Patent Description:
Recently, high power electronic circuits such as power modules, LEDs and the like have been used for various uses. An insulating substrate on which such a circuit is, for example, mountable is required. As such an insulating substrate, a ceramic substrate is generally used. Especially, a silicon nitride sintered substrate has a high mechanical strength. For example, Patent Document <NUM> discloses a silicon nitride sintered substrate having a low dielectric constant, a high air tightness and a high productivity.

Patent Document No. <NUM>: <CIT>. <CIT>, <CIT> and <CIT> disclose a silicon nitride sintered substrate.

For the above-described uses, a large-sized insulating substrate may be required. A large-sized silicon nitride sintered substrate is required such that a large number of electronic circuits are produced on one substrate to increase the productivity. The present invention provides a large-sized silicon nitride sintered substrate, a silicon nitride sintered substrate piece, a circuit board and a method for producing the silicon nitride sintered substrate.

A silicon nitride sintered substrate in an embodiment of the present disclosure has a main surface of a shape larger than a square having a side of a length of <NUM>. A ratio dc/de is <NUM> or higher where a central area of the main surface has a density dc and an end area of the main surface has a density de, the central area of the main surface has a void fraction vc of <NUM>% or lower, and the end area of the main surface has a void fraction ve of <NUM>% or lower.

The density dc of the central area may be <NUM>/cm<NUM> or higher, the density de of the end area may be <NUM>/cm<NUM> or higher, and a ratio ve/vc of the void fraction vc of the central area and the void fraction ve of the end area may be <NUM> or higher.

The density dc of the central area may be <NUM>/cm<NUM> or higher, the density de of the end area may be <NUM>/cm<NUM> or higher, and the void fraction vc of the central area may be <NUM>% or lower.

The silicon nitride sintered substrate may have a partial discharge inception voltage, defined by a voltage value when a discharge charge amount of <NUM> pC is reached, of <NUM> kV or higher.

The silicon nitride sintered substrate may have a thickness of <NUM> or greater and <NUM> or less.

The main surface may have a square shape having a side of a length of <NUM> or a shape smaller than the square shape.

The silicon nitride sintered substrate has a carbon content, and the carbon content is <NUM>% by mass or lower.

The main surface may have a shape larger than a rectangle of <NUM> × <NUM>.

A plurality of silicon nitride sintered substrate piece in an embodiment of the present disclosure is divided from the silicon nitride sintered substrate described in any of the above.

A circuit board in an embodiment of the present disclosure includes the silicon nitride sintered substrate described in any of the above. The circuit board has a dielectric breakdown voltage of <NUM> kV or higher and a Weibull coefficient of dielectric breakdown voltage of <NUM> or higher.

The main surface may have a square shape having a side of a length of <NUM> or a shape smaller than the square shape, and the circuit board has a Weibull coefficient of dielectric breakdown voltage of <NUM> or higher.

A method for producing the silicon nitride sintered substrate in an embodiment of the present disclosure includes step (a) of mixing Si<NUM>N<NUM> powder at <NUM>% by mass or higher and <NUM>% by mass or lower, Mg compound powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into an oxide, and at least one type of rare earth element compound powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into an oxide to provide mixed powder; step (b) of putting the mixed powder into a slurry and forming a plurality of greensheets by molding; step (c) of stacking the plurality of greensheets with a boron nitride powder layer being located between each two adjacent greensheets, among the plurality of greensheets, to form a stacked assembly; and step (d) of locating the stacked assembly in a sintering furnace and sintering the stacked assembly. In the step (c), the boron nitride powder layer has a thickness of <NUM> or greater and <NUM> or less. The step (d) includes step (d1) of removing carbon from the greensheets while maintaining an atmosphere temperature of <NUM> or higher and <NUM> or lower in a vacuum atmosphere of <NUM> Pa of lower; and step (d2) of, after the step (d1), sintering the greensheets at an atmosphere temperature of <NUM> or higher and <NUM> or lower in a nitrogen atmosphere.

A method for producing the silicon nitride sintered substrate in another embodiment of the present disclosure includes step (a) of mixing Si powder, or Si powder and Si<NUM>N<NUM> powder, at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into Si<NUM>N<NUM>, Mg compound powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into an oxide, and at least one type of rare earth element compound powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into an oxide to provide mixed powder; step (b) of putting the mixed powder into a slurry and forming a plurality of greensheets by molding; step (c) of stacking the plurality of greensheets with a boron nitride powder layer being located between each two adjacent greensheets, among the plurality of greensheets, to form a stacked assembly; and step (d) of locating the stacked assembly in a sintering furnace and sintering the stacked assembly. In the step (c), the boron nitride powder layer has a thickness of <NUM> or greater and <NUM> or less. The step (d) includes step (d1) of removing carbon from the greensheets while maintaining an atmosphere temperature of <NUM> or higher and <NUM> or lower in a vacuum atmosphere of <NUM> Pa of lower; step (d2) of, after the step (d1), nitriding the Si powder in the greensheets at an atmosphere temperature of <NUM> or higher and <NUM> or lower in a nitrogen atmosphere; and step (d3) of, after the step (d2), sintering the greensheets at an atmosphere temperature of <NUM> or higher and <NUM> or lower in the nitrogen atmosphere.

The silicon nitride sintered substrate has a main surface of a shape larger than a square having a side of a length of <NUM>. The main surface may have a shape larger than a rectangle of <NUM> × <NUM>. The main surface may have a square shape having a side of a length of <NUM> or a shape smaller than the square shape.

A silicon nitride sintered substrate and a method for producing the same according to the present application provide a large-sized silicon nitride sintered substrate having a high insulation reliability.

According to detailed studies made by the inventors of the present application, when the size of a silicon nitride sintered substrate is increased, various properties are made different between a central area and the vicinity thereof, and an end area, of the substrate; and as a result, the uniformity in each of the properties in the plane of the substrate is decreased. It has been found out that especially at the central area and the vicinity thereof of the substrate, a greensheet is not easily shrunk at the time of sintering, and therefore, the density is decreased and the void fraction is increased in the central area and the vicinity thereof.

In the case where a silicon nitride sintered substrate is used as a substrate on which a high power circuit such as a power module, an LED or the like is, for example, to be mounted, it is preferred that the silicon nitride sintered substrate has a high breakdown voltage and a high insulation reliability. It has been found out as a result of the studies that a partial discharge voltage is usable as a characteristic by which it is evaluated whether or not a substrate has a high insulation reliability, and that the partial discharge voltage is correlated with a void fraction of a silicon nitride sintered substrate. The "high insulation reliability" refers to that high insulation characteristics are maintained for a long time.

It has also been found out that in order to produce a silicon nitride sintered substrate having a high in-plane uniformity in the density and the void fraction and having a large external shape, it is important to control the thickness of a boron nitride powder layer stacked together with greensheets during the production of the silicon nitride sintered substrate and to control the carbon content at the time of sintering. Based on these results of the studies, the inventors of the present application conceived a large-sized silicon nitride sintered substrate having a high breakdown voltage and a high insulation reliability that are small in the in-plane variance, and a method for producing the same. Hereinafter, an example of silicon nitride sintered substrate and method for producing the same in this embodiment will be described. The present invention is not limited to any of the following embodiments and may be modified or altered in various manners.

As shown in <FIG>, a silicon nitride sintered substrate <NUM> in this embodiment has a main surface 101a. Herein, the term "main surface" refers to the broadest surface among surfaces of the silicon nitride sintered substrate <NUM>. In this embodiment, a surface 101b opposite to the main surface 101a has substantially the same size as that of the main surface 101a. The main surface 101a has a shape larger than a square having a side of a length of at least <NUM>. In the case where the main surface 101a is rectangular, both of two sides L1 and L2 thereof are longer than <NUM>. For example, the silicon nitride sintered substrate <NUM> may have a shape larger than a rectangle having a size of <NUM> × <NUM>. The main surface 101a may be polygonal or of a shape formed of a curved line with no apex or side such as, for example, a circle, an ellipse or the like. In the case of having a shape other than a square or a rectangle, the main surface 101a has a shape larger than a shape including(circumscribing) a square having a side of a length of <NUM>.

