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Immersion Lithography
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Virtually all advanced microelectronics devices are fabricated using projection lithography, shown
schematically in Figure 1. In this technique, a light image of the desired pattern, transmitted through a
mask, is reduced in size and precisely focused onto a resist-coated wafer using a system of lenses. Due to
diffraction and slight imperfections in the optical components, a nominally square wave pattern of light
intensity is presented as a sinusoidal pattern of light at the wafer plane. The minimum resolution Wmin
achievable with projection lithography is governed by the equation
Wmin = k1 λ/NA
where λ is the wavelength of exposing radiation, k1 is a process- and material-dependent parameter less
than unity, and NA is the numerical aperture, equal to the refractive index of the surrounding medium
(~1 for air) times the sine of the angle q subtended by the objective lens of the system. Either the
wavelength must be decreased or the NA of the system increased to improve tool resolution. In the most
advanced production exposure systems, the exposure wavelength is 193 nm and the NA is approaching 1, the
fundamental limit for imaging in air. However, if a fluid with a higher refractive index is interposed
between resist film and the objective lens, this limit is eased, paving the way to improved resolution.
There is today an intensive industry effort to adapt 193 nm projection exposure tools to such immersion
imaging, with the goals of improved process control and ultimately improved resolution compared to dry
imaging systems.
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| Resist Development for Immersion Lithography
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| | A Testbed for 193 nm Interferometric Immersion Lithography
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