JP6893593B2 - Bonding materials and structures using Ni nanoparticles - Google Patents

Description

The present invention relates to a joining material using Ni nanoparticles and a joining structure obtained by joining two members to be joined using this joining material, and in particular, two joints constituting the joining structure. Even when there is a difference in the amount of thermal expansion based on the difference in the linear thermal expansion coefficient between the members and the use at a high temperature (for example, 200 ° C. or higher) is required, the members to be joined The present invention relates to a bonding material and a bonding structure capable of suppressing peeling and cracking in the bonding layer.

Metal particles having an average particle size of less than 0.5 μm, particularly 10 to 100 nm, are called metal nanoparticles. It has been confirmed that the metal nanoparticles have high sinterability brought about by the fine particle size, and the particles are sintered at a temperature of 500 ° C. or lower, which is much lower than the melting point of the metal constituting the metal nanoparticles. Has been done. Further, it is expected that the structural strength of the obtained sintered body is maintained up to near the melting point of the metal. As the metal constituting the metal nanoparticles, Ag is a typical example, and Au, Cu, Ni and the like can be mentioned (for example, Patent Document 1).

Metal nanoparticles are generally covered with an organic substance. At room temperature, the organic material covering the particles prevents the metal nanoparticles from self-aggregating, and the metal nanoparticles maintain their independently dispersed morphology. Further, these metal nanoparticles are supplied to the surface of the member to be joined as organic-metal composite nanoparticles, and when they are heated to a predetermined temperature and fired, the organic substances are decomposed and removed, and the active surface of the metal nanoparticles is obtained. Is exposed and the low-temperature sintering function is exhibited, and the metal nanoparticles are bonded to each other and at the same time bonded to the surface of the member to be bonded (Non-Patent Document 1).

On the other hand, when the average particle size is larger than 0.5 μm, particularly when it is 1 to 10 μm, it is sometimes called a metal powder. A temperature higher than about 500 ° C., which is higher than the temperature at which the particles are sintered, is required. On the other hand, when metal nanoparticles and metal powder are used in combination, the metal powder can be sintered via the metal nanoparticles that are sintered at a low temperature of 500 ° C. or lower, so that the metal nanoparticles can be fired at a low temperature as in the case of only the metal nanoparticles. it can. Further, since metal powder can be produced at a lower cost than metal nanoparticles, metal nanoparticles and metal powder may be used in combination (for example, Non-Patent Document 2).

By the way, in the technical field of power semiconductors and the like, power semiconductor modules in which semiconductor elements and the like are bonded to an insulating circuit board and further added with base plates and terminals are used in various electronic devices and the like. Conventionally, solder bonding technology has been mainly used as an integrated technology used for bonding between a device and an insulating circuit board.

In this technical field, the demand for energy saving has been increasing in recent years, and the realization of energy-saving power devices using compound semiconductors such as SiC (silicon carbide) is expected. Since this energy-saving power element is used at a higher temperature (for example, 200 ° C. or higher) than the conventional semiconductor element, heat resistance at a higher temperature is also required for the joint portion of the power semiconductor module. However, the conventional solder joining technique has a problem that the joining strength at a high temperature cannot be secured.

In order to solve such problems in solder bonding technology, metal nanoparticles that can be sintered at low temperatures and are expected to have heat resistance close to that of bulk metal after sintering are bonded to conductor elements and the like. Technology to be used as a material has been proposed. However, when this bonding material is used as a bonding layer and two members to be bonded are bonded to each other through the bonding layer, thermal stress is applied to the bonding layer when the bonding structure rises and falls. , Defects such as cracks may occur near the bonding interface of the semiconductor element, and the bonding strength may decrease.

That is, conventionally, in the case of a bonded structure using metal nanoparticles, as shown in FIG. 7, the bonded surface (first bonded surface) 1a of the first bonded member 1 and the second bonded member 2 A bonding layer 3 made of only metal nanoparticles or a metal sintered body obtained by sintering metal nanoparticles and metal powder is formed between the surface to be bonded (second surface to be bonded) 2a. Has been done. However, in such a bonded structure, when the first member to be joined 1 and the second member to be joined 2 are made of materials having different linear thermal expansion coefficients, they are joined by on / off operation of the semiconductor element or the like. When the structure rises and falls in temperature, a difference in the amount of thermal expansion is inevitably generated between these two first bonded members 1 and the second bonded member 2, so that the bonding layer 3 for bonding between them is inevitably generated. Thermal stress due to thermal deformation is generated in.

For example, in the bonded structure shown in FIG. 7, the first bonded member 1 is a Si semiconductor element [Note that the back surface of the Si semiconductor element, that is, the first bonded surface 1a has Au, Ag, Al, Ti, Although a metallized layer such as Ni is formed, its thickness is as thin as several μm, so that it does not affect the coefficient of linear thermal expansion of the first member to be joined 1 so much, and the wire of the first member to be joined 1 is formed. The coefficient of thermal expansion is close to that of a semiconductor device, that is, Si. ], And the second member 2 to be joined is a Cu circuit layer [Note that the Cu circuit layer is plated with Au, Ag, Ni, etc. on the outermost surface, and the second surface to be joined 2a is Cu, Au, Ag. , Ni, etc., but even in that case, the thickness of the plating is as thin as several μm, so the coefficient of thermal expansion of the second member to be joined 2 is not significantly affected, and the second member to be joined 2 has. The coefficient of thermal expansion is close to that of Cu, which is the material of the circuit layer. ], In particular, the difference in thermal expansion between the bonding layer 3 obtained by sintering metal nanoparticles such as Ag, Au, Cu, and Ni and the first bonded member 1 is the second coating. It is larger than the difference in thermal expansion between the bonding member 2 and the bonding layer 3, and is generated when the temperature of the component having the bonding structure rises or falls due to the on / off operation of the semiconductor element or the like. The thermal stress could not be completely relaxed, and defects such as cracks occurred near the bonding interface (first surface to be bonded 1a) of the first member to be bonded 1, and the bonding strength may decrease. The reason is that when solder is used as the bonding material, the high diffusivity of the solder in the bonding layer absorbs the difference in the amount of thermal expansion between the first member to be bonded and the second member to be bonded, resulting in thermal stress. However, when metal nanoparticles are used as the bonding material, the diffusivity of the bonding layer made of a sintered body of metal nanoparticles is expected to be lower than that of solder. It is possible that the difference in the amount of thermal expansion of the joining member cannot be completely absorbed.

