Patent Description:
Silicon nitride ceramics are highly heat conductive and highly insulating. A bonded substrate in which a copper plate is bonded to a silicon nitride ceramic substrate through a bonding layer is thus suitable for use as an insulating heat-dissipating substrate on which a power semiconductor device is mounted.

Electrical breakdown of the bonded substrate occurs in two modes. In the first mode, due to application of an electric field exceeding a dielectric breakdown voltage in particles constituting the silicon nitride ceramic substrate or of grain boundaries between the particles to the bonded substrate, electrical breakdown starts to occur at an end of the copper plate on which the electric field is concentrated. In the second mode, as a result that a crack is formed by partial discharge starting at a defect, such as a void, of the silicon nitride ceramic substrate and the formed crack is developed, electrical breakdown starts to occur at the defect.

Electrical breakdown occurring in the above-mentioned first mode can easily be found by conducting a withstand voltage test on the bonded substrate before shipment of the bonded substrate as a product. Shipment of the bonded substrate wherein electrical breakdown might occur in the above-mentioned first mode is thus easily avoided.

Defects of the silicon nitride ceramic substrate can be classified into a defect isolated in the substrate and a defect exposed to the surface of the substrate.

The defect isolated in the substrate is formed by inclusion of foreign matter into a raw slurry, retention of coarse agglomerated particles in the raw slurry caused due to poor dispersion of a raw powder in the raw slurry, and the like in the process of manufacturing the silicon nitride ceramic substrate.

The defect exposed to the surface of the substrate is formed by adherence of foreign matter to the surface of an intermediate product manufactured in the process of manufacturing the silicon nitride ceramic substrate, volatilization of an aid from the surface of the intermediate product during sintering of the intermediate product, and the like.

From among these defects, the defect isolated in the substrate can easily be reduced by filtration of the raw slurry before molding and the like. It is thus easily avoided to ship the bonded substrate in which electrical breakdown due to partial discharge starting at the defect isolated in the silicon nitride ceramic substrate, among electrical breakdown in the above-mentioned second mode, might occur.

However, it is not easy to find partial discharge starting at the defect exposed to the surface of the silicon nitride ceramic substrate even when the withstand voltage test is conducted on the bonded substrate before shipment. A partial discharge test may be conducted on the bonded substrate before shipment, but it is not easy to measure a pA-order micro-current flowing due to partial discharge, and, even in a case of having equipment enabling such measurement in principle, it is necessary to take measures against noise for such measurement, and thus it may be necessary to introduce expensive equipment. It is thus not realistic to conduct the partial discharge test on all the bonded substrates before shipment.

It is also not easy to find the defect exposed to the surface of the silicon nitride ceramic substrate through imaging of the silicon nitride ceramic substrate to which the copper plate has not been bonded. This is because such a defect only has a diameter of approximately <NUM> and a depth of approximately <NUM>, it is necessary to introduce expensive equipment to find the defect, and it is not realistic to conduct imaging on all the silicon nitride ceramic substrates used for the bonded substrates to be shipped using such equipment.

The defect exposed to the surface of the silicon nitride ceramic substrate formed by a cause as described above cannot easily be reduced in the first place.

For reasons as described above, it cannot easily be avoided to ship the bonded substrate in which electrical breakdown due to partial discharge starting at the defect exposed to the surface of the silicon nitride ceramic substrate, from among electrical breakdown occurring in the second mode, might occur.

In technology disclosed in <CIT>, a brazing material pattern is printed on opposite sides of a silicon nitride substrate, and the opposite sides of the silicon nitride substrate and copper plates are bonded. In the technology disclosed in <CIT>, a form of the surface of a ceramic substrate is controlled by a method of introducing few defects in the surface to obtain the ceramic substrate having desirable partial discharge properties. In US Patent Application <CIT>, a brazing material is applied between a silicon nitride substrate and a copper plate filling the particle-defect holes of the substrate.

In Japanese Patent Application <CIT>, a plated brazing material is applied between a ceramic substrate and a metal layer.

In <CIT>, a brazing material is applied between a copper plate and a silicon nitride substrate.

As described above, in a case of a conventional bonded substrate, shipment of the bonded substrate in which electrical breakdown might occur due to partial discharge starting at the defect exposed to the surface of the silicon nitride ceramic substrate as a product cannot easily be avoided. The problem arises also when the silicon nitride ceramic substrate is replaced with another ceramic substrate.

The present invention has been conceived in view of the problem. It is an object of the present invention to provide a bonded substrate in which electrical breakdown occurring due to partial discharge starting at a defect exposed to the surface of a ceramic substrate is suppressed.

The present invention relates to a bonded substrate.

The bonded substrate includes a ceramic substrate, a copper plate, and a bonding layer.

