JP7487011B2 - Bonding material, manufacturing method for bonding material, and bonding method - Google Patents

Description

The present invention relates to a bonding material, a manufacturing method for a bonding material, and a bonding method.

In recent years, in the field of power devices such as power LEDs, high-frequency devices, and inverters, the problem of heat generation from devices has become more serious as devices become smaller and have higher output, and thermal management has become an active topic of discussion. In other semiconductor devices, too, there is a trend toward higher junction temperatures as studies are underway to switch from conventional silicon-based elements to compound semiconductor elements.

In light of these technical backgrounds, materials used to bond various semiconductor devices to substrates, etc., are required to have heat dissipation properties and bonding reliability. Recently, bonding materials using fine metal powders such as sintered silver have been attracting attention as materials that can achieve this heat dissipation properties and bonding reliability. In addition, because such bonding materials are sintered at low temperatures to form a metal bonding layer, they have also attracted attention because they allow the use of substrates with low heat resistance.

As mentioned above, the metal bonding layer formed from the bonding material is required to have high bonding strength and reliability. Reducing voids in the metal bonding layer is effective in improving these.

As a bonding method, there is a pressure bonding method in which a bonding material is applied to a substrate, a semiconductor element or the like is placed on the formed coating, and this is fired while being pressed with a strong force (for example, a pressure of 10 MPa) to sinter the metal particles in the bonding material and form a metal bonding layer (see, for example, Patent Documents 1 and 2). This makes it possible to form a metal bonding layer with few voids.

However, applying such a strong force to the semiconductor element and the substrate underneath it can damage them, raising concerns about the reliability of the product manufactured by bonding. Therefore, research is being conducted on bonding materials and methods that use weak pressure, or that bond without pressure at all (for example, Patent Documents 3 to 6).

JP 2011-046770 A JP 2019-149529 A JP 2017-214609 A JP 2020-035721 A JP 2016-054098 A JP 2011-175871 A

Due to concerns about damage caused by the above-mentioned pressure, the inventors also investigated a pressureless bonding method. The gaps between the metal bonding layer and the substrate, etc. are thought to be the areas where voids form during bonding (firing). With pressure-type bonding, the coating film (which is sintering to form the metal bonding layer) is pressed against the substrate, etc., and these gaps are crushed and eliminated, forming a metal bonding layer with reduced voids. However, with pressureless bonding, this cannot be expected, and voids did indeed form.

The present invention aims to provide a bonding material that can form a metal bonding layer with very few voids using a pressure-free bonding method.

The inventors conducted extensive research to solve the above problems and discovered that even bonding materials with the same viscosity measured at shear rates in the range widely used in viscosity measurements of bonding materials can have different viscosities when measured at very low shear rates, and that bonding materials with low viscosities measured at such low shear rates can form metal bonding layers with very few voids (between the metal bonding layer and the substrate, etc.) when used in pressureless bonding. This led to the completion of the present invention.

That is, the present invention is as follows.
[1] A bonding material comprising a metal particle powder and a solvent, the bonding material having a viscosity of 1000 Pa·s or less measured at 25° C. and 0.157 s ⁻¹ .

[2] The bonding material described in [1], in which a portion of the metal particle powder is a metal fine particle powder having an average primary particle diameter of 150 nm or less.

[3] The joining material according to [2], in which the content of metal fine particle powder in the joining material is 2 to 45 mass %.

[4] A bonding material according to any one of [1] to [3], in which the viscosity is 800 Pa·s or less.

[5] A bonding material according to any one of [1] to [4], in which the viscosity is 500 Pa·s or less.

[6] A joining material according to any one of [1] to [5], in which a portion of the metal particle powder is a large metal particle powder having a filling rate of 65.0% or more and a volume-based cumulative 50% particle diameter (D50) of 0.8 to 3.2 μm as measured by a laser diffraction particle size distribution measuring device.

[7] The joining material according to [6], in which the content of the large metal particle powder in the joining material is 40 to 88 mass %.

[8] The bonding material according to [6] or [7], in which the filling rate of the large metal particle powder is 66.5% or more.

[9] A bonding material according to any one of [1] to [8], wherein the metal is at least one selected from the group consisting of gold, silver, and copper.

[10] The bonding material according to any one of [1] to [9], wherein the metal is silver.

[11] A joining material according to any one of [1] to [10], used for pressureless joining.

[12] The joining material according to any one of [1] to [11], wherein the content of the metal particle powder in the joining material is 90 to 96 mass %.

[13] A method for manufacturing a bonding material, which comprises mixing a fine metal powder having an average primary particle diameter of 150 nm or less, a large metal particle powder having a packing rate of 65.0% or more and a cumulative 50% particle diameter (D50) of 0.8 to 3.2 μm on a volume basis as measured by a laser diffraction particle size distribution analyzer, and a solvent.

[14] The method for manufacturing a joining material described in [13], in which the amounts of the fine metal particle powder and the large metal particle powder used are such that the contents of the fine metal particle powder and the large metal particle powder in the joining material are 2 to 45 mass % and 40 to 88 mass %, respectively.

[15] The method for manufacturing a joining material according to [13] or [14], in which the total amount of the fine metal particle powder and the large metal particle powder used is such that the total content of the fine metal particle powder and the large metal particle powder in the joining material is 90 to 96 mass %.

