WO2015060274A1 - 窒化珪素基板およびそれを用いた窒化珪素回路基板 - Google Patents

窒化珪素基板およびそれを用いた窒化珪素回路基板 Download PDF

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WO2015060274A1
WO2015060274A1 PCT/JP2014/077889 JP2014077889W WO2015060274A1 WO 2015060274 A1 WO2015060274 A1 WO 2015060274A1 JP 2014077889 W JP2014077889 W JP 2014077889W WO 2015060274 A1 WO2015060274 A1 WO 2015060274A1
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Prior art keywords
silicon nitride
nitride substrate
grain boundary
substrate
boundary phase
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PCT/JP2014/077889
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English (en)
French (fr)
Inventor
中山　憲隆
青木　克之
佐野　孝
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株式会社東芝
東芝マテリアル株式会社
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Application filed by 株式会社東芝, 東芝マテリアル株式会社 filed Critical 株式会社東芝
Priority to US15/027,829 priority Critical patent/US9630846B2/en
Priority to JP2015543852A priority patent/JP6293772B2/ja
Priority to EP14856390.1A priority patent/EP3061739B1/en
Priority to CN201480058070.0A priority patent/CN105683129B/zh
Publication of WO2015060274A1 publication Critical patent/WO2015060274A1/ja
Priority to US15/430,920 priority patent/US9884762B2/en
Priority to US15/833,113 priority patent/US10322934B2/en

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Definitions

  • Embodiments described later generally relate to a silicon nitride substrate and a silicon nitride circuit substrate using the same.
  • a silicon nitride (Si 3 N 4 ) substrate As the semiconductor circuit substrate, an alumina (Al 2 O 3 ) substrate or an aluminum nitride (AlN) substrate is used.
  • the alumina substrate has a thermal conductivity of about 30 W / m ⁇ K, but the cost can be reduced.
  • the aluminum nitride substrate can have high thermal conductivity with a thermal conductivity of 160 W / m ⁇ K or more.
  • a silicon nitride substrate As a silicon nitride substrate, a substrate having a thermal conductivity of 50 W / m ⁇ K or more has been developed.
  • the silicon nitride substrate has a lower thermal conductivity than the aluminum nitride substrate, it has an excellent three-point bending strength of 500 MPa or more.
  • the three-point bending strength of an aluminum nitride substrate is usually about 300 to 400 MPa, and the strength tends to decrease as the thermal conductivity increases.
  • the silicon nitride substrate can be thinned by taking advantage of high strength. Since the thermal resistance can be lowered by reducing the thickness of the substrate, heat dissipation is improved.
  • silicon nitride substrates are widely used as circuit boards by providing circuit parts such as metal plates.
  • circuit parts such as metal plates.
  • Patent Document 1 There is also a method of using it as a circuit board for a pressure contact structure as shown in International Publication No. WO2011 / 010597 pamphlet (Patent Document 1).
  • the characteristics required for a silicon nitride substrate include thermal conductivity, strength, and insulation.
  • Patent Document 2 discloses a silicon nitride substrate having a current leakage value of 1000 nA or less when an AC voltage of 1.5 kV-100 Hz is applied between the front and back surfaces of the silicon nitride substrate under conditions of a temperature of 25 ° C. and a humidity of 70%. Has been. The smaller the current leak value, the higher the insulation between the front and back sides.
  • the silicon nitride substrate is composed of a silicon nitride sintered body having silicon nitride crystal grains and a grain boundary phase.
  • the silicon nitride crystal particles and the grain boundary phase are compared, the silicon nitride crystal particles have higher insulation. Therefore, portions having different insulating properties are formed in the silicon nitride substrate according to the abundance ratio of the grain boundary phase. For this reason, even if the current leak value is equal to or less than a certain value, a phenomenon has occurred in which the insulation is insufficient.
  • the silicon nitride substrate according to the embodiment includes a silicon nitride substrate having silicon nitride crystal grains and a grain boundary phase and having a thermal conductivity of 50 W / m ⁇ K or more.
  • the cross-sectional structure of the silicon nitride substrate is the thickness of the silicon nitride substrate.
  • the ratio of the total length T2 of the grain boundary phase to the thickness T1 (T2 / T1) is 0.01 to 0.30, and the dielectric strength when measured by the four-terminal method with electrodes in contact with the front and back of the substrate The variation from the average value is 20% or less.
  • the ratio (T2 / T1) of the total length T2 of the grain boundary phase to the thickness T1 of the silicon nitride substrate is defined within a predetermined range. The variation of is small. Therefore, when used for a circuit board or the like, a circuit board having excellent insulation and high reliability can be obtained.
  • the silicon nitride substrate according to the present embodiment is a silicon nitride substrate having a thermal conductivity of 50 W / m ⁇ K or more and having silicon nitride crystal grains and a grain boundary phase, and the cross-sectional structure of the silicon nitride substrate is that of the silicon nitride substrate.
  • the ratio of the total length T2 of the grain boundary phase to the thickness T1 (T2 / T1) is 0.01 to 0.30, and the dielectric strength when measured by the four-terminal method with electrodes in contact with the front and back of the substrate The variation from the average value is 20% or less.
  • the silicon nitride substrate is composed of a silicon nitride sintered body having a thermal conductivity of 50 W / m ⁇ K or more and comprising silicon nitride crystal grains and a grain boundary phase.
  • the thermal conductivity is preferably 50 W / m ⁇ K or more, more preferably 90 W / m ⁇ K or more.
  • the thermal conductivity is as low as less than 50 W / m ⁇ K, the heat dissipation is reduced.
  • FIG. 1 shows an example of a cross-sectional structure of the silicon nitride substrate according to the embodiment.
  • reference numeral 1 denotes a silicon nitride substrate
  • 2 denotes silicon nitride crystal particles
  • 3 denotes a grain boundary phase
  • T1 denotes the thickness of the silicon nitride substrate.
  • FIG. 2 is a cross-sectional view for explaining the ratio (T2 / T1) of the total length T2 of the grain boundary phase to the substrate thickness T1 in the silicon nitride substrate according to the embodiment.
  • 2 is silicon nitride crystal particles
  • 3 is the grain boundary phase
  • T2-1 to T4 are the lengths of the grain boundary phases in the thickness direction.
  • the silicon nitride substrate is composed of a silicon nitride sintered body having silicon nitride crystal grains and a grain boundary phase.
  • the silicon nitride crystal particles are preferably 95% to 100% in terms of the number ratio of ⁇ -Si 3 N 4 crystal particles. When the ⁇ -Si 3 N 4 crystal particles are 95% or more, the structure has silicon nitride crystal particles randomly present, and the strength is improved.
  • the grain boundary phase is mainly composed of a sintering aid.
  • the sintering aid is preferably at least one selected from rare earth elements, magnesium, titanium, and hafnium.
  • the sintering aid preferably contains a total of 2 to 14% by mass in terms of oxides.
  • the sintering aid is less than 2% by mass in terms of oxide, there is a possibility that a portion having a low abundance ratio of the grain boundary phase is generated.
  • the sintering aid exceeds 14% by mass in terms of oxide, the abundance ratio of the grain boundary phase may be excessively increased. Therefore, the sintering aid is preferably contained in the range of 4.0 to 12.0% by mass in terms of oxide.
  • the cross-sectional structure of the silicon nitride substrate of the embodiment is characterized in that the ratio (T2 / T1) of the total length T2 of the grain boundary phase to the thickness T1 of the silicon nitride substrate is 0.01 to 0.30. .
  • the thickness T1 of the silicon nitride substrate is the thickness of the substrate as shown in FIG.
  • the thickness T1 of the substrate is measured with a caliper.
  • FIG. 2 is a cross-sectional view for explaining the ratio (T2 / T1) of the total length T2 of the grain boundary phase to the substrate thickness T1 in the silicon nitride substrate.
  • reference numeral 2 denotes silicon nitride crystal grains
  • 3 denotes a grain boundary phase.
  • the enlarged photograph is preferably a scanning electron microscope (SEM) photograph.
  • SEM photograph has an advantage that it is easy to distinguish because there is a contrast difference between the silicon nitride crystal grains and the grain boundary phase.
  • the magnification is 2000 times or more, it is easy to distinguish between silicon nitride crystal grains and grain boundary phases.
  • a straight line is drawn in the substrate thickness direction with respect to the enlarged photograph of the cross-sectional structure, and the length of the grain boundary phase existing on the straight line is obtained. To do.
  • T2 (T2-1) + (T2-2) + (T2-3) + (T2- 4)
  • T2 (T2-1) + (T2-2) + (T2-3) + (T2- 4)
  • this operation is repeated until the substrate thickness T1 is reached.
  • an arbitrary cross section is mirror-polished to have a surface roughness Ra of 0.05 ⁇ m or less, and an image is taken after performing an etching process.
  • the etching process either chemical etching or plasma etching is effective. Also, pores present in the substrate are not counted in the length of the grain boundary phase.
  • the cross-sectional structure of the silicon nitride substrate according to the embodiment is characterized in that the ratio (T2 / T1) of the total length T2 of the grain boundary phase to the thickness T1 of the silicon nitride substrate is 0.01 to 0.30. Yes.
  • the ratio (T2 / T1) is less than 0.01, a region having a small grain boundary phase is partially formed, so that the insulating property is lowered.
  • the ratio (T2 / T1) is more than 0.30, a region having a large amount of grain boundary phase is formed, which causes variation in insulation.
  • the ratio (T2 / T1) is preferably in the range of 0.10 to 0.25 in order to ensure insulation and reduce variations.
  • the electrode is brought into contact with the front and back of the substrate and measured from the average value of the dielectric strength when measured by the four-terminal method.
  • the variation can be 20% or less, and further 15% or less.
  • Fig. 3 shows an example of a dielectric strength measurement method using the 4-terminal method.
  • reference numeral 1 is a silicon nitride substrate
  • 4 is a front side measurement terminal
  • 5 is a back side measurement terminal
  • 6 is a measuring instrument.
  • the tip shapes of the front surface side measurement terminal 4 and the back surface side measurement terminal 5 are spheres.