As shown in <FIG>, the silicon nitride sintered substrate <NUM> is provided as follows: a greensheet is sintered to provide a silicon nitride sintered body <NUM>', and an end portion <NUM> (represented by hatching), called an "extra portion", located along an outer perimeter is cut off from the silicon nitride sintered body <NUM>'. The end portion <NUM> has a width of, for example, about <NUM>. In <FIG>, the silicon nitride sintered substrate <NUM> provided after the end portion <NUM> is cut off is square. As described above, the silicon nitride sintered substrate <NUM> may be polygonal or of any other shape in accordance with the use thereof.

There is no limitation on the size of the main surface 101a as long as the density and the void fraction of the silicon nitride sintered substrate <NUM> fulfill conditions described below. However, as the main surface 101a is larger, the difference in the shrinking amount between the end area and the central area of the greensheet during the production of the silicon nitride sintered substrate <NUM> is larger. As a result, the breakdown voltage and the insulation reliability are more varied between the end area and the central area. In order to decrease the in-plane variance in the breakdown voltage and the insulation reliability, it is preferred that the main surface 101a has a square shape having a side of a length of <NUM> or a shape smaller than this square shape. It is more preferred that the main surface 101a has a square shape having a side of a length of <NUM> or a shape smaller than this square shape.

Thickness t of the silicon nitride sintered substrate <NUM> is preferably <NUM> or greater and <NUM> or less. If the thickness is less than <NUM>, the substrate may be cracked during the production of the silicon nitride sintered substrate <NUM>, more specifically, in a step in which each of the silicon nitride sintered substrates <NUM> are peeled off from a post-sintering stacked assembly. This increases the possibility that the quality and the yield of the substrates are decreased. If the thickness is greater than <NUM>, the difference in the density between the central area and the end area of the silicon nitride sintered substrate <NUM>, and the difference in the density in the thickness direction of the substrate, are increased. As a result, the difference in the density between the central area and the end area is made more conspicuous.

Regarding the main surface 101a of the silicon nitride sintered substrate <NUM>, the ratio between density dc of the central area and density de of the end area, namely, dc/de is <NUM> or higher. If the density ratio dc/de is lower than <NUM>, the density of the main surface 101a of the silicon nitride sintered substrate <NUM> is significantly varied, which is not preferable. Specifically, if the density ratio de/de of the main surface 101a is lower than <NUM>, the difference between the density dc of the central area and the density de of the end area is <NUM>/cm<NUM> or greater. It is preferred that the density dc of the central area is <NUM>/cm<NUM> or higher and that the density de of the end area is <NUM>/cm<NUM> or higher. In the case where dc/de is <NUM> or higher, the density difference between the central area and the end area of the silicon nitride sintered substrate <NUM> is small, and thus the density uniformity in the silicon nitride sintered substrate <NUM> is increased. In the case where the density dc of the central area is <NUM>/cm<NUM> or higher and the density de of the end area is <NUM>/cm<NUM> or higher, the silicon nitride sintered substrate <NUM> has a high density and a high density uniformity. In the case where the density dc of the central area is <NUM>/cm<NUM> or higher and the density de of the end area is <NUM>/cm<NUM> or higher, the density of the silicon nitride sintered substrate <NUM> is higher and the uniformity thereof is higher. As a result, the silicon nitride sintered substrate <NUM> has high insulation characteristics. The density of the silicon nitride sintered substrate <NUM> is correlated with the void fraction described below, and is also related with insulation characteristics of the substrate.

According to the studies made by the inventors of the present application, the void fraction of the silicon nitride sintered substrate <NUM> is correlated with the carbon content. As the remaining amount of carbon is larger, the void fraction is higher. It is preferred that the carbon content of the silicon nitride sintered substrate <NUM> is <NUM>% by mass or lower in the central area. If the carbon content exceeds <NUM>% by mass, the void fraction of the central area exceeds <NUM>%.

Patent Document No. <NUM> discloses that if carbon derived from an organic binder or the like remains in the greensheet during the production of a silicon nitride sintered substrate, silicon carbide is generated as a result of sintering, which causes a problem that the dielectric constant of the silicon nitride sintered substrate is increased or the insulation resistance thereof is decreased. In order to solve such a problem, Patent Document No. <NUM> discloses decreasing the carbon content of the silicon nitride sintered substrate to <NUM>% by mass or lower.

As described above, according to the studies made by the inventors of the present application, it is preferred that the carbon content is significantly lower than the above-described value in order to decrease the void fraction of the silicon nitride sintered substrate <NUM>. Patent Document No. <NUM> also describes a silicon nitride sintered substrate produced with no use of an organic binder. However, such a substrate has a size of <NUM> × <NUM> × <NUM>, and is not a large-sized substrate.

In order to produce a silicon nitride sintered substrate having a small amount of remaining carbon, it is conceivable forming a greensheet with no use of an organic binder. In this case, however, a large-sized greensheet cannot be formed by molding. Namely, in the case where no organic binder is used, it is highly difficult to produce a large-sized silicon nitride sintered substrate due to the issue of moldability.

The void fraction of the silicon nitride sintered substrate <NUM> is related with a partial discharge inception voltage. In the case of a large substrate, the difference in the shrinkage ratio between the end area and the central area of a greensheet is increased, which increases the void fraction and decreases the uniformity in the void fraction (specifically, increases the difference in the void fraction between the central area and the end area). As a result, the partial discharge voltage is decreased. For this reason, it is preferred that void fraction vc of the central area is <NUM>% or lower and that void fraction ve of the end area is <NUM>% or lower. It is more preferred that the void fraction vc of the central area is <NUM>% or lower. If the void fraction vc of the central area and the void fraction ve of the end area are higher than these values, the partial discharge inception voltage, more specifically, the partial discharge inception voltage and a partial discharge extinction voltage are both decreased. In this case, the silicon nitride sintered substrate <NUM> does not have a sufficiently high insulation reliability. Specifically, in the case where a predetermined high voltage is applied, the time duration until dielectric breakdown is shortened. In order to guarantee a higher insulation reliability, it is preferred that the ratio between the void fraction vc of the central area and the void fraction ve of the end area, namely, ve/ve, is <NUM> or higher. In order to provide the above-described appropriate void fractions of the central area and the end area, the thickness of the boron nitride powder layer is made appropriate as described below, namely, is made <NUM> or greater and <NUM> or less.

Now, the definitions of the above-described properties will be described. First, the size of the main surface 101a will be described. As described above, the main surface 101a has a shape larger than a square <NUM> having a side of a length of <NUM> (square drawn by phantom lines). In the case where the main surface 101a has a square shape, the external shape of the main surface 101a matches the square <NUM> drawn by the phantom lines. The square <NUM> (square drawn by phantom lines) may be set as a maximum square inscribed in the shape of the main surface 101a. As shown in <FIG>, the length of a side of the square <NUM> is defined by intervals L1 and L2, between two opposing sides, measured at the centers of the sides of the square.

In the square <NUM>, nine circles C11 through C13, C21 through C23 and C31 through C33, each having a diameter of <NUM>, are arrayed in a matrix of <NUM> rows by <NUM> columns. The circle C22 at the center is located such that the center of the square, namely, the intersection of two diagonal lines connecting two apexes located at diagonal positions, matches the center of the circle C22. The other eight circles C11 through C13, C21, C23, and C31 through C33 are located as follows. A square <NUM>' formed of sides located inner to the sides of the square <NUM> by <NUM> is set. The circles C11, C13, C31 and C33 each having a diameter of <NUM> are located in contact with two of the sides of the square <NUM>' in the vicinity of the apexes of the square <NUM>'. The circles C12, C21, C23 and C32 are located at the middle between these circles and in contact with the corresponding sides of the square <NUM>'.

