Synchrotrom radiation X-ray lithography

Abstract

X-ray lithography is a shadow printing technique which uses X-rays to transfer a pattern from a mask into a resist-coated wafer. Synchrotron radiation from a storage ring has advantages over the radiation from conventional X-ray sources for this purpose, since it is 2 to 4 orders of magnitude more intense and its natural collimation virtually eliminates penumbra and run-out errors. To take advantage of synchrotron radiation, we have built a beamline for X-ray lithography at the VUV storage ring of the NSLS. This beamline was designed to meet two major requirements: vacuum isolation from the UHV in the storage ring and uniform illumination over the exposure area, to achieve good linewidth control in the resist pattern. To achieve uniform illumination in the vertical direction, we use a scanning grazing-incidence mirror that sweeps the beam over the exposure area. This mirror also collimates horizontally to maximize the exposure flux. In this paper we will describe the basic design including optics, beryllium window and the calculated spectral power distribution. The performance of the beamline will be discussed in terms of experimentally achieved uniformity of illumination, mask contrast and mask substrate transmission, exposure flux and exposure flux and exposure times and resolution. With PMMA resist, features down to 0.25 μm have been copied with an aspect ratio of 16 to 1 using a 40 μm mask to wafer separation. The ultimate resolution is limited by diffraction. We have begun to study a variety of resists to evaluate their suitability for X-ray lithography.