Silicon nitride sintered substrate, silicon nitride sintered substrate sheet, circuit substrate, and production method for silicon nitride sintered substrate

1. A silicon nitride sintered substrate having a main surface of a shape larger than a square having a side of a length of 120 mm, wherein a ratio dc/de is 0.98 or higher where a central area of the main surface has a density dc and an end area of the main surface has a density de, the central area of the main surface has a void fraction vc of 1.80% or lower, and the end area of the main surface has a void fraction ve of 1.00% or lower.

2. The silicon nitride sintered substrate of claim 1 , wherein the density dc of the central area is 3.120 g/cm 3 or higher, the density de of the end area is 3.160 g/cm 3 or higher, and a ratio ve/vc of the void fraction vc of the central area and the void fraction ve of the end area is 0.50 or higher.

3. The silicon nitride sintered substrate of claim 1 wherein the density dc of the central area is 3.140 g/cm 3 or higher, the density de of the end area is 3.160 g/cm 3 or higher, and the void fraction vc of the central area is 1.3% or lower.

4. The silicon nitride sintered substrate of claim 1 , wherein the silicon nitride sintered substrate has a partial discharge inception voltage, defined by a voltage value when a discharge charge amount of 10 pC is reached, of 4.0 kV or higher.

5. The silicon nitride sintered substrate of claim 1 , wherein the silicon nitride sintered substrate has a partial discharge inception voltage, defined by a voltage value when a discharge charge amount of 10 pC is reached, of 5.0 kV or higher.

6. The silicon nitride sintered substrate of claim 1 , wherein the silicon nitride sintered substrate has a carbon content of 0.20% by mass or lower.

7. The silicon nitride sintered substrate of claim 1 , wherein the silicon nitride sintered substrate has a thickness of 0.15 mm or greater and 2.0 mm or less.

8. The silicon nitride sintered substrate of claim 1 , wherein the main surface has a shape larger than a rectangle of 150 mm×170 mm.

9. The silicon nitride sintered substrate of claim 1 , wherein the main surface has a square shape having a side of a length of 250 mm or a shape smaller than the square shape.

10. A plurality of silicon nitride sintered substrate piece divided from the silicon nitride sintered substrate of claim 1 .

11. A circuit board including the silicon nitride sintered substrate of claim 1 , the circuit board having a dielectric breakdown voltage of 8.0 kV or higher and a Weibull coefficient of dielectric breakdown voltage of 6 or higher.

12. The circuit board of claim 11 , wherein the main surface has a square shape having a side of a length of 220 mm or a shape smaller than the square shape, and the circuit board has a Weibull coefficient of dielectric breakdown voltage of 10 or higher.

13. A method for producing a silicon nitride sintered substrate, comprising:

step (a) of mixing Si 3 N 4 powder at 80% by mass or higher and 98.3% by mass or lower, Mg compound powder at 0.7% by mass or higher and 10% by mass or lower as converted into an oxide, and at least one type of rare earth element compound powder at 1% by mass or higher and 10% by mass or lower as converted into an oxide to provide mixed powder;

step (b) of putting the mixed powder into a slurry and forming a plurality of greensheets by molding;

step (c) of stacking the plurality of greensheets with a boron nitride powder layer being located between each two adjacent greensheets, among the plurality of greensheets, to form a stacked assembly; and

step (d) of locating the stacked assembly in a sintering furnace and sintering the stacked assembly;

wherein:

in the step (c), the boron nitride powder layer has a thickness of 3 μm or greater and 20 μm or less; and

the step (d) includes:

step (d1) of removing carbon from the greensheets while maintaining an atmosphere temperature of 900° C. or higher and 1300° C. or lower in a vacuum atmosphere of 80 Pa of lower; and

step (d2) of, after the step (d1), sintering the greensheets at an atmosphere temperature of 1600° C. or higher and 2000° C. or lower in a nitrogen atmosphere.

14. A method for producing a silicon nitride sintered substrate, comprising:

step (a) of mixing Si powder, or Si powder and Si 3 N 4 powder, at 80% by mass or higher and 98.3% by mass or lower as converted into Si 3 N 4 , Mg compound powder at 0.7% by mass or higher and 10% by mass or lower as converted into an oxide, and at least one type of rare earth element compound powder at 1% by mass or higher and 10% by mass or lower as converted into an oxide to provide mixed powder;

step (b) of putting the mixed powder into a slurry and forming a plurality of greensheets by molding;

step (c) of stacking the plurality of greensheets with a boron nitride powder layer being located between each two adjacent greensheets, among the plurality of greensheets, to form a stacked assembly; and

step (d) of locating the stacked assembly in a sintering furnace and sintering the stacked assembly;

wherein:

in the step (c), the boron nitride powder layer has a thickness of 3 μm or greater and 20 μm or less; and

the step (d) includes:

step (d1) of removing carbon from the greensheets while maintaining an atmosphere temperature of 900° C. or higher and 1300° C. or lower in a vacuum atmosphere of 80 Pa of lower;

step (d2) of, after the step (d1), nitriding the Si powder in the greensheets at an atmosphere temperature of 1350° C. or higher and 1450° C. or lower in a nitrogen atmosphere; and

step (d3) of, after the step (d2), sintering the greensheets at an atmosphere temperature of 1600° C. or higher and 2000° C. or lower in the nitrogen atmosphere.

15. The method for producing a silicon nitride sintered substrate of claim 13 , wherein the silicon nitride sintered substrate has a main surface of a shape larger than a square having a side of a length of 120 mm.

16. The method for producing a silicon nitride sintered substrate of claim 13 , wherein the main surface has a shape larger than a rectangle of 150 mm×170 mm.

17. The method for producing a silicon nitride sintered substrate of claim 13 , wherein the main surface has a square shape having a side of a length of 250 mm or a shape smaller than the square shape.

18. The method for producing a silicon nitride sintered substrate of claim 14 , wherein the silicon nitride sintered substrate has a main surface of a shape larger than a square having a side of a length of 120 mm.

19. The method for producing a silicon nitride sintered substrate of claim 14 , wherein the main surface has a shape larger than a rectangle of 150 mm×170 mm.

20. The method for producing a silicon nitride sintered substrate of claim 14 , wherein the main surface has a square shape having a side of a length of 250 mm or a shape smaller than the square shape.