discloses a method for producing a composite material by coating diamond particles with a metal layer and then sintering with Cu powder. Patent Document 3 discloses a heat dissipation substrate in which a metal layer is formed by plating on an alloy composite mainly composed of diamond powder, metal, and additive powder.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] The present disclosure provides a new composite substrate, a method for manufacturing the same, and a semiconductor device including the composite substrate.

MEANS FOR SOLVING THE PROBLEMS

[0007] In an exemplary embodiment, the composite substrate of the present disclosure is formed of a composite material including diamond and a metal, and includes a base layer having a first surface and a second surface on the opposite side of the first surface, a lower surface bonded to the first surface of the base layer, a flat layer having an upper surface with a surface roughness Ra of 10 nm or less, and an insulating layer directly bonded to the upper surface of the flat layer.

[0008] The semiconductor device of the present disclosure includes the above composite substrate and a semiconductor element supported on the insulating layer of the composite substrate and electrically connected to the substrate.

[0009] In an exemplary embodiment, the method for manufacturing the composite substrate of the present disclosure includes preparing a laminate having a base layer formed of a composite material including diamond and a metal, having a first surface and a second surface on the opposite side of the first surface, a lower surface bonded to the first surface of the base layer, and a flat layer having an upper surface with a surface roughness Ra of 10 nm or less, and directly bonding an insulating layer to the upper surface of the flat layer.

ADVANTAGES OF THE INVENTION

[0010] It is possible to provide a new composite substrate, a method for manufacturing the same, and a semiconductor device including the composite substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]

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DETAILED DESCRIPTION OF THE INVENTION

[0012] Before describing the embodiments of the present disclosure, the findings discovered by the present inventors and the technical background thereof will be described.

[0013] FIG. 1 is a graph plotting the coefficient of thermal expansion and the thermal conductivity for materials such as diamond and Cu. The horizontal axis of the graph is the coefficient of thermal expansion, and the vertical axis is the thermal conductivity. The coefficient of thermal expansion is the rate of thermal expansion per unit length, and the unit is [10 -6 / K]. The unit of thermal conductivity is [W / m·K (watts per meter per kelvin)].

[0014] Single-crystal diamond has a high thermal conductivity among substances and ideally shows an extremely high value exceeding 2000 [W / m·K]. The thermal conductivity of polycrystalline diamond particles is, for example, about 900 to 1800 [W / m·K]. The thermal conductivity of diamond particles produced by the CVD method or the like is, for example, about 900 to 1800 [W / m·K].

[0015] FIG. 2 is a perspective view schematically showing an example of diamond particles 10. The diamond particles 10 in the example of FIG. 2 have, for example, a polyhedral shape having hexagonal and square faces (facets) on the surface. In actual diamond particles, other polygonal faces may appear on the surface or a part may be missing, and they may have a more complex and diverse shape. The thermal conductivity of individual diamond particles may have different values depending on the presence of crystal defects or impurities present on the surface or inside.

[0016] On the one hand, the thermal conductivity of Cu, which is a metal, is about 400 [W / m·K], the thermal conductivity of silver (Ag) is about 420 [W / m·K], and that of aluminum (Al) is about 235 [W / m·K]. Therefore, the thermal conductivity of a composite material in which diamond particles are dispersed in a metal such as Cu shows an intermediate value between the thermal conductivity of the metal and that of the diamond particles. The higher the volume ratio of the diamond particles contained in the composite material, the higher the theoretical value of the thermal conductivity of the composite material. However, in reality, the thermal conductivity of the composite material is not simply determined by the volume ratio of diamond, and it is considered that the state of the interface between Cu and the diamond particles also has an influence. The interface between Cu and diamond can vary due to defects or scratches that can occur in the diamond particles during the manufacturing process.

[0017] When a semiconductor element is bonded to a substrate and used, if the difference in the coefficient of thermal expansion between the member in contact with the composite substrate on the semiconductor element side and the composite substrate is large, problems such as peeling may occur. Therefore, it is desirable that the coefficient of thermal expansion of the composite material used for the composite substrate be close to the coefficient of thermal expansion of the member to be bonded.

[0018] A composite material containing diamond particles and Cu has excellent properties of having a higher thermal conductivity than Cu and being close to the coefficient of thermal expansion of a semiconductor. Note that a composite material with a metal as a matrix is sometimes called a metal matrix composite (MMC). Therefore, in this specification, a composite material in which diamond particles are dispersed in Cu may be referred to as "Cu-diamond MMC". It may also simply be referred to as "Cu diamond composite material" or "composite material".

[0019] Examples of the material for the insulating layer provided on the composite material include aluminum nitride (AlN), silicon carbide (SiC), alumina (Al2O3), silicon nitride (Si3N4), and gallium nitride (GaN). Since the insulating layer provided on the composite material constitutes a part of the composite substrate and is located on the heat dissipation path, it is preferably formed of a material having a high thermal conductivity. Examples of insulating materials having a relatively high thermal conductivity are GaN, ceramic AlN, and SiC. However, there are the following problems in joining an insulating layer formed of such a material to the above diamond and metal composite material.

[0020] Generally, methods for joining two members include, for example, joining with an inorganic adhesive, joining with an intermediate layer interposed between the joining surfaces, and "direct joining" in which solid members are directly joined. Joining with an intermediate layer interposed between the joining surfaces uses an intermediate layer that changes from a liquid phase state to a solid phase state, such as brazing with a metal solder. Joining with an intermediate layer interposed between the joining surfaces can be performed even if there are fine irregularities on the joining surfaces of the two members to be joined. On the other hand, for direct joining, flatness is required such that the surface roughness (Ra) of the joining surface is 10 nm or less. If such flat surfaces are used, joining can be performed at room temperature. Direct joining includes surface activation joining, atomic diffusion joining, and the like.

[0021] In joining using a metal solder, it is necessary to heat the metal solder at a temperature equal to or higher than the melting point of the metal solder. In this case, there is a problem that the insulating layer is likely to peel off due to the difference in the coefficient of thermal expansion between the composite material and the material of the insulating layer. In addition, a composite material containing diamond particles and metal is generally formed by a sintering process, but the flatness of the surface of the sintered composite material may be insufficient when direct joining is performed. Moreover, since diamond particles are present on the surface of such a composite material or in its vicinity, even if grinding or polishing is performed, the diamond particles in the composite material may be degranulated or broken, and irregularities on the order of the particle size of the diamond particles may occur on the polished surface or the ground surface. For this reason, it is difficult to directly join, for example, an insulating layer made of an insulating ceramic to the polished surface of the composite material by direct joining.

[0022] Hereinafter, embodiments of the composite substrate, its manufacturing method, and the semiconductor device in the present disclosure will be described.

[0023] <Embodiment> First, with reference to FIGS. 3, 4A, and 4B, a configuration example of a composite substrate in an embodiment of the present disclosure will be described. FIG. 3 is a perspective view of a composite substrate 14 and a semiconductor element 12 in an embodiment of the present disclosure. FIG. 4A is a cross-sectional view schematically showing a configuration example of the composite substrate 14, and FIG. 4B is a cross-sectional view schematically showing a state before direct bonding.

[0024] As shown in FIG. 3, the composite substrate 14 includes a base layer 180, a flat layer 82, and an insulating layer (insulating member) 84. In the example of FIG. 3, a semiconductor element 12 is disposed on the insulating layer 84 of the composite substrate 14.

[0025] Next, referring to FIGS. 4A and 4B. The base layer 180 of the composite substrate 14 is formed of a composite material 80 containing diamond particles 10 and a metal 70, and has a first surface 180A and a second surface 180B on the opposite side of the first surface 180A. The specific structure of the composite material 80 constituting the base layer 180 of the composite substrate 14 and its manufacturing method are not particularly limited, but the composite material 80 is preferably Cu-diamond MMC, Al-diamond MMC, or Ag-diamond MMC having excellent thermal conductivity. Details of the structure and manufacturing method of the composite material 80 that can be used in an embodiment of the present disclosure will be described later.

[0026] The flat layer 82 has a lower surface 82B bonded to the first surface 180A of the base layer 180 and an upper surface 82A with a surface roughness Ra of 10 nm or less. Details of the method for forming the flat layer 82 will also be described later. The insulating layer 84 is directly bonded to the upper surface 82A of the flat layer 82. More specifically, the insulating layer 84 has an upper surface 84A and a lower surface 84B with a surface roughness of 10 nm or less, and the lower surface 84B is directly bonded to the upper surface 82A of the flat layer 82.

[0027] In one embodiment, the flat layer 82 is formed by including one or more materials selected from the group consisting of Cu, Ag, Au, Al, CuW, CuMo, AlN, SiN, and SiO2. In a preferred embodiment, the flat layer 82 is formed of a material having a coefficient of thermal expansion that is at least twice that of the coefficient of thermal expansion of the composite material 80. Since the flat layer and the insulating layer are directly bonded, even if the difference in the coefficient of thermal expansion is at least twice, they can be bonded without the need for intentional heating. Many such materials have high thermal conductivity, and thus, the decrease in thermal conductivity due to providing the flat layer can be reduced. Examples of materials having a coefficient of thermal expansion that is at least twice that of the coefficient of thermal expansion of the composite material include Cu, Ag, Au, and Al. The thickness of the flat layer 82 is, for example, 50 μm or more and 1000 μm or less. Preferably, it is 50 μm or more and 500 μm or less, and more preferably, it is 50 μm or more and 200 μm or less. Within these ranges, the decrease in thermal conductivity due to providing the flat layer can be reduced.

[0028] In one embodiment, the insulating layer 84 is preferably made of a material with excellent thermal conductivity. The insulating layer 84 is formed of, for example, GaN, AlN of ceramics, or SiC. The thickness of the insulating layer 84 is, for example, 200 μm or less. If the insulating layer 84 is too thick, it becomes difficult to exhibit the heat dissipation function of the composite substrate 14. Also, if the insulating layer 84 is too thin, sufficient electrical insulation cannot be ensured. The thickness of the insulating layer 84 is less than 200 μm, preferably 150 μm or less, more preferably 100 μm or less, and particularly preferably 50 μm or less. If the thickness of the insulating layer 84 is within the above range, the decrease in the thermal conductivity of the composite substrate can be reduced. Also, the insulating layer 84 is 5 μm or more. By setting it to 5 μm or more, electrical insulation can be ensured. Further, for example, when it is necessary to form an insulating layer 84 with a thickness of 100 μm or less, after manufacturing the composite substrate, an insulating layer 84 having a desired thickness can be formed by polishing or the like in order to improve the handleability of the insulating layer in the manufacturing process. However, it is also possible to prepare an insulating layer 84 having a desired thickness in advance and directly bond it to the flat layer.

[0029] The method of "direct bonding" is not limited to a specific method. For example, either surface activation bonding or atomic diffusion bonding can be adopted. In surface activation bonding, an active surface state is realized and bonding is performed by cleaning the bonding surface in a vacuum. The cleaning is performed, for example, by irradiating the bonding surface with an ion beam. In atomic diffusion bonding, for example, after forming a metal layer having a thickness of 1 nm or more and 500 nm or less on each bonding surface, the respective bonding surfaces are brought into contact with each other in a vacuum or an inert gas, so that metal atoms diffuse into each other and the metal atoms can be directly bonded to each other. Note that before bonding, the bonding surfaces may be irradiated with argon plasma, an ion beam, or an atomic beam in a vacuum to perform surface cleaning and activation of each bonding surface, and then the bonding surfaces may be brought into contact with each other. According to such direct bonding, strong bonding can be realized by the bonding of atoms without using a bonding member such as an adhesive. Therefore, a decrease in thermal conductivity at the bonding surface can be suppressed.

