JP2004231513A - Circuit board excellent in high strength/high heat conductivity - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a circuit board having excellent bonding strength and thermal cycle resistant property. <P>SOLUTION: The circuit board excellent in high strength/high heat conductivity has a silicon nitride substrate in which the surface area ratio of silicon nitride grain is 85-100% when the total surface area ratio of the silicon nitride grain and a grain boundary phase is defined as 100% in the silicon nitride sintered compact substrate composed of the silicon nitride grain and the grain boundary phase, the height difference (L) between the crest part of the maximum height of silicon nitride particle exposed to the surface and the root part of the minimum height of the silicon nitride grain or the grain boundary phase is 1.5-15 μm, the center line average surface roughness (Ra) is 0.2-5 μm and a Cu circuit plate or an Al circuit plate is bonded to at least one surface of the silicon nitride substrate through a brazing filler metal phase or an oxide film phase. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a circuit board excellent in high strength and high thermal conductivity suitable for mounting a power module or the like with high withstand voltage and high current.

  Silicon nitride sintered bodies have excellent heat resistance, low thermal expansion, thermal shock resistance, and corrosion resistance to metals, in addition to mechanical properties such as high-temperature strength properties and wear resistance. It is used for various structural members such as engine members, steelmaking mechanical members, and molten metal melt-resistant members. In addition, it is used as an electrical insulating material by utilizing high insulating properties.

  2. Description of the Related Art In recent years, with the development of semiconductor elements having a large amount of heat, such as high-frequency transistors and power ICs, the demand for ceramic substrates having good heat dissipation characteristics (high thermal conductivity) in addition to electrical insulation has been increasing. An aluminum nitride substrate is already used as such a ceramic substrate. However, the aluminum nitride substrate has low mechanical strength, low fracture toughness, and the like, and may be broken by tightening in the assembly process of the substrate unit. In addition, in a circuit board in which a silicon semiconductor element is mounted on an aluminum nitride substrate, the thermal expansion difference between silicon (Si) and the aluminum nitride substrate is large. Is reduced.

  Therefore, although thermal conductivity is inferior to that of aluminum nitride, the thermal expansion coefficient is close to that of Si, and a substrate made of a high thermal conductive silicon nitride sintered body having excellent mechanical strength, fracture toughness and thermal fatigue resistance has attracted attention. I have.

  Further, as the circuit board, there are various ones in which a copper circuit is formed on a silicon nitride substrate having excellent material properties as described above, and those in which an aluminum circuit is formed for the purpose of further improving the cooling / heating cycle resistance. Is being considered.

  For example, with respect to a copper circuit board, Patent Document 1 (Japanese Patent Application Laid-Open No. 6-216481) discloses a bonding metal layer containing an active metal on the surface of a silicon nitride substrate having a thermal conductivity of 60 to 180 W / m · K. Discloses a ceramic copper circuit board formed by integrally joining a copper circuit board via a substrate. This circuit board contains 15 to 35% by weight of Cu, and 1 to 10% by weight of at least one active metal selected from the group consisting of Ti, Zr, Hf and Nb, and the balance substantially consists of Ag By using a brazing material having a composition, the bonding strength between the copper circuit board and the silicon nitride substrate is improved.

  Patent Document 2 (Japanese Patent Application Laid-Open No. 8-319187) discloses that a copper circuit board on which a copper oxide layer is formed by surface oxidation at a temperature of 150 to 360 ° C. in the atmosphere is placed at a predetermined position on the surface of a silicon nitride substrate. And heated to a temperature higher than the eutectic temperature of copper-copper oxide (1065 ° C) at a temperature lower than the melting point of copper (1083 ° C), and using the generated Cu-O eutectic compound liquid phase as a bonding agent to form a copper circuit board A so-called DBC (Direct Bonding Cupper) circuit board which is directly bonded to a silicon nitride substrate surface is described. In this circuit board, since the copper plate is directly joined to the silicon nitride substrate, there is no inclusion such as an adhesive or a brazing material between the metal circuit board and the silicon nitride substrate. Therefore, there is an advantage that the heat resistance between the two is low, and the heat generated by the semiconductor element provided on the metal circuit board can be quickly radiated out of the system.

