Ceramic substrate, and semiconductor device substrate equipped therewith

Abstract

The ceramic substrate according to the present invention contains Al, Zr, and Y, with an Al content of 75-90 mass% in terms of Al2O3, a Zr content of 5-14 mass% in terms of ZrO2, and a Y content of 0.3-2.0 mass% in terms of Y2O3. The crystalline phase contains Al2O3 (alumina) particles and YSZ (yttria partially stabilized zirconia) particles, and the average distance between the nearest YSZ particles is 0.50-1.20 μm.

Description

Technical Field

[0006] , , , , , ,

[0005]

[0001] The present invention relates to a ceramic substrate and a substrate for a semiconductor device including the same.

Background Art

[0002] As a substrate for a semiconductor device used in a power transistor module or the like, a DBOC substrate (Direct Bonding of Copper Substrate) having a copper plate on the surface of a ceramic substrate is known (for example, Patent Documents 1 to 3).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, in the above-described substrate for a semiconductor device, a copper plate is joined to the surface of the ceramic substrate. However, the present inventor has found that when the oxygen ion conductivity of the ceramic substrate is increased, the bonding strength between the ceramic substrate and the copper plate decreases when a DC voltage is applied, and the copper plate may peel off.

[0005] The present invention has been made to solve the above problems, and an object thereof is to provide a ceramic substrate and a substrate for a semiconductor device that can suppress peeling of a copper plate when a DC voltage is applied when the copper plate is joined.

Means for Solving the Problems

[0006] Item 1. Al and Zr and, Y and, It contains, The Al content is 75-90% by mass, calculated as Al2O3. The Zr content is 5-14% by mass in terms of ZrO2. The Y content is 0.3 to 2.0 mass% in terms of Y2O3. The crystalline phase contains Al2O3 (alumina) particles and YSZ (yttria partially stabilized zirconia) particles. The average distance between the nearest YSZ particles is 0.50 to 1.20 μm. Ceramic substrate.

[0007] Item 2. The first grain boundary formed between the Al2O3 particles and the YSZ particles is amorphous. The second grain boundary formed between the Al2O3 particles is amorphous. The thickness of the first grain boundary is 2.0 to 5.0 nm. The ceramic substrate described in item 1.

[0008] Item 3. The thickness of the first grain boundary is greater than the thickness of the second grain boundary. The ceramic substrate described in item 2.

[0009] Item 4. The thickness of the second grain boundary is 0.5 to 1.5 nm. The ceramic substrate described in item 2.

[0010] Item 5. The first grain boundary and the second grain boundary contain at least one element selected from Al, Si, O, Ca, Y, Zr, Sr, and Ba. A ceramic substrate as described in any of items 2 to 4.

[0011] Item 6. Al and, Zr and, Y and, It contains, The Al content is 75-90% by mass, calculated as Al2O3. The Zr content is 5-14% by mass in terms of ZrO2. The content of Y is 0.3 to 2.0% by mass in terms of Y2O3, contains, as crystal phases, Al2O3 (alumina) particles and YSZ (yttria partially stabilized zirconia) particles, a first grain boundary formed between the Al2O3 particles and the YSZ particles is amorphous, a second grain boundary formed between the Al2O3 particles is amorphous, the thickness of the first grain boundary is 2.0 to 5.0 nm, a ceramic substrate.

[0012] The thickness of the first grain boundary according to item 7 is thicker than the thickness of the second grain boundary, The ceramic substrate according to item 6.

[0013] According to item 8, the thickness of the second grain boundary is 0.5 to 1.5 nm, The ceramic substrate according to item 6.

[0014] According to item 9, the first grain boundary and the second grain boundary contain at least one kind selected from Al, Si, O, Ca, Y, Zr, Sr, and Ba as elements, The ceramic substrate according to any one of items 6 to 8.

[0015] According to item 10, the average particle diameter of the YSZ particles is 0.50 to 0.90 μm, The ceramic substrate according to any one of items 1 to 9.

[0016] According to item 11, the flexural strength is 700 MPa or more, The ceramic substrate according to any one of items 1 to 10.

[0017] According to item 12, a substrate for a semiconductor device for mounting electronic components, the ceramic substrate according to items 1 to 11, and a copper plate joined to at least one surface of the ceramic substrate, A substrate for a semiconductor device comprising the above.

Advantages of the Invention

[0018] According to the present invention, when a copper plate is bonded to a ceramic substrate, it is possible to suppress the delamination of the ceramic substrate and the copper plate when a DC voltage is applied. [Brief explanation of the drawing]

[0019] [Figure 1] This is a cross-sectional view showing one embodiment of a semiconductor device comprising a substrate for a semiconductor device according to the present invention. [Figure 2] This diagram explains how to calculate the distance CR. [Figure 3] This figure shows the SEM image (EBSD) of Example 1 after image processing, and a graph showing the distribution of the distance between the centers of mass of each YSZ particle. [Figure 4] This image shows the thickness of the first grain boundary in Example 1, observed using STEM. [Figure 5] This image shows the thickness of the second grain boundary in Example 1, observed using STEM. [Figure 6] This shows the results of STEM-EDX analysis of the elements present at the first grain boundary in Example 1. [Figure 7] This shows the results of STEM-EDX analysis of the elements present at the second grain boundary in Example 1. [Modes for carrying out the invention]

[0020] Hereinafter, an embodiment of the ceramic substrate and the semiconductor device substrate using the same according to the present invention will be described with reference to the drawings. Figure 1 is a cross-sectional view of a semiconductor device having the semiconductor device substrate according to this embodiment.

