Substrate

Abstract

This substrate comprises: a nitride ceramic substrate that includes a first main surface; a copper plate that includes a second main surface which faces the first main surface; and a bonding layer that is in contact with the first main surface and the second main surface so as to bond the nitride ceramic substrate and the copper plate to each other, contains titanium and / or zirconium, and does not contain silver, indium, or tin. A plurality of linear voids that extend from the first main surface to the second main surface are formed in the bonding layer. With respect to a cross-section parallel to a first direction, which is the direction in which the first main surface and the second main surface face each other, the average pitch of the plurality of linear voids in a second direction, which is the direction orthogonal to the first direction, is 14 μm to 25 μm inclusive, the average number of linear voids in a region of 500 μm in the second direction is 18 to 37 inclusive, and the average number of linear voids in a region of 100 μm in the second direction is 4 to 7 inclusive.

Description

Substrate

[0001] The present disclosure relates to a substrate. This application claims priority based on Japanese Patent Application No. 2025-053042 filed on March 27, 2025, and incorporates all the contents described in the above Japanese application by reference.

[0002] There is known a substrate including a ceramic substrate, a copper plate, and a bonding layer that bonds the ceramic substrate and the copper plate (see, for example, Patent Document 1 below). In the substrate described in Patent Document 1, the ceramic substrate contains silicon nitride, and the bonding layer contains titanium.

[0003] Japanese Unexamined Patent Publication No. 2022-56203

[0004] The coefficient of thermal expansion of a ceramic substrate and the coefficient of thermal expansion of a copper plate are different, and there is a difference between them. Therefore, when a thermal cycle is applied to the substrate, there is a problem that the copper layer peels off from the ceramic substrate due to the above-mentioned difference in coefficient of thermal expansion.

[0005] The present disclosure provides a substrate that can prevent the copper plate from peeling off from the nitride ceramic substrate even when a thermal cycle is applied to the substrate.

[0006] A substrate according to the present disclosure includes: a nitride ceramic substrate including a first main surface; a copper plate including a second main surface facing the first main surface; and a bonding layer that is in contact with the first main surface and the second main surface to bond the nitride ceramic substrate and the copper plate, contains at least one of titanium and zirconium, and does not contain silver, indium or tin. A plurality of linear voids extending from the first main surface to the second main surface are formed in the bonding layer. Regarding a cross section parallel to a first direction that is a direction in which the first main surface and the second main surface face each other, an average pitch of the plurality of linear voids in a second direction orthogonal to the first direction is 14 μm or more and 25 μm or less, an average number of the linear voids in a 500 μm region in the second direction is 18 or more and 37 or less, and an average number of the linear voids in a 100 μm region in the second direction is 4 or more and 7 or less.

[0007] The substrate according to the present disclosure can prevent the copper plate from peeling off from the nitride ceramic substrate even when a thermal cycle is applied to the substrate.

[0008] Figure 1 is a plan view of an embodiment of a substrate according to the present disclosure. Figure 2 is a cross-sectional view of the substrate shown in Figure 1. Figure 3 is an enlarged view of the bonding layer in the substrate shown in Figure 2.

[0009] [Summary of Embodiments] A substrate according to the present disclosure comprises a nitride ceramic substrate including a first main surface, a copper plate including a second main surface facing the first main surface, and a bonding layer that contacts the first and second main surfaces to bond the nitride ceramic substrate and the copper plate, and contains at least one of titanium and zirconium, but does not contain silver, indium, or tin. A plurality of linear voids are formed in the bonding layer from the first main surface to the second main surface. With respect to a cross section parallel to the first direction, which is the direction in which the first and second main surfaces face each other, the average pitch of the plurality of linear voids in the second direction, which is the direction perpendicular to the first direction, is 14 μm or more and 25 μm or less, the average number of linear voids in a 500 μm region in the second direction is 18 or more and 37 or less, and the average number of linear voids in a 100 μm region in the second direction is 4 or more and 7 or less.

[0010] If the average pitch of multiple linear voids in the second direction falls below 14 μm, or if the average number of linear voids in the 500 μm region exceeds 37, or if the average number of linear voids in the 100 μm region exceeds 7, the linear voids in the bonding layer become overcrowded. As a result, the area partitioned by linear voids in the bonding layer becomes too small. Therefore, when the substrate is subjected to thermal cycling, the bond strength between the nitride ceramic substrate and the bonding layer decreases due to the stress on the bonding layer caused by the difference in thermal expansion coefficients between the nitride ceramic substrate and the copper plate. Consequently, the copper plate may peel off from the nitride ceramic substrate.