In the case where a rectangle (rectangle drawn by phantom lines) larger than the square <NUM> having a side of a length of <NUM> (square drawn by phantom lines) may be inscribed in the main surface 101a of the silicon nitride sintered substrate <NUM>, as shown in <FIG>, a rectangle <NUM>' is set and nine circles C11 through C13, C21 through C21 and C31 through C33 are located in substantially the same manner as described above.

The density and the void fraction are values found by cutting out the nine circles located under the above-described conditions by laser processing from the silicon nitride sintered substrate <NUM> and performing a measurement on the circles. The density of the central area is the density of the cut-out circle C22. The density of the end area is the minimum value among the values measured on the cut-out circles C11, C13, C13 and C33. The density is measured by the Archimedes' method.

The void fraction of the central area is the void fraction measured on the cut-out circle C22. The void fraction of the end area is the void fraction measured on the circle from which the density of the end area has been found by the above-described definition (circle having the minimum density value). In order to measure the void fraction, measurement samples are created as follows. A sample of <NUM> × <NUM> is cut out by laser processing from each of the cut-out circle at the center and the cut-out circle at the end. A gap of each of the samples is filled with a resin, and the surface thereof is polished. In this manner, the measurement samples are created. The created samples are each imaged by a 500x optical microscope, and the area size of the voids present within an area of <NUM> × <NUM> of the resultant image is found by image analysis. The void fraction is found by (area size of the void) / (<NUM> × <NUM>) × <NUM>. The observation of the central area and the vicinity thereof the substrate was made on a cross-section of the substrate.

The carbon content of the silicon nitride sintered substrate <NUM> is the value measured on the circle C22 at the center of the main surface. The carbon content is measured by a non-dispersive infrared absorption method. The carbon content may be measured by, for example, CS744-type carbon and sulfur analyzer produced by LECO Corporation.

The silicon nitride sintered substrate <NUM> has the above-described density and void fraction, and thus is high in the breakdown voltage and the insulation reliability and is high in the uniformity in the density and the void fraction of the main surface 101a. The partial discharge of the silicon nitride sintered substrate <NUM> is a precursor phenomenon for dielectric breakdown. As the partial discharge voltage is higher, the dielectric breakdown voltage is higher and the time duration until the dielectric breakdown is longer. The silicon nitride sintered substrate <NUM> in this embodiment has a partial discharge inception voltage of <NUM> kV or higher and a partial discharge extinction voltage of <NUM> kV or higher. Specifically, in the case where, regarding the silicon nitride sintered substrate <NUM>, the density dc of the central area is <NUM>/cm<NUM> or higher, the density de of the end area is <NUM>/cm<NUM> or higher, and the void fraction vc of the central area is <NUM>% or lower, the partial discharge inception voltage is <NUM> kV or higher. In this case, a higher insulation reliability is provided. The partial discharge inception voltage is defined by a voltage value when, while the voltage applied to the silicon nitride sintered substrate <NUM> is increased, a discharge amount of <NUM> pC is reached. The partial discharge extinction voltage is defined by a voltage value when, while the voltage applied to the silicon nitride sintered substrate <NUM> is decreased, the discharge amount of <NUM> pC is reached.

For example, the measurement may be performed by use of DAC-PD-<NUM> produced by Soken Electric Co. by setting the maximum applied voltage to <NUM> kV at a voltage increasing and decreasing rate of <NUM> V/sec. Any other device or any other measurement conditions may be used.

For the measurement, a measurement system shown in <FIG> is used. As shown in <FIG>, a rear electrode <NUM> having a size of <NUM> × <NUM> is located in a tank <NUM>, and the silicon nitride sintered substrate <NUM> as a measurement target is put thereon. A front electrode <NUM> having a diameter of <NUM> is located on the silicon nitride sintered substrate <NUM>, and one ends of lines <NUM> are respectively connected with the rear electrode <NUM> and the front electrode <NUM>. The other ends of the lines <NUM> are connected with the measurement device. The tank <NUM> is filled with a fluorine-based insulating liquid <NUM>, and the measurement is performed.

A circuit board produced to include the silicon nitride sintered substrate <NUM> in this embodiment has a dielectric breakdown voltage of <NUM> kV or higher and a Weibull coefficient of dielectric breakdown voltage of <NUM> or higher. The dielectric breakdown voltage is an average value found by a measurement performed on circular plates cut out from the central area and the end area, as defined above, of the main surface 101a of the silicon nitride sintered substrate <NUM>. Specifically, as shown in <FIG>, an Ag paste having a size of <NUM> × <NUM> is applied to a front surface and a rear surface of a circular plate <NUM> cut out from the silicon nitride sintered substrate <NUM> and is baked at <NUM>, and thus a measurement circuit board including electrodes <NUM> is produced. A DC voltage is applied between the electrodes <NUM> of the resultant measurement circuit board. The voltage at which dielectric breakdown occurs to the measurement circuit board, namely, at which a hole running through the substrate from the front surface to the rear surface is formed, is set as the dielectric breakdown voltage.

The circuit board in this embodiment includes the silicon nitride sintered substrate <NUM>, a metal circuit plate (e.g., copper circuit plate) provided on one surface of the silicon nitride sintered substrate <NUM>, and a metal heat sink plate (e.g., copper heat releasing plate) provided on the other surface of the silicon nitride sintered substrate <NUM>. The circuit board may further include a semiconductor element or the like provided on a top surface of the metal circuit plate. The silicon nitride sintered substrate may be joined with the metal circuit plate and the metal heat releasing plate by, for example, an active metal method by use of brazing material or by a copper direct bonding method of directly joining the copper plate.

The Weibull coefficient of dielectric breakdown voltage is found by plotting a Weibull distribution in which the dielectric breakdown voltage is represented by the horizontal axis and the breakdown probability is represented by the vertical axis. Specifically, where the natural logarithm is Ln, the dielectric breakdown probability (probability density function) is F and the dielectric breakdown voltage is V (kV), Ln (Ln(<NUM>/(<NUM>-F))) is set as the vertical axis, and Ln(V) is set as the horizontal axis regarding a circular plate cut out from the circuit board. From measurement points plotted, the Weibull coefficient of dielectric breakdown voltage is found by an approximation equation represented by Ln(Ln(<NUM>/(<NUM>-F))) = mLn(V) + constant. In this equation, m is the Weibull coefficient of dielectric breakdown voltage.

There is no specific limitation on the dielectric constant of the silicon nitride sintered substrate <NUM>, and the silicon nitride sintered substrate <NUM> may have any dielectric constant in accordance with the use thereof. For example, in the case where the silicon nitride sintered substrate <NUM> is used for a power module, it is preferred that the silicon nitride sintered substrate <NUM> has a dielectric constant of <NUM> or less, for example, about <NUM> or greater and about <NUM> or less.

The silicon nitride sintered substrate in this embodiment has, specifically, the density and the void fraction limited to the above-described ranges, and thus is large-sized and has a high breakdown voltage and a high insulation reliability that are highly uniform in the plane of the substrate. A silicon nitride sintered substrate that is large-sized and has superb characteristics for uses for a high power device, which is conventionally difficult to be produced, is now realized because of the above-described characteristics.

In the case where the silicon nitride sintered substrate in this embodiment is used as a substrate assembly to be divided, a large number of silicon nitride sintered substrate pieces are provided from one large-sized silicon nitride sintered substrate. Therefore, a high productivity is realized and the production cost of each silicon nitride sintered substrate piece is decreased. The large number of silicon nitride sintered substrate pieces provided as a result of the division are not much varied in the characteristics such as the density, the void fraction, the partial discharge voltage and the like. Two or more, namely, a plurality of silicon nitride sintered substrate pieces divided or cut out from one silicon nitride sintered substrate in this embodiment may be specified by, for example, reading identification information put on each of the silicon nitride sintered substrate pieces or measuring the continuity of change in the above-described properties of the substrate or the continuity of change in the composition or thickness thereof.

