Assembly and alignment of three-aspherical-mirror optics for extreme ultraviolet projection lithography

We have assembled and aligned projection optics for extreme ultraviolet (EUV) lithography. The projection optics consists of three aspherical mirrors. First, the positions of the mirrors were coarsely adjusted using the side and back surface of the mirrors. Next, the mirrors were finely aligned to minimize the wavefront errors which were measured by an interferometer. The adjustable axes were selected according to the results of the analysis of the allowable error range. The compensation values for each adjustable axis were calculated by commercially available ray-tracing software. After the alignment procedure, the wavefront error of 3 nm RMS was achieved.