The ceramic substrate has a main surface having a flat region having a maximum height Rz of <NUM> or less. The ceramic substrate has a particle-defect hole being exposed to the main surface, imparting flatness lower than flatness of the flat region to a part of the main surface, and having a depth of <NUM> or more and <NUM> or less.

The copper plate includes a first portion disposed over the flat region and a second portion filling the particle-defect hole.

The bonding layer includes a third portion covering the flat region and a fourth portion filling the particle-defect hole. The bonding layer bonds the copper plate to the main surface. The second portion of the copper plate and the fourth portion of the bonding layer fill <NUM>% or more of a volume of the particle-defect hole.

The present invention also relates to a bonded substrate manufacturing method.

The bonded substrate manufacturing method includes a step of preparing a ceramic substrate, a step of forming a brazing material layer, a step of disposing a copper plate on the brazing material layer to obtain an intermediate product including the ceramic substrate, the brazing material layer, and the copper plate, and a step of hot pressing the intermediate product.

In the step of hot pressing the intermediate product, a portion of the copper plate is caused to enter the particle-defect hole. The copper plate is thereby deformed to include a first portion disposed over the flat region and a second portion filling the particle-defect hole. Furthermore, a portion of the brazing material layer is caused to enter the particle-defect hole. The brazing material layer is thereby changed into a bonding layer including a third portion covering the flat region and a fourth portion filling the particle-defect hole, and bonding the copper plate to the main surface. In the step, the second portion of the copper plate and the fourth portion of the bonding layer fill <NUM>% or more of a volume of the particle-defect hole.

According to the present invention, the bonded substrate in which the portion of the copper plate and the portion of the bonding layer fill <NUM>% or more of the volume of the particle defect hole exposed to the main surface of the ceramic substrate and having a depth of <NUM> or more and <NUM> or less can be obtained. In the bonded substrate, partial discharge starting at the particle-defect hole is suppressed. The bonded substrate in which electrical breakdown occurring due to partial discharge starting at a defect exposed to the surface of the ceramic substrate is suppressed can thereby be provided.

The objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description and the accompanying drawings.

<FIG> is a cross-sectional view schematically showing a bonded substrate <NUM> according to an embodiment of the present invention.

The bonded substrate <NUM> shown in <FIG> includes a silicon nitride ceramic substrate <NUM>, copper plates <NUM>, and bonding layers <NUM>.

In <FIG> and the following figures, a direction from the silicon nitride ceramic substrate <NUM> to the copper plates <NUM> in <FIG> is simply referred to as an "upward direction", and an opposite direction thereof is simply referred to as a "downward direction". Thus, in a case of <FIG>, a direction from the silicon nitride ceramic substrate <NUM> to an upper copper plate <NUM> in <FIG> and a direction from the silicon nitride ceramic substrate <NUM> to a lower copper plate <NUM> in <FIG> each correspond to the "upward direction", and directions opposite the directions each correspond to the "downward direction".

The copper plates <NUM> and the bonding layers <NUM> are arranged over main surfaces <NUM> of the silicon nitride ceramic substrate <NUM>. The bonding layers <NUM> bond the copper plates <NUM> to the main surfaces <NUM> of the silicon nitride ceramic substrate <NUM>.

The bonded substrate <NUM> may be used in any way, and is used, for example, as an insulating heat-dissipating substrate on which a power semiconductor device is mounted.

<FIG> is an enlarged cross-sectional view schematically showing a portion of the bonded substrate <NUM>.

Each of the main surfaces <NUM> of the silicon nitride ceramic substrate <NUM> has a flat region 11f as shown in <FIG>. The flat region 11f is a region having high flatness, and is preferably a region having a maximum height Rz of <NUM> or less. The maximum height Rz is a maximum height in a case where a reference length is <NUM>.

The silicon nitride ceramic substrate <NUM> has a particle-defect hole <NUM>. The particle-defect hole <NUM> is exposed to the main surface <NUM> of the silicon nitride ceramic substrate <NUM>, and imparts flatness lower than flatness of the flat region 11f to a part of the main surface <NUM> of the silicon nitride ceramic substrate <NUM>. The particle-defect hole <NUM> thus has a depth greater than the maximum height Rz of the flat region 11f. The particle-defect hole <NUM> is formed by adherence of foreign matter to a main surface of an intermediate product manufactured in the process of manufacturing the silicon nitride ceramic substrate <NUM>, volatilization of an aid from the main surface of the intermediate product during sintering of the intermediate product, and the like.

In the present embodiment, the particle-defect hole <NUM> is shown as a typical example or a representative example of a defect exposed to the surface of the silicon nitride ceramic substrate <NUM>. Thus, an inner surface 11i of the particle-defect hole <NUM> and the flat region 11f of the main surface <NUM> form a smoothly continuous curve in <FIG>, but this is just a schematic example. For example, an upper end of the inner surface 11i and the flat region 11f may form an angle at least in a portion of an entrance of the particle-defect hole <NUM>. Even if the upper end of the inner surface 11i and the flat region 11f form an angle in the silicon nitride ceramic substrate <NUM> before bonding of each of the copper plates <NUM>, the angled portion may be deformed with hot pressing for bonding.

