Saddle Toroid Arrays: Novel Grazing Incidence Optics for Synchrotron X-Ray Lithography

Abstract

Replacing the scanning mirror in a synchrotron lithography beamline with a fixed saddle-toroid array mirror will give uniform illumination over wafer-size fields, without the problem of moving parts in the high vacuum section of the beamline, the constraints of oscillating the electron beam in the storage ring or the engineering difficulties of moving the entire wafer alignment system. Ray tracing results for one and two saddle-toroid systems are presented, showing that a single saddle-toroid mirror provides a satisfactory alternative to the scanning cylindrical mirrors used in existing lithography beamlines, if the input beam is uniform. A two element system, comprising a saddle-toroid and a cylindrical mirror, can be used to give a nearly rectangular patch of illumination, closely matched to the needs of lithography. However the total usable flux through the such a device is lower, because of the extra reflection. Both designs require a uniform input beam and are not suitable for the gaussian profiled synchrotron beam. By using the techniques developed to produce micro-lens arrays, saddle-toroid arrays can be made which spatially average the beam. These provide outputs that are uniform, even from a synchrotron beam. Though designed with storage ring x-ray lithography in mind, saddle-toroid arrays may have applications in expanding undulator and free-electron laser output. Patents are pending.