[0030] When performing atomic diffusion bonding, direct bonding can be realized at a relatively low temperature, for example, a maximum temperature reached by heating due to the heat generated during operation of the semiconductor element 12 in thermal contact with the composite substrate 14, such as about 200 °C or less. However, when performing direct bonding at such a temperature, the surface roughness (Ra) of the bonding surface needs to be 10 nm or less, preferably 5 nm or less, and particularly preferably 1 nm or less. According to the direct bonding that can be performed at such a relatively low temperature, when bonding the flat layer 82 and the insulating layer 84, a high-temperature environment is not required, so the bonding interface is less affected by stress due to the difference in thermal expansion coefficients of the two, and high bonding strength can be obtained. Also, this means that when selecting the materials of the flat layer 82 and the insulating layer 84, there may be a relatively large difference in the thermal expansion coefficients of the two, which increases the degree of freedom in material selection. Specifically, when forming the insulating layer 84 using AlN, a ceramic having high thermal conductivity, according to the embodiment of the present disclosure, it is possible to form the flat layer 82 from Cu, Ag, Au, or Al having a thermal expansion coefficient of at least twice that of AlN.

[0031] Next, with reference to FIG. 5, an example of the direct bonding process will be described. Here, a form using atomic diffusion bonding as the direct bonding will be described. In the example of FIG. 5, a metal layer 90 is formed on the lower surface 84B of the insulating layer 84. Similarly, a metal layer 92 is formed on the upper surface 82A of the flat layer 82. The metal layers 90 and 92 are formed from an arbitrary metal material, but preferably can be formed from a metal such as Ti, Al, Au, Cu, or Ta, or an alloy thereof. The metal layers 90 and 92 can be formed on the lower surface 84B of the insulating layer 84 and the upper surface 82A of the flat layer 82, respectively, by vacuum evaporation, sputtering, or the like. The thickness of each of the metal layers 90 and 92 is, for example, 1 nm or more and 500 nm or less.

[0032] The metal layers 90 and 92 have a flat surface reflecting the flatness of the underlying metal layer and insulating layer. During the direct bonding process, between the metal layer 90 and the metal layer 92 that are in contact with each other, atoms diffuse into each other and the bonding progresses even at, for example, 200°C or lower, 50°C or lower, or 40°C or lower. In order to achieve such room-temperature bonding, the surfaces of the metal layers 90 and 92 need to be kept clean from immediately after formation until the start of direct bonding. For this reason, it is preferable that the formation process of the metal layers 90 and 92 and the subsequent direct bonding are continuously performed in a vacuum chamber shielded from the atmosphere. However, when Au is used as the material of the metal layers 90 and 92, bonding may be performed in the atmosphere. Au is a relatively stable metal and hardly forms a natural oxide film on the surface even in the atmosphere. Also, even if a natural oxide film is formed on the Au surface, it is possible to perform bonding while removing the natural oxide film by heating under conditions of 200°C or lower, which is a relatively low temperature as a semiconductor process, so it is easy to handle.

[0033] Next, with reference to FIG. 6, another example of the direct bonding process will be described. In the example of FIG. 6, the metal layer 90 formed on the lower surface 84B of the insulating layer 84 includes a stacked first metal layer 90A and a second metal layer 90B. A metal layer 92 is formed on the upper surface 82A of the flat layer 82. When the insulating layer 84 is formed from a sintered body such as ceramics, fine irregularities may exist on its lower surface 84B. A method can be selected such that the flatness of the surface of the metal layer 90 is improved by making the metal layer 90 multilayer. For example, a sputtering film formation method combined with ion etching, or chemical mechanical polishing (CMP) can be mentioned. The first metal layer 90A is preferably formed from, for example, Ti and Cu. Cu has excellent thermal conductivity, and Ti has excellent adhesion to a sintered body such as Cu and AlN. Note that, similar to the metal layer 90, the metal layer 92 may also be made multilayer.

[0034] In order to realize the above direct bonding, in the embodiment of the present disclosure, the flat layer 82 plays an important role. The flat layer 82 in the embodiment is formed from a metal material such as Cu as described above, but in the embodiment of the present disclosure, the bonding between the flat layer 82 and the composite material 80 is realized by sintering. Note that, so far, the case of directly bonding the composite material 80 and the flat layer 82, and the insulating layer 84 using atomic diffusion bonding has been described, but the direct bonding may also use the above-described surface activation bonding method. That is, the surface roughness of the upper surface 82A of the flat layer 82, which is the bonding surface, and the lower surface 84B of the insulating layer are each made 10 nm or less, and these bonding surfaces are cleaned by an ion beam in a vacuum. By overlapping the cleaned bonding surfaces in a vacuum, the composite substrate 14 can be obtained.

[0035] Hereinafter, an embodiment of a method for manufacturing a composite substrate according to the present disclosure will be described.

[0036] First, refer to FIG. 7A. FIG. 7A is a flowchart showing the main steps in a method for manufacturing a composite substrate according to an embodiment of the present disclosure. The method for manufacturing a composite substrate according to this embodiment is formed from a composite material containing diamond and metal, and includes a base layer having a first surface and a second surface on the opposite side of the first surface, a lower surface bonded to the first surface of the base layer, and a flat layer having an upper surface with a surface roughness Ra of 10 nm or less. A step S400 of preparing a laminate is executed. The flat layer is bonded to the first surface of the base layer by sintering. A specific example of a method for manufacturing such a laminate will be described later. Next, a step S430 of directly bonding an insulating layer to the upper surface of the flat layer is executed. In the step of preparing the laminate, the surface of the metal layer in the laminate may be polished to form a flat layer having an upper surface with a surface roughness Ra of 10 nm or less.

[0037] Next, referring to FIG. 7B, an example of the step of preparing a laminate having the base layer and the metal layer of the composite material described above will be described. FIG. 7B is a flowchart showing the step of preparing a laminate in an embodiment of the present disclosure. As shown in FIG. 7B, the step of preparing the laminate includes a step S100 of preparing a composite material containing diamond and Cu by first pulse electric current sintering, and a step S300 of forming a metal layer on the base layer by second pulse electric current sintering. Details of the pulse electric current sintering will be described later.

[0038] The method for preparing the laminate is not limited to such an example. FIG. 7C is a flowchart showing another example of the step of preparing the laminate. The step of preparing the laminate in this example includes, as shown in FIG. 7C, a step S100 of preparing a composite material containing diamond and Cu by first pulse electric current sintering, a step S200 of thinning the surface of the composite material to form a processed surface on the composite material, and a step S300 of forming a metal layer on the base layer by second pulse electric current sintering. Hereinafter, the pulse electric current sintering performed after such a polishing step may be referred to as "re-pulse electric current sintering" to distinguish it from the pulse electric current sintering performed to manufacture the composite material.

[0039] Next, the process S100 of preparing the base layer of the composite material in this embodiment will be described in detail. FIG. 7D is a flowchart showing a specific example of the process S100 of preparing the composite material according to this embodiment. In this embodiment, various composite materials manufactured by known methods can be used, and for example, commercially available composite materials may be used. Examples of methods for manufacturing composite materials include a sintering method, a melting method, and the like. A preferred method for manufacturing composite materials is the pulse electric current sintering method. With this method, as will be described later, a composite material can be obtained under relatively low-temperature conditions compared to the prior art.

[0040] As shown in FIG. 7D, this process S100 includes a process S10 of preparing a mixed powder of diamond particles and Cu powder particles, and a process S20 of generating a composite material containing diamond and Cu from the mixed powder by holding the mixed powder at a temperature of 500° C. or higher and lower than 800° C. while applying pressure by the pulse electric current sintering method. This "temperature" is the "temperature" measured directly or indirectly by a temperature measuring device such as a radiation thermometer or a thermocouple. The "temperature" in the present disclosure is the measured value of the temperature in the die 30 of the SPS device as shown in FIG. 9 to be described later. Note that the sintering peak temperature Ts to be described later means the temperature that is held for a total of 1 minute or more at a predetermined pressure among the temperatures of the die 30 measured by a radiation thermometer or a thermocouple. For example, in the case of FIG. 10, in the sintering process, the temperature held at the highest temperature is called the sintering peak temperature Ts, and is distinguished from the temperature that transiently changes during the temperature rise and fall.

[0041] FIG. 8 is a photograph showing an example of diamond particles contained in the mixed powder in step S10. In one embodiment, the average particle size of the diamond particles is 40 μm or more and 500 μm or less. The particle size distribution of the diamond particles does not have to be a unimodal type with a single peak, and there may be a plurality of peaks. By setting the average particle size of the diamond particles to 40 μm or more, the thermal conductivity can be increased while manufacturing the Cu-diamond MMC at low cost. By setting the average particle size of the diamond particles to 500 μm or less, the manufacturing cost of the diamond particles themselves can be reduced. Further, the particle size distribution of the diamond particles is preferably 200 μm or more and 400 μm or less. If it is in this range, the thermal conductivity can be further improved. In one embodiment, the diamond particles in the mixed powder may include particles having an average particle size of 200 μm or more and 500 μm or less, and particles having an average particle size of 40 μm or more and 80 μm or less. In this specification, particles having an average particle size of 200 μm or more and 500 μm or less are referred to as "large particles", and particles having an average particle size of 40 μm or more and 80 μm or less are referred to as "small particles". Since particles having a relatively small average particle size can be arranged so as to fill the gaps formed by particles having a relatively large average particle size, the filling amount of diamond having a thermal conductivity higher than that of Cu can be increased. Thereby, the thermal conductivity of the Cu-diamond MMC can be further improved. Such a mixed powder having a bimodal particle size distribution is referred to as a "bimodal mixed powder" in this specification. In the bimodal mixed powder, the ratio of particles having an average particle size of 200 μm or more and 400 μm or less to the total diamond particles is preferably 50% or more by mass ratio. This is because the larger the number of particles having a large average particle size, the smaller the total surface area of the diamond particles, and the smaller the contribution of the thermal resistance at the interface between the diamond particles and Cu.

[0042] The average particle size of the Cu powder particles is, for example, 3 μm or more and 10 μm or less. The Cu powder may contain fine Cu powder with a particle size of 1 μm or less that can be generated by pulverization or the like. The Cu powder particles may contain inevitable impurities. However, since impurities such as oxygen and nitrogen cause a decrease in thermal conductivity, it is desirable to remove them as much as possible. The content of impurities in the Cu powder particles is preferably 2% by mass or less. Such Cu powder particles may be those manufactured by known manufacturing methods. Also, commercially available Cu powder particles may be used.

[0043] Note that the "average particle size" in the present disclosure means the "median diameter" in the particle size distribution measured by a laser diffraction particle size distribution measuring device.