The number of particle-defect holes <NUM> present in each of the main surfaces and the sizes of the particle-defect holes <NUM> vary among individual silicon nitride ceramic substrates <NUM>.

The copper plate <NUM> includes a first portion <NUM> disposed over the flat region 11f of the silicon nitride ceramic substrate <NUM> and a second portion <NUM> filling the particle-defect hole <NUM>. The second portion <NUM> is continuous with the first portion <NUM>, and is electrically connected to the first portion <NUM>.

Each of the bonding layers <NUM> includes a third portion <NUM> covering the flat region 11f and a fourth portion <NUM> filling the particle-defect hole <NUM>.

The copper plate <NUM> is bonded to the silicon nitride ceramic substrate <NUM> through the bonding layer <NUM>. The bonding layer <NUM> is generated from a brazing material layer containing active metal, and contains, as a material component, the active metal contained in the brazing material layer.

The active metal contained in the bonding layer <NUM> is at least one type of active metal selected from the group consisting of titanium and zirconium.

The bonding layer <NUM> contains metal other than the active metal. The metal other than the active metal contained in the bonding layer <NUM> is at least one type of metal selected from the group consisting of silver, copper, indium, and tin.

The bonding layer <NUM> contains nitrogen and/or silicon supplied from the silicon nitride ceramic substrate <NUM>. Nitrogen and/or silicon supplied from the silicon nitride ceramic substrate <NUM> and the active metal form a compound.

The bonding layer <NUM> may contain copper supplied from the copper plate <NUM>.

In the bonded substrate <NUM> having a configuration as described above, the second portion <NUM> of the copper plate <NUM> and the fourth portion <NUM> of the bonding layer <NUM> fill the particle-defect hole <NUM> to suppress partial discharge starting at the particle-defect hole <NUM>. A crack formed by partial discharge starting at the particle-defect hole <NUM> and, further, electrical breakdown occurring due to development of the crack (electrical breakdown occurring in the second mode) are thereby suppressed in the bonded substrate <NUM>. In description made below, the second portion <NUM> of the copper plate <NUM> and the fourth portion <NUM> of the bonding layer <NUM> filling the particle-defect hole <NUM> are generically referred to as a hole filling portion.

In the bonded substrate <NUM>, electrical breakdown not starting at the particle-defect hole <NUM> but starting at the hole filling portion filling the particle-defect hole <NUM> (electrical breakdown occurring in the first mode) might occur. This electrical breakdown, however, can be found by conducting a withstand voltage test on the bonded substrate <NUM>, and thus, by conducting the withstand voltage test on the bonded substrate <NUM> before shipment of the bonded substrate <NUM>, shipment of the bonded substrate <NUM> not meeting required specifications of a withstand voltage due to the electrical breakdown can be avoided. Thus, in the present embodiment, electrical breakdown occurring in the mode is considered to have no problem based on the assumption that the withstand voltage test is conducted before shipment.

As described above, in the bonded substrate <NUM> according to the present embodiment, occurrence of a failure in a manner that the crack starting at the particle-defect hole <NUM> is formed due to partial discharge occurring when an electric field is applied and then electrical breakdown occurs due to development of the crack, is suppressed. Furthermore, a failure caused by electrical breakdown starting at the hole filling portion can be avoided at least after shipment of the bonded substrate <NUM>.

More particularly, in the bonded substrate in which the particle-defect hole <NUM> has a depth of <NUM> or more, but the hole filling portion does not fill the particle-defect hole <NUM>, there is a possibility that the crack starting at the particle-defect hole <NUM> develops and electrical breakdown starting at the particle-defect hole <NUM> occurs. Thus, in a case where the particle-defect hole <NUM> has a depth of <NUM> or more, it is preferable to use a structure in which the hole filling portion fills the particle-defect hole <NUM> as in the present embodiment to avoid the occurrence of the electrical breakdown.

The particle-defect hole <NUM>, however, preferably has a depth of <NUM> or less. In a case where the particle-defect hole <NUM> has a depth of more than <NUM>, a recess might be formed in an upper main surface of the copper plate <NUM> when the second portion <NUM> of the copper plate <NUM> is formed. In a case where the particle-defect hole <NUM> has a depth of more than <NUM>, a crack might be formed in the silicon nitride ceramic substrate <NUM> when the copper plate <NUM> is bonded to the silicon nitride ceramic substrate <NUM>.