[0021] <1. Overview of Semiconductor Devices> The semiconductor device according to this embodiment can be used as a power module in various electronic devices such as smartphones, personal computers, large home appliances, railways, electric vehicles, power generation (wind power generation, solar power generation, fuel cells, etc.), air conditioners, industrial robots, commercial elevators, household microwave ovens, induction rice cookers, and UPS (uninterruptible power supply).

[0022] As shown in Figure 1, the semiconductor device 1 according to this embodiment comprises a semiconductor device substrate 2, a first bonding material 5, a second bonding material 5', a semiconductor chip 6, bonding wires 7, and a heat sink 8.

[0023] The semiconductor device substrate 2 is a so-called DBOC substrate (Direct Bonding of Copper Substrate), comprising a plate-shaped ceramic substrate 3 which is an insulator, a first copper plate 4 bonded to one side (top surface) of the ceramic substrate 3, and a second copper plate 4' bonded to the other side (bottom surface). Details of the ceramic substrate 3 will be described later.

[0024] A transmission circuit is formed on the first copper plate 4. On the other hand, the second copper plate 4' is formed in a flat plate shape.

[0025] A semiconductor chip 6 is bonded to the upper surface of the semiconductor device substrate 2, specifically a portion of the upper surface of the first copper plate 4, via a first bonding material 5. Furthermore, the semiconductor chip 6 and the first copper plate 4 are connected by bonding wires 7.

[0026] On the other hand, a heat sink 8 is bonded to the lower surface of the semiconductor device substrate 2, that is, the lower surface of the second copper plate 4', via a second bonding material 5'. The heat sink 8 is a known type and can be made of a metal such as copper.

[0027] <2. Ceramic Substrate Configuration> Next, the ceramic substrate 3 will be described in detail. In the following, a crystalline state that is not crystalline is referred to as amorphous (amorphous is also included in amorphous), and amorphous materials are referred to as glassy. The ceramic substrate 3 contains alumina (Al2O3), zirconia (ZrO2), and yttria (Y2O3), and may also contain glassy material and the remainder. Furthermore, the glassy material may contain a first component and a second component. The first component may contain Si, alkali metals, and O as elements, and the second component may contain at least one element selected from Ca, Sr, and Ba as elements. However, the glassy material may also contain components other than these. The content of the constituent elements of this ceramic substrate 3 will be described below.

[0028] <2-1. Components other than glassy material> The ceramic substrate 3 has a matrix component composed of alumina. Regarding the alumina content, for example, the Al content is preferably 75% to 90% by mass, and more preferably 85% to 90% by mass, in terms of Al2O3.

[0029] Regarding the zirconia content, it is preferable that the Zr content be 5% by mass or more and 14% by mass or less in terms of ZrO2, and more preferably 10% by mass or more and 14% by mass or less. By having a zirconia content of 5% by mass or more, the strength of the ceramic substrate 3 (for example, the flexural strength described later) can be improved. Furthermore, it is thought that the linear thermal expansion coefficient of the ceramic substrate 3 can be prevented from being too small, and the difference in the linear thermal expansion coefficient between the ceramic substrate 3 and the first and second copper plates 4,4' can be reduced. As a result, it is thought that the thermal stress generated at the bonding interface can be reduced, which contributes to suppressing the occurrence of cracks in the ceramic substrate 3 at the bonding interface.

[0030] Furthermore, Cu diffusing from the copper plates 4,4' bonded to the ceramic substrate 3 preferentially passes through the zirconia crystal grains rather than the alumina crystal grains constituting the matrix component, thereby diffusing into the ceramic substrate 3 and forming a Cu-containing region. Therefore, when the zirconia content is set as described above, the diffusion of Cu is promoted, and the bonding strength of the copper plates 4,4' is increased, as will be described later, which is preferable.

[0031] On the other hand, by limiting the zirconia content to 14% by mass or less, it is possible to suppress excessive reactions at the bonding interface during copper plate bonding, thereby suppressing the formation of voids at the bonding interface. This is because alumina and zirconia have different wettability with the Cu-O eutectic liquid phase during copper plate bonding. Furthermore, by limiting the zirconia content to 14% by mass or less, the impedance of the ceramic substrate 3 can be improved without increasing the silica content, as will be described later.

[0032] Regarding the yttria content, it is preferable that the Y content be between 0.3% and 2.0% by mass, and more preferably between 0.5% and 1.2% by mass, in terms of Y2O3. By setting the content to 0.3% by mass or more, it is thought that the proportion of the monoclinic phase in the zirconia crystal phase will be suppressed, while the proportion of the tetragonal phase will be increased. As a result, it is thought that the mechanical strength of the ceramic substrate 3 can be improved and that this will contribute to suppressing the occurrence of cracks in the ceramic substrate 3 at the bonding interface.

[0033] On the other hand, by limiting the yttria content to 2.0 mass% or less, it is thought that the proportion of cubic crystals in the zirconia crystal phase can be suppressed, while the proportion of tetragonal crystals can be increased. As a result, the mechanical strength of the ceramic substrate 3 can be improved, and this is thought to contribute to suppressing the occurrence of cracks in the ceramic substrate 3 at the bonding interface.