  • the front surface side measurement terminal 4 and the back surface side measurement terminal 5 are arranged to face each other with the silicon nitride substrate 1 interposed therebetween.
  • the variation from the average value is 20% or less even when the front side measurement terminal 4 and the back side measurement terminal 5 are measured at any position on the silicon nitride substrate 1.
  • the average value of the dielectric strength is determined by measuring at least five locations on the silicon nitride substrate 1 by the above-described measuring method and obtaining the average value.
  • FIG. 4 shows an example of measurement points of dielectric strength.
  • five measurement points S1, S2, S3, S4, and S5 are measurement targets when measuring five points on one substrate. That is, S1 that is the intersection (center) of the diagonal lines of the substrate 1 and four points S2 to S5 that are the midpoints of each corner from S1.
  • the average value of the dielectric strength at these five measurement points is taken as the average value of the dielectric strength of the silicon nitride substrate 1. That is, the dielectric strength at S1 is ES1, the dielectric strength at S2 is ES2, the dielectric strength at S3 is ES3, the dielectric strength at S4 is ES4, and the dielectric strength at S5 is ES5.
  • the average value ESA is obtained by the following formula. Moreover, there are at least 5 measurement points, and the number of measurement points may be 6 or more.
  • ESA (ES1 + ES2 + ES3 + ES4 + ES5) / 5
  • the variation (%) in the dielectric strength is expressed by (
  • / ESA) ⁇ 100 (%), n integer (measurement point number), and the ratio (%) of deviation from the average value.
  • the absolute value will be used.
  • the measurement conditions other than the above are measured according to JIS-C-2141.
  • the dielectric strength is measured in Fluorinert. Fluorinate is a perfluorocarbon (PFC) insulating solvent.
  • the silicon nitride substrate of the embodiment has a small variation in dielectric strength of 20% or less.
  • the silicon nitride substrate is a silicon nitride sintered body composed of silicon nitride crystal grains and a grain boundary phase. Further, when used as a substrate, it is used as a thin substrate having a plate thickness of 1.0 mm or less, further 0.4 mm or less. This is because by reducing the thickness of the substrate, the thermal resistance of the substrate is reduced and heat dissipation is improved.
  • the silicon nitride substrate according to the embodiment is effective for a thin substrate having a thickness T1 of 0.1 to 1.0 mm, further 0.1 to 0.4 mm.
  • the average value ESA of the dielectric strength is preferably 15 kV / mm or more. If the average value is less than 15 kV / mm, the insulation as a substrate is insufficient.
  • the average dielectric strength ESA is preferably 15 kV / mm or more, and more preferably 20 kV / mm or more. When the ratio (T2 / T1) is 0.15 or less, the average value of the dielectric strength tends to be 20 kV / mm or more.
  • the volume resistivity value at the time of 1000V application at room temperature (25 degreeC) is 60 * 10 ⁇ 12 > (ohm) m or more.
  • the ratio ( ⁇ v2 / ⁇ v1) of the volume resistivity value ⁇ v1 when 1000V is applied at room temperature (25 ° C) to the volume resistivity value ⁇ v2 when 1000V is applied at 250 ° C is 0.20 or more. preferable.
  • Fig. 5 shows the measurement method of volume resistivity.
  • reference numeral 1 is a silicon nitride substrate
  • 7 is a front surface side carbon electrode
  • 8 is a back surface side carbon electrode
  • 9 is a measuring device.
  • the silicon nitride substrate 1 is pressed and fixed by the front surface side carbon electrode 7 and the back surface side carbon electrode 8.
  • d is the diameter of the surface-side carbon electrode
  • t is the thickness of the silicon nitride substrate.
  • the volume resistivity measured at room temperature (25 ° C.) is measured in an atmosphere of ⁇ v1 and 250 ° C as ⁇ v2. Measurement conditions other than the above shall be performed in accordance with JIS-K-6911.
  • the volume specific resistance value when applying 1000 V at room temperature (25 ° C.) is 60 ⁇ 10 12 ⁇ m or more.
  • a silicon nitride circuit board in which a metal circuit board is provided on a silicon nitride substrate can be mounted with various semiconductor elements.
  • Some semiconductor devices have a high operating voltage of 500 to 800V. It is preferable that ⁇ v1 is as high as 60 ⁇ 10 12 ⁇ m or more, more preferably 90 ⁇ 10 12 ⁇ m or more. As described above, by reducing the variation in dielectric strength and increasing the volume resistivity value, it is possible to obtain excellent reliability that does not cause dielectric breakdown even when a semiconductor element having a high operating voltage is mounted.
  • the ratio ( ⁇ v2 / ⁇ v1) is as high as 0.20 or more, and further 0.40 or more, excellent insulation can be maintained even if the use environment is a high temperature of 200 to 300 ° C. .
  • semiconductor devices having an operating temperature of 150 to 250 ° C. such as SiC devices have been developed.
  • the silicon nitride substrate according to the embodiment as an insulating substrate for mounting such a semiconductor element, excellent reliability can be obtained as a semiconductor device.
  • the maximum length of the grain boundary phase is 50 ⁇ m or less.
  • the average particle diameter of the major axis of the silicon nitride crystal particles is preferably 1.5 to 10 ⁇ m.
  • the abundance ratio (T2 / T1) between the silicon nitride crystal grains and the grain boundary phase in the thickness direction is set within a predetermined range. It is effective.
  • the size of the grain boundary phase in order to set the volume resistivity value ⁇ v1 to a predetermined value or more and the ratio ( ⁇ v2 / ⁇ v1) to a predetermined value or more.
  • the cross section in the thickness direction of the silicon nitride substrate is observed with an enlarged photograph, it is preferable to reduce the maximum length of the grain boundary phase to 50 ⁇ m or less, further 20 ⁇ m or less, and further 10 ⁇ m or less.
  • the maximum length of the grain boundary phase in the thickness direction indicates that all of the above-described T2-1, T2-2, T2-3, and T2-4 are 50 ⁇ m or less.
  • the average particle diameter of the major axis of the silicon nitride crystal particles is 1.5 to 10 ⁇ m.
  • the major axis of the silicon nitride crystal particles is obtained by measuring the maximum diameter of each silicon nitride crystal particle appearing within a unit area of 100 ⁇ m ⁇ 100 ⁇ m in an enlarged photograph of an arbitrary cross-sectional structure and calculating the average value thereof. In the measurement of the maximum diameter, the longest diagonal line of the silicon nitride crystal particles shown in the enlarged photograph is determined as the long diameter. This operation is performed at three different locations with a unit area of 100 ⁇ m ⁇ 100 ⁇ m, and the average value is taken as the average particle size of the major axis of the silicon nitride crystal particles.
  • the average particle diameter of the major axis of the silicon nitride crystal particles is as small as less than 1.5 ⁇ m, the grain boundary between the silicon nitride crystal particles increases, so that a portion where the ratio (T2 / T1) exceeds 0.30 is formed. There is a fear. If the average particle size of the major axis of the silicon nitride crystal particles is larger than 10 ⁇ m, the number of grain boundaries between the silicon nitride crystal particles is reduced, but the length of the grain boundary between the silicon nitride crystal particles is increased. There is a possibility that a portion where the maximum length of the phase cannot be reduced to 50 ⁇ m or less is formed.
  • the average particle diameter of the major axis of the silicon nitride crystal particles is preferably in the range of 1.5 to 10 ⁇ m, more preferably 2 to 7 ⁇ m.
  • a magnified photograph shall use a thing of 2000 times or more. Further, when it is difficult to determine crystal grains and grain boundaries, a magnified photograph of 5000 times is used.
  • the porosity of the silicon nitride substrate is preferably 3% or less.
  • the maximum diameter of the pores is preferably 20 ⁇ m or less.
  • the variation in the dielectric strength is 20% or less even if the porosity is 3%. Further, it can be made 15% or less.
  • the pores are preferably as small as possible, and the porosity is preferably 1% or less, and more preferably 0.5% or less.
  • the maximum diameter of the pores is preferably 20 ⁇ m or less, more preferably 10 ⁇ m or less, and further preferably 3 ⁇ m or less (including 0). Further, the maximum diameter of the pores is obtained from an enlarged photograph in an arbitrary cross section.
  • the porosity is 1% or less (including 0) and the maximum pore diameter is 10 ⁇ m or less (including 0).
  • the above enlarged photo is an SEM photo.
  • the pores are distinguishable because they have different contrast differences from the silicon nitride crystal grains and the grain boundary phase.
  • Excellent volume resistivity even in high temperature environments (at 250 ° C atmosphere) by reducing the ratio or size of pores observed in SEM photographs at magnifications of 2000 times or more and even 5000 times A value can be obtained.
  • a grain boundary phase exists in 10% or more of the perimeter of the pores. There will be air in the pores.
  • the silicon nitride particles are an insulator.
  • the grain boundary phase component is formed by reaction of a sintering aid made of a metal oxide. For this reason, since the grain boundary phase component is an oxide, insulation is high.
  • air tends to be an electric path.
  • a large voltage of 600 V or more it tends to be an electric path.
  • the pore is a residual defect in the densification process by the sintering process, and the densification proceeds through the grain boundary phase.
  • the ⁇ -silicon nitride crystal particles have an elongated shape.
  • the strength of the silicon nitride substrate is improved by the random orientation in which the ⁇ -silicon nitride crystal particles are intertwined in a complicated manner.
  • gaps between silicon nitride crystal grains are likely to be formed.
  • the periphery of the pore is covered with a grain boundary phase component, which indicates a good densification process.
  • the grain boundary phase component be present in 10% or more of the outer peripheral length of the pore after the maximum diameter of the pore is set to 20 ⁇ m or less.
  • the proportion of the grain boundary phase component present in the outer peripheral length of the pore is preferably as large as possible and is preferably 50% or more and 100% or less.
  • the relative dielectric constant at 50 Hz is ⁇ r50 and the relative dielectric constant at 1 kHz is ⁇ r1000
  • ( ⁇ r50 ⁇ r1000 ) / ⁇ r50 ⁇ 0.1 is preferable.
