Optical Technology for EUV Lithography

A high-throughput optical system that has a bouncing number of only four, including a reflective mask, was designed. The illumination system is comprised of a single tilted-ellipsoidal mirror, that is suitable for a two-aspherical-mirror ring-field imaging system. A prototype optical system was evaluated using synchrotron radiation at the Photon Factory. The experimental results combined with calculations have indicated that a throughput of 15 to 20 wafers/h can be obtained by using the optical system in combination with a compact storage ring. In addition, 0.12-μm line-and-space patterns were delineated in parts of the ring field. The use of alternating and attenuated reflective phase-shifting masks is also discussed to enhance the resolution of EUV lithography.