Manufacturing method of the bonded body

Abstract

To make it possible to form a joining layer in a short time with less voids.SOLUTION: A conjugate manufacturing method includes an application step of applying a paste-like bonding composition containing metal powder to the surface of a first member, and a heating step of heating the bonding composition in a state in which a second member is laminated to sinter the bonding composition to bond the first member and the second member, and the heating step includes a volatilization step of heating at a volatilization rate of 0.5% by mass or more and 2% by mass or less / minute until the amount of volatile components in the bonding composition is 70% by mass or less with respect to the total amount of volatile components in the bonding composition before heating, and a sintering step of raising the temperature after the volatilization step to sinter the bonding composition.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a bonded assembly by bonding a semiconductor element to an insulating circuit board, in which a paste containing metal powder is interposed between the circuit layer of the insulating circuit board and the semiconductor element and sintered to bond them together. [Background technology]

[0002] BACKGROUND ART In insulating circuit boards and the like, a method is known in which a paste containing metal powder is used to bond two components together, and the two components are bonded by sintering the paste.

[0003] For example, Patent Document 1 discloses a method for manufacturing a power module in which Au or Ag plating is applied to the wiring of a substrate, a sintered Ag paste is interposed between the wiring and a semiconductor element, and the bonding is performed while oxidizing the sintered paste under conditions of 250 to 350°C, a temperature rise rate of 30°C / min or less, a holding time of 1 to 30 minutes, and a pressure of 0.1 to 10 MPa. This Ag sintered paste contains nano-order Ag filler, and a porous Ag layer with a porosity of 5% to 30% is formed as a bonding layer.

[0004] Furthermore, Patent Document 2 describes a method of preparing a silver paste using first silver particles and second silver particles having different particle sizes as raw materials for forming a bonding layer, and describes a method of laminating a silver-plated Cu plate on a gold-plated Si wafer with a silver paste layer interposed between them, and then sintering the silver paste by simply heating the laminate without applying pressure. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5936407 [Patent Document 2] Patent No. 6737381 Summary of the Invention [Problem to be solved by the invention]

[0006] This type of paste contains metal powders such as Ag, as well as resins and solvents. These resins and solvents, other than the metal powder, volatilize and disappear at the temperature during bonding, but if the gas generated during this process does not completely escape from the paste, it will remain as voids in the bonding layer after sintering, causing bonding failure. To avoid the formation of these voids, the heating rate is slowed down to allow sufficient time for the volatile components to volatilize before the metal powder is sintered, but this lengthens the firing time and reduces productivity.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a method for manufacturing a bonded body that enables the formation of a bonding layer with fewer voids in a short time. [Means for solving the problem]

[0008] The method for manufacturing a bonded body of the present invention is a method for manufacturing a bonded body through an application step of applying a paste-like bonding composition containing metal powder to the surface of a first member, and a heating step of heating a second member stacked on the bonding composition to sinter the bonding composition and bond the first member and the second member, wherein the heating step includes a volatilization step of heating at a volatilization rate of 0.5 mass% or more and 2 mass% or less until the amount of volatile components in the bonding composition becomes 70 mass% or less of the total amount of volatile components in the bonding composition before heating, and a sintering step of increasing the temperature after the volatilization step to sinter the bonding composition.

[0009] By controlling the heating profile during bonding and going through the volatilization and sintering processes, the volatile components are sufficiently volatilized in the volatilization process before proceeding to the sintering process, thereby effectively suppressing the occurrence of voids. If the volatilization rate in the volatilization step is too high, exceeding 2% by mass / min, the volatile components will gasify all at once and not completely escape from between the semiconductor element and the insulating circuit board, resulting in the formation of voids. Voids will also occur if the temperature is raised to near the sintering temperature before the total amount of volatile components has fallen to 70% by mass. If the volatilization rate is less than 0.5% by mass / min, the formation of voids can be suppressed, but it will take time, resulting in a loss of productivity. Preferably, the volatilization step should be carried out until the volatilization rate is 65% by mass or less, and a volatilization rate of 1% by mass or less is preferred.

