Composite substrate

A composite substrate consisting of an inorganic insulating sintered body substrate having a heat conductivity of at least 5 W/m·K and a volume resistivity of at least 1×108 Ω·cm, a single crystal semiconductor film, and a thin layer disposed between the inorganic insulating sintered body substrate, the single crystal semiconductor film and a layer of polycrystalline or amorphous silicon disposed on the back surface of the composite substrate, said thin layer being composed of silicon nitride deposited by a chemical vapor deposition method, and being formed to cover the inorganic insulating sintered body substrate in entirety, the single crystal semiconductor film being bonded to only the front surface of the inorganic insulating sintered body substrate via the thin layer,

• wherein the thin layer is a film to prevent metal impurities in the inorganic insulating sintered body substrate from leaching out or diffusing during a semiconductor device fabrication process using the composite substrate.