Lithography creates a resist image on a wafer. The subsequent etching, lift off, or ion implantation process is masked by the resist image at the areas dictated by the lithography mask. Hence, the thin-film material on the wafer is selectively removed, built up, or its characteristics are selectively altered. Replicating the mask pattern produces the resist image, except when mask making or direct writing on a wafer. Figure 1.1 depicts the mask replication process with an imaging lens. The condenser collects light from the source and illuminates the mask pattern. Light then passes through the imaging lens to form an aerial image to selectively expose the resist. After development, the resist image is produced. Figure 1.2 illustrates various forms of image transfer from the resist to the underlying thin film. The film can be isotropically or anisotropically etched, lifted off, plated, or implanted, using the patterned resist as the mask. Detailed descriptions of these transfer processes are given in Chapter 3.
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Introducing Optical Lithography