The particle-defect hole <NUM> more preferably has a depth of <NUM> or less. In a case where the particle-defect hole <NUM> has a depth of <NUM> or less, a withstand voltage of the bonded substrate <NUM> is equivalent to a withstand voltage of a bonded substrate in which a copper plate is bonded to a silicon nitride ceramic substrate not having the particle-defect hole <NUM>.

In a case where the particle-defect hole <NUM> has a depth of more than <NUM>, the withstand voltage of the bonded substrate <NUM> is lower than the withstand voltage of the bonded substrate in which the copper plate is bonded to the silicon nitride ceramic substrate not having the particle-defect hole <NUM>. Even in case of the bonded substrate <NUM>, however, the occurrence of the failure caused by electrical breakdown occurring due to development of the crack starting at the particle-defect hole <NUM> is suppressed by using the structure in which the hole filling portion fills the particle-defect hole <NUM>.

In the bonding structure in <FIG>, the hole filling portion fills the particle-defect hole <NUM> as a whole. However, there are some cases that partial discharge starting at the particle-defect hole <NUM> is sufficiently suppressed, even when the hole filling portion fills only a part of the particle-defect hole <NUM>. In this case, the hole filling portion preferably fills <NUM>% or more of the volume (capacity) of the particle-defect hole <NUM>, and more preferably fills <NUM>% or more of the volume of the particle-defect hole <NUM>.

Furthermore, in the bonding structure in <FIG>, the fourth portion <NUM> of the bonding layer <NUM> covers the inner surface 11i of the particle-defect hole <NUM>, and the second portion <NUM> of the copper plate <NUM> enters the particle-defect hole <NUM> on the fourth portion <NUM> of the bonding layer <NUM> to fill the particle-defect hole <NUM> as a whole.

In the bonding structure in <FIG>, the fourth portion <NUM> of the bonding layer <NUM> covers the inner surface 11i of the particle-defect hole <NUM> as a whole. Even in a case where the fourth portion <NUM> of the bonding layer <NUM> covers only a part of the inner surface 11i of the particle-defect hole <NUM>, however, the second portion <NUM> of the copper plate <NUM> may enter the particle-defect hole <NUM> on the fourth portion <NUM> of the bonding layer <NUM>. In this case, the fourth portion <NUM> of the bonding layer <NUM> preferably covers <NUM>% or more of the area of the inner surface 11i of the particle-defect hole <NUM>, and more preferably covers <NUM>% or more of the area of the inner surface 11i of the particle-defect hole <NUM>.

<FIG> is a flowchart showing a sequence in manufacturing the bonded substrate <NUM>. <FIG>, <FIG>, and <FIG> are cross-sectional views schematically showing intermediate products obtained in the process of manufacturing the bonded substrate <NUM>.

In the manufacture of the bonded substrate <NUM>, steps S101 to S105 shown in <FIG> are sequentially performed.

In step S101, the silicon nitride ceramic substrate <NUM> is prepared. The prepared silicon nitride ceramic substrate <NUM> has the main surfaces <NUM> and the particle-defect holes <NUM> exposed to the main surfaces <NUM> as described above.

In step S102, brazing material layers 13i are formed on the main surfaces <NUM> of the silicon nitride ceramic substrate <NUM> as shown in <FIG>.

When the brazing material layers 13i are formed, a paste containing an active metal brazing material and a solvent is prepared. The paste may further contain a binder, a dispersant, an antifoaming agent, and the like. The prepared paste is then screen printed on the main surfaces <NUM> of the silicon nitride ceramic substrate <NUM> to form screen printed films on the respective main surfaces <NUM> of the silicon nitride ceramic substrate <NUM>. The solvent contained in the formed screen printed films is then volatilized. The screen printed films are thereby changed into the respective brazing material layers 13i. The brazing material layers 13i contain the active metal brazing material. The brazing material layers 13i may be formed by a method different from this method.

The active metal brazing material contains an active metal hydride powder and a metal powder. The active metal hydride powder contains a hydride of at least one type of active metal selected from the group consisting of titanium and zirconium. The metal powder contains silver. The metal powder may contain metal other than silver. The metal other than silver is at least one type of metal selected from the group consisting of copper, indium, and tin. In a case where the active metal brazing material contains at least one type of metal selected from the group consisting of copper, indium, and tin and silver, the active metal brazing material has a lower melting point than silver.

The active metal brazing material preferably contains <NUM> wt% or more and <NUM> wt% or less silver.

The active metal brazing material is preferably formed of a powder having an average particle size of <NUM> or more and <NUM> or less. The average particle size can be obtained by measuring particle size distribution using a commercially available laser diffraction particle size distribution analyzer, and calculating a median diameter D50 from the measured particle size distribution. Use of the active metal brazing material formed of the powder having such a small average particle size for formation of the brazing material layers 13i enables the brazing material layers 13i to each have a small thickness.

