An assembly of a support plate and an exit window foil for use in an electron beam device. The support plate is designed to reduce wrinkles in said foil, which wrinkles may arise due to surplus foil arising in the assembly process. The foil is being bonded to the support plate along a closed bonding line bounding a substantially circular area in which the support plate is provided with apertures and foil support portions and in which area the foil is adapted to serve as a portion of a wall of a vacuum tight housing of the electron beam device. Another aspect involves a method for using the assembly in a filling machine, as well as a method of reducing wrinkles.