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Pellicle mounting is the last process step in the manufacture of photomask. Pellicle prevents or minimizes the impact of impurities on the mask surface to be transferred as undesirable pattern to the wafer surface during the lithography process. In the event that there is an impurity on the mask surface, the impurity will appear on the wafer surface after exposure. If the impurities are on the surface of pellicle, not on the mask surface, then, the same image does not get transferred on the wafer surface, due to different DOF (depth of focus). In general, pellicle is a transparent membrane with approximately 1 um in thickness.