Further, when the metal nanoparticles are sintered at a firing temperature of about 350 ° C., the Si semiconductor element of the first member to be joined 1 and the Cu circuit layer of the second member to be joined 2 each have heat corresponding to 350 ° C. The length is extended by the amount of expansion, and in this state, the sintering of the metal nanoparticles proceeds and the bonding layer 3 is formed. After that, when the temperature is lowered to room temperature, thermal stress due to thermal deformation is generated in the formed bonding layer 3 due to the difference in the amount of heat shrinkage between the first member to be bonded 1 and the second member to be bonded 2. Generally, the firing temperature is higher than the operating temperature when the semiconductor element is turned on and off, so even a single thermal stress during the fabrication of the bonded structure causes cracks in the bonding layer 3, and the bonding is made of a sintered body of metal nanoparticles. In some cases, the shear strength of layer 3 was insufficient.

Attempts have been made to solve various problems in the technique of using metal nanoparticles as a bonding material for such semiconductor elements. For example, Patent Document 2 proposes to eliminate the thermal stress generated in a bonding layer formed by using metal nanoparticles by increasing the thickness of the bonding layer. The thickness of the bonding layer is 100 μm or more. However, when the bonding layer is made thicker, when Ag, Au, Cu, or Ni nanoparticles are used as the metal nanoparticles, the coefficient of thermal expansion of the bonding layer itself formed by sintering the nanoparticles becomes too large. Another problem occurs. If the coefficient of thermal expansion of the bonding layer is large, the thermal stress generated by the thermal history associated with the heat treatment during bonding or the thermal history generated during the on / off operation of the semiconductor element increases, and the bonding strength may decrease. ..

That is, in the most common configuration of a power semiconductor module, the semiconductor element is Si (coefficient of linear thermal expansion = about 3 × ^10-6 / K) or SiC (coefficient of linear thermal expansion = about 5 × ^10-6 / K). Yes, and the circuit layer of the insulating circuit board is Cu (coefficient of linear thermal expansion = about 17 × ^10-6 / K). Then, between these, Ag (coefficient of linear thermal expansion = about 19 × ^10-6 / K), Au (coefficient of linear thermal expansion = about 14 × ^10-6 / K), Cu (as described above), Ni (wire). When bonding with metal nanoparticles made of metal such as thermal expansion coefficient = about 13 × ^10-6 / K), there is not much difference in linear thermal expansion coefficient between the Cu circuit layer and the metal nanoparticles, There is a large difference in the coefficient of linear thermal expansion between the semiconductor element and the metal nanoparticles. Therefore, when the semiconductor element and the substrate are firmly bonded by the bonding layer made of a sintered body of metal nanoparticles, a large thermal stress due to the difference in the amount of thermal expansion is generated especially at the bonding interface between the bonding layer and the semiconductor element. If it occurs, it may cause peeling of the bonding interface or destruction of the semiconductor element, and the bonding strength may decrease at a stage after bonding.

Japanese Unexamined Patent Publication No. 2013-012,693 Japanese Unexamined Patent Publication No. 2011-041,955

"Joining technology using metal nanoparticles" Surface technology Vol.59, No.7, 2008 "Improvement of silver nanopaste and application to silver hybrid paste" HARIMA TECHNOLOGY REPORT pp443-447

The present invention is a bonding material using Ni nanoparticles, and in particular, there is a difference in the amount of thermal expansion based on the difference in the coefficient of linear thermal expansion between the two members to be bonded that constitute the bonding structure. To provide a bonding material capable of suppressing peeling between members to be bonded and generation of cracks in the bonding layer even when use at a high temperature (for example, 200 ° C. or higher) is required. With the goal.
Another object of the present invention is to provide a joining structure in which two members to be joined are joined by using such a joining material.

That is, the gist of the present invention is as follows.
(1) A bonding material containing Ni nanoparticles, metal powder, low thermal expansion material powder, and an organic solvent.
The average particle size of the Ni nanoparticles is 20 nm or more and less than 100 nm.
Any one or a mixture of two or more selected from the group consisting of pure metals in which the metal powder is composed of Ag, Au, Cu, and Ni, and alloys containing the pure metals in a total ratio of 50% by mass or more. Yes, also
A bonding material characterized in that the linear thermal expansion coefficient of the low thermal expansion material powder ^{has a difference of 5 × 10 -6} / K or more and is lower than the linear thermal expansion coefficient of Ni.
(2) The metal powder has an average particle size of 0.5 μm or more and less than 20 μm, and the low thermal expansion material powder has an average particle size of 0.5 μm or more and less than 20 μm (1). ) Described in the bonding material.
(3) The bonding material according to (1) or (2 ) above, wherein the metal powder is Ni powder and / or Au powder.
(4) wherein the low thermal expansion material powder, W, Mo, Cr, wherein the TiB _2, and any one or two or more mixtures selected from the group consisting of ZrB ₂ (1) The bonding material according to any one of (3).
(5) The first member to be bonded and the second member to be bonded are made of a sintered body of Ni nanoparticles, metal powder, and low thermal expansion material powder in the bonding material according to any one of claims 1 to 4. A bonded structure characterized in that it is bonded to each other via a bonding layer.
(6) The bonding layer has a Ni phase derived from Ni nanoparticles, a metal phase derived from metal powder, and a low thermal expansion phase derived from low thermal expansion material powder, and the linear thermal expansion coefficient of the low thermal expansion phase is 5 × 10. The bonded structure according to (5) ^{above, which has a difference of -6} / K or more and is lower than the coefficient of linear thermal expansion of the Ni phase.
(7) The bonded structure according to (5) or (6 ) above, wherein the metal phase is a Ni phase derived from Ni powder.
(8) The low thermal expansion phase is a low thermal expansion phase derived from any one or a mixture of two or more selected from the group consisting of W, Mo, Cr, TiB ₂ and ZrB _2. The bonded structure according to any one of (5) to (7) above.