Material powder usable to produce the silicon nitride sintered substrate in this embodiment contains silicon nitride (Si<NUM>N<NUM>) as a main component and further includes a sintering additive. Specifically, the material powder contains Si<NUM>N<NUM> powder at <NUM>% by mass or higher and <NUM>% by mass or lower, Mg compound powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into an oxide, and at least one type of rare earth element compound powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into an oxide. It is preferred that the ratio at which the silicon nitride powder is of the α-phase is <NUM>% or higher and <NUM>% or lower from the points of view of the density of the silicon nitride sintered body, the flexural strength and the thermal conductivity.

If the content of Si<NUM>N<NUM> is lower than <NUM>% by mass, the flexural strength and the thermal conductivity of the resultant silicon nitride sintered substrate are too low. By contrast, if the content of Si<NUM>N<NUM> exceeds <NUM>% by mass, the amount of the sintering additive is insufficient and thus the resultant silicon nitride sintered substrate is not dense. If the content of Mg is lower than <NUM>% by mass as converted into an oxide, the amount of a liquid phase generated at a low temperature is insufficient. By contrast, if the content of Mg exceeds <NUM>% by mass as converted into an oxide, Mg is volatilized too much and thus the silicon nitride sintered substrate tends to have pores. If the content of the rare earth element is lower than <NUM>% by mass as converted into an oxide, the bonding of silicon nitride particles is too weak and thus cracks easily spread the boundary, which decreases the flexural strength. By contrast, if the content of the rare earth element exceeds <NUM>% by mass as converted into an oxide, the ratio of the boundary phase is too high and thus the thermal conductivity is decreased.

The content of Mg (as converted into an oxide) is preferably <NUM>% by mass or higher and <NUM>% by mass or lower, more preferably <NUM>% by mass or higher and <NUM>% by mass or lower, and most preferably <NUM>% by mass or higher and <NUM>% by mass or lower. The content of the rare earth element (as converted into an oxide) is preferably <NUM>% by mass or higher and <NUM>% by mass or lower, and more preferably <NUM>% by mass or higher and <NUM>% by mass or lower. Therefore, the content of Si<NUM>N<NUM> is preferably <NUM>% by mass or higher and <NUM>% by mass or lower, and more preferably <NUM>% by mass or higher and <NUM>% by mass or lower. Examples of the usable rare earth element include Y, La, Ce, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu and the like. Among these, Y is effective to increase the density of the silicon nitride sintered substrate and thus is preferred. Mg and the rare earth element may be in the form of an oxide or in the form of a compound other than oxygen. For example, a nitride such as Mg<NUM>N<NUM>, YN or the like, or a silicide such as Mg<NUM>Si or the like, may be used. More preferably, Mg and the rare earth element are each used in the form of oxide powder. Therefore, a preferred sintering additive is a combination of MgO powder and Y<NUM>O<NUM> powder.

Si powder may be used in the material powder to produce the silicon nitride sintered substrate in this embodiment. In this case, the Si powder may be nitrided before the greensheet is sintered, so that the silicon nitride sintered substrate is provided. In the case where the Si powder is used as a material component, the content ratio (first content ratio) of the Si powder is the ratio as converted into Si<NUM>N<NUM> in the case where the above-described silicon nitride powder is used. Specifically, the material powder contains the Si powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into Si<NUM>N<NUM>, the Mg compound powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into an oxide, and at least one type of rare earth element compound powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into an oxide.

Alternatively, the Si powder and the Si<NUM>N<NUM> powder may be used in the material powder to produce the silicon nitride sintered substrate in this embodiment. In the case where silicon nitride in the silicon nitride sintered substrate is entirety provided by the Si powder, there may be a possibility that Si is melted by rapid heat generation occurring at the time of nitridation thereof and is not sufficiently nitrided. By contrast, in the case where the Si<NUM>N<NUM> powder is used as a material component, the amount of heat generation and the heat generation density are decreased, so that Si is suppressed from being melted. In this case, the Si powder and the Si<NUM>N<NUM> powder may be mixed at any ratio. Specifically, the material powder contains the Si powder and the Si<NUM>N<NUM> powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into Si<NUM>N<NUM>, the Mg compound powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into an oxide, and at least one type of rare earth element compound powder at <NUM>% by mass or higher and <NUM>% by mass or lower as converted into an oxide.

A method for producing the silicon nitride sintered substrate by use of a stacked assembly of greensheets will be described below. Since the greensheets are stacked to form the stacked assembly and the stacked greensheets are sintered at the same time, the productivity is high. Herein, the "stacked assembly" refers to a temporary stack body used for sintering that includes a plurality of greensheets that are stacked so as not to be welded together. After the sintering, individual silicon nitride sintered substrates may be separated from the stacked assembly.

<FIG> is a flowchart showing an example of method for producing the silicon nitride sintered substrate in this embodiment in the case where the silicon nitride powder is used for the material powder. <FIG> is a flowchart showing an example of method for producing the silicon nitride sintered substrate in this embodiment in the case where the silicon powder, or both of the silicon powder and the silicon nitride powder, are used for the material powder. For the sake of simplicity, the silicon nitride powder will be referred to as the Si<NUM>N<NUM> powder, the silicon powder will be referred to as the Si powder, the Mg material will be referred to as the MgO powder, and the rare earth element material will be referred to as Y<NUM>O<NUM> powder. The oxidization state or the nitridation state of the material components are not limited to any of these compositions, and any other oxidation state or nitridation state may be used.

The material powder formed as a result of mixing so as to have the above-described composition is mixed with a plasticizer (e.g., phthalic acid-based plasticizer), an organic binder (e.g., polyethylenebutyral) and an organic solvent (e.g., ethylalcohol) in a ball mill or the like to form a slurry containing the raw materials. It is preferred that the slurry has a solid content of <NUM>% by mass or higher and <NUM>% by mass or lower. In the case where the Si powder is to be nitrided as described above, the Si powder, or the Si powder and the Si<NUM>N<NUM> powder, are used instead of the Si<NUM>N<NUM> powder.

After the slurry is defoamed and made viscous, the greensheet is formed by, for example, a doctor blade method. The thickness of the greensheet is appropriately set in consideration of the thickness of the silicon nitride sintered substrate to be formed and the shrinking ratio by sintering. The greensheet formed by the doctor blade method is usually lengthy strip-like, and thus is punched out or cut out into a predetermined shape and size. One greensheet has a shape larger than a square having a side of a length of <NUM> and has a size in consideration of the shrinking amount by sintering.

In order to efficiently produce the silicon nitride sintered substrate <NUM>, it is preferred to stack a plurality of the greensheets. As shown in <FIG>, a plurality of the greensheets <NUM> are stacked with a boron nitride powder layer <NUM> having a thickness of <NUM> or greater and <NUM> or less being located between each two adjacent greensheets <NUM>, among the plurality of greensheets <NUM>, to form a stacked assembly <NUM>. The boron nitride powder layer <NUM> is provided in order to make it easy to separate the post-sintering silicon nitride sintered substrate, and may be formed by, for example, spraying, applying by a brush, or screen-printing a slurry of boron nitride powder on one surface of each of the greensheets <NUM>. It is preferred that the boron nitride powder has a purity of <NUM>% or higher and an average particle diameter of <NUM> or longer and <NUM> or shorter. Herein, the "average particle diameter" refers to a value of D50 calculated from the particle diameter distribution measured by a laser diffraction-scattering method.

The boron nitride powder layer <NUM> is not sintered in a sintering step described below, and thus is not shrunk by the sintering. Therefore, if being thicker than <NUM>, the boron nitride powder layer <NUM> has a large influence of preventing the shrinkage of the greensheets. Since the shrinkage of the greensheet is prevented especially in the central area and the vicinity thereof, the resultant silicon nitride sintered substrate <NUM> tends to have the density decreased and the void fraction increased in the central area thereof. By contrast, if being thinner than <NUM>, the boron nitride powder layer <NUM> does not have a sufficient effect as a releasing agent, and as a result, it is made difficult to separate each of the silicon nitride sintered substrates from the stacked assembly after the sintering. It is more preferred that the boron nitride powder layer <NUM> has a thickness of <NUM> or greater and <NUM> or less. The thickness of the boron nitride powder layer <NUM> may be adjusted by, for example, the average particle diameter of the boron nitride powder used and/or the viscosity of the slurry. The thickness of the boron nitride powder layer <NUM> is the thickness in the state of being applied to the greensheet as the slurry.

