Materials design and development of fluoropolymers for use as pellicles in 157-nm photolithography

The introduction of 157 nm as the next optical lithography wavelength has created a need for new soft (polymeric) or hard (quartz) pellicle materials optimized for this wavelength. Materials design and development of ultra transparent fluoropolymers suitable for 157 nm soft pellicle applications has produced a number of promising candidate materials with absorbances below 0.03/micrometer as is necessary to achieve pellicle transmissions above 95%. We have developed 12 families of experimental TeflonAF R (TAFx) materials which have sufficient transparency to produce transmissions above 95%. For the successful fabrication of 157 nm pellicles from these materials, the fluoropolymers must have appropriate physical properties to permit the spin coating of thin polymer films and their lifting and adhesive mounting to pellicle frames, the processes which produce free standing pellicle membranes of micron scale thickness. Relevant physical properties include molecular weight, glass transition temperature, and mechanical strength and toughness. We have successfully developed various of the ultra transparent TAFx polymer families with these physical properties. Upon irradiation these 157 nm pellicle polymers undergo photochemical darkening, which reduces the 157 nm transmission of the material. Measurements of the photochemical darkening rate allow the estimation of the pellicle lifetime corresponding to a 10% drop in 157 nm transmission. Increasing the 157 nm lifetime of fluoropolymers involves simultaneous optimization of the materials, the pellicle and the end use. Similar optimization was essential to achieve the desired radiation durability lifetimes for pellicles successfully developed for use with KrF (248 nm) and ArF (193 nm) lithography.