A method for generating an electromagnetic radiation includes the following operations. A target material is introduced in a chamber. A light beam is irradiated on the target material in the chamber to generate plasma and an electromagnetic radiation. The electromagnetic radiation is collected with an optical device. A gas mixture is introduced in the chamber. The gas mixture includes a first buffer gas reactive to the target material, and a second buffer gas to slow down debris of the target material and/or plasma by-product, so as to increase an reaction efficiency of the target material and the first buffer gas, and to reduce deposition of the debris of the target material and/or the plasma by-product on the optical device.