Demagnification in proximity X-ray lithography and extensibility to 25 nm by optimizing Fresnel diffraction

Abstract

This new understanding and demonstration of features printed by proximity x-ray lithography allows a revolutionary extension and simplification of otherwise established processes for microfabrication. The ability to produce fine features is controlled predominantly by diffraction and photoelectron blur. The diffraction manifests itself as feature `bias'. In the classical approach the bias is minimized. Bias optimization in terms of mask/wafer gap and resist processing allows the formation, on a wafer, of features smaller than those on the mask: thus producing local `demagnification'. This demagnification (x3-x6) is achieved without lenses or mirrors, but it offers the same advantages as projection optical lithography in terms of critical dimension control. The photoelectron blur is more or less pronounced depending on exposure dose and development conditions. Resist exposure and process can be optimized to utilize a approximately 50% photoelectron energy loss range. In consequence proximity x-ray lithography is extensible to feature sizes below 25 nm, taking advantage of comparatively large mask features (>100 nm) and large gaps (30-15 μm). The method is demonstrated for demagnification values down to x3.5. To produce DRAM half-pitch fine features, techniques such as multiple exposures with a single development step are proposed.