Method for bonding between components of semiconductor device

In the present invention, for a power semiconductor device or the like, a metal paste containing metal nanoparticles is used between the conductor pattern of the insulating substrate and the semiconductor chip, and between the semiconductor chip and the heat spreader laminated on the upper surface thereof. The present invention relates to a method of joining parts of a semiconductor device to be joined.

First, FIG. 3 shows an assembly structure of a conventional example of the power semiconductor device described above, which is the object of the present invention. In the figure, 1 is a copper base for heat dissipation, and 2 is an insulating substrate in which conductor patterns (copper foil) 2b and 2c are formed on both upper and lower surfaces of a ceramic plate 2a such as alumina and mounted on the copper base 1 and joined. For example, Direct Bonding Copper substrate), 3 is a semiconductor chip (for example, IGBT) mounted on the upper surface conductor pattern (circuit pattern) 2c of the insulating substrate 2, 3a is a Ni / Au plating film metallized on the lower surface electrode surface of the semiconductor chip 3, and 3b is a chip. Metallized aluminum film on the upper surface, 4 is a heat spreader bonded to the upper surface of the semiconductor chip 3 (see, for example, Patent Document 1), 5 is a top electrode of the semiconductor chip 3 and a corresponding conductor pattern 2c via the heat spreader 4. Lead frames (copper and aluminum conductors) wired between them, 6 are externally derived leads joined to the conductor pattern 2c, and are the conductor pattern 2b of the copper base 1 / insulating substrate 2 and the conductor pattern 2c / semiconductor of the insulating substrate 2. The module is assembled by joining the conductor patterns 2c / leads 5 and 6 of the chip 3, the semiconductor chip 3, the upper surface / heat spreader 4, the heat spreader 4 / lead frame 5, and the insulating substrate 2 with solder 7, respectively.

By the way, regarding the solder that joins each component of the semiconductor device described above, the conversion to replace Sn-Pb-based eutectic solder with lead-free solder (for example, Sn-Ag-based solder) has recently been promoted due to the problem of environmental protection. ing. However, lead-free solder has a higher melting point than Sn-Pb solder, and the intermetallic compound generated at the bonding interface due to thermal history increases the stress concentration on the solder joint, and overcurrent causes solder to the semiconductor chip. If heat exceeding the melting point is generated, damage such as melting, peeling, and short circuit may occur at the solder joint, which causes major problems in the power cycle resistance and reliability of the semiconductor module. ing.

On the other hand, recently, research on metal nanoparticles has progressed, and even in the field of semiconductor device manufacturing technology, low-temperature sintering phenomenon due to the quantum size effect of metal nanoparticles and low-temperature firing type conductive paste utilizing high surface activity (hereinafter referred to as "nano") "Metal paste") has been developed (see, for example, Non-Patent Document 1). This nanometal paste reacts with metal nanoparticles such as Ag or Cu, an organic dispersant that suppresses the aggregation of the metal nanoparticles at room temperature and keeps the nanoparticles in an independently dispersed state, and an organic dispersant by heating. The dispersant trapping material that strips the metal nanoparticles and the volatile organic component (solvent) that captures and gasifies the reactants of the dispersant and the dispersant trapping material are kneaded with the metal powder (aggregate). It was made into a paste.

Further, there is also known that a metal paste containing metal nanoparticles is used instead of the conventional solder bonding to bond the parts of the semiconductor device (see, for example, Patent Documents 2 and 3). Here, when joining between metal members (bulk materials) using the nanometal paste, in the conventional method, the nanometal paste is applied to the joint surface of one of the metal members by a screen printing method or the like. In a temporary assembly state in which a metal member on the other side is superposed on this and a light pressing force is applied, the temporary assembly is carried into a reflow furnace for joining. Regarding the bonding mechanism between the metal nanoparticles and the metal member to be bonded due to heating, and the characteristics such as curing conditions, bonding strength, and heat resistant temperature, the above-mentioned Non-Patent Document 1 and As described in detail in Patent Document 2, welding / sintering (crosslinking) by low-temperature sintering proceeds between the metal nanoparticles activated by heating and between the metal nanoparticles and the metal powder or the bonded metal plate. Finally, a bonding layer having a matrix structure with a low resistance is formed.

