Demagnification-by-bias in proximity X-ray lithography

Abstract

The ability to produce fine features using X-ray proximity lithography is controlled predominantly by diffraction and photoelectron blur. The diffraction manifests itself as feature `bias'. The classical approach is to attempt to minimize the bias; that is, to print features which are 1:1 images of those on the mask. However, bias can also be exploited to print features smaller than those on the mask. This demagnification-by-bias technique can be optimized with respect to mask-wafer gap and resist processing, and can provide reductions of 3× to 6×. Demagnification offers many of the same advantages as projection optical lithography in terms of critical dimension control: relaxed mask features CD. In addition, it provides a very large `depth of focus' and wide dose latitude. In consequence proximity X-ray lithography is extendible to feature sizes below 25 nm, taking advantage of comparatively large mask features (>0.1 nm) and large gaps (10-25 μm). The method was demonstrated for demagnification values down to ×3.5. To produce DRAM half-pitch fine features techniques such as multiple exposures with a single development step are proposed.