[0034] Therefore, in the ceramic substrate 3 of this embodiment, zirconia and yttria exist as yttria-stabilized zirconia (YSZ), in which yttria is solid-solved in zirconia, but zirconia and yttria may also exist individually. However, in this invention, these two embodiments will be described as equivalent. Therefore, even when zirconia and yttria are described individually, they may exist in the ceramic substrate 3 as yttria-stabilized zirconia. The YSZ content in the ceramic substrate 3 is preferably 5.3 to 15% by mass, and more preferably 10 to 15% by mass.

[0035] Furthermore, it is preferable that the mass ratio of the total content of zirconia and yttria to the alumina content be 0.05 or higher. This is because a mass ratio of 0.05 or higher can improve flexural strength. On the other hand, it is preferable that this mass ratio be 0.30 or lower. This is because, since the thermal conductivity of zirconia is lower than that of alumina, if this mass ratio exceeds 0.30, it may not be possible to maintain sufficient thermal conductivity as an insulating heat dissipation circuit board. The presence of the above-mentioned alumina, zirconia, and yttria as oxides in the ceramic substrate 3 can be proven by XRD or the like.

[0036] <2-2. Components of glassy material> Next, we will explain the glassy material. As described above, the glassy material may contain a first component containing Si, alkali metals, and O as elements, and a second component containing at least one element selected from Ca, Sr, and Ba as elements. However, since these elements mainly exist as oxides such as silica (SiO2), alkali metal oxides, calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO), examples of these will be explained below. The first component may also contain Mg, which may exist as an oxide such as magnesia (MgO) in the glassy material. Furthermore, although the glassy component is not essential, the following explanation will be based on the assumption that a glassy component is present.

[0037] First, let's explain the first component. Regarding the silica content, it is preferable that the Si content be between 0.1% by mass and 2.5% by mass, calculated as SiO2. When the silica content is 0.1% by mass or more, as will be described later, the oxygen ion conductivity of the ceramic substrate 3 is suppressed, and the impedance can be improved. On the other hand, if the silica content is too high, the strength of the ceramic substrate 3 may decrease, so to suppress this, it is preferable that the silica content be 2.5% by mass or less.

[0038] Regarding the magnesia content, it is preferable that the Mg content be between 0.05% by mass and 0.5% by mass in terms of MgO, and more preferably between 0.10% by mass and 0.3% by mass. By setting the magnesia content to 0.05% by mass or more, it is thought that the ceramic substrate 3 can be sintered without excessively high firing temperatures, and the coarsening of alumina particles and YSZ particles can be suppressed. As a result, the mechanical strength of the ceramic substrate 3 can be improved, and it is thought that this contributes to suppressing the occurrence of cracks in the ceramic substrate 3 at the bonding interface with the copper plate 4. Furthermore, it is thought that a sufficient amount of MgAl2O4 crystals (hereinafter referred to as "spinel crystals") can be generated in the ceramic substrate 3, and the wettability with the Cu-O eutectic liquid phase during copper plate bonding can be improved. As a result, it is thought that this contributes to suppressing the occurrence of voids at the bonding interface.

[0039] On the other hand, by limiting the magnesia content to 0.5 mass% or less, it is possible to suppress the excessive formation of spinel crystals with low mechanical strength, thereby improving the mechanical strength of the ceramic substrate 3. As a result, this is expected to contribute to suppressing the occurrence of cracks in the ceramic substrate 3 at the bonding interface.

[0040] Alkali metal ions have the function of cleaving the Si-O bonds of SiO2 to form non-bridging oxygen (NBO), and when NBO becomes the diffusion site of ions, oxygen ion conductivity is generated. Examples of such alkali metals include sodium, potassium, lithium, etc., and two or more of these may be included. These may be contained in the ceramic substrate 3 as sodium oxide (Na2O), potassium oxide (K2O), and lithium oxide (Li2O). The inventor has confirmed that, regardless of which of these alkali metal oxides is used, the effects shown in the examples described later can be obtained in the same manner.

[0041] The second components are all glassy network-modifying oxides, which are incorporated into the skeleton of the network-forming oxide mainly composed of glassy silica (SiO2) and exhibit the function of changing the chemical and physical properties of the glass. In particular, when calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO) are contained in the glass containing alkali metal oxides as described above, the ionic conduction with NBO as the diffusion site is suppressed, resulting in a decrease in electrical conductivity. Also, the function of decreasing the electrical conductivity increases in the order of CaO < SrO < BaO.

[0042] This is because, as the radius difference between the monovalent alkali metal ions and the divalent ions increases, the blocking effect of inhibiting the movement of the alkali metal ions by the divalent ions increases, and the depolarization property with respect to the non-bridging oxygen ions decreases, strengthening the bond between adjacent alkali metal ions and oxygen ions, and decreasing the mobility of the alkali metal ions.

[0043] Also, based on this reason, by including CaO, SrO, and BaO in the glass containing alkali metal oxides, the insulation can be further enhanced. Therefore, it becomes possible to improve the property that, when a DC voltage is applied, which is the problem to be solved by the present invention, oxygen ions conduct from the negative electrode side to the positive electrode side inside the ceramic substrate, and the bonding strength between the electrode and the ceramic substrate decreases.

