A bonding structure and a semiconductor device having the bonding structure.

Abstract

Regarding a bonding structure using liquid-phase diffusion bonding, the present invention provides a bonding structure that ensures bonding strength while also exhibiting excellent heat resistance and durability. [Solution] The present invention relates to a bonding structure to be formed when bonding a pair of materials, such as a Si semiconductor and a substrate, by liquid-phase diffusion bonding. This bonding structure includes a bonding portion that exhibits a unique material structure. The metal composition of this bonding portion consists of 78.0% to 80.0% by mass of Ag, 20.0% to 22.0% by mass of Sn, and unavoidable impurity elements. When an arbitrary cross-section of the bonding portion is observed, it exhibits a characteristic material structure composed of island-shaped Ag phases containing 95% or more by mass of Ag, and Ag3Sn phases consisting of Ag3Sn intermetallic compounds surrounding the island-shaped Ag phases. The bonding portion of the bonding structure of the present invention has suitable bonding strength and excellent heat resistance and durability due to the aforementioned material structure.

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. Specifically, it relates to a bonding structure that is lead-free and has excellent bonding strength, durability against thermal stress, and heat resistance.

Background Art

[0002] Semiconductor devices such as power devices used in hybrid cars, EVs, power generation facilities, etc. are configured by bonding semiconductor elements to a circuit board provided with a heat sink made of copper or the like. In a semiconductor device having such a structure, high reliability is required for the joint between the semiconductor element and the circuit board. And in order to ensure the reliability of the joint, it is necessary to have many characteristics. Specifically, while exhibiting high bonding strength between the semiconductor element and the circuit board, high durability is required not to cause cracks even when subjected to repeated thermal stress due to the difference in thermal expansion. And high heat resistance that does not cause melting or softening against the heat generated during the manufacturing process or operation process of the semiconductor device is also required.

[0003] In recent years, in the above semiconductor devices, improvements in semiconductor elements aiming at improving energy efficiency and increasing output density have been progressing. Such improvements are factors causing temperature rise at the joint, and a joint capable of having the above various characteristics under such circumstances is required.

[0004] Also, as a trend in the joint of semiconductor devices, the demand for lead-free of constituent materials is increasing. For the formation of the joint of the above semiconductor devices accompanied by heat generation, for example, high melting point solder with a high Pb content such as Pb-10 mass% Sn, Pb-5 mass% Ag, etc. is used. These high melting point solders have a melting point (liquidus temperature) of 300 °C or higher and can form a joint that can withstand the high temperature of the joint due to an increase in output density or the like. However, considering recent regulations due to environmental problems, the use of Pb is tending to be avoided, and the demand for lead-free is increasing in the joint of semiconductor devices.

[0005] Therefore, development is underway to create joints that are Pb-free while maintaining joint strength and durability, and one such method is the formation of joints using the transient liquid diffusion bonding (TLP) method. Transient liquid diffusion bonding is a bonding method in which a bonding material (insert metal) made up of two or more predetermined metals with different melting points is placed between the materials to be bonded, and the materials are heated to a temperature near the melting point of the lower melting point of the insert metal to partially melt the insert metal and the materials to be bonded, thereby causing diffusion bonding.

[0006] One example of liquid-phase diffusion bonding is a method using an insert metal that combines Sn, a low-melting-point metal, with Ag, a high-melting-point metal. In this liquid-phase diffusion bonding using Sn and Ag as insert metals, the bonding temperature is set to a temperature near the melting point of Sn (200-250°C), causing the Sn to melt. If this state is maintained by keeping the heating temperature constant, the liquid-phase Sn diffuses into the interface with the materials to be joined and integrates, and also diffuses into Ag, forming an intermetallic compound (Ag3Sn) 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 characteristic of being able to form a high-melting-point joint while keeping the bonding temperature low by appropriately setting the composition of the insert metal. In conventional bonding methods using common bonding materials such as solder and brazing material, the bonding temperature at the time of bonding and the melting point of the formed joint are almost the same. In contrast, liquid-phase diffusion bonding can form a joint with a melting point higher than the bonding temperature. By including high-melting-point intermetallic compounds in this way, a highly heat-resistant joint can be obtained.

