Ceramic substrate and semiconductor device substrate comprising same

Abstract

A ceramic substrate according to the present invention comprises Al, Zr, and Y, wherein: the Al content is 75-90 mass% in terms of Al2O3; the Zr content is 5-14 mass% in terms of ZrO2; the Y content is 0.3-2.0 mass% in terms of Y2O3; alumina (Al2O3) particles and yttria partially stabilized zirconia (YSZ) particles are contained as a crystal phase; and the average distance between YSZ particles that are positioned closest to each other is 0.50-1.20 μm.

Description

Ceramic substrate and substrate for semiconductor device including same

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

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

[0003] JP 2022-515808 A, International Publication No. 2012 / 060341 A, International Publication No. 2020 / 115868 A

[0004] In the substrate for semiconductor device described above, a copper plate is bonded to the surface of the ceramic substrate. However, the present inventors have found that when the oxygen ion conductivity of the ceramic substrate increases, the bonding strength between the ceramic substrate and the copper plate decreases when a DC voltage is applied, and there is a risk that the copper plate may peel off.

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

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

[0007] Item 2. The ceramic substrate according to Item 1, wherein a first grain boundary formed between the Al2O3 particle and the YSZ particle is amorphous, a second grain boundary formed between the Al2O3 particle is amorphous, and a thickness of the first grain boundary is 2.0 to 5.0 nm.

[0008] Item 3. The ceramic substrate according to Item 2, wherein the thickness of the first grain boundary is greater than the thickness of the second grain boundary.

[0009] Item 4. The ceramic substrate according to Item 2, wherein the thickness of the second grain boundary is 0.5 to 1.5 nm.

[0010] Item 5. The ceramic substrate according to any one of Items 2 to 4, wherein 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.

[0011] Item 6. A ceramic substrate comprising Al, Zr, and Y, wherein the Al content is 75 to 90 mass % calculated as Al2O3, the Zr content is 5 to 14 mass % calculated as ZrO2, and the Y content is 0.3 to 2.0 mass % calculated as Y2O3, the substrate containing Al2O3 (alumina) particles and YSZ (yttria-partially stabilized zirconia) particles as a crystalline phase, wherein first grain boundaries formed between the Al2O3 particles and the YSZ particles are amorphous, and second grain boundaries formed between the Al2O3 particles are amorphous, and the thickness of the first grain boundaries is 2.0 to 5.0 nm.

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

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

[0014] Item 9. The ceramic substrate according to any one of Items 6 to 8, wherein 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.

[0015] Item 10. The ceramic substrate according to any one of Items 1 to 9, wherein the average particle size of the YSZ particles is 0.50 to 0.90 μm.

[0016] Item 11. The ceramic substrate according to any one of Items 1 to 10, having a bending strength of 700 MPa or more.

[0017] Item 12. A substrate for a semiconductor device for mounting electronic components, comprising: the ceramic substrate according to any one of Items 1 to 11; and a copper plate bonded to at least one surface of the ceramic substrate.

[0018] According to the present invention, when a copper plate is bonded to a ceramic substrate, peeling between the ceramic substrate and the copper plate can be suppressed when a DC voltage is applied.

[0019] FIG. 1 is a cross-sectional view showing an embodiment of a semiconductor device including a semiconductor device substrate according to the present invention. FIG. 2 is a diagram explaining a method for calculating distance CR. FIG. 3 is a diagram obtained by image processing an SEM image (EBSD) of Example 1, and a graph showing the distribution of the distance between the centers of gravity of each YSZ particle. FIG. 4 is a diagram obtained by observing the thickness of a first grain boundary in Example 1 using STEM. FIG. 5 is a diagram obtained by observing the thickness of a second grain boundary in Example 1 using STEM. FIG. 6 is a result of STEM-EDX analysis of elements present in the first grain boundary in Example 1. FIG. 7 is a result of STEM-EDX analysis of elements present in the second grain boundary in Example 1.

[0020] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A ceramic substrate and a substrate for a semiconductor device using the same according to an embodiment of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a cross-sectional view of a semiconductor device having a substrate for a semiconductor device according to this embodiment.

[0021] <1. Overview of Semiconductor Device> The semiconductor device according to this embodiment is 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, home microwave ovens, induction electric rice cookers, and UPS (uninterruptible power supplies).