[0010] In the method for manufacturing a bonded body of the present invention, the second member is a semiconductor element, and the bonding step can be performed in a pressure-free state. This can prevent damage to the semiconductor element made of a brittle material. The pressure-free state means that no pressure is actively applied in the stacking direction to the semiconductor element stacked on the insulating circuit board via the paste-like bonding composition. This does not exclude slight loads acting on the semiconductor element, such as the weight of the semiconductor element itself or a device for preventing misalignment of the semiconductor element placed on the semiconductor element. [Effects of the Invention]

[0011] The method for producing a bonded body of the present invention can form a bonding layer with few voids in a short time. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a flowchart illustrating a manufacturing method according to an embodiment of the present invention. [Figure 2] 2 is a cross-sectional view showing an example of a semiconductor device manufactured by the manufacturing method shown in FIG. 1. [Figure 3] 3 is a cross-sectional view showing the state of the semiconductor device shown in FIG. 2 before bonding a semiconductor element. [Figure 4] 1 is a graph showing the relationship between temperature change and thermal weight loss amount, and the relationship between solvent weight ratio and weight loss rate in Examples. [Figure 5] 3. This is a graph similar to FIG. [Figure 6]1 is a graph showing the relationship between the solvent weight ratio and the weight loss rate measured under various conditions. [Figure 7] 1 is an ultrasonic flaw detection image of a bonding layer in an example. [Figure 8] 10 is an ultrasonic flaw detection image of a bonded joint in a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the method for producing a bonded body according to the present invention will be described. 2, a bonded body according to one embodiment is a semiconductor device (power module) 30 in which a semiconductor element 20 made of a power semiconductor is mounted on an insulating circuit board 10. The insulating circuit board 10 has metal layers 12 and 13 formed on both sides of a ceramic substrate 11, with one of the metal layers 12 being a circuit layer (first member) on which a semiconductor element (second member) 20 is bonded via a bonding layer 15.

[0014] The ceramic substrate 11 constituting the insulating circuit board 10 can be made of nitride ceramics such as AlN (aluminum nitride) or Si3N4 (silicon nitride), or oxide ceramics such as Al2O3 (alumina), and the thickness is not particularly limited, but is set within the range of 0.2 mm to 1.5 mm, for example.

[0015] The metal layers 12, 13 are made of aluminum or an aluminum alloy, or copper or a copper alloy, and have a thickness of, for example, 0.1 mm to 5.0 mm. They are usually formed in a rectangular shape with a planar shape smaller than the ceramic substrate 11. These metal layers 12, 13 are stacked with a brazing filler metal (not shown) interposed between them and the ceramic substrate 11, and are joined by heating them while being pressurized in the stacking direction, melting the brazing filler metal, and solidifying it. When forming a metal layer made of aluminum or an aluminum alloy, an alloy such as an Al-Si, Al-Ge, Al-Cu, Al-Mg, or Al-Mn alloy is used as the brazing filler metal. When forming a metal layer made of copper or a copper alloy, an alloy such as Ag-Ti or Ag-Cu-Ti is used.

[0016] As the semiconductor element 20, various semiconductor elements such as an IGBT (Insulated Gate Bipolar Transistor), a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), or an FWD (Free Wheeling Diode) are selected depending on the required function.

[0017] This semiconductor element 20 is bonded onto the metal layer (circuit layer) 12 of the insulating circuit board 10 via a bonding layer 15. A metallized layer 21 made of gold, silver, nickel, or the like is formed on the bonding surface of the semiconductor element 20. The bonding layer 15 is formed by applying and sintering a paste-like bonding composition 16 (see FIG. 3) containing metal powder, resin, and solvent. The metal powder may be, for example, silver or copper.

[0018] In an example where silver powder is used as the paste-like bonding composition 16, the bonding composition is composed of silver powder, fatty acid silver, aliphatic amine, solvent, and resin. Specifically, the bonding composition is as follows.