[0044] In the embodiments of the present disclosure, no metal other than the Cu powder particles is intentionally added to enhance wettability. Conventionally, since the wettability between Cu and diamond is poor, it has been considered necessary to improve the wettability by adding a metal. However, in the embodiments of the present disclosure, such an added metal is unnecessary. By not intentionally adding a metal other than the Cu powder particles, sintering inhibition can be reduced. In one embodiment, the addition amount of a metal other than Cu in the mixed powder is 1% or less by mass ratio.

[0045] Also, for the purpose of enhancing the adhesion between the diamond particles and the Cu powder particles, there is a possibility that the individual diamond particles are coated with a metal layer such as Cu in advance as described above. However, in the embodiments of the present disclosure, the diamond particles are not coated with a metal layer in advance. This is because by not coating the diamond particles with a metal layer such as Cu in advance, the sintering activity can be enhanced as compared with the case where the diamond particles are coated with a metal layer such as Cu in advance. When the diamond particles are coated with a metal layer such as Cu in advance, the sintering activity decreases, and firing at a high temperature is required as in the case of adding a metal.

[0046] In this embodiment, the mass ratio of the Cu powder particles in the mixed powder is preferably, for example, 60% or more and 85% or less, and the mass ratio of the diamond particles in the mixed powder is preferably, for example, 15% or more and 40% or less. In other words, the mass ratio of the Cu powder particles to the diamond particles is selected within the range of 60:40 to 85:15, for example, 80:20. As the mass ratio of the diamond particles increases, the thermal conductivity of the composite material increases. By setting the mass ratio of the Cu powder particles to 60% or more and 85% or less, a sufficient amount of Cu is arranged around the diamond, and gaps that inhibit heat conduction are less likely to occur in the Cu-diamond MMC. In other words, if the mass ratio of the Cu powder particles is within the above range, Cu can have a volume sufficient to be arranged around the diamond, so that the gaps between the diamond particles can be effectively filled. Further, since the thermal conductivity of Cu is lower than that of diamond, if the mass ratio of Cu is too high, the thermal conductivity of the Cu-diamond MMC may decrease, but if it is within the above range, a Cu-diamond MMC having a high thermal conductivity can be manufactured. Further, as will be described later, according to the experiments of the present inventors, even when the mass ratio of the diamond particles is the same, the bimodal mixed powder is more likely to achieve a high thermal conductivity.

[0047] The pulse electric current sintering method in step S20 can be carried out using a sintering apparatus 100 as shown in FIG. 9, for example. The pulse electric current sintering method is sometimes referred to as a discharge plasma sintering method (spark plasma sintering: SPS) method. For this reason, the sintering apparatus 100 in FIG. 9 may be called an "SPS apparatus". The sintering apparatus 100 in FIG. 9 includes a die 30 having a through hole that forms a cavity 20, and an upper punch 40 and a lower punch 50 that can move relatively up and down along the through hole of the die 30. The sintering apparatus 100 in this example can perform sintering using self-heating under uniaxial vertical pressure. The upper punch 40 is electrically connected to the first electrode 51, and the lower punch 50 is electrically connected to the second electrode 52. The first electrode 51 and the second electrode 52 are electrically connected to a power supply unit 60.

[0048] The die 30 can be formed from a material with excellent heat resistance, such as graphite. The upper punch 40 and the lower punch 50 can be formed from a material having conductivity and heat resistance, such as graphite. In addition, in the measurement of thermal conductivity, the flatter the sample is, the higher the reliability of the measured thermal diffusivity can be. Therefore, when it is difficult to ensure the flatness of the sintered body to be manufactured, it is preferable to insert a hard material between the upper punch 40 and the sample and between the lower punch 50 and the sample to obtain a sintered body having a flat surface. The hard material is preferably cemented carbide, for example, tungsten carbide (WC) or titanium carbide (TiC). Also, the materials of the upper punch 40 and the lower punch 50 may be changed to hard materials. The cavity 20 is a space defined by the inner wall surface of the through hole of the die 30, the lower end surface of the upper punch 40, and the upper end surface of the lower punch 50. The above-mentioned mixed powder is loaded into the cavity 20. When at least one of the upper punch 40 and the lower punch 50 moves in the vertical direction, the distance between the upper punch 40 and the lower punch 50 decreases, and pressure is applied to the mixed powder in the cavity 20. The upper punch 40 and the lower punch 50 are driven by, for example, a hydraulic device (not shown). The pressure applied to the mixed powder in the cavity 20 can be arbitrarily adjusted, for example, in the range of 5 MPa or more and 100 MPa or less.

[0049] In the pulse electric current sintering method, while applying pressure to the mixed powder by the sintering apparatus 100, pulse electric current is passed between the upper punch 40 and the lower punch 50. The pulse electric current is applied by repeatedly passing a direct current pulse current from the power supply unit 60 between the first electrode 51 and the second electrode 52. In the mixed powder pressurized by the upper punch 40 and the lower punch 50, since the Cu powder particles are in contact with each other, several local current paths are formed in the mixed powder and the current flows. Due to such pulse electric current, Joule heat is generated, and the temperature of the mixed powder rises to a predetermined sintering temperature.

[0050] Figure 10 schematically shows an example of how the temperature and pressure of the mixed powder change over time from the start to the end of the pulse current sintering process. The temperature is indicated by a dashed line, and the pressure is indicated by a solid line. The horizontal axis represents time, the left vertical axis represents temperature, and the right vertical axis represents pressure. By moving the upper punch 40 and / or the lower punch 50, the pressure reaches the sintering pressure Ps in, for example, a few seconds, and the sintering pressure Ps is maintained for, for example, 60 to 1800 seconds. When the pulse current starts, the temperature of the mixed powder rises at a rate of, for example, 10 to 150 [°C / min] or more. After reaching the predetermined sintering peak temperature Ts, by adjusting the voltage application conditions of the pulse current, the sintering peak temperature Ts is maintained within ±5 °C of the target temperature. The control of the sintering temperature can be carried out by the feedback of the measurement by the temperature measuring device attached to the die 30 in Fig. 9.

[0051] After a predetermined sintering time has elapsed and the composite material is produced from the mixed powder, the pulse current is stopped and the temperature reduction is started. Also, by moving the upper punch 40 and / or the lower punch 50, the application of pressure to the composite material is stopped. After the temperature of the composite material has sufficiently decreased, for example, to 50 °C or less, the composite material is removed from the die 30.

[0052] In Fig. 10, the temperature and pressure are shown as rising and being maintained linearly over time for the sake of easy explanation. The actual transitions of the temperature and pressure may include slight overshoots, curved changes, small vibrations, etc.

[0053] In this embodiment, the voltage for maintaining the sintering temperature is adjusted in the range of 1.0 V or more and 3.0 V or less, and the pulse current is adjusted in the range of 400 A or more and 800 A or less. The duty ratio of the pulse current is, for example, 10 to 80%, and the pulse width is, for example, 1 to 500 milliseconds. After holding at the predetermined sintering peak temperature Ts for a predetermined time, the pulse current is stopped. Note that these values vary depending on various conditions such as the weight of the mixed powder, the sintering temperature, and the material of the punch, and are not limited to the above ranges and can be changed as appropriate.

[0054] When the sintering temperature is set to, for example, 600°C, the time from room temperature to reaching the sintering temperature is, for example, about several minutes to 20 minutes. The cavity 20 is located in the decompression chamber. The atmospheric pressure in the decompression chamber is, for example, 100 Pa or less. Thus, it is possible to prevent the powder particles from being oxidized or nitrided during the sintering process by pulse energization.

[0055] As described above, the sintering peak temperature Ts in the embodiment according to the present disclosure is 500°C or more and less than 800°C. Within this range, a Cu-diamond MMC having excellent thermal conductivity can be manufactured. Also, a preferable sintering peak temperature Ts is 500°C or more and 750°C or less. More preferably, it is 550°C or more and 700°C or less. Particularly preferably, it is 600°C or more and 700°C or less. Within the above range, a Cu-diamond MMC having further excellent thermal conductivity can be obtained. The range of the sintering time also depends on the sintering peak temperature Ts and is, for example, in the range of 1 minute or more and 30 minutes or less. When the sintering peak temperature Ts is, for example, 550°C or more and 650°C or less, the sintering time can be 5 minutes or more and 20 minutes or less, for example, about 10 minutes.

[0056] When the pressure applied to the mixed powder at the above sintering peak temperature Ts is maintained at a constant pressure, it is 5 MPa or more and 100 MPa or less. By setting the applied pressure to 5 MPa or more and 100 MPa or less, a Cu-diamond MMC having excellent thermal conductivity can be manufactured. The preferable pressure range is 10 MPa or more and 90 MPa. A more preferable pressure range is 20 MPa or more and 90 MPa or less. Even more preferably, it is 25 MPa or more and 75 MPa or less. Particularly preferably, it is 25 MPa or more and 50 MPa or less. Within these pressure ranges, a Cu-diamond MMC having excellent thermal conductivity can be manufactured. As described above, applying a constant pressure at the sintering peak temperature Ts is referred to as "continuous pressurization" in this specification.

[0057] So far, the case of continuous pressure application has been described. However, the above pressure does not necessarily have to be constant at all times. The applied pressure may be increased or decreased stepwise or continuously according to the progress of sintering. Also, while performing pulse energization, the first pressure and the second pressure higher than the first pressure may be repeatedly applied to the mixed powder. Such a pressure application form will be referred to as "cycle pressure application". In the above cycle pressure application, by repeatedly applying the first pressure and the second pressure higher than the first pressure, a Cu-diamond MMC having a high thermal conductivity can be manufactured. This is considered to be due to a mechanism as described below, for example.

[0058] There is a difference in the absolute value between the first pressure and the second pressure. When these pressures are repeatedly applied, there is a step of applying the first pressure after applying the second pressure. At this time, since the relatively low pressure is applied after the relatively high pressure, the degree of freedom of the diamond particles in the die increases compared to when the second pressure is applied. As a result, it is considered that the arrangement of the diamond particles or the state of the interface between Cu and the diamond particles can be more advantageous for heat conduction. There is also a possibility of removing oxygen present on the Cu surface that inhibits heat conduction. As a result, it is considered that the Cu-diamond MMC manufactured by cycle pressure application has an improved thermal diffusivity and a higher thermal conductivity compared to the case where only the first pressure is applied.

[0059] The first pressure and the second pressure applied to the mixture of the diamond particles and the Cu powder particles can be respectively set in the range of 5 MPa or more and 100 MPa or less under the condition that the second pressure is greater than the first pressure. For example, the first pressure may be 5 MPa or more and 60 MPa or less, and the second pressure may be 20 MPa or more and 100 MPa or less. Preferably, the first pressure is 5 MPa or more and less than 20 MPa, and the second pressure is 20 MPa or more and 40 MPa or less. By setting the first pressure and the second pressure within the above ranges, the thermal conductivity can be further improved.