[16] A method for manufacturing a joining material according to any one of [13] to [15], in which the filling rate of the large metal particle powder is 66.5% or more.

[17] The method for manufacturing a joining material according to any one of [13] to [16], wherein the metal is at least one selected from the group consisting of gold, silver, and copper.

[18] A method for joining two workpieces, comprising the steps of applying a joining material according to any one of [1] to [12] or a joining material manufactured by the manufacturing method for a joining material according to any one of [13] to [17] to one of the workpieces to form a coating film, placing the other workpiece on the coating film, and baking the coating film on which the other workpiece is placed at 160 to 350°C to form a metal joining layer from the coating film.

[19] The joining method described in [18], in which no pressure is applied to the two joined members and the coating film when the coating film is fired to form a metal joining layer.

[20] The joining method according to [18] or [19], wherein the one member to be joined is a substrate and the other member to be joined is a semiconductor element.

The present invention provides a bonding material that can form a metal bonding layer with very few voids using a pressure-free bonding method.

Hereinafter, an embodiment of the present invention will be described in detail.
[Joint material]
The embodiment of the bonding material of the present invention contains a metal particle powder and a solvent, and is characterized by a low viscosity measured at a very low shear rate (viscosity measured under conditions of 0.157 s ^-1 and 25°C, as described below; hereinafter, also referred to as "low shear viscosity").

<Low shear viscosity>
The viscosity of the bonding material according to the embodiment of the present invention measured at a shear rate of 0.157 s ^-1 and a temperature of 25°C is 1000 Pa·s or less. Such a very low shear rate is considered to roughly correspond to the gravity applied to the coating film formed by applying the bonding material to the bonded object such as a substrate. If the viscosity at such a shear force (corresponding shear rate) is relatively low, it is considered that the coating film will wet and spread on the bonded object due to its own weight. As a result, no gap is formed between the bonded object (substrate, etc.) and the coating film, and it is considered that voids due to the gap are not formed when the metal bonding layer is formed.

From the viewpoint of reducing voids, the low shear viscosity is preferably 800 Pa·s or less, more preferably 500 Pa·s or less, and particularly preferably 450 Pa·s or less. The low shear viscosity is preferably 10 Pa·s or more. The low shear viscosity is measured using an E-type rotational viscometer. As described above, the low shear viscosity is an index of the ease of flow when the bonding material is applied to a substrate or the like and self-flows to follow the surface shape of the substrate or the like, so it is necessary to be able to stably measure the viscosity using a viscometer in a low shear rate range. In the evaluation of the low shear viscosity, the low shear viscosity is calculated from the shear stress at 60 seconds after the start of rotation when the cone of the viscometer is rotated at a shear rate of 0.157 s ^-1 . From the viewpoint of ensuring the reliability of the measurement data, the measurement condition is that the shear rate is within ±1% of the set value of 0.157 s ^-1 for 30 seconds from 30 seconds after the start of rotation to 60 seconds after 30 seconds after the start of rotation.

<Metal Particle Powder>
The embodiment of the bonding material of the present invention contains a metal particle powder, which is basically sintered by firing to form a metal bonding layer.

The content of metal particle powder in the bonding material is preferably 70 to 98% by mass, more preferably 80 to 97% by mass, and even more preferably 90 to 96% by mass. If the content of metal particle powder in the bonding material is high, the content of other organic components in the bonding material (such as solvents, which either evaporate during firing or remain, and in either case the parts where the organic components were present can become voids) is low, which is advantageous in terms of reducing voids.

The constituent metals of the metal particle powder are preferably gold, silver, copper or an alloy of two or more of these metals from the viewpoints of electrical conductivity and thermal conductivity. Furthermore, from the viewpoints of cost and oxidation resistance, silver is particularly preferred. The same applies to the large metal particle powder and fine metal particle powder described below.

The shape of the metal particle powder is not particularly limited. It may be roughly spherical, flake-like, or irregular, but roughly spherical metal particle powder is preferred from the viewpoint of particle packing in the bonding material. The same applies to the large metal particle powder and fine metal particle powder described below.

(Large metal particle powder)
A portion of the metal particle powder described above is preferably a large metal particle powder having a volume-based cumulative 50% particle diameter (D50) of 0.8 to 3.2 μm as measured by a laser diffraction particle size distribution measurement device and a packing rate of 65.0% or more.

If the cumulative 50% particle diameter (D50) of the large metal particle powder is 0.8 μm or more, the viscosity of the bonding material (not the low shear viscosity, but the viscosity measured at a shear rate corresponding to the shear force of the bonding material when printed) will decrease, making it suitable for printing, and if the cumulative 50% particle diameter (D50) is 3.2 μm or less, the large metal particle powder will have some sinterability, making it possible to form a metal bonding layer with excellent bonding strength. From these perspectives, the cumulative 50% particle diameter (D50) of the large metal particle powder is preferably 1.0 to 2.2 μm.

Regarding the filling rate of the large metal particle powder, the low shear viscosity of the embodiment of the bonding material of the present invention can be adjusted to the range specified in the present invention described above by appropriate selection of the type and amount of the metal particle powder, solvent, and other components. However, if a large metal particle powder with a high filling rate (65.0% or more) is used, the low shear viscosity of the bonding material can be suitably adjusted to the above range.

