JP6713120B1 - Copper Sintered Substrate Nano Silver Impregnated Bonding Sheet, Manufacturing Method and Bonding Method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To use for high temperature heat resistant bonding of a power semiconductor and a copper substrate, easy to mount, excellent in strength, electrical conductivity and thermal conductivity of the formed bond, capable of being manufactured at low cost, and capable of being stored for a long period of time. Provided are a joining sheet for binding and joining, a manufacturing method thereof, and a joining method. A copper sintered substrate (2), which is a porous copper sintered structure composed of a plurality of copper nanoparticles (4) sintered and bonded, and is disposed on a surface (33) and micropores (21) of the copper sintered substrate (2). A bonding sheet 1 composed of a plurality of silver single nanoparticles 5 having an organic coating, wherein the silver single nanoparticles 5 are bonded to each other and also to the copper sintered structure, and the number density of the silver single nanoparticles 5 is Is high on the surface 33 of the bonding sheet 1 and low on the inside, and is made to have a gradient concentration, and the silver single nanoparticles 5 have a silver nucleus 84% particle diameter D84 of 1.4 to 2.0 nm and a particle diameter distribution width of 15% or less. A carboxylic acid-coated silver nanoparticle having an organic coating amount of 10 to 15% by mass. [Selection diagram] Figure 2

Description

The present invention relates to a joining sheet for sinter joining, a manufacturing method thereof, and a joining method, and more specifically, a sinter joining for forming a joining structure that can be used at high temperature, such as joining a power semiconductor and a copper substrate. TECHNICAL FIELD The present invention relates to a joining sheet for a car, a manufacturing method thereof and a joining method.

With the recent advancement of advanced technology, miniaturization and high performance of joint products such as semiconductors are demanded, and joint materials having heat resistance that can be used even in a high temperature environment are demanded. Lead alloy solder, which is currently on the market, is designated as an environmentally harmful substance because the lead metal ions of harmful substances are emitted when industrial products using the solder are discarded, and currently there is no alternative. It is the situation where the use is permitted exceptionally for the reason. The upper limit temperature for use of lead-free alloy solder is currently 230°C, and even the use temperature of lead solder is 320°C, which is far from the demand for heat resistance required by the market. There is a demand for a lead-free bonding material that can be used at higher temperatures and has various properties that are recognized by the market.

Since silver nanoparticles can be sintered at a relatively low temperature and have excellent electrical and thermal characteristics, various manufacturing methods have been developed as a semiconductor bonding material that replaces lead solder, and the dispersion of them has been performed. Glue pastes are well known. In addition, a nano-metal bonding sheet, which is made by kneading a high molecular weight organic material into the paste and excluding the viscosity adjusting solvent of the paste and processing it into a thin sheet, is developed, and the paste is applied, dried, and sintered. A nano metal bonding sheet (patent document 4), which is devised to be simpler when mounting than a process step, and a nano silver bonding sheet (sixth patent document 6) whose sintering is completed incompletely are also known. There is. However, these conventional nano-metal paste materials are superior in terms of bonding characteristics to lead solder and lead-free solder, but they have protrusions during die attachment, uneven paste coating thickness, and brittle sintered layers. There are various problems in the mounting process, such as compatibility. Nanometal bonding sheet, which is a sheet material of nanometal paste, has solved the problem of simplicity in the mounting process, but the temperature and temperature of the paste are higher than that of the paste during the firing process of the polymer material that is kneaded during sheet formation. Time baking is necessary, and improvement in this respect is indispensable.

In JP 2006-202944 A (Patent Document 1), as shown in FIG. 15, a copper porous plate 103, which is a porous metal layer, is interposed between a silver electrode 111 of a semiconductor element 101 and a copper substrate 121a. Then, the nano silver paste 104 is placed between the silver electrode 111 and the copper porous plate 103 and between the copper substrate 121a and the copper porous plate 103, and the invention is disclosed as a method of heating and bonding. .. This invention has the effects of reducing the amount of nano silver paste used and relaxing stress, but since the organic components contained in the binder of nano silver paste are gasified in large quantities during firing, the bonding strength of the joint structure formed is increased. , Heat resistance, electrical conductivity and thermal conductivity are adversely affected, and since the paste-like joining material has various complications in the mounting process as compared with the sheet-like joining material, and the heating and joining process However, there is a drawback in that a long holding time is required at the firing set temperature.

There are some conventional techniques for a method of applying a nano silver paste to a material to be bonded and bonding the material by firing, and a bonding structure resulting therefrom. Japanese Unexamined Patent Application Publication No. 2012-074627 (Patent Document 2) discloses a method for reducing the contact resistance, and Japanese Unexamined Patent Application Publication No. 2018-193604 (Patent Document 3) discloses a method for improving heat dissipation and suppressing damage. From the viewpoint of cost reduction, Japanese Patent Laid-Open Publication No. 021578 (Patent Document 5) discloses an invention of a bonding method or a bonding structure similar to that of Patent Document 1. However, since each uses a paste-like bonding material, there are various complications in the mounting process as compared with a sheet-shaped bonding material, and it takes a long time to perform heating and bonding at the firing set temperature. There are drawbacks.

Japanese Unexamined Patent Application Publication No. 2017-069560 (Patent Document 4) discloses an invention of a heating bonding sheet containing metal fine particles. However, in the present invention, since the heat-decomposable binder contained in the heat-bonding sheet generates a large amount of gas during firing, the bonding strength and the like are reduced, and it takes a long time for heat-bonding at the firing set temperature. Etc. In addition, International Publication No. 2015-056589 (Patent Document 6) discloses a silver sheet for heat bonding, which is produced by sintering a paste containing silver nanoparticles, and the sintering is stopped halfway. Certain silver sheet inventions are disclosed. However, in the present invention, it takes a further 3 to 30 minutes for complete sintering in heating and joining at a firing temperature of 170° C. to 250° C. under pressure, and a large amount of silver nanoparticles are contained. Therefore, there are drawbacks such as high cost.

Generally speaking, the problems required for high precision and miniaturization bonding in the field of semiconductor mounting using bonding sheets for heat bonding are summarized as follows: simplification of mounting process, shortening of time, cost reduction, In addition to maintaining high functionality, it also has long-term stability and at the same time guarantees mass productivity, and environmental safety at the time of disposal of products, which has become important in recent years.
More specifically, the joining sheet is required to have the following properties as an industrial product. (1) Before firing, the stability of the quality of the product as an industrial product during storage, the ease of cutting, such as ease of cutting, and the convenience of the laminate forming work. (2) When sintering proceeds, lower sintering temperature, shorter sintering. Long time, safety of decomposed components generated during sintering, (3) Bonding strength characteristics, electric conduction/heat conduction characteristics, proper response to changes in environmental temperature after completion of sintering, that is, stress relaxation characteristics Quality reliability is required. As described above, no product that solves all of these problems can be found, and it must have excellent characteristics and be optimized for stress relaxation as compared with current market products such as solder. An object of the present invention is to realize a product having performance characteristics which are superior in all the above points to a solder bonding agent and a bonding sheet which are commercial products.

JP, 2006-202944, A JP 2012-074627 A JP, 2018-193604, A JP, 2017-069560, A JP, 2016-021578, A International Publication No. 2015-056589

Ph. Buffat and JP. Borel Phys. Rev. A13, 2287 (1976)

Therefore, the subject of the present invention is lead-free, can be used for high temperature heat resistant bonding applications, is easy to mount, has excellent strength, electrical conductivity and thermal conductivity of the formed bond, and can be manufactured at low cost for a long time. A storable joining sheet for sinter joining, a method for producing the joining sheet, and a joining method using the joining sheet.

The present invention has been made in order to solve the above problems, and a combination of a copper sintered substrate composed of copper nanoparticles and a single nano silver monodispersed liquid applied to and dried on the copper sintered substrate, The object is to solve the above-mentioned problems by realizing a pressure/short-time sintering bonding type bonding sheet which is bonded and exhibits long-term reliability. That is, the metal component of the nano metal paste is replaced with copper nanoparticles to reduce the cost, and the nano copper paste is fired without pressure or under low pressure, and has a uniform thickness with a large number of fine pores and parallelism. Well, create a sintered copper substrate with a thickness of about several tens of microns that has an internal stress relaxation function, and use a single nanoparticle of uniform particle size on the surface to be the joint surface, preferably on both surfaces. The silver monodisperse liquid is soaked from the surface to the inside, coated, and dried. Preferably, the firing temperature of the coating and drying film (nanosilver-impregnated region) of a single nanosilver dispersion liquid in which silver single nanoparticles are monodispersed and coated with a low molecular weight carboxylic acid can be set to about 250° C. The particle size distribution width of the nanoparticles is small, and there is a feature that it is possible to design the firing to proceed at once in a short time.