[0060] FIG. 11A is a diagram showing an example of "cycle pressurization" during sintering. In this example, during the sintering time, the first pressure P1 and the second pressure P2 higher than the first pressure P1 are repeatedly applied to the mixed powder. As an example, when the first pressure P1 is 10 MPa and the second pressure P2 is 40 MPa, after applying the first pressure P1 for 10 seconds, the operation of applying the second pressure P2 for 20 seconds is defined as one cycle, and for example, the pressure application of 10 to 50 cycles can be repeated during the sintering process. As will be described later, by varying the pressure during the sintering time, the resulting composite material is densified and the relative density increases. This contributes to increasing the thermal conductivity of the composite material.

[0061] FIG. 11B is a diagram showing another example of "cycle pressurization" during sintering. As shown in FIG. 11B, the first pressure and the second pressure higher than the first pressure may be repeatedly applied to the mixed powder from the heating-up stage. Also, the first pressure and the second pressure can be repeatedly applied even during the holding time of the sintering peak temperature Ts.

[0062] FIG. 12 is a perspective view schematically showing an example of the outer shape of the composite material 80 manufactured according to the present embodiment. In this example, the composite material 80 has a disk shape with a thickness of T [mm] and a radius of R [mm]. The thickness T is, for example, 0.2 mm or more and 20.0 mm or less. Also, the radius R is, for example, 3 mm or more and 200 mm or less. The shape of the composite material 80 immediately after sintering is not limited to a disk shape, and it may have a rectangular parallelepiped or other polyhedron shape, or may have a shape with stripe grooves or a regular uneven pattern on the surface. The shape of the composite material 80 in a top view immediately after sintering is defined by the shape of the axial cross-section perpendicular to the cavity 20 in FIG. 9. For example, when the die 30 in FIG. 9 has a through-hole in the shape of a prism and the lower end surface of the upper punch 40 and the upper end surface of the lower punch 50 are flat rectangular surfaces, the composite material 80 taken out from the sintering apparatus 100 may have a thin plate shape with a rectangular top view. After being taken out from the sintering apparatus 100, the composite material 80 may be subjected to machining such as cutting and polishing, or various treatments such as laser processing. It is also possible to individualize a plurality of heat dissipation members from one composite material 80.

[0063] FIG. 13 is a schematic diagram showing an enlarged part of the cross-section of the composite material 80 manufactured in this way. FIG. 13 is based on cross-section observation by an optical microscope or a scanning electron microscope (SEM). The composite material 80 shown in FIG. 13 contains Cu70 which is a metal matrix, and a large number of diamond particles 10 dispersed in Cu70. Cu70 is a metal body formed by sintering and integrating Cu powder particles contained in the mixed powder by pulse energization. Each diamond particle 10 contained in the composite material 80 is a diamond particle contained in the mixed powder. Although a part of the diamond particles 10 may be chipped during the process of pulse energization sintering, the plurality of diamond particles do not combine and grow into a new single particle.

[0064] According to the above-described embodiments, by fabricating a Cu-diamond composite material at a lower temperature and a lower pressure compared to the prior art, the thermal conductivity of the composite material can be set to a value of, for example, 460 [W / m·K] or higher. Further, a composite material having a thermal conductivity of 500 [W / m·K] or higher can also be obtained. Furthermore, it is possible to set the value to 600 [W / m·K], preferably 690 to 710 [W / m·K] or higher. This is considered to be because by reducing the sintering temperature to a value lower than that conventionally considered necessary and reducing the pressure, it is possible to reduce or avoid the deterioration of the thermal conductivity at the diamond particles or the Cu / diamond interface that occurred during high-temperature and high-pressure sintering.

[0065] According to the above-described embodiments, in the process of cooling the Cu-diamond MMC to room temperature after pulse current sintering, the amounts of expansion and contraction of Cu and diamond respectively, which depend on the difference in the coefficient of thermal expansion, are small. That is, since Cu and diamond are less likely to peel due to the difference in the coefficient of thermal expansion, it is possible to reduce the decrease in thermal conductivity.

[0066] Referring again to FIG. 7C, step S200 will be described. In this step S200, the Cu-diamond composite material is thinned (processed to be thinner). The thinning process includes polishing and laser processing. The polishing can be performed by various polishing methods such as rough polishing, fine polishing, fixed abrasive grain method (grinding), and loose abrasive grain method (polishing).

[0067] "Polishing" in the present disclosure has a meaning including "grinding" in which the surface of the composite material 80 is scraped off by abrasive grains having a high hardness such as diamond. In the Cu-diamond composite material, since diamond is included, an ideally flat surface may not be formed even when polishing is performed. Due to the collision or friction with the abrasive grains, diamond particles are lost from the surface of the composite material, so that unevenness on the order of the size of the diamond particles may remain on the processed surface of the composite material even after polishing. Therefore, in the present disclosure, polishing and grinding are not distinguished, and the "polishing step" broadly means a step of performing polishing and / or grinding.

[0068] FIG. 14 is a cross-sectional view schematically showing the vicinity of the processed surface 80P of the composite material 80. The portion scraped off by the thinning process is marked with the reference numeral "80X". In the example of FIG. 14, the processed surface 80P has irregularities on the order of the size of the diamond particles 10. For example, when the thickness of the composite material 80 before the polishing process is 1 mm, the thickness of the composite material 80 after the polishing process can be, for example, 100 μm or more and 800 μm or less. Such a polishing process makes it possible to make the thickness of the composite material 80 immediately after pulse electric current sintering sufficiently larger than the thickness of the heat dissipation member finally required. According to the inventor's experiments, it was found that if the composite material 80 to be produced is too thin, the distribution of heat generated by pulse electric current sintering becomes non-uniform during the sintering process, etc., and the variation in the characteristics of the composite material 80 increases. Therefore, on the premise that the thickness is adjusted to the target value by the polishing process, if the composite material 80 thicker than the target value is formed, it becomes possible to suppress or reduce the variation in characteristics.

[0069] However, according to further studies by the present inventor, it was found that the thermal conductivity of the composite material 80 after the thinning process can decrease to, for example, about 80% of the thermal conductivity before the thinning process. This is due to the decrease in the relative density and thermal diffusivity of the composite material 80 due to the thinning process. The reason is considered to be, for example, when a polishing process is performed, stress is generated when a part of the composite material 80 is scraped off, and the thermal resistance at the interface between the diamond particles 10 and Cu70 increases. In addition, it is considered that the vibration during polishing also contributes to the decrease in the adhesion between diamond and Cu inside the composite material. The decrease in relative density means that a plurality of voids are generated inside the composite material 80. Also, when laser processing is performed, heat is locally applied to the composite material, and the temperature of the composite material rises. As a result, the adhesion between diamond and Cu inside the composite material can decrease due to the difference in thermal expansion coefficient between the diamond particles 10 and Cu70. The cause of the decrease in thermal conductivity that can occur due to such a thinning process will hereinafter be referred to as "defects" or "damage" of the composite material. Note that the decrease in thermal conductivity due to the thinning process as described above can also occur even if the matrix of the composite material is a metal other than Cu.

[0070] In this embodiment, as shown in FIG. 7C, after the step S200 of performing the thinning process, a step S300 of defect recovery by re-pulse electric current sintering is executed. In this step S300, it becomes possible to mitigate the degree of defects or damage generated by the thinning process or to recover to the original state. As a result, the thermal conductivity decreased by the thinning process can be increased. In other words, by re-pulse electric current sintering, at least the relative density of the composite material 80 can be made higher than the relative density before performing the re-pulse electric current sintering. The re-pulse electric current sintering can be performed while applying a pressure of less than 50 MPa to the composite material.

[0071] In a certain embodiment, the re-pulse electric current sintering is executed under low temperature and low pressure conditions as when producing the composite material 80. Specifically, for example, in a sintering apparatus 100 as shown in FIG. 9, the polished composite material 80 is loaded into a cavity 20 formed between an upper punch 40 and a lower punch 50. As the sintering apparatus 100, a sintering apparatus capable of performing pulse electric current sintering is used. When obtaining the composite material 80 by pulse electric current sintering a mixed powder of Cu and diamond, a sintering apparatus identical or of the same type as the sintering apparatus used when producing the composite material 80 may be preferably used. By re-pulse electric current sintering, the composite material 80 is held at a temperature of, for example, 500°C or higher and lower than 800°C while applying a pressure of, for example, 5 MPa or higher and lower than 50 MPa. The holding time depends on the holding temperature, but is, for example, 60 seconds or longer and 1800 seconds or shorter. The application of pressure may be the "cycle pressurization" described above. The method of pulse electric current sintering for increasing the thermal conductivity decreased by polishing may include various examples described with reference to FIGS. 10, 11A, and 11B, but it is not necessarily executed under the same conditions as the temperature and pressure when manufacturing the composite material 80 from the mixed powder.

[0072] FIG. 15 is a cross-sectional view schematically showing the state of the polished composite material 80 after re-pulse electric current sintering. As a result of the re-pulse electric current sintering, the degree of defects or damage caused by the polishing process is alleviated and can be restored to the original state. This is presumably because under the temperature and pressure in the re-pulse electric current sintering process, the relative density of the composite material 80 increases, the thermal resistance at the interface between the diamond particles and Cu decreases, or the heat dissipation path becomes shorter and the thermal diffusivity increases.

[0073] Also, when re-pulse electric current sintering is performed under the same conditions as the pulse electric current sintering carried out when manufacturing the composite material 80, it has been found that the thermal diffusivity of the composite material 80 becomes higher than the thermal diffusivity before the re-pulse electric current sintering. Here, there is the following relationship between the thermal conductivity and the thermal diffusivity. Thermal conductivity [W / m·K] = Thermal diffusivity [m 2 / s] × Specific heat [J / K·kg] × Density [kg / m 3

[0074] According to the results of the experiments described later, both the relative density and the thermal conductivity decreased by the thinning process increase due to the re-pulse electric current sintering. Also, for example, according to Experimental Example 1 described later, it has been found that the rate of increase in the thermal conductivity is greater than the rate of increase in the relative density due to the re-pulse electric current sintering. This means that the re-pulse electric current sintering increases the thermal diffusivity itself of the composite material 80 that has decreased due to polishing. In other words, it is considered that the re-pulse electric current sintering not only increases the relative density of the composite material after polishing but also contributes to recovering the defects and damage caused by polishing.

[0075] ​The re-pulse electric current sintering may be performed in a state where a metal powder or a metal plate is brought into contact with the processing surface 80P of the composite material 80. Examples of these metals are Cu, Al, CuW, CuMo, etc. From the viewpoints of thermal conductivity and ease of handling, it is preferable to use Cu. Also, from the viewpoint of reducing the difference in thermal expansion with the composite material 80, it is preferable to use CuW. The Cu powder or the Cu plate is easily joined and integrated with the Cu of the composite material 80. By the re-pulse electric current sintering, the metal powder can form a metal layer bonded to the processing surface 80P of the composite material 80. Also, the metal plate brought into contact with the processing surface 80P of the composite material 80 can also form a metal layer bonded to the processing surface 80P of the composite material 80 by the re-pulse electric current sintering. The "metal plate" does not need to have self-standing rigidity when its thickness is sufficiently thin. Such a thin metal plate may be called a metal thin film or foil.

