Composite nanometal paste containing copper filler and joining method

Abstract

The present invention addresses the problem of providing a composite nanometal paste which is relatively low in price and is excellent in terms of bonding characteristics, thermal conductivity, and electrical property. The present invention is a copper-filler-containing composite nanometal paste that contains composite nanometal particles each comprising a metal core and an organic coating layer formed thereon. The metal paste contains a copper filler and contains, as binders, first composite nanometal particles and second composite nanometal particles which differ from the first composite nanometal particles in the thermal decomposition temperature of the organic coating layer, wherein the mass proportion W1 of the organic coating layer in the first composite nanometal particles is in the range of 2-13 mass %, the mass proportion W2 of the organic coating layer in the second composite nanometal particles is in the range of 5-25 mass %, and these particles satisfy the relationships W1.

Claims

1. A copper-filler-containing composite nanometal paste comprising composite nanometal particles in which each particle has an organic coating layer being an organic matter formed around a metal core, and a copper filler which has no organic coating layer, wherein said composite nanometal particles comprise at least a first composite nanometal particle with a first organic coating layer thereon and a second composite nanometal particle with a second organic coating layer thereon in which the first and second organic coating layers have different compositions so that pyrolysis temperatures of the first and second organic coating layers differ from each other, and wherein, when a mass percent concentration of said organic coating layers in said composite nanometical particles is defined by a term of mass ratio, a mass ratio W1 of said first organic coating layer in said first composite nanometal particle exists within a range of 2 to 13 mass %, and a mass ratio W2 of said second organic coating layer in said second composite nanometal particle exists within a range of 5 to 25 mass %, and when T1 is a pyrolysis temperature of said first organic coating layer and T2 is a pyrolysis temperature of said second organic coating layer, relationships of T1<T2 and W1<W2 are satisfied for said first and second composite nanometal particles.

2. The copper-filler-containing composite nanometal paste according to claim 1, wherein a relationship of N1<N2 is satisfied where N1 is a carbon number of said organic matter included in said first composite nanometal particle and N2 is a carbon number of said organic matter included in said second composite nanometal particle.

3. The copper-filler-containing composite nanometal paste according to claim 2, wherein said carbon number N1 is selected from a range of 1 to 6, and said carbon number N2 is selected from a range of 7 to 12.

4. The copper-filler-containing composite nanometal paste according to any one of claims 1 to 3, wherein a metallic element forming said metal core is selected from one or more kinds among Ag, Au, Pt, Ru, Zn, Sn and Ni.

5. The copper-filler-containing composite nanometal paste according to any one of claims 1 to 3, wherein said copper filler is contained in a range of 10 mass % to 90 mass % of total mass of said copper-filler-containing composite nanometal paste.

6. The copper-filler-containing composite nanometal paste according to any one of claims 1 to 3, wherein said organic matter is selected from one or more kinds among an alcohol molecule, a cut residue of an alcohol molecule and an alcohol derivative derived from an alcohol molecule.

7. The copper-filler-containing composite nanometal paste according to any one of claims 1 to 3, wherein an addition agent comprising a viscosity grant agent and/or a viscosity adjustment solvent is added, and an addition amount of said addition agent is determined by a ratio of a total amount of organic matter contained in said composite nanometal particles and a total amount of all metal ingredients contained in said copper-filler-containing composite nanometal paste.

8. The copper-filler-containing composite nanometal paste according to claim 2 or 3, wherein a third composite nanometal particle in which a third organic coating layer comprising an organic matter of carbon number N3 is formed around said metal core is additionally contained, a mass ratio W3 of said third organic coating layer in said third composite nanometal particle is not more than 0.2 mass %; said carbon number N3 satisfies a relationship of N3<N2, and a pyrolysis temperature T3 of said third organic coating layer satisfies a relationship of T3<T2 because of a difference of composition.

9. A joining method comprising the steps of forming a paste layer by applying the copper-filler-containing composite nanometal paste according to any one of claims 1 to 3 between joined members, and sintering said paste layer by heating or heating with pressurization to join said joined members.

10. The joining method according to claim 9, wherein a sinter layer formed by sintering said paste layer absorbs a deformation caused due to difference of thermal expansion coefficients in said joined members.

