Streamlined cryogenic deep reactive ion etching protocol for hybrid micronozzle arrays

This paper describes a novel fabrication methodology for hybrid micronozzle arrays that markedly streamlines and simplifies process flow for cryogenic deep reactive ion etching (DRIE). Cryogenic DRIE utilizes SF6/O2-based high-density plasmas at cryogenic temperatures. A key innovation that we have developed and tested is the application of SU-8 negative resist as both the cryogenic etch mask, replacing hard masks, and as a means of defining micronozzle orifices. First, a thin layer of SU-8 is spun onto one side of the silicon wafer and is patterned to define the micronozzle exit orifices. Then a thick layer of SU-8 is spun onto the backside of wafer, aligned to the micro-patterns of the thin layer of SU-8 and is patterned to act as etch mask and define the micronozzle inlets. These parallel SU-8 coatings on the wafer simplify and shorten the fabrication process by eliminating multiple etching steps and mitigate common problems associated with wafer-wide etching rate non-uniformities and RIE lag. The potential benefits of the rapid cryogenic DRIE micronozzle array fabrication strategy include (1) accelerated throughput of micronozzle array fabrication, (2) enhanced feasibility of fabricating comparatively more complex and/or novel hybrid structures and (3) potential simplification of other through-silicon microfabrication processes.