Silicon nitride sintered body and circuit board

Abstract

To provide a silicon nitride sintered body with reduced etching unevenness on a substrate surface after etching treatment and improved dielectric voltage property.SOLUTION: A silicon nitride sintered body includes silicon nitride grains and grain boundary phases existing between silicon nitride grains and has an X-ray diffraction peak of β-Si3 N4. In a substrate after etching treatment obtained by soaking a substrate of a silicon nitride sintered body in a hydrochloric acid solution at 50°C and with mass fraction of 7.8% for 30 min., an etching depth ratio RD of a front / rear face substrate ratio is 10% or under.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a silicon nitride sintered body, a circuit board, and a method for manufacturing a silicon nitride sintered body.

Background Art

[0002] In recent years, with the increasing density and high power of electronic devices and semiconductor devices, the heat generation density of power modules has been increasing. The temperature rise of the power module causes malfunctions of elements and cracks in the insulating circuit board. Therefore, ceramic substrates such as alumina and aluminum nitride, which are materials with relatively high thermal conductivity, have been used for insulating circuit boards. However, alumina and aluminum nitride have the disadvantage of low mechanical strength. Therefore, it has not been possible to directly bond thick copper to the ceramic substrate where strong thermal stress is applied, which has restricted the structure of the power module. Specifically, since it is necessary to solder a heat sink such as copper or aluminum to the insulating circuit board, the problem is that the power module becomes larger. Therefore, the silicon nitride (Si3N4) material has been attracting attention as an insulating circuit board. Since the silicon nitride sintered body has higher strength and fracture toughness than alumina and aluminum nitride sintered bodies, it is possible to directly bond thick copper to the insulating circuit board, contributing to the miniaturization of the module. Therefore, the development of a silicon nitride sintered body with improved thermal conduction performance as well as mechanical strength has been carried out.

[0003] For example, Patent Document 1 discloses a method for manufacturing a silicon nitride sintered body substrate with improved mechanical properties and thermal conductivity. In this manufacturing method, a sintering aid of one or more elements selected from Mg, Ca, Sr, Ba, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Yb is added to silicon nitride powder with an Al content of 0.1 wt% or less in the range of 1 wt% or more and 15 wt% or less, and after molding, it is fired at a temperature of 1700 °C or more and 2300 °C or less under a nitrogen gas pressure of 1 atm or more and 500 atm or less. The silicon nitride sintered body substrate obtained by this manufacturing method is composed of β-type silicon nitride grains of 85 wt% or more and 99 wt% or less and the balance being a grain boundary phase of an oxide or oxynitride. Further, the grain boundary phase contains 0.5 wt% or more and 10 wt% or less of one or more metal elements selected from Mg, Ca, Sr, Ba, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Yb. And the Al atom content in the grain boundary phase is 1 wt% or less, the porosity is 5% or less, and the ratio of β-type silicon nitride grains having a minor axis diameter of 5 μm or more in the microstructure of the sintered body is 10 vol% or more and 60 vol% or less. That is, in order to obtain a high thermal conductivity silicon nitride sintered body substrate, it is known that rare earth compounds or magnesium oxide can be added as a sintering aid, and the thermal conductivity and mechanical strength can be improved by their mixing ratio and addition amount.

[0004] On the surface of a silicon nitride sintered body substrate having such high thermal conductivity and high mechanical strength, various metal plates are joined for circuit formation. For the method of joining various metal plates (mainly copper plates) to the surface of the silicon nitride sintered body substrate, a direct bonding method, an active metal method, etc. are used. The direct bonding method is to heat the copper plate etc. in contact with the substrate and directly bond the substrate and the copper plate. The active metal method is to bond the ceramic substrate and the copper plate through a brazing agent containing an active metal such as Ti, Zr, Hf. Then, an etching mask is formed in a circuit pattern shape on the surface of the copper plate after joining, and the entire substrate is immersed in an etching solution such as a copper chloride solution or a hydrochloric acid solution, and the unnecessary portion of the copper plate (where the etching mask is not formed) is etched. Since the characteristics of the surface of the silicon nitride sintered body substrate may be altered when it is exposed to the etching solution, it is desirable for the silicon nitride sintered body to have higher etching resistance.

[0005] On the other hand, Patent Document 2 discloses increasing the etching resistance to acids and alkalis by increasing the ratio of the crystalline phase among the amorphous phase and the crystalline phase in the grain boundary phase by utilizing the property that the grain boundary crystalline phase is difficult to be etched. Further, in Patent Document 2, rare earth elements that lower the crystallinity of the grain boundary phase of the silicon nitride sintered body are not included, and by adding particles containing elements of Ti, Zr, or Hf, the crystallization of the grain boundary phase is further promoted.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, in conventional silicon nitride sintered body substrates, (even in silicon nitride sintered bodies containing rare earth elements such as in Patent Document 1), the presence of a certain amount of grain boundary crystal phase in the grain boundary phase can cause uneven etching (or etching residue) in the grain boundary crystal phase portion of the silicon nitride sintered body, which can be a cause of poor appearance of the substrate surface after the etching process. In particular, as in Patent Document 2, when the proportion of the grain boundary crystal phase is increased to enhance the etching resistance, this etching unevenness becomes even more prominent. Such uneven etching in the grain boundary crystal phase portion causes non-uniformity in the particle distribution within the substrate and local reduction of the breakdown voltage of the substrate surface, which is a problem. In particular, in a circuit board, it is known that electric field concentration occurs at the end of the wiring pattern, and a discharge path is formed along the surface of the insulating substrate, leading to dielectric breakdown. This dielectric breakdown phenomenon is called creeping discharge and can cause damage to the device. Therefore, in order to alleviate such electric field concentration, it is required to improve the breakdown voltage of the surface of the silicon nitride sintered body substrate.

[0008] The inventors reexamined the manufacturing method of the conventional silicon nitride sintered body and focused on controlling the crystallization of the grain boundary phase, aiming to further improve the substrate characteristics of the silicon nitride sintered body.

[0009] In order to solve the above problems, an object of the present invention is to provide a silicon nitride sintered body with reduced etching unevenness on the substrate surface after the etching process and improved breakdown voltage performance, and a manufacturing method thereof.

Means for Solving the Problems

[0010] A silicon nitride sintered body according to one embodiment of the present invention is a silicon nitride sintered body having silicon nitride particles and a grain boundary phase existing between the silicon nitride particles, in the substrate after the etching process obtained by immersing the substrate of the silicon nitride sintered body in a hydrochloric acid solution at 50 ° C. and a mass fraction of 7.8% for 30 minutes, the breakdown electric field Vi on the substrate surface is 8 kV / mm or more at room temperature.