  • the relative dielectric constant indicates a value divided by the electric capacity when the electric capacity of the capacitor is vacuum when the medium between the electrodes is filled.
  • the medium this time is a silicon nitride substrate.
  • ( ⁇ r50 - ⁇ r1000) / ⁇ that r50 is ⁇ 0.1 shows that the dielectric constant of the silicon nitride substrate does not increase even if higher frequency. This indicates that the silicon nitride substrate is not easily polarized.
  • Examples of the state where polarization hardly occurs include small pores and few pores.
  • it is also effective to control the size of the grain boundary phase and to have a grain boundary phase component around the pores. Furthermore, it is also effective to reduce the segregation region described later.
  • the maximum length of the segregation region in the grain boundary phase is preferably 5 ⁇ m or less (including 0).
  • the grain boundary phase is a reaction phase mainly composed of a sintering aid.
  • the sintering aid is preferably at least one selected from rare earth elements, magnesium, titanium, and hafnium.
  • the segregation region indicates a region where a deviation of 30% or more from the average concentration of a specific element occurs when a unit area of 20 ⁇ m ⁇ 20 ⁇ m is color-mapped by EPMA (electron beam microanalyzer).
  • the specific element indicates a sintering aid component.
  • Y 2 O 3 yttrium oxide
  • Y element mapping is performed to obtain a region having a concentration deviation of 30% or more with respect to the average concentration.
  • the metal elements of the respective components are mapped.
  • a region where “Y”, “Mg”, and “Hf” deviate from the average concentration by 30% or more is obtained.
  • the deviation of 30% or more from the average density corresponds to the case where the deviation is large or small.
  • the segregation region is preferably small, and the maximum length of the segregation region is preferably 5 ⁇ m or less, more preferably 1 ⁇ m or less (including 0).
  • the volume resistivity value ⁇ v1 can be 90 ⁇ 10 12 ⁇ m or more, and the ratio ( ⁇ v2 / ⁇ v1) can be 0.40 or more.
  • the maximum length of the segregation region is set to 5 ⁇ m or less, and further to 1 ⁇ m or less (including 0)
  • the variation in dielectric strength can be reduced to 5% or less. The thinner the substrate, the greater the effect. Therefore, when the plate thickness T1 is 0.1 to 0.4 mm, the segregation region is preferably as small as 1 ⁇ m or less or not present.
  • the thickness T1 of the silicon nitride substrate is reduced to 0.1 to 1.0 mm, further 0.1 to 0.4 mm, the variation in dielectric strength is reduced.
  • the average value of dielectric strength can be improved.
  • the thermal conductivity of the silicon nitride substrate is set to 50 W / m ⁇ K or more, and the strength is increased. It can also be 600 MPa or more.
  • the thermal conductivity is set to 80 W / m ⁇ K or more, and further to 90 W / m ⁇ K or more. It becomes easy.
  • the silicon nitride substrate according to the embodiment is suitable for a silicon nitride circuit substrate.
  • the circuit board is provided with a metal plate and a metal layer as a circuit portion.
  • the metal plate include a metal plate having good conductivity such as a copper plate and an Al plate.
  • various joining methods such as an active metal joining method and a direct joining method can be applied to joining the metal plates.
  • a metal plate is provided on the back surface as necessary.
  • the metal layer include a metallized film formed by heating a metal paste, a metal thin film using a thin film forming technique such as a plating method, a sputtering method, and a thermal spraying method.
  • the silicon nitride substrate according to the embodiment has improved dielectric strength, it is also effective as a substrate for pressure contact structure.
  • the manufacturing method is not particularly limited, but examples of a method for efficiently manufacturing include the following.
  • silicon nitride powder and sintering aid powder are prepared as raw material powder.
  • the silicon nitride powder preferably has a ⁇ conversion rate of 80% by mass or more, an average particle size of 0.4 to 2.5 ⁇ m, and an impurity oxygen content of 2% by mass or less.
  • the impurity oxygen content is preferably 2% by mass or less, more preferably 1.0% by mass or less, and further preferably 0.1 to 0.8% by mass. If the content of impurity oxygen exceeds 2% by mass, the reaction between impurity oxygen and the sintering aid occurs, and there is a possibility that a grain boundary phase is formed more than necessary.
  • the sintering aid is preferably a metal oxide powder having an average particle size of 0.5 to 3.0 ⁇ m.
  • the metal oxide powder include oxides of rare earth elements, magnesium, titanium, hafnium, and the like.
  • the sintering aid one or more selected from rare earth elements, magnesium, titanium, and hafnium are added in a total amount of 2 to 14% by mass in terms of oxides. If this range is deviated, the grain growth of the silicon nitride crystal grains during the sintering process and the ratio of the grain boundary phase will deviate, making it difficult to achieve the target ratio (T2 / T1).
  • a predetermined amount of silicon nitride powder and sintering aid powder are mixed, and an organic binder is further added to prepare a raw material mixture.
  • an organic binder is further added to prepare a raw material mixture.
  • amorphous carbon, a plasticizer or the like may be added.
  • Amorphous carbon functions as a deoxidizer. That is, since amorphous carbon reacts with oxygen and is released to the outside as CO 2 or CO, it facilitates the liquid phase reaction in the sintering process.
  • a molding process for molding the raw material mixture is performed.
  • a forming method of the raw material mixture a general-purpose mold pressing method, a cold isostatic pressing (CIP) method, or a sheet forming method such as a doctor blade method or a roll forming method can be applied.
  • a raw material mixture shall be mixed with solvents, such as toluene, ethanol, butanol, as needed.
  • a degreasing step of the molded body is performed.
  • most of the organic binder previously added is degreased by heating in a non-oxidizing atmosphere at a temperature of 500 to 800 ° C. for 1 to 4 hours.
  • the non-oxidizing atmosphere include a nitrogen gas atmosphere and an argon gas atmosphere.
  • examples of the organic binder include butyl methacrylate, polyvinyl butyral, polymethyl methacrylate and the like. Further, when the total amount of the silicon nitride powder and the sintering aid powder is 100 parts by mass, the addition amount of the organic binder is preferably 3 to 17 parts by mass.
  • the amount of the organic binder added is less than 3 parts by mass, the amount of the binder is too small to maintain the shape of the molded body. In such a case, it becomes difficult to increase the productivity by increasing the number of layers.
  • the amount of the binder exceeds 17 parts by mass, the voids of the molded body (molded body after the degreasing process) are increased after the degreasing step, and the pores of the silicon nitride substrate are increased.
  • the degreased molded body is housed in a firing container, heated in a non-oxidizing atmosphere in a firing furnace to a temperature of 1400 to 1650 ° C., and subjected to a heat treatment step for 1 to 8 hours.
  • This treatment promotes the liquid phase reaction of the sintering aid powder.
  • the diffusion of the liquid phase component to the grain boundaries of the silicon nitride crystal particles is promoted, and the pores are reduced.
  • the holding temperature is less than 1400 ° C.
  • the liquid phase reaction is difficult to occur, and when the holding temperature is higher than 1650 ° C., the growth of silicon nitride crystal particles proceeds, so the effect of reducing pores due to diffusion of the liquid phase component is sufficient. It can no longer be obtained.
  • the non-oxidizing atmosphere include nitrogen gas (N 2 ) and argon gas (Ar). It is also effective to improve the mass productivity by laminating molded bodies in multiple stages. Moreover, by making it multistage, the temperature in a furnace becomes uniform and a liquid phase reaction can be made uniform.
  • the sintering step is performed by heating the compact to a temperature of 1800 to 1950 ° C. for 8 to 18 hours in a non-oxidizing atmosphere.
  • a non-oxidizing atmosphere a nitrogen gas atmosphere or a reducing atmosphere containing nitrogen gas is preferable.
  • the firing furnace pressure is preferably a pressurized atmosphere.
  • the sintering temperature is lower than 1800 ° C., the silicon nitride crystal grains are not sufficiently grown and it is difficult to obtain a dense sintered body.
  • the sintering temperature is higher than 1950 ° C., it may be decomposed into Si and N 2 when the furnace pressure is low, so the sintering temperature is preferably controlled within the above range.
  • the sintering temperature is preferably 1950 ° C. or lower because there is a risk of pressure variation in the furnace.
  • the sintering temperature is higher than 1950 ° C., silicon nitride crystal grains grow more than necessary, and the target ratio (T2 / T1) may not be obtained.
  • the cooling rate of the sintered body after the sintering step is 100 ° C./h or less.
  • the grain boundary phase can be crystallized by slowly cooling the cooling rate to 100 ° C./h or less, and further to 50 ° C./h or less.
  • the ratio of the crystalline compound in the grain boundary phase can be increased.
  • the heat treatment process promotes the liquid phase reaction of the grain boundary phase.
  • the grain boundary phase is crystallized, there is little aggregation and segregation of the liquid phase generated in the sintered body, and a grain boundary phase in which a fine crystal structure is uniformly dispersed is obtained.
  • the pores formed in the crystal structure can be reduced at the same time as they are refined.
  • the ratio of the crystalline compound phase in the grain boundary phase can be set to 20% or more, further 50% or more in terms of area ratio.
  • the thermal conductivity of the silicon nitride substrate can be 80 W / m ⁇ K or more.
  • the cooling rate after the sintering process is furnace cooling (natural cooling with the furnace switched off), it is usually about 600 ° C./h. Even in such a case, if the above-described heat treatment step is performed, the grain boundary phase can be made uniform. Therefore, the thermal conductivity is set to 50 W / m ⁇ K or more, and the ratio (T2 / T1) and the insulation are set. The variation in yield strength can be set within a predetermined range.
  • the additional heat treatment is desirably performed at a temperature higher than the liquid phase generation temperature and lower than the processing temperature in the sintering process.
  • the liquid phase component cooled from the surface active state of grain growth in the sintering process becomes a steady state and fixed at the grain boundary.
  • stabilization from the active region tends to proceed heterogeneously. Therefore, it is possible to more uniformly improve the stabilization of the grain boundary by subsequent cooling by heat-treating until the liquid phase is generated and fluidized again, while the grain growth does not proceed.
