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In photolithography, immersion lithography is a resolution enhancement technique that interposes a liquid medium between the optics and the wafer surface, replacing the usual air gap. This liquid has a refractive index greater than one. With the 193 nm wavelength, the typical liquid used is ultra-pure, degassed water. Immersion lithography increases the effective depth-of-focus for a given numerical aperture and permits the use of optics with numerical apertures above 1.0, thus raising the maximum resolution potential of extant wavelength technologies. For all numerical apertures, the immersion fluid confers the advantage of reducing reflections by virtue of reducing refractive index differences.

As of 2005, it is expected that immersion lithography at the 193 nm wavelength will be used in 2009 to print 45 nm lines and spaces . Following its aggressive introduction, it is speculated that enhancements will be used to prolong the use of the technology to smaller features. Such enhancements include the use of higher refractive index materials in the final lens, immersion fluid, and photoresist. Each of these materials puts a limit on the largest angle that the light makes with the optical axis normal to the image plane. For example, using LaF3 with a refractive index n=1.67 as the final lens material, along with a recently demonstrated immersion fluid with refractive index n=1.64, should enable 32 nm lines and spaces. Alternatively, by doping the water, a numerical aperture of 1.5 can be easily reached. This is the minimum value needed for the same 32 nm line-space resolution.

As numerical apertures increase, the degree of polarization of the light becomes critical to the image quality. This is because the interference of light inside the photoresist becomes polarization-dependent. Specifically, the imaging of straight lines near the resolution limit is best done with light polarized parallel to the lines. This requires special illumination preparation which is available on the most advanced lithography systems. Imaging of islands or holes is more problematic unless the holes or islands are closely spaced near the resolution limit in one direction and widely spaced in the orthogonal direction; otherwise, polarization effects will be a hindrance rather than a benefit. Due to these imaging constraints, integrated circuit (IC) layouts utilizing dimensions near the resolution limit will be required to be 'lithography-friendly'.

For immersion lithography, as the minimum resolvable half-pitch linewidth decreases, features on the photomask will eventually approach subwavelength sizes. Subwavelength features no longer obey the laws of classical imaging optics but need to be rigorously analyzed using electromagnetic theory (see for example, ). One way to delay this outcome would be to increase the magnification of the photomask image relative to the wafer image.

Ultimately, the upper limit of the numerical aperture will be the refractive index of the photoresist. At this theoretical point, there would be light traveling parallel to the photoresist surface on the wafer. For a wavelength of 193 nm and a refractive index of 1.7, this would correspond to 28 nm lines and spaces.

Once the maximum numerical aperture is reached, the only way immersion lithography can print denser features at the same wavelength would be to split a dense layer into two looser layers .

The other choice is to use shorter wavelengths, e.g., 157 nm. However, it would be necessary to consider the mechanisms by which these wavelengths interact with the materials used in photolithography. Absorption is greatly enhanced, and the ionization potential is exceeded. The success of 157 nm immersion, for example, would depend on the use of materials with minimal absorption. Water itself is not transparent enough to serve as an immersion fluid for this wavelength. It also is important to make sure that when the wavelength is divided by the refractive index, it gives a sufficiently smaller value than 118 nm, which is the 193 nm wavelength divided by the refractive index of the next-generation immersion fluid at that wavelength (n=1.64). This is the measure of the degree of resolution enhancement.

Other considerations which are important to immersion lithography systems are the elimination of bubbles in the immersion fluid, temperature and pressure variations in the immersion fluid, and immersion fluid absorption by the photoresist. Degassing the fluid, carefully constraining the fluid thermodynamics and carefully treating the top layer of photoresist are key to the implementation of immersion lithography.

References

1. M. LaPedus, "Litho race," EE Times, October 21, 2005.
2. D. Ristau et. al., Appl. Opt. 41, pp. 3196-3204 (2002).
3. A. Hand, "High-Index Fluids Look to 2nd-Generation Immersion," Semiconductor International, April 1, 2005.
4. B. W. Smith et. al., Proc. SPIE 5377, pp. 273-284 (2004).
5. C-W. Chang et. al., Laser Physics Letters 2, pp. 351-355 (2005).
6. G. Vandenberghe, "How Optical Lithography Prints a 32 nm Node 6T-SRAM Cell," Semiconductor International, June 1, 2005.
7. M. Switkes et. al., J. Vac. Sci. & Tech. B 21, pp. 2794-2799 (2003).

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