In the present invention, "the coefficient of linear thermal expansion of Ni nanoparticles, metal powder, and low thermal expansion material powder" and "Ni phase derived from Ni nanoparticles, metal phase derived from metal powder, and low thermal expansion material powder" are derived. The coefficient of linear thermal expansion of the low thermal expansion phase is the physical property value given to each powder and the element (or compound) that composes the phase (for example, "Metal Data Book" edited by the Japan Institute of Metals, published by Maruzen Publishing Co., Ltd. " It is the coefficient of linear thermal expansion as the physical property value) listed in the 3rd revised edition.
Further, the "sintered body of Ni nanoparticles, metal powder, and low thermal expansion material powder" forming the bonding layer means that the bonding material is applied between the first member to be bonded and the second member to be bonded, and is predetermined. When the bonding layer was formed by firing at the above temperature, at least between the Ni nanoparticles and the Ni nanoparticles, between the Ni nanoparticles and the metal powder, and between the Ni nanoparticles and the bonding surface of the first member to be bonded. The state in which the Ni nanoparticles are sintered and connected to each other between the Ni nanoparticles and the joint surface between the Ni nanoparticles and the second member to be bonded, and between the Ni nanoparticles and the low thermal expansion material powder, they are mutually connected. It may or may not be sintered.

According to the bonding material using Ni nanoparticles of the present invention, the presence of Ni nanoparticles not only enables bonding at a lower temperature, but also allows the two members to be bonded to be bonded to each other to form a bonded structure. There is a difference in the amount of thermal expansion based on the difference in the coefficient of linear thermal expansion between the two members, and even if the bonded structure is required to be used at a high temperature exceeding 200 ° C. It is possible to prevent the peeling of the particles and the generation of cracks in the bonding layer as much as possible.

Further, according to the bonding structure of the present invention, a low thermal expansion phase derived from the low thermal expansion material powder exists in the bonding layer formed between the two members to be bonded, and the thermal expansion characteristics of the bonding layer are improved by this low thermal expansion phase. It is adjusted to a suitable state between the thermal expansion characteristics of the two members to be joined, and the generation and growth of cracks in the joint layer are the metal phase derived from the metal powder in the joint layer and the low thermal expansion phase derived from the low thermal expansion material powder. As a result, the difference in the amount of thermal expansion inevitably generated between the two members to be joined can be reduced as much as possible when the thermal history acts on the bonded structure. As a result, peeling and cracking in the bonded structure can be suppressed.

FIG. 1 is a cross-sectional explanatory view showing a joint structure according to an example of carrying out the present invention. FIG. 2 is a cross-sectional explanatory view showing a joint structure according to another embodiment of the present invention. FIG. 3 is an SEM image obtained by observing the cross section of the joint layer of the joint structure obtained in Example 9 by SEM. FIG. 4 is an EDX analysis image corresponding to FIG. 3 targeting Ni in the bonding layer of the bonding structure of Example 9. FIG. 5 is an EDX analysis image corresponding to FIG. 3 targeting Zr in the bonding layer of the bonding structure of Example 9. FIG. 6 is an EDX analysis image corresponding to FIG. 3 targeting B in the bonding layer of the bonding structure of Example 9. FIG. 7 is a cross-sectional explanatory view showing a bonded structure (Comparative Example 1) using conventional metal nanoparticles. FIG. 8 is a cross-sectional explanatory view showing the joint structure of Comparative Example 2. FIG. 9 is a cross-sectional explanatory view showing the joint structure of Comparative Example 3.

The bonding material of the present invention is a bonding material containing Ni nanoparticles (hereinafter, may be simply referred to as “Ni nanoparticles”), metal powder, low thermal expansion material powder, and an organic solvent, and is the low thermal expansion material powder. The coefficient of linear thermal expansion of is lower than the coefficient of linear thermal expansion of Ni with a difference of 5 × 10 ^{-6 / K or more.}

In the present invention, Ni nanoparticles are particles having a particle size of less than 0.5 μm and an average particle size of 10 nm or more and less than 300 nm, preferably 20 nm or more and less than 100 nm. The smaller the average particle size, the lower the temperature at which it can be sintered. Therefore, the smaller the average particle size, the better. However, if the average particle size is smaller than 10 nm, the self-aggregation property increases, and it becomes difficult to control the production as a bonding material. On the other hand, if the average particle size is larger than 300 nm, the surface activity is lowered, so that sintering does not proceed sufficiently and the bonding strength is lowered. By using the Ni sintered body obtained by sintering such Ni nanoparticles as a bonding layer between the first bonded member and the second bonded member in the bonded structure, the temperature is much lower than the melting point of Ni. Sintering occurs between the Ni nanoparticles, between the Ni nanoparticles and the metal powder, or between the Ni nanoparticles and the surface to be bonded, and the first member to be bonded and the second member to be bonded can be bonded to each other. When sintering these Ni nanoparticles, the sintering temperature is preferably 400 ° C. or lower, more preferably 300 ° C. or lower, still more preferably 250 ° C. or lower, and the sintering temperature, that is, the bonding temperature is lower. If possible, the stress generated at the time of joining can be made smaller. These sintering temperatures can be adjusted by characteristics such as the method for producing Ni nanoparticles, the type of organic substance covering the surface of Ni nanoparticles, and the average particle size. As for the shape of the metal nanoparticles, a spherical shape is preferable in order to increase the packing rate of the particles, but a square shape, a flat shape, an elliptical shape, or the like may be used in addition to the spherical shape. In these cases, the longest side is defined as the particle size.