A first aspect of the present invention is a copper sintered substrate, which is a porous copper sintered structure composed of a plurality of copper nanoparticles bonded by sintering, and a copper sintered substrate disposed on the surface and fine pores of the copper sintered substrate. A bonding sheet composed of a plurality of silver single nanoparticles having an organic coating, wherein the silver single nanoparticles are bonded to each other and also to the copper sintered structure, and the number density of the silver single nanoparticles is The joining sheet is characterized in that it is high on at least one surface of the joining sheet, low inside, and has a gradient density.

In a second aspect of the present invention, the silver single nanoparticles are silver having a 84% particle size (D ₈₄ ) in the particle size distribution of silver nuclei of 1.4 to 2.0 nm and a particle size distribution width of 15% or less. The bonding sheet is nanoparticles.

A third aspect of the present invention is the bonding sheet, wherein the silver single nanoparticles are carboxylic acid-coated silver nanoparticles having an organic coating amount of 10 to 15% by mass.

A fourth aspect of the present invention is the bonding sheet, wherein the number density is lowered from both front and back surfaces of the bonding sheet toward the inside.

A fifth aspect of the present invention is the bonding sheet, wherein the number density decreases from one surface to the other surface of the front and back surfaces of the bonding sheet. However, even on the surface having the lower number density, the number density of the silver single nanoparticles is a number density having a bonding function.

A sixth aspect of the present invention is a single nanosilver dispersion liquid preparation step of dispersing silver single nanoparticles having an organic coating in a solvent to obtain a single nanosilver dispersion liquid, and a nanocopper paste containing copper nanoparticles. A copper sintered substrate forming step of forming a porous copper sintered substrate by firing, and applying or impregnating the single nano silver dispersion liquid on the surface of the copper sintered substrate and further drying the copper A bonding sheet, which comprises a step of imparting a gradient function by arranging gradient-concentrated silver single nanoparticles on the surface of the sintered substrate and a volume region adjacent to the surface, It is a manufacturing method.

The 7th form of this invention is a joining method which joins a 1st to-be-joined object and a 2nd to-be-joined object via the said joining sheet, and a 1st to-be-joined object and one or more said joining sheets. And a second object to be joined in this order to form a laminate, and a step of firing the laminate while applying pressure to form a joined body. is there.

An eighth aspect of the present invention is the joining method, wherein in the laminating step, an immobilization aid for ensuring immobilization at the time of mounting is added to the surface of the joining sheet prior to the lamination.

A ninth aspect of the present invention is a laminated body obtained by laminating and temporarily fixing the joining sheet on an article to be joined.

According to the first aspect of the present invention, a copper sintered substrate, which is a porous copper sintered structure composed of a plurality of copper nanoparticles bonded by sintering, and a copper sintered substrate arranged on the surface and fine pores of the copper sintered substrate. A plurality of silver single nanoparticles having an organic coating, wherein the silver single nanoparticles are bonded to each other, the copper sintered structure is also bonded, the number density of the silver single nanoparticles. Can provide a joining sheet characterized by being high in at least one surface of the joining sheet, low in the inside thereof, and having a gradient concentration.

The bonding sheet of this embodiment is composed of a sintered copper sintered substrate and an unsintered silver single nanoparticle group. The copper sintered substrate has a porous sintered structure of copper metal (copper sintered structure) having fine pores and interparticle-bonded to each other by sintering copper nanoparticles. The silver single nanoparticles having an organic coating are adsorbed and distributed on the surface of the copper sintered substrate and in the fine pores of the copper sintered structure. With reference to FIG. 2, such a configuration including the copper sintered substrate 2 and the silver single nanoparticles 5 is, for example, a nano copper paste containing the copper nanoparticles 4 is fired to produce the copper sintered substrate 2. It can be formed by coating and impregnating the surface of the copper sintered substrate 2 with a single nano silver dispersion liquid containing the silver single nano particles 5, and further removing the solvent of the single nano silver dispersion liquid by drying. "Comprised of a copper sintered substrate and a silver single nanoparticle group" means that the solvent used when arranging the silver single nanoparticles on the surface of the copper sintered substrate and on the micropores has been removed. Means Since the bonding sheet of this embodiment has a dry surface because the solvent has been removed, there is no problem in storing a plurality of sheets in a stack even without applying a film or the like for the cover in the air. Dust is not easily attached, is easy to handle and has excellent storability. In addition, since the solvent has been removed, no gas is generated due to thermal decomposition or oxidative decomposition of the solvent during pressure firing for bonding using the bonding sheet. There is no destruction or deterioration of the structure, and the low temperature melting phenomenon of the silver metal component of the silver single nanoparticles forms a strong joint structure by silver plating and silver-copper alloying of the porous sintered structure of copper metal. It

There are two types of joining sheets of this embodiment. One is type A in which silver single nanoparticles are densely distributed on both sides, as shown in Fig. (1A), and the other is silver single nanoparticles, as shown in Fig. (1B). Is a type B in which one side has a high density and the back side has a distribution controlled to a low concentration such that bonding during firing is possible. When the joining sheet of the present embodiment is used, for example, a type A main joining sheet is sandwiched between two objects to be joined to form a laminate, and the laminate is pressure-fired to be included in the main joining sheet. While oxidizing and removing the organic coating of the silver single nanoparticles, the silver nuclei of the silver single nanoparticles are sintered with the silver nuclei of other silver single nanoparticles, the copper nanoparticles already sintered and bonded, or the object to be bonded. They are joined together to form a joined body in which two objects to be joined are joined via the present joining sheet. When firing for bonding, the copper sintered substrate has already been sintered and only the silver single nanoparticles have to be sintered, so that the firing time at the firing setting temperature can be short. In addition, in this bonding sheet, the silver single nanoparticles that need to be sintered are concentrated near the surface where heat transfer is fast, and the time required for firing at the firing setting temperature can be as short as 10 to 60 seconds. , Organic coating of silver single nanoparticles that can be fired in about 1/5 of the time of bonding with conventional nano-silver paste, has no solvent that thermally decomposes to generate gas, and causes a decrease in bonding strength Since the amount of gas generated due to the thermal decomposition of is small, a large bonding strength can be secured. Furthermore, the time and effort required for coating, worries of squeeze-out, and the time required for drying, which are required with conventional nanosilver pastes, are unnecessary. In addition, the bonding sheet of this embodiment can be manufactured at low cost because the amount of silver used is small.

Furthermore, in the bonding sheet of the present embodiment, the concentration distribution of the silver single nanoparticles in the direction perpendicular to the surface becomes smaller as it goes inward (see FIG. (1c)), and the bonding interface with the object to be bonded is decreased. In the vicinity of 33, silver single nanoparticles are concentrated at almost 100%, that is, at a relative concentration of 95% or more (ratio of the density of the silver component to the total density of the silver component and the copper component), and firing for bonding. The bonding force at the bonding interface afterward is entirely borne by the melt of the silver nuclei whose melting point is lowered by the sintering of the silver single nanoparticles, and the strong bonding with excellent thermal conductivity and electrical conductivity is designed. There is. Further, inside the bonding sheet, the silver nuclei of silver single nanoparticles and the porous copper sintered structure are almost alloyed and integrated to form a strong bond.

In the present specification, the copper nanoparticles refer to copper particles that are not organically coated and have an average particle size of about 5×10 ¹ nm. In the bonding sheet of the present embodiment, the porous copper sintered substrate composed of a plurality of copper nanoparticles bonded by sintering exhibits stress relaxation characteristics, so that the difference in temperature expansion/contraction characteristics between the objects to be bonded during firing for bonding. It has the advantage of forming a strong joint that does not depend on the above, and has the advantages of high durability and long-term reliability of the formed joint after firing for joining. For example, by sandwiching the bonding sheet of the present embodiment, a metal semiconductor on the bonding surface and a copper substrate can be firmly bonded by pressure firing, and the formed bonding is resistant to thermal shock. Has long-term durability.

In the present specification, the silver single nanoparticles mean silver nanoparticles having an average particle size of about 2 nm. From the viewpoint of dispersibility in a solvent used when arranging silver single nanoparticles on a copper sintered substrate, the silver single nanoparticles have an organic coating. It means the average (number average) of the grain size (diameter) of the silver nuclei, which does not include the part.

According to the second aspect of the present invention, the silver single nanoparticles have a 84% particle size (D ₈₄ ) in the particle size distribution of silver nuclei of 1.4 to 2.0 nm and a particle size distribution width of 15% or less. It is possible to provide the bonding sheet, which is the silver nanoparticles. The 84% particle size (D ₈₄ ) is obtained by arranging a large number of particles in ascending order of particle size, and the particle with the smallest particle size is 0% and the particle with the largest particle size is 100%. When the particle size distribution is a normal distribution, it is a particle size corresponding to a value (m+σ) obtained by adding a standard deviation (σ) to the average particle size (m). In the silver single nanoparticles of this embodiment, 84% of the particles have a particle size of 2.0 nm or less, and the melting point of the particles is 250° C. or less. Therefore, sinter-bonding at a low temperature of about 250° C. to form silver. Is possible. Note that it is difficult to control the particle size of the silver single nanoparticle group having the 84% particle size of less than 1.4 nm during production. Further, it is preferable that the width of the particle size distribution (the ratio of the standard deviation of the particle size distribution to the average particle size) is as narrow as 15% or less. For example, if the average particle size is 1.7 nm±0.3 nm (standard deviation of particle size), the particle size distribution width is 15%, which satisfies this condition. When the particle size distribution width of the silver single nanoparticles contained in the bonding sheet is narrow, the silver single nanoparticles can be melted and bonded at almost the same temperature at the time of firing for bonding. There is an advantage that the process proceeds all at once, and the sinter bonding is efficiently performed in a short time, so that a strong bond is surely formed.

