Mounting structure and nanoparticle mounting material

Abstract

A mounting structure is used, which includes: a semiconductor element including an element electrode; a metal member; and a sintered body configured to bond the semiconductor element and the metal member is used, in which the sintered body contains a first metal and a second metal solid-dissolved in the first metal, the second metal is a metal having a diffusion coefficient in the first metal larger than a self-diffusion coefficient of the first metal, and a content ratio of the second metal relative to a total mass of the first metal and the second metal in the sintered body is equal to or lower than a solid solution limit of the second metal to the first metal.

Claims

1. A mounting structure comprising: a semiconductor element including an element electrode; a metal member; and a sintered body configured to bond the semiconductor element and the metal member, wherein the sintered body contains a first metal and a second metal solid-dissolved in the first metal, the second metal is a metal having a diffusion coefficient in the first metal larger than a self-diffusion coefficient of the first metal, and a content ratio of the second metal relative to a total mass of the first metal and the second metal in the sintered body is equal to or lower than a solid solution limit of the second metal to the first metal, and wherein (1) the first metal is Ag and the second metal is Pb, or (2) the first metal is Cu and the second metal is Ge.

2. The mounting structure according to claim 1, wherein the metal member is a lead frame or a heat sink.

3. The mounting structure according to claim 1, wherein the diffusion coefficient, the self-diffusion coefficient, and the solid solution limit are determined at 200° C.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a cross-sectional view in a direction parallel to a thickness direction of a bonding portion from a semiconductor element to a lead frame of a mounting structure in the present embodiment.

(2) FIG. 2 is a cross-sectional SEM image of a sintered body in the mounting structure of Example 1-4 in the present embodiment.

(3) FIG. 3 is a cross-sectional SEM image of a sintered body in the mounting structure of Comparative Example 1-1.

DESCRIPTION OF EMBODIMENTS

(4) Hereinafter, a mounting structure, a nanoparticle mounting material for manufacturing the mounting structure, and a method for manufacturing the mounting structure according to one embodiment of the invention will be described in detail.

(5) (Structure)

(6) FIG. 1 is a cross-sectional view in a direction parallel to a thickness direction of a bonding portion from a semiconductor element to a lead frame of a mounting structure in the present embodiment.

(7) Amounting structure 108 in the present embodiment is amounting structure in which a semiconductor element 101 including an element electrode 102 and a lead frame 105 including a lead frame electrode 104 are bonded via a sintered body 103.

(8) The mounting structure 108 has a structure in which the semiconductor element 101 such as a transistor is supported by the lead frame 105 and connected to an external wiring by the lead frame 105. The semiconductor element 101 and the lead frame 105 are electrically and mechanically connected by bonding the element electrode 102 of the semiconductor element 101 and the lead frame electrode 104 of the lead frame 105 via the sintered body 103.

(9) In the present embodiment, the sintered body 103 is a structure formed by sintering nano-sized metal particles, and is a sintered body containing a first metal and a second metal.

(10) The size of the nano-sized metal particles is preferably from 50 nm to 100 nm in average particle diameter.

(11) The average particle diameter is calculated using a laser particle diameter distribution meter. The average particle diameter is a median diameter d50% (μm, in terms of volume). The same applies below.

(12) The first metal is conductive, has high heat conductivity, and has heat resistance so as to withstand 200° C. to 250° C., a temperature Tj of a bonding portion (sintered body 103) between the semiconductor element 101 and the lead frame 105 when a SiC element and a GaN element are used. Examples of the first metal include Ag and Cu.

(13) Here, the temperature Tj of the bonding portion refers to a maximum temperature at a bonding portion interface when the semiconductor element operates.

(14) The second metal can be solid-dissolved in the first metal, and at least a part of the second metal is solid-dissolved in the first metal in the sintered body 103. The second metal is an element whose diffusion coefficient in the first metal is larger than a self-diffusion coefficient of the first metal. Here, the self-diffusion coefficient refers to a coefficient at which metal atoms diffuses in the metal.

