In-situ and real-time investigation of EUV pellicle mechanical stress within EUV inner pod

Extreme ultraviolet (EUV) pellicle, a thin (approximately few nanometers in scale) protective membrane, dominates the defectivity control for protecting the EUV mask from airborne debris. The EUV mask equipped with pellicle is typically stored within a EUV inner pod (EIP) until use. However, such pellicle is easily deformed due to its structural weakness, the risk of thermal stress and so on, thereby altering its transmission as well as impacting the yield of EUV fabrication. In this paper, we present a novel investigation approach via both a chromatic confocal sensor and a conductance tester to address the above issue through incorporating with Gudeng Precision Industrial Co., Ltd. A load-deflection membrane model based on Timoshenko beam theory and minimum energy method was applied to evaluate the residual stress of EUV pellicle. During the pump/vent cycle (from atmospheric pressure to 5 Pa and vice versa), the activity of ASML EUV pellicle inspired the nature breathing manner, was deflected from -275 μm (minimum deflection) to +200 μm (maximum deflection). A pellicle deflection of approximately +100 μm (toward EUV mask the front side) was present during all vacuum steady states (i.e. closing pump at 5 Pa). Furthermore, the 5th ASML pellicle was verified to be 5.56 times stronger mechanically than the previous 4th pellicle from our experiment. Taken together, the proposed approach has been successfully demonstrated to enable in-situ and real-time examination of EUV pellicle mechanics within EIP in vacuum, which should be amenable for worldwide EUV mask cores.