Nanoscale Fabrication       

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Nanoscale Fabrication - Immersion Lithography


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.

Schematic diagram of the optics of a projection exposure lithography system