An electromagnetic radiation generation apparatus includes a collector, a gas supplier and a gas pipeline. The collector has a reflection surface configured to reflect an electromagnetic radiation. The collector includes a bottom portion, a perimeter portion, and a middle portion between the bottom portion and the perimeter portion. The middle portion of the collector includes a plurality of openings. The gas supplier is configured to provide a buffer gas. The gas pipeline is in communication with the gas supplier and the collector, and configured to purge the buffer gas through the openings of the middle portion to form a gas protection layer near the reflection surface of the collector. The openings of the middle portion include a plurality of holes arranged in an array including a plurality of rows of holes, or a plurality of concentric gaps.