Bonding paste and method for manufacturing a bonded body

Abstract

To allow for appropriate application to members while preventing a decrease in the strength of a joint layer.SOLUTION: A joining paste 10 includes copper particles 12, a solvent 14, and an additive 16 composed of a phosphoester. The content of the additive 16 is 0.5% or more and 3.0% or less, by mass, relative to the total mass of the joining paste 10. The viscosity at 25°C is 10 Pa s or more and 200 Pa s or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a bonding paste and a method for manufacturing a bonded body. [Background technology]

[0002] Joining materials are sometimes used to connect two or more parts. In recent years, the use of a paste containing copper particles as a joining material has been considered. Patent Document 1 describes a method of joining a first member and a second member by sintering a copper paste applied between the two members. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-45514 [Overview of the project] [Problems that the invention aims to solve]

[0004] Here, in order to properly join the components, it is necessary to properly apply the bonding paste to the components. Furthermore, copper is easily oxidized, and if components are joined using copper paste in a non-reducing atmosphere, for example, the strength of the bonded layer may decrease. Therefore, it is necessary to ensure that the paste is properly applied to the components while suppressing the decrease in the strength of the bonded layer.

[0005] This disclosure aims to solve the above-mentioned problems and to provide a bonding paste that can be appropriately applied to a component while suppressing a decrease in the strength of the bonding layer, and a method for manufacturing a bonded body. [Means for solving the problem]

[0006] The bonding paste according to the present disclosure is a bonding paste containing copper particles, a solvent, and an additive composed of a phosphate ester, wherein the content of the additive is 0.5 mass% or more and 3.0 mass% or less relative to the entire bonding paste, and the viscosity at 25°C is 10 Pa·s or more and 200 Pa·s or less.

[0007] In the bonding paste according to the present disclosure, the average molecular weight of the phosphate ester is preferably 1000 or more and 2000 or less.

[0008] In the bonding paste according to the present disclosure, the Pb content is preferably 10 ppm or less.

[0009] A method for producing a bonded body according to the present disclosure produces a bonded body by bonding a first member and a second member using the bonding paste as a bonding layer.

[0010] In the method for producing a bonded body according to the present disclosure, the first member and the second member are semiconductor elements, and it is preferable that the bonding paste is disposed between a protruding electrode provided on the first member and a protruding electrode provided on the second member to bond the first member and the second member together. [Effects of the Invention]

[0011] According to the present disclosure, a decrease in strength of the bonding layer can be suppressed while the bonding paste is appropriately applied to members. Brief Description of the Drawings

[0012] [Figure 1] Figure 1 is a schematic diagram of the bonding paste according to the present embodiment. [Figure 2] Figure 2 is a schematic diagram illustrating a bonding method for the bonded body according to the present embodiment. [Figure 3] Figure 3 is a schematic diagram illustrating a bonding method for the bonded body according to the present embodiment. [Figure 4] Figure 4 is a schematic diagram illustrating a bonding method for the bonded body according to the present embodiment. [Figure 5]Figure 5 is a table showing the characteristics of the bonding paste for each example and the evaluation results for each example. [Modes for carrying out the invention]

[0013] The present invention will now be described in detail with reference to the drawings. However, the present invention is not limited to the embodiments described below. Furthermore, the components in the embodiments below include those easily conceivable by those skilled in the art, those substantially identical, and those within the so-called equivalent range. Moreover, the components disclosed in the embodiments below can be combined as appropriate. Numerical values ​​are rounded to the nearest whole number.

[0014] (Bonding paste) Figure 1 is a schematic diagram of the bonding paste according to this embodiment. The bonding paste of this embodiment is used to bond members together. As shown in Figure 1, the bonding paste 10 of this embodiment contains copper particles 12, a solvent 14, and an additive 16. Note that Figure 1 is a schematic diagram, and the actual shape of the bonding paste 10 is not limited to that shown in Figure 1.

[0015] (Copper particles) The copper particles 12 preferably have a BET diameter of 50 nm or more and 300 nm or less. The BET diameter is the particle size calculated from the BET specific surface area and true density of the copper particles, which are determined by the BET method, assuming the copper particles 12 are spheres or cubes. Specifically, it can be determined by the method described in the examples below.

[0016] If the BET diameter of the copper particles 12 is 50 nm or more, it is difficult to form a strong aggregate. Therefore, the surface of the copper particles 12 can be uniformly coated with the solvent 14. On the other hand, if the BET diameter of the copper particles 12 is 300 nm or less, the reaction area is large and the sinterability by heating is high, so a strong bonding layer can be formed. The BET diameter of the copper particles 12 is preferably in the range of 80 nm to 200 nm, and particularly preferably in the range of 80 nm to 170 nm.