[0044] Furthermore, the second component of the glassy material may contain one or more of the following: CaO, SrO, and BaO.

[0045] Furthermore, the glassy material may contain Al, Zr, and Y.

[0046] In this ceramic substrate, the mass ratio of the alkali metal oxide content to the content of the second component in terms of glassy oxide is preferably 1.0 or less, more preferably 0.5 or less, and particularly preferably 0.1 or less. This effectively suppresses oxygen ion conduction. As a result, it is possible to suppress a significant decrease in the bonding strength between the ceramic substrate 3 and the copper plate after applying a DC voltage.

[0047] Furthermore, the mass ratio of the total content of silica, magnesia, and glassy second component to the alkali metal oxide content is preferably 25.0 or higher, and more preferably 30.0 or higher. This allows for even more effective suppression of oxygen ion conduction. In particular, it has been found that when this value is less than 25.0, the bonding strength between the ceramic substrate 3 and the copper plates 4,4' decreases significantly as the value decreases after applying a DC voltage. The upper limit of this value can be, for example, 65.0 or lower.

[0048] Furthermore, since sodium affects electrical conductivity, it is preferable that the sodium content in the ceramic substrate 3, in terms of oxide, be 0% by mass or greater than 0% by mass and 0.05% by mass or less.

[0049] Furthermore, the content of glassy oxides in the ceramic substrate 3 can be, for example, 0.9% to 3% by mass. This is because if it is less than 0.9% by mass, oxygen ion conduction cannot be sufficiently suppressed, while if it exceeds 3% by mass, sufficient flexural strength cannot be obtained.

[0050] The content of the remaining material is preferably 0.5% by mass or less, and more preferably 0.05% by mass or less, in terms of oxides. This is thought to suppress excessive sintering of the ceramic substrate 3 even when the firing temperature is not excessively high, and to reduce the porosity of the ceramic substrate 3. As a result, the mechanical strength of the ceramic substrate 3 can be improved, and this is thought to contribute to suppressing the occurrence of cracks in the ceramic substrate 3 at the bonding interface with the copper plate 4.

[0051] In this embodiment, the content of the constituent elements of the ceramic substrate 3 is calculated in terms of oxides as described above. However, the constituent elements of the ceramic substrate 3 may or may not exist in the form of oxides. For example, at least one of Y, Mg, and Ca may not exist in the form of oxides but be dissolved in ZrO2. Similarly, some alkali metals may not exist in the form of oxides. Even for alkali metals that do not exist in the form of oxides, the content is calculated in terms of oxides.

[0052] The oxide content of the constituent elements of the ceramic substrate 3 is calculated as follows: First, the constituent elements of the ceramic substrate 3 are qualitatively analyzed using an XRF (X-ray fluorescence) analyzer or an EDX (Energy Dispersive X-ray Fluorescence) analyzer attached to a STEM (Scanning Transmission Electron Microscope). Next, each element detected by this qualitative analysis is quantitatively analyzed using an ICP (Integrated Cellular Emission Spectrometer). Then, the content of each element measured by this quantitative analysis is converted to oxide.

[0053] The elements included in the remainder may be elements that are intentionally added or elements that are inevitably mixed in. There are no particular restrictions on the elements included in the remainder, but examples include Fe (iron), Ti (titanium), and Mn (manganese).

[0054] Furthermore, the glassy material described above cannot exist within the alumina particles or YSZ particles themselves, but rather in the grain boundaries, which are regions surrounded by the alumina particles or YSZ particles. There are three types of grain boundaries, for example, Al2O3 / Al2O3, Al2O3 / YSZ, and YSZ / YSZ, and they are amorphous. From a macroscopic perspective, there are two types of grain boundaries: those inside the ceramic substrate 3 that are not exposed to the outside (the three types mentioned above), and those on the surface that are exposed to the outside. In addition, there is a possibility that trace amounts of glassy components may be dissolved in Al2O3 and YSZ. According to the inventors, of the three grain boundaries mentioned above, the YSZ / YSZ grain boundary has little effect on the bonding strength between the ceramic substrate 3 and the copper plates 4,4', so it will not be considered in the grain boundary analysis described later.

[0055] <3. Study on bonding strength between ceramic substrate and copper plate> Next, we will examine the bonding strength between the ceramic substrate 3 and the copper plates 4 and 4'. In the semiconductor device substrate 2 described above, copper plates 4 and 4' are bonded to both sides of the ceramic substrate 3. However, if the oxygen ion conductivity of the ceramic substrate 3 increases, the bonding strength between the ceramic substrate 3 and the copper plates 4 and 4' decreases when a DC voltage is applied, and there is a risk that the copper plates 4 and 4' may peel off. This point will be explained below.

[0056] First, Cu-O-Al bonds are formed at the interface between the first copper plate 4 and the second copper plate 4' and the ceramic substrate 3 during the bonding process involving the generation and solidification of the Cu-O eutectic liquid phase. Then, for example, when the first copper plate 4 is connected to the negative electrode and the second copper plate 4' is connected to the positive electrode, and a DC voltage is applied to them, the Cu-O-Al bonds are reduced near the interface between the first copper plate 4 and the ceramic substrate 3. This reduces the bonding strength between the first copper plate 4 and the ceramic substrate 3. The oxygen ions generated by this reduction move to the second copper plate 4' via the ceramic substrate 3. As a result, the second copper plate 4' is oxidized by the oxygen ions, reducing the bonding strength between the second copper plate 4' and the ceramic substrate 3.