11. The joining method according to claim 9, wherein said joined members comprise first copper specimen and second copper specimen, and a shear strength with which said first copper specimen and said second copper specimen are joined is not less than 20 MPa, when a joining area of said first copper specimen is the area of a circle with a diameter of 5 mm, a joining area of said second copper specimen is larger than the area of a circle with a diameter of 5 mm, a heating temperature of said heating is not lower than 250° C. and said pressurization is not less than 5 MPa.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph diagram showing results of thermal shock test by a high-heat resistant power semiconductor which functions by setting an appropriate electrode structure, where a heat sink and a ceramic substrate (27×21 mm) are joined by using the copper-filler-containing composite nanometal paste concerning the present invention in the heating and the pressurizing, so that an electrical insulation layer is formed, and at an upper part thereof, Si semiconductor (9.0×8.4 mm) is joined with rigid by the same method and the same paste.

(2) FIG. 2 is a graph diagram showing results of thermal shock test by a power semiconductor which functions by setting an appropriate electrode structure, where a heat sink and a ceramic substrate (27×21 mm) are joined by using the copper-filler-containing composite nanometal paste concerning the present invention in the heating and the pressurization, so that an electrical insulation layer is formed, and at an upper part thereof, Si semiconductor (9.0×8.4 mm) is joined with rigid by the same method and the same paste.

(3) FIG. 3 is a graph diagram showing thermal analysis spectra which is obtained by a thermal analysis of the composite nanosilver particle concerning the present invention under the atmosphere.

(4) FIG. 4 is a graph diagram showing a pressurization pressure dependence of shear strength where sample pieces of 10 mmφ×10 mmφ are joined due to the heating and the pressurization for 3 minutes under the atmosphere by using the copper-filler-containing composite nanometal paste concerning the present invention.

(5) FIG. 5 is a graph diagram showing a copper filler content dependence of shear strength where sample pieces of 5 mmφ×10 mmφ are joined due to the heating and the pressurization for 3 minutes under the atmosphere by using the composite nanometal pastes which have difference of the copper filler content concerning the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

(6) FIG. 1 is the graph diagram showing the results of thermal shock test of Si semiconductor module, where the heat sink and the ceramic substrate are joined by using the copper-filler-containing composite nanometal paste concerning the present invention in the heating and the pressurizing, so that the electrical insulation layer is formed, and at the upper part thereof, the Si semiconductor of 9.0 mm×8.4 mm is joined by using the same metal paste, so that the Si semiconductor module is produced. Namely, the heat sink, the ceramic substrate and the Si semiconductor are used as the joined members. In the case between the heat sink and the ceramic substrate and in the case between the ceramic substrate and the semiconductor, each sintering is performed for 180 seconds in the pressurization of 40 MPa by heating to 300° C. under the atmosphere. The horizontal axis is the number of cycles (cy) in which a cycle of module temperature comprises decreasing rapidly to −55° C., being held for 30 minutes, increasing rapidly to 150° C. and being held for 30 minutes, and the cycle of thermal-shock is repeated to 2000 times. The vertical axis is a thermal resistance characteristic showing a change amount of forward voltage before and after applying large pulse current in each thermal cycle, and if the temperature rise due to the forward voltage of semiconductor is effectively absorbed by the heat sink before and after applying the large pulse current, the change amount is small, so that it is given the characteristic being an index whether a joining structure preferable in the thermal conductivity is formed, and the thermal resistance characteristic is better as the value is smaller in each thermal shock. The thermal resistance characteristic is measured at room temperature, and the follow is normalized so that the initial value of high temperature solder described later becomes 100.

(7) In the FIG. 1, the embodiments of the present invention are represented by sample names of “copper filler content (30%)” and “copper filler content (60%)”. In the module which is joined by the pressure sintering (described by “heat press” in the figure) using the copper-filler-containing composite nanometal paste of the sample “copper filler content (30%)”, it is shown that a mass ratio of copper filler in a total mass of metal paste used for joining is about 30 mass %. Similarly, the sample name of “copper filler content (60%)” shows that a mass ratio of copper filler in the metal paste is about 60 mass %. An average particle diameter of the copper filler is 1.5 μm. In the metal pastes of “copper filler content (30%)” and “copper filler component (60%)”, as the first composite nanometal particle, there is used the composite nanometal particle which is produced from starting materials being hexanol and silver carbonate, and as the second composite nanometal particle, there is used the composite nanometal particle which is produced from starting materials being dodecanol and silver carbonate. Furthermore, the viscosity grant agent and the viscosity adjustment agent are contained within 5 to 7 mass % of total mass. As the viscosity grant agent, commercial thickeners can be used, and as the viscosity adjustment agent, higher alcohols may be used.