Patent Document 3 (Japanese Patent Application Laid-Open No. H10-65296) discloses an aluminum circuit board, in which a ceramic substrate formed of Si ₃ N ₄ and two surfaces of the ceramic substrate with Al-Si brazing material interposed therebetween. A circuit board with an adhered aluminum plate is described. This circuit board has a smaller deformation resistance than conventional metal members made of copper, copper alloy, etc., and the thermal stress acting on the ceramics substrate when a cooling cycle is added to the circuit board is small. This has the advantage that cracks do not occur in the ceramic substrate.

JP-A-6-216481 JP-A-8-319187 JP-A-10-65296

  However, in the above-described conventional circuit board, no study has been made on the surface morphology of the silicon nitride substrate which governs the bonding state between the metal circuit board such as copper or aluminum and the silicon nitride substrate. If the surface morphology of the silicon nitride substrate is not adjusted by any of the above-described bonding methods, variations occur in the bonding strength between the metal circuit board such as copper or aluminum and the silicon nitride substrate and the thermal cycling characteristics, and the high reliability circuit. There is a problem that a substrate cannot be obtained.

  The present invention has been made in view of the above-described conventional problems, and provides a circuit board using a silicon nitride substrate having a surface property suitable for manufacturing a circuit board having excellent bonding strength and thermal cycling resistance. With the goal.

  The present invention, in a silicon nitride sintered body substrate substantially consisting of silicon nitride particles and grain boundary phase, when the total area ratio of silicon nitride particles and grain boundary phase on the surface of the sintered body substrate is 100%, The area ratio of the silicon nitride particles is 85 to 100%, and the height difference between the peak of the maximum height of the silicon nitride particles exposed on the surface and the valley bottom of the minimum height of the silicon nitride particles or the grain boundary phase ( L) has a surface property of 1.5 to 15 μm and a center line average roughness (Ra) of 0.2 to 5 μm, and a Cu circuit board is formed on at least one surface of the silicon nitride substrate by a brazing material phase or This is a circuit board that is bonded through an oxide film phase and has excellent high strength and high thermal conductivity.

  The present invention, in a silicon nitride sintered body substrate substantially consisting of silicon nitride particles and grain boundary phase, when the total area ratio of silicon nitride particles and grain boundary phase on the surface of the sintered body substrate is 100%, The area ratio of the silicon nitride particles is 85 to 100%, and the height difference between the peak of the maximum height of the silicon nitride particles exposed on the surface and the valley bottom of the minimum height of the silicon nitride particles or the grain boundary phase ( L) having a surface property of 1.5 to 15 μm and a center line average roughness (Ra) of 0.2 to 5 μm, and an Al circuit board is formed on at least one surface of the silicon nitride substrate by a brazing material phase or This is a circuit board that is bonded through an oxide film phase and has excellent high strength and high thermal conductivity.

  In the circuit board of the present invention, when the area ratio of the silicon nitride particles is 90 to 100% and the center line average roughness (Ra) is 0.2 to 2 μm, the height difference (L) is 1.5 to 5 μm. It is desirable.

When the silicon nitride substrate used in the present invention has a center line average roughness (Ra) of more than 5 μm, voids are locally generated at the bonding interface when a metal circuit board is bonded to the silicon nitride substrate, and the unbonded area is large. This causes a significant decrease in bonding strength. On the other hand, when the center line average roughness (Ra) is less than 0.2 μm, generation of voids can be suppressed, but sufficient bonding strength cannot be obtained because the composite effect by anchoring cannot be obtained. Therefore, regarding the surface properties of the silicon nitride substrate, the center line average roughness (Ra) is desirably 0.2 to 5 μm. More preferably, it is 0.2 to 2 μm.
Further, the silicon nitride substrate used in the present invention is substantially composed of a silicon nitride sintered body composed of silicon nitride particles and a grain boundary phase, and has a total area ratio of the silicon nitride particles and the grain boundary phase on the substrate surface. As 100%, the area ratio of the silicon nitride particles is preferably 85 to 100%. A silicon nitride substrate satisfying this condition has excellent thermal shock resistance and thermal fatigue reliability. More preferably, it is 90 to 100%.
Further, the silicon nitride substrate used in the present invention has a height difference (L) between the peak of the maximum height of the silicon nitride particles exposed on the substrate surface and the bottom of the valley of the minimum height of the silicon nitride particles or the grain boundary phase. Has a surface property of 1.5 to 15 μm. If the height difference (L) exceeds 15 μm, voids are locally formed at the bonding interface between the silicon nitride substrate and the metal circuit board, and the unbonded area increases, resulting in a decrease in bonding strength. If the thickness is less than 1.5 μm, the generation of voids can be suppressed, but the combined effect of anchoring cannot be obtained, so that sufficient bonding strength cannot be obtained. More preferably, it is 1.5 to 5 μm.

