Of the many factors affecting the x-ray intensity distribution, the variables that can be controlled are the source spectrum, the proximity gap, the source spatial coherence, the mask linewidth bias, and the absorber thickness. To obtain the highest quality aerial image, all of these parameters must be optimized simultaneously. An, optimization of the spectrum of the synchrotron Helios, located at IBM's Advanced Lithography Facility is described. The optimum parameter space for proximity x-ray lithography at 0.1 mum minimum linewidth is then determined using the optimized spectrum by adjusting the free parameters. For maximum accuracy, a rigorous electromagnetic model that accounts for the dielectric properties of the absorber, the source partial coherence, and diffraction in the proximity gap is used to calculate the x-ray aerial image at the wafer. Descriptive figures-of-merit (FOMs) of the aerial image are the image contrast [(I(max) - I(min))/(I(max) + I(min)] and the exposure latitude. These two FOMs are maximized with respect to source spectrum, gap, source spatial coherence, feature size and bias, and mask absorber thickness. The global maximum of these FOMs is coarsely located in parameter space by determining the dependence of the FOMs on two variables at a time. The feature bias is then determined so that all feature types (gratings, lines, spaces) can be printed at the same dose with maximum average contrast and exposure latitude.