According to the third aspect of the present invention, it is possible to provide the bonding sheet in which the silver single nanoparticles are carboxylic acid-coated silver nanoparticles having an organic coating amount of 10 to 15% by mass. The silver single nanoparticles whose organic coating component is derived from carboxylic acid generate heat during the sintering due to the oxidative decomposition reaction of the carboxylic acid. Therefore, even when the object to be bonded and the bonding sheet of the present embodiment are bonded at a low sintering environment temperature of about 250° C., the silver single nano-particles existing near the bonding interface with the object to be bonded in the bonding sheet. The particles will be in a molten state in a chain with the progress of sintering, and the joining will proceed reliably with efficiency and acceleration. That is, the heating effect caused by the oxidative decomposition reaction of the organic coating of the silver single nanoparticles during heating and sintering causes the temperature around the bonding interface of the bonding sheet to spontaneously rise, and the sintering is designed to be accelerated at once. There is. Since the "functional silver-copper hybrid sintered layer" designed in this way can be formed at the time of firing, the joining sheet of the present embodiment can be sintered for a short time and to be strong even at a low sintering environment temperature. It has the advantage of forming bonds. In addition, when the organic coating amount of the silver single nanoparticles is less than 10% by mass, the dispersibility of the silver single nanoparticles in the solvent of the single nanosilver dispersion described later becomes poor and the silver single nanoparticles are easily aggregated. When the nanoparticles are arranged on the copper sintered substrate, it becomes difficult to arrange them uniformly in the direction along the surface thereof. Further, when the organic coating amount exceeds 15% by mass, it takes time to oxidize and decompose the organic coating, making it difficult to sinter for a short time.

In the present embodiment, from the viewpoint of ensuring the above-mentioned heat generation effect, it is preferable that the smaller the particle size of the silver nuclei of the silver single nanoparticles, the smaller the molecular weight and the carbon number of the carboxylic acid coated with the carboxylic acid. However, if the silver nuclei are too small, it is difficult to control the particle size when they are formed by the wet reduction method. For silver nuclei having a particle size of about 2.0 nm, octanoic acid having 8 carbon atoms is optimal as the carboxylic acid.

According to the fourth aspect of the present invention, it is possible to provide the joining sheet in which the number density decreases from both surfaces of the front and back surfaces of the joining sheet toward the inside. The joining sheet of the present invention is of two types, type A and type B. The present embodiment is a type A joining sheet among them. For example, a strong bond can be formed by laminating two materials to be bonded having different temperature expansion/contraction characteristics via a bonding sheet of type A, such as a silicon semiconductor and a copper substrate, and press-baking them at a low temperature. The formed joint has long-term durability against thermal shock.

In the case of the type A joining sheet, as shown in FIG. 2A, the number density of silver single nanoparticles is large on both the front and back surfaces of the joining sheet and is small inside, and the distance from one surface is large. Is plotted on the abscissa and the value of the ratio is plotted on the ordinate, a U-shaped graph is obtained. In the case of the type B joining sheet, as shown in FIG. 2B, the number density is large on the surface on one side of the joining sheet and decreases as it goes inward, and the distance x from the surface on one side is reduced. When a graph is drawn with the value of the ratio on the horizontal axis and the vertical axis, the graph decreases exponentially. When d is the thickness of the copper sintered substrate, C and L are positive parameters, and the value of the ratio is fitted with an exponential function C×exp (−x/L) in the range of 0<x≦d, the length is L×ln(2) is referred to as the half-thickness of the type B joining sheet. Further, in the case of the type A joint sheet, C, D, and L are positive parameters, and the value of the ratio is the sum of two exponential functions C×exp(−x/L) in the range of 0<x≦d. )+D×exp((x−d)/L), the length L×ln(2) is called half thickness. In any of the joining sheets, the relative concentration of silver single nanoparticles on the surface coated with nanosilver is almost 100%, that is, 95% or more.

The volume region included in the range (0≦x≦L) from the surface coated with nano silver of the bonding sheet to the depth L is referred to as a nano silver-impregnated region. For convenience, the nano-silver-impregnated region (3) is a defined volume region, and the silver single nanoparticles are also present in the portion other than the nano-silver-impregnated region of the bonding sheet. When the thickness of the joining sheet is thin, the entire joining sheet of both type A and type B becomes the nano silver-impregnated region.

From the viewpoint of ensuring sufficient bonding strength, the minimum content of silver single nanoparticles is necessary, and the half-thickness is preferably not too small. Although the half-thickness is not limited, it is generally 1 to 100 μm, and more preferably 5 to 20 μm. When the thickness of the joining sheet is thin, it is not necessary to set an upper limit on the half-thickness.

According to the fifth aspect of the present invention, it is possible to provide the bonding sheet in which the number density decreases from one surface to the other surface of the front and back surfaces of the bonding sheet. This form is a type B joining sheet. As the type B joining sheet, one having a small thickness and having the entire joining sheet being a nano silver-impregnated region is usually used, and two sheets having a half thickness are used in piles and used in place of the type A. By doing so, after firing and bonding, the "functional silver-copper hybrid sintered layer" is arranged instead of the most fragile inside of the copper sintered substrate in the bonding sheet of type A, and a large bonding strength is secured. You can

According to the sixth aspect of the present invention, a single nanosilver dispersion liquid preparation step of dispersing silver single nanoparticles having an organic coating in a solvent to obtain a single nanosilver dispersion liquid, and a nanocopper containing copper nanoparticles By firing the paste to form a porous copper sintered substrate, a copper sintered substrate forming step, and by coating or impregnating the single nano silver dispersion liquid on the surface of the copper sintered substrate, and further drying. A gradient function imparting step of imparting a gradient function by arranging gradient-concentrated silver single nanoparticles on the surface of the copper sintered substrate and a volume region adjacent to the surface. A method for manufacturing a joining sheet can be provided.

In the single nano silver dispersion liquid preparation step, silver single nanoparticles having an organic coating are dispersed in a solvent to obtain a single nano silver dispersion liquid. The solvent is not limited, but an alkane solvent such as methylcyclohexane can be used. A small amount of a low boiling point solvent may be added to adjust the viscosity. It is preferable that the silver single nanoparticles have a narrow particle size distribution width of about 15% or less and are monodispersed in a solvent. The concentration is adjusted using an evaporator to obtain a single nanosilver dispersion liquid containing silver single nanoparticles at a concentration of about 10% by mass.

In the step of forming a copper sintered substrate, a nano copper paste containing copper nanoparticles is fired to form a porous copper sintered substrate. A nano-copper paste is applied on a mirror-polished substrate, for example, a silicon substrate, with a uniform thickness of about 15 to 200 μm, and the solvent contained in the nano-copper paste is dried, and no pressure is applied in a pure nitrogen gas atmosphere. Alternatively, it is fired at a low pressure to separate it from the silicon substrate to obtain a self-standing sintered copper sintered substrate having a uniform thickness of about 10 to 150 μm.

In the gradient function imparting step, at least one surface of the copper sintered substrate that is a porous copper sintered structure is coated or impregnated with the single nanosilver dispersion liquid and further slowly dried at a low temperature, Bonding formed by forming a nano-silver impregnated region containing silver single nanoparticles having a gradient-concentrated organic coating on at least the surface side of a copper sintered substrate, and coating the copper sintered substrate with nano silver Get the sheet. By forming the nano-silver-impregnated region in this way, the concentration of silver single nanoparticles in the joining sheet becomes a high concentration on the surface and a concentration concentration distribution that decreases toward the inside.

In the step of imparting a gradient function, a single nano silver dispersion liquid is applied or impregnated on the surface of the copper sintered substrate, and further slowly dried at a low temperature, silver single nanoparticles are arranged on the surface and the fine pores of the copper sintered substrate, The silver single nanoparticle group having the organic coating is adsorbed and bonded to the surface of the copper sintered substrate and the fine pores of the porous copper sintered structure to form a bonding sheet. On the other hand, the solvent of the single nanosilver dispersion liquid and the optional low boiling point solvent are evaporated and removed during slow drying at low temperature. Therefore, since the surface of the bonding sheet is in a dry state, there is no problem in stacking and storing several bonding sheets, dust in the air is not easily attached, and handling is easy and storage stability is excellent. Further, during firing at a firing setting temperature (250° C. to 350° C.) for bonding, the solvent and the low boiling point solvent are not removed by evaporation as described above, so the solvent and the low boiling point solvent do not exist in the silver single nanoparticle. It does not interfere with the sintering of the particles.