  • the pores in the silicon nitride substrate can be eliminated, the pores can be reduced, or the grain boundary phase components can be present in the peripheral length of the pores.
  • the temperature of the heat treatment is preferably 1000 ° C. or higher and 1700 ° C. or lower.
  • the grain boundary phase component can be slightly moved while suppressing the grain growth of the silicon nitride crystal grains. At this time, it is easy to obtain the effect of eliminating the pores, reducing the pores, or having the grain boundary phase component in the peripheral length of the pores by pressing or turning over the front and back.
  • the silicon nitride substrate which concerns on embodiment can be obtained.
  • Example 1 (Examples 1 to 20 and Comparative Example 1) A silicon nitride powder having an average particle size of 1.0 ⁇ m, an impurity oxygen content of 1% by mass, and a pregelatinization rate of 98% was prepared. Moreover, what was shown in Table 1 and Table 2 was prepared as sintering auxiliary agent powder. A sintering aid powder having an average particle size of 0.8 to 1.6 ⁇ m was prepared.
  • Silicon nitride powder and sintering aid powder were mixed to prepare a raw material mixture. Further, the raw material mixture was mixed with a dispersant and an organic solvent, and ball mill mixing was performed. Next, 10 parts by mass of butyl methacrylate as the organic binder and 4 parts by mass of dibutyl phthalate as the plasticizer are mixed with 100 parts by mass of the raw material mixed powder, and an organic solvent is additionally added. This was carried out to prepare a slurry-like raw material mixture. After adjusting the viscosity of the slurry to 5000-15000 cps, the sheet was molded by a sheet molding method (doctor blade method) and dried to prepare a molded body (green sheet).
  • a sheet molding method doctor blade method
  • the degreasing step was performed by heating the compact in a nitrogen gas atmosphere at a temperature of 500 to 800 ° C. for 1 to 4 hours.
  • the thermal conductivity, the three-point bending strength, the cross-sectional structure in the substrate thickness direction were observed, the ratio (T2 / T1), the maximum diameter of the grain boundary phase in the thickness direction
  • the average particle diameter and porosity of the major axis of the silicon nitride crystal particles were investigated.
  • the pore size and segregation region in the grain boundary phase were also investigated.
  • the above thermal conductivity was obtained by a laser flash method.
  • the three-point bending strength was measured according to JIS-R-1601 (2008).
  • the substrate thickness T1 was measured with a caliper.
  • the porosity was determined by a mercury intrusion method. Further, an SEM photograph (2000 times) of an arbitrary cross-sectional structure with respect to the substrate thickness direction was taken, and the maximum diameter of the grain boundary phase in the thickness direction and the average particle diameter of the major axis of the silicon nitride crystal particles were investigated.
  • the pore size (maximum diameter) and the ratio of the presence of grain boundary phase components in the peripheral length of the pore were determined.
  • the pore size (maximum diameter) was determined from an SEM photograph (5000 times). Further, the existence ratio of the grain boundary phase component of the perimeter of the pore was obtained by EPMA. The results are shown in Table 6 below.
  • the silicon nitride substrate according to each example had small pores, and the grain boundary phase component was present at 10% or more in the peripheral length of the pores. Moreover, pores could be reduced (including the case where no pores exist) by performing additional heat treatment.
  • Dielectric strength and volume resistivity were measured for the silicon nitride substrates according to Examples and Comparative Examples as described above.
  • the dielectric strength was measured by the 4-terminal method according to JIS-C-2141.
  • the measurement terminal used was a spherical electrode having a tip of 20 mm in diameter.
  • the dielectric strength was measured in Fluorinert.
  • the average values and variations of the five measurement locations (S1 to S5) shown in FIG. 4 were obtained.
  • the volume resistivity value was measured according to JIS-K-6911.
  • the surface-side carbon electrode was formed into a disk shape with a diameter of 20 mm
  • the back-surface carbon electrode was formed into a disk shape with a diameter of 28 mm
