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Revision as of 15:23, 3 December 2005 by Guiding light (talk | contribs) (Described theoretical upper limit on numerical aperture a bit more)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)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.
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.
As numerical apertures increase, the degree of polarization of the light becomes critical to the image quality. 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.
Numerical aperture cannot be increased indefinitely, as features on the photomask 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 would be to split a dense layer into two looser layers .
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
- M. LaPedus, "Litho race," EE Times, October 21, 2005.
- D. Ristau et. al., Appl. Opt. 41, pp. 3196-3204 (2002).
- A. Hand, "High-Index Fluids Look to 2nd-Generation Immersion," Semiconductor International, April 1, 2005.
- C-W. Chang et. al., Laser Physics Letters 2, pp. 351-355 (2005).
- G. Vandenberghe, "How Optical Lithography Prints a 32 nm Node 6T-SRAM Cell," Semiconductor International, June 1, 2005.
- M. Switkes et. al., J. Vac. Sci. & Tech. B 21, pp. 2794-2799 (2003).
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