The substrate of a silicon nitride sintered body and its manufacturing method

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A process for producing a sintered silicon nitride substrate which comprises 80-98.3 mass% Si3N4, 0.7-10 mass% (in terms of oxide amount) Mg, and 1-10 mass% (in terms of oxide amount) at least one rare-earth element and which has length and width dimensions of 100 mm or larger each and a thickness of 0.7 mm or less, characterized by comprising a step in which a silicon nitride powder is mixed with a sintering aid powder comprising both Mg and at least one rare-earth element, a step in which the obtained mixed powder is molded, and a step in which the obtained green sheet is fired in a nitrogen atmosphere, the firing step being conducted while a weight plate is kept being placed on the green sheet so that a load of 10-600 Pa is imposed on the green sheet.