As shown in <FIG>, in order to suppress the silicon nitride sintered substrate to be formed from being curved or waved, a weight plate <NUM> is placed on a top surface of the stacked assembly <NUM>, so that a load acts on the greensheets <NUM>. The load acting on each greensheet <NUM> is in the range of <NUM> to <NUM> Pa. If the load is more lightweight than <NUM> Pa, each of the post-sintering silicon nitride sintered substrates tends to be curved. By contrast, if the load exceeds <NUM> Pa, each greensheet <NUM> is restricted by the load and is inhibited from being smoothly shrunk. As a result, the resultant silicon nitride sintered substrate tends not to be dense. The load acting on each greensheet <NUM> is preferably <NUM> to <NUM> Pa, more preferably <NUM> to <NUM> Pa, and most preferably <NUM> to <NUM> Pa.

It is now assumed that the weight of the weight plate <NUM> is W<NUM> g, the weight and the area size of each greensheet <NUM> is W<NUM> g and S sm<NUM>, and the number of the greensheets <NUM> in the stacked assembly <NUM> is n. In this case, the load acting on the uppermost greensheet 1a is <NUM> × (W<NUM>/S)Pa, and the load acting on the lowermost greensheet 1b is <NUM> × [(W<NUM> + W<NUM> × (n - <NUM>)]/SPa. In the case where, for example, a boron nitride plate having a thickness of <NUM> is used as the weight plate <NUM> and the number of the greensheets <NUM> in the stacked assembly <NUM> is <NUM>, the load acting on the lowermost greensheet 1b is about three to four times the load acting on the uppermost greensheet 1a. The weight of the weight plate <NUM> and the number of the greensheets <NUM> in the stacked assembly <NUM> are set in consideration of this point. It is preferred that the weight W<NUM> of the weight plate <NUM> is set such that even the lowermost greensheet 1b receives a load in the range of <NUM> to <NUM> Pa and is sintered without being curved or waved, with no restriction on the shrinkage thereof.

The greensheets <NUM> contain an organic binder and a plasticizer. Therefore, before a sintering step S5, the stacked assembly <NUM> is heated to <NUM> to <NUM> to be degreased. The post-degreasing greensheets <NUM> are brittle. Therefore, it is preferred that the degreasing is performed on the stacked assembly <NUM>.

<FIG> shows an example of container usable to sinter a plurality of stacked assemblies <NUM> at the same time. The container <NUM> includes a carrying plate assembly <NUM> including a stack of plurality of carrying plates <NUM> provided in a multi-stage manner, each accommodating the stacked assembly <NUM>, an inner container <NUM> accommodating the carrying plate assembly <NUM>, and an outer container <NUM> accommodating the inner container <NUM>. An interval between each two carrying plates <NUM> adjacent to each other in an up-down direction is maintained by a vertical frame member <NUM>.

The container <NUM> include a double-wall structure of the inner container <NUM> and the outer container <NUM>, so that decomposition of Si<NUM>N<NUM> in the greensheets <NUM> and volatilization of MgO in the greensheets <NUM> are suppressed and thus the silicon nitride sintered substrates are denser and less curved. It is preferred that the inner container <NUM> and the outer container <NUM> are both formed of boron nitride. Alternatively, the outer container <NUM> may be formed of carbon coated with p-boron nitride by CVD. In the case where the outer container <NUM> is formed of carbon coated with p-boron nitride, the following advantages are provided. Such a carbon substrate, which has a high thermal conductivity, easily uniformizes the temperature distribution during the temperature rise. The silicon nitride sintered substrates are suppressed from being curved or waved. Generation of a reducing atmosphere, which might be otherwise provided by the carbon substrate, is prevented by the p-boron nitride coating. The inner container <NUM> includes a bottom plate 40a, a side plate 40b and a top plate 40c. The outer container <NUM> includes a bottom plate 50a, a side plate 50b and a top plate 50c.

In the case where top surfaces of the carrying plates <NUM> have warpages or undulations, the lowermost greensheet 1b in contact with each carrying plate <NUM> has a contact portion in contact with the top surface of the carrying plate <NUM> and a non-contact portion not in contact with the top surface of the carrying plate <NUM>. In this case, at the time of sintering, the non-contact portion of the greensheet 1b is easily shrunk, whereas the contact portion of the greensheet 1b is not easily shrunk. As a result, the shrinkage of the greensheet 1b is not uniform, and thus the greensheet 1b is curved or waved. The curve or wave of the lowermost greensheet 1b is transmitted to the upper greensheets <NUM>, resulting in all the silicon nitride sintered substrates being curved or waved. For this reason, it is preferred that the top surface of each carrying plate <NUM> is as flat as possible. Specifically, it is preferred that the curve is within <NUM>/mm and that the wave is within <NUM>. The curve and wave of the carrying plate <NUM> may be measured by the same method as the curve and wave of the silicon nitride sintered substrate.

As shown in <FIG>, it is preferred to locate packing powder <NUM> in the inner container <NUM>. The packing powder <NUM> is, for example, mixed powder of <NUM> to <NUM>% by mass of magnesia (MgO) powder, <NUM> to <NUM>% by mass of silicon nitride (Si<NUM>N<NUM>) powder, and <NUM> to <NUM>% by mass of boron nitride powder. The silicon nitride powder and the magnesia powder in the packing powder <NUM> are volatilized at a high temperature of <NUM> or higher, adjust the partial pressure of Mg and Si in the sintering atmosphere, and suppress silicon nitride and magnesia (magnesium oxide) from being volatilized from the greensheets <NUM>. The boron nitride powder in the packing powder <NUM> prevents the adhesion of the silicon nitride powder and the magnesia powder. Use of the packing powder <NUM> allows the silicon nitride sintered substrates to be dense and not to be much curved. In order to allow the packing powder <NUM> to be handled easily and to prevent the packing powder <NUM> from contacting the greensheets <NUM>, it is preferred to locate the packing powder <NUM> on the uppermost carrying plate 21a.

As shown in <FIG>, the bottom plate 40a of the inner container <NUM> is placed on a top surface of the bottom plate 50a of the outer container <NUM>, the carrying plate <NUM> is placed on a top surface of the bottom plate 40a of the inner container <NUM>, and the stacked assembly <NUM> including the plurality of greensheets <NUM> and the weight plate <NUM> are placed on the carrying plate <NUM>. As shown in <FIG>, the vertical frame member <NUM> is set on an outer circumference of the carrying plate <NUM>, the next-stage carrying plate <NUM> is placed thereon, and the stacked assembly <NUM> and the weight plate <NUM> are placed thereon. In this manner, the carrying plate assembly <NUM> including a desired number of the stacked assemblies <NUM> and weight plates <NUM> is formed. Then, the packing powder <NUM> is located on a top surface of the uppermost carrying plate 21a. Next, the side plate 40b and the top plate 40c of the inner container <NUM> are assembled, and the side plate 50b and the top plate 50c of the outer container <NUM> are assembled. In this manner, the container <NUM> accommodating the stacked assemblies <NUM> is completed. A desired number of (e.g., five) such containers <NUM> are located in a sintering furnace (not shown).

In the case where the Si<NUM>N<NUM> powder is used, the greensheets <NUM> are sintered in accordance with a temperature profile P shown in <FIG>. In the case where the Si powder, or the Si powder and the Si<NUM>N<NUM> powder, are used, the greensheets <NUM> are sintered in accordance with a temperature profile P' shown in <FIG>. The temperature profile P includes a temperature raising region including a decarbonization region Pc of removing carbon from the greensheets <NUM> and a gradually heating region P<NUM>, a temperature maintaining region including a first temperature maintaining region P<NUM> and a second temperature maintaining region P<NUM>, and a cooling region. In <FIG>, the temperature represented by the vertical axis is the atmosphere temperature in the sintering furnace. Namely, the sintering step includes a decarbonization step of using the decarbonization region Pc of the temperature profile P and a sintering step of using the first temperature maintaining region P<NUM> of the temperature profile P. In the case where the Si powder, or the Si powder and the Si<NUM>N<NUM> powder, are used, a nitridation step is included between the decarbonization step and the sintering step. Therefore, the temperature profile P' includes a nitridation region Pn between the decarbonization region Pc and the first temperature maintaining region P<NUM>.

