40 keV Shaped electron beam lithography for LIGA intermediate mask fabrication

High precision LIGA masks require a soft X-ray pattern transfer from intermediate masks by means of electron beam lithography. Such a process has been realized using an upgraded Leica ZBA 23 machine with an acceleration voltage of 40 kV. Three process variations of the developer system, so called GG developer which contains butoxy-ethoxy-ethanol as main component, for PMMA are investigated. The pattern accuracy meets an absolute tolerance of ±100 nm for features down to 2 µm with sidewall angles towards 90 degrees. However, the process offers pattern resolution down to 0.4 µm. The best results for absorber patterning have been achieved with gold seed layers. The main advantages of this technology are the stringent shape control with corner radii smaller than 0.5 µm. Using direct intermediate mask fabrication a fourteen-step process including photolithography can be replaced by a seven-step direct additive process onto mask blanks