Abstract

The design aspects of candidate reflective optical systems for projection lithography using extreme-ultraviolet (XUV) exposure wavelengths from approximately 10 to 20 nm have been discussed by various authors.^1-3 The very demanding requirements for XUV-projection systems include the ability to resolve 0.1-μm features over at least a 25 mm x 25 mm image field with less than 0.01-μm distortion while providing a minimum of 1-μm total depth of focus. Other practical considerations affecting system imaging capability include thermal distortion of the mirror surfaces due to absorption of a fraction of the incident radiation beam and alignment tolerances. We present our analyses of the magnitude of these latter two effects on two types of reflective projection optical designs. We use the usual criteria for manufacturing and alignment errors⁴ and Zernike polynomials⁵ to represent the thermal and other distortions induced by the incident flux. We find that concentric, symmetric two-mirror systems² are significantly less sensitive to thermal and alignment errors than off-axis, four-mirror systems.

© 1992 Optical Society of America