Junction structure and semiconductor device having the junction structure

Abstract

The present invention pertains to a bonding structure to be formed when, for example, a Si semiconductor or the like and a substrate are materials to be bonded and said pair of materials to be bonded are bonded by liquid phase diffusion bonding. This bonding structure encompasses a bonding part that exhibits a specific material construction. The metal composition of this bonding part comprises: 78.0% to 80.0% by mass, inclusive, of Ag; 20.0% to 22.0% by mass, inclusive, of Sn; and unavoidable impurity elements. Additionally, when any cross-section of the bonding part is observed, a characteristic material construction is exhibited which is constituted by: an island-shaped Ag phase containing 95% by mass or more of Ag; and an Ag3Sn phase comprising an Ag3Sn intermetallic compound surrounding the island-shaped Ag phase. The bonding part provided to this bonding structure has suitable bonding strength and is excellent in heat resistance and durability due to the aforementioned material construction.

Description

[Technical Field]

[0001] The present invention relates to a bonding structure for bonding a semiconductor element such as a Si semiconductor to a substrate, and more particularly to a lead-free bonding structure that has high bonding strength, excellent durability against thermal stress, and excellent heat resistance. [Background technology]

[0002] Semiconductor devices, such as power devices used in hybrid cars, electric vehicles, power generation facilities, and the like, are constructed by bonding a semiconductor element to a circuit board equipped with a heat sink made of copper or the like. Semiconductor devices with such a structure require high reliability for the joint between the semiconductor element and the circuit board. Furthermore, ensuring the reliability of the joint requires the material to possess many characteristics. Specifically, the material must exhibit high bonding strength between the semiconductor element and the circuit board while also being highly durable enough to prevent cracks even when subjected to repeated thermal stress due to differences in thermal expansion. Furthermore, the material must also have high heat resistance to prevent melting or softening due to heat generated during the manufacturing process and operation of the semiconductor device.

[0003] In recent years, semiconductor elements in the above-mentioned semiconductor devices have been improved to improve energy efficiency and increase power density. These improvements have led to temperature increases at the junctions, and there is a demand for junctions that can maintain the various characteristics described above even under such conditions.

[0004] Furthermore, there is a growing trend in the joints of semiconductor devices toward Pb-free constituent materials. High-melting-point solders with a high Pb content, such as Pb-10% by mass Sn and Pb-5% by mass Ag, are used to form the joints of the above-mentioned semiconductor devices that generate heat. These high-melting-point solders have melting points (liquidus temperatures) of 300°C or higher, enabling the formation of joints that can withstand high temperatures due to increased power density, etc. However, in light of recent regulations due to environmental concerns, the use of Pb is being avoided, and there is a growing demand for Pb-free joints in semiconductor devices as well.

[0005] For this reason, efforts are being made to develop joints that are Pb-free while still maintaining the required strength and durability, and one such method is the formation of joints using transient liquid diffusion bonding (TLP).TLP is a joining method in which a joining material (insertion metal) made up of a combination of two or more metals with different melting points is placed between the materials to be joined, and the insert metal and the materials to be joined are partially melted by heating the material to a temperature close to the melting point of the lower melting point of the insert metal, thereby creating a diffusion bond.

[0006] An example of liquid phase diffusion bonding is a method in which a combination of Sn, a low-melting-point metal, and Ag, a high-melting-point metal, is used as the insert metal. In liquid phase diffusion bonding using Sn and Ag as the insert metals, Sn melts when the bonding temperature is set to a temperature near the melting point of Sn (200-250°C). Then, by maintaining a constant heating temperature, the liquid Sn diffuses to the interface with the joined materials and integrates with them, and also diffuses into Ag, forming an intermetallic compound (AgSn) with Ag. This intermetallic compound has a high melting point of approximately 480°C, which can raise the melting point of the joint. Thus, liquid phase diffusion bonding has the advantage that it can form a high-melting-point joint while maintaining a low bonding temperature by appropriately selecting the composition of the insert metal. Until now, with common bonding methods using solder or brazing filler metal, the bonding temperature during bonding and the melting point of the resulting joint are nearly the same. In contrast, liquid phase diffusion bonding can form a joint with a melting point higher than the bonding temperature. By including such a high melting point intermetallic compound, a highly heat-resistant joint can be obtained.