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

[0008] [Patent Document 1] Japanese Patent Publication No. 2018-98265 [Overview of the project] [Problems that the invention aims to solve]

[0009] The prior art described above can be said to possess various properties required for a joint, as it forms characteristic intermetallic compounds in liquid-phase diffusion bonding while considering durability due to stress-relaxing materials such as Sn. 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. In other words, although an intermetallic compound of Sn is formed in the joint of this prior art, most of the Sn remains as stress-relaxing material. Therefore, although the melting point of the joint is high in the region where the Sn intermetallic compound is present, the melting point is low in the region where Sn remains, and overall, it cannot be said to have high heat resistance. For this reason, there is a concern that this joint may soften or partially melt at high temperatures. Softening at high temperatures can lead to a decrease in the strength of the joint layer itself, and there is a risk that the joint layer itself may break. For these reasons, it is predicted that the above-mentioned conventional liquid-phase diffusion bonding will be difficult to use for semiconductor devices whose power density is expected to continue to increase.

[0010] The present invention was made against the background described above, and provides a Pb-free bonding structure by liquid-phase diffusion bonding that ensures bonding strength, exhibits the heat resistance intended by liquid-phase diffusion bonding, and provides a bonding structure with excellent durability. [Means for solving the problem]

[0011] The technical background to the present invention, as described above, is the challenge of how to form a joint that can replace high-lead solders such as Pb-10 mass%Sn and Pb-5 mass%Ag, which have 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 described above uses Sn as the main component of the joint while utilizing the action of liquid-phase diffusion bonding. Using Sn, a low-melting-point metal, as the main component is effective in joint formation, but as mentioned above, the heat resistance of the joint is insufficient.

[0012] Unlike the conventional approach described above, the inventors have decided to investigate a joint formed by liquid-phase diffusion bonding, in which Ag is used as the main component instead of Pb, and which appropriately contains Sn, which can form an intermetallic compound with Ag. This is because Ag is a relatively soft metal but has a high melting point, and is therefore expected to be able to form a suitable joint. However, when Ag is the main component, it is expected that the desired properties may not be easily achieved depending on the form in which Ag exists after the joint is formed.

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

[0014] In other words, the present invention relates to a joining structure comprising a pair of materials to be joined and a joint formed between the pair of materials to be joined, wherein the joint consists of 78.0% to 80.0% by mass of Ag, 20.0% to 22.0% by mass of Sn, and unavoidable impurity elements as constituent elements, and when an arbitrary cross-section of the joint is observed, a material structure is observed that consists of island-shaped Ag phases containing 95% or more by mass of Ag, and Ag3Sn intermetallic compounds, with Ag3Sn phases surrounding the island-shaped Ag phases. The configuration of the joining structure according to the present invention and the method of forming it 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 has as an essential component a pair of materials to be joined and a joint formed between them. Here, the joint formed between a pair of materials to be joined means a joint that is sandwiched between the pair of materials, either in contact with or without contact with them. Sandwiching in a non-contact state by a pair of materials to be joined means that a diffusion layer with a different composition and structure from them exists at the interface between the joint and the materials to be joined, and the joint and the materials to be joined are not in direct contact. As described later, in the present invention, depending on the manufacturing conditions of the joint (composition of the joining material), a diffusion layer may be formed at the joining interface. Since this diffusion layer is an optional component, it has been defined as described above. The details of each component of the joining structure of the present invention (materials to be joined, joint, optional diffusion layer) will be described below.

