Composite substrate and method for producing same

A method for producing a composite substrate, comprising:

• a preparation step of preparing a supporting substrate comprising a single crystal of an insulating material and a single-crystal silicon semiconductor substrate;
• an activation step of individually irradiating a main surface of the supporting substrate and a main surface of the silicon semiconductor substrate using a fast atom beam (FAB) gun to activate both of the main surfaces with the silicon the semiconductor substrate and the supporting substrate not facing each other to form an activated main surface of the silicon semiconductor substrate and an activated main surface of the supporting substrate;
• a metal supply step of supplying a metal containing Ni and Fe to at least one of the activated main surface of the supporting substrate and the activated main surface of the silicon semiconductor substrate, the metal being composed of a metal element other than main components contained in the supporting substrate and the silicon semiconductor substrate;
• a bonding step of bringing the activated main surface of the silicon semiconductor substrate and the activated main surface of the supporting substrate into contact with each other at normal temperature to bond the silicon semiconductor substrate and the supporting substrate together;
• and
• a thickness reduction step of reducing the thickness of the silicon semiconductor substrate from the other main surface of the silicon semiconductor substrate to form the silicon semiconductor substrate to a silicon semiconductor portion to form a composite substrate comprising a supporting substrate comprising a single crystal of an insulating material, a silicon semiconductor portion on the supporting substrate, and interfacial inclusions located at an interface between the supporting substrate and the silicon semiconductor portion in a density of 1012 atoms/cm2 or less, the interfacial inclusions containing Ni and Fe, and the ratio of Ni to Fe being 0.4 or more.