Copper/ceramic bonded body, insulating circuit board, method for producing copper/ceramic bonded body, and method for producing insulating circuit board

An insulating circuit substrate comprising:

• a copper sheet made of copper or a copper alloy;
• and
• a ceramic substrate made of nitrogen-containing ceramics, the copper sheet being bonded to a surface of the ceramic substrate,
• wherein, between the copper sheet and the ceramic substrate, an active metal nitride layer containing nitrides of one or more active metals selected from Ti, Zr, Nb, and Hf is formed on a ceramic substrate side, and a Mg solid solution layer in which Mg is solid-dissolved in a Cu matrix is formed between the active metal nitride layer and the copper sheet,
• Cu-containing particles composed of either one or both of Cu particles and compound particles of Cu and the active metal are dispersed in an interior of the active metal nitride layer, and
• a ratio C2/C1 of an average copper concentration C1 (atomic %) in a region from an interface on a ceramic substrate side to a position of 25% of a total thickness of the active metal nitride layer to an average copper concentration C2 (atomic %) in a region from an interface on a copper sheet side to a position of 25% of the total thickness of the active metal nitride layer is 0.8 or less.