According to the 7th form of this invention, with reference to FIG. 3A or FIG. 3B, the 1st to-be-joined object (6a) and the 2nd to-be-joined object via the said joining sheet (1a or 1b). A method for joining objects (6b), wherein a first article to be joined, one or more of the joining sheets (1a or 1b), and a second article to be joined are laminated in this order, and a laminate (10). It is possible to provide a joining method characterized by including a laminating step of forming a laminated body and a step of firing the laminated body while applying pressure to form a joined body. The article to be joined is not limited, but is, for example, a semiconductor such as a power semiconductor, an electronic component, a metal substrate, a circuit board, or a heat dissipation plate, and a power semiconductor and a copper substrate are particularly preferable. The joining sheet may be used as a cut piece that is freely cut according to the shape of the article to be joined.

The joint sheet according to the present invention can be designed such that the thickness thereof is preferably in the range of 10 to 150 μm, more preferably 20 to 100 μm, from the viewpoint of ease of handling during manufacturing and mounting. It is thick for large area bonding such as a heat dissipation substrate and thin for small area semiconductor bonding, and it is also possible to stack and join a large number of sheets. When the joint structure formed by this joint sheet is shear-ruptured, it is destroyed inside the copper sintered substrate, not near the joint interface. Therefore, the “functional silver-copper hybrid” formed in the nano-silver impregnated region near the joint interface is destroyed. It can be seen that the "sintered layer" has a higher strength than the inside. Therefore, when the thickness of one of the main bonding sheets is as thin as about 30 μm or less, it is better to stack the main bonding sheets and press and fire to bond the main bonding sheets to each other so that the total thickness of the bonding portion is only one. Compared to the case where the joining sheet is pressed and fired for joining, the same joining strength can be secured by firing at a lower temperature and lower pressure.

According to the eighth aspect of the present invention, it is possible to provide the joining method in which, in the laminating step, an immobilization aid for ensuring immobilization at the time of mounting is added to the surface of the joining sheet before the lamination. .. According to the joining method of the present embodiment, since the immobilization aid is added to the surface of the joining sheet that comes into contact with the article to be joined at the time of stacking, the main joining sheet can be reliably fixed to the article to be joined by simply holding it lightly. It has the advantage that it can be realized. The immobilization aid may be of any type as long as it satisfies certain conditions. The conditions are (1) having the viscosity necessary for the above-mentioned immobilization, and (2) when a small amount is added to the surface of the nano-silver impregnated region of the copper sintered substrate, wetting that spreads over the entire surface with a uniform thickness. (3) It evaporates at a temperature lower than the preset firing temperature (250°C to 350°C), so that it does not hinder the sintering of silver single nanoparticles during firing for bonding, and (4) a small amount. Since it is sufficient to use, the amount of gas generated by the above evaporation is also small. For example, as a fixing aid, a solution of isobornylcyclohexanol dissolved in methylcyclohexane at a concentration of 10 to 30% by mass, more preferably 20% by mass is used, and the solution is impregnated with nano silver on a copper sintered substrate. Immobilization satisfying the above conditions (1) to (4) can be achieved by dropping one drop or a few drops on the surface of the region.

According to a ninth aspect of the present invention, with reference to FIG. 4A or FIG. 4B, a laminate obtained by stacking the joining sheet (1b) on the article (6) to be joined and temporarily fixing the same. Can be provided. The article to be joined is not limited, but is, for example, a semiconductor such as a power semiconductor, an electronic component, a metal substrate, a circuit board, or a radiator plate, and a radiator plate is particularly preferable. The joining sheet may be used as a cut piece that is freely cut according to the shape of the article to be joined.

An example of using this embodiment will be described. When joining and arranging a semiconductor, such as a rectifying element, which generates a lot of heat with a current specification and a general semiconductor on the same substrate, it is necessary to add a heat sink to the upper surface of the rectifying element and join them. In one mounting method, first, a general semiconductor and a rectifying element are fired and joined and arranged on a substrate by a joining sheet according to the present invention, and then a rectifying element and a heat sink are fired through the joining sheet according to the present invention. It is a method of joining. In this case, when the intermediate laminate in which the joining sheet according to the present invention is temporarily fastened to the heat sink is prepared in advance as in the present embodiment, the rectifying element, the joining sheet according to the present invention, and the heat sink are laminated in this order. It is possible to save the time and labor for positioning when forming the body. The method of temporary fixing is not limited, and various methods such as local bonding with an adhesive and local pressure sintering can be used. The bonding sheet used may be type A or type B, the front and back of type B may be any depending on the properties of the objects to be bonded, and a plurality of bonding sheets may be temporarily fixed.

FIG. 1 is an explanatory diagram of a cross-sectional structure of a joining sheet. FIG. 2 is a schematic diagram for explaining the gradient concentration distribution of silver single nanoparticles in the bonding sheet. FIG. 3 is an explanatory diagram of a laminated body in which two objects to be joined are laminated via a joining sheet. FIG. 4 is an explanatory diagram of an intermediate laminated body in which an article to be joined and a joining sheet are laminated. FIG. 5 is a flow chart of a method for manufacturing a joining sheet. FIG. 6 is a graph showing the results of thermogravimetric differential thermal analysis for a single nanosilver dispersion liquid. FIG. 7 is an electron micrograph of silver single nanoparticles. FIG. 8 is a photograph showing a state in which the copper nano paste is applied on the silicon substrate. FIG. 9 is a photograph showing a copper sintered substrate formed on a silicon substrate. FIG. 10 is an SEM photograph showing the state of the sintered surface of the copper sintered substrate. FIG. 11 is a photograph showing a state in which a single nanosilver dispersion liquid is spray-coated on the surface of a copper sintered substrate and dried. FIG. 12 is an explanatory diagram showing a cut piece of the cut joining sheet. FIG. 13 is a photographic diagram for explaining a method of performing a bonding strength test on a copper test piece. FIG. 14 is a graph showing the results of the heat cycle load test. FIG. 15 is an explanatory diagram of a conventional joining method using a conductive paste.

Embodiments of a copper sintered substrate nano-silver-impregnated joining sheet according to the present invention, a method for producing the same, and a joining method will be described in detail below with reference to the drawings.

<1. Background and main points of the present invention>
The present invention is a pressure bonding/short time sintering showing strong bonding and long-term reliability, which is obtained by combining a copper sintered substrate made of copper nanoparticles and coating/drying a single nano silver monodisperse liquid on the copper sintered substrate. A joining type joining sheet, a method for producing the joining sheet, and a joining method using the joining sheet.
Porous copper sintering, in which copper nanoparticles have completed sintering as the sheet base material of the joining sheet, in order to solve all the above-mentioned problems required for high precision and miniaturization joining in the field of semiconductor mounting A single nano silver dispersion liquid in which silver single nanoparticles are mono-dispersed in a solvent is used to provide a bonding function when bonding a semiconductor and a copper substrate, etc. by using a substrate, and the mounting process is simplified and the mounting time is shortened. .. That is, a nano-copper-based copper sintered substrate having a particle size of about 50 nm is used as the sheet base material, and a stress relaxation function is provided while achieving cost reduction. The copper sintered substrate is produced by firing the nano-copper paste under no pressure or low pressure, has a uniform thickness and good parallelism, and has a large number of fine holes to have a stress relaxation function. On one surface, or preferably on both surfaces, which is the bonding surface, the average grain size is as small as about 1 nm, and the grain size distribution width is as narrow as 15% or less, and the grain size is uniform. A single nanosilver dispersion containing nanoparticles is dipped from the surface into the interior, applied and dried to achieve a high concentration on the surface and a graded arrangement distribution with a low concentration toward the inside, resulting in a gradient in bonding strength. Aim for functionalization. That is, the organic coating of silver single nanoparticles, preferably a low-molecular carboxylic acid coating during heating and sintering, causes the exothermic effect of the oxidative decomposition reaction to raise the temperature of the entire bonding layer to a spontaneously high temperature, so that sintering is accelerated at once. The designed "functional silver-copper hybrid sintered layer" is composed, and the bonding surface is metallized at a low temperature range of about 250°C. A dispersion liquid of silver single nanoparticles with an extremely narrow particle size distribution is bonded from the bonding surface. A bonding sheet, which is an inexpensive sheet-shaped bonding material suitable for semiconductor bonding, which has strong bonding properties and internal stress relaxation properties, and is applied so as to reduce the concentration of nanoparticles toward the inside of the layer. A manufacturing method and a joining method are provided. At the time of firing, since the inside of the bonding layer is a copper sintered body that has already been sintered, the portion that needs to be sintered is only near the joint interface where heat transfer is the fastest, and this is necessary for firing for bonding. It is possible to make the time very short.