[0017] The BET specific surface area of the copper particles 12 is 2.0 m 2 / g or more and 8.0 m 2 / g or less, more preferably 3.5 m 2 / g or more and 8.0 m 2 / g or less, and particularly preferably 4.0 m 2 / g or more and 8.0 m 2 / g or less. In addition, the shape of the copper particles 12 is not limited to a spherical shape, and may be acicular or flat plate-shaped.

[0018] Preferably, the surface of the copper particles 12 is covered with an organic protective film that is an organic film. By being covered with the organic protective film, oxidation of the copper particles 12 is suppressed, and a decrease in sinterability caused by oxidation of the copper particles 12 is further less likely to occur. It should be noted that the organic protective film covering the copper particles 12 is not formed by the solvent 14, and can be said not to be derived from the solvent 14. It can also be said that the organic protective film covering the copper particles 12 is not a copper oxide film formed by oxidation of copper.

[0019] That the copper particles 12 are covered with an organic protective film can be confirmed by analyzing the surface of the copper particles 12 using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Therefore, in the present embodiment, the copper particles 12 are detected by analyzing the surface using time-of-flight secondary ion mass spectrometry, and the Cu + ratio of the detected amount of C3H3O3 ions to the detected amount of - ions (C3H3O3 - / Cu + ratio) is preferably 0.001 or more. The C3H3O3 - / Cu + ratio is more preferably in the range of 0.05 or more and 0.2 or less. Note that the surface of the copper particles 12 in the present analysis does not refer to the surface of the copper particles 12 when the organic protective film is removed therefrom, but refers to the surface of the copper particles 12 including the covering organic protective film (that is, the surface of the organic protective film).

[0020] The copper particle 12 was analyzed on its surface using time-of-flight secondary ion mass spectrometry to determine the presence of C3H4O2. - Ions and ions of C5 or higher may be detected. + C3H4O2 relative to the amount of ions detected - Ratio of detected ions (C3H4O2) - / Cu + The ratio is preferably 0.001 or higher. Also, Cu + Ratio of the amount of C5 or greater ions detected to the amount of ions detected (C5 or greater ions / Cu + The ratio is preferably less than 0.005.

[0021] C3H3O3 detected by time-of-flight secondary ion mass spectrometry - Ions and C3H4O2 - The ions and ions with C5 or higher originate from the organic protective film covering the surface of the copper particles 12. Therefore, C3H3O3 - / Cu + Ratio and C3H4O2 - / Cu + When each of the ratios is 0.001 or higher, the surface of the copper particles 12 becomes less susceptible to oxidation, and the copper particles 12 become less susceptible to aggregation. Also, C3H3O3 - / Cu + Ratio and C3H4O2 - / Cu + When the ratio is 0.2 or less, oxidation and aggregation of the copper particles 12 can be suppressed without excessively reducing the sinterability of the copper particles 12, and the generation of decomposition gases of the organic protective film during heating can be suppressed, so that a bonding layer with fewer voids can be formed. To further improve the oxidation resistance of the copper particles 12 during storage and to further improve the sinterability at low temperatures, C3H3O3 - / Cu + Ratio and C3H4O2 - / Cu + The ratio is preferably within the range of 0.08 to 0.16. Also, C5 or higher ions / Cu + If the ratio is 0.005 or higher, a large amount of organic protective film with a relatively high desorption temperature is present on the particle surface, resulting in insufficient sinterability and difficulty in obtaining a strong bonding layer. C5 or higher ions / Cu+ The ratio is preferably less than 0.003 times.

[0022] The organic protective film is preferably derived from citric acid. A method for producing copper particles 12 coated with a citric acid-derived organic protective film will be described later. The amount of organic protective film coating on the copper particles 12 is preferably in the range of 0.5% to 2.0% by mass, more preferably in the range of 0.8% to 1.8% by mass, and even more preferably in the range of 0.8% to 1.5% by mass, based on 100% by mass of copper particles. By having an organic protective film coating amount of 0.5% by mass or more, the copper particles 12 can be uniformly coated with the organic protective film, and the oxidation of the copper particles 12 can be more reliably suppressed. Furthermore, by having an organic protective film coating amount of 2.0% by mass or less, it is possible to suppress the generation of voids in the sintered body (bonding layer) of copper particles due to gases generated by the decomposition of the organic protective film by heating. The amount of organic protective film coating can be measured using commercially available equipment. For example, the amount of coating can be measured using a differential thermal balance TG8120-SL (manufactured by RIGAKU Corporation). In this case, for example, copper particles from which moisture has been removed by freeze-drying are used as the sample. To suppress oxidation of the copper particles, the measurement is performed in nitrogen (G2 grade) gas, the heating rate is set to 10°C / min, and the weight loss rate when heated from 250°C to 300°C can be defined as the amount of organic protective film coating. That is, coating amount = (weight of sample after measurement) / (weight of sample before measurement) × 100 (wt%). The measurement is performed three times for each batch of copper particles from the same lot, and the arithmetic mean may be used as the coating amount.