[0011] In a further form of the silicon nitride sintered body of the present invention, more preferably, the weight loss amount wd per unit surface area after the etching treatment of the silicon nitride sintered body under predetermined etching conditions is 12 μg / mm 2 or less, wherein the weight loss amount wd is obtained by dividing the difference between the weight of the substrate after the etching treatment obtained by immersing the substrate of the silicon nitride sintered body in a hydrochloric acid solution at 50 °C and a mass fraction of 7.8% for 30 minutes and the weight of the substrate before the etching treatment by the surface area of the substrate.

[0012] A further form of the silicon nitride sintered body of the present invention has an X-ray diffraction peak of β-Si3N4. More preferably, the grain boundary phase consists of an amorphous phase and a grain boundary crystal phase, the integrated intensity ratio R of the grain boundary crystal phase is 3% or less, the integrated intensity ratio R is represented by the following formula: R = (1 - Ig / Ia) × 100 (%) where Ig is the sum of the integrated intensities of all X-ray diffraction peaks indexed to β-Si3N4, and Ia is the sum of the integrated intensities of all observed X-ray diffraction peaks.

[0013] A silicon nitride sintered body according to one form of the present invention includes silicon nitride particles and a grain boundary phase existing between the silicon nitride particles, and is a silicon nitride sintered body having an X-ray diffraction peak of β-Si3N4, the grain boundary phase consists of an amorphous phase and a grain boundary crystal phase, the integrated intensity ratio R of the grain boundary crystal phase is 3% or less, the integrated intensity ratio R is represented by the following formula: R = (1 - Ig / Ia) × 100 (%) where Ig is the sum of the integrated intensities of all X-ray diffraction peaks indexed to β-Si3N4, and Ia is the sum of the integrated intensities of all observed X-ray diffraction peaks.

[0014] The silicon nitride sintered body according to one embodiment of the present invention is a silicon nitride sintered body having silicon nitride particles and a grain boundary phase existing between the silicon nitride particles, wherein the grain boundary phase is composed of an amorphous phase and a grain boundary crystal phase, the weight reduction amount wd per unit surface area of the substrate after the etching treatment of the silicon nitride sintered body under predetermined etching conditions is 12 μg / mm 2 or less, and the weight reduction amount wd is obtained by dividing the difference between the weight of the substrate after the etching treatment obtained by immersing the substrate of the silicon nitride sintered body in a hydrochloric acid solution at 50 °C and a mass fraction of 7.8% for 30 minutes and the weight of the substrate before the etching by the surface area of the substrate.

[0015] The silicon nitride sintered body according to a further embodiment of the present invention is more preferably further characterized in that the integrated intensity ratio R of the grain boundary crystal phase is 0.3 to 3%.

[0016] The silicon nitride sintered body according to a further embodiment of the present invention is more preferably further characterized in that it contains 0.02 to 0.2% by weight of calcium element.

[0017] The silicon nitride sintered body according to a further embodiment of the present invention is more preferably further characterized in that it contains 0.05 to 0.2% by weight of fluorine element.

[0018] The silicon nitride sintered body according to a further embodiment of the present invention is more preferably further characterized in that in the cross-sectional SEM image of the substrate after the etching treatment, the grain boundary phase at a depth of less than 40 μm from the substrate surface layer is removed, and no grain boundary crystal phase remains in the region where the grain boundary phase is removed.

[0019] The silicon nitride sintered body according to a further embodiment of the present invention is more preferably further characterized in that it is composed of 1 to 5% by weight of yttrium oxide, 1.5 to 3.5% by weight of magnesium oxide, and 0.05 to 0.3% by weight of a Group 2 element fluoride. The Group 2 element fluoride is more preferably selected from the group consisting of CaF2 and MgF2.

[0020] A method for manufacturing a silicon nitride sintered body according to one embodiment of the present invention is a method for manufacturing a silicon nitride sintered body, comprising: mixing silicon nitride powder, rare earth oxide powder, magnesium oxide powder, and group 2 element fluoride powder at a predetermined mixing ratio, and adding a solvent to form a slurry; forming the slurry into a sheet-shaped body having a predetermined thickness; sintering the sheet-shaped body in a non-oxidizing atmosphere to obtain a silicon nitride sintered body.

[0021] A method for manufacturing a silicon nitride sintered body according to a further embodiment of the present invention is more preferably characterized in that the fluorine content of the silicon nitride powder is 800 ppm or less.

[0022] A method for manufacturing a silicon nitride sintered body according to a further embodiment of the present invention is more preferably characterized in that the fluorine content of the mixture of raw material powders is 600 ppm or more.

[0023] A method for manufacturing a silicon nitride sintered body according to a further embodiment of the present invention is more preferably characterized in that the group 2 element fluoride is 0.05% by weight or more in the mixing ratio of the raw material powders.

[0024] A method for manufacturing a silicon nitride sintered body according to a further embodiment of the present invention is more preferably characterized in that the group 2 element fluoride is selected from the group consisting of CaF2 and MgF2.

[0025] A method for manufacturing a silicon nitride sintered body according to a further embodiment of the present invention is more preferably characterized by including mixing 1 to 5% by weight of yttrium oxide powder, 1.5 to 3.5% by weight of magnesium oxide powder, and 0.05 to 0.3% by weight of group 2 element fluoride powder.

[0026] A silicon nitride sintered body according to one embodiment of the present invention is a silicon nitride sintered body having silicon nitride particles and a grain boundary phase existing between the silicon nitride particles, In the substrate of the silicon nitride sintered body, the etching depth ratio R of the front and back surfaces of the substrate in the etched substrate obtained by immersing the substrate in a hydrochloric acid solution at 50 °C and a mass fraction of 7.8% for 30 minutes D is 10% or less.

[0027] The silicon nitride sintered body according to one embodiment of the present invention is a silicon nitride sintered body having silicon nitride particles and a grain boundary phase existing between the silicon nitride particles, In the substrate of the silicon nitride sintered body, the boundary line length ratio R indicating the ratio of the length of the boundary line between the non-etched layer and the etched layer to the evaluation length L in the etched substrate obtained by immersing the substrate in a hydrochloric acid solution at 50 °C and a mass fraction of 7.8% for 30 minutes L is less than 2.0.

Advantages of the Invention

[0028] The silicon nitride sintered body and its manufacturing method of the present invention can reduce the unevenness of etching on the surface of the substrate of the silicon nitride sintered body after the etching treatment, and can improve the breakdown voltage insulation performance.

