Ceramic substrate and ceramic circuit board using same

Abstract

A ceramic substrate according to an embodiment of the present invention has compressive stress of different values in a first direction and a second direction parallel to the surface and orthogonal to each other. When the compressive stress in the first direction is defined as compressive stress A and the compressive stress in the second direction is defined as compressive stress B, 4 / 6 ≤ compressive stress A / compressive stress B ≤ 6 / 4 is satisfied.

Description

Ceramic substrate and ceramic circuit board using the same

[0001] The embodiments described below generally relate to a ceramic substrate and a ceramic circuit board using the same.

[0002] Ceramic substrates are used in ceramic circuit boards on which semiconductor elements are mounted, and include silicon nitride substrates, aluminum nitride substrates, aluminum oxide substrates, and zirconium oxide substrates.

[0003] For example, Japanese Patent No. 6293772 (Patent Document 1) discloses a silicon nitride substrate having a thermal conductivity of 50 W / m·K or more and a three-point bending strength of 600 MPa or more. The silicon nitride substrate of Patent Document 1 has excellent insulating properties even when the substrate thickness is reduced. Furthermore, this silicon nitride substrate can also obtain excellent insulating properties when used as a silicon nitride circuit substrate to which a metal plate is bonded.

[0004] On the other hand, variations occurred in thermal cycling test (TCT) characteristics. For example, Japanese Patent No. 6789955 (Patent Document 2) discloses controlling the size and hardness of the protruding portion of the bonding layer. By performing the control described in Patent Document 2, it is possible to improve TCT characteristics. However, some products had insufficient TCT characteristics. Investigation into the cause revealed that the cause was due to residual stress in the ceramic substrate. For example, International Publication No. 2014 / 080536 (Patent Document 3) discloses an aluminum nitride substrate with a surface residual stress of -60 MPa or less.

[0005] Japanese Patent No. 6293772 Japanese Patent No. 6789955 International Publication No. 2014 / 080536

[0006] In Patent Document 3, the residual stress on the surface of the aluminum nitride substrate is controlled to -60 MPa or less. Negative residual stress indicates compressive stress. In Patent Document 3, the TCT characteristics of the aluminum nitride circuit board are improved by controlling the compressive stress.

[0007] In recent years, there has been a trend toward increasing the thickness of the metal plate in ceramic circuit boards to 0.6 mm or more, as in Patent Document 2. When the metal plate is thickened, a phenomenon has occurred in which the TCT does not necessarily improve simply by controlling the residual stress to a predetermined value or less.

[0008] The ceramic substrate according to the embodiment is intended to address such issues, and is characterized by providing a ceramic substrate in which residual stress is appropriately controlled.

[0009] The ceramic substrate according to the embodiment has compressive stresses of different values ​​in a first direction and a second direction that are parallel to the surface and perpendicular to each other, where the compressive stress in the first direction is compressive stress A and the compressive stress in the second direction is compressive stress B, and the relationship 4 / 6≦compressive stress A / compressive stress B≦6 / 4 is satisfied.

[0010] Fig. 1 is a perspective view showing an example of a ceramic substrate according to an embodiment. Fig. 2 is a plan view showing an example of a measurement position during X-ray residual stress measurement. Fig. 3 is a side view showing an example of a ceramic circuit substrate according to an embodiment. Figs. 4(a) and 4(b) are plan views showing an example of a ceramic circuit substrate according to an embodiment. Figs. 5(a) and 5(b) are side views showing an example of dry blasting. Fig. 6 is a flowchart showing a manufacturing process of a ceramic substrate according to an embodiment.

[0011] The ceramic substrate according to the embodiment has compressive stresses of different values ​​in a first direction and a second direction that are parallel to the surface and perpendicular to each other, where the compressive stress in the first direction is compressive stress A and the compressive stress in the second direction is compressive stress B, and the relationship 4 / 6≦compressive stress A / compressive stress B≦6 / 4 is satisfied.

[0012] The ceramic substrate according to the embodiment may have various shapes such as a quadrilateral, a circular (including an ellipse), a triangular, a pentagonal, a hexagonal, an L-shape, or a U-shape. The quadrilateral shape may be either a square or a rectangle. The shape of the ceramic substrate is the shape when the surface on which the circuit portion is provided is viewed from above.

[0013] The first direction and the second direction are any directions parallel to the surface. For example, if the ceramic substrate has a rectangular shape, the first direction may be parallel to the long side direction, and the second direction may be parallel to the short side direction. If the first direction is oblique to the long side direction, the second direction is a direction perpendicular to the oblique direction.

[0014] Compressive stress is the stress that occurs in an object when an external force is applied in a direction that compresses the object. The opposite of compressive stress is tensile stress. Tensile stress is the stress that occurs in an object when an external force is applied in a direction that pulls the object. Compressive stress is expressed as a negative value. Tensile stress is expressed as a positive value.

[0015] Hereinafter, an embodiment of the present invention will be described using a square (rectangular) ceramic substrate as an example.

[0016] 1 is a perspective view showing an example of a ceramic substrate according to an embodiment, in which reference numeral 1 denotes a ceramic substrate, reference numeral 2 denotes a direction parallel to a long side (long side direction), and reference numeral 3 denotes a direction parallel to a short side (short side direction).

[0017] The compressive stress has different values ​​in the first direction and the second direction, and the compressive stress in the first direction is compressive stress A, and the compressive stress parallel to the other side is compressive stress B. Figure 1 illustrates a rectangular ceramic substrate. For example, the first direction is parallel to one of the long side direction and the short side direction. The second direction is parallel to the other of the long side direction and the short side direction. Compressive stress A and compressive stress B are compressive stresses in two directions that are perpendicular to the sides. If the compressive stress in the long side direction is "compressive stress A," the compressive stress in the short side direction is "compressive stress B." Conversely, if the compressive stress in the short side direction is "compressive stress A," the compressive stress in the long side direction is "compressive stress B."

[0018] In the following examples, the first direction is parallel to the long side direction, and the compressive stress in the first direction is referred to as "compressive stress A." The second direction is parallel to the short side direction, and the compressive stress in the second direction is referred to as "compressive stress B."

