Fabrication of a complex-shaped mirror for an extreme ultraviolet lithography illumination system

We propose and discuss several fabrication processes for a complex-shaped mirror, which is a fly-eye mirror, used in an extreme ultraviolet lithography (EUVL) illumination system. The mirror has a complex reflective surface consisting of many concave mirror elements that are sections of a sphere; the top of each element is arc shaped. In the present study, we focus on one process in which all elements are fabricated individually and then arranged side-by-side to form the mirror. Thus, as the first step in this process, we fabricate the arc-shaped elements made of invar with electroless nickel plating. The resultant reflective surfaces have a peak-to-valley (PV) surface accuracy of 0.31 mum. The surfaces have the rms roughnesses of about 0.23 and 0.35 nm in areas of 110x 140 mum and 1 x 1 mum, respectively. The slope accuracies of the surfaces relative to the bottom surfaces are - 166 and 43 arc-sec in the y and x directions, respectively. Thus, the mirror elements for the fly-eye mirror can be fabricated very accurately with smooth surfaces, although the mirror elements have a special shape compared to that of general optics. (C) 2003 Society of Photo-Optical Instrumentation Engineers.