Semiconductor device, wiring substrate forming method, and substrate processing apparatus

A substrate support (201) having a flat supporting surface (201a) is prepared, and a semiconductor substrate (1) is fixed to the substrate supporting surface (201) by attaching a wiring forming surface (1a) to the supporting surface (201a) by suction, for example, by vacuum suction. On this occasion, the wiring forming surface (1a) is forcibly flattened by being attached to the supporting surface (201a) by suction, and therefore the wiring forming surface (1a) becomes a reference plane for planarization of a back surface (1b). In this state, planarization processing is performed by mechanically grinding the back surface (1b) to grind away projecting portions (12) of the back surface (1b). Hence, variations in the thickness of the substrate (especially, semiconductor substrate) are made uniform, and high-speed planarization is realized easily and inexpensively without disadvantages such as dishing and without any limitation on a wiring design.