[0019] The ceramic substrate according to the embodiment has different compressive stress values ​​in the first direction and the second direction. This means that compressive stress A and compressive stress B are different. Furthermore, the ceramic substrate according to the embodiment satisfies 4 / 6≦compressive stress A / compressive stress B≦6 / 4. It is preferable that compressive stress A and compressive stress B are within the range of −250 MPa or more and −60 MPa or less. Hereinafter, the ratio of “compressive stress A / compressive stress B” will also be simply referred to as “ratio A / B.”

[0020] With respect to the ratio A / B, since compressive stress A and compressive stress B are different from each other, a ratio A / B=1 is excluded. A ratio A / B=1 means that the ratio A / B is in the range of 0.995 or more and 1.004 or less. Furthermore, a ratio A / B in the range of 4 / 6 or more and 6 / 4 or less means that the ratio A / B is in the range of 0.667 or more and 1.500 or less. In other words, a ratio A / B in the range of 4 / 6 or more and 6 / 4 or less and A / B≠1 means that the ratio A / B is in the range of 0.667 or more and 1.500 or less, but outside the range of 0.995 or more and 1.004 or less.

[0021] By setting the ratio A / B within the above range, the TCT characteristics can be improved when the ceramic substrate is used in a ceramic circuit board. As will be described later, in a ceramic circuit board, a circuit portion is bonded to the ceramic substrate. When the circuit portion repeatedly expands and contracts during TCT, if the compressive stress of the ceramic substrate is controlled, the occurrence of defects can be suppressed.

[0022] The defect is the occurrence of defects such as cracks in the ceramic substrate or the bonding layer. When the circuit part expands, tensile stress is generated in the ceramic substrate and the bonding layer. When the circuit part contracts, compressive stress is generated in the ceramic substrate and the bonding layer. Cracks are particularly likely to occur when the circuit part expands, that is, when tensile stress is generated. By applying a predetermined compressive stress to the surface of the ceramic substrate in advance, the effects of tensile stress caused by the expansion of the circuit part can be mitigated.

[0023] Furthermore, the larger the size of the circuit section, the greater the amount of deformation of the circuit section due to heat. By aligning the long dimension of the circuit section with the direction in which compressive stress is greatest, the effects of tensile stress due to expansion of the circuit section can be further mitigated.

[0024] Compressive stress A and compressive stress B are preferably within the range of -250 MPa or more and -60 MPa or less. In other words, both the compressive stress in the long side direction and the compressive stress in the short side direction are within the range of -250 MPa or more and -60 MPa or less. If compressive stress A and compressive stress B are less than -60 MPa, the effect of alleviating the tensile stress associated with the expansion of the circuit portion may be insufficient. Furthermore, if compressive stress A and compressive stress B are greater than -250 MPa, cracks may occur due to the synergistic effect with the compressive stress when the circuit portion contracts. For this reason, compressive stress A and compressive stress B are preferably within the range of -250 MPa or more and -60 MPa or less, and more preferably within the range of -200 MPa or more and -80 MPa or less. Note that a large compressive stress indicates a large negative value. Furthermore, a small compressive stress indicates a small negative value. For example, when comparing a stress of -250 MPa with a stress of -60 MPa, the stress of -250 MPa is a larger compressive stress.

[0025] Furthermore, by making the ratio A / B ≠ 1 and the ratio A / B 4 / 6≦ratio A / B≦6 / 4, it is possible to mitigate the effects of tensile stress due to expansion of the circuit portion and compressive stress due to contraction of the circuit portion. Circuit portions have various shapes. In the circuit portion, tensile stress due to expansion and compressive stress due to contraction occur depending on the shape of the circuit portion. By satisfying the above conditions, it is possible to mitigate stress occurring in various directions.

[0026] The ceramic substrate 1 is subjected to different stresses in different directions depending on the shape of the circuit portion. If the ratio A / B=1, i.e., if compressive stress A and compressive stress B are the same, the different stresses in different directions cannot be effectively alleviated. Furthermore, if the ratio A / B is smaller than 4 / 6 or larger than 6 / 4, the difference in compressive stress between the first and second directions becomes too large, and the effect of stress alleviation in a specific direction may be insufficient.

[0027] The shape of the circuit part varies. Therefore, when the ratio A / B is 4 / 6 or more and 6 / 4 or less, a stress relaxation effect according to the shape of the circuit part can be obtained. Furthermore, for ceramic substrates on which semiconductor elements are mounted, rectangular ceramic substrates are often used. The circuit part is often provided along the sides of the rectangular ceramic substrate. Therefore, it is preferable to control the ratio of compressive stress A in the long side direction of the ceramic substrate to compressive stress B in the short side direction.

[0028] For example, when a circuit unit bonded to the ceramic substrate 1 expands or contracts, stress is applied from the circuit unit to the ceramic substrate 1. If the circuit unit is rectangular in plan view, the amount of thermal expansion of the circuit unit in the longitudinal direction is greater than the amount of thermal expansion of the circuit unit in the lateral direction. Therefore, when the circuit unit expands, the tensile stress applied to the ceramic substrate 1 in the longitudinal direction is greater than the tensile stress applied to the ceramic substrate 1 in the lateral direction. When the circuit unit contracts, the compressive stress applied to the ceramic substrate 1 in the lateral direction is greater than the compressive stress applied to the ceramic substrate 1 in the longitudinal direction.

[0029] When joining a circuit part to the ceramic substrate 1 according to the embodiment, it is effective to align the longitudinal direction of the circuit part with the direction of the largest compressive stress of the ceramic substrate 1. For example, when the size and orientation of the circuit part to be joined to the ceramic substrate 1 are known in advance, compressive stress is applied to the surface of the ceramic substrate 1 in each of the first and second directions according to the size and orientation of the circuit part.

[0030] In this way, it is effective to control the compressive stress of the ceramic substrate 1 in accordance with the shape of the circuit portion. In other words, the embodiment is suitable for a ceramic substrate on which a circuit portion having different lengths in the first direction and the second direction is mounted.