Brief Description of the Drawings

[0029]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Mode for Carrying Out the Invention

[0030] The silicon nitride sintered body according to an embodiment of the present invention has a substrate shape with a predetermined thickness, and mainly, a metal plate such as a copper plate is brazed (brazed or soldered) to the substrate surface, and the metal plate is etched in a circuit shape, so that it can be used as a circuit board for mounting electronic components for mounting electronic components. In particular, the entire substrate of the silicon nitride sintered body in a state where the metal plate is joined is immersed in an etching solution such as a copper chloride solution or a hydrochloric acid solution for a predetermined time, so that the metal plate at the location excluding the etching mask such as a resist material is removed, and a predetermined circuit is formed on the substrate surface. By mounting electronic components such as semiconductor chips on this circuit board of the silicon nitride sintered body, electronic devices for various applications are configured. Generally, it is known that the silicon nitride sintered body is affected by the etching solution over its surface area, and the dielectric breakdown field Vi on the substrate surface decreases due to the etching treatment. The silicon nitride sintered body of the present embodiment has improved etching resistance in terms of etching unevenness and / or dielectric breakdown field compared to the conventional one.

[0031] Note that the thickness of the substrate of the silicon nitride sintered body of the present embodiment is preferably 0.1 to 1.0 mm. That is, the surface area of the side surface of the substrate is configured to be sufficiently small with respect to the surface area of the front and back surfaces of the substrate, and in the performance evaluation, the influence of etching on the thickness of the substrate can be ignored.

[0032] The silicon nitride sintered body of this embodiment is composed of a silicon nitride sintered body containing 1 to 5% by weight of a rare earth oxide (first sintering aid), 1.5 to 3.5% by weight of magnesium oxide (second sintering aid), 0.05 to 0.3% by weight of a Group 2 element fluoride, and the balance being Si3N4. In other words, the silicon nitride sintered body of this embodiment is obtained by firing a slurry of a mixed raw material powder in which silicon nitride powder, rare earth oxide powder, magnesium oxide powder, and Group 2 element fluoride powder are mixed at a predetermined mixing ratio.

[0033] In this embodiment, the sintering aid is a combination of an oxide of a rare earth element (Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Yb) as the first sintering aid and magnesium oxide (MgO) as the second sintering aid. In particular, by adding an Mg component together with a rare earth oxide as a sintering aid component, the liquid phase formation temperature during sintering can be lowered, and the sinterability can be improved. Furthermore, it is more preferable that the first sintering aid is Y2O3 at 1 to 5% by weight and the second sintering aid is MgO at 1.5 to 3.5% by weight. It has been known from past findings that by using Mg and Y as sintering aid components, the silicon nitride sintered body can be densified, and as a result, relatively high thermal conductivity and mechanical strength can be achieved simultaneously. In this embodiment, MgO and Y2O3 are employed as the sintering aid. It should be noted that since it has been obtained from past findings that the blending composition ratio of the rare earth oxide does not significantly affect the characteristics of the silicon nitride sintered body substrate as long as it is within the range of 1 to 5% by weight, those skilled in the art can arbitrarily select the type and blending ratio thereof.

[0034] In the sintering process of the silicon nitride sintered body, a liquid phase is formed by the surface silica layer (SiO2) of the silicon nitride powder and Y2O3 and MgO added as sintering aids. After α-type silicon nitride dissolves in the generated liquid phase, when the nitrogen concentration in the liquid crystal increases, it reprecipitates as β-type silicon nitride, and particle rearrangement occurs in the liquid phase, resulting in densification. The sintering process is generally carried out in a temperature range of 1800°C to 1900°C, and crystallization of the grain boundary phase occurs in the cooling process after the sintering process. The inventors focused on suppressing the crystallization of the grain boundary phase in the cooling process in order to suppress the decrease in the breakdown electric field after the etching treatment (or after the etching process). However, in the prior art, it was difficult to effectively control the ratio of crystallization of the grain boundary phase in the cooling process.

[0035] The present invention makes it possible to effectively control the crystallization of the grain boundary phase of the sintered body in order to suppress uneven etching on the substrate surface regardless of the fluorine content (impurity content) of the silicon nitride powder by adding a Group 2 element fluoride as an additive to the raw material. That is, the silicon nitride sintered body has reduced uneven etching and improved dielectric breakdown voltage performance and etching resistance on the substrate surface of the silicon nitride sintered body after the etching treatment. Here, the Group 2 element fluoride is one or more selected from the group consisting of fluorides of Mg, Ca, Sr, and Ba. In particular, the Group 2 element fluoride is preferably CaF2, MgF2, or a combination thereof. When the Group 2 element fluoride contains CaF2, it is considered that Ca ions and F ions introduced into the liquid phase during firing remain in the grain boundary phase even after sintering, lowering the solidification temperature of the grain boundary phase and facilitating suppression of grain boundary crystallization. On the other hand, when the Group 2 element fluoride contains MgF2, Mg ions promote densification of the sintered body in the same manner as MgO. The silicon nitride sintered body of the present embodiment is preferably configured to contain 0.05 to 0.2% by weight of fluorine element. Also, the silicon nitride sintered body of the present embodiment more preferably contains 0.02 to 0.2% by weight of calcium element. The content of the calcium element can be calculated based on the blending amount of CaF2.

[0036] The silicon nitride sintered body of the present embodiment includes a large number of silicon nitride particles and a grain boundary phase existing between the silicon nitride particles. Further, the silicon nitride sintered body has an X-ray diffraction peak of β-Si3N4 in the X-ray diffraction pattern. The grain boundary phase consists of an amorphous phase and a grain boundary crystal phase. As an index indicating the ratio of the grain boundary crystal phase in the grain boundary phase (that is, the degree of crystallization), the integrated intensity ratio R of the grain boundary crystal phase is 3% or less. The integrated intensity ratio R is represented by the following formula: R = (1 - Ig / Ia) × 100 (%). Ig is the sum of the integrated intensities of all X-ray diffraction peaks indexed to β-Si3N4, and Ia is the sum of the integrated intensities of all observed X-ray diffraction peaks. The integrated intensity ratio R of the grain boundary crystal phase is preferably 0.3 to 3%.

[0037] Further, the silicon nitride sintered body of the present embodiment can achieve both high thermal conductivity and high mechanical strength. Specifically, in the silicon nitride sintered body of the present embodiment, it is preferable that the thermal conductivity is 80 W / mK or more and the three-point bending strength is 700 MPa or more.