[0057] On the other hand, when an AC voltage is applied, it is thought that oxygen ions hardly move between the alumina and zirconia particles inside the ceramic substrate 3. This is because even when an AC voltage is applied, peeling of the first copper plate 4 and the second copper plate 4' due to reduction of the Cu-O-Al bond does not occur. However, since there is some movement of oxygen ions, the ease of oxygen ion movement in the ceramic substrate can be evaluated by measuring the impedance when an AC voltage is applied. In other words, if oxygen ions do not move easily, the impedance will be high.

[0058] In response to this, the inventors have obtained the following findings. First, the pathways through which the oxygen ions generated as described above conduct will be explained. Oxygen ions mainly conduct within the YSZ particles, which are their conductors. Therefore, as described above, although the flexural strength improves when the zirconia and yttria content increases, the conduction pathways for oxygen ions increase, which may reduce the bonding strength between the ceramic substrate 3 and the copper plates 4,4'. In response to this, the inventors have obtained the following findings.

[0059] <3-1. Examination of interparticle distance> As described above, oxygen ions conduct through YSZ particles, so the conductivity of oxygen ions decreases as the distance between YSZ particles increases. Therefore, the inventors obtained the following findings regarding the interparticle distance. First, an image of the cross-section of the ceramic substrate (for example, an image acquired by EBSD) was taken. For example, the surface of the ceramic substrate was polished using a polishing method such as ion milling polishing to expose the cross-section near the center of the thickness of the ceramic substrate, and the exposed cross-section was photographed. Next, the projected area circle equivalent diameter (so-called Haywood diameter, the diameter of a circle with the same area as the projected area of ​​the particle) of all YSZ particles observed within a predetermined range in the captured image (for example, within a range of 30 μm × 30 μm) was calculated. At this time, image processing was applied to make it easier to distinguish the YSZ particles. For example, as shown in Figure 3 described later, the region of YSZ particles extracted by EBSD and the region of other compositions were binarized, and all YSZ particles were made white. The software used for the binarization process was OIM Analysis 7.3 (TSL Solutions Co., Ltd.). Then, using the processed image, the distance CR between each YSZ particle and the nearest YSZ particle is calculated using the following equation (1) (see Figure 2). Distance CR=r-(R1+R2) (1) r is the distance between the centers of mass of adjacent YSZ particles. The center of mass is determined, for example, from the contour of the YSZ particle extracted by the image processing described above, according to Green's theorem (for example, the "+" attached to the region of the YSZ particle in Figure 3, described later, corresponds to the center of mass). Furthermore, the YSZ particles defined by the binarization process also include aggregates formed by the collection of YSZ particles. Also, R1 and R2 are the radii of adjacent YSZ particles (half of the equivalent diameter of the projected area circle), respectively.

[0060] In this way, the average of the distance CR calculated for each YSZ particle is determined. The inventors confirmed that the average distance CR of YSZ particles observed within the above-mentioned size range was approximately the same at any position on the ceramic substrate.

[0061] Furthermore, according to the inventors, the average distance CR is preferably 0.50 to 1.20 μm, and more preferably 0.70 to 1.1 μm. If the average distance CR is 0.50 μm or more, the distance between adjacent YSZ particles increases, resulting in lower oxygen ion conductivity. That is, the decrease in bonding strength between the ceramic substrate 3 and the copper plates 4,4' when a DC voltage is applied can be suppressed. On the other hand, if the average distance CR is too large, the flexural strength may decrease, so it is preferably 1.20 μm or less. The average particle size of the YSZ particles is, for example, 0.50 to 0.90 μm. By keeping the average particle size within the range of 0.50 to 0.90 μm, the YSZ exhibits good dispersibility, which hinders crack propagation and is particularly effective in improving fracture toughness. On the other hand, if the average grain size of YSZ falls outside the above range, there is a risk of a decrease in mechanical strength due to a reduction in the degree of sintering caused by the deterioration of YSZ dispersibility, or a decrease in fracture toughness because YSZ is not included in the crack propagation path.

[0062] <3-2. Examination of grain boundaries> Next, we will explain the grain boundaries of Al2O3 particles and YSZ particles, which are crystalline phases. We will examine the grain boundaries between Al2O3 particles and between Al2O3 particles and YSZ particles. Since Al2O3 particles do not conduct oxygen ions easily, the grain boundaries between Al2O3 particles serve as conduction pathways. However, it is known that oxygen ions do not conduct easily when these grain boundaries are amorphous. Similarly, oxygen ions do not conduct easily when the grain boundaries between Al2O3 particles and YSZ particles are amorphous.

[0063] In response to this, the inventors have obtained the following findings. First, the pathways through which the oxygen ions generated as described above conduct will be explained. Oxygen ions mainly conduct within the YSZ particles, which are their conductors. Therefore, as described above, although the flexural strength improves when the zirconia and yttria content increases, the conduction pathways for oxygen ions increase, which may reduce the bonding strength between the ceramic substrate 3 and the copper plates 4,4'.