(8) In the FIG. 1, as the comparative examples, a result of the module joined by the conventional lead-free solder for joining (Sn-3.0Ag-0.5Cu) is represented by “lead free solder”, and a result of the module joined by the composite nanosilver paste containing silver filler (Applied Nanoparticle Laboratory Corporation) also is represented by “silver filler content (60%)”. The composite nanosilver paste containing silver filler contains the silver filler of 60 mass % instead of the copper filler.

(9) As shown in the FIG. 1, when the lead-free solder of the comparative example exceeds 1000 cycles, the thermal resistance characteristic suddenly rises. In the result shown in FIG. 1, when the module is joined by the silver filler content (60%) of the comparative example and the copper filler content (30%) or the copper filler content (60%) of the embodiment, it is shown that the thermal resistance characteristic almost does not change at 2000 cycles, so that it is found that the paste design is performed effectively.

(10) FIG. 2 is the graph diagram showing the results of the thermal shock test under higher temperature of 200° C. in the power semiconductor module produced by using Si (9×8.4 mm) as the power semiconductor. The heating temperature, the time and the joining condition under pressurization are same as the Si semiconductor, but the thermal shock condition differs, and the horizontal axis is the number of cycles (cy) in which a cycle of module temperature comprises decreasing rapidly to −40° C., being held for 30 minutes, increasing rapidly to 200° C. and being held for 30 minutes, and the cycle of high temperature thermal-shock is repeated to 2000 times. Similarly, the vertical axis is the thermal resistance characteristic which is the change amount of forward voltage before and after applying large pulse current in each thermal cycle, and the thermal resistance characteristic is better as the value is smaller in each thermal shock. The thermal resistance characteristic is measured at room temperature, same as FIG. 1, and is normalized so that the initial value of high temperature solder becomes 100.

(11) As the embodiment of the present invention, similarly to the FIG. 1, FIG. 2 shows a test result of module joined by using the copper-filler-containing composite nanometal paste containing which is represented by the sample name of “copper filler content (60%)”. As indicated previously, the metal paste used in the “copper filler content (60%)” contains the copper filler of about 60 mass % to the total mass of metal paste where the average particle diameter is 1.5 μm. Similarly, in the FIG. 2, as the comparative example, it is shown test results of modules which are joined by using each of the high temperature solder, the lead-free solder and the silver-filler-containing composite nanosilver paste (“silver filler containing”). In addition, said silver filler containing composite nanosilver paste is same as the paste described in the FIG. 1. Furthermore, in the copper-filler-containing composite nanometal paste used for “copper filler content (60%)” being the embodiment, the composite nanometal particle is contained and is produced by the production method same as FIG. 1.

(12) In the FIG. 2, as compared with the joining by the high temperature solder or the lead-free solder, it is found that in the joining by the copper-filler-containing composite nanometal paste of “copper filler content (60%)”, the thermal conductivity is superior because the initial value is 87 and small. In the thermal shock test, the lead-free solder same as the FIG. 1 is used, and when the high temperature side of thermal shock is changed from 50° C. to 200° C., the thermal resistance characteristic changes largely at 100 cycles, and in the high temperature solder, changes at 300 cycles. In the high temperature thermal shock test of the module joined by the silver-filler-containing composite nanosilver paste (“silver filler containing”) being the comparative sample, in the case exceeding 1000 cycles, the thermal resistance characteristic begins to slightly rise, and then as result, it is suggested that the structural change is generated in the micro structure of joining state so as to obstruct the thermal conductivity. On the other hand, in the case that the module is joined by the copper-filler-containing composite nanometal paste (“copper filler content (60%)”), the same value as the initial value is shown at the 2000 cycles, so that it is indicated that the value of thermal resistance characteristic is not changed at all. Namely, it is found that the copper-filler-containing composite nanometal paste is not only effective because of low cost but the joining property also is superior to the solder and the silver-filler-containing composite nanosilver paste.

