Silicon nitride sintered body, method of producing the same, and silicon nitride circuit substrate and semiconductor module using the same

1. A silicon nitride sintered body, consisting of crystal grains of β-type silicon nitride and a grain boundary phase containing at least one type of rare earth element (RE), magnesium (Mg) and silicon (Si), wherein:

the grain boundary phase comprises an amorphous phase and a MgSiN 2 crystal phase;

the X-ray diffraction line peak intensity of any crystal plane of a crystal phase containing the rare earth element (RE) is less than 0.0005 times the sum of the diffraction peak intensities of (110), (200), (101), (210), (201), (310), (320) and (002) of the crystal grains of the β-type silicon nitride;

the X-ray diffraction peak intensity of (121) of the MgSiN 2 crystal phase is 0.0005 to 0.0019 times the sum of the X-ray diffraction peak intensities of (110), (200), (101), (210), (201), (310), (320) and (002) of the crystal grains of the β-type silicon nitride, and

the silicon nitride sintered body contains magnesium (Mg) with 6.7 to 9.9 mol % of magnesium oxide (MgO), and at least one type of rare earth element (RE) with 1.2 to 2.3 mol % of rare-earth element oxide (RE 2 O 3 ), the total amount of MgO and RE 2 O 3 contained is 7.9 to 12.2 mol %, and a molar ratio of (RE 2 O 3 )/(MgO) is 0.1 to 0.3.

2. The silicon nitride sintered body according to claim 1 , wherein thermal conductivity is 84 W/m·K or more.

3. The silicon nitride sintered body according to claim 1 , wherein the silicon nitride sintered body contains magnesium (Mg) with 6.7 to 9.8 mol % of magnesium oxide (MgO), and at least one type of rare earth element (RE) with 1.2 to 1.8 mol % of rare-earth element oxide (RE 2 O 3 ), the total amount of MgO and RE 2 O 3 contained is 7.9 to 11.0 mol %.

4. A method of producing the silicon nitride sintered body according to claim 1 , comprising:

mixing 6.7 to 9.9 mol % of magnesium oxide (MgO) and 1.2 to 2.3 mol % of at least one type of rare-earth element oxide (RE 2 O 3 ) into a base powder of silicon nitride whose oxygen content is less than or equal to 2.0 percent by mass so that the total amount comes to 7.9 to 12.2 mol % and the molar ratio of (RE 2 O 3 )/MgO) to 0.1 to 0.3;

forming a sheet formed body that is 40 mm or less in total thickness;

heating the sheet formed body from 1,600 degrees Celsius to a temperature between 1800 degrees Celsius and 2000 degrees Celsius, at a heating rate of 300 degrees Celsius/h or less; and

cooling the sheet formed body down to 1,500 degrees Celsius at a cooling rate of 100 degrees Celsius/h or more for sintering after keeping the sheet formed body for 2 to 5 hours.

5. A silicon nitride circuit substrate comprising:

a silicon nitride substrate made of the silicon nitride sintered body according to claim 1 ;

a metallic circuit plate that is bonded to one side of the silicon nitride substrate; and

a metallic radiator plate that is bonded to the other side of the silicon nitride substrate.

6. A semiconductor module comprising:

the silicon nitride circuit substrate according to claim 5 ; and

a semiconductor element mounted on the silicon nitride circuit substrate.

7. The silicon nitride sintered body according to claim 1 , wherein bending strength is 829 MPa or higher.

8. The method of producing a silicon nitride sintered body according to claim 4 ,

wherein mixing 6.7 to 9.8 mol % of the MgO and 1.2 to 1.8 mol % of the RE 2 O 3 ; so that the total amount comes to 7.9 to 11.0 mol %.