Bonded substrate

A bonded substrate, comprising:

• a silicon nitride ceramic substrate;
• a copper plate disposed on the silicon nitride ceramic substrate;
• and
• a bonding layer having a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm disposed on the silicon nitride ceramic substrate, bonding the copper plate to the silicon nitride ceramic substrate,
• the bonding layer including:
• an interplate portion between the silicon nitride ceramic substrate and the copper plate;
• and
• a protruding portion protruding from between the silicon nitride ceramic substrate and the copper plate,
• wherein
• the bonding layer covers the silicon nitride ceramic substrate to prevent exposure of the silicon nitride ceramic substrate at a position where the protruding portion is disposed,
• an upper surface of the bonding layer has an arithmetic average roughness Ra less than or equal to 0.7 μm, and a maximum height Rz less than 5 μm, and
• the protruding portion contains at least one of titanium and zirconium, does not contain silver, and does not contain copper.