Structure quality of high aspect ratio sub-micron polymer structures patterned at the electron storage ring ANKA

Abstract

We describe the fabrication of polymer structures with lateral dimensions in the sub-micron regime using hard x rays (λ≈0.4nm) from the electron storage ring ANKA. PMMA and Novolak resists have been analyzed with respect to development rates and contrast. Films with a thickness from 2μmto11μm have been patterned using a high resolution x-ray mask consisting of 2μm thick gold absorbers on a suspended silicon nitride membrane. The fabrication of those sub-micron x-ray lithography structures is confined by the mask absorber sizes of down to 400nm and by the process conditions. The yield of resist structures with aspect ratios of 9 and above is limited by bending of the structures. An intermediate buffer layer of polyimide enhances the resist adhesion and reduces cracking in the microstructures. Diffraction at mask absorber edges biases the resist feature size on the order of tens of nanometers. It may also result in surface attack of periodic resist structures if the proximity gap between mask and resist chosen is too high. Resist surfaces are subject to rounding which can locally diminish the sidewall verticality. PMMA structures have been successfully used as a template for electroplating of 1μm thick gold to demonstrate the fabrication capability of sub-micron scale metal parts.