[0011] In contrast, in this substrate, the average pitch of multiple linear voids is 14 μm or more, the average number of linear voids in the 500 μm region is 37 or less, and the average number of linear voids in the 100 μm region is 7 or less. Therefore, the density of the areas partitioned by linear voids in the bonding layer, and the average number of such areas, are appropriate. As a result, when the substrate is subjected to thermal cycling, the stress on the bonding layer is relieved in each of the multiple areas. Consequently, it is possible to prevent the copper plate from peeling off the nitride ceramic substrate.

[0012] On the other hand, if the average pitch of multiple linear voids exceeds 25 μm, or if the average number of linear voids in the 500 μm region falls below 18, or if the average number of linear voids in the 100 μm region falls below 4, the linear voids in the bonding layer become sparse. As a result, the aforementioned portion becomes excessive. Therefore, when the substrate is subjected to thermal cycling, the stress on the bonding layer, caused by the difference in thermal expansion coefficients between the nitride ceramic substrate and the copper plate, is not relieved in each of the multiple portions. Consequently, the copper plate may peel off from the nitride ceramic substrate.

[0013] In contrast, in this substrate, the average pitch of multiple linear voids is 25 μm or less, the average number of linear voids in the 500 μm region is 18 or more, and the average number of linear voids in the 100 μm region is 4 or more. Therefore, the density and average number of linear voids in the bonding layer are appropriate. As a result, when the substrate is subjected to thermal cycling, the stress on the bonding layer caused by the difference in thermal expansion coefficients between the nitride ceramic substrate and the copper plate is relieved in each of the multiple parts. Consequently, it is possible to prevent the copper plate from peeling off the nitride ceramic substrate.

[0014] In particular, since the average pitch of the linear voids and the average number of linear voids in the 500 μm and 100 μm regions are within the above-mentioned ranges, multiple linear voids exist in the cross-section with little variation in the second direction, i.e., in a well-dispersed state. Therefore, even when the substrate is subjected to thermal cycling in the area partitioned by the linear voids, the stress due to the difference in thermal expansion coefficients between the nitride ceramic substrate and the copper plate can be relieved in a balanced manner. As a result, the delamination of the copper plate from the nitride ceramic substrate can be reliably prevented.

[0015] In the substrate described above, the nitride in the nitride ceramic substrate may be silicon nitride. A nitride ceramic substrate in which the nitride is silicon nitride exhibits excellent rigidity, thereby increasing the strength of the substrate. On the other hand, the difference in thermal expansion coefficients between a nitride ceramic substrate in which the nitride is silicon nitride and a copper plate is greater than the difference in thermal expansion coefficients between a nitride ceramic substrate in which the nitride is aluminum nitride and a copper plate. Therefore, when a thermal cycle is applied to a substrate in which the nitride is silicon nitride, the copper layer tends to peel off from the nitride ceramic substrate due to the large thermal expansion coefficient. However, in this substrate, since the average pitch of linear voids and the average number of linear voids in the 500 μm and 100 μm regions are within the range described above, the stress on the bonding layer is relieved in each of the multiple parts, preventing the copper layer from peeling off from the hard substrate.

[0016] [Specific Examples of Embodiments] Specific embodiments of the substrate according to this disclosure will be described with reference to Figures 1 to 3. Figure 1 is a plan view of an embodiment of the substrate according to this disclosure. Figure 2 is a cross-sectional view of the substrate shown in Figure 1. Figure 2 is a cross-sectional view of the substrate shown in Figure 1 along line XX. Figure 3 is an enlarged view of the bonding layer in the substrate shown in Figure 2.

[0017] [Basic configuration of substrate 1] As shown in Figure 1, substrate 1 has a plate shape. Substrate 1 has, for example, a rectangular plate shape. As shown in Figure 2, substrate 1 has a thickness. The thickness of substrate 1 is 0.2 mm or more and 5 mm or less, and also 0.5 mm or more and 2 mm or less. Substrate 1 comprises a nitride ceramic substrate 2, two copper plates 3, and two bonding layers 4. In the thickness direction TD, the copper plates 3, bonding layers 4, nitride ceramic substrate 2, bonding layers 4, and copper plates 3 are arranged in this order.