[0023] When the copper particles 12 are heated at 300°C for 30 minutes under an inert gas atmosphere such as argon gas, it is preferable that 50% or more of the organic protective film decomposes. The organic protective film derived from citric acid generates carbon dioxide gas, nitrogen gas, acetone vapor, and water vapor during decomposition.

[0024] Copper particles 12 coated with an organic protective film derived from citric acid can be manufactured, for example, as follows. First, an aqueous dispersion of copper citrate is prepared, and a pH adjusting agent is added to this aqueous dispersion to adjust the pH to between 2.0 and 7.5. Next, under an inert gas atmosphere, a hydrazine compound in an amount of 1.0 to 1.2 equivalents, which can reduce copper ions, is added to this pH-adjusted aqueous dispersion of copper citrate and mixed. The resulting mixture is heated to a temperature of 60°C to 80°C under an inert gas atmosphere and held for 1.5 to 2.5 hours. This reduces the copper ions eluted from the copper citrate to produce copper particles 12, and also forms an organic protective film derived from citric acid on the surface of these copper particles 12.

[0025] Aqueous dispersions of copper citrate can be prepared by adding powdered copper citrate to pure water such as distilled water or deionized water at a concentration of 25% to 40% by mass, stirring with a stirring blade, and dispersing it uniformly. Examples of pH adjusters include triammonium citrate, ammonium hydrogen citrate, and citric acid. Among these, triammonium citrate is preferred because it allows for mild pH adjustment. The pH of the aqueous copper citrate dispersion is set to 2.0 or higher to increase the elution rate of copper ions eluted from copper citrate, thereby accelerating the formation of copper particles and obtaining the target fine copper particles 12. The pH is set to 7.5 or lower to suppress the formation of copper(II) hydroxide from the eluted copper ions, thereby increasing the yield of copper particles 12. Furthermore, by setting the pH to 7.5 or lower, the reducing power of the hydrazine compound can be suppressed, making it easier to obtain the target copper particles 12. It is preferable to adjust the pH of the copper citrate dispersion to a range of 4 to 6.

[0026] The reduction of copper citrate with hydrazine compounds is carried out under an inert gas atmosphere. This is to prevent the oxidation of copper ions dissolved in the liquid. Examples of inert gases include nitrogen gas and argon gas. Hydrazine compounds have advantages such as not producing residue after the reduction reaction when reducing copper citrate under acidic conditions, being relatively safe, and being easy to handle. Examples of hydrazine compounds include hydrazine monohydrate, anhydrous hydrazine, hydrazine hydrochloride, and hydrazine sulfate. Among these hydrazine compounds, hydrazine monohydrate and anhydrous hydrazine, which do not contain components that can become impurities such as sulfur and chlorine, are preferred.

[0027] Generally, copper generated in acidic solutions with a pH of less than 7 dissolves. However, in this embodiment, a hydrazine compound, which is a reducing agent, is added to an acidic solution with a pH of less than 7 and mixed, generating copper particles 12 in the resulting mixture. As a result, the citric acid-derived components generated from copper citrate quickly coat the surface of the copper particles 12, thereby suppressing the dissolution of the copper particles 12. It is preferable to raise the aqueous dispersion of copper citrate, after adjusting the pH, to a temperature of 50°C to 70°C to facilitate the reduction reaction.

[0028] The mixture of copper particles 12 and hydrazine compounds is heated to a temperature of 60°C to 80°C under an inert gas atmosphere and held for 1.5 hours to 2.5 hours in order to generate copper particles 12 and to form and coat the surface of the generated copper particles 12 with an organic protective film. The heating and holding under an inert gas atmosphere is to prevent oxidation of the generated copper particles 12. The starting material, copper citrate, usually contains about 35% by mass of copper. By adding a hydrazine compound, which is a reducing agent, to a copper citrate aqueous dispersion containing this amount of copper, heating it to the above temperature, and holding it for the above time, the generation of copper particles 12 and the formation of an organic protective film on the surface of the copper particles 12 proceed in a balanced manner, so that copper particles 12 can be obtained in which the amount of organic protective film covering is in the range of 0.5% by mass to 2.0% by mass per 100% by mass of copper particles. If the heating temperature is below 60°C and the holding time is less than 1.5 hours, the copper citrate may not be completely reduced, resulting in a slower rate of copper particle 12 formation and potentially an excessive amount of organic protective film coating the copper particles 12. Conversely, if the heating temperature exceeds 80°C and the holding time exceeds 2.5 hours, the rate of copper particle 12 formation may be too fast, potentially resulting in an insufficient amount of organic protective film coating the copper particles 12. The preferred heating temperature is between 65°C and 75°C, and the preferred holding time is between 2 hours and 2.5 hours.