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

[0023] The substrate 2 for a semiconductor device is a so-called DBOC (Direct Bonding of Copper Substrate) substrate, and includes a plate-shaped ceramic substrate 3, which is an insulator, a first copper plate 4 bonded to one surface (top surface) of the ceramic substrate 3, and a second copper plate 4' bonded to the other surface (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, i.e., to a part of the upper surface of the first copper plate 4, via a first bonding material 5. The semiconductor chip 6 and the first copper plate 4 are also 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, i.e., the lower surface of the second copper plate 4′, via a second bonding material 5′. The heat sink 8 is a known material and can be made of a metal such as copper.

[0027] 2. Structure of the Ceramic Substrate Next, the ceramic substrate 3 will be described in detail. Hereinafter, a non-crystalline state will be referred to as amorphous (including amorphous), and an amorphous material will be referred to as vitreous. The ceramic substrate 3 contains alumina (Al2O3), zirconia (ZrO2), and yttria (YO3), and may also contain a vitreous material and the remainder. The vitreous material may also contain a first component and a second component. The first component may contain, as elements, Si, an alkali metal, and O, and the second component may contain, as elements, at least one element selected from Ca, Sr, and Ba. However, the vitreous material may also contain other components. The content of the constituent elements of the ceramic substrate 3 will be described below.

[0028] <2-1. Components other than glass> The matrix component of the ceramic substrate 3 is made of alumina. The content of alumina, for example, is preferably 75% by mass or more and 90% by mass or less, and more preferably 85% by mass or more and 90% by mass or less, calculated as Al2O3.

[0029] Regarding the zirconia content, the Zr content, calculated as ZrO2, is preferably 5% by mass or more and 14% by mass or less, and more preferably 10% by mass or more and 14% by mass or less. By setting the zirconia content to 5% by mass or more, the strength of the ceramic substrate 3 (e.g., the flexural strength described below) can be improved. Furthermore, it is believed that the linear thermal expansion coefficient of the ceramic substrate 3 can be prevented from becoming too small, thereby reducing the difference in the linear thermal expansion coefficient between the ceramic substrate 3 and the first and second copper plates 4, 4'. As a result, it is believed that the thermal stress generated at the bonding interface can be reduced, contributing to the prevention of cracks occurring in the ceramic substrate 3 at the bonding interface.

[0030] Furthermore, Cu diffused from the copper plates 4, 4' bonded to the ceramic substrate 3 diffuses into the ceramic substrate 3 preferentially through the zirconia crystal grains rather than through the alumina crystal grains constituting the matrix component, thereby 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 setting the zirconia content to 14% by mass or less, it is believed that excessive reaction at the bonding interface during copper plate bonding can be suppressed, and the generation of voids at the bonding interface can be suppressed. This is due to the difference in wettability between alumina and zirconia with the Cu-O eutectic liquid phase during copper plate bonding. Furthermore, by setting 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 content of yttria, the Y content, calculated as YO, is preferably 0.3% by mass or more and 2.0% by mass or less, and more preferably 0.5% by mass or more and 1.2% by mass or less. By setting the content to 0.3% by mass or more, it is believed that the proportion of the monoclinic phase in the zirconia crystal phase can be prevented from becoming excessive, while the proportion of the tetragonal phase can be increased. As a result, it is believed that the mechanical strength of the ceramic substrate 3 can be improved, and that this contributes to the prevention of cracks occurring in the ceramic substrate 3 at the bonding interface.

[0033] On the other hand, by setting 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 prevented from becoming excessive, while the proportion of tetragonal crystals can be increased, which is thought to improve the mechanical strength of the ceramic substrate 3 and contribute to preventing cracks from occurring 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-partially stabilized zirconia (YSZ), in which yttria is solid-dissolved in zirconia. However, zirconia and yttria may exist independently. However, in the present invention, these two forms will be described as equivalent. Therefore, even when zirconia and yttria are described individually, they may exist as yttria-partially stabilized zirconia in the ceramic substrate 3. The content of YSZ in the ceramic substrate 3 is preferably 5.3 to 15 mass%, and more preferably 10 to 15 mass%.