[0038] The silicon nitride sintered body was subjected to an etching treatment under specific etching treatment conditions (immersion in a hydrochloric acid solution at 50 °C and a mass fraction of 7.8% for 30 minutes) in order to evaluate the etching resistance. The weight reduction amount wd per unit surface area of the silicon nitride sintered body after the etching treatment is 12 μg / mm 2 or less. The weight reduction amount wd is obtained by dividing the difference between the weight of the substrate of the silicon nitride sintered body after the etching treatment obtained by immersing the substrate in a hydrochloric acid solution at 50 °C and a mass fraction of 7.8% for 30 minutes and the weight of the substrate before the etching treatment by the surface area of the substrate. As will be described later, the weight reduction amount wd indicating the etching amount is reduced compared to a conventional silicon nitride sintered body that does not contain a Group 2 element fluoride. That is, the silicon nitride sintered body of the present embodiment has higher etching resistance.

[0039] Also, in the cross-sectional SEM image of the silicon nitride sintered body substrate after the etching treatment (see Figure 2), the grain boundary phase at a depth of less than 40 μm from the substrate surface layer has been removed. On the other hand, in a conventional silicon nitride sintered body that does not contain a Group 2 element fluoride, it has been found that the grain boundary phase at a depth of 40 μm or more from the substrate surface layer has been removed (see Figures 3 and 4). As shown in Figure 6, this etching depth D (μm) can also be quantitatively evaluated by the average value of the maximum distance (D1max, D2max) and the minimum distance (D1min, D2min) from the front and back surfaces of the substrate to the boundary line of the remaining grain boundary phase in the evaluation length L of the substrate cross-section of the silicon nitride sintered body. Also, as shown in Figure 2, in the surface layer region where the grain boundary phase of the silicon nitride sintered body of this embodiment has been removed, no grain boundary crystal phase remains. On the other hand, as shown in Figures 3 and 4, in a conventional silicon nitride sintered body that does not contain a Group 2 element fluoride, etching residue can be confirmed in its surface layer region. That is, the silicon nitride sintered body of this embodiment has reduced non-uniformity of etching in the surface layer region of the substrate surface after the etching treatment.

[0040] Furthermore, as one index for evaluating the non-uniformity of etching of the substrate of the silicon nitride sintered body after the etching treatment, the etching depth ratio R D of the front and back surfaces of the substrate, and / or the boundary line length ratio R L can be used.

[0041] The etching depth ratio R D represents the ratio (%) of the difference between the etching depth D1 on the substrate surface side and the etching depth D2 on the substrate back surface side to the average etching depth D of the front and back surfaces of the substrate, and is expressed by the following formula, R D =|D1 - D2| / D The smaller the etching depth ratio R D , the smaller the variation in etching between the front and back surfaces of the substrate, and it is estimated that the etching non-uniformity is small. In the silicon nitride sintered body after the etching treatment of this embodiment, the etching depth ratio R D is preferably 10% or less.

[0042] Boundary line length ratio R L represents the ratio of the length of the boundary line between the non-etching layer and the etching layer to the evaluation length L, and is given by the following formula: R L =(L1 + L2) / 2L where L1 represents the boundary line length on the substrate surface side, and L2 represents the boundary line length on the substrate back surface side. The smaller the boundary line length ratio R L , the less unevenness there is on the boundary surface between the non-etching layer and the etching layer, and it is presumed that there is less etching unevenness. In the silicon nitride sintered body after the etching process of the present embodiment, the boundary line length ratio R L is preferably less than 2.0.

[0043] And, the silicon nitride sintered body has a dielectric breakdown electric field Vi on the substrate surface at normal temperature or room temperature (in a measurement environment of 25°C) of 8 kV / mm or more after the etching process of the substrate. On the other hand, in a conventional silicon nitride sintered body that does not contain a Group 2 element fluoride as shown in FIG. 3, it has been found that the dielectric breakdown electric field Vi on the substrate surface at normal temperature or room temperature (in a measurement environment of 25°C) is less than 8 kV / mm. That is, the silicon nitride sintered body of the present embodiment has improved dielectric breakdown voltage performance compared to the prior art. Here, the dielectric breakdown electric field Vi (V / mm) was evaluated as follows. Electrodes A and B of two independent copper plates were printed on the substrate surface of the silicon nitride sintered body after the etching process by sputtering so as to be separated at the minimum distance (see FIG. 5). Then, an alternating current was applied to the electrodes in a fluorine-based inert liquid (fluorinate) at 25°C, the voltage at which the insulation was broken was measured, and the dielectric breakdown electric field Vi (V / mm) was obtained by dividing the measured value by the minimum distance.

[0044] Next, the changes in the properties of the silicon nitride sintered body due to the addition of Group 2 element fluoride (CaF2) will be considered. During the sintering process of the mixture (its sheet molded body) to which CaF2 powder is added as an additive in a predetermined addition amount, Ca ions and F ions are added to the liquid-phase composition of the oxynitride glass. Ca ions have the effect of lowering the melting point of the glass by forming a eutectic compound with the oxynitride glass containing Si and Mg. Also, by introducing F ions into the oxynitride glass, the nitrogen solubility in the liquid phase is increased, the glass melting point is lowered, and the degree of polymerization of the glass molecules is lowered by two F ions substituting one O ion, so that the viscosity of the liquid phase can be lowered. These effects lower the starting temperature of densification at the initial stage of liquid-phase sintering, improve the uniformity of the grain boundary glass layer, and have the effect of promoting densification at the initial stage of sintering. If densification proceeds in the initial stage of sintering, the shrinkage ratio observed during the heating process from the middle to the final stage of sintering with respect to the shrinkage ratio of the molded body observed during the entire sintering process from the initial to the final stage of sintering becomes smaller. In the heating process from the middle to the final stage of sintering, due to the influence of the volatilization of MgO and SiO2, a decrease in the liquid-phase amount and an increase in viscosity are likely to occur. Promoting densification in the initial stage of sintering is effective for the refinement of the sintered body, the uniformity and property improvement on the front and back surfaces of the substrate and within the substrate surface.

[0045] Regarding the effect on suppressing grain boundary crystallization, Ca ions and F ions introduced into the liquid phase remain in the grain boundary phase even after sintering, lowering the solidification temperature of the grain boundary phase and facilitating the suppression of grain boundary crystallization.

[0046] Regarding the effect on etching resistance, CaF2 dissolves in the liquid phase formed by Si3N4 - SiO2 - Y2O3, increasing the nitrogen concentration in the liquid phase. The grain boundary phase in the sintered body produced under this condition has a low oxygen concentration, and it is considered that the etching resistance is improved by the formation of passive chlorides on the surface of the grain boundary glass phase during acid treatment with hydrochloric acid.