Here, the surface of Ni nanoparticles is usually coated with an organic substance such as a fatty acid or an aliphatic amine in order to maintain dispersibility as nanoparticles or to prevent oxidation of the particles, and is contained in an organic solvent. Metal nanoparticles other than Ni nanoparticles, such as Ag, Au, depending on the material of the surface to be bonded in the two members to be bonded, which are supplied as a slurry dispersed in a highly dispersed state. , Cu, and any one or more metal nanoparticles selected from an alloy containing any of these metals, etc., or may contain an oxygen atom or a carbon atom. These metal nanoparticles other than Ni nanoparticles have good electrical conductivity and thermal conductivity that are essential for the bonding layer of a semiconductor device, and also have an electrode structure on the back surface of the semiconductor device (this corresponds to the surface to be bonded). It is often used from the viewpoint of correlation, and the surface of these metal nanoparticles is usually coated with an organic substance such as a fatty acid or an aliphatic amine, and the nanoparticles are supplied as a slurry dispersed in an organic solvent in a highly dispersed state.

Here, in general, a bonding layer made of a metal sintered body of metal nanoparticles is a sintered body containing voids called microporous and tends to have poor ductility. Therefore, the bonding reliability of the bonding layer is improved. In order to maintain it, it is desirable that the bonded structure itself does not generate thermal stress as much as possible, and in the present invention, the linear thermal expansion coefficient is the first to be bonded as the metal of the metal nanoparticles. Ni nanoparticles whose main metal is Ni between the member and the second member to be bonded are selected. In particular, the first member to be bonded is a Si semiconductor element or a SiC semiconductor element, and the second member to be bonded is the second metal. When the member is a copper substrate, the metal of the other metal nanoparticles used in combination with the Ni nanoparticles is preferably Au having a linear thermal expansion coefficient between the member and the metal, and more preferably. Ni nanoparticles are used alone. When metal nanoparticles other than Ni nanoparticles are blended, it is preferably less than 50% by volume with respect to the entire Ni nanoparticles so as not to impair the advantage of the small linear thermal expansion coefficient of the Ni nanoparticles. Is less than 30% by volume, more preferably less than 10% by volume.

Here, the particle size and the average particle size of the Ni nanoparticles can be measured by the following method.
[Measurement method of particle size and average particle size of Ni nanoparticles]
A slurry in which Ni nanoparticles are highly dispersed in a solvent such as ethanol or water is applied to an observation sample table and sufficiently dried by a method such as vacuum drying to achieve high resolution SEM (Scanning Electron Microscope) or TEM (Transmission Electron Microscope). ) Prepare a sample for observation. The observation sample prepared in this manner is observed in a viewing range of about 10 times the particle diameter (for example, a SEM image having a viewing angle of 1270 nm × 950 nm), and an SEM image or a TEM image is acquired to obtain the image. Print on paper, measure the length of the scale bar in the image and the diameter of each particle, and convert it to the actual particle size from the scale bar to obtain the particle size. Then, when obtaining the average particle size, the same operation as the above-mentioned operation for obtaining the particle size is repeated 10 times, and the average particle size of the particles is calculated by the arithmetic mean.

Further, in the present invention, the metal powder is made of a metal having a coefficient of linear thermal expansion of Ni having a particle size of 0.5 μm or more and having a coefficient of linear thermal expansion of Ni having a particle size of 0.5 μm or more, and preferably made of a metal having excellent conductivity and thermal conductivity. More preferably, it is a metal having a coefficient of linear thermal expansion between the coefficient of linear thermal expansion of the first member to be joined and the coefficient of linear thermal expansion of the second member to be joined. As such a metal powder, specifically, any one selected from the group consisting of pure metals composed of Ag, Au, Cu, and Ni, and alloys containing the pure metals in a total ratio of 50% by mass or more. An example consisting of a seed or a mixture of two or more kinds can be exemplified, preferably Ni or Au, and more preferably Ni. Moreover, this metal powder may contain an oxygen atom or a carbon atom. The average particle size of this metal powder is usually 0.5 μm or more and less than 20 μm, preferably 1 μm or more and less than 10 μm. If the average particle size is 20 μm or more, the workability of applying the bonding material may be deteriorated, or the surface unevenness of the coating layer may become large, which may make control difficult. The average particle size of the metal powder is preferably smaller, but if it is smaller than 1 μm, the viscosity tends to increase and the paste property may decrease when the paste-like bonding material is prepared.

Here, the particle size and the average particle size of the metal powder can also be measured by the same method as in the case of the Ni nanoparticles [method for measuring the particle size and the average particle size of the Ni nanoparticles]. The shape of the metal powder may be a square shape, a flat shape, an elliptical shape, or the like, in addition to the spherical shape. In these cases, the longest side is defined as the particle size. Even when this metal powder is used in combination with Ni nanoparticles, sintering can be performed between the particles at a relatively low temperature by interposing the Ni nanoparticles, as in the case of using only Ni nanoparticles. Further, the metal powder can usually be produced at a lower cost than the Ni nanoparticles, and the cost of the bonding material can be significantly reduced by using the metal nanoparticles and the metal powder in combination.

Further, the reason why the metal powder is used in the bonding material in addition to the Ni nanoparticles is as follows. That is, with only Ni nanoparticles, the amount of sintering shrinkage due to sintering is large, cracks are generated in the formed bonding layer, and the cracks are likely to grow to generate voids, but the particles are contained in the bonding material. When a metal powder with a relatively large diameter is mixed, this metal powder acts like a stone for the generation and growth of cracks, plays a role of dividing the sintering shrinkage of Ni nanoparticles, and is relatively in the bonding layer. This is because it has the effect of preventing the generation and growth of large cracks and suppressing the generation of voids, and further, due to the size of the particle size, the thickness of the bonding layer can be ensured even when pressure bonding is performed. ..