[0016] A-1. Parts to be joined In the bonding structure according to the present invention, the type, application, structure, dimensions, etc., of the materials to be bonded are not particularly limited. A typical application of the present invention is a bonding structure between a semiconductor element (such as Si) and a circuit board, in which case they constitute a pair of materials to be bonded. However, there are no limitations on the constituent materials, structure, dimensions, etc., of the semiconductor element. In addition, in general semiconductor devices, the circuit board is 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 the presence or absence of a Cu plate.

[0017] A-2.Joint part In the bonding structure of the present invention, the bonding portion is a major essential component. The bonding portion of the bonding structure according to the present invention mainly consists of Ag as a constituent element. A portion of Ag is used as an Ag3Sn intermetallic compound, while the excess Ag that does not form an intermetallic compound is dispersed as island-shaped metallic phases. As a result, the bonding portion of the present invention exhibits a material structure consisting of island-shaped Ag phases and Ag3Sn phases that fill the gaps between the island-shaped Ag phases and surround them. To facilitate understanding of this material structure, Figure 1 shows an example of a cross-section of the bonding portion of the bonding structure according to the present invention.

[0018] Since Ag is a metal with a higher melting point than Sn, etc., by constructing the joint with Ag phase and Ag3Sn phase, the joint as a whole will have a high melting point, ensuring heat resistance. Furthermore, Ag is a metal with high thermal conductivity, and by distributing it in island-like formations, it is expected that the overall thermal conductivity of the joint will be improved. Moreover, Ag is a relatively soft metal and has more flexibility than intermetallic compounds, which tend to be hard and brittle. The island-like Ag phases in the joint of the joint structure of the present invention are thought to act as a buffer when the joint is subjected to thermal stress. Therefore, the joint structure of the present invention is thought to have good durability and to suppress the occurrence of cracks due to stress loading. As described above, the joint structure according to the present invention exhibits a characteristic material structure, and can exhibit properties suitable for use as a joint in semiconductor devices.

[0019] A-2-1. Composition of the joint The overall composition of the bonding structure according to the present invention consists of 78.0% to 80.0% by mass of Ag, 20.0% to 22.0% by mass of Sn, and unavoidable impurity elements. The reason for using Ag as the main component is, as described above, to take into consideration the advantageous properties of Ag, such as its high melting point and high thermal conductivity. Furthermore, since Ag can form the intermetallic compound Ag3Sn with Sn, it can be used for liquid-phase diffusion bonding.

[0020] The Ag content of the joint of the present invention is Ag of 78.0% by mass or more and 80.0% by mass or less. According to the study by the present inventors, the joint based on the liquid-phase diffusion bonding of the present inventors described later exhibits suitable bonding strength when the Ag content is within the narrow range as described above. When the Ag content of the joint is less than 78.0% by mass or more than 80.0% by mass, at least one of the bonding strengths at room temperature or high temperature is insufficient, and the durability is also inferior. Such a narrow composition range in the joint is a feature of the joint structure of the present invention together with the material structure.

[0021] Note that the Ag content is the Ag content of the entire joint and is the sum of the Ag content in the Ag phase and the Ag content in the Ag3Sn phase. As described above, in the joint structure of the present invention, optionally, a diffusion layer may be provided at the interface between the joined material and the joint. However, the above overall composition means that the diffusion layer is not included, and the total of the Ag content and the Sn content of only the joint is taken as the overall composition.

[0022] The constituent elements of the joint essentially include Sn together with Ag and are generally composed of only these. However, the inclusion of inevitable impurity elements is allowed. Inevitable impurities may include Cu, Pb, etc. It is preferable that the total of these inevitable impurities is 0.1% by mass or less with respect to the entire joint.