The joining sheet according to the present invention has a stress relaxation function because it includes a porous copper sintered substrate as a base material. Since the lead solder or the lead-free alloy solder is used at a temperature equal to or higher than the melting point of the constituent metal components at the time of joining, it is used in a temperature range of 350°C to 230°C. Due to the difference in thermal characteristics between the materials to be joined, internal stress is generated inside the joining material joining the materials to be joined depending on the shape and temperature when the environmental temperature changes. , A mechanism to alleviate it is needed. In lead, there is a stress relaxation mechanism called creep between atoms. But,
The nanometal paste in which the nanoparticles of silver or copper metal are arranged is solid-state sintering bonding, and the metal has a high melting point and strong interatomic interaction, and stress relaxation due to the occurrence of creep phenomenon cannot be expected. In the formation of a bond using a bonding material containing sinterable metal nanoparticles, the elucidation of the expression of the relaxation mechanism of the internal stress caused by the environmental change after sintering by a mechanism other than the creep phenomenon, and the development of such a stress relaxation mechanism. It is necessary to design the optimum composition of the bonding material to have. In the present invention, stress relaxation is realized by using a porous copper sintered substrate formed by sintering-bonding copper nanoparticles as a base material.

The characteristics of the bonding sheet according to the present invention, the manufacturing method thereof, and the bonding method from the viewpoint of the mounting process of a semiconductor or the like for forming the bonding are (1) Use the completed copper sintered substrate as a base material. (2) Sintering shortens the time. That is, since the organic coating component of the silver single nanoparticles with a small particle size distribution width is derived from the low molecular weight carboxylic acid, the silver nanoparticles with an organic coating amount of about 13% by mass are generated by the oxidative decomposition reaction of the carboxylic acid during sintering. Heat is generated, and nano-silver becomes molten between the copper sintered substrate and the material to be bonded at the sintering environment temperature, and the bonding process surely progresses efficiently and at an accelerated rate. (3) After the sintering is completed, since the microstructure of the copper sintered substrate, which is the base material, can provide a highly functional sinterable bonding sheet that exhibits stress relaxation characteristics in the bonding layer, long-term reliability That is, it is possible to form a bonded structure having properties.

In order to achieve the present invention, first, a method for producing silver single nanoparticles having a narrow particle size distribution width is established, and further, a method for producing a single nanosilver dispersion liquid in which the silver single nanoparticles are dispersed is established. Secondly, to establish a method for producing a porous copper sintered substrate in which copper nanoparticles are sintered and bonded. Third, to establish a method for the gradient coating of silver single nanoparticles on copper sintered substrates. Fourthly, it is necessary to dispose the obtained bonding sheet on an object to be bonded, evaluate the functionality by short-time mounting, and prove the long-term stability of the formed bonding structure by an appropriate evaluation method. It was Hereinafter, one embodiment of the present invention will be described in order with reference to FIG. 5, which is a flowchart showing a manufacturing method.

<2. Preparation of Single Nano Silver Dispersion>
The single nano silver dispersion liquid preparation step S1 for preparing the single nano silver dispersion liquid includes a silver single nano particle formation step S11 and a silver single nano particle dispersion step S12. Important conditions required for silver single nanoparticles are (1) particle size distribution width of 15% or less, (2) average particle size of about 2 nm, (3) organic coating amount of 15% by mass or less, 4) The oxidative decomposition reaction of the organic coating during firing produces heat, and (5) the monodispersity in the alkane solvent is excellent. In addition, in order to achieve the firing set temperature of 250° C., the condition (6) 84% particle size (D ₈₄ ) is required to be 2.0 nm or less.

<2-1. Silver single nanoparticle formation process>
In the silver single nanoparticle forming step S11 for producing silver single nanoparticles having a narrow particle size distribution width, for example, the carboxylic acid of the coating reagent and the silver carbonate are added to an alcohol solution heated to 180° C. After stirring, and the generation of carbon dioxide gas, which is a decomposition gas generated in the reaction, is finished, the reaction solution is immediately discharged into isopropyl alcohol cooled to -40°C, and ultra-rapid cooling is performed to stop the particle size growth. The reaction liquid after cooling is washed with isopropyl alcohol heated to 60° C. in the process of forming by filtration to obtain purified silver single nano silver particles.
This time, sample #C8AgSN (Example) of silver single nanoparticles was obtained using octanol as the alcohol and octanoic acid as the coating reagent, and silver single nanoparticles using decanol as the alcohol and decanoic acid as the coating reagent. Sample #C10AgSN (Comparative Example) was obtained. The reaction conditions and the characteristics of the sample are shown in Table 1. Among the characteristics, the method of measuring the silvering temperature will be described later. The sample #C8AgSN satisfies all the above six conditions, but the sample #C10AgSN does not satisfy the conditions (3), (4) and (6).

<2-2. Silver single nanoparticles dispersion process>
Next, in the silver single nanoparticle dispersion step S12 in which the silver single nanoparticles are monodispersed in a solvent, preferably an alkane-based solvent, for example, methylcyclohexane is used as a solvent, and the silver single nanosilver particles are dispersed in the solvent. Then, a monodispersed state of silver single nanoparticles is prepared, and the solvent is evaporated using an evaporator to obtain a single nanosilver dispersion liquid in which the concentration of silver single nanoparticles is about 10% by mass. As a result of particle size distribution observation using a high resolution transmission electron microscope (HRTEM), (1.7±0.3) nm was obtained as described later, and the particle size distribution width was extremely small at about 15%.

<2-3. Thermal analysis results of silver single nanoparticles>
Thermogravimetric differential thermal analysis (TG-DTA) was performed by using a single nanosilver dispersion liquid formed using silver single nanoparticles as a sample and changing the temperature from 30°C to 400°C using a thermogravimetric differential thermal analyzer. It was As an example, the results of thermogravimetric differential thermal analysis of a single nanosilver dispersion formed using silver single nanoparticles of sample #C8AgSN (see <Note> below) are shown in FIG. The heating rate was 5° C./min. Prior to analysis, the solvent of the single nanosilver dispersion was fully evaporated using an evaporator. A curve 81 shows a differential heat flow rate (μV), which is measured by converting the temperature difference between the reference substance and the sample into a voltage using a thermocouple. A curve 82 shows the mass ratio (%), which represents the ratio of the mass to the initial value. The mass decreases between 30°C and 198°C due to evaporation of the solvent. The mass at 198° C. is 97.8% of the initial value (see point B2 in FIG. 6). Thereafter, the carboxylic acid coating of the silver single nanoparticles decomposes with excessive heat generation due to the oxidation reaction between 198° C. and 260° C., and the mass gradually decreases (points A2 to A3 to A4 and point B2 to FIG. 6). See B4). Furthermore, the silver nuclei of the silver single nanoparticles start to sinter at around 240° C., reach a peak of exotherm at about 250° C. (see point A3), and complete sintering at about 260° C. Therefore, the sintering temperature (silvering temperature) of the silver single nanoparticles of sample #C8AgSN is about 250°C. Since the mass at the time of completion of sintering is 85.3% of the initial value (see point B4), the organic coating amount of silver single nanoparticles of sample #C8AgSN is approximately (97.8%-85.3%)/ 0.978 = 12.8 mass%
I understand.

Thermogravimetric differential thermal analysis (TG-DTA) was also performed on sample #C10AgSN, and the organic coating amount of silver single nanoparticles in the sample was about 18% by mass, and the sintering temperature (silvering temperature) was about 275°C. I got the result.

When the particle size of metal nanoparticles decreases, it is observed that the melting point of the metal decreases depending on the particle size. With the particle size of single nano particles, the melting point decreases to about 1/3 of the bulk value. It is described in Patent Document 1. The firing temperatures of sinterable bonding materials using silver or copper nanoparticles are all based on this principle. If the melting point of the silver single nanoparticles is 250° C., the particle size of the silver nuclei of the silver single nanoparticles is about 2.0 nm. In the range of 200° C. to 300° C., the melting point drops by about 10° C. every time the particle size of silver nuclei decreases by 0.1 nm.

<2-4. TEM image of silver single nanoparticles>
A single nanosilver dispersion liquid formed using silver single nanoparticles of Sample #C8AgSN was applied to a silicon substrate, the solvent was evaporated, and then the observation was performed using a high resolution transmission electron microscope (HRTEM). The TEM image is shown in FIG. There are some clusters formed of several silver single nanoparticles and having a diameter of about 4 to 6 nm. The silver single nanoparticles in each cluster are composed of silver nuclei having a particle size of about 1.2 to 2.3 nm and an organic coating. The crystal structure of metallic silver of each silver nucleus can be observed. In this TEM image, since the solvent of the single nanosilver dispersion liquid was observed to be evaporated, the silver single nanoparticles aggregated to form clusters, but in the state before the solvent was evaporated, it was almost the same. It is considered to be monodisperse. As a particle size distribution of silver nuclei based on the analysis of the TEM image, an average particle size of 1.7 nm±0.3 nm (standard deviation) was obtained. The size distribution width of the silver nuclei of the silver single nanoparticles of sample #C8AgSN is about 15%, which is very narrow. The average particle size and the particle size distribution width are consistent with the results of the TG-DTA analysis described above.