[0007] An example of the application of liquid phase diffusion bonding to semiconductor devices is the joint structure described in Patent Document 1. In this prior art, a joint (joining material layer) is formed by liquid phase diffusion bonding using a solder material composed of a low-melting-point metal primarily composed of Sn and high-melting-point metals such as Ni, Cu, and Ag. The joint structure exhibits a material structure in which spherical, columnar, and elliptical intermetallic compounds ((Cu, Ni)6Sn5, Ag3Sn) are dispersed in a stress reliever primarily composed of Sn. According to this prior art, the stress reliever filled between the metal compounds suppresses the generation and propagation of cracks, ensuring the durability of the joint. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2018-98265 Summary of the Invention [Problem to be solved by the invention]

[0009] The above-mentioned prior art can be said to have various characteristics required for a joint by forming a characteristic intermetallic compound using liquid phase diffusion bonding while taking into account durability due to the Sn stress reliever. However, the joint formed by this prior art is considered to have insufficient heat resistance because it is mainly composed of Sn, a low-melting-point metal. That is, although an Sn intermetallic compound is formed in the joint of this prior art, most of the Sn remains as a stress reliever. Therefore, although the melting point of the region where the Sn intermetallic compound is present is high, the melting point of the region where the Sn remains is low, and it is difficult to say that the joint has high heat resistance overall. Therefore, there is a concern that the joint may soften or partially melt at high temperatures. Softening at high temperatures may lead to a decrease in the strength of the joint layer itself, which may result in the joint layer itself being destroyed. For these reasons, it is predicted that the above-mentioned conventional liquid phase diffusion bonding will have difficulty in adapting to semiconductor devices whose power density is expected to continue to increase.

[0010] The present invention has been made in light of the above-mentioned background, and provides a Pb-free bonding structure using liquid phase diffusion bonding, which ensures bonding strength, exhibits the heat resistance that liquid phase diffusion bonding aims for, and has excellent durability. [Means for solving the problem]

[0011] The technical background of the present invention described above is the problem of how to form a joint that can replace high-Pb solder, such as Pb-10% by mass Sn or Pb-5% by mass Ag, which has excellent heat resistance. To achieve this, it is necessary to change the main component of the joint from Pb to another metal. Regarding this change in the main component of the joint, Patent Document 1 uses the action of liquid phase diffusion bonding to make Sn the main component of the joint. Using Sn, a low-melting point metal, as the main component is effective in forming the joint, but as mentioned above, the heat resistance of the joint is insufficient.

[0012] The present inventors decided to develop a joint formed by liquid phase diffusion bonding that uses Ag as the main component instead of Pb and appropriately contains Sn, which can form an intermetallic compound with Ag. Ag is a relatively soft metal with a high melting point, and thus it is expected that a suitable joint can be formed. However, when Ag is used as the main component, it is expected that the desired properties may not be achieved depending on the form of Ag present after the joint is formed.

[0013] Therefore, the present inventors decided to reconsider the method of forming a joint and to investigate a technique for effectively promoting liquid phase diffusion bonding. As a result, they found that a compression-molded body made of Ag powder and Sn powder, which will be described later, can be used as a joining material (insert metal) for liquid phase diffusion bonding, and that a joint formed by using the joining material under appropriate conditions exhibits effective heat resistance, joining strength, and durability. The present inventors investigated the structure of the joint formed in this way and came up with the present invention, which has a joint with a distinctive material structure.

[0014] That is, the present invention provides a joint structure including a pair of workpieces and a joint formed between the pair of workpieces, wherein the joint is composed of 78.0 to 80.0 mass% Ag, 20.0 to 22.0 mass% Sn, and inevitable impurities as constituent elements, and wherein, when an arbitrary cross section of the joint is observed, a material structure is observed that is composed of island-shaped Ag phases containing 95 mass% or more of Ag and an Ag3Sn intermetallic compound phase surrounding the island-shaped Ag phases. The configuration of the joint structure according to the present invention and a method for forming the same will be described below.

[0015] A. Configuration of the joint structure according to the present invention As described above, the joining structure according to the present invention essentially comprises a pair of workpieces and a joining portion formed therebetween. Here, the joining portion formed between a pair of workpieces refers to a joining portion sandwiched between the pair of workpieces, either in contact or out of contact with the pair of workpieces. Non-contact sandwiching between a pair of workpieces refers to a state in which a diffusion layer with a different composition and structure from the workpieces exists at the interface between the joining portion and the workpieces, and the joining portion and the workpieces are not in direct contact with each other. As described below, in the present invention, a diffusion layer may be formed at the joining interface depending on the manufacturing conditions of the joining portion (the composition of the joining material). This diffusion layer is an optional component, and therefore is defined as above. Below, the details of each component of the joining structure according to the present invention (workpieces, joining portion, optional diffusion layer) are described.

[0016] A-1. Parts to be joined In the joining structure according to the present invention, there are no particular limitations on the type, use, structure, dimensions, etc. of the joined materials. A typical use of the present invention is a joining structure between a semiconductor element (such as Si) and a circuit board, in which case they constitute a pair of joined materials. However, there are no limitations on the constituent material, structure, dimensions, etc. of the semiconductor element. Furthermore, in general semiconductor devices, the circuit board has a ceramic substrate on which a Cu plate or the like is placed to dissipate heat from the semiconductor element, but there are no limitations on the material of the substrate or whether or not a Cu plate is present.