[0031] In this embodiment, it is preferable to satisfy the relationship 4 / 6≦ratio B / A≦6 / 4. That is, the magnitude relationship between the compressive stress A in the first direction and the compressive stress B in the second direction can be freely set. The first direction and the second direction can also be freely set. Even if the magnitude relationship between the compressive stress A in the first direction and the compressive stress B in the second direction changes, the effect of stress relaxation can be improved by setting the ratio A / B and the ratio B / A within the range of 4 / 6 or more and 6 / 4 or less.

[0032] The compressive stress of the ceramic substrate 1 is controlled on the surface to which the circuit section or the heat dissipation section is bonded. When the circuit section and the heat dissipation section are bonded to both surfaces of the ceramic substrate 1, respectively, the compressive stress on both surfaces is controlled separately. When the circuit section is bonded to only one surface of the ceramic substrate 1, the compressive stress is controlled only on the surface to which the circuit section is bonded.

[0033] The method for measuring residual stress will now be described. Residual stress is measured by the single incidence method using an X-ray residual stress measurement device. The X-ray residual stress measurement device used is a μX360s manufactured by Pulstec Industrial Co., Ltd., or a device with equivalent performance. When the ceramic substrate 1 is a silicon nitride substrate, the measurement conditions are set as follows: vanadium target (V-Kα), incident angle 30°, and sample measurement distance 45 mm. Compressive stress is measured using the (411) diffraction peak.

[0034] Figure 2 is a plan view illustrating measurement positions when performing X-ray residual stress measurement. Residual stress is measured at two points on the surface of ceramic substrate 1. In Figure 2, reference numeral 1 denotes the ceramic substrate, reference numeral 7 denotes a first measurement point, and reference numeral 8 denotes a second measurement point. The first measurement point 7 is located at the center of ceramic substrate 1. The center of ceramic substrate 1 is the intersection of the lines connecting opposing corners. The second measurement point 8 is the midpoint between the first measurement point 7 and one of the short sides.

[0035] First, the residual stress on the surface in each of the first and second directions is measured at one of the first measurement point 7 and the second measurement point 8. Next, the residual stress on the surface in each of the first and second directions is measured at the other of the first measurement point 7 and the second measurement point 8. In the ceramic substrate 1 according to the embodiment, compressive stress is measured. The average of the compressive stress in the first direction at the first measurement point 7 and the compressive stress in the first direction at the second measurement point 8 is defined as the compressive stress in the first direction of the ceramic substrate 1. The average of the compressive stress in the second direction at the first measurement point 7 and the compressive stress in the second direction at the second measurement point 8 is defined as the compressive stress in the second direction of the ceramic substrate 1. The cos α method is used to calculate the compressive stress.

[0036] In the case of an aluminum oxide substrate, the measurement conditions are set as follows: iron target (Fe-Kα), incident angle 30°, sample measurement distance 45 mm. Compressive stress is measured using the (2.1.10) diffraction peak.

[0037] In the case of an aluminum nitride substrate, the measurement conditions are set as follows: chromium target (Cr-Kα), incident angle 30°, sample measurement distance 45 mm. Compressive stress is measured using the (112) diffraction peak.

[0038] The X-ray target and diffraction peak are selected according to the material of the ceramic substrate, referring to the method recommended by the Ceramics Subcommittee of the X-ray Materials Strength Division Committee of the Japan Society for Materials Science (for example, JSMS-SD-1-00 X-ray Stress Measurement Standards, Ceramics Edition).

[0039] In the ceramic substrate according to the embodiment, the compressive stress in the direction parallel to the long side or the direction parallel to the short side is preferably within the range of -250 MPa or more and -60 MPa or less, regardless of where on the surface it is measured.

[0040] Furthermore, the maximum height Sz on the surface of the ceramic substrate is preferably 60 μm or less. The maximum height Sz is the distance between the highest point (highest convex portion) and the lowest point (lowest concave portion) on the surface. The distance is measured in the thickness direction of the ceramic substrate 1. The maximum height Sz is defined in ISO 25178 and is measured according to a method conforming to ISO 25178. As the measuring device, a Keyence VR-5000 series one-shot 3D shape measuring instrument or a device with equivalent performance is used. The measurement range is set to 9 mm long x 12 mm wide. When measuring any 9 mm x 12 mm area on the surface of the ceramic substrate 1 according to the embodiment, the maximum height Sz is preferably 60 μm or less.

[0041] When the maximum height Sz is 60 μm or less, it is easy to control the compressive stress within a predetermined range. Furthermore, the bonding strength between the ceramic substrate 1 and the metal plate can be improved. While the lower limit of the maximum height Sz is not particularly limited, it is preferably 30 μm or more. As described below, blasting or the like is used to control the compressive stress. When blasting is used, controlling the maximum height Sz to fall within a predetermined range is effective in controlling the residual stress. If the maximum height Sz exceeds 60 μm, there is a possibility that a portion with a residual stress smaller than −60 MPa will be formed. Furthermore, if the maximum height Sz is less than 30 μm, the residual stress on the surface will be homogenized, and the ratio A / B will tend to decrease. As a result, there is a possibility that a portion with a ratio A / B = 1 will be formed on the surface. Therefore, it is preferable that the maximum height Sz be 60 μm or less, and more preferably within the range of 30 μm to 60 μm.

[0042] Furthermore, it is preferable that the difference between the maximum convex height and the maximum concave depth of the surface waviness on the surface of the ceramic substrate 1 is 1.6 mm or less. The surface waviness refers to gentle irregularities on the surface of the ceramic substrate. The surface waviness is to be distinguished from the above-mentioned maximum height Sz.

[0043] The method for measuring the maximum convex height and maximum concave depth of the surface waviness will be described. First, the ceramic substrate 1 is placed on a planar stage. Next, the height of the ceramic substrate 1 is measured using a laser. A measurement area is set at an arbitrary position on the surface, with a size of 132 mm x 106 mm. A total of 143 measurement points (13 x 11 points) are set within the measurement area, and the height is measured at each measurement point. The 13 x 11 measurement points are set evenly without bias. The largest value among the measurement results is taken as the maximum convex height, and the smallest value is taken as the maximum concave depth. If the ceramic substrate 1 is smaller than 132 mm x 106 mm, the largest possible measurement area is set. Specifically, the ratio of the area of ​​the set measurement area to the area of ​​132 mm x 106 mm is calculated. The number of measurement points obtained by multiplying 143 by the ratio is set evenly within the set measurement area.

