Requirements for reticle and reticle material for 157-nm lithography: requirements for hard pellicle

Since conventional pellicle material is not transparent to 157nm light, we are developing a thick pellicle made of fused silica. The effect of aberration due to the pellicle on the optics of an exposure tool has been estimated by simulation, and requirements for the pellicle dimensions have been proposed. It was found that a thick pellicle generates spherical aberration, and this has to be corrected in the system optics. It was indicated that the tilt of the pellicle gives an image shift and coma aberration. Sagging of the membrane causes tilting, and the use of a thicker membrane improves the degree of sagging. However, the requirements for tilt angle are then tighter. It was also noted that wedge due to the thickness change generates an image shift and a coma aberration. The effect of wedge is reduced by having a lower pellicle stand-off, but this also increases the printability of particles. We need to consider the total effect of using a thick pellicle as a component of an exposure tool or an inspection tool in order to define the specification of a hard pellicle for 157nm lithography.