Studying stacked ultra deep X-ray lithography exposures: Preliminary results

The Center for Advanced Microstructures and Devices (CAMD) at Louisiana State University (LSU) supports one of the strongest programs in synchrotron radiation based microfabrication in particular, in deep X-ray lithography (DXRL) in the USA. For taller microstructures above 500μm height, a harder source has been made available at CAMD using a 5-pole 7T super-conducting wiggler that has been installed in one of the straight sections of the synchrotron ring. A beamline and exposure station designed for ultra deep X-ray lithography (UDXRL) has been constructed and connected to the wiggler. An in-air scanner system has been built and installed at the beamline in approximately 10m distance to the source point. The scanner system features a fully water-cooled mask and substrate assembly to allow accurate patterning of high aspect ratio microstructures. The performance of the entire exposure system has been qualified and being proved compatible to standard exposure tools.[1] Simultaneous exposure of a stack of four graphite substrates with 500μm thick PMMA resist layers illustrate the potential for a cost-effective mass production of LIGA microstructures at hard UDXLR sources. The samples have been exposed using a 600μm thick beryllium mask with 50μm gold absorber. Dose calculations for the stacked exposures and preliminary exposure results as well as measurements of patterning accuracy over structure height and structure quality are presented.