DIFFRACTION EFFECTS AND IMAGE BLURRING IN X-RAY PROXIMITY PRINTING

The influence of diffraction on the shape and size of features printed using x-ray proximity printing is reviewed, and the effect of image blurring on these results is described. Diffraction can alter the shape of a printed feature, and the systematic shape changes observed in resist images can be explained using simple scaling based on Fresnel diffraction. In addition, the linewidth change with exposure dose is independent of feature type and size, and depends only on the square root of the mask to wafer gap. The shape of printed features and the linewidth change with dose can be modified by smearing the aerial image at the wafer plane. This can be achieved by adding beam divergence or by varying the angle of incidence of the x-ray beam onto the mask (wobbling). A technique for incorporating wobble into an exposure system is described, and exposures of contact holes, spaces, lines, and line-space arrays using this technique are presented. For example, 0.35 mum square contact holes normally print diamond shaped at a 40 mum gap. However, the same contact holes are round in resist when 4 mrad of wobble is incorporated into the exposure. The linewidth change for a 10% increase in dose is 24 nm at a 40 mum gap with 5 mrad of wobble. This linewidth change with exposure dose is larger than the 20 nm measured for exposures at a 40 mum gap without wobble. Finally, wobbling during exposure can either increase or decrease the absolute linewidth of a feature in resist at a given exposure dose.