[0035] Furthermore, the mass ratio of the total content of zirconia and yttria to the content of alumina is preferably 0.05 or more. This is because a mass ratio of 0.05 or more can improve the flexural strength. On the other hand, this mass ratio is preferably 0.30 or less. This is because zirconia has a lower thermal conductivity than alumina, and if this mass ratio exceeds 0.30, there is a possibility that the insulating heat dissipation circuit board will not be able to maintain sufficient thermal conductivity. Note that the presence of the above-mentioned alumina, zirconia, and yttria as oxides themselves in the ceramic substrate 3 can be verified by XRD or the like.

[0036] <2-2. Vitreous Components> Next, the vitreous will be described. As described above, the vitreous may contain a first component including elements such as Si, an alkali metal, and O, and a second component including at least one element selected from Ca, Sr, and Ba. However, since these elements exist primarily as oxides such as silica (SiO), alkali metal oxides, calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO), examples of these will be described below. Note that the first component may contain Mg, which may exist in the vitreous as an oxide such as magnesia (MgO). Furthermore, while a vitreous component is not essential, the following description will be given assuming that a vitreous component is included.

[0037] First, the first component will be described. Regarding the silica content, the Si content, calculated as SiO2, is preferably 0.1 mass % or more and 2.5 mass % or less. As will be described later, a silica content of 0.1 mass % or more suppresses the oxygen ion conductivity of the ceramic substrate 3, thereby improving impedance. On the other hand, a high silica content may result in a decrease in the strength of the ceramic substrate 3. To prevent this, the silica content is preferably 2.5 mass % or less.

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

[0039] On the other hand, it is believed that by setting the magnesia content to 0.5 mass % or less, it is possible to suppress the excessive formation of spinel crystals, which have low mechanical strength, and improve the mechanical strength of the ceramic substrate 3. As a result, it is believed that this contributes to suppressing the occurrence of cracks in the ceramic substrate 3 at the bonding interface.

[0040] Alkali metal ions break the Si-O bonds of SiO2 and form non-bridging oxygen (NBO), which then serves as a diffusion site for ions, resulting in oxygen ion conductivity. Examples of such alkali metals include sodium, potassium, and lithium, and two or more of these may be included. These may be contained in the ceramic substrate 3 as sodium oxide (Na2O), potassium oxide (KO2O), and lithium oxide (Li2O). The inventors have confirmed that the effects shown in the examples below can be obtained regardless of the use of any of these alkali metal oxides.

[0041] The second components are all glassy network-modifying oxides that are incorporated into the framework of the glassy network-forming oxides, primarily silica (SiO2), and act to change the chemical and physical properties of the glassy material. In particular, when calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO) are contained in a glassy material containing an alkali metal oxide as described above, ionic conduction through NBOs as diffusion sites is suppressed, thereby reducing electrical conductivity. Furthermore, the effect of reducing electrical conductivity increases in the order of CaO < SrO < BaO.

[0042] The reason for this is that the larger the difference in radius between a monovalent alkali metal ion and a divalent ion, the greater the blocking effect of the divalent ion that inhibits the migration of the alkali metal ion, and the lower the polarizability of the non-bridging oxygen ion, which strengthens the bond between adjacent alkali metal ions and oxygen ions, thereby reducing the mobility of the alkali metal ions.

[0043] For this reason, the insulating properties can be further improved by adding CaO, SrO, or BaO to a glassy material containing an alkali metal oxide, which makes it possible to improve the problem to be solved by the present invention, namely, the property that oxygen ions are conducted from the negative electrode side to the positive electrode side inside the ceramic substrate by applying a DC voltage, resulting in a decrease in the bonding strength between the electrode and the ceramic substrate.

[0044] The second vitreous component may contain one or more of CaO, SrO, and BaO.

[0045] The glass may also contain Al, Zr, and Y.

[0046] In this ceramic substrate, the mass ratio of the alkali metal oxide content to the second component content, calculated as a vitreous 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, a significant decrease in the bonding strength between the ceramic substrate 3 and the copper plate after application of a DC voltage can be suppressed.

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

[0048] Furthermore, since sodium affects electrical conductivity, the content of sodium in the ceramic substrate 3 in terms of oxide is preferably 0 mass % or more than 0 mass % but not more than 0.05 mass %.

[0049] The content of the glassy oxide in the ceramic substrate 3 can be, for example, 0.9% by mass to 3% by mass, 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 bending strength cannot be obtained.