  As described above, the circuit board of the present invention has a surface morphology suitable for bonding a metal circuit board such as copper or aluminum, and can significantly improve the bonding strength between the metal circuit board and the silicon nitride substrate. . Further, the silicon nitride substrate used in the present invention has a high thermal conductivity in addition to the inherently high strength / high toughness, so that when a circuit board for a semiconductor element is formed, it can be subjected to repeated cooling / heating cycles accompanying the operation of the semiconductor element. It is possible to provide a circuit board which can suppress the occurrence of cracking of the board and peeling of the metal circuit board due to the occurrence thereof, and can significantly improve the thermal shock resistance and the thermal cycling resistance.

  Hereinafter, embodiments of the present invention will be described in detail. First, as a method of adjusting the surface properties of the silicon nitride substrate of the present invention, there is a method of mechanically removing a grain boundary phase by machining such as sand blasting, shot blasting, grid blasting or hydroblasting.

  In order to join a metal plate such as copper or aluminum to a silicon nitride substrate, a brazing method is preferable. As described above, an Ag-Cu alloy containing an active metal such as Ti, Zr or Hf is preferably used as a brazing material for a copper plate, and an Al-Si alloy is preferably used as a brazing material for an aluminum plate. Furthermore, a Cu—O and Al—O eutectic compound liquid phase may be used as an adhesive layer for a copper plate and an aluminum plate, respectively, and these metal circuit boards may be directly bonded to a silicon nitride substrate.

  Factors governing the bonding strength between a metal circuit board and a silicon nitride substrate include (1) wetting and diffusion between bonding materials, (2) strength of interface products, and (3) interface structure. For example, in the active metal method in which a metal plate is bonded to a silicon nitride substrate using an Ag-Cu alloy containing active metal Ti as a brazing material, the bonding strength at the interface greatly affects the factors (2) and (3). Is done. In this case, the interface product is a TiN phase formed at the brazing material phase / silicon nitride interface. The generation process of the TiN phase will be described in detail. When silicon nitride comes into contact with the brazing material phase in the heat treatment process, Si and N are melted into the brazing material phase and a liquid phase mixed phase is formed. Next, TiN particles are nucleated from the liquid phase region generated by the decomposition, and grow along the bonding interface between the silicon nitride and the brazing material. Since TiN particles are generated and grown at a crystal grain boundary having a specific crystallographic orientation relationship, the TiN phase and silicon nitride have a crystalline consistency, and the bonding strength increases. Therefore, in order to obtain a strong bonding strength, it is important to sufficiently precipitate TiN particles at the brazing filler metal phase / silicon nitride interface.

Also, in the direct bonding method using a eutectic oxide liquid phase of copper or aluminum as an adhesive, it is necessary to optimize an oxide film phase that is an interface product at a bonding interface. Here, in the case of a silicon nitride substrate, the oxide film phase comprises a silicate crystal phase of a sintering aid component and SiO ₂ and a glass phase. Specifically, when Y ₂ O ₃ is used as a sintering aid, a Y ₂ O ₃ .2SiO ₂ phase and a Y ₂ O ₃ —SiO ₂ glass are generated. The bonding strength between the metal circuit board and the silicon nitride substrate largely depends on the form of formation of these silicate phase and glass phase. Therefore, in the direct bonding method, it is important to control the generation form of the oxide film phase.

  However, the generation of a TiN phase or an oxide film phase that governs the bonding strength between the metal circuit board and the silicon nitride substrate is formed only when the surface properties of the substrate are appropriate. In the former case, if there are large irregularities on the surface of the silicon nitride substrate, the brazing material phase does not penetrate the entire surface, and voids are formed at the brazing material phase / silicon nitride interface, resulting in poor bonding. In the latter case, although an oxide film phase is generated, the oxide film phase does not penetrate the entire surface, and a bonding failure similarly occurs. On the other hand, when the surface of the silicon nitride substrate has very few irregularities, interface products are precipitated, but on the other hand, the anchoring effect due to the brazing material phase being caught between the silicon nitride particles is lost, and as a result, the bonding strength is reduced. descend. In order to obtain such a sufficient bonding strength, the surface morphology of the silicon nitride substrate needs to satisfy predetermined conditions.