[0064] Furthermore, the inventors found that increasing the thickness of the amorphous grain boundary between Al2O3 particles and YSZ particles suppresses the penetration of oxygen ions attempting to pass through the YSZ particles, thus resulting in a high effect in suppressing oxygen ion conduction. In other words, it is possible to suppress the decrease in bonding strength between the ceramic substrate 3 and the copper plates 4,4' when a DC voltage is applied. Note that regions with irregular structural arrangements were defined as amorphous using STEM (Scanning Transmission Electron Microscope) observation images.

[0065] Specifically, the thickness of the grain boundary between Al2O3 particles and YSZ particles (hereinafter referred to as the first grain boundary) is preferably 2.0 to 5.0 nm, and more preferably 2.5 to 4.0 nm. When the thickness of the grain boundary between Al2O3 and YSZ is 2.0 nm or more, the effect of suppressing the penetration of oxygen ions into YSZ particles is enhanced. When the thickness of the grain boundary between Al2O3 and YSZ exceeds 5.0 nm, there is a risk that the flexural strength will decrease. The thickness of the first grain boundary was measured using the method described in the examples below. The same applies to the second grain boundary.

[0066] Furthermore, amorphous grain boundaries between Al2O3 particles (hereinafter referred to as second grain boundaries) also contribute to suppressing the penetration of oxygen ions, but if the thickness of the second grain boundaries becomes too large, it may lead to a decrease in flexural strength. Therefore, it is preferable that the thickness of the second grain boundaries is at least smaller than that of the first grain boundaries. Specifically, the thickness of the second grain boundaries is preferably 0.5 to 1.5 nm. As described above, the inventors focused on the distance CR and the thickness of the grain boundaries in order to suppress the decrease in bonding strength between the ceramic substrate 3 and the copper plates 4, 4' when a DC voltage is applied. They found that if either of these values ​​is measured and satisfies the above-mentioned range, the decrease in bonding strength can be suppressed.

[0067] <4. Method for manufacturing ceramic substrates and substrates for semiconductor devices> Next, a method for manufacturing ceramic substrates and semiconductor device substrates will be described. First, the powder materials of the constituent elements mentioned above are prepared. Next, the prepared powder materials are crushed and mixed using, for example, a planetary mill or a bead mill.

[0068] Next, the crushed and mixed powder material is packed into a graphite die and fired using an SPS (discharge plasma sintering) apparatus to complete the ceramic substrate. In this case, the applied pressure is preferably 35 to 45 MPa, and more preferably 40 to 45 MPa. The maximum sintering temperature is preferably 1400 to 1700°C, and more preferably 1580 to 1700°C. The heating rate is preferably 50 to 70°C / min, and more preferably 60 to 70°C / min. The holding time at the maximum temperature is preferably 5 to 15 minutes, and more preferably 10 to 15 minutes. After that, it is slowly cooled to room temperature over 3 to 5 hours.

[0069] Then, the ceramic substrate is completed by processing the fired ceramic body to a predetermined thickness. While processing methods such as double-sided lapping are possible, the method is not particularly limited.

[0070] It has been empirically observed that when the heating rate is increased relative to the above settings, the proportion of voids within the ceramic substrate increases proportionally, leading to a reduction in mechanical strength. Furthermore, since the above maximum temperature is in a temperature range exceeding the melting point of typical glass, as the time spent above the glass melting point, including the heating process and the maximum temperature, increases, grain growth in the matrix base material is promoted, and the network of glass particles present in a mesh-like structure at the grain boundaries tends to break down, resulting in a more scattered behavior. This weakens the electrical insulation effect. Therefore, it is preferable to shorten the heating time by increasing the heating rate as described above, and also shorten the holding time at the maximum temperature. Based on this mechanism, the above firing conditions were optimized.

[0071] Next, a method for manufacturing a semiconductor device substrate will be described. First, a laminate is formed by placing copper plates 4 and 4' with oxidized surfaces on the upper and lower surfaces of a ceramic substrate 3, and heating it for about 5 to 20 minutes (for example, about 10 minutes) under a nitrogen atmosphere at 1065°C to 1083°C. Due to the heating, a Cu-O eutectic liquid phase is formed at the interface where the ceramic substrate 3 and the copper plates 4 and 4' are joined. The thickness of each copper plate 4 and 4' can be, for example, 0.1 to 2.0 mm.

[0072] Next, the Cu-O eutectic liquid phase solidifies by cooling this laminate, and the copper plates 4 and 4' are bonded to the ceramic substrate 3. In this way, the semiconductor device substrate 2 is completed. The transmission circuit formed on the copper plate 4 on the surface to which the semiconductor chip 6 is bonded can be formed, for example, by the subtractive method or the additive method. [Examples]

[0073] The following describes embodiments of the present invention. However, the present invention is not limited to the following embodiments.