[0076] FIG. 16 is a cross-sectional view schematically showing a part of the composite material 80 having the metal layer 81 formed in this way. Integral sintering is achieved between the metal layer 81 formed by the re-pulse electric current sintering and the underlying composite material 80. Particularly when the metal layer 81 is formed of Cu, it is continuous with the Cu constituting the composite material 80. According to such integral sintering, the composite material 80 and the metal layer 81 can be more firmly bonded as compared with the case where the metal is joined to the processing surface 80P of the composite material 80 by a physical or chemical method, or the case where the metal layer is joined to the composite material 80 via a joining material.

[0077] During the process of re-pulse energization sintering, the surface of the metal layer 81 is pressed against the lower end surface of the upper punch 40 in the sintering apparatus 100 shown in FIG. 9, for example. At this time, the pressure is, as described above, for example, 5 MPa or more and less than 50 MPa, and the temperature is, for example, 500 °C or more and less than 800 °C. Therefore, the softened metal can be strongly bonded to the processed surface 80P. Since the thermal conductivity of the metal layer 81 is lower than that of diamond, if the metal layer 81 is too thick, it is considered that the thermal conductivity of the composite substrate in which the composite material 80 and the metal layer 81 are integrally sintered will decrease. However, if the thickness of the metal layer 81 is, for example, 50 μm or more and 1000 μm or less, it is possible to reduce the decrease in the thermal conductivity of the composite substrate.

[0078] When pulse energization sintering is performed in a state where metal powder is in contact with the processed surface of the composite material 80 to form the metal layer 81, it is easy to enhance the adhesion of the metal layer 81 to the composite material 80.

[0079] In addition, when re-pulse energization sintering is performed in a state where a metal plate is in contact with the processed surface of the composite material 80 to form the metal layer 81, a metal plate having a predetermined thickness can be used, so there is an advantage that it is easy to make the thickness of the metal layer 81 approach the target value. Also, when using a metal plate, the thermal conductivity of the metal plate can be maintained close to the thermal conductivity of the metal that is the material. Therefore, by making the composite material 80 thicker than a predetermined thickness, it is possible to reduce the decrease in the thermal conductivity of the composite substrate in which the composite material 80 and the metal layer 81 are integrally sintered.

[0080] Since the metal layer 81 formed in this way does not substantially contain diamond particles 10, it can form a flatter surface than the processed surface 80P of the composite material 80. A flat surface contributes to expanding the effective contact area and reducing the thermal resistance when a heat source such as a semiconductor light-emitting element and a heat dissipation member are in thermal contact. Also, since the metal layer 81 is located on the surface of the composite material 80, there is an effect that plating and the like become easier.

[0081] In an embodiment of the present disclosure, in order to perform direct bonding by atomic diffusion bonding, the upper surface of the metal layer 81 is polished to form a flat metal layer 82. The polishing can be performed by various methods. Since the metal layer 81 does not contain diamond particles, it is possible to form a flat surface suitable for direct bonding.

[0082] FIG. 17 is a cross-sectional view showing a state in which an insulating layer 84 is formed on the flat layer 82. Since the polished flat layer 82 has a flatter surface than the composite material 80, good adhesion can be maintained with the insulating layer 84. In particular, in order to realize "direct bonding" by the surface activation bonding method or the atomic diffusion bonding method described above, it is required to prepare a flat bonding surface. For example, by polishing the metal layer 81 to form the flat layer 82, the requirement can be satisfied. The surface roughness (Ra) of the upper surface of the flat layer 82 is, for example, 10 nm or less, preferably 5 nm or less, and particularly preferably 1 nm or less. According to the surface activation bonding method or the atomic diffusion bonding method described above, it is possible to bond two materials even at room temperature. Here, "room temperature" is, for example, 50°C or less, preferably 40°C or less. These methods can bond at a temperature lower than the maximum temperature reached by heating with the heat generated during the driving of the semiconductor element in thermal contact with the composite substrate when bonding the metal layer 81 and the insulating layer 84. Therefore, the bonding interface is less affected by the stress due to the difference in the thermal expansion coefficients of the two, and it is also preferable in that high bonding strength can be obtained.

[0083] By fragmenting the composite material 80 in which the flat layer 82 and the insulating layer 84 thus laminated are firmly bonded, a plurality of composite substrates can also be obtained.

[0084] Also, as shown in FIG. 18, a metal member having a thickness greater than the surface roughness of the metal layer 81 may be provided on the metal layer 81, and the metal member may be polished until the surface roughness becomes 10 nm or less to form a flat layer 82. Examples of the metal member for the flat layer 82 include plating layers such as Cu. The metal member may be a Cu film formed by other methods. The Cu film can be formed, for example, by sputtering. In addition, in order to improve the adhesion between the metal layer 81 and the metal member, an adhesion layer such as titanium, nickel, or gold may be further provided on the surface of the metal layer 81. Since the metal member deposited by a thin film deposition technique such as a plating method or a sputtering method is easier to align the crystal orientation than the metal layer 81 formed by the method described above, a flat surface can be easily formed by polishing. Further, when a metal member having a thickness greater than the surface roughness of the metal layer 81 is formed, the unevenness of the metal layer 81 is absorbed by the metal member, so that the flatness can be improved by polishing. The metal member may be formed before or after polishing the metal layer 81, but it is preferable to form the metal member after polishing the metal layer 81. Thereby, the flat layer 82 can be formed efficiently. The composite material after the thinning process may have unevenness on the order of diamond particles. It is more efficient to form the metal member after previously forming and planarizing the metal layer 81 than to form a metal member having a thickness sufficient to absorb this unevenness by a plating method or a sputtering method.

[0085] Note that the step of preparing a laminate having a base layer formed from a composite material and a metal layer on this base layer includes a step of bringing a metal powder or a metal plate into contact with a layer of a mixed powder containing a plurality of diamond particles and a plurality of metal powder particles, and a step of generating a composite material from the mixed powder by a pulse electric current sintering method to form a base layer and bonding a metal layer made of the metal powder or the metal plate to the first surface of the base layer. Further, the step of preparing this laminate may include, as described above, a step of generating a composite material from a mixed powder containing a plurality of diamond particles and a plurality of metal powder particles by first pulse electric current sintering to form a base layer, and a step of bonding the metal powder or the metal plate to the first surface of the base layer by second pulse electric current sintering to obtain the metal layer.

[0086] Thus, in the embodiments of the present disclosure, thinning the composite material itself is not an essential step. However, as described above, when it is necessary to adjust the thickness of the composite material by polishing, the step of preparing the laminate includes, after the step of forming the base layer by first pulse electric current sintering, a step of thinning the first surface of the base layer, and a step of obtaining a metal layer by bonding a metal powder or a metal plate to the thinned first surface of the base layer by second pulse electric current sintering.

[0087] Note that the diamond included in the base layer described so far may be obtained by performing the following steps. That is, in the step of forming the base layer, as a plurality of diamond particles included in the mixed powder, a group of diamond particles having L * a * b * of 53 or more in the CIE1976L * color system may be prepared. The "group of diamond particles" is an aggregate of a plurality of diamond particles, and the "group of diamond particles having L * a * b * of 53 or more in the CIE1976L * color system" means that L * of the entire group of diamond particles is 53 or more. By forming the base layer using such a group of diamond particles, the thermal conductivity of the base layer can be improved. L * represents the lightness in the CIE1976L * a * b * color system. The CIE1976L * a * b * color system is one of the color spaces (uniform color spaces) standardized by the CIE (International Commission on Illumination). L * takes a value from 0 to 100, and the larger the value, the higher the lightness. The L *A low value means that the diamond particle group contains many diamond particles close to black. The reason why some of the diamond particles become black in this way is considered to be due to the presence of carbon components in a state different from diamond inside or on the surface of the diamond particles as impurities. And an increase in the concentration of such impurities can cause a decrease in brightness and thermal conductivity. L * is preferably 54 or more. Also, L * is more preferably 55 or more, and particularly preferably 56 or more. Thereby, the thermal conductivity of the composite material can be further improved. L * may be 60 or less.

[0088] In addition, diamond particles may be contaminated with diamond particles having a high Fe impurity concentration (relatively low brightness). Inside or on the surface of diamond particles with a relatively high Fe concentration, it is considered that Fe forms a ferromagnetic metal (which may include an alloy) or a ferromagnetic compound (such as a metal oxide). Such diamond particles are more likely to be attracted by magnetic force compared to diamond particles with a relatively low Fe concentration. Therefore, in the step of preparing the above-described diamond particle group, a step of removing diamond particles containing ferromagnetic impurities by magnetic force can be included. Thereby, diamond particles containing a large amount of ferromagnetic metal or ferromagnetic compound such as Fe can be efficiently removed. For example, by bringing a permanent magnet close to a plurality of diamond particles, it is possible to attract diamond particles with a relatively high Fe concentration to the permanent magnet and remove them from the plurality of diamond particles. Such magnetic separation (magnetic force separation) can be efficiently performed, for example, by arranging a permanent magnet with a gap of about several millimeters from the upper surface of the belt of a belt conveyor and dispersing a large number of diamond particles on the running belt. Instead of the belt conveyor, a cylindrical drum device may also be used. In addition, to facilitate magnetic separation, magnetic separation may be performed not only by separating diamond particles in a powder state but also by bringing a large number of diamond particles in a state of being dispersed in water or a state of freely falling from above into direct or indirect contact with a magnet. Furthermore, ultrasonic vibration may be applied to the diamond particle powder.

[0089] Such magnetic separation may be performed on any diamond used in the composite material, but it is preferable to perform magnetic separation on a group of diamond particles having an average particle size of 400 μm or more and 600 μm or less. By performing magnetic separation on relatively large diamond particles, the thermal conductivity of the composite material is likely to be improved.

[0090] In magnetic separation, not only diamonds with a high Fe concentration but also other ferromagnetic metals such as Ni and Co or ferromagnetic compounds containing them can be used for separation. The content of impurities contained in the magnetically separated diamond particle group can be analyzed, for example, by X-ray fluorescence analysis (XRF analysis). The higher the Fe impurity concentration, the higher the content of ferromagnetic alloys or ferromagnetic compounds formed by Fe inside or on the surface of diamond particles, and due to the difference in thermal expansion coefficients between diamond and the ferromagnetic alloy or ferromagnetic compound, defects are likely to be formed in the diamond particles. Therefore, it is considered that by reducing the concentration of Fe impurities, the crystallinity of diamond particles can be increased and the thermal conductivity can be improved.

[0091] Hereinafter, an example of a method for manufacturing a composite material in the present disclosure will be described.

[0092] <Experimental Example 1> First, a Cu-diamond MMC composite material was prepared. The composite material was obtained by the following method. Cu powder particles with an average particle size of 5 μm and diamond particles with an average particle size of 250 μm were mixed to obtain a mixed powder. The total weight of the mixed powder was 6.24 grams. With respect to the entire mixed powder, the Cu powder particles were 60 mass percent, the diamond particles with an average particle size of 250 μm were 20 mass percent, and the diamond particles with an average particle size of 60 μm were 20 mass percent. Next, a Cu-diamond MMC with a thickness of 3 mm was fabricated using a sintering apparatus (model number SPS-515S) manufactured by SPS Syntex. The sintering peak temperature Ts was 670 °C, and the applied pressure was 36 MPa. The holding time of the sintering peak temperature Ts was 10 minutes. Also, the vacuum degree at the start of sintering was 10 Pa. The size of the composite material was diameter 20 mm × thickness 2.73 mm. After obtaining the composite material, the density and thermal diffusivity were measured. The manufacturing conditions of the above composite material are shown in Table 1.

