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X-ray lithography was invented in the early 70s and, almost 25 years later, has not yet reached a manufacturing stage. Clearly, the cause of this delay is the explosive development of optical lithography. Optical lithography has however exhausted its initial “resolution reserve” (i.e., the distance from the diffraction limit) that it originally enjoyed: several generations of devices were manufactured with H-line and I-line tools, but only one or at most two will be fabricated using 248 nm (DUV) before the move to 193 nm become necessary; it is unlikely that 193 nm will be used beyond the 4 Gbit DRAM. After 193 nm the horizon is dark. In parallel, X-ray lithography has continued to evolve, trying to reach a continuously changing target. With the 100% accuracy of hindsight, we can say that it would have been better to concentrate the development efforts on the most advanced target (1 Gbit and beyond) rather than playing catch-up to the faster moving optical efforts. This notwithstanding, today the tooling for X-ray lithography has reached significant levels; it is the only technology that can support multi-generation of chips fabrication, all the way to the production of nanostructures.
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