The filling rate is the ratio of the tap density of the large metal particle powder to the bulk density (true density) of the metal (tap density ÷ bulk density × 100 (%)). When a large metal particle powder with a filling rate of 65.0% or more is added to the joining material, the low shear viscosity can be set to 1000 Pa·s or less, preferably 800 Pa·s or less. When a large metal particle powder with a filling rate of 66.5% or more is added to the joining material, the low shear viscosity can be set to a value of 500 Pa·s or less or 450 Pa·s or less, which is particularly preferable from the viewpoint of reducing voids in the metal joining layer. The filling rate of the large metal particle powder is preferably 85.8% or less.

The powder filling rate is an indicator of powder fluidity (affected by factors such as the smoothness of the particle surface), and it is believed that by using powders with a high filling rate in joining materials used in pressureless joining, the low shear viscosity of the joining material can be reduced.

Many known metal powders with sizes similar to those of large metal particle powders have a packing ratio of 64.0% or less, but some have packing ratios of over 65.0%. In the present invention, as an example of how to achieve the above-mentioned low shear viscosity, a large metal particle powder with a packing ratio of 65.0% or more is selectively used. Details of the method for measuring tap density to determine the packing ratio are explained in the Examples section.

The content of the large metal particle powder in the bonding material is preferably 40 to 88 mass %, more preferably 50 to 85 mass %, and even more preferably 60 to 82 mass %, from the viewpoint of realizing a low low-shear viscosity of the bonding material.

From the viewpoint of increasing the dispersibility in the bonding material and thereby increasing the filling property, the large metal particle powder may be coated with an organic compound. Any known organic compound capable of coating the particle surface of the large metal particle powder may be used as this organic compound without any particular limitation. Examples of the organic compound include organic compounds having 12 to 24 carbon atoms and at least one functional group selected from the group consisting of hydroxyl groups, carboxyl groups, amino groups, thiol groups, and disulfide groups. This organic compound may be branched and may be saturated or unsaturated.

(Fine metal powder)
In an embodiment of the bonding material of the present invention, it is preferable that the metal particle powder contains a metal fine particle powder having an average primary particle size of 150 nm or less as part of the metal particle powder. Such a fine-sized metal fine particle powder has excellent sinterability, and a metal bonding layer having excellent bonding strength is formed from the bonding material containing the metal fine particle powder.

From the viewpoint of sinterability by low-temperature firing, the average primary particle diameter of the metal microparticle powder is preferably 130 nm or less, and more preferably 100 nm or less. In addition, the average primary particle diameter of the metal microparticle powder is preferably 1 nm or more.

In this specification, the average primary particle size refers to the average value of the primary particle size (average primary particle size based on number) obtained from a transmission electron microscope photograph (TEM image) or a scanning electron microscope photograph (SEM image) of the particles. More specifically, for example, the average primary particle size can be calculated from the primary particle size (diameter of a circle (area equivalent circle) having the same area as the particle) of 100 or more, preferably 250, arbitrary particles in an image (SEM image or TEM image) obtained by observing the particles at a predetermined magnification using a transmission electron microscope (TEM) (JEM-1011 manufactured by JEOL Ltd.) or a scanning electron microscope (SEM) (S-4700 manufactured by Hitachi High-Technologies Corporation). The calculation of the diameter of the area equivalent circle can be performed, for example, using image analysis software (A-zo-kun (registered trademark) manufactured by Asahi Kasei Engineering Co., Ltd.).

The content of the metal fine particle powder in the bonding material is preferably 2 to 45 mass %, more preferably 5 to 35 mass %, and even more preferably 8 to 30 mass %, from the viewpoint of forming a metal bonding layer with excellent bonding strength and preventing the low shear viscosity of the bonding material from becoming high.

Metal microparticle powders have a tendency to easily aggregate due to their small particle size. To prevent this, it is preferable that the metal microparticle powder is coated with an organic compound. Any known organic compound capable of coating the particle surface of metal microparticle powder can be used without any particular restrictions. Examples of the organic compound include organic compounds having 1 to 18 carbon atoms and at least one functional group selected from the group consisting of hydroxyl groups, carboxyl groups, amino groups, thiol groups, and disulfide groups. The organic compound may be branched and may be saturated or unsaturated.

The organic compound is preferably one having 12 or less carbon atoms, and more preferably a saturated or unsaturated fatty acid or a saturated or unsaturated amine having 2 to 8 carbon atoms, so that it can be sufficiently separated from the metal fine particle powder by firing at about 160 to 350°C and does not inhibit sintering between the particles of the metal fine particle powder.

The constituent metal of the fine metal powder is preferably the same as the constituent metal of the large metal particle powder, since the constituent metal of the metal bonding layer formed from the bonding material is a single type of metal, and the thermal expansion coefficient is substantially constant throughout the entire bonding layer, thereby improving bonding reliability.

As described above, in an embodiment of the bonding material of the present invention, it is preferable that a large metal particle powder is included as part of the metal particle powder, and a fine metal powder is included as part of the metal particle powder. The total of the large metal particle powder and the fine metal powder in the entire metal particle powder (100% by mass) is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 100% by mass.