The atmosphere temperature in the sintering furnace in the sintering step may be, for example, a temperature of a target (carbon) in the furnace measured through a window provided in the sintering furnace by an infrared thermometer. Specifically, the container for sintering that accommodates the greensheets, a carbon cylindrical wall having the container for sintering in an area inner thereto, and the target located in the vicinity of an outer circumference of the cylindrical wall may be provided in the sintering furnace. The measurement of the temperature of the target may be substantially considered as the measurement of a temperature corresponding to the atmosphere temperature in the furnace in the temperature raising region or the temperature maintaining region.

First, the atmosphere temperature in the sintering furnace is raised from room temperature to a temperature range of the decarbonization region Pc. The heating rate is, for example, <NUM>/hr. When the atmosphere temperature in the sintering furnace reaches a temperature of <NUM> or higher and <NUM> or lower, the temperature is maintained in this temperature range for <NUM> minute or longer and <NUM> hours or shorter (maintaining time tc). It is preferred that the pressure in the sintering furnace is a reduced pressure, specifically, <NUM> Pa or lower. As described above, if carbon remains at the time of sintering, voids are easily generated in the sintered body. Therefore, the greensheets <NUM> are maintained at the reduced pressure to remove carbon from the greensheets <NUM>. This step is the decarbonization step of removing carbon more completely by use of a condition under which carbon is more easily volatilized than in the degreasing step S4. If the atmosphere temperature is lower than <NUM>, carbon may not be removed sufficiently. If the atmosphere temperature is higher than <NUM>, the sintering additive may also be removed. It is more preferred that the atmosphere temperature in the sintering furnace is <NUM> or higher and <NUM> or lower.

In general, in the production of a silicon nitride sintered substrate, the sintering is performed in a nitrogen atmosphere in order to suppress volatilization of nitrogen. However, it has been found by the studies made by the inventors of the present application that the greensheets <NUM> may be heated at a temperature and in an atmosphere where nitrogen is not volatilized, so that carbon is removed more completely to suppress generation of voids while the volatilization of nitrogen is suppressed.

After the heating by use of the decarbonization region Pc is finished, the atmosphere temperature in the sintering furnace is controlled by the temperature profile in the gradually heating region Po. The gradually heating region P<NUM> is a temperature region in which the sintering additive contained in the greensheets <NUM> reacts with an oxide layer at a surface of the silicon nitride particles to generate a liquid phase. In the gradually heating region P<NUM>, the silicon nitride particles are suppressed from growing, and are realigned and densified in the sintering additive in the form of the liquid phase. As a result, the silicon nitride sintered substrate provided after the first and second temperature maintaining regions P<NUM> and P<NUM> has a short pore diameter, a low porosity, a high flexural strength and a high thermal conductivity. It is preferred that temperature T<NUM> in the gradually heating region P<NUM> is in the range of <NUM> or higher and <NUM> or lower, which is lower than temperature T<NUM> in the first temperature maintaining region P<NUM>, that the heating rate in the gradually heating region P<NUM> is <NUM>/hr or higher, and that heating time t<NUM> in the gradually heating region P<NUM> is <NUM> hours or longer and <NUM> hours or shorter. The heating rate may include <NUM>/hr; namely, the gradually heating region P<NUM> may be a temperature maintaining region in which the temperature is maintained at a certain level. The heating rate in the gradually heating region P<NUM> is more preferably <NUM> to <NUM>/hr, and most preferably <NUM> to <NUM>/hr. The heating time t<NUM> is more preferably <NUM> to <NUM> hours, and most preferably <NUM> to <NUM> hours.

It is preferred that in the gradually heating region and steps thereafter, the sintering furnace is filled with a nitrogen atmosphere. Specifically, nitrogen, a mixed gas containing nitrogen as a main component and also containing inert gas such as argon or the like, or a mixed gas containing nitrogen gas and about <NUM>% or less of hydrogen is usable. It is preferred that the pressure in the sintering furnace is <NUM> atmospheric pressure or higher and about <NUM> atmospheric pressure or lower.

In the case where the Si powder, or the Si powder and the Si<NUM>N<NUM> powder, are used, as shown in <FIG>, the nitridation region Pn is provided in the gradually heating region P<NUM>, so that a nitridation step is performed to nitride the Si powder. For example, after the heating by use of the decarbonization region Pc is finished, the atmosphere temperature in the sintering furnace is raised and is maintained at temperature Tn in the range of <NUM> or higher and <NUM> or lower. It is preferred that maintaining time tn is <NUM> hours or longer and <NUM> hours or shorter. The greensheets <NUM> are maintained at this temperature in the nitrogen atmosphere, so that the Si powder reacts with nitrogen, which is the atmosphere in the sintering furnace, to generate silicon nitride.

After the heating by use of the gradually heating region P<NUM> is finished, the atmosphere temperature in the sintering furnace is controlled by the temperature profile in the first temperature maintaining region P<NUM>. In this step, the material components in the greensheets <NUM> are sintered.

The first temperature maintaining region P<NUM> is a temperature region in which the liquid phase generated in the gradually heating region P<NUM> is used to promote the realignment of the silicon nitride particles, phase transformation from α-phase silicon nitride crystal into β-phase silicon nitride crystal, and particle growth of the silicon nitride crystal, and thus to further densify the sintered body. As can be seen, the α to β phase transformation is one cause to promote the densification of the sintered body. Therefore, it is preferred that the silicon nitride powder as a material component contains the α-type crystal, and that the ratio at which the silicon nitride powder is of the α-type is <NUM>% or higher and <NUM>% or lower. In consideration of the size and the aspect ratio (ratio between a longer axis and a shorter axis) of the β-type silicon nitride particles, formation of pores by the volatilization of the sintering additive, and the like, it is preferred that the temperature T<NUM> in the first temperature maintaining region P<NUM> is in the range of <NUM> or higher and <NUM> or lower and that maintaining time t<NUM> is about <NUM> hour to <NUM> hours. More preferably, the temperature T<NUM> is in the range of <NUM> or higher and <NUM> or lower. If the temperature T<NUM> in the first temperature maintaining region P<NUM> is lower than <NUM>, it is difficult to sufficiently densify the silicon nitride sintered body. By contrast, if the temperature T<NUM> exceeds <NUM>, the sintering additive is volatilized too much and silicon nitride is decomposed too much, and as a result, it is difficult to sufficiently densify the silicon nitride sintered body. The heating temperature T<NUM> in the first temperature maintaining region P<NUM> may be changed (e.g., gradually raised) within the range of <NUM> to <NUM>.

The temperature T<NUM> in the first temperature maintaining region P<NUM> is more preferably in the range of <NUM> or higher and <NUM> or lower, and most preferably in the range of <NUM> or higher and <NUM> or lower. The temperature T<NUM> in the first temperature maintaining region P<NUM> is preferably higher by <NUM> or more, more preferably higher by <NUM> or more and <NUM> or less, than the upper limit of the temperature T<NUM> in the gradually heating region P<NUM>. The maintaining time t<NUM> is <NUM> hours or longer and <NUM> hours or shorter, and most preferably <NUM> hours or longer and <NUM> hours or shorter.

The second temperature maintaining region P<NUM> after the first temperature maintaining region P<NUM> is a temperature region in which the sintered body is maintained at temperature T<NUM> slightly lower than the temperature T<NUM> in the first temperature maintaining region P<NUM> to maintain the liquid phase treated in the first temperature maintaining region P<NUM> as it is or in a state where the solid phase and the liquid phase coexist. It is preferred that the temperature region T<NUM> in the second temperature maintaining region P<NUM> is in the range of <NUM> to <NUM> and is lower than the temperature T<NUM> in the first temperature maintaining region P<NUM>. Maintaining time t<NUM> in the second temperature maintaining region P<NUM> is <NUM> to <NUM> hours. The second temperature maintaining region P<NUM> is provided after the first temperature maintaining region P<NUM>, so that, for example, the curve of the silicon nitride sintered substrate may be within <NUM>/mm.