[0019] (silver powder) For example, it preferably contains first silver particles (particle size of 100 nm or more but less than 500 nm), second silver particles (particle size of 50 nm or more but less than 100 nm), and third silver particles (particle size less than 50 nm), all of which have different particle sizes. These first to third silver particles all aggregate together as primary particles to form aggregates (silver powder). When the total amount of the first to third silver particles is taken as 100% by volume, it is preferable that the first silver particles are contained in an amount of 55% by volume or more but less than 95% by volume, the second silver particles are contained in an amount of 5% by volume or more but less than 40% by volume, and the third silver particles are contained in an amount of 5% by volume or less. The purity of silver in the first to third silver particles is preferably 90% by mass or more, more preferably 99% by mass or more, and may contain Au, Cu, Pd, etc.

[0020] (fatty acid silver) Examples of the fatty acid silver salts of the present embodiment include silver acetate, silver oxalate, silver propionate, silver myristate, and silver butyrate.

[0021] (Aliphatic amines) The aliphatic amine of the present embodiment may be a primary amine, a secondary amine, a tertiary amine, etc. The aliphatic amine preferably has 8 to 12 carbon atoms. In the bonding composition, the molar ratio of the aliphatic amine to the fatty acid silver, ie, the molar amount of the aliphatic amine / the molar amount of the fatty acid silver, is preferably in the range of 1.5-3.

[0022] (solvent) The solvent of this embodiment is an alcohol-based solvent containing a monohydric alcohol and a high-dielectric-constant alcohol having a dielectric constant of 30 or more. In addition to the alcohol-based solvent, the solvent may contain water, an acetate-based solvent, a hydrocarbon-based solvent, a mixture thereof, or the like.

[0023] (resin) The bonding composition may further contain a resin, such as an epoxy resin, a silicone resin, an acrylic resin, or a mixture thereof.

[0024] The bonding composition has a viscosity of 20 Pa·s or more and 250 Pa·s or less. More preferably, the viscosity is 50 Pa·s or more and 200 Pa·s or less. The above viscosity values are measured at 25°C.

[0025] Next, a method for manufacturing this bonded body (semiconductor device 30) will be described. The method includes an application step (S11) of applying the above-mentioned paste-like bonding composition 16 to the surface of the circuit layer 12 of the insulating circuit board 10, a placement step (S12) of placing the semiconductor element 20 on the bonding composition 16, and a heating step (S13) of heating the laminate of the semiconductor element 20 and the insulating circuit board 10 to sinter the bonding composition 16 and form a bonding layer 15, which bonds the circuit layer 12 of the insulating circuit board 10 and the semiconductor element 20 together.

[0026] (Coating process: S11) First, as shown in Fig. 3, a paste-like bonding composition 16 is applied to the circuit layer 12 of the insulating circuit board 10. The thickness of the applied bonding composition 16 is not particularly limited, but is, for example, 30 µm or more and 140 µm or less. The area to which the bonding composition 16 is applied is set to be approximately the same as or slightly larger than the area of the metallized layer 21 of the semiconductor element 20 and to have the same shape or a similar shape. The bonding composition 16 is generally applied by a metal mask method, a screen printing method, or the like, but is not limited to these, and may also be applied by dispensing using a dispenser or the like.

[0027] (Placement step: S12) A semiconductor element 20 is placed on the bonding composition 16 applied to the circuit layer 12 of the insulating circuit board 10 . (Heating process: S13) The heating step (S13) includes a volatilization step (S14) in which the laminate of the insulating circuit board 10 and the semiconductor element 20 is heated in a stacked state to volatilize the volatile components in the bonding composition 16, and a sintering step (S15) in which the temperature is increased after the volatilization step to sinter the bonding composition 16 and form the bonding layer 15. The semiconductor element 20 is stacked with its metallized layer 21 in contact with the bonding composition 16.

[0028] (Volatilization step: S14) In the volatilization step (S14), the paste-like bonding composition 16 is heated at a volatilization rate of 0.5 mass% / min or more and 2 mass% / min or less until the amount of volatile components in the bonding composition 16 becomes 70 mass% or less of the total amount of volatile components in the bonding composition 16 before heating. The volatile components in the bonding composition 16 are solvents, resins, etc. other than the metal powder, and in the case of the bonding composition using the silver powder described above, these are part of the solvent and resin.