[0044] In the ceramic substrate 1, the difference between the maximum convex height and the maximum concave depth of the surface waviness being 1.6 mm or less indicates that (maximum convex height - maximum concave depth)≦1.6 mm. If the difference between the maximum convex height and the maximum concave depth exceeds 1.6 mm, the surface waviness will be too large, which may make it difficult to control the ratio A / B.

[0045] The lower limit of the difference between the maximum convex height and the maximum concave depth of the surface waviness is not particularly limited, but is preferably 0.2 mm or more. Ideally, this difference is 0 mm, but mirror polishing is required to make the difference between the maximum convex height and the maximum concave depth 0 mm. Mirror polishing increases costs. By controlling the compressive stress, a stress relaxation effect can be obtained even if there is a difference between the maximum convex height and the maximum concave depth of the surface waviness. For this reason, the difference between the maximum convex height and the maximum concave depth of the surface waviness is preferably 1.6 mm or less, and more preferably within the range of 0.2 mm to 1 mm.

[0046] The ceramic substrate 1 may be a silicon nitride substrate, an aluminum nitride substrate, an aluminum oxide substrate, a zirconium oxide substrate, or the like. Ceramic substrates are classified based on the component they contain most abundantly. For example, a silicon nitride substrate contains silicon nitride most abundantly. An aluminum nitride substrate contains aluminum nitride most abundantly. The ceramic substrate may contain a sintering aid component as needed.

[0047] The thermal conductivity of the silicon nitride substrate is 40 W / m·K or more, and can even be 80 W / m·K or more. The three-point bending strength of the silicon nitride substrate is 600 MPa or more, and can even be 700 MPa or more.

[0048] The thermal conductivity of aluminum nitride substrates is 160 W / m·K or higher, and can even be 200 W / m·K or higher. The three-point bending strength of aluminum nitride substrates is approximately 300 to 450 MPa. The three-point bending strength of aluminum oxide substrates is approximately 300 to 450 MPa, but aluminum oxide substrates are less expensive than other substrates. The thermal conductivity of aluminum oxide substrates is approximately 20 to 30 W / m·K. The three-point bending strength of zirconium oxide substrates is 500 MPa or higher, but the thermal conductivity is approximately 20 to 30 W / m·K.

[0049] The ceramic substrate 1 is preferably a silicon nitride substrate. Silicon nitride substrates have both high strength and high thermal conductivity. On the other hand, silicon nitride substrates are mainly composed of elongated silicon nitride crystal grains with an aspect ratio of 2 or more. Aluminum nitride crystal grains, aluminum oxide crystal grains, and zirconium oxide crystal grains are mainly round crystal grains with an aspect ratio of less than 2.

[0050] In silicon nitride substrates, high strength is achieved by the complex entanglement of elongated silicon nitride crystal grains. Conventionally, the structure of elongated silicon nitride crystal grains intricately entangled results in random directionality of compressive stress, resulting in variations in TCT characteristics. With the silicon nitride substrate according to the embodiment, the compressive stress is controlled in each direction, thereby achieving high strength and high thermal conductivity while improving TCT characteristics.

[0051] The thickness of the ceramic substrate 1 is preferably in the range of 0.2 mm to 3 mm. If the thickness is less than 0.2 mm, the insulating properties of the ceramic substrate 1 may be insufficient. If the thickness exceeds 3 mm, the insulating properties of the ceramic substrate 1 may be good, but the heat dissipation properties may be reduced.

[0052] Since silicon nitride substrates have high strength, the thickness of the substrate can be set within the range of 0.2 mm to 0.8 mm, and further within the range of 0.2 mm to 0.4 mm.

[0053] The length of the short side of the ceramic substrate 1 may be 100 mm or more. Even if the substrate is large and the short side is 100 mm or more, the characteristics can be improved by controlling the compressive stress. The length of the long side is not particularly limited, but is preferably 300 mm or less. If the length of the long side exceeds 300 mm, it may be difficult to control the difference between the maximum convex height and the maximum concave depth of the surface waviness.

[0054] A large substrate may be divided into multiple pieces. Multi-piece production is a method of dividing a large substrate by scribing or the like to obtain multiple ceramic substrates. Multi-piece production may also be performed on a bonded body in which metal plates are bonded. In the ceramic substrate 1 according to the embodiment, the compressive stress in each of the arbitrary orthogonal directions is controlled. Whether it is a large substrate or multiple ceramic substrates after division, the compressive stress in each of the arbitrary orthogonal directions is controlled.

[0055] In the multi-piece production, a large rectangular substrate is divided. By controlling the compressive stress A in the long side direction of the large substrate and the compressive stress B in the short side direction of the large substrate, the residual stress of each ceramic substrate after division can be controlled. According to the embodiment, a large substrate suitable for multi-piece production can be provided.

[0056] A ceramic circuit substrate can be obtained by providing a circuit portion on the ceramic substrate according to the embodiment. Fig. 3 is a side view showing an example of a ceramic circuit substrate according to the embodiment. In Fig. 3, reference numeral 1 denotes a ceramic substrate, reference numeral 4 denotes a metal plate (front metal plate), reference numeral 5 denotes a metal plate (back metal plate), reference numeral 6 denotes a bonding layer, and reference numeral 10 denotes a ceramic circuit substrate.

[0057] In the example shown in FIG. 3 , metal plates 4 and 5 are bonded to ceramic substrate 1 via bonding layer 6. Metal plate 4 is used as a circuit portion, and metal plate 5 is used as a heat sink. Two metal plates 4 are bonded to ceramic substrate 1. The number of metal plates 4 is not limited to two, and any required number may be provided. The metal plates on both sides of ceramic substrate 1 may each be used as a circuit portion. Here, for convenience of explanation, metal plate 4 may be referred to as a front metal plate, and metal plate 5 as a back metal plate. Furthermore, in the ceramic circuit board according to the embodiment, the circuit portion may be a metallized layer or a thin film instead of a metal plate.