(15) Examples of the second metal include Sn, Cu, Pb and Ge when the first metal is Ag, and Ge when the first metal is Cu.

(16) As will be described later, the sintered body 103 is formed by sintering a nanoparticle mounting material containing nanoparticles of the first metal and nanoparticles of the second metal. During sintering, since the diffusion coefficient of the nanoparticles of the second metal in the first metal is larger than the self-diffusion coefficient of the first metal, the nanoparticles of the second metal diffuse more quickly than the first metal, which makes it easy to bond the nanoparticles of the first metal. Therefore, the second metal can shorten a time required for bonding the nanoparticles of the first metal and shorten a time required for bonding by sintering. Accordingly, thermal damage to a peripheral member during the sintering can be reduced. Further, at this time, since the nanoparticles of the second metal decreases by solid-dissolving the second metal in the first metal by the diffusion, the phase of the second metal is not formed in the sintered body 103, and properties such as conductivity, heat conductivity, and heat resistance of the first metal are not easily damaged.

(17) The diffusion coefficient of the second metal in the first metal and the self-diffusion coefficient of the first metal can be known values, and may be determined based on a value at a temperature, for example, 200° C. for preparing the sintered body 103.

(18) In the sintered body 103, a content ratio of the second metal relative to the total mass of the first metal and the second metal is less than or equal to a solid solution limit of the second metal to the first metal. Accordingly, an intermetallic compound of the first metal and the second metal is formed, and thus it is possible to prevent changes in the properties such as conductivity, heat conductivity, and heat resistance of the first metal.

(19) On the other hand, in order to sufficiently bond the nanoparticles of the first metal during sintering in a low temperature and in a short time and to prevent deterioration in conductivity, heat conductivity, and heat resistance due to remaining unsintered nanoparticles, it is preferable to increase the content ratio of the second metal as much as possible to equal to or lower than the solid solution limit. Specifically, the content ratio of the second metal can be 70% or more and 100% or less relative to the solid solution limit. For example, when the first metal is Ag and the second metal is Sn (the solid solution limit of Sn is 10 mass %), the content ratio of Sn as the second metal can be 7 mass % or more and 10 mass % or less relative to the total mass of the first metal and the second metal.

(20) The solid solution limit of the second metal to the first metal can be obtained from an equilibrium state diagram of the first metal and the second metal, and may be a solid solution limit at a temperature, for example, 200° C. for preparing the sintered body 103.

(21) (Nanoparticle Mounting Material)

(22) The nanoparticle mounting material in the present embodiment is a mounting material containing at least the nanoparticles of the first metal and the nanoparticles of the second metal.

(23) In the present embodiment, the first metal is also conductive, has high heat conductivity, and has heat resistance so as to withstand 200° C. to 250° C., the temperature Tj of the bonding portion (sintered body 103) between the semiconductor element 101 and the lead frame 105 when the SiC element and the GaN element are used. Examples of the first metal include Ag and Cu.

(24) In the present embodiment, the second metal can also be solid-dissolved in the first metal, and is an element whose diffusion coefficient in the first metal is larger than the self-diffusion coefficient of the first metal. Examples of the second metal include Sn, Cu, Pb and Ge when the first metal is Ag, and Ge when the first metal is Cu.

(25) In the present embodiment, the proportion of the mass of the second metal relative to the total mass of the first metal and the second metal is also less than or equal to the solid solution limit of the second metal to the first metal. The content ratio of the second metal can be 70% or more and 100% or less relative to the solid solution limit. When the first metal is Ag and the second metal is Sn (the solid solution limit of Sn is 10 mass %), the content ratio of Sn as the second metal can be 7 mass % or more and 10 mass % or less relative to the total mass of the first metal and the second metal.

(26) In the present embodiment, a proportion of nanoparticles of the first metal having a particle diameter of 100 nm or less relative to the total number of the nanoparticles of the first metal is 50% or more. The specific surface area of the nanoparticles increases as the particle diameter of the nanoparticles of the first metal gets smaller, which facilitates sintering at a lower temperature.

