Bonded substrate and manufacturing method of bonded substrate

A bonded substrate manufacturing method comprising:

• a) forming, on a silicon nitride ceramic substrate, a brazing material layer containing an active metal brazing material and having a thickness of 0.1 μm or more and 10 μm or less, the active metal brazing material including metal powder containing silver and at least one metal hydride powder selected from the group consisting of titanium hydride powder and zirconium hydride powder;
• b) disposing a copper plate on the brazing material layer to obtain an intermediate product including the silicon nitride ceramic substrate, the brazing material layer, and the copper plate;
• c) thermally treating the intermediate product to change the brazing material layer into a bonding layer to bond the copper plate to the silicon nitride ceramic substrate, and diffusing silver contained in the brazing material layer into the copper plate to form, in the copper plate, a first thickness-direction region containing silver and having a first silver concentration and a second thickness-direction region having a second silver concentration lower than the first silver concentration;
• and
• d) etching the first thickness-direction region at a first etching rate, and etching the second thickness-direction region at a second etching rate faster than the first etching rate.