[0029] The copper particles 12 produced in the mixture are separated from the mixture using, for example, a centrifuge under an inert gas atmosphere, and then dried by freeze-drying or vacuum drying to obtain copper particles 12 whose surface is coated with an organic protective film. Because the surface of these copper particles 12 is coated with an organic protective film, they are less susceptible to oxidation even when stored in the atmosphere until they are used as a bonding paste 10.

[0030] (solvent) Solvent 14 acts as a binder for copper particles 12. Solvent 14 is an organic solvent. Any solvent can be used as solvent 14, but examples include alcohol-based solvents, glycol-based solvents, acetate-based solvents, hydrocarbon-based solvents, and amine-based solvents. Specific examples of alcohol-based solvents include α-terpineol and isopropyl alcohol. Specific examples of glycol-based solvents include ethylene glycol, diethylene glycol, and polyethylene glycol. Specific examples of acetate-based solvents include butyltol acetate carbitate. Specific examples of hydrocarbon-based solvents include decane, dodecane, and tetradecane. Specific examples of amine-based solvents include hexylamine, octylamine, and dodecylamine.

[0031] (Additives) Additive 16 is a phosphate ester. Among organophosphate compounds, esters formed by the dehydration condensation of phosphoric acid and alcohol may be called phosphate esters. The phosphate ester used as additive 16 preferably has an average molecular weight of 1000 to 2000, more preferably 1200 to 1800, and even more preferably 1400 to 1600. An average molecular weight of 1000 or more suppresses decomposition at room temperature and improves storage stability, while an average molecular weight of 2000 or less enables decomposition and reaction at the target heating temperature (approximately 200 to 350°C). The average molecular weight here refers to the weight-average molecular weight. The average molecular weight can be measured, for example, by size exclusion chromatography.

[0032] The phosphate ester used in additive 16 can be any type, but examples include laureth-n phosphate, oleth-n phosphate, and steareth-n phosphate (where n is an integer). One of these may be used as additive 16, or two or more may be used.

[0033] (Characteristics of bonding paste) In this embodiment, it is preferable that the bonding paste 10 contains no substances other than copper particles 12, solvent 14, and additive 16 consisting of phosphate ester, excluding unavoidable impurities. However, it is not limited to this, and the bonding paste 10 may contain additives other than copper particles 12, solvent 14, and additive 16 consisting of phosphate ester. The bonding paste 10 preferably contains 10 ppm or less of lead (Pb) by mass, more preferably 5 ppm or less, and even more preferably 1 ppm or less, relative to the total mass of the bonding paste 10. A Pb content within this range makes it suitable for bonding semiconductor devices, for example. Note that the Pb referred to here does not only refer to elemental Pb metal contained in the bonding paste 10, but may also include elemental Pb metal and compounds. In other words, the above-mentioned Pb content includes both the elemental Pb metal content and the Pb ion content in the compounds. The Pb content can be measured using ICP (plasma emission spectrometer).

[0034] The bonding paste 10 preferably contains an additive 16 in a mass ratio of 0.5% to 3.0% of the total bonding paste 10, more preferably 0.5% to 2.0%, and more preferably 1.0% to 1.5%. By having an additive 16 content within this range, oxidation of copper particles 12 is suppressed, and as a result, the reduction in the strength of the bonding layer when bonding members together using the bonding paste 10 in a non-reducing atmosphere can be appropriately suppressed.

[0035] The bonding paste 10 preferably contains solvent 14 in a mass ratio of 5% to 20% of the total bonding paste 10, more preferably 5% to 15%, and even more preferably 8% to 13%. This range of solvent 14 content allows for proper dispersion of copper particles 12.