(13) Furthermore, a working condition of Si power semiconductor does not become a design to withstand operation in high temperature environment of 200° C., but in the next generation power semiconductor, it is confirmed that it is possible to perform stable normal operations under high temperature environment of 200° C. by the next generation power semiconductor. In the point, it is shown that in the heat generation of element, it is shown that the superior thermal conductivity is assured by the joining due to the copper-filler-containing composite nanometal paste.

(14) TABLE-US-00001 TABLE 1 Packing Surface Organic Number Atom Coating C12- Particle of Atoms Number Group Coating C6-Coating Diameter An Asn Gn Amount Amount (nm) (Number) (Number) (Number) (mass %) (mass %) 20 245649 20619 5162 3.47 1.92 15 103633 11486 2873 4.53 2.53 10 30706 5006 1253 6.53 3.67 5 3838 1180 295 11.6 6.70 3 836 157 98 16.8 9.95 2 249 157 39 21.5 13.0 2 31 28 7 24.7 15.2

(15) Table 1 is a calculation result estimating the mass ratio of organic coating layer in the composite nanometal particle concerning the present invention. In the case that it is assumed that the metal core is formed by silver atoms, since the crystal structure of silver single crystal is face-centered cubic lattice, the bulk density is 74%. The lattice constant of silver single crystal is a=b=c=0.40862 nm, α=β=γ=90°, and the chemical unit number Z=4. Therefore, volume Vg of one silver atom becomes the follow by calculation.
Vg=0.74×0.40862.sup.3/4=about 1.262×10.sup.−2nm.sup.3

(16) A sphere volume of radius r is (4/3)π r.sup.3 and is regarded as about 4.19 r, and then radius Rg of silver atom is calculated as follows from the value of said volume Vg.
Rg=(1.262×10.sup.−2/4.19).sup.1/3=0.144nm

(17) When the metal core is assumed as a sphere of particle radius R, the packing number An of silver atoms in the sphere by the follow formula as the atomic packing factor of 74%.
An=0.74×4.19R.sup.3/Vg=0.246R.sup.3  (1)

(18) Therefore, as shown in the Table 1, when the particle diameter is 20 nm, the particle radius R is 10 nm, and the packing number An of atoms becomes An=246000 from said formula (1). In the case being the particle diameter of 15 nm to 2 nm, the packing number An of atoms is shown in the Table 1. Furthermore, the thickness Ta of surface layer in the metal core sphere becomes Ta=2 Rg=0.288 nm, because the radius Rg of silver atom is Rg=0.144 nm. Volume Vs of said surface layer is calculated as follows.
Vs=4.19(R.sup.3−(R−0.288).sup.3)

(19) When the closest packing factor of this case also is 74%, the surface atom number Asn of the sphere surface is obtained by the follow formula using the volume Vg of said one silver atom and the volume Vs of said surface layer.
Asn=0.74Vs/Vg

(20) Where since the organic group forming the organic coating layer has a volume, it is not considered that all of surface atoms existing on the sphere surface binds th organic groups. Accordingly, it is reasonably assumed that atoms neighboring to the silver atoms which bind the organic coating groups are not bound to the organic coating groups. Therefore, the organic coating groups are bound to ¼ of the surface atom number Asn, so that the organic coating group number Gn is expressed by the following formula.
Gn=Asn/4  (2)

(21) The value of the organic coating number Gn in a metal core sphere of 20 nm to 2 nm is calculated from the formula (2), and is described in the Table 1. When Avogadro's number N.sub.A is used, the molar number ng of the organic coating layer is represented by ng=Gn/N.sub.A and the molar number ns of the metal core sphere is expressed by ns=An/NA, and when the molecular weight of organic coating groups and the molecular weight of silver are assumed to be Mg and Ms, respectively, the mass ratio Mc of the organic coating layer comprising the organic coating groups is by the following formula.
Mc=ng.Math.Mg/(ng.Math.Mg+ns.Math.Ms)  (3)

(22) In the case that the molecular weight Ms of silver is 108 g and the organic coating group is C.sub.12H.sub.25O, the molecular weight Mg (C12) is 185 g. In addition, when the organic coating group is C.sub.6H.sub.13O, the molecular weight Mg (C6) is 101 g. The calculated result of said formula (3) by using their values is shown in the Table 1, where the case that the organic coating ground is C.sub.12H.sub.25O is represented as “C12-coating amount” and the case that the organic coating ground is C.sub.6H.sub.13O is represented as “C6-coating amount”.

