Immersion Lithography

Scott Hafeman
(Professor Andrew R. Neureuther)

Immersion lithography offers the capability to further reduce the resolution of printed features. For example, placing deionized water (n=1.44) between the projection optics and the wafer, the current 193 nm stepper could achieve the same resolution as a 157 nm stepper while improving the depth of focus by 8%. Immersion liquids such as Fomblin pump-oil (n=1.36) and other liquids have been examined for suitability in 157 nm steppers [1]. The immersion layer also allows larger amounts of energy to be coupled into the resist due to a lower reflection coefficient and lower effective NA.

This research focuses on quantifying potential problems by performing analytical estimation and simulation. Intrinsic characteristics of the liquid likely to contribute to aerial image degradation include homogeneity and reactivity with optical and resist surfaces. Liquid dynamic effects such as local heating, relative motion of the liquid and projection lens, convection heating, and resist outgassing are being investigated.

The high resolution and superior coupling of light into photoresist are being investigated through generalizing the thin-film formulation developed by Michael Yeung for SPLAT [2]. This approach consists of modeling thin-film effects through viewing them as amplitude and phase effects in the lens pupil.

M. Switkes and M. Rothchild, "Resoultion Enhancement of 157 nm Lithography by Liquid Immersion," SPIE, Vol. 4691., 2002.
M. S. Yeung, D. Lee, R. Lee, and A. R. Neureuther, "Extension of the Hopkins' Theory of Partially Coherent Imaging to Include Thin-Film Interference Effects," SPIE, Vol. 1927, 1993.

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