<Solvent>
The bonding material according to the embodiment of the present invention includes a solvent. As the solvent, any solvent that can disperse the metal particle powder and has substantially no reactivity with the components in the bonding material can be used.

The content of the solvent in the bonding material is preferably 1.5 to 18 mass%, more preferably 2.5 to 16 mass%, and even more preferably 3 to 9.5 mass%. This solvent can be a polar or non-polar solvent, but it is preferable to use a polar solvent from the viewpoint of compatibility with other components in the bonding material and environmental load.

Examples of polar solvents include water;
Monoalcohols such as terpineol, texanol, phenoxypropanol, 1-octanol, 1-decanol, 1-dodecanol, 1-tetradecanol, Tersolve MTPH (manufactured by Nippon Terpene Chemical Co., Ltd.), dihydroterpinyloxyethanol (manufactured by Nippon Terpene Chemical Co., Ltd.), Tersolve TOE-100 (manufactured by Nippon Terpene Chemical Co., Ltd.), and Tersolve DTO-210 (manufactured by Nippon Terpene Chemical Co., Ltd.);
Polyols such as 3-methyl-1,3-butanediol, 2-ethyl-1,3-hexanediol (octanediol), hexyl diglycol, 2-ethylhexyl glycol, dibutyl diglycol, glycerin, dihydroxyterpineol, 3-methylbutane-1,2,3-triol (isoprene triol A (IPTL-A), manufactured by Nippon Terpene Chemical Co., Ltd.), and 2-methylbutane-1,2,4-triol (isoprene triol B (IPTL-B), manufactured by Nippon Terpene Chemical Co., Ltd.);
Ether compounds such as butyl carbitol, diethylene glycol monobutyl ether, terpinyl methyl ether (manufactured by Nippon Terpene Chemical Co., Ltd.), and dihydroterpinyl methyl ether (manufactured by Nippon Terpene Chemical Co., Ltd.);
Glycol ether acetates such as butyl carbitol acetate, diethylene glycol monobutyl ether acetate, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, and diethylene glycol monoethyl ether acetate;
Nitrogen-containing cyclic compounds such as 1-methylpyrrolidinone and pyridine;
Ester compounds such as γ-butyrolactone, methoxybutyl acetate, methoxypropyl acetate, ethyl lactate, 3-hydroxy-3-methylbutyl acetate, dihydroterpinyl acetate, Tersolve IPG-2Ac (manufactured by Nippon Terpene Chemical Co., Ltd.), Tersolve THA-90 (manufactured by Nippon Terpene Chemical Co., Ltd.), and Tersolve THA-70 (manufactured by Nippon Terpene Chemical Co., Ltd.);
These may be used alone or in combination of two or more.

<Other ingredients (additives)>
The embodiment of the bonding material of the present invention may contain known additives as other components. Specific examples of additives include dispersants such as acid dispersants and phosphate dispersants, sintering accelerators such as glass frit, antioxidants, viscosity adjusters, pH adjusters, buffers, defoamers, leveling agents, and volatilization inhibitors. The content of the additives in the bonding material is preferably 2% by mass or less (when multiple types of additives are included, the total content is 2% by mass or less) (when the bonding material includes additives, the content is preferably 0.005% by mass or more (when multiple types of additives are included, the content of each additive is 0.005% by mass or more)).

Some bonding materials contain resin to act as a binder between metal particle powders, but this type of resin remains in the metal bonding layer formed from the bonding material and may adversely affect heat dissipation and conductivity. In addition, resin has a significantly different thermal expansion coefficient from metal, so when the metal bonding layer is subjected to a thermal cycle, stress is generated due to this difference, adversely affecting the reliability of the bond.

In view of the above, it is preferable that the embodiment of the bonding material of the present invention does not substantially contain resin. Specifically, the resin content in the bonding material is preferably 0.3 mass% or less, more preferably 0.1 mass% or less, and particularly preferably 0.05 mass% or less.

[Method of manufacturing the bonding material]
The embodiment of the bonding material of the present invention can be manufactured by kneading the metal particle powder (preferably including large metal particle powder and fine metal particle powder as described above), solvent, and other optional components (additives, etc.) described above by a known method. It is preferable that the amount of each component used is such that the content of each component in the bonding material is calculated from the charged amount of each component and is the amount described above as the preferred content of each component. In particular, from the viewpoint of reducing voids, it is preferable that the total amount of the fine metal particle powder and the large metal particle powder used is such that the total content of the fine metal particle powder and the large metal particle powder in the bonding material is 90 to 96 mass %.

The kneading method is not particularly limited. For example, the bonding material can be produced by preparing each component separately and kneading them in any order using an ultrasonic disperser, a disperser, a triple roll mill, a ball mill, a bead mill, a two-shaft kneader, a planetary mixer, or a revolving agitator.

[Joining method]
The embodiment of the bonding method of the present invention is a method for bonding two members to be bonded using a bonding material manufactured by the embodiment of the bonding material of the present invention or the embodiment of the manufacturing method of the bonding material of the present invention. The embodiment of the bonding method of the present invention has a coating film forming step, a placing step, and a metal bonding layer forming step, and may also include a preliminary drying step. Each of these steps will be described below.

