Synchrotron radiation for lithography and optical materials processing

The need for deep sub-micron resolution has driven the development of various 'next generation' lithographic techniques, amongst which X-ray lithography (XRL) has emerged as a leading candidate. Although the relatively weak interaction between X-rays and optical elements and photoresists presents a major challenge, XRL techniques based on the collimated flux from a synchrotron storage ring are gaining in popularity. In particular, high aspect ratio structures with very smooth sidewalls can be produced by exposing relatively thick layers of resist with energetic X-rays (7-20 keV). These structural characteristics are particularly useful for the fabrication of photonic devices and a range of recent applications are reviewed. We also discuss the potential for the fabrication of Bragg gratings in optical waveguide devices by means of synchrotron radiation. Although it has been known since at least 1953 that the refractive index of silica can be modified by energetic radiation, it appears that synchrotron light has not yet been used to fabricate optical devices in this way. The refractive index modification has been attributed to a compaction process similar to that observed in silicate glasses exposed to UV radiation with energy greater than the 8 eV band gap. The compaction process offers a number of advantages for the fabrication of Bragg gratings and waveguides. Finally, we discuss some preliminary modelling results relevant to the direct writing of refractive index structures with synchrotron radiation