After the heating by use of the first Lemperature maintaining region P<NUM> is finished, the atmosphere temperature in the sintering furnace is controlled by the temperature profile in the second temperature maintaining region P<NUM>. If the temperature T<NUM> in the second temperature maintaining region P<NUM> is lower than <NUM>, the boundary phase is easily crystallized and thus the flexural strength of the resultant silicon nitride sintered substrate is low. By contrast, if the temperature T<NUM> exceeds <NUM>, the liquid phase has too high a fluidity and thus the above-described effect is not provided. The temperature T<NUM> is more preferably <NUM> or higher and <NUM> or lower, and most preferably <NUM> or higher and <NUM> or lower. It is preferred that the maintaining time t<NUM> in the second temperature maintaining region P<NUM> is <NUM> hour or longer and <NUM> hours or shorter. If the maintaining time t<NUM> in the second temperature maintaining region P<NUM> is shorter than <NUM> hours, the boundary phase is not sufficiently uniformized. In order to suppress the volatilization of the sintering additive to prevent the mechanical characteristics and the thermal conductivity of the silicon nitride sintered substrate from being lowered, the maintaining time t<NUM> in the second temperature maintaining region P<NUM> is set to <NUM> hours or shorter.

Use of the above-described conditions allows the Mg to be distributed uniformly in the boundary phase in the thickness direction of the silicon nitride sintered substrate without being eccentrically distributed. Therefore, the silicon nitride sintered substrate has a high mechanical strength (flexural strength and fracture toughness) and is suppressed from being curved.

After the temperature control by use of the second temperature maintaining region P<NUM> is finished, the atmosphere temperature in the sintering furnace is controlled by the temperature profile in the cooling region P<NUM>. The cooling region P<NUM> is a temperature region in which the liquid phase maintained in the second temperature maintaining region P<NUM> is cooled and solidified to secure the position of the resultant boundary phase. In order to solidify the liquid phase quickly to maintain the uniformity in the distribution of the boundary phase, the cooling rate in the cooling region P<NUM> is preferably <NUM>/hr or higher, more preferably <NUM>/hr or higher, and most preferably <NUM>/hr or higher. Practically, it is preferred that the cooling rate is <NUM>/hr or higher and <NUM>/hr or lower. The liquid phase is cooled at such a cooling rate, so that the crystallization of the sintering additive to be solidified is suppressed and thus the resultant grain boundary phase is mainly formed of a glassy phase. Therefore, the flexural strength of the silicon nitride sintered substrate is increased. The cooling rate in the cooling region P<NUM> is maintained until the temperature is lowered to <NUM>, and there is no specific limitation on the cooling rate at which the temperature is decreased to a lower temperature.

As a result of the above-described steps, the silicon nitride sintered bodies <NUM>' are produced. In the stacked assemblies <NUM>, the silicon nitride sintered bodies <NUM>' are separated from each other by the boron nitride powder layer <NUM>. Therefore, each silicon nitride sintered body <NUM>' is easily separated from the cooled stacked assemblies <NUM>. As described above with reference to <FIG>, the end portion <NUM> located along the outer perimeter is cut off from the separated silicon nitride sintered body <NUM>'. As a result, the silicon nitride sintered substrate <NUM> is provided.

According to the method for producing the silicon nitride sintered substrate in this embodiment, the thickness of the boron nitride powder layer located between the greensheets <NUM> while the greensheets <NUM> are stacked is selected to be an appropriate range, so that the greensheets are suppressed from being restricted by the boron nitride powder layer during the sintering. Thus, in the central area, the density is suppressed from being decreased and the void fraction is suppressed from being increased. While the atmosphere temperature is rising during the sintering, carbon is removed at a reduced pressure, so that the amount of carbon remaining in the greensheets is decreased. This suppresses the generation of the voids during the sintering. Therefore, the resultant silicon nitride sintered substrate has a high in-plane uniformity in the density and of the void fraction and has a large external size.

Silicon nitride sintered substrates were produced under various conditions and characteristics thereof were examined. Hereinafter, the results will be described.

A greensheet <NUM> was formed, by a doctor blade method, of a material powder slurry (solid content concentration: <NUM>% by mass) containing <NUM>% by mass of MgO powder, <NUM>% by mass of Y<NUM>O<NUM> powder and a remaining part of Si<NUM>N<NUM> powder and unavoidable impurities. Twenty such greensheets <NUM> were stacked with a boron nitride powder layer being located between each two adjacent greensheets <NUM> to form a stacked assembly <NUM>. The size of the greensheets <NUM> was varied by example as shown in Table <NUM>. The thickness of the boron nitride powder layer was varied by example as shown in Table <NUM>.

Weight plates <NUM> were respectively located on such stacked assemblies <NUM>, and the resultant assemblies including the weight plates <NUM> and the stacked assemblies <NUM> were respectively located on carrying plates <NUM>. The resultant assemblies including the weight plates <NUM>, the stacked assemblies <NUM> and the carrying plates <NUM> were set in a container <NUM> (double-wall container) shown in <FIG>. The load on the uppermost greensheet 1a provided by the weight plate <NUM> was <NUM> Pa. A packing powder containing <NUM>% by mass of magnesia powder, <NUM>% by mass of silicon nitride powder and <NUM>% by mass of boron nitride powder was located on a top surface of the uppermost carrying plate 21a.

The container <NUM> was put into a sintering furnace, and the pressure of the sintering furnace was decreased to <NUM>-<NUM> Pa by use of a vacuum pump. As shown in Table <NUM>, the maintaining temperature in the decarbonization region Pc was varied by example. The maintaining time tc in each example was set to <NUM> hour.

Then, the atmosphere in the sintering furnace was changed to, for example, nitrogen of <NUM> atmospheric pressure, and the atmosphere temperature was raised at a temperature raising rate of <NUM>/hr (<NUM>/min) for <NUM> hours as a temperature profile in the gradually heating region P<NUM>. Next, the atmosphere temperature was maintained at the temperature T<NUM> of <NUM> for <NUM> hours as a temperature profile in the first temperature maintaining region P<NUM>, and the atmosphere temperature was maintained at the temperature T<NUM> of <NUM> for <NUM> hours as a temperature profile in the second temperature maintaining region P<NUM>. Then, the atmosphere temperature was lowered at a cooling rate of <NUM>/hr as a temperature profile in the cooing region P<NUM>. An extra portion was cut off from the resultant silicon nitride sintered substrate to provide a silicon nitride sintered substrate <NUM>, which has a square shape having a side of a length shown in Table <NUM>. The thickness of the silicon nitride sintered substrate <NUM> was <NUM>.

Square silicon nitride sintered substrates respectively having sides of lengths of <NUM> and <NUM> were produced. The other conditions than those shown in Table <NUM> were the same as those in the examples.

Silicon nitride sintered substrates were produced under the same conditions as those in the examples except that the maintaining temperature in the decarbonization region Pc, the atmosphere in the decarbonization step, and the thickness of the boron nitride powder layer were different as shown in Table <NUM>.

The compositions of the silicon nitride sintered substrates in the examples, the reference examples and the comparative examples produced under the conditions described in the above embodiment were checked. Specifically, the silicon nitride sintered substrates were subjected to a microwave decomposition process to be put into a solution state, and then an Mg amount and an RE amount were measured by ICP spectrometry. The measured values were respectively converted into a magnesium oxide (MgO) content and a rare earth element oxide (RE<NUM>O<NUM>) content. The resultant contents were confirmed to be substantially the same (same in the % by mass until the first decimal place) as the amounts thereof initially contained (amounts in the composition).