[0047] Regarding the effect on the breakdown electric field, there is an effect that leakage current is generated due to the influence of chloride formed on the surface of the grain boundary glass phase, and the voltage applied between the electrodes on the substrate surface is relaxed. In addition, the etching residue generated when the crystallization ratio of the grain boundary crystal phase is high causes non-uniformity in the particle distribution in the substrate and is a factor that partially causes electric field concentration. Therefore, by suppressing the crystallization ratio of the grain boundary crystal phase to a low level, a decrease in the local dielectric breakdown electric field can be suppressed.

[0048] Next, a method for manufacturing the silicon nitride sintered body of the present embodiment will be described. The method for manufacturing a silicon nitride sintered body mainly includes a step of mixing silicon nitride powder, rare earth oxide powder, magnesium oxide powder, and group 2 element fluoride powder at a predetermined mixing ratio and adding a solvent to form a slurry, a step of forming the slurry into a sheet molded body having a predetermined thickness, and a step of sintering the sheet molded body in a non-oxidizing atmosphere to obtain a silicon nitride sintered body. Hereinafter, each step will be described in detail.

[0049] (1) Slurry formation step 1 to 5 wt% of yttrium oxide powder, 1.5 to 3.5 wt% of magnesium oxide powder, and 0.05 to 0.3 wt% of group 2 element fluoride powder were wet-mixed by a ball mill in a mixed solvent of toluene and ethanol using a silicon nitride ball of φ10 and a nylon pot mill. In wet mixing, an appropriate amount of a dispersant may be added. Usually, polycarboxylic acid-based or amine-based dispersants are 0.2 mg / m with respect to the surface area of the raw material powder. 2 to 1.5 mg / m 2It is added within the range. In this mixing step, the pulverization and mixing time was adjusted so that the D50 diameter (median diameter) of the Group 2 element fluoride powder would be 5 μm or less. Once the particle size of the Group 2 element fluoride powder could be adjusted, 92 to 97.5 wt% of silicon nitride powder was charged into the mill and mixed for 12 to 24 hours. Then, a binder, a plasticizer, and an organic solvent were added to form a slurry. As a method for adjusting the slurry used for molding, wet mixing using an organic solvent is desirable in order to suppress productivity and an increase in the amount of oxygen during mixing. As a specific example, a dispersant, toluene, and an organic solvent mixed with ethanol are added to the raw material powder and adjusted by a commonly used mixing and pulverization method. Then, the mixture is uniformly mixed and the particle size is adjusted by methods such as a ball mill, a bead mill, and a vibration mill. The slurry is defoamed and the viscosity is adjusted in a vacuum, and the viscosity is adjusted to a level at which a green sheet can be molded. As the material of the mill and media used in the mixing and pulverization method, resin-made materials such as urethane and nylon, or ceramic-made materials such as silicon nitride and zirconium oxide can be used, but in order to prevent impurity contamination of the slurry, it is preferable to use resin or silicon nitride as the material.

[0050] Here, the silicon nitride powder is preferably a high-purity silicon nitride powder produced by a direct nitridation method, an imide pyrolysis method, etc., and having a low oxygen content. On the other hand, the silicon nitride powder contains a certain amount of fluorine as an impurity, but the fluorine amount in the silicon nitride powder is preferably 800 ppm or less. Also, the fluorine amount contained in the mixture of the raw material powders is preferably 600 ppm or more.

[0051] The rare earth oxide powder can be selected from oxides of Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Yb, or combinations thereof. Preferably, the rare earth oxide powder is yttrium oxide (Y2O3).

[0052] The Group 2 element fluoride powder can be selected from fluorides of Mg, Ca, Sr, and Ba or combinations thereof. Preferably, the Group 2 element fluoride powder is selected from CaF2, MgF2, and combinations thereof.

[0053] (2) Forming step In the forming step, a high-viscosity slurry after viscosity adjustment by vacuum degassing is formed into a sheet shape, and a green sheet with a predetermined thickness is obtained by the doctor blade method. The thickness of the green sheet can be appropriately changed according to the required thickness of the sintered body, but is usually in the range of about 0.1 to 1.3 mm. The formed green sheet is processed into a desired shape by a mold press or a cutting machine to obtain a formed body.

[0054] (3) Sintering step A spray of boron nitride powder, which is a release agent, is applied to the surface of the formed body obtained in the forming step, and a block body in which 15 to 30 formed bodies are laminated is prepared. This block body is placed in a boron nitride sheath, and debinding is performed in a temperature range of 500 to 600 °C in dry air. The atmosphere during debinding may be nitrogen or vacuum. By sintering the debound block body at a high temperature for a predetermined time, a silicon nitride sintered body, which is the final target product of this manufacturing method, is obtained. The sintering treatment is performed in a non-oxidizing (nitrogen) atmosphere in a firing furnace in a temperature range of about 1750 to 2000 °C. Also, in order to prevent the volatilization of Si3N4 and sintering aids (for example, MgO), it is preferable to perform pressure sintering under a pressure of 5 atm or more.

[0055] By going through the firing process after the slurry formation process described above, a substrate of silicon nitride sintered body having high etching resistance can be obtained. A metal plate such as a copper plate is brazed (brazed or soldered) to the surface of the substrate of the silicon nitride sintered body thus produced, an etching mask in the shape of a circuit pattern is formed on the surface of the copper plate after bonding, and unnecessary portions of the copper plate are etched by an etching solution such as a copper chloride solution or a hydrochloric acid solution, whereby a circuit board can be manufactured. That is, the circuit board can be configured to include a substrate made of a silicon nitride sintered body that has been etched (under the etching conditions of the copper plate), and a metal plate joined on the surface of the substrate and formed in a predetermined circuit shape by the etching process. In other words, the substrate of the circuit board is a silicon nitride sintered body of the present embodiment that has undergone an etching process during circuit formation.

Example

[0056] Hereinafter, the present invention will be described more specifically based on examples and comparative examples, but the present invention is not to be construed as being limited by the following examples.

