The basic problem of imaging is shown in Fig. 2.1. Light from an illumination source passes through a photomask (often referred to as a reticle), which defines the patterns. The simple photomask illustrated here consists of two types of complementary areas—one type that is opaque, while the other is transparent. In this example, the transparent (or “clear”) area is a long space of uniform width, and the optical and resist profiles shown in Fig. 2.1 are cross sections for this geometry. Some of the light that passes through the mask continues through a lens, which projects an image of the mask pattern onto a wafer. The wafer is coated with a photosensitive film, a photoresist that undergoes a chemical reaction upon exposure to light. After exposure, the wafer is baked and developed, leaving regions covered by photoresist and complementary regions that are not covered. The patterning objective of microlithography is to produce well-defined resist features, sized within specifications. This is a challenge because of the shape of the light-intensity distribution produced at the wafer plane by a lens of finite resolution. This distribution lacks a clearly defined edge (Fig. 2.1). From the light-intensity distribution alone, it is not possible to know where the edges of the feature are. If the light-intensity distribution had the shape shown in Fig. 2.2, there would be no such problem, because a clear delineation would exist between areas of the resist exposed to light and areas unexposed to light.
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