Aspheric collimator for a point source x-ray lithography system

Abstract

X-ray point sources are an interesting alternative to synchrotrons for small to medium scale production. These sources are by nature highly divergent, and thus require the use of collimation for delivering an acceptable lithographic illumination. We present the design of an aspheric collimator for a point source such as a dense plasma focus system. Designing a collimating mirror for a point source presents different challenges than designing one for a synchrotron source, although both cases require that the radiation be condensed and collimated to deliver the radiation with 1%-2% uniformity and acceptable runout over a large field area. The collimator is 3 cm wide and 31 cm long, accepts 20 mrad in the vertical and 70 mrad in the horizontal, and delivers the x-ray flux to a 3 cmϫ3 cm field on a mask located 3 m from the source with a runout of 6 mrad. The figure is an asphere, not dissimilar to those developed previously. The scanning mechanism, however, is radically different, since the virtual rotation point is located at the source, rather than at the mirror pole. This design provides excellent uniformity since the mirror is always scanned inside the radiation cone while maintaining a constant nominal incidence angle. The design principles and predicted performance are discussed in detail, together with a consideration of the manufacturing challenge for such a mirror. The collimator design presented has the capability of delivering the required power density and collimation from an x-ray point source to satisfy the requirements of x-ray lithography.