[0018] [Nitride Ceramic Substrate 2] As shown in Figure 1, the nitride ceramic substrate 2 has, for example, a rectangular plate shape. As shown in Figure 2, the nitride ceramic substrate 2 includes two first main surfaces 21. Each of the two first main surfaces 21 is a main surface perpendicular to the thickness direction TD. Examples of nitrides in the nitride ceramic substrate 2 include silicon nitride and aluminum nitride. These can be used alone or in combination. That is, the nitride ceramic substrate 2 includes at least one of silicon nitride and aluminum nitride. If the nitride is silicon nitride, the mechanical strength of the nitride ceramic substrate 2 can be increased. If the nitride is aluminum nitride, the thermal conductivity of the nitride ceramic substrate 2 can be increased. The thickness of the nitride ceramic substrate 2 is 0.1 mm or more and 1 mm or less, and may be 0.2 mm or more and 0.5 mm or less.

[0019] [Copper Plate 3] As shown in Figure 1, each of the two copper plates 3 has, for example, a rectangular plate shape. Each of the two copper plates 3 has a shape smaller than the nitride ceramic substrate 2 when viewed in the thickness direction TD. As shown in Figure 2, each of the two copper plates 3 includes a second main surface 31. The second main surface 31 faces the first main surface 21 in the first direction D1. The first direction D1 corresponds to the thickness direction TD. The second main surface 31 is located away from the first main surface 21 in the thickness direction TD. The second main surface 31 is a main surface perpendicular to the first direction D1. The thickness of the copper plate 3 is 0.1 mm or more and 3 mm or less, and may be 0.3 mm or more and 2 mm or less.

[0020] [Bonding Layer 4] Each of the two bonding layers 4 contacts the first main surface 21 and the second main surface 31 to bond the nitride ceramic substrate 2 and the two copper plates 3, respectively. The bonding layer 4 may contact a portion of the first main surface 21 or the entire first main surface 21. In this disclosure, the bonding layer 4 contacts a portion of the first main surface 21 and the entire second main surface 31. The bonding layer 4 extends in a direction perpendicular to the first direction D1. The bonding layer 4 contains at least one of titanium and zirconium. Titanium and zirconium are the main components in the bonding layer 4. On the other hand, the bonding layer 4 may further contain a migration component in addition to the main component. The migration component includes one selected from the group consisting of nitrogen, silicon, aluminum, and copper. The migration component is a component that migrates from at least one of the nitride ceramic substrate 2 and the copper plates 3 to the bonding layer 4 during bonding by heating (described later).

[0021] On the other hand, the bonding layer 4 does not contain silver, indium, or tin. Since silver, indium, and tin are the main components of the brazing material, the bonding layer 4 is brazing material-free. However, the bonding layer 4 may contain at least one selected from the group consisting of silver, indium, and tin as an unavoidable impurity. The proportion of unavoidable impurities in the bonding layer 4 is 0.03% by mass or less. The proportion of silver in the bonding layer 4 is 0.01% by mass or less. The proportion of indium in the bonding layer 4 is 0.01% by mass or less. The proportion of tin in the bonding layer 4 is 0.01% by mass or less. The thickness of the bonding layer 4 is 0.1 μm or more and 20 μm or less. The bonding layer 4 may be a single layer or a multi-layered layer.

[0022] [Linear voids 5] Multiple linear voids 5 are formed in each of the two joining layers 4. The two joining layers 4 include the first joining layer 4A and the second joining layer 4B. Below, the details of the first joining layer 4A shown in Figure 3 will be described, but the structure of the second joining layer 4B is the same as that of the first joining layer 4A, so its description will be omitted. Each of the multiple linear voids 5 extends from the first main surface 21 to the second main surface 31. The linear voids 5 extend along the first direction D1. The linear voids 5 in cross-section may have a shape that includes multiple bending points. The multiple bending points are aligned in the first direction D1. The linear voids 5 may have a portion that is inclined with respect to the first direction D1. Also, the linear voids 5 in cross-section may have a straight shape. A linear void 5 is a space (gap) in cross-section that does not contain either the main component or the transitional component of the bonding layer 4 as described above. Among a plurality of linear voids 5, adjacent linear voids 5 are located at positions far apart from each other in the second direction D2. The second direction D2 is perpendicular to the first direction D1. As shown in Figure 1, in this disclosure, the linear void 5 may extend in a direction intersecting (perpendicular to) the XX line. The linear void 5 may also be a through hole penetrating the bonding layer 4 in the first direction D1. The width W of the linear void 5 is 1 nm or more, may be 10 nm or more, may be 1000 nm or less, or may be 100 nm or less.