The brazing material layers 13i each are preferably formed to have a thickness of <NUM> or more and <NUM> or less, and are more preferably formed to have a thickness of <NUM> or more and <NUM> or less.

In step S103, copper plates 12i are disposed on the respective formed brazing material layers 13i as shown in <FIG>. An intermediate product 1i including the silicon nitride ceramic substrate <NUM>, the copper plates 12i, and the brazing material layers 13i is thereby obtained.

In step S104, the obtained intermediate product 1i is hot pressed. During hot pressing, the active metal contained in the active metal brazing material constituting the brazing material layers 13i reacts with nitrogen contained in the silicon nitride ceramic substrate <NUM>. A nitride of the active metal is thereby generated. A melt of the active metal brazing material is wet to the generated nitride of the active metal. Furthermore, the melt of the active metal brazing material is wet to the copper plates 12i. As a result, a substance constituent the melt of the active metal brazing material combines with substances constituent the silicon nitride ceramic substrate <NUM> and the copper plates 12i to generate bonding layers 13j bonding copper plates 12j and the silicon nitride ceramic substrate <NUM> as shown in <FIG>.

In this case, the majority of each of the generated bonding layers 13j becomes the third portion <NUM> covering the flat region 11f (see <FIG>), but a portion of each of the brazing material layers 13i enters the particle-defect hole <NUM> exposed to the main surface <NUM> of the silicon nitride ceramic substrate <NUM> with the progress of hot pressing. The fourth portion <NUM> of each of the bonding layers 13j filling the particle-defect hole <NUM> is thereby formed. Furthermore, in this situation, a portion of each of the softened copper plates 12i enters the particle-defect hole <NUM>. The second portion <NUM> of each of the copper plates 12j filling the particle-defect hole <NUM> is thereby formed in addition to the first portion <NUM> disposed over the flat region 11f. An intermediate product 1j including the silicon nitride ceramic substrate <NUM>, the copper plates 12j, and the bonding layers 13j is thereby obtained.

In a case where the intermediate product 1i is hot pressed, the intermediate product 1i is preferably heated in accordance with a temperature profile having a maximum temperature of <NUM> or more and <NUM> or less while being pressurized in a vacuum along a thickness direction of the silicon nitride ceramic substrate <NUM> in accordance with a contact pressure profile having a maximum contact pressure of <NUM> MPa or more and <NUM> MPa or less. The copper plates 12i can thereby be bonded to the silicon nitride ceramic substrate <NUM> even in a case where the brazing material layers 13i each have a small thickness of <NUM> or more and <NUM> or less.

<FIG> is an enlarged cross-sectional view schematically showing a portion of a bonded substrate manufactured under conditions of insufficient pressurization of the intermediate product 1i compared to pressurization in accordance with the above-mentioned contact pressure profile, which is shown for comparison with the bonded substrate <NUM> according to the present embodiment.

The bonded substrate shown in <FIG> differs from the bonded substrate <NUM> according to the present embodiment shown in <FIG> in that the bonding layer <NUM> does not include the fourth portion <NUM> filling the particle-defect hole <NUM> to cover the inner surface 11i of the particle-defect hole <NUM>, and the copper plate <NUM> does not include the second portion <NUM> filling the particle-defect hole <NUM>.

The melt of the active metal brazing material has high wettability to each of the copper plates 12i. The melt of the active metal brazing material present over the particle-defect hole <NUM> is thus likely to adhere to the copper plate 12i. In a case where pressurization of the intermediate product 1i is insufficient, even when a portion of the melt of the active metal brazing material present in the flat region 11f around the particle-defect hole <NUM> enters the particle-defect hole <NUM>, the entering melt can easily be brought back to the flat region 11f around the particle-defect hole <NUM>. Thus, in a case where pressurization of the intermediate product 1i is insufficient, the melt of the active metal brazing material present near the particle-defect hole <NUM> tends to have difficulty entering the particle-defect hole <NUM>.

A space filling ratio of the brazing material powder is only approximately <NUM>% to <NUM>% in the first place. Thus, when pressurization of the intermediate product 1i is insufficient in a case where the active metal brazing material is in a melting state due to hot pressing, at most only approximately <NUM>% to <NUM>% of the particle-defect hole <NUM> is filled, even if the active metal brazing material in the form of a powder completely fills the particle-defect hole <NUM>.

For these reasons, the bonding layer <NUM> and the copper plate <NUM> do not sufficiently fill the particle-defect hole <NUM> when contact pressure to the intermediate product 1i is insufficient. For example, only less than <NUM>% of the volume of the particle-defect hole <NUM> is filled. In an extreme case, the active metal brazing material does not enter the particle-defect hole <NUM>, and thus the copper plate does not enter the particle-defect hole <NUM>, and, as a result, the particle-defect hole <NUM> can remain as a void V as shown in <FIG>. For these reasons, it is necessary to pressurize the intermediate product 1i under sufficient contact pressure in manufacturing the bonded substrate <NUM> to push the active metal brazing material and, further, the copper plate, into the particle-defect hole <NUM>.