Further, in the present invention, in addition to the Ni nanoparticles and the metal powder, the coefficient of linear thermal expansion of Ni is 5 × ^10-6 / K or more, which is lower than that of Ni. A low thermal expansion material powder having an expansion coefficient is used.
Here, the low thermal expansion material powder is a particle having a particle size of 0.5 μm or more, and is a low thermal expansion inorganic material having an average particle size of 0.5 μm or more and less than 20 μm, preferably 1 μm or more and less than 10 μm. If the average particle size exceeds 20 μm, the fluidity of the bonding material may decrease, and conversely, if the average particle size is smaller than 0.5 μm, the effect of reducing the amount of thermal expansion / contraction of the bonding layer may decrease. .. Further, the low thermal expansion material powder used in the present invention facilitates control for ensuring the uniformity of the conductivity and thermal conductivity of the formed bonding layer and further for improving the packing coefficient of the particles. Therefore, it is preferable that the particle size distribution is narrow, and further, it is preferable that the particle size is spherical. Then, the particle size and the average particle size of the low thermal expansion material powder can be measured by the same method as the above-mentioned [Measuring method of the particle size and the average particle size of Ni nanoparticles]. The shape of the low thermal expansion material powder may be a square shape, a flat shape, an elliptical shape, or the like, in addition to the spherical shape. In these cases, the longest side is defined as the particle size.

Further, the material constituting the low thermal expansion material powder is particularly limited as long as the material has a ^{coefficient of linear thermal expansion lower than that of Ni by a difference of 5 × 10-6 / K or more than the coefficient of linear thermal expansion of Ni.} However, it can be appropriately selected depending on the type of metal powder and the material of the two members to be joined to each other by the joining layer, particularly the material of the surface to be joined in the member to be joined. Specifically, for example, when preparing a bonding material for manufacturing a power semiconductor module, W (coefficient of linear thermal expansion: about 4.5 × ^10-6 / K), Mo (coefficient of linear thermal expansion: about 4.8 × 10) ^{Metals such as -6} / K) and Cr (coefficient of linear thermal expansion: about 4.9 × 10 ^-6 / K) and TiB ₂ [coefficient of linear thermal expansion: about (6.2-7.2) × 10 ^-6 / K] and ZrB ₂ [coefficient of linear thermal expansion: about (6.8 to 7.9) × ^10-6 / K] can be exemplified as metal borides, etc., and these are any one of them. Only the seeds can be used alone, or two or more seeds can be used in combination. When the difference in the coefficient of linear thermal expansion of the low thermal expansion material powder with respect to the coefficient of linear thermal expansion of Ni (hereinafter, may be referred to as "difference in coefficient of linear thermal expansion with respect to Ni") is ^{smaller than 5 × 10-6} / K, There is a risk that the effect of reducing the amount of thermal expansion / contraction in the formed bonding layer cannot be sufficiently achieved.

These low thermal expansion material powders are materials having a smaller coefficient of linear thermal expansion than the Ni nanoparticles and metal powders, and the coefficient of linear thermal expansion, average particle size, blending ratio, etc. of these low thermal expansion material powders are taken into consideration. If necessary, the coefficient of linear thermal expansion, the average particle size, the blending ratio, and the like of the metal powder to be used may be taken into consideration, and the metal powder may be appropriately combined and used so as to obtain the amount of thermal expansion of the bonding layer to be controlled. Even if it is an element other than these, the effect of reducing thermal expansion / contraction can be expected if the particles are made of a material having a smaller coefficient of linear thermal expansion than Ni. In the present invention, the low thermal expansion material powder composed of the above metal, metal boride, etc. means that the purity is 95% by mass or more, and if it is less than 0.5% by mass, unspecified impurities (metal or metal). Metal compounds, etc.), especially unavoidable impurities, etc. may be present.

The bonding material of the present invention contains, in addition to the above-mentioned Ni nanoparticles, metal powder, and low thermal expansion material powder, an organic solvent that disperses them, depending on the type of organic substance that coats the Ni nanoparticles. It is selected from alcohol-based, glycol ether-based, and hydrocarbon-based solvents. These organic solvents can be identified by thermal analysis (gas chromatography, etc.). Further, if necessary, various conventionally known dispersion aids, binders and the like may be selected and added to the bonding material to impart desired fluidity and the like to the bonding material. Can be done.

Regarding the composition of the bonding material of the present invention, the above-mentioned Ni nanoparticles are the Ni nanoparticles themselves (here, simply referred to as "Ni nanoparticles"), organic substances added for the purpose of coating the Ni nanoparticles, and organic substances. Considering it separately from other additives (here, these are simply referred to as "additives"), it varies depending on the type and average particle size of each material used, but preferably Ni nanoparticles are 10. ~ 52% by volume, metal powder 5 to 26% by volume, low thermal expansion material powder 5 to 26% by volume, organic solvent 30 to 60% by volume, and additives 0.1 to 10% by volume. Here, since some of the organic solvents and additives may volatilize or decompose and separate from the bonding layer during firing, and some of them may decompose and carbonize and remain in the bonding layer. If the amount is too small, the amount is better, but if the amount is too small, the fluidity of the bonding material is lost and the coatability is deteriorated. Furthermore, the lifetime of the bonding material is shortened. .1% by volume or more is preferable, and on the contrary, if too much organic solvent or additive is used, the bonding material may be spread and spread when printing the bonding material, and the bonding material is applied in an amount exceeding the range where the bonding material can exist. Further, the amount of the organic substance increases and the possibility of carbonization remaining increases. Therefore, it is preferable that the organic solvent is 60% by volume or less and the additive is 10% by volume or less.