[0023] [Average pitch and number of linear voids 5] With respect to a cross section parallel to the first direction D1, the average pitch of multiple linear voids 5 in the second direction D2 is 14 μm or more and 25 μm or less. The average pitch is the average of the pitch P, which is the distance between adjacent linear voids 5 in the cross section. The average pitch is also the average of the distance (pitch P) between the starting points 51 of adjacent linear voids 5 on the first main surface 21. The average pitch of multiple linear voids 5 may be 17 μm or more and 22 μm or less. If the average pitch of multiple linear voids 5 is within the above range, the density of the portions 52 partitioned by the linear voids 5 in the cross section and the average number of portions 52 will be appropriate. Therefore, when a thermal cycle is applied to the substrate 1, the stress on the bonding layer 4 is relieved by the portions 52.

[0024] With respect to a cross section parallel to the first direction D1, in the first region which is a 500 μm area in the second direction D2, the average number of linear voids 5 is 18 or more and 37 or less. With respect to a cross section parallel to the first direction D1, in the second region which is a 100 μm area in the second direction, the average number of linear voids 5 is 4 or more and 7 or less. In the above cross section, the second region may be within the first region or outside the first region. A part of the second region may overlap with a part of the first region. The cross section relating to the first region and the cross section relating to the second region may be the same or different from each other. The average number of linear voids 5 in the first region relating to one cross section may be 18 or more and 37 or less, and the average number of linear voids 5 in the second region relating to a different cross section may be 4 or more and 7 or less. In the first region, the average number of linear voids 5 may be 21 or more and 33 or less. In the second region, the average number of linear voids 5 may be 5 or more and 6 or less.

[0025] If the average number of linear voids 5 in the first region and the average number of linear voids 5 in the second region are within the above-described range, the density of the portion 52 and the average number of portions 52 will be appropriate. Therefore, when a thermal cycle is applied to the substrate 1, the stress on the bonding layer 4 is relieved in each of the multiple portions 52. As a result, even when a thermal cycle is applied to the substrate 1, the copper plate 3 can be prevented from peeling off from the nitride ceramic substrate 2.

[0026] As shown in Figure 1, the linear void 5 is identified by cutting along the second direction D2 of the substrate 1, and observing the central portion 25 and both ends 26, 27 of the cut surface (cross section) using a scanning electron microscope (SEM). In this disclosure, for example, the substrate 1 is cut along a line connecting two corners in the rectangle of the bonding layer 4, as shown by line XX in Figure 1. This identifies the linear void 5. Subsequently, the pitch P of the linear void 5 is determined at each of the central portion 25 and both ends 26, 27, and the average of the pitches P at the three locations (25, 26, 27) is obtained as the average pitch. Each of the two ends 26, 27 is a region that includes each of the two corners of the bonding layer 4. The central portion 25 is a region that includes the midpoint of both ends 26, 27.

[0027] Furthermore, the average number of linear voids 5 is counted in each of the first regions between the central part 25 and both ends 26, 27, and the average number of linear voids 5 in the first region is obtained by averaging the numbers at the three locations (25, 26, 27). The average number of linear voids 5 is counted in each of the second regions between the central part 25 and both ends 26, 27, and the average number of linear voids 5 in the second region is obtained by averaging the numbers at the three locations (25, 26, 27).

[0028] [Method for Manufacturing Substrate 1] To manufacture substrate 1, first, prepare the nitride ceramic substrate 2 and the two copper plates 3. Next, the nitride ceramic substrate 2 and the two copper plates 3 are joined using two bonding layers 4. The method of joining is not limited. Examples of joining methods include using a sheet-like bonding material or a paste-like bonding composition.