[0058] The metal plate is, for example, a copper plate (including a copper alloy) or an aluminum plate (including an aluminum alloy). If necessary, the metal plate may be bonded to the ceramic substrate 1 via a bonding layer. For example, the bonding layer is formed by an active metal bonding method using an active metal. When bonding a copper plate by the active metal bonding method, an active metal brazing material containing an active metal such as Ti is used. When bonding an aluminum plate, an active metal brazing material containing an active metal such as Si is used.

[0059] The metallized layer is formed by applying a metal paste onto the ceramic substrate 1 and firing the paste. The metallized layer is mainly composed of one selected from the group consisting of Ag (silver), Cu (copper), Mo (molybdenum), and W (tungsten). The thin film is conductive and is formed by sputtering or vapor deposition.

[0060] The circuit section is preferably a metal plate. The thickness of the metal plate is, for example, 0.2 mm or more. The thickness of the metal plate is preferably 0.6 mm or more. By increasing the thickness of the metal plate, it is possible to improve the current carrying capacity and heat dissipation properties.

[0061] The circuit portion is formed of a metal plate, a metallized layer, a thin film, or the like. When TCT is performed, the metal plate generates the largest stress. In particular, when a metal plate with a thickness of 0.6 mm or more is used, a larger stress is generated. In the ceramic substrate 1 according to the embodiment, since the compressive stress is controlled, the TCT characteristics are good even when a metal plate with a thickness of 0.6 mm or more is bonded.

[0062] The upper limit of the thickness of the metal plate is not particularly limited, but is preferably 5 mm or less. If the thickness of the metal plate exceeds 5 mm, it may be difficult to impart a pattern shape to the metal plate. Therefore, the thickness of the metal plate is preferably in the range of 0.2 mm to 5 mm, and more preferably in the range of 0.6 mm to 2 mm.

[0063] 4(a) and 4(b) are plan views showing an example of a ceramic circuit substrate according to an embodiment, in which reference numeral 1 denotes a ceramic substrate, reference numeral 2 denotes a long side direction, reference numeral 3 denotes a short side direction, reference numeral 4 denotes a metal plate (front metal plate), reference numeral 6 denotes a bonding layer, and reference numeral 10 denotes a ceramic circuit substrate.

[0064] 4(a), the length of the metal plate 4 in the long side direction 2 is longer than the length of the metal plate 4 in the short side direction 3. When the metal plate 4 expands, the tensile stress in the long side direction 2 is greater than the tensile stress in the short side direction 3. In this case, the ceramic substrate 1 has a greater compressive stress in the long side direction 2 than in the short side direction 3.

[0065] 4(b), the length of the metal plate 4 in the short side direction 3 is longer than the length of the metal plate 4 in the long side direction 2. When the metal plate 4 expands, the tensile stress in the short side direction 3 is greater than the tensile stress in the long side direction 2. In this case, the ceramic substrate 1 has a greater compressive stress in the short side direction 3 than in the long side direction 2.

[0066] 4(a) and 4(b), the bonding layer 6 has a protruding portion. The protruding portion is a portion that protrudes outward from the end of the metal plate 4 and does not overlap with the metal plate 4 in the thickness direction of the substrate. Although the protruding portion can be omitted, the TCT characteristics can be further improved by having the bonding layer 6 have a protruding portion.

[0067] A method for manufacturing the ceramic substrate 1 according to the embodiment will be described. The ceramic substrate according to the embodiment may be manufactured by any method as long as it has the above-described configuration. Here, a method for obtaining the ceramic substrate 1 with a high yield will be described.

[0068] First, a ceramic substrate is prepared. The ceramic substrate may be produced by an existing technique, or a commercially available ceramic substrate may be used. A step of applying compressive stress to the surface of the ceramic substrate is carried out. The compressive stress is applied by processing or heat treatment. The processing is, for example, dry blasting or surface polishing.

[0069] Dry blasting is a method of processing the surface of a ceramic substrate by spraying abrasive grains onto the surface. Alumina abrasive grains can be used for dry blasting. For example, the grain size of the abrasive grains is in the range of 10 μm to 0.2 mm, and the spray pressure of the abrasive grains is in the range of 0.1 MPa to 1 MPa. The larger the grain size of the abrasive grains or the higher the spray pressure of the abrasive grains, the larger the compressive stress applied (the larger the negative value). The smaller the grain size or the lower the spray pressure of the abrasive grains, the smaller the compressive stress applied (the smaller the negative value).

[0070] Adjusting the angle at which the abrasive grains collide with the surface of the ceramic substrate is also effective in controlling the compressive stress. In a typical dry blasting process, the abrasive grains collide with the surface of the ceramic substrate from a 90° angle (perpendicular direction). In order to control the compressive stress, it is effective to collide the abrasive grains with the surface of the ceramic substrate from an oblique direction.

[0071] The oblique direction refers to the direction in which the abrasive grains are sprayed relative to the surface of the ceramic substrate, within the range of 20° to 70°. The larger the angle of the abrasive grains relative to the surface, the more likely it is that areas with high compressive stress (large negative value) will be formed. Here, colliding the abrasive grains with the ceramic substrate from an oblique direction is referred to as obliquely colliding the abrasive grains.

[0072] 5(a) and 5(b) are side views showing an example of dry blasting, in which reference numeral 1 denotes a ceramic substrate, reference numeral 2 denotes a direction parallel to the long side, reference numeral 3 denotes a direction parallel to the short side, and reference numeral 9 denotes abrasive grains.

[0073] 5(a) shows a state in which abrasive grains 9 are collided obliquely with respect to the long side direction 2 of the ceramic substrate 1. In the process shown in FIG. 5(a), the angle θ1 between the long side direction 2 and the direction of the abrasive grains 9 is controlled to be 20° or more and 70° or less on a plane parallel to the long side direction 2 and the thickness direction. This process imparts compressive stress to the surface of the ceramic substrate 1 in the long side direction 2.

