(647b) Development of a Novel Technique for Functionalization of Nanoporous Silicon Membranes

A novel technique has been developed to assemble molecules with functional end groups inside pores of a nanoporous silicon membrane. A 20-um thick p-doped silicon membrane was anodized in HF electrolyte to produce 5-7 nm in diameter pores through the entire membrane thickness, with the exception of a 50-nm thick layer at the backside of the membrane that was later etched using reactive ion etching process. A recipe was developed to hydroxylate the surface of the pores. It was shown that a simple process of soaking the membrane in self-assembly solution, as done in prior studies, only results in self-assembly in a very shallow depth inside the pores. Thus, a new method was developed to conduct self-assembly down to the bottom of the pores (the aspect ratio of the pores is approximately four thousand). The molecule assembled on the pores' wall is (3-mercaptopropyl)trimethoxysilane (MPTS). The thiol end group of this molecule was later oxidized to sulfonate group in nitric acid. Penetration depth of the functional group inside the membrane was verified using Time of Flight-Secondary Ion Mass Spectroscopy (TOF-SIMS) and Auger Electron Spectroscopy (AES). In TOF-SIMS, a spot on the membrane was carved down gradually and its elemental composition was measured. The sulfur concentration was found to stay relatively constant through the membrane thickness. Analysis of the membrane cross section using AES also showed uniform coverage. The developed membrane is the first of its kind, with uniformly covered covalently bonded sulfonate groups, that can be used as proton exchange membrane (PEM). This development has a significant impact on silicon-based micro fuel cell technology where a silicon-based MEMS/CMOS compatible PEM membrane has been sought.