(23) As shown in the Table 1, in the “C12-coating amount” that the organic coating group is C.sub.12H.sub.25O, it is found that when the particle diameter is 20 nm to 1.5 nm, the mass ratio of organic coating layer becomes about 25 mss % to 3 mass %. In the composite nanometal particle produced by using dodecanol and silver carbonate as starting materials (represented by “C12 nanosilver particle” as follows), it is confirmed that the particle diameter is several nanometers nm, and it is found that it is possible to be coated to the mass ratio of organic coating layer about 25 mass %. The mass ratio of organic coating layer is measured by the thermal analysis measurement (TG-DTA), and said C12 nanosilver particle indicates preferable dispersibility not less than 5 mass % and indicates more preferable dispersibility not less than 8 mass %. Namely, in the C12 nanosilver particle, it is preferred that the mass ratio exists within 25 mass % to 5 mass %, and when the mass ratio of organic coating layer is exceeded, it is considered that the organic ingredient ratio I too much, and the metal ingredient ratio of metal paste decreases.

(24) As shown in the Table 1, it is found that the “C6-coating amount” in which the organic coating group is C.sub.6H.sub.13O has the particle diameter of 20 nm to 1.5 nm and the mass ratio of organic coating layer is 15 mass % to 2 mass %. In the composite nanometal particle produced by using hexanol and silver carbonate as starting materials (represented by “C6 nanosilver particle” as follows), it is confirmed that the particles of average particle diameter which is not larger than 20 nm can be produced. In the “C6 nanosilver particle” of Table 1, the mass ratio of 20 nm becomes 1.92 mass %, and in the particle diameter of 20 nm to 1.5 nm, the mass ratio becomes about 2 mass % to 15 mass %. The C6 nanosilver particle can be produced in the price lower than the C12 nanosilver particle, and it is preferred that the content is lager than the C12 nanosilver particle and the C12 nanosilver particle is contained so as to be relatively little. Accordingly, when the mass ratio of organic coating layer exceeds 13 mass %, the ratio of organic ingredient is too much, so that the ratio of metal ingredient in the metal paste decreases. Namely, it is preferred that the mass ratio of organic coating layer in the C6 nanosilver particle exists within 2 mass % to 13 mass %.

(25) FIG. 3 is the graph diagram showing the thermal analysis spectra that the thermal analysis of composite nanosilver particle concerning the present invention is performed under the atmosphere, and the temperature rising rate 5° C./min. The composite nanosilver particle is produced by the method same as the production method described in the patent document 1, and the starting materials are the hexanol and the silver carbonate. Therefore, the organic coating layer derived from hexanol is formed where the carbon number N is not less 6, and is represented by “C6 composite nanosilver particle” as follows. Values of thermogravimetry shown in bold line (represented by “TG (%)” as follows) decrease with rising of temperature, and become constant values at 3.28 mass % shown by the arrow a. Accordingly, the mass ratio of organic coating layer shows becoming 3.28 mass %, and it is guessed that the particle diameter of metal core is larger than 10 nm in the produced composite nanosilver particle. The values described in the Table 1 are the values which are calculated by the model what is the maximum of organic coating group coating the metal core, and even if the mass ratio of organic coating layer is 3.28 mass %, it can be considered that the particle diameter is 5 nm to 10 nm. In addition, due to the change of the reaction temperature and the reaction time, the mass ratio of organic coating layer can be adjusted.

