Silicon nitride substrate, silicon nitride circuit substrate, and evaluation method, evaluation device, and evaluation system for silicon nitride substrate and silicon nitride circuit substrate

A silicon nitride circuit substrate comprising:

• a sintered silicon nitride substrate comprising:
• a plane having a shape that is rectangular;
• a plurality of sides, each side having a length equal to or longer than 100 mm;
• a sintered substrate material manufactured by nitriding a sheet-shaped molded body that contains silicon;
• a first surface;
• and
• a second surface disposed opposite the first surface,
• one of the first surface or the second surface being a measurement surface,
• on the measurement surface, a value of an average full width at half maximum (FWHM) Cave being broader than 0 cm−1 and narrower than 5.32 cm−1 as measured by using a measurement method below:
• a central portion and first, second, third and fourth edge portions on the measurement surface are determined as measurement points;
• the first, second, third and fourth measurement points are each located on the sintered silicon nitride substrate at a distance of 10 mm to an adjacent long side of the sintered silicon nitride substrate and 15 mm to an adjacent short side of the sintered silicon nitride substrate;
• a Raman spectrum is measured at each of the measurement points;
• a full width at half maximum (FWHM) C of a spectral peak having the maximum strength within a range from 850 cm−1 or greater to 875 cm−1 or less is calculated in each Raman spectrum;
• and
• an average value of thus calculated FWHMs C is determined as the average FWHM Cave;
• a metallic circuit disposed on the first surface of the sintered silicon nitride substrate;
• and
• a metallic radiator plate disposed on the second surface of the sintered silicon nitride substrate.