[0050] The content of the remainder is preferably 0.5% by mass or less, and more preferably 0.05% by mass or less, calculated as oxide. This is thought to prevent 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, it is thought to improve the mechanical strength of the ceramic substrate 3 and contribute to preventing cracks from occurring in the ceramic substrate 3 at the bonding interface with the copper plate 4.

[0051] In this embodiment, the contents of the constituent elements of the ceramic substrate 3 are calculated in terms of oxides as described above, but 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 may be solid-solved in ZrO. Some alkali metals may not exist in the form of oxides. The contents of all alkali metals not existing in the form of oxides are also calculated in terms of oxides.

[0052] The content of the constituent elements of the ceramic substrate 3 in terms of oxide is calculated as follows. First, the constituent elements of the ceramic substrate 3 are qualitatively analyzed using an X-ray fluorescence analyzer (XRF) or an energy dispersive X-ray fluorescence analyzer (EDX) attached to a scanning transmission electron microscope (STEM). Next, each element detected by this qualitative analysis is quantitatively analyzed using an ICP optical emission spectrometer. Next, the content of each element measured by this quantitative analysis is converted into its oxide.

[0053] The element contained in the balance may be an element that is intentionally added or an element that is unavoidably mixed in. The element contained in the balance is not particularly limited, but examples thereof include Fe (iron), Ti (titanium), and Mn (manganese).

[0054] Furthermore, the glassy substance described above does not exist within alumina particles or YSZ particles, but exists at grain boundaries, which are regions surrounded by alumina particles or YSZ particles. The grain boundaries are amorphous, and there are three types, for example, Al2O3 / Al2O3, Al2O3 / YSZ, and YSZ / YSZ. From a macroscopic perspective, there are two types of grain boundaries: the interior of the ceramic substrate 3 that is not exposed to the outside (the three types described above), and the surface layer that is exposed to the outside. Furthermore, there is a possibility that trace amounts of glassy components may be dissolved in Al2O3 and YSZ. According to the present inventors, of the three grain boundaries described above, the YSZ / YSZ grain boundary has little effect on the bonding strength between the ceramic substrate 3 and the copper plates 4, 4', and therefore will not be considered in the grain boundary discussion described below.

[0055] <3. Consideration of Bonding Strength Between Ceramic Substrate and Copper Plates> Next, the bonding strength between the ceramic substrate 3 and the copper plates 4, 4' will be considered. In the substrate 2 for a semiconductor device described above, the copper plates 4, 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, 4' decreases when a DC voltage is applied, and there is a risk that the copper plates 4, 4' may peel off. This point will be explained.

[0056] First, Cu-O-Al bonds are formed at the interfaces between the first copper plate 4 and the second copper plate 4' and the ceramic substrate 3 during the bonding process due to the generation and solidification of a 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' to the positive electrode and a DC voltage is applied between 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. Oxygen ions generated by this reduction migrate 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 or zirconia particles inside the ceramic substrate 3. This is because 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 even when an AC voltage is applied. However, some oxygen ions do move, so the ease with which oxygen ions move 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 present inventors have made the following discovery. First, the pathways along which oxygen ions generated as described above are conducted will be explained. Oxygen ions are mainly conducted within the YSZ grains, which are their conductors. Therefore, as described above, an increase in the content of zirconia and yttria improves the bending strength, but the number of oxygen ion conduction pathways increases, which may result in a decrease in the bonding strength between the ceramic substrate 3 and the copper plates 4, 4'. In response to this, the present inventors have made the following discovery.

[0059] <3-1. Consideration of Interparticle Distance> As described above, oxygen ions conduct through YSZ particles. Therefore, as the distance between YSZ particles increases, the conductivity of oxygen ions decreases. Therefore, the present inventors have made the following findings regarding interparticle distance. First, an image of a cross section of a ceramic substrate (e.g., an image acquired by EBSD) was captured. For example, the surface of the ceramic substrate was polished using a polishing method such as ion milling to expose a cross section near the center of the ceramic substrate's thickness, and the exposed cross section was then photographed. Next, the projected area circle equivalent diameter (so-called Heywood diameter, the diameter of a circle having the same area as the projected area of ​​the particle) of all YSZ particles observed within a predetermined range (e.g., within a 30 μm × 30 μm range) in the captured image was calculated. At this time, image processing was performed to make it easier to distinguish the YSZ particles. For example, as shown in Figure 3 (described later), the YSZ particle region extracted by EBSD and the regions of other compositions were binarized, and all YSZ particles were displayed in white. The software used for the binarization process was OIM Analysis 7.3 (TSL Solutions, Inc.). Then, using the processed image, the distance CR between each YSZ particle and the nearest YSZ particle was calculated using the following formula (1) (see Figure 2): Distance CR = r - (R1 + R2) (1) where r is the distance between the centers of gravity of adjacent YSZ particles. The center of gravity was set, for example, according to Green's theorem from the outline of the YSZ particle extracted by the image processing (for example, the "+" next to the YSZ particle area in Figure 3, described later, corresponds to the center of gravity). The YSZ particles defined by the binarization process also include aggregates formed by the aggregation of YSZ particles. R1 and R2 are the radii of adjacent YSZ particles (half the diameter of the circle equivalent to the projected area).