[0057] The silicon nitride sintered bodies according to Examples 1 to 7 and Comparative Examples 1 to 3 were produced by the following conditions and procedures. First, high-purity silicon nitride powder produced by the direct nitridation method was prepared. The silicon nitride powder had an average particle diameter (D50) of about 0.8 μm and an oxygen content of about 0.8% by weight. In accordance with the blending composition ratio of each powder, an appropriate amount of MgO powder and Y2O3 powder was added to the silicon nitride powder (Examples 1 to 7, Comparative Examples 1 to 3), and further, an appropriate amount of CaF2 powder and / or MgF2 powder (Examples 1 to 7, Comparative Example 2) was added. With respect to 100 parts by weight of this mixture, a polycarboxylic acid-based surfactant-type dispersant was 0.3 mg / m 2About 50 parts by weight of a mixed solvent of toluene and ethanol was added, and pulverization and mixing were performed using silicon nitride jade. Then, 18 parts by weight of polyvinyl butyral as a binder, 6 parts by weight of dioctyl adipate as a plasticizer, and about 20 parts by weight of a mixed solvent of toluene and ethanol were added. After stirring and mixing with a ball mill until the binder was completely dissolved and mixed, a slurry was prepared. Then, the slurry was left in a vacuum for defoaming and volatilization to adjust the viscosity. Next, a sheet molded body was obtained from the prepared slurry by the doctor blade method. The obtained sheet molded body was die-cut into a shape of 260 mm × 200 mm by die pressing. Then, a spray of boron nitride powder as a release agent was applied to the surface of the die-cut sheet, and a laminate of 25 sheets per block was prepared and placed in a sheath made of boron nitride. It was heated at 500 °C for about 4 hours in dry air to remove organic components such as the binder. Then, the sheet molded body was heated at 1860 °C for 4 hours in a nitrogen atmosphere of 9 atmospheres to obtain a laminate of silicon nitride substrates. The substrates of the silicon nitride laminate were separated, the uppermost and lowermost substrates were removed, and the outer peripheries of the remaining 23 substrates were subjected to break treatment by laser. By removing the boron nitride powder remaining on the substrate surface and the droplets attached by laser processing by honing treatment, a substrate of a silicon nitride sintered body of 190 mm × 140 mm × 0.32 mm was obtained.

[0058] For each sample, the fluorine content of the mixture of raw material powders before firing was measured. Also, for each sample (sintered body) of Examples 1 to 7 and Comparative Examples 1 to 3 prepared, the crystal phase of each sample was identified by X-ray diffraction measurement, and the integrated intensity ratio R of the grain boundary crystal phase was derived by analyzing the X-ray diffraction pattern. Furthermore, for each sample (sintered body), the fluorine content, thermal conductivity (W / mK), and three-point bending strength (MPa) were measured.

[0059] Also, for each of the samples of Examples 1 to 7 and Comparative Examples 1 to 3 prepared, after performing an etching process under the same test conditions, the etching resistance performance was evaluated. Specifically, for the samples after the etching process, the weight reduction amount wd per unit surface area before and after the etching process, and the dielectric breakdown field Vi on the substrate surface were measured. Further, an SEM image obtained by photographing the cut cross-section of the sample after the etching process at 250 times magnification was acquired. This SEM image was analyzed and observed to evaluate the etching depth D from the substrate surface, the etching depth ratio R between the front and back surfaces of the substrate D , the boundary line length ratio R L , and the presence or absence of etching unevenness.

[0060] Various measurements and evaluations were carried out under the following conditions.

[0061] · X-ray diffraction measurement and its analysis Using a type UltimaIV manufactured by Rigaku Corporation, the X-ray diffraction intensity of each sample was measured by the powder X-ray diffraction method using Cu-Kα rays. For the measurement, individual pieces cut to 15 mm × 15 mm were used. The substrate surface was polished to 20 μm or more to obtain a measurement surface. The measurement conditions are as follows. Scan range: 10 degrees to 85 degrees Sampling width: 0.02 degrees Scan speed: 10 degrees / min Divergence slit: 2 / 3 degrees Divergence vertical slit: 10 mm Scattering slit: 8 mm Receiving slit: Open Tube voltage / current: 40 kV / 40 mA Detector: Semiconductor detector In the X-ray diffraction pattern of the substrate plane obtained by X-ray incidence on the substrate plane, the integrated intensity of the diffraction peaks corresponding to all Miller indices (hkl) of the β-type silicon nitride particles was calculated (see the lower column of Fig. 1), and the sum thereof was defined as Ig. The Miller indices of the β-type silicon nitride particles are (100), (110), (200), (101), (120), (111), (300), (201), (220), (121), (310), (301), (221), (311), (320), (002), (410), (401), (102), (112), (321), (202), (500), (141), (330), (122), (420), (302), (501), (150), (331), (222), (421). Also, in the X-ray diffraction pattern, the integrated intensity of all the observed X-ray diffraction peaks was calculated, and the sum thereof was defined as Ia. Then, the integrated intensity ratio R of the grain boundary crystal phase was derived using the relational expression of R = (1 - Ig / Ia) × 100 (%). The peak search of the measurement data was carried out using the integrated powder X-ray analysis software PDXL2.8 (ver. 2.8.4.0) manufactured by Rigaku Corporation under the following conditions. Use template peak list and background: On Remove kβ line and filter edge: On σ cut value: 3.00 Minimum value of σ cut range: 0.50 Maximum value of σ cut range: 20

[0062] · Fluorine content of the mixture of raw material powders Measured by the thermal hydrolysis separation-ion chromatography method described in JIS R 1603 (2018) using the ion chromatograph Dionex Integrion RFIC manufactured by Thermo Fisher Scientific K.K. and the automatic sample combustion device AQF-2100H manufactured by Nitto Seiko Analytic Co., Ltd.

[0063] · Fluorine content of the sintered body Qualitative analysis was performed by EZ Scan using the Primus IV model manufactured by Rigaku Corporation. For the measurement, specimens cut into pieces of 25 mm × 25 mm were used. The substrate surface was polished to 20 μm or more to obtain a measurement surface, and the measurement diameter was 20 mm.

[0064] ·Thermal conductivity For the measurement method of the thermal conductivity in the thickness direction of the substrate, the flash method was adopted. For the measurement, a thermal conductivity measuring device LFA467 manufactured by NETZSCH Geratebau GmbH was used. For the measurement, specimens cut from the substrate into pieces of 10 mm × 10 mm were used. To suppress the transmission of the flash light, sputtered films of gold were formed on both sides of the specimen, and graphene spray was used on both sides of the specimen to uniformly absorb the pulsed light, and blackening treatment was performed. When calculating the thermal conductivity, a value of 0.68 J / (g·K) was used as the specific heat of the obtained sintered body.

[0065] ·Three-point bending strength The measuring device was the AG-IS model manufactured by Shimadzu Corporation. The measurement conditions were a crosshead speed of 0.5 mm / min and a distance between supports of 30 mm. The size of the test piece was a width of 20 mm and a thickness of 0.3 to 0.4 mm.

[0066] ·Weight reduction per unit surface area after etching treatment The weight of a 40 mm square substrate of the produced silicon nitride sintered body was measured. Then, it was immersed in a hydrochloric acid solution with a mass fraction of 7.8% heated to 50 °C for 30 minutes, washed with water and dried, and then the weight of the substrate was measured again. The value obtained by subtracting the weight after the etching treatment from the weight before the etching treatment was divided by the surface area (3200 mm 2 ) to calculate the weight reduction per unit surface area. Here, since the ratio of the surface area occupied by the side surface (thickness 0.32 mm) is extremely small with respect to the areas of the front and back surfaces, the thickness was ignored in the calculation formula.