[0029] The sheet-like bonding material includes at least one of sheet-like titanium and sheet-like zirconium. The thickness of the sheet-like bonding material is 0.1 μm or more and 20 μm or less. The sheet-like bonding material is sandwiched between the nitride ceramic substrate 2 and the copper plate 3 so that the sheet-like bonding material is in contact with the first main surface 21 and the second main surface 31. Alternatively, a copper plate 3 (sheet-laminated copper plate) with the sheet-like bonding material pre-placed on the two second main surfaces 31 can be used. The laminated sheet-like bonding material, nitride ceramic substrate 2, and copper plate 3 are placed in a furnace and heated and pressurized. As a result, the sheet-like bonding material reacts with the nitride ceramic substrate 2 and the two copper plates 3 respectively, forming two bonding layers 4. The pressure applied in the thickness direction TD due to pressurization is 0.1 MPa or more and 30 MPa or less. The maximum heating temperature is 875°C or higher, and may be 900°C or higher. The maximum heating temperature is the temperature inside the furnace into which the sheet-like bonding material, the nitride ceramic substrate 2, and the two copper plates 3 are placed. The heating time is between 3 and 6 hours. The heating rate (rate of temperature increase) is not limited.

[0030] After heating, the substrate 1 is cooled. Pressurization of the substrate 1 is continued during cooling. The first cooling rate (first cooling rate), which is the average cooling rate from the maximum heating temperature down to 250°C, is between 13°C / min and 20°C / min.

[0031] If the first cooling rate falls below 13°C / min, the average number of linear voids 5 in the 500 μm region may fall below 18, the average number of linear voids 5 in the 100 μm region may fall below 4, and the average pitch of linear voids 5 may exceed 25 μm. Furthermore, if the first cooling rate falls below 13°C / min, linear voids 5 may not be formed in the bonding layer 4.

[0032] On the other hand, if the first cooling rate exceeds 20°C / min, the average number of linear voids 5 in the 500 μm region may exceed 37, the average number of linear voids 5 in the 100 μm region may exceed 7, and the average pitch of linear voids 5 may fall below 14 μm.

[0033] In any of the above cases, when a thermal cycle is applied to the substrate 1, the copper plate 3 may peel off from the nitride ceramic substrate 2 due to the difference in thermal expansion coefficients between the nitride ceramic substrate 2 and the copper plate 3. On the other hand, if the first cooling rate is 13°C / min or more and 20°C / min or less, a specific average pitch and number of linear voids 5 are formed in the bonding layer 4. The second cooling rate, which is the average cooling rate from 250°C to 25°C, is not limited.

[0034] The paste-like bonding composition contains at least one of powdered titanium and powdered zirconium (active metal powder), a binder, and a solvent. The median diameters of the powdered titanium and powdered zirconium are 0.1 μm or more and 20 μm or less, respectively. Examples of the binder include polymers. The paste-like bonding composition is prepared by mixing the active metal powder, the binder, and the solvent.

[0035] Next, a paste-like bonding composition is placed in a layer on at least one of the first main surface 21 and the second main surface 31. Then, the paste-like bonding composition is heated to remove the solvent, and subsequently, the bonding composition is heated further to remove (degrease) the binder. This forms a bonding precursor layer made of activated metal powder. Subsequently, the bonding precursor layer is sandwiched between the nitride ceramic substrate 2 and the copper plate 3 so that the bonding precursor layer is in contact with both the first main surface 21 and the second main surface 31, and these are subjected to pressurized heat treatment. This causes the bonding precursor layer to react with the nitride ceramic substrate 2 and the copper plate 3, respectively, and a bonding layer 4 is formed. The pressurized heat conditions using the bonding precursor layer are the same as those using the sheet-like bonding material. The cooling conditions are the same as the cooling conditions (including the first cooling rate) when using the sheet-like bonding material.

[0036] As described above, by setting the first cooling rate within a specific range, a specific average pitch and number of linear voids 5 are formed. However, the method for forming the specific average pitch and number of linear voids 5 is not limited to controlling the first cooling rate as described above. For example, a method that utilizes the difference in thermal expansion of the materials to control the type and thickness of the nitride ceramic substrate 2 and the copper plate 3 to be joined may also be used.