[0036] The bonding paste 10 has a viscosity at 25°C of 10 Pa·s or more and 200 Pa·s or less, preferably 20 Pa·s or more and 150 Pa·s or less, and more preferably 30 Pa·s or more and 100 Pa·s or less. A viscosity of 10 Pa·s or more makes it less likely to flow off the surface of the material, allowing it to be properly retained on the surface and applied properly. Furthermore, a viscosity of 200 Pa·s or less allows for proper sintering and suppresses a decrease in the strength of the bonded layer. Moreover, the bonding paste 10 has a viscosity at 25°C of 10 Pa·s or more and less than 50 Pa·s, more preferably 20 Pa·s or more and less than 50 Pa·s, and even more preferably 30 Pa·s or more and less than 50 Pa·s. A viscosity of less than 50 Pa·s at 25°C allows for more proper sintering and suppresses a decrease in the strength of the bonded layer. The viscosity can be measured using a Malcolm PCU-02V micro spiral viscometer.

[0037] (Method for manufacturing bonding paste) The bonding paste 10 is manufactured by performing a mixing step of mixing copper particles 12, solvent 14, and additive 16. In this mixing step, the copper particles 12, solvent 14, and additive 16 are mixed such that the content of additive 16 is 0.5% or more and 3.0% or less by mass ratio of the total bonding paste 10. In addition, in the mixing step, the copper particles 12, solvent 14, and additive 16 are mixed such that the content of solvent 14 and copper particles 12 relative to the total bonding paste 10 is also within the above range. Furthermore, in the mixing step, a kneading device may be used to mix the copper particles 12, solvent 14, and additive 16. For example, a three-roll mill can be used as a kneading device.

[0038] (Method of manufacturing the joint) Figures 2 to 4 are schematic diagrams illustrating the joining method of the joint according to this embodiment. In this embodiment, a joining paste 10 is used as a joining layer 10A to join a first member 20 and a second member 30 to manufacture a joint 100. The first member 20 and the second member 30 may be any material, but in this embodiment, they are preferably semiconductor devices. Examples of semiconductor devices include IGBTs (Insulated Gate Bipolar Transistors), diodes, Schottky barrier diodes, MOS-FETs (Metal Oxide Semiconductor Field Effect Transistors), thyristors, logic circuits, sensors, analog integrated circuits, LEDs (Light Emitting Diodes), semiconductor lasers, and oscillators.

[0039] In this embodiment, the first member 20 and the second member 30 are provided with protruding electrodes (bumps) connected to semiconductor elements. Specifically, as shown in Figure 2, the first member 20 has an element layer 22 which is a layer of a semiconductor element, and a protruding electrode 24 provided on the surface of the element layer 22 and electrically connected to the element layer 22. Similarly, as shown in Figure 3, the second member 30 has an element layer 32 which is a layer of a semiconductor element, and a protruding electrode 34 provided on the surface of the element layer 32 and electrically connected to the element layer 32. In this embodiment, a bonding paste 10 is applied between the protruding electrode 24 of the first member 20 and the protruding electrode 34 of the second member 30, and the bonding paste 10 is sintered to bond the protruding electrode 24 of the first member 20 and the protruding electrode 34 of the second member 30, thereby forming a bonded body 100.

[0040] The protruding electrodes 24 and 34 are conductive members, and in this embodiment, it is preferable that at least their surfaces are made of copper. For example, the protruding electrodes 24 and 34 may have a base material and a copper layer (copper plating layer) formed on the surface of the base material, or the entire electrode may be a copper layer. It is preferable that the copper layer of the protruding electrodes 24 and 34 is not a sintered copper body, in other words, it is preferable that it does not have voids inside. The diameter D of the protruding electrodes 24 and 34 (see Figure 3) is preferably 5 μm to 100 μm, more preferably 10 μm to 50 μm, and even more preferably 10 μm to 20 μm. The bonding paste 10 in this embodiment can appropriately bond protruding electrodes of such size together. Note that diameter D refers to the diameter of the tip surfaces 24a and 34a of the protruding electrodes 24 and 34. If the surfaces 24a and 34a are not circular, the diameter D may be the diameter obtained by converting the surfaces 24a and 34a to circular. That is, the diameter D may be the diameter of a circle with the same area as the surfaces 24a and 34a. Note that the number of protruding electrodes 24 and 34 may be arbitrary.

[0041] The method for manufacturing the joined body of this embodiment will now be described in detail. In this joining method, a coating layer formation step is performed in which a joining paste 10 is applied to at least one of the surfaces 24a of the protruding electrode 24 of the first member 20 and the surface 34a of the protruding electrode 34 of the second member 30 to form a coating layer consisting of the joining paste 10. In the example shown in Figure 2, an example is shown in which the joining paste 10 is applied to the surface 24a of the protruding electrode 24 of the first member 20, but the method is not limited to this, and the joining paste 10 may also be applied to the surface 34a of the protruding electrode 34 of the second member 30, or to both of them.