[0067] ·Dielectric breakdown field of the substrate surface after etching treatment The fabricated silicon nitride sintered body was immersed in a hydrochloric acid solution with a mass fraction of 7.8% heated to 50°C for 30 minutes, followed by washing with water and drying. Then, an L / S pattern with a width of 1 mm and a spacing (minimum distance) of 2 mm was formed on the substrate surface by sputtering (see Fig. 5). An alternating voltage was applied in Florinate at 25°C (room temperature), and the breakdown voltage was measured. Measurement terminals were attached to electrodes A and B in Fig. 5, and a partial discharge measuring device (partial discharge measuring device A006 manufactured by Fujikura Cable) was used to measure the breakdown voltage. Electrodes A and B form the minimum distance at five locations, and the voltage value at the time when breakdown occurs at any one of these locations is measured as the breakdown voltage. Then, the breakdown electric field was derived by dividing the breakdown voltage value by the pattern pitch (2 mm).

[0068] ·Observation of the substrate cross-section after etching treatment by SEM After the fabricated silicon nitride sintered body was immersed in a hydrochloric acid solution with a mass fraction of 7.8% heated to 50°C for 30 minutes, washed with water and dried, the substrate was cut with a diamond cutter or the like, embedded in an epoxy resin, and mirror-polished so that the substrate cross-section became the observation surface. A cross-sectional photograph was taken at a magnification of 250 times using a scanning electron microscope JSM-IT700HR manufactured by JEOL Ltd. In the cross-sectional BSE image of the substrate, the areas that appear white indicate the grain boundary phase. For each sample, the depth from the substrate surface in the surface layer region where the white part was removed was defined as the etching depth. Also, in the surface layer region with a depth less than D / 2 μm from the front or back surface of the substrate, if white lumps were observed, it was determined that there was etching unevenness (×), and if no white lumps were observed, it was determined that there was no etching unevenness (〇).

[0069] ·Evaluation of the boundary line length ratio between the etched layer and the non-etched layer, the etching depth, and the etching depth ratio of the front and back surfaces of the substrate in the substrate cross-section Using the BSE images of the substrate cross-section at 250 times magnification taken for each sample, the boundary line between the etching layer and the non-etching layer was determined using the image analysis and measurement software WinROOF2021 (ver. 5.7.4) manufactured by Mitani Shoji Co., Ltd., and the boundary line length and etching depth were calculated based on the boundary line. Specifically, the BSE image was imported into WinROOF2021, and various image processes such as filtering, edge processing, and monochrome imaging were performed. Then, binary processing such as binarization and morphology was performed to separate the non-etching layer. The analysis of the image in which the non-etching layer was separated was performed for each of the front and back surfaces of the substrate of each sample. Fig. 6 is a schematic diagram for explaining the said analysis. For the image in which the non-etching layer was separated, the boundary line lengths L1 and L2 of the front and back surfaces of the substrate were measured over an arbitrarily set evaluation length L (1000 μm or more). The boundary line lengths L1 and L2 represent the length when the boundary line between the non-etching layer and the etching layer separated by image processing is extended into a single line segment. By dividing the average value of the boundary line lengths L1 and L2 of the front and back surfaces of the substrate by the evaluation length L, the boundary line length ratio R L was calculated. This boundary line length ratio R L is an index indicating the degree of undulation of the boundary line with respect to the planar direction of the substrate, and it can be interpreted that the greater this value, the lower the uniformity of etching. The formula for calculating the boundary line length ratio R L is as follows. R L =(L1 + L2) / 2L Also, for the front and back surfaces of the substrate, the depth direction distance between the front and back surfaces of the substrate and the boundary line was measured over the evaluation length L. From the measured depth direction distances, the maximum distance D1max and the minimum distance D1min on the substrate surface side were obtained, and their average value was calculated as the etching depth D1 of the substrate surface. Similarly, the maximum distance D2max and the minimum distance D2min on the substrate back surface side were obtained, and their average value was calculated as the etching depth D2 of the substrate back surface. Then, the etching depth D was calculated as the average value of the etching depths D1 and D2 of the front and back surfaces of the substrate. Furthermore, as an index indicating the variation in the etching degree of the front and back surfaces of the substrate, the etching depth ratio R D of the front and back surfaces of the substrate was calculated. R DThe formula for calculating it is as follows. R D =|D1 - D2| / D Note that the total evaluation length L is desirably 1000 μm or more. When it is difficult to measure in a single field of view, it may be calculated by combining a plurality of fields of view.

[0070] The conditions and various measurement results for each sample of Examples 1 to 7 and Comparative Examples 1 to 3 are shown in Tables 1 and 2. Table 1 mainly shows the characteristics of the sintered body, and Table 2 mainly shows the etching resistance characteristics. Figure 1 typically shows the X-ray diffraction patterns of Example 1, Comparative Examples 1 and 3. For the X-ray diffraction patterns, each diffraction peak indexed with the Miller indices of β-type silicon nitride particles is described in the lower column of Figure 1. Further, Figures 2 to 4 typically show SEM photographs of the substrate surfaces of the samples of Example 1, Comparative Examples 1 and 3 taken at 250 times magnification after the etching treatment.

[0071]

Table 1

[0072]

Table 2

[0073] According to Tables 1 and 2, Examples 1 to 7 differ from Comparative Examples 1 to 3 in that they contain a Group 2 element fluoride selected from 0.05 wt% or more of CaF2 and / or MgF2. Specifically, Examples 1 to 5 contain 0.10 to 0.30 wt% of CaF2, Example 6 contains 0.05 wt% of CaF2 and 0.05 wt% of MgF2, and Example 7 contains 0.20 wt% of MgF2. On the other hand, Comparative Examples 1 and 3 do not contain a Group 2 element fluoride, and Comparative Example 2 contains a trace amount (0.03 wt%) of CaF2.

[0074] As shown in Table 1, the fluorine content of the mixture of raw material powders before firing is 600 ppm or more in Examples 1 to 7, while it is less than 600 ppm in Comparative Examples 1 to 3. The fluorine content in Comparative Examples 1 to 3 is mainly due to the impurities in the raw material powder of silicon nitride, and the fluorine content caused by this raw material powder is 800 ppm or less. Regarding the characteristics of the sintered body, the fluorine content (fluorine element) contained in the sintered body substrate was 0.05 wt% or more in Examples 1 to 7, while it was not detected (i.e., 0 wt%) in Comparative Examples 1 to 3. That is, it is considered that the fluorine element effectively remained in the sintered body by adding fluorine as an appropriate amount of Group 2 element fluoride.

