EUV optical characterization of alternative membrane materials for EUV pellicles

Pellicles are an important part of the IC-manufacturing supply chain, keeping particles away from the imaging plane of the photomask to preserve wafer yield. EUV lithography poses new challenges on the pellicle membrane because the radiation must pass twice due to the reflective mask. Additionally, there are no transparent materials for EUV so the EUV pellicle must be extremely thin to keep the transmission high. Present continuous-membrane pellicle solutions will not be sufficient for source powers greater than 250 W that are anticipated for HVM EUV lithography. A possible approach to maintain strength and high transmittance is to use nano-structured materials. We report here on the EUV optical characterization of a variety of alternative membrane materials. The fine structure of etched holes or membranes made of carbon nano-tubes introduces interesting optical effects. We, therefore, not only address specular reflectance or transmittance by the optical characterization but also investigate off-specular diffuse scatter. We compare the respective optical properties of homogeneous reference membranes with etched membranes and carbon nano-tubes. Particularly the latter show a very high EUV transmittance of more than 95 % and are therefore considered being a highly promising candidate for alternative EUV pellicles.