[0023] A-2-2. Material Structure of Joint (1) Island-like Ag phase The Ag phase is a metal phase with an Ag concentration of 95% by mass or more. The Ag phase may be pure Ag (Ag concentration 100% by mass), but since it may be affected by Sn or inevitable impurities during the formation process of the bonding structure, the Ag content is set to 95% by mass or more. This Ag phase is distributed in the bonding 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 that has an independent shape individually and is not a metal phase that is connected and continuous in a columnar or layered state in the cross-sectional structure. In the present invention, it is considered that the heat resistance and durability are improved by the distribution of the Ag phase having an independent shape individually in the cross-sectional structure. If the Ag phase is in a columnar or layered form, there is a possibility that anisotropy may occur in the characteristics of the joint portion. Incidentally, the shape of the island-like Ag phase is not particularly defined and exhibits shapes such as spherical, elliptical, and irregular shapes.

[0024] (2) Ag3Sn phase The Ag3Sn phase consists of Ag3Sn, which is an intermetallic compound of Ag and Sn, and is a phase surrounding the island-like Ag phase in the material structure of the cross-section of the joint. The Ag3Sn phase is a main phase constituting the joint together with the island-like Ag phase. Since Ag3Sn has a melting point of about 480°C, it contributes to the heat resistance of the joint. Also, since Ag3Sn is an intermetallic compound and is excellent in terms of strength, it can also contribute to the bonding strength. And since the thermal stress received by the Ag3Sn phase is absorbed by the relatively soft Ag phase, ensuring the durability against the thermal stress of the entire joint is also expected. Such improvement in durability is a characteristic that is difficult to expect for a joint composed of the Ag3Sn phase alone or mostly of the Ag3Sn phase. Incidentally, since the composition of the Ag3Sn intermetallic compound is 73% by mass Ag - 27% by mass Sn from its atomic ratio (Ag:Sn = З:1), the Ag3Sn phase of the present invention is also approximated to this composition.

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

[0026] When measuring the area fraction of the Ag phase as described above, the cross-sectional structure can be observed by appropriate means, and the measurement can be calculated from the observation photographs / images. It is preferable to observe the longitudinal cross-section at any position of the joint. There are no particular limitations on the size of the observation area set for observing the cross-sectional structure, but it is preferable to observe so as to include the interface (upper and lower ends) between the non-jointed material and the joint. The area fraction of the Ag phase can be calculated using appropriate image analysis software based on the observation photographs / images. For example, the area fraction can be calculated by observing the cross-section with an SEM and acquiring elemental mapping images by EDS, referring to the image outputting only the areas with high Ag concentration, and analyzing the image.

[0027] Furthermore, 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, it is 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 a joint, they not only act as a medium for crack propagation but can also become the starting point for cracks. Therefore, it is preferable that the joint structure according to the present invention reduces the presence of voids. Specifically, it is preferable that the void ratio in any cross-section of the joint is 4% or less in terms of area ratio. In the present invention, liquid-phase diffusion bonding is applied, and as described later, a compressed molded body of Ag powder and Sn powder is used as the bonding material. This makes it possible to form a dense joint in which the generation of voids is suppressed. As for the lower limit of the void ratio, 0% is optimal, but since it is practically difficult to completely eliminate voids, it is preferable to set the lower limit at 1%.

[0029] A-3. Diffusion layer at the bonding interface with the materials to be bonded (arbitrary configuration) The joining structure according to the present invention has the above-described composition and the joining portion having a material structure composed of island-like Ag phases and Ag3Sn phases as an essential main component. However, the presence of a diffusion layer at the interface between the materials to be joined and the joining portion is optionally permitted. This diffusion layer is composed of the constituent materials of the materials to be joined and the constituent materials of the joining portion (especially Sn). In liquid-phase diffusion joining, the joining portion is formed by the melting and diffusion of the low-melting-point metal constituting the joining material (insert metal). At this time, the low-melting-point metal often diffuses into the materials to be joined as well. In the joining structure according to the present invention, while Sn, which is a low-melting-point metal, forms an Ag3Sn phase with Ag, some of the Sn may diffuse into the materials to be joined and form a diffusion layer made of an Sn alloy at the joining interface.