[0075] Figure 1 shows the X-ray diffraction patterns of each sample (sintered body substrate). Diffraction peaks of β-type silicon nitride particles were confirmed in all samples. The diffraction peaks of β-type silicon nitride particles are shown in the lower part of Figure 1. On the other hand, in the X-ray diffraction patterns of each sample, diffraction peaks also appear at locations other than the diffraction peaks of β-type silicon nitride particles. When visually comparing the X-ray diffraction patterns of Example 1 and Comparative Examples 1 and 3, diffraction peaks are confirmed in Comparative Examples 1 and 3 in the vicinity of diffraction angles 2θ of 20°, 30°, and 45°, while the diffraction peaks of Example 1 are smaller or cannot be confirmed. Since Examples 2 to 7 showed diffraction patterns with the same tendency as Example 1, the illustration is omitted. Furthermore, when quantitatively analyzing the X-ray diffraction pattern, as shown in Table 1, the integrated intensity ratio R of the grain boundary crystal phase is 3% or less in Examples 1 to 7, while it is larger than 3% in Comparative Examples 1 to 3. More specifically, the integrated intensity ratio R of the grain boundary crystal phase is 0.3 to 3% in Examples 1 to 7. That is, in the silicon nitride sintered bodies of Examples 1 to 7, crystallization in the grain boundary phase is suppressed.

[0076] Also, according to Table 1, it was confirmed that in all samples of Examples 1 to 7 and Comparative Examples 1 to 3, the thermal conductivity was 80 W / mK or more and the three-point bending strength was 700 MPa or more. That is, it was found that the characteristics of thermal conductivity and mechanical strength did not deteriorate due to the addition of an appropriate amount of Group 2 element fluoride.

[0077] Table 2 and FIGS. 2 to 4 show the etching resistance characteristics of each sample. As shown in Table 2, the weight loss amount wd indicating the etching amount per unit surface area under predetermined conditions is 12 μg / mm 2 or less in Examples 1 to 7, whereas it is 12 μg / mm 2 or more in Comparative Examples 1 to 3. Also, the dielectric breakdown field Vi at room temperature is 8 kV / mm or more in Examples 1 to 7, whereas it is less than 8 kV / mm in Comparative Examples 1 to 3. That is, it was confirmed that the silicon nitride sintered bodies of Examples 1 to 7 have suppressed crystallization in the grain boundary phase, while the etching amount is reduced, and further, the dielectric breakdown field Vi of the substrate surface after the etching treatment is improved.

[0078] FIGS. 2 to 4 are SEM images (BSE images at a magnification of 250 times) of the cross section of the sintered body substrate after the etching treatment. The white portions indicate the grain boundary crystal phase, and the grain boundary crystal phase does not exist in the other portions. It can be seen that the white portions uniformly exist in the deep part of the unetched substrate, and the grain boundary crystal phase is not etched. On the other hand, it can be seen that the white portions are removed by the etching treatment in the surface layer region at a predetermined depth from the substrate surface. And according to Table 2 and FIGS. 2 to 4, the etching depth D of the comparative example is significantly larger than the etching depth D of the example. Furthermore, there is a significant difference in the etching depth ratio R D between the front and back surfaces of the substrate of the comparative example. More specifically, the etching depth D of the silicon nitride sintered bodies of Examples 1 to 7 is less than 40 μm (about 32 to 36 μm), and the etching depth ratio R D between the front and back surfaces of the substrate is 10% or less, whereas in Comparative Examples 1 to 3, the etching depth D is larger than 40 μm (about 43 to 46 μm), and the etching depth ratio R D between the front and back surfaces of the substrate is larger than 10%. Also, regarding the boundary line length ratio R L , the boundary line length ratio R L of the silicon nitride sintered bodies of Examples 1 to 7 is less than 2.0 (about 1.5 to 1.6), whereas in Comparative Examples 1 to 3, the boundary line length ratio R Lis 2.0 or more (about 2.0 to 2.1). Further, in Examples 1 to 7, it was confirmed that no grain boundary crystal phase remained in the surface layer region less than D / 2 in surface depth, whereas in Comparative Examples 1 to 3, the grain boundary crystal phase remained without being etched. These SEM observation and analysis results also support the improvement in the etching resistance and the improvement in the dielectric breakdown field Vi of the substrate surface in the silicon nitride sintered body of the present invention.

[0079] The present invention is not limited to the above-described examples, and can be implemented in various modes as long as it belongs to the technical scope of the present invention.

Claims

1. A silicon nitride sintered body having silicon nitride particles and a grain boundary phase present between the silicon nitride particles, The silicon nitride sintered body substrate is immersed in a hydrochloric acid solution at 50° C. and a mass fraction of 7.8% for 30 minutes, and the etching depth ratio R Ｄ A silicon nitride sintered body, characterized in that the content of SiO2 is 10% or less.

2. A silicon nitride sintered body having silicon nitride particles and a grain boundary phase present between the silicon nitride particles, The silicon nitride sintered body substrate is immersed in a hydrochloric acid solution at 50° C. and a mass fraction of 7.8% for 30 minutes, and the substrate after etching is subjected to an etching treatment. The boundary length ratio R indicates the ratio of the length of the boundary between the non-etched layer and the etched layer to the evaluation length L. Ｌ A silicon nitride sintered body, characterized in that the refractive index is less than 2.

0.

3. 2. The silicon nitride sintered body according to claim 1, characterized in that in a SEM cross-sectional image of the substrate after the etching treatment, the grain boundary phase has been removed to a depth of less than 40 μm from the substrate surface, and no grain boundary crystal phase remains in the region from which the grain boundary phase has been removed.

4. The β-Si alloy comprises silicon nitride particles and a grain boundary phase present between the silicon nitride particles. ３ N ４ A silicon nitride sintered body having an X-ray diffraction peak of the grain boundary phase is composed of an amorphous phase and a grain boundary crystalline phase, The integrated intensity ratio R of the grain boundary crystal phase is 3% or less, The integrated intensity ratio R is expressed by the following formula: R=(1-Ig / Ia)×100(%) and Ig is β-Si ３ N ４ and Ia is the sum of the integrated intensities of all X-ray diffraction peaks indexed by Ib, and Ib is the sum of the integrated intensities of all X-ray diffraction peaks observed.

5. 5. The silicon nitride sintered body according to claim 4, wherein the integrated intensity ratio R of the grain boundary crystal phase is 0.3 to 3%.

6. 6. A circuit board comprising: a substrate of the silicon nitride sintered body according to claim 1; and a metal plate bonded onto a surface of the substrate and formed into a predetermined circuit shape.

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