Abstract: A method for reducing contamination of a photomask that has a pellicle frame holding a pellicle film to cover the photomask, and a sealed space defined by the pellicle frame, the pellicle film and the photomask, includes: (a) providing a pressure regulating hole in the pellicle frame; (b) disposing the photomask into a receiving space of a chamber to fluidly connect the sealed space with the receiving space of the chamber; and (c) varying a temperature of the photomask to change a gas pressure within the sealed space and to thereby produce a gas transfer between the sealed space and the receiving space through the pressure regulating hole.