(26) When TG (%) decreases to about 3.2 mass %, in the values (represented by “DTA (μV)” as follows) of the differential thermal analysis measurement indicated by the thin line, peak Tm appears. The peak Tm is an exothermic peak with the metalation of said C6 composite nanosilver particle due to the pyrolysis of organic coating layer. The temperature of peak Tm is named by the pyrolysis temperature. The peak Tr in the DTA (μV) is guessed as an exothermic peak due to beginning the pyrolysis of organic coating layer, but the metalation temperature in which the organic coating layer is completely pyrolyzed is determined by the pyrolysis temperature of peak Tm.

(27) Furthermore, in the composite nanosilver particle which is produced by using the alcohol and the silver carbonate as shown in FIG. 3, the alcohol is selected among methanol having the carbon number N of 1, ethanol having the carbon number N of 2, propanol having the carbon number N of 3, butanol having the carbon number N of 4, pentanol having the carbon number N of 5, hexanol having the carbon number N of 6, heptanol having the carbon number N of 7, octanol having the carbon number N of 8, nonanol having the carbon number N of 9, decanol having the carbon number N of 10, sea urchin decanol having the carbon number N of 11 and dodecanol having the carbon number N of 12, so that it is possible to adjust the carbon number of organic coating group (also described simply by “organic matter”) forming the organic coating layer derived from alcohol. The pyrolysis temperature tends to rise with increasing of carbon number N, but depends on the reaction temperature and the reaction time, so that the pyrolysis temperature of composite nanosilver particle is set.

(28) As the composite nanosilver particles concerning the present invention, it is possible to use the composite nanosilver particle produced from the starting materials such as silver nitrate, reducing agent and solvent or the composite nanosilver particle produced from the starting materials such as stearic acid silver, reducing agent and solvent, besides the composite nanosilver particle using the alcohol described above. However, since the reducing agent is not needed, it is preferred to use the composite nanosilver particle produced by using the alcohol, so that it is possible to be produced relatively easily.

(29) FIG. 4 is the graph diagram showing the pressurization pressure dependence of shear strength where the sample pieces of 10 mmφ×10 mmφ are joined due to the heating and the pressurization for 3 minutes under the atmosphere by using the copper-filler-containing composite nanometal paste concerning the present invention. As inserted in graph diagram below, the joined members comprises first copper specimen 1 in which the diameter is 10 mnφ and the thickness is 2 mm and second copper specimen 2 in which the diameter is 10 mnφ and the thickness is 5 mm, and after the copper-filler-containing composite nanosilver particle is applied between the first copper specimen and the second copper specimen, the pressure sintering with heating is performed for 3 minutes under the atmosphere, and there is measured the shear strength of joining layer 3 in the sintered body which is formed after sintering. The maximum stress (MPa) of joining layer 3 tends to increase linearly with increasing of load (MPa) added during the sintering. When the load is 10 MPa, the maximum stress approximates 20 MPa, the share strength has practical values. When the load is not less than 20 MPa, it is practically preferred that the maximum stress surely exceeds 20 MPa.

(30) TABLE-US-00002 TABLE 2 Shear Strength of Joined Members 10 mm φ × 10 mml: Copper Filler (1.5 μm) Content 60 mass % Sintering Sintering Time Temperature Shear Strength Sample Number (sec) (° C.) (MPa) #1 180 250 22.7 #2 60 300 21.7 #3 120 300 29.1 #4 180 300 31.5 #5 60 350 41.6 #6 180 350 49.7

(31) In Table 2, when the joined members shown in FIG. 4 are joined by the copper-filler-containing composite nanosilver paste, the share strength (means “the maximum stress” described previously) is measured with changing the sintering time and the sintering temperature. As described previously, the joined members comprises the first copper specimen in which the diameter is 10 mnφ and the thickness is 2 mm and the second copper specimen in which the diameter is 10 mnφ and the thickness is 5 mm, and the load during the pressure sintering is 40 MPa. In addition, the copper-filler-containing composite nanometal paste is produced by the method same as the paste which employed in tests of FIGS. 1 to 3. In the sample #1, the sintering temperature is 250° C. being comparatively low, but the shear strength more than 20 MPa is obtained by heating for 180 sec. In the sample #2, the sintering time is 60 sec being short time, but since the sintering temperature is 300° C., the shear strength more than 20 MPa is obtained. When the sintering temperature is 300° C., in the sample #2 to 4, the shear strength (MPa) increases with increasing of the sintering time. Even if the sintering temperature is 350° C., a tendency similar to the samples #5 and 6 is shown, and it is found that the shear strength becomes more than 40 MPa.