  • the volume resistivity value ⁇ v1 at room temperature (25 ° C.) and the volume resistivity value ⁇ v2 at 250 ° C. were obtained with an applied voltage of 1000 V. .
  • the silicon nitride substrate according to each example showed excellent characteristics in dielectric strength and volume resistivity. In addition, it showed excellent characteristics with respect to frequency dependence of relative permittivity.
  • Such a silicon nitride substrate has excellent insulating properties even if it is thinned. Therefore, excellent reliability can be ensured even when applied to a silicon nitride circuit substrate or a pressure contact structure substrate.

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Abstract

　窒化珪素結晶粒子と粒界相を具備する熱伝導率が５０Ｗ／ｍ・Ｋ以上の窒化珪素基板において、窒化珪素基板の断面組織は、窒化珪素基板の厚さＴ１に対し厚さ方向の粒界相の合計長さＴ２の比（Ｔ２／Ｔ１）が０．０１～０．３０であり、基板の表裏に電極を接触して４端子法で測定したときの絶縁耐力の平均値からのばらつきが１５％以下であることを特徴とする。また、絶縁耐力の平均値が１５ｋｖ／ｍｍ以上であることが好ましい。上記構成によれば絶縁耐力のばらつきが小さい窒化珪素基板およびそれを用いた窒化珪素回路基板が得られる。

Description

窒化珪素基板およびそれを用いた窒化珪素回路基板
　後述する実施形態は、概ね、窒化珪素基板およびそれを用いた窒化珪素回路基板に関する。
　近年、窒化珪素（Ｓｉ３Ｎ４）基板を半導体回路基板に適用することが試みられている。半導体回路基板としては、アルミナ（Ａｌ２Ｏ３）基板、窒化アルミニウム（ＡｌＮ）基板が使用されている。アルミナ基板は熱伝導率が３０Ｗ／ｍ・Ｋ程度であるが、低コスト化が可能である。また、窒化アルミニウム基板は熱伝導率が１６０Ｗ／ｍ・Ｋ以上となる高熱伝導化が可能である。一方、窒化珪素基板としては、熱伝導率が５０Ｗ／ｍ・Ｋ以上の基板が開発されている。
　窒化珪素基板は、窒化アルミニウム基板と比較して熱伝導率は低いが、３点曲げ強度が５００ＭＰａ以上と優れている。窒化アルミニウム基板の３点曲げ強度は通常３００～４００ＭＰａ程度であり、熱伝導率が高くなるほどに強度が下がる傾向にある。高強度の利点を生かすことにより窒化珪素基板は薄型化が可能である。基板の薄型化により熱抵抗を下げることが可能になるので放熱性が向上する。
　このような特性を生かして窒化珪素基板は、金属板などの回路部を設けて回路基板として広く使用されている。また、国際公開番号ＷＯ２０１１／０１０５９７号パンフレット（特許文献１）に示したような圧接構造用の回路基板として使用する方法もある。
　上記のような様々な使い方をするにあたり、窒化珪素基板に求められる特性は熱伝導率、強度、さらには絶縁性が挙げられる。
　絶縁性が良い窒化珪素基板に関して、特開２００２－２０１０７５号公報（特許文献２）に提案されている。特許文献２には、温度２５ ℃、湿度７０％の条件下で窒化珪素基板の表裏間に１．５ｋＶ－１００Ｈｚの交流電圧を印加したときの電流リーク値が１０００ｎＡ以下である窒化珪素基板が開示されている。この電流リーク値は小さいほど表裏間の絶縁性が高いことを示している。
国際公開番号ＷＯ２０１１／０１０５９７号パンフレット 特開２００２－２０１０７５号公報
　しかしながら、特許文献２のように電流リーク値が一定値以下であったとしても絶縁性が不十分となる場合があった。この原因を追究した結果、基板の厚さ方向の窒化珪素結晶粒子と粒界相との存在比が大きく関係することが判明した。窒化珪素基板は、窒化珪素結晶粒子と粒界相とを具備する窒化珪素焼結体から構成されている。窒化珪素結晶粒子と粒界相とを比較した場合、窒化珪素結晶粒子の方が、絶縁性が高い。そのため、窒化珪素基板内で粒界相の存在比に応じて絶縁性が異なる部分が形成される。このため、電流リーク値が一定値以下であったとしても絶縁性が不十分となる現象が発生していた。
　実施形態に係る窒化珪素基板は、窒化珪素結晶粒子と粒界相とを具備する熱伝導率が５０Ｗ／ｍ・Ｋ以上の窒化珪素基板において、窒化珪素基板の断面組織は、窒化珪素基板の厚さＴ１に対し粒界相の合計長さＴ２の比（Ｔ２／Ｔ１）が０．０１～０．３０であり、基板の表裏に電極を接触して４端子法で測定したときの絶縁耐力の平均値からのばらつきが２０％以下であることを特徴とするものである。
　実施形態に係る窒化珪素基板は、窒化珪素基板の厚さＴ１に対し粒界相の合計長さＴ２の比（Ｔ２／Ｔ１）を所定の範囲に規定しているので、厚さ方向における絶縁性のばらつきが小さい。そのため、回路基板などに使用した場合に、絶縁性が優れ信頼性が高い回路基板を得ることができる。
実施形態に係る窒化珪素基板の断面組織の一例を示す断面図である。 実施形態に係る窒化珪素基板において、基板厚さＴ１に対する粒界相の合計の長さＴ２の比（Ｔ２／Ｔ１）の比を説明するための断面図である。 窒化珪素基板について、４端子法によって絶縁耐力を測定する方法の一例を示す断面図である。 絶縁耐力の測定箇所の一例を示す平面図である。 窒化珪素基板の体積固有抵抗値を測定する方法の一例を示す側面図である。
　本実施形態に係る窒化珪素基板は、窒化珪素結晶粒子と粒界相とを具備する熱伝導率が５０Ｗ／ｍ・Ｋ以上の窒化珪素基板において、窒化珪素基板の断面組織は、窒化珪素基板の厚さＴ１に対し粒界相の合計長さＴ２の比（Ｔ２／Ｔ１）が０．０１～０．３０であり、基板の表裏に電極を接触して４端子法で測定したときの絶縁耐力の平均値からのばらつきが２０％以下であることを特徴とするものである。
　まず、窒化珪素基板は、窒化珪素結晶粒子と粒界相とを具備する熱伝導率が５０Ｗ／ｍ・Ｋ以上の窒化珪素焼結体から構成されている。また、熱伝導率は５０Ｗ／ｍ・Ｋ以上、さらには９０Ｗ／ｍ・Ｋ以上であることが好ましい。熱伝導率が５０Ｗ／ｍ・Ｋ未満と低い場合には放熱性が低下する。
　図１に実施形態に係る窒化珪素基板の断面組織の一例を示す。図中、符号１は窒化珪素基板、２は窒化珪素結晶粒子、３は粒界相、Ｔ１は窒化珪素基板の厚さ、である。また、図２は、実施形態に係る窒化珪素基板において、基板厚さＴ１に対する粒界相の合計の長さＴ２の比（Ｔ２／Ｔ１）を説明するための断面図である。図中、２は窒化珪素結晶粒子、３は粒界相、Ｔ２－１～４は厚さ方向の粒界相の長さである。
　窒化珪素基板は、窒化珪素結晶粒子と粒界相とを具備する窒化珪素焼結体から構成されている。窒化珪素結晶粒子は β－Ｓｉ３Ｎ４結晶粒子が個数割合で９５％以上１００％以下であることが好ましい。 β－Ｓｉ３Ｎ４結晶粒子が９５％以上となることにより、窒化珪素結晶粒子がランダムに存在した組織となり、強度が向上する。
　また、粒界相は、焼結助剤を主として構成されるものである。また、焼結助剤としては、希土類元素、マグネシウム、チタン、ハフニウムから選択される１種以上が好ましい。また、焼結助剤はそれぞれ酸化物換算で合計２～１４質量％が含有されていることが好ましい。燒結助剤が酸化物換算で２質量％未満では、粒界相の存在比が少ない部分が生じるおそれがある。また、焼結助剤が酸化物換算で１４質量％を超えて過量であると、粒界相の存在比が多くなり過ぎるおそれがある。そのため、焼結助剤は酸化物換算で４．０～１２．０質量％の範囲で含有されることが好ましい。
　実施形態の窒化珪素基板の断面組織は、窒化珪素基板の厚さＴ１に対し粒界相の合計長さＴ２の比（Ｔ２／Ｔ１）が０．０１～０．３０であることを特徴としている。窒化珪素基板の厚さＴ１は、図１に示したように基板の厚さである。基板の厚さＴ１はノギスにより測定するものとする。
　また、粒界相の合計長さＴ２の測定方法に関しては、図２を参照して説明する。図２は窒化珪素基板において、基板厚さＴ１に対する粒界相の合計の長さＴ２の比（Ｔ２／Ｔ１）を説明するための断面図である。図中、符号２は窒化珪素結晶粒子であり、３は粒界相である。まず、窒化珪素基板の厚さ方向の任意の断面組織を拡大写真に撮る。一視野で厚さ方向の断面組織が観察できないときは、複数回に分けて撮影してもよいものとする。
　また、拡大写真は走査型電子顕微鏡（ＳＥＭ）写真であることが好ましい。ＳＥＭ写真であれば、窒化珪素結晶粒子と粒界相とでコントラスト差が付くため区別し易い利点がある。
　また、倍率については２０００倍以上であれば窒化珪素結晶粒子と粒界相との区別を行い易い。粒界相の合計の長さＴ２を求めるには、断面組織の拡大写真に対して、基板厚さ方向に直線を引き、その直線上に存在する粒界相の長さを求めていくものとする。
　図２の場合、Ｔ２－１、Ｔ２－２、Ｔ２－３、Ｔ２－４の合計がＴ２となる（Ｔ２＝（Ｔ２－１）＋（Ｔ２－２）＋（Ｔ２－３）＋（Ｔ２－４））。拡大写真を分けて撮影する場合は、基板厚さＴ１になるまで、この作業を繰り返すものする。なお、拡大写真の撮影するに際しては、任意の断面を表面粗さＲａが０．０５ μｍ以下に鏡面研磨して、エッチング処理してから撮影を実施するものとする。なお、エッチング処理としては、ケミカルエッチング、プラズマエッチングのどちらでも有効である。また、基板中に存在するポアは粒界相の長さにカウントしないものとする。
　実施形態に係る窒化珪素基板の断面組織は、窒化珪素基板の厚さＴ１に対し粒界相の合計長さＴ２の比（Ｔ２／Ｔ１）が０．０１～０．３０であることを特徴としている。上記比（Ｔ２／Ｔ１）が０．０１未満では、部分的に粒界相の少ない領域ができてしまうために絶縁性が低下する。一方、上記比（Ｔ２／Ｔ１）が０．３０を超えて多いと部分的に粒界相が多い領域が形成されてしまうために、絶縁性のばらつきが発生する原因となる。絶縁性の確保と、そのばらつきの低減のためには、上記比（Ｔ２／Ｔ１）が０．１０～０．２５の範囲であることが好ましい。