(27) In addition, in the present embodiment, the average particle diameter of the nanoparticles of the second metal is 100 nm or less. As the average particle diameter of the nanoparticles of the second metal gets smaller, the second metal can be solid-dissolved into the first metal more easily and the time required for sintering can also be shortened. In addition, as the average particle diameter of the nanoparticles of the second metal gets smaller, the second metal can be solid-dissolved in the first metal more easily and it is possible to prevent the change in the properties such as conductivity, heat conductivity, and heat resistance of the first metal due to the nanoparticles of the second metal remaining in the sintered body 103 after sintering.

(28) The nanoparticle mounting material may contain an organic solvent for imparting a coating property to the electrode or imparting dispersibility to the nanoparticles, a dispersant for preventing aggregation of the nanoparticles during storage, and a reducing agent for removing an oxide film formed on the surface of the nanoparticles.

(29) (Process)

(30) The method of manufacturing the mounting structure according to the present embodiment includes a step of bonding the element electrode of the semiconductor element and the lead frame electrode of the lead frame by sintering the nanoparticles of the first metal and the nanoparticles of the second metal using the above nanoparticle mounting material.

(31) (1) Nanoparticle Mounting Material Preparation Step

(32) Firstly, a nanoparticle mounting material containing nanoparticles of the first metal and nanoparticles of the second metal is prepared. The nanoparticle mounting material may contain an organic solvent, a dispersant, and a reducing agent.

(33) (2) Nanoparticle Mounting Material Supply Step

(34) Then, the nanoparticle mounting material is supplied to the lead frame electrode 104 on the lead frame 105 by using a metal mask or the like. The lead frame electrode 104 may be formed of Cu or the like.

(35) (3) Element Placement Step

(36) Further, the semiconductor element 101 is placed on the supplied nanoparticle mounting material such that the element electrode 102 is in contact with the nanoparticle mounting material. The element electrode 102 may have an outermost layer formed of Au or the like.

(37) (4) Bonding Step

(38) Finally, the lead frame 105 on which the semiconductor element 101 and the nanoparticle mounting material are placed is put into an air flow type heating furnace and heated in an air atmosphere. The heating condition is not particularly limited, and may be, for example, about 20 min at 200° C.

(39) As a result, growth of the particles occurs by sintering the nanoparticles of the first metal and the nanoparticles of the second metal in the nanoparticle mounting material, and the sintered body 103 composed of a set of aggregates with 200 nm or more is formed. At this time, the respective elements mutually diffuse in a range of about several nanometers in thickness from an interface between the element constituting the element electrode 102 and the first metal and the second metal, and the respective elements mutually diffuse in a range of several nanometers in thickness from an interface between the elements constituting the lead frame electrode 104 and the first metal and the second metal, so that the element electrode 102 and the lead frame electrode 104 are bonded together and the mounting structure 108 is prepared.

EXAMPLES

(40) The invention will be described in detail based on Examples, but the invention is not limited to these Examples.

Example 1

(41) 1. Preparation of Mounting Structure

(42) Nanoparticles of Ag having an average particle diameter of 100 nm were prepared as the nanoparticles of the first metal, and nanoparticles of Sn having an average particle diameter of 100 nm, nanoparticles of Sb having an average particle diameter of 100 nm, and Pd nanoparticles having an average particle diameter of 100 nm were prepared as the nanoparticles of the second metal. When the nanoparticles of the first metal were observed with a scanning electron microscope (SEM), a proportion of the nanoparticles having a particle diameter of 100 nm or less was 50% relative to the number of the nanoparticles of the first metal observed.

(43) The nanoparticles of the first metal and the nanoparticles of the second metal in a total amount of 70 mass %, polyethylene glycol (average molecular weight is 200) in an amount of 29 mass % as an organic solvent, and a dispersant and a reducing agent in a total amount of 1 mass % were mixed to prepare a nanoparticle mounting material. At this time, the nanoparticles of the first metal and the nanoparticles of the second metal were mixed at different ratios in the total amount of 70 mass %, to obtain a plurality of nanoparticle mounting materials.

