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Chapter 8:
Microlithographic Projection Optics
Microlithographic projection optics image a pattern on a reticle, or mask, onto the surface of a silicon wafer. This pattern selectively exposes a photoresist on the wafer, which is subsequently etched to form one of many layers that build the structure of a microchip. The image on the wafer may be either the same size as the reticle, or a reduced copy of it, depending on the magnification of the projection lens. The minimum feature size that can be resolved is given by a modified form of Rayleigh's equation: Linewidth=k1λNA. This differs from the formula for microscopy because microlithography is primarily concerned with the imaging of extended patterns of lines, rather than adjacent points. The empirical variable k1 depends on the details of the lithographic process, such as photoresist, illumination conditions, periodicity of the lines, and whether the imaged features include phase objects. In the early decades of microlithography, it had a lower limit of 0.8. However, lithographic technology in the 1990s has been able to utilize lower image contrasts, allowing a reduction of k1 toward 0.25.
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