　このように上記比（Ｔ２／Ｔ１）を０．０１～０．３０の範囲に規定することにより、基板の表裏に電極を接触して４端子法で測定したときの絶縁耐力の平均値からのばらつきが２０％以下、さらには１５％以下とすることができる。
　図３に４端子法を使用した絶縁耐力の測定方法の一例を示す。図中、符号１は窒化珪素基板であり、４は表面側測定端子であり、５は裏面側測定端子であり、６は測定器である。表面側測定端子４および裏面側測定端子５の先端形状は球体とする。測定端子の先端形状を球体とすることにより、窒化珪素基板１への面圧を一定にすることが可能になり、測定誤差を無くすことができる。
　また、表面側測定端子４と裏面側測定端子５は窒化珪素基板１を挟むように対向して配置するものとする。実施形態の窒化珪素基板１は、表面側測定端子４と裏面側測定端子５を窒化珪素基板１のどの位置に配置して測定した場合でも、平均値からのばらつきが２０％以下になる。
　上記絶縁耐力の平均値は、前述の測定方法により、窒化珪素基板１において少なくとも５か所を測定し、その平均値を求めるものとする。図４に絶縁耐力の測定箇所の一例を示す。例えば、1枚の基板について５か所を測定する場合の測定箇所は、図４に示すようにＳ１、Ｓ２、Ｓ３、Ｓ４、Ｓ５の５か所を測定対象とする。すなわち、基板１の対角線同士の交点（中心）となるＳ１と、Ｓ１からそれぞれの角部の中点となる４点のＳ２～Ｓ５とした。
　このような５か所の測定点における絶縁耐力の平均値を窒化珪素基板１の絶縁耐力の平均値とする。すなわち、Ｓ１での絶縁耐力をＥＳ１、Ｓ２での絶縁耐力をＥＳ２、Ｓ３での絶縁耐力をＥＳ３、Ｓ４での絶縁耐力をＥＳ４、Ｓ５での絶縁耐力をＥＳ５、としたときに、絶縁耐力の平均値ＥＳＡは、下記算式により求めるものとする。また、測定点は少なくとも５点であり、測定点を６か所以上にしてもよい。
　　　　　ＥＳＡ＝（ＥＳ１＋ＥＳ２＋ＥＳ３＋ＥＳ４＋ＥＳ５）／５
　また、絶縁耐力のばらつき（％）は、（｜平均値ＥＳＡ－ＥＳｎ｜／ＥＳＡ） ×１００（％）、ｎ＝整数（測定点の番号）、により平均値からのずれの割合（％）を絶対値で求めるものとする。なお、上記に示した測定条件以外はＪＩＳ－Ｃ－２１４１に準じて測定するものとする。なお、絶縁耐力の測定はフロリナート中で行うものとする。フロリナートは、パーフルオロカーボン（ＰＦＣ）系の絶縁性溶剤である。
　実施形態の窒化珪素基板は絶縁耐力のばらつきが２０％以下と小さい。窒化珪素基板は、窒化珪素結晶粒子と粒界相とから成る窒化珪素焼結体である。また、基板として使用する場合、板厚が１．０ｍｍ以下、さらには０．４ｍｍ以下となるような薄型基板として使用される。これは基板を薄型化することにより基板の熱抵抗を低減して放熱性を上げるためである。
　上記基板の厚さＴ１が１．０ｍｍ以下である薄型基板において、部分的な絶縁耐力のばらつきが大きいと絶縁耐力の低い部分に電界集中が起き易くなる。その結果、絶縁耐力の低い部分が絶縁破壊を起こし易くなる恐れがある。実施形態の窒化珪素基板では、絶縁耐力のばらつきを低減しているので、絶縁耐力が低い部分に電界が集中することを効果的に防止することができる。そのため、基板厚さＴ１を０．１ｍｍまで薄くすることも可能である。言い換えると、実施形態に係る窒化珪素基板は、厚さＴ１が０．１～１．０ｍｍ、さらには０．１～０．４ｍｍと薄い基板に有効である。
　また、絶縁耐力の平均値ＥＳＡは１５ｋＶ／ｍｍ以上であることが好ましい。平均値が１５ｋＶ／ｍｍ未満では基板としての絶縁性が不足する。絶縁耐力の平均値ＥＳＡは１５ｋＶ／ｍｍ以上、さらには２０ｋＶ／ｍｍ以上であることが好ましい。前記の比（Ｔ２／Ｔ１）を０．１５以下にすると、絶縁耐力の平均値は２０ｋＶ／ｍｍ以上となり易い。
　また、室温（２５ ℃）での１０００Ｖ印加時の体積固有抵抗値が６０ ×１０１２Ωｍ以上であることが好ましい。また、室温（２５ ℃）での１０００Ｖ印加時の体積固有抵抗値ρｖ１と、２５０ ℃での１０００Ｖ印加時の体積固有抵抗値 ρｖ２との比（ρｖ２／ρｖ１）が０．２０以上であることが好ましい。
　図５に体積固有抵抗値の測定方法を示す。図中、符号１は窒化珪素基板、７は表面側カーボン電極、８は裏面側カーボン電極、９は測定装置、である。なお、体積固有抵抗値の測定にあたっては、表面側カーボン電極７と裏面側カーボン電極８で窒化珪素基板１を押圧して固定する。また、印加電圧は直流１０００Ｖとし、６０秒間印加後の体積抵抗Ｒｖを測定する。体積固有抵抗値 ρｖ＝Ｒｖ・ πｄ２／４ｔ、により体積固有抵抗値を求めるものとする。ここで πは円周率（＝３．１４）、ｄは表面側カーボン電極の直径、ｔは窒化珪素基板の厚さである。このような体積固有抵抗値の測定を、室温（２５℃）で行ったものを ρｖ１、２５０℃の雰囲気中で行ったものを ρｖ２とする。また、上記以外の測定条件はＪＩＳ－Ｋ－６９１１に準じて行うものとする。
　室温（２５ ℃）での１０００Ｖ印加時の体積固有抵抗値が６０ ×１０１２Ωｍ以上であることが好ましい。窒化珪素基板に金属回路板を設けた窒化珪素回路基板は様々な半導体素子を搭載することが可能である。
　半導体素子の中には動作電圧が５００～８００Ｖと高いものもある。 ρｖ１が６０×１０１２Ωｍ以上、さらには９０×１０１２Ωｍ以上と高いことが好ましい。前述のように絶縁耐力のばらつきを低減した上で体積固有抵抗値を高くすることにより、動作電圧の高い半導体素子を実装しても絶縁破壊が起きない優れた信頼性を得ることができる。
　また、比（ρｖ２／ρｖ１）が０．２０以上、さらには０．４０以上と高いことにより、使用環境が２００～３００℃の高温下であったとしても優れた絶縁性を維持することができる。近年、ＳｉＣ素子などのように動作温度が１５０～２５０℃になる半導体素子が開発されている。このような半導体素子を実装する絶縁基板として実施形態に係る窒化珪素基板を使うことにより、半導体装置としても優れた信頼性を得ることができる。
　また、窒化珪素基板の厚さ方向の断面を拡大写真にて観察したとき、粒界相の最大長が５０μｍ以下であることが好ましい。また、窒化珪素結晶粒子の長径の平均粒径は１．５～１０μｍであることが好ましい。絶縁耐力の平均値を高くし、かつ、そのばらつきを２０％以下にするには、厚さ方向における窒化珪素結晶粒子と粒界相との存在比（Ｔ２／Ｔ１）を所定の範囲内にすることが有効である。
　その上で、体積固有抵抗値ρｖ１を所定の値以上、比（ρｖ２／ρｖ１）を所定の値以上にするには、粒界相のサイズを制御することが有効である。窒化珪素基板の厚さ方向の断面を拡大写真にて観察したとき、粒界相の最大長を５０μｍ以下、さらには２０μｍ以下、さらには１０μｍ以下と小さくすることが好ましい。厚さ方向の粒界相の最大長とは、前述のＴ２－１、Ｔ２－２、Ｔ２－３、Ｔ２－４のいずれもが５０μｍ以下であることを示している。
　また、粒界相の最大長を５０μｍ以下にするには、窒化珪素結晶粒子の長径の平均粒径は１．５～１０μｍであることが好ましい。窒化珪素結晶粒子の長径は任意の断面組織の拡大写真において、単位面積１００ μｍ×１００μｍ内に写る窒化珪素結晶粒子個々の最大径を測定し、その平均値により求めるものとする。最大径の測定は、拡大写真に写る窒化珪素結晶粒子の最も長い対角線を長径として求めるものとする。この作業を単位面積１００μｍ ×１００μｍを異なる３か所で行い、その平均値を窒化珪素結晶粒子の長径の平均粒径とする。
　上記窒化珪素結晶粒子の長径の平均粒径が１．５μｍ未満と小さいと、窒化珪素結晶粒子同士の粒界が増えるために、比（Ｔ２／Ｔ１）が０．３０を超える部分が形成されるおそれがある。窒化珪素結晶粒子の長径の平均粒径が１０μｍを超えて大きいと、窒化珪素結晶粒子間の粒界の数は減るものの、窒化珪素結晶粒子間の粒界の長さが大きくなってしまい粒界相の最大長を５０μｍ以下にできない部分が形成されるおそれがある。そのため、窒化珪素結晶粒子の長径の平均粒径は１．５～１０μｍ、さらには２～７μｍの範囲であることが好ましい。なお、拡大写真は２０００倍以上のものを用いるものとする。また、結晶粒子および粒界を判断し難いときは５０００倍の拡大写真を使用するものとする。
　また、窒化珪素基板の気孔率が３％以下であることが好ましい。また、気孔（ポア）の最大径は２０μｍ以下であることが好ましい。実施形態に係る窒化珪素基板は基板厚み方向の窒化珪素結晶粒子と粒界相の比（Ｔ２／Ｔ１）を制御してあるので気孔率３％まで存在したとしても絶縁耐力のばらつきは２０％以下、さらには１５％以下にすることができる。
　なお、気孔（ポア）はできるだけ少ないことが好ましく、気孔率は１％以下、さらには０．５％以下であることが好ましい。また、気孔の最大径は２０μｍ以下、さらには１０μｍ以下、さらには３μｍ以下（０含む）であることが好ましい。また、気孔の最大径は任意の断面における拡大写真から求めるものとする。
　また、前述のように体積固有抵抗値ρｖ１が６０×１０１２Ωｍ以上、比（ρｖ２／ρｖ１）が０．２０以上にするには、窒化珪素基板の任意の表面または断面を拡大写真（２０００倍以上）にて観察したとき、気孔率１％以下（０含む）かつ気孔の最大径が１０μｍ以下（０含む）であることが好ましい。
　上記拡大写真とはＳＥＭ写真のことである。ＳＥＭ写真において、ポアは窒化珪素結晶粒子および粒界相とは異なるコントラスト差が生じるので区別可能である。倍率を２０００倍以上、さらには５０００倍に拡大したＳＥＭ写真にて観察されるポアの割合またはサイズを小さくすることにより、高温環境下（２５０℃雰囲気下）であったとしても優れた体積固有抵抗値を得ることができる。
　また、任意の断面を拡大写真にて観察したときにポアが存在する場合、ポアの周囲長の１０％以上に粒界相が存在することが好ましい。ポアとなっている部分は空気が存在することになる。窒化珪素粒子は絶縁物である。また、粒界相成分は金属酸化物からなる焼結助剤が反応して形成されるものである。このため粒界相成分は酸化物であるため絶縁性が高い。
　一方、空気は電気の通り道になり易い。特に６００Ｖ以上の大きな電圧を印可したときには電気の通り道になり易い。ポアは焼結工程による緻密化過程での残存欠陥であり、その緻密化は粒界相を介して進行する。
　また、β－窒化珪素結晶粒子は細長い形状を有する。β－窒化珪素結晶粒子が複雑に絡み合ったランダム配向することにより、窒化珪素基板の強度が向上する。一方、ランダム配向すると窒化珪素結晶粒子同士の隙間が形成され易い。窒化珪素結晶粒子同士の隙間を粒界相成分で埋めることによりポアが形成され難くなる。またポアが形成されても、その周辺の緻密化阻害により生じる構造欠陥を含み難くなる。そのため、ポアの周囲が粒界相成分で覆われていることが、良好な緻密化過程を示唆することとなり好ましい。