Regarding Ni nanoparticles, metal powder, and low thermal expansion material powder, the Ni nanoparticles play a role of bonding with the metal powder, low thermal expansion material powder, or with the member to be bonded. Further, if the amount of Ni nanoparticles is too large, the effect of adding the metal powder or the low thermal expansion material powder becomes small, and it is preferable that the amount of the metal powder and the low thermal expansion material powder is large for the low thermal expansion of the bonding layer. However, since it is necessary to ensure the bondability of the Ni nanoparticles, the blending ratio (A: B: C) of these Ni nanoparticles (A), the metal powder (B), and the low thermal expansion material powder (C) It is preferable that Ni nanoparticles are 10 to 52% by volume, metal powder is 5 to 26% by volume, and low thermal expansion material powder is 5 to 26% by volume. In order to reduce the thermal expansion of the bonding layer, it is preferable to increase the proportion of the low thermal expansion material powder rather than the metal powder.

When the first member to be bonded and the second member to be bonded are bonded using the bonding material of the present invention prepared in this manner, the composition of the first member to be bonded and the second member to be bonded can be adjusted to form a slurry, a paste, or a grease. It is prepared into a shape such as a shape or a wax shape, and the prepared bonding material is used, for example, by an air spray coater, a roll coater, an electrostatic spray coater, a squeegee method, a mask printing method, etc. 2. It may be applied to each surface to be joined of the two members to be joined, and the first member to be joined and the second member to be joined may be overlapped and heated integrally with the joining material. At this time, the joining material is fired and joined. Some of the organic solvents and additives in the material volatilize or decompose to separate from the bonding layer, and some decompose and carbonize to remain in the bonding layer. The heating temperature at the time of firing of this bonding material is usually 200 ° C. or higher and 450 ° C. or lower, preferably 200 ° C. or higher and 300 ° C. or lower, and if this heating temperature is lower than 200 ° C., sintering due to Ni nanoparticles is sufficient. In addition to the possibility that it will not proceed, there is a risk that sufficient volatilization and decomposition of organic substances will not proceed and sintering of the bonding material will be hindered, and as a result, sufficient bonding strength may not be obtained. If the temperature exceeds 450 ° C., for example, when manufacturing a power semiconductor module, there is a concern that the semiconductor element may be damaged or the thermal stress generated in the process of cooling from the firing temperature to room temperature may increase, resulting in damage to the bonded structure. Further, at the time of joining using the joining material of the present invention, an appropriate pressure between the first joining member, the joining material and the second joined member, preferably 0.1 MPa or more and 50 MPa or less, more preferably 1 MPa or more and 10 MPa or less. Pressure may be applied.

Further, regarding the bonding structure formed by using the bonding material of the present invention, for example, as shown in FIG. 1, the bonded surface (first bonded surface) 1a of the first bonded member 1 and the second bonded surface 1a are bonded. The bonding material is sintered between the member 2 and the surface to be bonded (second surface to be bonded) 2a to form a bonding layer 3, and Ni nanoparticles and metal powder in the bonding material are formed on the bonding layer 3. , And a Ni phase 3a derived from Ni nanoparticles formed by sintering the low thermal expansion material powder, a metal phase 3b derived from the metal powder, and a low thermal expansion phase 3c derived from the low thermal expansion material powder. Then, when the difference in linear thermal expansion coefficient between the Ni phase 3a and the low thermal expansion phase 3c formed in the bonding layer 3 is 5 × 10 ^-6 / K or more, for example, the first bonding member 1 is a semiconductor. When a power semiconductor module, which is an element and the second bonding member 2 is a Cu circuit layer of an insulating circuit board, is manufactured, the following effects are exhibited. When the metal powder in the bonding material is Ni powder, the metal phase 3b derived from the metal powder becomes a Ni phase in the bonding layer 3 and can be distinguished from the Ni phase 3a derived from Ni nanoparticles integrally. Although it becomes difficult, it can be considered conceptually, and the Ni phase 3a derived from Ni nanoparticles usually has voids of less than 0.5 μm, whereas it is derived from a metal powder of 0.5 μm or more. Since such voids do not exist in the metal phase, they can be visually distinguished by an SEM image or a TEM image of the cross section of the bonding layer.

That is, since the difference in the coefficient of linear thermal expansion between the semiconductor element of the first joining member 1 and the Cu circuit layer of the second joining member 2 is large, the semiconductor element is joined according to the temperature history generated by ON / OFF during operation. Thermal stress is applied to the structure, and if this thermal stress is large, it will lead to damage to the joint structure. However, in the joint structure of the present invention, the linear heat in the joint layer 3 is higher than that of the Ni phase 3a and the metal phase 3b. There is a low thermal expansion phase 3c with a small coefficient of expansion, and the amount of expansion / contraction due to the thermal history of this low thermal expansion phase 3c is small, the amount of expansion / contraction of the entire bonding layer 3 is small, and the Ni phase in the bonding layer 3 is also small. The influence of the expansion / contraction amount of the metal phase 3a and the metal phase 3b is divided by the low thermal expansion phase 3c, and the influence of the expansion / contraction due to the thermal history associated with ON / OFF during the operation of the semiconductor element is reduced as a whole, and as a result. It is possible to suppress peeling and cracking during operation of the semiconductor element.

When the power semiconductor module is configured as described above as the bonded structure of the present invention, the second member to be bonded is a metal substrate such as an aluminum substrate, an iron substrate, a copper substrate, or a stainless steel substrate, a copper-coated alumina substrate, or a copper-coated carbonized substrate. It is conceivable to arrange an insulating circuit board such as a silicon substrate or a copper-coated nitride-based substrate. For example, when the second member to be bonded is a copper substrate and the metal phase component in the bonding layer is Au or Ni, these are used. Since the coefficient of linear thermal expansion of Au or Ni is smaller than that of Cu, if the amount of expansion and contraction of the bonding layer during the thermal history is reduced by forming a low thermal expansion phase, it is the second bonding surface of the second member to be bonded. On the contrary, the difference in the amount of expansion / contraction between Cu and the bonding layer becomes large.

