A little light magic [optical lithography]

At first glance, it appears that optical lithography has hit a dead end. Wavelengths shorter than 193 nm can't be used without a drastic redesign of lithographic systems because the shorter wavelengths are simply absorbed by the quartz lenses that direct the light onto the wafer. So is this the end? Not quite. There are a few more tricks that IC manufacturers can play. Lumped together, they are called resolution enhancement techniques (RETs), and in one way or another, they all coax the light into resolving shapes much smaller than its wavelength. The main techniques are optical proximity correction, phase-shifting masks, and modified, or "off-axis," illumination. With these tricks, the existing-and already paid for-optical lithography equipment can create patterns much smaller than the wavelength of the light used to produce them, these techniques can be easily extended to one-eighth the wavelength of the light-that is, to less than 25 nm for 193 nm light. Using these techniques, scientists at the Massachusetts Institute of Technology's (MIT's) Lincoln Laboratory, in Lexington, have built prototype transistors with a gate length of only 9 run-smaller than the smallest virus. Other tricks can also come into play. For example, by immersing the focusing lenses in a liquid with an index of refraction greater than that of air, optical lithography may be extended indefinitely. Ever since the 180 nm generation of devices, in 1999, RETs are making advanced optical lithography possible.