The absence of obstructions is an essential requirement for reflective ring-field projection systems to be used in EUV lithography. However, due to the large number of variables and constraints involved, choosing unobstructed starting configurations for subsequent optimization is a nontrivial issue in EUV system design. For this purpose we have developed a systematic method based on paraxial obstruction analysis. Despite the fact that in the parameter space of all possible system configurations unobstructed domains are very small, we can identify the unobstructed regions very effectively. The paraxial results are validated through comparison with ray tracing. We can convert even most of the paraxial solutions that lead to ray failure into ray-traceable starting points for optimization. We also introduce a new classification method based on the relative arrangement of the mirrors in a EUV system - a feature which in typical situations is unaffected by optimization. We present some new possible design forms for EUV imaging systems belonging to different classes and show the remarkable flexibility of the obscuration borders.

Date Published: 20 December 2001
PDF: 7 pages
Proc. SPIE 4506, Soft X-Ray and EUV Imaging Systems II, (20 December 2001); doi: 10.1117/12.450960

Published in SPIE Proceedings Vol. 4506:
Soft X-Ray and EUV Imaging Systems II
Daniel A. Tichenor; James A. Folta, Editor(s)