In the silicon nitride substrate of the present invention, the silicon nitride particles preferably have an area ratio of 70 to 100%. In the case of the active metal method described above, the TiN phase formed as a result of the contact between the brazing filler metal phase and the silicon nitride particles governs the bonding strength. However, if there are many grain boundary phases composed of the sintering aid component, this TiN phase Si components dissolved in the grain boundary phase in addition to diffuse through the grain boundary phase to form a Ti silicide reacts with excess Ti component by reaction _{5Ti + 3Si → Ti 5 Si 3} . This Ti silicide has not only low strength but also a thermal expansion coefficient of 9.5 × 10 ⁻⁶ / K, which is about three times as large as a thermal expansion coefficient of 3.2 × 10 ⁻⁶ / K of Si ₃ N ₄ . For this reason, interface delamination between Si ₃ N ₄ / Ti ₅ Si ₃ due to a difference in thermal expansion coefficient occurs, which causes a significant decrease in bonding strength. Therefore, it is important to reduce the amount of the grain boundary phase in order to obtain sufficient bonding strength.

In the case of direct bonding method, oxide film phase generated at the joint interface dominates the bonding strength, the oxide film silicate with sintering aid component and SiO ₂ - consists preparative crystalline phase and glass phase, Y ₂ the O ₃ If a sintering agent to produce a _Y 2 _O 3 · 2SiO ₂ phase and _Y 2 _O 3 -SiO ₂ based glass phase. When the amount of the grain boundary phase composed of the auxiliary component at the bonding interface increases, the generation ratio of the glass phase increases, and accordingly, the bonding strength improves. However, when the amount of the grain boundary phase is further increased, the generation rate of the silicate phase having a low strength is increased, and the strength is significantly reduced. Therefore, when any of the joining methods is used, there is an appropriate ratio range of the silicon nitride particles and the grain boundary phase. In the present invention, it has been newly found that the area ratio of silicon nitride particles on the silicon nitride substrate is preferably 70 to 100%.

  The circuit board formed by using the silicon nitride substrate of the present invention is a substrate for a power semiconductor and a multi-chip module by taking advantage of an excellent bonding strength between a metal and a ceramic substrate and a strength reliability such as a high resistance to heat and heat cycle. It is suitable as a member for electronic components such as various substrates such as a substrate, a heat transfer plate for a Peltier element, or a heat sink for various heating elements.

  When a substrate for a semiconductor device is constituted by the silicon nitride substrate of the present invention, the occurrence of cracks in the substrate due to repeated cooling and heating cycling accompanying the operation of the semiconductor device is suppressed, and the thermal shock resistance and the cooling and heat cycling resistance are remarkably improved. It is possible to provide products excellent in performance and reliability. Further, even when a semiconductor element for high output and high integration is mounted, deterioration of thermal resistance characteristics is small and excellent heat radiation characteristics are exhibited. In addition, since the silicon nitride substrate of the present invention can also serve as a structural member by reflecting the excellent mechanical properties of the silicon nitride substrate, it has a practical advantage that the circuit substrate structure can be simplified.

Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to the examples.
(Example 1)
A mixed powder was prepared by adding 96 parts by weight of silicon nitride powder having an average particle diameter of 0.2 to 3.0 μm and 3 parts by weight of MgO and 1 part by weight of Y ₂ O ₃ . Next, into a resin pot of a ball mill filled with a toluene / butanol solution to which 2% by weight of an amine-based dispersant was added, the mixed powder and the silicon nitride ball as a pulverizing medium were charged and wet-mixed for 48 hours. Next, 12.5 parts by weight of a polyvinyl organic binder and 4.2 parts by weight of a plasticizer (dimethyl phthalate) were added to 83.3 parts by weight of the mixed powder in the pot, and then wet-mixed for 48 hours to obtain a slurry for sheet forming. Got. After adjusting the molding slurry, a green sheet was formed by a doctor blade method. Next, the formed green sheet is heated in air at 400 to 600 ° C. for 2 to 5 hours to sufficiently remove the organic binder component, and the degreased green sheet is placed in a nitrogen atmosphere of 0.9 MPa (9 atm) for 1850 hours. C. for 5 hours, heat treated at 1900.degree. C. for 24 hours in the same nitrogen atmosphere, and then cooled to room temperature. The obtained sheet-shaped silicon nitride sintered body was machined, and the surface properties were adjusted by sand blasting to obtain a silicon nitride substrate for a semiconductor module having a length of 50 mm, a width of 50 mm and a thickness of 0.6 mm. Sandblast conditions were as follows.
Substrate feed rate: 20 cm / min Processing zone length: 80 cm
Number of nozzles: 4 Nozzle ejection pressure: 0.35 Mpa
Spray angle to substrate surface: 30 °
Abrasive: Alumina # 240