As described above, as a method for joining parts of a semiconductor device, by joining using a nanometal paste instead of the conventional solder joining, low resistance, high heat resistance, heat transfer, and joining strength can be obtained, and nano Higher reliability than solder bonding can be expected, as the joint does not melt below the melting temperature of metal particles (silver, copper).<nplcit num="1"><text>Masaaki Oda, "Nanometal Particles", Journal of Electronics Packaging Society, 2002, vol5, No6, p523-528</text></nplcit><patcit num="1"><text>Japanese Unexamined Patent Publication No. 2004-241734 (Fig. 1)</text></patcit><patcit num="2"><text>Japanese Unexamined Patent Publication No. 2004-128357 (Figs. 3 and 6)</text></patcit><patcit num="3"><text>Japanese Unexamined Patent Publication No. 2004-2130371</text></patcit>

<p> By the way, regarding the parts bonding of the semiconductor device shown in Fig. 3, the inventors conducted various experiments and verifications on the bonding conditions when the nanometal paste was applied, and found that the method was the same as that of the conventional reflow soldering. After superimposing the parts to be joined on the nanometal paste applied to the joint surface of the parts, lightly apply pressure to temporarily assemble them, and if they are carried into a reflow furnace in this temporarily assembled state and joined, they will be joined. It was found that many unbonded defects occur in the central part of the surface.</p><p> Therefore, as a result of investigating the cause of the occurrence of this joint defect, the inventors have clarified that the cause of the defect is as follows. That is, in order to properly proceed with fusion / sintering between the particles of the metal nanoparticles and between the metal nanoparticles and the metal member to be bonded (conductor pattern of the insulating substrate, semiconductor chip, heat spreader in FIG. 3), the metal In addition to promoting the reaction between the dispersant and the dispersant trapping material, which holds the nanoparticles in an independently dispersed state, the volatile substances that trapped the reactants are evaporated and removed from the junction surface area to remove the metal nanoparticles. It is an essential condition to leave it naked to increase its surface activity.</p><p> However, when the parts to be joined such as the semiconductor chip 3, the insulating substrate 2, and the heat spreader 4 are stacked and heated from both sides with the nanometal paste applied to the joint surface, especially in the central surface area of the joint surface. Since the nanometal paste in the above is contained between the parts, the free evaporation (degassing) of the heat-decomposed organic component to the surroundings is hindered. For this reason, it is presumed that the fusion / sintering of the metal nanoparticles does not proceed sufficiently in the central surface region of the joint surface, and as a result, an unbonded portion remains in the joint portion.</p><p> The present invention has been made in view of the above points, and is intended for a semiconductor device or the like in which components such as an insulating substrate and a heat spreader are superposed on both main surfaces of a semiconductor chip and surface-bonded to each other as shown in FIG. An object of the present invention is to provide an improved method for joining parts of a semiconductor device so that parts can be properly joined over the entire joint surface without leaving an unbonded portion in the joint layer when joining parts using a nanometal paste. And.</p>

<p> In order to achieve the above object, according to the present invention, metal nanoparticles, an organic dispersant that suppresses aggregation of metal nanoparticles at room temperature, a dispersant trapping material that reacts with an organic dispersant by heating, and the dispersant. A method for joining parts of a semiconductor device using a metal paste containing a volatile organic component that captures and volatilizes a reactant with a dispersant capturing material, and is a joining surface area of a part to be joined whose surface is a metal base material. In the joining method in which the metal paste is applied to the metal paste and the parts to be joined are heated in a state of being overlapped with each other, and the metal nanoparticles are sintered at a low temperature by this heating so that the parts are conductively and heat-conductingly joined. In a state where the metal paste is applied to the joint surface of the part to be joined, the metal core material is dispersed and inserted between the facing surfaces of the parts and superposed, and the metal core material is joined through a subsequent heating step (claim 1).</p><p> Here, the metal nanoparticles contained in the metal paste are Ag nanoparticles or Cu nanoparticles (claim 2), and the metal core material has a wire diameter corresponding to the thickness of the bonding layer between the parts to be bonded. (Claim 3). Further, in the present invention, a semiconductor chip bonded to an insulating substrate of a semiconductor device and its conductor pattern, or a heat spreader bonded to the semiconductor chip and its upper surface is used as a bonded component, and the semiconductor device is assembled by applying the bonding method described above. (Claim 4).</p>

<p> In the temporary assembly state in which the nanometal paste is applied to the joint surface of the part to be joined, the metal core material is dispersedly arranged on the coated surface, and the parts are stacked and pressed as in the above joining method, the metal core material is used. The space between the parts is maintained as a spacer, and a evaporation path for the volatile organic components that are thermally decomposed in the subsequent heating step is secured. Since the cure condition of the metal paste for low-temperature sintering of metal nanoparticles is around 200 ° C, the metal core material (copper material) does not melt in the heating process.</p><p> Therefore, the volatile organic components of the nanometal paste, which are thermally decomposed as the bonding reaction by heating progresses, do not remain confined between the parts, but are surrounded through the evaporation path secured by the insertion of the metal core. Evaporate to. As a result, the reaction between the dispersant of the metal nanoparticles and the dispersant trapping material is promoted over the entire area of the nanometal paste applied to the joint surface, and the evaporation of the pyrolysis gas proceeds without being hindered by the parts to be joined. As a result, the surface activity of the metal nanoparticles is increased, and bonding between parts without unbonding (defects) is achieved over the entire bonding surface.</p><p> Moreover, by using a columnar copper material having a wire diameter corresponding to the thickness of the joint layer as the metal core material, the core material rolls or moves in the process of temporary assembly in which the parts to be joined are overlapped and inserted. Even if the position is slightly displaced, the distance and parallelism between the parts can be stably maintained.</p>

