"Liquid lenses" may shrink feature sizes on microchips
New data from the National Institute of Standards and Technology (NIST) will assist in the design of optics for liquid immersion lithography, an old idea that recently has attracted new interest as a possible means of improving image resolution and thereby shrinking feature sizes of computer chips.
Conventional optical lithography has advanced sufficiently to achieve a resolution of 100 nanometers (billionths of a meter), but there are physical and technical limits to how much better it can get. By placing certain liquids between the final optical element and the silicon wafer, it may be possible to extend the resolution to 65 nanometers for state-of-the-art lithography using the 193-nanometer wavelength of light, or even 45 nanometers or below for future systems using the 157-nanometer wavelength.
A key characteristic of liquids to be used in immersion lithography is their refractive index, which affects how light bends as it crosses an interface, such as that between the liquid and a lens or a silicon wafer. Air has an index close to one. By contrast, water has a refractive index almost 50 percent higher. Placing this higher-index fluid between the lens and the silicon wafer reduces the resolution-limiting effects of diffraction, enabling imaging of smaller feature sizes.
NIST physicists recently presented preliminary measurements of the refractive index for high-purity water, considered the best candidate for a “liquid lens” at the 193-nanometer wavelength. They also determined that the index is very sensitive to temperature changes, a critical issue for optics design. A forthcoming paper describes the techniques used to make these technically difficult measurements.* Future work will focus on refining these measurements and collecting data on other liquids that may be useful in 157-nanometer lithography.
*Burnett, John H., and Simon Kaplan. In press. “Measurement of the refractive index and thermooptic coefficient of water near 193 nanometers.” To appear in Proceedings of SPIE Optical Microlithography XVI (2003) 5040-188.0
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