[0030] The configuration of the diffusion layer at this bonding interface is determined based on the constituent materials of the surfaces of the materials to be bonded. In semiconductor devices, one material to be bonded is a circuit board equipped with a Cu heat sink, and the other material to be bonded is a semiconductor element (such as 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 properties, a single or multiple layers of metal films such as Ni and Ti may be formed on the semiconductor element. In this case, in the bonding structure according to the present invention, a single or multiple layers of a 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] Furthermore, the diffusion layer does not necessarily have to be a continuous layer in the planar direction; it may be partially fragmented. Also, it is not limited to a layer with a clearly defined thickness; a point-like Sn alloy can also be referred to as a diffusion layer. However, in the bonding structure of the present invention, the diffusion layer is an optional component and not an essential component, because the diffusion layer has little influence on the properties of the bonding 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 stated above, liquid-phase diffusion bonding is a method in which a joining material (insert metal) consisting of a low-melting-point metal and a high-melting-point metal is placed between the materials to be joined and heated.

[0033] In this invention, a joint is formed mainly of Ag, using Sn as the low-melting-point metal and Ag as the high-melting-point metal. Here, in order to form a joint that has the above-described material structure while suppressing the formation of voids and the Sn phase, it is important to efficiently promote the diffusion of molten Sn into Ag in the joining material and the alloying of the diffused Sn and Ag. The inventors, through studies considering this point, have found that a joining material consisting of a compressed molded body of a mixture of Ag powder and Sn powder is applicable as a joining material for forming the joint structure according to the present invention.

[0034] This compression-molded bonding material is a solid bonding material formed into a sheet (foil) by pressing and 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, followed by rolling. The reason for using such a solid bonding material is to suppress the formation of voids in the joint. A common method for handling the mixed Ag and Sn powder is to disperse them in a solvent to form a paste. However, such a paste cannot sufficiently suppress the formation of voids due to the evaporation of the solvent during bonding. In addition, there is a possibility that solvent may remain in the joint, which also contributes to void formation. Considering this point, a solvent-free solid bonding material is used.

[0035] Furthermore, the bonding material used in this invention consists of a compressed molded body obtained by pressurizing and highly compressing and rolling a mixture of Ag powder and Sn powder of the above average particle size. By highly compressing and rolling the Ag powder and Sn powder of the above average particle size, diffusion of molten Sn and formation of intermetallic compounds can be effectively produced during liquid-phase diffusion bonding.

[0036] This compression-molded body has a relative density of 95% or higher, preferably 98% or higher, relative to the bulk metal (alloy) assumed based on the compositional mixing ratio of each metal. Relative density can be calculated from 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 methods such as the Archimedes method.

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

[0038] The joint of the present invention can be formed by liquid-phase diffusion bonding using a bonding material consisting of a compressed molded body of Ag powder and Sn powder as described above. In this bonding method, it is preferable to place the bonding material between a pair of materials to be bonded and then heat it at a temperature of 200°C to 300°C. If the bonding temperature is below 200°C, Sn is difficult to melt, making it difficult for liquid-phase diffusion bonding to proceed, and it is not possible to achieve a suitable composition for the joint. On the other hand, if the bonding temperature is too high, molten Sn is more likely to move to the outside of the joint, and the compositional variation of the joint due to the lack of Sn around the Ag powder becomes large. Also, since the amount of Sn diffusion into the Ag powder decreases due to the lack of Sn, the quantitative balance between the island Ag phase and the Ag3Sn phase tends to be disrupted. For these reasons, if the bonding temperature is too high, the joint often does not have a suitable configuration. Furthermore, bonding at high temperatures may cause the semiconductor device to be destroyed by thermal shock after bonding. For this reason, the upper limit of the bonding temperature is preferably 300°C. The heating time depends on the thickness and volume of the bonding material, which is made of a compression molded body, but it is preferable to heat it for at least 1 minute and no more than 1 hour. Furthermore, a nitrogen atmosphere is recommended to prevent oxidation of the bonding material and substrate.

