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Moore¿s Law holds that every 18 months, semiconductor companies can put twice the amount of transistors on a microchip. But each time the industry squeezes in more transistors, the process becomes more costly and difficult. The challenges are so great that Moore¿s Law is threatened because of soaring costs and the inability of the technology to make smaller circuit features. Photolithography, today¿s standard for fabricating microchips, has difficulty making features any smaller than 130 nanometers on a side. The technique focuses an image of the circuits on a chip onto a photosensitive polymer. Then either the exposed or unexposed regions of the polymer are etched away. At these dimensions, however, photolithography has trouble resolving the tiny circuit elements.
Several alternatives to photolithography have been proposed. Stephen Chou and colleagues from Princeton University have designed one new candidate, called LADI, or laser-assisted direct imprint, that may be able to print faster and more cheaply at a smaller scale. The technique, described today in the journal Nature, uses a printing mask made of quartz that contains the raised features desired on the silicon chip. The mask first contacts the silicon surface. Then a laser fires through the clear printing mask, which temporarily melts the surface of the silicon, allowing the surface to take on the complementary image of the mask. The laser is then switched off and the mask is lifted off of the silicon, where a permanent shape remains. This process imprints features as small as 10 nanometers in a few seconds. By one estimate, it would cost only about 50 cents a chip for an area of 100 square centimeters. The LADI process avoids many of the steps required in conventional chip making and does not require use of polluting chemicals. It still has some drawbacks, however. For instance, it hasn't been demonstrated that it can imprint multiple layers on a chip. LADI has nonetheless received good reviews. In a commentary accompanying the Nature report, R. Fabian Pease of Stanford University¿s department of electrical engineering remarks that "on grounds of cost, speed and resolution, LADI, or some other form of mechanical printing, may displace optical projection as the preferred manufacturing technology for fashioning silicon chips."