[0074] 5(b) shows a state in which abrasive grains 9 are collided obliquely with the short side direction 3 of the ceramic substrate 1. In the process shown in FIG. 5(b), the angle θ2 between the short side direction 3 and the direction of the abrasive grains 9 is controlled to be 20° or more and 70° or less in a plane parallel to the short side direction 3 and the thickness direction. This process imparts compressive stress to the surface of the ceramic substrate 1 in the short side direction 3.

[0075] By adjusting the angles θ1 and θ2, the ratio of compressive stress A to compressive stress B can be controlled. As mentioned above, angle adjustment may be combined with adjustment of the jetting pressure. When the abrasive grain jetting direction is constant when applying compressive stress in the long side direction 2 or the short side direction 3, it is preferable to perform dry blasting while transporting the ceramic substrate. By passing the ceramic substrate through the area where the abrasive grains are jetted while maintaining a constant jetting direction, the abrasive grains can be impinged obliquely on the ceramic substrate. For example, if the relative positions of the jet nozzle and the ceramic substrate are fixed and the area where the abrasive grains are jetted is small relative to the ceramic substrate, it is necessary to change the direction of the jet nozzle while applying the abrasive grains to the entire surface of the ceramic substrate. In this case, there may be times when the jetting direction is not oblique while the jet nozzle is being moved, or the angle at which the abrasive grains hit the surface of the ceramic substrate may vary. Therefore, to apply compressive stress uniformly across the entire surface of the ceramic substrate, it is preferable to set the abrasive grain jetting direction to a constant value and apply the abrasive grains obliquely to the ceramic substrate while transporting the ceramic substrate.

[0076] In addition, either or both of conveyance for applying compressive stress in the long side direction and conveyance for applying compressive stress in the short side direction may be used. Note that a belt conveyor or the like may be used for the conveyance process. By controlling the conveyance speed, the angle at which the abrasive grains collide can be controlled.

[0077] Surface polishing is a method of surface processing using a grindstone. The larger the grain size of the grindstone used in surface polishing, the greater the compressive stress applied. The smaller the grain size of the grindstone, the smaller the compressive stress applied.

[0078] For example, when polishing is performed along the long side direction 2 of the ceramic substrate 1, compressive stress can be applied in the long side direction 2. When polishing is performed along the short side direction 3 of the ceramic substrate 1, compressive stress can be applied in the short side direction 3. It is also effective to polish both the long side direction 2 and the short side direction 3. Furthermore, by alternately polishing the long side direction 2 and the short side direction 3, it becomes easier to control the ratio of compressive stresses.

[0079] The particle sizes of the abrasive grains used in dry blasting and the grinding stones used in surface polishing are specified in JIS-R-6001-1 and JIS-R-6001-2. JIS-R-6001-1 corresponds to ISO 8486-1. JIS-R-6001-2 corresponds to ISO 8486-2.

[0080] When compressive stress is applied by heat treatment, the pressure during the heat treatment is controlled. For example, the ceramic substrate 1 is manufactured by mixing a ceramic powder as a base material with a sintering aid powder, molding, degreasing, and sintering the mixture. The sintering process is performed at a temperature ranging from 1500°C to 2000°C. If the pressure during the sintering process is set high, the compressive stress can be increased. If the pressure during the sintering process is set low, the compressive stress can be decreased.

[0081] It is also effective to perform a reheat treatment while applying pressure to the ceramic substrate 1 obtained by the sintering process. The temperature of the reheat treatment is, for example, in the range of 1000°C to 2000°C. In the reheat treatment, if the pressure is set high, the compressive stress can be increased. If the pressure is set low, the compressive stress can be decreased.

[0082] The pressure in the sintering step or reheat treatment is preferably 0.3 MPa or more. For example, if the pressure is 0.5 MPa or less, the compressive stress can be reduced. If the pressure exceeds 0.5 MPa, the compressive stress can be increased. There is no particular upper limit to the pressure, but it is preferably 2 MPa or less. If the pressure exceeds 2 MPa, the load on the manufacturing equipment may increase.

[0083] Adjusting the cooling rate after the sintering step is also effective in controlling the compressive stress. By increasing the cooling rate, the applied compressive stress can be increased. By decreasing the cooling rate, the applied compressive stress can be decreased. Similarly, in the re-heat treatment, by increasing the cooling rate, the applied compressive stress can be increased. By decreasing the cooling rate, the applied compressive stress can be decreased. For example, if the cooling rate is 120°C / hour or more, the compressive stress can be increased. If the cooling rate is less than 120°C / hour, the compressive stress can be decreased.

[0084] As explained above, methods for controlling compressive stress include blasting, surface processing, pressure adjustment in the sintering process, pressure adjustment in the reheating process, adjustment of the cooling rate in the sintering process, adjustment of the cooling rate in the reheating process, etc. These methods may be used alone or in combination.

[0085] When multiple pieces are to be obtained, these methods may be performed on the large substrate before division, or on the ceramic substrate after division. By using at least one of these methods, the compressive stress is controlled to be within the range of -250 MPa or more and -60 MPa or less, and the ratio A / B is controlled.

[0086] The blasting treatment, surface processing treatment, and reheating treatment are performed on the ceramic substrate after the sintering process. By measuring the residual stress of the ceramic substrate and then performing any of these treatments, it becomes easier to control the compressive stress in each direction. Therefore, when manufacturing the ceramic substrate 1 according to the embodiment, it is preferable to perform one or more treatments selected from the blasting treatment, surface processing treatment, and reheating treatment.

[0087] In particular, the blasting treatment and the surface treatment are effective for varying the compressive stress in each direction on the surface of the ceramic substrate. Therefore, when manufacturing the ceramic substrate 1 according to the embodiment, it is preferable to perform one or more treatments selected from the blasting treatment and the surface treatment.

[0088] FIG. 6 is a flowchart showing an example of a method for manufacturing a ceramic substrate according to an embodiment. First, a sintering process is performed to prepare a ceramic substrate (step S1). In the sintering process, pressure may be adjusted to control residual stress. Next, the ceramic substrate is subjected to one or more treatments selected from blasting, surface processing, and reheating (step S2). Before step S2, the residual stress on the surface of the ceramic substrate 1 may be measured to determine whether or not to perform step S2. The conditions in step S2 may be adjusted depending on the residual stress. When multiple treatments are performed in step S2, the residual stress may be measured between treatments. Depending on the residual stress, it may be determined whether or not to perform a subsequent treatment, or the conditions for the subsequent treatment may be adjusted.

