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Revision as of 02:37, 28 November 2005 by Guiding light (talk | contribs) (change stub from photo to tech (different from photography))(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)In photolithography, immersion lithography is a variant technique that interposes a liquid medium between the optics and the wafer surface, replacing the usual air gap. This liquid will have a refractive index greater than 1. 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 technology 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.
An important consideration for numerical apertures approaching and exceeding unity is that the 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 immersion 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, ).
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 .
Aside from numerical aperture, 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 therefore key to the implementation of immersion lithography.
References:
1. M. LaPedus, "Litho race," EE Times, October 21, 2005.
2. C-W. Chang et. al., Laser Physics Letters 2, pp. 351-355 (2005).
3. G. Vandenberghe, "How Optical Lithography Prints a 32 nm Node 6T-SRAM Cell," Semiconductor International, June 1, 2005.
4. M. Switkes et. al., J. Vac. Sci. & Tech. B 21, pp. 2794-2799 (2003).
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