[0060] In this way, the average of the distances CR calculated for each YSZ particle is calculated. Note that, according to confirmation by the present inventors, the average distances CR of YSZ particles observed at least in the above-mentioned size range are generally the same at any position on the ceramic substrate.

[0061] The inventors have found that 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 greater, the distance between adjacent YSZ particles increases, resulting in lower oxygen ion conductivity. In other words, a decrease in the 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, there is a risk of a decrease in flexural strength, so a distance of 1.20 m or less is preferable. The average particle size of the YSZ particles is, for example, 0.50 to 0.90 μm. By setting the average particle size within the range of 0.50 to 0.90 μm, YSZ exhibits good dispersibility and acts as an obstacle to crack propagation, which is particularly effective in improving fracture toughness. On the other hand, if the average particle size of YSZ is outside the above range, there is a risk that the mechanical strength will decrease due to a decrease in the degree of sintering caused by a deterioration in the dispersibility of YSZ, or that the fracture toughness will decrease because YSZ is not included in the crack propagation path.

[0062] <3-2. Consideration of Grain Boundaries> Next, the grain boundaries of Al2O3 particles and YSZ particles, which are crystalline phases, will be explained. The grain boundaries between Al2O3 particles and between Al2O3 particles and YSZ particles will be examined. Since Al2O3 particles do not conduct oxygen ions well, the grain boundaries between Al2O3 particles serve as conduction paths. However, it is known that oxygen ions do not conduct well when these grain boundaries are amorphous. Furthermore, oxygen ions do not conduct well when the grain boundaries between Al2O3 particles and YSZ particles are amorphous.

[0063] In response to this, the present inventors have made the following discovery. First, the pathways along which oxygen ions generated as described above are conducted will be explained. Oxygen ions are mainly conducted within the YSZ grains, which are their conductors. Therefore, as described above, an increase in the content of zirconia and yttria improves the bending strength, but the number of oxygen ion conduction pathways increases, which may result in a decrease in the bonding strength between the ceramic substrate 3 and the copper plates 4, 4′.

[0064] The inventors have found that increasing the thickness of the amorphous grain boundaries between Al2O3 particles and YSZ particles in particular can suppress the penetration of oxygen ions that attempt to pass through the YSZ particles, thereby effectively suppressing oxygen ion conduction. In other words, it is possible to suppress a decrease in the bonding strength between the ceramic substrate 3 and the copper plates 4, 4' when a DC voltage is applied. Using STEM (Scanning Transmission Electron Microscope) observation images, regions with irregular structural arrangements were defined as amorphous.

[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 the YSZ particles is enhanced. When the thickness of the grain boundary between Al2O3 and YSZ exceeds 5.0 nm, the bending strength may decrease. The thickness of the first grain boundary was measured by 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 oxygen ion penetration, 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 at least the second grain boundaries is 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 grain boundary thickness in order to suppress a decrease in the bonding strength between the ceramic substrate 3 and the copper plates 4, 4' when a DC voltage is applied, and found that by measuring either one of them and ensuring that the above-mentioned range is satisfied, a decrease in bonding strength can be suppressed.

[0067] 4. Manufacturing Method of Ceramic Substrate and Substrate for Semiconductor Device> Next, a manufacturing method of a ceramic substrate and a substrate for semiconductor device will be described. First, powder materials of the above-described constituent elements are mixed. Next, the mixed powder materials are pulverized and mixed using, for example, a planetary mill or a bead mill.