[0089] A ceramic circuit board is produced by forming a circuit portion made of a metal plate, a metallized layer, or a thin film on a ceramic substrate 1 to which a predetermined compressive stress has been applied. If necessary, a circuit shape may be imparted to the metal plate, metallized layer, or thin film by an etching process or the like. An overhanging portion may be formed in the bonding layer by the etching process. After the circuit portion is formed, the ceramic substrate 1 may be divided into multiple pieces. Furthermore, a semiconductor device can be formed by mounting semiconductor elements on the ceramic circuit board.

[0090] (Examples 1 to 7, Comparative Examples 1 to 3) Silicon nitride substrates, aluminum nitride substrates, and aluminum oxide substrates were prepared as ceramic substrates. The silicon nitride substrate had a thermal conductivity of 90 W / m·K and a three-point bending strength of 700 MPa. The aluminum nitride substrate had a thermal conductivity of 170 W / m·K and a three-point bending strength of 370 MPa. The aluminum oxide substrate had a thermal conductivity of 25 W / m·K and a three-point bending strength of 400 MPa.

[0091] After measuring the compressive stress of the ceramic substrate, a process for controlling the compressive stress was performed. Specifically, a blasting treatment or a surface processing treatment was performed to control the compressive stress. In Examples 1, 3, and 4, the surface of the ceramic substrate was blasted from an oblique direction (within the range of 20° to 70°). In Examples 2 and 5, the surface was polished alternately in the long and short side directions. In Example 6, a blasting treatment was performed from an oblique direction (within the range of 20° to 70°) followed by a reheating treatment. In Example 7, the surface was polished alternately in the long and short side directions followed by a reheating treatment. For the reheating treatment in Examples 6 and 7, the residual stress on the surface of the ceramic substrate was measured before the heat treatment, and the heat treatment conditions were selected based on the measurement results. If the residual stress was low, the reheating treatment was performed at a high pressure or a fast cooling rate. If the residual stress was high, the reheating treatment was performed at a low pressure or a slow cooling rate.

[0092] In Comparative Example 1, no surface treatment was performed on the ceramic substrate. In Comparative Example 2, blasting was performed in a direction perpendicular to the surface of the ceramic substrate (at an angle of 90°). In Comparative Example 3, polishing was performed only along the long side direction. The conditions in the examples and comparative examples are shown in Table 1.

[0093]

[0094] The compressive stress on the surface of the obtained ceramic substrate was measured. The compressive stress measurement method was as described above. The compressive stress in the long side direction was designated "compressive stress A," and the compressive stress in the short side direction was designated "compressive stress B." In addition, the maximum height Sz and the difference between the maximum convex height and the maximum concave depth of the surface waviness were measured. The respective measurement methods were as described above. The results are shown in Table 2.

[0095]

[0096] As can be seen from Table 2, in the ceramic substrates according to the examples, the compressive stresses A and B were within the range of -250 MPa or more and -60 MPa or less. The compressive stresses A and B were different, and the ratio A / B was 4 / 6 or less and 6 / 4 was satisfied. The ratio B / A was also 4 / 6 or less and 6 / 4 was satisfied. In addition, in the ceramic substrates according to the examples, the difference between the maximum convex height and the maximum concave depth of the surface waviness was 1.6 mm or less. The maximum height Sz was 60 μm or less.

[0097] In contrast, in Comparative Example 1, the ratio A / B was 1. The ratio A / B = 1 indicates that the ratio A / B was within the range of 0.995 to 1.004. In Comparative Examples 2 and 3, one of the compressive stresses A and B was outside the preferred range, and the ratio A / B was outside the preferred range.

[0098] Next, the ceramic substrate according to the example was scribed and divided into a plurality of ceramic substrates. In the division process for obtaining multiple pieces, the large substrate was divided into three equal parts along the long side and into two equal parts along the short side, thereby obtaining six ceramic substrates.

[0099] The compressive stress, maximum convex height, maximum concave depth, and maximum height Sz were measured for each of the divided ceramic substrates. As a result, in the examples, the compressive stress A and compressive stress B were within the range of -250 MPa or more and -60 MPa or less for all ceramic substrates. The compressive stresses A and B were different from each other, and the ratio A / B was 4 / 6 or less and 6 / 4 was satisfied. In addition, the difference between the maximum convex height and maximum concave depth of the surface waviness was 1.6 mm or less. The maximum height Sz was 60 μm or less. From the examples, it was found that measurement results similar to those of a large substrate could be obtained even when multiple pieces were obtained.

[0100] Next, metal plates were bonded to both sides of the ceramic substrates according to the examples and comparative examples. The ceramic substrates were used after multiple-cavity processing. Copper plates were used as the metal plates. Three types of copper plates with thicknesses of 0.3 mm, 0.6 mm, and 1.0 mm were prepared. An Ag-Cu-Sn-Ti-based active metal brazing material was used as the brazing material. Each copper plate was subjected to an etching process to produce a ceramic circuit board. The shape of the copper plate after etching was rectangular in plan view.

[0101] The TCT characteristics of each ceramic circuit substrate were investigated. For silicon nitride circuit substrates, 3,000 cycles were performed, with one cycle consisting of -30°C x 30 minutes, room temperature x 10 minutes, 170°C x 30 minutes, and room temperature x 10 minutes. For aluminum nitride circuit substrates and aluminum oxide circuit substrates, 3,000 cycles were performed, with one cycle consisting of -30 x 30 minutes, room temperature x 10 minutes, 125°C x 30 minutes, and room temperature x 10 minutes. For each of the examples and comparative examples, the TCT was performed on 100 ceramic circuit substrates.

[0102] After TCT, the presence or absence of cracks in the ceramic substrate or bonding layer was measured. The presence or absence of cracks was measured using ultrasonic flaw detection (SAT). Cases where the crack occurrence was 0% or more and 1% or less were rated as best (circle), cases where the crack occurrence was more than 1% and 3% or less were rated as good (triangle), and cases where the crack occurrence was more than 3% were rated as poor (cross). The results are shown in Table 3.

