The present invention refers to an assembly of a support plate (22) and an exit window foil (20) for use in an electron beam generating device. Said support plate (22) is designed to reduce wrinkles in said foil (20). The foil (20) is bonded to the support plate (22) along a closed bonding line bounding an area in which the support plate (22) is provided with a pattern of apertures and foil support portions alternately. When vacuum is created in the housing said pattern is adapted to form a topographical profile of the foil (20) substantially absorbing any surplus foil. The invention also refers to methods, one being a method in a filling machine for sterilizing a packaging material web.