The photomask manufacturing method comprises at least one step of cleaning the photomask and at least one step of placing a protective pellicle onto the photomask at the end of manufacturing. The inventive method further comprises at least one step of removing ammonia and sulfate residue between the cleaning step and the pellicle placement step. This step comprises the following operations: placing the photomask into a sealed chamber, creating a low pressure within the sealed chamber by pumping out the gases that it contains, exposing the photomask to infrared radiation, stopping the infrared radiation, checking that the temperature of the photomask does not exceed 50.degree. C., restoring the atmospheric pressure within the chamber, and removing the photomask from the chamber. The device for implementing the inventive method comprises a sealed chamber containing at least one photomask, a pumping unit for creating and maintaining a vacuum inside the chamber, a system for holding at least one photomask, placed inside the sealed chamber, infrared radiation means and a gas injection system.