  Since the grain boundary phase on the surface of the sintered body is removed by sand blasting, the sand blasting conditions (plate feed speed, length of processing zone, number of nozzles and ejection pressure, spray angle with respect to the substrate surface, type of abrasive grains, By adjusting the grain size, etc., the center line average surface roughness (Ra) of the sintered body, the silicon nitride particles, the area ratio of the grain boundary phase, and the height difference (L) between the top and the bottom of the grain boundary phase can be freely adjusted. Can be obtained. FIG. 1 shows the results of measuring the center line average roughness (Ra) of a typical silicon nitride substrate for a semiconductor module of the present invention obtained by processing under the above sandblasting conditions using a stylus type surface roughness measuring instrument. Show. In FIG. 1, the horizontal axis represents the measured length (30 mm) of the surface of the silicon nitride substrate, and the vertical axis represents Ra. The origin of the measurement is 0, and the scale of Ra and the measurement length are shown at the lower left. As a result, Ra of the silicon nitride substrate of this example was 0.6 μm, the area ratio of silicon nitride particles was 81.0%, and the area ratio of the grain boundary phase was 19.0%.

  FIG. 2A is a photograph of the surface structure of the obtained silicon nitride substrate taken at a magnification of 2000 times with a scanning electron microscope (trade name: S4500, manufactured by Hitachi, Ltd.). FIG. 2B is a schematic view corresponding to the scanning electron micrograph of FIG. 2A, in which silicon nitride particles are denoted by 21 and grain boundary phases are denoted by 22. FIG. 7 shows a photograph of the surface structure of a silicon nitride substrate having a silicon nitride particle area ratio of 5% as a comparative example. Next, FIGS. 3A and 3B show a typical cross-sectional structure of a circuit board of the present invention constituted by using the silicon nitride substrate having Ra = 0.6 μm by a scanning electron microscope at a magnification of 50 × and 4000 ×. The Cu circuit board 31 and the silicon nitride substrate 33 are joined via the brazing material layer 32. FIG. 4 is a schematic cross-sectional view showing a state in which the Cu circuit board 31 and the brazing material layer 32 in FIG. 3B have been removed, that is, a surface state of the silicon nitride substrate 33 before bonding to the Cu circuit board 31. Here, (L) in the silicon nitride substrate 33 corresponds to a height difference 43 between the peak 41 of the maximum height of the silicon nitride particles 21 and the valley bottom 42 of the silicon nitride particles 21 or the minimum height of the grain boundary phase 22. I do.

  Next, representative samples (samples Nos. 1 to 10) of the silicon nitride substrates produced by appropriately changing the sandblasting conditions were sampled, and their cross-sectional micrographs were taken with the scanning electron microscope at a magnification of 2000 times. In the 200 μm × 500 μm field of view of the obtained cross-sectional structure photograph, the height: the height between the peak of the maximum height of the silicon nitride particles and the bottom of the minimum height of the silicon nitride particles or the grain boundary phase over 500 μm The difference (L) was measured. Further, the visual field of 200 μm × 500 μm of the obtained cross-sectional structure photograph was analyzed by an image analyzer to determine the average area ratio of the silicon nitride particles and the grain boundary phase. Further, a peel strength test was performed to evaluate the bonding strength between the metal circuit board and the silicon nitride substrate (Sample Nos. 1 to 10). In the peel strength test, one end of the copper or aluminum circuit board 52 of the circuit board 50 shown in FIG. 5 was joined in advance so that it protruded 5 mm from the side surface of the silicon nitride board 51, and this was pulled upward by 90 degrees. Evaluation was based on the force per unit length required for raising.

