Ion projection lithography

In spite of the comparatively modest level of effort devoted to ion projection lithography (IPL), the results obtained so far indicate that the technology is highly promising. Accordingly, a $36M program has been launched in Europe (MEDEA) to develop a full field, LPL process tool. In ion projection lithography a stencil mask, consisting of a Si membrane with a fine pattern of holes, is "illuminated" with a uniform beam of hydrogen or helium ions at about 10 kV. The image of this mask is projected through an ion optical column and demagnified by 4x on the wafer to expose a full die with energies of similar to 75 keV. Ions are very efficient at exposing resists so that the exposure time per die is well under a second, even for comparatively insensitive (1 mu C/cm(2)) resists. Well formed lines down to 80 MI have been exposed in 370 nm thick very sensitive (0.16 mu C/cm(2)) resists. Comparable features have been exposed in a number of high contrast, common resists. The IPL stepper will be similar to an optical stepper and will be suitable for "mix-and-match" insertion into an optical fab. Ion lithography has negligible proximity effects. Diffraction is also totally negligible (wavelength similar to 10(-4) nm), permitting a very large depth of focus(similar to 100 mu m). The ion lenses in the optical column are "tunable" so that the image can be servo-locked on to special alignment marks during exposure, and it can be adjusted when the machine is set up by imaging a special metrology plate placed at the position of the wafer. The principal challenge that the building of the next tool will address is overlay. The main inherent factors affecting overlay in IPL are: uncorrected distortion in the lenses, non-uniform global distortion due to ion-ion Coulomb repulsion, and distortion in the mask. The experiments and the modeling done so far both on the ion columns and the masks indicate that the distortions will be within tolerable limits. The tool that is being designed and built by the European program is expected to have a throughput of up to 85, 300 mm wafers/hour.