[0017] A-2.Joint part In the joint structure of the present invention, the joint is a major essential component. The joint of the joint structure of the present invention is composed primarily of Ag as a constituent element. Some of the Ag is converted into an Ag3Sn intermetallic compound, while the excess Ag that does not form an intermetallic compound is dispersed as an island-like metallic phase. As a result, the joint of the present invention exhibits a material structure consisting of an island-like Ag phase and an Ag3Sn phase that fills the gaps between the island-like Ag phases and surrounds them. To facilitate understanding of this material structure, FIG. 1 shows an example cross-section of the joint of the joint structure of the present invention.

[0018] Since Ag has a higher melting point than Sn and other metals, forming a joint from Ag and Ag3Sn phases ensures a high melting point throughout the joint, ensuring heat resistance. Ag is also a metal with high thermal conductivity, and distributing it in island-like formations is expected to improve the thermal conductivity of the entire joint. Furthermore, Ag is a relatively soft metal and is more flexible than intermetallic compounds, which tend to be hard and brittle. The island-like Ag phases in the joints of the joint structure of the present invention are believed to act as a buffer when the joint is subjected to thermal stress. Therefore, the joint structure of the present invention is also believed to have good durability and to be able to suppress cracks due to stress loads. As described above, the joint structure of the present invention exhibits a distinctive material structure, enabling it to exhibit properties suitable for use in semiconductor device joints.

[0019] A-2-1. Composition of joint The overall composition of the bonded structure according to the present invention is composed of 78.0 to 80.0 mass% Ag, and 20.0 to 22.0 mass% Sn and inevitable impurity elements. As mentioned above, Ag is used as the main component because of its advantageous properties such as high melting point and high thermal conductivity. Furthermore, Ag can form an intermetallic compound, Ag3Sn, with Sn, making it suitable for liquid phase diffusion bonding.

[0020] The Ag content of the joint of the present invention is 78.0 mass % or more and 80.0 mass % or less. According to the studies of the present inventors, the joint based on the liquid phase diffusion bonding of the present inventors described below exhibits suitable joint strength within the narrow Ag content range mentioned above. If the Ag content of the joint is less than 78.0 mass % or more than 80.0 mass %, at least one of the joint strength at room temperature and high temperature will be insufficient, and durability will also be poor. This narrow composition range of the joint, together with the material structure, is a characteristic of the joint structure of the present invention.

[0021] The Ag content is the Ag content of the entire joint, and is the sum of the Ag content of the Ag phase and the Ag content of the Ag3Sn phase. As mentioned above, the joint structure of the present invention may optionally have a diffusion layer at the interface between the joined materials and the joint, but the above overall composition does not include the diffusion layer and is the sum of the Ag content and Sn content of only the joint.

[0022] The constituent elements of the joint are essentially Ag and Sn, and in principle, consist of only these. However, the inclusion of unavoidable impurity elements is permitted. The unavoidable impurities may include Cu, Pb, etc. The total amount of these unavoidable impurities is preferably 0.1 mass% or less of the entire joint.

[0023] A-2-2.Material structure of the joint (1) Island-like Ag phase The Ag phase is a metallic phase with an Ag concentration of 95% by mass or more. The Ag phase may be pure Ag (Ag concentration 100% by mass), but the Ag content is set to 95% by mass or more because there is a risk of being affected by Sn and unavoidable impurities during the formation of the joint structure. This Ag phase is distributed in the joint structure in an island-like form in the material structure of the cross section of the joint. The island-like Ag phase is an Ag phase distributed with individually independent shapes, and is a metallic phase that is not connected or continuous in a row-like or layer-like form in the cross-sectional structure. In the present invention, it is believed that the distribution of individually independent Ag phases in the cross-sectional structure improves heat resistance and durability. If the Ag phase is in a row-like or layer-like form, anisotropy may occur in the properties of the joint. The shape of the island-like Ag phase is not particularly specified, and may include spherical, elliptical, or irregular shapes.

[0024] (2)Ag3Sn phase The Ag3Sn phase is an intermetallic compound of Ag and Sn, and is the phase surrounding the island-shaped Ag phase in the material structure of the joint cross section. The Ag3Sn phase, along with the island-shaped Ag phase, is the main phase that constitutes the joint. Ag3Sn has a melting point of approximately 480°C, contributing to the heat resistance of the joint. Furthermore, Ag3Sn is an intermetallic compound with excellent strength, which can contribute to joint strength. Furthermore, because the thermal stress experienced by the Ag3Sn phase is absorbed by the relatively soft Ag phase, the overall joint is expected to be durable against thermal stress. Such improved durability is difficult to achieve with joints composed solely or predominantly of the Ag3Sn phase. The composition of the Ag3Sn intermetallic compound is 73% Ag by mass and 27% Sn by mass, based on its atomic ratio (Ag:Sn = 3:1). The Ag3Sn phase of the present invention is similar to this composition.

