Design and performance of an x-ray lithography beam line at a storage ring

Abstract

A beam line for x-ray lithography using synchrotron radiation (SR) has been built at the VUV storage ring of the National Synchrotron Light Source and is now in operation. The high intensity and good collimation of SR overcome the problems of low throughput and resolution-limiting penumbra associated with using conventional x-ray sources. The design of a beam line for SR has to deal with two problems. First, the beam line must meet the ultrahigh vacuum standards of the storage ring, and second, the emitted radiation is horizontally uniform but very nonuniform vertically. The beam line optics consist of an oscillating cylindrical mirror that scans the reflected light vertically over the exposure area and collimates the incident radiation horizontally, thus increasing the intensity by a factor of 3. We will describe the vacuum system, the beryllium window that separates the exposure chamber from the beam line, and the vacuum protection of the ring. We will show the measured uniformity of illumination. The energy density incident onto the resist as well as the energy absorbed in the resist have been determined and will be compared with the calculated power distribution. First exposures show that features down to 0.25 μm can be copied into PMMA with an aspect ratio of 10 to 1 using a 40 μm mask-to-wafer separation.