[0042] The method for applying the bonding paste 10 is not particularly limited, but examples include spin coating, metal mask coating, spray coating, dispenser coating, knife coating, slit coating, inkjet coating, screen printing, offset printing, die coating, etc. Furthermore, a preheating step may be performed after the coating layer formation step and before the subsequent overlapping step to preheat the applied bonding paste 10. The preheating step is performed to volatilize the solvent in the bonding paste 10. In the preheating step, the applied bonding paste 10 may be preheated at a temperature of 50°C to 150°C for a period of 1 to 30 minutes. Note that the preheating step may be performed after the overlapping step and before the subsequent heating step.

[0043] Next, as shown in Figure 3, an overlapping step is performed in which the first member 20 and the second member 30 are overlapped via the bonding paste 10 (coating layer). In this step, the surface 24a of the protruding electrode 24 of the first member 20 and the surface 34a of the protruding electrode 34 of the second member 30 are bonded together via the bonding paste 10.

[0044] Next, as shown in Figure 4, a heating step is performed to heat the first member 20 and the second member 30, which are stacked on top of each other via the bonding paste 10 (coating layer). In the heating step, the first member 20 and the second member 30, which are stacked on top of each other via the coating layer, are heated at a predetermined temperature for a predetermined time. By performing the heating step, the copper particles 12 in the bonding paste 10 are sintered to form a bonding layer 10A, and a bonded body 100 is manufactured in which the protruding electrode 24 of the first member 20 and the protruding electrode 34 of the second member 30 are joined by the bonding layer 10A.

[0045] In the heating process, it is preferable to heat the first member 20 and the second member 30, which are stacked together via the bonding paste 10, in a non-reducing atmosphere. Here, a non-reducing atmosphere refers to a state filled with a non-reducing gas, and can also be called an inert gas atmosphere filled with an inert gas. Examples of non-reducing gases include nitrogen and noble gases such as argon. For example, in this embodiment, the heating process may be carried out in a nitrogen atmosphere with an oxygen concentration of 1000 ppm. By carrying out the heating process in a non-reducing atmosphere, it becomes unnecessary to use a reducing gas, and the heating process can be carried out easily.

[0046] In the heating process, it is preferable to heat at least one of the first member 20 and the second member 30, which are stacked together via the bonding paste 10, while applying a predetermined pressure. The predetermined pressure is preferably 0.5 MPa to 10 MPa, more preferably 1 MPa to 5 MPa, and even more preferably 2 MPa to 5 MPa. By applying a relatively low pressure in this manner, it is possible to properly bond the first member 20 and the second member 30 while suppressing defects in the shape of the bonding layer 10A. The pressure here is applied in a direction that causes the first member 20 and the second member 30 to press against each other relatively through the coating layer.

[0047] Furthermore, the predetermined heating temperature in the heating process is preferably 200°C to 300°C, more preferably 230°C to 300°C, and even more preferably 250°C to 300°C. By setting the heating temperature in such a relatively low range, it is possible to properly sinter the copper particles 12 while suppressing shape defects in the bonding layer 10A.

[0048] Furthermore, the predetermined heating time in the heating process is preferably 1 minute or more and 10 minutes or less, more preferably 1 minute or more and 5 minutes or less, and even more preferably 1 minute or more and 3 minutes or less. By setting the heating time within this range, the copper particles 12 can be properly sintered.

[0049] As described above, in this embodiment, the bonded body 100 is manufactured by joining the protruding electrode 24 and the protruding electrode 34 with the bonding paste 10. However, it is not limited to this, and for example, the first member 20 and the second member 30 do not have protruding electrodes, and the first member 20 and the second member 30 may be joined at any position with the bonding paste 10. Also, in the example of Figure 4, the bonded body 100 has two members (semiconductor elements in this case), the first member 20 and the second member 30, but the number of members joined to the bonding paste 10 in one bonded body 100 is not limited to two, but may be three or more.

[0050] (Joining layer) As described above, the bonding layer 10A in this embodiment is formed when the bonding paste 10 is heated and the copper particles 12 are sintered. The bonding layer 10A is located between the first member 20 and the second member 30 and bonds the first member 20 and the second member 30. The bonding layer 10A can also be described as a sintered copper body. The bonding layer 10A preferably has a sintering density of 80% or more of copper particles, more preferably 85% to 95%, and even more preferably 85% to 90%. By having a sintering density within this range, the bonding layer 10A can ensure electrical conductivity and thermal conductivity. Note that sintering density refers to the ratio of the volume of the bonding layer 10A excluding open and closed pores to the total volume of the bonding layer 10A including open and closed pores. The sintering density can be calculated by randomly acquiring images of the cross-section of the bonding layer at a magnification of 30,000x using a Scanning Electron Microscope (SEM), binarizing them using image processing software (ImageJ from the National Institutes of Health, USA), separating them into particle and void regions, and then calculating the sintering density using the following formula (1).