[0025] (3) Area ratio of the island Ag phase in the joint The characteristic material structure of the present invention is observed in any cross section of the joint. In the present invention, the area ratio of the island Ag phase in this material structure is preferably 18% or more. It is estimated that the proportion of the Ag phase can contribute to the bonding strength and durability of the joint. If the area ratio of the Ag phase is small, the bonding strength at high temperatures will be insufficient, so the lower limit of the area ratio of the island Ag phase is preferably 18% or more. The lower limit of the area ratio of the island Ag phase is more preferably 10% or more, even more preferably 15% or more, and particularly preferably 25% or more. Furthermore, if the proportion of the island Ag phase is too high, the bonding itself will become unstable, making it difficult to obtain the required bonding strength even at room temperature. Therefore, the upper limit of the area ratio of the island Ag phase is preferably 35% or less. The area (grain size) of each island Ag phase is not particularly limited as long as it is a size that can be recognized in the cross-sectional structure.

[0026] The area ratio of the Ag phase can be measured by observing the cross-sectional structure using appropriate means and calculating it from the observed photographs and images. It is preferable to observe the longitudinal section of the joint at any position. The size of the observation area set for this cross-sectional structure observation is not particularly limited, but it is preferable to observe it so that it includes the interface (top and bottom edges) between the non-joined material and the joint. The area ratio of the Ag phase can be calculated using appropriate image analysis software based on the observed photographs and images. For example, the cross section can be observed using an SEM and an elemental mapping image obtained by EDS, and the image showing only the area with a high Ag concentration can be referenced and the area ratio can be calculated by image analysis.

[0027] The thickness of the joint portion of the joint structure according to the present invention is preferably 10 μm or more and 200 μm or less, more preferably 20 μm or more and 150 μm or less, and even more preferably 20 μm or more and 100 μm or less.

[0028] (4) Porosity If voids exist within the bonded portion, they not only serve as a medium for crack propagation but can also themselves become the starting point of cracks. Therefore, it is preferable that the presence of voids is reduced in the bonded structure of the present invention. Specifically, it is preferable that the porosity in any cross section of the bonded portion is 4% or less in terms of area ratio. In the present invention, liquid phase diffusion bonding is applied, and as described below, a compression molded body of Ag powder and Sn powder is used as the bonding material. This makes it possible to form a dense bonded portion in which void generation is suppressed. Note that, although 0% is the optimum lower limit for the void ratio, since it is difficult to completely eliminate voids in reality, it is preferable to set the lower limit to 1%.

[0029] A-3. Diffusion layer at the bonding interface with the bonded material (optional configuration) The joining structure according to the present invention has the above-described composition and essentially comprises a joining portion having a material structure composed of island-shaped Ag phases and Ag3Sn phases. However, the presence of a diffusion layer at the interface between the joined materials and the joining portion is optional. This diffusion layer is composed of the constituent material of the joined materials and the constituent material of the joining portion (especially Sn). In liquid phase diffusion bonding, the joining portion is formed by melting and diffusing the low-melting-point metal that constitutes the joining material (insert metal). During this process, the low-melting-point metal often diffuses into the joined materials. In the joining structure according to the present invention, the low-melting-point metal Sn forms an Ag3Sn phase with Ag, while some Sn diffuses into the joined materials and forms a diffusion layer made of Sn alloy at the joining interface.

[0030] The composition of the diffusion layer at the bonding interface is determined based on the constituent materials of the surfaces of the materials to be bonded. In a semiconductor device, one of the materials to be bonded is a circuit board equipped with a Cu heat sink, and the other is a semiconductor element (e.g., Si). In this case, in the bonding structure according to the present invention, an alloy of Cu and Sn is formed as a diffusion layer at the bonding interface on the circuit board side (between the bonding portion and the Cu heat sink). Furthermore, in order to ensure bonding with respect to the semiconductor element, a single or multiple layer of metal film such as Ni or Ti may be formed on the semiconductor element. In this case, in the bonding structure according to the present invention, a single or multiple layer of diffusion layer made of an alloy of these metals and Sn is formed at the bonding interface on the semiconductor element side (between the bonding portion and the metal film).

[0031] The diffusion layer does not necessarily have to be a continuous layer in the planar direction, but may be partially divided. Furthermore, it is not limited to a layer with a clear thickness, and a dot-like Sn alloy can also be called a diffusion layer. However, in the joining structure of the present invention, the diffusion layer is an optional component, and is not an essential component. This is because the diffusion layer has little effect on the characteristics of the joining portion itself.

[0032] B. Method for manufacturing a joint structure according to the present invention As described above, the joining structure according to the present invention can be formed by liquid phase diffusion bonding. As described above, liquid phase diffusion bonding is a method in which a joining material (insertion metal) made of a low-melting point metal and a high-melting point metal is placed between materials to be joined and heated.