(44) A lead frame electrode made of Cu was disposed on the lead frame. Further, the nanoparticle mounting material was poured onto a metal mask having a length of 1 mm, a width of 1 mm, and a thickness of 50 μm disposed on the surface of the lead frame electrode. Further, a semiconductor element including an element electrode having the outermost layer formed of Au was placed such that the element electrode was in contact with the nanoparticle mounting material. Thereafter, the lead frame on which the semiconductor element was placed was put into the air flow type heating furnace, and heated in an air atmosphere at 200° C. for 20 min to obtain a mounting structure.

(45) The self-diffusion coefficient of Ag in Ag at 200° C. is 10.sup.−24 m.sup.2/sec, while the diffusion coefficient of Sn in Ag at 200° C. is 10.sup.−22 m.sup.2/sec. The diffusion coefficient of Sb in Ag at 200° C. is 10.sup.−25 m.sup.2/sec, and the diffusion coefficient of Pd in Ag at 200° C. is 10.sup.−34 m.sup.2/sec.

(46) The solid solution limit of Sn to Ag is 10 mass %.

(47) 2. Evaluation

(48) The prepared mounting structure was embedded in an epoxy resin, and then was mirror-polished from a semiconductor element side using a polishing paper and a buff to expose a cross section near the center in the thickness direction of the sintered body 103 formed by heating the nanoparticle mounting material. The cross section was observed by SEM, and the proportion of the unsintered nanoparticles, the presence of the intermetallic compound and the remaining state of the nanoparticles of the second metal were observed.

(49) (Proportion of Unsintered Nanoparticles)

(50) An image of the cross section was taken with the SEM at 10,000 times, and a proportion of the particles having a particle diameter of 100 nm or less relative to the total number of particles observed in the field of view was calculated to be the proportion of the unsintered nanoparticles.

(51) The proportion of the unsintered nanoparticles being smaller than a value in a case of not adding Sn as the second metal was evaluated as passed.

(52) (Presence of Intermetallic Compound)

(53) When a phase having a different lightness was confirmed the SEM image, an energy dispersion X-ray analysis (EDX analysis) of phase having a different brightness was performed. When the phase having a different lightness was a phase containing the first metal and the second metal, it was determined that the intermetallic compound was formed.

(54) Those with no intermetallic compound were evaluated as passed.

(55) (Remaining of Nanoparticles of Second Metal)

(56) When a phase having a different lightness was confirmed in the SEM image, the EDX analysis (energy dispersion X-ray analysis) of a phase having a different brightness was performed. When the phase having a different lightness was a phase substantially composed of only the second metal, it was determined that the nanoparticles of the second metal remained without sintering.

(57) Those with no remaining second metal were evaluated as passed.

(58) Table 1 shows, in Example 1, the types of the first metal and the second metal, the proportion (content ratio) of the mass of the nanoparticles of the second metal (Sn) relative to the total mass of the nanoparticles of the first metal (Ag) and the nanoparticles of the second metal (Sn), and the results of the above evaluation.

(59) TABLE-US-00001 TABLE 1 Nanoparticle mounting material Evaluation Content ratio of Unsintered First Second second metal proportion Intermetallic Remaining of metal metal (%) (%) compound second metal Acceptability Example 1-1 Ag Sn 2.5 28 No No ◯ Example 1-2 Ag Sn 5.0 25 No No ◯ Example 1-3 Ag Sn 7.5 16 No No ◯ Example 1-4 Ag Sn 10.0 2 No No ◯ Comparative Ag — 0.0 33 No No X Example 1-1 Comparative Ag Sn 12.5 0 Yes No X Example 1-2 Comparative Ag Sb 5.0 36 No Yes X Example 1-3 Comparative Ag Pd 5.0 40 No Yes X Example 1-4

(60) FIG. 2 shows a cross-sectional SEM image of the sintered body 103 in the mounting structure of Example 1-4. For comparison, FIG. 3 shows a cross-sectional SEM image of the sintered body 103 in the mounting structure of Comparative Example 1-1.

