SIMULTANEOUS-OPTIMIZATION OF SPECTRUM, SPATIAL COHERENCE, GAP, FEATURE BIAS, AND ABSORBER THICKNESS IN SYNCHROTRON-BASED X-RAY-LITHOGRAPHY

Of the many factors affecting the x-ray intensity distribution, the va riables that can be controlled are the source spectrum, the proximity gap, the source spatial coherence, the mask linewidth bias, and the ab sorber thickness. To obtain the highest quality aerial image, all of t hese parameters must be optimized simultaneously. An, optimization of the spectrum of the synchrotron Helios, located at IBM's Advanced Lith ography Facility is described. The optimum parameter space for proximi ty x-ray lithography at 0.1 mum minimum linewidth is then determined u sing the optimized spectrum by adjusting the free parameters. For maxi mum accuracy, a rigorous electromagnetic model that accounts for the d ielectric properties of the absorber, the source partial coherence, an d diffraction in the proximity gap is used to calculate the x-ray aeri al image at the wafer. Descriptive figures-of-merit (FOMs) of the aeri al image are the image contrast [(I(max) - I(min))/(I(max) + I(min)] a nd 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 coa rsely located in parameter space by determining the dependence of the FOMs on two variables at a time. The feature bias is then determined s o that all feature types (gratings, lines, spaces) can be printed at t he same dose with maximum average contrast and exposure latitude.