Design of an aspheric mirror for synchrotron radiation X-ray lithography beamline

Abstract

The illumination system for synchrotron radiation X-ray lithography consists of a storage ring to produce the X-rays, and a beamline to transport the X-rays to the exposure stage. Because the wavelengths are short, only grazing incident mirrors are used in the beamlines. The beamline optics need to deliver a beam with high transmission in the X-ray region. A condenser is a key component in the X-ray lithography beamline. For a scanning X-ray lithography system, the condenser is used to focus X-rays vertically while collimating horizontally. The lack of symmetry in grazing incident optics results in large aberrations if simple conical mirrors are used. In the past, we have used two toroidal mirrors to compensate each other's aberrations. Here we present the design for a novel aspherical mirror. It is calculated using numerical methods and described by polynomials. Ray-tracing with a finite synchrotron radiation source shows that the novel mirror shape gives superior performance in the synchrotron radiation collecting angle, and beam uniformity at the exposure stage. The design method is useful in designing aspheric surfaces in other beamlines.