[0033] In the present invention, Sn is used as the low-melting-point metal and Ag is used as the high-melting-point metal, and a joint mainly made of Ag is formed. To form a joint having the above-described material structure while suppressing the formation of voids and Sn phases, it is important to efficiently diffuse the molten Sn into Ag in the joining material and to efficiently alloy the diffused Sn with Ag. Based on this consideration, the inventors have found that a joining material made of a compression-molten mixture of Ag powder and Sn powder can be used to form the joining structure according to the present invention.

[0034] This compression-molded bonding material is a solid bonding material obtained by compressing a mixed powder of Ag powder with an average particle size of 30 μm to 75 μm and Sn powder with an average particle size of 1 μm to 20 μm, and then rolling it into a sheet (foil). The reason for using such a solid bonding material is to prevent voids from forming in the bonding area. A common method for handling a mixture of Ag powder and Sn powder is to use a paste in which both powders are dispersed in a solvent. However, this paste cannot sufficiently prevent voids from forming due to the evaporation of the solvent during bonding. Furthermore, there is a possibility that the solvent may remain in the bonding area, which also contributes to void formation. Taking this into consideration, a solvent-free solid bonding material is used.

[0035] The bonding material used in the present invention is a compression-molded body obtained by compressing and rolling a mixture of Ag powder and Sn powder having the above-mentioned average particle size. By compressing and rolling the Ag powder and Sn powder having the above-mentioned average particle size to a high degree, the diffusion of molten Sn and the generation of intermetallic compounds can be effectively caused during liquid phase diffusion bonding.

[0036] This compression-molded body has a relative density of 95% or more, preferably 98% or more, based on the bulk metal (alloy) assumed based on the compositional mixing ratio of each metal. The relative density can be calculated using the formula "density of compression-molded body / density of bulk metal (alloy)". The density of the compression-molded body and the density of the bulk metal (alloy) can be determined from the volume and mass measured by Archimedes' method or the like.

[0037] The mixture ratio of Ag powder and Sn powder in the compression-molded body that serves as the bonding material does not need to be the same as the composition of the bonding part described above. This is because consideration must be given to the formation of a diffusion layer due to the diffusion of Sn, a low-melting-point metal in the bonding material, to the bonding interface. The amount of Sn in the bonding material that forms the diffusion layer varies depending on the mixture ratio of Ag powder and Sn powder and the liquid-phase diffusion bonding conditions (temperature, pressure). Therefore, unlike the narrow range of the composition of the bonding part (Ag: 78.0 mass% to 80.0 mass%; Sn: 20.0 mass% to 22.0 mass%), the mixture ratio of the bonding material can be set within a relatively wide range. Specifically, a compression-molded body with a mixture ratio of Sn 5 mass% to 20 mass% higher than the composition of the bonding part can be used as a bonding material.

[0038] The bonded joint of the present invention can be formed by liquid-phase diffusion bonding using a bonding material composed of a compression-molded body of Ag powder and Sn powder. In this bonding method, the bonding material is preferably placed between a pair of bonded materials and then heated to 200°C or higher and 300°C or lower. If the bonding temperature is below 200°C, Sn is less likely to melt, making liquid-phase diffusion bonding less likely to proceed, and the bonded joint cannot achieve a desirable composition. On the other hand, if the bonding temperature is too high, molten Sn is more likely to migrate outside the bonded joint, resulting in significant compositional variation in the bonded joint due to a lack of Sn around the Ag powder. Furthermore, the lack of Sn reduces the amount of Sn diffusing into the Ag powder, which tends to disrupt the quantitative balance between the island Ag phase and the Ag3Sn phase. For these reasons, if the bonding temperature is too high, the bonded joint often does not have a desirable configuration. Furthermore, bonding at high temperatures may result in damage to the semiconductor element due to heat shock after bonding. Therefore, the upper limit of the bonding temperature is preferably 300°C. The heating time is preferably from 1 minute to 1 hour, depending on the thickness and volume of the compression-molded bonding material. A nitrogen atmosphere is recommended to prevent oxidation of the bonding material, substrate, etc.

[0039] Furthermore, when forming a bond using the above-mentioned bonding material, it is preferable to heat the bonding material while applying pressure. This is to promote the diffusion of molten Sn into the Ag powder. The pressure applied to the bonding material is preferably 1 MPa or more. It is also preferable to apply pressure from one or both of the materials to be bonded.