<Coating film formation process>
In this process, a bonding material manufactured by an embodiment of the bonding material of the present invention or an embodiment of the manufacturing method of a bonding material of the present invention is applied (by printing (e.g., metal mask printing, screen printing, pin transfer, etc.)) onto one of the members to be joined to form a coating film. Since the embodiment of the bonding material of the present invention has a low low shear viscosity, it is believed that the coating film wets and spreads over the one of the members to be joined, making it difficult for a gap to form between them.

An example of one of the members to be joined is a substrate. Examples of substrates include metal substrates such as copper substrates, alloy substrates of copper and some other metal (e.g., W (tungsten) or Mo (molybdenum)), ceramic substrates in which SiN (silicon nitride) or AlN (aluminum nitride) is sandwiched between copper plates, plastic substrates such as PET (polyethylene terephthalate) substrates, and PCB substrates such as FR4. Furthermore, laminate substrates in which these are stacked together can also be used in the joining method of the present invention.

The portion of the one of the members to be joined where the joining material is applied may be plated with a metal. From the viewpoint of compatibility with the metal particle powder in the coating, it is preferable that the metal plating is the same as the constituent metal of the metal particle powder.

<Placement process>
Next, the other member to be joined is placed on the coating film formed on the one member to be joined. Examples of the other member to be joined include semiconductor elements such as Si chips, SiC, and GaN chips, and substrates similar to those given as examples of the one member to be joined. Since a metal bonding layer with reduced voids is formed from the coating film, the embodiment of the bonding method of the present invention is preferably used to bond a substrate and a semiconductor element. That is, the other member to be joined is preferably a semiconductor element.

In addition, since the embodiment of the bonding material of the present invention (the coating film formed from it) has a low low shear viscosity, when the other member to be bonded is placed on the coating film, the coating film deforms to conform to the surface shape of the part (bottom surface) where it comes into contact with the coating film of the other member to be bonded, and it is believed that a gap is unlikely to form between the coating film and the other member to be bonded.

The part (bottom surface) of the other member to be joined that comes into contact with the coating may be plated with metal. From the viewpoint of compatibility with the metal particle powder in the coating, it is preferable that the metal plating of the other member to be joined is plated with the same metal as the constituent metal of the metal particle powder. When placing the other member to be joined on the coating, pressure (for example, a pressure of about 0.03 to 0.2 MPa) may or may not be applied between the two members to be joined in a direction that compresses the coating between the two members to be joined.

<Pre-drying process>
In order to reduce voids in the formed metal bonding layer when the coating film on which the other member to be bonded is placed is fired to sinter the metal particle powder, a pre-drying step may be performed after placing the other member to be bonded on the coating film (after the placing step). The purpose of the pre-drying is to remove the solvent from the coating film, and the coating film is dried under conditions in which the solvent volatilizes and the metal particle powder does not substantially sinter. For this reason, the pre-drying is preferably performed by heating the coating film at 60 to 150°C. This drying by heating may be performed under atmospheric pressure, or under reduced pressure or vacuum. In addition, in the metal bonding layer forming step described below, if the rate of temperature rise to the firing temperature is about 7°C/min or less, the pre-drying step can be performed by raising the temperature to the firing temperature.

<Metal bonding layer forming process>
After carrying out the placing step and, if necessary, a preliminary drying step, the coating film sandwiched between the two members to be joined is fired at 160 to 350°C to sinter the metal particle powder (particularly fine metal microparticle powder), thereby forming a metal joining layer and joining the two members to be joined.

In the metal bonding layer forming process, the temperature is raised to the baking temperature of 160 to 350°C, and the baking temperature is maintained for, for example, 1 minute to 2 hours, to form a metal bonding layer from the coating of the bonding material. The rate of temperature rise is not particularly limited, but can be, for example, 1.5°C/min to 12°C/min, and is preferably 2°C/min to 6°C/min.

The firing temperature is preferably 175 to 280°C from the viewpoint of the bonding strength of the resulting metal bonding layer and costs.

The time for holding at the firing temperature is preferably 10 to 90 minutes from the viewpoint of the bonding strength of the metal bonding layer formed and heat costs. Note that if the firing temperature is higher within the firing temperature range shown above, for example, 280°C or higher, the metal bonding layer may be formed before the temperature is raised to the firing temperature. In such cases, the holding time at the firing temperature may be 0 minutes.

As mentioned above, the low shear viscosity of the embodiment of the bonding material of the present invention (the coating film formed from it) is low, so it is believed that the formation of gaps between the coating film and one of the bonded members is suppressed. Therefore, in this metal bonding layer formation process, even if pressure is not applied between the bonded members (in the direction in which the coating film that is sintering to form the metal bonding layer is compressed by the two bonded members), voids are extremely unlikely to form originating from the interface between the coating film (that is forming the metal bonding layer) and one of the bonded members.