[0029] If the volatilization rate in this volatilization step is too high, exceeding 2% by mass / min, the volatile components will gasify all at once and not completely escape from between the semiconductor element 20 and the insulating circuit board 10, resulting in the formation of voids. Voids will also occur if the temperature is raised to near the sintering temperature before the total amount of volatile components has fallen to 70% by mass. If the volatilization rate is less than 0.5% by mass / min, the formation of voids can be suppressed, but it will take time, resulting in a loss of productivity. Preferably, the volatilization step is carried out until the volatilization rate falls to 65% by mass or less, and a volatilization rate of 1% by mass or less is preferred.

[0030] The amount of this volatile component can be measured by simultaneous thermogravimetry and differential thermal analysis (TG-DTA). This method involves simultaneously measuring the weight change of a sample (thermogravimetry: TG) and the temperature difference between the sample and a reference material while varying the temperature of the sample and a reference material under the same conditions according to a program. An empty container or α-alumina is used as the reference material, as it does not absorb or generate heat in the measurement temperature range. By configuring the program for simultaneous thermogravimetry and differential thermal analysis to mimic the temperature profile of the actual volatilization process, the measurement results can be estimated as the weight change during the actual volatilization process.

[0031] This volatilization step may be carried out in an air atmosphere using, for example, a continuous heating furnace, or may be carried out in a nitrogen atmosphere or a low-oxygen atmosphere.

[0032] In the heating process, including the volatilization process and the sintering process described later, the laminate is heated in a pressure-free state. The pressure-free state means that no pressure is actively applied in the stacking direction to the semiconductor element 20 stacked on the insulating circuit board 10 via the paste-like bonding composition 16. This does not exclude slight loads acting on the semiconductor element 20, such as the weight of the semiconductor element 20 itself or a misalignment prevention tool for the semiconductor element 20 placed on the semiconductor element 20.

[0033] (Sintering process: S15) The laminate after the volatilization step is placed in, for example, a batch-type heating furnace (not shown) and heated to a temperature above 150°C and below 300°C to sinter the bonding composition 16 and form the bonding layer 15. In this case, the laminate 40 is also in an unpressurized state. The atmosphere is preferably a nitrogen atmosphere or a low-oxygen atmosphere with an oxygen concentration of 1000 ppm or less. The heating time is preferably maintained at the peak heating temperature for 10 minutes or more and 240 minutes or less. The volatilization step and the sintering step may be performed in separate devices, or may be performed in the same device by program operation.

[0034] In this sintering step, the silver in the complex formed by the reaction of the silver powder, fatty acid silver, and aliphatic amine contained in the bonding composition 16 is reduced by the high-dielectric-constant alcohol to form silver nanoparticles. These silver nanoparticles and silver powder are sintered to form a bonding layer, and a semiconductor device 30 is produced in which the semiconductor element 20 is bonded to the circuit layer 12 via this bonding layer 15.

[0035] In the semiconductor device 30 manufactured as described above, the bonding layer 15 that bonds the semiconductor element 20 and the insulating circuit board 10 is sintered in the sintering process after volatilizing organic components in the volatilization process, so that the generation of voids in the bonding layer 15 can be suppressed and a good bonding state can be obtained. The bonding composition 16 is formed by adding fatty acid silver, aliphatic amine, solvent, and resin to silver powder, but is not limited to this composition and may be formed by other metal powders. [Example]

[0036] An insulated circuit board was prepared, consisting of a silicon nitride (Si3N4) ceramic substrate with a copper circuit layer (first component) formed on its surface, and a silicon semiconductor element (second component) with a gold metallized layer formed on its edge. The semiconductor element was a square plate with sides measuring 10 mm. The semiconductor element was bonded onto one of the circuit layers of the insulated circuit board.

[0037] In addition, a silver powder containing first silver particles having a particle size of 100 nm or more but less than 500 nm, second silver particles having a particle size of 50 nm or more but less than 100 nm, and third silver particles having a particle size less than 50 nm in a volume ratio of 80:17.5:2.5 was prepared. A mixed solution containing 22.0 mass% silver acetate as a fatty acid silver salt, 41.3 mass% aminodecane as an aliphatic amine, and 36.7 mass% butyl carbitol acetate as a solvent was prepared. This mixed solution (24.00 mass%), silver powder (75.00 mass%), and 1,2,4-butanetriol (D1) (1.00 mass%) as a high-dielectric-constant alcohol were mixed, and the mixture was stirred and kneaded to obtain a paste-like joining composition.