[0068] The pulverized and mixed powder material is then loaded into a graphite die and sintered using an SPS (spark plasma sintering) apparatus to produce a completed ceramic substrate. In this case, the pressure is preferably 35 to 45 MPa, more preferably 40 to 45 MPa. The maximum sintering temperature is preferably 1400 to 1700°C, more preferably 1580 to 1700°C. The heating rate is preferably 50 to 70°C / min, more preferably 60 to 70°C / min. The holding time at the maximum temperature is preferably 5 to 15 minutes, more preferably 10 to 15 minutes. The substrate is then slowly cooled to room temperature over 3 to 5 hours.

[0069] The fired ceramic body is then processed to a predetermined thickness to produce a ceramic substrate. Examples of the processing include double-sided lapping, but the method is not particularly limited.

[0070] Empirically, if the heating rate is higher than the set value, the number of voids inside the ceramic substrate increases proportionally, leading to a decrease in mechanical strength. Furthermore, since the maximum temperature is above the melting point of typical glass, the longer the time held above the glass melting point, including the heating process and the maximum temperature, the more grain growth in the matrix base material is promoted, leading to a tendency for the mesh-like glass network at the grain boundaries to break down and exhibit scattered behavior. This weakens the electrical insulating effect. Therefore, it is preferable to shorten the heating time by increasing the heating rate as described above and shorten the holding time at the maximum temperature. The firing conditions described above were optimized based on this mechanism.

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

[0072] Subsequently, the laminate is cooled to solidify the Cu-O eutectic liquid phase, and the copper plates 4, 4' are bonded to the ceramic substrate 3. This completes the semiconductor device substrate 2. The transmission circuit formed on the copper plate 4 on the surface to which the semiconductor chip 6 is bonded can be formed by, for example, a subtractive method or an additive method.

[0073] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0074] <1. Preparation of Ceramic Substrates> Ceramic substrates according to Examples 1 to 3 and Comparative Examples 1 and 2 were prepared, consisting primarily of the following materials. The compositions of these ceramic substrates are shown in Table 1. Specifically, powder materials prepared in predetermined proportions were first pulverized 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 qualitative and quantitative analysis of the constituent elements contained in the sintered body, and the values ​​can be calculated in terms of oxides (Al2O3, SiO2, CaO, SrO, BaO, MgO, Na2O). The YSZ content is the sum of the ZrO2 and YO3 contents.

[0075] Next, the pulverized and mixed powder materials were loaded into a graphite die and sintered using an SPS apparatus while the die was pressurized at 45 MPa. The die was heated to 1600°C at a heating rate of approximately 50°C / min. The maximum temperature of 1600°C was then maintained for 15 minutes, after which it was slowly cooled over approximately 1 hour. This resulted in a ceramic sintered body. This ceramic sintered body was then formed into a 0.32 mm thick, 40 mm long, and 40 mm wide ceramic substrate.

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

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

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

[0079] The bonding strength was measured as follows. First, a predetermined length of the bonded rectangular copper plates was peeled off from the end in the long-side direction 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 obtained at this time was defined as the measured strength. That is, bonding strength (kg / cm) = maximum pulling load (kg) / length of short side of rectangle (cm). Then, the ratio of the bonding strength after DC application to the bonding strength before DC application was calculated, and this was defined as the bonding strength ratio (%). Note that this bonding strength ratio is preferably 95% or more, and more preferably 98% or more.

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

[0081] (3) Interparticle Distance CR For Examples 1 to 3 and Comparative Example 1, SEM images were taken within a 30 μm x 30 μm area. Using the method described above, the distance CR between each YSZ particle within this area and 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 between all YSZ particles was calculated. As an example, an SEM image of Example 1 is shown. Figure 3 is a graph showing the SEM image of Example 1 and 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 were subjected to STEM observation and STEM-EDX analysis. The test conditions were as follows. Note that for Examples 1 to 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 taken as the average value. The radius R, center-of-gravity 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 positions: (N=1) Center of cross section at the center of the ceramic sintered body (first field of view) (N=2) Center of cross section 20 mm from the center of the ceramic sintered body (second field of view)

[0083] (5) Discussion of bonding strength and bending strength The results are shown in Table 2. FIG. 4 shows the thickness of the first grain boundary in Example 1, and FIG. 5 shows the thickness of the second grain boundary. FIG. 6 shows the elements present in the first grain boundary in Example 1, and FIG. 7 shows the elements present in the second grain boundary. In FIGS. 4 and 5, BF-STEM refers to bright field STEM observation images, and HAADF-STEM refers to high-angle annular dark field STEM observation images.

