Immersion lithography

Immersion lithography is a photolithography resolution enhancement technique for manufacturing integrated circuits (ICs) that replaces the usual air gap between the final lens and the wafer surface with a liquid medium that has a refractive index greater than one. The resolution is increased by a factor equal to the refractive index of the liquid. Current immersion lithography tools use highly purified water for this liquid, achieving feature sizes below 45 nanometers.ASML, Canon, and Nikon are currently the only manufacturers of immersion lithography systems. The idea for Immersion lithography was first proposed and realized in the 1980s.

The ability to resolve features in optical lithography is directly related to the numerical aperture of the imaging equipment, the numerical aperture being the sine of the maximum refraction angle multiplied by the refractive index of the medium through which the light travels. The lenses in the highest resolution "dry" photolithography scanners focus light in a cone whose boundary is nearly parallel to the wafer surface. As it is impossible to increase resolution by further refraction, additional resolution is obtained by inserting an immersion medium with a higher index of refraction between the lens and the wafer. The blurriness is reduced by a factor equal to the refractive index of the medium. For example, for water immersion using ultraviolet light at 193 nm wavelength, the index of refraction is 1.44.

The resolution enhancement from immersion lithography is about 30-40% (depending on materials used). However, the depth of focus, or tolerance in wafer topography flatness, is reduced compared to the corresponding "dry" tool at the same resolution.

The successful emergence of immersion lithography comes not just from its ability to extend resolution and depth of focus, but also from its timely introduction to the industry (e.g., IBM, AMD) between 65 nm and 45 nm nodes.

Intel's 32 nm process uses second-generation high-k, metal gate technology, but this will be the first time Intel has deployed immersion lithography.

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