Deep sub micron high aspect ratio polymer structures produced by hard X-ray lithography

Microfabrication using X-ray lithography is a well established process in many laboratories worldwide. Radiation spectra, mask technology and process conditions are optimized for patterning resist thicknesses of several hundred micrometers with lateral dimensions of a few microns. This article provides first results as to how far these technologies can be extended to form structures with sub-micrometer lateral dimensions in resist layers of a few micrometers thickness. Available equipment and processes of the 2.5 GeV electron storage ring ANKA and the process technology of the Institut fur Mikrostrukturtechnik (IMT) in Karlsruhe, Germany, have been applied. An X-ray mask with a 2.7 mum Ti-membrane and 20 mum thick Au-absorbers is used to expose 1.6 mum PMMA resist on silicon wafers. As the smallest features on the mask are 2 mum, a double exposure with an intermediate deliberate relative movement between mask and resist generated sub-micron resist test structures. Smallest feature sizes are between 300 and 400 nm. They are not yet limited by diffraction, but by different process conditions that lead to adhesion loss, resist cracking and surface rounding. It has been shown that an intermediate layer of 1.2 mum thick polyimide between resist and substrate significantly enhances adhesion and reduces resist cracking. Reducing the beam power impinging onto the sample from typically 21 to 0.4 W leads to a 50% reduction of the surface rounding at the top of the microstructures. To demonstrate the capability to pattern thicker resist layers, first samples with an increased resist thickness of 4.1 mum and an aspect ratio of up to 8 were processed. Finally, a metal mesh with 2 mum thick Au-absorbers and 900 nm hexagonal holes was applied to pattern showpieces of sub micron features using a reduced electron energy of 1.3 GeV.