[0051] Sintered density (%) = (Total area of ​​particles / (Total area of ​​particles + Total area of ​​voids)) × 100 ... (1)

[0052] Furthermore, it is preferable that the bonding layer 10A contains phosphorus. Here, phosphorus refers to phosphorus as an element, and includes not only elemental phosphorus but also phosphorus contained in any compound. The phosphorus content of the bonding layer 10A is preferably 10 ppm to 1000 ppm by mass ratio, more preferably 50 ppm to 500 ppm, and even more preferably 100 ppm to 500 ppm. Having a phosphorus content within this range suppresses a decrease in strength of the bonding layer 10A, even when it is formed, for example, under a non-reducing atmosphere. The phosphorus content can be measured using ICP-OES (Inductivity Coupled Plasma Optical Emission Spectrometer). In this embodiment, the phosphorus contained in the bonding layer 10A is derived from the phosphate ester contained in the bonding paste 10.

[0053] Furthermore, the bonding layer 10A preferably has a thickness of 10 μm to 200 μm, more preferably 20 μm to 150 μm, and even more preferably 50 μm to 100 μm. By having a thickness within this range, the bonding layer 10A can maintain high heat dissipation while mitigating stress caused by differences in the coefficient of linear expansion between members due to temperature differences.

[0054] As described above, the bonding layer 10A according to this embodiment is a copper sintered body containing phosphorus and having a sintering density of 80% or more. Because the bonding layer 10A according to this embodiment has a sintering density of 80% or more, a decrease in strength is suppressed. Furthermore, the oxidation of copper particles 12 is suppressed by the phosphorus-derived product (phosphate ester in this embodiment) contained in the bonding layer 10A, and as a result, a decrease in strength is suppressed in the bonding layer 10A containing phosphorus. In this embodiment, the bonding layer 10A is formed by heating the bonding paste 10 as described above, but the method of forming the bonding layer 10A may be arbitrary as long as it satisfies the above characteristics.

[0055] (effect) As described above, the bonding paste 10 according to this embodiment contains copper particles 12, a solvent 14, and an additive 16 consisting of a phosphate ester, with the content of the additive 16 being 0.5% to 3.0% by mass relative to the total bonding paste 10. The bonding paste 10 has a viscosity of 10 Pa·s to 200 Pa·s at 25°C. In this embodiment, the bonding paste 10 has a phosphate ester added as an additive, which suppresses the oxidation of copper particles 12. As a result, even when joining members in a non-reducing atmosphere, a decrease in the strength of the bonded layer can be suppressed. Furthermore, because the viscosity is within the above range, the bonding paste 10 can be properly applied to the members and properly sintered, thus suppressing a decrease in the strength of the bonded layer.

[0056] In the bonding paste 10 according to this embodiment, the average molecular weight of the phosphate ester is preferably 1000 or more and 2000 or less. By using a phosphate ester with such a molecular weight, the decrease in the strength of the bonding layer can be more effectively suppressed.

[0057] In this embodiment, the bonding paste 10 preferably has a Pb content of 10 ppm or less. By reducing the Pb content in this way, it becomes suitable for bonding semiconductor devices, for example.

[0058] The method for joining a joint according to this embodiment involves joining a first member 20 and a second member 30 using a joining paste 10 as a joining layer 10A to manufacture a joint 100. According to this embodiment, by using the joining paste 10, the joining paste 10 can be appropriately applied to the members, and a decrease in the strength of the joining layer can be suppressed.

[0059] In the joining method for a joined body according to this embodiment, the first member 20 and the second member 30 are semiconductor elements, and the joining paste 10 is placed between a protruding electrode 24 provided on the first member 20 and a protruding electrode 34 provided on the second member 30 to join the first member 20 and the second member 30. According to this embodiment, the protruding electrodes can be properly joined together.

[0060] (Examples) Next, we will describe the examples. Figure 5 is a table showing the characteristics of the bonding paste for each example and the evaluation results for each example.

[0061] (Example 1) In Example 1, copper particles with a BET diameter of 153 nm were prepared. The BET diameter was determined by measuring the amount of nitrogen gas adsorbed by the copper particles using a specific surface area analyzer (QUANTACHROME AUTOSORB-1, manufactured by Quantachrome Instruments), and then determining the specific surface area of ​​the copper particles by the BET method. The obtained specific surface area S(m²) 2 ( / g) and the density of copper particles ρ (g / cm³) 3 Using the above, the BET diameter was calculated from the following equation (2). BET diameter (nm) = 6000 / (ρ(g / cm) 3 ) × S(m 2 / g))···(2) In Example 1, oleth-10 phosphate (molecular weight 1492) was prepared as the phosphate ester for the additive, and ethylene glycol was prepared as the solvent. In Example 1, a bonding paste was obtained by mixing copper particles, an additive, and a solvent such that the additive content was 1% by mass, the solvent content was 10% by mass, and the remainder was copper particles. Furthermore, the viscosity of the bonding paste obtained in Example 1 was measured at 25°C using the method described in this embodiment. The measurement results are shown in Figure 5. In Example 1, the Pb content was 10 ppm or less.