During hot pressing of the intermediate product 1i, all or some of silver, copper, indium, and tin contained in the brazing material layer 13i may be diffused into the silicon nitride ceramic substrate <NUM> and/or the copper plate 12i. During hot pressing of the intermediate product 1i, nitrogen and/or silicon contained in the silicon nitride ceramic substrate <NUM> may be diffused into the brazing material layer 13i. Copper contained in the copper plate 12i may be diffused into the brazing material layer 13i.

In step S105, the copper plates 12j and the bonding layers 13j are patterned. The bonded substrate <NUM> in which the copper plates <NUM> and the bonding layers <NUM> are patterned to expose portions of the main surfaces <NUM> of the silicon nitride ceramic substrate <NUM> as shown in <FIG> is thereby obtained.

An experiment was conducted to evaluate the influence of the maximum contact pressure of the contact pressure profile and the maximum temperature of the temperature profile used for hot pressing performed in the process of manufacturing the bonded substrate <NUM> on an internal state of the particle-defect hole <NUM>.

First, various bonded substrates <NUM> were manufactured by the above-mentioned method for manufacturing the bonded substrate <NUM>. The depth of the particle-defect hole <NUM> present in the silicon nitride ceramic substrate <NUM> varied from <NUM> to <NUM>. A titanium hydride was used as an active metal hydride contained in the active metal brazing material. Silver was used as the metal other than the active metal contained in the active metal brazing material.

As for conditions of hot pressing, the maximum contact pressures in the contact pressure profiles were varied in six levels of <NUM> MPa, <NUM> MPa, <NUM> MPa, <NUM> MPa, <NUM> MPa, and <NUM> MPa, and the maximum temperatures in the temperature profiles were varied in four levels of <NUM>, <NUM>, <NUM>, and <NUM> as shown in Table <NUM>. That is to say, the intermediate product 1i was heated in accordance with the temperature profile having any of the maximum temperatures shown in Table <NUM> while being pressurized along the thickness direction of the silicon nitride ceramic substrate <NUM> in accordance with the contact pressure profile having any of the maximum contact pressures shown in Table <NUM> to manufacture each of the bonded substrates <NUM>.

A cross section of each of the manufactured bonded substrates <NUM> was observed with a scanning electron microscope (SEM) to obtain an SEM image. The internal state of the particle-defect hole <NUM> was checked with reference to the obtained SEM image. Results of evaluation of the internal state of the particle-defect hole <NUM> are shown in Table <NUM>.

In Table <NUM>, results of evaluation of the internal state of the particle-defect hole <NUM> marked with circles each indicate that all the following requirements are met:.

In Table <NUM>, results of evaluation of the internal state of the particle-defect hole <NUM> marked with circles with asterisks each indicate that all the following requirements are met:.

In Table <NUM>, results of evaluation of the internal state of the particle-defect hole <NUM> marked with crosses each indicate that all the following requirements are met:.

As shown in Table <NUM>, in each of the bonded substrates <NUM> manufactured under a maximum contact pressure of <NUM> MPa or more and <NUM> MPa or less and at a maximum temperature of <NUM> or more and <NUM> or less, the fourth portion <NUM> of the bonding layer <NUM> covered <NUM>% or more of the area of the inner surface 11i of the particle-defect hole <NUM>, the hole filling portion filled <NUM>% or more of the volume of the particle-defect hole <NUM>, and no crack was formed in the silicon nitride ceramic substrate <NUM>.

On the other hand, in each of the bonded substrates <NUM> manufactured under a maximum contact pressure of less than <NUM> MPa or at a maximum temperature of less than <NUM>, the fourth portion <NUM> of the bonding layer <NUM> covered only <NUM>% or less of the area of the inner surface 11i of the particle-defect hole <NUM>, and the hole filling portion filled only <NUM>% or less of the volume of the particle-defect hole <NUM>.

In each of the bonded substrates <NUM> manufactured under a maximum contact pressure of more than <NUM> MPa or at a maximum temperature of more than <NUM>, the fourth portion <NUM> of the bonding layer <NUM> covered <NUM>% or more of the area of the inner surface 11i of the particle-defect hole <NUM>, and the hole filling portion filled <NUM>% or more of the volume of the particle-defect hole <NUM>, but one or more cracks were formed in the silicon nitride ceramic substrate <NUM>. The reason why the cracks are formed in the silicon nitride ceramic substrate <NUM> in each of the bonded substrates <NUM> manufactured at a maximum temperature of more than <NUM> is that the copper plates 12i are excessively softened and flare during hot pressing, and, as a result, the cracks are formed in the silicon nitride ceramic substrate <NUM>.

