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The main task of a step-and-scan system is to transfer an image as present on a mask into a pattern on a wafer. The mask holds a magnified version (typical 4x) of the desired pattern. Figure 9.1 illustrates the principle of such a system. A source generates EUV light. This light is radiated into a so-called illuminator and reshaped to illuminate the pattern on the mask. Note that the illuminator is projected in a curved shape in order to match the shape of the entrance slit shape of the projection optics box (POB). The reason for this shape is explained in Chapter 5. The light reflected from the mask is collected by the POB, and a demagnified image is projected on the wafer. Note that current high-end commercial lithography tools scan the mask while the wafer moves synchronously with the mask. This way, relatively small projection optics can be used to expose a large area on the wafer. Because of the strong absorption of EUV light in air, the core parts of the system are enclosed in a vacuum vessel. This puts several constraints on the system design. The component on the lower left in Fig. 9.1 is the 13.5nm EUV light source. More details on these topics will be given in section 5.3.
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