[0039] Furthermore, when forming a joint using the above-mentioned bonding material, it is preferable to heat the bonding material under pressure. This is to promote the diffusion of molten Sn into 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 joined.

[0040] The joint structure according to the present invention is formed by a joining process using a joining material consisting of a compressed molded body of Ag powder and Sn powder as described above. [Effects of the Invention]

[0041] As described above, the joint structure according to the present invention is composed of an Ag phase and an Ag3Sn phase in the joint portion, and by utilizing the characteristics of each phase, it provides a joint portion with good joint strength and heat resistance. This joint portion also has excellent durability and can maintain a sound joint portion without cracking under repeated thermal stress. The joint structure having the joint portion of the present invention can be formed by liquid phase diffusion bonding using an appropriate bonding material. [Brief explanation of the drawing]

[0042] [Figure 1] A diagram illustrating the material structure of the joint in the joint structure according to the present invention. [Figure 2] A photograph showing the material structure of the joint portion of the joint structure of each sample manufactured in this embodiment. [Figure 3] EDS mapping images of the joints of the joint structures of each sample manufactured in this embodiment. [Modes for carrying out the invention]

[0043] The embodiments of the present invention will be described below based on the following examples. In this embodiment, a compression molded body was manufactured 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 and its properties such as heat resistance were then evaluated.

[0044] First, Ag powder (purity 99.9% by mass, average particle size 54.6 μm) 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. For measuring the average particle size of the Ag and Sn powders, each powder was observed using SEM (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 particles using the SEM's built-in functions, and its equivalent circle diameter was defined as the average particle size. In this embodiment, the Ag and Sn powders with average particle sizes of 54.6 μm and 3.7 μm were primarily used, but for some 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 were also manufactured using the atomization method.

[0045] In this embodiment, the mixing ratio of Ag powder and Sn powder was changed to produce mixtures with Sn content of 20% by mass, 25% by mass, 27% by mass, 30% by mass, 35% by mass, 40% by mass, 45% by mass, and 50% by mass. The mixture of Ag powder and Sn powder was poured into a cylindrical mold and compressed with a hydraulic press to obtain a flat disc-shaped molded body. The pressing pressure was set to 5 tons. The resulting compressed body was rolled with a benchtop rolling mill to obtain a sheet-like compressed molded body with a thickness of 0.1 mm, which was used as a bonding material. Furthermore, when the relative density of the bonding materials produced in this embodiment was measured using the Archimedes method against AgSn alloy (bulk metal) with the same composition as the mixing ratio, it was confirmed that all were within the range of 95% to 100%, even considering measurement errors.

[0046] Next, a bonded structure was formed by joining a semiconductor chip and a circuit board using the multiple bonding materials manufactured as described above. The semiconductor chip was a Si chip, and it was bonded to a Cu substrate (KFC material) as the circuit board.

[0047] In this embodiment, for the production of samples for the bonding strength evaluation test (die shear test) described later, Si chips (2 mm x 2 mm) with Ti (0.01 μm) / Ni (0.3 μm) / Ag (0.2 μm) metallized on one side were prepared. Four of these Si chips were placed on a carbon jig with the metallized surface facing upwards. Next, one bonding material (10 mm x 10 mm) manufactured above was placed on top of the four Si chips, and then a Cu substrate (11 mm x 11 mm, 0.2 mm thick) was placed on top of the bonding material. Then, the carbon jig was placed on top, sandwiching the Si chips / bonding material / Cu substrate, and then fixed with a spring-loaded jig (pressure 2 MPa).

[0048] Furthermore, for the production of samples for cross-sectional observation of the joint and heat cycle testing described later, Si chips (10mm x 10mm) with the same configuration as above were prepared. Then, following the same procedure as above, one Si chip was placed, one bonding material (10mm x 10mm) was placed on top of it, and then a Cu substrate (11mm x 11mm, 0.2mm thick) was placed on top of that, and the carbon jig was fixed in place (pressure applied: 2MPa).