When pressure is applied between the members to be joined, i.e., the two members to be joined and the coating, there is a risk of damaging them as explained in the [Background Art] section. However, by using the embodiment of the bonding material of the present invention, it is possible to perform bonding without the above-mentioned pressurization while suppressing the occurrence of voids. In addition, when applying pressure, from the viewpoint of bonding productivity, if multiple bonding operations are performed simultaneously, the same pressure is applied simultaneously in the same direction to multiple sandwich structures of bonded members-coating-bonded members, which is not easy, and if multiple pressure bonding operations are performed simultaneously, there is a concern about the uniformity of the quality of the resulting product. With bonding that does not use the above-mentioned pressure, there is no such concern. For the above reasons, in the present invention, the principle is to perform bonding using a non-pressurized method in which a metal bonding layer is formed by performing a metal bonding layer formation process without applying pressure.

The metal bonding layer formation process may be carried out in an air atmosphere or in an inert atmosphere such as a nitrogen atmosphere, but it is preferable to carry out the process in an inert atmosphere from the viewpoint of preventing oxidation. Furthermore, from the viewpoint of cost, it is more preferable to carry out this process in a nitrogen atmosphere.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[Fine silver particle powder and large silver particle powder 1 to 4]
The physical properties of the fine silver particle powders and large silver particle powders 1 to 4 used in the examples and comparative examples described below are as shown in Table 1 below.

The methods for measuring various physical properties are as follows:

<Average primary particle size>
The average primary particle diameter was calculated from the primary particle diameter (diameter of a circle having the same area as the particle (area equivalent circle)) of 250 random particles on an image obtained by observing the particles at a magnification of 50,000 times using a transmission electron microscope (TEM) (JEM-1011 manufactured by JEOL Ltd.). The calculation of the diameter of the area equivalent circle was performed using image analysis software (Azo-kun (registered trademark) manufactured by Asahi Kasei Engineering Co., Ltd.). At this time, the particle shape was also observed. The shapes of large silver particle powders 1 to 4 were observed at a magnification of 20,000 times using a field emission scanning electron microscope (JSM-7200M, manufactured by JEOL Ltd.).

<Particle size distribution>
The particle size distribution was measured using a laser diffraction particle size distribution measuring device (SYMPATEC's HELOS particle size distribution measuring device (HELOS & RODOS (airflow type dispersion module))) at a dispersion pressure of 5 bar and a lens with a focal length of 20 mm to determine the volumetric particle size distribution of the sample powder, thereby determining the cumulative 10% particle diameter (D10), cumulative 50% particle diameter (D50), and cumulative 90% particle diameter (D90).

<Tap density>
The tap density was measured in the same manner as described in JP 2007-263860 A, by filling a bottomed cylindrical die having an inner diameter of 6 mm and a height of 11.9 mm to 80% of the volume with the sample powder to form a powder layer, applying a pressure of 0.160 N/ ^m2 uniformly to the upper surface of this powder layer, compressing the powder layer with this pressure until the powder could no longer be packed densely, and then measuring the height of the powder layer. The density of the powder was calculated from the measured height of the powder layer and the weight of the filled sample powder, and this was taken as the tap density of the powder.

<Filling rate (%)>
The tap density was calculated as a percentage relative to the bulk density of silver (10.49 g/cm ³ ).

<Specific surface area>
The specific surface area was measured by a BET specific surface area measuring device (Macsorb, manufactured by Mountec Co., Ltd.) by flowing nitrogen gas into the measuring device at 105° C. for 20 minutes to remove any substances adhering to the particle surfaces of the sample powder, and then by the BET single-point method while flowing a mixed gas of nitrogen and helium (N2: 30 vol.%, He: 70 vol.%).

[Comparative Example (Preparation of Bonding Material)]
A silver paste was prepared by kneading 17.2 mass% of fine silver powder, 75.8 mass% of large silver particle powder 1, 0.2 mass% of SOLPLUSD-540 manufactured by Lubrizol as a dispersant, 2.35 mass% of Decanol manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 0.6 mass% of Tersolve IPTL-A manufactured by Nippon Terpene Chemical Co., Ltd., and 2.35 mass% of TOE-100 manufactured by Nippon Terpene Chemical Co., Ltd. as a solvent.

To adjust the viscosity of this silver paste to be suitable for printing, 0.75 wt% each of decanol and TOE-100 were added to dilute it, and a bonding material for the comparative example was obtained. The silver concentration in the resulting bonding material was determined by the ignition loss method to be 93.0 mass%.

[Examples 1 to 4 (Preparation of bonding materials)]
Example 1
A silver paste was prepared by kneading 16.3 mass% of fine silver powder, 76.8 mass% of large silver particle powder 2, 0.2 mass% of SOLPLUSD-540 manufactured by Lubrizol as a dispersant, 2.35 mass% of Decanol manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 0.6 mass% of Tersolve IPTL-A manufactured by Nippon Terpene Chemical Co., Ltd., and 2.35 mass% of TOE-100 manufactured by Nippon Terpene Chemical Co., Ltd. as solvents.

To adjust the viscosity of this silver paste to be suitable for printing, 0.70 wt% each of decanol and TOE-100 were added to dilute it, and the bonding material of Example 1 was obtained. The silver concentration in the resulting bonding material was determined by the ignition loss method and was found to be 93.1 mass%.

