Three-dimensional microfabrication in negative resist using printed masks

We present a process based on contact printed shadow masks for three-dimensional microfabrication of soft and sensitive overhanging membranes in SU-8. A metal mask composed of a 50 nm thick gold and 20 nm thick chromium film is transferred onto unexposed SU-8 from an elastomer stamp made of polydimethylsiloxane. The mask transfer is mediated by the strong bonding of the reactive chromium surface to the SU-8 and the low adhesion of the gold surface to the rubber stamp. The printed mask is embedded into the negative resist by layering an additional SU-8 film on the top. The mask protects buried resist from subsequent UV exposure enabling monolithic fabrication of cavities in SU-8. Unlike direct evaporation–deposition of a mask onto the SU-8, printing avoids high stress and radiation, thus preventing resist wrinkling and prepolymerization. The effective fabrication of soft, 4 µm thick and 100 µm long cantilevers integrated into a microfluidic system is demonstrated. The process yields very flat and well-defined membrane surfaces, paving the way for three-dimensional fabrication of lab-on-a-chip devices with integrated fine filters and highly sensitive cantilevered probes, valves and pressure sensors useful in research and diagnostics.