[0040] The joining structure according to the present invention is formed by the joining process using the joining material made of the compression-molded body of Ag powder and Sn powder described above. [Effects of the Invention]

[0041] As described above, the bonded structure according to the present invention has a bonded portion composed of an Ag phase and an Ag3Sn phase, and utilizes the characteristics of each phase to provide a bonded portion with excellent bond strength and heat resistance. This bonded portion also has excellent durability, and can maintain a sound bond without cracking due to repeated thermal stress. The bonded structure according to the present invention can be formed by liquid phase diffusion bonding using an appropriate bonding material. [Brief explanation of the drawings]

[0042] [Figure 1] 3A and 3B are diagrams illustrating the material structure of a joint portion in a joint structure according to the present invention; [Figure 2] Photographs showing the material structure of the joint portion of the joint structure of each sample manufactured in this embodiment. [Figure 3] 10 shows EDS mapping images of the joints of the joint structures of the samples manufactured in this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0043] Hereinafter, embodiments of the present invention will be described based on the following examples. In this embodiment, a compression molded body was produced from Ag powder and Sn powder, and this was used as a bonding material to bond a semiconductor chip and a substrate. The material structure of the bonded portion was observed, and characteristics such as heat resistance were evaluated.

[0044] First, Ag powder (purity 99.9% by mass, average particle size 54.6 μm) produced by atomization and Sn powder (purity 99.9% by mass, average particle size 3.7 μm) produced by atomization were mixed in a shaker to form a mixture. The average particle sizes of the Ag powder and Sn powder were measured by SEM observation (magnification: 200x (Ag powder), 2000x (Sn powder), image resolution: 1024 × 768, acceleration voltage: 15 kV). The average area of ​​each powder was calculated from the area and number of powder particles using the attached function of the SEM, and the equivalent circle diameter was used as the average particle size. In this embodiment, Ag powder and Sn powder with the above-mentioned average particle sizes of 54.6 μm and 3.7 μm were mainly used. However, as comparative examples, Ag powder with an average particle size of 90 μm and Sn powder with an average particle size of 24 μm were also used. These powders are also produced by the atomization method.

[0045] In this embodiment, the mixing ratio of Ag powder and Sn powder was changed to produce mixtures with Sn contents of 20 mass%, 25 mass%, 27 mass%, 30 mass%, 35 mass%, 40 mass%, 45 mass%, and 50 mass%. The mixture of Ag powder and Sn powder was then poured into a cylindrical mold and compressed using a hydraulic press to obtain a disk-shaped compact. The pressing pressure was 5 tons. The resulting compact was then rolled using a bench rolling mill to obtain a sheet-shaped compression compact with a thickness of 0.1 mm, which was used as the bonding material. The relative densities of the bonding materials produced in this embodiment to AgSn alloys (bulk metals) with the same composition and mixing ratio were measured using the Archimedes method. Even taking into account measurement error, all were confirmed to be within the range of 95% to 100%.

[0046] Next, a semiconductor chip and a circuit board were bonded using the bonding materials produced above to form a bonded structure. The semiconductor chip was a Si chip, and was bonded to a Cu substrate (KFC material) as the circuit board.

[0047] In this embodiment, to manufacture samples for the bonding strength evaluation test (die shear test) described below, Si chips (2 mm × 2 mm) metallized on one side with Ti (0.01 μm), Ni (0.3 μm), and Ag (0.2 μm) were prepared. Four of these Si chips were placed on a carbon jig with the metallized surface facing up. Next, one of the bonding materials (10 mm × 10 mm) manufactured as described above was placed on top of the four Si chips, and then a Cu substrate (11 mm × 11 mm, 0.2 mm thick) was placed on top of the bonding material. A carbon jig was then placed on top of the Si chips, sandwiching the Si chip / bonding material / Cu substrate and then fixing them in place with a spring-loaded jig (pressure of 2 MPa).

[0048] In addition, for the fabrication of samples for the cross-sectional observation of the bonded portion and the heat cycle test described below, Si chips (10 mm × 10 mm) with the same configuration as above were prepared. Then, using the same procedure as above, one Si chip was placed, one bonding material (10 mm × 10 mm) was placed on top of it, and then a Cu substrate (11 mm × 11 mm, thickness 0.2 mm) was placed on top, and a carbon jig was fixed in place (pressure pressure 2 MPa).

[0049] The Si chip / bonding material / Cu substrate sample, together with the jig, was placed in a low-oxygen oven (IPHH-202MS manufactured by Espec Corporation) and heated to form a bond. The oven was purged with nitrogen for 30 minutes to reduce the oxygen concentration to 70 ppm or less, and then heating was initiated. The temperature was increased at a rate of 5°C / min while flowing nitrogen until the set bonding temperature was reached. Once the temperature inside the oven reached the bonding temperature, the sample was maintained at the bonding temperature for 30 minutes while flowing nitrogen. After 30 minutes, the oven blower was turned on and the temperature was decreased for 30 minutes under nitrogen flow (temperature decrease rate: approximately -5°C / min). When the temperature inside the oven reached 40°C, the sample was removed and the jig was removed. In this embodiment, the bonding temperature was set to 250°C for the eight bonding materials. For some bonding materials (with an Sn content of 27% by mass), bonding temperatures of 185°C, 225°C, 250°C, 275°C, and 315°C were used to form bonds.