[0084] The thickness of each grain boundary was measured as follows: (i) The measurement field of view was adjusted so that the grain boundaries were aligned in a straight line at a magnification of 2-3 million times. The field of view was approximately 20 nm square. The grain boundaries were identified based on the contrast difference between the Al2O3 and YSZ on either side of the boundary. At this stage, the variation in grain boundary thickness within the field of view was extremely small, so the grain boundary thickness can be said to be constant regardless of the measurement position. (ii) HAADF-STEM images were acquired (in this example, two fields of view (two observation positions) per sample). (iii) The grain boundary thickness of an arbitrary region within each field of view was measured, and the average was taken as the grain boundary thickness.

[0085] As shown in Figures 4 and 5, the STEM observation images revealed that the grain boundaries were amorphous. Furthermore, it was found that in Example 1, the thickness of the first grain boundary was greater than the thickness of the second grain boundary. This tendency was also observed 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 from each grain boundary. As shown in Figure 6, the abundance ratios of elements in the first grain boundary were in the order Zr > Y > Si > Ca > Mg. On the other hand, the abundance ratios of elements in the second grain boundary were in the order Si > Zr > Ca > Mg > Y.

[0086] The reason why Na, as shown in Table 1, was not detected at the first and second grain boundaries 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 easily contained in network-forming oxides primarily composed of silica (SiO2). An example is soda-lime glass. From the above, it is highly likely that the first and second components described above are contained at the first and second grain boundaries, which is thought to contribute to suppressing the decrease in bonding strength after DC application.

[0087] In Examples 1 to 3, the decrease in bond strength was less than 2% before and after DC application, indicating that a high level of bond 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, each contributing to the maintenance of bond strength. On the other hand, in Comparative Example 1, the bond strength decreased by approximately 99% before and after DC application. This is thought to be due to the thickness of the grain boundary between Al2O3 and YSZ being less than 2 nm. Furthermore, the lower flexural strength compared to Examples 1 to 3 and Comparative Example 2 is thought to be due to the average distance CR being greater than 1.20 μm. In Comparative Example 2, 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, resulting in a low bond strength ratio of 2%. Note that Comparative Example 2's flexural strength is thought to be higher than Comparative Example 1 due to the higher YSZ content.

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

[0089] 2... Substrate for semiconductor device 3... Ceramic substrate 4, 4'... Copper plate

Claims

1. A ceramic substrate comprising Al, Zr, and Y, wherein the Al content is 75 to 90 mass % calculated as Al2O3, the Zr content is 5 to 14 mass % calculated as ZrO2, and the Y content is 0.3 to 2.0 mass % calculated as Y2O3, and the substrate contains Al2O3 (alumina) particles and YSZ (yttria-partially stabilized zirconia) particles as a crystalline phase, and the average distance between the nearest YSZ particles is 0.50 to 1.20 μm.

2. The ceramic substrate according to claim 1, wherein first grain boundaries formed between the Al2O3 particles and the YSZ particles are amorphous, second grain boundaries formed between the Al2O3 particles are amorphous, and the thickness of the first grain boundaries is 2.0 to 5.0 nm.

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

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

5. The ceramic substrate according to claim 2, wherein 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.

6. A ceramic substrate comprising Al, Zr, and Y, wherein the Al content is 75 to 90 mass % calculated as Al2O3, the Zr content is 5 to 14 mass % calculated as ZrO2, and the Y content is 0.3 to 2.0 mass % calculated as Y2O3, the substrate containing Al2O3 (alumina) particles and YSZ (yttria-partially stabilized zirconia) particles as a crystalline phase, wherein first grain boundaries formed between the Al2O3 particles and the YSZ particles are amorphous, and second grain boundaries formed between the Al2O3 particles are amorphous, and the thickness of the first grain boundaries is 2.0 to 5.0 nm.

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

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

9. The ceramic substrate according to claim 6, wherein 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.

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

11. The ceramic substrate according to claim 1 or 6, having a bending strength of 700 MPa or more.

12. A substrate for a semiconductor device for mounting electronic components, comprising: the ceramic substrate according to claim 1 or 6; and a copper plate bonded to at least one surface of the ceramic substrate.

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