Proximity-effect correction for EPL by using multiple pattern-area-density maps and pattern classification

摘要

In electron-beam projection lithography (EPL), the proximity effect is more complicated than in conventional direct electron-beam writing. The correction of this effect uses pattern-shape modification because dose adjustment is not possible. Moreover, large sub-field transcription produces non-uniform beam blur due to optical aberration and local and global Coulomb effects. This large beam blur requires sophisticated correction depending on pattern features and layout over a very short range as well as pattern density over a backscattering range. In response to these issues, the authors have developed a flexible and precise correction method for the proximity effect under various conditions of beam blur, LSI process, and pattern arrangement. It features (1) multiple pattern-area-density maps, (2) look-up tables classified by pattern features and layout, and (3) a fast calculation algorithm for the iteration process of optimal bias. The developed method (running on four PCs with 2.4-GHz CPUs) attains a processing time of 55 min by using a bias map for 12-GB LSI flat data in 2000 sub-fields. An example of pattern classification by this method showed the usefulness of pattern bias for each individual pattern edge over a short range. It is concluded that the developed correction method is useful not only for proximity effects, including the Coulomb effect, but also for various process effects in mask making with precise CD control.