(61) In FIG. 2, particles having a particle diameter of 100 nm or less are not found in the aggregate after the bonding except for a portion indicated by the dotted line. It was confirmed that, in the sintered body 103 in the mounting structure of Example 1-4, the sintering of the nanoparticle mounting material proceeded well. On the other hand, in FIG. 3, a proportion of the aggregate containing particles having a particle diameter of 100 nm or less indicated by the dotted line is clearly larger than that in FIG. 2. It was confirmed that, in the sintered body 103 in the mounting structure of Comparative Example 1-1, the sintering of the nanoparticle mounting material proceeded not well.

(62) As was clear from Table 1, in the mounting structures of Examples 1-1 to 1-4, the proportion of unsintered nanoparticles was lower than that of the mounting structure of Comparative Example 1-1. Further, generation of the intermetallic compound (Ag.sub.5Sn or the like) and remaining of the nanoparticles of the second metal (Sn) was not observed.

(63) The nanoparticle mounting materials used in the preparation of the mounting structures of Examples 1-1 to 1-4 have the following common points. First, a nanoparticle mounting material containing the nanoparticles of the first metal and the nanoparticles of the second metal solid-dissolved in the first metal is used. The second metal is a metal having a diffusion coefficient in the first metal larger than the self-diffusion coefficient of the first metal. Further, the proportion of the mass of the nanoparticles of the second metal relative to the total mass of the first metal and the second metal is less than or equal to the solid solution limit of the second metal to the first metal.

(64) On the other hand, in Comparative Example 1-2 having a content ratio of the second metal (Sn) relative to the total mass of the nanoparticles of the first metal (Ag) and the nanoparticles of the second metal (Sn) of 12.5 mass % and equal to or larger than the solid solution limit of the Sn in Ag, no unsintered nanoparticles were observed, but the generation of Ag.sub.5Sn, an intermetallic compound, was observed in the sintered body 103.

(65) In Comparative Examples 1-3 and 1-4 having a diffusion coefficient in Ag smaller than the self-diffusion coefficient of Ag in Ag, the proportion of the unsintered nanoparticles was larger than that of Comparative Example 1-1, and when the unsintered nanoparticles were measured, the remaining of Sb and Pd was observed.

Example 2

(66) In Example 2, amounting structure was prepared and evaluated in the same manner as in Example 1 except that the second metal was Cu, Pb, and Ge.

(67) The diffusion coefficient of Cu in Ag at 200° C. is 10.sup.−17 m.sup.2/sec, the diffusion coefficient of Pb in Ag at 200° C. is 10.sup.−22 m.sup.2/sec, and the diffusion coefficient of Ge in Ag at 200° C. is 10.sup.−22 m.sup.2/sec. These diffusion coefficients are all larger than the self-diffusion coefficient of Ag in Ag at 200° C.

(68) A mixing ratio of the nanoparticles of the second metal was adjusted such that a mass of Cu, Pb, and Ge relative to the total mass of Ag was half the solid solution limit at 200° C. in the equilibrium state diagram of Ag and each element.

(69) Table 2 shows, in Example 2, the types of the first metal and the second metal, the proportion (content ratio) of the mass of the nanoparticles of the second metal (Sn) and the total mass of the nanoparticles of the first metal (Ag) and the mass of the nanoparticles of the second metal (Sn), and the results of the above evaluation. In order to facilitate the comparison, the nanoparticle mounting materials used in Example 1-2 and Comparative Example 1-1 and evaluation results are also shown in Table 2.

(70) TABLE-US-00002 TABLE 2 Nanoparticle mounting material Evaluation Second metal mass/ Unsintered First Second first metal mass proportion Intermetallic Remaining of metal metal (%) (%) compound second metal Acceptability Example 1-2 Ag Sn 5.0 25 No No ◯ Example 2-1 Ag Cu 0.17 29 No No ◯ Example 2-2 Ag Pb 0.4 31 No No ◯ Example 2-2 Ag Ge 0.7 31 No No ◯ Comparative Ag Sn 0.0 33 No No X Example 1-1

(71) As was clear from Table 2, even when a metal other than Sn was used as the second metal, in the mounting structures of Examples 2-1 to 2-3, the proportion of unsintered nanoparticles was lower than that of the mounting structure of Comparative Example 1-1. Further, the generation of the intermetallic compound and the remaining of the nanoparticles of the second metal were not observed.