Regarding the produced silicon nitride sintered substrates, the density, the density ratio, the void fraction, the void fraction ratio, the carbon content, the partial discharge inception voltage, and the partial discharge extinction voltage were measured. The dielectric breakdown voltage was measured, and the Weibull coefficient of dielectric breakdown voltage was found. The results are shown in Table <NUM>. In a graph shown in <FIG> regarding the silicon nitride sintered substrates in examples <NUM> through <NUM>, reference examples <NUM> and <NUM>, and comparative examples <NUM> through <NUM>, the horizontal axis represents the carbon content and the vertical axis represents the void fraction (in the central area). Regarding the silicon nitride sintered substrates in examples <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> and reference examples <NUM> and <NUM>, graphs were created that show the external size of the substrate represented by the horizontal axis and the above-described measurement results represented by the vertical axis. The graphs are shown in <FIG>, <FIG>.

In each of the silicon nitride sintered substrates in examples <NUM> through <NUM>, the atmosphere in the decarbonization step is vacuum (<NUM> Pa or lower) and the maintaining temperature is <NUM> to <NUM>. It is considered that under such conditions, the carbon content of the silicon nitride sintered substrate is <NUM>% by mass or lower. It is also considered that since the carbon content is low and the thickness of the boron nitride powder layer is in the range of <NUM> to <NUM>, the density and the void fraction are low and the uniformity thereof at the main surface is high. Specifically, the ratio in the density between the central area and the end area is <NUM> or higher, the void fraction of the central area of the main surface is <NUM>% or lower, and the void fraction of the end area of the main surface is <NUM>% or lower. It is understood from <FIG> that when the carbon content is <NUM>% by mass or lower, the void fraction of the central area is <NUM>% or lower.

It is considered that since the void fraction is low, the partial discharge inception voltage and the partial discharge extinction voltage are each <NUM> kV or higher. It is considered that since the void fraction is low, the dielectric breakdown voltage is <NUM> V or higher and the Weibull coefficient of dielectric breakdown voltage is <NUM> or higher.

Especially in each of the silicon nitride sintered substrates in examples <NUM> through <NUM> and <NUM> through <NUM>, the maintaining temperature in the decarbonization step is <NUM> or higher, and the thickness of the applied boron nitride powder layer is about <NUM> or less. Therefore, the removal of carbon from the greensheets and the shrinkage of the greensheets in the stacked assemblies progress sufficiently, and as a result, the density of the silicon nitride sintered substrate is increased and the uniformity in the density is high. Specifically, the density dc of the central area of each silicon nitride sintered substrate is <NUM>/cm<NUM> or higher, and the density de of the end area of each silicon nitride sintered substrate is <NUM>/cm<NUM> or higher. It is considered that as a result of this, the partial discharge inception voltage is <NUM> kV or higher.

The silicon nitride sintered substrates in examples <NUM> through <NUM> each have a square shape having a side of a length of <NUM> or longer and <NUM> or shorter. It is understood that such a silicon nitride sintered substrates is large and has a high breakdown voltage and a high insulation reliability. The silicon nitride sintered substrates in reference examples <NUM> and <NUM> respectively have square shapes having sides of lengths of <NUM> and <NUM>. It is understood that the density, the void fraction, the partial discharge inception voltage, the dielectric breakdown voltage and the Weibull coefficient of dielectric breakdown voltage thereof are substantially the same as those in examples <NUM> through <NUM>. From a comparison of these substrates, it is understood that the silicon nitride sintered substrates in examples <NUM> through <NUM>, although being large, have a high in-plane uniformity in the partial discharge inception voltage and the dielectric breakdown voltage, like the silicon nitride sintered substrates having a side as short as <NUM>.

By contrast, in comparative example <NUM>, the atmosphere in the decarbonization step is nitrogen. In comparative example <NUM> through <NUM>, the atmosphere in the decarbonization step is vacuum, but the maintaining temperature is low. Therefore, carbon is not sufficiently removed in the decarbonization step, and the resultant silicon nitride sintered substrates contain a high carbon content. It is considered that as a result of these, the density and the void fraction are high, and the partial discharge voltage, the dielectric breakdown voltage and the Weibull coefficient of dielectric breakdown voltage are low.

In comparative example <NUM>, it is considered that since the atmosphere temperature in the decarbonization step is too high, the sintering additive is evaporated. It is considered that because of this, the sintering is not performed at a high density, and thus the density and the void fraction are high and the partial discharge voltage, the dielectric breakdown voltage and the Weibull coefficient of dielectric breakdown voltage are low.

In comparative examples <NUM> and <NUM>, it is considered that since the boron nitride powder layer is too thick, the shrinkage of the central area of the silicon nitride sintered substrate is inhibited. It is considered that as a result of this, the void fraction is especially high in the central area, and the partial discharge voltage, the dielectric breakdown voltage and the Weibull coefficient of dielectric breakdown voltage are low.

From the above-described results, it is understood that the silicon nitride sintered substrates in examples <NUM> through <NUM> are large and have a high breakdown voltage and a high insulation reliability because the maintaining temperature and the atmosphere in the decarbonization step fulfill the above-described conditions and the boron nitride powder layer has a thickness in the predetermined range.

<FIG> shows the relationship between the length of the side and the density of the silicon nitride sintered substrates in examples <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> and reference examples <NUM> and <NUM>. <FIG> shows the relationship between the length of the side and the void fraction of the silicon nitride sintered substrates in examples <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> and reference examples <NUM> and <NUM>. <FIG> also shows approximate straight lines found from the measurement data on the density of the central area and the density of the end area. <FIG> also shows approximate straight lines found from the measurement data on the void fraction of the central area and the void fraction of the end area. As can be seen from these results, although the length of the side of each of the silicon nitride sintered substrates in the examples is <NUM> at the maximum, it is presumed that even in a case of a silicon nitride sintered substrate of a square shape having a side of a length of <NUM>, the void fraction vc of the central area may be <NUM>% or lower, the void fraction ve of the end area may be <NUM>% or lower, the density dc of the central area may be <NUM>/cm<NUM> or higher, and the density de of the end area may be <NUM>/cm<NUM> or higher. It is understood that dc/de is <NUM> or higher and ve/vc is <NUM> or higher.

<FIG> shows the relationship between the length of the side and the partial discharge voltage of the silicon nitride sintered substrates in examples <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> and reference examples <NUM> and <NUM>. <FIG> shows the relationship between the length of the side and the Weibull coefficient of dielectric breakdown voltage of the silicon nitride sintered substrates in examples <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> and reference examples <NUM> and <NUM>. <FIG> also shows approximate straight lines found from the measurement data on the dielectric breakdown voltage, the partial discharge extinction voltage and the partial discharge inception voltage. <FIG> also shows an approximate straight line found from the measurement data on the Weibull coefficient of dielectric breakdown voltage. Also as can be seen from these results, it is presumed that in a case of a silicon nitride sintered substrate of a square shape having a side of a length of <NUM>, the dielectric breakdown voltage (dielectric withstand voltage) is <NUM> kV or higher, and the partial discharge extinction voltage and the partial discharge inception voltage are each <NUM> kV or higher. It is presumed that the Weibull coefficient of dielectric breakdown voltage is <NUM> or higher.

Therefore, even in the case where a silicon nitride sintered substrate of a square shape having a side of a length of <NUM> or longer and <NUM> or shorter is produced, it is presumed that the breakdown voltage and the insulation reliability are high and the in-plane uniformity thereof is also high.

Claim 1:
A silicon nitride sintered substrate having a main surface of a shape larger than a square having a side of a length of <NUM>, wherein a ratio dc/de is <NUM> or higher where a central area of the main surface has a density dc and an end area of the main surface has a density de, the central area of the main surface has a void fraction vc of <NUM>% or lower, and the end area of the main surface has a void fraction ve of <NUM>% or lower
wherein the density dc, density de, void fraction vc and void fraction ve are defined as described in the description, and
wherein the silicon nitride sintered substrate has a carbon content and the carbon content is <NUM>% by mass or lower that is measured as described in the description by a non-dispersive infrared absorption method on the circle (C22) at the center of the main surface.