An apparatus for processing a substrate may include a chamber, a substrate support, a showerhead structure and a purge ring structure. The purge ring structure may be arranged at an edge portion of the substrate support to inject a deposition-preventing gas, which may be supplied from the substrate support, to an edge portion of an upper surface of the substrate. The purge ring structure may include a purge ring and a plurality of bosses. The purge ring may be configured to surround the substrate. The bosses may be protruded from an inner surface of the purge ring in a radius direction of the substrate to form a gap between the inner surface of the purge ring and the edge portion of the substrate. The deposition-preventing gas may be supplied to the upper surface of the substrate through the gap.