<3. Formation of copper sintered substrate>
The copper sintered substrate forming step S2 for forming a porous copper sintered substrate formed by sintering-bonding copper nanoparticles includes a nano copper paste applying step S21 of applying a nano copper paste on the substrate, and a copper A nano-copper paste firing step S22 for firing the nano-copper paste applied to the substrate to obtain a copper sintered substrate is included.

<3-1. Nano copper paste>
The nano-copper paste is prepared by kneading copper nanoparticles, a thickener and a viscosity adjusting solvent. Using copper nanoparticles having an average particle diameter of 50 nm and having no organic coating, isobornyl cyclohexanol (trade name: Tersolv MTPH, manufactured by Nippon Terpene Chemical Co., Ltd.) as a thickener, and Nippon Terpene Chemical Co., Ltd. as a viscosity adjusting solvent The Tersolve THA-90 manufactured by the company was used at the mass ratios shown in Table 2 to obtain three types of nano-copper paste: #high viscosity type, #medium viscosity type, #low viscosity type.

<3-2. Application and firing of nano copper paste>
In the nano-copper paste applying step S21 of applying the nano-copper paste onto the substrate, for example, first, the nano-copper paste is applied onto a substrate such as a clean mirror-polished silicon substrate with a uniform thickness of about 15 to 200 μm. .. Next, in the nano-copper paste firing step S22 in which the nano-copper paste applied on the substrate is fired to obtain a copper-sintered substrate, in a high-purity nitrogen gas flow, a heating rate of about 2° C./s up to about 350° C. And is calcined for about 3 minutes under no pressure or low pressure to obtain a peeled and self-standing copper sintered substrate having a thickness of about 10 to 150 μm. When firing without pressure, the low-viscosity paste can be applied by spin coating. When firing under low pressure, a solvent evaporation step at a temperature of about 200° C. is provided during firing, and a pressure of a few MPa is applied by pressing with a mirror surface within a temperature/time that does not completely dry, The surface of the applied nano copper paste is flattened to make the thickness of the sintered copper substrate uniform after firing. By flattening the surface of the applied nano-copper paste, the sintered copper substrate after firing and the bonding sheet obtained by coating the surface with silver single nanoparticles have a uniform thickness. At the time of pressure baking and joining, the contact area between the article to be joined and the joining sheet is increased, and a strong joining strength is secured. This time, samples #Cu1 to #Cu5 of the copper sintered substrate were obtained by applying and baking the copper nanopaste corresponding to the five types of sheet thicknesses and firing conditions shown in Table 3. Table 3 also shows the sheet thickness of the copper sintered substrate after firing. The planar size of the produced copper sintered substrate is 100 mm×100 mm.

FIG. 8 is a photograph showing a state in which the copper nano paste (#high viscosity type) 42 is applied on the silicon substrate 7.
In FIG. 9, a copper nanopaste (#high-viscosity type) is applied on a silicon substrate 7, and pressureless firing is performed in high-purity nitrogen gas under conditions of a temperature rising rate of 100° C./min and 350° C. for 3 minutes. It is a photograph figure of the obtained copper sintered substrate 2. Two strip-shaped (planar size 10 mm×40 mm, thickness 50 μm) copper sintered substrate 2 is shown.
FIG. 10A is an SEM photograph of a sintered surface state showing the sintered copper nanoparticles 4 in the copper sintered substrate 2. The magnification is 30,000 times. Further, FIG. 10B is an enlarged view of the SEM photograph, and the magnification is 100,000 times. It can be seen that the copper sintered substrate 2 is porous and has a large number of fine holes 21, and the size of the fine holes 21 is several hundred nm to 1 μm.

In the bonding sheet according to the present invention, since the copper sintered substrate obtained by sintering the nano-copper paste containing copper nanoparticles having an average particle size of about 5×10 ¹ nm as the main component occupies most of the mass, It can be manufactured at a lower cost than a nano silver paste composed of silver and a silver filler. When the mass productivity of the bonding sheet is taken into consideration, the copper nanoparticles are required to have a considerable mass production capability.

<4. Formation of nano silver impregnated region>
In at least one surface of the copper sintered substrate, in at least one surface of the copper sintered substrate in the gradient function imparting step S3 for forming the nano-silver impregnated region containing the gradient-concentrated silver single nanoparticles. The purpose is achieved by applying or impregnating a single nano silver dispersion liquid on the above, and further drying.
For example, (1) a copper sintered substrate is placed on a thin SUS mesh plate of about 2 mm square, the single nano silver dispersion liquid prepared in advance is slowly spray-coated on the surface, and a hot plate of about 80° C. The above is slowly dried to obtain a nano silver-coated copper sintered substrate (bonding sheet) having a graded concentration. Alternatively, (2) the single nanosilver dispersion liquid is placed in a flat shallow container, the copper sintered substrate is dipped in the single nanosilver dispersion liquid, pulled out and dried to form a gradient-concentrated nanosilver-coated copper calcination. It is also possible to obtain a bonding substrate (bonding sheet).
This time, with respect to each of the samples #Cu1 to #Cu5 of the copper sintered substrate, the single nano silver dispersion liquid (#C8AgSN) was spray-coated by the method (1) on both surfaces, and then dried. By doing so, joining sheets (#1 to #5) having nano silver-impregnated regions on both surface sides were produced. Table 4 shows the sheet thickness of the produced type A joining sheet. Even if a copper sintered substrate is impregnated with silver single nanoparticles to form a bonding sheet, its thickness is almost unchanged, and the increase in thickness is submicron or less. The planar shape of each joining sheet is 100 mm×100 mm.

FIG. 11 shows the surface of a sintered copper substrate (#Cu3) 2 that has been dried by spraying a single nanosilver dispersion liquid (prepared using silver single nanoparticles (#C8AgSN)) in a strip shape on the surface of the substrate. It is a photograph figure of. Of the surface of the copper sintered substrate, the region where the nano-silver-impregnated region 3 is formed has a bluish color, and the other region is reddish because the surface of the copper sintered substrate 2 is exposed. It has a color. The high brightness at the top of FIG. 11 is due to the reflection of the light from the light source.

The joining sheet is cut to an appropriate size and placed on the article to be joined before use. FIG. 12 shows cut pieces of the joining sheet cut into plane sizes of 10 mm×10 mm and 5 mm×5 mm. The cut pieces of the six bonding sheets arranged in two rows on the left side in the figure are the bonding sheets of type B having a thickness of about 12 μm obtained by applying a single nano silver dispersion liquid (prepared using #C8AgSN) on one surface and drying. It is a cut piece of and has a purplish color. The cut pieces of the six bonding sheets arranged in two rows on the right side of the figure are the cutting of the type A bonding sheet (sample #1) having a thickness of about 12 μm, which was obtained by applying the same single nanosilver dispersion liquid on both surfaces and drying. It is a piece and has a bluish color. The difference in color is due to the difference in surface concentration of silver single nanoparticles. When the thickness of the copper sintered substrate is as thin as about 10 μm, the silver single nanoparticles penetrate through the fine pores even if the single nano silver dispersion liquid is applied to only one surface of the copper sintered substrate and dried. Some silver single nanoparticles are also arranged on the other surface side of the copper sintered substrate, and in the case of double-sided coating, the surface concentration is generally higher than in the case of single-sided coating, so that the above color difference occurs.

<5. Various characteristics of bonding sheet>
The joining sheet thus obtained is cut into an appropriate size according to the size of the article to be joined such as the electrodes of the electronic circuit component or the circuit board used for mounting, placed on the article to be joined, and Depending on the heat capacity and heat resistance of the object, in a high-purity nitrogen atmosphere, under a pressure environment of a temperature of 250° C. to 350° C. and a pressure of 20 MPa or more, about 10 seconds to 3 minutes, more preferably about 10 seconds to 60 seconds. The above temperature is maintained and baked for a short time, about 10 seconds at the shortest, to form a bonded structure and complete the mounting process. The functionality of this short-time mounting was evaluated, and the long-term stability of the joint structure formed was evaluated.

<5-1. Joining strength in joining copper test pieces>
As shown in FIG. 13, two copper test pieces 61 were joined using the joining sheet 1a of type A having nano silver-impregnated regions on both sides of the copper sintered substrate, and a test for examining the joining strength was conducted. .. The copper test pieces are a cylindrical copper test piece (large) 61a having a diameter of 10 mm and a thickness of 5 mm, and a cylindrical copper test piece (small) 61b having a diameter of 5 mm and a thickness of 2 mm. A copper test piece (large) 61a, a bonding sheet 1a, and a copper test piece (small) 61b were laminated in this order to form a laminate 10, which was fired for a short time while applying pressure to obtain a joined body. The following Table 5 shows the sample name and thickness of the bonding sheet used, the firing temperature setting, the pressure applied, and the holding time (holding the firing temperature setting). The heating rate was about 2° C./sec. The shear strength and maximum displacement of the obtained joint were measured using a die shear tester. Table 5 shows the measurement results.