[0050] [Observation of material structure and composition analysis of joints] After the above-mentioned joining process, the material structure of the cross section of the joint was observed, and then the composition (composition analysis of the island-like Ag phase and Ag3Sn phase), area ratio of the island-like Ag phase, and porosity of the cross section of the joint were measured.

[0051] In these observations and measurements, a sample (Si chip / bonding material / Cu substrate) of the bonded structure manufactured above was embedded in resin, cut near the center, and the cross section was polished. The cross section of the sample was then observed with a scanning electron microscope (SEM: JSM-IT500HR manufactured by JEOL Ltd.) and a cross-sectional photograph was taken. The magnification for the SEM observation was set to a magnification that allowed observation of the upper and lower ends of the bonded portion. In this embodiment, the SEM acceleration voltage was set to 15 kV and the magnification was set to 1000x. In addition, in these observations and measurements, the entire bonded portion (excluding the bonded interface) was observed.

[0052] To analyze the composition of the joints, an energy dispersive X-ray analyzer (EDS) attached to the SEM was used (accelerating voltage: 15 kV). First, for the areas estimated to be island Ag and Ag3Sn phases from SEM observation, spot analysis was performed at three points for each phase, with 10,000 samplings per point (measurement time: 30 seconds). As a result, it was confirmed that the composition of the island Ag phase was 100% Ag by mass, and the composition of the Ag3Sn phase was 73% Ag by mass and 27% Sn by mass.

[0053] Next, mapping was performed using EDS to measure the area ratio of the island-shaped Ag phase. First, element mapping was performed on the observation area (magnification 10,000 times) of the joint cross section. In this embodiment, the mapping resolution was set to 256 × 192 pixels, and the analyzed elements were set to Ag, Sn, Cu, Si, and Ni. In this EDS mapping analysis, in order to analyze the composition at each pixel as accurately as possible, it is preferable to perform sufficient sampling (counting) at each pixel. As an example of a suitable index, it is preferable to perform sampling 2,000 times or more per pixel, and in this embodiment, each analyzed element was measured by sampling 2,500 times per pixel.

[0054] Mapping analysis of each sample confirmed the presence of a region with an Ag concentration of approximately 100% by mass and a region with an Ag concentration of approximately 73% by mass in the cross section of the joint. Therefore, the obtained mapping images were subjected to two-color processing using a predetermined Ag concentration as a threshold to create a mapping image (Ag mapping image) for distinguishing the Ag phase from the Ag3Sn phase. In this embodiment, the threshold Ag concentration was set to 85% by mass. Regions above this threshold were identified as the Ag phase, and regions below this threshold were identified as the Ag3Sn phase. The area ratio of the Ag phase was calculated for the created Ag mapping images using image analysis software (product name: MIPAR). Figure 3 shows the mapping images of each sample created based on the Ag concentration. In each image, the light-colored areas represent the island-shaped Ag phase, and the dark-colored areas represent the Ag3Sn phase.

[0055] The porosity was measured by measuring the area ratio of the SEM images taken during the SEM observation using the same image analysis software as above. In this image analysis, the color tone of the areas in the SEM images that were clearly judged to be voids (areas close to black in the photograph) was used as the standard, and the area ratio of the areas with the same color tone as these voids was measured.

[0056] [Evaluation of bonding strength (die shear test)] Next, the bond strength of each sample was measured. The bond strength evaluation test (die shear test) was conducted on the prepared samples at room temperature and after heating to 260°C. The bond strength was measured by placing the sample with the Cu substrate facing downwards in a bond tester (Try Precision MFM1200L), hooking the tester's tool (claw) onto the edge of the chip, and applying a shear load of 200 kg to measure the die shear strength. A shear strength of 20 MPa or greater was evaluated as "pass (good)," and a bond that passed both at room temperature and at high temperatures was evaluated as being suitable.

[0057] [Evaluation of thermal cycle durability] Furthermore, a thermal cycle test was conducted on the bonded joints of the samples to confirm their durability against heating and cooling cycles. Each sample was subjected to 250 cycles of a thermal cycle consisting of a 30-minute hold at -50°C and a 30-minute hold at 175°C. The presence or absence of peeling or cracks in the bonded joints after 250 cycles was then confirmed. The thermal cycle test was not conducted on samples whose room temperature or high-temperature bond strength was below the lower limit of measurement (5 MPa or less) in the die shear test.

[0058] The test results are shown in Table 1. Table 1 also shows the composition and area ratio of the island Ag phase in the joint of each sample, as well as the measurement results of the porosity. As an example of the observed material structure of the joint in each sample, SEM photographs of the joints (joining temperature 250°C) of Nos. 1, 2, 5, 10, 11, 12, 13, and 14 in Table 1 are shown in Figure 2, and their mapping images are shown in Figure 3.

