Investigation of porous silicon properties for applications in microphotonics and electrochemical nanolithography

We have investigated the fundamental properties of porous silicon thin films and multilayered structures for technologically relevant uses in microphotonics and nanolithographic imaging. The fabrication of porous silicon is demonstrated to be simple, inexpensive and easily controlled. During the fabrication process the optical properties of porous silicon are tuned by the electrochemical etching parameters to yield a nanostructured dielectric film with controllable refractive index and thickness. By careful understanding of the destructive and constructive interference effects of a multilayered dielectric stack, a 1-dimensional photonic crystal is fabricated with nanoscale thickness. This dielectric stack, or Bragg mirror, exhibits unique reflective properties that are dependent on the individual layers and overall stack height. While large overall dimensions generally improve the optical performance of a Bragg mirror, it is sometimes not feasible to integrate large structures into microphotonic applications. Therefore, the process of constructing and optimizing nanoscale thickness porous silicon Bragg mirrors that are readily compatible with chip-platform technologies is addressed in this dissertation.;In particular, a method for reducing the spatial dimensions of porous silicon Bragg mirrors is investigated for effectiveness and potential applications. This method works by dry-removal soft-lithography using a robust, reusable elastomer stamp to pattern the surface of a porous silicon Bragg mirror into micron-sized dimensions. We utilized confocal microscopy integrated with white-light reflection spectroscopy to optically characterize these micro-patterned structures. Additionally, the patterning technique is further developed to transfer micro-patterned Bragg mirrors to a flexible polymer film. Collectively, this patterning technique produces optically functional porous silicon microstructures which can be investigated for potential applications in microphotonics.;In addition, the formation mechanism of porous silicon is utilized to image nanoscale hydrophilic domains of the polymer electrolyte membrane Nafion used in commercial fuel cells. The nanoscale imaging method works by electrochemical pore-mediated nanolithography of a silicon wafer to directly map the arrangement of hydrophilic channels at a silicon-Nafion layer interface. Specifically, the silicon surface undergoes electrochemical etching by HF electrolyte that diffuses through the hydrophilic domains for the overlying Nafion layer. Subsequent scan probe microscopy of the etched silicon surface reveals the distribution and size of the hydrophilic channel openings at the silicon/Nafion interface. Finally, using statistical analysis the average distance between features on a silicon surface etched through a Nafion mask, as well as the average distance between hydrophilic domains on the Nafion surface, is calculated. The ratio of these distances represents the connectivity of hydrophilic channels comprising the Nafion membrane.