In the joining test of the copper test piece, a large holding temperature of 80 to 113 MPa was obtained regardless of the thickness (20 to 50 μm) of the joining sheet with a short holding time of 30 seconds to 3 minutes under a preset firing temperature of 350° C. and a pressure of 20 to 40 MPa. It was found that a strong bond having bond strength can be formed. In the joining with the Pb5Sn solder of the comparative example, the joining strength of 35 MPa is retained in the holding time of 30 minutes. This bonding sheet can form a strong bond in a shorter time than solder bonding. The maximum displacement is within about 1 mm, and it is considered that the porous copper sintered substrate of the bonding sheet exerts a stress relaxation function and contributes to the stability of bonding. Three joining structure No. 1, No. 2, No. In No. 3, the firing conditions other than the thickness of the joining sheet are common, but the joining strengths are about 110 MPa and are almost equal. Further, when the fracture surface is observed with an optical microscope, it is found that the fracture occurs not inside the nano-silver-impregnated region near the surface of the bonding sheet but inside the bonding sheet (copper sintered substrate). The bonding strength of the bonding formed by the pressure firing bonding using the present bonding sheet does not depend much on the thickness of the bonding sheet, and is determined by the strength of the copper sintered substrate.

<5-2. Thermal conductivity of bonding sheet>
High thermal conductivity and high electrical conductivity are required for a joint structure for joining electronic components such as power semiconductor elements. Therefore, the thermal conductivity of the copper sintered substrate and the bonding sheet (before bonding) was measured using a thermal conductivity measuring device. The thermal conductivity was measured by the laser flash method. this is,
In this method, the surface of the sample is instantaneously irradiated with laser light to be heated, the thermal diffusivity and the specific heat capacity are measured from the temperature gradient transmitted to the back surface, and the thermal conductivity is calculated based on the density of the sample. For this measurement, a nano copper paste (#high-viscosity type) was fired to produce a 1.22 mm-thick copper sintered substrate (#Cu6). Similarly, a 1.25-mm-thick copper sintered substrate was also prepared. The substrate was fired to prepare a bonding sheet (#6) having nano silver-impregnated regions on both sides thereof. In each measurement, a cut piece cut into a circle having a plane size of 8.81 mm in diameter was used, and the measurement was performed using a substrate measurement attachment for measuring the thermal conductivity of a thin sample. The results are shown in Table 6. For reference, Table 6 also shows numerical values of thermal conductivity and specific resistance of lead eutectic solder, silver, and copper. The value of the specific resistance was calculated from the measured values of the thermal conductivity based on the relationship between the thermal conductivity and the electrical conductivity in the metal by Wiedemann and Franz.

Both the copper sintered substrate (#Cu6) and the bonding sheet (#6) have high thermal conductivity and electrical conductivity (reciprocal of specific resistance), which are as high as those of bulk metal. This is because the copper sintered substrate and the bonding sheet have a high metal content, and the copper nanoparticles are bonded by sintering to form an electric conduction path network with a low specific resistance, and thermal energy is also along the path. It is easy to transport. On the other hand, the thermal conductivity and the electrical conductivity of the lead eutectic solder are low values that are smaller by one digit than those of the bulk metal.

<5-3. Long-term stability of bonding sheet>
A thermal cycle load test (power cycle test) was performed in order to investigate the stability of the bonded structure formed using the bonding sheet against thermal shock.
<5-3-1. Preparation of semiconductor mounting sample>
First, a power diode (made of doped Si, surface mount type NP junction, plane size 10 mm×10 mm, thickness 150 μm), a cut piece of a bonding sheet (#4, thickness 40 μm) cut into a plane size of about 10 mm×10 mm, and a copper plate. (Plane size 20 mm×20 mm, thickness 2 mm) are laminated in this order from the top, and the bonded sheet bonded body (semiconductor mounted sample) is baked by holding at a baking setting temperature of 350° C. for 1 minute while applying pressure at 40 MPa. It was made. The temperature rising rate was about 2° C./sec.

<5-3-2. Preparation of solder joints>
As a comparative example, the same power diode and a copper plate of the same size as above were joined with a commercially available lead-free solder (Sn3.0Ag0.5Cu) to form a lead-free solder joint body. Similarly, a lead eutectic solder joint body was formed using a commercially available lead eutectic solder. The thickness of each bonding layer was about 50 μm.

<5-3-3. Thermal cycle load test>
Using the above bonded sheet assembly (semiconductor mounted sample) as a sample, a thermal cycle load of 200° C. to −40° C. was applied to the sample using a thermal cycle load tester. The sample is placed with the above-mentioned copper plate in contact with the upper surface of the heat sink at -40°C. Electrodes are connected to the upper surface of the power diode and the copper plate so that a voltage can be applied between the electrodes. One cycle is 30 minutes. In one cycle, a constant large forward current (forward current pulse) is applied to the sample for several seconds to raise the temperature of the junction to 200° C., and then the current is cut off to set the sample to −40. Cooling with a heat sink at ℃. The thermal resistance value of the sample for each cycle number is measured based on the rate of change in the electrical resistance between the electrodes after application of a high forward current pulse. The measurement ends when the thermal resistance value increases by 30% from the initial value.
Similarly, the above-mentioned lead-free solder joint body and lead eutectic solder joint body were used as samples, and the thermal resistance value of each sample was measured. The results are shown in Table 7. The thermal resistance values of the lead-free solder joint body and the lead eutectic solder joint body are shown by standardizing the initial thermal resistance value. Further, the thermal resistance value of the bonded sheet bonded body is shown by standardizing the initial value of the thermal resistance value of the lead eutectic solder bonded body. FIG. 14 is a graph chart in which the state of change in thermal resistance value with the number of cycles shown in Table 7 is standardized (standardized) with respect to each of the bonded sheet bonded body and the lead eutectic solder bonded body, and is a graph. is there.

In the lead-free solder joint, as shown by the curve 83, the thermal resistance value increased by 80% after 200 cycles, and in the lead eutectic solder joint, the thermal resistance value increased by 60% after 500 cycles. In the joined sheet joined body according to the invention, as shown by the curve 84, the thermal resistance value only increases by about 3% even after 2000 cycles of loading. It was confirmed that the joining using the joining sheet according to the present invention has long-term stability against thermal shock and cold shock.

Generally, in the case of solder bonding, pressurization is required during cooling after heating for bonding formation, and further cooling requires time. This is because if the cooling is performed for a short time without applying pressure during cooling, cracks are likely to occur at the joint. In the bonding sheet according to the present invention, the copper sintered substrate is sintered from the beginning, and the portion requiring sintering is only the thin nano-silver impregnated region, and the melting point of bulk silver or silver-copper alloy formed by sintering. Is high, and since the porous copper sintered substrate has a stress relaxation function, there is an advantage that large pressurization is not required during cooling and cooling can be completed in a short time.

<5-4. Comparison of bonding strength in short-time firing with nano silver paste>
Using the bonding sheet according to the present invention and the nano silver paste as a comparative example, a bonding body was formed by short-time baking while changing the baking conditions (baking set temperature, pressurizing pressure, holding time), and the shear strength was measured. To compare.
<5-4-1. Preparation of nano silver paste>
As a comparative example, nano silver paste 1 and nano silver paste 2 containing a copper filler were produced in order to form a joined body by firing the paste. The nano-silver paste 1 contains silver single nanoparticles #C8AgSN and a copper filler in a mass ratio of 4:6, and the copper filler is a copper particle having an average particle diameter of 0.10 μm and having no organic coating. The agent (isobornylcyclohexanol) and the viscosity adjusting solvent (dodecane) are contained in a total amount of about 7% by mass based on the total mass. The nano-silver paste 2 contains silver single nanoparticles #C10AgSN and a copper filler in a mass ratio of 4:6, and the copper filler is a copper particle having an average particle diameter of 0.45 μm and having no organic coating. The agent (isobornylcyclohexanol) and the viscosity adjusting solvent (dodecane) are contained in a total amount of about 7% by mass based on the total mass.

<5-4-2. Comparison of bonding strength>
Using the above-mentioned nano silver paste or the bonding sheet according to the present invention, the copper test piece (large) and the copper test piece (small) were baked under pressure for 10 to 60 seconds under the firing conditions shown in Table 8 below. Firing was performed for a short holding time (holding at the firing setting temperature) to form a joined body in which two copper test pieces were joined, and the maximum shear strength was measured in the same manner as above. The results are shown in Table 8. Here, the temperature rising rate was about 2° C./sec. When laminating two copper test pieces via the bonding sheet according to the present invention to form a laminate, an immobilization aid for ensuring immobilization during mounting is provided prior to lamination. One or a few drops were added to the surface of the bonding sheet with a spoid. As the immobilization aid, a solution in which isobornyl cyclohexanol was dissolved in methylcyclohexane at a concentration of 20 mass% was used.