[0074] <1. Fabrication of ceramic substrates> Ceramic substrates for Examples 1-3 and Comparative Examples 1 and 2 were prepared, consisting mainly of the following materials. The compositions of these ceramic substrates are shown in Table 1. Specifically, powder materials mixed in predetermined proportions were first crushed and mixed using a planetary mill. In Table 1, mass% is expressed as wt%. The values ​​shown in Tables 1 and 2 are the oxide equivalent values ​​of each element in the sintered body. ICP analysis can be used to determine the composition and content of the sintered body. ICP analysis allows for qualitative and quantitative analysis of the constituent elements contained in the sintered body, which can then be determined by converting them to oxides (Al2O3, SiO2, CaO, SrO, BaO, MgO, Na2O). The YSZ content is the sum of the ZrO2 content and the Y2O3 content. [Table 1]

[0075] Next, the crushed and mixed powder material was packed into a graphite die, and the die was fired under pressure of 45 MPa using an SPS apparatus. At this time, the die was heated to 1600°C at a heating rate of approximately 50°C / min. It was then held at the maximum temperature of 1600°C for 15 minutes, and then slowly cooled over approximately 1 hour. In this way, a ceramic sintered body was formed. This ceramic sintered body was then formed to a thickness of 0.32 mm, a length of 40 mm, and a width of 40 mm to obtain a ceramic substrate.

[0076] Then, copper plates with an oxidized surface and a thickness of 0.3 mm were placed on the top and bottom surfaces of the ceramic substrate, and the substrate was heated at atmospheric pressure under a nitrogen atmosphere at 1070°C for about 10 minutes. In this way, semiconductor device substrates according to Examples 1 to 3 and Comparative Examples 1 and 2 were obtained.

[0077] <2. Evaluation> The following evaluations were conducted.

[0078] (1) Bonding strength For semiconductor device substrates according to Examples 1-3 and Comparative Examples 1 and 2, the bonding strength between the ceramic substrate and the copper plate (bonding strength before DC (Direct Current) application) was measured. Subsequently, the bonding strength after applying DC 1360V for 1000 hours while heating each semiconductor device substrate at 275°C (bonding strength after DC application) was measured.

[0079] The joint strength was measured as follows: First, the rectangular copper plates being joined were peeled off from the end by a predetermined length along the longer side and bent at a 90° angle. Next, the end of the bent copper plate was clamped with a clip and pulled up vertically. The maximum value at this point was defined as the measured strength. That is, joint strength (kg / cm) = maximum lifting load (kg) / length of the shorter side of the rectangle (cm). Then, the ratio of the joint strength after DC application to the joint strength before DC application was calculated and defined as the joint strength ratio (%). This joint strength ratio is preferably 95% or higher, and more preferably 98% or higher.

[0080] (2) Method for measuring flexural strength The flexural strength of the ceramic substrates in Examples 1-3 and Comparative Example 1 was measured by a three-point bending test in accordance with JIS R1601:2008. A higher flexural strength is preferable, but for example, 700 MPa or higher is preferred, 730 MPa or higher is more preferred, and 750 MPa or higher is particularly preferred.

[0081] (3) Interparticle distance CR For Examples 1-3 and Comparative Example 1, SEM imaging was performed within a 30 μm × 30 μm area. Using the method described above, for each YSZ particle within this area, the distance CR to the nearest YSZ particle was calculated from the radius R of the YSZ particle and the distance r between the centers of gravity of the nearest YSZ particle. The average distance CR of all YSZ particles was then calculated. As an example, the SEM image of Example 1 is shown. Figure 3 shows the SEM image in Example 1 and a graph showing the distribution of the distance r between the centers of gravity of each YSZ particle.

[0082] (4) Observation of grain boundaries The first and second grain boundaries described above were observed using STEM and analyzed using STEM-EDX. The test conditions were as follows. For Examples 1-3 and Comparative Examples 1 and 2, observations were performed with N=2, and the thickness of each grain boundary shown in Table 2 was the average value of these observations. In addition, the radius R, centroidal distance r, and distance CR of the YSZ particles in Table 2 are the average values ​​obtained as described above. • Sample preparation: Ar ion milling method • Observation and analysis equipment: Atomic resolution analytical electron microscope • EDX detector: Silicon drift detector (SDD) • Sample observation location: (N=1) Center of the cross-section at the center of the ceramic sintered body (first field of view) (N=2) Cross-sectional area at a position 20 mm from the center of the ceramic sintered body (second field of view)

[0083] (5) Consideration of joint strength and flexural strength The results are shown in Table 2. Figure 4 shows the thickness of the first grain boundary in Example 1, and Figure 5 shows the thickness of the second grain boundary. Figure 6 shows the elements present at the first grain boundary in Example 1, and Figure 7 shows the elements present at the second grain boundary. In Figures 4 and 5, BF-STEM refers to bright-field STEM observations, and HAADF-STEM refers to high-angle annular dark-field STEM observations.

[0084] The thickness of each grain boundary was measured as follows: (i) Adjust the measurement field of view so that the grain boundaries are aligned in a straight line in the magnification range of 2 to 3 million times. The size of the field of view should be approximately 20 nm square. The grain boundaries were determined by the contrast difference between Al2O3 and YSZ on either side of the boundary. At the time of this adjustment, the variation in grain boundary thickness within the field of view is extremely small, so it can be said that the grain boundary thickness is constant regardless of the measurement location. (ii) Acquire HAADF-STEM images. (In this example, two fields of view (two observation positions) per sample.) (iii) Measure the grain boundary thickness of any region within each field of view, and take the average of these measurements as the grain boundary thickness.

