NOVELTY - Silicon nitride powder has specific surface area of 5-40 m2/g. The ratio of specific surface area (FS) of silicon nitride powder and content (in %mass) of oxygen in particle surface layer is 8-30, and ratio of FS and content of oxygen in the particles is 22 or more. The ratio (D10/D90) between the 10 vol.% diameter (D10) and 90 vol.% diameter (D90) of silicon nitride powder is 0.05-0.2 measured by laser-diffraction-type particle size distribution using particle size distribution analyzer.    USE - Silicon nitride powder is used for forming slurry for mold release agent of casting molds used for forming polycrystal silicon ingots (all claimed) for semiconductor substrates used for solar cells.    ADVANTAGE - The silicon nitride powder economically provides firm polycrystal silicon ingot with excellent mold release property and high yield, without producing notch or damage to mold even at low temperatures, by a simple process.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) manufacture of silicon nitride powder, which involves baking amorphous silicon nitride (Si-N(-H)-type) compound having specific surface area of 300-1200 m2/g at 1400-1700 degrees C in nitrogen-containing inert gas atmosphere or nitrogen-containing reducible gas atmosphere by flowing in continuous furnace. The ratio (RS/RO) is 100 or more, where RS (m2/g) is specific surface area of amorphous Si-N(-H)-type compound and RO (%mass) is oxygen-containing ratio. At the time of baking, the amorphous Si-N(-H)-type compound is heated with temperature increase rate of 12-500 degrees C/minute to 1100-1400 degrees C;    (2) silicon nitride powder-containing slurry, which is obtained by mixing silicon nitride powder with water;    (3) manufacture of polycrystal silicon ingot casting mold (1), which involves mixing silicon nitride powder with water to form slurry, applying slurry to the casting mold surface, drying and heating in the atmosphere containing oxygen; and    (4) manufacture of polycrystal silicon ingot, which involves casting polycrystal silicon ingot from silicon melt solution (3) using polycrystal silicon ingot casting mold, and taking out the polycrystal silicon ingot cast from the casting mold.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the polycrystal silicon ingot casting mold.    Casting mold (1)    Mold release layer (2)    Silicon melt solution (3)