As can be seen from Table 8, with the nano silver paste, the drying process for removing the organic components such as the binder contained in the paste before firing requires at least about 3 minutes, but the bonding sheet according to the present invention Then, since the copper sintered substrate has already been sintered, the drying step is unnecessary. If the present bonding sheet is used, the nano silver paste can be used by maintaining the firing set temperature for a short time of about 10 to 60 seconds and a minimum of 10 seconds regardless of the thickness (20 to 50 μm) of the bonding sheet. A strong bond having a shear strength equal to or higher than that of the existing bond can be formed.
The shear strength of the joining structure formed by holding the firing temperature for about 10 to 60 seconds using the joining sheet according to the present invention is set at the firing setting temperature, the pressure applied during firing, and the holding time. Also depends on. By linear regression analysis (multiple correlation coefficient R=0.84) of the joint strengths of the six joint structures (No. 11 to No. 16) shown in Table 8, the shear strength is about 3 MPa when the firing set temperature is 10° C. higher. It can be seen that when the pressure applied during firing is 10 MPa higher, the shear strength increases by about 13 MPa, and when the holding time is 10 seconds longer, the shear strength increases by about 3.5 MPa. According to the regression equation, the shear strength is as high as about 65 MPa for a firing setting temperature of 300° C., a pressure of 30 MPa and a holding time of 30 seconds. Further, the joint structure (No. 16) has a shear strength of about 38 MPa with respect to a firing set temperature of 250° C., a pressure of 20 MPa, and a holding time of 10 seconds, which is the joint strength of lead eutectic solder ( Above about 35 MPa).

<5-5. Summary of features of the main bonding sheet>
In the joining sheet according to the present invention, the melting point of the metal (silver, copper, silver-copper alloy) formed after sintering for joining is 779° C. or higher, and therefore 700 even if the effect of eutectic formation is taken into consideration. It does not melt at °C. Therefore, the present bonding sheet can be used for high temperature heat resistant bonding. Although many similar metal nano pastes and some bonding sheets have been proposed, the present invention provides excellent bonding functionality in all respects, comparing the physical properties of the bonded state and its long-term stability. it can. The long-term stability of the bonding is due to the fact that the relaxation of internal stress caused by a large change in the ambient temperature is realized by the porous copper sintered substrate of the bonding sheet. As shown in the comparison table in the examples, the advantages of the present joining sheet are summarized as follows: (1) Long-term storage stability: storage at room temperature, possible for a period of more than a year, (2) Metal content: 99 mass% or more (∵ Copper sintered substrate is 100% by mass. Single nanosilver with a metal content of 85% by mass or more is 99% by mass or more because the total amount used in the bonding sheet is 5% by mass or less), (3) Electricity Conductivity: Specific resistance 3 μΩ·cm or less, (4) Thermal conductivity: 200 W/(m·K) or more, (5) Bonding strength (shear strength): 78 MPa or more (350° C., 20 MPa under pressure for 1 minute firing) Bonding strength: 2.4 times or more of lead eutectic solder), (6) Long-term stability: in the result of thermal cycle load test ((200°C (several seconds), -40°C (about 30 minutes))/cycle) , The thermal resistance value after 2000 cycles of loading is about 3% or less higher than the initial value. On the other hand, lead eutectic solder is 60% more than the initial value after 500 cycles under the same condition load, and lacks long-term stability in a high temperature environmental load test. (7) Economical effect due to low cost manufacturing is there.

A joined body can be formed by arranging a cut piece obtained by cutting the present joining sheet into an appropriate size between two objects to be joined such as a semiconductor element and a copper substrate and pressurizing and firing. Compared with the conventional joining method by applying various nano metal pastes, the application process and drying time are not required, and the holding time for holding at the firing set temperature is as short as 1/5 or less, so the mass productivity of mounting is improved 10 times or more. There is a feature that can be done. In addition, even when compared with a conventional bonding sheet having a composition in which a resin is arranged in a nano metal paste, the firing setting temperature and the holding time of the main bonding sheet are 10 to 60 seconds at the lower temperature, and 10 seconds at the shortest. In addition, the amount of silver nanoparticles used is small, and it is characterized by being more economical. Furthermore, when mounting with high-temperature semiconductor elements, heat generation during operation caused by miniaturization and large current load is avoided by sufficient thermal conductivity similar to bulk metal, and the junction characteristics that do not deteriorate the semiconductor function are exhibited, and heat resistance is high. Greatly exceeds conventional lead eutectic solder, thermal conductivity is more than 5 times, and electrical conductivity is as good as bulk metal. Bonding characteristics greatly exceed conventional solder products. ing. In addition, because it does not contain lead, which is harmful to the environment, it also has environmental load safety.

The bonding sheet according to the present invention can be easily used, can be manufactured at low cost, can form a strong bond by firing for a short holding time, and the bond formed has bonding properties that greatly exceed conventional solder products, It also has environmental safety. As a substitute for conventional solder products and bonding sheets, and as a bonding material for bonding formation that can withstand high-temperature use, which cannot be realized with conventional solder products, the bonding sheet, its manufacturing method, and the bonding sheet The joining method used has wide industrial applicability.

1 Bonding Sheet 1a Type A Bonding Sheet 1b Type B Bonding Sheet 2 Copper Sintered Substrate 21 Micropores 3 Nano Silver Impregnation Area 31 1st Nano Silver Impregnation Area 32 2nd Nano Silver Impregnation Area 33 Surface (bonding surface, bonding interface) )
4 Copper Nanoparticles 42 Nanocopper Paste 5 Silver Single Nanoparticles 5c Silver Nucleus 6 Objects 6a First Objects 6b Second Objects 61 Copper Specimens 61a Copper Specimens (Large) 61b Copper Specimens (Small)
7 Silicon Substrate 10 Laminates 81-84 Curve

Claims (9)

A copper sintered substrate which is a porous copper sintered structure composed of a plurality of copper nanoparticles sintered and bonded,
A plurality of silver single nanoparticles having an organic coating, which are arranged on the surface and the micropores of the copper sintered substrate ,
It is a joint sheet consisting of only
The silver single nanoparticles are bonded to each other, and also bonded to the copper sintered structure,
The number density of the silver single nanoparticles is high on at least one surface of the bonding sheet, low inside, and has a graded concentration ,
The solvent used when the silver single nanoparticles are dispersed and placed on the copper sintered substrate is removed by evaporation, and has a surface in a dry state,
The silver single nanoparticles, and wherein 84% particle diameter in the particle size distribution of the silver nuclei (D ₈₄₎ is 1.4～2.0Nm, the particle size distribution width is a Oh Rukoto silver nanoparticles 15% or less Bonding sheet. The bonding sheet according to claim 1 , wherein the silver single nanoparticles are carboxylic acid-coated silver nanoparticles having an organic coating amount of 10 to 15% by mass. The copper nanoparticles have an organic coating and an average particle size of 5×10 5. ¹ The bonding sheet according to claim 1 or 2, which is a copper particle of nm. The joining sheet according to any one of claims 1 to 3, wherein the number density decreases inward from both front and back surfaces of the joining sheet. The joining sheet according to any one of claims 1 to 3, wherein the number density decreases from one surface to the other surface of the front and back surfaces of the joining sheet. Single nano silver dispersion liquid preparation step of dispersing silver single nano particles having an organic coating in a solvent to obtain a single nano silver dispersion liquid, and sintering a nano copper paste containing copper nanoparticles to sinter porous copper a sintered copper substrate forming step of forming a substrate, the surface of the sintered copper substrate, wherein is coated or impregnated with a single nanosilver dispersion by Rukoto to remove the solvent and further dried, the sintered copper a volumetric region adjacent the surface and the surface of the substrate, by arranging the inclined concentration of silver single nanoparticles, FGM step of applying a gradient function, a method of joining sheet that have a,
The bonding sheet has a dry surface,
The silver single nanoparticles, and wherein 84% particle diameter in the particle size distribution of the silver nuclei (D ₈₄₎ is 1.4～2.0Nm, the particle size distribution width is a Oh Rukoto silver nanoparticles 15% or less Manufacturing method of joining sheet. It is a joining method which joins a 1st to-be-joined object and a 2nd to-be-joined object via a joining sheet in any one of Claims 1-5, A 1st to-be-joined object and one or more said joining sheets. And a second object to be laminated in this order to form a laminated body, and a step of firing the laminated body while applying pressure to form a joined body. Method. In the stacking step, prior to stacking, an immobilization aid for ensuring immobilization during mounting is added to the surface of the bonding sheet ,
The bonding method according to claim 7, wherein the immobilization aid is a solution in which isobornyl cyclohexanol is dissolved in methylcyclohexane at a concentration of 10 to 30 mass % . A laminate obtained by stacking the joining sheet according to any one of claims 1 to 5 on a material to be joined and temporarily fixing the same.