Immersion Lithography Theory

Immersion lithography incorporates a high refractive index liquid into the gap between the final lens element of an exposure tool and the photoresist film.  This high index liquid would allow the NA to be increased above the previously accepted theoretical limit of NA = 1 because of Snell’s Law of Refraction,
where nlens is the index of refraction of the lens, qlens is the angle at which the light is traveling through the lens and NA is the numerical aperture in the Rayleigh Equation.  Lithographic exposures are done in air, for which n = 1; so the maximum achievable value for the lens NA is the maximum value of sin q, which is also one.  By increasing the value of n through the use of an immersion fluid, larger lenses with the appropriate lens would enable imaging of smaller features than could be achieved with a “dry” system.  Light that would usually be lost to total internal reflectance in the lens would now be focused into the photoresist.  Also, an immediate benefit of improved focus could be realized without new lenses.  Currently, water is the best candidate for a first generation immersion fluid because of its high index, n = 1.435, and low absorbance, A = 0.036 cm-1, at 193 nm.[1][2]
Ray tracing of exposure radiation through film stack representing lens, immersion medium and photoresist showing improved resolution and focus.
For more details >> Continue to Resist Extraction Measurement


1. Burnett, J. H.; Kaplan, S. Proc. SPIE 2003, 5040, 1742.
2. Switkes, M.; Rothschild, M. Proc. SPIE 2002, 4691, 459.