Smaller chips? Just add water

Researchers at the Rochester Institute of Technology (RIT) are refining a method that uses the current generation of chip-making tools to produce smaller chips than previously thought possible with those tools. The key ingredient happens to be one of the most commonly available substances on the planet.

By running a small stream of water across a silicon wafer as the circuit lines are being etched into the chip, the current generation of lithography tools can produce circuits down to the 45 nanometer level due to the refractive properties of water, said Bruce Smith, professor of microelectronics engineering and associate dean for graduate studies at RIT, in Rochester, New York.

Lithography tools are used to draw the lines of a circuit on to a semiconductor wafer. The current generation of tools use wavelengths of light that measure 193 nanometers long to produce 90 nanometer chips such as Intel's Prescott Pentium 4 processor and IBM's PowerPC 970FX.

Microscopes have taken advantage of the refractive properties of liquids to improve the resolution of specimens for over 100 years, Smith said. A material's refractive property refers to its ability to bend light.

Oily liquids have been used in biological labs for this purpose, and many researchers have postulated that certain liquids could be used for the practice of immersion lithography, Smith said. Lithography is basically the reverse of microscopy, in that the idea is to take a large image and shrink it. However, the necessity for cleanliness in a semiconductor manufacturing environment eliminated many liquids from consideration, Smith said.

Water turned out to be an ideal liquid because it does not produce a reaction with many commonly used semiconductor manufacturing materials and liquids, and because its refractive properties peak at the same wavelengths currently used by etching tools in the manufacturing process, Smith said.

Most semiconductor researchers had thought that tools with smaller wavelengths were necessary to produce chips for the 65 nanometer process generation and below, Smith said. Each time the industry moves to a new lithography tool, it must go through an expensive and time-consuming process to ensure that the new tools will work with existing materials and to iron out any problems that arise.

Chip makers will need to invest in new tools that can deliver the water in a steady stream and purification systems to ensure the water does not contaminate the silicon wafer, said Peter Silverman, an Intel fellow and director of lithography capital equipment at the Santa Clara, Calif., company. But they will still be able to use the same generation of lasers and optical equipment, he said.

Several challenges remain, such as the necessity to eliminate defects in the manufacturing process, said Michael Lercel, manager of the advanced imaging department at IBM. At this point, there simply isn't enough data about immersion lithography to know exactly what needs to be done to take this technique out of the lab and into manufacturing plants, he said.

Intel and IBM are waiting for immersion lithography research tools due out by the end of this year from vendors such as Nikon Precision, Canon and ASML Lithography Holding, both researchers said. The companies will be able to conduct experiments with immersion lithography during the first part of 2005, and decide if and how it will fit into their future manufacturing plans.