It is understood from the experiment that, by performing hot pressing under a maximum contact pressure of <NUM> MPa or more and <NUM> MPa or less and at a maximum temperature of <NUM> or more and <NUM> or less in the process of manufacturing the bonded substrate <NUM>, the fourth portion <NUM> of the bonding layer <NUM> can cover <NUM>% or more of the area of the inner surface 11i of the particle-defect hole <NUM>, the hole filling portion can fill <NUM>% or more of the volume of the particle-defect hole <NUM>, and, further, formation of any cracks in the bonded substrate <NUM> can be prevented.

An experiment was conducted to evaluate the influence of the depth of the particle-defect hole in the bonded substrate <NUM> on how electrical breakdown occurs.

First, bonded substrates <NUM> were manufactured by the above-mentioned method for manufacturing the bonded substrate <NUM>. Five silicon nitride ceramic substrates <NUM> were prepared for each of four ranges of a maximum depth t of the particle-defect hole <NUM> of t ≤ <NUM>, <NUM> < t ≤ <NUM>, <NUM> < t ≤ <NUM>, and <NUM> < t ≤ <NUM> shown in Table <NUM>. Titanium was used as the active metal contained in the active metal brazing material. Silver was used as the metal other than the active metal contained in the active metal brazing material.

As for conditions of hot pressing, the maximum contact pressures in the contact pressure profiles were varied in two levels of <NUM> MPa and <NUM> MPa shown in Table <NUM>, and a maximum temperature in the temperature profile was set to <NUM>. That is to say, the intermediate product 1i was manufactured using the silicon nitride ceramic substrate <NUM> having the particle-defect hole <NUM> having the maximum depth t in any of the ranges shown in Table <NUM>, and the manufactured intermediate product 1i was heated in accordance with the temperature profile having a maximum temperature of <NUM> while being pressurized along the thickness direction of the silicon nitride ceramic substrate <NUM> in accordance with the contact pressure profile having one of the maximum contact pressures shown in Table <NUM> to manufacture each of the bonded substrates <NUM>.

A cross section of each of the manufactured bonded substrates <NUM> was observed with the SEM to obtain an SEM image. The internal state of the particle-defect hole <NUM> was checked with reference to the obtained SEM image.

<FIG> is a cross-sectional SEM image of the bonded substrate <NUM> manufactured by hot pressing in accordance with the contact pressure profile having a maximum contact pressure of <NUM> MPa and the temperature profile having a maximum temperature of <NUM> shown as one example. It is confirmed from <FIG> that the hole filling portion fills almost <NUM>% of the volume of the particle-defect hole <NUM>.

As a result, as shown in Table <NUM>, the hole filling portion filled <NUM>% to <NUM>% of the volume of the particle-defect hole <NUM> in a case where the maximum contact pressure was <NUM> MPa. The hole filling portion filled <NUM>% to <NUM>% of the volume of the particle-defect hole <NUM> in a case where the maximum contact pressure was <NUM> MPa.

Each of the bonded substrates <NUM> in which <NUM>% to <NUM>% of the volume of the particle-defect hole <NUM> is filled obtained in a case where the maximum contact pressure was <NUM> MPa is hereinafter referred to as a high filling degree substrate, and each of the bonded substrates <NUM> in which <NUM>% to <NUM>% of the volume of the particle-defect hole <NUM> is filled obtained in a case where the maximum contact pressure was <NUM> MPa is hereinafter referred to as a low filling degree substrate.

Furthermore, a withstand voltage test was conducted on each of the manufactured bonded substrates <NUM> to measure a withstand voltage of the bonded substrate <NUM> for electrical breakdown. The withstand voltage was evaluated by increasing a voltage in steps of <NUM> kV, and keeping the voltage for ten minutes. The measured withstand voltage of the bonded substrate <NUM> is shown in Table <NUM>. Table <NUM> shows an average value of withstand voltages of the bonded substrates <NUM> belonging to each of the four ranges of the maximum depth t of the particle-defect hole <NUM>.

Furthermore, a partial discharge test was conducted on each of the manufactured bonded substrates <NUM> to measure a partial discharge voltage of the bonded substrate <NUM>. More specifically, a voltage was increased in steps of <NUM> kV/sec, and a discharge voltage when the amount of discharge of the bonded substrate <NUM> reached <NUM> pC was obtained as the partial discharge voltage. The partial discharge voltage was measured using a partial discharge test system from SOKEN ELECTRIC CO. The measured partial discharge voltage of the bonded substrate <NUM> is shown in Table <NUM>.