Therefore, in such a case, for example, as shown in FIG. 2, the bonded surface (second bonded surface) 2a of the second bonded member 2 contains only Ni nanoparticles that do not contain the low thermal expansion material powder. A bonding material or a bonding material containing Ni nanoparticles and metal powder is applied and dried to form a dry layer, and the bonding material of the present invention is applied to the surface of the dry layer and fired to be derived from the bonding material of the present invention. A bonding layer 3 composed of the bonding layers 3a, 3b, 3c and the bonding layer 3d derived from the dry layer may be formed, and the first bonded member 1 and the second bonded member 2 may be bonded. As a result, the amount of expansion / contraction between the first member to be joined 1 and the joint layers 3a, 3b, 3c is mainly increased without increasing the amount of expansion / contraction between the second member to be joined 2 and the joint layer 3d. It can also be reduced.

The Ni phase, the metal phase, and the low thermal expansion phase in the bonding layer can be confirmed by the following method.
[Method for detecting Ni phase, metal phase, and low thermal expansion phase in the bonding layer]
First, a sample in which the bonded structure is embedded in a resin such as a curable epoxy resin, the resin is cured, and then the sample is cut perpendicularly in the stacking direction from the first member to be bonded to the second member to be bonded via the bonding layer. A piece is cut out, the cut surface of this sample piece is polished, and if necessary, CP (Cross Section Polisher) processing is performed by an ion etching method using Ar ions or the like to prepare an observation sample piece for SEM observation.
Next, the prepared observation sample piece is set on the SEM observation sample table, the cut surface is observed in a range where the entire thickness direction of the bonding layer can be observed, for example, in a field of 24 μm × 33 μm, and the cut surface image is observed. At the same time, quantitative analysis of elements is performed by EDX (Energy Dispersive X-ray Spectroscopy) or the like attached to the SEM device. Here, if the Ni phase derived from Ni nanoparticles, the metal phase derived from metal powder, and the low thermal expansion phase derived from low thermal expansion material powder can be visually distinguished from the SEM image, point analysis is performed in each phase. Even if the analysis result is a compound, the origin of the phase can be determined from the main component and the atomic number concentration%. On the other hand, when it is difficult to distinguish each phase, the point analysis is repeated at a plurality of places, and the origin of the phase is determined from the result. These operations are performed on 3 to 10 cut surfaces to determine the origin of the Ni phase, metal phase, and low thermal expansion phase contained in the bonding layer.

In the present invention, since the bonding layer forms the entire bonding force by sintering particles starting from Ni nanoparticles, the Ni phase, the metal phase, or the low thermal expansion is intentionally formed in the bonding layer. It is not essential to include components other than the phase. If the bonding layer contains components other than the components constituting the Ni phase, metal phase, and low thermal expansion phase, or the C component derived from the organic solvent, organic substance, or other additives contained in the bonding material. When is contained, the ratio is preferably 20% by mass or less, whereby the effect of the present invention can be fully exhibited. Further, since the bonding layer is formed by sintering metal particles, voids or the like may be present in the bonding layer as long as the effects of the present invention can be sufficiently exhibited.

[ Examples 1 to 15 and 17, Reference Examples 16 and 18, and Comparative Examples 1 to 3 ]
Butyl carbitol (A) using Ni nanoparticles (A) coated with the organic substance octanoic acid, metal powder (B), and low thermal expansion material powder (C), and containing a viscosity adjusting additive as an organic solvent (butyl carbitol). After stirring and defoaming at the same time using (diethylene glycol monobutyl ether), kneading was performed to prepare the bonding material shown in Table 1.

Next, as the first member to be joined, a dummy silicon chip having a size of 0.4 mm in thickness × 5 mm in length × 5 mm in width is used, and Ti / Ni / Au having a total thickness of 1.1 μm on one surface by a sputtering method. A film was formed and used as the first surface to be bonded. Further, as the second bonded member, a copper substrate having a thickness of 1 mm × length 20 mm × width 20 mm was plated with Ni / Au having a total thickness of 5 μm to obtain a second bonded surface.

The joining materials of the above Examples, Reference Examples, and Comparative Examples are applied to the joined surface (first joined surface) of the first joined member by the squeegee method, and then the first joined surface is applied. The previously applied bonding material is sandwiched on the first surface to be bonded of the member, and the surface to be bonded (second surface to be bonded) of the second member to be bonded is overlapped, and the peak is applied under a pressure of 5 MPa. The temperature is 300 ° C., the holding time is 30 min, the temperature _{is reduced to 3% H 2} + 97% N ₂ , and the particles in the bonding material are heated and sintered to form a first bonded member and a second bonded member. A bonding layer was formed between the two, and cooled to room temperature to form a bonding structure of each Example, Reference Example, and Comparative Example.
Here, the joint structures of the obtained Examples and Reference Examples are as shown in FIG. 1, and for the joint structures of each Comparative Example, Comparative Example 1 is shown in FIG. 7 and Comparative Example 2 is shown. FIG. 8 and Comparative Example 3 are as shown in FIG. In FIGS. 7 to 9, the same reference numerals are used for the same parts as in FIG. 1.

[Joint strength test]
A bond tester (Dage Series 4000) was used to die the bonding structures of each Example and Comparative Example immediately after being produced using the bonding materials obtained in each Example , Reference Example, and Comparative Example. -The shear strength (n = 10) of the dummy Si chip was measured in the share mode, the shear strength was obtained by the arithmetic mean, and the initial joint strength was evaluated.

[Temperature cycle test]
For each of the example, reference example, and comparative example joint structures immediately after the above bonding was completed, a vapor phase thermal shock tester (TSA-ES72-W manufactured by ESPEC) was used, and- A 60-minute temperature cycle test is performed between 40 ° C and 175 ° C to evaluate reliability (reliability 1), and a 60-minute temperature cycle is performed between -40 ° C and 250 ° C. A test was performed to evaluate the reliability (reliability 2). During this temperature cycle test, the joint structure was taken out after 100 cycles, and the joint area (%) between the first member to be joined and the joint layer was investigated by an ultrasonic imaging device (FineSAT manufactured by Hitachi Power Solutions Co., Ltd.).

