CNT EUV pellicle: moving towards a full-size solution

Development of a pellicle membrane to protect the reticle from particles for EUV source powers beyond 250 W is a subject of intensive research and is in great demand to support high volume manufacturing with EUV lithography. Identifying a membrane with high EUV transmission, mechanical integrity, thermal stability and chemical resistance within the scanner environment is extremely challenging; yet these properties are required to realize next-generation EUV pellicle solutions. This paper proposes free-standing carbon nanotube (CNT) film as an alternative next generation core pellicle material. Most of the desired pellicle characteristics can be achieved by tuning the properties of freestanding CNT films. We demonstrate that free-standing CNT films possess very high EUV transmission (up to 99%) and good transmission uniformity (0.4% half range), mechanical stability (maximum deflection ~0.08 mm at 2 Pa), thermal stability (no change under greater than 500 W equivalent EUV exposure in vacuum without hydrogen radicals) and scalability to a full pellicle size. Other important CNT membrane properties are presented and are favourable for the pellicle application: low EUV scattering, low EUV reflectivity and high transmission under DUV. The ability of the CNT film to stop particles is analysed. The only known failure of the CNT membrane is instability to hydrogen radical/ion environment within the current reticle chamber of the scanner. If changing that environment to limit hydrogen radicals near the pellicle surface is not an option, there is a need to coat the CNT structures for protection. The challenges and considerations for coating the free-standing CNT membranes are discussed.