Evaluation of 32nm Advanced immersion lithography pellicles

Advanced immersion lithography utilizes higher numerical aperture (NA) stepper lenses resulting in higher angles of light illumination through photomasks. Transmission in conventional pellicles (830 nm thickness) is generally maximized at 0 degree illumination and decreases significantly at the higher angles. Most pellicle suppliers have developed thinner pellicle membranes (~280 nm) which allow considerably improved transmission of light at angles up to 20 degrees. In addition, aluminum frames have been shortened, potentially allowing inspection closer to the inside of the frame and reduced mask flatness distortion upon pellicle mount. Suppliers have also developed advanced adhesives which reduce outgassing even beyond the low levels obtained with current 45 nm pellicles. In this paper, advanced immersion pellicles from several suppliers are evaluated and compared with conventional 45 nm pellicles for the following quality parameters: physical durability, foreign material, ease of demounting and glue removal, chemical outgassing, mask flatness distortion and susceptibility to radiation damage. Improvements in mask inspection and pellicle optical transmission at higher incident angles are also evaluated and are discussed. © 2008 SPIE.