[0093] Next, mechanical polishing was performed on the prepared composite material to form a processed surface on the composite material. As a result of this polishing, the thickness decreased by 0.18 mm. After forming the processed surface, the density and thermal diffusivity were measured.

[0094] Next, pulse current sintering was performed on the composite material with the processed surface using the above sintering apparatus to obtain a composite substrate. The sintering peak temperature Ts was 660 °C, and the applied pressure was 36 MPa. The holding time of the sintering peak temperature Ts was 10 minutes. The vacuum degree at the start of sintering was 10 Pa. As a result of this pulse current sintering, the thickness of the composite material decreased by 0.12 mm. After obtaining the composite substrate, the density and thermal diffusivity were measured.

[0095] In addition, in Experimental Example 1 and Experimental Examples 2 and 3 described later, the current value during the maintenance of the sintering peak temperature Ts was in the range of 350 A or more and 700 A. The pulse width of the applied voltage was 3.3 milliseconds.

[0096] <Experimental Example 2> A composite substrate was obtained in the same manner as in Experimental Example 1 except for the following points.

[0097] When preparing the composite material, the mixing ratio of Cu powder particles and diamond particles was changed. With respect to the entire mixed powder, the Cu powder particles were 65 mass percent, the diamond particles with an average particle size of 250 μm were 20 mass percent, and the diamond particles with an average particle size of 60 μm were 15 mass percent. Pulse current sintering was performed with the sintering peak temperature Ts set at 660 °C. The production conditions of the above composite material are shown in Table 1.

[0098] The composite material was polished to form a processed surface, and then pulse current sintering was performed with a Cu plate in contact with the processed surface to obtain a composite substrate. The conditions for pulse current sintering are the same as the sintering conditions when preparing the composite material.

[0099] <Experimental Example 3> A composite substrate was obtained in the same manner as in Experimental Example 1 except for the following points.

[0100] When preparing the composite material, the first pressure was set at 10 MPa and the second pressure at 36 MPa, and these were repeatedly applied. The holding time of the applied pressure was 15 seconds for the first pressure, 20 seconds for the second pressure, the total pressure switching time was 50 seconds, and the total time for one cycle was 85 seconds. The number of repeated cycles was 25. The holding time of the sintering peak temperature Ts was 20 minutes. The production conditions of the above composite material are shown in Table 1.

[0101] After performing a thinning process on the composite material to form a processed surface, pulse current sintering was performed with Cu powder particles in contact with the processed surface to obtain a composite substrate. The conditions for pulse current sintering were the same as the sintering conditions when preparing the composite material.

[0102]

Table 1

[0103] <Thermal conductivity> The thermal conductivity was determined for each of Experimental Examples 1 to 3. The thermal conductivity can be determined as the product of specific heat, density, and thermal diffusivity. The specific heat was obtained by weighting the literature values of the specific heat of diamond and Cu according to the mixed mass ratio. The density was measured by the Archimedes method. The thermal diffusivity was obtained by the flash method using a xenon lamp with a measuring device (model number LFA-447) manufactured by Netzsch Japan Co., Ltd. The measurement temperature was 25°C. The thickness of the composite material in Experimental Example 1 was 2.73 mm, the thickness after polishing was 2.55 mm, and the thickness after pulse electric current sintering was 2.43 mm. The thickness of the composite material in Experimental Example 2 was 2.64 mm, the thickness after polishing was 2.49 mm, and the thickness after pulse electric current sintering was 2.56 mm. The thickness of the composite material in Experimental Example 3 was 2.64 mm, the thickness after polishing was 2.45 mm, and the thickness after pulse electric current sintering was 2.51 mm. Table 2 shows the "relative density" before polishing, after polishing, and after re-pulse electric current sintering, Table 3 shows the "thermal diffusivity", and Table 4 shows the "thermal conductivity". Note that the "relative density" in Table 2 represents the relative ratio of the actual density to the true specific gravity of the composite material. In Experimental Examples 2 and 3, since the true density after re-pulse electric current sintering is the density of the composite substrate obtained by sintering the composite material and Cu powder particles or the composite material and Cu plate, it becomes difficult to understand the influence of re-pulse electric current sintering on the density. Therefore, the transition of the relative density was determined for each of "after preparing the composite material", "after forming the processed surface", and "after re-pulse electric current sintering".

[0104]

Table 2

[0105]

Table 3

[0106]

Table 4

[0107] As can be seen from Tables 2 and 3, in any of the experimental examples, the relative density and the thermal diffusivity decreased due to the polishing process. However, the relative density of the composite substrates according to Experimental Examples 1 to 3 in which re-pulse energization sintering was performed after the formation of the processed surface was higher than the relative density after the formation of the processed surface. Similarly, the thermal conductivity was also higher after re-pulse energization sintering than after the formation of the processed surface.

[0108] In Experimental Examples 2 and 3 described above, a laminate having a base layer formed of a composite material and a metal layer bonded to the polished surface of this base layer was obtained.

[0109] Thus, according to Experimental Examples 2 and 3, even if a thinning process is performed on a composite material containing diamond and metal to adjust the thickness, the decrease in thermal conductivity due to the polishing process is recovered, and a metal layer that can be used for planarization is formed. Therefore, the insulation layer can be joined by direct bonding at room temperature thereon. According to such direct bonding at room temperature, it becomes possible to select, as the material of the flat layer, a metal having excellent thermal conductivity but a coefficient of thermal expansion that is, for example, twice or more that of the composite material and the insulation layer, and a composite substrate having excellent heat dissipation can be realized. As such a material of the flat layer, for example, Cu can be mentioned.

[0110] In the embodiment of the present disclosure, as the step of preparing a composite material containing diamond and Cu, various known methods can be adopted. Hereinafter, experimental examples of the step of preparing a composite material containing diamond and Cu will be described. After preparing the composite material according to the following experimental examples, the step of forming the above-described flat layer and the step of forming the insulation layer by direct bonding can be performed.

[0111] <Experimental Example 4> Cu powder particles with an average particle size of 5 μm and diamond particles with an average particle size of 250 μm were mixed to obtain a mixed powder. The total weight of the mixed powder was 6.24 grams. With respect to the entire mixed powder, the Cu powder particles were 80 mass percent, and the diamond particles were 20 mass percent. Next, a Cu-diamond MMC with a thickness of 3 mm was fabricated using a sintering apparatus (model number SPS-515S) manufactured by SPS Syntex. The sintering peak temperature Ts was 500 °C, and the applied pressure was 36 MPa. The holding time at the sintering peak temperature Ts was 10 minutes. Also, the degree of vacuum at the start of sintering was 10 Pa. The production conditions and the estimated thermal conductivity are shown in Table 5.

[0112] <Experimental Example 5> A Cu-diamond MMC was obtained in the same manner as in Experimental Example 4, except that the sintering temperature was changed to 600 °C and the applied pressure was changed to 10 MPa with respect to Experimental Example 4.

[0113] <Experimental Example 6> A Cu-diamond MMC was obtained in the same manner as in Experimental Example 4, except that the sintering temperature was changed to 600 °C with respect to Experimental Example 4.

[0114] <Experimental Example 7> A Cu-diamond MMC was obtained in the same manner as in Experimental Example 4, except that the sintering temperature was changed to 600 °C and the applied pressure was changed to 90 MPa with respect to Experimental Example 4.

[0115] <Experimental Example 8> A Cu-diamond MMC was obtained in the same manner as in Experimental Example 4, except that the sintering temperature was changed to 640 °C and 10 mass percent of diamond fine particles with an average particle size of 60 μm and 20 mass percent of diamond large particles with an average particle size of 250 μm were mixed to form a bimodal mixed powder with respect to the entire mixed powder.

[0116] <Experimental Example 9> A Cu-diamond MMC was obtained in the same manner as in Experimental Example 4, except that the sintering temperature was changed to 750 °C with respect to Experimental Example 4.

[0117] <Experimental Example 10> A composite material was obtained in the same manner as in Experimental Example 8 except for the following points. The first pressure was set to 10 MPa, and the second pressure was set to 36 MPa, and these were repeatedly applied. The holding time of the applied pressure was such that the first pressure was 15 seconds, the second pressure was 20 seconds, the total pressure switching time was 50 seconds, and the total time for one cycle was 85 seconds. The number of repeated cycles was 25. Also, the holding time of the sintering peak temperature Ts was 20 minutes.

[0118] <Experimental Example 11> For Experimental Example 10, the sintering temperature was changed to 660 °C. Also, 15 mass percent of diamond fine particles with an average particle size of 60 μm and 20 mass percent of diamond large particles with an average particle size of 250 μm were mixed with the entire mixed powder to form a bimodal mixed powder. A Cu-diamond MMC was obtained in the same manner as in Experimental Example 10 except for these.

[0119] <Experimental Example 12> A Cu-diamond MMC was obtained in the same manner as in Experimental Example 10 except that the first pressure was set to 50 MPa and the second pressure was set to 90 MPa for Experimental Example 10.

[0120] In the process of preparing the composite material, in Experimental Examples 1 to 12 where a mixed powder of diamond particles and Cu powder particles was prepared and a pressure of 5 MPa or more and 100 MPa or less was applied to the mixed powder and held at a temperature of 500 °C or more and less than 800 °C by pulse electric current sintering to generate a composite material from the mixed powder, a composite material having a higher thermal conductivity than pure Cu was generated. The composite materials in these Experimental Examples 4 to 12 can be used as the composite materials in the above Experimental Examples 1 to 3.

[0121]

Table 5

[0122] FIG. 19 is a diagram showing an optical micrograph of a cross section of a part of a composite material manufactured in the same manner as in Experimental Example 6 of Table 5. FIG. 20 is a diagram showing an optical micrograph of a cross section of a part of a composite material manufactured in the same manner as in Experimental Example 10 of Table 5. In each photograph, the region with relatively high brightness is the Cu part, and the region with relatively low brightness is the diamond particles. In the composite material of FIG. 20, relatively small diamond particles with an average particle size of 60 μm are located in the gaps between relatively large diamond particles with an average particle size of 250 μm. Compared with the composite material of FIG. 19, in the composite material of FIG. 20, the number of diamond particles increases, and the total surface area of the surface also increases. This means that the area of the interface between Cu and diamond increases. When the area of this interface increases, there is a possibility that the thermal resistance increases and the thermal conductivity decreases because the adhesion between diamond and Cu is poor. However, in this Experimental Example 10, such a phenomenon was not observed. As can be seen from FIGS. 19 and 20, the composite materials in Experimental Examples 6 and 10 are dense.

[0123] <Experimental Example 13> In this experimental example, first, a plurality of diamond particles with an average particle size of 500 μm were prepared. Then, a step (sorting step) of visually selecting and removing diamond particles with particularly low brightness (for example, close to black) was performed on these diamond particles. After removing the diamond particles with low brightness, L * was measured for the remaining diamond particles.

