The principle of the ENFOL technique is illustrated in Fig. 17.2. A conformable membrane mask is held in intimate contact with a photoresist-coated substrate, such that the mask-substrate gap g (including the photoresist thickness) is much less than the wavelength of the exposing light source. Under these conditions the optical field in the thin resist contains high spatial frequency evanescent components together with propagating diffracted components. The presence of these high frequency components makes resolution beyond the diffraction limit of projection lithography possible.
As can be seen from Fig. 17.2, the two main requirements for ENFOL are intimate mask-substrate contact over a large area and ultra-thin (sub-100 nm) resists. Using a conformable mask held in contact with the resist by external pressure or the use of a vacuum can achieve the former, however the exact requirements for the resist thickness are not easy to determine. Trade-offs exist between the required resolution and the ease of performing pattern transfer with the resist after exposure and development. The process latitude for the exposure can also be severely degraded if the resist becomes too thin.
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