Abstract: A thermal reactor for processing semiconductor wafers includes a ring-like reactor body having a cylindrical outer surface and having a central depressed portion to define an inner circular space at the cross section of the reactor body along a longitudinal direction of the reactor body. A wafer holder for mounting the wafers vertically is positioned within the inner circular space and is rotated when a reaction gas is passed from an open bottom end of the reactor body toward a top end of the reactor body. The reaction gas is exhausted through a conduit placed along the outer surface of the reactor body. A plurality of plasma generating electrodes are positioned around the outer surface and within the central depressed portion of the reactor body in order to clean an inner side wall of the reactor body.