Example 2
A silver paste was prepared by kneading 17.6 mass% of fine silver powder, 75.5 mass% of large silver particle powder 2, 0.2 mass% of SOLPLUSD-540 manufactured by Lubrizol as a dispersant, 2.35 mass% of Decanol manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 0.6 mass% of Tersolve IPTL-A manufactured by Nippon Terpene Chemical Co., Ltd., and 2.35 mass% of TOE-100 manufactured by Nippon Terpene Chemical Co., Ltd. as a solvent.

To adjust the viscosity of this silver paste to be suitable for printing, 0.70 wt% each of decanol and TOE-100 were added to dilute it, and the bonding material of Example 2 was obtained. The silver concentration in the resulting bonding material was determined by the ignition loss method and was found to be 93.1 mass%.

Example 3
A silver paste was prepared by kneading 16.7% by mass of fine silver powder, 76.4% by mass of large silver particle powder 3, 0.2% by mass of SOLPLUSD-540 manufactured by Lubrizol as a dispersant, 2.35% by mass of Decanol manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 0.6% by mass of Tersolve IPTL-A manufactured by Nippon Terpene Chemical Co., Ltd., and 2.35% by mass of TOE-100 manufactured by Nippon Terpene Chemical Co., Ltd. as a solvent.

To adjust the viscosity of this silver paste to be suitable for printing, 0.70 wt% each of decanol and TOE-100 were added to dilute it, and the bonding material of Example 3 was obtained. The silver concentration in the resulting bonding material was determined by the ignition loss method and was found to be 93.1 mass%.

Example 4
A silver paste was prepared by kneading 17.2 mass% of fine silver powder, 75.8 mass% of large silver particle powder 4, 0.2 mass% of SOLPLUSD-540 manufactured by Lubrizol as a dispersant, 0.6 mass% of Tersolve IPTL-A manufactured by Nippon Terpene Chemical Co., Ltd., 2.35 mass% of Decanol manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., and 2.35 mass% of TOE-100 manufactured by Nippon Terpene Chemical Co., Ltd. as a solvent.

To adjust the viscosity of this silver paste to be suitable for printing, 0.70 wt% each of decanol and TOE-100 were added to dilute it, and the bonding material of Example 4 was obtained. The silver concentration in the resulting bonding material was determined by the ignition loss method and was found to be 93.0 mass%.

The component compositions of the bonding materials prepared above for Comparative Example 1 and Examples 1 to 4 are shown in Table 2 below.

[Viscosity measurement]
<Low shear viscosity>
For the bonding materials of the comparative example and Examples 1 to 4, a rheometer (rotational dynamic viscoelasticity measuring device) (a cone with a cone diameter of 35 mm and a cone angle of 2°, manufactured by Thermo Corporation) was used to evaluate the viscosity (low shear viscosity) at 25°C and a shear rate of 0.157 s ^-1 . The viscosity measurement was performed as follows. The bonding material was injected into the gap between the stage and the cone, and the cone was rotated at 0.157 ^s-1 . The low shear viscosity was calculated from the shear stress at 60 seconds after the start of rotation when the bonding material was injected into the gap between the stage and the cone and the cone was rotated at 0.157 s ^{-1. The low shear viscosity was calculated after confirming that the shear rate at that time was within ±1% of the set value of 0.157 s-1} for 30 seconds from 30 seconds after the start of rotation to 60 seconds after the start of rotation.

<Viscosity at 1 rpm and 5 rpm>
Furthermore, for the bonding materials of the comparative example and Examples 1 to 4, the viscosity and thixotropy ratio (viscosity at 1 rpm/viscosity at 5 rpm) were determined at 25° C. and rotation speeds of 1 rpm (3.1 s ⁻¹ ) and 5 rpm (15.7 s ⁻¹ ) using the same rheometer as above.

The results of the above viscosity measurements are summarized in Table 3 below.

As shown in Table 3, the bonding materials of the comparative example and Examples 1 to 4 did not differ significantly in 1 rpm viscosity, 5 rpm viscosity, or thixotropy ratio, but the low shear viscosity was significantly different.

<Bonding test (void evaluation)>
A DBC substrate (a ceramic substrate consisting of SiN (silicon nitride) sandwiched between copper plates) measuring 30 mm × 36.6 mm × 2.32 mm (Cu/SiN/Cu = 1.0 mm/0.32 mm/1.0 mm) that had been degreased with ethanol and then treated with 10% by mass sulfuric acid, and a semiconductor element measuring 8 mm × 8 mm × 0.1 mm with Ag plating applied to the bonding surface (entire bottom surface) were prepared.

Next, the bonding materials of the comparative example and examples 1 to 4 were applied to the DBC substrate by metal mask printing. The semiconductor element was placed on the bonding material applied to the DBC substrate so that the Ag-plated portion of the semiconductor element was in contact with the bonding material. A load of 5 N (a pressure of about 0.08 MPa) was applied between the bonding material and the semiconductor element by pressing the entire upper surface of the element between the bonding material and the semiconductor element, and the temperature was then raised from 25°C to 250°C at a heating rate of 3°C/min in a nitrogen atmosphere (oxygen concentration 500 ppm or less) in a hot air circulation baking furnace, and the temperature was held at 250°C for 60 minutes to form a silver bonding layer, and the semiconductor element was bonded to the DBC substrate by this silver bonding layer.