In a case where the partial discharge voltage has an extremely high measured value of <NUM> kV or more, the bonded substrate <NUM> can be evaluated so that virtually no partial discharge occurs in the bonded substrate <NUM>. In contrast, in a case where the partial discharge voltage of the bonded substrate <NUM> has a measured value of less than <NUM> kV, the bonded substrate <NUM> can be evaluated so that the partial discharge might occur in the bonded substrate <NUM>.

As shown in Table <NUM>, in a case of the high filling degree substrate, the withstand voltage was <NUM> kV when the particle-defect hole <NUM> had a maximum depth t of <NUM> or less, and was <NUM> kV when the particle-defect hole <NUM> had a maximum depth t of more than <NUM>. That is to say, it is confirmed, for the high filling degree substrate, that the withstand voltage tends to be reduced when the particle-defect hole <NUM> has a maximum depth t of more than <NUM>.

In contrast, the partial discharge voltage of the high filling degree substrate was <NUM> kV regardless of whether the particle-defect hole <NUM> had a maximum depth t of <NUM> or less. That is to say, each high filling degree substrate was evaluated so that partial discharge does not occur in the high filling degree substrate.

On the other hand, in a case of the low filling degree substrate, the withstand voltage was <NUM> kV when the particle-defect hole <NUM> had a maximum depth of <NUM> or less, and was <NUM> kV when the particle-defect hole <NUM> had a maximum depth of more than <NUM>. That is to say, it is confirmed, for the low filling degree substrate, that the withstand voltage tends to be reduced when the particle-defect hole <NUM> has a maximum depth t of more than <NUM>.

The partial discharge voltage of the low filling degree substrate had a value varying with the maximum depth t of the particle-defect hole <NUM>. Specifically, the partial discharge voltage was <NUM> kV when the maximum depth t was <NUM> or less, was <NUM> kV when the maximum depth t was more than <NUM> and <NUM> or less, was <NUM> kV when the maximum depth t was more than <NUM> and <NUM> or less, and was <NUM> kV when the maximum depth t was more than <NUM>. That is to say, the low filling degree substrate was evaluated so that partial discharge might occur when the particle-defect hole <NUM> has a maximum depth of more than <NUM>.

The low filling degree substrate in which the maximum depth t was more than <NUM> had a higher withstand voltage than the high filling degree substrate in which the maximum depth t had substantially the same value, but had a lower partial discharge voltage than the high filling degree substrate.

It can be said, from the above-mentioned results, that there is little difference in voltage performance between the high filling degree substrate and the low filling degree substrate in each of which the particle-defect hole <NUM> has a maximum depth t of <NUM> or less, and they each have a sufficient withstand voltage, and partial discharge does not occur in each of them. On the other hand, it can be said that, in a case of the high filling degree substrate and the low filling degree substrate in each of which the maximum depth t is more than <NUM> and <NUM> or less, partial discharge might occur in the low filling degree substrate, whereas partial discharge does not occur in the high filling degree substrate. It can thus be said that, in a case where the bonded substrate is the high filling degree substrate in which the particle-defect hole has a maximum depth of <NUM> or more and <NUM> or less, and the hole filling portion fills <NUM>% or more of the volume of the particle-defect hole, partial discharge starting at the particle-defect hole does not occur, and thus it is not necessary to conduct the partial discharge test prior to shipment as a product. In other words, it can be said that the bonded substrate can be shipped as a product when only passing the withstand voltage test, which is conducted more easily than the partial discharge test.

In particular, the bonded substrate having a higher withstand voltage can be achieved in a case where the particle-defect hole has a maximum depth of <NUM> or less.

Claim 1:
A bonded substrate (<NUM>) suitable for use as an insulating heat-dissipating substrate for mounting a power semiconductor device, the bonded substrate (<NUM>) comprising:
a silicon nitride ceramic substrate (<NUM>) having a main surface (<NUM>) and a particle-defect hole (<NUM>), the main surface having a flat region (11f) having a maximum height Rz of <NUM> or less, the particle-defect hole (<NUM>) being exposed to the main surface, imparting flatness lower than flatness of the flat region to a part of the main surface, and having a depth of <NUM> or more and <NUM> or less;
a copper plate (<NUM>) including a first portion (<NUM>) disposed over the flat region (11f) and a second portion (<NUM>) filling the particle-defect hole (<NUM>); and
a bonding layer (<NUM>) containing a compound of at least one type of active metal selected from the group consisting of titanium and zirconium with nitrogen and/or silicon, further containing at least one type of metal selected from the group consisting of silver, copper, indium, and tin, and including a third portion (<NUM>) covering the flat region (11f) and a fourth portion (<NUM>) filling the particle-defect hole (<NUM>), and bonding the copper plate (<NUM>) to the main surface (<NUM>), wherein
the second portion (<NUM>) and the fourth portion (<NUM>) fill <NUM>% or more of a volume of the particle-defect hole (<NUM>).