(72) The nanoparticle mounting materials used in the preparation of the mounting structures of Examples 2-1 to 2-3 have the following common points. First, a nanoparticle mounting material containing the nanoparticles of the first metal and the nanoparticles of the second metal solid-dissolved in the first metal is used. The second metal is a metal having a diffusion coefficient in the first metal larger than the self-diffusion coefficient of the first metal. Further, the proportion of the mass of the nanoparticles of the second metal relative to the total mass of the first metal and the second metal is less than or equal to the solid solution limit of the second metal to the first metal.

(73) In Example 2-1 using Cu as the second metal, despite a small weight ratio of the second metal, the proportion of the unsintered nanoparticles decreased. This is because the diffusion coefficient of Cu in Ag is considerably larger than other elements.

Example 3

(74) The mounting structure was prepared and evaluated in the same manner as in Example 1 except that the first metal was Cu and the second element was Ge.

(75) The diffusion coefficient of Ge in Cu at 200° C. is 10.sup.−23 m.sup.2/sec, and the self-diffusion coefficient of Cu in Cu at 200° C. is 10.sup.−29 m.sup.2/sec. The diffusion coefficient of Ge in Cu at 200° C. is larger than the self-diffusion coefficient of Cu in Cu at 200° C.

(76) The mixing ratio of the nanoparticles of the second metal (Ge) was adjusted such that a mass of Ge relative to a total mass of Cu is half the solid solution limit at 200° C. in the equilibrium state diagram of Cu and Ge.

(77) Table 3 shows, in Example 3, types of the first metal and the second metal, the proportion (content ratio) of the mass of the nanoparticles of the second metal (Ge) relative to the total mass of the nanoparticles of the first metal (Cu) and the nanoparticles of the second metal (Ge), and the results of the above evaluation.

(78) TABLE-US-00003 TABLE 3 Nanoparticle mounting material Evaluation Second metal mass/ Unsintered First Second first metal mass proportion Intermetallic Remaining of metal metal (%) (%) compound second metal Acceptability Example 3-1 Cu Ge 6.0 41 No No ◯ Comparative Cu — 0.0 62 No No ◯ Example 3-1

(79) As was clear from Table 3, even when a metal other than Ag was used as the first metal, in the mounting structure of Example 3-1, the proportion of unsintered nanoparticles was lower than that of the mounting structure of Comparative Example 3-1. Further, the generation of the intermetallic compound and the remaining of the nanoparticles of the second metal were not observed.

(80) The nanoparticle mounting material for preparing the mounting structure of Example 3-1 has the following features. First, the nanoparticle mounting material containing the nanoparticles of the first metal and the nanoparticles of the second metal solid-dissolved in the first metal is used. The second metal is a metal having a diffusion coefficient in the first metal larger than the self-diffusion coefficient of the first metal. Further, the proportion of the mass of the nanoparticles of the second metal relative to the total mass of the first metal and the second metal is less than or equal to the solid solution limit of the second metal to the first metal.

(81) (Summary)

(82) Although the lead frame 105 has been described, the lead frame 105 may be a metal member. For example, a heat sink may be used.

INDUSTRIAL APPLICABILITY

(83) According to the mounting structure of the invention, it is possible to provide a mounting structure in which the time required for bonding is shortened, thermal damage to the peripheral member is small, and physical properties of the nanoparticles the metal of are secured, which is required for a device with large heat generation such as a power device.

REFERENCE SIGNS LIST

(84) 101: SEMICONDUCTOR ELEMENT

(85) 102: ELEMENT ELECTRODE

(86) 103: SINTERED BODY

(87) 104: LEAD FRAME ELECTRODE

(88) 105: LEAD FRAME

(89) 108: MOUNTING STRUCTURE