The resulting DBC substrate-silver bonding layer-semiconductor element bonded body was observed from the top of the semiconductor element at the bonded portion (the semiconductor element-silver bonding layer-DBC substrate stacked portion from above) using an ultrasonic flaw detector (C-SAM: D9500 manufactured by SONOSCAN). The resulting image (C-SAM image) was observed for the presence or absence of voids at the interface with the silver bonding layer of the DBC substrate. In addition, the analysis software attached to the device was used to calculate the proportion (percentage) of voids in the total observed area, which was defined as the void rate. The void area is the area that appears white in the C-SAM image, and the total observed area is a square area of 8 mm x 8 mm that corresponds to the bonding surface of the semiconductor element.

The evaluation results of the void ratio are shown in Table 4 below. The low shear viscosity of the bonding material and the tap density and filling rate of the large silver particle powder in the bonding material are also shown in Table 4.

As shown in Table 4, in the comparative example using a bonding material with a high low shear viscosity, the void ratio was 1.24%, whereas in Examples 1 to 4, by using a bonding material with a low shear viscosity of 1000 Pa·s or less, the void ratio was kept lower than in the comparative example at 0.77% or less, confirming that it was possible to reduce voids. It was also confirmed that there is a tendency for the void ratio to be reduced as the low shear viscosity is reduced. In this way, voids can be reduced by reducing the low shear viscosity of the bonding material.

Claims (23)

A bonding material comprising a metal particle powder and a solvent, the bonding material having a viscosity of 500 Pa·s or less measured at 25° C. and 0.157 s ⁻¹ . The bonding material according to claim 1, wherein a portion of the metal particle powder is a metal fine particle powder having an average primary particle diameter of 150 nm or less. The joining material according to claim 2, wherein the content of the metal microparticle powder in the joining material is 2 to 45 mass %. The joining material according to any one of claims 1 to 3, wherein a portion of the metal particle powder is a large metal particle powder having a filling rate of 65.0% or more and a cumulative 50% particle diameter (D50) on a volume basis measured by a laser diffraction type particle size distribution measuring device of 0.8 to 3.2 μm. The bonding material according to claim 4 , wherein the content of the large particle metal powder in the bonding material is 40 to 88 mass %. The bonding material according to claim 4 or 5 , wherein the filling rate of the large metal particle powder is 66.5% or more. A bonding material containing a metal particle powder and a solvent, and having a bonding strength of 0.157 s at 25°C. ^-1 a part of the metal particle powder is a large metal particle powder having a filling rate of 66.5% or more and a volume-based cumulative 50% particle diameter (D50) of 0.8 to 3.2 μm as measured by a laser diffraction particle size distribution measurement device. The bonding material according to claim 7 , wherein a part of the metal particle powder is a fine metal powder having an average primary particle size of 150 nm or less. The bonding material according to claim 8, wherein the content of the metal fine particle powder in the bonding material is 2 to 45 mass %. The bonding material according to any one of claims 7 to 9, wherein the viscosity is 800 Pa·s or less. The bonding material according to any one of claims 7 to 10, wherein the viscosity is 500 Pa·s or less. The bonding material according to any one of claims 7 to 11, wherein the content of the large particle metal powder in the bonding material is 40 to 88 mass %. The bonding material according to any one of claims 1 to 12 , wherein the metal is at least one selected from the group consisting of gold, silver, and copper. The bonding material according to any one of claims 1 to 13 , wherein the metal is silver. The bonding material according to any one of claims 1 to 14 , which is used for pressureless bonding. The bonding material according to any one of claims 1 to 15 , wherein the content of the metal particle powder in the bonding material is 90 to 96 mass %. A manufacturing method for a bonding material includes mixing a fine metal particle powder having an average primary particle diameter of 150 nm or less, a large metal particle powder having a filling rate of 66.5 % or more and a volume-based cumulative 50% particle diameter (D50) of 0.8 to 3.2 μm as measured by a laser diffraction type particle size distribution measuring device, and a solvent. The method for producing a bonding material according to claim 17, wherein the amounts of the fine metal particle powder and the large metal particle powder used are such that the contents of the fine metal particle powder and the large metal particle powder in the bonding material are 2 to 45 mass% and 40 to 88 mass%, respectively. The method for manufacturing a bonding material according to claim 17 or 18 , wherein a total amount of the fine metal particle powder and the large metal particle powder used is such that a total content of the fine metal particle powder and the large metal particle powder in the bonding material is 90 to 96 mass%. The method for manufacturing a bonding material according to any one of claims 17 to 19 , wherein the metal is at least one selected from the group consisting of gold, silver, and copper. A method for joining two members to be joined, comprising the steps of: applying a bonding material according to any one of claims 1 to 16 or a bonding material manufactured by the method for manufacturing a bonding material according to any one of claims 17 to 20 onto one of the members to be joined to form a coating film; placing the other member to be joined on the coating film; and baking the coating film on which the other member to be joined is placed at 160 to 350°C to form a metal bonding layer from the coating film. The bonding method according to claim 21 , wherein no pressure is applied to the two bonded members and the coating film when the coating film is fired to form the metal bonding layer. 23. The bonding method according to claim 21 or 22 , wherein the one member to be bonded is a substrate, and the other member to be bonded is a semiconductor element.