[0124] The L of the diamond particle group after performing the sorting step of this experimental example *It was 55.63. This measurement was performed by filling the opening 620 of the jig 600 shown in Fig. 21A with diamond particles. Specifically, as shown in Fig. 21B, the jig 600 was placed on 20 stacked white papers 640, diamond particles were filled inside the opening 620 of the jig 600, and then the spectrocolor difference measuring device 660 was pressed against the upper surface of the jig 600, and the brightness was measured with external light not entering the opening 620. The jig 600 was made of metal (specifically SUS), the depth of the opening 620 was 4 mm, and the diameter was 15 mm. The spectrocolor difference measuring device 660 was a product named "Handy-type Spectrocolorimeter NF555" manufactured by Nippon Denshoku Industries Co., Ltd.

[0125] L * To obtain the first reference value (ref.1) of L, for the 20 stacked white papers 640, the above-mentioned spectrocolor difference measuring device 660 was used to measure L. * In this measurement, the jig 600 shown in Fig. 21A was not used, the spectrocolor difference measuring device was directly pressed against it, and the brightness was measured with external light not entering the opening 620. The L obtained by the measurement, * that is, the first reference value (ref.1), was 93.62. Also, * To obtain the second reference value (ref.2) of L, the jig 600 in Fig. 21A was placed on 20 stacked white papers, and for the white paper located inside the opening 620, with external light not entering the inside of the opening 620, the spectrocolor difference measuring device 660 was used to measure L. * The L obtained by this measurement, * that is, the second reference value (ref.2), was 61.58.

[0126] Regarding the L * (55.63) of the diamond particle group after performing the sorting process of this experimental example, when calculating the relative value of L with respect to the reference value (ref.2), it was 0.903. *

[0127] Thus, the L in this disclosure *The measurement is carried out using a spectroscopic color difference measuring device on a group of diamond particles filled in a space where external light does not enter. Also, when obtaining relative values, the measured value of L of the white paper laminated in the space filled with diamond particles may be used as a reference value. *

[0128] Next, a group of diamond particles with L * of 55.63 was mixed with Cu powder particles with an average particle size of 5 μm to obtain a mixed powder. The total weight of the mixed powder was 6.24 grams. With respect to the entire mixed powder, the Cu powder particles were 80 mass percent and the diamond particles were 20 mass percent. Next, a Cu-diamond MMC with a thickness of 4 mm was fabricated using a sintering device (model number SPS-515S) manufactured by SPS Syntex. Specifically, a pressure of 36 MPa was applied. The sintering peak temperature Ts was 620 °C and the holding time was 10 to 15 minutes. The sintering was carried out in a vacuum atmosphere.

[0129] Also, a group of diamond particles with L * of 55.63 was mixed with diamond particles with an average particle size of 50 μm and Cu powder particles with a particle size of 5 μm to obtain another mixed powder (bimodal mixed powder). The total weight of this mixed powder was 5.76 grams. With respect to the entire mixed powder, the Cu powder particles were 70 mass percent, the diamond particles with a particle size of 500 μm were 20 mass percent, and the diamond particles with a particle size of 50 μm were 10 mass percent. Next, a Cu-diamond MMC with a thickness of 3 mm was fabricated using a sintering device (model number SPS-515S) manufactured by SPS Syntex. Specifically, the first pressure was set to 10 MPa and the second pressure was set to 36 MPa, and these were repeatedly applied. The holding time of the applied pressure was 15 seconds for the first pressure, 20 seconds for the second pressure, the total switching time of the pressure was 50 seconds, and the total time for one cycle was 85 seconds. The number of repeated cycles was 25 times. Also, the sintering peak temperature Ts was 660 °C and the holding time was 40 minutes. During the temperature increase from room temperature to 430 °C, the sintering atmosphere was vacuum, and above 430 °C, it was a mixed gas atmosphere of nitrogen and hydrogen.

[0130] For each of the Cu-diamond MMCs obtained by the above sintering, when the thermal conductivity was measured, the results shown in Table 6 were obtained.

[0131]

Table 6

[0132] <Experimental Example 14> In this experimental example, only the content of the diamond particle sorting process was different, and samples were prepared under the same other conditions. The diamond particles targeted in the sorting process were a diamond particle group (powder) with an average particle size of 500 μm, which is the same as in Experimental Example 13. From Experimental Example 15 to Experimental Example 18 described later, there is no difference in the diamond particles targeted in the sorting process, and there is only a difference in the sorting process.

[0133] In this experimental example, the above diamond particles were sorted by magnetic force. For the diamond particle group remaining without being attracted to the magnet, when the concentration of Fe impurities was measured by X-ray fluorescence analysis (XRF analysis; X-ray Fluorescence analysis), a value of 104 ppm was obtained. For the XRF analysis, the product name "ZSX Primus II" manufactured by Rigaku Corporation was used. The measurement conditions were a tube voltage of 50 kV and a tube current of 60 mA when confirming Fe as an impurity. Also, the X-ray source was Rh (Kα1 line), and a scintillation counter was used when confirming the Fe (Kα1 line) with the X-ray detector. The energy resolution was about 20 eV (the half-value width of Mn, 2θ corresponding to 1.5 degrees).

[0134] In this experimental example, a mixed powder and a bimodal mixed powder were each prepared using the diamond particle group that remained without being attracted to the magnet, and Cu-diamond MMC was produced in the same manner as in Experimental Example 13. For the diamond particle group that remained without being attracted to the magnet, the brightness was measured in the same manner as in Experimental Example 13. Further, the thermal conductivity of the Cu-diamond MMC obtained by sintering was measured. The results are shown in Table 6.

[0135] <Experimental Example 15> In this experimental example, a sorting process was further performed to remove black diamond particles by visual inspection on the diamond particle group that remained without being attracted to the magnet in the magnetic separation in Experimental Example 14. In this regard, this experimental example is different from Experimental Example 14. When the brightness measurement was performed on the diamond particle group after this sorting process, L * was 56.65 (relative value was 0.920). The L * of the diamond particle group after the sorting process in Experimental Example 14 was 56.38 (relative value was 0.916), so an increase in brightness was observed. Also, in Experimental Example 15, the measured value of the Fe concentration of the diamond particles remaining after such magnetic separation and visual sorting was 111 ppm.

[0136] Using the diamond particle group after the sorting process, the bimodal mixed powder described in Experimental Example 13 was prepared, and Cu-diamond MMC was produced in the same manner as in Experimental Example 13. For the Cu-diamond MMC thus obtained, the thermal conductivity was measured. The results are shown in Table 6.

[0137] <Experimental Example 16> In this experimental example, the brightness measurement was performed on the diamond particle group (that is, the diamond particles attracted to the magnet and recovered) removed from the diamond particles in the magnetic separation in Experimental Example 14. L * was 53.16 (relative value was 0.863). The L *Since it was 56.38 (relative value: 0.916), it was confirmed that diamond particles with relatively low brightness were recovered by magnetic separation.

[0138] Using the group of diamond particles recovered by magnetic force, the mixed powder (80 mass percent Cu powder particles and 20 mass percent diamond particles) described in Experimental Example 13 was prepared, and Cu-diamond MMC was produced in the same manner as in Experimental Example 13. The thermal conductivity of the thus obtained Cu-diamond MMC was measured. The results are shown in Table 6.

[0139] <Experimental Example 17> In this experimental example, after performing magnetic separation (the first magnetic separation) in Experimental Example 14, the second magnetic separation was performed on the group of diamond particles remaining without being attracted to the magnet. For the group of diamond particles attracted to the magnet and recovered by performing the second magnetic separation, the above-mentioned brightness measurement was performed. L * was 53.07 (relative value: 0.862).

[0140] Using the group of diamond particles recovered by the second magnetic separation, the bimodal mixed powder described in Experimental Example 13 was prepared, and Cu-diamond MMC was produced in the same manner as in Experimental Example 13. The thermal conductivity of the thus obtained Cu-diamond MMC was measured. The results are shown in Table 6.

[0141] <Experimental Example 18> In this experimental example, the same experiment as in Experimental Example 13 was conducted, except that diamond particles in a state where the above-mentioned separation process was not performed were used.

[0142] When the above-mentioned brightness measurement was performed on the group of diamond particles for which the separation process was not performed, L *It was 54.73 (relative value: 0.889). Also, the measured Fe concentration of these diamond particles was 113 ppm. Using such a group of diamond particles, mixed powders similar to the mixed powder and bimodal mixed powder described in Experimental Example 13 were prepared respectively, and Cu-diamond MMCs were produced in the same manner as in Experimental Example 13. The thermal conductivity of the thus obtained Cu-diamond MMCs was measured. The results are shown in Table 6.

[0143] Figure 21C is a graph showing the L * obtained in Experimental Examples 13 - 18 and the measurement results of thermal conductivity. Ex.13 - Ex.18 in the graph indicate the data of Experimental Examples 13 - 18 respectively. In Figure 21C, the black circles represent samples using a mixed powder with 80 mass percent of Cu powder particles and 20 mass percent of diamond particles. Also, the black squares represent samples using a bimodal powder of diamond particles as the mixed powder.

[0144] As can be seen from the graph in Figure 21C, a high thermal conductivity exceeding 575 [W / m·K] is achieved in all experimental examples. Therefore, it can be understood that if the L * of the diamond particle group contained in the base layer is 53 or more (relative value: 0.86 or more), a high thermal conductivity exceeding 575 [W / m·K] can be obtained. Also, when L * increases, a tendency for the thermal conductivity to increase is observed. It is preferable that L * is 54 or more (relative value: 0.88 or more). Also, it is more preferable that L * is 55 or more (relative value: 0.89 or more), and particularly preferably 56 or more (relative value: 0.91 or more). Thereby, the thermal conductivity of the composite material can be further improved.

[0145] Also, from Table 6, when comparing Experimental Example 18 where magnetic separation was not performed with Experimental Examples 14 and 15 where magnetic separation was performed, it was confirmed that the amount of Fe impurities in Experimental Examples 14 and 15 was lower than that in Experimental Example 18. Also, it was confirmed that the thermal conductivity of Experimental Examples 14 and 15 was higher than that of Experimental Example 18. L * Moreover, it was speculated that Fe impurities are one of the factors improving the thermal conductivity.

[0146] After preparing these composite materials, as described for Experimental Examples 1 to 3, by performing a polishing process, a metal layer formation process, a metal layer polishing process, and a direct bonding process of an insulating layer, experimental examples of the composite substrate according to the present disclosure can be obtained.

[0147] <Semiconductor device> Hereinafter, an example of a semiconductor device using the composite material in the present disclosure will be described.

[0148] A composite material containing diamond particles and a metal has conductivity as a whole because the continuous metal matrix can conduct an electric current. Therefore, when a semiconductor element is bonded and used with the composite material as described above, by providing an insulating layer (insulating member) on the surface of the composite material, a conductive portion of the semiconductor element, for example, a semiconductor substrate, can be mounted in a state of contacting the insulating layer of the composite material. Also, if the thermal expansion coefficient of the insulating layer is selected to be an intermediate value between that of the composite material and the semiconductor element, peeling between the composite material and the semiconductor element due to the difference in thermal expansion coefficient can be reduced.

