Comparative experimental study of single- and multi-walled carbon nanotube pellicles for EUV/soft X-ray space filters

Freestanding carbon nanotube (CNT) thin films are attractive as highly transmissive, mechanically robust supports for extreme ultraviolet (EUV) and soft X-ray space mission applications. We compare single-walled (SW) and multi-walled (MW) carbon nanotube (CNT) pellicles grown by floating-catalyst chemical vapor deposition, combining X-ray absorption spectroscopy (XAS), EUV transmittance, UV–VIS–IR spectroscopy, and spherical indentation to link composition to transmittance, and explore mechanical properties. In the 10–14 nm range, MW pellicles exhibit higher EUV transmission than SW pellicles with comparable visible transparency. At 13.5 nm, relevant for EUV lithography, representative MW samples reach transmittance T of about 0.80, while SW films with similar optical density reach T of about 0.70. XAS analyses show that this performance difference arises from compositional variations: SW pellicles contain a higher Fe:C atomic ratio, and the increased absorption, limited to the energy region above the Fe M2,3 photoabsorption edge, accounts for the reduced EUV transmission. Both types of pellicles display similar effective per-carbon extinction cross sections in the UV–VIS and low pellicle densities (respectively about 0.41 and 0.31 g cm−3 for SW and MW). Mechanical tests yield higher elastic moduli for SW films (6.0–7.6 GPa) than for MW films (4.2–4.7 GPa), but comparable ultimate tensile strengths (276–320 MPa). At fixed pellicle thicknesses, the lower stiffness of MW films results in higher rupture loads under out-of-plane indentation, as experimentally observed. The higher mechanical resistance to rupture and higher transmission make MW pellicles promising as very light substrates for EUV filters and/or pellicles in high-energy astrophysics applications, as well as for EUV lithography.