〔Evaluation method〕
Paste property: When the bonding materials of the above Examples, Reference Examples, and Comparative Examples were produced, those that could be easily produced as the bonding material were ◎, and the bonding materials that could be produced but obtained were handled. Those that were difficult to manufacture were evaluated as ◯, and those that could not be manufactured were evaluated as x.
Applicability: When applying the bonding material by the squeegee method, the one with good workability of the application work is ◎, the one with improved workability by kneading the paste immediately before application is ○, and the workability is poor. Those that did not exist were evaluated on the basis of x.
Application state: When the bonding material was applied by the squeegee method, the one that could be applied to the specified size was ◎, and the one that was less than or exceeded the specified size but was within the acceptable size was ○, the acceptable range. The outside was evaluated on the basis of x.
Initial joint strength: Regarding the joint strength measured by performing the joint strength test on the joint structures of each Example, Reference Example, and Comparative Example, those having a strength of 10 MPa or more are ◎, 1 MPa or more and less than 10 MPa. Those with a value of less than 1 MPa were evaluated as ◯, and those with a value of less than 1 MPa were evaluated as x.
[Comprehensive evaluation method]
Reliability 1: In the temperature cycle test, the joint area (%) was measured by an ultrasonic imaging device after 100 cycles, and those with a joint area of 60% or more were designated as ⊚, and those with a joint area of less than 60% were designated as ×. evaluated.
Reliability 2: In the temperature cycle test, the joint area (%) was measured by an ultrasonic imaging device after 100 cycles, and those with a joint area of 60% or more were designated as ⊚, and those with a joint area of less than 60% were designated as ×. evaluated.

The results are shown in Table 1.
In both the case of only the Ni nanoparticles of Comparative Example 1 and the case of only the Ni nanoparticles of Comparative Example 3 and the low thermal expansion material powder (ZrB ₂ ) , the reliability 1 and 2 and the evaluation of the initial bonding strength were ×. In the case of only the Ni nanoparticles and the metal powder (Ni powder) of Comparative Example 2, the increase coefficient of the peeling area near the interface between the Si chip and the bonding layer was increased by 100 cycles in the temperature cycle test. When it was 20% or more, the evaluations of reliability 1 and 2 were x, whereas in each embodiment of the present invention, the initial bonding strength including the evaluation of paste property, coatability, and coating state was included. And the evaluation of reliability 1 was both ⊚ or 〇.

FIG. 3 is an SEM image showing a cross section of the bonding layer in the bonding structure obtained in Example 9, and in this SEM image, the bonding layer has a Ni phase derived from Ni nanoparticles and a metal powder (Ni powder). ) Derived Ni phase and low thermal expansion phase (ZrB ₂ phase) derived from low thermal expansion material powder (ZrB ₂ powder) are observed.
Further, FIG. 4 is an EDX analysis image corresponding to FIG. 3 targeting Ni in the bonding layer of the bonding structure of Example 9, and FIG. 5 targets Zr in the bonding layer of the bonding structure of Example 9. FIG. 6 is an EDX analysis image corresponding to FIG. 3, and FIG. 6 is an EDX analysis image corresponding to FIG. 3 targeting B in the joint layer of the joint structure of Example 9, and is a joint structure of Example 9. A Ni phase derived from Ni nanoparticles, a Ni phase derived from metal powder (Ni powder), and a low thermal expansion phase (ZrB ₂ phase) derived from low thermal expansion material powder are confirmed in the bonding layer of the body, respectively.

1 ... 1st bonded member, 1a ... 1st bonded surface, 2 ... 2nd bonded member, 2a ... 2nd bonded surface, 3,3d ... bonded layer, 3a ... Ni phase, 3b ... derived from metal powder Metal phase, 3c ... Low thermal expansion phase derived from low thermal expansion material powder.

Claims (8)

A bonding material containing Ni nanoparticles, metal powder, low thermal expansion material powder, and an organic solvent.
The average particle size of the Ni nanoparticles is 20 nm or more and less than 100 nm.
Any one or a mixture of two or more selected from the group consisting of pure metals in which the metal powder is composed of Ag, Au, Cu, and Ni, and alloys containing the pure metals in a total ratio of 50% by mass or more. Yes, also
A bonding material characterized in that the linear thermal expansion coefficient of the low thermal expansion material powder ^{has a difference of 5 × 10 -6} / K or more and is lower than the linear thermal expansion coefficient of Ni. The first aspect of claim 1 , wherein the metal powder has an average particle size of 0.5 μm or more and less than 20 μm, and the low thermal expansion material powder has an average particle size of 0.5 μm or more and less than 20 μm. Joining material. The bonding material according to claim 1 or 2 , wherein the metal powder is Ni powder and / or Au powder. Any one of claims 1 to 3 , wherein the low thermal expansion material powder is any one or a mixture of two or more selected from the group consisting of _{W, Mo, Cr, TiB 2} , and ZrB _2. Joining material described in Crab. A bonding layer in which the first member to be bonded and the second member to be bonded are made of a sintered body of Ni nanoparticles, metal powder, and low thermal expansion material powder in the bonding material according to any one of claims 1 to 4. A bonded structure characterized by being bonded to each other via. The bonding layer has a Ni phase derived from Ni nanoparticles, a metal phase derived from metal powder, and a low thermal expansion phase derived from low thermal expansion material powder, and the linear thermal expansion coefficient of the low thermal expansion phase is 5 × ^10-6 /. The junction structure according to claim 5 , wherein the junction structure has a difference of K or more and is lower than the coefficient of linear thermal expansion of the Ni phase. The bonded structure according to claim 5 or 6 , wherein the metal phase is a Ni phase derived from Ni powder. The claim is characterized in that the low thermal expansion phase is a low thermal expansion phase derived from any one or a mixture of two or more selected from the group consisting of W, Mo, Cr, TiB ₂ , and ZrB _2. The bonded structure according to any one of 5 to 7.

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