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A pellicle is a photomask protective cover used in the projection printer or wafer/mask stepper to increase the yield. It is composed of a thin transparent membrane adhered to a metal frame which seals off a photomask or reticle from harmful particle contamination. The pellicle is permanently attached to a photomask or reticle and the membrane is held at a fixed distance from the photomask or reticle surface.

The Use of a Pellicle
During the printing process, the image of any particle on the pellicle film will be out of focus on the wafer plane and therefore has only a blurry shadow which has a minimal effect on the photomask's image on a wafer. Without pellicle protection, a photomask can easily get a particle on its surface and form a distorted image on the wafer, creating a defect on a chip. Before a time when pellicle was used, a photomask would require daily cleaning and inspection. Consequently, the photomask would easily become contaminated from environment or damaged from cleaning process, resulting in a low die yield and high replacement cost. The use of a pellicle in an optical projection system is illustrated below.

Figure 1: The Use of a Pellicle

Once a pellicle is properly attached, the surface of the photomask or reticle stays free from future particle contamination. The original quality of the photomask or reticle is therefore preserved. Now only a brief inspection of the pellicle membrane and mask surface is required to insure the quality of the photomask or reticle. Using a pellicle on a mask has become an integral part of IC manufacturing.