Multilayer substrate manufacturing method and wiring substrate

A method for manufacturing a multilayer substrate, comprising:

• providing a first substrate being a rigid substrate comprising a plurality of first bumps on a surface in a predetermined arrangement, and a second substrate or semiconductor device comprising a plurality of second bumps on a surface in an arrangement corresponding to the predetermined arrangement, each of the first bumps and the second bumps being composed of a metal or alloy having a melting point of 600° C. or more, and having a height of 0.3 μm or more;
• performing cleaning treatment on bonding surfaces of the first bumps and bonding surfaces of the second bumps in an atmosphere at a pressure of 1×10−3 Pa or less;
• and
• subsequently stacking the first substrate and the second substrate or semiconductor device in an atmosphere at a pressure of 1×10−3 Pa or less so that the bonding surfaces of the first bumps and the bonding surfaces of the second bumps abut each other, and pressure-welding the first bumps and the second bumps at a temperature of 90° C. or less to form a multilayer substrate.