Hereinafter, embodiments of the present invention will be described with reference to the examples shown in FIGS. 1 and 2. It should be noted that FIG. 1 is an assembly structural diagram of a semiconductor device assembled by applying the bonding method of the present invention, and FIG. 2 is a schematic diagram showing metal cores dispersed and arranged in the bonding surface region of the parts to be bonded. The members corresponding to FIG. 3 are designated by the same reference numerals, and the description thereof will be omitted. That is, in the semiconductor device of the illustrated embodiment, the nanometal paste described above is used between the conductor pattern 2b of the insulating substrate 2 and the semiconductor chip 3, and between the semiconductor chip 2 and the heat spreader 4 laminated on the upper surface side thereof. The joint layer is represented by reference numeral 5, and the metal core dispersedly interspersed in the joint surface region between the parts to be joined is represented by reference numeral 6 as described later.

Here, the illustrated semiconductor chip 3 is, for example, an IGBT chip having a side length of 7.5 mm square. In addition, as the nanometal paste, Ag nanoparticles or Cu nanoparticles are the main components, and organic dispersants, dispersant trapping materials, volatile organic components (solvents), and aggregates such as silver powder and copper powder are added thereto. In addition, a kneaded nanometal paste (for example, nanopaste (trade name): Harima Kasei Co., Ltd. disclosed in Non-Patent Document 1) is used, and the metal core 6 described above has a layer thickness of a bonding layer 5. A columnar copper material with a wire diameter of 100 μm and a length of 2 mm is used.

Next, the method of assembling the semiconductor device shown in FIG. 1 and the method of joining parts using the nanometal paste will be described. First, the nanometal paste is applied to the insulating substrate 2 to a uniform thickness by a screen printing method on the joint surface region of the conductor pattern 2b. Next, the rod-shaped metal core 6 is dispersed around the four sides around the joint surface area and placed on the nanometal paste applied thereto, and in this state, the semiconductor chip 3 is then superposed in accordance with the joint position from above. Then, a light pressing force is applied from above to push the metal core 6 into the coated surface of the nanometal paste so that the metal core 6 comes into contact with the conductor pattern 2b of the insulating substrate 2. Further, in the same manner, the nanometal paste is uniformly applied to the upper surface of the semiconductor chip 3, the heat spreader 4 is superposed on the metal core 6 and the metal core 6 is lightly pressed to make the metal core 6 of the nanometal paste. Temporarily assemble by pushing into the painted surface. As a result, a space corresponding to the layer thickness of the bonding layer 5 is secured between the components of the insulating substrate 2 / semiconductor chip 3 and the semiconductor chip 3 / heat spreader 4 by using the metal core 6 as a spacer member.

After that, the temporary assembly is carried into a reflow furnace and heated under the specified cure conditions (heating temperature 180 to 220 ° C, 30 to 60 min) of the nanometal paste to conduct and transfer heat between the parts. To join. In this heating step, as described above, the metal nanoparticles that have been exposed and activated by the reaction between the organic dispersant and the dispersant capture material, and the metal nanoparticles and the metal powder, and the parts to be bonded are welded together. Sintering (crosslinking) proceeds, and finally, a matrix structure bonding layer 5 is formed between the components of the insulating substrate 2 / semiconductor chip 3 / heat spreader 4 with a thickness corresponding to the wire diameter of the metal core 6. To. Moreover, in this joining process, a transpiration path leading to the surroundings is secured between the parts by the intervention of the metal core 6. As a result, the organic components thermally decomposed in the process of welding and sintering of the metal nanoparticles are not confined between the parts to be joined, especially in the central surface region of the joint surface, and are connected to the surroundings through the above-mentioned evaporation path. It will evaporate. As a result, the joint layer 5 can properly join the parts over the entire joint surface without leaving an unbonded portion in the layer, and the reliability of the semiconductor device is improved.

<figref num="1">Assembly structure diagram of a semiconductor device assembled by applying the joining method of the present invention.</figref><figref num="2">Top view schematically showing the distributed arrangement of metal cores inserted between the parts to be joined in FIG.</figref><figref num="3">Assembly structure diagram of a conventional semiconductor device in which parts are solder-bonded to each other</figref>

Code description

2 Insulated substrate 2b Conductor pattern 3 Semiconductor chip 4 Heat spreader 5 Bonding layer (nanometal paste) 6 Metal core