Effect of Pellicle Frame and Adhesive Material on Final Photomask Flatness

Previous work has shown that photomask blank flatness as well as photomask patterning and pelliclization all play an important role in finished photomask flatness. Additional studies have shown that pellicle mounting techniques, pellicle adhesives, frame flatness and shape and pellicle mounting tools play a role as well. It has become clear that frame flexibility, coupled with frame mounting surface flatness and shape are the principal factors influencing the pellicle effect on the mask distortion. Pellicle suppliers have begun to evaluate various polymers as potential replacements for the standard aluminum frame in current use. The elasticity of the frame adhesive has also been adjusted to evaluate its effect on the pellicle influence on mask flatness. This paper describes some joint evaluations between IBM, Toppan and ShinEtsu, performed to determine the effect of pellicle frame composition,, mount surface flatness, adhesive elasticity and adhesive surface flatness on the distortion of photolithography masks. It demonstrates that polymer pellicle frames with more flexible adhesive improve finished mask flatness approximately the same amount as reducing the total frame standoff height of aluminum frames with conventional adhesive.