CNTs in the context of EUV pellicle history

In the early 2000s, membranes both thin enough to transmit EUV light and strong enough to be free-standing at mask dimensions did not exist. The lithography community assumed that defect control for photomasks would be achieved, not with a pellicle, but with a clean scanner environment, thermophoretic protection and a removable pellicle. 1 In 2006, Intel published their research on an EUV pellicle. 2 Since then, an international development effort on EUV pellicle membranes has spanned a range of materials and fabrication approaches. Not only materials, but also the requirements of the EUV pellicle membrane have evolved over time. Imec’s pellicle work based on carbon nanotubes (CNTs) started in 2015, and is placed in relation to the rich history of EUV pellicles. CNTs are one-atom-thick carbon sheets rolled into tubes. The CNTs can be single- or multi-walled and can vary in diameter and in length. These engineered CNTs can be arranged in different configurations to form membranes of different densities. Thus, the CNT membrane’s properties can be fundamentally changed to meet the EUV pellicle targets for properties like transmittance. The historical trends in EUV pellicle membrane development are presented and the CNT membranes are described in that context.