For more than three decades, the number of transistors on a chip has grown exponentially, doubling on the average of every 18 months. With each new technology generation, the role of lithography has increased in importance not only because of the requirements for smaller feature sizes and tighter overlay, but also because of the increasing costs of lithography tools. Optical projection lithography and its extensions, e.g., water immersion, are expected to remain the lithographic technologies of choice until at least 2010. Extreme ultraviolet lithography (EUVL) extends optical lithography to a higher resolution and provides a larger depth of focus because it utilizes a shorter imaging wavelength (13.5 nm versus 193-248 nm) and employs a smaller numerical aperture (NA) imaging system (0.25-0.45 NA versus 0.93-1.35 NA). This chapter recounts the early years of EUVL development, from the first imaging with normal incidence multilayer (ML)-coated mirrors in 1981 to the beginning of EUVL commercialization efforts at the end of 1996.
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