[0049] The Si chip / bonding material / Cu substrate samples fixed as described above were introduced into a low-oxygen oven (ESPEC Corporation IPHH-202MS) along with a jig, and heated to form the bond. The heating process began after purging the oven with nitrogen for 30 minutes to reduce the oxygen concentration to 70 ppm or less. The heating rate was set to 5°C / min, and heating was carried out with nitrogen flow until the set bonding temperature was reached. Once the oven temperature reached the bonding temperature, it was heated and maintained at the bonding temperature for 30 minutes with nitrogen flow. After 30 minutes, the oven's blower was turned on and the temperature was cooled for 30 minutes under nitrogen flow (cooling rate: approximately -5°C / min). When the oven temperature reached 40°C, the samples were removed and the jig was removed. In this embodiment, the bonding temperature was set to 250°C for the eight types of bonding materials described above. In addition, for some bonding materials (Sn content 27 mass%), bonding temperatures of 185°C, 225°C, 250°C, 275°C, and 315°C were set to form the bond.

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

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

[0052] Next, to analyze the composition of the junction, EDS analysis was performed using an energy-dispersive X-ray analyzer (EDS) attached to the SEM (acceleration voltage 15kV). First, for the parts that were estimated to be island-like Ag phases and Ag3Sn phases from the SEM observation, spot analysis was performed at 3 points for each phase, with 10,000 samples per point (measurement time 30 seconds). As a result, it was confirmed that the composition of the island-like Ag phase was 100 mass% Ag, and the composition of the Ag3Sn phase was 73 mass% Ag - 27 mass% Sn.

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

[0054] Mapping analysis of each sample revealed that the cross-section of the junction contained regions with an Ag concentration of approximately 100% by mass and regions with an Ag concentration of approximately 73% by mass. Therefore, the obtained mapping images were bicolored using a predetermined Ag concentration as a threshold to create mapping images (Ag mapping images) that distinguish between the Ag phase and the Ag3Sn phase. In this embodiment, the threshold Ag concentration was set to 85% by mass, and the region above this was considered the Ag phase, while the region below this was considered the Ag3Sn phase, resulting in the Ag mapping image. Then, image analysis software (product name: MIPAR) was applied to the created Ag mapping image to calculate the area ratio of the Ag phase. Figure 3 shows the mapping images of each sample created based on the Ag concentration. In each image, the light-colored areas are island-like Ag phases, and the dark-colored areas are Ag3Sn phases.

[0055] Furthermore, the porosity was measured by using the same image analysis software as described above on the SEM images acquired during the SEM observation. In this image analysis, the color tone of areas that were clearly identified as voids in the SEM image (areas close to black in the photograph) was used as a reference, and the area ratio of areas with the same color tone as these void areas was measured.

[0056] [Evaluation of joint strength (die shear test)] Next, the bonding strength of the joints of each sample was measured. For the bonding strength evaluation test (die shear test), the samples prepared above were tested at room temperature and at a heated state of 260°C. To measure the bonding strength, the sample was placed in a bond tester (TRY PRECISION MFM1200L) with the Cu substrate side facing down, the tester's tool (claw) was hooked onto the end of the chip, and a shear load of 200 kg was applied to measure the die shear strength. A shear strength of 20 MPa or higher was judged as "pass (〇)", and a joint that passed both at room temperature and high temperature was judged to be a suitable joint.

[0057] [Evaluation of thermal cycling durability] Furthermore, to confirm the durability of the sample joints under heating and cooling cycles, a thermal cycling test was conducted. Each sample was subjected to 250 thermal cycling cycles, consisting of 30 minutes at -50°C followed by 30 minutes at 175°C. The presence or absence of delamination or cracks in the joints after 250 cycles was then checked. Note that the thermal cycling test was not performed on samples where the room temperature or high-temperature joint strength in the die shear test was below the detection limit (5 MPa or less).