[0062] (Comparative Example 1) In Comparative Example 1, a bonding paste was obtained in the same manner as in Example 1, except that the solvent content was 15% by mass. The viscosity measurement results for the bonding paste obtained in Comparative Example 1 at 25°C are shown in Figure 5.

[0063] (Manufacturing of joints) A bonded body was manufactured using the bonding paste obtained in each example. Specifically, two protruding electrodes with a diameter of 75 μm were prepared, and the bonding paste was printed onto the surface of one of the protruding electrodes to a target film thickness of 5 μm. After drying on a hot plate at 90°C for 5 minutes, the surfaces of the two protruding electrodes were placed facing each other with the bonding paste in between, and the bond was achieved by heating at 250°C for 3 minutes under a nitrogen atmosphere while pressurizing at 5 MPa.

[0064] (evaluation) In the evaluation, we assessed the applicability of the bonding paste to the surface of the protruding electrode and the shear strength of the resulting bonded structure. In terms of coatability, a coating thickness of ±30% of the target thickness was considered acceptable (○), while any other result was considered unacceptable (×). For shear strength, a value of 20 MPa or higher was considered a pass (〇), and a value of less than 20 MPa was considered a fail (×). The shear strength of the resulting joint was measured using a method compliant with JIS Z 3198-7 (Test methods for lead-free solder - Part 7: Shear test methods for solder joints of chip components). Specifically, a load was applied to the protruding electrode using a bond tester (Nordson DAGE, SERIES 4000), and the load at which the protruding electrode separated from the copper bonding layer (maximum shear load) was measured. The tool's movement speed was set to 50 μm / sec, and the gap between the tool tip and the protruding electrode was set to 50 μm. The obtained maximum shear load was converted to Newtons and divided by the area of ​​the copper bonding layer (2.5 mm × 2.5 mm) to obtain the shear strength (unit: MPa). Seven joints were fabricated, and the shear strength of each joint was measured. The evaluation results for each are shown in Figure 5.

[0065] As shown in Figure 5, in Example 1, by using a bonding paste containing 0.5% to 3.0% by mass ratio of phosphate ester and having a viscosity of 10 Pa·s to 200 Pa·s, it is possible to properly apply the paste and suppress the reduction in the strength of the bonded body. On the other hand, in Comparative Example 1, the viscosity requirement of 10 Pa·s to 200 Pa·s was not met, and both the applicability and shear strength were unsatisfactory. Therefore, it is not possible to satisfy both the requirement of properly applying the paste and suppressing the reduction in the strength of the bonded body.

[0066] Although embodiments of the present invention have been described above, the embodiments are not limited to those described herein. Furthermore, the aforementioned components include those that can be easily conceived by those skilled in the art, those that are substantially the same, and those that fall within the so-called equivalent range. Moreover, the aforementioned components can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the components can be made without departing from the spirit of the embodiments described above. [Explanation of Symbols]

[0067] 10 Bonding paste 10A bonding layer 12 copper particles 14 Solvents 16 Additives 20 First component 24, 34 protruding electrode 30 Second component 100 zygote

Claims

1. A bonding paste comprising copper particles, a solvent, and an additive consisting of one or more phosphate esters, including laureth-n phosphate, oleth-n phosphate, and steareth-n phosphate (where n is an integer), The content of the additive is 0.5% or more and 3.0% or less by mass ratio with respect to the total amount of the bonding paste. The viscosity at 25°C is between 10 Pa·s and 200 Pa·s. Bonding paste.

2. The bonding paste according to claim 1, wherein the average molecular weight of the phosphate ester is 1,000 or more and 2,000 or less.

3. The bonding paste according to claim 1 or claim 2, wherein the Pb content is 10 ppm or less.

4. A method for manufacturing a joined body, comprising joining a first member and a second member using the joining paste described in claim 1 or claim 2 as a joining layer.

5. The method for manufacturing a bonded body according to claim 4, wherein the first member and the second member are semiconductor elements, and the bonding paste is placed between a protruding electrode provided on the first member and a protruding electrode provided on the second member to bond the first member and the second member.

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