Development of a silicon-based electron beam transmission window for use in a KrF excimer laser system

Princeton Plasma Physics Laboratory, in collaboration with the Naval Research Laboratory, is currently investigating various novel materials (single-crystal silicon, 〈100〉, 〈110〉, and 〈111〉) for use as electron beam transmission windows in a krypton fluoride (KrF) excimer laser system. The primary function of the window is to isolate the active medium (excimer gas) from the excitation mechanism (field-emission diodes). The chosen window geometry must accommodate electron energy transfer >80% (750 keV) while maintaining the structural integrity during the mechanical load (1.3- to 2.0-atm base pressure differential, ∼0.5-atm cyclic pressure amplitude, 5-Hz repetition rate) and the thermal load across the entire hibachi area (∼0.9 W-cm^-2). In addition, the window must be chemically resistant to attack by fluorine free radicals (hydrofluoric acid, secondary). In accordance with these structural, functional, and operational parameters, a 22.4-mm square silicon prototype window, coated with 500-nm thin-film silicon nitride (Si₃N₄), has been fabricated. The window consists of 81 square panes 0.019 ± 0.001 mm thick. The stiffened (orthogonal) sections are 0.065 mm wide and 0.500 mm thick (approximate). Assessment of silicon (and silicon nitride) material properties and computer-aided design modeling/analysis of the window design suggest that silicon may be a viable solution to inherent parameters and constraints.