[0149] On the other hand, by utilizing the fact that the composite material has conductivity, the semiconductor element and the composite material can be electrically connected, and the composite material can be made a part of the current path. In this case, the composite material can serve two roles: a heat dissipation path and a current path. The composite material directly joined to the insulating layer can further include a conductive layer on the insulating layer. The conductive layer and the semiconductor element can be electrically connected by a wire. By using the conductive layer as a relay point, the length of each wire can be shortened, and the disconnection of the wire can be reduced. Also, by appropriately setting the area of the conductive layer, the handling for connection to the external electrode is improved. When, for example, a semiconductor laser element is used as the semiconductor element, it becomes easy to wire a plurality of wires from the semiconductor laser element to the conductive layer, and thus uniform current injection into the semiconductor element becomes possible.

[0150] Hereinafter, as a semiconductor device including a composite substrate manufactured in an embodiment of the present disclosure, a configuration example of a light-emitting device will be described.

[0151] FIG. 22 is a perspective view schematically showing a configuration example of a light-emitting device 200 including a light-emitting diode (LED) element 22 and a composite substrate 14 that supports the LED element 22. The composite substrate 14 includes a base layer 180, a flat layer 82, and an insulating layer 84, and is fabricated by the manufacturing method in the above-described embodiment. The insulating layer 84 is formed of, for example, ceramic AlN. On the insulating layer 84, a p-side wiring 28p and an n-side wiring 28n are formed, and are electrically connected to the p-side electrode and the n-side electrode of the LED element 22, respectively. The lower surface of the composite substrate 14 is in thermal contact with a heat sink 190. Heat generated in the LED element 22 during operation spreads in the in-plane direction by the insulating layer 84 and then dissipates to the outside through the base layer 180.

[0152] FIG. 23 is a cross-sectional view schematically showing a configuration example of a light-emitting device 300 including a laser diode (LD) element 32, a submount 34 that supports the LD element 32, and a substrate 42 that supports the submount 34. The submount 34 is formed from the base layer in the above-described embodiment. The substrate 42 may be formed from the same material as the submount 34, or may be formed from another material, such as metal or ceramics. A conductive layer 37 is provided on the upper surface of the substrate 42. The submount 34 is disposed on this conductive layer 37.

[0153] A part of the upper surface of the submount 34 is covered by an insulating layer 36 such as AlN, for example, but the conductive surface of the submount 34 appears in other parts of the upper surface. Thus, in the embodiment of the present disclosure, it is not necessary for the entire upper surface of the composite substrate to be covered by the insulating layer 36. This insulating layer 36 is directly bonded to the submount 34 by the direct bonding method described above. That is, the insulating layer 36 is directly bonded to the flat layer constituting the upper surface of the submount 34.

[0154] The substrate 42 has a p-side terminal 48p and an n-side terminal 48n that are electrically separated from the main body by an insulating member 44. The insulating member 44 and the substrate 42 are bonded by resin, solder, etc., or are bonded by diffusion bonding. The p-side terminal 48p and the n-side terminal 48n are electrically connected to conductive vias 49 that penetrate the insulating member 44 in the vertical direction, respectively.

[0155] The p-side electrode located on the lower surface of the LD element 32 is electrically connected to the conductive surface on the upper surface of the submount 34. The p-side terminal 48p of the substrate 42 is electrically connected to a conductive layer 38 provided on the conductive layer 37 located on the upper surface of the substrate 42 by a wire 33a. Since the submount 34 has conductivity, the p-side terminal 48p is electrically connected to the p-side electrode of the LD element 32 via the wire 33a, the conductive layer 38, the conductive layer 37, and the submount 34.

[0156] On one hand, the n-side electrode located on the upper surface of the LD element 32 is electrically connected to the conductive layer 39 formed on the insulating layer 36 by the wire 33b. This conductive layer 39 is electrically connected to the n-side terminal 48n by the wire 33c.

[0157] The lower surface of the substrate 42 may be in thermal contact with a heat sink or a cooling device. The heat generated by the LD element 32 during operation spreads in the in-plane direction by the submount 34 and then dissipates to the outside through the substrate 42.

[0158] FIG. 24 is a cross-sectional view showing a configuration example of a light-emitting device 400, which is another embodiment of the semiconductor device of the present disclosure. The difference between the light-emitting device 400 and the aforementioned light-emitting device 300 is mainly that an Au layer 41 is disposed in the upper surface region of the submount 34 that is not covered by the insulating layer 36. Since the Au layer 41 is disposed in the upper surface region of the submount 34 that is not covered by the insulating layer 36, the wire 33a can be electrically connected to the p-side electrode of the LD element 32 on the submount 34 through this Au layer 41. Such an Au layer 41 can be realized, for example, by the metal layer 92 shown in FIG. 5. That is, in the configuration example of the light-emitting device 400, the metal layer 92 can serve as a role of directly bonding the submount 34 and the insulating layer 36 and a role as a current path for electrically connecting the wire 33a and the submount 34.

[0159] FIG. 25 is a cross-sectional view showing a configuration example of a light-emitting device 500 which is another embodiment of the semiconductor device of the present disclosure. The light-emitting device 500 includes a base 110, a lid 120, a light reflecting member 130, a submount 140, a semiconductor laser element 150, and a wire 160. Also, inside the closed space of the package formed by joining the base 110 and the lid 120, the light reflecting member 130 and the submount 140 on which the semiconductor laser element 150 is disposed are arranged. Inside the closed space of the package, further, a wire 160 is provided which is electrically connected to the semiconductor laser element 150 disposed on the base 110. The lid 120 can be formed of, for example, glass or sapphire provided with a metal film on the surface. In particular, it is preferable to use sapphire provided with a metal film on the surface. Sapphire has a relatively high refractive index and can suppress the spread of light. Also, sapphire has relatively high strength and is difficult to break, so the airtightness of the closed space and the reliability of the light-emitting device 500 can be enhanced.

[0160] As shown in FIG. 25, the base 110 has a frame portion 112 and a bottom portion 114. The bottom portion 114 can be formed of the composite material according to the present disclosure. Examples of the composite material include Cu diamond, Ag diamond, and Al diamond. The frame portion 112 can be formed of ceramics such as alumina and AlN. The material of the base 110 is not limited to this. It is desirable that a material having a higher thermal conductivity than the frame portion 112 be selected for the bottom portion 114. The bottom portion 114 and the frame portion 112 can be joined by a direct bonding method. By direct bonding, a base 110 having high bonding strength can be obtained. Since the bottom portion 114 formed of the composite material has high thermal conductivity, the heat generated by driving the semiconductor laser element 150 easily dissipates. Since the light reflecting member 130 is also disposed on the bottom portion 114 formed of the composite material, the heat generated in the light reflecting member also easily dissipates.

[0161] Note that the above-described light-emitting devices 200, 300, 400, and 500 are examples of semiconductor devices including a heat dissipation substrate according to the embodiments of the present disclosure, and this heat dissipation substrate can be used for various applications.

Industrial Applicability

[0162] The composite substrate according to the present disclosure can be used as various heat dissipation members. Examples of the composite substrate include a substrate, a submount, a heat spreader, a package, and a heat sink that are in thermal contact with elements such as a semiconductor device, a semiconductor light-emitting element, a semiconductor integrated circuit element, and a monolithic microwave integrated circuit element.

Description of Symbols

[0163] 10... diamond particles, 20... cavity, 30... die, 40... upper punch, 50... lower punch, 60... power supply unit, 100... sintering apparatus

Claims

1. A base layer formed from a composite material containing diamond and metal, having a first surface and a second surface on the opposite side of the first surface, A lower surface bonded to the first surface of the base layer, and a flat layer having an upper surface with a surface roughness Ra of 10 nm or less, An insulating layer directly bonded to the upper surface of the flat layer, A composite substrate comprising.

2. The flat layer is formed by including one or more materials selected from the group consisting of Cu, Ag, Au, Al, CuW, CuMo, AlN, SiN, SiO 2 The composite substrate according to claim 1, which is formed.

3. The flat layer is formed from a material having a coefficient of thermal expansion of 2 times or more the coefficient of thermal expansion of the composite material, the composite substrate according to claim 1 or 2.

4. The thickness of the flat layer is 1000 μm or less, the composite substrate according to any one of claims 1 to 3.

5. The thickness of the insulating layer is 200 μm or less, the composite substrate according to any one of claims 1 to 4.

6. The flat layer includes a metal layer having a lower surface bonded to the first surface, and a metal member formed on the metal layer, thicker than the surface roughness of the metal layer, and having the upper surface, the composite substrate according to any one of claims 1 to 5.

7. A composite substrate according to any one of claims 1 to 6, A semiconductor element supported on the insulating layer of the composite substrate and electrically connected to the composite substrate, A semiconductor device comprising.

8. A step of preparing a laminate having a base layer formed from a composite material containing diamond and metal, having a first surface and a second surface on the opposite side of the first surface, a lower surface bonded to the first surface of the base layer, and a flat layer having an upper surface with a surface roughness Ra of 10 nm or less, A step of directly bonding an insulating layer to the upper surface of the flat layer; A method for manufacturing a composite substrate, comprising:

9. In the step of preparing the laminate, A step of forming a metal layer on the first surface of the base layer; The method for manufacturing a composite substrate according to claim 8, further comprising a step of polishing the surface of the metal layer to form the flat layer.

10. The step of preparing the laminate includes: A step of bringing a metal powder or a metal plate into contact with a layer of a mixed powder containing a plurality of diamond particles and a plurality of metal powder particles; A step of generating the composite material from the mixed powder by pulse electric current sintering to form the base layer, and bonding the metal layer made of the metal powder or the metal plate to the first surface of the base layer; The method for manufacturing a composite substrate according to claim 9, comprising:

11. The step of preparing the laminate includes: A step of forming the base layer by generating the composite material from a mixed powder containing a plurality of diamond particles and a plurality of metal powder particles by first pulse electric current sintering; A step of bonding a metal powder or a metal plate to the first surface of the base layer by second pulse electric current sintering to obtain the metal layer; The method for manufacturing a composite substrate according to claim 9, comprising:

12. The step of preparing the laminate includes: After the step of forming the base layer by the first pulse electric current sintering, A step of thinning the first surface of the base layer; A step of bonding the metal powder or the metal plate to the thinned first surface of the base layer by second pulse electric current sintering to obtain the metal layer; The method for manufacturing a composite substrate according to claim 11, comprising:

13. The method for manufacturing a composite substrate according to claim 12, wherein in the step of forming the flat layer, the flat layer is formed such that the surface roughness Ra of the upper surface is smaller than the surface roughness Ra of the first surface in the step of preparing the laminate.

14. The method for manufacturing a composite substrate according to any one of claims 11 to 13, wherein in the step of preparing the laminate, in the first pulse current sintering and / or the second pulse current sintering, a first pressure and a second pressure higher than the first pressure are repeatedly applied to the mixed powder.

15. The step of forming the base layer is As the plurality of diamond particles contained in the mixed powder, CIE1976L * a * b * in the color system, L * The method for manufacturing a composite substrate according to any one of claims 11 to 14, including a step of preparing a diamond particle group having an L value of 53 or more.

16. The method for manufacturing a composite substrate according to claim 15, wherein the step of preparing the diamond particle group includes a step of removing diamond particles containing ferromagnetic impurities by magnetic force.

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