　このため、ポアの最大径を２０μｍ以下にしたうえで、ポアの外周長の１０％以上に粒界相成分を存在させることが好ましい。ポアの外周長に粒界相成分が存在する割合は大きいほどよく５０％以上１００％以下であることが好ましい。ポアの外周長が５０％以上と大きくすることにより、絶縁耐力を向上させると共に、そのばらつきを低減することができる。言い換えれば、ポアが存在したとしても、その外周を粒界相成分で覆うことにより絶縁耐力を向上させることができる。
　また、５０Ｈｚでの比誘電率をεｒ５０、１ｋＨｚでの比誘電率をεｒ１０００としたとき、（εｒ５０－εｒ１０００）／εｒ５０≦０．１であることが好ましい。比誘電率とは、電極間の媒質が充填しているときの蓄電器の電気容量が真空であるときの電気容量で割った値を示す。今回の媒質とは、窒化珪素基板である。（εｒ５０－εｒ１０００）／εｒ５０≦０．１であるということは、窒化珪素基板の比誘電率は周波数が高くなっても大きくならないことを示している。これは、窒化珪素基板の分極が起き難い構成になっていることを示す。分極が起き難い状態としては、ポアが小さいこと、ポアが少ないこと、などが挙げられる。また、前述のように、粒界相のサイズを制御すること、ポアの周囲に粒界相成分を存在させることも有効である。さらに、後述する偏析領域を低減することも有効である。
　また、窒化珪素基板の任意の断面を観察したとき、粒界相中の偏析領域の最大長が５μｍ以下（０含む）であることが好ましい。粒界相は焼結助剤を主成分とする反応相である。燒結助剤としては、前述のように、希土類元素、マグネシウム、チタン、ハフニウムから選ばれる１種以上が好ましい。
　ここで偏析領域とは、ＥＰＭＡ（電子線マイクロアナライザ）により単位面積２０μｍ×２０μｍをカラーマッピングしたとき、特定元素の平均濃度に対し３０％以上ずれが生じている領域を示す。特定元素とは、焼結助剤成分を示す。例えば、焼結助剤成分として酸化イットリウム（Ｙ２Ｏ３）を用いた場合、Ｙ元素のマッピングを行い平均濃度に対して３０％以上濃度ずれた領域を求めるものとする。
　また、複数の焼結助剤成分を用いている場合は、それぞれの成分の金属元素をマッピングするものとする。例えば、焼結助剤成分としてＹ２Ｏ３、ＭｇＯ、ＨｆＯ２の３種類用いた場合は、「Ｙ」「Ｍｇ」「Ｈｆ」について平均濃度から３０％以上ずれている領域を求めるものとする。なお、平均濃度から３０％以上のずれとは多い場合も少ない場合も該当するものとする。
　上記偏析領域は小さいことが好ましく、偏析領域の最大長は５μｍ以下、さらには１μｍ以下（０含む）が好ましい。偏析領域を小さくすることにより、体積固有抵抗値 ρｖ１を９０×１０１２Ωｍ以上、前記比（ρｖ２／ρｖ１）を０．４０以上にすることができる。また、偏析領域の最大長を５μｍ以下、さらには１μｍ以下（０含む）の状態とすることにより、絶縁耐力のばらつきを５％以下にすることもできる。板厚が薄い基板であるほど影響が大きくなる。そのため、板厚Ｔ１が０．１～０．４ｍｍのものである場合は、偏析領域は１μｍ以下と小さい状態もしくは存在しない状態であることが好ましい。
　以上のような構成とすることにより、窒化珪素基板の板厚Ｔ１を０．１～１．０ｍｍ、さらには０．１～０．４ｍｍと薄型化しても絶縁耐力のばらつきを低減した上で、絶縁耐力の平均値を向上させることができる。
　また、粒界相の最大長、窒化珪素結晶粒子のサイズを制御することにより、絶縁耐力の向上のみならず、窒化珪素基板の熱伝導率を５０Ｗ／ｍ・Ｋ以上とした上で、強度を６００ＭＰａ以上とすることもできる。
　また、気孔率、ポアサイズや偏析領域サイズ（焼結助剤の偏析部サイズ）を制御することにより、さらなる絶縁耐力の向上、体積固有抵抗値の向上を図ることができる。
　また、特許文献２に示したように粒界相中における結晶化合物相を面積比で２０％以上とすることにより、熱伝導率を８０Ｗ／ｍ・Ｋ以上、さらには９０Ｗ／ｍ・Ｋ以上とし易くなる。
　実施形態に係る窒化珪素基板は、窒化珪素回路基板に好適である。回路基板は、回路部として金属板、金属層を設けたものである。金属板は、銅板、Ａｌ板などの導電性のよい金属板が例示できる。また、金属板の接合は活性金属接合法、直接接合法など様々な接合法が適用できる。また、必要に応じ、裏面にも金属板を設けるものとする。また、金属層は、金属ペーストを加熱して形成されるメタライズ膜や、メッキ法、スパッタリング法、溶射法など薄膜形成技術を使用した金属薄膜などが挙げられる。
　また、特許文献１に示したような圧接構造用基板としても使用できる。特に実施形態に係る窒化珪素基板は絶縁耐力を改善しているので、圧接構造用基板としても有効である。
　次に、実施形態に係る窒化珪素基板の製造方法について説明する。実施形態に係る窒化珪素基板は前述の構成を有している限り、製造方法は特に限定されるものではないが、効率よく製造するための方法として次のものが挙げられる。
　まず、原料粉末として、窒化珪素粉末、焼結助剤粉末を用意する。窒化珪素粉末は、 α化率が８０質量％以上であり、平均粒径が０．４～２．５μｍであり、不純物酸素含有量が２質量％以下であることが好ましい。また、不純物酸素含有量は２質量％以下、さらには１．０質量％以下、さらには０．１～０．８質量％であることが好ましい。不純物酸素含有量が２質量％を超えて多いと、不純物酸素と焼結助剤との反応が起きて、必要以上に粒界相が形成されてしまうおそれがある。
　また、焼結助剤は、平均粒径が０．５～３．０μｍの金属酸化物粉末であることが好ましい。金属酸化物粉末としては、希土類元素、マグネシウム、チタン、ハフニウムなどの酸化物が挙げられる。焼結助剤を金属酸化物として添加することにより、焼結工程中に液相成分を形成し易くなる。
　また、焼結助剤は、希土類元素、マグネシウム、チタン、ハフニウムから選択される１種または２種以上を酸化物換算で合計２～１４質量％添加するものとする。この範囲をずれると焼結工程中の窒化珪素結晶粒子の粒成長や粒界相の割合がずれて、目的とする比（Ｔ２／Ｔ１）の範囲内にすることが困難となる。
　次に、窒化珪素粉末および焼結助剤粉末を所定量混合し、さらに有機バインダを添加して原料混合体を調製する。このとき、必要に応じて、非晶質炭素、可塑剤等を添加してもよい。非晶質炭素は脱酸剤として機能する。すなわち非晶質炭素は酸素と反応してＣＯ２やＣＯとして外部に放出されるため、焼結工程の液相反応を促進し易くなる。
　次に、原料混合体を成形する成形工程を行う。原料混合体の成形法としては、汎用の金型プレス法、冷間静水圧プレス（ＣＩＰ）法、あるいはドクターブレード法、ロール成形法のようなシート成形法などが適用できる。また、必要に応じ、原料混合体を、トルエン、エタノール、ブタノールなどの溶媒と混合するものとする。
　次に上記成形工程の後、成形体の脱脂工程を行う。脱脂工程は、非酸化性雰囲気中、温度５００～８００ ℃で１～４時間加熱して、予め添加していた大部分の有機バインダの脱脂を行うものとする。非酸化性雰囲気としては、窒素ガス雰囲気中、アルゴンガス雰囲気中などが挙げられる。
　また、有機バインダとしては、ブチルメタクリレート、ポリビニルブチラール、ポリメチルメタクリレートなどが挙げられる。また、窒化珪素粉末と焼結助剤粉末との合計量を１００質量部としたとき、有機バインダの添加量は３～１７質量部であることが好ましい。
　有機バインダの添加量が３質量部未満ではバインダ量が少なすぎて成形体の形状を維持するのが困難となる。このような場合、多層化して量産性を向上することが困難となる。一方、バインダ量が１７質量部を超えて多いと、脱脂工程後に成形体（脱脂処理後の成形体）の空隙が大きくなり窒化珪素基板のポアが大きくなってしまう。
　次に脱脂処理された成形体は、焼成容器内に収容され焼成炉内において非酸化性雰囲気中で温度１４００～１６５０℃に加熱され、１～８時間保持される熱処理工程が実施される。この処理により、焼結助剤粉末の液相反応が促進される。液相反応の促進により、液相成分が窒化珪素結晶粒子の粒界への拡散が促進され、気孔が低減される。
　保持温度が１４００℃未満では液相反応が起き難く、保持温度が１６５０℃を超えて高いと、窒化珪素結晶粒子の粒成長が進んでしまうために、液相成分の拡散による気孔低減効果が十分得られなくなる。また、非酸化性雰囲気としては、窒素ガス（Ｎ２）やアルゴンガス（Ａｒ）などが挙げられる。また、成形体を多段に積層して量産性を向上させることも有効である。また、多段にすることにより、炉内の温度が均一になり、液相反応が均一にすることができる。
　次に、焼結工程を行う。焼結工程は、非酸化性雰囲気中で成形体を温度１８００～１９５０℃に８～１８時間加熱して実施されるものとする。非酸化性雰囲気としては、窒素ガス雰囲気、または窒素ガスを含む還元性雰囲気が好ましい。また、焼成炉内圧力は加圧雰囲気であることが好ましい。
　焼結温度が１８００℃未満と低温状態で焼成すると、窒化珪素結晶粒子の粒成長が十分でなく、緻密な焼結体が得難い。一方、焼結温度が１９５０℃より高温度で焼成すると、炉内雰囲気圧力が低い場合にはＳｉとＮ２に分解するおそれがあるため、焼結温度は上記範囲に制御することが好ましい。
　前述のように成形体を多段にした場合、炉内の圧力ばらつきが生じるおそれがあるため焼結温度は１９５０℃以下が好ましい。また、焼結温度が１９５０℃より高いと窒化珪素結晶粒子が必要以上に粒成長してしまい目的とする比（Ｔ２／Ｔ１）が得られなくなるおそれがある。
　また、焼結工程後における焼結体の冷却速度を１００℃／ｈ以下にすることが好ましい。冷却速度を１００℃／ｈ以下、さらには５０℃／ｈ以下とゆっくり冷却することにより、粒界相を結晶化することができる。粒界相中の結晶化合物の割合を大きくすることができる。前記熱処理工程により、粒界相の液相反応を促進している。
　このため、粒界相の結晶化を行ったとき、焼結体に生成した液相の凝集偏析が少なく、微細な結晶組織が均一に分散した粒界相が得られる。また結晶組織に形成される気孔も微細化すると同時に減少させることができる。
　また、焼結工程後の冷却速度を１００℃／ｈ以下とすることにより、粒界相中の結晶化合物相の割合を面積率で２０％以上、さらには５０％以上とすることができる。粒界相を結晶化することにより、窒化珪素基板の熱伝導率を８０Ｗ／ｍ・Ｋ以上とすることができる。