(32) FIG. 5 is the graph diagram showing dependence of the shear strength on the copper filler content where the composite nanometal paste concerning the present invention is used and the sample pieces of 5 mm×10 mm is joined by the pressure sintering with the heating for 3 minutes under the atmosphere. In the pressure sintering, the load is 40 MPa and the sintering temperature is 300° C. The average particle diameter of copper filler is 1.5 μm, and in the composite nanometal paste containing the copper filler, the copper filler content increases from 0 mass % to 75 mass %. When the copper filler content are 45 mass %, 60 mass % and 75 mass %, the maximum stress becomes more than 70 mass %, and then it is found that the strength increases especially. According to the dependence shown in FIG. 5, it is easily found that a preferred range of the copper filler content is 40 mass % to 80 mass %. Actually, in the case more than 80 mass %, it is difficult to produce preferred paste, and it is difficult to obtain the appointed shear strength.

INDUSTRIAL APPLICABILITY

(33) As indicated previously, in the case mass-producing industrially the composite nanometal paste containing the composite nanometal particle which the organic coating layer is formed around the metal core and the copper filler, it is possible to mass-produce the composite nanometal based upon physical quantities which can evaluate a characteristic of the composite nanometal paste. Namely, in the composite nanometal particle, the pyrolysis temperature T of the organic coating layer can be specified by the mass ratio W (mass %) of organic coating layer in the composite nanometal particle which has correlation with respect to the average particle diameter. In other words, according to the mass ratios W1 and W2 of the organic coating layer in the first and the second composite nanometal particles, it is possible to be designed so that the pyrolysis temperature T1 of said first composite nanometal particle and the pyrolysis temperature T2 of said second composite nanometal particle satisfy the relationship T2>T1. Accordingly, since the first composite nanometal particle and the second composite nanometal particle are contained as a binder and the copper filler having a large particle diameter is contained as an aggregate, a strong binding power after sintering is obtained and the total amount of organic matter to the metal ingredient decreases, so that it is possible to provide the composite nanometal paste having a preferable paste characteristic. In other words, in the metal paste, when the total amount of organic matter is set less than the predetermined value, it is possible to obtain the composite nanometal paste having the preferable paste characteristic.

(34) Furthermore, according to the present invention, the copper filler which is comparatively low-priced and have superior thermal conductivity and electrical characteristic is contained with the first and the second composite nanometal particles, and the organic ingredient ratio can be set to relatively low value, so that it is possible to provide the composite nanometal paste which have joining strength and the superior thermal conductivity and electrical characteristic by the sintering in comparatively low temperature and can be produced at low cost.

(35) The present inventors perform production and tests of the composite nanometal paste containing particles formed by copper, zinc, tin or cobalt and a filler which is coated by silver besides the copper filler. However, as to the joining strength, the thermal conductivity and the electrical characteristic besides cost of raw materials, when the copper filler is contained, the composite nanometal paste having the most superior paste characteristic is obtained. Namely, since the first composite nanometal particle and the second nanometal composite particle concerning the present invention are contained and sintered strongly with the copper filler, superiority of copper filler in the superior thermal conductivity and the electrical characteristic is given, and a good result in the thermal shock test. In other words, as a result of intensive researches by the present inventors, it is discovered that the composite nanometal paste containing the first composite nanometal particle, the second composite nanometal particle and the copper filler is a preferable combination in both sides of performance and cost.

(36) In addition, when the pyrolysis temperature and the mass ratio are different as the organic coating layer of composite nanometal particle, it can be used by selecting among carboxylic acid function, alkoxide, other alcohol derivatives and amide group. Especially, the carboxylic acid, the alkoxide and other alcohol derivatives can be comparatively easily produced and in the case that the alcohol solvent and the metal compound are reacted, the reducing agent is unnecessary because the alcohol itself has a reducing function, so that producing process is simplified.

DENOTATIONS OF REFERENCE NUMERALS

(37) 1 first copper specimen 2 Second copper specimen 3 Joining layer