[0037] The copper plate 3 of the manufactured substrate 1 is patterned together with the bonding layer 4, depending on the application. For example, the copper plate 3 and the bonding layer 4 are etched to form a copper wire circuit on the first main surface 21 of the nitride ceramic substrate 2. Applications of the substrate 1 include, for example, circuit boards and heat dissipation substrates.

[0038] The substrate 1 of this disclosure will be described in more detail below with reference to examples and comparative examples. [Example 1] A nitride ceramic substrate 2 made of silicon nitride and two copper plates 3 were prepared. Subsequently, a paste-like bonding composition containing powdered titanium with a median diameter of 5 μm, a binder, and a solvent was prepared. The paste-like bonding composition was placed on the two first main surfaces 21, and then the paste-like bonding composition was heated to remove the solvent, and then the bonding composition was heated to remove (degrease) the binder. This formed a bonding precursor layer made of titanium powder. Subsequently, the bonding precursor layer was sandwiched between the nitride ceramic substrate 2 and the copper plates 3 so that the bonding precursor layer was in contact with both the first main surface 21 and the second main surface 31, and they were subjected to pressurized heat treatment in a furnace at 10 MPa and 900°C. The maximum heating temperature was 900°C. After that, the nitride ceramic substrate 2, the copper plates 3, and the bonding layer 4 were cooled at a first cooling rate of 13°C / min. This is how circuit board 1 was manufactured.

[0039] [Examples 2 to Comparative Example 2] Substrates 1 for each of Examples 2 to Comparative Example 2 were manufactured in the same manner as in Example 1. However, the first cooling rate was changed according to the information in Table 1.

[0040] [Analysis] The substrate 1 was cut along the second direction D2, and the central portion 25 and both ends 26, 27 of the cut surface (cross section) were observed using a scanning electron microscope. The average number of linear voids 5 in the first region, the average number of linear voids 5 in the second region, and the average pitch of the linear voids 5 were determined. The results are shown in Table 1. Note that in Comparative Example 1, the formation of linear voids 5 was not observed in the bonding layer 4.

[0041] [Evaluation] First, the substrate 1 was subjected to the following thermal cycle. Thermal cycle: Temperature profiles of -55°C and 200°C, 5000 cycles Thermal cycle device: Thermal shock device manufactured by ESPEC Corporation (model number TSA-73ES) Subsequently, ultrasonic testing was performed on substrate 1 after the thermal cycle was subjected. In the ultrasonic testing, an ultrasonic flaw detector (manufactured by Hitachi Power Solutions, model number "FSesIII") was used to ultrasonically inspect more than 100 areas (2 mm square areas) to check for the presence or absence of delamination of the copper plate 3 from the nitride ceramic substrate 2. The delamination rate was then calculated. The results are shown in Table 1. If the delamination rate was 1% or more, it was evaluated as "unacceptable", and if it was 0%, it was evaluated as "good".

[0042]

[0043] The embodiments disclosed herein should be understood to be illustrative in all respects and not restrictive in any way. The scope of this disclosure is defined not by the foregoing description but by the claims, and all modifications within the meaning and scope equivalent to the claims are intended.

[0044] 1 Substrate, 2 Nitride ceramic substrate, 3 Copper plate, 4 Bonding layer, 4A First bonding layer, 4B Second bonding layer, 5 Linear void, 21 First main surface, 25 Central part, 26 End, 27 End, 31 Second main surface, 51 Starting point, 52 Part, D1 First direction, D2 Second direction, P Pitch, TD Thickness direction, W Width.

Claims

1. A substrate comprising: a nitride ceramic substrate including a first main surface; a copper plate including a second main surface facing the first main surface; and a bonding layer that contacts the first main surface and the second main surface to bond the nitride ceramic substrate and the copper plate, and contains at least one of titanium and zirconium, but does not contain silver, indium, or tin, wherein the bonding layer has a plurality of linear voids formed from the first main surface to the second main surface, and with respect to a cross section parallel to a first direction which is the direction in which the first main surface and the second main surface face each other, the average pitch of the plurality of linear voids in a second direction which is perpendicular to the first direction is 14 μm or more and 25 μm or less, the average number of the linear voids in a 500 μm region in the second direction is 18 or more and 37 or less, and the average number of the linear voids in a 100 μm region in the second direction is 4 or more and 7 or less.

2. The substrate according to claim 1, wherein the nitride in the nitride ceramic substrate is silicon nitride.

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