[0038] This bonding composition was applied to the circuit layer of a ceramic substrate, and a semiconductor element was placed on the applied bonding composition. The components were bonded through a volatilization step in which the component was held at 80°C for 60 minutes, followed by a sintering step in which the component was heated to 250°C. Both steps were carried out in an atmosphere with an oxygen concentration of 1000 ppm or less. As a comparative example, a sample was also produced in which the component was sintered by heating to 250°C at a constant rate of 2°C / min without going through the volatilization step.

[0039] Then, using a thermogravimetry and differential thermal simultaneous measurement device (STA2500Regulus) manufactured by Netsch Japan Co., Ltd., thermogravimetry and differential thermal were measured simultaneously, simulating the temperature profiles of the volatilization and sintering processes. Figure 4 shows an example, and Figure 5 shows a comparative example. On the left side of each figure, the horizontal axis represents elapsed time, and the temperature (°C) and the amount of weight loss (mass%) at that time are plotted. The temperature is shown as a solid line, and the amount of weight loss as a dashed line. On the right side of the figure, the amount of weight loss is converted into the solvent weight ratio (mass%) and plotted as the horizontal axis, and the weight loss rate (mass% / min) is plotted.

[0040] In the example shown in Figure 4, the temperature is maintained at around 80°C for approximately 60 minutes during the volatilization process, during which time the weight gradually decreases, reaching approximately 97% to 98% by the end of the volatilization process, and then decreasing to approximately 91% as the temperature rises to the sintering process. Looking at the solvent weight ratio graph on the right, we can see that the weight loss rate is small, at less than 1% by mass / min (approximately 0.5% by mass / min), until the solvent weight ratio is 70%, but once it falls below 70%, the weight loss rate increases sharply and then decreases at a constant rate. In contrast, in the comparative example of Figure 5, the graph on the left showing the time course shows a rapid weight loss of about 92% in the first 60 minutes of heating. In the graph on the right showing the solvent weight ratio, the rate of weight loss increases at a nearly constant rate up to about 60%, and then decreases at a constant rate once it falls below 60%.

[0041] In addition, Figure 6 shows plots of the solvent weight ratios shown on the right side of Figure 4, primarily for temperature conditions during the volatilization step. It was confirmed that void formation can be suppressed by performing the volatilization step at a weight loss rate of 1% by mass / min or less until the solvent weight ratio reaches 70% by mass or less. It appears that void formation can be suppressed even with a weight loss rate of up to 2% by mass / min if the volatilization step is performed until the solvent weight ratio reaches 70% by mass or less. While void formation can be suppressed at a rate of less than 0.5% by mass / min, it takes too long and reduces productivity, so a rate of 0.5% by mass / min or more is preferred.

[0042] Figures 7 and 8 are ultrasonic flaw detection images of the bonding layer, with Figure 7 showing the working example and Figure 8 showing the comparative example. No voids were observed in Figure 7, but it can be seen that voids have occurred in the comparative example in Figure 8. [Explanation of symbols]

[0043] 10. Insulated circuit board 11 Ceramic substrate 12 Metal layer (circuit layer, first member) 13 Metal layer 15 Bonding layer 16 Bonding composition 20 Semiconductor element (second component) 21 Metallized layer 30 Semiconductor devices 40 laminate

Claims

1. A method for manufacturing a bonded body through a coating process in which a paste-like bonding composition containing metal powder is applied to the surface of a first member, and a heating process in which a second member is stacked on the bonding composition and heated to sinter the bonding composition, thereby bonding the first member and the second member, wherein the heating process includes a volatilization process in which the bonding composition is heated at a volatilization rate of 0.5 mass% or more and 2 mass% or less until the amount of volatile components in the bonding composition becomes 70 mass% or less of the total amount of volatile components in the bonding composition before heating, and a sintering process in which the temperature is increased after the volatilization process to sinter the bonding composition.

2. 2. The method for manufacturing a bonded body according to claim 1, wherein the second member is a semiconductor element, and the volatilization step and the sintering step are carried out in a pressureless state.

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