[0058] The results of the above tests are shown in Table 1. Table 1 also shows the composition at the joint of each sample, the area ratio of island-like Ag phases, and the measured porosity. Furthermore, as examples of the observed material structure at the joint of each sample, SEM images of the joints (joining temperature 250°C) of No. 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 body of Ag powder and Sn powder prepared in this embodiment as a bonding material forms a joint having a material structure composed of Ag phase and Ag3Sn phase. However, depending on the composition of the joint, some samples showed insufficient bonding strength or poor thermal cycle durability. Specifically, when the Ag content of the joint was less than 78.0 mass% (No. 3, 13, 14), the bonding strength at high temperatures was poor. Also, when the Ag content exceeded 80.0 mass% (No. 1, No. 7~9), although the bonding strength tended to be higher than the samples with an Ag content of less than 78.0 mass%, the bonding strength at high temperatures was unacceptable. Furthermore, in thermal cycle tests, delamination and cracking occurred in these joints outside the preferred composition range. Therefore, it is considered that in the bonding structure of the liquid-phase diffusion bonding of the present invention, bonding strength and durability can be ensured by optimizing the composition range along with the material structure of the Ag phase and Ag3Sn phase.

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

[0062] Furthermore, in this embodiment, a joint was formed using Ag powder, Sn powder, and a compressed molded body as the bonding material. The composition of this bonding material and the bonding conditions (bonding temperature) also affect the composition of the joint. Regarding the bonding material, in No. 8 and 9, a bonding material using coarse Ag powder or Sn powder was applied. These joints had a higher Ag content and a higher area ratio of island-like Ag phases compared to joints using bonding materials with the same mixing ratio. Also, regarding the bonding temperature, No. 3 (bonding temperature 185°C), which deviated from the condition of 200°C to 300°C, had a low Ag content, while No. 7 (bonding temperature 315°C) had a high Ag content. This is presumed to be because the bonding temperature affects the diffusion of Sn that melts during bonding. In addition, the joints formed in this embodiment, except for No. 13, have a low porosity of 4% or less. This is thought to be the effect of liquid-phase diffusion bonding using a solid bonding material (compressed molded body of Ag powder and Sn powder) that differs from conventional pastes. [Industrial applicability]

[0063] The bonding structure according to the present invention comprises a bonding portion having a unique material structure composed of an Ag phase and an Ag3Sn phase. This bonding portion exhibits excellent bonding strength, heat resistance, and durability due to the good heat resistance of the Ag phase and Ag3Sn phase, as well as the flexibility of the Ag phase. The present invention can be suitably applied to bonding device elements in semiconductor devices such as power devices for hybrid cars and electric vehicles.

Claims

1. In a joining structure including a pair of materials to be joined and a joint formed between the pair of materials to be joined, The aforementioned joint consists of 78.0% to 80.0% by mass of Ag, 20.0% to 22.0% by mass of Sn, and unavoidable impurity elements as constituent elements. When an arbitrary cross-section of the joint is observed, island-shaped Ag phases containing 95% or more by mass of Ag and Ag ３ It consists of an intermetallic compound of Sn, and the island-shaped Ag phase surrounds the Ag ３ A bonding structure characterized by the observation of a material structure composed of the Sn phase.

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

3. The joint structure according to claim 1 or claim 2, wherein the void ratio in any cross-section of the joint is 4% or less in terms of area ratio.

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

5. In a joining material used to join a pair of materials to be joined by liquid-phase diffusion bonding, It consists of a sheet-like compressed molded body obtained by compressing and rolling 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. The compression molded body is a bonding material for liquid-phase diffusion bonding, characterized in that its relative density, based on the density of a bulk metal having the same composition as the mixing ratio of the mixed powder, is 95% or more.

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