　なお、焼結工程後の冷却速度を炉冷（炉のスイッチを切った自然冷却）にすると通常は６００℃／ｈ程度になる。このような場合でも、前述の熱処理工程を行っていれば、粒界相の均一化が図れるため、熱伝導率を５０Ｗ／ｍ・Ｋ以上とした上で、前記比（Ｔ２／Ｔ１）および絶縁耐力のばらつきを所定の範囲とすることができる。
　また、焼結工程後に、再度、追加の熱処理することも有効である。追加の熱処理は、液相生成温度以上でかつ焼結工程の処理温度より低温とすることが望ましい。また、加圧条件化で行うことが望ましい。焼結工程において粒成長という表面活性状態から冷却された液相成分は、粒界において定常状態となり固定化される。しかし、活性域からの安定化は不均質に進行しやすい。よって再度液相が生成し流動する状態、一方で粒成長は進行しない状態まで熱処理することで、その後の冷却による粒界の安定化をより均質に改善することが可能となる。
　また、追加の熱処理を行う際に、押圧すること、焼結体の表裏をひっくり返して行うこと、などが有効である。追加の熱処理を行うことにより、窒化珪素基板中のポアを無くしたり、ポアを小さくしたり、ポアの周囲長に粒界相成分を存在した状態とすることができる。上記熱処理の温度は１０００℃以上１７００℃以下が好ましい。
　上記の１０００℃以上１７００℃以下で熱処理することにより、窒化珪素結晶粒子の粒成長を抑制したうえで、粒界相成分を若干移動させることができる。このとき、押圧することや表裏をひっくり返して行うことにより、ポアを無くしたり、ポアを小さくしたり、ポアの周囲長に粒界相成分を存在した状態とする効果を得易くなる。
　以上のような製造方法であれば、実施形態に係る窒化珪素基板を得ることができる。
　（実施例）
　（実施例１～２０および比較例１）
　窒化珪素粉末として、平均粒径が１．０ μｍであり、不純物酸素含有量が１質量％であり、 α化率が９８％のものを用意した。また焼結助剤粉末として表１および表２に示したものを用意した。なお、焼結助剤粉末は平均粒径が０．８～１．６ μｍのものを用意した。
　窒化珪素粉末と焼結助剤粉末とを混合し、原料混合体を調製した。また、原料混合体に、分散剤、有機溶媒を混合してボールミル混合を行った。次に、原料混合粉１００質量部に対し、有機バインダとしてのブチルメタクリレートを１０質量部と、可塑剤としてジブチルフタレートを４質量部、混合し、有機溶媒を追加添加し、さらにボールミル混合を十分に実施してスラリー状の原料混合体を調製した。スラリーの粘度を５０００～１５０００ｃｐｓに調整した後に、シート成形法（ドクターブレード法）によりシート成形して乾燥し、成形体（グリーンシート）を調製した。
　成形体に対し、窒素ガス雰囲気中で温度５００～８００ ℃で１～４時間加熱して脱脂工程を行った。
　次に脱脂処理した成形体に対し、表１および表２に示した熱処理工程および焼結工程を実施した。この工程を実施した後、表１および表２に示した条件で実施例および比較例の窒化珪素基板を作製した。また、熱処理工程および焼結工程は多段（１０段重ね）にして実施した。
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
　各実施例および比較例に係る窒化珪素基板について、熱伝導率、３点曲げ強度、基板厚さ方向の断面組織を観察し、比（Ｔ２／Ｔ１），厚さ方向の粒界相の最大径、窒化珪素結晶粒子の長径の平均粒径、気孔率を調査した。また、ポアサイズ、粒界相中の偏析領域についても調査した。
　なお上記熱伝導率はレーザフラッシュ法により求めた。また、３点曲げ強度はＪＩＳ－Ｒ－１６０１（２００８）に準じて測定した。また、基板厚さＴ１はノギスで測定した。また、気孔率は水銀圧入法により求めた。また、基板厚さ方向に対して任意の断面組織をＳＥＭ写真（２０００倍）を撮影し、厚さ方向の粒界相の最大径、窒化珪素結晶粒子の長径の平均粒径を調査した。
　また、基板厚さ方向の任意の断面において単位面積２０μｍ×２０μｍの拡大写真（５０００倍のＳＥＭ写真）を１０か所分撮影し、ポアサイズ（最大径）を求めた。また、単位面積２０μｍ×２０μｍをＥＰＭＡにより焼結助剤成分の金属元素に関してカラーマッピングを行った。単位面積２０μｍ×２０μｍを５箇所測定し、平均濃度、さらには偏析領域（金属元素の濃度が３０％以上ずれている領域）のサイズを求めた。その結果を下記表３および表４に示す。
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
（実施例１１～２０）
　次に、実施例１～１０の窒化珪素基板に対し、表５に示す追加熱処理を行った。
Figure JPOXMLDOC01-appb-T000005
　実施例１～２０および比較例１に係る窒化珪素基板について、ポアサイズ（最大径）とポアの周囲長に粒界相成分が存在する割合を求めた。ポアサイズ（最大径）はＳＥＭ写真（５０００倍）にて求めた。また、ポアの周囲長の粒界相成分の存在割合はＥＰＭＡにより求めた。その結果を下記表６に示す。
Figure JPOXMLDOC01-appb-T000006
　上記表６に示す結果から明らかなように、各実施例に係る窒化珪素基板ではポアが小さく、ポアの周囲長に粒界相成分が１０％以上存在していた。また、追加熱処理を行うことにより、ポアを小さく（ポアが存在しない場合を含む）することができた。
　以上のような実施例および比較例に係る窒化珪素基板に対して、絶縁耐力、体積固有抵抗値を測定した。なお、上記絶縁耐力はＪＩＳ－Ｃ－２１４１に準じて４端子法にて測定した。測定端子は先端が直径２０ｍｍの球形電極のものを使用した。また、絶縁耐力の測定はフロリナート中で行った。また、測定箇所は図４に示した５か所（Ｓ１～５）の平均値、ばらつきを求めた。
　また、体積固有抵抗値は、ＪＩＳ－Ｋ－６９１１に準じて測定した。表面側カーボン電極を直径２０ｍｍの円盤状、裏面カーボン電極を直径２８ｍｍの円盤状とし、印加電圧１０００Ｖとして、室温（２５℃）における体積固有抵抗値 ρｖ１と２５０℃における体積固有抵抗値ρｖ２を求めた。
　また、比誘電率の周波数依存性を調べた。比誘電率の周波数依存性としては、５０Ｈｚでの比誘電率をεｒ５０、１ｋＨｚでの比誘電率をεｒ１０００としたとき、（εｒ５０－εｒ１０００）／εｒ５０により求めた。その結果を表７、表８に示す。
Figure JPOXMLDOC01-appb-T000007
Figure JPOXMLDOC01-appb-T000008
　以上のように各実施例に係る窒化珪素基板は絶縁耐力および体積固有抵抗値に優れた特性を示した。また、比誘電率の周波数依存性に関しても優れた特性を示した。
　このような窒化珪素基板であれば、薄型化しても優れた絶縁性を有している。そのため、窒化珪素回路基板や圧接構造用基板に適用したとしても優れた信頼性を確保することができる。
　以上、本発明のいくつかの実施形態を例示したが、これらの実施形態は、例として提示したものであり、発明の範囲を限定することは意図していない。これら新規な実施形態は、その他の様々な形態で実施されることが可能であり、発明の要旨を逸脱しない範囲で、種々の省略、置き換え、変更などを行うことができる。これら実施形態やその変形例は、発明の範囲や要旨に含まれるとともに、特許請求の範囲に記載された発明とその均等の範囲に含まれる。また、前述の各実施形態は、相互に組み合わせて実施することができる。
１ …窒化珪素基板
２ …窒化珪素結晶粒子
３ …粒界相
４，５ …４端子法による測定端子
６ …絶縁耐力測定器
７，８ …カーボン電極
９ …体積固有抵抗値測定器

Claims (15)

  1. 窒化珪素結晶粒子と粒界相とを具備する熱伝導率が５０Ｗ／ｍ・Ｋ以上の窒化珪素基板において、窒化珪素基板の断面組織は、窒化珪素基板の厚さＴ１に対し厚さ方向の粒界相の合計長さＴ２の比（Ｔ２／Ｔ１）が０．０１～０．３０であり、基板の表裏に電極を接触して４端子法で測定したときの絶縁耐力の平均値からのばらつきが２０％以下であることを特徴とする窒化珪素基板。
  2. 絶縁耐力のばらつきが１５％以下であることを特徴とする請求項１記載の窒化珪素基板。
  3. 絶縁耐力の平均値が１５ｋｖ／ｍｍ以上であることを特徴とする請求項１ないし請求項２のいずれか１項に記載の窒化珪素基板。
  4. 室温（２５ ℃）での１０００Ｖ印加時の体積固有抵抗値が６０ ×１０１２Ωｍ以上であることを特徴とする請求項１ないし請求項３のいずれか１項に記載の窒化珪素基板。
  5. 室温（２５ ℃）での１０００Ｖ印加時の体積固有抵抗値ρｖ１と、２５０℃での１０００Ｖ印加時の体積固有抵抗値ρｖ２と比（ρｖ２／ρｖ１）が０．２０以上であることを特徴とする請求項１ないし請求項４のいずれか１項に記載の窒化珪素基板。
  6. ５０Ｈｚでの比誘電率をεｒ５０、１ｋＨｚでの比誘電率をεｒ１０００としたとき、
    （εｒ５０－εｒ１０００）／εｒ５０≦０．１であることを特徴とする請求項１ないし請求項５のいずれか１項に窒化珪素基板。
  7. 窒化珪素基板の厚さ方向の断面を拡大写真にて観察したとき、粒界相の最大長が５０μｍ以下であることを特徴とする請求項１ないし請求項６のいずれか１項に記載の窒化珪素基板。
  8. 窒化珪素結晶粒子の長径の平均粒径は１．５～１０μｍであることを特徴とする請求項１ないし請求項７のいずれか１項に記載の窒化珪素基板。
  9. 窒化珪素基板の気孔率が３％以下であることを特徴とする請求項１ないし請求項８のいずれか１項に記載の窒化珪素基板。
  10. 窒化珪素基板の任意の表面または断面を拡大写真にて観察したとき、ポアの最大径が２０μｍ以下（０含む）であることを特徴とする請求項１ないし請求項９のいずれか１項に記載の窒化珪素基板。
  11. 窒化珪素基板の任意の断面を拡大写真にて観察したとき、ポアの最大径が２０μｍ以下であり、ポアの周囲長の１０％以上に粒界相成分が存在することを特徴とする請求項１ないし請求項１０のいずれか１項に記載の窒化珪素基板。
  12. 窒化珪素基板の任意の断面を観察したとき、粒界相中の偏析領域の最大長が５μｍ以下（０含む）であることを特徴とする請求項１ないし請求項１１のいずれか１項に記載の窒化珪素基板。
  13. 窒化珪素基板の厚さＴ１が０．１～１．０ｍｍであることを特徴とする請求項１ないし請求項１２のいずれか１項に記載の窒化珪素基板。
  14. 粒界相は面積率２０％以上が結晶化合物相であることを特徴とする請求項１ないし請求項１３のいずれか１項に記載の窒化珪素基板。
  15. 請求項１ないし請求項１４のいずれか１項に記載の窒化珪素基板に回路部を設けたことを特徴とする窒化珪素回路基板。
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