Ultralarge‐scale integrated circuits are patterned by optical lithography, which can now imprint linewidths narrower than 0.8 micron in commercial production. With some promising innovations, optical lithography may be able to push its limiting linewidths below 0.25 microns, narrow enough to produce the next two generations of circuits—16‐Mbit and 64‐Mbit dynamic random‐access memory chips. To cram still more electronic features onto a given area, as is envisioned for the manufacture of chips with 256 Mbits or more, will probably require wavelengths typical of soft x rays rather than visible or ultraviolet light. Thus for nearly 20 years researchers have been developing the sources, masks, aligners, resists and other components of x‐ray lithographic systems.

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