A silicon wafer is coated with 50 nm of LSN and windows are patterned on the backside of the wafer using photolithography and reactive ion etching. These windows are then etched through the silicon wafer by either KOH wet etching or Bosch-process directional reactive ion etching. Next the LSN is subjected to an organochlorosilane vapor, modifying the LSN surface to enhance polymer adhesion.
A poly(styrene)-poly(isoprene)-poly(lactide) (PS-PI-PLA) triblock terpolymer is then spin-coated onto this LSN surface and annealed under vacuum at 150 °C for 15 hrs. The PLA block self-assembles into cylinders perpendicular to the coating, nearly spanning it. The PLA cylinders are then removed with 0.05M NaOH in a 60:40 v/v water methanol solution, leaving densely packed 30 nm pores in the polymer film. The membrane is then subjected to reactive ion etching with an oxygen-free plasma for several minutes, during which time the nitride beneath the now-vacant cylinders is removed, while the rest of the LSN is protected by the polymer film. A subsequent oxygen plasma etch removes residual polymer, leaving a nanoporous silicon nitride film.
We have demonstrated effective patterning of high porosity silicon nitride films up to 50 nm thick. Such membranes could provide immuno-isolation without retarding small molecule transport and should integrate well with the growing number of BioMEMS devices under development.