[0103]

[0104] For silicon nitride circuit boards, the TCT characteristics were good even when the copper plate thickness was increased to 0.3 mm, 0.6 mm, and 1.0 mm. For Example 4 (aluminum nitride circuit board) and Example 5 (aluminum oxide circuit board), the TCT characteristics were good at a thickness of 0.3 mm. Because the strength of aluminum nitride substrates and aluminum oxide substrates is low, the TCT characteristics deteriorated at thicknesses of 0.6 mm or more.

[0105] In contrast, in Comparative Examples 1 to 3, many defects occurred when the copper plate thickness was 0.6 mm or more. It was found that when the copper plate became thicker and the stress increased, the effect of stress relaxation was insufficient, resulting in defects. For this reason, it can be said that the Examples are suitable for ceramic circuit boards using copper plates with a thickness of 0.6 mm or more.

[0106] Embodiments of the present invention include the following features. (Feature 1) A ceramic substrate having different compressive stress values ​​in a first direction and a second direction parallel to the surface and perpendicular to each other, where the compressive stress in the first direction is compressive stress A and the compressive stress in the second direction is compressive stress B, and the relationship 4 / 6≦compressive stress A / compressive stress B≦6 / 4 is satisfied. (Feature 2) The ceramic substrate according to Feature 1, wherein the compressive stress A and the compressive stress B are within a range of −250 MPa or more and −60 MPa or less. (Feature 3) The ceramic substrate according to Feature 1 or Feature 2, wherein the maximum height Sz of the surface is 60 μm or less. (Feature 4) The ceramic substrate according to any one of Features 1 to 3, wherein the difference between the maximum convex height and the maximum concave depth of the surface waviness on the surface is 1.6 mm or less. (Feature 5) The ceramic substrate according to any one of Features 1 to 4, wherein the surface is quadrangular, wherein the first direction is parallel to one side, and the second direction is parallel to another side. (Feature 6) The ceramic substrate according to Feature 5, wherein the length of the long side is 100 mm or more. (Feature 7) The ceramic substrate according to any one of Features 1 to 6, which is a silicon nitride substrate. (Feature 8) The ceramic substrate according to any one of Features 1 to 7, wherein the thickness is in the range of 0.2 mm or more and 1 mm or less. Substrate. (Feature 9) A ceramic circuit board comprising the ceramic substrate according to any one of Features 1 to 8, and a circuit unit provided on the surface. (Feature 10) The ceramic circuit board according to Feature 9, wherein the circuit unit is a metal plate having a thickness of 0.6 mm or more. (Feature 11) The ceramic circuit board according to Feature 9, wherein the circuit unit is a metal plate having a thickness of 0.6 mm or more, the length of the metal plate in the second direction is longer than the length of the metal plate in the first direction, and the compressive stress B is greater than the compressive stress A.

[0107] Although several embodiments of the present invention have been described above, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. Modifications of these embodiments are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. Furthermore, the above-described embodiments can be implemented in combination with each other.

[0108] REFERENCE SIGNS LIST 1... ceramic substrate 2... direction parallel to long side 3... direction parallel to short side 4... metal plate (front metal plate) 5... metal plate (back metal plate) 6... bonding layer 7... first measurement point 8... second measurement point 9... abrasive grain 10... ceramic circuit substrate

Claims

1. A ceramic substrate having different compressive stress values ​​in a first direction and a second direction which are parallel to the surface and perpendicular to each other, and satisfying 4 / 6≦compressive stress A / compressive stress B≦6 / 4, where the compressive stress in the first direction is compressive stress A and the compressive stress in the second direction is compressive stress B.

2. The ceramic substrate according to claim 1, wherein the compressive stress A and the compressive stress B are within a range of -250 MPa or more and -60 MPa or less.

3. The ceramic substrate according to claim 1 or 2, wherein the maximum height Sz of the surface is 60 μm or less.

4. A ceramic substrate according to claim 1 or 2, wherein the difference between the maximum convex height and the maximum concave depth of the surface waviness on the surface is 1.6 mm or less.

5. The ceramic substrate according to claim 3, wherein the difference between the maximum convex height and the maximum concave depth of the surface waviness on the surface is 1.6 mm or less.

6. The ceramic substrate according to claim 1 or 2, having a quadrangular surface, the first direction being parallel to one side, and the second direction being parallel to another side.

7. The ceramic substrate according to claim 5, wherein the surface is rectangular, the first direction is parallel to one side, and the second direction is parallel to another side.

8. The ceramic substrate according to claim 6, wherein the length of the long side is 100 mm or more.

9. The ceramic substrate according to claim 7, wherein the length of the long side is 100 mm or more.

10. The ceramic substrate according to claim 1 or 2, which is a silicon nitride substrate.

11. The ceramic substrate according to claim 7, which is a silicon nitride substrate.

12. The ceramic substrate according to claim 9, which is a silicon nitride substrate.

13. The ceramic substrate according to claim 1 or 2, having a thickness in the range of 0.2 mm or more and 1 mm or less.

14. The ceramic substrate according to claim 7, having a thickness in the range of 0.2 mm to 1 mm.

15. The ceramic substrate according to claim 11, having a thickness in the range of 0.2 mm to 1 mm.

16. A ceramic circuit board comprising: the ceramic substrate according to claim 1; and a circuit portion provided on the surface.

17. A ceramic circuit board comprising: the ceramic substrate according to claim 7; and a circuit portion provided on the surface.

18. A ceramic circuit board comprising: the ceramic substrate according to claim 1; and a circuit portion provided on the surface.

19. The ceramic circuit board according to claim 17, wherein the circuit portion is a metal plate having a thickness of 0.6 mm or more.

20. A ceramic circuit board as described in claim 18, wherein the circuit portion is a metal plate having a thickness of 0.6 mm or more, the length of the metal plate in the second direction is longer than the length of the metal plate in the first direction, and the compressive stress B is greater than the compressive stress A.