  Next, a circuit board 60 shown in FIG. 6 was produced from the produced silicon nitride substrate (sample Nos. 1 to 10). The circuit board 60 is provided by bonding a copper or aluminum circuit board 61 to the surface of a silicon nitride substrate 51 having the dimensions of 50 mm × 50 mm × 0.6 mm by brazing material 53, and a copper plate on the back of the board 51. Alternatively, an aluminum plate 62 is joined by a brazing material 53. Table 1 shows the center line average roughness Ra, the silicon nitride particles, the area ratio of the grain boundary phase, and L of each of the silicon nitride substrates of Sample Nos. 1 to 10. In addition, Table 1 shows the peel strength and the breakage occurrence position (breakage mode) when the bonding metal plate is made of copper or aluminum using the silicon nitride substrate and brazed or directly bonded. In the breakdown mode items in Table 1, Cu indicates that the bonding metal was broken from copper, Al indicates that the bonding metal was broken from aluminum, and the bonding interface was broken from the bonding interface between the substrate and the bonding metal. ing.

(Comparative Example 1)
A silicon nitride substrate and a circuit board were prepared and evaluated in the same manner as in Example 1 except that the sandblasting conditions were changed. The results are shown in sample Nos. 21 to 28 of Table 1.

  The following findings were obtained from Sample Nos. 1 to 10 of the examples in Table 1. The center line average roughness (Ra) has a surface property of 0.2 to 5 μm, the area ratio of the silicon nitride particles on the surface is 85 to 100%, the peak of the maximum height of the silicon nitride particles and the silicon nitride particles Alternatively, a circuit board in which a metal plate made of copper or aluminum is bonded to a silicon nitride substrate having a height difference (L) of 1.5 to 15 μm from the bottom of the valley having the lowest height of the grain boundary phase, and a peel strength is obtained. When measured, all of the obtained circuit boards had a high peel strength of 22.0 kN / m or more, and it was confirmed that destruction did not occur from the joint.

In contrast, the following findings were obtained from Sample Nos. 21 to 23 of Comparative Example 1 in Table 1.
(1) The center line average roughness (Ra) of No. 21 was less than 0.2 μm, the peel strength was as low as 8.5 kN / m, and the fracture occurred from the joint interface.
(2) The center line average roughness (Ra) of No. 22 was more than 20 μm, the peel strength was as low as 9.5 kN / m, and the fracture occurred from the joint interface.
(3) In No. 23, the area ratio of the silicon nitride particles was less than 70% and the area ratio of the grain boundary phase was more than 30%, the peel strength was as low as 5.5 kN / m, and the fracture occurred from the joint interface.
(4) The height difference (L) between the peak of the maximum height of the silicon nitride particles of No. 24 and the valley bottom of the silicon nitride particles or the minimum height of the grain boundary phase is less than 1 μm, and the peel strength is 7.0 kN. / m, which was low, and fracture occurred from the joint interface.
(5) The height difference (L) between the peak of the maximum height of the silicon nitride particles of No. 25 and the valley bottom of the silicon nitride particles or the minimum height of the grain boundary phase is 45 μm, and the peel strength is 6.5 kN. / m, which was low, and fracture occurred from the joint interface.
(6) No. 26 was the same as No. 22 except that the joining method was changed from brazing to direct joining. In this case, too, the peel strength was as low as 7.0 kN / m, and destruction occurred from the joining interface.
(7) No. 27 was the same as No. 22 except that the metal circuit board was changed from copper to aluminum, but also in this case, the peel strength was as low as 6.5 kN / m, and destruction occurred from the joint interface.
(8) No. 28 was the same as No. 27 except that the joining method was changed from brazing to direct joining, but also in this case, the peel strength was as low as 6.2 kN / m, and the fracture occurred from the joining interface.