[0059] [Table 1]

[0060] Referring to Table 1 and Figures 2 and 3, it can be seen that liquid phase diffusion bonding using a compression-molded Ag powder and Sn powder composite as the bonding material formed a joint having a material structure composed of Ag and Ag3Sn phases. However, depending on the composition of the joint, some samples exhibited insufficient bond strength or poor thermal cycle durability. Specifically, when the Ag content of the joint was less than 78.0 mass% (Nos. 3, 13, and 14), the bond strength at high temperatures was poor. Furthermore, when the Ag content exceeded 80.0 mass% (Nos. 1 and 7-9), although the bond strength tended to be higher than that of samples with an Ag content of less than 78.0 mass%, the bond strength at high temperatures was unacceptable. Furthermore, these joints outside the preferred composition range also exhibited peeling and cracking during thermal cycle testing. Therefore, it is believed that the bond strength and durability of the joints formed by liquid phase diffusion bonding of the present invention are ensured by optimizing the composition range as well as the material structure of the Ag and Ag3Sn phases.

[0061] From the viewpoint of optimizing the material structure, it is preferable to optimize the area ratio of the island Ag phase. Regarding the area ratio of the island Ag phase, there is not much difference in the values ​​of the area ratios for the joints with suitable joint compositions (No. 2, Nos. 4 to 6, and Nos. 10 to 12). These joints have suitable joint strength and thermal cycle durability. On the other hand, when the joint composition is outside the range of the present invention, the area ratio of the island Ag phase tends to be less than 18% or more than 35%. In particular, for joint No. 13, although the joint composition is very close to the range of the present invention, the joint strength is still slightly insufficient. This is thought to be due to the lack of island Ag phases.

[0062] In this embodiment, the joints were formed using Ag powder, Sn powder, and a compression-molded body as the joining material. However, the composition of the joining material and the joining conditions (joining temperature) also affect the composition of the joint. Regarding the joining material, Nos. 8 and 9 used joining materials containing coarse Ag or Sn powder. These joints had higher Ag contents and a higher area ratio of island-shaped Ag phases than joints using joining materials of the same mixture ratio. Furthermore, the joining temperature exceeded the 200°C to 300°C condition. No. 3 (joining temperature 185°C) had a low Ag content, while No. 7 (joining temperature 315°C) had a high Ag content. This is presumably because the joining temperature affects the diffusion of molten Sn during joining. The joints formed in this embodiment, except for No. 13, had a low porosity of 4% or less. This is thought to be due to the liquid-phase diffusion bonding using a solid joining material (a compression-molded body of Ag and Sn powders) that differs from conventional pastes. [Industrial Applicability]

[0063] The joint structure according to the present invention has a joint having a unique material structure composed of an Ag phase and an Ag3Sn phase. This joint has excellent joint strength, heat resistance, and durability due to the excellent heat resistance of the Ag and Ag3Sn phases and the flexibility of the Ag phase. The present invention is suitable for use in joining device elements in semiconductor devices such as power devices for hybrid cars and EVs.

Claims

1. A joining structure including a pair of workpieces and a joining portion formed between the pair of workpieces, The joint contains, as constituent elements, 78.0 mass% or more and 80.0 mass% or less of Ag, 20.0 mass% or more and 22.0 mass% or less of Sn, and inevitable impurity elements, When an arbitrary cross section of the joint is observed, island-shaped Ag phases containing 95 mass % or more of Ag and Ag ３ The Ag phase is composed of an Sn intermetallic compound and surrounds the island-shaped Ag phase. ３ A joining structure characterized in that a material structure consisting only of the Sn phase is observed.

2. 2. The joint structure according to claim 1, wherein the area ratio of the island-shaped Ag phase in any cross section of the joint is 18% or more and 35% or less.

3. 3. The joint structure according to claim 1, wherein the porosity in any cross section of the joint is 4% or less in terms of area ratio.

4. In a semiconductor device formed by bonding a semiconductor element to a substrate, A semiconductor device comprising the junction structure according to any one of claims 1 to 3 between the semiconductor element and the substrate.

5. A bonding material for liquid phase diffusion bonding to form the bonding structure according to claim 1, The composite is a sheet-shaped compression molded body obtained by compressing and rolling a mixed powder of Ag powder and Sn powder, the compression-molded body has a relative density of 95% or more based on the density of a bulk metal having the same composition as the mixed powder in the same mixing ratio; The joining material is placed between a pair of workpieces and then heated at 200°C or higher and 300°C or lower to form a joint, and when this is done, the joining material forms a material structure in any cross section of the joint that consists only of island-shaped Ag phases containing 95% or more by mass of Ag and Ag3Sn intermetallic compounds, surrounding the island-shaped Ag phases.

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