Method of manufacturing bonded substrate, method of manufacturing circuit substrate, and circuit substrate

A method of manufacturing a circuit substrate comprising steps of:

• manufacturing a bonded substrate with steps of:
• (a) preparing one or a plurality of products to be bonded, each including a brazing material layer and a copper plate laminated on both main surfaces of a ceramic substrate;
• (b) laminating the one or the plurality products to be bonded and a pair of clamping members that clamp the one or the plurality products to be bonded so that the one or the plurality products to be bonded are clamped by the pair of clamping members with a mold releasing layer being provided between each thereof;
• (c) heating the one or the plurality of products while pressing the one or the plurality of products in the pair of clamping members, to obtain the one or the plurality of bonded substrates in which the ceramic substrate and the copper plate are bonded with a bonding layer;
• and
• (d) removing the mold releasing layer from the bonded substrate by dissolving a portion in contact with the mold releasing layer of the copper plate included in the bonded substrate by wet etching,
• forming a predetermined circuit pattern on the bonded substrate;
• and
• performing immersion silver plating on a surface of the copper plate of the bonded substrate after the patterning.

A circuit substrate comprising:

• a ceramic substrate;
• copper plates which are bonded to two main surfaces of the ceramic substrate, respectively;
• and
• silver-plating films formed on surfaces of the copper plates, wherein
• a number of facets existing on a surface of the copper plate in an interface between the copper plate and the silver-plating film is 3000 or less per mm2.