(Example 2)
A circuit board 60 shown in FIG. 6 was manufactured from a silicon nitride substrate manufactured under the same manufacturing conditions as in Example 1. The circuit board 60 is provided with a copper circuit board 61 via a brazing material 53 on the surface of a manufactured silicon nitride substrate 51 having a size of 50 mm × 50 mm × 0.6 mm in thickness, and a brazing material 53 on the back surface of the silicon nitride substrate 51. The circuit board 62 made of copper is joined through the intermediary of. Ra of the obtained silicon nitride substrate 51 is 5 μm, the area ratio of silicon nitride particles is 85%, the area ratio of the grain boundary phase is 15%, and the height difference L between the top and the valley bottom of the surface is 5 μm. Met. The circuit board 60 was subjected to a three-point bending strength test and a cold / heat resistance test. As a result, the flexural strength was as large as 600 MPa or more, and no crack was generated due to tightening cracks in the mounting process of the circuit board 60 and thermal stress in the soldering process. Further, it has been demonstrated that the manufacturing yield of a semiconductor device (not shown) on which the circuit board 60 is mounted is remarkably improved. In addition, the cooling / heating cycle test was performed by repeating a heating / cooling cycle in which cooling at −40 ° C. for 30 minutes, holding at room temperature for 10 minutes, and heating at 125 ° C. for 30 minutes were one cycle. The number of cycles until a crack or the like occurred on the circuit board 60 was measured. As a result, there was no cracking of the silicon nitride substrate 51 and no peeling of the copper circuit boards 61 and 62 even after the lapse of 1000 cycles, and it was confirmed that both the durability and the reliability were excellent. Also, even after 1000 cycles, the withstand voltage characteristics of the circuit board 60 did not decrease. In addition, 1000 circuit boards 60 were manufactured, and the variation in the number of failures was investigated by the above-mentioned cooling and heat cycle test, but none of the circuit boards showed cracking of the silicon nitride board or peeling of the copper circuit board. It was confirmed that the thermal shock resistance and thermal fatigue reliability were maintained.

FIG. 4 is a diagram showing an example of Ra measurement results of a typical surface portion of the silicon nitride substrate of the present invention. FIG. 3 is a schematic diagram (b) corresponding to micrographs (a) and (a) of a typical surface structure of a silicon nitride substrate of the present invention taken by a scanning electron microscope. 3A and 3B are micrographs (a) and (b) of a typical cross-sectional structure of the silicon nitride substrate of the present invention taken by a scanning electron microscope. FIG. 4 is a schematic diagram corresponding to FIG. It is sectional drawing which shows the sample for a peel strength test. 1 is a cross-sectional view illustrating a typical circuit board of the present invention. 5 is a photograph taken by a scanning electron microscope of a surface structure of a silicon nitride substrate of a comparative example.

Explanation of reference numerals

21: Silicon nitride particles 22: Grain boundary phase 31: Cu circuit board 32: Brazing material layer 33: Silicon nitride substrate 41: Peak height of maximum height of silicon nitride particles 42: Minimum height of silicon nitride particles or grain boundary phase Valley bottom 43: Height difference (L) between the peak of the maximum height of the silicon nitride particles and the valley bottom of the minimum height of the silicon nitride particles or the grain boundary phase
50, 60: circuit board 51: silicon nitride substrate 52: copper alloy or aluminum alloy circuit board 53: brazing material 61, 62: copper alloy circuit board

Claims (3)

In a silicon nitride sintered body substrate substantially consisting of silicon nitride particles and a grain boundary phase, when the total area ratio of the silicon nitride particles and the grain boundary phase on the surface of the sintered body substrate is 100%, the silicon nitride particles And the height difference (L) between the peak of the maximum height of the silicon nitride particles exposed on the surface and the bottom of the valley of the lowest height of the silicon nitride particles or the grain boundary phase is 1.5 to 100%. And a center line average roughness (Ra) having a surface property of 0.2 to 5 μm. A Cu circuit board is formed on at least one surface of the silicon nitride substrate with a brazing material phase or an oxide film phase. A circuit board excellent in high strength and high thermal conductivity, characterized by being joined through a substrate. In a silicon nitride sintered body substrate substantially consisting of silicon nitride particles and a grain boundary phase, when the total area ratio of the silicon nitride particles and the grain boundary phase on the surface of the sintered body substrate is 100%, the silicon nitride particles And the height difference (L) between the peak of the maximum height of the silicon nitride particles exposed on the surface and the bottom of the valley of the lowest height of the silicon nitride particles or the grain boundary phase is 1.5 to 100%. And a center line average roughness (Ra) having a surface property of 0.2 to 5 μm. The Al circuit board is formed on at least one surface of the silicon nitride substrate with a brazing material phase or an oxide film phase. A circuit board excellent in high strength and high thermal conductivity, characterized by being joined through a substrate. The height difference (L) is 1.5 to 5 μm when the area ratio of the silicon nitride particles is 90 to 100% and the center line average roughness (Ra) is 0.2 to 2 μm. 3. The circuit board according to 1 or 2, which is excellent in high strength and high thermal conductivity.

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