[0085] As shown in Figures 4 and 5, STEM observations revealed that the grain boundaries were amorphous. In Example 1, the thickness of the first grain boundary was greater than that of the second grain boundary. This trend was similar in Examples 2 and 3 and Comparative Examples 1 and 2, as shown in Table 2. Furthermore, as shown in Figures 6 and 7, Al, O, Mg, Ca, Si, Zr, and Y were detected at each grain boundary. As shown in Figure 6, the relative abundance of elements at the first grain boundary was in the order Zr > Y > Si > Ca > Mg. On the other hand, the relative abundance of elements at the second grain boundary was in the order Si > Zr > Ca > Mg > Y.

[0086] Furthermore, the absence of Na at the first and second grain boundaries, as shown in Table 1, is thought to be because the Na content was below the detection limit of STEM-EDX analysis. It is well known that alkali metals such as Na, K, and Li are readily present in network-forming oxides mainly composed of silica (SiO2). Soda-lime glass is an example. Therefore, it is highly probable that the first and second grain boundaries contain the aforementioned first and second components, and this is thought to contribute to suppressing the decrease in bonding strength after DC application.

[0087] Examples 1-3 showed that the rate of decrease in bonding strength was less than 2% before and after DC application, indicating that a high level of bonding strength was maintained. This is thought to be due to the fact that the thickness of the grain boundary between Al2O3 and YSZ was 2 nm or more, and the average distance CR was 0.50 μm or more. On the other hand, Comparative Example 1 showed a decrease in bonding strength of about 99% before and after DC application. This is thought to be due to the fact that the thickness of the grain boundary between Al2O3 and YSZ was less than 2 nm. Furthermore, the lower flexural strength compared to Examples 1-3 and Comparative Example 2 is thought to be because the average distance CR was greater than 1.20 μm. Comparative Example 2 had a low bonding strength ratio of 2% because the thickness of the grain boundary between Al2O3 and YSZ was less than 2 nm and the average distance CR was less than 0.50 μm. It is thought that Comparative Example 2 had a higher flexural strength than Comparative Example 1 because of its higher YSZ content.

[0088] Furthermore, regarding flexural strength, all of Examples 1-3 and Comparative Example 2 showed good values ​​of 700 MPa or higher. This is thought to be due to the higher YSZ content and the average distance CR being 1.20 μm or less compared to Comparative Example 1. However, as in Comparative Examples 1 and 2, when the thickness of the grain boundary between Al2O3 and YSZ was less than 2 nm, the bonding strength after DC application decreased. On the other hand, in Examples 1-3, because the thickness of the grain boundary between Al2O3 and YSZ was greater than 2 nm, it was found that the flexural strength could be increased while suppressing the decrease in bonding strength after DC application. [Table 2] [Explanation of symbols]

[0089] 2… Circuit boards for semiconductor equipment 3…Ceramic substrate 4,4'...Copper plate

Claims

1. Al and, Zr and, Y and, It contains, Al content 2 O 3 Converted to 75-90% by mass, Zr content is ZrO 2 Converted to 5-14% by mass, The amount of Y is Y 2 O 3 This is equivalent to 0.3 to 2.0 mass percent. As a crystalline phase, Al 2 O 3 It contains (alumina) particles and YSZ (yttria partially stabilized zirconia) particles, The average distance between the nearest YSZ particles is 0.50 to 1.20 μm. Ceramic substrate.

2. The above-mentioned Al 2 O 3 a first grain boundary formed between the particles and the YSZ particles is amorphous, The aforementioned Al 2 O 3 The second grain boundary formed between the particles is amorphous. The thickness of the first grain boundary is 2.0 to 5.0 nm. The ceramic substrate according to claim 1.

3. The thickness of the first grain boundary is greater than the thickness of the second grain boundary. The ceramic substrate according to claim 2.

4. The thickness of the second grain boundary is 0.5 to 1.5 nm. The ceramic substrate according to claim 2.

5. The first grain boundary and the second grain boundary contain at least one element selected from Al, Si, O, Ca, Y, Zr, Sr, and Ba. The ceramic substrate according to claim 2.

6. Al and, Zr and, Y and, It contains, Al content 2 O 3 Converted to 75-90% by mass, The Zr content is 5 to 14% by mass, in terms of ZrO2. The amount of Y is Y 2 O 3 This is equivalent to 0.3 to 2.0 mass percent. As a crystalline phase, Al 2 O 3 It contains (alumina) particles and YSZ (yttria partially stabilized zirconia) particles, The aforementioned Al 2 O 3 The first grain boundary formed between the particle and the YSZ particle is amorphous. The aforementioned Al 2 O 3 The second grain boundary formed between the particles is amorphous. The thickness of the first grain boundary is 2.0 to 5.0 nm. Ceramic substrate.

7. The thickness of the first grain boundary is greater than the thickness of the second grain boundary. The ceramic substrate according to claim 6.

8. The thickness of the second grain boundary is 0.5 to 1.5 nm. The ceramic substrate according to claim 6.

9. The first grain boundary and the second grain boundary contain at least one element selected from Al, Si, O, Ca, Y, Zr, Sr, and Ba. The ceramic substrate according to claim 6.

10. The average particle size of the YSZ particles is 0.50 to 0.90 μm. A ceramic substrate according to claim 1 or 6.

11. The bending strength is 700 MPa or more. A ceramic substrate according to claim 1 or 6.

12. A substrate for semiconductor devices for mounting electronic components, A ceramic substrate according to claim 1 or 6, A copper plate bonded to at least one surface of the ceramic substrate, A substrate for semiconductor devices, equipped with the following features.