Abstract

Extreme ultraviolet projection lithography operating at a wavelength of 13nm is based on all reflective imaging systems made up of three to five multilayer coated mirrors. If these imaging systems are to achieve diffraction limited performance, the deviation of the wavefront from spherical in the exit pupil must satisfy the Maréchal condition of λ/14 rms where λ is the operating wavelength. The wavefront error must therefore be less than l.0nm rms. If the imaging system is made up of four mirrors, the error contribution from each mirror is then less than 0.5nm rms (assuming uncorrelated errors) which means the surface figure must be less than 0.25nm rms (due to the doubling of the error on reflection).

© 1996 Optical Society of America