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Chapter 2:
Optical Imaging and Resolution
The requirements of large field size, low aberrations, and wafer-specific adjustment necessitate complex optical systems in photolithography [34–37]. An example of a system is shown in Fig. 2.1. Such systems may consist of more than 40 lens elements, and the whole assembly can weigh more than six tons. Exact analysis of their operation is complicated, but the basic imaging process can be approximated by the schematic in Fig. 2.2, which is slightly modified from Fig. 1.21. Let us begin by examining coherent imaging with a monochromatic point source. With Kohler's illumination method incident light uniformly illuminates the mask. Because of the Fourier transforming property of lenses [2], energy transmitted through the photomask forms a distribution in the pupil plane that is proportional to the mask spectrum. Low-spatial-frequency components pass closer to the center of the pupil, whereas higher-frequency components are nearer the peripheral of the pupil. The highest frequencies are cut off by the pupil. The unblocked frequency components are combined at the wafer surface to form the reticle image.
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